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

[0001] 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.

[0002] 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

[0003] 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.

[0004] 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.

[0005] 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.

[0006] 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.

[0007] Patent Document 2 discloses a technique for controlling burr height after punching by controlling a ratio ds / db of the ferrite grain size ds of the surface layer to the ferrite grain db of an inside to 0.95 or less.PRIOR ART DOCUMENTPatent Document

[0008] [Patent Document 1] Japanese Unexamined Patent Application, First Publication No. 2005-179703

[0009] [Patent Document 2] Japanese Unexamined Patent Application, First Publication No. 1110-168544Non-Patent Document

[0010] [Non-Patent Document 1]J. Webel, J. Gola, D. Britz, F. Mucklich, Materials Characterization 144 (2018) 584-596

[0011] [Non-Patent Document 2]D. L. Naik, H. U. Sajid, R. Kiran, Metals 2019, 9, 546

[0012] [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. 474-485DISCLOSURE OF THE INVENTIONProblems to be Solved by the Invention

[0013] 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.

[0014] In addition, hot-rolled steel sheet having high strength may be required to have better fatigue property.

[0015] 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

[0016] The gist of the present invention made based on the above findings is as follows.

[0017] (1) A hot-rolled steel sheet according to one aspect of the present invention comprising, in terms of mass %, as a chemical composition,

[0018] C: 0.050% to 0.250%,

[0019] Si: 0.05% to 3.00%,

[0020] Mn: 1.00% to 4.00%,

[0021] one or two or more of Ti, Nb, and V: 0.060% to 0.500% in total,

[0022] sol. Al: 0.001% to 2.000%,

[0023] P: 0.100% or less,

[0024] S: 0.0300% or less,

[0025] N: 0.1000% or less,

[0026] O: 0.0100% or less,

[0027] Cu: 0% to 2.00%,

[0028] Cr: 0% to 2.00%,

[0029] Mo: 0% to 1.00%,

[0030] Ni: 0% to 2.00%,

[0031] B: 0% to 0.0100%,

[0032] Ca: 0% to 0.0200%,

[0033] Mg: 0% to 0.0200%,

[0034] REM: 0% to 0.1000%,

[0035] Bi: 0% to 0.020%,

[0036] one or two or more of Zr, Co, Zn, and W: 0% to 1.00% in total,

[0037] Sn: 0% to 0.05%, and

[0038] a remainder comprising Fe and impurities,

[0039] in which, in a microstructure,

[0040] in terms of area %,

[0041] residual austenite is less than 3.0%,

[0042] ferrite is 15.0% or more and less than 60.0%, and

[0043] pearlite is less than 5.0%,

[0044] an average sphere equivalent radius of alloy carbides in the ferrite is 0.5 nm or more and less than 5.0 nm,

[0045] an average number density of the alloy carbides in the ferrite is 3.5×1016 / cm3 or more,

[0046] an E value that indicates periodicity of the microstructure is 10.7 or more,

[0047] an I value that indicates uniformity of the microstructure is 1.020 or more,

[0048] a standard deviation of a Mn concentration is 0.60 mass % or less, and

[0049] a tensile strength is 980 MPa or more.

[0050] (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

[0051] Cu: 0.01% to 2.00%,

[0052] Cr: 0.01% to 2.00%,

[0053] Mo: 0.01% to 1.00%,

[0054] Ni: 0.02% to 2.00%,

[0055] B: 0.0001% to 0.0100%,

[0056] Ca: 0.0005% to 0.0200%,

[0057] Mg: 0.0005% to 0.0200%,

[0058] REM: 0.0005% to 0.1000%, and

[0059] Bi: 0.0005% to 0.020%.Effects of the Invention

[0060] 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.

[0061] 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

[0062] FIG. 1 is an example of a sheared end surface of a hot-rolled steel sheet according to a present invention example.

[0063] 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

[0064] 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.

[0065] 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

[0066] 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.

[0067] C: 0.050% to 0.250%

[0068] 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.

[0069] 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.

[0070] Si: 0.05% to 3.00%

[0071] 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.

[0072] 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.

[0073] Mn: 1.00% to 4.00%

[0074] 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.

[0075] 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.

[0076] One or two or more of Ti, Nb, and V: 0.060% to 0.500% in total

[0077] 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.

[0078] 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.

[0079] sol. Al: 0.001% to 2.000%

[0080] 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.

[0081] 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.

[0082] The sol. Al means acid-soluble Al and refers to solid solution Al present in steel in a solid solution state.

[0083] P: 0.100% or less

[0084] 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.

[0085] S: 0.0300% or less

[0086] 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.

[0087] N: 0.1000% or less

[0088] 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.

[0089] O: 0.0100% or less

[0090] 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.

[0091] 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.

[0092] 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.

[0093] Cu: 0.01% to 2.00%

[0094] Cr: 0.01% to 2.00%

[0095] Mo: 0.01% to 1.00%

[0096] Ni: 0.02% to 2.00%

[0097] B: 0.0001% to 0.0100%

[0098] 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.

[0099] 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.

[0100] 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.

[0101] 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.

[0102] 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.

[0103] 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.

[0104] Ca: 0.0005% to 0.0200%

[0105] Mg: 0.0005% to 0.0200%

[0106] REM: 0.0005% to 0.1000%

[0107] Bi: 0.0005% to 0.020%

[0108] 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.

[0109] 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.

[0110] One or two or more of Zr, Co, Zn, or W: 0% to 1.00% in total

[0111] Sn: 0% to 0.05%

[0112] 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.

[0113] 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.

[0114] 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

[0115] Next, the microstructure of the hot-rolled steel sheet according to the present embodiment will be described.

[0116] 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.

[0117] 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.

[0118] 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.

[0119] Area Ratio of Residual Austenite: Less than 3.0%

[0120] 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%.

[0121] 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.

[0122] 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.

[0123] Area Ratio of Ferrite: 15.0% or more and less than 60.0%

[0124] 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.

[0125] 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.

[0126] Area Ratio of Pearlite: Less than 5.0%

[0127] 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%.

[0128] 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.

[0129] 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.

[0130] 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.

[0131] Average Sphere Equivalent Radius of Alloy Carbides in Ferrite: 0.5 nm or more and less than 5.0 nm

[0132] 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.

[0133] Average Number Density of Alloy Carbides in Ferrite: 3.5×1016 / cm3 or more

[0134] 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.

[0135] 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.

[0136] In the present embodiment, the alloy carbides refer to carbides containing one or two or more of Ti, Nb, Mo, and V.

[0137] 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.

[0138] 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.

[0139] 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.

[0140] 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.).

[0141] 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.

[0142] E Value: 10.7 or more

[0143] I Value: 1.020 or more

[0144] 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.

[0145] 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.

[0146] 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.

[0147] The E value and the I value can be obtained by the following method.

[0148] 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.

[0149] 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.

[0150] 179 matrixes of the frequency values sampled by the above method are expressed as pk (k=0 . . . 89, 91, . . . 179) where k is a rotation angle from the original image. pk'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 ith row in the jth column of the matrix P is expressed as Pij.E=-Σi=1,j=1i=2⁢5⁢6,j=2⁢5⁢6⁢Pi⁢j⁢log⁢Pij(1)I=Σi=1,j=1i=2⁢5⁢6,j=2⁢5⁢6⁢Pi⁢j / (1+|i-j| / 2⁢5⁢6)(2)Standard Deviation of Mn Concentration: 0.60 mass % or Less

[0152] 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.

[0153] 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

[0154] 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.

[0155] 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.

[0156] 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.

[0157] 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

[0158] 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.

[0159] 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

[0160] 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

[0161] 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

[0162] 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.

[0163] 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.

[0164] 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.

[0165] (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.

[0166] (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.

[0167] (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.

[0168] (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.

[0169] (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.

[0170] (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.

[0171] (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.

[0172] (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.

[0173] (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.

[0174] (10) The steel sheet is coiled in a temperature range of 350° C. or lower.

[0175] 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

[0176] 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.

[0177] 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.

[0178] 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.

[0179] 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.

[0180] 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.

[0181] 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

[0182] 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.

[0183] 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.

[0184] 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.

[0185] 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.

[0186] 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

[0187] 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

[0188] 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.

[0189] 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.

[0190] 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.

[0191] 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

[0192] 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.

[0193] 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

[0194] 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

[0195] 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

[0196] 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.

[0197] 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.

[0198] 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.

[0199] 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.

[0200] 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.

[0201] 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 SheetsTensile Properties

[0202] 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

[0203] 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)

[0204] 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.

[0205] 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 1SteelMass %, remainder is Fe and impuritiesNo.CSiMnTiNbVTi + Nb + Vsol. AlPSNONoteA0.0551.181.690.0960.0960.0380.0270.00160.00420.0036Steel of Present InventionB0.0871.141.750.1050.1050.0340.0090.00280.00220.0035Steel of Present InventionC0.1531.361.960.1350.1350.0740.0220.00490.00370.0025Steel of Present InventionD0.1120.321.560.0850.0850.0260.0110.00290.00300.0014Steel of Present InventionE0.0882.711.880.1030.1030.0330.0240.00230.00180.0020Steel of Present InventionF0.0920.861.200.0970.0970.0470.0020.00330.00240.0007Steel of Present InventionG0.0881.253.620.1100.0500.1600.0370.0340.00170.00350.0051Steel of Present InventionH0.0950.751.730.0600.0600.0350.0210.00300.00200.0013Steel of Present InventionI0.0921.001.700.1350.1350.0280.0020.00480.00230.0030Steel of Present InventionJ0.0911.151.810.0430.0300.0720.1450.0330.0180.00490.00360.0050Steel of Present InventionK0.0850.941.920.1360.1360.0530.0250.00440.00340.0051Steel of Present InventionL0.0770.831.720.1140.1140.0530.0270.00270.00380.0020Steel of Present InventionM0.0821.061.600.1010.0480.1490.0380.0180.00170.00210.0030Steel of Present InventionN0.0741.001.510.0920.0920.0350.0110.00370.00430.0021Steel of Present InventionO0.1011.171.870.2090.2090.0600.0210.00320.00440.0044Steel of Present InventionP0.0771.532.170.1210.1210.3180.0200.00350.00590.0041Steel of Present InventionQ0.0621.762.280.0850.0850.3780.0170.00070.00150.0012Steel of Present InventionR0.0471.001.970.1340.1340.0370.0160.00470.00470.0027Comparative ExampleS0.2580.911.780.1130.1130.0580.0090.00350.00390.0050Comparative ExampleT0.0923.211.750.1010.1010.0320.0250.00250.00310.0035Comparative ExampleU0.0950.950.820.1330.1330.0750.0280.00140.00780.0020Comparative ExampleV0.1011.181.930.0440.0100.0540.0490.0240.00620.00280.0054Comparative ExampleUnderlines indicate that corresponding values are outside the range of the present invention.TABLE 2SteelMass %, remainder is Fe and impuritiesNo.CuCrMoNiBCaMgREMBiZrCoZnWSnNoteA0.00180.0018Steel of Present InventionBSteel of Present InventionC0.0032Steel of Present InventionD0.350.310.24Steel of Present InventionE0.0050.18Steel of Present InventionFSteel of Present InventionGSteel of Present InventionH0.02Steel of Present InventionISteel of Present InventionJSteel of Present InventionK0.170.17Steel of Present InventionL0.28Steel of Present InventionM0.080.03Steel of Present InventionN0.29Steel of Present InventionO0.00210.02Steel of Present InventionPSteel of Present InventionQSteel of Present InventionRComparative ExampleSComparative ExampleTComparative ExampleUComparative ExampleVComparative ExampleTABLE 3-1SheetLoaded stressRetentionRetentionthicknessafter rollingtime intime inreduction inat one standtemperaturetemperaturetemperaturebefore finalRollingrangerangerangestand and beforeFinishingreductionManufac-of 700° C.Heatingof 1100° C.of 850° C.rolling attemperatureratio atturingSteelto 850° C.temperatureor higherto 1100° C.final standTffinal standNo.No.s° C.s%kPa° C.%Note1A1486126192699520293910Present Invention Example2B1481123491819621796911Present Invention Example3B 855124190259623293610Comparative Example4B1126123492618720493510Comparative Example5B12171234558092198949 8Comparative Example6B1467122196409416598010Comparative Example7B151512438893942161023 10Comparative Example8B14601215810593197982 7Comparative Example9B12931218913294229942 9Comparative Example10B12511242956795234961 8Present Invention Example11B13501235903096238971 8Comparative Example12B1392120694069119592310Comparative Example13B1114121689549420697210Comparative Example14B15061249934596230931 9Comparative Example15B13201221890192194931 9Comparative Example16C12681218903394213948 8Present Invention Example17D14771236959194195971 9Present Invention Example18E1466120991239021894210Present Invention Example19F1492125492209219794411Present Invention Example20G144012239376982241002 28Present Invention Example21H1595122995929818396911Present Invention Example22I14731213902896209920 8Present Invention Example23J16071244960897215971 9Present Invention ExampleUnderlines indicate that corresponding values are not preferable manufacturing conditions.TABLE 3-2SheetLoaded stressRetentionRetentionthicknessafter rollingtime intime inreduction inat one standtemperaturetemperaturetemperaturebefore finalRollingrangerangerangestand and beforeFinishingreductionManufac-of 700° C.Heatingof 1100° C.of 850° C.rolling oftemperatureratio ofturingSteelto 850° C.temperatureor higherto 1100° C.final standTffinal standNo.No.s° C.s%kPa° C.%Note24K1331123094329822494610Present Invention Example25L1464120993199420792611Present Invention Example26M1344123089809722596510Present Invention Example27N1241123196229822194710Present Invention Example28O12111251958195231956 8Present Invention Example29P 847122990109223294310Comparative Example30P2272121894469220891410Present Invention Example31P1462120511880 9019693711Present Invention Example32P1293121858909420695111Comparative Example33P13271238941692206928 9Present Invention Example34P1515126391399824098510Present Invention Example35P13041229886497223944 6Comparative Example36P1096122792439021092910Comparative Example37P1301126789789823795111Comparative Example38P 9681231924896218951 8Present Invention Example39P1310123366039121296110Present Invention Example40Q1192122594999423295110Present Invention Example41R1455122991829121296010Comparative Example42S11661217881293200921 8Comparative Example43T16121248917896237941 9Comparative Example44U1298121393139321097110Comparative Example45V14591219958395220953 9Comparative Example46Q13841253889494215937 9Comparative Example47Q14251238874195207896 9Comparative ExampleUnderlines indicate that corresponding values are not preferable manufacturing conditions.TABLE 4-1Sheet thicknessreduction of lightLoaded stress afterLoaded stress afterrolling at temperaturerolling at final standrolling at final standAveragerange of 840° C. orof hot rolling andof light rolling untilcooling ratehigher and lower thanbefore first rolling ofsteel sheet beingof acceleratedManufacturing900° C.light rollingcooled to 800° C.coolingNo.Steel No.%kPakPa° C. / s1A6 96 471122B6100152973B6 81 50524B6117182565B6102137886B6119180647B6 90114728B6 63 94639B4 80 929210B6 88 789311B612618415212B6202 925413B61082017614B61181787915B9 951338516C6 82 515317D6 96 858818E610712712419F6 89 539120G6113 7514221H611714810122I6103 8214723J6102 1895Slow coolingAverageCooling stoptime incoolingtemperature oftemperaturerate untilacceleratedrange of 680° C.coilingCoilingManufacturingcoolingto 730°temperaturetemperatureNo.° C.s° C. / s° C.Note16943.110850Present Invention Example27103.811450Present Invention Example36994.410750Comparative Example47283.0 8650Comparative Example57293.813850Comparative Example67223.811450Comparative Example77263.815150Comparative Example87232.510850Comparative Example97223.3 8950Comparative Example106934.4 8950Present Invention Example117021.511150Comparative Example126943.911150Comparative Example137244.4 8550Comparative Example147154.4 2350Comparative Example157242.7 9950Comparative Example166983.014950Present Invention Example176973.513450Present Invention Example187053.1 9650Present Invention Example196913.214750Present Invention Example206984.6 9350Present Invention Example217013.814950Present Invention Example226972.514150Present Invention Example236953.5 9250Present Invention ExampleUnderlines indicate that corresponding values are not preferable manufacturing conditions.TABLE 4-2Sheet thicknessreduction of lightLoaded stress afterLoaded stress afterrolling at temperaturerolling of final standrolling of final standAveragerange of 840° C. orof hot rolling andof light rolling andcooling ratehigher and lower thanbefore first rolling ofuntil steel sheet isof acceleratedManufacturing900° C.light rollingcooled to 800° C.coolingNo.Steel No.%kPakPa° C. / s24K611210314425L61124913326M611016115027N6128180 8528O612117715729P69226 5130P611214315231P610698 7232P6125104 5333P6101150 9334P65315014835P69788 5636P610985 4237P61154515038P61134710539P61275713340Q611319311041R610753 9642S681171 5043T6983012744U610018713245V611875 5046Q——— 9847Q694103 85Slow coolingAverageCooling stoptime incoolingtemperature oftemperaturerate toacceleratedrange of 680° C.coilingCoilingcoolingto 730°temperaturetemperatureManufacturing° C.s° C. / s° C.Note247104.010450Present Invention Example256913.69150Present Invention Example267003.38950Present Invention Example277092.314250Present Invention Example287073.413450Present Invention Example297003.310850Comparative Example307113.3130200Present Invention Example316922.814150Present Invention Example327214.310750Comparative Example337183.410550Present Invention Example347003.710850Present Invention Example357254.714050Comparative Example367264.011550Comparative Example377364.610450Comparative Example387004.513150Present Invention Example396953.7128130Present Invention Example407003.311550Present Invention Example416943.111350Comparative Example427113.611350Comparative Example436903.613750Comparative Example447033.49050Comparative Example456883.412750Comparative Example466822.413750Comparative Example477253.610950Comparative ExampleUnderlines indicate that corresponding values are not preferable manufacturing conditions.TABLE 5-1AverageAveragespherenumberequivalentdensityradius ofof alloySheetRemainderalloycarbidesMnManufac-thick-Residualin micro-carbidesin ferriteEIstandardturingSteelnessFerriteaustenitePearlitestructurein ferrite×1016valuevaluedeviationNo.No.mmArea %Area %Area %Area %nmnumber / cm3——Mass %Note 1A2.656.80.00.043.21.130.411.31.0440.44Present Invention Example 2B2.627.30.00.072.71.217.410.81.0290.49Present Invention Example 3B2.634.30.00.065.71.620.411.31.0340.67Comparative Example 4B2.628.30.00.071.71.418.410.81.0220.61Comparative Example 5B2.623.70.00.076.32.114.711.11.0280.63Comparative Example 6B2.620.60.00.079.43.4 3.610.41.0320.48Comparative Example 7B2.634.20.00.065.81.028.810.51.0360.42Comparative Example 8B2.628.50.00.071.54.8 3.510.31.0330.46Comparative Example 9B2.618.20.00.081.80.4 3.411.21.0430.41Comparative Example10B2.626.40.00.073.62.4 9.811.11.0290.51Present Invention Example11B2.6 9.20.00.090.81.811.810.91.0540.46Comparative Example12B2.654.10.00.045.91.136.510.21.0930.48Comparative Example13B2.658.50.00.041.51.813.110.51.0550.37Comparative Example14B2.646.00.05.448.62.2 8.710.71.0740.43Comparative Example15B2.657.70.00.042.35.1 1.511.11.0300.37Comparative Example16C6.015.91.10.083.02.6 5.211.01.0520.45Present Invention Example17D2.617.50.00.082.51.715.411.21.0280.49Present Invention Example18E2.650.52.00.047.51.433.011.01.0310.39Present Invention Example19F1.645.50.00.054.53.2 5.610.71.0310.48Present Invention Example20G2.616.20.00.083.80.727.011.11.0310.59Present Invention Example21H2.628.00.00.072.02.215.010.81.0390.50Present Invention Example.22I2.617.10.00.082.91.340.511.01.0410.49Present Invention Example23J2.628.50.00.071.53.7 3.511.11.0290.44Present Invention ExampleUnderlines indicate that corresponding values are outside the range of the present invention.TABLE 5-2AverageAveragespherenumberequivalentdensityradius ofof alloySheetRemainderalloycarbidesMnManufac-thick-Residualin micro-carbidesin ferriteEIstandardturingSteelnessFerriteaustenitePearlitestructurein ferrite×1016valuevaluedeviationNo.No.mmArea %Area %Area %Area %nmnumber / cm3——Mass %Note24K2.630.20.00.069.81.523.511.01.0390.53Present Invention Example25L2.629.60.00.070.41.338.011.01.0310.40Present Invention Example26M2.636.10.00.063.91.226.310.81.0270.52Present Invention Example27N2.617.60.00.082.42.8 6.611.11.0340.42Present Invention Example28O2.629.20.00.070.81.710.411.11.0260.41Present Invention Example29P3.254.20.00.045.81.1 3.611.11.0230.63Comparative Example30P3.248.80.00.051.24.9 3.610.91.0240.38Present Invention Example31P3.234.00.00.066.01.032.411.01.0310.42Present Invention Example32P3.255.40.00.044.60.810.410.91.0270.62Comparative Example33P3.252.30.00.047.71.031.711.21.0350.52Present Invention Example34P3.236.90.00.063.11.135.111.01.0310.46Present Invention Example35P3.259.40.00.040.61.420.210.61.0310.49Comparative Example36P3.239.63.25.152.12.2 9.110.81.0310.46Comparative Example37P3.214.33.60.082.11.215.411.11.0220.44Comparative Example38P3.257.50.00.042.51.153.011.01.0220.55Present Invention Example39P3.258.40.00.041.61.533.511.11.0370.53Present Invention Example40Q2.944.10.00.055.91.139.011.21.0200.51Present Invention Example41R2.680.10.00.019.93.8 3.810.91.0950.48Comparative Example42S2.6 1.51.80.096.71.919.710.81.0170.43Comparative Example43T2.659.22.30.038.51.029.710.91.0550.50Comparative Example44U2.672.80.00.027.20.968.411.11.0810.44Comparative Example45V2.625.20.00.074.80.4 0.311.01.0450.44Comparative Example46Q2.916.70.00.083.30.4 3.210.81.0250.47Comparative Example47Q2.663.20.00.036.81.320.111.41.0320.52Comparative ExampleUnderlines indicate that corresponding values are outside the range of the present invention.TABLE 6-1TotalMinimum valuePresence or absenceTensileelongationof repetitionof secondary shearedManufacturingSteelstrength TSElTS × Elhardening ratiosurfaceNo.No.MPa%MPa · %——Note 1A 98316.5162201.02AbsentPresent Invention Example 2B102315.9162661.00AbsentPresent Invention Example 3B101515.9161391.00PresentComparative Example 4B100815.8159261.03PresentComparative Example 5B103215.1155831.02PresentComparative Example 6B103814.9154661.00PresentComparative Example 7B102714.0143781.01PresentComparative Example 8B101215.7158881.02PresentComparative Example 9B108612.6136840.99AbsentComparative Example10B103815.0155701.01AbsentPresent Invention Example11B108511.8128031.00AbsentComparative Example12B 92113.4123411.03PresentComparative Example13B 91514.0128101.00PresentComparative Example14B 96413.0125321.03AbsentComparative Example15B 91114.1128450.99AbsentComparative Example16C158312.2193131.01AbsentPresent Invention Example17D 98913.8136481.01AbsentPresent Invention Example18B113012.2137861.02AbsentPresent Invention Example19F 99016.0158401.00AbsentPresent Invention Example20G130410.2133011.03AbsentPresent Invention Example21H 98116.6162851.02AbsentPresent Invention Example22I102813.5138781.00AbsentPresent Invention Example23J100714.2142991.03AbsentPresent Invention ExampleUnderlines indicate that corresponding values are outside the range of the present invention or not preferable properties.TABLE 6-2TotalMinimum valuePresence or absenceTensileelongationof repetitionof secondary shearedManufacturingSteelstrength TSElTS × Elhardening ratiosurfaceNo.No.MPa%MPa · %——Note24K103915.6162081.00AbsentPresent Invention Example25L 99815.9158681.02AbsentPresent Invention Example26M103815.5160891.00AbsentPresent Invention Example27N101613.5137161.03AbsentPresent Invention Example28O 99315.7155901.01AbsentPresent Invention Example29P104415.5161821.03PresentComparative Example30P103515.3158361.02AbsentPresent Invention Example31P106014.8156881.01AbsentPresent Invention Example32P103814.9154661.02PresentComparative Example33P106615.0159901.03AbsentPresent Invention Example34P105514.7155091.03AbsentPresent Invention Example35P105215.0157801.02PresentComparative Example36P 95715.6149291.02PresentComparative Example37P108711.5125011.01AbsentComparative Example38P103015.9163771.02AbsentPresent Invention Example39P103915.1156891.01AbsentPresent Invention Example40Q104214.9155261.00AbsentPresent Invention Example41R 90516.3147521.01AbsentComparative Example42S1893 9.0170371.03PresentComparative Example43T1350 9.8132301.02AbsentComparative Example44U 96816.9163591.00AbsentComparative Example45V 94316.3153710.99AbsentComparative Example46Q100813.0131040.98AbsentComparative Example47Q 90215.8142521.01AbsentComparative ExampleUnderlines 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 APPLICABILITYAccording 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%; anda 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%, andpearlite 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, anda 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%; andBi: 0.0005% to 0.020%.

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