STEEL SHEET FOR HOT STAMPING AND HOT STAMPED PART

- NIPPON STEEL CORPORATION

Provided is a steel sheet for hot stamping having a predetermined chemical composition and a metallographic structure comprising, by area ratio, ferrite: 10% or more and pearlite: 10% or more, wherein a total of ferrite and pearlite is 80% or more, and a dispersion index of pearlite is 0.50 or more. Further, provided is a hot stamped part having a predetermined chemical composition and a metallographic structure comprising, by area ratio, at least one of martensite, bainite, and tempered martensite in a total of 90% or more, wherein a standard deviation in a hardness distribution of prior austenite grains at a sheet thickness ¼ position is 150 Hv or less.

Skip to: Description  ·  Claims  · Patent History  ·  Patent History
Description
FIELD

The present invention relates to a steel sheet for hot stamping and a hot stamped part produced using the same.

BACKGROUND

In recent years, in the automobile industry, lighter weight of car bodies has been sought from the viewpoint of improvement of fuel economy. To achieve both lighter weight of car bodies and collision safety, one effective method is to increase the strength of the steel sheet used. A high strength steel sheet is being developed due to such a background. On the other hand, if making a steel sheet high in strength, the formability falls, and therefore it is generally difficult to achieve both strength and formability in the steel sheet.

In relation to this, PTL 1 describes a cold rolled steel sheet having a predetermined chemical composition and a metallographic structure comprising, by area ratio, polygonal ferrite in 40.0% or more and less than 60.0%, bainitic ferrite in 30.0% or more, retained austenite in 10.0% or more and 25.0% or less, and martensite in 15.0% or less, wherein in the retained austenite, a ratio of the retained austenite, in which an aspect ratio is 2.0 or less, a length of a long axis is 1.0 um or less, and a length of a short axis is 1.0 μm or less, is 80.0% or more, wherein in the bainitic ferrite, a ratio of the bainitic ferrite, in which an aspect ratio is 1.7 or less and an average value of crystal orientation difference of a region surrounded by grain boundaries with a crystal orientation difference of 15° or more is 0.5° or more and less than 3.0°, is 80.0% or more, and wherein a connectivity D value of the martensite, bainitic ferrite, and retained austenite is 0.70 or less. Further, PTL 1 describes that according to the above constitution, it is possible to provide a high strength cold rolled steel sheet having a tensile strength of 980 MPa or more, a 0.2% yield strength of 600 MPa or more, and excellent punching fatigue properties, elongation, and hole expandability suitable as a structural member of an automobile, etc.

CITATIONS LIST Patent Literature

[PTL 1] WO 2016/136810

SUMMARY Technical Problem

Hot stamping (hot pressing) is known as a technique for press-forming a material, which is difficult to form, such as a high strength steel sheet. Hot stamping is a technique of hot forming which heats then forms a material to be formed. This technique heats then forms the material, and therefore at the time of forming, the steel material is soft and has good formability. Therefore, even a high strength steel material can be formed into a complex shape with a good precision. Further, it is hardened at the same time as being formed by the press dies, and therefore a formed steel material is known to have sufficient strength.

In a hot stamped part having such a high strength, sometimes hydrogen embrittlement cracking (also referred to as “delayed fracture”, etc.) becomes a problem. “Hydrogen embrittlement cracking” is the phenomenon where a steel member which is acted on by a high stress under conditions of use suddenly fractures due to hydrogen penetrating the steel from the environment. In general, it is known that hydrogen embrittlement cracking occurs more easily the higher the strength of the steel material. On the other hand, in the automobile industry, etc., further reduction of weight of the steel material is sought. To achieve such lighter weight, a need arises to raise the strength of the steel material more than the past. Therefore, there is a great need for a steel material, more specifically a hot stamped part, able to solve the problem of hydrogen embrittlement even if raising the strength equal to the past or more than the same. Therefore, the present invention has as its object to provide a hot stamped part which is high in strength and able to suppress hydrogen embrittlement and a steel sheet for hot stamping for producing such a hot stamped part by a novel constitution.

Solution to Problem

The inventors engaged in studies focusing on the metallographic structures at a steel sheet before hot stamping and a hot stamped part after hot stamping so as to achieve the above object. As a result, the inventors discovered that by making the pearlite forming starting points of austenite grains at the time of heating in hot stamping homogeneously disperse in a steel sheet before hot stamping, the prior austenite grains are made uniform at the finally obtained hot stamped part, and accordingly it is possible to reduce the variation in hardness of the prior austenite grains at the metallographic structure of the hot stamped part. Further, the inventors discovered that by reducing the variation in hardness of prior austenite grains at the metallographic structure of the hot stamped part, it is possible to suppress a rise in local hardness and as a result possible to remarkably improve the hydrogen embrittlement resistance regardless of having a high tensile strength and thereby completed the present invention.

The present invention able to achieve this object is as follows:

    • (1) A steel sheet for hot stamping having a chemical composition comprising, by mass %,
      • C: 0.40 to 0.70%,
      • Si: 0.010 to 1.300%,
      • Mn: 0.60 to 3.00%,
      • P: 0.100% or less,
      • S: 0.0100% or less,
      • N: 0.0200% or less,
      • O: 0.0200% or less,
      • Al: 0.0010 to 0.5000%,
      • Nb: 0.0010 to 0.100%,
      • Ti: 0.010 to 0.200%,
      • B: 0.0005 to 0.0200%,
      • Cr: 0.010 to 0.80%,
      • Mo: 0.0010 to 1.000%,
      • Co: 0 to 2.00%,
      • Ni: 0 to 3.00%,
      • Cu: 0 to 1.00%,
      • V: 0 to 1.00%,
      • W: 0 to 1.000%,
      • Ca: 0 to 0.010%,
      • Mg: 0 to 1.000%,
      • REM: 0 to 1.000%,
      • Sb: 0 to 1.000%,
      • Zr: 0 to 1.000%,
      • Sn: 0 to 1.000%,
      • As: 0 to 0.100%, and
      • balance: Fe and impurities, and
      • a metallographic structure comprising, by area ratio,
      • ferrite: 10% or more and
      • pearlite: 10% or more, wherein
      • a total of ferrite and pearlite is 80% or more, and
      • a dispersion index of pearlite is 0.50 or more.
    • (2) The steel sheet for hot stamping according to the above (1), wherein the chemical composition contains, by mass %, one or more selected from the group consisting of
      • Co: 0.001 to 2.00%,
      • Ni: 0.001 to 3.00%,
      • Cu: 0.001 to 1.00%,
      • V: 0.001 to 1.00%,
      • W: 0.001 to 1.000%,
      • Ca: 0.0001 to 0.010%,
      • Mg: 0.0001 to 1.000%,
      • REM: 0.0001 to 1.000%,
      • Sb: 0.001 to 1.000%,
      • Zr: 0.001 to 1.000%,
      • Sn: 0.001 to 1.000%, and
      • As: 0.001 to 0.100%.
    • (3) A hot stamped part having a chemical composition comprising, by mass %,
      • C: 0.40 to 0.70%,
      • Si: 0.010 to 1.300%,
      • Mn: 0.60 to 3.00%,
      • P: 0.100% or less,
      • S: 0.0100% or less,
      • N: 0.0200% or less,
      • O: 0.0200% or less,
      • Al: 0.0010 to 0.5000%,
      • Nb: 0.0010 to 0.100%,
      • Ti: 0.010 to 0.200%,
      • B: 0.0005 to 0.0200%,
      • Cr: 0.010 to 0.80%,
      • Mo: 0.0010 to 1.000%,
      • Co: 0 to 2.00%,
      • Ni: 0 to 3.00%,
      • Cu: 0 to 1.00%,
      • V: 0 to 1.00%,
      • W: 0 to 1.000%,
      • Ca: 0 to 0.010%,
      • Mg: 0 to 1.000%,
      • REM: 0 to 1.000%,
      • Sb: 0 to 1.000%,
      • Zr: 0 to 1.000%,
      • Sn: 0 to 1.000%,
      • As: 0 to 0.100%, and
      • balance: Fe and impurities, and
      • a metallographic structure comprising, by area ratio, at least one of martensite, bainite, and tempered martensite in a total of 90% or more, wherein
      • a standard deviation in a hardness distribution of prior austenite grains at a sheet thickness ¼ position is 150 Hv or less.
    • (4) The hot stamped part according to the above (3), wherein the chemical composition contains, by mass %, one or more selected from the group consisting of
      • Co: 0.001 to 2.00%,
      • Ni: 0.001 to 3.00%,
      • Cu: 0.001 to 1.00%,
      • V: 0.001 to 1.00%,
      • W: 0.001 to 1.000%,
      • Ca: 0.0001 to 0.010%,
      • Mg: 0.0001 to 1.000%,
      • REM: 0.0001 to 1.000%,
      • Sb: 0.001 to 1.000%,
      • Zr: 0.001 to 1.000%,
      • Sn: 0.001 to 1.000%, and
      • As: 0.001 to 0.100%.

Advantageous Effects of Invention

According to the present invention, it is possible to provide a hot stamped part which is high in strength and able to suppress hydrogen embrittlement and a steel sheet for hot stamping for producing such a hot stamped part.

DESCRIPTION OF EMBODIMENTS Steel Sheet for Hot Stamping

The steel sheet for hot stamping according to an embodiment of the present invention has a chemical composition comprising, by mass %,

    • C: 0.40 to 0.70%,
    • Si: 0.010 to 1.300%,
    • Mn: 0.60 to 3.00%,
    • P: 0.100% or less,
    • S: 0.0100% or less,
    • N: 0.0200% or less,
    • O: 0.0200% or less,
    • Al: 0.0010 to 0.5000%,
    • Nb: 0.0010 to 0.100%,
    • Ti: 0.010 to 0.200%,
    • B: 0.0005 to 0.0200%,
    • Cr: 0.010 to 0.80%,
    • Mo: 0.0010 to 1.000%,
    • Co: 0 to 2.00%,
    • Ni: 0 to 3.00%,
    • Cu: 0 to 1.00%,
    • V: 0 to 1.00%,
    • W: 0 to 1.000%,
    • Ca: 0 to 0.010%,
    • Mg: 0 to 1.000%,
    • REM: 0 to 1.000%,
    • Sb: 0 to 1.000%,
    • Zr: 0 to 1.000%,
    • Sn: 0 to 1.000%,
    • As: 0 to 0.100%, and
    • balance: Fe and impurities, and
    • a metallographic structure comprising, by area ratio,
    • ferrite: 10% or more and
    • pearlite: 10% or more, wherein
    • a total of ferrite and pearlite is 80% or more, and
    • a dispersion index of pearlite is 0.50 or more.

As stated above, it is known that hydrogen embrittlement cracking occurs more easily the higher the strength of the steel material. Therefore, the inventors engaged in studies focusing on the metallographic structures of a steel sheet before hot stamping and a hot stamped part after hot stamping from the viewpoint of reducing or suppressing regions which could form starting points of hydrogen embrittlement cracking in such a high strength steel material. More specifically, first, the inventors discovered that if the variation in prior austenite grain size in the metallographic structure of a hot stamped part is large, the hardness rises in regions with smaller prior austenite grain size and that such local high hardness regions can form starting points of hydrogen embrittlement cracking. As opposed to this, the inventors discovered that by reducing the variation in prior austenite grain size and as a result reducing the variation in hardness at the prior austenite grains, more specifically controlling the standard deviation in hardness distribution of prior austenite grains to 150 Hv or less, it is possible to reliably suppress such a rise in local hardness.

While not intending to be bound to any specific theory, it is believed that at the time of hot stamping, the starting temperature of martensite transformation changes in accordance with the size of the austenite grains. If explained in more detail, it is believed that austenite grains having a larger grain size become lower in hardness since the starting temperature of martensite transformation is higher compared with austenite grains having a smaller grain size. Austenite grains having a smaller grain size rise in hardness due to martensite transformation at a lower temperature than large grains. Therefore, to suppress or reduce such a rise in local hardness, it is important to reduce the variation in austenite grain size before martensite transformation. In other words, it is believed that by reducing the variation in austenite grain size before martensite transformation, it is possible to reduce the variation in prior austenite grain size after martensite transformation and as a result possible to reduce the variation in hardness of prior austenite grains in the metallographic structure of the hot stamped part. From such a reason, it is believed that by controlling the standard deviation in hardness distribution of prior austenite grains in the metallographic structure of the hot stamped part to 150 Hv or less to reduce the variation in hardness of the prior austenite grains, it is possible to remarkably suppress the rise in local hardness based on the difference in timing of martensite transformation. If there are regions with high hardness locally, it is believed that there would be a high possibility of causing hydrogen embrittlement cracking particularly at the interfaces of prior austenite grains with differences in hardness, and therefore in the metallographic structure of the hot stamped part, reducing the variation in hardness of the prior austenite grains is extremely effective in improving the hydrogen embrittlement resistance.

In relation to this, as explained later in detail regarding the method of production of a steel sheet for hot stamping, the inventors focused on the metallographic structure of a steel sheet before hot stamping, for example, a hot rolled steel sheet, and discovered that by making the pearlite homogeneously disperse in the metallographic structure, it is possible to reduce the variation in prior austenite grain size at the final metallographic structure of the hot stamped part and in relation to this is possible to control the standard deviation in hardness distribution of prior austenite grains to 150 Hv or less. Along with the higher strength of steel materials, sometimes a relatively large amount of Mn is added for improving the hardenability of the steel material, but this time, in research by the inventors, it was learned that with such a high Mn content (for example, 0.60 mass % or more), pearlite forms relatively easily, and therefore it is extremely difficult to make the pearlite formed in larger amounts in the metallographic structure of a hot rolled steel sheet compared with the case of low Mn content homogeneously disperse and as a result the variation in prior austenite grain size becomes greater in the metallographic structure after hot stamping. However, the inventors discovered, in relation to such a problem, that by applying relatively high reduction at the final stage of finish rolling and further by suitably controlling the cooling after that, it is possible to homogeneously disperse pearlite in the metallographic structure of the hot rolled steel sheet and as a result possible to reduce the variation in prior austenite grain size in the final metallographic structure of the hot stamped part and thereby remarkably reduce the variation in hardness distribution of prior austenite grains. More specifically, the inventors discovered that controlling a dispersion index of pearlite, obtained by dividing the number of A/B boundaries (boundaries of ferrite phases and pearlite phases) in an electron micrograph of the metallographic structure of the steel sheet for hot stamping mainly comprised of ferrite and pearlite by the total of the number of A/A boundaries (boundaries of ferrite phases and ferrite phases), the number of B/B boundaries (boundaries of pearlite phases and pearlite phases), and the number of A/B boundaries (boundaries of ferrite phases and pearlite phases), to 0.5 or more is important and further that controlling the dispersion index of pearlite to 0.5 or more enables achievement of a standard deviation in hardness distribution of prior austenite grains of 150 Hv or less in the final metallographic structure of the hot stamped part due to homogeneous dispersion of pearlite.

The fact that by hot stamping a steel sheet for hot stamping in which pearlite is homogeneously dispersed in a predetermined range, it is possible to control the variation in the hardness distribution of the prior austenite grains in the metallographic structure of the obtained hot stamped part to within a predetermined range was first clarified this time by the inventors. In addition, according to the hot stamped part according to an embodiment of the present invention, by controlling the variation in the hardness distribution of the prior austenite grains to within a predetermined range, it is possible to remarkably suppress a rise in local hardness, and therefore it is possible to remarkably improve the hydrogen embrittlement resistance regardless of having a high tensile strength, for example a high tensile strength of 2200 MPa or more.

Below, the steel sheet for hot stamping according to the embodiment of the present invention will be explained in more detail. In the following explanation, the “%” of the units of content of the elements, unless otherwise indicated, means “mass %”. Further, in this Description, “to” showing a numerical range, unless otherwise indicated, is used in the sense including the numerical values described before and after it as the lower limit value and upper limit value.

C: 0.40 to 0.70%

C is an element improving the strength of a hot stamped part. If the C content is less than 0.40%, it is not possible to obtain the desired strength at the hot stamped part. For this reason, the C content is 0.40% or more. The C content is preferably more than 0.40%, 0.42% or more, 0.43% or more, 0.44% or more, 0.45% or more, or 0.46% or more.

On the other hand, if the C content is more than 0.70%, the strength becomes too high and sometimes excellent hydrogen embrittlement resistance cannot be obtained. For this reason, the C content is 0.70% or less. Preferably, the C content is 0.68% or less, 0.67% or less, 0.65% or less, or 0.60% or less.

Si: 0.010 to 1.300%

Si is an element improving the strength of a hot stamped part by solid solution strengthening. If the Si content is less than 0.010%, the desired strength cannot be obtained. For this reason, the Si content is 0.010% or more. The Si content is preferably 0.050% or more, 0.100% or more, 0.200% or more, more than 0.250%, 0.255% or more, 0.260% or more, 0.270% or more, 0.280% or more, 0.300% or more, or 0.400% or more.

On the other hand, if the Si content is more than 1.300%, in the steel sheet for hot stamping, the amount of ferrite increases and sometimes the desired metallographic structure cannot be obtained. For this reason, the Si content is 1.300% or less. The Si content is preferably 1.200% or less, 1.000% or less, 0.800% or less, 0.600% or less, or 0.500% or less.

Mn: 0.60 to 3.00%

Mn is an element promoting transformation of austenite to pearlite in a hot rolled steel sheet in the process of production of the hot stamped part and contributing to control of the dispersion index of pearlite in a steel sheet for hot stamping and further the hardness distribution of prior austenite grains in a hot stamped part. To control the dispersion index of pearlite and standard deviation in hardness distribution of prior austenite grains to the desired ranges, the Mn content is 0.60% or more. The Mn content is preferably more than 0.60%, 0.70% or more, 0.80% or more, 1.00% or more, or 1.30% or more.

On the other hand, if the Mn content is more than 3.00%, in the hot rolled steel sheet, transformation from austenite to pearlite is promoted too much and the dispersion index of pearlite and standard deviation in hardness distribution of prior austenite grains cannot be rendered the desired ranges. For this reason, the Mn content is 3.00% or less. Preferably, the Mn content is 2.90% or less, 2.70% or less, 2.50% or less, 2.30% or less, or 2.00% or less.

P: 0.100% or Less

P is an impurity element and segregates at the grain boundaries to cause the hydrogen embrittlement resistance to deteriorate. For this reason, the P content is 0.100% or less. The P content is preferably 0.070% or less, 0.050% or less, or 0.010% or less.

The lower limit of the P content is not particularly prescribed, but if less than 0.0001%, the dephosphorization cost greatly rises making this not preferable economically. For this reason, the P content may also be 0.0001% or more.

S: 0.0100% or Less

S is an impurity element and forms inclusions in the steel. The inclusions cause the hydrogen embrittlement resistance to deteriorate, and therefore the S content is 0.0100% or less. The S content is preferably 0.0080% or less, 0.0050% or less, 0.0030% or less, or 0.0020% or less.

The lower limit of the S content is not particularly prescribed, but if less than 0.0001%, the desulfurization cost greatly rises making this not preferable economically. For this reason, the S content may also be 0.0001% or more.

N: 0.0200% or Less

N is an impurity element and forms nitrides in the steel. The nitrides cause the hydrogen embrittlement resistance to deteriorate, and therefore the N content is 0.0200% or less. The N content is preferably 0.0180% or less, 0.0150% or less, 0.0100% or less, 0.0060% or less, or 0.0040% or less.

The lower limit of the N content is not particularly prescribed, but if reducing this to less than 0.0001%, the denitridation cost greatly rises making this not preferable economically. For this reason, the N content may also be 0.0001% or more.

O: 0.0200% or Less

O, if contained in a large amount in the steel, forms coarse oxides and causes the hydrogen embrittlement resistance to deteriorate. For this reason, the O content is 0.0200% or less. The O content is preferably 0.0150% or less, 0.0100% or less, 0.0070% or less, or 0.0040% or less.

From the viewpoint of reducing the refining costs, the O content may also be 0.0001% or more. To make a large number of fine oxides disperse at the time of deoxidation of the molten steel, the O content may be 0.0005% or more.

Al: 0.0010 to 0.5000%

Al is an element having the action of deoxidizing the molten steel and making the steel sounder. If the Al content is less than 0.0010%, the deoxidation will not sufficiently proceed and coarse oxides will be formed causing the hydrogen embrittlement resistance to deteriorate. For this reason, the Al content is 0.0010% or more. The Al content is preferably 0.0030% or more, 0.0050% or more, 0.0100% or more, or 0.0300% or more.

On the other hand, if the Al content is more than 0.5000%, coarse oxides will form in the steel causing the hydrogen embrittlement resistance of the hot stamped part to fall. For this reason, the Al content is 0.5000% or less. The Al content is preferably 0.4000% or less, 0.3000% or less, 0.2000% or less, 0.1500% or less, or 0.1000% or less.

Nb: 0.0010 to 0.100%

Nb is an element forming carbonitrides in steel and improving the strength of the hot stamped part by precipitation strengthening. If the Nb content is less than 0.0010%, the desired strength cannot be obtained. For this reason, the Nb content is 0.0010% or more. The Nb content is preferably 0.005% or more, 0.009% or more, or 0.015% or more.

On the other hand, if the Nb content is more than 0.100%, carbonitrides are formed in the steel in a large amount and the hydrogen embrittlement resistance of the hot stamped part falls. For this reason, the Nb content is 0.100% or less. The Nb content is preferably 0.080% or less, 0.060% or less, or 0.050% or less.

Ti: 0.010 to 0.200%

Ti is an element forming carbonitrides in steel and improving the strength of the hot stamped part by precipitation strengthening. If the Ti content is less than 0.010%, the desired strength cannot be obtained. For this reason, the Ti content is 0.010% or more. The Ti content is preferably 0.015% or more, 0.020% or more, or 0.025% or more.

On the other hand, if the Ti content is more than 0.200%, carbonitrides are formed in large amounts in the steel and the hydrogen embrittlement resistance of the hot stamped part falls. For this reason, the Ti content is 0.200% or less. The Ti content is preferably 0.180% or less, 0.150% or less, 0.100% or less, 0.080% or less, 0.060% or less, or 0.050% or less.

B: 0.0005 to 0.0200%

B is an element improving the hardenability of steel. If the B content is less than 0.0005%, the desired strength cannot be obtained. For this reason, the B content is 0.0005% or more. The B content is preferably 0.0010% or more, 0.0015% or more, or 0.0020% or more.

On the other hand, if the B content is more than 0.0200%, coarse intermetallic compounds are formed at the hot stamped part and the hydrogen embrittlement resistance of the hot stamped part falls. For this reason, the B content is 0.0200% or less. The B content is preferably 0.0150% or less, 0.0100% or less, 0.0080% or less, 0.0060% or less, or 0.0040% or less.

Cr: 0.010 to 0.80%

Cr is an element raising the strength of the hot stamped part by dissolving in the prior austenite grains at the time of heating before hot stamping. If the Cr content is less than 0.010%, the desired strength cannot be obtained. For this reason, the Cr content is 0.010% or more. The Cr content is preferably 0.05% or more, 0.10% or more, 0.15% or more, or 0.20% or more.

On the other hand, if the Cr content is more than 0.80%, coarse intermetallic compounds are formed in the hot stamped part and the hydrogen embrittlement resistance of the hot stamped part deteriorates. For this reason, the Cr content is 0.80% or less. The Cr content is preferably 0.70% or less, 0.60% or less, 0.50% or less, or 0.40% or less.

Mo: 0.0010 to 1.000%

Mo is an element improving the hardenability of steel. If the Mo content is less than 0.0010%, the desired strength cannot be obtained. For this reason, the Mo content is 0.0010% or more. The Mo content is preferably 0.005% or more, 0.010% or more, 0.050% or more, or 0.100% or more.

On the other hand, if the Mo content is more than 1.000%, coarse intermetallic compounds are formed in the hot stamped part and the hydrogen embrittlement resistance of the hot stamped part deteriorates. For this reason, the Mo content is 1.000% or less. The Mo content is preferably 0.800% or less, 0.600% or less, 0.500% or less, or 0.300% or less.

The basic chemical composition of the steel sheet for hot stamping according to an embodiment of the present invention is as explained above. Furthermore, the steel sheet for hot stamping may, in accordance with need, contain at least one of the following optional elements in place of part of the Fe of the balance. For example, the steel sheet for hot stamping may contain at least one element selected from the group consisting of Co: 0 to 2.00%, Ni: 0 to 3.00%, Cu: 0 to 1.00%, V: 0 to 1.00%, and W: 0 to 1.000%. Further, the steel sheet for hot stamping may contain at least one selected from the group consisting of Ca: 0 to 0.010%, Mg: 0 to 1.000%, and REM: 0 to 1.000%. Further, the steel sheet for hot stamping may contain at least one selected from the group consisting of Sb: 0 to 1.000%, Zr: 0 to 1.000%, and Sn: 0 to 1.000%. Further, the steel sheet for hot stamping may contain As: 0 to 0.100%. Below, these optional elements will be explained in detail.

Co: 0 to 2.00%

Co is an element improving the strength of the hot stamped part by solid solution strengthening. The Co content may be 0.001% or more, but to reliably obtain this effect, the Co content is preferably 0.01% or more or 0.05% or more.

On the other hand, even if contained in a large amount, the effect becomes saturated, and therefore the Co content is preferably 2.00% or less. The Co content may also be 1.80% or less, 1.50% or less, 1.00% or less, 0.80% or less, or 0.60% or less.

Ni: 0 to 3.00%

Ni has the action of dissolving in the austenite grains at the time of heating in the hot stamping step and thereby raising the strength of the hot stamped part. The Ni content may be 0.001% or more, but to reliably obtain this effect, the Ni content is preferably 0.01% or more.

On the other hand, even if contained in a large amount, the effect becomes saturated, and therefore the Ni content is preferably 3.00% or less. The Ni content may also be less than 3.00%, 2.80% or less, 2.50% or less, 2.00% or less, 1.50% or less, 1.00% or less, or 0.80% or less.

Cu: 0 to 1.00%

Cu has the action of dissolving in the austenite grains at the time of heating in the hot stamping step and thereby raising the strength of the hot stamped part. The Cu content may be 0.001% or more, but to reliably obtain this effect, the Cu content is preferably 0.01% or more or 0.05% or more.

On the other hand, even if contained in a large amount, the effect becomes saturated, and therefore the Cu content is preferably 1.00% or less. The Cu content may also be 0.80% or less, 0.60% or less, 0.50% or less, or 0.30% or less.

V: 0 to 1.00%

V has the effect of forming carbonitrides in the steel to thereby improve the strength of the hot stamped part by precipitation strengthening. The V content may be 0.001% or more, but to reliably obtain this effect, the V content is preferably 0.01% or more or 0.05% or more.

On the other hand, even if contained in a large amount, the effect becomes saturated, and therefore the V content is preferably 1.00% or less. The V content may also be 0.80% or less, 0.60% or less, 0.50% or less, or 0.30% or less.

W: 0 to 1.000%

W is an element improving the hardenability of steel. The W content may be 0.001% or more, but to reliably obtain this effect, the W content is preferably 0.005% or more or 0.010% or more.

On the other hand, even if included in a large amount, the above effect becomes saturated, and therefore the W content is preferably 1.000% or less. The W content may also be 0.800% or less, 0.600% or less, 0.500% or less, or 0.300% or less.

Ca: 0 to 0.010%

Ca is an element able to suppress the formation of oxides. The Ca content may be 0.0001% or more, but to reliably obtain this effect, the Ca content is preferably 0.0005% or more or 0.001% or more.

On the other hand, even if contained in a large amount, the effect becomes saturated, and therefore the Ca content is preferably 0.010% or less. The Ca content may also be 0.008% or less, 0.006% or less, 0.004% or less, 0.003% or less, or 0.002% or less.

Mg: 0 to 1.000%

Mg forms oxides and sulfides in the molten steel to suppress the formation of coarse MnS, causes dispersion of large number of fine oxides, and contributes to increased fineness of the metallographic structure. The Mg content may be 0.0001% or more, but to reliably obtain this effect, the Mg content is preferably 0.0005% or more or 0.001% or more.

On the other hand, even if contained in a large amount, the effect becomes saturated, and therefore the Mg content is preferably 1.000% or less. The Mg content may also be 0.500% or less, 0.100% or less, 0.050% or less, 0.010% or less, 0.005% or less, or 0.002% or less.

REM: 0 to 1.000%

REM is an element suppressing the formation of oxides. The REM content may be 0.0001% or more, but to reliably obtain this effect, the REM content is preferably 0.0005% or more or 0.001% or more.

On the other hand, even if contained in a large amount, the effect becomes saturated, and therefore the REM content is preferably 1.000% or less. The REM content may be 0.500% or less, 0.100% or less, 0.050% or less, 0.010% or less, 0.005% or less, or 0.002% or less.

In the present embodiment, “REM” is the general term for the 17 elements of atomic number 21 scandium (Sc), atomic number 39 yttrium (Y), and the lanthanoids of atomic number 57 lanthanum (La) to atomic number 71 lutetium (Lu). The REM content is the total content of these elements.

Sb: 0 to 1.000%

Sb is an element inhibiting the formation of oxides. To reliably obtain this effect, the Sb content is preferably 0.001% or more.

On the other hand, even if contained in a large amount, the effect becomes saturated, and therefore the Sb content is preferably 1.000% or less. The Sb content may also be 0.800% or less, 0.500% or less, 0.200% or less, or 0.100% or less.

Zr: 0 to 1.000%

Zr is an element suppressing the formation of oxides. If reliably obtaining this effect, the Zr content is preferably 0.001% or more.

On the other hand, even if included in a large amount, the above effect becomes saturated, and therefore the Zr content is preferably 1.000% or less. The Zr content may also be 0.800% or less, 0.500% or less, 0.200% or less, or 0.100% or less.

Sn: 0 to 1.000%

Sn is an element suppressing the formation of oxides. If reliably obtaining this effect, the Sn content is preferably 0.001% or more.

On the other hand, even if included in a large amount, the above effect becomes saturated, and therefore the Sn content is preferably 1.000% or less. The Sn content may also be 0.800% or less, 0.500% or less, 0.200% or less, or 0.100% or less.

As: 0 to 0.100%

As causes the temperature for forming an austenite single phase to fall and thereby contributes to refinement of the prior austenite grains. If reliably obtaining this effect, the As content is preferably 0.001% or more.

On the other hand, even if contained in a large amount, the above effect is saturated, and therefore the As content is preferably 0.100% or less. The As content may be 0.080% or less, 0.050% or less, 0.020% or less, or 0.010% or less.

In the steel sheet for hot stamping according to an embodiment of the present invention, the balance besides the above elements is comprised of Fe and impurities. The “impurities” are constituents, etc., entering due to various factors in the production process starting from materials such as ore and scrap, etc., when industrially producing the steel sheet for hot stamping.

The chemical composition of the above-mentioned steel sheet for hot stamping may be measured by a general analysis method. For example, it may be measured using ICP-AES (Inductively Coupled Plasma-Atomic Emission Spectrometry). C and S may be measured using the combustion-infrared absorption method, N may be measured using the inert gas melting-thermal conductivity method, and O may be measured by the inert gas melting-nondispersion type infrared absorption method.

If the surface of the steel sheet for above-mentioned hot stamping is provided with a plating layer, mechanical polishing may be used to remove the plating layer, then the chemical composition may be analyzed.

Ferrite: 10% or More, Pearlite: 10% or More, Total of Ferrite and Pearlite: 80% or More

The metallographic structure of the steel sheet for hot stamping according to an embodiment of the present invention contains, by area ratio, ferrite: 10% or more and pearlite: 10% or more. The total of ferrite and pearlite is 80% or more. Pearlite forms the starting points of austenite grains at the time of heating in hot stamping, and therefore has to be present in the metallographic structure by an area ratio of 10% or more. Further, in the present embodiment, inclusion of pearlite in combination with ferrite enables homogeneous dispersion of pearlite. The area ratios of ferrite and pearlite may respectively be any values of 10% or more in the range where the total of the same becomes 80% or more. For example, they may respectively independently be 20% or more, 30% or more, 40% or more, 50% or more, or 60% or more. The upper limit is not particularly prescribed, but the area ratios of ferrite and pearlite may, for example, be respectively independently 85% or less, 80% or less, or 70% or less. The total of the area ratios of ferrite and pearlite may be 85% or more, 90% or more, or 95% or more. The upper limit is not particularly prescribed, but the total of the area ratios of ferrite and pearlite may be 100%, for example, may be 99% or less or 98% or less. The remaining structure is not particularly limited, but may be comprised of at least one of bainite, martensite, retained austenite, and carbides. The carbides are, for example, Fe carbides. Sometimes trace amounts are formed at the boundaries of the ferrite phases and ferrite phases. The area ratio of the remaining structure is 20% or less, for example, may be 17% or less, 15% or less, 12% or less, 10% or less, 8% or less, 5% or less, or 3% or less.

Dispersion Index of Pearlite: 0.50 or More

In the metallographic structure of the steel sheet for hot stamping according to an embodiment of the present invention, pearlite has to be homogeneously dispersed. In the present embodiment, such a homogeneous dispersion of pearlite is achieved by controlling the dispersion index of pearlite to become 0.50 or more. The dispersion index of pearlite is obtained by dividing the number of A/B boundaries (boundaries of ferrite phases and pearlite phases) in an electron micrograph of the metallographic structure of the steel sheet for hot stamping mainly comprised of ferrite and pearlite by the total of the number of A/A boundaries (boundaries of ferrite phases and ferrite phases), the number of B/B boundaries (boundaries of pearlite phases and pearlite phases), and the number of A/B boundaries (boundaries of ferrite phases and pearlite phases). The dispersion index of pearlite being high means that in the metallographic structure mainly comprised of ferrite and pearlite, the ratio of the A/B boundaries is high, i.e., the number of boundaries of ferrite phases and pearlite phases is large. Therefore, by controlling the dispersion index of pearlite to a high value, it is possible to reduce more the parts where pearlite is present connected and possible to achieve more homogeneous dispersion of pearlite. According to the present embodiment, by controlling the dispersion index of pearlite to 0.50 or more, it is possible to achieve a standard deviation in hardness distribution of prior austenite grains of 150 Hv or less in the final metallographic structure of the hot stamped part due to such homogeneous dispersion of pearlite. As a result, it is possible to reduce the variation in hardness of the prior austenite grains in the metallographic structure of the hot stamped part and in turn possible to remarkably suppress a rise in local hardness, and therefore it is possible to remarkably improve the hydrogen embrittlement resistance despite having a high tensile strength, for example, a high tensile strength of 2200 MPa or more. The higher the dispersion index of pearlite, the more preferable. For example, it may be 0.52 or more, 0.55 or more, 0.58 or more, 0.60 or more, 0.62 or more, or 0.65 or more. The upper limit is not particularly prescribed, but the dispersion index of pearlite may, for example, be 0.80 or less, 0.75 or less, or 0.70 or less.

The dispersion index of pearlite is determined in the following way. First, using a scan type electron microscope, an electron channeling contrast image centered at the sheet thickness 1/4 position in the cross-section vertical to the surface in a range of 35 um in the direction vertical to the sheet thickness direction and 10 um in the sheet thickness direction is obtained. For this measurement, specifically an EBSD analysis apparatus comprised of a thermal field emission type scan electron microscope and EBSD detector may be used. For example, an EBSD analysis apparatus comprised of a JSM-7001F made by JEOL and DVC5 type detector made by TSL may be used. Next, in the obtained electron channeling contrast image, 10 lines vertical to the sheet thickness direction are drawn at 1 um intervals. Next, the phase boundaries where these lines cross are classified as A/A boundaries (boundaries of ferrite phases and ferrite phases), B/B boundaries (boundaries of pearlite phases and pearlite phases), and A/B boundaries (boundaries of ferrite phases and pearlite phases) and the intersecting points of the boundaries are calculated. Next, the number of intersecting points of the A/B boundaries are divided by the total number of intersecting points, i.e., the total of the number of intersecting points of the A/A boundaries, the number of intersecting points of the B/B boundaries, and the number of intersecting points of the A/B boundaries, to obtain the ratio of the A/B boundaries in the field. The same procedure is performed on the same sample at five fields and the average value of the ratios of the A/B boundaries at the five fields is determined as the dispersion index of pearlite.

Sheet Thickness

The steel sheet for hot stamping according to an embodiment of the present invention is not particularly limited, but, for example, has a sheet thickness of 0.1 to 4.0 mm. The sheet thickness may also be 0.2 mm or more, 0.4 mm or more, 0.6 mm or more, 0.8 mm or more, or 1.0 mm or more. Similarly, the sheet thickness may be 3.6 mm or less, 3.2 mm or less, 2.8 mm or less, 2.4 mm or less, or 2.0 mm or less. If the steel sheet for hot stamping is a hot rolled steel sheet, the sheet thickness may, for example, be 1.0 to 4.0 mm. On the other hand, if the steel sheet for hot stamping is a cold rolled steel sheet, the sheet thickness may, for example, be 0.1 to 2.0 mm.

Plating

The steel sheet for hot stamping according to an embodiment of the present invention may have a plating layer on its surface. By having a plating layer on its surface, it is possible to improve the corrosion resistance after hot stamping. As the plating layer, an aluminum plating layer, aluminum-zinc plating layer, aluminum-silicon plating layer, hot dip galvanized layer, electrogalvanized layer, hot dip galvannealed plating layer, zinc-nickel plating layer, aluminum-magnesium-zinc plating layer, etc., may be illustrated.

Hot Stamped Part

In the present invention, in addition to the above steel sheet for hot stamping, a hot stamped part produced using the steel sheet for hot stamping is further provided. Therefore, below, a hot stamped part according to an embodiment of the present invention will be explained in more detail. The hot stamped part has a chemical composition comprising, by mass %,

    • C: 0.40 to 0.70%,
    • Si: 0.010 to 1.300%,
    • Mn: 0.60 to 3.00%,
    • P: 0.100% or less,
    • S: 0.0100% or less,
    • N: 0.0200% or less,
    • O: 0.0200% or less,
    • Al: 0.0010 to 0.5000%,
    • Nb: 0.0010 to 0.100%,
    • Ti: 0.010 to 0.200%,
    • B: 0.0005 to 0.0200%,
    • Cr: 0.010 to 0.80%,
    • Mo: 0.0010 to 1.000%,
    • Co: 0 to 2.00%,
    • Ni: 0 to 3.00%,
    • Cu: 0 to 1.00%,
    • V: 0 to 1.00%,
    • W: 0 to 1.000%,
    • Ca: 0 to 0.010%,
    • Mg: 0 to 1.000%,
    • REM: 0 to 1.000%,
    • Sb: 0 to 1.000%,
    • Zr: 0 to 1.000%,
    • Sn: 0 to 1.000%,
    • As: 0 to 0.100%, and
    • balance: Fe and impurities, and
    • a metallographic structure comprising, by area ratio, at least one of martensite, bainite, and tempered martensite in a total of 90% or more, wherein
    • a standard deviation in a hardness distribution of prior austenite grains at a sheet thickness ¼ position is 150 Hv or less.

Chemical Composition of Hot Stamped Part

The chemical composition does not substantially change in hot stamping, and therefore the chemical composition of the hot stamped part has the same chemical composition as the steel sheet for hot stamping described above. Therefore, the explanations of the elements and balance relating to the chemical composition of the steel sheet for hot stamping described above apply to not only the steel sheet for hot stamping, but also the hot stamped part.

At Least One of Martensite, Bainite, and Tempered Martensite: Total of 90% or More

The metallographic structure of the hot stamped part includes, by area ratio, at least one of martensite, bainite, and tempered martensite in a total of 90% or more. The remaining structure is not particularly limited, but may also be comprised of at least one of ferrite, retained austenite and pearlite in 10% or more. Martensite, bainite, and tempered martensite are extremely hard structures, and therefore by the hot stamped part containing at least one of martensite, bainite, and tempered martensite in an area ratio of a total of 90% or more, a high tensile strength, specifically a tensile strength of 2200 MPa or more, can be achieved. The total of the area ratios of the least one of martensite, bainite, and tempered martensite is preferably 92% or more or 94% or more, more preferably 95% or more or 97% or more. The upper limit of the total of the area ratios of the at least one of martensite, bainite, and tempered martensite is not particularly prescribed and may also be 100%.

Identification of Metallographic Structure and Calculation of Area Ratios

The metallographic structures in the hot stamped part and the previously explained steel sheet for hot stamping are identified and the area ratios are calculated in the following way. First, a sample is cut out from any position 50 mm or more away from the ends of the steel material (if not possible to obtain a sample from this position, a position away from the ends) so as to enable a cross-section of thickness vertical to the surface to be examined. The size of the sample depends on the measurement device, but is a size enabling 10 mm or so to be examined in a direction vertical to the thickness direction.

The cross-section of the sample is polished using #600 to #1500 silicon carbide paper, then a liquid comprised of particle size 1 to 6 um diamond powder dispersed in alcohol or other diluent or pure water is used to polish the surface to a mirror finish. Next, the examined surface is finished by electrolytic polishing. An area of a length 50 μm and 50 μm in the sheet thickness direction centered at a ¼ depth position of the sheet thickness at any position in the long direction of the sample cross-section is measured at 0.1 μm measurement intervals by electron backscatter diffraction to obtain crystal orientation information. For the measurement, an EBSD analysis apparatus comprised of a thermal field emission type scan electron microscope and EBSD detector may be used. For example, an EBSD analysis apparatus comprised of a JSM-7001F made by JEOL and a DVC5 model detector made by TSL may be used. At that time, the vacuum degree inside the EBSD analysis apparatus may be 9.6×10−5 Pa or less, the acceleration voltage may be 15 kV, and the beam current level may be 13.

The obtained crystal orientation information is analyzed using the “Phase Map” function included in the software “OIM Analysis®” attached to the EBSD analysis apparatus. Structures with fcc crystal structures are judged to be retained austenite. The area ratio of the retained austenite is obtained by calculating the area ratio of this retained austenite. Next, regions with bcc crystal structures are judged to be bainite, tempered martensite, martensite, and ferrite. In these regions, using the “Grain Average Misorientation” function included in the software “OIM Analysis®” attached to the EBSD analysis apparatus, under conditions deeming a 5° grain boundary as a crystal grain boundary, a region having a “Grain Average Misorientation” of 0.5° or less is extracted as ferrite. The area ratio of ferrite is obtained by calculating the area ratio of the extracted ferrite.

Next, the remaining region (region with “Grain Average Misorientation” of more than 0.5°) is made the area ratio of the total of martensite, tempered martensite, and bainite. As for the carbides, regions having grain-like shapes which are bright in contrast in the secondary electron image captured using a scan type electron microscope for the same field as the EBSD examination are judged to be carbides. The area ratio of the corresponding regions is calculated to obtain the area ratio of carbides. The area ratio of pearlite is calculated by subtracting from 100% the area ratio of the retained austenite, the area ratios of the bainite, tempered martensite, martensite, and ferrite, and the area ratio of the carbides.

Standard Deviation in Hardness Distribution of Prior Austenite Grains at Sheet Thickness ¼ Position: 150 Hv or Less

In an embodiment of the present invention, the standard deviation in hardness distribution of prior austenite grains at the sheet thickness ¼ position of the hot stamped part is 150 Hv or less. If the variation in hardness of the prior austenite grains is large, a rise in local hardness is invited and sometimes hydrogen embrittlement cracking is triggered. According to an embodiment of the present invention, by controlling the standard deviation in hardness distribution of prior austenite grains at the sheet thickness ¼ position of the hot stamped part to 150 Hv or less and reducing the variation in hardness of the prior austenite grains, it is possible to reliably suppress a rise in local hardness forming starting points of hydrogen embrittlement cracking. Preferably, the standard deviation is 140 Hv or less, 130 Hv or less, 120 Hv or less, or 110 Hv or less. The lower limit is not particularly prescribed, but the standard deviation in hardness distribution of prior austenite grains at the sheet thickness ¼ position of the hot stamped part may, for example, be 50 Hv or more, 60 Hv or more, or 80 Hv or more.

In an embodiment of the present invention, as explained above, it is important to control the standard deviation in hardness distribution of prior austenite grains at the sheet thickness ¼ position of the hot stamped part (not hardness distribution of metallographic structure as a whole at sheet thickness ¼ position, but hardness distribution of prior austenite grains present in the metallographic structure) to 150 Hv or less to reduce the variation in hardness of the prior austenite grains. For this reason, it is not necessary to control the hardness itself of the prior austenite grains to a specific range. Therefore, the hardness of the prior austenite grains at the sheet thickness ¼ position of the hot stamped part is not particularly limited, but, for example, may be 500 Hv or more and/or may be 1000 Hv or less. The “hardness of the prior austenite grains at the sheet thickness ¼ position of the hot stamped part” means the average of all measured values of hardness measured in the method of determination of the standard deviation in hardness distribution of prior austenite grains explained below.

The standard deviation in hardness distribution of prior austenite grains is determined in the following way. First, a sample is cut out from any position 50 mm or more away from the end faces of the hot stamped part so that the cross-section vertical to the surface (sheet thickness cross-section) can be examined. The sample, while depending on the measurement apparatus, is made a size enabling examination of 10 mm in a direction vertical to the sheet thickness direction. The cross-section of the sample is polished using #600 to #1500 silicon carbide paper, then is finished to a mirror surface using a liquid comprised of diamond powder of a granularity of 1 to 6 μm dispersed in alcohol or other diluent and pure water. The cross-section finished to a mirror surface is measured for Vickers hardness at a ¼ depth position of the sheet thickness from the surface using a micro Vickers hardness tester by a load of 1 gf at intervals of 3 times or more of the indentations in the direction parallel to the sheet surface. Measurement values of a total of 100 points or more are obtained. Next, an EBSD analysis apparatus was used to measure the same sample. Referring to the obtained results of analysis of the structure, only measurement points with indentations inside the prior austenite grains (i.e., where indentations do not cover the grain boundaries) are extracted. Finally, the standard deviation obtained based on the measurement values of Vickers hardness relating to 20 or more different prior austenite grains extracted is determined as the standard deviation in hardness distribution of prior austenite grains at the sheet thickness ¼ position of the hot stamped part. For the EBSD measurement, an EBSD analysis apparatus comprised of a thermal field emission type scan electron microscope and EBSD detector may be used. For example, an EBSD analysis apparatus comprised of a JSM-7001F made by JEOL and DVC5 type detector made by TSL may be used. At that time, the vacuum degree inside the EBSD analysis apparatus may be 9.6×10−5 Pa or less, the acceleration voltage may be 15 kV, and the beam current level may be 13. The method described in Acta Materialia, 58 (2010), 6393-6403 is used to prepare a crystal orientation map of the prior austenite grains and the prior austenite grains are identified based on this crystal orientation map.

Plating

The hot stamped part according to the present embodiment may have a plating layer on its surface. By having a plating layer on its surface, it is possible to improve the corrosion resistance. As the plating layer, an aluminum plating layer, aluminum-zinc plating layer, aluminum-silicon plating layer, hot dip galvanized layer, electrogalvanized layer, hot dip galvannealed plating layer, zinc-nickel plating layer, aluminum-magnesium-zinc plating layer, etc., may be illustrated.

Mechanical Properties

According to the hot stamped part of an embodiment of the present invention, excellent mechanical properties, for example, a tensile strength of 2200 MPa or more, can be achieved.

The tensile strength is preferably 2300 MPa or more, more preferably 2400 MPa or more, most preferably 2500 MPa or more or 2600 MPa or more. The upper limit is not particularly prescribed, but, for example, the tensile strength may be 3500 MPa or less, 3300 MPa or less, or 3000 MPa or less. The tensile strength of the hot stamped part is measured by preparing a No. 5 test piece and conducting a tensile test based on JIS Z 2241:2011.

The hot stamped part according to an embodiment of the present invention, despite as explained above having a high tensile strength of, for example, 2200 MPa or more, is excellent in hydrogen embrittlement resistance, and therefore is extremely useful for use as, for example, a frame member or bumper of an automobile or other structural member and reinforcing member where strength is required.

Method of Production

Next, a preferable method of production of the steel sheet for hot stamping and the hot stamped part according to an embodiment of the present invention will be explained. The following explanation is intended to illustrate the characteristic method for producing the steel sheet for hot stamping and the hot stamped part according to the embodiment of the present invention and is not intended to limit the steel sheet for hot stamping and hot stamped part to one produced by the method of production such as explained below.

Method of Production of Steel Sheet for Hot Stamping

In the method of production of steel sheet for hot stamping according to an embodiment of the present invention, in particular controlling the finish rolling conditions and later cooling conditions is effective. Specifically, the method of production of steel sheet for hot stamping according to an embodiment of the present invention comprises

    • hot rolling a slab having a chemical composition explained above in relation to the steel sheet for hot stamping, wherein the hot rolling includes heating the slab, then finish rolling it, and a rolling reduction of a final stage in the finish rolling is 40% or more (hot rolling step),
    • rapidly cooling the obtained hot rolled steel sheet within 1.0 second after the end of finish rolling, then cooling it by an average cooling speed of 90° C./s or more (cooling step), and
    • coiling the hot rolled steel sheet at a temperature of 500 to 700° C. (coiling step). Below, the steps will be explained in detail.

Hot Rolling Step Heating of Slab

First, a slab having the chemical composition explained above in relation to the steel sheet for hot stamping is heated. The method of casting the molten steel is not particularly limited. The slab may be produced by continuous casting, ingot forming, or thin slab casting. The heating before the hot rolling is not particularly limited, but the slab used contains a relatively large amount of alloying elements for obtaining a high strength steel sheet. For this reason, the slab may also be heated before being sent on for hot rolling. For the purpose of making the alloying elements dissolve in the slab, the heating temperature may be 1100° C. or more.

Rough Rolling

In the present method, for example, the heated slab may be rough rolled before the finish rolling so as to adjust the sheet thickness, etc. The rough rolling need only secure the desired sheet bar dimensions. The conditions are not particularly limited.

Finish Rolling

The heated slab, or the slab additionally rough rolled in accordance with need, is next finish rolled. In the present method, making the rolling reduction of the final stage at the finish rolling 40% or more is important. By making the rolling reduction of the final stage at the finish rolling 40% or more, pearlite is homogeneously dispersed in the hot rolled steel sheet after rolling. This pearlite forms starting points for austenite at the time of heating in hot stamping step explained in detail later relating to the method of production of the hot stamped part. For this reason, if pearlite is homogeneously dispersed, it is possible to reduce the variation in prior austenite grain size in the hot stamped part. As a result, it is possible to reduce the variation in hardness of the prior austenite grains in the metallographic structure of the hot stamped part and in turn possible to remarkably suppress a rise in local hardness. Therefore, it is possible to remarkably improve the hydrogen embrittlement resistance regardless of having a high tensile strength, for example a high tensile strength of 2200 MPa or more. More preferably, the rolling reduction of the final stage in the finish rolling is 45% or more or 50% or more.

In the steel sheet for hot stamping, the amount of Mn added tends to be increased for the purpose of securing a high hardenability. For example, 0.60% or more of Mn is added. In relation to this, in the current research of the inventors, it was learned that with such a high Mn content, pearlite tends to be arranged relatively connected at the hot rolled steel sheet, and therefore compared with the case of a low Mn content, it is extremely difficult to make the pearlite homogeneously disperse in the metallographic structure of the hot rolled steel sheet. Therefore, if finish rolling a steel material with such a high Mn content by a relatively low rolling reduction of less than 40%, it is believed that in the metallographic structure, the presence of parts where pearlite is connected will become particularly remarkable.

However, by making the rolling reduction of the final stage in the finish rolling 40% or more, despite the high Mn content of 0.60% or more, it is possible to arrange the pearlite sufficiently dispersed in the hot rolled steel sheet after the hot rolling step and the subsequent cooling step and coiling step. Therefore, in the metallographic structure of the hot rolled steel sheet rolled in this way, either there will be no parts where pearlite is present connected or they will be sufficiently reduced, and therefore in the structure after hot stamping, the variation in prior austenite grain size can be reduced. As a result, it is possible to reduce the variation in hardness of the prior austenite grains in the metallographic structure of the hot stamped part. The upper limit of the rolling reduction of the final stage in the finish rolling is not particularly prescribed. In this way, even in a steel material having a high Mn content, by particularly suitably controlling the rolling reduction of the final stage in the finish rolling, it is possible to arrange the pearlite sufficiently dispersed in the metallographic structure of the hot rolled steel sheet and in turn possible to reduce the variation in prior austenite grain size and suppress a rise in local hardness.

In such a form of metallographic structure, the rolling reduction of the final stage in the finish rolling and the average cooling speed in the subsequent cooling step and coiling temperature in the coiling step are dominant factors. For example, no particularly great effect is affected even by an optional cold rolling or subsequent annealing, etc. This is because if making the rolling reduction of the final stage in the finish rolling 40% or more to form the hot rolled steel sheet, even if the hot rolled steel sheet is cold rolled and then annealed under a relatively high temperature, there is a high tendency that a metallographic structure will be formed in which carbides, grain boundaries and retained austenite forming starting points of austenite after cooling are arranged dispersed. In general, if overly increasing the rolling reduction of the final stage in the finish rolling, cracking of the steel sheet at the time of rolling will be a concern. In particular, in the case of a high strength steel sheet with a C content of 0.40% or more, if overly raising the rolling reduction of the final stage, in addition to the concern over cracking of the steel sheet, the rolling load of the rolling mill will also remarkably increase. For this reason, in a steel material having a chemical composition similar to that of the steel sheet for hot stamping according to an embodiment of the present invention, finish rolling resulting in a rolling reduction in the final stage of 40% or more has not been performed in the past. Therefore, the fact that by making the rolling reduction of the final stage in the finish rolling 40% or more and further suitably controlling the average cooling speed in the subsequent cooling step and coiling temperature in the coiling step and combining the same, it is possible to make the pearlite forming starting points of austenite grains at the time of heating in the hot stamping homogeneously disperse was not known in the past. Therefore, only naturally, in relation to this, the fact that in the finally obtained hot stamped part, the prior austenite grains are uniform, and accordingly the variation in hardness of the prior austenite grains in the metallographic structure of the hot stamped part can be reduced was not known in the past. These facts were first clarified this time by the inventors.

Cooling Step

Next, the finish rolled hot rolled steel sheet is rapidly cooled within 1.0 second after the end of the finish rolling. Ferrite is generally formed from the grain boundaries of austenite grains, and therefore if the austenite grains become large, the number of grain boundaries forming starting points of ferrite will be reduced. In such a case, preventing connection of pearlite and making it homogeneously disperse becomes difficult. Therefore, rapidly cooling the hot rolled steel sheet immediately after the end of finish rolling, specifically rapidly cooling the hot rolled steel sheet within 1.0 second after the end of the finish rolling, preferably within 0.8 second, to suppress the growth of austenite grains is extremely important in making pearlite form homogeneously dispersed in the hot rolled steel sheet. The average cooling speed and cooling time period at the time of rapid cooling are not particularly limited, but, for example, the average cooling speed is preferably 200 to 1000° C./s and the cooling time period is preferably 0.2 to 2.0 seconds.

Next, the rapidly cooled hot rolled steel sheet is cooled by an average cooling speed of 90° C./s or more. If the cooling speed is slow, bainite is greatly formed, austenite remains as retained austenite, and formation of a metallographic structure mainly comprised of ferrite and pearlite becomes no longer possible. As a result, controlling the arrangement of ferrite and pearlite becomes difficult. In particular, achieving homogeneous dispersion of pearlite becomes difficult. On the other hand, by cooling at an average cooling speed of 90° C./s or more, it is possible to form a metallographic structure comprised mainly of ferrite and pearlite, more specifically, possible to form a metallographic structure having a total of the area ratios of ferrite and pearlite of 80% or more. The average cooling speed is preferably 95° C./s or more. The upper limit is not particularly prescribed, but the average cooling speed may, for example, be 200° C./s or less or 150° C./s or less.

Coiling Step

Next, the finish rolled hot rolled steel sheet is coiled at a temperature of 500 to 700° C. If the coiling temperature is high, grain growth occurs and sometimes homogeneous dispersion of pearlite is obstructed. On the other hand, if the coiling temperature is low, bainite and martensite are formed and formation of a metallographic structure mainly comprised of ferrite and pearlite becomes no longer possible. As opposed to this, by controlling the coiling temperature to 500 to 700° C., it is possible to suppress grain growth and keep ferrite from being arranged connected at the hot rolled steel sheet after rolling and possible to make the pearlite homogeneously disperse. Preferably, the coiling temperature is 505 to 650° C. or 550 to 650° C. Further, after coiling, the coil may be treated to soften it for the purpose of softening the hot rolled steel sheet. The method of heat treatment for softening is not particularly limited. General conditions may be used.

Pickling Step

After the coiling step and before the cold rolling step, pickling may be performed for removing the oxide scale formed on the surface of the hot rolled steel sheet. The pickling may be performed under conditions suitable for removing oxide scale. It may be performed at one time or may be performed divided into several times so as to reliably remove the oxide scale.

Cold Rolling Step

After the coiling step, the steel sheet may be optionally cold rolled. The cold rolling is not particularly limited and may be performed under any suitable conditions. For example, the rolling reduction of the cold rolling may be 30 to 80%. The number of rolling passes and the rolling reduction per pass are not particularly limited and may be suitable set so that the rolling reduction of the cold rolling as a whole becomes the above range.

Annealing Step

For example, after the cold rolling step, annealing may optionally be performed to adjust the metallographic structure and/or properties. The heating temperature of the annealing step is not particularly limited, but may for example be 800° C. or less.

Plating Step

For the purpose of improving the corrosion resistance, etc., the surface of the hot rolled steel sheet or cold rolled steel sheet may be treated to plate it. The plating treatment may be hot dip coating, hot dip alloyed coating, electroplating, or other treatment. For example, the steel sheet may be hot dip galvanized as plating treatment or may be hot dip galvanized and then alloyed. As the plating layer, an aluminum plating layer, aluminum-zinc plating layer, aluminum-silicon plating layer, hot dip galvanized layer, electrogalvanized layer, hot dip galvannealed layer, zinc-nickel plating layer, aluminum-magnesium-zinc plating layer, etc., may be illustrated. The specific conditions of the plating treatment and alloying treatment are not particularly limited and may be any suitable conditions known to persons skilled in the art.

Temper Rolling Step

To correct the shape of the steel sheet or adjust the surface roughness, etc., it is possible, for example, to temper roll the steel sheet after the annealing step, or after the plating step.

Method of Production of Hot Stamped Part

Next, the method of production of the hot stamped part according to an embodiment of the present invention will be explained. Specifically, this method of production includes hot stamping a steel sheet for hot stamping obtained by the method of production of a steel sheet for hot stamping explained above, wherein the hot stamping includes heating the steel sheet for hot stamping to a temperature range of 800° C. to 1000° C., then holding it there for 60 to 600 seconds.

Hot Stamping Step

The steel sheet for hot stamping is hot stamped in the hot stamping step so as to produce a hot stamped part having the desired chemical composition and metallographic structure. In the present embodiment, at the time of heating in the hot stamping, the homogeneously dispersed pearlite in the metallographic structure of the steel sheet forms starting points for formation of austenite. In the subsequent forming and cooling operations, the desired hard structures are formed, the desired prior austenite grain size distribution with reduced variations is obtained, and therefore a hot stamped part having a metallographic structure in which variation in hardness of the prior austenite grains is reduced is produced. From the viewpoint of obtaining such desired hard structures and hardness distribution of prior austenite grains, it is preferable to heat the steel sheet for hot stamping to a temperature range of 800° C. to 1000° C. and hold it at the temperature range for 60 to 600 seconds. If the heating temperature is less than 800° C., sometimes the austenization will become insufficient, the desired area ratio of the hard structures (at least one of martensite, bainite, and tempered martensite) will not be able to be obtained, and the tensile strength will deteriorate. On the other hand, if the heating temperature is more than 1000° C., the austenite grains will excessively grow, the desired prior austenite grain size distribution will not be able to be obtained, and, as a result, sometimes the desired hardness distribution of the prior austenite grains will not be able to be obtained and the hydrogen embrittlement resistance will deteriorate. If the holding time period is less than 60 seconds, in the same way as the case where the heating temperature is less than 800° C., sometimes the austenization will become insufficient, the desired area ratio of the hard structures (at least one of martensite, bainite, and tempered martensite) will not be able to be obtained, and the tensile strength will deteriorate. If the holding time period is more than 600 seconds, the austenite grains will excessively grow, the desired prior austenite grain size distribution will not be able to be obtained, and, as a result, sometimes the desired hardness distribution of the prior austenite grains will not be able to be obtained, and the hydrogen embrittlement resistance will deteriorate.

The heating atmosphere is not particularly limited. Usual conditions are enough. For example, it may be an air atmosphere, a gas combustion atmosphere controlled in ratio of air and fuel, and a nitrogen atmosphere. The dew points may also be controlled in these gases. The steel sheet is held at a temperature range of 800° C. to 1000° C., then hot stamped. After hot stamping, it may be cooled down to a temperature region of 250° C. or less by an average cooling speed of 20° C./s or more.

As the heating method before hot stamping, for example, furnace heating by an electric furnace, gas furnace, etc., flame heating, ohmic heating, high frequency heating, induction heating, etc., may be mentioned.

The hot stamped part according to the present embodiment is obtained by the above method. After hot stamping, it may be tempered at 130 to 600° C. or coated, then bake hardened (BH). Further, part of the hot stamped part may be tempered by being irradiated by a laser, etc. to partially provide softened regions.

Below, examples will be used to explain the present invention in more detail, but the present invention is not limited to these examples in any way.

EXAMPLES

In the following examples, hot stamped parts according to an embodiment of the present invention were produced under various conditions and the tensile strength and hydrogen embrittlement resistance of the obtained hot stamped parts were investigated.

First, molten steels having the chemical compositions shown in Table 1 were cast by continuous casting to produce slabs. The balances besides the constituents shown in Table 1 were Fe and impurities. These slabs were heated to a temperature of 1100° C. or more and rough rolled under predetermined conditions, then were finish rolled, cooled, and coiled under conditions shown in Table 2. In all of the invention examples and comparative examples, the average cooling speed at the time of rapid cooling after the end of finish rolling was within a range of 200 to 1000° C./s and the cooling time period was within a range of 0.2 to 2.0 seconds. Next, the obtained hot rolled steel sheets were cold rolled by predetermined rolling reductions of 30 to 80%. Next, some of the steel sheets were subjected to annealing, plating, or temper rolling under predetermined conditions. Next, the obtained steel sheets were hot stamped under the conditions shown in Table 2. The heating atmosphere and heating method in the hot stamping step, except when clearly indicated otherwise, were a gas combustion atmosphere (air-fuel ratio 0.85) and furnace heating. After the hot stamping, some of the hot stamped parts were tempered or partially softened.

TABLE 1 Steel Chemical composition (mass %, balance: Fe and impurities) type C Si Mn F S N O Al Nb Ti B Cr Mo A1 0.37 0.440 1.30 0.007 0.0018 0.0022 0.0022 0.0410 0.020 0.034 0.0031 0.21 0.153 A2 0.41 0.410 1.32 0.005 0.0018 0.0033 0.0014 0.0610 0.041 0.027 0.0034 0.22 0.229 A3 0.43 0.450 1.32 0.005 0.0004 0.0021 0.0014 0.0540 0.019 0.045 0.0032 0.31 0.228 A4 0.44 0.410 1.36 0.010 0.0011 0.0026 0.0029 0.0550 0.016 0.044 0.0031 0.21 0.223 A5 0.45 0.400 1.27 0.005 0.0018 0.0028 0.0025 0.0530 0.031 0.032 0.0023 0.25 0.165 A6 0.46 0.410 1.24 0.004 0.0016 0.0023 0.0021 0.0480 0.018 0.040 0.0029 0.23 0.211 A7 0.48 0.450 1.34 0.005 0.0014 0.0036 0.0029 0.0600 0.019 0.025 0.0021 0.24 0.132 A8 0.50 0.420 1.24 0.008 0.0017 0.0040 0.0021 0.0430 0.021 0.024 0.0019 0.31 0.221 A9 0.55 0.410 1.36 0.005 0.0007 0.0028 0.0012 0.0560 0.029 0.036 0.0018 0.25 0.204 A10 0.61 0.410 1.25 0.005 0.0007 0.0022 0.0011 0.0470 0.041 0.048 0.0018 0.30 0.191 A11 0.66 0.430 1.25 0.006 0.0019 0.0040 0.0011 0.0480 0.036 0.044 0.0024 0.23 0.191 A12 0.68 0.440 1.26 0.010 0.0009 0.0033 0.0011 0.0530 0.034 0.041 0.0031 0.21 0.153 A13 0.73 0.400 1.30 0.004 0.0008 0.0031 0.0020 0.0590 0.019 0.029 0.0031 0.31 0.219 B1 0.45 0.007 1.32 0.004 0.0002 0.0023 0.0017 0.0600 0.019 0.041 0.0032 0.24 0.134 B2 0.44 0.014 1.33 0.006 0.0016 0.0026 0.0011 0.0550 0.029 0.036 0.0028 0.24 0.137 B3 0.47 0.060 1.27 0.010 0.0005 0.0035 0.0017 0.0540 0.033 0.023 0.0025 0.31 0.231 B4 0.47 0.150 1.27 0.006 0.0007 0.0029 0.0013 0.0430 0.031 0.032 0.0022 0.25 0.206 B5 0.47 0.240 1.32 0.009 0.0003 0.0023 0.0013 0.0450 0.022 0.046 0.0020 0.25 0.150 B6 0.45 0.370 1.28 0.010 0.0017 0.0039 0.0030 0.0510 0.030 0.038 0.0030 0.33 0.171 B7 0.44 0.560 1.36 0.009 0.0018 0.0035 0.0021 0.0440 0.032 0.044 0.0034 0.23 0.222 B8 0.44 0.750 1.36 0.004 0.0017 0.0027 0.0018 0.0600 0.031 0.048 0.0019 0.30 0.157 B9 0.46 0.940 1.28 0.008 0.0006 0.0033 0.0019 0.0520 0.036 0.035 0.0025 0.25 0.145 B10 0.46 1.080 1.36 0.006 0.0009 0.0025 0.0026 0.0420 0.025 0.035 0.0016 0.30 0.131 B11 0.47 1.250 1.27 0.008 0.0015 0.0028 0.0016 0.0530 0.024 0.036 0.0023 0.32 0.230 B12 0.44 1.400 1.24 0.007 0.0003 0.0030 0.0010 0.0590 0.015 0.028 0.0033 0.23 0.139 C1 0.46 0.470 0.55 0.007 0.0015 0.0033 0.0020 0.0570 0.014 0.045 0.0028 0.29 0.175 C2 0.45 0.400 0.62 0.004 0.0007 0.0034 0.0026 0.0540 0.027 0.021 0.0026 0.25 0.164 C3 0.44 0.450 0.69 0.006 0.0003 0.0031 0.0022 0.0520 0.023 0.035 0.0024 0.22 0.213 C4 0.46 0.400 0.75 0.010 0.0010 0.0022 0.0020 0.0550 0.038 0.027 0.0018 0.22 0.152 C5 0.44 0.440 0.88 0.008 0.0003 0.0023 0.0027 0.0470 0.034 0.039 0.0027 0.23 0.198 C6 0.44 0.390 1.10 0.010 0.0007 0.0030 0.0011 0.0410 0.041 0.037 0.0027 0.25 0.214 C7 0.45 0.390 1.30 0.006 0.0013 0.0026 0.0016 0.0410 0.016 0.042 0.0019 0.31 0.130 C8 0.44 0.470 1.60 0.007 0.0007 0.0033 0.0020 0.0390 0.020 0.037 0.0018 0.33 0.146 C9 0.45 0.470 1.70 0.009 0.0004 0.0024 0.0020 0.0470 0.028 0.043 0.0019 0.26 0.231 C10 0.45 0.450 1.90 0.010 0.0020 0.0036 0.0021 0.0610 0.033 0.028 0.0025 0.30 0.167 C11 0.45 0.440 2.20 0.009 0.0004 0.0027 0.0030 0.0470 0.019 0.029 0.0032 0.22 0.171 C12 0.44 0.430 2.40 0.008 0.0018 0.0030 0.0029 0.0450 0.031 0.037 0.0025 0.24 0.134 C13 0.44 0.440 2.60 0.006 0.0019 0.0039 0.0022 0.0600 0.035 0.040 0.0022 0.30 0.136 C14 0.45 0.420 2.80 0.010 0.0008 0.0038 0.0017 0.0600 0.022 0.022 0.0030 0.21 0.219 C15 0.45 0.420 2.90 0.007 0.0019 0.0034 0.0013 0.0400 0.037 0.025 0.0020 0.25 0.138 C16 0.46 0.420 3.10 0.007 0.0015 0.0033 0.0024 0.0520 0.014 0.024 0.0025 0.23 0.209 D1 0.45 0.410 1.25 0.001 0.0004 0.0026 0.0016 0.0500 0.022 0.032 0.0016 0.30 0.175 D2 0.47 0.470 1.29 0.002 0.0008 0.0021 0.0012 0.0550 0.032 0.041 0.0021 0.26 0.177 D3 0.44 0.390 1.36 0.009 0.0008 0.0022 0.0015 0.0480 0.015 0.042 0.0029 0.25 0.164 D4 0.46 0.410 1.25 0.017 0.0015 0.0035 0.0026 0.0490 0.041 0.042 0.0020 0.31 0.181 D5 0.46 0.470 1.36 0.032 0.0005 0.0032 0.0016 0.0600 0.022 0.027 0.0023 0.27 0.178 D6 0.45 0.450 1.36 0.046 0.0010 0.0029 0.0023 0.0420 0.017 0.021 0.0029 0.29 0.223 D7 0.47 0.440 1.36 0.065 0.0007 0.0031 0.0013 0.0600 0.033 0.020 0.0022 0.21 0.230 D8 0.47 0.390 1.26 0.083 0.0010 0.0038 0.0015 0.0460 0.029 0.042 0.0029 0.27 0.219 D9 0.44 0.460 1.26 0.130 0.0007 0.0036 0.0029 0.0570 0.033 0.031 0.0030 0.33 0.178 E1 0.47 0.420 1.26 0.010 0.0006 0.0025 0.0028 0.0440 0.019 0.036 0.0024 0.29 0.188 E2 0.44 0.440 1.33 0.005 0.0010 0.0029 0.0015 0.0410 0.028 0.025 0.0031 0.27 0.131 E3 0.44 0.430 1.34 0.005 0.0020 0.0028 0.0023 0.0560 0.021 0.031 0.0031 0.21 0.160 E4 0.47 0.390 1.31 0.009 0.0040 0.0030 0.0027 0.0480 0.016 0.044 0.0018 0.24 0.206 E5 0.45 0.420 1.28 0.008 0.0060 0.0034 0.0026 0.0450 0.032 0.036 0.0025 0.27 0.220 E6 0.46 0.400 1.34 0.010 0.0070 0.0035 0.0024 0.0400 0.041 0.040 0.0033 0.23 0.192 E7 0.44 0.430 1.32 0.008 0.0080 0.0026 0.0012 0.0510 0.024 0.032 0.0028 0.24 0.188 E8 0.45 0.400 1.30 0.009 0.0090 0.0026 0.0027 0.0430 0.028 0.028 0.0017 0.23 0.186 E9 0.47 0.440 1.30 0.010 0.0120 0.0030 0.0027 0.0580 0.026 0.021 0.0024 0.21 0.195 F1 0.47 0.420 1.36 0.004 0.0019 0.0008 0.0021 0.0420 0.029 0.034 0.0016 0.30 0.163 F2 0.47 0.460 1.28 0.004 0.0004 0.0020 0.0022 0.0550 0.025 0.022 0.0032 0.28 0.138 F3 0.47 0.460 1.26 0.006 0.0006 0.0030 0.0020 0.0420 0.029 0.031 0.0028 0.32 0.161 F4 0.45 0.420 1.28 0.007 0.0013 0.0050 0.0016 0.0520 0.034 0.038 0.0028 0.29 0.172 F5 0.46 0.390 1.24 0.004 0.0005 0.0080 0.0010 0.0570 0.039 0.025 0.0029 0.21 0.170 F6 0.44 0.450 1.24 0.008 0.0007 0.0130 0.0027 0.0500 0.036 0.040 0.0023 0.32 0.218 F7 0.46 0.410 1.32 0.004 0.0002 0.0170 0.0014 0.0590 0.026 0.045 0.0024 0.22 0.178 F8 0.47 0.450 1.32 0.005 0.0018 0.0180 0.0026 0.0420 0.019 0.024 0.0029 0.21 0.216 F9 0.47 0.430 1.32 0.009 0.0002 0.0240 0.0011 0.0600 0.014 0.035 0.0023 0.25 0.162 G1 0.44 0.400 1.31 0.009 0.0019 0.0028 0.0009 0.0390 0.023 0.028 0.0023 0.31 0.177 G2 0.47 0.400 1.26 0.004 0.0019 0.0022 0.0020 0.0490 0.017 0.038 0.0023 0.21 0.160 G3 0.45 0.430 1.32 0.008 0.0013 0.0026 0.0030 0.0570 0.020 0.041 0.0030 0.33 0.131 G4 0.46 0.460 1.29 0.010 0.0013 0.0023 0.0060 0.0560 0.014 0.042 0.0030 0.27 0.173 G5 0.47 0.450 1.31 0.004 0.0003 0.0023 0.0090 0.0570 0.017 0.046 0.0020 0.27 0.217 G6 0.47 0.400 1.36 0.010 0.0005 0.0027 0.0120 0.0600 0.027 0.048 0.0031 0.25 0.204 G7 0.46 0.470 1.36 0.010 0.0020 0.0027 0.0150 0.0480 0.034 0.040 0.0018 0.23 0.151 G8 0.45 0.450 1.24 0.007 0.0012 0.0027 0.0190 0.0610 0.037 0.044 0.0024 0.23 0.152 G9 0.45 0.470 1.24 0.005 0.0004 0.0035 0.0220 0.0400 0.027 0.032 0.0016 0.28 0.160 H1 0.45 0.420 1.32 0.009 0.0002 0.0031 0.0027 0.0008 0.031 0.021 0.0023 0.32 0.211 H2 0.45 0.460 1.28 0.010 0.0010 0.0040 0.0024 0.0018 0.028 0.023 0.0027 0.26 0.216 H3 0.47 0.400 1.31 0.006 0.0016 0.0024 0.0014 0.0039 0.027 0.032 0.0021 0.26 0.228 H4 0.45 0.460 1.32 0.009 0.0016 0.0028 0.0010 0.0058 0.031 0.031 0.0027 0.26 0.169 H5 0.44 0.420 1.34 0.010 0.0011 0.0037 0.0018 0.0120 0.026 0.032 0.0020 0.23 0.141 H6 0.44 0.460 1.30 0.005 0.0018 0.0024 0.0013 0.0330 0.024 0.023 0.0034 0.33 0.145 H7 0.46 0.400 1.24 0.006 0.0005 0.0032 0.0021 0.0820 0.037 0.041 0.0034 0.27 0.165 H8 0.47 0.410 1.32 0.009 0.0011 0.0024 0.0018 0.1100 0.036 0.039 0.0026 0.24 0.166 H9 0.47 0.450 1.25 0.005 0.0008 0.0040 0.0016 0.1600 0.036 0.029 0.0021 0.32 0.166 H10 0.46 0.390 1.36 0.010 0.0015 0.0035 0.0026 0.2900 0.030 0.036 0.0021 0.21 0.178 H11 0.47 0.390 1.33 0.007 0.0014 0.0023 0.0014 0.3500 0.025 0.038 0.0025 0.21 0.188 H12 0.46 0.390 1.24 0.009 0.0018 0.0039 0.0011 0.4400 0.021 0.041 0.0019 0.25 0.130 H13 0.45 0.410 1.26 0.005 0.0017 0.0032 0.0023 0.5300 0.016 0.034 0.0024 0.29 0.130 I1 0.44 0.450 1.32 0.004 0.0008 0.0038 0.0028 0.0440  0.0007 0.027 0.0017 0.24 0.178 I2 0.45 0.390 1.28 0.010 0.0019 0.0027 0.0023 0.0520 0.002 0.032 0.0026 0.24 0.154 I3 0.46 0.430 1.29 0.009 0.0012 0.0039 0.0020 0.0560 0.006 0.025 0.0021 0.29 0.136 I4 0.44 0.400 1.28 0.008 0.0010 0.0028 0.0025 0.0490 0.015 0.027 0.0016 0.21 0.178 I5 0.47 0.440 1.32 0.010 0.0002 0.0024 0.0011 0.0520 0.024 0.039 0.0032 0.29 0.195 I6 0.45 0.450 1.26 0.007 0.0018 0.0030 0.0017 0.0550 0.038 0.019 0.0019 0.25 0.165 I7 0.44 0.410 1.25 0.009 0.0006 0.0037 0.0030 0.0440 0.045 0.037 0.0030 0.28 0.176 I8 0.45 0.440 1.32 0.006 0.0008 0.0024 0.0026 0.0410 0.052 0.038 0.0031 0.28 0.180 I9 0.44 0.440 1.35 0.006 0.0002 0.0025 0.0021 0.0480 0.068 0.038 0.0033 0.24 0.167 I10 0.47 0.390 1.28 0.005 0.0014 0.0025 0.0011 0.0490 0.083 0.047 0.0033 0.33 0.188 I11 0.47 0.430 1.36 0.005 0.0015 0.0039 0.0014 0.0510 0.096 0.021 0.0033 0.33 0.173 I12 0.44 0.440 1.36 0.009 0.0011 0.0024 0.0019 0.0530 0.121 0.039 0.0025 0.29 0.186 J1 0.46 0.440 1.33 0.007 0.0002 0.0026 0.0029 0.0470 0.038 0.008 0.0024 0.24 0.228 J2 0.46 0.390 1.26 0.010 0.0003 0.0039 0.0015 0.0590 0.039 0.013 0.0023 0.33 0.208 J3 0.44 0.470 1.31 0.009 0.0003 0.0035 0.0015 0.0560 0.018 0.023 0.0033 0.29 0.138 J4 0.46 0.460 1.33 0.004 0.0015 0.0020 0.0014 0.0510 0.033 0.045 0.0030 0.23 0.150 J5 0.46 0.450 1.32 0.006 0.0013 0.0039 0.0013 0.0440 0.031 0.068 0.0030 0.33 0.152 J6 0.45 0.390 1.30 0.009 0.0011 0.0035 0.0023 0.0440 0.022 0.075 0.0017 0.24 0.174 J7 0.46 0.400 1.34 0.005 0.0009 0.0029 0.0022 0.0500 0.021 0.092 0.0022 0.22 0.204 J8 0.44 0.450 1.32 0.010 0.0014 0.0032 0.0029 0.0610 0.018 0.111 0.0023 0.27 0.169 J9 0.45 0.470 1.36 0.010 0.0012 0.0030 0.0013 0.0410 0.017 0.133 0.0030 0.28 0.150 J10 0.46 0.420 1.30 0.006 0.0016 0.0031 0.0015 0.0480 0.031 0.159 0.0023 0.32 0.158 J11 0.44 0.420 1.26 0.010 0.0011 0.0020 0.0024 0.0400 0.027 0.187 0.0025 0.27 0.142 J12 0.45 0.450 1.27 0.006 0.0016 0.0020 0.0016 0.0520 0.028 0.213 0.0025 0.22 0.195 K1 0.44 0.400 1.31 0.004 0.0017 0.0022 0.0026 0.0580 0.041 0.019 0.0004 0.32 0.225 K2 0.46 0.420 1.33 0.004 0.0011 0.0024 0.0010 0.0470 0.027 0.033 0.0009 0.33 0.130 K3 0.45 0.420 1.29 0.010 0.0008 0.0027 0.0018 0.0460 0.024 0.022 0.0012 0.31 0.218 K4 0.46 0.440 1.35 0.008 0.0015 0.0029 0.0012 0.0530 0.016 0.024 0.0028 0.29 0.136 K5 0.47 0.400 1.33 0.007 0.0007 0.0033 0.0016 0.0540 0.019 0.041 0.0039 0.30 0.193 K6 0.45 0.430 1.25 0.006 0.0009 0.0026 0.0014 0.0420 0.022 0.042 0.0051 0.31 0.149 K7 0.47 0.470 1.24 0.007 0.0005 0.0021 0.0026 0.0500 0.016 0.037 0.0067 0.30 0.230 K8 0.44 0.400 1.34 0.009 0.0006 0.0037 0.0010 0.0520 0.019 0.034 0.0089 0.25 0.198 K9 0.44 0.430 1.34 0.010 0.0015 0.0036 0.0022 0.0410 0.016 0.044 0.0123 0.26 0.148 K10 0.47 0.410 1.30 0.009 0.0003 0.0034 0.0022 0.0500 0.019 0.047 0.0149 0.28 0.216 K11 0.45 0.450 1.34 0.010 0.0004 0.0037 0.0028 0.0470 0.025 0.032 0.0186 0.29 0.179 K12 0.46 0.430 1.30 0.006 0.0016 0.0026 0.0013 0.0540 0.039 0.024 0.0227 0.22 0.222 L1 0.47 0.430 1.26 0.010 0.0014 0.0023 0.0012 0.0490 0.026 0.041 0.0020  0.006 0.213 L2 0.47 0.470 1.30 0.008 0.0004 0.0023 0.0027 0.0500 0.019 0.043 0.0030 0.02 0.174 L3 0.44 0.460 1.29 0.008 0.0014 0.0021 0.0010 0.0490 0.031 0.028 0.0029 0.08 0.179 L4 0.47 0.450 1.25 0.006 0.0016 0.0029 0.0024 0.0470 0.025 0.037 0.0021 0.13 0.202 L5 0.47 0.420 1.32 0.005 0.0011 0.0022 0.0025 0.0570 0.031 0.028 0.0026 0.19 0.157 L6 0.44 0.450 1.26 0.006 0.0016 0.0036 0.0019 0.0590 0.031 0.026 0.0025 0.24 0.193 L7 0.45 0.390 1.27 0.007 0.0002 0.0021 0.0030 0.0410 0.037 0.028 0.0029 0.36 0.169 L8 0.47 0.460 1.35 0.010 0.0008 0.0039 0.0017 0.0570 0.027 0.020 0.0021 0.44 0.133 L9 0.45 0.400 1.33 0.008 0.0002 0.0031 0.0024 0.0490 0.034 0.037 0.0026 0.53 0.156 L10 0.46 0.410 1.29 0.009 0.0015 0.0028 0.0023 0.0470 0.014 0.041 0.0022 0.65 0.191 L11 0.45 0.410 1.27 0.004 0.0009 0.0028 0.0011 0.0410 0.021 0.033 0.0019 0.78 0.150 L12 0.45 0.400 1.34 0.008 0.0002 0.0036 0.0012 0.0500 0.021 0.040 0.0024 0.89 0.146 M1 0.45 0.410 1.24 0.004 0.0012 0.0022 0.0013 0.0540 0.034 0.030 0.0021 0.33  0.0008 M2 0.45 0.430 1.34 0.008 0.0009 0.0038 0.0023 0.0460 0.015 0.034 0.0024 0.26 0.002 M3 0.47 0.430 1.25 0.008 0.0013 0.0023 0.0011 0.0570 0.031 0.039 0.0018 0.24 0.005 M4 0.45 0.450 1.25 0.005 0.0007 0.0037 0.0012 0.0520 0.040 0.027 0.0024 0.31 0.018 M5 0.44 0.430 1.36 0.005 0.0012 0.0020 0.0026 0.0430 0.027 0.032 0.0029 0.22 0.056 M6 0.47 0.420 1.29 0.008 0.0006 0.0031 0.0022 0.0610 0.026 0.043 0.0017 0.32 0.085 M7 0.47 0.420 1.31 0.009 0.0005 0.0026 0.0015 0.0530 0.040 0.029 0.0027 0.22 0.213 M8 0.44 0.420 1.30 0.007 0.0020 0.0029 0.0030 0.0420 0.023 0.020 0.0034 0.23 0.441 M9 0.44 0.460 1.36 0.010 0.0009 0.0038 0.0026 0.0420 0.016 0.038 0.0034 0.26 0.592 M10 0.44 0.450 1.27 0.007 0.0005 0.0029 0.0024 0.0470 0.037 0.026 0.0022 0.32 0.773 M11 0.47 0.440 1.32 0.006 0.0009 0.0027 0.0015 0.0450 0.036 0.032 0.0017 0.31 0.938 M12 0.44 0.420 1.25 0.005 0.0019 0.0021 0.0022 0.0520 0.017 0.024 0.0034 0.27 1.227 N1 0.46 0.400 1.34 0.004 0.0016 0.0029 0.0028 0.0410 0.013 0.044 0.0031 0.25 0.225 N2 0.45 0.460 1.35 0.005 0.0010 0.0033 0.0016 0.0570 0.020 0.042 0.0017 0.22 0.184 N3 0.45 0.390 1.34 0.006 0.0002 0.0024 0.0027 0.0460 0.035 0.041 0.0031 0.24 0.227 N4 0.46 0.390 1.26 0.005 0.0018 0.0040 0.0018 0.0600 0.040 0.043 0.0022 0.30 0.185 N5 0.44 0.420 1.24 0.008 0.0008 0.0027 0.0018 0.0550 0.032 0.028 0.0032 0.30 0.193 N6 0.45 0.400 1.28 0.006 0.0009 0.0028 0.0013 0.0600 0.033 0.035 0.0032 0.21 0.156 N7 0.47 0.450 1.34 0.009 0.0009 0.0025 0.0021 0.0570 0.015 0.028 0.0030 0.32 0.206 N8 0.45 0.450 1.24 0.005 0.0005 0.0031 0.0030 0.0560 0.035 0.033 0.0019 0.32 0.230 N9 0.47 0.420 1.26 0.009 0.0001 0.0020 0.0025 0.0520 0.022 0.041 0.0022 0.27 0.195 N10 0.47 0.410 1.31 0.008 0.0002 0.0030 0.0021 0.0580 0.022 0.047 0.0022 0.30 0.186 N11 0.44 0.400 1.36 0.004 0.0007 0.0033 0.0028 0.0390 0.020 0.025 0.0023 0.29 0.190 N12 0.44 0.440 1.25 0.010 0.0008 0.0028 0.0018 0.0540 0.024 0.034 0.0020 0.22 0.213 O1 0.44 0.450 1.28 0.006 0.0007 0.0020 0.0026 0.0530 0.016 0.031 0.0021 0.21 0.147 O2 0.46 0.390 1.34 0.008 0.0015 0.0022 0.0014 0.0440 0.019 0.019 0.0025 0.33 0.190 O3 0.44 0.450 1.24 0.009 0.0015 0.0029 0.0013 0.0410 0.017 0.043 0.0033 0.24 0.221 O4 0.44 0.390 1.31 0.005 0.0005 0.0021 0.0027 0.0400 0.018 0.035 0.0016 0.29 0.169 O5 0.46 0.410 1.30 0.007 0.0014 0.0024 0.0020 0.0510 0.020 0.026 0.0017 0.24 0.199 O6 0.44 0.430 1.25 0.004 0.0018 0.0020 0.0012 0.0570 0.018 0.027 0.0017 0.25 0.188 O7 0.47 0.420 1.25 0.007 0.0020 0.0035 0.0024 0.0540 0.034 0.024 0.0025 0.32 0.212 O8 0.46 0.460 1.27 0.007 0.0010 0.0032 0.0017 0.0590 0.021 0.039 0.0026 0.24 0.228 O9 0.46 0.470 1.29 0.006 0.0005 0.0028 0.0022 0.0590 0.024 0.038 0.0023 0.28 0.145 O10 0.46 0.400 1.35 0.007 0.0005 0.0025 0.0025 0.0580 0.025 0.022 0.0028 0.26 0.219 O11 0.44 0.460 1.32 0.009 0.0009 0.0035 0.0016 0.0450 0.041 0.033 0.0032 0.28 0.135 P1 0.47 0.460 1.30 0.006 0.0008 0.0024 0.0013 0.0600 0.037 0.033 0.0022 0.27 0.199 P2 0.46 0.400 1.35 0.009 0.0017 0.0028 0.0024 0.0580 0.023 0.033 0.0028 0.26 0.185 P3 0.44 0.420 1.32 0.006 0.0009 0.0036 0.0018 0.0610 0.039 0.023 0.0025 0.21 0.189 P4 0.47 0.390 1.29 0.004 0.0016 0.0038 0.0016 0.0480 0.040 0.047 0.0030 0.22 0.224 P5 0.46 0.430 1.33 0.007 0.0012 0.0034 0.0013 0.0440 0.037 0.034 0.0026 0.32 0.206 P6 0.45 0.440 1.33 0.004 0.0003 0.0032 0.0025 0.0510 0.035 0.043 0.0021 0.26 0.178 P7 0.45 0.440 1.25 0.006 0.0015 0.0021 0.0019 0.0480 0.024 0.025 0.0017 0.29 0.206 P8 0.46 0.390 1.24 0.004 0.0011 0.0024 0.0023 0.0550 0.028 0.026 0.0033 0.29 0.171 Q1 0.45 0.460 1.24 0.010 0.0010 0.0035 0.0022 0.0440 0.031 0.045 0.0030 0.32 0.203 Q2 0.44 0.410 1.34 0.009 0.0003 0.0040 0.0021 0.0480 0.028 0.033 0.0024 0.31 0.171 Q3 0.45 0.420 1.29 0.005 0.0020 0.0022 0.0027 0.0600 0.037 0.027 0.0019 0.25 0.134 Q4 0.47 0.440 1.34 0.007 0.0003 0.0039 0.0013 0.0420 0.024 0.026 0.0033 0.30 0.194 Q5 0.46 0.410 1.30 0.004 0.0017 0.0033 0.0023 0.0560 0.015 0.040 0.0025 0.32 0.167 Q6 0.47 0.470 1.30 0.010 0.0010 0.0020 0.0011 0.0560 0.037 0.029 0.0028 0.23 0.217 Q7 0.44 0.450 1.31 0.005 0.0006 0.0038 0.0025 0.0560 0.032 0.041 0.0017 0.24 0.215 Q8 0.45 0.470 1.36 0.006 0.0020 0.0028 0.0021 0.0550 0.036 0.032 0.0021 0.30 0.219 Q9 0.46 0.410 1.33 0.010 0.0011 0.0025 0.0016 0.0510 0.026 0.027 0.0020 0.27 0.166 Q10 0.45 0.440 1.33 0.010 0.0012 0.0035 0.0027 0.0540 0.041 0.019 0.0028 0.31 0.201 R1 0.44 0.400 1.25 0.009 0.0004 0.0034 0.0023 0.0610 0.031 0.025 0.0030 0.32 0.203 R2 0.47 0.440 1.30 0.008 0.0003 0.0029 0.0025 0.0440 0.038 0.019 0.0028 0.30 0.194 R3 0.47 0.470 1.24 0.010 0.0004 0.0037 0.0018 0.0610 0.018 0.035 0.0027 0.28 0.144 R4 0.45 0.430 1.33 0.004 0.0011 0.0036 0.0017 0.0510 0.035 0.028 0.0024 0.24 0.136 R5 0.44 0.410 1.35 0.004 0.0004 0.0020 0.0023 0.0410 0.030 0.027 0.0019 0.24 0.216 R6 0.44 0.460 1.24 0.009 0.0014 0.0031 0.0025 0.0480 0.018 0.047 0.0022 0.25 0.214 R7 0.46 0.390 1.34 0.007 0.0003 0.0022 0.0017 0.0590 0.023 0.029 0.0017 0.28 0.165 R8 0.46 0.440 1.34 0.005 0.0004 0.0021 0.0014 0.0600 0.020 0.029 0.0017 0.31 0.161 R9 0.45 0.460 1.36 0.006 0.0014 0.0028 0.0015 0.0540 0.021 0.036 0.0018 0.22 0.201 S1 0.46 0.460 1.30 0.004 0.0014 0.0030 0.0016 0.0400 0.031 0.029 0.0020 0.30 0.178 S2 0.46 0.420 1.36 0.004 0.0008 0.0027 0.0022 0.0490 0.017 0.033 0.0030 0.27 0.155 S3 0.45 0.410 1.35 0.008 0.0012 0.0039 0.0024 0.0390 0.031 0.045 0.0019 0.21 0.193 S4 0.45 0.460 1.31 0.005 0.0003 0.0040 0.0010 0.0520 0.013 0.019 0.0023 0.25 0.223 S5 0.46 0.430 1.29 0.008 0.0004 0.0024 0.0026 0.0440 0.037 0.026 0.0031 0.28 0.221 S6 0.47 0.460 1.24 0.009 0.0016 0.0032 0.0013 0.0470 0.020 0.028 0.0023 0.28 0.140 S7 0.46 0.430 1.34 0.010 0.0008 0.0028 0.0025 0.0600 0.036 0.033 0.0031 0.26 0.183 S8 0.46 0.470 1.35 0.009 0.0020 0.0034 0.0014 0.0420 0.032 0.048 0.0023 0.33 0.184 T1 0.46 0.400 1.29 0.009 0.0001 0.0038 0.0028 0.0580 0.040 0.040 0.0033 0.30 0.218 T2 0.47 0.430 1.35 0.004 0.0008 0.0025 0.0010 0.0440 0.032 0.043 0.0025 0.24 0.200 T3 0.47 0.410 1.28 0.009 0.0009 0.0022 0.0021 0.0490 0.024 0.026 0.0019 0.29 0.231 T4 0.47 0.470 1.36 0.007 0.0015 0.0040 0.0018 0.0420 0.027 0.020 0.0034 0.24 0.215 T5 0.45 0.420 1.28 0.006 0.0014 0.0022 0.0017 0.0400 0.030 0.030 0.0029 0.22 0.150 T6 0.47 0.450 1.31 0.010 0.0013 0.0028 0.0020 0.0560 0.020 0.033 0.0019 0.24 0.152 T7 0.45 0.460 1.31 0.007 0.0017 0.0034 0.0017 0.0400 0.040 0.039 0.0027 0.27 0.167 T8 0.47 0.410 1.26 0.007 0.0005 0.0028 0.0010 0.0390 0.039 0.030 0.0017 0.24 0.204 T9 0.46 0.420 1.33 0.005 0.0013 0.0025 0.0022 0.0460 0.013 0.035 0.0021 0.30 0.192 T10 0.45 0.440 1.28 0.009 0.0014 0.0031 0.0024 0.0590 0.028 0.023 0.0024 0.32 0.178 U1 0.46 0.440 1.27 0.009 0.0011 0.0022 0.0010 0.0570 0.030 0.028 0.0030 0.31 0.146 U2 0.47 0.420 1.26 0.005 0.0017 0.0026 0.0029 0.0600 0.041 0.030 0.0024 0.27 0.142 U3 0.45 0.430 1.24 0.005 0.0014 0.0029 0.0019 0.0600 0.031 0.042 0.0018 0.23 0.135 U4 0.44 0.420 1.32 0.008 0.0012 0.0039 0.0027 0.0580 0.029 0.029 0.0022 0.25 0.135 U5 0.46 0.450 1.35 0.004 0.0015 0.0021 0.0014 0.0390 0.016 0.025 0.0024 0.30 0.131 U6 0.44 0.460 1.24 0.004 0.0017 0.0021 0.0029 0.0430 0.014 0.031 0.0018 0.21 0.149 U7 0.44 0.390 1.28 0.009 0.0015 0.0024 0.0021 0.0610 0.039 0.044 0.0017 0.22 0.152 U8 0.46 0.460 1.25 0.010 0.0006 0.0023 0.0025 0.0480 0.019 0.019 0.0028 0.33 0.205 V1 0.46 0.400 1.28 0.004 0.0010 0.0040 0.0017 0.0430 0.029 0.019 0.0026 0.30 0.213 V2 0.47 0.390 1.25 0.005 0.0018 0.0035 0.0013 0.0420 0.017 0.027 0.0026 0.22 0.168 V3 0.44 0.420 1.31 0.010 0.0006 0.0021 0.0027 0.0600 0.019 0.028 0.0034 0.27 0.204 V4 0.44 0.440 1.24 0.010 0.0003 0.0036 0.0025 0.0530 0.027 0.038 0.0020 0.28 0.184 V5 0.44 0.460 1.27 0.007 0.0007 0.0035 0.0018 0.0520 0.039 0.023 0.0024 0.24 0.171 V6 0.45 0.430 1.32 0.006 0.0002 0.0024 0.0017 0.0580 0.036 0.037 0.0032 0.21 0.210 V7 0.44 0.430 1.25 0.007 0.0018 0.0027 0.0030 0.0600 0.016 0.042 0.0023 0.28 0.230 V8 0.44 0.430 1.32 0.008 0.0020 0.0035 0.0026 0.0390 0.018 0.024 0.0027 0.32 0.220 W1 0.45 0.470 1.30 0.004 0.0006 0.0021 0.0012 0.0550 0.017 0.031 0.0018 0.24 0.193 W2 0.47 0.470 1.29 0.005 0.0017 0.0025 0.0029 0.0490 0.023 0.032 0.0019 0.28 0.145 W3 0.47 0.460 1.27 0.007 0.0015 0.0036 0.0010 0.0460 0.024 0.031 0.0029 0.22 0.135 W4 0.46 0.430 1.33 0.010 0.0010 0.0035 0.0018 0.0590 0.029 0.023 0.0026 0.25 0.206 W5 0.45 0.470 1.30 0.005 0.0003 0.0020 0.0023 0.0560 0.020 0.032 0.0021 0.23 0.206 W6 0.47 0.430 1.36 0.008 0.0019 0.0021 0.0018 0.0390 0.015 0.034 0.0030 0.29 0.136 W7 0.44 0.390 1.31 0.008 0.0008 0.0023 0.0014 0.0560 0.024 0.042 0.0027 0.26 0.214 W8 0.45 0.420 1.28 0.004 0.0005 0.0035 0.0017 0.0530 0.041 0.030 0.0016 0.23 0.184 X1 0.44 0.470 1.30 0.004 0.0008 0.0022 0.0015 0.0600 0.016 0.030 0.0032 0.29 0.131 X2 0.46 0.430 1.35 0.006 0.0010 0.0040 0.0021 0.0490 0.024 0.048 0.0031 0.32 0.148 X3 0.47 0.470 1.36 0.007 0.0016 0.0026 0.0023 0.0570 0.030 0.030 0.0025 0.22 0.184 X4 0.44 0.410 1.26 0.004 0.0010 0.0032 0.0030 0.0440 0.041 0.037 0.0029 0.33 0.224 X5 0.45 0.470 1.26 0.008 0.0001 0.0033 0.0021 0.0400 0.015 0.032 0.0018 0.30 0.181 X6 0.47 0.440 1.26 0.006 0.0015 0.0040 0.0020 0.0500 0.041 0.035 0.0018 0.22 0.207 X7 0.45 0.450 1.24 0.007 0.0015 0.0037 0.0022 0.0470 0.030 0.030 0.0030 0.27 0.138 X8 0.45 0.450 1.24 0.006 0.0016 0.0035 0.0011 0.0410 0.019 0.030 0.0032 0.22 0.165 Y1 0.47 0.390 1.36 0.007 0.0020 0.0037 0.0013 0.0470 0.015 0.023 0.0028 0.23 0.207 Y2 0.47 0.410 1.34 0.007 0.0017 0.0021 0.0013 0.0470 0.026 0.044 0.0033 0.30 0.219 Y3 0.45 0.390 1.35 0.010 0.0003 0.0039 0.0023 0.0420 0.015 0.047 0.0029 0.27 0.216 Y4 0.46 0.460 1.24 0.008 0.0012 0.0035 0.0028 0.0530 0.017 0.020 0.0022 0.24 0.206 Y5 0.47 0.430 1.32 0.005 0.0002 0.0036 0.0030 0.0450 0.019 0.019 0.0025 0.24 0.188 Y6 0.47 0.410 1.36 0.007 0.0019 0.0031 0.0020 0.0480 0.019 0.037 0.0025 0.27 0.153 Y7 0.47 0.440 1.24 0.008 0.0006 0.0024 0.0014 0.0510 0.026 0.035 0.0027 0.33 0.147 Y8 0.47 0.430 1.34 0.010 0.0004 0.0034 0.0023 0.0610 0.015 0.046 0.0019 0.29 0.149 Z1 0.46 0.420 1.24 0.009 0.0015 0.0037 0.0018 0.0580 0.018 0.027 0.0025 0.22 0.131 Steel Chemical composition (mass %, balance: Fe and impurities) type Co Ni Cu V W Ca Mg REM Sb Zr Sn As Remarks A1 Comp. steel A2 Inv. steel A3 Inv. steel A4 Inv. steel A5 Inv. steel A6 Inv. steel A7 Inv. steel A8 Inv. steel A9 Inv. steel A10 Inv. steel A11 Inv. steel A12 Inv. steel A13 Comp. steel B1 Comp. steel B2 Inv. steel B3 Inv. steel B4 Inv. steel B5 Inv. steel B6 Inv. steel B7 Inv. steel B8 Inv. steel B9 Inv. steel B10 Inv. steel B11 Inv. steel B12 Comp. steel C1 Comp. steel C2 Inv. steel C3 Inv. steel C4 Inv. steel C5 Inv. steel C6 Inv. steel C7 Inv. steel C8 Inv. steel C9 Inv. steel C10 Inv. steel C11 Inv. steel C12 Inv. steel C13 Inv. steel C14 Inv. steel C15 Inv. steel C16 Comp. steel D1 Inv. steel D2 Inv. steel D3 Inv. steel D4 Inv. steel D5 Inv. steel D6 Inv. steel D7 Inv. steel D8 Inv. steel D9 Comp. steel E1 Inv. steel E2 Inv. steel E3 Inv. steel E4 Inv. steel E5 Inv. steel E6 Inv. steel E7 Inv. steel E8 Inv. steel E9 Comp. steel F1 Inv. steel F2 Inv. steel F3 Inv. steel F4 Inv. steel F5 Inv. steel F6 Inv. steel F7 Inv. steel F8 Inv. steel F9 Comp. steel G1 Inv. steel G2 Inv. steel G3 Inv. steel G4 Inv. steel G5 Inv. steel G6 Inv. steel G7 Inv. steel G8 Inv. steel G9 Comp. steel H1 Comp. steel H2 Inv. steel H3 Inv. steel H4 Inv. steel H5 Inv. steel H6 Inv. steel H7 Inv. steel H8 Inv. steel H9 Inv. steel H10 Inv. steel H11 Inv. steel H12 Inv. steel H13 Comp. steel I1 Comp. steel I2 Inv. steel I3 Inv. steel I4 Inv. steel I5 Inv. steel I6 Inv. steel I7 Inv. steel I8 Inv. steel I9 Inv. steel I10 Inv. steel I11 Inv. steel I12 Comp. steel J1 Comp. steel J2 Inv. steel J3 Inv. steel J4 Inv. steel J5 Inv. steel J6 Inv. steel J7 Inv. steel J8 Inv. steel J9 Inv. steel J10 Inv. steel J11 Inv. steel J12 Comp. steel K1 Comp. steel K2 Inv. steel K3 Inv. steel K4 Inv. steel K5 Inv. steel K6 Inv. steel K7 Inv. steel K8 Inv. steel K9 Inv. steel K10 Inv. steel K11 Inv. steel K12 Comp. steel L1 Comp. steel L2 Inv. steel L3 Inv. steel L4 Inv. steel L5 Inv. steel L6 Inv. steel L7 Inv. steel L8 Inv. steel L9 Inv. steel L10 Inv. steel L11 Inv. steel L12 Comp. steel M1 Comp. steel M2 Inv. steel M3 Inv. steel M4 Inv. steel M5 Inv. steel M6 Inv. steel M7 Inv. steel M8 Inv. steel M9 Inv. steel M10 Inv. steel M11 Inv. steel M12 Comp. steel N1 0.001 Inv. steel N2 0.003 Inv. steel N3 0.01 Inv. steel N4 0.02 Inv. steel N5 0.05 Inv. steel N6 0.09 Inv. steel N7 0.20 Inv. steel N8 0.50 Inv. steel N9 0.70 Inv. steel N10 0.90 Inv. steel N11 1.30 Inv. steel N12 1.80 Inv. steel O1 0.001 Inv. steel O2 0.004 Inv. steel O3 0.07 Inv. steel O4 0.20 Inv. steel O5 0.40 Inv. steel O6 0.60 Inv. steel O7 0.90 Inv. steel O8 1.30 Inv. steel O9 1.90 Inv. steel O10 2.30 Inv. steel O11 2.80 Inv. steel P1 0.001 Inv. steel P2 0.003 Inv. steel P3 0.03 Inv. steel P4 0.12 Inv. steel P5 0.38 Inv. steel PE 0.54 Inv. steel P7 0.71 Inv. steel P8 0.93 Inv. steel Q1 0.001 Inv. steel Q2 0.002 Inv. steel Q3 0.01 Inv. steel Q4 0.03 Inv. steel Q5 0.07 Inv. steel Q6 0.25 Inv. steel Q7 0.44 Inv. steel Q8 0.58 Inv. steel Q9 0.77 Inv. steel Q10 0.89 Inv. steel R1 0.001 Inv. steel R2 0.005 Inv. steel R3 0.030 Inv. steel R4 0.110 Inv. steel R5 0.280 Inv. steel R6 0.530 Inv. steel R7 0.650 Inv. steel R8 0.740 Inv. steel R9 0.910 Inv. steel S1 0.0001 Inv. steel S2 0.0004 Inv. steel S3 0.002 Inv. steel S4 0.004 Inv. steel S5 0.006 Inv. steel S6 0.007 Inv. steel S7 0.009 Inv. steel S8 0.010 Inv. steel T1 0.0001 Inv. steel T2 0.0005 Inv. steel T3 0.001 Inv. steel T4 0.004 Inv. steel T5 0.008 Inv. steel T6 0.040 Inv. steel T7 0.080 Inv. steel T8 0.280 Inv. steel T9 0.510 Inv. steel T10 0.730 Inv. steel U1 0.0006 Inv. steel U2 0.002 Inv. steel U3 0.003 Inv. steel U4 0.007 Inv. steel U5 0.030 Inv. steel U6 0.080 Inv. steel U7 0.210 Inv. steel U8 0.620 Inv. steel V1 Inv. steel V2 0.001 Inv. steel V3 0.009 Inv. steel V4 0.060 Inv. steel V5 0.180 Inv. steel V6 0.480 Inv. steel V7 0.750 Inv. steel V8 0.920 Inv. steel W1 0.001 Inv. steel W2 0.003 Inv. steel W3 0.008 Inv. steel W4 0.040 Inv. steel W5 0.130 Inv. steel W6 0.440 Inv. steel W7 0.710 Inv. steel W8 0.950 Inv. steel X1 0.001 Inv. steel X2 0.009 Inv. steel X3 0.030 Inv. steel X4 0.080 Inv. steel X5 0.140 Inv. steel X6 0.310 Inv. steel X7 0.640 Inv. steel X8 0.880 Inv. steel Y1 0.001 Inv. steel Y2 0.003 Inv. steel Y3 0.007 Inv. steel Y4 0.021 Inv. steel Y5 0.038 Inv. steel Y6 0.055 Inv. steel Y7 0.073 Inv. steel Y8 0.082 Inv. steel Z1 Inv. steel Underlines indicate outside scope of present invention.

TABLE 2 Hot rolling step Cooling step Coiling step Hot stamping step Finish rolling final Time period Average cooling Coiling Heating Holding Steel stage rolling until rapid speed temp. temp. time No. type reduction % cooling starts ° C./s ° C. ° C. periods Others Remarks 1 A1 48 0.66 118 582 913 488 Comp. ex. 2 A2 50 0.54 112 667 886 481 Inv. ex. 3 A3 52 0.73  97 666 898 483 Inv. ex. 4 A4 52 0.65 102 654 905 491 Inv. ex. 5 A5 46 0.72  99 641 881 476 Inv. ex. 6 A6 45 0.2  112 661 911 476 Inv. ex. 7 A7 45 0.28 103 608 892 480 Inv. ex. 8 A8 46 0.31  95 664 887 478 Inv. ex. 9 A9 48 0.42  95 612 906 483 Inv. ex. 10 A10 52 0.8  116 586 911 492 Inv. ex. 11 A11 52 0.54 107 645 900 485 Inv. ex. 12 A12 46 0.52 106 544 908 490 Inv. ex. 13 A13 49 0.23 100 631 918 469 Comp. ex. 14 B1 52 0.63 101 658 904 486 Comp. ex. 15 B2 47 0.65  96 542 917 466 Inv. ex. 16 B3 45 0.51 106 629 901 495 Inv. ex. 17 B4 50 0.22 116 535 895 491 Inv. ex. 18 B5 48 0.66 118 578 912 490 Inv. ex. 19 B6 48 0.63  99 646 908 468 Inv. ex. 20 B7 52 0.8  103 530 890 472 Inv. ex. 21 B8 50 0.3   98 587 880 492 Inv. ex. 22 B9 48 0.69 110 597 883 482 Inv. ex. 23 B10 49 0.39 116 647 880 493 Inv. ex. 24 B11 50 0.3  111 642 889 478 Inv. ex. 25 B12 48 0.72 101 584 887 473 Comp. ex. 26 C1 46 0.22 120 613 898 490 Comp. ex. 27 C2 47 0.38 120 604 887 494 Inv. ex. 28 C3 48 0.41 116 530 911 482 Inv. ex. 29 C4 52 0.66 102 602 918 480 Inv. ex. 30 C5 47 0.3  107 628 908 465 Inv. ex. 31 C6 48 0.54 101 587 907 482 Inv. ex. 32 C7 46 0.42  96 652 905 493 Inv. ex. 33 C8 49 0.7  102 595 912 485 Inv. ex. 34 C9 51 0.24 118 646 912 467 Inv. ex. 35 C10 47 0.65  98 550 908 472 Inv. ex. 36 C11 51 0.41 114 542 905 478 Inv. ex. 37 C12 50 0.71  96 589 919 482 Inv. ex. 38 C13 51 0.64 104 550 898 471 Inv. ex. 39 C14 52 0.39 110 614 908 478 Inv. ex. 40 C15 48 0.25 103 580 886 472 Inv. ex. 41 C16 52 0.72 109 538 904 470 Comp. ex. 42 D1 45 0.59 106 610 891 472 Inv. ex. 43 D2 48 0.34 111 592 892 478 Inv. ex. 44 D3 48 0.64 117 581 886 489 Inv. ex. 45 D4 50 0.63 102 540 886 483 Inv. ex. 46 D5 45 0.35 116 610 894 465 Inv. ex. 47 D6 50 0.33 105 591 915 490 Inv. ex. 48 D7 49 0.67  99 573 882 485 Inv. ex. 49 D8 52 0.74 106 666 894 472 Inv. ex. 50 D9 50 0.72 119 530 904 488 Comp. ex. 51 E1 52 0.62 103 584 918 488 Inv. ex. 52 E2 49 0.71  96 606 912 475 Inv. ex. 53 E3 47 0.75 119 636 887 478 Inv. ex. 54 E4 52 0.35 107 574 883 466 Inv. ex. 55 ES 51 0.42 103 592 893 484 Inv. ex. 56 E6 50 0.8  104 606 916 469 Inv. ex. 57 E7 48 0.52 112 646 904 473 Inv. ex. 58 E8 51 0.35 107 666 892 476 Inv. ex. 59 E9 51 0.69 108 612 913 473 Comp. ex. 60 F1 47 0.4  101 584 910 483 Inv. ex. 61 F2 46 0.68 107 669 893 475 Inv. ex. 62 F3 51 0.44 114 577 886 489 Inv. ex. 63 F4 46 0.34 117 574 913 493 Inv. ex. 64 F5 45 0.3   96 624 886 487 Inv. ex. 65 F6 50 0.26 107 545 917 484 Inv. ex. 66 F7 50 0.68 101 635 889 492 Inv. ex. 67 F8 50 0.57 111 623 913 490 Inv. ex. 68 F9 49 0.5  118 544 904 482 Comp. ex. 69 G1 52 0.36 105 632 916 475 Inv. ex. 70 G2 50 0.57 105 625 880 492 Inv. ex. 71 G3 51 0.36  99 638 890 494 Inv. ex. 72 G4 46 0.25  95 593 914 470 Inv. ex. 73 G5 47 0.35 120 544 881 492 Inv. ex. 74 G6 50 0.29  96 651 905 491 Inv. ex. 75 G7 48 0.41 105 615 901 473 Inv. ex. 76 G8 51 0.33  99 612 911 485 Inv. ex. 77 G9 45 0.45 103 600 913 487 Comp. ex. 78 H1 51 0.43  95 655 886 468 Comp. ex. 79 H2 45 0.53  98 545 881 484 Inv. ex. 80 H3 47 0.6   99 643 911 487 Inv. ex. 81 H4 52 0.5  114 559 893 465 Inv. ex. 82 H5 52 0.54 114 603 890 484 Inv. ex. 83 H6 47 0.64 114 630 905 479 Inv. ex. 84 H7 49 0.64 117 648 896 481 Inv. ex. 85 H8 52 0.55 106 539 892 486 Inv. ex. 86 H9 46 0.22 115 637 913 491 Inv. ex. 87 H10 45 0.78 105 594 889 468 Inv. ex. 88 H11 51 0.58  95 533 918 483 Inv. ex. 89 H12 51 0.49 105 573 915 494 Inv. ex. 90 H13 50 0.34 102 664 919 483 Comp. ex. 91 I1 52 0.55 101 634 906 493 Comp. ex. 92 I2 51 0.76 117 607 899 474 Inv. ex. 93 I3 48 0.57 113 611 914 482 Inv. ex. 94 I4 47 0.57 117 621 881 471 Inv. ex. 95 I5 50 0.28 112 557 890 489 Inv. ex. 96 I6 52 0.58 110 635 880 478 Inv. ex. 97 I7 45 0.62 119 646 919 465 Inv. ex. 98 I8 51 0.58 108 638 899 483 Inv. ex. 99 I9 48 0.43 108 588 894 480 Inv. ex. 100 I10 45 0.32 110 667 916 495 Inv. ex. 101 I11 46 0.42 111 663 897 473 Inv. ex. 102 I12 45 0.79 103 610 903 493 Comp. ex. 103 J1 48 0.59 116 612 915 475 Comp. ex. 104 J2 49 0.54 105 596 918 492 Inv. ex. 105 J3 50 0.48 104 579 901 486 Inv. ex. 106 J4 52 0.3  106 659 915 467 Inv. ex. 107 J5 51 0.47 106 640 903 475 Inv. ex. 108 J6 50 0.36 109 572 905 493 Inv. ex. 109 J7 45 0.6  120 598 886 495 Inv. ex. 110 J8 48 0.34 111 569 915 467 Inv. ex. 111 J9 45 0.35 102 614 906 486 Inv. ex. 112 J10 46 0.55  97 665 916 477 Inv. ex. 113 J11 47 0.22 115 613 914 490 Inv. ex. 114 J12 45 0.8  110 607 912 472 Comp. ex. 115 K1 45 0.68 101 606 915 490 Comp. ex. 116 K2 51 0.7  116 666 910 470 Inv. ex. 117 K3 50 0.8  101 568 912 495 Inv. ex. 118 K4 50 0.62 107 634 901 482 Inv. ex. 119 K5 51 0.66 113 587 905 490 Inv. ex. 120 K6 52 0.47 109 653 918 494 Inv. ex. 121 K7 51 0.49 108 594 903 466 Inv. ex. 122 K8 46 0.62 105 583 883 486 Inv. ex. 123 K9 50 0.54 100 568 897 495 Inv. ex. 124 K10 47 0.51 113 612 897 482 Inv. ex. 125 K11 47 0.39 103 670 908 488 Inv. ex. 126 K12 47 0.36 104 601 898 492 Comp. ex. 127 L1 52 0.4  100 578 890 495 Comp. ex. 128 L2 48 0.28 113 626 898 490 Inv. ex. 129 L3 49 0.48 101 546 902 483 Inv. ex. 130 L4 49 0.71 105 622 913 482 Inv. ex. 131 L5 45 0.4  112 556 904 484 Inv. ex. 132 L6 47 0.53 101 659 906 474 Inv. ex. 133 L7 50 0.21 105 657 892 494 Inv. ex. 134 L8 48 0.37 117 645 887 473 Inv. ex. 135 L9 52 0.45 111 598 893 480 Inv. ex. 136 L10 45 0.43 100 582 911 474 Inv. ex. 137 L11 45 0.24 101 532 914 495 Inv. ex. 138 L12 52 0.38 118 652 884 478 Comp. ex. 139 M1 47 0.66 106 533 906 483 Comp. ex. 140 M2 48 0.74 103 668 903 473 Inv. ex. 141 M3 52 0.44 115 669 899 487 Inv. ex. 142 M4 50 0.34  97 585 906 469 Inv. ex. 143 M5 49 0.64  95 643 910 493 Inv. ex. 144 M6 45 0.36 113 610 909 467 Inv. ex. 145 M7 48 0.27 108 597 915 479 Inv. ex. 146 M8 50 0.53 112 618 880 465 Inv. ex. 147 M9 46 0.26 119 572 888 494 Inv. ex. 148 M10 48 0.57  98 584 904 494 Inv. ex. 149 M11 52 0.58 100 561 914 488 Inv. ex. 150 M12 47 0.34 105 597 893 468 Comp. ex. 151 N1 49 0.73 115 605 909 483 Inv. ex. 152 N2 47 0.75  95 630 894 465 Inv. ex. 153 N3 48 0.71 107 547 900 465 Inv. ex. 154 N4 51 0.35 110 538 907 490 Inv. ex. 155 N5 48 0.31 100 546 899 487 Inv. ex. 156 N6 47 0.56 114 650 919 489 Inv. ex. 157 N7 49 0.58 115 537 880 469 Inv. ex. 158 N8 51 0.68 104 573 897 476 Inv. ex. 159 N9 47 0.71 105 668 896 489 Inv. ex. 160 N10 50 0.37 106 530 915 495 Inv. ex. 161 N11 47 0.41 108 550 892 484 Inv. ex. 162 N12 51 0.68  97 561 895 470 Inv. ex. 163 O1 48 0.23 109 635 907 483 Inv. ex. 164 O2 47 0.32 113 630 918 480 Inv. ex. 165 O3 50 0.79 107 582 916 492 Inv. ex. 166 O4 50 0.63 104 642 908 474 Inv. ex. 167 O5 45 0.75 102 565 895 485 Inv. ex. 168 O6 49 0.2  120 570 881 485 Inv. ex. 169 O7 52 0.26 113 559 885 470 Inv. ex. 170 O8 48 0.26 120 589 907 473 Inv. ex. 171 O9 46 0.43  98 578 896 492 Inv. ex. 172 O10 47 0.74 116 584 901 473 Inv. ex. 173 O11 52 0.21 115 531 889 484 Inv. ex. 174 P1 45 0.75  95 665 880 473 Inv. ex. 175 P2 52 0.45 107 664 888 490 Inv. ex. 176 P3 48 0.72 110 567 901 469 Inv. ex. 177 P4 45 0.55 109 613 895 482 Inv. ex. 178 P5 52 0.66  96 544 920 468 Inv. ex. 179 P6 45 0.35 104 657 881 484 Inv. ex. 180 P7 49 0.35 118 561 886 470 Inv. ex. 181 P8 52 0.45 114 534 909 485 Inv. ex. 182 Q1 48 0.27 114 532 915 489 Inv. ex. 183 Q2 47 0.74 103 668 905 473 Inv. ex. 184 Q3 47 0.71 112 621 913 468 Inv. ex. 185 Q4 50 0.43 113 573 913 489 Inv. ex. 186 Q5 47 0.67 115 606 882 471 Inv. ex. 187 Q6 51 0.73  98 614 918 473 Inv. ex. 188 Q7 49 0.31 108 633 885 485 Inv. ex. 189 Q8 47 0.64 109 587 888 493 Inv. ex. 190 Q9 49 0.37 112 580 907 468 Inv. ex. 191 Q10 47 0.25 103 597 905 482 Inv. ex. 192 R1 45 0.2  103 630 880 490 Inv. ex. 193 R2 52 0.29 110 621 890 490 Inv. ex. 194 R3 50 0.52 110 602 903 468 Inv. ex. 195 R4 51 0.74  95 543 883 466 Inv. ex. 196 R5 49 0.69 110 591 887 484 Inv. ex. 197 R6 45 0.76  96 531 881 485 Inv. ex. 198 R7 50 0.22 106 549 893 466 Inv. ex. 199 R8 51 0.77  98 652 884 479 Inv. ex. 200 R9 48 0.31 101 647 892 493 Inv. ex. 201 S1 51 0.34 112 577 908 492 Inv. ex. 202 S2 52 0.39 111 545 898 489 Inv. ex. 203 S3 45 0.69 108 646 914 492 Inv. ex. 204 S4 47 0.55  97 636 904 494 Inv. ex. 205 S5 45 0.69 112 550 916 481 Inv. ex. 206 S6 49 0.26 114 532 919 492 Inv. ex. 207 S7 49 0.75 109 610 889 478 Inv. ex. 208 S8 47 0.62 108 622 911 475 Inv. ex. 209 T1 50 0.66 116 636 884 481 Inv. ex. 210 T2 47 0.36 115 539 885 492 Inv. ex. 211 T3 51 0.65 116 532 917 465 Inv. ex. 212 T4 49 0.23 111 565 916 487 Inv. ex. 213 T5 51 0.41 100 546 908 472 Inv. ex. 214 T6 50 0.61 114 582 901 480 Inv. ex. 215 T7 50 0.4  103 583 892 485 Inv. ex. 216 T8 50 0.49  96 605 898 488 Inv. ex. 217 T9 47 0.8  119 572 914 491 Inv. ex. 218 T10 45 0.38 115 650 902 490 Inv. ex. 219 U1 46 0.68 110 658 885 470 Inv. ex. 220 U2 51 0.62  95 617 888 468 Inv. ex. 221 U3 48 0.61 118 653 903 466 Inv. ex. 222 U4 47 0.42 102 622 899 493 Inv. ex. 223 U5 47 0.2  116 559 912 468 Inv. ex. 224 U6 51 0.53 104 624 909 491 Inv. ex. 225 U7 45 0.22 110 641 901 492 Inv. ex. 226 U8 46 0.79 105 626 919 471 Inv. ex. 227 V1 49 0.6   98 575 899 495 Inv. ex. 228 V2 51 0.67 100 532 881 493 Inv. ex. 229 V3 46 0.22 105 570 915 488 Inv. ex. 230 V4 48 0.72 104 543 896 487 Inv. ex. 231 V5 47 0.54  96 647 896 495 Inv. ex. 232 V6 51 0.6  114 648 900 490 Inv. ex. 233 V7 45 0.55  99 538 905 489 Inv. ex. 234 V8 50 0.32 102 623 895 489 Inv. ex. 235 W1 47 0.27 103 632 899 485 Inv. ex. 236 W2 49 0.45 117 613 909 469 Inv. ex. 237 W3 46 0.77 109 600 889 470 Inv. ex. 238 W4 52 0.36  95 569 882 469 Inv. ex. 239 W5 45 0.62  98 594 903 481 Inv. ex. 240 W6 50 0.52 102 618 915 473 Inv. ex. 241 W7 49 0.53 100 653 884 465 Inv. ex. 242 W8 46 0.68 106 606 910 478 Inv. ex. 243 X1 48 0.48 109 651 919 480 Inv. ex. 244 X2 47 0.63 115 645 885 483 Inv. ex. 245 X3 48 0.56 103 614 907 484 Inv. ex. 246 X4 50 0.39 101 545 912 482 Inv. ex. 247 X5 46 0.38 106 557 882 481 Inv. ex. 248 X6 46 0.45 109 648 881 481 Inv. ex. 249 X7 46 0.3  100 566 882 479 Inv. ex. 250 X8 48 0.34 118 605 891 491 Inv. ex. 251 Y1 49 0.79 111 536 892 471 Inv. ex. 252 Y2 51 0.78 109 570 911 473 Inv. ex. 253 Y3 48 0.28 110 568 904 470 Inv. ex. 254 Y4 45 0.49 105 582 919 489 Inv. ex. 255 Y5 46 0.8  111 594 899 478 Inv. ex. 256 Y6 48 0.38  96 531 886 489 Inv. ex. 257 Y7 49 0.5  111 628 916 489 Inv. ex. 258 Y8 49 0.38  99 582 910 471 Inv. ex. 259 Z1 35 0.74 117 637 914 484 Comp. ex. 260 Z1 40 0.25 119 662 914 469 Inv. ex. 261 Z1 41 0.26 103 533 919 485 Inv. ex. 262 Z1 43 0.42 118 592 892 479 Inv. ex. 263 Z1 46 0.69 110 577 896 482 Inv. ex. 264 Z1 52 0.34 118 651 905 485 Inv. ex. 265 ZI 50 0.22 105 641 913 489 Inv. ex. 266 Z1 52 0.41 100 594 896 473 Inv. ex. 267 Z1 45 0.59 110 583 917 490 Inv. ex. 268 Z1 52 0.83 104 585 908 471 Inv. ex. 269 Z1 47 0.98 101 602 889 481 Inv. ex. 270 Z1 46 1.23 112 544 889 487 Comp. ex. 271 Z1 49 0.22  72 605 887 494 Comp. ex. 272 Z1 48 0.22  91 600 880 490 Inv. ex. 273 Z1 50 0.38  93 542 917 488 Inv. ex. 274 Z1 52 0.44  99 670 900 474 Inv. ex. 275 Z1 45 0.43 121 557 903 482 Inv. ex. 276 Z1 51 0.2  109 485 900 480 Comp. ex. 277 Z1 47 0.7   98 506 912 495 Inv. ex. 278 Z1 47 0.29  97 538 904 483 Inv. ex. 279 Z1 50 0.63 111 577 887 486 Inv. ex. 280 Z1 51 0.57 109 615 908 465 Inv. ex. 281 Z1 47 0.28  99 642 881 476 Inv. ex. 282 Z1 49 0.49 101 678 916 478 Inv. ex. 283 Z1 50 0.8  114 695 883 477 Inv. ex. 284 Z1 52 0.21 115 731 885 491 Comp. ex. 285 Z1 49 0.79  95 607 906 478 Pickling Inv. ex. 286 Z1 47 0.8  112 616 919 474 No cold rolling Inv. ex. 287 Z1 50 0.64 107 632 908 493 Annealing Inv. ex. 288 Z1 51 0.7  110 547 906 478 Al plating Inv. ex. 289 Z1 47 0.46 116 628 883 475 Al—Zn plating Inv. ex. 290 Z1 48 0.27 101 598 918 466 Al—Si plating Inv. ex. 291 Z1 51 0.57  98 607 880 466 Hot dip galvanization Inv. ex 292 Z1 49 0.59 107 579 900 467 Electrogalvanization Inv. ex. 293 Z1 46 0.78  97 588 894 484 Hot dip galvannealing Inv. ex. 294 Z1 48 0.39 118 645 915 480 Zn—Ni plating Inv. ex. 295 Z1 51 0.68 115 551 883 478 Al—Mg—Zn plating Inv. ex. 296 Z1 45 0.24 115 604 880 490 Temper rolling Inv. ex. 297 Z1 51 0.59  98 669 805 478 Inv. ex. 298 Z1 47 0.25 107 656 813 475 Inv. ex. 299 Z1 46 0.78 112 596 832 494 Inv. ex. 300 Z1 51 0.34 107 665 845 488 Inv. ex. 301 Z1 45 0.49 117 649 876 466 Inv. ex. 302 Z1 51 0.57 119 615 903 483 Inv. ex. 303 Z1 50 0.38 120 597 924 467 Inv. ex. 304 Z1 50 0.43 107 567 951 471 Inv. ex. 305 Z1 45 0.71 114 570 975 472 Inv. ex. 306 Z1 52 0.56 111 574 993 490 Inv. ex. 307 Z1 49 0.23 107 534 897 64 Inv. ex. 308 Z1 51 0.4  117 551 893 91 Inv. ex. 309 Z1 48 0.51 105 563 890 146 Inv. ex. 310 Z1 45 0.69 106 594 912 195 Inv. ex. 311 Z1 46 0.25 114 629 904 287 Inv. ex. 312 Z1 46 0.63  98 665 909 366 Inv. ex. 313 Z1 49 0.37 104 647 913 425 Inv. ex. 314 Z1 50 0.46 115 621 881 496 Inv. ex. 315 Z1 47 0.22 111 594 896 521 Inv. ex. 316 Z1 52 0.21 117 544 902 563 Inv. ex. 317 Z1 48 0.56  95 550 892 595 Inv. ex. 318 Z1 47 0.8  115 648 894 489 Gas combustion atmosphere Inv. ex. (air-fuel ratio 0.80) 319 Z1 46 0.41 111 612 898 495 Gas combustion atmosphere Inv. ex. (air-fuel ratio 0.85) 320 Z1 52 0.4  113 608 899 481 Gas combustion atmosphere Inv. ex. (air-fuel ratio 1.1) 321 Z1 51 0.72  96 641 884 475 Air Inv. ex. 322 Z1 49 0.53  95 592 885 467 Nitrogen gas Inv. ex. (dew point −30° C.) 323 Z1 52 0.71 110 542 917 472 Nitrogen gas Inv. ex. (dew point 0° C.) 324 Z1 45 0.3  114 598 883 472 Nitrogen dew point +10° C.) Inv. ex. 325 Z1 47 0.36 102 584 887 490 Ohmic heating temp. Inv. ex. 326 Z1 52 0.55 108 616 904 466 Tempering temp. 152° C. Inv. ex. 327 Z1 47 0.8  107 558 913 488 Tempering temp. 170° C. Inv. ex. 328 Z1 45 0.44  95 612 908 492 Tempering temp. 201° C. Inv. ex. 329 Z1 46 0.31 112 591 884 474 Tempering temp. 341° C. Inv. ex. 330 Z1 49 0.42 105 553 885 473 Tempering temp. 433° C. Inv. ex. 331 Z1 46 0.5  114 563 909 491 Tempering temp. 521° C. Inv. ex. 332 Z1 49 0.76 107 543 903 481 Tempering temp. 591° C. Inv. ex. 333 Z1 52 0.26 102 605 910 486 Partial softening treatment Inv. ex. Underlines indicate production conditions not preferable.

The properties of the obtained hot stamped part were measured and evaluated by the following methods:

Tensile Strength (TS)

The tensile strength (TS) of the hot stamped part was obtained from any position of the hot stamped part by preparing a No. 5 test piece and conducting a tensile test based on JIS Z 2241:2011. The crosshead speed was 1 mm/min.

Hydrogen Embrittlement Resistance

The hydrogen embrittlement resistance of the hot stamped part was evaluated as follows. First, a 1.2t×7.0W×68L (mm) test piece was fabricated. A four-point bending jig was used to impart various strain (stress) to the test piece. Next, this was dipped in hydrochloric acid (room temperature, pH-4) for 48 hours and the limit amount of strain where cracking occurred was investigated. Cases where the limit amount of strain was 0.6% or more were evaluated as passing (P) and cases where the limit amount of strain was less than 0.6% were evaluated as failing (F).

Cases where the tensile strength was 2200 MPa or more and the hydrogen embrittlement resistance was evaluated as passing were evaluated as a hot stamped part which is high in strength and able to suppress hydrogen embrittlement. The results are shown in Table 3. In Table 3, the area ratios of ferrite and pearlite and the dispersion index of pearlite at the steel sheet for hot stamping after the coiling step are shown. The remaining structure besides ferrite and pearlite was comprised of bainite, martensite, retained austenite, and/or trace amounts of carbides. Similarly, in Table 3, the area ratio of hard structures at the hot stamped part and the standard deviation in hardness distribution of the prior austenite grains (former γ grains) at the sheet thickness ¼ position are shown. The “area ratio of hard structures” means the total of the area ratios of martensite, bainite, and tempered martensite. Further, the remaining structure other than the hard structures was comprised of ferrite, retained austenite, and/or pearlite.

TABLE 3-1 Hot stamped part Former γ Steel sheet for hot stamping grain A B Hard hardness Ferrite Pearlite structure distribution area area A + Pearlite area standard Tensile Hydrogen Steel ratio ratio B dispersion ratio deviation strength embrittlement No. type % % % index % Hv MPa resistance Remarks 1 A1 52 43 95 0.62 97 113 2174 P Comp. ex. 2 A2 48 51 99 0.60 98 101 2220 P Inv. ex. 3 A3 32 58 90 0.65 95 128 2330 P Inv. ex. 4 A4 35 60 95 0.66 94 109 2464 P Inv. ex. 5 A5 38 53 91 0.64 96 111 2481 P Inv. ex. 6 A6 41 57 98 0.60 97 110 2540 P Inv. ex. 7 A7 30 61 91 0.65 95 125 2574 P Inv. ex. 8 A8 45 46 91 0.65 94 126 2727 P Inv. ex. 9 A9 24 65 89 0.65 98 121 2795 P Inv. ex. 10 A10 46 48 94 0.66 97 127 2897 P Inv. ex. 11 A11 54 42 96 0.66 95 108 2941 P Inv. ex. 12 A12 32 58 90 0.61 98  96 3043 P Inv. ex. 13 A13 51 44 95 0.65 96 122 3115 F Comp. ex. 14 B1 40 53 93 0.65 96 101 2192 P Comp. ex. 15 B2 40 59 99 0.66 99 126 2243 P Inv. ex. 16 B3 37 56 93 0.65 97  98 2305 P Inv. ex. 17 B4 54 43 97 0.63 97 118 2503 P Inv. ex. 18 B5 33 61 94 0.61 97 119 2523 P Inv. ex. 19 B6 47 50 97 0.62 99 130 2546 P Inv. ex. 20 B7 51 44 95 0.65 96 116 2470 P Inv. ex. 21 B8 55 42 97 0.60 98 121 2507 P Inv. ex. 22 B9 58 36 94 0.61 99 103 2534 P Inv. ex. 23 B10 68 30 98 0.56 96 132 2534 P Inv. ex. 24 B11 78 18 96 0.52 94 147 2431 P Inv. ex. 25 B12 91  7 98 0.38 96 188 2476 F Comp. ex. 26 C1 51 47 98 0.41 94 179 2525 F Comp. ex. 27 C2 42 52 94 0.51 99 146 2445 P Inv. ex. 28 C3 52 45 97 0.57 95 134 2548 P Inv. ex. 29 C4 47 43 90 0.65 95 123 2527 P Inv. ex. 30 C5 38 61 99 0.65 96 117 2513 P Inv. ex. 31 C6 33 63 96 0.67 98  99 2452 P Inv. ex. 32 C7 39 41 80 0.65 98 118 2431 P Inv. ex. 33 C8 43 44 87 0.60 96 119 2541 P Inv. ex. 34 C9 41 45 86 0.60 99 111 2489 P Inv. ex. 35 C10 42 47 89 0.64 95  99 2425 P Inv. ex. 36 C11 38 53 91 0.63 99 107 2430 P Inv. ex. 37 C12 37 55 92 0.67 97 126 2462 P Inv. ex. 38 C13 33 58 91 0.63 96 105 2504 P Inv. ex. 39 C14 29 68 97 0.58 97 137 2512 P Inv. ex. 40 C15 16 82 98 0.53 95 146 2515 P Inv. ex. 41 C16  6 92 98 0.43 95 201 2447 F Comp. ex. 42 D1 53 46 99 0.64 98 125 2530 P Inv. ex. 43 D2 47 49 96 0.61 98 116 2470 P Inv. ex. 44 D3 52 40 92 0.66 96 105 2468 P Inv. ex. 45 D4 53 42 95 0.66 99 130 2486 P Inv. ex. 46 D5 56 40 96 0.62 95 115 2476 P Inv. ex. 47 D6 28 65 93 0.62 99  99 2432 P Inv. ex. 48 D7 42 50 92 0.63 98 113 2548 P Inv. ex. 49 D8 49 45 94 0.64 96 119 2449 P Inv. ex. 50 D9 37 62 99 0.65 98 124 2517 F Comp. ex. 51 E1 36 54 90 0.60 98 100 2466 P Inv. ex. 52 E2 43 49 92 0.63 96 125 2424 P Inv. ex. 53 E3 34 55 89 0.63 95 109 2433 P Inv. ex. 54 E4 26 64 90 0.66 98  98 2474 P Inv. ex. 55 E5 30 60 90 0.65 94 114 2424 P Inv. ex. 56 E6 34 65 99 0.67 96 110 2432 P Inv. ex. 57 E7 42 50 92 0.61 99 109 2521 P Inv. ex. 58 E8 39 59 98 0.60 99 109 2521 P Inv. ex. 59 E9 43 55 98 0.61 99 110 2483 F Comp. ex. 60 F1 54 44 98 0.59 95 103 2423 P Inv. ex. 61 F2 27 63 90 0.59 96 124 2477 P Inv. ex. 62 F3 36 62 98 0.60 98 123 2500 P Inv. ex. 63 F4 46 48 94 0.66 99 105 2548 P Inv. ex. 64 F5 41 58 99 0.65 97 125 2488 P Inv. ex. 65 F6 56 41 97 0.62 97 101 2525 P Inv. ex. 66 F7 51 47 98 0.64 97  95 2501 P Inv. ex. 67 F8 43 51 94 0.63 94 110 2494 P Inv. ex. 68 F9 49 41 90 0.60 96  98 2495 F Comp. ex. 69 G1 48 45 93 0.65 94 122 2444 P Inv. ex. 70 G2 53 43 96 0.61 94 119 2517 P Inv. ex. 71 G3 57 42 99 0.64 98 108 2455 P Inv. ex. 72 G4 52 42 94 0.59 98 124 2518 P Inv. ex. 73 G5 29 61 90 0.65 98 129 2505 P Inv. ex. 74 G6 28 62 90 0.66 95 109 2454 P Inv. ex. 75 G7 55 43 98 0.63 95 106 2534 P Inv. ex. 76 G8 49 48 97 0.62 97 100 2538 P Inv. ex. 77 G9 42 54 96 0.64 95  99 2464 F Comp. ex. 78 H1 57 40 97 0.59 96 118 2488 F Comp. ex. 79 H2 29 63 92 0.64 99 116 2437 P Inv. ex. 80 H3 52 46 98 0.62 99 110 2488 P Inv. ex. 81 H4 59 40 99 0.60 94  99 2536 P Inv. ex. 82 H5 47 50 97 0.67 97 116 2533 P Inv. ex. 83 H6 51 44 95 0.67 96 109 2475 P Inv. ex. 84 H7 51 47 98 0.63 98 104 2453 P Inv. ex. 85 H8 39 52 91 0.66 97 116 2535 P Inv. ex. 86 H9 56 43 99 0.61 99 111 2484 P Inv. ex. 87 H10 50 42 92 0.65 95 115 2476 P Inv. ex. 88 H11 54 43 97 0.66 99 118 2496 P Inv. ex. 89 H12 36 59 95 0.63 94 115 2549 P Inv. ex. 90 H13 45 45 90 0.60 97 104 2484 F Comp. ex. 91 I1 36 56 92 0.65 95 130 2171 P Comp. ex. 92 I2 39 52 91 0.64 96 100 2203 P Inv. ex. 93 I3 56 43 99 0.63 99 121 2372 P Inv. ex. 94 I4 37 62 99 0.61 99 105 2543 P Inv. ex. 95 I5 46 44 90 0.63 97 110 2542 P Inv. ex. 96 I6 56 43 99 0.64 94  99 2480 P Inv. ex. 97 I7 50 45 95 0.59 94 106 2495 P Inv. ex. 98 I8 32 57 89 0.65 98 122 2526 P Inv. ex. 99 I9 42 53 95 0.65 98 102 2532 P Inv. ex. 100 I10 37 58 95 0.61 95 128 2466 P Inv. ex. 101 I11 43 53 96 0.65 95 102 2428 P Inv. ex. 102 I12 36 53 89 0.59 98 110 2493 F Comp. ex. 103 J1 39 50 89 0.64 99 130 2152 P Comp. ex. 104 J2 43 47 90 0.64 99 116 2234 P Inv. ex. 105 J3 41 53 94 0.66 96 110 2377 P Inv. ex. 106 J4 42 57 99 0.64 97  95 2430 P Inv. ex. 107 J5 39 58 97 0.67 98 105 2505 P Inv. ex. 108 J6 34 60 94 0.64 94 119 2467 P Inv. ex. 109 J7 30 64 94 0.60 99 127 2466 P Inv. ex. 110 J8 54 43 97 0.64 95 107 2431 P Inv. ex. 111 J9 30 60 90 0.59 97 110 2480 P Inv. ex. 112 J10 50 46 96 0.61 94 127 2429 P Inv. ex. 113 J11 41 53 94 0.60 99 117 2428 P Inv. ex. 114 J12 28 65 93 0.62 98 106 2499 F Comp. ex. 115 K1 51 46 97 0.65 97 120 2190 P Comp. ex. 116 K2 41 50 91 0.63 98 129 2207 P Inv. ex. 117 K3 46 48 94 0.63 98 125 2351 P Inv. ex. 118 K4 28 61 89 0.60 99  98 2546 P Inv. ex. 119 K5 41 51 92 0.60 99 129 2494 P Inv. ex. 120 K6 31 59 90 0.63 98 109 2504 P Inv. ex. 121 K7 37 59 96 0.60 98 117 2501 P Inv. ex. 122 K8 30 59 89 0.66 97 130 2466 P Inv. ex. 123 K9 45 49 94 0.66 99 105 2465 P Inv. ex. 124 K10 37 55 92 0.62 99 124 2521 P Inv. ex. 125 K11 42 54 96 0.61 97  98 2520 P Inv. ex. 126 K12 30 60 90 0.61 99 102 2454 F Comp. ex. 127 L1 38 57 95 0.61 98 100 2191 P Comp. ex. 128 L2 36 55 91 0.60 99 104 2218 P Inv. ex. 129 L3 53 42 95 0.63 99 128 2309 P Inv. ex. 130 L4 30 65 95 0.65 97 127 2442 P Inv. ex. 131 L5 44 48 92 0.67 96 109 2540 P Inv. ex. 132 L6 44 51 95 0.61 98 100 2510 P Inv. ex. 133 L7 27 65 92 0.63 95 111 2523 P Inv. ex. 134 L8 45 46 91 0.61 99 104 2522 P Inv. ex. 135 L9 57 41 98 0.64 99  95 2541 P Inv. ex. 136 L10 54 45 99 0.64 95 101 2482 P Inv. ex. 137 L11 41 49 90 0.62 97 103 2530 P Inv. ex. 138 L12 39 50 89 0.60 97 128 2487 F Comp. ex. 139 M1 48 46 94 0.66 99 123 2176 P Comp. ex. 140 M2 41 58 99 0.62 98 115 2230 P Inv. ex. 141 M3 45 51 96 0.67 99 126 2314 P Inv. ex. 142 M4 40 56 96 0.62 98 118 2423 P Inv. ex. 143 M5 35 58 93 0.66 98  98 2495 P Inv. ex. 144 M6 52 45 97 0.59 99 118 2454 P Inv. ex. 145 M7 44 55 99 0.61 98  98 2444 P Inv. ex. 146 M8 42 55 97 0.65 98 122 2541 P Inv. ex. 147 M9 48 45 93 0.65 97 109 2441 P Inv. ex. 148 M10 35 63 98 0.66 98 114 2423 P Inv. ex. 149 M11 38 58 96 0.59 98 115 2548 P Inv. ex. 150 M12 40 51 91 0.67 99  95 2460 F Comp. ex. 151 N1 49 41 90 0.59 98  98 2590 P Inv. ex. 152 N2 45 50 95 0.67 95 104 2580 P Inv. ex. 153 N3 44 49 93 0.59 98  98 2558 P Inv. ex. 154 N4 35 59 94 0.61 94 125 2582 P Inv. ex. 155 N5 41 50 91 0.63 97 119 2611 P Inv. ex. 156 N6 36 55 91 0.64 99 115 2570 P Inv. ex. 157 N7 34 62 96 0.62 97 120 2583 P Inv. ex. 158 N8 46 50 96 0.65 95 109 2577 P Inv. ex. 159 N9 35 64 99 0.60 94 118 2592 P Inv. ex. 160 N10 29 63 92 0.59 96 102 2553 P Inv. ex. 161 N11 46 52 98 0.61 96 119 2583 P Inv. ex. 162 N12 44 51 95 0.64 95 102 2611 P Inv. ex. 163 O1 38 60 98 0.65 94 118 2583 P Inv. ex. 164 O2 43 55 98 0.62 94 117 2604 P Inv. ex. 165 O3 51 46 97 0.64 95 104 2603 P Inv. ex. 166 O4 43 46 89 0.64 97 105 2587 P Inv. ex. 167 O5 36 61 97 0.60 94 120 2594 P Inv. ex. 168 O6 48 41 89 0.59 95 117 2607 P Inv. ex. 169 O7 46 52 98 0.64 99 130 2591 P Inv. ex. 170 O8 38 52 90 0.60 94  99 2594 P Inv. ex. 171 O9 46 53 99 0.67 96 111 2574 P Inv. ex. 172 O10 53 41 94 0.64 99 113 2568 P Inv. ex. 173 O11 49 42 91 0.65 94 120 2567 P Inv. ex. 174 P1 52 47 99 0.60 99  98 2588 P Inv. ex. 175 P2 52 47 99 0.66 94 121 2567 P Inv. ex. 176 P3 43 49 92 0.59 94 109 2571 P Inv. ex. 177 P4 54 41 95 0.61 97  98 2568 P Inv. ex. 178 P5 46 53 99 0.60 96 104 2569 P Inv. ex. 179 P6 51 41 92 0.65 96 110 2552 P Inv. ex. 180 P7 47 44 91 0.61 95 126 2557 P Inv. ex. 181 P8 39 54 93 0.61 95 101 2603 P Inv. ex. 182 Q1 32 64 96 0.65 94 109 2565 P Inv. ex. 183 Q2 40 50 90 0.61 98 122 2562 P Inv. ex. 184 Q3 49 49 98 0.67 94 113 2575 P Inv. ex. 185 Q4 58 40 98 0.66 99 117 2570 P Inv. ex. 186 Q5 29 63 92 0.62 98 102 2559 P Inv. ex. 187 Q6 30 65 95 0.64 98 103 2572 P Inv. ex. 188 Q7 52 45 97 0.59 96 127 2589 P Inv. ex. 189 Q8 41 50 91 0.66 97 112 2583 P Inv. ex. 190 Q9 45 53 98 0.60 95 127 2590 P Inv. ex. 191 Q10 54 44 98 0.64 94 108 2553 P Inv. ex. 192 R1 41 56 97 0.59 97 126 2448 P Inv. ex. 193 R2 50 47 97 0.59 97 112 2503 P Inv. ex. 194 R3 33 65 98 0.62 98  96 2481 P Inv. ex. 195 R4 44 45 89 0.64 98 118 2467 P Inv. ex. 196 R5 40 53 93 0.60 98 120 2545 P Inv. ex. 197 R6 50 46 96 0.67 98 124 2424 P Inv. ex. 198 R7 27 62 89 0.59 99 108 2497 P Inv. ex. 199 R8 34 56 90 0.63 98 121 2503 P Inv. ex. 200 R9 52 44 96 0.67 97 117 2538 P Inv. ex. 201 S1 45 54 99 0.65 99 123 2441 P Inv. ex. 202 S2 33 61 94 0.61 99 129 2505 P Inv. ex. 203 S3 34 56 90 0.61 94 112 2450 P Inv. ex. 204 S4 36 63 99 0.63 98 113 2491 P Inv. ex. 205 S5 37 56 93 0.67 97 112 2497 P Inv. ex. 206 S6 45 52 97 0.62 96  99 2470 P Inv. ex. 207 S7 43 53 96 0.61 94 103 2453 P Inv. ex. 208 S8 33 57 90 0.60 99 102 2455 P Inv. ex. 209 T1 47 47 94 0.64 97 104 2581 P Inv. ex. 210 T2 55 43 98 0.63 96 117 2598 P Inv. ex. 211 T3 47 47 94 0.59 97 110 2559 P Inv. ex. 212 T4 41 58 99 0.65 99 122 2561 P Inv. ex. 213 T5 54 44 98 0.64 95 125 2554 P Inv. ex. 214 T6 54 45 99 0.62 97 114 2613 P Inv. ex. 215 T7 39 56 95 0.65 94 111 2616 P Inv. ex. 216 T8 39 50 89 0.59 95 128 2601 P Inv. ex. 217 T9 39 57 96 0.61 96 104 2579 P Inv. ex. 218 T10 48 45 93 0.60 99  99 2611 P Inv. ex. 219 U1 33 58 91 0.64 98 122 2449 P Inv. ex. 220 U2 29 61 90 0.66 94  99 2530 P Inv. ex. 221 U3 44 49 93 0.65 95 116 2525 P Inv. ex. 222 U4 46 43 89 0.67 97 104 2424 P Inv. ex. 223 U5 36 58 94 0.60 99 102 2548 P Inv. ex. 224 U6 48 46 94 0.63 98 112 2507 P Inv. ex. 225 U7 30 63 93 0.61 97 125 2480 P Inv. ex. 226 U8 36 57 93 0.60 98 118 2527 P Inv. ex. 227 V1 39 55 94 0.64 95  97 2446 P Inv. ex. 228 V2 35 56 91 0.59 94 108 2434 P Inv. ex. 229 V3 38 51 89 0.60 96  95 2423 P Inv. ex. 230 V4 49 42 91 0.67 99 126 2485 P Inv. ex. 231 V5 39 51 90 0.61 96 124 2494 P Inv. ex. 232 V6 41 51 92 0.65 96 107 2506 P Inv. ex. 233 V7 28 62 90 0.59 96 106 2437 P Inv. ex. 234 V8 36 56 92 0.60 95 127 2432 P Inv. ex. 235 W1 26 64 90 0.66 95 112 2507 P Inv. ex. 236 W2 54 43 97 0.61 96 126 2541 P Inv. ex. 237 W3 43 53 96 0.61 96 109 2537 P Inv. ex. 238 W4 34 62 96 0.60 96 121 2482 P Inv. ex. 239 W5 39 55 94 0.63 95 107 2501 P Inv. ex. 240 W6 38 51 89 0.65 94 113 2447 P Inv. ex. 241 W7 43 53 96 0.59 94 119 2500 P Inv. ex. 242 W8 48 50 98 0.64 98 101 2516 P Inv. ex. 243 X1 45 52 97 0.67 97 104 2493 P Inv. ex. 244 X2 44 45 89 0.67 96 119 2421 P Inv. ex. 245 X3 42 54 96 0.64 95 124 2429 P Inv. ex. 246 X4 33 59 92 0.61 99 112 2538 P Inv. ex. 247 X5 55 41 96 0.59 97  95 2523 P Inv. ex. 248 X6 43 53 96 0.62 95  97 2457 P Inv. ex. 249 X7 40 50 90 0.66 94 102 2536 P Inv. ex. 250 X8 35 62 97 0.67 96 105 2478 P Inv. ex. 251 Y1 44 48 92 0.60 99 110 2551 P Inv. ex. 252 Y2 32 63 95 0.64 98 122 2595 P Inv. ex. 253 Y3 54 45 99 0.65 98 102 2591 P Inv. ex. 254 Y4 43 54 97 0.66 99 121 2585 P Inv. ex. 255 Y5 42 55 97 0.63 95 101 2566 P Inv. ex. 256 Y6 33 65 98 0.59 97 107 2560 P Inv. ex. 257 Y7 48 50 98 0.63 95 100 2599 P Inv. ex. 258 Y8 41 57 98 0.62 94 114 2600 P Inv. ex. 259 Z1 42 47 89 0.41 95 183 2517 F Comp. ex. 260 Z1 49 46 95 0.52 96 148 2437 P Inv. ex. 261 Z1 45 53 98 0.55 97 135 2447 P Inv. ex. 262 Z1 38 53 91 0.58 97 138 2523 P Inv. ex. 263 Z1 39 56 95 0.66 97 122 2441 P Inv. ex. 264 Z1 44 48 92 0.66 97 119 2510 P Inv. ex. 265 Z1 56 41 97 0.67 95 101 2534 P Inv. ex. 266 Z1 49 43 92 0.62 98 106 2490 P Inv. ex. 267 Z1 30 62 92 0.62 99 122 2486 P Inv. ex. 268 Z1 31 65 96 0.57 94 135 2539 P Inv. ex. 269 Z1 27 63 90 0.51 94 143 2513 P Inv. ex. 270 Z1 35 61 96 0.39 98 199 2530 F Comp. ex. 271 Z1 39 38 77 0.42 93 202 2518 F Comp. ex. 272 Z1 51 46 97 0.52 98 147 2492 P Inv. ex. 273 Z1 50 46 96 0.56 97 133 2487 P Inv. ex. 274 Z1 42 53 95 0.59 99 111 2485 P Inv. ex. 275 Z1 57 40 97 0.65 97  98 2428 P Inv. ex. 276 Z1 27 36 63 0.59 94 186 2421 F Comp. ex. 277 Z1 33 50 83 0.66 96 148 2546 P Inv. ex. 278 Z1 31 57 88 0.62 97 133 2443 P Inv. ex. 279 Z1 43 47 90 0.66 99 107 2481 P Inv. ex. 280 Z1 40 57 97 0.66 94  98 2522 P Inv. ex. 281 Z1 46 51 97 0.62 98 114 2448 P Inv. ex. 282 Z1 57 42 99 0.57 96 139 2448 P Inv. ex. 283 Z1 53 41 94 0.53 97 141 2536 P Inv. ex. 284 Z1 31 59 90 0.44 98 207 2474 F Comp. ex. 285 Z1 30 63 93 0.60 94 104 2462 P Inv. ex. 286 Z1 32 65 97 0.62 98  96 2509 P Inv. ex. 287 Z1 37 55 92 0.67 96 112 2544 P Inv. ex. 288 Z1 35 64 99 0.65 96 112 2474 P Inv. ex. 289 Z1 46 49 95 0.61 95 107 2462 P Inv. ex. 290 Z1 36 55 91 0.62 98 110 2517 P Inv. ex. 291 Z1 33 61 94 0.62 96 129 2465 P Inv. ex. 292 Z1 45 52 97 0.67 97 122 2466 P Inv. ex. 293 Z1 44 52 96 0.62 96 124 2494 P Inv. ex. 294 Z1 40 51 91 0.59 95  99 2471 P Inv. ex. 295 Z1 37 60 97 0.66 98 102 2498 P Inv. ex. 296 Z1 42 56 98 0.60 99 121 2451 P Inv. ex. 297 Z1 44 50 94 0.61 91 101 2247 P Inv. ex. 298 Z1 30 63 93 0.66 93  99 2367 P Inv. ex. 299 Z1 31 65 96 0.60 96 126 2529 P Inv. ex. 300 Z1 50 46 96 0.66 95 127 2459 P Inv. ex. 301 Z1 28 65 93 0.67 99 127 2468 P Inv. ex. 302 Z1 34 57 91 0.61 96 102 2480 P Inv. ex. 303 Z1 50 44 94 0.67 98 128 2451 P Inv. ex. 304 Z1 53 42 95 0.62 98 111 2522 P Inv. ex. 305 Z1 57 41 98 0.59 98 136 2512 P Inv. ex. 306 Z1 45 46 91 0.63 94 143 2432 P Inv. ex. 307 Z1 40 53 93 0.61 98 142 2462 P Inv. ex. 308 Z1 52 43 95 0.67 99 137 2465 P Inv. ex. 309 Z1 46 45 91 0.65 96  95 2533 P Inv. ex. 310 Z1 54 42 96 0.59 97 123 2445 P Inv. ex. 311 Z1 47 45 92 0.61 94 129 2539 P Inv. ex. 312 Z1 45 47 92 0.59 96 102 2445 P Inv. ex. 313 Z1 35 58 93 0.59 96 103 2434 P Inv. ex. 314 Z1 48 45 93 0.63 96  95 2467 P Inv. ex. 315 Z1 34 59 93 0.64 95 120 2460 P Inv. ex. 316 Z1 55 44 99 0.66 94 137 2498 P Inv. ex. 317 Z1 41 56 97 0.63 99 142 2456 P Inv. ex. 318 Z1 38 52 90 0.60 96 125 2547 P Inv. ex. 319 Z1 27 62 89 0.62 98 100 2485 P Inv. ex. 320 Z1 48 51 99 0.66 94 111 2437 P Inv. ex. 321 Z1 46 46 92 0.63 94 126 2494 P Inv. ex. 322 Z1 44 50 94 0.65 95 112 2448 P Inv. ex. 323 Z1 45 53 98 0.63 98 109 2544 P Inv. ex. 324 Z1 44 48 92 0.65 98 126 2497 P Inv. ex. 325 Z1 48 47 95 0.65 96  99 2497 P Inv. ex. 326 Z1 32 59 91 0.66 95 127 2470 P Inv. ex. 327 Z1 28 62 90 0.66 98 122 2425 P Inv. ex. 328 Z1 30 60 90 0.67 97 109 2471 P Inv. ex. 329 Z1 49 44 93 0.66 98  98 2487 P Inv. ex. 330 Z1 53 45 98 0.65 98 128 2546 P Inv. ex. 331 Z1 30 62 92 0.62 94  96 2486 P Inv. ex. 332 Z1 40 58 98 0.63 99 111 2440 P Inv. ex. 333 Z1 34 55 89 0.63 99 127 2443 P Inv. ex. Underlines indicate outside scope of present invention or values of properties not preferable.

Referring to Table 3, in Comparative Example 1, the C content was low, and therefore the tensile strength fell. In Comparative Example 13, the C content was high, and therefore the strength became too high and the hydrogen embrittlement resistance fell. In Comparative Example 14, the Si content was low, and therefore the tensile strength fell. In Comparative Example 25, the Si content was high, and therefore in the steel sheet for hot stamping, the amount of ferrite increased, the desired metallographic structure was not obtained, and the dispersion index of pearlite became less than 0.50. As a result, in the hot stamped part, it was not possible to control the standard deviation in hardness distribution of prior austenite grains to the desired range and the hydrogen embrittlement resistance fell. In Comparative Example 26, the Mn content was low, and therefore the dispersion index of pearlite in the steel sheet for hot stamping and the standard deviation in hardness distribution of prior austenite grains in the hot stamped part could not be controlled to within the desired ranges and the hydrogen embrittlement resistance fell. In Comparative Example 41, the Mn content was high, and therefore it is believed that in the hot rolled steel sheet, transformation from austenite to pearlite was promoted too much. As a result, the dispersion index of pearlite in the steel sheet for hot stamping and the standard deviation in hardness distribution of prior austenite grains in the hot stamped part could not be controlled to within the desired ranges and the hydrogen embrittlement resistance fell. In each of Comparative Examples 50, 59, 68, 77, 78, and 90, the P, S, N, O, or Al content was not suitable, and therefore the hydrogen embrittlement resistance fell. In each of Comparative Examples 91, 103, 115, 127, and 139, the respective Nb, Ti, B, Cr, and Mo contents were low, and therefore the strength could not be sufficiently improved and the tensile strength fell. In each of Comparative Examples 102, 114, 126, 138, and 150, the respective Nb, Ti, B, Cr, and Mo contents were high, and therefore it is believed that large amounts of carbonitrides were formed in the steel or coarse intermetallic compounds were formed. As a result, the hydrogen embrittlement resistance fell.

In Comparative Example 259, the rolling reduction of the final stage in the finish rolling of the hot rolling step was low, and therefore it is believed pearlite could not be made to homogeneously disperse in the hot rolled steel sheet after rolling. As a result, the dispersion index of pearlite in the steel sheet for hot stamping and the standard deviation in hardness distribution of prior austenite grains in the hot stamped part could not be controlled to within the desired ranges and the hydrogen embrittlement resistance fell. In Comparative Example 270, the time period from the end of the finish rolling to the start of rapid cooling was long, and therefore it is believed that growth of the austenite grains could not be sufficiently suppressed and ferrite was arranged connected and pearlite could not be made to homogeneously disperse. As a result, the dispersion index of pearlite in the steel sheet for hot stamping and the standard deviation in hardness distribution of prior austenite grains in the hot stamped part could not be controlled to within the desired ranges and the hydrogen embrittlement resistance fell. In Comparative Example 271, the average cooling speed after rapid cooling in the cooling step was slow, and therefore in the steel sheet for hot stamping, the desired metallographic structure could not be formed. As a result, the dispersion index of pearlite in the steel sheet for hot stamping and the standard deviation in hardness distribution of prior austenite grains in the hot stamped part could not be controlled to within the desired ranges and the hydrogen embrittlement resistance fell. In Comparative Example 276, the coiling temperature was low, and therefore in the steel sheet for hot stamping, the desired metallographic structure could not be formed. As a result, the standard deviation in hardness distribution of prior austenite grains in the hot stamped part could not be controlled to within the desired ranges and the hydrogen embrittlement resistance fell. In Comparative Example 284, the coiling temperature was high, and therefore it is believed grain growth occurred and homogeneous dispersion of the pearlite was obstructed. As a result, the dispersion index of pearlite in the steel sheet for hot stamping and the standard deviation in hardness distribution of prior austenite grains in the hot stamped part could not be controlled to within the desired ranges and the hydrogen embrittlement resistance fell.

In contrast to this, in the steel sheet for hot stamping and hot stamped part of all of the invention examples, by having the predetermined chemical composition and metallographic structure, controlling the dispersion index of pearlite in the steel sheet for hot stamping to 0.50 or more, and, in the hot stamped part, controlling the standard deviation in hardness distribution of the prior austenite grains to 150 Hv or less, it was possible to reliably suppress hydrogen embrittlement regardless of having a high tensile strength of 2200 MPa or more. Further, in the hot stamped part of all of the invention examples, the hardness of the prior austenite grains at the sheet thickness ¼ position (average of all measurement values of hardness in invention examples) was controlled to a range of 500 to 1000 Hv.

Claims

1. A steel sheet for hot stamping having a chemical composition comprising, by mass %,

C: 0.40 to 0.70%,
Si: 0.010 to 1.300%,
Mn: 0.60 to 3.00%,
P: 0.100% or less,
S: 0.0100% or less,
N: 0.0200% or less,
O: 0.0200% or less,
Al: 0.0010 to 0.5000%,
Nb: 0.0010 to 0.100%,
Ti: 0.010 to 0.200%,
B: 0.0005 to 0.0200%,
Cr: 0.010 to 0.80%,
Mo: 0.0010 to 1.000%,
Co: 0 to 2.00%,
Ni: 0 to 3.00%,
Cu: 0 to 1.00%,
V: 0 to 1.00%,
W: 0 to 1.000%,
Ca: 0 to 0.010%,
Mg: 0 to 1.000%,
REM: 0 to 1.000%,
Sb: 0 to 1.000%,
Zr: 0 to 1.000%,
Sn: 0 to 1.000%,
As: 0 to 0.100%, and
balance: Fe and impurities, and
a metallographic structure comprising, by area ratio,
ferrite: 10% or more and
pearlite: 10% or more, wherein
a total of ferrite and pearlite is 80% or more, and
a dispersion index of pearlite is 0.50 or more.

2. The steel sheet for hot stamping according to claim 1, wherein the chemical composition contains, by mass %, one or more of

Co: 0.001 to 2.00%,
Ni: 0.001 to 3.00%,
Cu: 0.001 to 1.00%,
V: 0.001 to 1.00%,
W: 0.001 to 1.000%,
Ca: 0.0001 to 0.010%,
Mg: 0.0001 to 1.000%,
REM: 0.0001 to 1.000%,
Sb: 0.001 to 1.000%,
Zr: 0.001 to 1.000%,
Sn: 0.001 to 1.000%, and
As: 0.001 to 0.100%.

3. A hot stamped part having a chemical composition comprising, by mass %,

C: 0.40 to 0.70%,
Si: 0.010 to 1.300%,
Mn: 0.60 to 3.00%,
P: 0.100% or less,
S: 0.0100% or less,
N: 0.0200% or less,
O: 0.0200% or less,
Al: 0.0010 to 0.5000%,
Nb: 0.0010 to 0.100%,
Ti: 0.010 to 0.200%,
B: 0.0005 to 0.0200%,
Cr: 0.010 to 0.80%,
Mo: 0.0010 to 1.000%,
Co: 0 to 2.00%,
Ni: 0 to 3.00%,
Cu: 0 to 1.00%,
V: 0 to 1.00%,
W: 0 to 1.000%,
Ca: 0 to 0.010%,
Mg: 0 to 1.000%,
REM: 0 to 1.000%,
Sb: 0 to 1.000%,
Zr: 0 to 1.000%,
Sn: 0 to 1.000%,
As: 0 to 0.100%, and
balance: Fe and impurities, and
a metallographic structure comprising, by area ratio, at least one of martensite, bainite, and tempered martensite in a total of 90% or more, wherein
a standard deviation in a hardness distribution of prior austenite grains at a sheet thickness ¼ position is 150 Hv or less.

4. The hot stamped part according to claim 3, wherein the chemical composition contains, by mass %, one or more of

Co: 0.001 to 2.00%,
Ni: 0.001 to 3.00%,
Cu: 0.001 to 1.00%,
V: 0.001 to 1.00%,
W: 0.001 to 1.000%,
Ca: 0.0001 to 0.010%,
Mg: 0.0001 to 1.000%,
REM: 0.0001 to 1.000%,
Sb: 0.001 to 1.000%,
Zr: 0.001 to 1.000%,
Sn: 0.001 to 1.000%, and
As: 0.001 to 0.100%.
Patent History
Publication number: 20250146111
Type: Application
Filed: Mar 2, 2023
Publication Date: May 8, 2025
Applicant: NIPPON STEEL CORPORATION (Tokyo)
Inventors: Yuma ASADA (Tokyo), Yuri TODA (Tokyo), Yasuyuki OGISU (Tokyo), Tamaki SUZUKI (Tokyo)
Application Number: 18/837,633
Classifications
International Classification: C22C 38/38 (20060101); C22C 38/00 (20060101); C22C 38/02 (20060101); C22C 38/04 (20060101); C22C 38/06 (20060101); C22C 38/22 (20060101); C22C 38/26 (20060101); C22C 38/28 (20060101); C22C 38/32 (20060101);