HOT-ROLLED STEEL SHEET

Abstract

This hot-rolled steel sheet has a desired chemical composition and microstructure, an average sphere equivalent radius of alloy carbides in the ferrite is 0.5 nm or more and less than 5.0 nm, an average number density of the alloy carbides in the ferrite is 3.5×1016/cm3 or more, an E value that indicates periodicity of the microstructure is 10.7 or more, and an I value that indicates uniformity of the microstructure is 1.020 or more, a standard deviation of a Mn concentration is 0.60 mass % or less, and a tensile strength is 980 MPa or more.

Description

TECHNICAL FIELD OF THE INVENTION

The present invention relates to a hot-rolled steel sheet. Specifically, the present invention relates to a hot-rolled steel sheet that is formed into various shapes by press working or the like to be used, and particularly relates to a hot-rolled steel sheet that has high strength and excellent ductility, fatigue property and shearing property.

Priority is claimed on Japanese Patent Application No. 2022-015116, filed on Feb. 2, 2022, the content of which is incorporated herein by reference.

BACKGROUND ART

In recent years, from the viewpoint of protecting the global environment, efforts have been made to reduce the amount of carbon dioxide gas emitted in many fields. Vehicle manufacturers are also actively developing techniques for reducing the weight of vehicle bodies for the purpose of reducing fuel consumption. However, it is not easy to reduce the weight of vehicle bodies since the emphasis is placed on improvement in collision resistance to secure the safety of the occupants.

In order to achieve both vehicle body weight reduction and collision resistance, an investigation has been conducted to make a member thin by using a high-strength steel sheet. Therefore, there is a strong demand for a steel sheet having both high strength and excellent formability. In order to meet this demand, several techniques have been conventionally proposed. Since there are various working methods for vehicle members, the required formability differs depending on members to which the working methods are applied, but among these, ductility is placed as important indices for formability.

In addition, vehicle members are formed by press forming, and the press-formed blank sheet is often manufactured by highly productive shearing working. A blank sheet manufactured by shearing working needs to be excellent in terms of the end surface accuracy after shearing working. For example, when a secondary sheared surface consisting of a sheared surface, a fractured surface, and a sheared surface is generated in the appearance of the end surface after shearing working (sheared end surface), the accuracy of the sheared end surface significantly deteriorates.

For example, Patent Document I discloses a high-strength steel sheet having excellent ductility and stretch flangeability and having a tensile strength of 980 MPa or more, in which a second phase consisting of residual austenite and/or martensite is finely dispersed in crystal grains.

Patent Document 2 discloses a technique for controlling burr height after punching by controlling a ratio d_s/d_b of the ferrite grain size d_s of the surface layer to the ferrite grain d_b of an inside to 0.95 or less.

PRIOR ART DOCUMENT

Patent Document

• [Patent Document 1] Japanese Unexamined Patent Application, First Publication No. 2005-179703
• [Patent Document 2] Japanese Unexamined Patent Application, First Publication No. 1110-168544

Non-Patent Document

• [Non-Patent Document 1]J. Webel, J. Gola, D. Britz, F. Mucklich, Materials Characterization 144 (2018) 584-596
• [Non-Patent Document 2]D. L. Naik, H. U. Sajid, R. Kiran, Metals 2019, 9, 546
• [Non-Patent Document 3]K. Zuiderveld, Contrast Limited Adaptive Histogram Equalization, Chapter VIII. 5, Graphics Gems IV. P. S. Heckbert (Eds.), Cambridge, MA, Academic Press, 1994, pp. 474-485

DISCLOSURE OF THE INVENTION

Problems to be Solved by the Invention

The techniques disclosed in Patent Documents 1 and 2 are all techniques for improving either ductility or an end surface property after shearing working. However, Patent Documents 1 and 2 do not refer to a technique for achieving both of the properties.

In addition, hot-rolled steel sheet having high strength may be required to have better fatigue property.

The present invention has been made in view of the above problems of the related art, and an object of the present invention is to provide a hot-rolled steel sheet having high strength and excellent ductility, fatigue property and shearing property.

Means for Solving the Problem

The gist of the present invention made based on the above findings is as follows.

• • (1) A hot-rolled steel sheet according to one aspect of the present invention comprising, in terms of mass %, as a chemical composition,
  • C: 0.050% to 0.250%,
  • Si: 0.05% to 3.00%,
  • Mn: 1.00% to 4.00%,
  • one or two or more of Ti, Nb, and V: 0.060% to 0.500% in total,
  • sol. Al: 0.001% to 2.000%,
  • P: 0.100% or less,
  • S: 0.0300% or less,
  • N: 0.1000% or less,
  • O: 0.0100% or less,
  • Cu: 0% to 2.00%,
  • Cr: 0% to 2.00%,
  • Mo: 0% to 1.00%,
  • Ni: 0% to 2.00%,
  • B: 0% to 0.0100%,
  • Ca: 0% to 0.0200%,
  • Mg: 0% to 0.0200%,
  • REM: 0% to 0.1000%,
  • Bi: 0% to 0.020%,
  • one or two or more of Zr, Co, Zn, and W: 0% to 1.00% in total,
  • Sn: 0% to 0.05%, and
  • a remainder comprising Fe and impurities,
  • in which, in a microstructure,
  • in terms of area %,
  • residual austenite is less than 3.0%,
  • ferrite is 15.0% or more and less than 60.0%, and
  • pearlite is less than 5.0%,
  • an average sphere equivalent radius of alloy carbides in the ferrite is 0.5 nm or more and less than 5.0 nm,
  • an average number density of the alloy carbides in the ferrite is 3.5×10¹⁶/cm³ or more,
  • an E value that indicates periodicity of the microstructure is 10.7 or more,
  • an I value that indicates uniformity of the microstructure is 1.020 or more,
  • a standard deviation of a Mn concentration is 0.60 mass % or less, and
  • a tensile strength is 980 MPa or more.
  • (2) The hot-rolled steel sheet according to (1) may further comprise, in terms of mass %, one or two or more selected from the group consisting of, as the chemical composition
  • Cu: 0.01% to 2.00%,
  • Cr: 0.01% to 2.00%,
  • Mo: 0.01% to 1.00%,
  • Ni: 0.02% to 2.00%,
  • B: 0.0001% to 0.0100%,
  • Ca: 0.0005% to 0.0200%,
  • Mg: 0.0005% to 0.0200%,
  • REM: 0.0005% to 0.1000%, and
  • Bi: 0.0005% to 0.020%.

Effects of the Invention

According to the above aspect according to the present invention, it is possible to obtain a hot-rolled steel sheet having high strength and excellent ductility, fatigue property and shearing property.

The hot-rolled steel sheet according to the above aspect of the present invention is suitable as an industrial material used for vehicle members, mechanical structural members, and building members.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an example of a sheared end surface of a hot-rolled steel sheet according to a present invention example.

FIG. 2 is an example of a sheared end surface of a hot-rolled steel sheet according to a comparative example.

EMBODIMENTS OF THE INVENTION

The chemical composition and microstructure of a hot-rolled steel sheet according to the present embodiment will be more specifically described below. However, the present invention is not limited only to a configuration disclosed in the present embodiment, and various modifications can be made without departing from the scope of the gist of the present invention.

The numerical limit range described below with “to” in between includes the lower limit and the upper limit. Regarding the numerical value indicated by “less than” or “more than”, the value does not fall within the numerical range. In the following description, % regarding the chemical composition of the hot-rolled steel sheet is mass % unless particularly otherwise specified.

Chemical Composition

The hot-rolled steel sheet according to the present embodiment includes, in terms of mass %, C: 0.050% to 0.250%, Si: 0.05% to 3.00%, Mn: 1.00% to 4.00%, one or two or more of Ti, Nb, and V: 0.060% to 0.500% in total, sol. Al: 0.001% to 2.000%, P: 0.100% or less, S: 0.0300% or less, N: 0.1000% or less, O: 0.0100% or less, and a remainder of Fe and impurities. Each element will be described in detail below.

• • C: 0.050% to 0.250%

C increases the area ratio of a hard phase and increases the strength of ferrite by bonding to a precipitation hardening element such as Ti, Nb, or V. When the C content is less than 0.050%, a desired strength cannot be obtained. Therefore, the C content is set to 0.050% or more. The C content is preferably 0.060% or more, more preferably 0.070% or more, and still more preferably 0.080% or more.

On the other hand, when the C content is more than 0.250%, the ductility of the hot-rolled steel sheet deteriorates due to a decrease in the area ratio of ferrite. Therefore, the C content is set to 0.250% or less. The C content is preferably 0.200% or less, 0.180% or less or 0.150% or less.

• • Si: 0.05% to 3.00%

Si has an action of improving the ductility of the hot-rolled steel sheet by promoting the formation of ferrite and has an action of increasing the strength of the hot-rolled steel sheet by the solid solution strengthening of ferrite. In addition, Si has an action of making steel sound by deoxidation (suppressing the occurrence of a defect such as a blowhole in steel). When the Si content is less than 0.05%, an effect by the action cannot be obtained. Therefore, the Si content is set to 0.05% or more. The Si content is preferably 0.50% or more and more preferably 0.80% or more.

On the other hand, when the Si content is more than 3.00%, the surface properties, chemical convertibility, furthermore, ductility, and weldability of the steel sheet significantly deteriorate, and the A₃ transformation point significantly increases. Therefore, it becomes difficult to perform hot rolling in a stable manner. Therefore, the Si content is set to 3.00% or less. The Si content is preferably 2.50% or less, and more preferably 2.00% or less or 1.50% or less.

• • Mn: 1.00% to 4.00%

Mn has an action of suppressing ferritic transformation to enhance strength of the hot-rolled steel sheet. When the Mn content is less than 1.00%, a desired strength cannot be obtained. Therefore, the Mn content is set to 1.00% or more. The Mn content is preferably 1.30% or more and more preferably 1.50% or more.

On the other hand, when the Mn content is more than 4.00%, due to the segregation of Mn, the form of the hard phase becomes a periodic band shape, and it becomes difficult to obtain a desired shearing property. Therefore, the Mn content is set to 4.00% or less. The Mn content is preferably 3.50% or less and more preferably 3.00% or less or 2.50% or less.

One or two or more of Ti, Nb, and V: 0.060% to 0.500% in total

Ti, Nb, and V are elements that are finely precipitated in steel as a carbide and a nitride and improve the strength of steel by precipitation hardening. Furthermore, these elements are essential elements to obtain a desired fatigue property. When the total amount of Ti, Nb, and V is less than 0.060%, these effects cannot be obtained. Therefore, the total amount of Ti, Nb, and V is set to 0.060% or more. Not all of Ti, Nb, and V need to be contained, and any one thereof may be contained, and the amount thereof may be 0.060% or more. The total amount of Ti, Nb, and V is preferably 0.080% or more and more preferably 0.100% or more.

On the other hand, when the total amount of Ti, Nb, and V exceeds 0.500%, the workability of the hot-rolled steel sheet deteriorates. Therefore, the total amount of Ti, Nb, and V is set to 0.500% or less. The total amount of Ti, Nb, and V is preferably 0.300% or less, more preferably 0.250% or less, and still more preferably 0.200% or less.

• • sol. Al: 0.001% to 2.000%

Similar to Si, Al has an action of making steel sound by deoxidizing and has an action of enhancing the ductility of the hot-rolled steel sheet by promoting the formation of ferrite. When the sol. Al content is less than 0.001%, an effect by the action cannot be obtained. Therefore, the sol. Al content is set to 0.001% or more. The sol. Al content is preferably 0.010% or more, and more preferably 0.020% or more or 0.030% or more.

On the other hand, when the sol. Al content is more than 2.000%, the above effects are saturated, which is not economically preferable, and thus the sol. Al content is set to 2.000% or less. The sol. Al content is preferably 1.500% v or less, more preferably 1.000% or less, and still more preferably 0.500% or less.

The sol. Al means acid-soluble Al and refers to solid solution Al present in steel in a solid solution state.

• • P: 0.100% or less

P is an element that is generally contained as an impurity, and has an action of increasing the strength of the hot-rolled steel sheet by solid solution strengthening. Therefore, P may be positively contained. However, P is an element that is easily segregated, and, when the P content exceeds 0.100%, the deterioration of ductility attributed to boundary segregation becomes significant. Therefore, the P content is set to 0.100% or less. The P content is preferably 0.030% or less. The lower limit of the P content does not need to be particularly specified, but may be 0%. The P content is preferably set to 0.001% from the viewpoint of the refining cost.

• • S: 0.0300% or less

S is an element that is contained as an impurity and forms a sulfide-based inclusion in steel to degrade the ductility of the hot-rolled steel sheet. When the S content is more than 0.0300%, the ductility of the hot-rolled steel sheet significantly deteriorates. Therefore, the S content is set to 0.0300% or less. The S content is preferably 0.0050% or less. The lower limit of the S content does not need to be particularly specified, but may be 0%. The S content is preferably set to 0.0001% from the viewpoint of the refining cost.

• • N: 0.1000% or less

N is an element that is contained in steel as an impurity and has an action of degrading the ductility of the hot-rolled steel sheet. When the N content is more than 0.1000%, the ductility of the hot-rolled steel sheet significantly deteriorates. Therefore, the N content is set to 0.1000% or less. The N content is preferably 0.0800% or less, more preferably 0.0700% or less, and still more preferably 0.0100% or less or 0.0050% or less. Although the lower limit of the N content does not need to be particularly specified, but may be 0%. In a case where one or two or more of Ti, Nb, and V are contained to further refine the microstructure, the N content is preferably set to 0.0010% or more and more preferably set to 0.0020% or more to promote the precipitation of a carbonitride.

• • O: 0.0100% or less

When a large amount of O is contained in steel, O forms a coarse oxide that becomes the starting point of fracture and causes brittle fracture and hydrogen-induced cracks. Therefore, the O content is set to 0.0100% or less. The O content is preferably 0.0080% or less and more preferably 0.0055% or less or 0.0050% or less. The O content may be set to 0.0005% or more or 0.0010% or more to disperse a large number of fine oxides when molten steel is deoxidized.

The remainder of the chemical composition of the hot-rolled steel sheet according to the present embodiment may be Fe and an impurity. In the present embodiment, the impurities mean substances that are incorporated from ore as a raw material, a scrap, manufacturing environment, or the like and/or substances that are permitted to an extent that the hot-rolled steel sheet according to the present embodiment is not adversely affected.

Instead of a part of Fe, the hot-rolled steel sheet according to the present embodiment may contain the following elements as optional elements. In a case where the optional elements are not contained, the lower limit of the content thereof is 0%. Hereinafter, the optional elements will be described in detail.

• • Cu: 0.01% to 2.00%
  • Cr: 0.01% to 2.00%
  • Mo: 0.01% to 1.00%
  • Ni: 0.02% to 2.00%
  • B: 0.0001% to 0.0100%

All of Cu, Cr, Mo, Ni, and B have an action of enhancing the hardenability of the hot-rolled steel sheet. In addition, Cu and Mo have an action of being precipitated as a carbide in steel to increase the strength of the hot-rolled steel sheet. Furthermore, in a case where Cu is contained, Ni has an action of effectively suppressing the grain boundary cracking of a slab caused by Cu. Therefore, one or two or more of these elements may be contained.

As described above, Cu has an action of enhancing the hardenability of the hot-rolled steel sheet and an action of being precipitated as a carbide in steel at a low temperature to increase the strength of the hot-rolled steel sheet. In order to more reliably obtain the effect by the action, the Cu content is preferably set to 0.01% or more and more preferably set to 0.05% or more. However, when the Cu content is more than 2.00%, grain boundary cracking may occur in the slab in some cases. Therefore, the Cu content is set to 2.00% or less. The Cu content is preferably 1.50% or less and more preferably 1.00% or less.

As described above, Cr has an action of enhancing the hardenability of the hot-rolled steel sheet. In order to more reliably obtain the effect by the action, the Cr content is preferably set to 0.01% or more and more preferably set to 0.05% or more. However, when the Cr content is more than 2.00%, the chemical convertibility of the hot-rolled steel sheet significantly deteriorates. Therefore, the Cr content is set to 2.00% or less.

As described above, Mo has an action of enhancing the hardenability of the hot-rolled steel sheet and an action of being precipitated as a carbide in steel to increase the strength of the hot-rolled steel sheet. In order to more reliably obtain the effect by the action, the Mo content is preferably set to 0.01% or more and more preferably set to 0.02% or more. However, even when the Mo content is set to more than 1.00%, the effect by the action is saturated, which is not economically preferable. Therefore, the Mo content is set to 1.00% or less. The Mo content is preferably 0.50% or less and more preferably 0.20% or less.

As described above, Ni has an action of enhancing the hardenability of the hot-rolled steel sheet. In addition, in a case where Cu is contained, Ni has an action of effectively suppressing the grain boundary cracking of the slab caused by Cu. In order to more reliably obtain the effect by the action, the Ni content is preferably set to 0.02% or more. Since Ni is an expensive element, it is not economically preferable to contain a large amount of Ni. Therefore, the Ni content is set to 2.00% or less.

As described above, B has an action of enhancing the hardenability of the hot-rolled steel sheet. In order to more reliably obtain the effect by this action, the B content is preferably set to 0.0001% or more and more preferably set to 0.0002% or more. However, when the B content is more than 0.0100%, the formability of the hot-rolled steel sheet significantly deteriorates, and thus the B content is set to 0.0100% or less. The B content is preferably 0.0050% or less.

• • Ca: 0.0005% to 0.0200%
  • Mg: 0.0005% to 0.0200%
  • REM: 0.0005% to 0.1000%
  • Bi: 0.0005% to 0.020%

All of Ca, Mg, and REM have an action of enhancing the ductility of the hot-rolled steel sheet by adjusting the shape of inclusions in steel to a preferable shape. In addition, Bi has an action of enhancing the ductility of the hot-rolled steel sheet by refining the solidification structure. Therefore, one or two or more of these elements may be contained. In order to more reliably obtain the effect by the action, it is preferable that any one or more of Ca, Mg, REM, and Bi are set to 0.0005% or more. However, when the Ca content or Mg content is more than 0.0200% or when the REM content is more than 0.1000%, an inclusion is excessively formed in steel, and thus the ductility of the hot-rolled steel sheet may be conversely degraded in some cases. In addition, even when the Bi content is set to more than 0.020%, the above effect by the action is saturated, which is not economically preferable. Therefore, the Ca content and the Mg content are set to 0.0200% or less, the REM content is set to 0.1000% or less, and the Bi content is set to 0.020% or less. The Bi content is preferably 0.010% or less.

Here, REM refers to a total of 17 elements consisting of Sc, Y, and lanthanoids, and the REM content refers to the total amount of these elements. In the case of the lanthanoids, the lanthanoids are industrially added in the form of misch metal.

One or two or more of Zr, Co, Zn, or W: 0% to 1.00% in total

• • Sn: 0% to 0.05%

Regarding Zr, Co, Zn, and W, the present inventors have confirmed that, even when a total of 1.00% or less of these elements are contained, the effect of the hot-rolled steel sheet according to the present embodiment is not impaired. Therefore, one or two or more of Zr, Co, Zn, or W may be contained in a total of 1.00% or less.

In addition, the present inventors have confirmed that, even when a small amount of Sn is contained, the effect of the hot-rolled steel sheet according to the present embodiment is not impaired. However, when a large amount of Sn is contained, a defect may be generated during hot rolling, and thus the Sn content is set to 0.05% or less.

The chemical composition of the above hot-rolled steel sheet may be measured by a general analytical method. For example, inductively coupled plasma-atomic emission spectrometry (ICP-AES) may be used for measurement. sol. Al may be measured by the ICP-AES using a filtrate after a sample is decomposed with an acid by heating. C and S may be measured by using a combustion-infrared absorption method, N may be measured by using the inert gas melting-thermal conductivity method, and O may be measured using an inert gas melting-non-dispersive infrared absorption method.

Microstructure of Hot-Rolled Steel Sheet

Next, the microstructure of the hot-rolled steel sheet according to the present embodiment will be described.

In the microstructure of the hot-rolled steel sheet according to the present embodiment, in terms of area %, residual austenite is less than 3.0%, ferrite is 15.0% or more and less than 60.0%, and pearlite is less than 5.0%, an average sphere equivalent radius of alloy carbides in the ferrite is 0.5 nm or more and less than 5.0 nm, an average number density of the alloy carbides in the ferrite is 3.5×10¹⁶/cm³ or more, the E value that indicates the periodicity of the microstructure is 10.7 or more, the I value that indicates the uniformity of the microstructure is 1.020 or more, and the standard deviation of the Mn concentration is 0.60 mass % or less.

Since the hot-rolled steel sheet according to the present embodiment has the above microstructure, high strength and excellent ductility, fatigue property and shearing property can be obtained.

In the present embodiment, the microstructural ratios, the average sphere equivalent radius and the average number density of the alloy carbides, the E value, the I value, and the standard deviation of the Mn concentration in the microstructure at a depth of ¼ of the sheet thickness from the surface (a region between a depth of ¼ of the sheet thickness from the surface and a depth of ¾ of the sheet thickness from the surface) and the center position in the sheet width direction in a cross section parallel to the rolling direction are specified. The reason therefor is that the microstructure at this position indicates a typical microstructure of the steel sheet.

• • Area Ratio of Residual Austenite: Less than 3.0%

Residual austenite is a microstructure that is present as a face-centered cubic lattice even at room temperature. Residual austenite has an action of enhancing the ductility of the hot-rolled steel sheet by transformation-induced plasticity (TRIP). On the other hand, residual austenite transforms into high-carbon martensite during shearing working, which inhibits the stable occurrence of cracking and causes the formation of a secondary sheared surface. When the area ratio of the residual austenite is 3.0% or more, the action is actualized, and the shearing property of the hot-rolled steel sheet deteriorates. Therefore, the area ratio of the residual austenite is set to less than 3.0%. The area ratio of the residual austenite is preferably less than 1.5% and more preferably less than 1.0%. Since residual austenite is preferably as little as possible, the area ratio of the residual austenite may be 0%.

As the measurement method of the area ratio of the residual austenite, there are methods by X-ray diffraction, EBSP (electron back scattering diffraction pattern) analysis, and magnetic measurement and the like. In the present embodiment, the area ratio of the residual austenite is measured by X-ray diffraction.

In the measurement of the area ratio of the residual austenite by X-ray diffraction in the present embodiment, first, the integrated intensities of a total of 6 peaks of α(110), α(200), α(211), γ(111), γ(200), and γ(220) are obtained in the cross section parallel to the rolling direction at a depth of ¼ of the sheet thickness (a region between a depth of ⅛ of the sheet thickness from the surface to a depth of ⅜ of the sheet thickness from the surface) and the center position in the sheet width direction of the hot-rolled steel sheet using Co-Kα rays, and the volume ratio of the residual austenite is obtained by calculation using the strength averaging method. The obtained volume ratio of the residual austenite is regarded as an area ratio of the residual austenite.

• • Area Ratio of Ferrite: 15.0% or more and less than 60.0%

Ferrite is a structure formed when fcc transforms into bcc at a relatively high temperature. Ferrite has a high work hardening rate and thus has an action of enhancing the strength-ductility balance of the hot-rolled steel sheet. In order to obtain the above action, the area ratio of the ferrite is set to 15.0% or more. The area ratio of the ferrite is preferably 20.0% or more, more preferably 25.0% or more, and still more preferably 30.0% or more.

Since ferrite has a low strength, when the area ratio is excessive, a desired tensile strength cannot be obtained. Therefore, the area ratio of the ferrite is set to less than 60.0%. The area ratio of the ferrite is preferably 50.0% or less and more preferably 45.0% or less.

• • Area Ratio of Pearlite: Less than 5.0%

Pearlite is a lamellar microstructure in which cementite is precipitated in layers between ferrite and is a soft microstructure as compared with bainite and martensite. When the area ratio of the pearlite is 5.0% or more, carbon is consumed by cementite that is contained in pearlite, and the strengths of martensite and bainite, which are the remainder in microstructure, decrease, and a desired strength cannot be obtained. Therefore, the area ratio of the pearlite is set to less than 5.0%. The area ratio of the pearlite is preferably 3.0% or less. In order to improve the stretch flangeability of the steel sheet, the area ratio of the pearlite is preferably reduced as much as possible, and the area ratio of the pearlite is more preferably 0%.

The steel sheet according to the present embodiment contains a full hard structure consisting of one or two or more of bainite, martensite, and tempered martensite in a total area ratio of 32.0% or more and less than 85.0% as the remainder in microstructure other than residual austenite, ferrite, and pearlite.

Measurement of the area ratios of the microstructure is conducted by the following method. A sheet thickness cross section parallel to the rolling direction is mirror-finished and, furthermore, polished at room temperature with colloidal silica not containing an alkaline solution for 8 minutes, thereby removing strain introduced into the surface layer of a sample. In a random position of the sample cross section in a longitudinal direction, a region with a length of 50 μm and at a ¼ depth position of the sheet thickness from the surface (a region between a depth of ⅛ of the sheet thickness from the surface to a depth of ⅜ of the sheet thickness from the surface) and the center position in the sheet width direction is measured by electron backscatter diffraction at a measurement interval of 0.1 μm to obtain crystal orientation information. For the measurement, an EBSD analyzer configured of a thermal field emission scanning electron microscope (JSM-7001F manufactured by JEOL) and an EBSD detector (DVC5 type detector manufactured by TSL) is used. At this time, the degree of vacuum inside the EBSD analyzer is set to 9.6×10⁻⁵ Pa or less, the acceleration voltage is set to 15 kV, the irradiation current level is set to 13, and the electron beam irradiation level is set to 62.

Furthermore, a reflected electron image is photographed at the same visual field. First, crystal grains where ferrite and cementite are precipitated in layers are specified from the reflected electron image, and the area ratio of the crystal grains is calculated, thereby obtaining the area ratio of pearlite. After that, for crystal grains except the crystal grains determined as pearlite, from the obtained crystal orientation information, regions where the grain average misorientation value is 1.0° or less are determined as ferrite using a “Grain Average Misorientation” function installed in software “OIM Analysis (registered trademark)” included in the EBSD analyzer. At this time, the Grain Tolerance Angle is set to 15°, the area ratio of the region determined as the ferrite is obtained, thereby obtaining the area ratio of the ferrite.

• • Average Sphere Equivalent Radius of Alloy Carbides in Ferrite: 0.5 nm or more and less than 5.0 nm

The hot-rolled steel sheet according to the present embodiment has excellent fatigue property since the average sphere equivalent radius and the average number density of the alloy carbides in the ferrite is preferably controlled. When the average sphere equivalent radius of alloy carbides in the ferrite is less than 0.5 mu, the strength against repeated deformation of ferrite cannot be increased and a desired fatigue property cannot be obtained. Therefore, the average sphere equivalent radius of alloy carbides in the ferrite is set to 0.5 nm or more. On the other hand, when the average sphere equivalent radius of alloy carbides in the ferrite is 5.0 nm or more, the strength of ferrite cannot be sufficiently increased, cracks from the cutting edge of the shearing tool occur very early in the shearing process and a fractured surface is formed due to the hardness difference between grains, and then a sheared surface is formed again. As a result, since secondary sheared surface are more likely to be formed, and a desired shearing property cannot be obtained in the hot-rolled steel sheet. Therefore, the average sphere equivalent radius of alloy carbides in the ferrite is set to less than 5.0 nm. The average sphere equivalent radius of alloy carbides in the ferrite is preferably 4.0 nm or less, 3.0 nm or less or 2.0 nm or less, and more preferably less than 1.5 nm.

• • Average Number Density of Alloy Carbides in Ferrite: 3.5×10¹⁶/cm³ or more

When the average number density of the alloy carbides in the ferrite is less than 3.5×10¹⁶/cm³, the strength against repeated deformation of ferrite cannot be increased and a desired fatigue property cannot be obtained. Therefore, the average number density of the alloy carbides in the ferrite is set to 3.5×10¹⁶/cm³ or more. The average number density of the alloy carbides in the ferrite is preferably 5.0×10¹⁶/cm³ or more, 10.0×10¹⁶/cm³ or more or 20.0×10¹⁶/cm³ or more.

Although the upper limit of the average number density of the alloy carbides in the ferrite is particularly limited, the more the better. However, in the chemical composition and the microstructure of hot-rolled steel sheet according to the present embodiment, it is difficult to set the average number density of the alloy carbides in the ferrite to more than 1.0×10¹⁹/cm³. Therefore, the average number density of the alloy carbides in the ferrite may set to 1.0×10¹⁹/cm³ or less.

In the present embodiment, the alloy carbides refer to carbides containing one or two or more of Ti, Nb, Mo, and V.

A sphere equivalent radius and a number density of alloy carbides in ferrite are measured by three-dimensional atom probe. In three-dimensional atom probe measurement, the laser wavelength (λ) is set to 355 nm, the laser power is set to 30 pJ, and the temperature of the needle-shaped test piece is set to 50K. The device used for three-dimensional atom probe measurement is not particularly limited. The three-dimensional atom probe measuring device is, for example, LEAP4000XHR manufactured by AMETEK Corporation.

From the ferrite grains within the observation field by the above-mentioned EBSD, in which the area ratio of each structure was measured, a sample is taken using an FIB (focused ion beam) device. By processing the taken sample into a needle shape using a well-known method and using a three-dimensional atom probe, the equivalent sphere radius and number density of fine precipitates ranging from less than I nm to several tens of nanometers in equivalent sphere radius can be accurately measured. The number density of precipitates can be obtained by dividing the number of precipitates included in the area measured with the three-dimensional atom probe by the volume of the measurement area at precipitates identified as alloy carbides by the method described below.

The total volume of precipitates in the measurement area is obtained by dividing the total number of atoms of alloying elements (Ti, Nb, Mo, V, C) contained in all the precipitates in the measurement area by the atomic density of the alloy carbide. The volume of precipitates is obtained by dividing the total volume of precipitates by the number of precipitates. From the obtained volume of precipitates, the spherical equivalent radius is calculated assuming that the precipitate is spherical.

The average number density and the average sphere equivalent radius are obtained by performing the above-described method on five or more of measurement data having a measurement area volume of 30000 nm³ or more. The region where Ga introduced during FIB processing is less than 0.025 at % is defined as the observation region, and the region where Ga is mixed in at 0.025 at % or more is excluded from the measurement area. To confirm the amount of Ga, the amount of Ga in the longitudinal direction of the needle sample can be confirmed using the ID Concentration Profile function of the data analysis software IVAS 3.6.14 (manufactured by CAMECA Instruments Inc.).

Whether or not the observed precipitate is an alloy carbide is determined by using the data acquired by the three-dimensional atom probe with the Cluster Analysis function of the analysis software IVAS 3.6.14. For the analysis, dmax=1.2 nm, Order=10, Nmin=10, L=0.5 nm, d erosion=0.5 nm are used as analysis parameters, and the precipitates recognized as clusters are determined as alloy carbides.

• • E Value: 10.7 or more
  • I Value: 1.020 or more

In order to suppress the generation of a secondary sheared surface, it is important to form a fractured surface after a sheared surface is sufficiently formed, and there is a need to suppress the early occurrence of cracking from the cutting edge of the tool during shearing working. In order for that, it is important that the periodicity of the microstructure is low and the uniformity of the microstructure is high. In the present embodiment, the generation of a secondary sheared surface is suppressed by controlling the E (Entropy) value that indicates the periodicity of the microstructure and the I (inverse difference normalized) value that indicates the uniformity of the microstructure.

The E value represents the periodicity of the microstructure. In a case where the brightness is periodically arranged due to an influence of the formation of a band-like structure or the like, that is, the periodicity of the microstructure is high, the E value decreases. In the present embodiment, since there is a need to make the microstructure poorly periodic, it is necessary to increase the E value. When the E value is less than 10.7, a secondary sheared surface is likely to be generated. From periodically arranged structures as starting points, cracking occurs from the cutting edge of a shearing tool in an extremely early stage of shearing working to form a fractured surface, and then a sheared surface is formed again. It is presumed that this makes it likely for a secondary sheared surface to be generated. Therefore, the E value is set to 10.7 or more. The E value is preferably 10.8 or more and more preferably 11.0 or more. The E value is preferably as high as possible, and the upper limit is not particularly specified and may be set to 13.0 or less, 12.5 or less, or 12.0 or less.

The I value represents the uniformity of the microstructure and increases as the area of a region having certain brightness increases. A high I value means that the uniformity of the microstructure is high. In the present embodiment, since there is a need to make the microstructure highly uniform, it is necessary to increase the I value. When the I value is less than 1.020, due to an influence of the hardness distribution attributed to precipitates in crystal grains and an element concentration difference, cracking occurs from the cutting edge of a shearing tool in an extremely early stage of shearing working to form a fractured surface, and then a sheared surface is formed again. It is presumed that this makes it likely for a secondary sheared surface to be generated. Therefore, the I value is set to 1.020 or more. The I value is preferably 1.025 or more and more preferably 1.030 or more. The I value is preferably as high as possible, and the upper limit is not particularly specified and may be set to 1.200 or less, 1.150 or less, or 1.100 or less.

The E value and the I value can be obtained by the following method.

In the present embodiment, the photographing region of a SEM image photographed for calculating the E value and the I value is a ¼ depth position of the sheet thickness from the surface of the steel sheet (a region between a depth of ⅛ of the sheet thickness from the surface and a depth of ⅜ of the sheet thickness from the surface) and the center position in the sheet width direction in a sheet thickness cross section parallel to the rolling direction. The SEM image is photographed using an SU-6600 Schottky electron gun manufactured by Hitachi High-Technologies Corporation with a tungsten emitter and an acceleration voltage of 1.5 kV. Based on the above settings, the SEM image is output at a magnification of 1000 times and a gray scale of 256 gradations.

Next, on an image obtained by cutting out the obtained SEM image into a 880×880-pixel region, a smoothing treatment described in Non-Patent Document 3, in which the contrast-enhanced limit magnification is set to 2.0 and the tile grid size is 8×8 is performed. The smoothed SEM image is rotated counterclockwise from 0 degrees to 179 degrees in increments of 1 degree, excluding 90 degrees, and an image is created at each angle, thereby obtaining a total of 179 images. Next, from each of these 179 images, the frequency values of brightness between adjacent pixels are sampled in a matrix form using the GLCM method described in Non-Patent Document 1.

179 matrixes of the frequency values sampled by the above method are expressed as p_k (k=0 . . . 89, 91, . . . 179) where k is a rotation angle from the original image. p_k's generated for individual images are summed for all k's (k=0 . . . 89, 91, . . . 179), and then 256×256 matrixes P standardized such that the total of individual components becomes 1 are calculated. Furthermore, the E value and the I value are each calculated using the following formula (1) and formula (2) described in Non-Patent Document 2. In the following formula (1) and formula (2), the value at the i^th row in the j^th column of the matrix P is expressed as P_ij.

$\begin{array}{cc}E=-{\Sigma }_{i=1,j=1}^{i=256,j=256}{P}_{ij}\log P_{\mathrm{ij}}& \left(1\right)\end{array}$ $\begin{array}{cc}I={\Sigma }_{i=1,j=1}^{i=256,j=256}{P}_{ij}/\left(1+\left|i-j\right|/256\right)& \left(2\right)\end{array}$

• • Standard Deviation of Mn Concentration: 0.60 mass % or Less

The standard deviation of the Mn concentration at the depth of ¼ of the sheet thickness from the surface (a region between a depth of ¼ of the sheet thickness from the surface and a depth of ¾ of the sheet thickness from the surface) of the hot-rolled steel sheet according to the present embodiment and the center position in the sheet width direction is 0.60 mass % or less. This makes it possible to uniformly disperse the hard phase and makes it possible to prevent the occurrence of cracking from the cutting edge of the shearing tool in an extremely early stage of shearing working. As a result, the generation of a secondary sheared surface can be suppressed. The standard deviation of the Mn concentration is preferably 0.50 mass % or less and more preferably 0.47 mass % or less. The value of the lower limit of the standard deviation of the Mn concentration is desirably as small as possible from the viewpoint of suppressing excessively large burrs, but the substantial lower limit is 0.10 mass % due to restrictions in the manufacturing process.

After a sheet thickness cross section parallel to the rolling direction of the hot-rolled steel sheet is mirror polished, and then a depth of ¼ of the sheet thickness from the surface (a region between a depth of ⅛ of the sheet thickness from the surface and a depth of ⅜ of the sheet thickness from the surface) and the center position in the sheet width direction is measured with an electron probe microanalyzer (EPMA) to measure the standard deviation of the Mn concentration. As the measurement conditions, the acceleration voltage is set to 15 kV, the magnification is set to 5000 times, and the distribution image of a range that is 20 μm long in the sample rolling direction and 20 μm long in the sample sheet thickness direction is measured. More specifically, the measurement interval is set to 0.1 μm, and the Mn concentrations at 40000 or more points are measured. Next, the standard deviation is calculated based on the Mn concentrations obtained from all of the measurement points, thereby obtaining the standard deviation of the Mn concentration.

Tensile Properties

Among the mechanical properties of the hot-rolled steel sheets, the tensile strength properties (tensile strength and total elongation) were evaluated according to JIS Z 2241: 2011. A test piece is a No. 5 test piece of JIS Z 2241: 2011. The sampling position of the tensile test piece may be a ¼ portion from the end portion in the sheet width direction, and a direction perpendicular to the rolling direction may be the longitudinal direction.

In the hot-rolled steel sheet according to the present embodiment, the tensile (maximum) strength is 980 MPa or more. The tensile strength is preferably 1000 MPa or more. By setting the tensile strength to less than 980 MPa, it is possible to significantly contribute to vehicle body weight reduction without limiting applicable components. The upper limit does not need to be particularly limited and may be set to 1780 MPa from the viewpoint of suppressing the wearing of a die.

The total elongation is preferably set to 10.0% or more, and the product of the tensile strength and the total elongation (TS×El) is preferably set to 13000 MPa-% or more. The total elongation is more preferably set to 11.0% or more and still more preferably set to 13.0% or more. In addition, the product of the tensile strength and the total elongation is more preferably set to 14000 MPa-% or more and still more preferably 15000 MPa-% or more.

By setting the total elongation to 10.0% or more and the product of the tensile strength and the total elongation to 13000 MPa-% or more, it is possible to significantly contribute to vehicle body weight reduction without limiting applicable components.

Fatigue Property

If softening occurs during repeated deformation, fatigue life may be significantly reduced. Therefore, it is preferable that no softening occurs during repeated deformation. Whether or not softening occurs during repeated deformation can be determined by the following method.

A test piece is taken from a position of ¼ in the width direction of the hot-rolled steel sheet in accordance with JIS Z 2275-1978 so that the longitudinal direction is perpendicular to the rolling direction (C direction). Using this test piece, a plane bending fatigue test is performed in accordance with JIS Z 2275-1978. The plane bending fatigue test is performed with the repeated stress so that the fracture repeated counts becomes 1×10⁵ or more and less than 3×10⁵ times, the repeated stress so that the facture repeated counts becomes 3×10⁵ or more and less than 3×10⁶ times, and the repeated stress so that the facture repeated counts becomes 3×10⁶ or more and less than 1×10⁷ times. The torque during the fatigue test or the value of the strain gauge attached to the test piece is measured to evaluate the change of the repeated stress. The repetition hardening rate is determined in the plane bending fatigue test at each repeated stress. Repetition hardening rate is defined as (minimum value of repeated stress in more than 100 times of repeated counts/repeated stress in 100 times of repeated counts). When the minimum value of the repetition hardening rate under each repeated stress is 1.00 or more, the hot-rolled steel sheet can be determined that repeated softening does not occur and the hot-rolled steel sheet has excellent fatigue property.

Sheet Thickness

The sheet thickness of the hot-rolled steel sheet according to the present embodiment is not particularly limited and may be set to 0.5 to 8.0 mm. When the sheet thickness of the hot-rolled steel sheet is less than 0.5 mm, it may become difficult to secure the rolling finishing temperature and the rolling force may become excessive, which makes hot rolling difficult. Therefore, the sheet thickness of the hot-rolled steel sheet according to the present embodiment may be set to 0.5 mm or more. The sheet thickness is preferably 1.2 mm or more or 1.4 mm or more. On the other hand, when the sheet thickness is more than 8.0 mm, it becomes difficult to refine the microstructure, and it may be difficult to obtain the above microstructure. Therefore, the sheet thickness may be set to 8.0 mm or less. The sheet thickness is preferably 6.0 mm or less.

Plating Layer

The hot-rolled steel sheet according to the present embodiment having the above-described chemical composition and microstructure may be provided with a plating layer on the surface for the purpose of improving corrosion resistance and the like and thereby made into a surface-treated steel sheet. The plating layer may be an electro plating layer or a hot-dip plating layer. Examples of the electro plating layer include electrogalvanizing, electro Zn—Ni alloy plating, and the like. Examples of the hot-dip plating layer include hot-dip galvanizing, hot-dip galvannealing, hot-dip aluminum plating, hot-dip Zn—Al alloy plating, hot-dip Zn—Al—Mg alloy plating, hot-dip Zn—Al—Mg—Si alloy plating, and the like. The plating adhesion amount is not particularly limited and may be the same as before. In addition, it is also possible to further enhance the corrosion resistance by performing an appropriate chemical conversion treatment (for example, the application and drying of a silicate-based chromium-free chemical conversion treatment liquid) after plating.

Manufacturing Conditions

A suitable method for manufacturing the hot-rolled steel sheet according to the present embodiment having the above-described chemical composition and microstructure is as follows.

In order to obtain the hot-rolled steel sheet according to the present embodiment, it is effective to perform hot rolling after heating a slab under predetermined conditions, perform accelerated cooling to a predetermined temperature range, then, slowly cool the slab, and control the cooling history until coiling.

In the suitable method for manufacturing the hot-rolled steel sheet according to the present embodiment, the following steps (1) to (10) are sequentially performed. The temperature of the slab and the temperature of the steel sheet in the present embodiment refer to the surface temperature of the slab and the surface temperature of the steel sheet. In addition, stress refers to stress that is loaded in the rolling direction of the steel sheet.

• • (1) The slab is held in a temperature range of 700° C. to 850° C. for 900 seconds or longer, then, further heated, and held in a temperature range of 1100° C. or higher for 6000 seconds or longer.
  • (2) Hot rolling is performed in a temperature range of 850° C. to 1100° C. so that the sheet thickness is reduced by a total of 90% or more.
  • (3) Rolling of at one stand before a final stand is performed at a temperature range of 900° C. or higher and lower than 1010° C., stress of 170 kPa or more is loaded to the steel sheet after rolling at the one stand before the final stand of the hot rolling and before rolling of the final stand.
  • (4) The rolling reduction ratio at the final stand of the hot rolling is set to 8% or more, and the hot rolling is finished so that a finishing temperature Tf is 900° C. or higher and lower than 1010° C.
  • (5) Light rolling is performed so that the sheet thickness reduction is 5% or more and less than 8% in total at a temperature range of 840° C. or higher and lower than 900° C.
  • (6) Stress that is loaded to the steel sheet after the rolling of the final stand of the hot rolling and before a first rolling of the light rolling, and stress that is loaded to the steel sheet after a rolling of a final stand of the light rolling and until the steel sheet is cooled to 800° C., is set to less than 200 kPa.
  • (7) After the light rolling, the steel sheet is rapidly cooled to a temperature range of 680° C. to 730° C. at an average cooling rate of 50° C./s or faster.
  • (8) Slow cooling at an average cooling rate of slower than 5° C./s is performed in a temperature range of 680° C. to 730° C. for 2.0 seconds or longer.
  • (9) The steel sheet is cooled to a temperature range of 350° C. or lower at an average cooling rate of 50° C./s or faster.
  • (10) The steel sheet is coiled in a temperature range of 350° C. or lower.

A hot-rolled steel sheet with high strength, excellent ductility, fatigue property and shearing property can be stably manufactured by adopting the above manufacturing method. That is, when the slab heating conditions and the hot rolling conditions are appropriately controlled, the reduction of Mn segregation and equiaxed austenite before transformation are achieved, and, in cooperation with the cooling conditions after the hot rolling to be described below, a hot-rolled steel sheet having a desired microstructure can be stably manufactured.

(1) Slab, Slab Temperature and Retention Time on Hot Rolling

As the slab that is subjected to hot rolling, a slab obtained by continuous casting, a slab obtained by casting and blooming, or the like can be used, and, if necessary, it is possible to use the above slabs after hot working or cold working. The slab that is subjected to hot rolling is preferably held in a temperature range of 700° C. to 850° C. for 900 seconds or longer during slab heating, then, further heated, and held in a temperature range of 1100° C. or higher for 6000 seconds or longer. The upper limit of the heating temperature during slab heating is particularly limited, but may be 1350° C. or lower from the view point of thermal efficiency.

During retention in the temperature range of 700° C. to 850° C., the steel sheet temperature may be fluctuated or be maintained constant in this temperature range. In addition, during retention at 1100° C. or higher, the steel sheet temperature may be fluctuated or be maintained constant in the temperature range of 1100° C. or higher.

In austenite transformation in the temperature range of 700° C. to 850° C., Mn is distributed between ferrite and austenite, and Mn can be diffused into the ferrite region by extending the transformation time. Accordingly, the Mn microsegregation unevenly distributed in the slab can be eliminated, and the standard deviation of the Mn concentration can be significantly reduced. Therefore, it is preferable to retain the slab in the temperature range of 700° C. to 850° C. for 900 seconds or longer. In addition, by retention the slab in the temperature range of 1100° C. or higher for 6000 seconds or longer, the Mn concentration can be significantly reduced.

In the hot rolling, it is preferable to use a reverse mill or a tandem mill for multi-pass rolling. Particularly, from the viewpoint of industrial productivity and the viewpoint of stress loading on the steel sheet during the rolling, at least the final two stands are more preferably hot rolling in which a tandem mill is used.

(2) Rolling Reduction Ratio of Hot Rolling: Sheet Thickness Reduction of 90% or More in Total in Temperature Range of 850° C. to 1100° C.

When the hot rolling is performed so that the sheet thickness reduction is 90% or more in total in a temperature range of 850° C. to 1100° C., mainly recrystallized austenite grains are refined, and accumulation of strain energy into the unrecrystallized austenite grains is promoted. In addition, the recrystallization of austenite is promoted, and the atomic diffusion of Mn is promoted, which makes it possible to reduce the standard deviation of the Mn concentration. Therefore, it is preferable to perform the hot rolling so that the sheet thickness reduction is 90% or more in total in the temperature range of 850° C. to 1100° C.

The sheet thickness reduction in total in the temperature range of 850° C. to 1100° C. can be expressed as {(t₀−t₁)/t₀}×100(%) where an inlet sheet thickness before the rolling of the first rolling in this temperature range is to and an outlet sheet thickness after the rolling of the final stand in this temperature range is t₁.

(3) Rolling Temperature at One Stand Before Final Stand: 900° C. or Higher and Lower than 1010° C., Stress after Rolling of Rolling at One Stand Before Final Stand of Hot Rolling and Before Rolling at Final Stand: 170 kPa or More

Rolling at the one stand before the final stand is preferably performed at a temperature range of 900° C. or higher and lower than 1010° C., and the stress that is loaded to the steel sheet after the rolling at the one stand before the final stand of hot rolling and before the rolling at the final stand is preferably set to 170 kPa or more. These make it possible to reduce the number of crystal grains having a {110}<001> crystal orientation in the recrystallized austenite after the rolling at the one stand before the final stand. Since {110}<001> is a crystal orientation that is difficult to recrystallize, recrystallization by the rolling of the final stand can be effectively promoted by suppressing the formation of this crystal orientation. As a result, the band-like structure of the hot-rolled steel sheet is improved, the periodicity of the microstructure is reduced, and the E value increases. When the stress that is loaded to the steel sheet is less than 170 kPa, it may be impossible to obtain a desired E value. The stress that is loaded to the steel sheet is more preferably 190 kPa or more. The upper limit of the stress that is loaded to the steel sheet is not particularly limited, but can be 350 kPa or lower. The stress that is loaded to the steel sheet refers tension of a rolling direction, and can be controlled by adjusting the roll rotation speed during tandem rolling.

(4) Rolling Reduction Ratio at Final Stand of Hot Rolling: 8% or More, Finishing Temperature Tf: 900° C. or Higher and Lower than 1010° C.

It is preferable that the rolling reduction ratio at the final stand of the hot rolling is set to 8% or more and the finishing temperature Tf is set to 900° C. or higher. When the rolling reduction ratio at the final stand of the hot rolling is set to 8% or more, it is possible to promote recrystallization caused by the final stand rolling. As a result, the band-like structure of the hot-rolled steel sheet is improved, the periodicity of the microstructure is reduced, and the E value increases. When the finishing temperature Tf is set to 900° C. or higher, it is possible to suppress an excessive increase in the number of ferrite nucleation sites in austenite. As a result, the formation of ferrite in the final structure (the microstructure of the hot-rolled steel sheet after manufacturing) is suppressed, and a desired strength of the hot-rolled steel sheet can be obtained. The upper limit of the rolling reduction ratio at the final stand of the hot rolling is not particularly limited, but can be 40% or lower. In addition, when Tf is set to lower than 1010° C., it is possible to suppress the coarsening of the austenite grain sizes and to obtain a desired E value by reducing the periodicity of the microstructure.

(5) Light Rolling is Performed so that the Sheet Thickness Reduction is 5% or More and Less than 8% in Total at a Temperature Range of 840° C. or Higher and Lower than 900° C.

After the rolling of the final stand of the hot rolling, it is preferable to perform the light rolling so that the sheet thickness reduction is 5% or more and less than 8% in total at a temperature range of 840° C. or higher and lower than 900° C. This can make it possible to control the average sphere equivalent radius and the average number density of the alloy carbides in the ferrite in desired amounts.

Light rolling may be performed, for example, at the final stand of the final stand of the finishing mill, or by introducing new rolling equipment between the finishing mill and the cooling bed. Light rolling may be performed in multiple stands using multiple rolls.

The sheet thickness reduction in total in light rolling can be expressed as {(t₀−t₁)/t₀}×100(%) where an inlet sheet thickness before the first rolling in the light rolling is to and an outlet sheet thickness after the rolling of the final stand in the light rolling.

(6) Stress Loaded to Steel Sheet after Rolling at Final Stand of Hot Rolling and Before First Rolling of Light Rolling, and Stress Loaded to Steel Sheet after Rolling of Final Stand of Light Rolling Until Steel Sheet being Cooled to 800° C.: Less than 200 KPa

Stress that is loaded to the steel sheet after the rolling at the final stand of the hot rolling and before the first rolling of the light rolling, and stress that is loaded to the steel sheet after the rolling of the final stand of the light rolling and until the steel sheet is cooled to 800° C. is preferably set to less than 200 kPa respectively. When the stresses that are loaded to the steel sheet at the above positions are set to less than 200 kPa, the recrystallization of austenite preferentially proceeds in the rolling direction, and an increase in the periodicity of the microstructure can be suppressed. As a result, a desired E value can be obtained. The stresses that are loaded to the steel sheet at the above positions are more preferably 180 kPa or less respectively.

(7) After Light Rolling, Accelerated Cooling to Temperature Range of 680° C. to 730° C. at Average Cooling Rate of 50° C./See or Faster

In order to suppress the growth of austenite grain refined by the hot rolling, accelerated cooling is preferably performed to a temperature range of 730° C. or lower at the average cooling rate of 50° C./sec or faster after the light rolling. By performing the accelerated cooling to the temperature range of 730° C. or lower, it is possible to suppress the formation of ferrite or pearlite with a small amount of precipitation hardening. Accordingly, the strength of the hot-rolled steel sheet improves.

The average cooling rate referred to herein is a value obtained by dividing the temperature drop width of the steel sheet from the start of accelerated cooling (when introducing the steel sheet into cooling equipment) to the completion of accelerated cooling (when deriving the steel sheet from the cooling equipment) by the time required from the start of accelerated cooling to the completion of accelerated cooling.

The upper limit of the cooling rate is not particularly specified, but when the cooling rate is increased, the cooling equipment becomes large and the equipment cost increases. Therefore, considering the equipment cost, the average cooling rate is preferably 300° C./sec or slower. In addition, in order to obtain a desired amount of ferrite, the cooling stop temperature of the accelerated cooling is preferably set to 680° C. or higher.

In order to achieve the average cooling rate such as described above, cooling with a high average cooling rate may be performed after completion of the light rolling, for example, by injecting cooling water onto the surface of the steel sheet.

(8) Slow Cooling at Average Cooling Rate of Slower than 5° C./s being Performed in Temperature Range of 680° C. to 730° C. for 2.0 Seconds or Longer

When slow cooling at an average cooling rate of slower than 5° C./s is performed in a temperature range of 680° C. to 730° C. for 2.0 seconds or longer, it is possible to sufficiently precipitate the precipitation-hardened ferrite. This makes it possible to achieve both strength and ductility of the hot-rolled steel sheet. The average cooling rate referred to herein refers to a value obtained by dividing the temperature drop width of the steel sheet from the cooling stop temperature of the accelerated cooling to the stop temperature of the slow cooling by the time required from the stop of the accelerated cooling to the stop of the slow cooling.

The slow cooling time is preferably 3.0 seconds or longer. The upper limit of the slow cooling time is determined by the equipment layout and may be set to approximately shorter than 10.0 seconds. In addition, the lower limit of the average cooling rate of the slow cooling is not particularly provided and may be set to 0° C./s or faster since heating the steel sheet without cooling accompanies a huge equipment investment.

(9) Average Cooling Rate to Coiling Temperature: 50° C./Sec or Faster

In order to suppress the area ratio of the pearlite and obtain a desired strength, the average cooling rate from the cooling stop temperature of the slow cooling to the coiling temperature is preferably set to 50° C./sec or faster. In such a case, the primary phase structure can be made full hard. The average cooling rate referred to herein refers to a value obtained by dividing the temperature drop width of the steel sheet from the cooling stop temperature of the slow cooling where the average cooling rate is slower than 5° C./s to the coiling temperature by the time required from the stop of the slow cooling where the average cooling rate is slower than 5° C./s to coiling.

(10) Coiling Temperature: 350° C. or Lower

The coiling temperature is preferably set to 350° C. or lower. When the coiling temperature is set to 350° C. or lower, the amount of an iron carbide precipitated is reduced, and the variation in the hardness distribution in the hard phase can be reduced. As a result, it is possible to obtain a desired I value. The lower limit of the coiling temperature is not particularly limited, but can be a room temperature.

EXAMPLES

Next, the effects of one aspect of the present invention will be described more specifically by way of examples, but the conditions in the examples are condition examples adopted for confirming the feasibility and effects of the present invention. The present invention is not limited to these condition examples. The present invention can adopt various conditions as long as the object of the present invention is achieved without departing from the gist of the present invention.

Steels having a chemical composition shown in Tables 1 and 2 were melted and continuously cast to manufacture slabs having a thickness of 240 to 300 mm. The obtained slabs were used to obtain hot-rolled steel sheets shown in Table 5-1 to Table 6-2 under the manufacturing conditions shown in Table 3-1 to Table 4-2.

The average cooling rate of slow cooling was set to slower than 5° C./s. In addition, since the measurement lower limit of the coiling temperature shown in Table 4-1 and Table 4-2 is 50° C., the actual coiling temperatures of the examples with a value of 50° C. are 50° C. or lower. Except the manufacturing No. 7, the rolling at the one stand before the final stand of hot rolling was performed at a temperature of 900° C. or higher and lower than 1010° C.

The Hot-rolled steel sheet of manufacturing No. 46 was manufactured under the conditions without the above-mentioned light rolling. In manufacturing No. 46, the stress that is loaded after the rolling of the final stand of the hot rolling and until the steel sheet was cooled to 800° C. was set to 170 kPa.

The area ratio of the microstructure, the E value, the I value, the standard deviation of the Mn concentration, the average sphere equivalent radius and the average number density of the alloy carbides in the ferrite, the tensile strength TS, and the total elongation El of each the obtained hot-rolled steel sheets were obtained by the above methods. In addition, by the above methods, the fatigue property was evaluated by performing the plane bending fatigue test. The obtained measurement results are shown in Table 5-1 to Table 6-2.

Based on the chemical composition and the manufacturing method of the present steel, the remainder in microstructure was determined to be a hard structure of one or two or more of bainite, martensite, and tempered martensite.

Evaluation Method of Properties of Hot-Rolled Steel Sheets

Tensile Properties

In a case where the tensile strength TS was 980 MPa or more, the total elongation El was 10.0% or more, and the tensile strength TS×total elongation El was 13000 MPa·% or more, the hot-rolled steel sheet was determined as having high strength and excellent ductility, and being successful. In a case where any one was not satisfied, the hot-rolled steel sheet was determined as not having high strength and excellent ductility, and not being successful.

Fatigue Property

By performing the plane bending fatigue test using the method described above, repetition hardening rate at each repeated stress was obtained. When the minimum values of the repetition hardening rates under each cyclic stress was 1.00 or more, the hot-rolled steel sheet was determined as not occurring repeated softening, having excellent fatigue property and being successful. On the other hand, when the minimum values of the repetition hardening rates under each cyclic stress was less than 1.00, the hot-rolled steel sheet was determined as not having excellent fatigue property and not being successful.

Shearing Property (Evaluation of Secondary Sheared Surface)

The shearing property of the hot-rolled steel sheet was evaluated by a punching test. Three punched holes were produced in each example with a hole diameter of 10 mm, a clearance of 10%, and a punching speed of 3 m/s. Next, a sheet thickness cross section perpendicular to the rolling direction and a sheet thickness cross section parallel to the rolling direction of the punched hole were each embedded in a resin, and the cross-sectional profile was photographed with a scanning electron microscope. In the obtained observation photographs, the sheared end surfaces as shown in FIG. 1 or FIG. 2 can be observed. FIG. 1 is an example of a sheared end surface of a hot-rolled steel sheet according to the present invention example, and FIG. 2 is an example of a sheared end surface of a hot-rolled steel sheet according to a comparative example. In FIG. 1, the sheared end surface is a sheared end surface with a shear droop, a sheared surface, a fractured surface, and a burr. On the other hand, in FIG. 2, the sheared end surface is a sheared end surface with a shear droop, a sheared surface, a fractured surface, a sheared surface, a fractured surface, and a burr. Here, the shear droop is an R-like smooth surface region, the sheared surface is the region of a punched end surface separated by shear deformation, the fractured surface is the region of a punched end surface separated by a crack initiated from the vicinity of the cutting edge, and a burr is a surface having projections protruding from the lower surface of the hot-rolled steel sheet.

In a case where, for example, a sheared surface, a fractured surface, and a sheared surface as shown in FIG. 2 appeared on two surfaces perpendicular to the rolling direction and two surfaces parallel to the rolling direction in the obtained sheared end surface, a secondary sheared surface was determined to be formed. 4 surfaces for each punched hole, that is, a total of 12 surfaces were observed, and, in a case where there was no surface on which a secondary sheared surface appeared, the hot-rolled steel sheet was determined as having excellent shearing property and being successful, and a value “Absent” was entered into Tables. On the other hand, in a case where even a single secondary sheared surface was formed, the hot-rolled steel sheet was determined as not having excellent shearing property and not being successful, and a value “Present” was entered into Tables.

TABLE 1
Steel	Mass %, remainder is Fe and impurities
No.	C	Si	Mn	Ti	Nb	V	Ti + Nb + V	sol. Al	P	S	N	O	Note
A	0.055	1.18	1.69	0.096			0.096	0.038	0.027	0.0016	0.0042	0.0036	Steel of Present Invention
B	0.087	1.14	1.75	0.105			0.105	0.034	0.009	0.0028	0.0022	0.0035	Steel of Present Invention
C	0.153	1.36	1.96	0.135			0.135	0.074	0.022	0.0049	0.0037	0.0025	Steel of Present Invention
D	0.112	0.32	1.56	0.085			0.085	0.026	0.011	0.0029	0.0030	0.0014	Steel of Present Invention
E	0.088	2.71	1.88	0.103			0.103	0.033	0.024	0.0023	0.0018	0.0020	Steel of Present Invention
F	0.092	0.86	1.20	0.097			0.097	0.047	0.002	0.0033	0.0024	0.0007	Steel of Present Invention
G	0.088	1.25	3.62	0.110	0.050		0.160	0.037	0.034	0.0017	0.0035	0.0051	Steel of Present Invention
H	0.095	0.75	1.73	0.060			0.060	0.035	0.021	0.0030	0.0020	0.0013	Steel of Present Invention
I	0.092	1.00	1.70			0.135	0.135	0.028	0.002	0.0048	0.0023	0.0030	Steel of Present Invention
J	0.091	1.15	1.81	0.043	0.030	0.072	0.145	0.033	0.018	0.0049	0.0036	0.0050	Steel of Present Invention
K	0.085	0.94	1.92	0.136			0.136	0.053	0.025	0.0044	0.0034	0.0051	Steel of Present Invention
L	0.077	0.83	1.72	0.114			0.114	0.053	0.027	0.0027	0.0038	0.0020	Steel of Present Invention
M	0.082	1.06	1.60	0.101		0.048	0.149	0.038	0.018	0.0017	0.0021	0.0030	Steel of Present Invention
N	0.074	1.00	1.51	0.092			0.092	0.035	0.011	0.0037	0.0043	0.0021	Steel of Present Invention
O	0.101	1.17	1.87		0.209		0.209	0.060	0.021	0.0032	0.0044	0.0044	Steel of Present Invention
P	0.077	1.53	2.17	0.121			0.121	0.318	0.020	0.0035	0.0059	0.0041	Steel of Present Invention
Q	0.062	1.76	2.28	0.085			0.085	0.378	0.017	0.0007	0.0015	0.0012	Steel of Present Invention
R	0.047	1.00	1.97	0.134			0.134	0.037	0.016	0.0047	0.0047	0.0027	Comparative Example
S	0.258	0.91	1.78	0.113			0.113	0.058	0.009	0.0035	0.0039	0.0050	Comparative Example
T	0.092	3.21	1.75	0.101			0.101	0.032	0.025	0.0025	0.0031	0.0035	Comparative Example
U	0.095	0.95	0.82	0.133			0.133	0.075	0.028	0.0014	0.0078	0.0020	Comparative Example
V	0.101	1.18	1.93	0.044	0.010		0.054	0.049	0.024	0.0062	0.0028	0.0054	Comparative Example
Underlines indicate that corresponding values are outside the range of the present invention.

TABLE 2

Steel

Mass %, remainder is Fe and impurities

No.

Cu

Cr

Mo

Ni

B

Ca

Mg

REM

Bi

Zr

Co

Zn

W

Sn

Note

A

0.0018

0.0018

Steel of Present Invention

B

Steel of Present Invention

C

0.0032

Steel of Present Invention

D

0.35

0.31

0.24

Steel of Present Invention

E

0.005

0.18

Steel of Present Invention

F

Steel of Present Invention

G

Steel of Present Invention

H

0.02

Steel of Present Invention

I

Steel of Present Invention

J

Steel of Present Invention

K

0.17

0.17

Steel of Present Invention

L

0.28

Steel of Present Invention

M

0.08

0.03

Steel of Present Invention

N

0.29

Steel of Present Invention

O

0.0021

0.02

Steel of Present Invention

P

Steel of Present Invention

Q

Steel of Present Invention

R

Comparative Example

S

Comparative Example

T

Comparative Example

U

Comparative Example

V

Comparative Example

TABLE 3-1
					Sheet	Loaded stress
		Retention		Retention	thickness	after rolling
		time in		time in	reduction in	at one stand
		temperature		temperature	temperature	before final		Rolling
		range		range	range	stand and before	Finishing	reduction
Manufac-		of 700° C.	Heating	of 1100° C.	of 850° C.	rolling at	temperature	ratio at
turing	Steel	to 850° C.	temperature	or higher	to 1100° C.	final stand	Tf	final stand
No.	No.	s	° C.	s	%	kPa	° C.	%	Note
1	A	1486	1261	9269	95	202	939	10	Present Invention Example
2	B	1481	1234	9181	96	217	969	11	Present Invention Example
3	B	855	1241	9025	96	232	936	10	Comparative Example
4	B	1126	1234	9261	87	204	935	10	Comparative Example
5	B	1217	1234	5580	92	198	949	8	Comparative Example
6	B	1467	1221	9640	94	165	980	10	Comparative Example
7	B	1515	1243	8893	94	216	1023	10	Comparative Example
8	B	1460	1215	8105	93	197	982	7	Comparative Example
9	B	1293	1218	9132	94	229	942	9	Comparative Example
10	B	1251	1242	9567	95	234	961	8	Present Invention Example
11	B	1350	1235	9030	96	238	971	8	Comparative Example
12	B	1392	1206	9406	91	195	923	10	Comparative Example
13	B	1114	1216	8954	94	206	972	10	Comparative Example
14	B	1506	1249	9345	96	230	931	9	Comparative Example
15	B	1320	1221	8901	92	194	931	9	Comparative Example
16	C	1268	1218	9033	94	213	948	8	Present Invention Example
17	D	1477	1236	9591	94	195	971	9	Present Invention Example
18	E	1466	1209	9123	90	218	942	10	Present Invention Example
19	F	1492	1254	9220	92	197	944	11	Present Invention Example
20	G	1440	1223	9376	98	224	1002	28	Present Invention Example
21	H	1595	1229	9592	98	183	969	11	Present Invention Example
22	I	1473	1213	9028	96	209	920	8	Present Invention Example
23	J	1607	1244	9608	97	215	971	9	Present Invention Example
Underlines indicate that corresponding values are not preferable manufacturing conditions.

TABLE 3-2
					Sheet	Loaded stress
		Retention		Retention	thickness	after rolling
		time in		time in	reduction in	at one stand
		temperature		temperature	temperature	before final		Rolling
		range		range	range	stand and before	Finishing	reduction
Manufac-		of 700° C.	Heating	of 1100° C.	of 850° C.	rolling of	temperature	ratio of
turing	Steel	to 850° C.	temperature	or higher	to 1100° C.	final stand	Tf	final stand
No.	No.	s	° C.	s	%	kPa	° C.	%	Note
24	K	1331	1230	9432	98	224	946	10	Present Invention Example
25	L	1464	1209	9319	94	207	926	11	Present Invention Example
26	M	1344	1230	8980	97	225	965	10	Present Invention Example
27	N	1241	1231	9622	98	221	947	10	Present Invention Example
28	O	1211	1251	9581	95	231	956	8	Present Invention Example
29	P	847	1229	9010	92	232	943	10	Comparative Example
30	P	2272	1218	9446	92	208	914	10	Present Invention Example
31	P	1462	1205	11880	90	196	937	11	Present Invention Example
32	P	1293	1218	5890	94	206	951	11	Comparative Example
33	P	1327	1238	9416	92	206	928	9	Present Invention Example
34	P	1515	1263	9139	98	240	985	10	Present Invention Example
35	P	1304	1229	8864	97	223	944	6	Comparative Example
36	P	1096	1227	9243	90	210	929	10	Comparative Example
37	P	1301	1267	8978	98	237	951	11	Comparative Example
38	P	968	1231	9248	96	218	951	8	Present Invention Example
39	P	1310	1233	6603	91	212	961	10	Present Invention Example
40	Q	1192	1225	9499	94	232	951	10	Present Invention Example
41	R	1455	1229	9182	91	212	960	10	Comparative Example
42	S	1166	1217	8812	93	200	921	8	Comparative Example
43	T	1612	1248	9178	96	237	941	9	Comparative Example
44	U	1298	1213	9313	93	210	971	10	Comparative Example
45	V	1459	1219	9583	95	220	953	9	Comparative Example
46	Q	1384	1253	8894	94	215	937	9	Comparative Example
47	Q	1425	1238	8741	95	207	896	9	Comparative Example
Underlines indicate that corresponding values are not preferable manufacturing conditions.

TABLE 4-1
		Sheet thickness
		reduction of light	Loaded stress after	Loaded stress after
		rolling at temperature	rolling at final stand	rolling at final stand	Average
		range of 840° C. or	of hot rolling and	of light rolling until	cooling rate
		higher and lower than	before first rolling of	steel sheet being	of accelerated
Manufacturing		900° C.	light rolling	cooled to 800° C.	cooling
No.	Steel No.	%	kPa	kPa	° C./s
1	A	6	96	47	112
2	B	6	100	152	97
3	B	6	81	50	52
4	B	6	117	182	56
5	B	6	102	137	88
6	B	6	119	180	64
7	B	6	90	114	72
8	B	6	63	94	63
9	B	4	80	92	92
10	B	6	88	78	93
11	B	6	126	184	152
12	B	6	202	92	54
13	B	6	108	201	76
14	B	6	118	178	79
15	B	9	95	133	85
16	C	6	82	51	53
17	D	6	96	85	88
18	E	6	107	127	124
19	F	6	89	53	91
20	G	6	113	75	142
21	H	6	117	148	101
22	I	6	103	82	147
23	J	6	102	18	95
			Slow cooling	Average
		Cooling stop	time in	cooling
		temperature of	temperature	rate until
		accelerated	range of 680° C.	coiling	Coiling
	Manufacturing	cooling	to 730°	temperature	temperature
	No.	° C.	s	° C./s	° C.	Note
	1	694	3.1	108	50	Present Invention Example
	2	710	3.8	114	50	Present Invention Example
	3	699	4.4	107	50	Comparative Example
	4	728	3.0	86	50	Comparative Example
	5	729	3.8	138	50	Comparative Example
	6	722	3.8	114	50	Comparative Example
	7	726	3.8	151	50	Comparative Example
	8	723	2.5	108	50	Comparative Example
	9	722	3.3	89	50	Comparative Example
	10	693	4.4	89	50	Present Invention Example
	11	702	1.5	111	50	Comparative Example
	12	694	3.9	111	50	Comparative Example
	13	724	4.4	85	50	Comparative Example
	14	715	4.4	23	50	Comparative Example
	15	724	2.7	99	50	Comparative Example
	16	698	3.0	149	50	Present Invention Example
	17	697	3.5	134	50	Present Invention Example
	18	705	3.1	96	50	Present Invention Example
	19	691	3.2	147	50	Present Invention Example
	20	698	4.6	93	50	Present Invention Example
	21	701	3.8	149	50	Present Invention Example
	22	697	2.5	141	50	Present Invention Example
	23	695	3.5	92	50	Present Invention Example
	Underlines indicate that corresponding values are not preferable manufacturing conditions.

TABLE 4-2
		Sheet thickness
		reduction of light	Loaded stress after	Loaded stress after
		rolling at temperature	rolling of final stand	rolling of final stand	Average
		range of 840° C. or	of hot rolling and	of light rolling and	cooling rate
		higher and lower than	before first rolling of	until steel sheet is	of accelerated
Manufacturing		900° C.	light rolling	cooled to 800° C.	cooling
No.	Steel No.	%	kPa	kPa	° C./s
24	K	6	112	103	144
25	L	6	112	49	133
26	M	6	110	161	150
27	N	6	128	180	85
28	O	6	121	177	157
29	P	6	92	26	51
30	P	6	112	143	152
31	P	6	106	98	72
32	P	6	125	104	53
33	P	6	101	150	93
34	P	6	53	150	148
35	P	6	97	88	56
36	P	6	109	85	42
37	P	6	115	45	150
38	P	6	113	47	105
39	P	6	127	57	133
40	Q	6	113	193	110
41	R	6	107	53	96
42	S	6	81	171	50
43	T	6	98	30	127
44	U	6	100	187	132
45	V	6	118	75	50
46	Q	—	—	—	98
47	Q	6	94	103	85
			Slow cooling	Average
		Cooling stop	time in	cooling
		temperature of	temperature	rate to
		accelerated	range of 680° C.	coiling	Coiling
		cooling	to 730°	temperature	temperature
	Manufacturing	° C.	s	° C./s	° C.	Note
	24	710	4.0	104	50	Present Invention Example
	25	691	3.6	91	50	Present Invention Example
	26	700	3.3	89	50	Present Invention Example
	27	709	2.3	142	50	Present Invention Example
	28	707	3.4	134	50	Present Invention Example
	29	700	3.3	108	50	Comparative Example
	30	711	3.3	130	200	Present Invention Example
	31	692	2.8	141	50	Present Invention Example
	32	721	4.3	107	50	Comparative Example
	33	718	3.4	105	50	Present Invention Example
	34	700	3.7	108	50	Present Invention Example
	35	725	4.7	140	50	Comparative Example
	36	726	4.0	115	50	Comparative Example
	37	736	4.6	104	50	Comparative Example
	38	700	4.5	131	50	Present Invention Example
	39	695	3.7	128	130	Present Invention Example
	40	700	3.3	115	50	Present Invention Example
	41	694	3.1	113	50	Comparative Example
	42	711	3.6	113	50	Comparative Example
	43	690	3.6	137	50	Comparative Example
	44	703	3.4	90	50	Comparative Example
	45	688	3.4	127	50	Comparative Example
	46	682	2.4	137	50	Comparative Example
	47	725	3.6	109	50	Comparative Example
	Underlines indicate that corresponding values are not preferable manufacturing conditions.

TABLE 5-1
							Average	Average
							sphere	number
							equivalent	density
							radius of	of alloy
		Sheet				Remainder	alloy	carbides			Mn
Manufac-		thick-		Residual		in micro-	carbides	in ferrite	E	I	standard
turing	Steel	ness	Ferrite	austenite	Pearlite	structure	in ferrite	×1016	value	value	deviation
No.	No.	mm	Area %	Area %	Area %	Area %	nm	number/cm3	—	—	Mass %	Note
1	A	2.6	56.8	0.0	0.0	43.2	1.1	30.4	11.3	1.044	0.44	Present Invention Example
2	B	2.6	27.3	0.0	0.0	72.7	1.2	17.4	10.8	1.029	0.49	Present Invention Example
3	B	2.6	34.3	0.0	0.0	65.7	1.6	20.4	11.3	1.034	0.67	Comparative Example
4	B	2.6	28.3	0.0	0.0	71.7	1.4	18.4	10.8	1.022	0.61	Comparative Example
5	B	2.6	23.7	0.0	0.0	76.3	2.1	14.7	11.1	1.028	0.63	Comparative Example
6	B	2.6	20.6	0.0	0.0	79.4	3.4	3.6	10.4	1.032	0.48	Comparative Example
7	B	2.6	34.2	0.0	0.0	65.8	1.0	28.8	10.5	1.036	0.42	Comparative Example
8	B	2.6	28.5	0.0	0.0	71.5	4.8	3.5	10.3	1.033	0.46	Comparative Example
9	B	2.6	18.2	0.0	0.0	81.8	0.4	3.4	11.2	1.043	0.41	Comparative Example
10	B	2.6	26.4	0.0	0.0	73.6	2.4	9.8	11.1	1.029	0.51	Present Invention Example
11	B	2.6	9.2	0.0	0.0	90.8	1.8	11.8	10.9	1.054	0.46	Comparative Example
12	B	2.6	54.1	0.0	0.0	45.9	1.1	36.5	10.2	1.093	0.48	Comparative Example
13	B	2.6	58.5	0.0	0.0	41.5	1.8	13.1	10.5	1.055	0.37	Comparative Example
14	B	2.6	46.0	0.0	5.4	48.6	2.2	8.7	10.7	1.074	0.43	Comparative Example
15	B	2.6	57.7	0.0	0.0	42.3	5.1	1.5	11.1	1.030	0.37	Comparative Example
16	C	6.0	15.9	1.1	0.0	83.0	2.6	5.2	11.0	1.052	0.45	Present Invention Example
17	D	2.6	17.5	0.0	0.0	82.5	1.7	15.4	11.2	1.028	0.49	Present Invention Example
18	E	2.6	50.5	2.0	0.0	47.5	1.4	33.0	11.0	1.031	0.39	Present Invention Example
19	F	1.6	45.5	0.0	0.0	54.5	3.2	5.6	10.7	1.031	0.48	Present Invention Example
20	G	2.6	16.2	0.0	0.0	83.8	0.7	27.0	11.1	1.031	0.59	Present Invention Example
21	H	2.6	28.0	0.0	0.0	72.0	2.2	15.0	10.8	1.039	0.50	Present Invention Example.
22	I	2.6	17.1	0.0	0.0	82.9	1.3	40.5	11.0	1.041	0.49	Present Invention Example
23	J	2.6	28.5	0.0	0.0	71.5	3.7	3.5	11.1	1.029	0.44	Present Invention Example
Underlines indicate that corresponding values are outside the range of the present invention.

TABLE 5-2
							Average	Average
							sphere	number
							equivalent	density
							radius of	of alloy
		Sheet				Remainder	alloy	carbides			Mn
Manufac-		thick-		Residual		in micro-	carbides	in ferrite	E	I	standard
turing	Steel	ness	Ferrite	austenite	Pearlite	structure	in ferrite	×1016	value	value	deviation
No.	No.	mm	Area %	Area %	Area %	Area %	nm	number/cm3	—	—	Mass %	Note
24	K	2.6	30.2	0.0	0.0	69.8	1.5	23.5	11.0	1.039	0.53	Present Invention Example
25	L	2.6	29.6	0.0	0.0	70.4	1.3	38.0	11.0	1.031	0.40	Present Invention Example
26	M	2.6	36.1	0.0	0.0	63.9	1.2	26.3	10.8	1.027	0.52	Present Invention Example
27	N	2.6	17.6	0.0	0.0	82.4	2.8	6.6	11.1	1.034	0.42	Present Invention Example
28	O	2.6	29.2	0.0	0.0	70.8	1.7	10.4	11.1	1.026	0.41	Present Invention Example
29	P	3.2	54.2	0.0	0.0	45.8	1.1	3.6	11.1	1.023	0.63	Comparative Example
30	P	3.2	48.8	0.0	0.0	51.2	4.9	3.6	10.9	1.024	0.38	Present Invention Example
31	P	3.2	34.0	0.0	0.0	66.0	1.0	32.4	11.0	1.031	0.42	Present Invention Example
32	P	3.2	55.4	0.0	0.0	44.6	0.8	10.4	10.9	1.027	0.62	Comparative Example
33	P	3.2	52.3	0.0	0.0	47.7	1.0	31.7	11.2	1.035	0.52	Present Invention Example
34	P	3.2	36.9	0.0	0.0	63.1	1.1	35.1	11.0	1.031	0.46	Present Invention Example
35	P	3.2	59.4	0.0	0.0	40.6	1.4	20.2	10.6	1.031	0.49	Comparative Example
36	P	3.2	39.6	3.2	5.1	52.1	2.2	9.1	10.8	1.031	0.46	Comparative Example
37	P	3.2	14.3	3.6	0.0	82.1	1.2	15.4	11.1	1.022	0.44	Comparative Example
38	P	3.2	57.5	0.0	0.0	42.5	1.1	53.0	11.0	1.022	0.55	Present Invention Example
39	P	3.2	58.4	0.0	0.0	41.6	1.5	33.5	11.1	1.037	0.53	Present Invention Example
40	Q	2.9	44.1	0.0	0.0	55.9	1.1	39.0	11.2	1.020	0.51	Present Invention Example
41	R	2.6	80.1	0.0	0.0	19.9	3.8	3.8	10.9	1.095	0.48	Comparative Example
42	S	2.6	1.5	1.8	0.0	96.7	1.9	19.7	10.8	1.017	0.43	Comparative Example
43	T	2.6	59.2	2.3	0.0	38.5	1.0	29.7	10.9	1.055	0.50	Comparative Example
44	U	2.6	72.8	0.0	0.0	27.2	0.9	68.4	11.1	1.081	0.44	Comparative Example
45	V	2.6	25.2	0.0	0.0	74.8	0.4	0.3	11.0	1.045	0.44	Comparative Example
46	Q	2.9	16.7	0.0	0.0	83.3	0.4	3.2	10.8	1.025	0.47	Comparative Example
47	Q	2.6	63.2	0.0	0.0	36.8	1.3	20.1	11.4	1.032	0.52	Comparative Example
Underlines indicate that corresponding values are outside the range of the present invention.

TABLE 6-1
			Total		Minimum value	Presence or absence
		Tensile	elongation		of repetition	of secondary sheared
Manufacturing	Steel	strength TS	El	TS × El	hardening ratio	surface
No.	No.	MPa	%	MPa · %	—	—	Note
1	A	983	16.5	16220	1.02	Absent	Present Invention Example
2	B	1023	15.9	16266	1.00	Absent	Present Invention Example
3	B	1015	15.9	16139	1.00	Present	Comparative Example
4	B	1008	15.8	15926	1.03	Present	Comparative Example
5	B	1032	15.1	15583	1.02	Present	Comparative Example
6	B	1038	14.9	15466	1.00	Present	Comparative Example
7	B	1027	14.0	14378	1.01	Present	Comparative Example
8	B	1012	15.7	15888	1.02	Present	Comparative Example
9	B	1086	12.6	13684	0.99	Absent	Comparative Example
10	B	1038	15.0	15570	1.01	Absent	Present Invention Example
11	B	1085	11.8	12803	1.00	Absent	Comparative Example
12	B	921	13.4	12341	1.03	Present	Comparative Example
13	B	915	14.0	12810	1.00	Present	Comparative Example
14	B	964	13.0	12532	1.03	Absent	Comparative Example
15	B	911	14.1	12845	0.99	Absent	Comparative Example
16	C	1583	12.2	19313	1.01	Absent	Present Invention Example
17	D	989	13.8	13648	1.01	Absent	Present Invention Example
18	B	1130	12.2	13786	1.02	Absent	Present Invention Example
19	F	990	16.0	15840	1.00	Absent	Present Invention Example
20	G	1304	10.2	13301	1.03	Absent	Present Invention Example
21	H	981	16.6	16285	1.02	Absent	Present Invention Example
22	I	1028	13.5	13878	1.00	Absent	Present Invention Example
23	J	1007	14.2	14299	1.03	Absent	Present Invention Example
Underlines indicate that corresponding values are outside the range of the present invention or not preferable properties.

TABLE 6-2
			Total		Minimum value	Presence or absence
		Tensile	elongation		of repetition	of secondary sheared
Manufacturing	Steel	strength TS	El	TS × El	hardening ratio	surface
No.	No.	MPa	%	MPa · %	—	—	Note
24	K	1039	15.6	16208	1.00	Absent	Present Invention Example
25	L	998	15.9	15868	1.02	Absent	Present Invention Example
26	M	1038	15.5	16089	1.00	Absent	Present Invention Example
27	N	1016	13.5	13716	1.03	Absent	Present Invention Example
28	O	993	15.7	15590	1.01	Absent	Present Invention Example
29	P	1044	15.5	16182	1.03	Present	Comparative Example
30	P	1035	15.3	15836	1.02	Absent	Present Invention Example
31	P	1060	14.8	15688	1.01	Absent	Present Invention Example
32	P	1038	14.9	15466	1.02	Present	Comparative Example
33	P	1066	15.0	15990	1.03	Absent	Present Invention Example
34	P	1055	14.7	15509	1.03	Absent	Present Invention Example
35	P	1052	15.0	15780	1.02	Present	Comparative Example
36	P	957	15.6	14929	1.02	Present	Comparative Example
37	P	1087	11.5	12501	1.01	Absent	Comparative Example
38	P	1030	15.9	16377	1.02	Absent	Present Invention Example
39	P	1039	15.1	15689	1.01	Absent	Present Invention Example
40	Q	1042	14.9	15526	1.00	Absent	Present Invention Example
41	R	905	16.3	14752	1.01	Absent	Comparative Example
42	S	1893	9.0	17037	1.03	Present	Comparative Example
43	T	1350	9.8	13230	1.02	Absent	Comparative Example
44	U	968	16.9	16359	1.00	Absent	Comparative Example
45	V	943	16.3	15371	0.99	Absent	Comparative Example
46	Q	1008	13.0	13104	0.98	Absent	Comparative Example
47	Q	902	15.8	14252	1.01	Absent	Comparative Example
Underlines indicate that corresponding values are outside the range of the present invention or not preferable properties.

From Table 5-i to Table 6-2, it is found that the hot-rolled steel sheets according to the present invention examples have high strength and excellent ductility, fatigue property and shearing property.

On the other hand, it is found that the hot-rolled steel sheets according to the comparative examples did not have any one or more of the above properties.

INDUSTRIAL APPLICABILITY

According to the above aspect of the present invention, it is possible to provide a hot-rolled steel sheet having high strength and excellent ductility, fatigue property and shearing property.

The hot-rolled steel sheet according to the present invention is suitable as an industrial material used for vehicle members, mechanical structural members, and building members.

Claims

1. A hot-rolled steel sheet comprising, in terms of mass %, as a chemical composition:

C: 0.050% to 0.250%;

Si: 0.05% to 3.00%;

Mn: 1.00% to 4.00%;

one or more of Ti, Nb, and V: 0.060% to 0.500% in total;

sol. Al: 0.001% to 2.000%;

P: 0.100% or less;

S: 0.0300% or less;

N: 0.1000% or less;

O: 0.0100% or less;

Cu: 0% to 2.00%;

Cr: 0% to 2.00%;

Mo: 0% to 1.00%;

Ni: 0% to 2.00%;

B: 0% to 0.0100%;

Ca: 0% to 0.0200%;

Mg: 0% to 0.0200%;

REM: 0% to 0.1000%;

Bi: 0% to 0.020%;

one or more of Zr, Co, Zn, and W: 0% to 1.00% in total;

Sn: 0% to 0.05%; and

a remainder comprising Fe and impurities,

wherein, a microstructure having, in terms of area %, residual austenite at less than 3.0%, ferrite at 15.0% or more and less than 60.0%, and pearlite at less than 5.0%, an average sphere equivalent radius of alloy carbides in the ferrite of 0.5 nm or more and less than 5.0 nm, an average number density of the alloy carbides in the ferrite of 3.5×1016/cm3 or more, an E value that indicates periodicity of the microstructure of 10.7 or more, an I value that indicates uniformity of the microstructure of 1.020 or more, a standard deviation of a Mn concentration of 0.60 mass % or less, and a tensile strength of 980 MPa or more.

2. The hot-rolled steel sheet according to claim 1, further comprising, in terms of mass %, one or more of, as the chemical composition:

Cu: 0.01% to 2.00%;

Cr: 0.01% to 2.00%;

Mo: 0.01% to 1.00%;

Ni: 0.02% to 2.00%;

B: 0.0001% to 0.0100%;

Ca: 0.0005% to 0.0200%;

Mg: 0.0005% to 0.0200%;

REM: 0.0005% to 0.1000%; and

Bi: 0.0005% to 0.020%.