HOT STAMPED BODY

Info

Publication number: 20200230681
Type: Application
Filed: Feb 20, 2018
Publication Date: Jul 23, 2020
Applicant: NIPPON STEEL CORPORATION (Tokyo)
Inventors: Yuri TODA (Tokyo), Genki ABUKAWA (Tokyo), Daisuke MAEDA (Tokyo), Kazuo HIKIDA (Tokyo), Shingo FUJINAKA (Tokyo)
Application Number: 16/487,036

Abstract

The present invention, in consideration of the problems in the prior art, provides a hot stamped body simultaneously achieving the high bendability and high ductility for realizing impact resistance and also hydrogen embrittlement resistance and kept down in scattering in hardness. The hot stamped body according to the present invention is provided with a middle part in sheet thickness and a softened layer arranged at both sides or one side of the middle part in sheet thickness. The middle part in sheet thickness has a hardness of 500Hv to 800Hv and has metal structures from a depth of 20 μm below the surface of the softened layer to a depth of ½ of the thickness of the softened layer with an area rate of a total of crystal grains with a maximum crystal orientation difference inside the crystal grains of 1° or less and crystal grains with a maximum crystal orientation difference inside the crystal grains of 8° or more and 15° or less of 50% or more and less than 85%, when a region surrounded by grain boundaries having an orientation difference of 15° or more in a cross-section parallel to the sheet thickness direction is defined as a “crystal grain”.

Description

Description

FIELD

The present invention relates to a hot stamped body used for structural members or reinforcing members of automobiles or structures where strength is required, in particular a hot stamped body excellent in strength, impact resistance, ductility, and hydrogen embrittlement resistance after hot stamping and small in scattering in hardness.

BACKGROUND

In recent years, from the viewpoints of environmental protection and resource saving, lighter weight of automobile bodies is being sought. For this reason, application of high strength steel sheet to automobile members has been accelerating. However, along with the increase in strength of steel sheets, the formability deteriorates, and therefore in high strength steel sheets, formability into members with complicated shapes is a problem.

To solve this problem, hot stamping, where the steel sheet is heated to a high temperature of the austenite region, then is press-formed, is increasingly being applied. Since hot stamping performs press-forming and simultaneously quenching in the die, it is possible to obtain a strength corresponding to the C amount of the steel sheet. This is being taken note of as a technique achieving both formation of a material into an automobile member and securing strength.

However, since in conventional hot pressed parts which were produced by press hardening, the entire sheet thickness is formed by hard structures (mainly martensite), if bending deformation occurs at the time of collision of the automobile, the largest strain will be applied to the bent portion of the part, cracks will advance starting from the vicinity of the surface layer of the steel sheet, and finally fracture will easily be caused.

For example, in a conventional hat-shaped member or other hot stamped body produced by press hardening, if bending deformation occurs at the time of collision of an automobile, the hat-shaped member will buckle and thereby deformation will become localized and the load resistance of the member will fall. That is, the maximum load of a member of a hot stamped body is affected not only by the strength of the member, but also the ease of buckling. If the ductility of the steel sheet is high, in the state of a member formed into a certain shape, it becomes harder for localization of the deformation region to occur. That is, the member becomes resistant to buckling.

Further, in a hot stamped body, the way of contact with the die is not necessarily uniform. For example, at the vertical wall parts of a hat-shaped member etc., the cooling rate easily falls. For this reason, steel sheet is sometimes locally formed with regions with low hardnesses. Deformation concentrates in a local soft part at the time of collision and becomes a cause of cracking, so a small scattering in hardness of the body, that is, securing stable strength, is important in securing impact resistance.

Therefore, in a hot stamped part as well, ductility is important, but in general the ductility of martensite is low. Further, the density of lattice defects of the surface layer of the steel sheet is high, so there is the problem that penetration by hydrogen is promoted and the part becomes poor in hydrogen embrittlement resistance. Due to such reasons, hot stamped parts produced by press hardening have been limited in locations of use in auto parts.

To deal with this problem, art has been proposed for raising the deformability of hot pressed parts to suppress cracking. PTL 1 discloses making the hardness of the middle in sheet thickness of a hot pressed part 400 Hv or more and forming a softened layer with a thickness of 20 μm to 200 μm and a hardness of 300 Hv or less on a surface layer so as to secure a strength of a tensile strength of 1300 MPa or more while suppressing cracking at the time of automobile collision. PTL 2 discloses controlling the concentration of carbon at a surface layer in sheet thickness to ⅕ or less of the concentration of carbon of the middle part in sheet thickness so as to reduce the density of lattice defects of the surface layer and improve the hydrogen embrittlement resistance. PTL 3 discloses to make the middle part in sheet thickness a dual phase structure of ferrite and martensite and raise the structural fraction of ferrite of a surface layer portion so as to ease the stress even if the surface layer part receives severe bending deformation.

However, in the members described in PTL 1 and PTL 2, by making a surface layer portion in sheet thickness by soft structures and making a middle part in sheet thickness by hard structures, a sharp gradient in hardness ends up being formed in the sheet thickness direction. For this reason, when subjected to bending deformation, there is the issue that cracking easily occurs near the boundary between the soft structures and hard structures where this sharp gradient of hardness occurs. Further, in PTL 3, a surface layer portion in sheet thickness is made by soft structures and the middle part in sheet thickness is made by a dual phase structure of hard structures and soft structures so as to reduce the sharp gradient in hardness in the sheet thickness direction. However, since making the middle part in sheet thickness a dual phase structure, the upper limit of tensile strength ends up becoming 1300 MPa or so. It is difficult to secure the tensile strength of 1500 MPa or more sought for hot pressed parts.

CITATION LIST Patent Literature

[PTL 1] Japanese Unexamined Patent Publication No. 2015-30890

[PTL 2] Japanese Unexamined Patent Publication No. 2006-104546

[PTL 3] WO 2015/097882

SUMMARY Technical Problem

The present invention, in consideration of the technical issues in the prior art, has as its object to provide a hot stamped body achieving both a high bendability and high ductility for realizing impact resistance and hydrogen embrittlement resistance and keeping down the scattering in hardness.

Solution to Problem

The inventors engaged in an in-depth study of a method for solving the above technical issues. As a result, to improve the hydrogen embrittlement resistance, it is effective to reduce the density of lattice defects at the surface layer in sheet thickness. For this reason, it is necessary to form soft structures at the surface layer. On the other hand, to secure a 1500 MPa or more tensile strength, it is necessary to form the middle part in sheet thickness by only hard structures. In this way, the inventors thought that if forming the surface layer in sheet thickness by soft structures and forming the middle part in sheet thickness by hard structures, if it were possible to reduce the sharp gradient of hardness in the sheet thickness direction occurring near the boundary of the hard structures and soft structures, a strength of a tensile strength of 1500 MPa or more and excellent hydrogen embrittlement resistance could be secured while excellent bendability could be obtained.

Therefore, the inventors investigated and engaged in intensive studies on metal structures of steel sheets where good bendability was obtained by controlling the structures of a surface layer of soft structures. As a result, it was discovered that the metal structures forming the surface layer should be comprised of crystal grains with a maximum crystal orientation difference inside the crystal grains of 1° or less and crystal grains with a maximum crystal orientation difference inside the crystal grains of 8° or more and less than 15° when a region surrounded by grain boundaries having an orientation difference of 15° or more in the sheet thickness cross-section is defined as a “crystal grain”. These measurements were performed in the region from a position of a depth of 20 μm below the surface of the surface layer to a position of a depth of ½ of the thickness of the surface layer (center of surface layer). It was discovered that the effects of the surface properties of the hot stamped body and the effects of the transitional part from the middle part in sheet thickness to the surface layer can be eliminated by such metal structures.

Further, by controlling the amounts of addition of Mn and Si at the middle part in sheet thickness, the inventors raised the ductility and raised the hardenability to stably secure high strength. As a result, it is possible to keep down the occurrence of cracking at the time of bending deformation. The inventors succeeded in securing a 1500 MPa or more tensile strength and good hydrogen embrittlement resistance while realizing excellent bendability, ductility, and stability of strength and were able to obtain a hot stamped body excellent in impact resistance and hydrogen embrittlement resistance.

The present invention was completed based on the above discovery and has as its gist the following:

(1) A hot stamped body comprising a middle part in sheet thickness and a softened layer arranged at both sides or one side of the middle part in sheet thickness, wherein

the middle part in sheet thickness comprises, by mass %,

C: 0.20% or more and less than 0.70%,

Si: less than 3.00%,

Mn: 0.20% or more and less than 3.00%,

P: 0.10% or less,

S: 0.10% or less,

sol. Al: 0.0002% or more and 3.0000% or less,

N: 0.01% or less, and

a balance of Fe and unavoidable impurities, and has a hardness of 500 Hv or more and 800 Hv or less,

in the metal structures from a depth of 20 μm below the surface of the softened layer to a depth of ½ of the thickness of the softened layer, when defining a region surrounded by grain boundaries having a 15° or higher orientation difference in a cross-section parallel to the sheet thickness direction as a “crystal grain”, the area rate of the total of crystal grains with a maximum crystal orientation difference inside the crystal grains of 1° or less and crystal grains with a maximum crystal orientation difference inside the crystal grains of 8° or more and less than 15° is 50% or more and less than 85%,

the tensile strength is 1500 MPa or more.

(2) The hot stamped body according to (1), wherein the Si content is 0.50% or less and the Mn content is 0.20% or more and less than 1.50%.
(3) The hot stamped body according to (1), wherein the Si content is 0.50% or less and the Mn content is 1.50% or more and less than 3.00%.
(4) The hot stamped body according to (1), wherein the Si content is more than 0.50% and less than 3.00%, the Mn content is 0.20% or more and less than 1.50%, and the middle part in sheet thickness comprises, by area percent, 1.0% or more and less than 5.0% of residual austenite.
(5) The hot stamped body according to (1), wherein the Si content is more than 0.50% and less than 3.00%, the Mn content is 1.50% or more and less than 3.00%, and the middle part in sheet thickness comprises, by area percent, 1.0% or more and less than 5.0% of residual austenite.
(6) The hot stamped body according to any one of (1) to (5), where the middle part in sheet thickness further comprises, by mass %, Ni: 0.01% or more and 3.00% or less.
(7) The hot stamped body according to any one of (1) to (6), where the middle part in sheet thickness further comprises, by mass %, one or more of Nb: 0.010% or more and 0.150% or less, Ti: 0.010% or more and 0.150% or less, Mo: 0.005% or more and 1.000% or less, and B: 0.0005% or more and 0.0100% or less.
(8) The hot stamped body according to any one of (1) to (7), where a plated layer is formed on the softened layer.

Advantageous Effects of Invention

According to the present invention, it is possible to provide a hot stamped body excellent in bendability, ductility, impact resistance, and hydrogen embrittlement resistance and with small scattering in hardness.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a schematic view for explaining the diffusion of C atoms when producing a hot stamped body of the present invention.

FIG. 2 is a graph showing the change in dislocation density after a rolling pass relating to rough rolling used in the method for producing the hot stamped body of the present invention.

DESCRIPTION OF EMBODIMENTS (Structure of Hot Stamped Body According to Present Invention)

The hot stamped body according to the present invention is a structure with a softened layer arranged on the surface at both sides or one side. The softened layer has a region having a hardness 10 Hv or more lower than the hardness of the middle part in sheet thickness.

(Middle Part in Sheet Thickness)

The middle part in sheet thickness of the hot stamped body according to the present invention must have a hardness of 500 Hv to 800 Hv. The reasons for limiting the composition of constituents at the middle part in sheet thickness to make the hardness of the middle part in sheet thickness the above-mentioned range are explained below. Below, the % relating to the component of constituents means mass %.

(C: 0.20% or More and Less than 0.70%))

C is an important element for obtaining a 500 Hv to 800 Hv hardness at the middle part in sheet thickness. With less than 0.20%, it is difficult to secure 500 Hv or more at the middle part in sheet thickness, and therefore C is 0.20% or more. Preferably it is 0.30% or more. On the other hand, with more than 0.70%, the hardness of the middle part in sheet thickness exceeds 800 Hv and the bendability falls, and therefore C is 0.70% or less. Preferably, it is 0.50% or less.

(Si: Less than 3.00%)

Si is an element contributing to improvement of strength by solution strengthening. The amount of addition of Si for obtaining the effect of improvement of strength of the steel sheet by formation of a solid solution of Si in the metal structures is preferably 0.30% or more, but even if adding more than 0.5% of Si, the effect becomes saturated.

Si also has the effect of causing the formation of residual austenite and raising the ductility. To obtain this effect, addition of more than 0.50% is at least necessary. On the other hand, even if adding more than 3.00%, the effect becomes saturated, and therefore the amount of addition of Si is one with an upper limit of less than 3.00%. Preferably, the amount is less than 2.0%.

(Mn: 0.20% or More and Less than 3.00%)

Mn is an element contributing to improvement of strength by solution strengthening. The effect of improving the strength of the steel sheet by solid solution of Mn in the metal structures cannot be obtained with an amount of addition of less than 0.20%, so 0.20% or more is added. Preferably the content is 0.70% or more. On the other hand, even if adding 1.50% or more, the effect becomes saturated.

Mn, further, has the effect of raising the hardenability. By adding 1.50% or more, it is possible to raise the hardenability and stably obtain high strength. The preferable amount of addition for obtaining the effect of raising the hardenability is 1.70% or more. Even if adding 3.00% or more, the effect becomes saturated, and therefore the upper limit of the amount of addition of Mn is 3.00%. Preferably, the content is less than 2.00%.

(P: 0.10% or Less)

P is an element segregating at the grain boundaries and impairing the strength of the grain boundaries. If more than 0.10%, the strength of the grain boundaries remarkably falls and the hydrogen embrittlement resistance and bendability fall, and therefore P is 0.10% or less. Preferably, it is 0.05% or less. The lower limit is not particularly prescribed, but if reducing this to less than 0.0001%, the dephosphorizing cost greatly rises and the result becomes economically disadvantageous, so in practical steel sheet, 0.0001% is the substantive lower limit.

(S: 0.10% or Less)

S is an element forming inclusions. If more than 0.10%, inclusions are formed and the hydrogen embrittlement resistance and bendability fall, and therefore S is 0.10% or less. Preferably, it is 0.0025% or less. The lower limit is not particularly prescribed, but if reducing this to less than 0.0015%, the desulfurizing cost greatly rises and the result becomes economically disadvantageous, so in practical steel sheet, 0.0001% is the substantive lower limit.

(Sol. Al: 0.0002% or More and 3.0000% or Less)

Al is an element acting to deoxidize the molten steel and make the steel sounder. In the present invention, to obtain the deoxidizing action, the range of content of not all of the Al contained in the steel, but the content of so-called “acid soluble aluminum” (sol. Al) is prescribed. With a sol. Al content of less than 0.0002%, the deoxidizing is insufficient, and therefore sol. Al is 0.0002% or more. Preferably the content is 0.0010% or more. On the other hand, even if adding more than 3.0%, the effect becomes saturated, and therefore the content is 3.0000% or less.

(N: 0.01% or Less)

N is an impurity element and is an element which forms nitrides and impairs bendability. If more than 0.01%, coarse nitrides are formed and the bendability remarkably falls, and therefore N is 0.01% or less. Preferably the content is 0.0075% or less. The lower limit is not particularly prescribed, but if reducing this to less than 0.0001%, the denitriding cost greatly rises and the result becomes economically disadvantageous, so in practical steel sheet, 0.0001% is the substantive lower limit.

(Ni: 0.01% or More and 3.00% or Less)

Ni is an element contributing to improvement of strength by solution strengthening, so may be added as needed. With less than 0.010%, the effect is not obtained, so 0.010% or more is added. Preferably, the content is 0.5% or more. On the other hand, even if added in more than 3.00%, the effect becomes saturated, and therefore the content is 3.00% or less. Preferably, the content is 2.50% or less.

(Nb: 0.010% or More and 0.150% or Less)

Nb is an element contributing to improvement of strength by solution strengthening, so may be added as needed. With less than 0.010%, the effect is not obtained, so 0.010% or more is added. Preferably, the content is 0.035% or more. On the other hand, even if added in more than 0.150%, the effect becomes saturated, and therefore the content is 0.150% or less. Preferably, the content is 0.120% or less.

(Ti: 0.010% or More and 0.150% or Less)

Ti is an element contributing to improvement of strength by solution strengthening, so may be added as needed. With less than 0.010%, the effect is not obtained, and therefore the content is 0.010% or more. Preferably, the content is 0.020%. On the other hand, even if added in more than 0.150%, the effect becomes saturated, and therefore the content is 0.150% or less. Preferably, the content is 0.120% or less.

(Mo: 0.005% or More and 1.0% or Less)

Mo is an element contributing to improvement of strength by solution strengthening, so may be added as needed. With less than 0.005%, the effect is not obtained, and therefore the content is 0.005% or more. Preferably, the content is 0.0100% or more. On the other hand, even if added in more than 1.000%, the effect becomes saturated, and therefore the content is 1.000% or less. Preferably, the content is 0.800% or less.

(B: 0.0005% or More and 0.0100% or Less)

B is an element segregating at the grain boundaries and improving the strength of the grain boundaries, so may be added as needed. With less than 0.0005%, the effect of addition is not sufficiently obtained, so 0.0005% or more is added. Preferably, the content is 0.0010% or more. On the other hand, even if added in more than 0.0100%, the effect becomes saturated, and therefore the content is 0.0100% or less. Preferably, the content is 0.0075% or less.

The balance of the composition of constituents of the middle part in sheet thickness consists of Fe and unavoidable impurities. The unavoidable impurities are elements which unavoidably enter from the steel raw materials and/or in the steelmaking process and are allowed in ranges not impairing the characteristics of the hot stamped body of the present invention.

(Hardness of Middle Part in Sheet Thickness is 500 Hv or More and 800 Hv or Less)

If the hardness of the middle part in sheet thickness is 500 Hv or more, as the tensile strength of the hot stamped body of the present invention, 1500 MPa or more can be secured. Preferably, it is 600 Hv or more. On the other hand, if the hardness of the middle part in sheet thickness is more than 800 Hv, since the difference in hardness with the softened layer becomes too large and deterioration of the bendability is invited, 800 Hv is the upper limit. Preferably the hardness is 720 Hv or less.

The method of measurement of the hardness of the middle part in sheet thickness is as follows: A cross-section vertical to the sheet surface of the hot stamped body is taken to prepare a sample of the measurement surface. This is supplied to a hardness test. The method of preparing the measurement surface may be based on JIS Z 2244. For example, #600 to #1500 silicon carbide paper may be used to polish the measurement surface, then a solution of particle size 1 μm to 6 μm diamond powder dispersed in alcohol or another diluent or pure water may be used to finish the sample to a mirror surface. The hardness test may be performed by the method described in JIS Z 2244. A micro-Vickers hardness tester is used to measure 10 points at the ½ position of thickness of the hot stamped body by a load of 1 kgf and intervals of 3 times or more of the dents. The average value was defined as the hardness of the middle part in sheet thickness.

(Metal Structures at Middle Part in Sheet Thickness)

The middle part in sheet thickness can be improved in ductility by including residual austenite in an area percent of 1% or more. The area percent of residual austenite at the middle part in sheet thickness is preferably 2% or more. However, if making the area percent of the residual austenite 5% or more, since deterioration of the bendability is invited, the upper limit is less than 5.0%. Preferably, the fraction is less than 4.5%.

The area percent of the residual austenite can be measured by the following method. A sample is taken from a hot stamped member and ground down at its surface to a depth of ½ of the sheet thickness from the normal direction of the rolling surface. The ground down surface is used for X-ray diffraction measurement. From the image obtained by the X-ray diffraction method using Kα rays of Mo, the area rate Vγ of residual austenite can be determined using the following formula:

Vγ=(⅔){100/(0.7×α(211)/γ(220)+1)}+(⅓){100/(0.78×α(211)/γ(311)+1)}

Here, α(211) is the X-ray diffraction intensity at the (211) face of ferrite, γ(220) is the X-ray diffraction intensity at the (220) face of austenite, and γ(311) is the X-ray diffraction intensity at the (311) face of austenite.

(Softened Layer)

As explained above, in the present invention, the “softened layer” is the region in the sheet thickness direction of the cross-section of sheet thickness of the hot stamped body from the position where the hardness falls by 10 Hv or more from hardness of the middle part in sheet thickness (hardness at position of ½ of sheet thickness) to the surface of the stamped body.

(Metal Structures of Softened Layer)

The inventors investigated the metal structures of steel sheets where good bendability was obtained and as a result discovered that the metal structures forming the softened layer should be comprised of crystal grains with a maximum crystal orientation difference inside the crystal grains of 1° or less and crystal grains with a maximum crystal orientation difference inside the crystal grains of 8° or more and less than 15° when defining a region surrounded by grain boundaries having a 15° or higher orientation difference in a cross-section of sheet thickness as a “crystal grain”. These measurements were performed in the region from a position of a depth of 20 μm below the surface of the softened layer to a position of a depth of ½ of the thickness of the softened layer (center of softened layer). The inventors engaged in intensive studies and as a result discovered that from the viewpoint of the bendability and other effects, the fractions of structures from a position of 20 μm from the surface of the softened layer to a position of a depth of ½ of the thickness of the softened layer are important. The effects of the surface properties of the hot stamped body and the effects of the transitional part from the middle part in sheet thickness to the softened layer can be eliminated by such metal structures.

In the above-mentioned metal structures of the softened layer, the area rate of the total of crystal grains with a maximum crystal orientation difference inside the crystal grains of 1° or less and crystal grains with a maximum crystal orientation difference inside the crystal grains of 8° or more and less than 15° should be 50% or more, more preferably 55% or more. On the other hand, with an area rate of the total of the metal structures of the softened layer of 85% or more, the difference in hardness of the softened layer and the middle part in sheet thickness becomes too great and the effect of reduction of the sharp gradient of hardness in the sheet thickness direction occurring at the time of bending deformation cannot be obtained, and therefore the area rate is less than 85%. More preferably, it is 80% or less.

Between the position of a depth of ½ of the thickness of the softened layer (center of softened layer) to the middle part in sheet thickness, if the hardness at the sheet thickness middle part side of the softened layer (boundary with middle part in sheet thickness) is HvA and the hardness of the center of the softened layer is HvB, they are in the relationship of HvA−HvB≥10 Hv.

The method of determining the region from 20 μm below the surface of the softened layer to a position of ½ of the thickness of the softened layer will be explained below. A cross-section vertical to the surface of the hot stamped body being measured (cross-section of sheet thickness) is taken to prepare a sample of the measurement surface. This is used for a hardness test. The method of preparing the measurement surface may be based on JIS Z 2244. For example, #600 to #1500 silicon carbide paper may be used to polish the measurement surface, then a solution of particle size 1 μm to 6 μm diamond powder dispersed in alcohol or another diluent or pure water may be used to finish the sample to a mirror surface. The sample with the prepared measurement surface is measured two times based on the method described in JIS Z 2244 using a micro Vickers hardness tester. The first time measures the hardness from the region within 20 μm from the surface of the hot stamped body in the sheet thickness direction to the middle part in sheet thickness (position of ½ of sheet thickness) in the direction vertical to the surface (sheet thickness direction) by a load of 0.3 kgf at intervals of 3 times or more the dents. However, if there is a plated layer, this is measured from the region within 20 μm right under the plating or coating or the alloy layer of the plating or coating and material of the softened layer. The position where the hardness starts to drop by 10 Hv or more from the hardness of the middle part in sheet thickness (hardness at position of ½ of sheet thickness) is determined and the layer from that sheet thickness position to the surface of the hot stamped body is defined as the “softened layer”. If the softened layer is present at both surfaces, the second measurement is performed at the surface at the opposite side to the first one (back surface) by a similar method to determine the position where the hardness starts to drop by 10 Hv or more from the hardness of the middle part in sheet thickness.

Next, the method of calculating the area rates of metal structures of the softened layer will be explained. A sample is cut out from a hot stamped body to enable examination of a cross-section vertical to its surface (sheet thickness direction). The length of the sample depends on the measuring device, but may be about 50 μm. The region in the sheet thickness direction of the sample from the surface of the softened layer to the position of ½ of the thickness of the softened layer (center of softened layer) is analyzed at 0.2 μm measurement intervals by EBSD to obtain information on the crystal orientation. Here, this EBSD analysis is performed using an apparatus comprised of a thermal field emission type scan electron microscope (JSM-7001F made by JEOL) and EBSD detector (DVCS type detector made by TSL) at an analysis speed of 200 to 300 points/second.

Next, based on the obtained crystal orientation information, a region surrounded by grain boundaries having an orientation difference of 15° or more is defined as one crystal grain and a crystal orientation map in the sheet surface direction is prepared. The obtained crystal orientation map is used to find the crossing points of the long axis of one crystal grain and the crystal grain boundaries. Among the two crossing points, one is designated as the starting point and the other is designated as the end point and the difference in orientation among all measurement points contained on the long axis of the crystal grain is calculated. The maximum value of the orientation difference obtained was defined as the maximum crystal orientation difference at that crystal grain. The above analysis was performed for all crystal grains included in the measurement region, then the average of these values was defined as the maximum crystal orientation difference inside a region surrounded by grain boundaries of 15° or more.

The above-defined maximum crystal orientation difference can be simply calculated, for example, if using the “Inverse Pole Figure Map” and “Profile Vector” functions included in the software (OIM Analysis®) attached to the EBSD analysis system. With the “Inverse Pole Figure Map” function, it is possible to draw grain boundaries having slants of 15° or more as large angle grain boundaries and further possible to prepare a crystal orientation map in the sheet surface direction. With the “Profile Vector” function, it is possible to calculate the misorientation angle (difference in crystal orientations) between all measurement points included on any line. All crystal grains contained in the measurement region (crystal grains at end parts of measurement region not included) are analyzed as explained above and the area rate of the total of the crystal grains with a maximum crystal orientation difference inside the regions surrounded by grain boundaries of 15° or more of 1° or less and the crystal grains with a crystal orientation difference of 8° or more and less than 15° is calculated. If the softened layer is formed on both surfaces, the above procedure is performed at the back surface side of the hot stamped body as well and the average value of the area rates obtained from the front surface side and the back surface side is employed.

(Composition of Softened Layer)

The composition of the softened layer is not particularly limited other than regarding the unavoidable impurity elements of P, S, and N impairing the strength and/or bendability, but the layer is preferably the following composition so as to secure the strength of the hot stamped body and steel exhibiting excellent bendability.

In the composition of the softened layer, one or more of the C content, Si content, and Mn content are preferably respectively 0.6 time the corresponding contents of elements of the middle part in sheet thickness. The preferable ranges of the constituents in this case are as follows:

(C: 0.05% or More and Less than 0.42%)

C may be added in 0.05% or more so as to raise the strength. From the viewpoint of raising the load resistance as a member and improving the impact characteristics, preferably the content is 0.10% or more. To make the hardness of the softened layer lower than the hardness of the middle part in sheet thickness, it is preferable to make the content smaller than the middle part in sheet thickness. For this reason, the preferable C content of the softened layer is less than 0.42%. Preferably the content is 0.35% or less.

(Si: Less than 2.00%)

Si is an element contributing to improvement of strength by solution strengthening, so is added for raising the strength. However, to make the hardness of the softened layer lower than the hardness of the middle part in sheet thickness, it is preferable to make this smaller in content than the middle part in sheet thickness.

If the Si content of the middle part in sheet thickness is 0.50% or less, the preferable Si content of the softened layer is 0.30% or less, more preferably 0.20% or less. Further, if the Si content of the middle part in sheet thickness is more than 0.50% and less than 3.00%, the preferable Si content of the softened layer is less than 2.00%, more preferably 1.50% or less.

(Mn: 0.12% or More and Less than 1.80%)

Mn is an element contributing to improvement of strength by solution strengthening, so may be added in 0.12% or more for raising the strength. However, to make the hardness of the softened layer lower than the hardness of the middle part in sheet thickness, it is preferably smaller in content than the middle part in sheet thickness.

If the Mn content at the middle part in sheet thickness is 0.20% to less than 1.50%, the preferable Mn content of the softened layer is less than 0.90%, more preferably is 0.70% or less. Further, if the Mn content of the middle part in sheet thickness is 1.50% to less than 3.00%, the preferable Mn content of the softened layer is less than 1.80%, preferably 1.40% or less.

(P: 0.10% or Less)

P is an element segregating at the grain boundaries and impairing the strength of the grain boundaries. If more than 0.10%, the strength of the grain boundaries remarkably falls and the hydrogen embrittlement resistance and bendability fall, and therefore P is 0.1% or less. Preferably, it is 0.05% or less. The lower limit is not particularly prescribed, but if reducing this to less than 0.0001%, the dephosphorizing cost greatly rises and the result becomes economically disadvantageous, so in practical steel sheet, 0.0001% is the substantive lower limit.

(S: 0.10% or Less)

S is an element forming inclusions. If more than 0.10%, inclusions are formed and the hydrogen embrittlement resistance and bendability fall, and therefore S is 0.10% or less. Preferably, it is 0.0025% or less. The lower limit is not particularly prescribed, but if reducing this to less than 0.0015%, the desulfurizing cost greatly rises and the result becomes economically disadvantageous, so in practical steel sheet, 0.0001% is the substantive lower limit.

(Sol. Al: 0.0002% or More and 3.0000% or Less)

Al is an element acting to deoxidize the molten steel and make the steel sounder. In the present invention, to obtain this deoxidizing action, the range of content of not all of the Al contained in the steel, but the so-called “acid soluble aluminum” (sol. Al) is prescribed. With a sol. Al content of less than 0.0002%, the deoxidizing is insufficient, and therefore the sol. Al is preferably 0.0002% or more. More preferably the content is 0.0010% or more. On the other hand, even if adding more than 3.0000%, the effect becomes saturated, and therefore the content is 3.0000% or less.

(N: 0.01% or Less)

N is an impurity element and is an element which forms nitrides and impairs bendability. If more than 0.01%, coarse nitrides are formed and the bendability remarkably falls, and therefore N is 0.01% or less. Preferably the content is 0.0075% or less. The lower limit is not particularly prescribed, but if reducing this to less than 0.0001%, the denitriding cost greatly rises and the result becomes economically disadvantageous, so in practical steel sheet, 0.0001% is the substantive lower limit.

Regarding the constituents of the softened layer, one or more of the C content, Si content, and Mn content are preferably respectively 0.6 time or less the C content, Si content, and Mn content of the middle part in sheet thickness. Other than the upper limits of the unavoidable impurity elements of P, S, and N impairing the strength and/or bendability being prescribed, the other constituents are not particularly limited. In general, the softened layer may optionally and selectively include one or more of the following constituents besides C, Si, and Mn.

(Ni: 0.01% or More and 3.00% or Less)

Ni is an element contributing to improvement of strength by solution strengthening, so may be added as needed. With less than 0.01%, the effect is not obtained, and therefore preferably 0.01% or more is added. More preferably, the content is 0.50% or more. On the other hand, even if added in more than 3.00%, the effect becomes saturated, and therefore the content is 3.00% or less. Preferably, the content is 2.50% or less.

(Nb: 0.010% or More and 0.150% or Less)

Nb is an element contributing to improvement of strength by solution strengthening, so may be added as needed. With less than 0.010%, the effect is not obtained, so preferably 0.010% or more is added. More preferably, the content is 0.035% or more. On the other hand, even if added in more than 0.150%, the effect becomes saturated, and therefore the content is 0.150% or less. Preferably, the content is 0.120% or less.

(Ti: 0.010% or More and 0.150% or Less)

Ti is an element contributing to improvement of strength by solution strengthening, so may be added as needed. With less than 0.010%, the effect is not obtained, and therefore preferably the content is 0.010% or more. More preferably, the content is 0.020%. On the other hand, even if added in more than 0.150%, the effect becomes saturated, and therefore the content is 0.150% or less. Preferably, the content is 0.120% or less.

(Mo: 0.005% or More and 1.000% or Less)

Mo is an element contributing to improvement of strength by solution strengthening, so may be added as needed. With less than 0.005%, the effect is not obtained, and therefore preferably the content is 0.005% or more. More preferably, the content is 0.010% or more. On the other hand, even if added in more than 1.000%, the effect becomes saturated, and therefore the content is 1.000% or less. Preferably, the content is 0.800% or less.

(B: 0.0005% or More and 0.0100% or Less)

B is an element segregating at the grain boundaries and improving the strength of the grain boundaries, so may be added as needed. With less than 0.0005%, the effect of addition is not sufficiently obtained, and therefore preferably 0.0005% or more is added. More preferably, the content is 0.0010% or more. On the other hand, even if added in more than 0.0100%, since the effect becomes saturated, the content is 0.0100% or less. Preferably, the content is 0.0075% or less.

(Cross-Sectional Distribution of Hardness of Hot Stamped Body)

At the cross-section vertical to the surface of the hot stamped body, the distribution of hardness at the middle part in sheet thickness is preferably uniform with no scattering. In a hat-shaped structure, at the vertical wall parts, contact with the die is difficult and the cooling rate becomes low, so sometimes the hardness falls. If there is a region where the hardness falls by 100 Hv or more from the average hardness of the cross-section vertical to the longitudinal direction of the hat-shaped member, at the time of impact, the deformation will concentrate at the softened part and the part will fracture early, so a high impact resistance cannot be obtained. For this reason, there must not be a point with a hardness more than 100 HV below the average value of the distribution of hardness in the cross-section vertical to the surface of the hot stamped body (below, referred to as the “average hardness of cross-section”). The distribution of hardness at the cross-section and the average hardness of the cross-section are obtained by obtaining a cross-section vertical to the longitudinal direction of a long hot stamped body at any position in the longitudinal direction and measuring the Vickers hardness between the end parts of the cross-section at equal intervals of 1 mm pitch or less at the middle position of sheet thickness of the entire cross-sectional region including the vertical walls using a Vickers hardness tester (load of 1 kgf).

(Formation of Plated Layer)

The surface of the softened layer may be formed with a plated layer for the purpose of improving the corrosion resistance. The plated layer may be either an electroplated layer or a hot dip coated layer. An electroplated layer includes, for example, an electrogalvanized layer, electro Zn—Ni alloy plated layer, etc. As a hot dip coated layer, a hot dip galvanized layer, a hot dip galvannealed layer, a hot dip aluminum coated layer, a hot dip Zn—Al alloy coated layer, a hot dip Zn—Al—Mg alloy coated layer, a hot dip Zn—Al—Mg—Si alloy coated layer, etc., may be mentioned. The amount of deposition of the layer is not particularly limited and may be a general amount of deposition.

(Method of Production of Hot Stamped Body According to Present Invention)

Next, the method of production for obtaining the hot stamped body according to the present invention will be explained, but the present invention is not limited to the form of the double layer steel sheet explained below.

As one embodiment of the method of production of the present invention, first, a steel sheet satisfying the requirements of the composition of constituents of the middle part in sheet thickness explained above is ground down at its front surface and/or back surface to remove surface oxides, then a steel sheet for softened layer formation use (below, referred to as a “steel sheet for surface layer”) is superposed on each ground down surface side. The method of joining the steel sheet for surface layer and the steel sheet for sheet thickness middle part is not particularly limited, but they may be joined by arc welding. A steel sheet for surface layer wherein one or more of the C content, Si content, and Mn content are 0.6 time or less the content of the corresponding element of the steel sheet for sheet thickness middle part is preferably superposed.

Further, by controlling the casting rate to ton/min or more in the continuous casting process of the steel sheet for surface layer, it is possible to keep down microsegregation of Mn in the steel sheet for surface layer and possible to make the distribution of concentration of Mn at the steel sheet for surface layer uniform. Mn raises the yield strength of austenite to thereby affect the behavior in formation of grain boundaries in the transformed structures, so when defining a region surrounded with grain boundaries having orientation differences of 15° or more as a “crystal grain”, it has the effect of promoting the formation of crystal grains with a maximum crystal orientation difference inside the crystal grains of 8° or more and less than 15°. For this reason, it is also possible to control the casting rate to 6 ton/min or more in the continuous casting process of steel sheet for surface layer for the purpose of promoting the formation of the above microstructures.

Further, a double layer steel sheet fabricated by the above method is preferably held at 1100° C. or more and 1350° C. or less in temperature for 20 minutes to less than 60 minutes. The held sheet is preferably used as the steel sheet for hot stamped body according to the present invention. The inventors studied this and as a result learned that by performing heat treatment holding the steel sheet at 1100° C. or more and 1350° C. or less for 20 minutes to less than 60 minutes, in the metal structures in the region from a position of a depth of 20 μm below the surface of the softened layer to the center of the softened layer, the area rate of the total of crystal grains with a maximum crystal orientation difference inside the crystal grains of 1° or less and crystal grains with a maximum crystal orientation difference inside the crystal grains of 8° or more and less than 15° becomes 50% to less than 85% when a region surrounded by grain boundaries having an orientation difference of 15° or more is defined as a “crystal grain” and that excellent bendability and hydrogen embrittlement resistance can be obtained.

The multilayer member produced by the above method of production (double layer steel sheet) can be treated by hot rolling, cold rolling, hot stamping, continuous hot dip coating, etc., to obtain the hot stamped body according to the present invention.

The hot rolling may be hot rolling performed under usual conditions. For example, the finishing temperature may also be in the temperature range of 810° C. or more. The subsequent following cooling conditions also do not have to be particularly prescribed. The steel sheet is coiled in the temperature region of 750° C. or less. Further, it may be reheated for the purpose of softening the double layer steel sheet after hot rolling.

Further, to promote more the formation of the middle part in sheet thickness, the hot rolling after the above heat treatment of the double layer steel sheet preferably includes rough rolling and finish rolling with the rough rolling being performed twice under conditions of a temperature of 1100° C. or more, a sheet thickness reduction rate per pass of 5% or more and less than 50%, and a time between passes of 3 seconds or more.

Specifically, to promote more the formation of the middle part in sheet thickness in the present invention, the concentrations of alloy elements, in particular C atoms, have to be controlled to become more moderately distributed. The distribution of concentration of C is obtained by diffusion of C atoms. The diffusion frequency of C atoms increases the higher the temperature. Therefore, to control the C concentration, control in the rough rolling from the hot rolling heating becomes important. In hot rolling heating, to promote the diffusion of C atoms, the heating temperature has to be high. Preferably, it is 1100° C. or more and 1350° C. or less, more preferably more than 1150° C. and 1350° C. or less. With hot rolled heating, the changes of (i) and (ii) shown in FIG. 1 occur. (i) shows the diffusion of C atoms from the middle part in sheet thickness to the surface layer, while (ii) shows the decarburization reaction of C being desorbed from the surface layer to the outside. A distribution occurs in the concentration of C due to the balance between this diffusion of C atoms and the desorption reaction of (i) and (ii). With less than 1100° C., the reaction of (i) is insufficient, so the preferable distribution of the concentration of C cannot be obtained. On the other hand, with more than 1350° C., the reaction of (ii) excessively occurs, so similarly a preferable distribution of concentration cannot be obtained.

After adjusting the hot rolling heating temperature to obtain the preferable distribution of concentration of C, to obtain a further optimum distribution of concentration of C, pass control in rough rolling becomes extremely important. Rough rolling is performed two times or more under conditions of a rough rolling temperature of 1100° C. or more, a sheet thickness reduction rate per pass of 5% or more and less than 50%, and a time between passes of 3 seconds or more. This is so as to promote the diffusion of C atoms of (i) in FIG. 1 by the strain introduced in the rough rolling. Even if using an ordinary method to rough roll and finish roll a slab controlled in concentration of C to a preferable state by hot rolling heating, the sheet thickness will be reduced without the C atoms sufficiently diffusing in the surface layer. Therefore, if manufacturing hot rolled steel sheet of a thickness of several mm from a slab having a thickness more than 200 mm through a general hot rolling process, the result will be a steel sheet changing rapidly in concentration of C at the surface layer. A moderate hardness change will no longer be able to be obtained. The method discovered to solve this is the above pass control of the rough rolling. The diffusion of C atoms is greatly affected by not only the temperature, but also the strain (dislocation density). In particular, compared with lattice diffusion, with dislocation diffusion, the diffusion frequency becomes 10 times or more higher, so steps have to be taken to leave the dislocation density while rolling to reduce the sheet thickness. Curve 1 of FIG. 2 shows the change in the dislocation density after a rolling pass in the case where the sheet thickness reduction rate per pass in the rough rolling is small. It will be understood that strain remains over a long time period. By causing strain to remain at the surface layer over a long time period in this way, C atoms sufficiently diffuse in the surface layer and the optimum distribution of concentration of C can be obtained. On the other hand, curve 2 shows the change in dislocation density in the case where the sheet thickness reduction rate is large. If the amount of strain introduced by the rolling rises, recovery is easily promoted and the dislocation density rapidly falls. For this reason, to obtain the optimal distribution of concentration of C, it is necessary to prevent the occurrence of a change in dislocation density like the curve 2. From such a viewpoint, the upper limit of the sheet thickness reduction rate per pass becomes less than 50%. To promote the diffusion of C atoms at the surface layer, certain amounts of dislocation density and holding time have to be secured, so the lower limit of the sheet thickness reduction rate becomes 5%. As the time between passes, 3 seconds or more has to be secured.

The cold rolling may be cold rolling performed by a usual rolling reduction, for example, 30 to 90%. The hot rolled steel sheet and the cold rolled steel sheet include steel sheets as hot rolled and cold rolled and also steel sheets obtained by recrystallization annealing hot rolled steel sheet or cold rolled steel sheet under usual conditions and steel sheets obtained by skin pass rolling under usual conditions.

The heating, shaping, and cooling steps at the time of hot stamping may also be performed under usual conditions. For example, hot rolled steel sheet obtained by uncoiling hot rolled steel sheet coiled in the hot rolling step, cold rolled steel sheet obtained by uncoiling and cold rolling coiled hot rolled steel sheet, or steel sheet obtained by plating or coating cold rolled steel sheet, heating this by a 0.1° C./s to 200° C./s heating rate up to 810° C. or more and 1000° C. or less in temperature, and holding it at this temperature is formed into the required shape by the usual hot stamping.

The holding time may be set according to the mode of forming, so is not particularly limited. For example, if 30 seconds or more and 600 seconds or less, a good hot stamped body is cooled to room temperature.

The cooling rate may also be set to a usual condition. For example, the average cooling rate in the temperature region from the heating temperature to more than 400° C. may be 50° C./s or more. In the case of steel sheet with an Si content at the middle part in sheet thickness of more than 0.50% and less than 3.00% and an Mn content at the middle part in sheet thickness of 0.20% or more and less than 1.50% and steel sheet with an Si content at the middle part in sheet thickness of more than 0.50% and less than 3.00% and an Mn content at the middle part in sheet thickness of 1.50% or more and less than 3.00%, for the purpose of increasing the amount of formation of residual austenite to improve the ductility, it is preferable to control the average cooling rate at the cooling after heating and holding at the 200° C. to 400° C. temperature region to less than 50° C./s.

Further, for the purpose of adjusting the strength etc., it is possible to temper the body cooled down to room temperature in the range of 150° C. to 600° C.

In the method of production of the hot stamped body of the above-mentioned embodiment, the middle part in sheet thickness and the softened layer were configured by separate steel sheets. However, the hot stamped body of the present invention is not limited to double layer steel sheet comprised of two of the above-mentioned steel sheets superposed. The middle part in sheet thickness and the softened layer may be formed inside a single material steel sheet. For example, it is possible to treat a single layer steel sheet to decarburize it and soften the surface layer part to thereby produce high strength steel sheet comprised of a softened layer and a middle part in sheet thickness.

EXAMPLES

Next, examples of the present invention will be explained, but the conditions in the examples are just illustrations of conditions employed for confirming the workability and advantageous effects of the present invention. The present invention is not limited to the illustration of conditions. The present invention can employ various conditions so long as not departing from the gist of the present invention and achieving the object of the present invention.

Manufacturing Example A

The Nos. 1 to 18 steel sheets for sheet thickness middle part having the chemical compositions shown in Table A-1-1 (in the table, “Steel Nos. 1 to 18”) were ground down at their surfaces to remove the surface oxides. After that, the respective steel sheets for sheet thickness middle part were welded with steel sheets for surface layer having the chemical compositions shown in Table A-1-2 at both surfaces or single surfaces by arc welding to fabricate the Nos. 1 to 43 multilayer steel sheets for hot stamped body. The total of the sheet thicknesses of the steel sheet for surface layer and the steel sheet for sheet thickness middle part after arc welding is 200 mm to 300 mm and the thickness of the steel sheet for surface layer is ⅓ or so of the thickness of the steel sheet for sheet thickness middle part (¼ or so in case of single side). The No. 37 multilayer steel sheet is steel with the steel sheet for surface layer welded to only one surface. In the Nos. 1 to 43 multilayer steel sheets of Table A-1-1 to Table A-1-2, ones with a steel sheet for sheet thickness middle part not satisfying the requirement for composition of the middle part in sheet thickness of the hot stamped body according to the present invention are indicated as “comparative steel” in the remarks column.

The Nos. 1 to 43 multilayer steel sheets were respectively treated under the conditions of the Nos. 1 to 43 manufacturing conditions shown in Table A-2-1 to Table A-2-2 by heat treatment before hot rolling, rough rolling, hot rolling, and cold rolling to obtain steel sheets. Next, the steel sheets were heat treated as shown in Table A-2-1 and Table A-2-2 (in the tables, “heat treatment of hot stamped body”) for hot stamping to manufacture the Nos. 1A to 43A hot stamped bodies (“stamped bodies” of Table A-3). Further, the Nos. 35A and 36A hot stamped bodies were coated on a hot dip coating line at the surfaces with 120 to 160 g/m²amounts of aluminum.

In the tables, the item “sheet thickness reduction rate” of the “rough rolling” means the sheet thickness reduction rate per pass of the rough rolling. The item “number of rolling operations” means the number of rolling operations under the conditions of a time between passes of 3 seconds or more. Further, the item in the tables of “heating rate (° C./s)” means the rate of temperature rise until reaching the heating temperature of the “heat treatment at the time of hot stamping” after the cold rolling process. Further, in the tables, the item “heating temperature (° C.)” of the “heat treatment at the time of hot stamping” is the temperature at the time of hot stamping, the “average cooling rate (° C./s) (more than 400° C.)” means the average cooling rate (° C./s) in the temperature region from the heating temperature to more than 400° C., and the “average cooling rate (° C./s) (400° C. or less)” means the average cooling rate (° C./s) in the temperature region from 200° C. to 400° C. Further, in the tables, the fields with the notations “-” indicate no corresponding treatment performed.

Table A-3 shows the metal structures and characteristics of the Nos. 1A to 43A hot stamped bodies. The constituents obtained by analyzing the positions of ½ of the sheet thicknesses of the samples taken from the hot stamped bodies and positions of 20 μm from the surfaces of the softened layers were equivalent to the constituents of the steel sheets for sheet thickness middle part and steel sheets for surface layer of the Nos. 1 to 43 multilayer steel sheets of Table A-1-1 to Table A-1-2.

The metal structures of the hot stamped steel sheets were measured by the above-mentioned method. The hardness of the steel sheet for sheet thickness middle part forming the middle part in sheet thickness and the area rate of the total of the crystal grains with a maximum crystal orientation difference inside the regions surrounded by grain boundaries of 15° or more of 1° or less and the crystal grains with a crystal orientation difference of 8° or more and less than 15° in the metal structures from the surface of the steel sheet for surface layer forming the softened layer to ½ of the thickness were calculated. The calculated values of the area rate are shown in the item “area rate (%) of total of crystal grains with maximum crystal orientation difference inside large angle grain boundaries of 1° or less and crystal grains with maximum crystal orientation difference of 8° or more and less than 15°” of Table A-3.

Further, a tensile test of the hot stamped body was performed. The results are shown in Table A-3. The tensile test was performed by preparing a No. 5 test piece described in JIS Z 2201 and following the test method described in JIS Z 2241.

The hydrogen embrittlement resistance of the hot stamped body was evaluated using a test piece cut out from the stamped body. In general, a hot stamped body is joined with other parts using spot welding or another joining method. Depending upon the precision of the shape of the part, the hot stamped body will be subjected to twisting and stress will be applied. The stress differs depending on the position of the part. Accurately calculating this is difficult, but if there is no delayed fracture at the yield stress, it is believed there is no problem in practical use. Therefore, a sheet thickness 1.2 mm×width 6 mm×length 68 mm test piece was cut out from the stamped body, a strain corresponding to the yield stress was imparted in a four-point bending test, then the body was immersed in pH3 hydrochloric acid for 100 hours. The presence of any cracking was used to evaluate the hydrogen embrittlement resistance. A case of no cracking was indicated as passing (“good”) and a case with cracking was indicated as failing (“poor”).

For the purpose of evaluating the impact resistance of the hot stamped body, the body was evaluated based on the VDA standard (VDA238-100) prescribed by the German Association of the Automotive Industry under the following measurement conditions. In the present invention, the displacement at the time of maximum load obtained in the bending test was converted to angle by the VDA standard to find maximum bending angle and thereby evaluate the impact resistance of the hot stamped body.

Test piece dimensions: 60 mm (rolling direction)×60 mm (direction vertical to rolling) or 30 mm (rolling direction)×60 mm (direction vertical to rolling)

Bending ridgeline: direction perpendicular to rolling

Test method: roll support, punch pressing

Roll diameter: φ30 mm

Punch shape: tip R=0.4 mm

Distance between rolls: 2.0×sheet thickness (mm)+0.5 mm

Indentation rate: 20 mm/min

Tester: SHIMAZU AUTOGRAPH 20 kN

If the tensile strength is 1500 MPa or more, the maximum bending angle (°) was 70(°) or more, and the hydrogen embrittlement resistance was a passing level, it was judged that the impact resistance and hydrogen embrittlement resistance were excellent and the case was indicated as an “invention example”. If even one of the three aspects of performance is not satisfied, the case was indicated as a “comparative example”.

In each hot stamped body of the invention examples, the area rate of the total of the crystal grains with a maximum crystal orientation difference inside the regions surrounded by grain boundaries of 15° or more of 1° or less and the crystal grains with a crystal orientation difference of 8° or more and less than 15° in the metal structures from the surface of the steel sheet for surface layer to ½ of the thickness was 50% to less than 85%. Further, each hot stamped body of the invention examples was excellent in tensile strength, bendability, and hydrogen embrittlement resistance.

As opposed to this, the No. 5A hot stamped body was low in carbon content of the steel sheet for sheet thickness middle part, so the hardness of the middle part in sheet thickness became insufficient and the tensile strength became insufficient. The No. 9A hot stamped body was excessive in carbon content of the steel sheet for sheet thickness middle part, so the hardness of the middle part in sheet thickness became excessive and the targeted bendability could not be obtained. Further, the No. 11A hot stamped body was low in Mn content at the steel sheet for sheet thickness middle part, so the hardness of the middle part in sheet thickness became insufficient and the tensile strength became insufficient.

The Nos. 30A to 32A hot stamped bodies are comparative examples produced using the multilayer steel sheets for hot stamped body to which the desirable heat treatment had not been applied before the hot stamping process. The No. 30A hot stamped body was low in heat treatment temperature before the hot stamping process, while the No. 31A hot stamped body was short in heat treatment time before the hot stamping process, so in the metal structures from the surface of the softened layer to ½ of the thickness, the soft structures and metal structures with intermediate hardnesses insufficiently grew and the target bendability could not be obtained. Further, the No. 32A hot stamped body was excessively high in heat treatment temperature before the hot stamping process, so the effect of reduction of the sharp gradient in hardness in the sheet thickness direction occurring at the time of bending deformation could not be obtained.

The No. 40A hot stamped body was low in rolling temperature of the rough rolling. Further, the No. 41A hot stamped body was low in sheet thickness reduction rate of the rough rolling. Further, the No. 42A hot stamped body was low in number of rolling operations under conditions of a time between passes of 3 seconds or more. These hot stamped bodies were not manufactured under the suitable rough rolling conditions, so the soft structures and metal structures with intermediate hardnesses insufficiently grew, it was not possible to ease the strain occurring due to bending deformation, and the targeted bendability could not be obtained.

The No. 43A hot stamped body is a steel sheet controlled in casting rate to 6 ton/min or more in the continuous casting process of steel sheet for surface layer. It can raise the area rate (%) of the total of the crystal grains with a maximum crystal orientation difference inside the regions surrounded by grain boundaries of 15° or more of 1° or less and the crystal grains with a crystal orientation difference of 8° or more and less than 15° in the metal structures from the surface of the steel sheet for surface layer to ½ of the thickness and is excellent in bendability.

TABLE A-1-1 Multilayer Chemical constituents of steel sheet for sheet thickness middle part (mass %) steel sheet Steel no. no. C Si Mn P S sol.Al N Ni Nb Ti Mo B Remarks 1 1 0.25 0.24 1.22 0.016 0.0025 0.047 0.0037 0 0 0 0 0 2 2 0.30 0.23 1.25 0.015 0.0007 0.039 0.0036 0 0 0 0 0 3 3 0.38 0.13 1.31 0.011 0.0010 0.045 0.0033 0 0 0 0 0 4 4 0.45 0.17 1.35 0.009 0.0001 0.040 0.0028 0 0 0 0 0 5 5 0.12 0.17 1.31 0.016 0.0011 0.037 0.0039 0 0 0 0 0 Comp. steel 6 6 0.23 0.17 1.28 0.015 0.0012 0.043 0.0037 0 0 0 0 0 7 7 0.33 0.12 1.27 0.006 0.0014 0.037 0.0032 0 0 0 0 0 8 8 0.32 0.11 1.34 0.007 0.0016 0.049 0.0033 0 0 0 0 0 9 9 0.82 0.14 1.26 0.015 0.0007 0.043 0.0027 0 0 0 0 0 Comp. steel 10 10 0.35 0.43 1.29 0.010 0.0020 0.049 0.0028 0 0 0 0 0 11 11 0.30 0.22 0.07 0.016 0.0016 0.042 0.0041 0 0 0 0 0 Comp. steel 12 12 0.29 0.22 0.76 0.015 0.0011 0.044 0.0027 0 0 0 0 0 13 13 0.27 0.24 1.29 0.005 0.0014 0.045 0.0028 0.37 0 0 0 0 14 14 0.35 0.18 1.29 0.018 0.0003 0.044 0.0034 0 0.042 0 0 0 15 15 0.29 0.14 1.23 0.014 0.0013 0.050 0.0031 0 0 0.018 0 0 16 16 0.32 0.12 1.40 0.010 0.0011 0.049 0.0032 0 0 0 0.06 0 17 17 0.34 0.20 1.38 0.013 0.0018 0.047 0.0033 0 0 0 0 0.0021 18 1 0.25 0.24 1.22 0.016 0.0025 0.047 0.0037 0 0 0 0 0 19 1 0.25 0.24 1.22 0.016 0.0025 0.047 0.0037 0 0 0 0 0 20 1 0.25 0.24 1.22 0.016 0.0025 0.047 0.0037 0 0 0 0 0 21 2 0.30 0.23 1.25 0.015 0.0007 0.039 0.0036 0 0 0 0 0 22 2 0.30 0.23 1.25 0.015 0.0007 0.039 0.0036 0 0 0 0 0 23 2 0.30 0.23 1.25 0.015 0.0007 0.039 0.0036 0 0 0 0 0 24 3 0.38 0.13 1.31 0.011 0.0010 0.045 0.0033 0 0 0 0 0 25 3 0.38 0.13 1.31 0.011 0.0010 0.045 0.0033 0 0 0 0 0 26 3 0.38 0.13 1.31 0.011 0.0010 0.045 0.0033 0 0 0 0 0 27 4 0.45 0.17 1.35 0.009 0.0001 0.040 0.0028 0 0 0 0 0 28 4 0.45 0.17 1.35 0.009 0.0001 0.040 0.0028 0 0 0 0 0 29 4 0.45 0.17 1.35 0.009 0.0001 0.040 0.0028 0 0 0 0 0 30 2 0.30 0.23 1.25 0.015 0.0007 0.039 0.0036 0 0 0 0 0 31 2 0.30 0.23 1.25 0.015 0.0007 0.039 0.0036 0 0 0 0 0 32 2 0.30 0.23 1.25 0.015 0.0007 0.039 0.0036 0 0 0 0 0 33 2 0.30 0.23 1.25 0.015 0.0007 0.039 0.0036 0 0 0 0 0 34 18 0.69 0.22 1.34 0.005 0.0004 0.051 0.0027 0 0 0 0 0 35 18 0.69 0.22 1.34 0.005 0.0004 0.051 0.0027 0 0 0 0 0 In the table, fields with compositions of constituents of 0 indicate corresponding constituents not intentionally added.

TABLE A-1-2 Multilayer steel sheet Composition of constituents of steel sheet for surface layer (mass %) no. C Si Mn P S sol.Al N Ni Nb Ti Mo B Remarks 1 0.095 0.098 0.476 0.017 0.0034 0.049 0.0036 0 0 0 0 0 2 0.141 0.104 0.613 0.014 0.0012 0.037 0.0034 0 0 0 0 0 3 0.175 0.069 0.655 0.011 0.0014 0.046 0.0033 0 0 0 0 0 4 0.230 0.090 0.702 0.009 0.0001 0.040 0.0028 0 0 0 0 0 5 0.043 0.071 0.655 0.015 0.0017 0.039 0.0041 0 0 0 0 0 Comp. steel 6 0.104 0.082 0.563 0.015 0.0018 0.044 0.0035 0 0 0 0 0 7 0.162 0.052 0.584 0.004 0.0019 0.039 0.0032 0 0 0 0 0 8 0.176 0.056 0.509 0.007 0.0024 0.051 0.0033 0 0 0 0 0 9 0.328 0.064 0.517 0.016 0.0017 0.042 0.0028 0 0 0 0 0 Comp. steel 10 0.158 0.224 0.697 0.008 0.0029 0.051 0.0027 0 0 0 0 0 11 0.147 0.119 0.029 0.014 0.0026 0.040 0.0042 0 0 0 0 0 Comp. steel 12 0.145 0.119 0.365 0.015 0.0016 0.045 0.0025 0 0 0 0 0 13 0.124 0.127 0.684 0.003 0.002 0.045 0.0027 0.34 0 0 0 0 14 0.172 0.085 0.555 0.018 0.0006 0.044 0.0033 0 0.0038 0 0 0 15 0.136 0.064 0.492 0.016 0.0022 0.05 0.0032 0 0 0.0025 0 0 16 0.166 0.061 0.63 0.008 0.0015 0.048 0.0031 0 0 0 0.05 0 17 0.153 0.102 0.607 0.012 0.0021 0.047 0.0031 0 0 0 0 0.0018 18 0.110 0.211 1.183 0.016 0.0029 0.045 0.0036 0 0 0 0 0 19 0.105 0.214 0.622 0.016 0.0031 0.049 0.0038 0 0 0 0 0 20 0.143 0.11 1.049 0.016 0.0031 0.048 0.0036 0 0 0 0 0 21 0.273 0.101 0.638 0.014 0.0013 0.038 0.0038 0 0 0 0 0 22 0.270 0.124 1.088 0.013 0.0014 0.041 0.0038 0 0 0 0 0 23 0.264 0.205 0.575 0.015 0.0014 0.038 0.0038 0 0 0 0 0 24 0.296 0.057 0.616 0.009 0.0018 0.044 0.0033 0 0 0 0 0 25 0.190 0.124 0.655 0.009 0.0018 0.047 0.0034 0 0 0 0 0 26 0.175 0.061 1.009 0.010 0.0017 0.044 0.0032 0 0 0 0 0 27 0.401 0.092 0.689 0.011 0.0004 0.038 0.0027 0 0 0 0 0 28 0.207 0.153 0.594 0.007 0.0007 0.041 0.0028 0 0 0 0 0 29 0.203 0.075 1.175 0.009 0.0007 0.040 0.0027 0 0 0 0 0 30 0.144 0.127 0.550 0.014 0.0014 0.039 0.0036 0 0 0 0 0 31 0.144 0.113 0.588 0.013 0.0009 0.038 0.0037 0 0 0 0 0 32 0.135 0.113 0.563 0.017 0.0012 0.039 0.0036 0 0 0 0 0 33 0.156 0.110 0.513 0.015 0.0016 0.039 0.0036 0 0 0 0 0 34 0.290 0.125 0.63 0.004 0.0009 0.052 0.0025 0 0 0 0 0 35 0.352 0.112 0.63 0.006 0.0004 0.053 0.0028 0 0 0 0 0 36 0.132 0.12 0.575 0.017 0.0015 0.039 0.0036 0 0 0 0 0 37 0.150 0.124 0.563 0.013 0.0010 0.038 0.0035 0 0 0 0 0 38 0.221 0.092 0.689 0.011 0.0004 0.038 0.0027 0 0 0 0 0 In the table, fields with compositions of constituents of 0 indeicate corresponding constituents not intentionally added.

TABLE A-2-1 Heat Heat treatment at hot stamping treatment Average before hot Rough rolling cooling Average Multi- rolling Rate of Hot rolling Cold Heat- rate cooling layer Manu- Heat- Hold- Roll- reduction No. of Finish Coil- rolling ing Heat- (° C./s) rate Temper- Sheet steel facturing ing ing ing of sheet rolling rolling ing Rolling rate ing (more (° C./s) ing thick- sheet condition temp. time temp. thickness operations temp. temp. rate (° C./ temp. than (400° C. temp. Plat- ness no. no. (° C.) (min) (° C.) (%) (times) (° C.) (° C.) (%) s) (° C.) 400° C.) or less) (° C.) ing (mm) 1 1 1320 35 1149 34 3 838 586 53 32 903 70 53 — None 1.3 2 2 1293 31 1171 32 3 840 538 58 34 872 97 94 — None 1.2 3 3 1257 41 1138 20 3 917 604 53 51 859 73 66 — None 1.3 4 4 1285 36 1157 42 3 840 567 48 61 885 100 86 — None 1.5 5 5 1318 36 1133 38 3 856 712 60 54 884 96 84 — None 1.1 6 6 1275 35 1165 30 3 865 578 57 74 822 70 62 — None 1.2 7 7 1275 32 1184 47 3 872 685 49 61 909 82 77 — None 1.4 8 8 1268 41 1130 42 3 847 680 37 51 817 81 79 — None 1.8 9 9 1338 50 1167 29 3 865 693 58 29 843 69 60 — None 1.2 10 10 1270 39 1132 37 3 838 602 51 40 913 77 69 — None 1.4 11 11 1272 45 1130 34 3 900 620 62 57 831 99 98 — None 1.1 12 12 1301 49 1174 37 3 844 536 61 59 880 75 77 — None 1.1 13 13 1297 52 1176 28 3 860 744 38 26 823 100 93 — None 1.7 14 14 1287 42 1143 42 3 927 536 53 42 905 73 63 — None 1.3 15 15 1341 58 1154 43 3 915 545 40 28 912 89 77 — None 1.7 16 16 1268 54 1142 35 3 845 722 50 34 887 104 95 — None 1.4 17 17 1321 43 1141 43 3 865 569 34 63 849 113 109 — None 1.8 18 18 1315 55 1146 34 3 868 570 49 49 902 96 83 — None 1.4 19 19 1306 47 1157 42 3 862 624 63 20 844 73 63 — None 1 20 20 1322 34 1126 42 3 867 652 40 22 887 106 86 — None 1.7 21 21 1316 58 1159 48 3 847 589 57 19 845 102 93 — None 1.2 22 22 1287 56 1171 34 3 856 562 44 29 812 99 92 — None 1.6 23 23 1333 58 1132 40 3 856 707 47 61 834 97 80 — None 1.5 24 24 1306 47 1167 34 3 861 574 42 70 940 79 67 — None 1.6 25 25 1339 54 1143 40 3 882 697 48 63 899 91 83 — None 1.5

TABLE A-2-2 Heat Heat treatment at hot stamping treatment Average before hot Rough rolling cooling Average Multi- rolling Rate of Hot rolling Cold rate cooling layer Manu- Heat- Hold- Roll- reduction No. of Finish Coil- rolling Heat- Heat- (° C./s) rate Temper- Sheet steel facturing ing ing ing of sheet rolling rolling ing Rolling ing ing (more (° C./s) ing thick- sheet condition temp. time temp. thickness operations temp temp. rate rate temp. than (400° C. temp. Plat- ness no. no. (° C.) (min) (° C.) (%) (times) (° C.) (° C.) (%) (° C./s) (° C.) 400° C.) or less) (° C.) ing (mm) 26 26 1267 58 1131 24 3 884 587 40 21 869 91 84 — None 1.7 27 27 1251 46 1176 22 3 861 659 47 70 893 97 93 — None 1.5 28 28 1267 53 1183 22 3 893 699 59 28 877 105 100 — None 1.1 29 29 1270 49 1140 35 3 896 634 61 44 877 87 69 — None 1.1 30 30 981 58 970 30 3 841 612 44 52 933 83 82 — None 1.6 31 31 1320 7 1141 47 3 906 713 42 73 893 71 62 — None 1.6 32 32 1387 48 1135 29 3 858 578 50 69 843 85 69 — None 1.4 33 33 1298 42 1127 20 3 899 604 0 69 839 84 67 — None 2.8 34 34 1281 55 1122 22 3 835 667 56 50 903 70 71 267 None 1.2 35 35 1289 49 1132 46 3 831 641 51 58 927 95 87 274 Yes 1.4 36 36 1318 49 1171 38 3 844 745 53 53 864 101 88 — Yes 1.3 37 37 1239 48 1138 42 3 829 551 51 24 879 80 67 None None 1.3 38 38 1250 44 1127 39 3 885 661 47 30 856 103 90 — None 1.5 39 39 1251 49 1170 39 3 891 700 59 68 871 85 72 — None 1.1 40 40 1322 52 1008 44 3 882 697 40 69 917 104 103 — None 1.6 41 41 1333 43 1152 3 2 893 634 59 31 934 74 76 — None 1.4 42 42 1267 58 1141 40 1 906 713 50 65 903 83 78 — None 1.2 43 43 1339 22 1111 38 3 899 667 51 63 892 102 93 — None 1.3

TABLE A-3 Metal structures Area rate (%) of total of crystal grains with maximum difference of crystal orientation inside large angle grain boundaries of 1° or Hardness less and crystal of middle grains with Mechanical properties part in maximum difference Max. Multilayer sheet of crystal orientation Tensile bending Hydrogen Stamped steel sheet Manufacturing thickness of 8° or more and strength angle embrittlement body no. no. condition no. (Hv) less than 15° (MPa) (°) resistance Remarks 1A 1 1 564 80 1674 87.5 Good Inv. ex. 2A 2 2 632 72 1884 79.1 Good Inv. ex. 3A 3 3 751 65 2259 72.6 Good Inv. ex. 4A 4 4 771 57 2309 75.8 Good Inv. ex. 5A 5 5 377 82 1119 89.9 Good Comp. ex. 6A 6 6 528 76 1586 86.8 Good Inv. ex. 7A 7 7 678 73 2034 79.4 Good Inv. ex. 8A 8 8 663 68 1994 75.7 Good Inv. ex. 9A 9 9 973 52 2915 63.4 Good Comp. ex. 10A 10 10 700 69 2100 80.8 Good Inv. ex. 11A 11 11 490 83 1467 81.7 Good Comp. ex. 12A 12 12 630 71 1883 89.5 Good Inv. ex. 13A 13 13 640 70 1927 88.4 Good Inv. ex. 14A 14 14 653 68 1940 84.4 Good Inv. ex. 15A 15 15 640 69 1918 89.3 Good Inv. ex. 16A 16 16 642 69 1905 85.5 Good Inv. ex. 17A 17 17 657 68 1963 83.7 Good Inv. ex. 18A 18 18 505 80 1511 86.9 Good Inv. ex. 19A 19 19 504 80 1502 87.4 Good Inv. ex. 20A 20 20 500 80 1532 88.1 Good Inv. ex. 21A 21 21 634 72 1909 78.9 Good Inv. ex. 22A 22 22 631 72 1900 79 Good Inv. ex. 23A 23 23 633 72 1905 77.1 Good Inv. ex. 24A 24 24 710 65 2132 72.4 Good Inv. ex. 25A 25 25 702 65 2084 71.9 Good Inv. ex. 26A 26 26 703 65 2110 70.9 Good Inv. ex. 27A 27 27 768 57 2290 74.3 Good Inv. ex. 28A 28 28 767 57 2297 76.8 Good Inv. ex. 29A 29 29 770 57 2308 76.2 Good Inv. ex. 30A 30 30 631 16 1895 68.1 Poor Comp. ex. 31A 31 31 630 18 1880 61.7 Poor Comp. ex. 32A 32 32 632 95 1885 68.8 Good Comp. ex. 33A 33 33 628 70 1878 83.1 Good Inv. ex. 34A 34 34 721 63 2168 73 Good Inv. ex. 35A 35 35 715 63 2145 78.3 Good Inv. ex. 36A 36 36 631 70 1892 84 Good Inv. ex. 37A 37 37 641 72 2153 72.6 Good Inv. ex. 38A 38 38 776 56 2297 73.9 Good Inv. ex. 39A 39 39 781 55 2381 70.2 Good Inv. ex. 40A 40 40 627 10 2069 60.2 Poor Comp. ex. 41A 41 41 635 11 2096 60.1 Poor Comp. ex. 42A 42 42 625 13 2063 59.2 Poor Comp. ex. 43A 43 43 634 46 2092 109.5 Good Inv. ex.

Manufacturing Example B

The Nos. 1 to 18 steel sheets for sheet thickness middle part having the chemical compositions shown in Table B-1-1 (“Steel Nos. 1 to 18” in Table B-1-1) were ground down at their surfaces to remove the surface oxides. After that, the respective steel sheets for sheet thickness middle part were welded with steel sheets for surface layer having the chemical compositions shown in Table B-1-2 at both surfaces or single surfaces by arc welding to fabricate the Nos. 1 to 41 multilayer steel sheets for hot stamped body. The sheet thickness of the total of the steel sheet for surface layer and the steel sheet for sheet thickness middle part after arc welding was 200 mm to 300 mm and the thickness of the steel sheet for surface layer was ⅓ or so of the thickness of the steel sheet for sheet thickness middle part (in case of single side, ¼ or so). The No. 37 multilayer steel sheet was steel with steel sheet for surface layer welded to only one side. The multilayer steel sheets other than No. 37 respectively had steel sheets for surface layer welded to both sides of the steel sheet for sheet thickness middle part. Among the Nos. 1 to 41 multilayer steel sheets of Table B-1-3, ones where the steel sheet for sheet thickness middle part did not satisfy the requirements of composition of the middle part in sheet thickness of the hot stamped body according to the present invention are indicated as “comparative steels” in the remarks columns.

The Nos. 1 to 41 multilayer steel sheets were respectively treated under the conditions of the Nos. 1 to 41 manufacturing conditions shown in Table B-2-1 to Table B-2-2 by heat treatment before hot rolling, rough rolling, hot rolling, and cold rolling to obtain steel sheets. Next, the steel sheets were heat treated as shown in Table B-2-1 and Table B-2-2 (in the tables, “heat treatment of hot stamped body”) for hot stamping to manufacture the Nos. 1B to 41B hot stamped bodies (“stamped bodies” of Table B-3-1 and Table B-3-2). Further, the Nos. 35B and 36B hot stamped bodies were coated on a hot dip coating line at their surfaces with 120 to 160 g/m²amounts of aluminum. Further, the items in Table B-2-1 to Table B-2-2 correspond to the items in Table A-2-1 to Table A-2-2. Further, in the tables, the fields with the notations “-” indicate no corresponding treatment performed.

Table B-3-1 and Table B-3-2 show the metal structures and characteristics of the Nos. 1B to 41B hot stamped bodies. The constituents obtained by analyzing the positions of ½ of the sheet thicknesses of the samples taken from the hot stamped bodies (middle parts in sheet thickness) and positions of 20 μm from the surfaces of the softened layers were equivalent to the constituents of the steel sheets for sheet thickness middle part and steel sheets for surface layer of the Nos. 1 to 41 multilayer steel sheets of Table B-1-1 to Table B-1-3.

The metal structures of the hot stamped steel sheets were measured by the above-mentioned method. The hardness of the steel sheet for sheet thickness middle part forming the middle part in sheet thickness and the area rate (%) of the total of the crystal grains with a maximum crystal orientation difference inside the regions surrounded by grain boundaries of 15° or more of 1° or less and the crystal grains with a crystal orientation difference of 8° or more and less than 15° in the metal structures from the surface of the steel sheet for surface layer forming the softened layer to ½ of the thickness of that softened layer were calculated. The calculated values of the area rate are shown in the items “area rate (%) of total of crystal grains with maximum crystal orientation difference inside large angle grain boundaries of 1° or less and crystal grains with maximum crystal orientation difference of 8° or more and less than 15°” of Tables B-3-1 to Table B-3-2.

Further, the Nos. 1B to 41B hot stamped bodies were respectively measured for average hardness (HV) and minimum hardness (HV) at the middle part in sheet thickness (position of ½ of sheet thickness) by the above method. The measurement results are shown in Table B-3-1 to Table B-3-2. The Nos. 1B to 41B hot stamped bodies had differences of the average hardness (HV) and minimum hardness (HV) shown in the “scattering in cross-sectional hardness” of Table B-3-1 to Table B-3-2. Further, cases with a scattering in cross-sectional hardness of 100 HV or more were indicated as failing.

The hot stamped bodies were subjected to tensile tests. The results are shown in Table B-3-1 to Table B-3-2. The tensile tests were performed by fabricating No. 5 test pieces described in JIS Z 2201 and testing them by the method described in JIS Z 2241.

The hydrogen embrittlement resistance of the hot stamped body, in the same way as Manufacturing Example A, was evaluated using a test piece cut out from the stamped body. That is, a test piece of a sheet thickness of 1.2 mm×width 6 mm×length 68 mm was cut out from the stamped body, given a strain corresponding to the yield stress in a four-point bending test, then immersed in pH3 hydrochloric acid for 100 hours and evaluated for hydrogen embrittlement resistance by the presence of any cracks. The case of no cracks was indicated as passing (“Good”) and the case of cracks was evaluated as failing (“Poor”).

For the purpose of evaluating the impact resistance of the hot stamped body, the body was evaluated based on the VDA standard (VDA238-100) prescribed by the German Association of the Automotive Industry under the same measurement conditions as Manufacturing Example A. In the present invention, the displacement at the time of maximum load obtained in the bending test was converted to angle by the VDA standard to find maximum bending angle and thereby evaluate the impact resistance of the hot stamped body.

If the tensile strength is 1500 MPa or more, the maximum bending angle (°) was 70(°) or more, and the hydrogen embrittlement resistance was a passing level, it was judged that the impact resistance and hydrogen embrittlement resistance were excellent and the case was indicated as an “invention example”. If even one of the three aspects of performance is not satisfied, the case was indicated as a “comparative example”.

In each hot stamped body of the invention examples, the area rate (%) of the total of the crystal grains with a maximum crystal orientation difference inside the regions surrounded by grain boundaries of 15° or more of 1° or less and the crystal grains with a crystal orientation difference of 8° or more and less than 15° in the metal structures from the surface of the steel sheet for surface layer to ½ of the thickness was 50% to less than 85%. Further, each hot stamped body of the invention examples was excellent in tensile strength, bendability, and hydrogen embrittlement resistance.

As opposed to this, the No. 5B hot stamped body was low in carbon content at the steel sheet for sheet thickness middle part, so the hardness of the middle part in sheet thickness became insufficient and the tensile strength became insufficient. The No. 9B hot stamped body was excessive in carbon content of steel sheet for sheet thickness middle part, so the hardness of the middle part in sheet thickness also became excessive and the targeted bendability could not be obtained. Further, the No. 11B hot stamped body was sparse in Mn content at the steel sheet for sheet thickness middle part, so the hardness of the middle part in sheet thickness became insufficient and the tensile strength became insufficient.

The Nos. 30B to 32B hot stamped bodies are comparative examples produced using the multilayer steel sheets for hot stamped body to which the desirable heat treatment had not been applied before the hot stamping process. The No. 30B hot stamped body was low in heat treatment temperature before the hot stamping process, while the No. 31B hot stamped body was short in heat treatment time before the hot stamping process, so in the metal structures of the softened layer from the surface of the softened layer to ½ of the thickness, the soft structures and metal structures with intermediate hardnesses insufficiently grew and the target bendability could not be obtained. Further, the No. 32B hot stamped body was excessively high in heat treatment temperature before the hot stamping process, so the effect of reduction of the sharp gradient in hardness in the sheet thickness direction occurring at the time of bending deformation could not be obtained.

The No. 38B hot stamped body was low in rolling temperature of the rough rolling. Further, the No. 39B hot stamped body was low in sheet thickness reduction rate of the rough rolling. Further, the No. 40B hot stamped body was low in number of rolling operations under conditions of a time between passes of 3 seconds or more. These hot stamped bodies were not manufactured under the suitable rough rolling conditions, so the soft structures and metal structures with intermediate hardnesses insufficiently grew, it was not possible to ease the strain occurring due to bending deformation, and the targeted bendability could not be obtained.

The No. 41B hot stamped body is a steel sheet controlled in casting rate to 6 ton/min or more in the continuous casting process of steel sheet for surface layer. It can raise the area rate of the total of the crystal grains with a maximum crystal orientation difference inside the regions surrounded by grain boundaries of 15° or more of 1° or less and the crystal grains with a crystal orientation difference of 8° or more and less than 15° in the metal structures from the surface of the steel sheet for surface layer to ½ of the thickness and is excellent in bendability.

TABLE B-1-1 Multilayer Composition of constituents of steel sheet for sheet thickness middle part (mass %) steel sheet Steel no. no. C Si Mn P S sol.Al N Ni Nb Ti Mo B 1 1 0.24 0.25 1.57 0.012 0.0005 0.042 0.0037 2 2 0.31 0.25 1.78 0.015 0.0009 0.032 0.0031 3 3 0.38 0.13 1.79 0.006 0.0018 0.045 0.004 4 4 0.45 0.21 1.69 0.009 0.0015 0.025 0.0044 5 5 0.17 0.17 1.51 0.006 0.0018 0.025 0.0035 6 6 0.24 0.20 1.64 0.014 0.0014 0.043 0.0027 7 7 0.33 0.15 1.75 0.016 0.0014 0.031 0.0027 8 8 0.32 0.11 1.57 0.012 0.0020 0.045 0.0031 9 9 0.81 0.14 1.96 0.013 0.0016 0.034 0.003 10 10 0.35 0.27 1.75 0.008 0.0013 0.033 0.003 11 11 0.3 0.22 1.05 0.014 0.0013 0.026 0.003 12 12 0.29 0.22 1.74 0.016 0.0007 0.032 0.004 13 13 0.27 0.24 1.63 0.01 0.0007 0.045 0.0043 0.20 14 14 0.35 0.20 1.55 0.012 0.0014 0.043 0.004 0.050 15 15 0.29 0.15 2.05 0.006 0.0015 0.033 0.0039 0.015 16 16 0.32 0.12 1.84 0.012 0.0007 0.032 0.0044 0.050 17 17 0.34 0.20 1.79 0.009 0.0007 0.033 0.0043 0.0019 18 1 0.24 0.25 1.57 0.012 0.0005 0.042 0.0037 19 1 0.24 0.25 1.57 0.012 0.0005 0.042 0.0037 20 1 0.24 0.25 1.57 0.012 0.0005 0.042 0.0037 21 2 0.31 0.25 1.78 0.015 0.0009 0.032 0.0031 22 2 0.31 0.25 1.78 0.015 0.0009 0.032 0.0031 23 2 0.31 0.25 1.78 0.015 0.0009 0.032 0.0031 24 3 0.38 0.13 1.79 0.006 0.0018 0.045 0.0040 25 3 0.38 0.13 1.79 0.006 0.0018 0.045 0.0040 26 3 0.38 0.13 1.79 0.006 0.0018 0.045 0.0040 27 4 0.45 0.21 1.69 0.009 0.0015 0.025 0.0044 28 4 0.45 0.21 1.69 0.009 0.0015 0.025 0.0044 29 4 0.45 0.21 1.69 0.009 0.0015 0.025 0.0044 30 2 0.31 0.25 1.78 0.015 0.0009 0.032 0.0031 31 2 0.31 0.25 1.78 0.015 0.0009 0.032 0.0031 32 2 0.31 0.25 1.78 0.015 0.0009 0.032 0.0031 33 2 0.31 0.25 1.78 0.015 0.0009 0.032 0.0031 34 18 0.68 0.23 1.81 0.008 0.0019 0.041 0.0036 35 18 0.68 0.23 1.81 0.008 0.0019 0.041 0.0036 36 2 0.31 0.25 1.78 0.015 0.0009 0.032 0.0031 37 2 0.31 0.25 1.78 0.015 0.0009 0.032 0.0031 38 2 0.31 0.25 1.78 0.015 0.0009 0.032 0.0031 39 2 0.31 0.25 1.78 0.015 0.0009 0.032 0.0031 40 2 0.31 0.25 1.78 0.015 0.0009 0.032 0.0031 41 2 0.31 0.25 1.78 0.015 0.0009 0.032 0.0031 In the table, blanks indicate corresponding constituents not intentionally added.

TABLE B-1-2 Multilayer steel sheet Composition of constituents of steel sheet for surface layer (mass %) no. C Si Mn P S sol.Al N Ni Nb Ti Mo B 1 0.10 0.10 0.48 0.005 0.0018 0.030 0.0036 2 0.13 0.10 0.60 0.016 0.0008 0.033 0.0030 3 0.11 0.07 0.75 0.015 0.0010 0.041 0.0029 4 0.22 0.09 0.70 0.006 0.0020 0.029 0.0031 5 0.10 0.07 0.66 0.006 0.0019 0.028 0.0041 6 0.14 0.08 0.56 0.014 0.0019 0.033 0.0028 7 0.11 0.05 0.58 0.008 0.0016 0.030 0.0040 8 0.12 0.06 0.51 0.006 0.0012 0.033 0.0027 9 0.36 0.06 0.52 0.018 0.0017 0.036 0.0032 10 0.15 0.22 0.70 0.018 0.0005 0.033 0.0042 11 0.15 0.12 0.03 0.013 0.0008 0.027 0.0044 12 0.11 0.12 0.36 0.008 0.0017 0.038 0.0041 13 0.12 0.13 0.68 0.010 0.0013 0.032 0.0027 0.15 14 0.14 0.08 0.55 0.018 0.0009 0.038 0.0040 0.010 15 0.11 0.06 0.49 0.013 0.0008 0.034 0.0032 0.010 16 0.10 0.06 0.63 0.018 0.0008 0.028 0.0039 0.050 17 0.25 0.10 0.61 0.017 0.0010 0.035 0.0039 0.0018 18 0.12 0.21 1.18 0.012 0.0008 0.029 0.0030 19 0.10 0.21 0.62 0.012 0.0007 0.037 0.0042 20 0.10 0.11 1.05 0.016 0.0011 0.033 0.0039 21 0.20 0.10 1.25 0.007 0.0007 0.029 0.0030 22 0.13 0.16 1.09 0.009 0.0014 0.033 0.0036 23 0.15 0.20 0.58 0.018 0.0019 0.027 0.0033 24 0.11 0.06 0.62 0.007 0.0005 0.041 0.0032 25 0.10 0.12 0.66 0.011 0.0017 0.030 0.0042 26 0.14 0.06 1.01 0.008 0.0016 0.027 0.0040 27 0.30 0.09 0.69 0.014 0.0011 0.041 0.0029 28 0.23 0.15 0.59 0.005 0.0013 0.040 0.0033 29 0.23 0.07 1.17 0.007 0.0010 0.030 0.0031 30 0.26 0.20 0.55 0.018 0.0020 0.043 0.0044 31 0.17 0.11 0.59 0.017 0.0011 0.035 0.0029 32 0.17 0.11 0.56 0.008 0.0014 0.032 0.0039 33 0.17 0.11 0.51 0.008 0.0018 0.036 0.0027 34 0.36 0.13 0.63 0.007 0.0005 0.042 0.0035 35 0.37 0.11 0.63 0.007 0.0011 0.029 0.0031 36 0.16 0.12 0.58 0.015 0.0005 0.033 0.0032 37 0.17 0.12 0.56 0.008 0.0009 0.030 0.0044 38 0.13 0.10 0.60 0.016 0.0008 0.033 0.0030 39 0.13 0.10 0.60 0.016 0.0008 0.033 0.0030 40 0.13 0.10 0.60 0.016 0.0008 0.033 0.0030 41 0.13 0.10 0.60 0.016 0.0008 0.033 0.0030 In the table, blanks indicate corresponding constituents not intentionally added.

TABLE B-1-3 Steel sheet for Sheet thickness Multilayer sheet thickness of steel sheet steel sheet middle part for surface no. Steel no. layer (mm) Remarks 1 1 85 2 2 83 3 3 84 4 4 97 5 5 94 Comp. steel 6 6 82 7 7 88 8 8 94 9 9 83 Comp. steel 10 10 88 11 11 83 Comp. steel 12 12 92 13 13 82 14 14 86 15 15 95 16 16 96 17 17 96 18 1 95 19 1 99 20 1 98 21 2 88 22 2 84 23 2 85 24 3 83 25 3 91 26 3 88 27 4 96 28 4 82 29 4 91 30 2 93 31 2 92 32 2 94 33 2 84 34 18 92 35 18 92 36 2 84 37 2 93 38 2 95 39 2 98 40 2 85 41 2 88

TABLE B-2-1 Heat Heat treatment at hot stamping treatment Average before hot Rough rolling cooling Average Multi- rolling Rate of Hot rolling Cold rate cooling layer Manu- Heat- Hold- Roll- reduction No. of Finish Coil- rolling Heat- Heat- (° C./s) rate Temper- Sheet steel facturing ing ing ing of sheet rolling rolling ing Rolling ing ing (more (° C./s) ing thick- sheet condition temp. time temp. thickness operations temp. temp. rate rate temp. than (400° C. temp. Plat- ness no. no. (° C.) (min) (° C.) (%) (times) (° C.) (° C.) (%) (° C./s) (° C.) 400° C.) or less) (° C.) ing (mm) 1 1 1290 35 1151 33 3 917 565 50 35 858 72 49 None None 1.4 2 2 1280 50 1167 31 3 911 688 44 35 849 96 89 None None 1.6 3 3 1255 40 1141 22 3 915 515 57 46 861 77 65 None None 1.2 4 4 1285 40 1153 39 3 887 661 40 64 896 96 86 None None 1.7 5 5 1318 35 1129 40 3 895 513 55 57 916 101 81 None None 1.3 6 6 1275 35 1162 33 3 885 527 57 70 880 72 61 None None 1.2 7 7 1275 35 1180 51 3 896 591 49 56 912 82 78 None None 1.4 8 8 1250 40 1135 47 3 895 628 35 47 902 76 76 None None 1.8 9 9 1350 55 1168 30 3 920 701 35 25 863 65 60 None None 1.8 10 10 1290 35 1136 35 3 899 611 51 35 854 73 71 None None 1.4 11 11 1250 40 1125 38 3 894 688 50 52 871 100 102 None None 1.4 12 12 1300 40 1171 41 3 907 652 50 63 884 78 77 None None 1.4 13 13 1250 50 1175 26 3 896 687 38 30 895 101 93 None None 1.7 14 14 1300 55 1142 41 3 900 714 53 43 910 68 67 None None 1.3 15 15 1330 50 1157 46 3 906 559 40 27 909 87 78 None None 1.7 16 16 1270 60 1146 34 3 895 710 50 31 891 107 97 None None 1.4 17 17 1310 45 1137 45 3 899 672 34 65 901 111 114 None None 1.8 18 18 1300 55 1151 34 3 888 664 50 50 855 95 83 None None 1.4 19 19 1300 40 1156 40 3 917 564 50 15 915 75 66 None None 1.4 20 20 1290 50 1122 44 3 903 666 50 23 871 108 88 None None 1.4 21 21 1280 55 1163 46 3 907 614 57 22 868 101 89 None None 1.2 22 22 1280 40 1171 30 3 914 514 44 34 855 96 93 None None 1.6 23 23 1300 40 1136 42 3 888 562 47 66 878 93 79 None None 1.5 24 24 1255 40 1164 32 3 893 524 57 70 857 75 71 None None 1.2 25 25 1255 55 1143 39 3 910 520 57 67 899 91 83 None None 1.2

TABLE B-2-2 Heat Heat treatment at hot stamping treatment Average before hot Rough rolling cooling Average Multi- rolling Rate of Hot rolling Cold rate cooling layer Manu- Heat- Hold- Roll- reduction No. of Finish Coil- rolling Heat Heat- (° C./s) rate Temper- Sheet steel facturing ing ing ing of sheet rolling rolling ing Rolling ing ing (more (° C./s) ing thick- sheet condition temp. time temp. thickness operations temp. temp. rate rate temp. than (400 C. temp. Plat- ness no. no. (° C.) (min) (° C.) (%) (times) (° C.) (° C.) (%) (° C./s) (° C.) 400° C.) or less) (° C.) ing (mm) 26 26 1300 35 1127 20 3 893 527 57 23 901 88 81 None None 1.2 27 27 1285 40 1174 27 3 901 533 40 71 869 92 90 None None 1.7 28 28 1285 40 1178 24 3 913 522 40 31 879 102 100 None None 1.7 29 29 1350 55 1142 33 3 905 551 40 40 912 92 66 None None 1.7 30 30 1070 50 1010 32 3 908 638 44 55 847 88 83 None None 1.6 31 31 1300 10 1137 48 3 892 587 44 74 862 76 66 None None 1.6 32 32 1400 50 1140 26 3 911 642 44 68 871 82 71 None None 1.6 33 33 1300 40 1123 16 3 916 534 0 66 891 80 63 None None 2.8 34 34 1280 55 1124 18 3 881 665 56 45 900 67 76 267 None 1.2 35 35 1300 40 1127 41 3 883 650 51 63 874 91 85 274 Yes 1.4 36 36 1290 50 1166 43 3 909 541 44 52 891 101 86 None Yes 1.6 37 37 1280 50 1143 47 3 910 704 44 29 863 80 72 None None 1.6 38 38 1330 40 1005 48 3 882 697 40 67 917 103 108 — None 1.2 39 39 1310 45 1157 3 2 893 634 59 26 934 69 74 — None 1.2 40 40 1290 40 1137 41 1 906 713 50 62 903 84 82 — None 1.6 41 41 1280 23 1112 41 3 899 667 51 65 892 100 90 — None 1.6

TABLE B-3-1 Metal structures Area rate (%) of total of crystal grains with maximum difference of crystal orientation inside large angle grain boundaries of 1° or less and crystal grains Hardness with maximum Mechanical properties of middle difference Average Scattering part in of crystal Maximum cross- in cross- Multilayer sheet orientation of Tensile bending Hydrogen sectional Minimum sectional Stamped steel sheet Manufacturing thickness 8° or more and strength angle embrittlement hardness hardness hardness body no. no. condition no. (Hv) less than 15° (MPa) (°) resistance (Hv) (Hv) (Hv) Remarks 1B 1 1 546 75 1621 89 Good 519 480 39 Inv. ex. 2B 2 2 647 64 1836 78.6 Good 607 575 32 Inv. ex. 3B 3 3 748 65 2187 72.4 Good 714 705 9 Inv. ex. 4B 4 4 785 57 2328 70.7 Good 742 724 18 Inv. ex. 5B 5 5 446 80 1210 89.7 Good 450 357 93 Comp. ex. 6B 6 6 528 66 1579 88.1 Good 499 469 30 Inv. ex. 7B 7 7 678 53 2027 79.6 Good 643 628 15 Inv. ex. 8B 8 8 663 71 1982 77.3 Good 615 581 34 Inv. ex. 9B 9 9 973 56 2746 57.8 Good 915 899 16 Comp. ex. 10B 10 10 700 65 2093 75.1 Good 668 655 13 Inv. ex. 11B 11 11 490 57 1362 88.5 Good 458 327 131 Comp. ex. 12B 12 12 618 54 1847 81.3 Good 588 562 26 Inv. ex. 13B 13 13 590 65 1760 82.5 Good 568 540 28 Inv. ex. 14B 14 14 705 77 2085 73.5 Good 655 631 24 Inv. ex. 15B 15 15 618 60 1832 80.2 Good 591 580 11 Inv. ex. 16B 16 16 662 65 1975 81.7 Good 625 618 7 Inv. ex. 17B 17 17 683 71 2034 74.3 Good 649 638 11 Inv. ex. 18B 18 18 534 80 1564 86.1 Good 504 461 43 Inv. ex. 19B 19 19 537 78 1596 89.2 Good 522 473 49 Inv. ex. 20B 20 20 541 60 1621 89.5 Good 518 497 21 Inv. ex. 21B 21 21 639 82 1901 77.4 Good 598 588 10 Inv. ex. 22B 22 22 630 75 1874 79.2 Good 581 567 14 Inv. ex. 23B 23 23 625 68 1850 80.8 Good 578 559 19 Inv. ex. 24B 24 24 740 60 2105 73.5 Good 691 681 10 Inv. ex. 25B 25 25 735 76 2195 71.1 Good 667 656 11 Inv. ex.

TABLE B-3-2 Metal structures Area rate (%) of total of crystal 1 grains with maximum difference of crystal orientation inside large angle grain boundaries of 1° or less Hardness and crystal grains Mechanical properties of middle with maximum Average Scattering part in difference of Maximum cross- in cross- Multilayer sheet crystal orientation Tensile bending Hydrogen sectional Minimum sectional Stamped steel sheet Manufacturing thickness of 8° or more and strength angle embrittlement hardness hardness hardness body no. no. condition no. (Hv) less than 15° (MPa) (°) resistance (Hv) (Hv) (Hv) Remarks 26B 26 26 731 78 2187 70.8 Good 671 654 17 Inv. ex. 27B 27 27 791 70 2387 72.5 Good 754 737 17 Inv. ex. 28B 28 28 775 63 2317 73 Good 739 717 22 Inv. ex. 29B 29 29 780 78 2271 74.5 Good 734 721 13 Inv. ex. 30B 30 30 657 15 1975 61.6 Poor 627 605 22 Comp. ex. 31B 31 31 647 18 1931 62.4 Poor 620 600 20 Comp. ex. 32B 32 32 644 90 1957 65.1 Good 611 599 12 Comp. ex. 33B 33 33 645 84 1930 75.8 Good 605 551 54 Inv. ex. 34B 34 34 745 64 2180 78.5 Good 717 702 15 Inv. ex. 35B 35 35 740 75 2260 79.3 Good 726 707 19 Inv. ex. 36B 36 36 647 71 1941 77.7 Good 611 581 30 Inv. ex. 37B 37 37 647 65 1920 78.1 Good 614 587 27 Inv. ex. 38B 38 38 643 9 2122 59.9 Poor 643 614 29 Comp. ex. 39B 39 39 637 10 2102 59.8 Poor 637 608 29 Comp. ex. 40B 40 40 645 12 2129 60.1 Poor 645 615 30 Comp. ex. 41B 41 41 641 45 2115 111.2 Good 641 611 30 Inv. ex.

Manufacturing Example C

Steel sheets for sheet thickness middle part having the chemical compositions shown in Table C-1-1 to Table C-1-2 were ground down at their surfaces to remove the surface oxides. After that, the respective steel sheets for sheet thickness middle part were welded with steel sheets for surface layer having the chemical compositions shown in Table C-1-3 to Table C-1-4 at both surfaces or single surfaces by arc welding to fabricate the Nos. 1 to 49 multilayer steel sheets for hot stamped body. The sheet thickness of the total of the steel sheet for surface layer and the steel sheet for sheet thickness middle part after arc welding was 200 mm to 300 mm and the thickness of the steel sheet for surface layer was ⅓ or so of the thickness of the steel sheet for sheet thickness middle part (in case of single side, ¼ or so). The No. 31 multilayer steel sheet was steel with steel sheet for surface layer welded to only one side. Among the Nos. 1 to 53 multilayer steel sheets of Table C-1-1 to Table C-1-4, ones where the steel sheet for sheet thickness middle part did not satisfy the requirements of composition of the middle part in sheet thickness of the hot stamped body according to the present invention are indicated as “comparative steels” in the remarks columns.

The “ratio of C, Si, and Mn contents of steel sheet for surface layer to steel sheet for sheet thickness middle part” of Table C-1-3 to Table C-1-4 show the ratios of C, Si, and Mn contents of steel sheet for surface layer to the C, Si, and Mn contents of steel sheet for sheet thickness middle part in the Nos. 1 to 53 multilayer steel sheets for hot stamped body.

The Nos. 1 to 53 multilayer steel sheets were respectively treated under the conditions of the Nos. 1 to 53 manufacturing conditions shown in Table C-2-1 to Table C-2-2 by heat treatment before hot rolling, rough rolling, hot rolling, and cold rolling to obtain steel sheets. Next, the steel sheets were heat treated as shown in Table C-2-1 to Table C-2-2 (in the tables, “heat treatment of hot stamped body”) for hot stamping to manufacture the Nos. 1C to 53C hot stamped bodies (“stamped bodies” of Table C-3-1 to Table C-3-2). Further, the No. 30C hot stamped body was coated on a hot dip coating line at the surface with a 120 to 160 g/m²amount of aluminum. Further, the items in Table C-2-1 to Table C-2-2 correspond to the items in Table A-2-1 to Table A-2-2. Further, in the tables, the fields with the notations “-” indicate no corresponding treatment performed.

Table C-3-1 to Table C-3-2 show the metal structures and characteristics of the Nos. 1C to 53C hot stamped bodies. The constituents obtained by analyzing the positions of ½ of the sheet thicknesses of the samples taken from the hot stamped bodies (middle parts in sheet thickness) and positions of 20 μm from the surfaces of the softened layers were equivalent to the constituents of the steel sheets for sheet thickness middle part and the steel sheets for surface layer of the Nos. 1 to 53 multilayer steel sheets of Table C-1-1 to Table C-1-4.

The metal structures of the hot stamped steel sheets were measured by the above-mentioned method. The hardness of the steel sheet for sheet thickness middle part forming the middle part in sheet thickness and the area rate of the total of the crystal grains with a maximum crystal orientation difference inside the regions surrounded by grain boundaries of 15° or more of 1° or less and the crystal grains with a crystal orientation difference of 8° or more and less than 15° in the metal structures from the surface of the steel sheet for surface layer forming the softened layer to ½ of the thickness of that softened layer were calculated. The calculated values of the area rate are shown in the items “area rate (%) of total of crystal grains with maximum crystal orientation difference inside large angle grain boundaries of 1° or less and crystal grains with maximum crystal orientation difference of 8° or more and less than 15°” of Tables C-3-1 to C-3-2.

The hot stamped bodies were subjected to tensile tests. The results are shown in Table C-3-1 to Table C-3-2. The tensile tests were performed by fabricating No. 5 test pieces described in JIS Z 2201 and testing them by the method described in JIS Z 2241.

The hydrogen embrittlement resistance of the hot stamped body, in the same way as Manufacturing Example A, was evaluated using a test piece cut out from the stamped body. That is, a test piece of a sheet thickness of 1.2 mm×width 6 mm×length 68 mm was cut out from the stamped body, given a strain corresponding to the yield stress in a four-point bending test, then immersed in pH3 hydrochloric acid for 100 hours and evaluated for hydrogen embrittlement resistance by the presence of any cracks. The case of no cracks was indicated as passing (“Good”) and the case of cracks was evaluated as failing (“Poor”).

For the purpose of evaluating the impact resistance of the hot stamped body, the body was evaluated based on the VDA standard (VDA238-100) prescribed by the German Association of the Automotive Industry under the same measurement conditions as Manufacturing Example A. In the present invention, the displacement at the time of maximum load obtained in the bending test was converted to angle by the VDA standard to find maximum bending angle and thereby evaluate the impact resistance of the hot stamped body.

If the tensile strength is 1500 MPa or more, the maximum bending angle (°) was 70(°) or more, the uniform elongation was 5% or more, and the hydrogen embrittlement resistance was a passing level, it was judged that the impact resistance, hydrogen embrittlement resistance, and ductility were excellent and the case was indicated as an “invention example”. If even one of the three aspects of performance is not satisfied, the case was indicated as a “comparative example”.

In each hot stamped body of the invention examples, the area rate of the total of the crystal grains with a maximum crystal orientation difference inside the regions surrounded by grain boundaries of 15° or more of 1° or less and the crystal grains with a crystal orientation difference of 8° or more and less than 15° in the metal structures from the surface of the steel sheet for surface layer to ½ of the thickness of the steel sheet for surface layer was 50% to less than 85%. Further, each hot stamped body of the invention examples was excellent in tensile strength, bendability, and hydrogen embrittlement resistance.

As opposed to this, the No. 5C hot stamped body was low in carbon content of the steel sheet for sheet thickness middle part, so became insufficient in hardness of the middle part in sheet thickness and became insufficient in tensile strength. The No. 9C hot stamped body was excessive in carbon content of the steel sheet for sheet thickness middle part, so also became excessive in hardness of the middle part in sheet thickness and could not be given the targeted bendability. Further, the No. 11C hot stamped body was low in Si content of the steel sheet for sheet thickness middle part, so the area percent of the residual austenite of the metal structures at the middle part in sheet thickness was less than 1.0% and the uniform elongation was low.

The Nos. 25C to 27C and 49C hot stamped bodies are comparative examples manufactured using the multilayer steel sheets for hot stamped body to which the preferable heat treatment is not applied before the hot stamping process. The No. 25C hot stamped body is too low in heat treatment temperature before the hot stamping process, so the soft structures and metal structures with intermediate hardnesses insufficiently grew, the effect of surface properties of the hot stamped body and effect of the transitional part from the middle part in sheet thickness to the softened layer could not be eliminated, and excellent bendability could not be obtained.

Further, the No. 26C hot stamped body was excessively high in heat treatment time before the hot stamping process, so the soft structures and metal structures with intermediate hardnesses excessively grew, the difference in hardness between the softened layer and the middle part in sheet thickness became too large, and the effect of reducing the sharp gradient of hardness in the sheet thickness direction occurring at the time of bending deformation could not be obtained. For this reason, the No. 26C hot stamped body could not be given the targeted bendability.

Further, the Nos. 27C and 49C hot stamped bodies were too long in heat treatment time before the hot stamping process, the difference in hardness between the softened layer and the middle part in sheet thickness become too great. Further, the heat treatment temperature was excessively high, so the effect of reducing the sharp gradient of hardness in the sheet thickness direction occurring at the time of bending deformation could not be obtained. For this reason, the Nos. 27C and 49C hot stamped bodies could not be given excellent bendability.

The No. 50C hot stamped body was low in rolling temperature of the rough rolling. Further, the No. 51C hot stamped body was low in sheet thickness reduction rate of the rough rolling. Further, the No. 52C hot stamped body was low in number of rolling operations under conditions of a time between passes of 3 seconds or more. These hot stamped bodies were not manufactured under the suitable rough rolling conditions, so the soft structures and metal structures with intermediate hardnesses insufficiently grew, it was not possible to ease the strain occurring due to bending deformation, and the targeted bendability could not be obtained.

The No. 53C hot stamped body is a steel sheet controlled in casting rate to 6 ton/min or more in the continuous casting process of steel sheet for surface layer. It can raise the area rate of the total of the crystal grains with a maximum crystal orientation difference inside the regions surrounded by grain boundaries of 15° or more of 1° or less and the crystal grains with a crystal orientation difference of 8° or more and less than 15° in the metal structures from the surface of the steel sheet for surface layer to ½ of the thickness and is excellent in bendability.

TABLE C-1-1 Multilayer steel sheet Composition of constituents of steel sheet for sheet thickness middle part (mass %) no. C Si Mn P S sol.Al N Ni Nb Ti Mo B Remarks 1 0.22 1.67 1.23 0.007 0.0025 0.036 0.0041 0 0 0 0 0 2 0.36 1.84 0.68 0.011 0.0028 0.027 0.0053 0 0 0 0 0 3 0.28 2.29 1.33 0.012 0.0015 0.056 0.0030 0 0 0 0 0 4 0.50 1.33 1.15 0.006 0.0043 0.032 0.0068 0 0 0 0 0 5 0.19 2.23 1.48 0.006 0.0033 0.059 0.0044 0 0 0 0 0 Comp. steel 6 0.20 0.71 1.21 0.005 0.0029 0.025 0.0053 0 0 0 0 0 7 0.22 2.70 1.21 0.010 0.0045 0.046 0.0055 0 0 0 0 0 8 0.24 1.14 0.65 0.009 0.0049 0.036 0.0032 0 0 0 0 0 9 0.72 1.45 0.68 0.007 0.0007 0.038 0.0060 0 0 0 0 0 Comp. steel 10 0.22 1.50 0.58 0.006 0.0059 0.052 0.0036 0 0 0 0 0 11 0.34 0.42 1.18 0.010 0.0020 0.030 0.0038 0 0 0 0 0 Comp. steel 12 0.36 1.29 1.25 0.010 0.0016 0.041 0.0019 0 0 0 0 0 13 0.25 0.85 0.80 0.006 0.0004 0.034 0.0023 0.10 0 0 0 0 14 0.27 1.12 1.49 0.005 0.0007 0.031 0.0011 0 0 0 0 0.0015 15 0.26 1.71 1.34 0.010 0.0051 0.034 0.0047 0 0.045 0.025 0 0.0020 16 0.22 0.72 1.42 0.010 0.0041 0.047 0.0066 0 0 0 0 0 17 0.23 0.58 1.13 0.012 0.0001 0.041 0.0062 0 0 0 0 0 18 0.37 2.64 0.58 0.004 0.006 0.035 0.0044 0 0 0 0 0 19 0.32 1.03 1.21 0.008 0.0038 0.030 0.0021 0 0 0 0 0 20 0.37 2.35 0.56 0.007 0.0036 0.059 0.0053 0 0 0 0 0 21 0.30 0.95 0.89 0.011 0.0035 0.054 0.0031 0 0 0 0 0 22 0.55 1.03 1.00 0.004 0.0004 0.028 0.0055 0 0.020 0.025 0 0.0015 23 0.56 1.80 1.04 0.009 0.0060 0.023 0.0053 0 0 0 0 0 24 0.51 0.65 0.76 0.008 0.0028 0.036 0.0028 0 0 0 0 0 25 0.36 2.06 1.42 0.005 0.0020 0.041 0.0013 0 0 0 0 0 26 0.33 1.01 1.27 0.011 0.0027 0.049 0.0041 0 0 0 0 0 29 0.61 2.88 0.60 0.011 0.0047 0.058 0.0047 0 0 0 0 0 30 0.64 2.41 1.34 0.004 0.0006 0.038 0.0064 0 0 0 0 0 In the table, fields with compositions of constituents of 0 indicate corresponding constituents not intentionally added.

TABLE C-1-2 Multilayer steel sheet Chemical consttuents of steel sheet for sheet thickness middle part (mass %) no. C Si Mn P S sol.Al N Ni Nb Ti Mo B Remarks 31 0.32 1.13 0.84 0.006 0.0032 0.043 0.0014 0 0 0 0 0 32 0.31 2.44 0.81 0.100 0.0060 2.600 0.0052 0 0 0 0 0 33 0.28 2.08 0.67 0.083 0.0040 0.057 0.0030 2.50 0 0 0 0 34 0.25 1.81 0.82 0.108 0.0030 0.053 0.0053 0.05 0 0 0 0 35 0.34 2.48 1.08 0.072 0.0020 0.042 0.0033 0 0.120 0 0 0 36 0.40 2.78 1.33 0.112 0.0030 0.059 0.0034 0 0 0.130 0 0 37 0.28 1.89 0.80 0.088 0.0020 0.046 0.0034 0 0 0 0.600 0 38 0.36 2.04 0.96 0.066 0.0050 0.059 0.0031 0 0 0 0.100 0 39 0.38 2.48 1.05 0.084 0.0060 0.054 0.0057 0 0 0 0 0.0070 40 0.31 2.43 1.11 0.100 0.0020 0.039 0.0036 0 0 0 0 0 41 0.36 2.37 1.36 0.066 0.0040 0.035 0.0034 0 0 0 0 0 42 0.32 1.93 1.08 0.119 0.0060 0.055 0.0055 0 0 0 0 0 43 0.31 1.93 0.63 0.082 0.0040 0.053 0.0038 0 0 0 0 0 44 0.27 2.13 1.09 0.103 0.0040 0.049 0.0034 0 0 0 0 0 45 0.37 1.96 0.64 0.074 0.0050 0.055 0.0044 0 0 0 0 0 46 0.25 2.80 0.60 0.080 0.0030 0.053 0.0037 0 0 0 0 0 47 0.34 2.00 1.38 0.095 0.0040 0.034 0.0054 0 0 0 0 0 48 0.33 2.41 1.26 0.071 0.0060 0.055 0.0032 0 0 0 0 0 49 0.35 2.55 1.32 0.066 0.0050 0.037 0.0038 0 0 0 0 0 50 0.36 1.84 0.68 0.011 0.0028 0.027 0.0053 0 0 0 0 0 51 0.36 1.84 0.68 0.011 0.0028 0.027 0.0053 0 0 0 0 0 52 0.36 1.84 0.68 0.011 0.0028 0.027 0.0053 0 0 0 0 0 53 0.36 1.84 0.68 0.011 0.0028 0.027 0.0053 0 0 0 0 0 In the table, fields with compositions of constituents of 0 indicate corresponding constituents not intentionally added.

TABLE C-1-3 Thickness of steel sheet for Multilayer surface steel sheet Composition of constituents of steel sheet for surface layer (mass %) layer no. C Si Mn P S sol.Al N Ni Nb Ti Mo B (mm) Remarks 1 0.106 0.735 0.517 0.006 0.0024 0.048 0.0046 0 0 0 0 0 96 2 0.173 1.030 0.360 0.006 0.0065 0.036 0.0016 0 0 0 0 0 91 3 0.099 0.847 0.399 0.006 0.0032 0.036 0.0035 0 0 0 0 0 95 4 0.294 0.479 0.414 0.006 0.0056 0.039 0.0070 0 0 0 0 0 96 5 0.082 1.115 0.592 0.003 0.0070 0.034 0.0020 0 0 0 0 0 78 Comp. steel 6 0.075 0.355 0.520 0.006 0.0018 0.034 0.0031 0 0 0 0 0 82 7 0.100 1.485 0.387 0.009 0.0025 0.034 0.0068 0 0 0 0 0 84 8 0.143 0.684 0.390 0.009 0.0038 0.027 0.0027 0 0 0 0 0 106 9 0.310 0.827 0.252 0.009 0.0078 0.035 0.0059 0 0 0 0 0 85 Comp. steel 10 0.106 0.795 0.313 0.006 0.0080 0.029 0.0058 0 0 0 0 0 85 11 0.174 0.151 0.578 0.004 0.0062 0.049 0.0048 0 0 0 0 0 103 Comp. steel 12 0.182 0.606 0.713 0.011 0.0039 0.032 0.0031 0 0 0 0 0 75 13 0.095 0.357 0.408 0.005 0.0019 0.043 0.0010 0 0 0 0 0 94 14 0.155 0.437 0.760 0.006 0.0023 0.031 0.0024 0 0 0 0 0.0017 89 15 0.153 1.163 0.965 0.012 0.0071 0.040 0.0035 0 0.045 0.025 0 0.0015 83 16 0.096 0.518 0.809 0.009 0.0025 0.050 0.0034 0 0 0 0 0 87 17 0.086 0.197 0.848 0.005 0.0071 0.032 0.0017 0 0 0 0 0 86 18 0.260 1.320 0.209 0.003 0.0027 0.032 0.0070 0 0 0 0 0 90 19 0.287 0.453 1.041 0.006 0.0062 0.035 0.0020 0 0 0 0 0 102 20 0.296 2.021 0.235 0.008 0.0056 0.026 0.0054 0 0 0 0 0 101 21 0.189 0.352 0.472 0.004 0.0045 0.040 0.0036 0 0 0 0 0 105 22 0.456 0.474 0.570 0.004 0.0069 0.034 0.0054 0 0 0 0 0 84 23 0.373 1.278 0.530 0.004 0.0051 0.043 0.0051 0 0 0 0 0 102 24 0.224 0.371 0.631 0.010 0.0028 0.029 0.0038 0.10 0 0 0 0 88 25 0.155 1.030 0.667 0.011 0.0049 0.021 0.0061 0 0 0 0 0 84 26 0.184 0.313 0.762 0.011 0.0075 0.031 0.0044 0 0 0 0 0 89 In the table, fields with compositions of constituents of 0 indicate corresponding constituents not intentionally added.

TABLE C-1-4 Thickness of steel Multilayer sheet for steel Composition of constituents of steel sheet for surface layer (mass %) surface sheet no. C Si Mn P S sol.Al N Ni Nb Ti Mo B layer (mm) Remarks 29 0.262 1.699 0.180 0.009 0.0045 0.038 0.0027 0 0 0 0 0 86 30 0.230 0.940 0.590 0.007 0.0076 0.034 0.0046 0 0 0 0 0 81 31 0.188 0.350 0.454 0.007 0.0048 0.049 0.0064 0 0 0 0 0 162 32 0.146 0.625 1.127 0.006 0.0040 0.040 0.0045 0 0 0 0 0 103 33 0.118 0.545 1.120 0.007 0.0050 0.039 0.0088 0 0 0 0 0 88 34 0.130 0.484 1.128 0.010 0.0030 0.033 0.0077 0 0 0 0 0 107 35 0.136 0.588 0.893 0.012 0.0020 0.021 0.0073 0 0 0 0 0 81 36 0.200 0.673 0.680 0.012 0.0060 0.043 0.0051 0 0 0 0 0 102 37 0.120 0.530 0.966 0.012 0.0050 0.023 0.0056 0 0 0 0 0 96 38 0.180 0.650 1.680 0.007 0.0020 0.050 0.0099 0 0 0 0 0 100 39 0.163 0.383 1.250 0.007 0.0030 0.036 0.0053 0 0 0 0 0 92 40 0.177 0.519 0.800 0.009 0.0030 0.047 0.0097 2.20 0 0 0 0 91 41 0.209 0.573 1.081 0.009 0.0060 0.042 0.0041 0.05 0 0 0 0 86 42 0.157 0.522 1.368 0.011 0.0040 0.023 0.0067 0 0.110 0 0 0 103 43 0.183 0.447 1.566 0.012 0.0040 0.046 0.0064 0 0 0.110 0 0 89 44 0.130 0.423 1.260 0.010 0.0020 0.048 0.0065 0 0 0 0.600 0 92 45 0.215 0.567 1.120 0.013 0.0050 0.045 0.0052 0 0 0 0.100 0 109 46 0.130 0.672 1.653 0.009 0.0050 0.036 0.0059 0 0 0 0 0.010 82 47 0.160 0.452 1.276 0.007 0.0050 0.028 0.0049 0 0 0 0 0.001 88 48 0.211 0.595 1.612 0.011 0.0030 0.020 0.0070 0 0 0 0 0 97 49 0.172 0.656 0.846 0.010 0.0030 0.029 0.0094 0 0 0 0 0 91 50 0.173 1.030 0.360 0.006 0.0065 0.036 0.0016 0 0 0 0 0 91 51 0.173 1.030 0.360 0.006 0.0065 0.036 0.0016 0 0 0 0 0 91 52 0.173 1.030 0.360 0.006 0.0065 0.036 0.0016 0 0 0 0 0 91 53 0.173 1.030 0.360 0.006 0.0065 0.036 0.0016 0 0 0 0 0 91 In the table, fields with compositions of constituents of 0 indicate corresponding constituents not intentionally added.

TABLE C-2-1 Heat treatment at hot stamping Average Heat treatment Rough rolling cooling Average before hot Rate of No. of Hot rolling Cold rate cooling Manu- rolling Roll- reduction rolling Finish Coil- rolling Heat- Heat- (° C./s) rate Tem- Sheet facturing Heating Holding ing of sheet opera- rolling ing Rolling ing ing (more (° C./s) pering thick- condition temp. time temp. thickness tions temp. temp. rate rate temp. than (400° C. temp. ness no. (° C.) (min) (° C.) (%) (times) (° C.) (° C.) (%) (° C./s) (° C.) 400° C.) or less) (° C.) Plating (mm) 1 1275 45 1144 36 3 948 609 69 39 847 70 44 — None 1.6 2 1274 21 1166 31 3 872 510 66 39 848 98 42 — None 1.4 3 1300 23 1145 26 3 924 665 63 41 882 79 29 — None 1.4 4 1278 31 1162 39 3 869 741 50 68 916 94 30 — None 1.2 5 1333 26 1129 45 3 925 635 41 61 849 100 29 — None 1.0 6 1315 55 1161 32 3 915 581 50 68 891 75 25 — None 1.2 7 1193 40 1180 52 3 920 678 51 55 822 87 28 — None 1.2 8 1239 27 1137 52 3 919 606 61 48 838 80 37 — None 1.4 9 1329 53 1169 28 3 905 550 52 23 872 63 30 — None 1.1 10 1191 52 1145 39 3 909 686 45 32 836 76 27 — None 1.0 11 1143 56 1135 43 3 921 747 66 50 903 96 30 — None 1.4 12 1171 50 1150 36 3 942 546 49 62 873 74 38 — None 1.1 13 1108 33 1102 23 3 883 619 45 35 898 101 33 — None 1.0 14 1323 44 1149 37 3 886 633 46 41 869 69 25 — None 1.0 15 1169 35 1150 46 3 856 500 50 24 925 88 27 — None 1.0 16 1127 43 1112 31 3 863 500 69 36 904 104 25 — None 1.6 17 1135 38 1128 49 3 881 590 60 64 850 113 44 — None 1.4 18 1111 50 1103 38 3 948 541 60 49 826 100 32 — None 1.4 19 1194 39 1159 43 3 939 606 55 15 900 71 28 — None 1.2 20 1277 23 1131 40 3 920 563 49 25 917 104 32 — None 1.0 21 1131 31 1121 48 3 867 693 42 25 889 97 41 — None 1.0 22 1204 32 1167 33 3 938 514 57 35 886 98 44 — None 1.2 23 1231 22 1129 44 3 876 668 60 71 896 90 32 — None 1.4 24 1160 23 1142 32 3 936 657 53 75 855 74 26 — None 1.2 25 1070 20 1020 40 3 943 523 53 65 892 96 27 — None 1.2 26 1370 43 1123 16 3 912 666 49 20 907 86 43 — None 1.1 In the table, fields with notations — indicate corresponding treatment not performed.

TABLE C-2-2 Heat treatment at hot stamping Average Heat treatment Rough rolling cooling Average before hot Rate of No. of Hot rolling Cold rate cooling Manu- rolling Roll- reduction rolling Finish Coil- rolling Heat- Heat- (° C./s) rate Tem- Sheet facturing Heating Holding ing of sheet opera- rolling ing Rolling ing ing (more (° C./s) pering thick- condition temp. time temp. thickness tions temp. temp. rate rate temp. than (400° C. temp. ness no. (° C.) (min) (° C.) (%) (times) (° C.) (° C.) (%) (° C./s) (° C.) 400° C.) or less) (° C.) Plating (mm) 29 1192 21 1145 35 3 941 659 57 35 919 89 33 322 None 1.2 30 1336 54 1136 37 3 864 649 60 55 926 92 27 395 Yes 1.4 31 1178 25 1135 50 3 861 708 44 74 852 72 38 — None 1 32 1258 37 1137 23 3 900 630 45 65 947 80 34 — None 1.7 33 1246 47 1128 13 3 868 576 45 62 841 85 44 — None 1.2 34 1226 32 1121 22 3 868 576 56 41 891 67 27 — None 1.1 35 1208 23 1123 38 3 895 588 47 62 937 88 41 — None 1.5 36 1256 46 1160 46 3 879 616 52 53 906 100 29 — None 1.1 37 1196 50 1152 52 3 881 621 56 33 895 81 43 — None 1.3 38 1191 58 1152 44 3 894 563 53 33 856 104 38 — None 1.1 39 1245 34 1121 22 3 893 618 52 69 848 72 37 — None 1.6 40 1236 51 1166 22 3 937 622 45 67 873 83 30 — None 1.6 41 1217 32 1170 29 3 945 628 45 70 913 98 30 — None 1.7 42 1248 23 1143 35 3 864 554 46 28 861 70 35 — None 1.7 43 1237 24 1150 35 3 918 620 57 67 842 93 42 — None 1.2 44 1188 33 1132 51 3 924 621 55 28 925 97 35 — None 1.7 45 1183 23 1131 21 3 909 621 48 39 835 82 36 — None 1 46 1236 24 1126 14 3 910 590 52 56 846 102 39 — None 1.1 47 1239 42 1125 22 3 917 595 50 67 838 72 37 — None 1.4 48 1195 56 1124 39 3 896 642 57 67 893 88 39 — None 1.8 49 1214 15 1182 22 3 853 583 51 66 910 99 33 — None 1.1 50 1208 32 1013 48 3 882 697 40 71 917 107 26 — None 1.7 51 1191 50 1161 4 2 893 634 59 28 934 71 36 — None 1 52 1236 34 1137 46 1 906 713 50 63 903 84 32 — None 1 53 1248 22 1103 46 3 899 667 51 60 892 96 34 — None 1.4 In the table, fields with notations — indicate corresponding treatment not performed.

TABLE C-3-1 Metal structure Any rate (%) of total of crystal grains with maximum difference of crystal orientation inside large angle grain boundaries of 1° or less and crystal grains with Hard- maximum ness difference of of middle crystal Multi- Manu- part in orientation Mechanical properties layer facturing sheet of 8° Maximum Residual Stamped steel condi- thick- or more Tensile Uniform bending Hydrogen γ area body sheet tion ness and less strength elongation angle embrittlement rate no. no. no. (Hv) than 15° (MPa) (%) (°) resistance (%) Remarks 1C 1 1 576 70 1547 5.2 87.5 Good 4.5 Inv. ex. 2C 2 2 738 50 2083 7.5 95.6 Good 2.6 Inv. ex. 3C 3 3 639 58 2156 5.6 95.6 Good 3 Inv. ex. 4C 4 4 785 68 2212 6 71.1 Good 1.7 Inv. ex. 5C 5 5 402 55 1116 6.5 91.9 Good 4 Comp. ex. 6C 6 6 554 54 1770 6.2 88.5 Good 3.8 Inv. ex. 7C 7 7 567 65 1644 7.2 85.5 Good 2.9 Inv. ex. 8C 8 8 592 79 1963 6.6 82.2 Good 4.4 Inv. ex. 9C 9 9 823 64 2442 5.6 39.4 Good 1 Comp. ex. 10C 10 10 694 71 1805 7.5 80.4 Good 2.5 Inv. ex. 11C 11 11 664 55 2145 4.2 97.7 Good 0.8 Comp. ex. 12C 12 12 727 82 2062 6.3 88.2 Good 3.1 Inv. ex. 13C 13 13 622 84 1901 5.7 89.8 Good 2 Inv. ex. 14C 14 14 667 85 1969 7.4 84.7 Good 4.8 Inv. ex. 15C 15 15 542 54 1545 5.2 81 Good 2 Inv. ex. 16C 16 16 590 80 1536 7.5 83.9 Good 2.6 Inv. ex. 17C 17 17 513 80 1549 6.8 92.8 Good 2.6 Inv. ex. 18C 18 18 759 59 2024 6.1 88 Good 2.3 Inv. ex. 19C 19 19 603 77 2006 6.4 87 Good 2.9 Inv. ex. 20C 20 20 578 81 1895 6.3 92.3 Good 4.6 Inv. ex. 21C 21 21 713 79 2035 5.8 87.6 Good 4.9 Inv. ex. 22C 22 22 683 74 2690 7.2 75.6 Good 3.6 Inv. ex. 23C 23 23 726 69 2475 6.4 78.1 Good 4.3 Inv. ex. 24C 24 24 771 55 2450 5.5 73.2 Good 1.9 Inv. ex. 25C 25 25 700 15 1964 7.5 54.3 Poor 1 Comp. ex. 26C 26 26 728 92 1832 7.1 54.2 Good 3 Comp. ex.

TABLE C-3-2 Metal structure Any rate (%) of total of crystal grains with maximum difference of crystal orientation inside large angle grain boundaries of 1° or less and crystal grains with Hard- maximum ness difference of of middle crystal Multi- Manu- part in orientation Mechanical properties layer facturing sheet of 8° Maximum Residual Stamped steel condi- thick- or more Tensile Uniform bending Hydrogen γ area body sheet tion ness and less strength elongation angle embrittlement rate no. no. no. (Hv) than 15° (MPa) (%) (°) resistance (%) Remarks 29C 29 29 792 51 2102 5.8 89.1 Good 4.7 Inv. ex. 30C 30 30 698 70 2317 5.1 81.8 Good 3.5 Inv. ex. 31C 31 31 695 56 2139 6.6 82 Good 3 Inv. ex. 32C 32 32 633 72 2018 5.5 74 Good 3.7 Inv. ex. 33C 33 33 671 62 2261 6.9 90 Good 2.6 Inv. ex. 34C 34 34 612 64 2240 6.2 73 Good 3.3 Inv. ex. 35C 35 35 682 71 2055 7.5 82 Good 4.3 Inv. ex. 36C 36 36 622 61 2118 5.8 86 Good 2.5 Inv. ex. 37C 37 37 659 50 2003 5.7 75 Good 2.4 Inv. ex. 38C 38 38 647 72 2253 6.5 87 Good 2.9 Inv. ex. 39C 39 39 614 73 2072 7.4 75 Good 3.4 Inv. ex. 40C 40 40 649 56 2115 5.8 83 Good 2.6 Inv. ex. 41C 41 41 606 62 2167 6.6 90 Good 2.9 Inv. ex. 42C 42 42 639 61 2032 6.4 83 Good 4.5 Inv. ex. 43C 43 43 670 53 2140 8 75 Good 4.1 Inv. ex. 44C 44 44 669 67 2055 7 88 Good 2.9 Inv. ex. 45C 45 45 692 53 2246 6.5 78 Good 4.4 Inv. ex. 46C 46 46 673 62 2222 6.5 90 Good 2.1 Inv. ex. 47C 47 47 659 56 2156 6.5 90 Good 2.7 Inv. ex. 48C 48 48 625 52 2177 7.8 85 Good 2.3 Inv. ex. 49C 49 49 656 13 2222 6.5 62 Poor 2.8 Comp. ex. 50C 50 50 630 9 2079 6.7 59.1 Poor 3.1 Comp. ex. 51C 51 51 644 10 2125 6.2 59.8 Poor 3.1 Comp. ex. 52C 52 52 635 13 2096 6.3 60.4 Poor 3.2 Comp. ex. 53C 53 53 638 47 2105 6.3 112.5 Good 3.1 Inv. ex.

Manufacturing Example D

Steel sheets for sheet thickness middle part having the Nos. 1 to 37 chemical compositions shown in Table D-1-1 to Table D-1-2 (in the tables, “Steel Nos. 1 to 37”) were ground down at their surfaces to remove the surface oxides. After that, the respective steel sheets for sheet thickness middle part were welded with steel sheets for surface layer having the chemical compositions shown in Table D-1-3 to Table D-1-4 at both surfaces or single surfaces by arc welding to fabricate the Nos. 1 to 60 multilayer steel sheets for hot stamped body. The sheet thickness of the total of the steel sheet for surface layer and the steel sheet for sheet thickness middle part after arc welding was 200 mm to 300 mm and the thickness of the steel sheet for surface layer was ⅓ or so of the thickness of the steel sheet for sheet thickness middle part (in case of single side, ¼ or so). The No. 37 multilayer steel sheet is steel with the steel sheet for surface layer welded to only one surface. The multilayer steel sheets other than No. 37 have steel sheets for surface layer welded to both surfaces of the steel sheet for sheet thickness middle part. In the Nos. 1 to 60 multilayer steel sheets of Table D-1-1 to Table D-1-4, cases where the steel sheet for sheet thickness middle part does not satisfy the requirement of the composition of the middle part in sheet thickness of the hot stamped body according to the present invention are indicated as “comparative steels” in the remarks column.

The Nos. 1 to 60 multilayer steel sheets were treated under the conditions of the Nos. 1 to 60 manufacturing conditions shown in Table D-2-1 to Table D-2-3 by heat treatment before hot rolling, rough rolling, hot rolling, and cold rolling to obtain steel sheets. Next, the steel sheets were heat treated as shown in Table D-2-1 to Table D-2-3 (in the tables, “heat treatment of hot stamped bodies”) for hot stamping to produce the Nos. 1D to 60D hot stamped bodies (“stamped bodies” of Tables D-3-1 to D-3-3). Further, the Nos. 38D and 39D hot stamped bodies were coated on a hot dip coating line at the surfaces with 120 to 160 g/m²amounts of aluminum. Further, the items of Table D-2-1 to Table D-2-3 correspond to the items of Table A-2-1 to Table A-2-2. Further, in the tables, the fields with the notations “-” indicate no corresponding treatment performed.

Tables D-3-1 to D-3-3 show the metal structures and characteristics of the Nos. 1D to 60D hot stamped bodies. The constituents obtained by analyzing the positions of ½ of the sheet thicknesses of the samples taken from hot stamped bodies (middle parts in sheet thickness) and positions of 20 μm from the surfaces of the softened layers were equivalent to the constituents of the steel sheets for sheet thickness middle part and the steel sheets for surface layer of the Nos. 1 to 60 multilayer steel sheets of Table D-1-1 to Table D-1-3.

The metal structures of the hot stamped steel sheets were measured by the above-mentioned method. The hardness of the steel sheet for sheet thickness middle part forming the middle part in sheet thickness and the area rate of the total of the crystal grains with a maximum crystal orientation difference inside the regions surrounded by grain boundaries of 15° or more of 1° or less and the crystal grains with a crystal orientation difference of 8° or more and less than 15° in the metal structures from the surface of the steel sheet for surface layer forming the softened layer to ½ of the thickness of that softened layer were calculated. The calculated values of the area rate are shown in the items “area rate (%) of total of crystal grains with maximum crystal orientation difference inside large angle grain boundaries of 1° or less and crystal grains with maximum crystal orientation difference of 8° or more and less than 15°” of Tables D-3-1 to D-3-3.

The Nos. 1D to 60D hot stamped bodies were subjected to tensile tests. The results are shown in Tables D-3-1 to D-3-3. The tensile tests were performed by fabricating No. 5 test pieces described in JIS Z 2201 and testing them by the method described in JIS Z 2241.

The hot stamped bodies were evaluated for hydrogen embrittlement resistance in the same way as Manufacturing Example A using test pieces cut out from the stamped bodies. That is, test pieces of a sheet thickness of 1.2 mm×width 6 mm×length 68 mm were cut out from the stamped bodies, given strain corresponding to the yield stress in four-point bending tests, then immersed in pH3 hydrochloric acid for 100 hours and evaluated for hydrogen embrittlement resistance by the presence of any cracks. Cases of no fracture were evaluated as passing (“good”) and cases of fracture were evaluated as failing (“Poor”).

For the purpose of evaluating the impact resistance of the hot stamped body, the body was evaluated based on the VDA standard (VDA238-100) prescribed by the German Association of the Automotive Industry under the same measurement conditions as Manufacturing Example A. In the present invention, the displacement at the time of maximum load obtained in the bending test was converted to angle by the VDA standard to find maximum bending angle and thereby evaluate the impact resistance of the hot stamped body.

The hot stamped bodies were also evaluated for impact resistance from the viewpoint of ductility. Specifically, the hot stamped steel sheets were subjected to tensile tests to find the uniform elongations of the steel sheet to evaluate the impact resistance. The tensile tests were performed by fabricating No. 5 test pieces described in JIS Z 2201 and testing them by the method described in JIS Z 2241. The elongations where the maximum tensile loads were obtained were defined as the uniform elongations.

Deformation concentrates at a local softened part at the time of collision and becomes a cause of cracking, so a small scattering in hardness at the stamped body, that is, securing stable strength, is important in securing impact resistance. Therefore, the impact resistance of a hot stamped body was also evaluated from the viewpoint of the scattering in hardness. A cross-section vertical to the longitudinal direction of a long hot stamped body was taken at any position in that longitudinal direction and measured for hardness at the middle position in sheet thickness at the entire cross-sectional region including the vertical walls. For the measurement, use was made of a Vickers hardness tester. The measurement load was 1 kgf, 10 points were measured, and the measurement interval was 1 mm. The difference between the average cross-sectional hardness and the minimum hardness is shown in Table D-3-1 to Table D-3-3. Cases with no measurement points of below 100 Hv from the average value of all measurement points were evaluated as being small in scattering in hardness, that is, excellent in stability of strength and, as a result, were evaluated as excellent in impact resistance and therefore passing, while cases with measurement points below 100 Hv were evaluated as failing.

Cases where the tensile strength was 1500 MPa or more, the uniform elongation was 5% or more, the scattering in hardness was a passing level, the maximum bending angle (°) was 70.0(°) or more, and the hydrogen embrittlement resistance was passing were evaluated as hot stamped bodies excellent in impact resistance and hydrogen embrittlement resistance (“invention examples” in Table D-3-1 to Table D-3-3). On the other hand, cases where even one of the above five aspects of performance was not satisfied are indicated as “comparative examples”.

In each of the hot stamped bodies of the invention examples, the area rate of the total of crystal grains with a maximum crystal orientation difference inside regions surrounded by grain boundaries of 15° or higher of 1° or less and crystal grains with a crystal orientation difference of 8° or more and less than 15° in the metal structures from the surface of the steel sheet for surface layer to ½ of the thickness was 50% to less than 85%. Further, in each of the hot stamped bodies of the invention examples, the tensile strength, bendability, and hydrogen embrittlement resistance were excellent.

As opposed to this, the No. 5D hot stamped body was low in carbon content of the steel sheet for sheet thickness middle part, so became insufficient in hardness of the middle part in sheet thickness and became insufficient in tensile strength. The No. 9D hot stamped body was excessive in carbon content of the steel sheet for sheet thickness middle part, so became excessive in hardness of the middle part in sheet thickness as well and could not be given the targeted bendability. Further, the Nos. 10D and 11D hot stamped bodies were sparse in Si content of the steel sheet for sheet thickness middle part, so were insufficient in uniform elongation. Further, the No. 12D hot stamped body was insufficient in Mn content, so became insufficient in hardness of the middle part in sheet thickness and were insufficient in tensile strength. The No. 14D and the No. 15D hot stamped bodies were sparse in Si content and Mn content, so had an area percent of residual austenite of less than 1.0% and an insufficient uniform elongation. Further, the No. 12D to No. 15D hot stamped bodies were large in scattering in hardness and deemed failing.

The Nos. 33D to 35D hot stamped bodies are comparative examples produced using multilayer steel sheets for hot stamped body which were not subjected to the desirable heat treatment before the hot stamping process. The No. 33D hot stamped body was low in heat treatment temperature before the hot stamping process, so became insufficient in growth of soft structures and metal structures of intermediate hardnesses in the metal structures of the softened layer from the surface of the softened layer to ½ of the thickness and was not able to be given the targeted bendability. The No. 34D hot stamped body was excessively high in heat treatment temperature before the hot stamping process, so the fraction of structures from a position of 20 μm from the surface of the softened layer to a position of a depth of ½ of the thickness of the softened layer exceeded 85%. For this reason, in the No. 34D hot stamped body, the difference in hardness between the softened layer and the middle part in sheet thickness became too large, and the effect of reduction of the sharp gradient in hardness in the sheet thickness direction occurring at the time of bending deformation could not be obtained. Further, the No. 35D hot stamped body was short in heat treatment time before the hot stamping process, so in the metal structures from the surface of the softened layer to ½ of the thickness, the soft structures and metal structures with intermediate hardnesses insufficiently grew and the target bendability could not be obtained.

The No. 40D hot stamped body was excessive in Si content, so residual austenite was excessively produced exceeding an area percent of 5%. For this reason, the No. 40D hot stamped body was inferior in bendability. The No. 41D hot stamped body was excessive in Mn content, so became the greatest in tensile strength among the Nos. 1D to 56D hot stamped bodies and was inferior in bendability. The No. 42D hot stamped body was poor in content of acid soluble aluminum, so inclusions containing oxygen were excessively produced and bendability was inferior. Further, the No. 45D hot stamped body included excessive aluminum, so oxides mainly comprised of aluminum were excessively produced and bendability was inferior.

The No. 57D hot stamped body was low in rolling temperature of the rough rolling. Further, the No. 58D hot stamped body was low in sheet thickness reduction rate of the rough rolling. Further, the No. 59D hot stamped body was low in number of rolling operations under conditions of a time between passes of 3 seconds or more. These hot stamped bodies were not produced under optimal rough rolling conditions, so were insufficient in growth of soft structures and metal structures of intermediate hardnesses, were not able to be eased in strain caused by bending deformation, and were not able to be given the targeted bendability.

The No. 60D hot stamped body is steel sheet with a casting rate controlled to 6 ton/min or more in a continuous casting process of steel sheet for surface layer. It can raise the area rate of the total of crystal grains with a maximum crystal orientation difference inside regions surrounded by grain boundaries of 15° or higher of 1° or less and crystal grains with a crystal orientation difference of 8° or more and less than 15° in the metal structures from the surface of the steel sheet for surface layer to ½ of the thickness and is excellent in bendability.

TABLE D-1-1 Multi- layer steel Chemical constituents of steel sheet for sheet thickness middle part (mass %) sheet Steel no. no. C Si Mn P S sol.Al N Ni Nb Ti Mo B Remarks 1 1 0.26 1.32 1.82 0.013 0.0028 0.049 0.0036 0 0 0 0 0 2 2 0.27 1.29 1.84 0.012 0.0011 0.045 0.0035 0 0 0 0 0 3 3 0.35 1.54 1.68 0.013 0.0011 0.043 0.0036 0 0 0 0 0 4 4 0.48 1.5 2.08 0.005 0.0002 0.035 0.0026 0 0 0 0 0 5 5 0.08 1.25 1.77 0.016 0.0010 0.029 0.0036 0 0 0 0 0 Comp. steel 6 6 0.23 1.45 1.98 0.016 0.0014 0.035 0.0037 0 0 0 0 0 7 7 0.36 1.81 1.89 0.010 0.0018 0.044 0.0032 0 0 0 0 0 8 8 0.28 1.75 1.90 0.011 0.0017 0.054 0.0029 0 0 0 0 0 9 9 0.83 1.65 1.84 0.017 0.0008 0.034 0.0027 0 0 0 0 0 Comp. steel 10 10 0.38 0.13 1.87 0.009 0.0022 0.058 0.003 0 0 0 0 0 Comp. steel 11 11 0.32 0.41 1.90 0.020 0.0016 0.048 0.0038 0 0 0 0 0 Comp. steel 12 12 0.25 1.21 0.19 0.012 0.0009 0.046 0.0023 0 0 0 0 0 Comp. steel 13 13 0.31 1.27 0.90 0.006 0.0021 0.052 0.0028 0 0 0 0 0 Comp. steel 14 14 0.34 0.48 1.34 0.018 0.0016 0.051 0.0041 0 0 0 0 0 Comp. steel 15 15 0.27 0.27 1.18 0.016 0.0008 0.053 0.0031 0 0 0 0 0 Comp. steel 16 16 0.30 1.59 1.75 0.004 0.0012 0.052 0.0030 0.33 0 0 0 0 17 17 0.36 1.00 1.78 0.022 0.0007 0.045 0.0032 0 0.078 0 0 0 18 18 0.27 1.63 1.97 0.016 0.0012 0.051 0.0029 0 0 0.032 0 0 19 19 0.29 1.27 2.01 0.013 0.0013 0.057 0.0030 0 0 0 0.040 0 20 20 0.30 1.45 1.72 0.014 0.0016 0.043 0.0032 0 0 0 0 0.0020 21 1 0.26 1.32 1.82 0.013 0.0028 0.049 0.0036 0 0 0 0 0 22 1 0.26 1.32 1.82 0.013 0.0028 0.049 0.0036 0 0 0 0 0 23 1 0.26 1.32 1.82 0.013 0.0028 0.049 0.0036 0 0 0 0 0 24 2 0.27 1.29 1.84 0.012 0.0011 0.045 0.0035 0 0 0 0 0 25 2 0.27 1.29 1.84 0.012 0.0011 0.045 0.0035 0 0 0 0 0 26 2 0.27 1.29 1.84 0.012 0.0011 0.045 0.0035 0 0 0 0 0 27 3 0.35 1.54 1.68 0.013 0.0011 0.043 0.0036 0 0 0 0 0 28 3 0.35 1.54 1.68 0.013 0.0011 0.043 0.0036 0 0 0 0 0 29 3 0.35 1.54 1.68 0.013 0.0011 0.043 0.0036 0 0 0 0 0 30 4 0.48 1.5 2.08 0.005 0.0002 0.035 0.0026 0 0 0 0 0 In the table, fields with compositions of constituents of 0 indicate corresponding constituents not intentionally added.

TABLE D-1-2 Multi- layer steel Chemical constituents of steel sheet for sheet thickness middle part (mass %) sheet Steel no. no. C Si Mn P S sol.Al N Ni Nb Ti Mo B Remarks 31 4 0.48 1.50 2.08 0.005 0.0002 0.035 0.0026 0 0 0 0 0 32 4 0.48 1.50 2.08 0.005 0.0002 0.035 0.0026 0 0 0 0 0 33 2 0.27 1.29 1.84 0.012 0.0011 0.045 0.0035 0 0 0 0 0 34 2 0.27 1.29 1.84 0.012 0.0011 0.045 0.0035 0 0 0 0 0 35 2 0.27 1.29 1.84 0.012 0.0011 0.045 0.0035 0 0 0 0 0 36 2 0.27 1.29 1.84 0.012 0.0011 0.045 0.0035 0 0 0 0 0 37 21 0.65 1.29 1.84 0.008 0.0005 0.058 0.003 0 0 0 0 0 38 21 0.65 1.29 1.84 0.008 0.0005 0.058 0.003 0 0 0 0 0 39 2 0.27 1.29 1.84 0.012 0.0011 0.045 0.0035 0 0 0 0 0 40 22 0.38 5.30 1.87 0.009 0.0022 0.058 0.003 0 0 0 0 0 Comp. steel 41 23 0.25 1.21 4.90 0.012 0.0009 0.046 0.0023 0 0 0 0 0 Comp. steel 42 24 0.26 1.32 1.82 0.013 0.0028 0.0001 0.0036 0 0 0 0 0 Comp. steel 43 25 0.26 1.32 1.82 0.013 0.0028 0.001 0.0036 0 0 0 0 0 44 26 0.26 1.32 1.82 0.013 0.0028 2.600 0.0036 0 0 0 0 0 45 27 0.26 1.32 1.82 0.013 0.0028 4.200 0.0036 0 0 0 0 0 Comp. steel 46 28 0.30 1.59 1.75 0.004 0.0012 0.052 0.0030 0.03 0 0 0 0 47 29 0.30 1.59 1.75 0.004 0.0012 0.052 0.0030 2.70 0 0 0 0 48 30 0.36 1.00 1.78 0.022 0.0007 0.045 0.0032 0 0.020 0 0 0 49 31 0.36 1.00 1.78 0.022 0.0007 0.045 0.0032 0 0.130 0 0 0 50 32 0.27 1.63 1.97 0.016 0.0012 0.051 0.0029 0 0 0.040 0 0 51 33 0.27 1.63 1.97 0.016 0.0012 0.051 0.0029 0 0 0.120 0 0 52 34 0.29 1.27 2.01 0.013 0.0013 0.057 0.003 0 0 0 0.009 0 53 35 0.29 1.27 2.01 0.013 0.0013 0.057 0.003 0 0 0 0.900 0 54 36 0.30 1.45 1.72 0.014 0.0016 0.043 0.0032 0 0 0 0 0.0009 55 37 0.30 1.45 1.72 0.014 0.0016 0.043 0.0032 0 0 0 0 0.0070 56 4 0.48 1.50 2.08 0.005 0.0002 0.035 0.0026 0 0 0 0 0 57 2 0.27 1.29 1.84 0.012 0.0011 0.045 0.0035 0 0 0 0 0 58 2 0.27 1.29 1.84 0.012 0.0011 0.045 0.0035 0 0 0 0 0 59 2 0.27 1.29 1.84 0.012 0.0011 0.045 0.0035 0 0 0 0 0 60 2 0.27 1.29 1.84 0.012 0.0011 0.045 0.0035 0 0 0 0 0 In the table, fields with compositions of constituents of 0 indicate corresponding constituents not intentionally added.

TABLE D-1-3 Multi- layer steel sheet Chemical constituents of steel sheet for sheet thickness middle part (mass %) no. C Si Mn P S sol.Al N Ni Nb Ti Mo B Remarks 1 0.15 0.66 0.91 0.011 0.0026 0.045 0.0031 0 0 0 0 0 2 0.12 0.72 0.94 0.01 0.0008 0.041 0.003 0 0 0 0 0 3 0.19 0.88 0.96 0.009 0.0008 0.038 0.0031 0 0 0 0 0 4 0.28 0.78 1.14 0.004 0.0001 0.029 0.0021 0 0 0 0 0 5 0.05 0.60 0.90 0.014 0.0006 0.024 0.0034 0 0 0 0 0 Comp. steel 6 0.11 0.80 1.09 0.013 0.0011 0.029 0.0035 0 0 0 0 0 7 0.17 0.81 0.91 0.009 0.0015 0.041 0.0029 0 0 0 0 0 8 0.15 0.86 0.86 0.007 0.0014 0.050 0.0027 0 0 0 0 0 9 0.39 0.79 0.92 0.015 0.0005 0.029 0.0024 0 0 0 0 0 Comp. steel 10 0.21 0.06 0.94 0.004 0.0021 0.053 0.0027 0 0 0 0 0 Comp. steel 11 0.16 0.24 0.93 0.019 0.0015 0.042 0.0035 0 0 0 0 0 Comp. steel 12 0.14 0.64 0.11 0.009 0.0005 0.041 0.0021 0 0 0 0 0 Comp. steel 13 0.17 0.64 0.41 0.002 0.0017 0.046 0.0024 0 0 0 0 0 Comp. steel 14 0.16 0.24 0.72 0.014 0.0015 0.046 0.0036 0 0 0 0 0 Comp. steel 15 0.14 0.15 0.58 0.013 0.0006 0.047 0.0029 0 0 0 0 0 Comp. steel 16 0.15 0.78 0.95 0.003 0.0008 0.046 0.0028 0.21 0 0 0 0 17 0.17 0.55 0.91 0.020 0.0003 0.040 0.0027 0 0.036 0 0 0 18 0.15 0.85 1.04 0.013 0.0009 0.046 0.0026 0 0 0.028 0 0 19 0.15 0.74 1.13 0.008 0.0012 0.054 0.0027 0 0 0 0.030 0 20 0.17 0.78 0.81 0.013 0.0012 0.039 0.0027 0 0 0 0 0.0020 21 0.21 0.59 0.87 0.009 0.0027 0.044 0.0031 0 0 0 0 0 22 0.12 0.91 0.82 0.009 0.0027 0.043 0.0032 0 0 0 0 0 23 0.14 0.73 1.55 0.010 0.0025 0.044 0.0032 0 0 0 0 0 24 0.21 0.59 1.03 0.009 0.0009 0.041 0.0033 0 0 0 0 0 25 0.14 0.95 0.83 0.009 0.0007 0.039 0.0031 0 0 0 0 0 26 0.15 0.66 1.69 0.011 0.0008 0.041 0.0032 0 0 0 0 0 27 0.28 0.75 0.81 0.010 0.001 0.039 0.0032 0 0 0 0 0 28 0.20 1.40 0.96 0.008 0.0009 0.037 0.0032 0 0 0 0 0 29 0.17 0.89 1.46 0.009 0.0009 0.037 0.0031 0 0 0 0 0 30 0.41 0.75 1.1 0.002 0.0009 0.032 0.0021 0 0 0 0 0 In the table, fields with compositions of constituents of 0 indicate corresponding constituents not intentionally added.

TABLE D-1-4 Multi- layer steel sheet Chemical constituents of steel sheet for sheet thickness middle part (mass %) no. C Si Mn P S sol.Al N Ni Nb Ti Mo B Remarks 31 0.24 1.26 1.02 0.003 0.0009 0.029 0.0021 0 0 0 0 0 32 0.27 0.84 1.68 0.003 0.0009 0.030 0.0024 0 0 0 0 0 33 0.13 0.62 1.01 0.010 0.0009 0.039 0.0032 0 0 0 0 0 34 0.16 0.62 0.83 0.007 0.0010 0.041 0.0031 0 0 0 0 0 35 0.12 0.63 0.96 0.007 0.0010 0.040 0.0033 0 0 0 0 0 36 0.15 0.66 0.85 0.011 0.0009 0.040 0.0031 0 0 0 0 0 37 0.3 0.59 1.03 0.006 0.0003 0.053 0.0027 0 0 0 0 0 38 0.32 0.74 0.98 0.007 0.0003 0.054 0.0028 0 0 0 0 0 39 0.14 0.58 0.86 0.008 0.0008 0.042 0.0033 0 0 0 0 0 40 0.21 0.06 0.94 0.007 0.002 0.054 0.0026 0 0 0 0 0 Comp. steel 41 0.14 0.64 0.11 0.011 0.0006 0.043 0.0018 0 0 0 0 0 Comp. steel 42 0.15 0.66 0.91 0.01 0.0026 0.042 0.0032 0 0 0 0 0 Comp. steel 43 0.15 0.66 0.91 0.01 0.0026 0.043 0.0032 0 0 0 0 0 44 0.15 0.66 0.91 0.011 0.0026 2.595 0.0032 0 0 0 0 0 45 0.15 0.66 0.91 0.009 0.0026 2.819 0.0031 0 0 0 0 0 Comp. steel 46 0.15 0.78 0.95 0.002 0.0011 0.047 0.0027 0.04 0 0 0 0 47 0.15 0.78 0.95 0.002 0.0008 0.049 0.0025 2.60 0 0 0 0 48 0.17 0.55 0.91 0.021 0.0004 0.041 0.0028 0 0.030 0 0 0 49 0.17 0.55 0.91 0.019 0.0006 0.042 0.0030 0 0.120 0 0 0 50 0.15 0.85 1.04 0.011 0.0010 0.046 0.0027 0 0 0.060 0 0 51 0.15 0.85 1.04 0.012 0.0009 0.046 0.0025 0 0 0.110 0 0 52 0.15 0.74 1.13 0.009 0.0011 0.052 0.0025 0 0 0 0.010 0 53 0.15 0.74 1.13 0.008 0.0011 0.054 0.0027 0 0 0 0.800 0 54 0.17 0.78 0.81 0.009 0.0015 0.037 0.0027 0 0 0 0 0.001 55 0.17 0.78 0.81 0.009 0.0014 0.037 0.0029 0 0 0 0 0.006 56 0.2 0.98 1.44 0.003 0.0009 0.030 0.0024 0 0 0 0 0 57 0.12 0.72 0.94 0.01 0.0008 0.041 0.003 0 0 0 0 0 58 0.12 0.72 0.94 0.01 0.0008 0.041 0.003 0 0 0 0 0 59 0.12 0.72 0.94 0.01 0.0008 0.041 0.003 0 0 0 0 0 60 0.12 0.72 0.94 0.01 0.0008 0.041 0.003 0 0 0 0 0 In the table, fields with compositions of constituents of 0 indicate corresponding constituents not intentionally added.

TABLE D-2-1 Heat Rough rolling Heat treatment at hot stamping treatment Rate of Cold Average before hot reduc- Hot rolling roll- cooling Average Multi- Manu- rolling tion of No. of Finish ing rate cooling layer facturing Heat- Hold- Roll- sheet rolling roll- Coil- Roll- Heat- Heat- (° C./s) rate Temper- Sheet steel condi- ing ing ing thick- opera- ing ing ing ing ing (more (° C./s) ing thick- sheet tion temp. time temp. ness tions temp. temp. rate rate temp. than (400° C. temp. ness no. no. (° C.) (min) (° C.) (%) (times) (° C.) (° C.) (%) (° C./s) (° C.) 400° C.) or less) (° C.) Plating (mm) 1 1 1220 47 1147 33 3 854 556 45 39 895 69 41 None None 1.5 2 2 1205 43 1156 26 3 855 674 54 35 899 94 42 None None 1.3 3 3 1218 43 1141 24 3 840 602 48 37 910 78 28 None None 1.5 4 4 1254 38 1164 35 3 834 601 50 69 898 99 33 None None 1.4 5 5 1268 23 1123 44 3 831 675 53 61 886 97 27 None None 1.3 6 6 1260 23 1153 31 3 856 614 42 70 904 76 21 None None 1.6 7 7 1264 46 1170 47 3 862 553 54 57 891 88 32 None None 1.3 8 8 1224 32 1141 52 3 870 680 49 52 908 83 37 None None 1.4 9 9 1153 38 1123 23 3 843 600 46 28 893 68 31 None None 1.5 10 10 1263 23 1140 36 3 845 564 50 37 891 72 22 None None 1.4 11 11 1239 38 1132 42 3 865 627 54 55 910 101 30 None None 1.3 12 12 1249 45 1180 31 3 831 582 48 58 883 69 41 None None 1.5 13 13 1177 57 1155 18 3 840 684 54 30 910 100 38 None None 1.3 14 14 1223 36 1150 36 3 856 578 43 39 898 65 22 None None 1.6 15 15 1240 34 1149 45 3 853 714 49 20 884 90 31 None None 1.4 16 16 1229 25 1154 27 3 849 637 52 36 897 101 22 None None 1.3 17 17 1151 39 1127 46 3 868 582 54 64 883 111 41 None None 1.3 18 18 1192 28 1162 36 3 857 643 49 52 884 95 35 None None 1.4 19 19 1316 32 1159 43 3 832 677 47 14 892 66 26 None None 1.5 20 20 1264 44 1140 39 3 859 628 45 29 904 107 37 None None 1.5

TABLE D-2-2 Heat Rough rolling Heat treatment at hot stamping treatment Rate of Cold Average before hot reduc- roll- cooling Average Multi- Manu- rolling tion of No. of Hot rolling ing rate cooling layer facturing Heat- Hold- Roll- sheet rolling Finish Coil- Roll- Heat- Heat- (° C./s) rate Temper- Sheet steel condi- ing ing ing thick- opera- rolling ing ing ing ing (more (° C./s) ing thick- sheet tion temp. time temp. ness tions temp. temp. rate rate temp. than (400° C. temp. ness no. no. (° C.) (min) (° C.) (%) (times) (° C.) (° C.) (%) (° C./s) (° C.) 400° C.) or less) (° C.) Plating (mm) 21 21 1217 23 1170 46 3 858 601 47 21 890 99 38 None None 1.5 22 22 1165 40 1158 31 3 840 621 48 36 895 95 40 None None 1.5 23 23 1259 39 1127 44 3 854 552 51 66 881 85 33 None None 1.4 24 24 1176 32 1171 28 3 849 643 42 78 909 77 25 None None 1.6 25 25 1153 29 1137 40 3 834 597 51 62 883 92 30 None None 1.4 26 26 1193 39 1128 11 3 856 608 45 20 898 83 47 None None 1.5 27 27 1250 54 1181 23 3 861 651 42 72 909 95 41 None None 1.6 28 28 1304 32 1169 23 3 842 707 43 40 910 108 26 None None 1.6 29 29 1226 27 1153 31 3 864 633 43 33 894 89 29 None None 1.6 30 30 1188 38 1141 33 3 861 566 49 50 897 91 28 None None 1.4 31 31 1267 36 1144 47 3 853 597 44 77 893 69 40 None None 1.6 32 32 1262 36 1128 20 3 859 687 51 64 885 83 37 None None 1.4 33 33 1084 37 1052 12 3 836 647 43 59 892 87 43 None None 1.6 34 34 1372 50 1118 17 3 861 623 52 40 899 63 31 None None 1.3 35 35 1204 14 1127 34 3 850 555 49 57 910 84 43 None None 1.4 36 36 1274 29 1153 44 3 865 713 0 53 892 105 26 None None 2.8 37 37 1294 51 1145 52 3 868 671 54 35 897 78 43 258 None 1.3 38 38 1211 43 1160 43 3 855 562 48 34 890 99 33 271 Yes 1.5 39 39 1305 51 1111 17 3 840 679 46 72 901 71 37 None Yes 1.5 40 40 1229 47 1159 21 3 847 555 50 62 892 85 26 None None 1.4

TABLE D-2-3 Heat Rough rolling Heat treatment at hot stamping treatment Rate of Cold Average before hot reduc- roll- cooling Average Multi- rolling tion of No. of Hot rolling ing rate cooling layer Manu- Heat- Hold- Roll- sheet rolling Finish Coil- Roll- Heat- Heat- (° C./s) rate Temper- Sheet steel facturing ing ing ing thick- opera- rolling ing ing ing ing (more (° C./s) ing thick- sheet condition temp. time temp. ness tions temp. temp. rate rate temp. than (400° C. temp. ness no. no. (° C.) (min) (° C.) (%) (times) (° C.) (° C.) (%) (° C./s) (° C.) 400° C.) or less) (° C.) Plating (mm) 41 41 1270 30 1161 29 3 848 596 48 70 897 100 29 None None 1.5 42 42 1233 56 1145 33 3 839 608 45 32 879 66 34 None None 1.5 43 43 1247 40 1158 35 3 850 576 45 69 888 88 40 None None 1.5 44 44 1237 36 1141 47 3 841 583 45 31 885 98 40 None None 1.5 45 45 1246 54 1134 17 3 851 591 45 36 894 80 38 None None 1.5 46 46 1257 51 1136 9 3 838 636 52 59 887 105 36 None None 1.3 47 47 1239 33 1126 21 3 847 565 52 72 875 70 35 None None 1.3 48 48 1265 57 1130 37 3 852 565 54 65 876 83 42 None None 1.3 49 49 1235 56 1183 19 3 849 638 54 66 877 98 34 None None 1.3 50 50 1276 45 1155 31 3 842 624 49 72 883 107 27 None None 1.4 51 51 1232 38 1138 50 3 846 565 49 25 889 69 37 None None 1.4 52 52 1257 26 1128 18 3 848 564 47 66 890 89 36 None None 1.5 53 53 1220 31 1132 12 3 845 628 47 64 893 94 31 None None 1.5 54 54 1215 38 1130 17 3 842 555 45 29 877 81 31 None None 1.5 55 55 1264 33 1132 39 3 843 644 45 68 890 93 29 None None 1.5 56 56 1261 37 1192 18 3 856 681 51 59 855 91 35 None None 1.4 57 57 1246 40 1019 47 3 841 596 48 69 885 106 28 None None 1.5 58 58 1265 54 1163 1 2 852 591 45 32 875 75 39 None None 1.5 59 59 1232 56 1143 41 1 842 565 54 61 877 88 36 None None 1.3 60 60 1215 21 1102 43 3 848 638 49 57 893 96 32 None None 1.5

TABLE D-3-1 Metal structure Any rate (%) of total of crystal grains with maximum difference of crystal orientation inside large angle grain boundaries of 1° or less and crystal Mechanical properties grains with Average Hard- maximum cross ness difference sec- of of tional middle crystal hard- Multi- Manu- part in orientation ness layer facturing sheet of 8° Residual mini- Maximum Stamped steel condi- thick- or more γ area Tensile Uniform mum bending Hydrogen body sheet tion ness and less rate strength elongation hard- angle embrittlement no. no. no. (Hv) than 15° (%) (MPa) (%) ness (°) resistance Remarks 1D 1 1 600 82 2.5 1740 5.2 43 84.8 Good Inv. ex. 2D 2 2 671 78 2.2 1945 5.1 26 75.6 Good Inv. ex. 3D 3 3 768 71 3.3 2227 6.3 44 73.4 Good Inv. ex. 4D 4 4 793 63 4.7 2300 6.8 68 74.9 Good Inv. ex. 5D 5 5 420 84 3.2 1218 6.1 53 88.9 Good Comp. ex. 6D 6 6 570 66 4.4 1653 6.8 44 86.6 Good Inv. ex. 7D 7 7 720 83 4 2088 6.4 60 76.2 Good Inv. ex. 8D 8 8 699 78 2.3 2026 5.8 61 76.2 Good Inv. ex. 9D 9 9 1014 52 2.8 2941 5.8 58 61.2 Good Comp. ex. 10D 10 10 741 71 0.2 2148 2.6 35 80 Good Comp. ex. 11D 11 11 751 81 0.5 2478 4.3 43 83.6 Good Comp. ex. 12D 12 12 443 84 3.5 1285 6.4 167 89.2 Good Comp. ex. 13D 13 13 492 53 4.6 1427 6.8 155 86.8 Good Comp. ex. 14D 14 14 632 63 0.7 1832 4.8 115 76.8 Good Comp. ex. 15D 15 15 629 83 0.3 1824 3.7 127 74.1 Good Comp. ex. 16D 16 16 681 78 4.3 1975 6.9 41 89.6 Good Inv. ex. 17D 17 17 688 65 2.6 1995 5 49 88.3 Good Inv. ex. 18D 18 18 685 63 3.2 1987 6.1 72 86.2 Good Inv. ex. 19D 19 19 678 79 3.8 1966 6.9 38 82.9 Good Inv. ex. 20D 20 20 697 67 2.7 2021 5.4 64 84.5 Good Inv. ex.

TABLE D-3-2 Metal structure Any rate (%) of total of crystal grains with maximum difference of crystal orientation inside large angle grain boundaries of 1° or less and crystal Mechanical properties grains with Average Hard- maximum cross ness difference sec- of of tional middle crystal hard- Multi- Manu- part in orientation ness layer facturing sheet of 8° Residual mini- Maximum Stamped steel condi- thick- or more γ area Tensile Uniform mum bending Hydrogen body sheet tion ness and less rate strength elongation hard- angle embrittlement no. no. no. (Hv) than 15° (%) (MPa) (%) ness (°) resistance Remarks 21D 21 21 542 72 2.5 1573 5.6 57 79.5 Good Inv. ex. 22D 22 22 546 80 4.5 1584 6.8 56 78.6 Good Inv. ex. 23D 23 23 540 64 4.5 1567 6.9 27 87.6 Good Inv. ex. 24D 24 24 669 68 3.1 1940 6.1 32 75.3 Good Inv. ex. 25D 25 25 671 73 3 1945 5.9 69 80.5 Good Inv. ex. 26D 26 26 676 82 3 1960 5.3 39 74.6 Good Inv. ex. 27D 27 27 749 66 4.3 2171 6.6 32 82.3 Good Inv. ex. 28D 28 28 747 80 3.6 2166 5.7 36 81.2 Good Inv. ex. 29D 29 29 742 72 2.4 2151 5.7 56 76.7 Good Inv. ex. 30D 30 30 781 67 3.9 2265 5.4 27 83.7 Good Inv. ex. 31D 31 31 792 79 2.8 2297 5.5 69 75.7 Good Inv. ex. 32D 32 32 788 77 4.1 2285 6.7 64 84.9 Good Inv. ex. 33D 33 33 668 14 4.6 1937 6.9 44 66.8 Poor Comp. ex. 34D 34 34 667 95 4.6 1934 6.9 52 67.4 Good Comp. ex. 35D 35 35 673 17 4.2 1951 6.8 26 61.9 Poor Comp. ex. 36D 36 36 671 61 3.1 1945 6.1 28 82.4 Good Inv. ex. 37D 37 37 763 69 4.4 2213 6.6 44 71.2 Good Inv. ex. 38D 38 38 753 71 4.2 2184 5.9 26 78.5 Good Inv. ex. 39D 39 39 669 77 2.9 1940 5.8 54 82.5 Good Inv. ex. 40D 40 40 743 71 12.5 2148 8 67 61.8 Good Comp. ex.

TABLE D-3-3 Metal structure Any rate (%) of total of crystal grains with maximum difference of crystal orientation inside large angle grain boundaries of 1° or less and crystal Mechanical properties grains with Average Hard- maximum cross ness difference sec- of of tional middle crystal hard- Multi- Manu- part in orientation ness layer facturing sheet of 8° Residual mini- Maximum Stamped steel condi- thick- or more γ area Tensile Uniform mum bending Hydrogen body sheet tion ness and less rate strength elongation hard- angle embrittlement no. no. no. (Hv) than 15° (%) (MPa) (%) ness (°) resistance Remarks 41D 41 41 792 84 3.5 2610 6.3 26 51.2 Good Comp. ex. 42D 42 42 605 83 2.1 1997 5.2 42 64.1 Good Comp. ex. 43D 43 43 612 75 2.5 2020 5.4 71 84.5 Good Inv. ex. 44D 44 44 608 76 2.2 2006 5.5 65 84.4 Good Inv. ex. 45D 45 45 605 84 2.3 1997 5.3 40 56.1 Good Comp. ex. 46D 46 46 651 76 4.3 2148 6.8 39 88.7 Good Inv. ex. 47D 47 47 701 79 4.4 2313 6.9 60 89.8 Good Inv. ex. 48D 48 48 661 63 2.5 2181 5.1 57 88.1 Good Inv. ex. 49D 49 49 703 65 2.6 2320 5.3 41 88.7 Good Inv. ex. 50D 50 50 658 61 3 2171 5.9 34 86.7 Good Inv. ex. 51D 51 51 709 64 3.3 2340 6.2 60 85.1 Good Inv. ex. 52D 52 52 661 77 3.7 2181 6.7 65 81.6 Good Inv. ex. 53D 53 53 682 80 3.8 2251 7 61 82.5 Good Inv. ex. 54D 54 54 683 65 2.4 2254 5.3 25 81.8 Good Inv. ex. 55D 55 55 705 67 2.8 2327 5.5 68 84.4 Good Inv. ex. 56D 56 56 781 76 4 2577 6.6 61 85.3 Good Inv. ex. 57D 57 57 642 10 2.9 2119 6.3 30 59.2 Poor Comp. ex. 58D 58 58 634 11 3.0 2092 6.7 26 61.4 Poor Comp. ex. 59D 59 59 637 12 3.0 2102 6.6 22 61.9 Poor Comp. ex. 60D 60 60 636 46 3.0 2099 6.7 27 111.8 Good Inv. ex.

INDUSTRIAL APPLICABILITY

The hot stamped body of the present invention is excellent in strength, ductility, bendability, impact resistance, and hydrogen embrittlement resistance and is small in scattering in hardness, so can be suitably used for structural members or reinforcing members for automobiles or structures requiring strength.

Claims

1-8. (canceled)

9. A hot stamped body comprising a middle part in sheet thickness and a softened layer arranged at both sides or one side of the middle part in sheet thickness, wherein

the middle part in sheet thickness comprises, by mass %,

C: 0.20% or more and less than 0.70%,

Si: less than 3.00%,

Mn: 0.20% or more and less than 3.00%,

P: 0.10% or less,

S: 0.10% or less,

sol. Al: 0.0002% or more and 3.0000% or less,

N: 0.01% or less, and

a balance of Fe and unavoidable impurities, and has a hardness of 500 Hv or more and 800 Hv or less,

in the metal structures from a depth of 20 μm below the surface of the softened layer to a depth of ½ of the thickness of the softened layer, when defining a region surrounded by grain boundaries having a 15° or higher orientation difference in a cross-section parallel to the sheet thickness direction as a “crystal grain”, the area rate of the total of crystal grains with a maximum crystal orientation difference inside the crystal grains of 1° or less and crystal grains with a maximum crystal orientation difference inside the crystal grains of 8° or more and less than 15° is 50% or more and less than 85%,

the tensile strength is 1500 MPa or more.

10. The hot stamped body according to claim 9, wherein the Si content is 0.50% or less and the Mn content is 0.20% or more and less than 1.50%.

11. The hot stamped body according to claim 9, wherein the Si content is 0.50% or less and the Mn content is 1.50% or more and less than 3.00%.

12. The hot stamped body according to claim 9, wherein the Si content is more than 0.50% and less than 3.00%, the Mn content is 0.20% or more and less than 1.50%, and the middle part in sheet thickness comprises, by area percent, 1.0% or more and less than 5.0% of residual austenite.

13. The hot stamped body according to claim 9, wherein the Si content is more than 0.50% and less than 3.00%, the Mn content is 1.50% or more and less than 3.00%, and the middle part in sheet thickness comprises, by area percent, 1.0% or more and less than 5.0% of residual austenite.

14. The hot stamped body according to claim 9, where the middle part in sheet thickness further comprises, by mass %, one or more of Ni: 0.01% or more and 3.00% or less, Nb: 0.010% or more and 0.150% or less, Ti: 0.010% or more and 0.150% or less, Mo: 0.005% or more and 1.000% or less, and B: 0.0005% or more and 0.0100% or less.

15. The hot stamped body according to claim 10, where the middle part in sheet thickness further comprises, by mass %, one or more of Ni: 0.01% or more and 3.00% or less, Nb: 0.010% or more and 0.150% or less, Ti: 0.010% or more and 0.150% or less, Mo: 0.005% or more and 1.000% or less, and B: 0.0005% or more and 0.0100% or less.

16. The hot stamped body according to claim 11, where the middle part in sheet thickness further comprises, by mass %, one or more of Ni: 0.01% or more and 3.00% or less, Nb: 0.010% or more and 0.150% or less, Ti: 0.010% or more and 0.150% or less, Mo: 0.005% or more and 1.000% or less, and B: 0.0005% or more and 0.0100% or less.

17. The hot stamped body according to claim 12, where the middle part in sheet thickness further comprises, by mass %, one or more of Ni: 0.01% or more and 3.00% or less, Nb: 0.010% or more and 0.150% or less, Ti: 0.010% or more and 0.150% or less, Mo: 0.005% or more and 1.000% or less, and B: 0.0005% or more and 0.0100% or less.

18. The hot stamped body according to claim 13, where the middle part in sheet thickness further comprises, by mass %, one or more of Ni: 0.01% or more and 3.00% or less, Nb: 0.010% or more and 0.150% or less, Ti: 0.010% or more and 0.150% or less, Mo: 0.005% or more and 1.000% or less, and B: 0.0005% or more and 0.0100% or less.

19. The hot stamped body according to claim 9, where a plated layer is formed on the softened layer.

20. The hot stamped body according to claim 10, where a plated layer is formed on the softened layer.

21. The hot stamped body according to claim 11, where a plated layer is formed on the softened layer.

22. The hot stamped body according to claim 12, where a plated layer is formed on the softened layer.

23. The hot stamped body according to claim 13, where a plated layer is formed on the softened layer.

24. The hot stamped body according to claim 14, where a plated layer is formed on the softened layer.

25. The hot stamped body according to claim 15, where a plated layer is formed on the softened layer.

26. The hot stamped body according to claim 16, where a plated layer is formed on the softened layer.

27. The hot stamped body according to claim 17, where a plated layer is formed on the softened layer.

28. The hot stamped body according to claim 18, where a plated layer is formed on the softened layer.