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 10.0 nm, an average number density of the alloy carbides in the ferrite is 0.10×1016 pieces / cm3 or more and less than 1.45×1016 pieces / cm3, 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, and a standard deviation of Mn concentration is 0.60 mass % or less.

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. 2022-135960, filed on Aug. 29, 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 1 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. H10-168544Non-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 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] Q: 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 0.0200%,

[0036] As: 0% to 0.100%,

[0037] one or two or more of Zr, Co, Zn, and W: 0% to 1.00% in total,

[0038] Sn: 0% to 0.05%, and

[0039] a remainder comprising Fe and impurities,

[0040] in which, in a microstructure,

[0041] in terms of area %,

[0042] residual austenite is less than 3.0%,

[0043] ferrite is 15.0% or more and less than 60.0%, and

[0044] pearlite is less than 5.0%,

[0045] an average sphere equivalent radius of alloy carbides in the ferrite is 0.5 nm or more and less than 10.0 nm,

[0046] an average number density of the alloy carbides in the ferrite is 0.10×1016 pieces / cm3 or more and less than 1.45×1016 pieces / cm3,

[0047] an E value that indicates periodicity of the microstructure is 10.7 or more,

[0048] an I value that indicates uniformity of the microstructure is 1.020 or more, and

[0049] a standard deviation of Mn concentration is 0.60 mass % or less.

[0050] (2) The hot-rolled steel sheet according to (1), in which the chemical composition may comprise, in terms of mass %, one or two or more selected from the group consisting of:

[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%,

[0059] Bi: 0.0005% to 0.0200%, and

[0060] As: 0.001% to 0.100%.Effects of the Invention

[0061] 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.

[0062] 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

[0063] FIG. 1 is an example of a sheared end surface of a hot-rolled steel sheet according to a present invention example.

[0064] 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

[0065] 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.

[0066] 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

[0067] 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%

[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 more than 0.070%, 0.75% or more or 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.Si: 0.05% to 3.00%

[0070] 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.

[0071] 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%

[0072] 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.

[0073] 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

[0074] 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, more preferably 0.100% or more, and still more preferably 0.120% or more.

[0075] 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%

[0076] 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.

[0077] 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 0.400% or less, more preferably 0.300% or less, and still more preferably 0.250% or less.

[0078] 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

[0079] P 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 the P content is preferably set to 0.001% from the viewpoint of the refining cost.S: 0.0300% or less

[0080] S 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 the S content is preferably set to 0.0001% from the viewpoint of the refining cost.N: 0.1000% or less

[0081] N 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 0.0050% or less. Although the lower limit of the N content does not need to be particularly specified, but 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 in a case where one or two or more of Ti, Nb, and V are contained to further refine the microstructure.O: 0.0100% or Less

[0082] 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, and still more preferably 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.

[0083] 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.

[0084] 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.

[0085] Cu: 0.01% to 2.00%

[0086] Cr: 0.01% to 2.00%

[0087] Mo: 0.01% to 1.00%

[0088] Ni: 0.02% to 2.00%

[0089] B: 0.0001% to 0.0100%

[0090] 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.

[0091] 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.

[0092] 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.

[0093] 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.

[0094] 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.

[0095] 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.

[0096] Ca: 0.0005% to 0.0200%

[0097] Mg: 0.0005% to 0.0200%

[0098] REM: 0.0005% to 0.1000%

[0099] Bi: 0.0005% to 0.0200%

[0100] 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 0.0200%, 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 0.0200% or less. The Bi content is preferably 0.0100% or less.

[0101] 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.As: 0.001% to 0.100%

[0102] As lowers an austenitizing temperature and thus refines the prior austenite grains, thereby contributing for an improvement of ductility of the hot-rolled steel sheet. In order to reliably obtain the effects, the As content is preferably set to 0.001% or more.

[0103] On the other hand, since the above effects are saturated even in a case where a large amount of As is contained, the As content is set to 0.100% or less.One or Two or More of Zr, Co, Zn, or W: 0% to 1.00% in TotalSn: 0% to 0.05%

[0104] 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.

[0105] 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.

[0106] 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.

[0107] When a plating layer or a coating film is provided on the surface of the hot-rolled steel sheet, the chemical composition is analyzed after the plating layer or the coating film is removed by mechanical grinding or the like as necessary.Microstructure of Hot-Rolled Steel Sheet

[0108] Next, the microstructure of the hot-rolled steel sheet according to the present embodiment will be described.

[0109] 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 10.0 nm, an average number density of the alloy carbides in the ferrite is 0.10×1016 pieces / cm3 or more and less than 1.45×1016 pieces / cm3, 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.

[0110] 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.

[0111] 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 region of a position of ¼ from the surface in a sheet thickness direction of the hot-rolled steel sheet and a position of ¼ from an end surface in a direction (a sheet width direction) perpendicular to a rolling direction and the sheet thickness direction (the rolling direction is an arbitrary position) are specified. The reason therefor is that the microstructure at this position indicates a typical microstructure of the steel sheet. When the hot-rolled steel sheet has the plating layer or the coating film, the “surface” refers to the interface of the plating layer or the coating film and the steel sheet.Area Ratio of Residual Austenite: Less than 3.0%

[0112] 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%.

[0113] 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.

[0114] In the measurement of the area ratio of the residual austenite by X-ray diffraction in the present embodiment, first, in a cross section at a position of ¼ from the surface in the sheet thickness direction of the hot-rolled steel sheet, a sample is collected so that the microstructure at a region of 1 mm or more in an arbitrary position of the rolling direction and 1 mm or more centered in a position of ¼ from the end surface in the sheet width direction can be observed. For the sample, 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 using Co-Kαrays. Next, the volume ratio of the residual austenite is obtained using the strength averaging method from the integrated intensities, and the obtained volume ratio is regarded as an area ratio of the residual austenite.Area Ratio of Ferrite: 15.0% or More and Less than 60.0%

[0115] 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.

[0116] 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%

[0117] 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. Furthermore, in the interface of ferrite and cementite that are contained in pearlite, micro voids which deteriorates ductility generate in early stage, and therefore a desired ductility and fatigue property cannot be obtained when the area ratio of pearlite is 5.0% or more. 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 still more preferably 0%.

[0118] 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.

[0119] Measurement of the area ratios of the microstructure other than residual austenite is conducted by the following method. First, in a position of ¼ from the end surface in the sheet width direction of the hot-rolled steel sheet, a sample is collected so that the microstructure in a region of a position of ¼ from the surface in the sheet thickness direction in a cross section parallel to the rolling direction can be observed. The size of the sample depends on a measurement device, but is set to a size that can be observed by about 10 mm in the rolling direction. Next, after mirror finishing the observed cross section of the sample by polishing, the observed cross section of the sample is polished at room temperature with colloidal silica not containing an alkaline solution for 8 minutes, thereby removing strain introduced into the surface layer of the sample. For a region of 200 μm or more at an arbitrary position in the rolling direction and 200 μm or more centered in a position of ¼ from the surface in the sheet thickness direction of the observed cross section, crystal orientation information is obtained by a measurement using electron backscatter diffraction at a measurement interval of 0.1 μm. 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.

[0120] 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.

[0121] The area ratio of a remainder of the microstructure is obtained by subtracting the area ratios of residual austenite, pearlite, and ferrite from 100%. Note that the remainder of the microstructure can be estimated to be a hard structure consisting of one or two or more of bainite, martensite, and tempered martensite from the chemical composition of the hot-rolled steel sheet and the manufacturing conditions.

[0122] Note that the rolling direction of the hot-rolled steel sheet is determined by the following method.

[0123] First, a test piece is collected so that a sheet thickness cross section of the hot-rolled steel sheet can be observed. The sheet thickness cross section of the collected test piece is observed using an optical microscope after mirror polishing and corrosion with a picric acid-saturated aqueous solution. An observation range is set to an entire thickness of the sheet thickness, and an extending direction of grains is determined. Here, an angle difference between the extending direction and the sheet thickness direction is set to θ. Furthermore, a surface at a position of ¼ of the sheet thickness that is perpendicular to the sheet thickness direction and parallel to the above-mentioned extending direction is polished similar to the sheet thickness cross section, and the extending direction of grains is determined. Here, an angle difference between the sheet thickness cross section and the extending direction is set to φ. In a spherical coordinate system in accordance with ISO 3785 with the sheet thickness direction as the Z axis, the direction with θ and φ, which are obtained from the extending direction of the grains in the above-mentioned two cross sections, as deflection angles is determined as the rolling direction. To determine the extending direction θ and φ, the “Analyze Particles” function of the image analysis software “ImageJ” can be used to obtain the extending direction of the grains by setting the circularity to 0.7 or less.Average Sphere Equivalent Radius of Alloy Carbides in Ferrite: 0.5 nm or More and Less than 10.0 nm

[0124] In the hot-rolled steel sheet according to the present embodiment, 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 nm, the strength against repeated deformation of ferrite cannot be increased and a desired fatigue property cannot be obtained.

[0125] Therefore, the average sphere equivalent radius of alloy carbides in the ferrite is set to 0.5 nm or more. The average sphere equivalent radius of alloy carbides in the ferrite is preferably 1.0 nm or more.

[0126] On the other hand, when the average sphere equivalent radius of alloy carbides in the ferrite is 10.0 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 10.0 nm. The average sphere equivalent radius of alloy carbides in the ferrite is preferably 8.0 nm or less, 6.0 nm or less or 4.0 nm or less, and more preferably less than 2.0 nm.Average Number Density of Alloy Carbides in Ferrite: 0.10×1016 Pieces / cm3 or More and Less than 1.45×1016 Pieces / cm3

[0127] When the average number density of the alloy carbides in the ferrite is less than 0.10×1016 pieces / cm3 or 1.45×1016 pieces / cm3 or more, 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 0.10×1016 pieces / cm3 or more and less than 1.45×1016 pieces / cm3. The average number density of the alloy carbides in the ferrite is preferably 0.50×1016 pieces / cm3 or more, and more preferably 1.00×1016 pieces / cm3 or more. In addition, the average number density of the alloy carbides in the ferrite is preferably 1.40×1016 pieces / cm3 or less, more preferably 1.20×1016 pieces / cm3 or less, and still more preferably 1.10×1016 pieces / cm3 or less.

[0128] In the present embodiment, the alloy carbides refer to carbides containing one or two or more of Ti, Nb, Mo, and V.

[0129] 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.

[0130] 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 1 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.

[0131] 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, and 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.

[0132] 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 1D Concentration Profile function of the data analysis software IVAS 3.6.14 (manufactured by CAMECA Instruments Inc.).

[0133] 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.

[0134] E Value: 10.7 or more

[0135] I Value: 1.020 or more

[0136] 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.

[0137] 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.

[0138] 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.

[0139] In the present embodiment, the photographing region of a SEM image photographed for calculating the E value and the I value is set to, in a cross section parallel to the rolling direction at a position of ¼ from the end surface in the sheet width direction, 200 μm×200 μm centered in a position of ¼ from the surface in the sheet thickness direction, and the number of the observation fields is set to 5. 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.

[0140] 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 gray level co-occurrence matrices method (the GLCM method) described in Non-Patent Document 1.

[0141] 179 matrices 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 matrices P standardized so 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. Note that the average value obtained by measuring the entire visual fields is calculated.

[0142] P(i,j) in the following formula (1) and formula (2) is the gray level co-occurrence matrix, the value at the ith row and jth column of the matrix P is expressed as P(i,j). As described above, since the calculation is performed using the 256×256 matrices P, when this point is emphasized, the following formula (1) can be modified to the following formula (1′), and the following formula (2) can be modified to the following formula (2′). In the following formula (1′) and formula (2′), the value at the ith row and the jth column of the matrix P is expressed as Pij.[Formula⁢ 1]E=-∑ i?p⁡(i,j)·log⁢ (p⁡(i,j))(1)[Formula⁢ 2]I=?∑jp⁡(i,j)1+<semantics definitionURL="">❘<annotation encoding="Mathematica">"\[LeftBracketingBar]"< / annotation>< / semantics>i-j<semantics definitionURL="">❘<annotation encoding="Mathematica">"\[RightBracketingBar]"< / annotation>< / semantics>L(2)[Formula⁢ 3]E=-?⁢Pij⁢ log⁢ Pij(1′)[Formula⁢ 4]I=?Pij / (1+<semantics definitionURL="">❘<annotation encoding="Mathematica">"\[LeftBracketingBar]"< / annotation>< / semantics>i-j<semantics definitionURL="">❘<annotation encoding="Mathematica">"\[RightBracketingBar]"< / annotation>< / semantics> / 256(2′)?indicates text missing or illegible when filedStandard Deviation of Mn Concentration: 0.60 Mass % or Less

[0143] The standard deviation of the Mn concentration of the hot-rolled steel sheet according to the present embodiment 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.The Standard Deviation of the Mn Concentration can be Obtained by the Following Method.

[0144] First, in a position of ¼ from the end surface in the sheet width direction, a sample is collected so that a region of ¼ from the surface in the sheet thickness direction in a cross section parallel to the rolling direction can be observed. The size of the sample depends on a measurement device, but is set to a size that can be observed by about 10 mm in the rolling direction. Next, after mirror polishing the sample, the standard deviation of the Mn concentration is measured using an electron probe micro analyzer (EPMA). As the measurement conditions, the acceleration voltage is set to 15 kV and the magnification is set to 5000 times, and the distribution image of 40000 or more points in a range that is 20 μm in the rolling direction and 20 μm in the sheet thickness direction are measured with the measurement interval of to 0.1 μm. 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

[0145] 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 test piece may be set to a position of ¼ from the end surface in the sheet width direction of the hot-rolled steel sheet, and the sheet width direction may be set to the longitudinal direction.

[0146] In the hot-rolled steel sheet according to the present embodiment, the tensile (maximum) strength is preferably 980 MPa or more. The tensile strength is more preferably 1000 MPa or more. By setting the tensile strength to 980 MPa or more, 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.

[0147] 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.

[0148] 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

[0149] When hardening of materials occurs during repeated deformation, since an amount of plastic deformation during repeated deformation become small, fatigue life may be longer. Therefore, it is preferable that hardening of materials occurs during repeated deformation. Whether or not hardening occurs during repeated deformation can be determined by the following method.

[0150] In a position of ¼ from the end surface in the sheet width direction of the hot-rolled steel sheet, in accordance with JIS Z 2275-1978, the test piece of the symbol “1-15” is collected so that the sheet width direction is the longitudinal direction of the test piece. Using this test piece, a plane bending fatigue test is performed in accordance with JIS Z 2275-1978 with the repeated stress so that the fracture repeated counts becomes 3 million or more and less than 10 million 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 repeated stress at 100 repeated counts is defined as standard stress, in a range of repeated counts of 100 thousand to 1 million, when the repeated stress becomes 5% or more higher than the standard stress, the hot-rolled steel sheet can be determined that repeated hardening occurs and has excellent fatigue property.Sheet Thickness

[0151] 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.

[0152] 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

[0153] 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

[0154] 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.

[0155] 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.

[0156] 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 tension that is loaded in the rolling direction of the steel sheet. The stress can be controlled by adjusting the rotation speed of the rolling stand and the coiling device, and can be determined by dividing the measured load in the rolling direction by the cross-sectional area of the passing sheet.

[0157] (1) The slab is retained in a temperature range of 700° C. to 850° C. for 900 seconds or longer, then, further heated, and retained in a temperature range of 1100° C. or higher for 6000 seconds or longer.

[0158] (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.

[0159] (3) Rolling 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.

[0160] (4) The rolling reduction 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.

[0161] (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.

[0162] (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.

[0163] (7) After the light rolling, the steel sheet is rapidly cooled to a temperature range of 600° C. or higher and lower than 680° C. at an average cooling rate of 50° C. / s or faster.

[0164] (8) Slow cooling at an average cooling rate of slower than 5° C. / s is performed in a temperature range of 600° C. or higher and lower than 680° C. for 2.0 seconds or longer.

[0165] (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.

[0166] (10) The steel sheet is coiled in a temperature range of 350° C. or lower.

[0167] 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.

[0168] (1) Slab, Slab Temperature and Retention Time on Hot Rolling

[0169] 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 retained in a temperature range of 700° C. to 850° C. for 900 seconds or longer during slab heating, then, further heated, and retained in a temperature range of 1100° C. or higher for 6000 seconds or longer.

[0170] Note that 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.

[0171] 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.

[0172] 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.

[0173] (2) Rolling Reduction of Hot Rolling: Sheet Thickness Reduction of 90% or more in Total in Temperature Range of 850° C. to 1100° C.

[0174] 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.

[0175] 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 t0 and an outlet sheet thickness after the rolling of the final stand in this temperature range is t1.

[0176] (3) Rolling Temperature at One Stand before Final Stand: 900° C. or Higher and Lower Than 1010° C., Stress After Rolling at One Stand before Final Stand of Hot Rolling and Before Rolling at Final Stand: 170 kPa or More

[0177] 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. Note that the stress that is loaded to the steel sheet refers to tension in the longitudinal direction of the steel sheet, and can be controlled by adjusting the roll rotation speed during tandem rolling.

[0178] The upper limit of the stress that is loaded to the steel sheet is not particularly limited, but may be 350 kPa or lower.

[0179] (4) Rolling Reduction at Final Stand of Hot Rolling: 8% or more, Finishing Temperature Tf: 900° C. or higher and lower than 1010° C.

[0180] It is preferable that the rolling reduction 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 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. 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. Note that the upper limit of the rolling reduction at the final stand of the hot rolling is not particularly limited, but can be 30% or lower, 20% or lower, and preferably set to 15% or lower.

[0181] (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.

[0182] 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.

[0183] Light rolling may be performed, for example, at the final stand of the finishing mill, or by introducing new rolling equipment between the finishing mill and the cooling bed.

[0184] 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 t0 and an outlet sheet thickness after the rolling of the final stand in the light rolling.

[0185] (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

[0186] 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.

[0187] (7) After Light Rolling. Accelerated Cooling to Temperature Range of 600° C. or Higher and Lower than 680° C. at Average Cooling Rate of 50° C. / s 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 lower than 680° C. at the average cooling rate of 50° C. / s or faster after the light rolling. By performing the accelerated cooling to the temperature range of lower than 680° C., the average number density of the alloy carbides in the ferrite can be set to less than 1.45×1016 pieces / cm3. In addition, by setting the average cooling rate in the temperature range of 600° C. or higher and lower than 680° C. to 50° C. / s or faster, excessive generation of pearlite can be suppressed.

[0189] Note that 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. / s or slower. In addition, in order to set the area ratio of ferrite to 15.0% or more, the average number density of the alloy carbides in the ferrite to 0.10×1016 pieces / cm3 or more, and the average sphere equivalent radius of alloy carbides in the ferrite to 0.5 nm or more, the cooling stop temperature of the accelerated cooling is preferably set to 600° 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.

[0192] (8) Slow Cooling at Average Cooling Rate of Slower Than 5° C. / s Being Performed in Temperature Range of 600° C. or Higher and Lower than 680° C. for 2.0 Seconds or Longer

[0193] When slow cooling at an average cooling rate of slower than 5° C. / s is performed in a temperature range of 600° C. or higher and lower than 680° C. for 2.0 seconds or longer, a desired amount of ferrite can be obtained. In addition, the average number density of the alloy carbides in the ferrite can be controlled in a desired amount.

[0194] Note that 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.

[0195] 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.

[0196] (9) Average Cooling Rate to Coiling Temperature: 50° C. / s or Faster

[0197] 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. / s or faster. In such a case, the primary phase structure can be made full hard, and the average sphere equivalent radius and the average number density of the alloy carbides in the ferrite can be controlled in a desired amount.

[0198] Note that 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.

[0199] (10) Coiling Temperature: 350° C. or Lower

[0200] 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.Examples

[0201] 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.

[0202] 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 5A to Table 6B under the manufacturing conditions shown in Table 3A to Table 4B.

[0203] 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 4A and Table 4B is 50° C. the actual coiling temperatures of the examples with a value of 50° C. are 50° C. or lower. In addition, 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.

[0204] 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 5A to Table 6B.Evaluation Method of Properties of Hot-Rolled Steel SheetsTensile Properties

[0205] 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

[0206] By performing the plane bending fatigue test by the above-described method. in a range of repeated counts of 100 thousand to 1 million, when the repeated stress became 5% or more higher than the standard stress, it was determined that repeated hardening occurred, and as being the hot-rolled steel sheet having excellent fatigue property and being successful.

[0207] On the other hand, in a range of repeated counts of 100 thousand to 1 million, when the repeated stress did not become 5% or more higher than the standard stress, it was determined that repeated hardening did not occur, and as not being the hot-rolled steel sheet having excellent fatigue property and not being successful.

[0208] The examples that were determined as being successful were mentioned with “Good” in the column of fatigue property in Tables, and the examples that were determined as not being successful were mentioned with “NG” in Tables.Shearing Property (Evaluation of Secondary Sheared Surface)

[0209] The shearing property of the hot-rolled steel sheet was evaluated by a punching test.

[0210] 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.

[0211] 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. 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. The examples that were determined as being successful were mentioned with “Good” in the column of shearing property in Tables, and the examples that were determined as not being successful were mentioned with “NG” in Tables.TABLE 1SteelMass %, remainder is Fe and impuritiesNo.CSiMnTiNbVTi + Nb + Vsol. AlPSNONoteA0.0611.451.920.0870.0870.0290.0130.00050.00300.0024Present Invention ExampleB0.0981.271.600.0930.0930.0260.0020.00250.00150.0028Present Invention ExampleC0.1591.341.910.1260.1260.0710.0190.00460.00270.0015Present Invention ExampleD0.1100.511.660.0860.0860.0390.0240.00420.00350.0019Present Invention ExampleB0.0832.801.960.1110.1110.0350.0260.00250.00290.0031Present Invention ExampleF0.1020.981.350.0980.0980.0630.0180.00490.00300.0013Present Invention ExampleG0.1031.223.630.1030.0390.1420.0260.0230.00060.00300.0046Present Invention ExampleH0.1020.821.870.0600.0600.0570.0190.00280.00350.0028Present Invention ExampleI0.1041.191.850.1400.1400.0330.0070.00530.00230.0030Present Invention ExampleJ0.0961.131.870.0300.0330.0750.1380.0360.0210.00520.00280.0042Present Invention ExampleK0.0960.941.920.1240.1240.0500.0220.00410.00190.0036Present Invention ExampleL0.0820.901.820.1090.1090.0570.0310.00310.00470.0029Present Invention ExampleM0.0901.201.700.1070.0610.1680.0510.0310.00300.00370.0046Present Invention ExampleN0.0851.131.590.1020.1020.0430.0190.00450.00490.0027Present Invention ExampleO0.1061.131.860.2030.2030.0540.0150.00260.00310.0031Present Invention ExampleP0.0441.021.890.1230.1230.0220.0010.00320.00470.0027Comparative ExampleQ0.2620.951.810.1180.1180.0620.0130.00390.00490.0060Comparative ExampleR0.0953.191.750.1000.1000.0280.0210.00210.00340.0038Comparative ExampleS0.0960.930.730.1290.1290.0570.0100.00060.00620.0004Comparative ExampleT0.1151.181.890.0370.0110.0480.0400.0150.00530.00250.0051Comparative ExampleU0.0781.482.100.1050.1050.3090.0110.00260.00420.0024Present Invention ExampleV0.0651.882.400.0860.0860.3840.0230.00130.00270.0024Present Invention ExampleW0.2371.071.880.0700.0700.0630.0140.00300.00260.0025Present Invention ExampleX0.2151.351.520.1320.1300.1100.3720.0520.0210.00240.00320.0024Present Invention ExampleUnderlines indicate that values are outside the range of the present invention.TABLE 2SteelMass %, remainder is Fe and impuritiesNo.CuCrMoNiBCaMgREMBiA0.00360.0014BC0.0035D0.380.180.39E0.0050FGHIJK0.22L0.27M0.15N0.36O0.0041UVWXSteelMass %, remainder is Fe and impuritiesNo.AsZrCoZnWSnNoteAPresent Invention ExampleBPresent Invention ExampleCPresent Invention ExampleDPresent Invention ExampleE0.27Present Invention ExampleFPresent Invention ExampleGPresent Invention ExampleH0.03Present Invention ExampleIPresent Invention ExampleJPresent Invention ExampleK0.13Present Invention ExampleLPresent Invention ExampleM0.03Present Invention ExampleNPresent Invention ExampleO0.02Present Invention ExamplePComparative ExampleQComparative ExampleRComparative ExampleSComparative ExampleTComparative ExampleU0.095Present Invention ExampleVPresent Invention ExampleWPresent Invention ExampleXPresent Invention ExampleTABLE 3ASheetLoaded stressRetentionRetentionthicknessafter rollingtime intime inreduction inat one standtemperaturetemperaturetemperaturebefore finalRollingrange ofrange ofrange ofstand andFinishingreductionManu-700° C. toHeating1100° C. or850° C. tobefore rollingtemperatureat finalfacturingSteel850° C.temperaturehigher1100° C.at final standTfstandNo.No.s° C.s%kPa° C.%Note 1A15561219932997200943 9Present Invention Example 2B14261219910194212973 9Present Invention Example 3B 800127289259122393110Comparative Example 4B1096122292718921295112Comparative Example 5B1232120756109520195911Comparative Example 6B1402120895609616298612Comparative Example 7B153512098943932221035  8Comparative Example 8B14201255806591197986 6Comparative Example 9B1283125091329723194710Comparative Example10B1276128795679522995112Present Invention Example11B1315122689509322996210Comparative Example12B1467120595269620494110Comparative Example13B1119125189849721197011Comparative Example14B15011286932594227932 8Comparative Example15B14051280903196203935 9Comparative Example16C13531289912394214946 8Present Invention Example17D15921233974195202987 8Present Invention Example18E1596119792939322694811Present Invention Example19F15921188934090201941 8Present Invention Example20G141012039296922141002 28Present Invention Example21H15401272950294176957 9Present Invention Example22I1458121490189421092111Present Invention Example23J1547128095289621196111Present Invention Example24K1356129394229921794611Present Invention Example25L1434127593199721393812Present Invention ExampleUnderlines indicate that values are outside the range of the present invention or are not preferable manufacturing conditions.TABLE 3BSheetLoaded stressRetentionRetentionthicknessafter rollingtime intime inreduction inat one standtemperaturetemperaturetemperaturebefore finalRollingrange ofrange ofrange ofstand andFinishingreductionManu-700° C. toHeating1100° C. or850° C. tobefore rollingtemperatureat finalfacturingSteel850° C.temperaturehigher1100° C.at final standTfstandNo.No.s° C.s%kPa° C.%Note26M129412688920952239679Present Invention Example27N1166120295029721293611Present Invention Example28O1201124195719623196510Present Invention Example29P1450127492029521796912Comparative Example30Q121112928882952059338Comparative Example31R1582129890989622793711Comparative Example32S1398128594239521298011Comparative Example33T1424127095539722195912Comparative Example34U 887124490609623794912Comparative Example35U2227123794469721791412Present Invention Example36U1472124311930 9520494813Present Invention Example37U123312305840942089449Comparative Example38U1357120494269320293112Present Invention Example39U1495126590799723297812Present Invention Example40U1319121688649622094010Comparative Example41U1196125793939422093411Comparative Example42U131112698948922299349Comparative Example43U 908126791689421494111Present Invention Example44U1335124066639621996813Present Invention Example45V1313125591249620397410Comparative Example46V126212159569932329509Present Invention Example47V1255123790539424294312Comparative Example48W1501128791239021293711Present Invention Example49X1495128595539522795911Present Invention Example50G14051203920297208100812Present Invention ExampleUnderlines indicate that values are outside the range of the present invention or are not preferable manufacturing conditions.TABLE 4ASheet thickness reductionLoaded stress afterLoaded stress afterof light rolling atrolling at final standrolling at final stand ofAveragetemperature range ofof hot rolling andlight rolling until steelcooling rate of840° C. or higher andbefore first rolling ofsheet being cooled toacceleratedManufacturingSteellower than 900° C.light rolling800° C.coolingNo.No.%kPakPa° C. / s 1A6 9647104 2B6100152  97 3B6 8150 80 4B6117182 110 5B6102137 102 6B6119180 114 7B6 90114  89 8B6 6394 59 9B4 8092 7810B6 8878 9311B6126184 12712B620292 4113B6108201 10614B6118178 11915B6 95133  9916C6 8251 9017D6 968510418E6107127 11619F6 8953 9720G61137511521H6117148 11522I61038210123J610218 9824K6112103 11825L611249106Cooling stopSlow cooling time inAveragetemperature oftemperature range ofcooling rateaccelerated600° C. or higher anduntil coilingCoilingManufacturingcoolinglower than 680° C.temperaturetemperatureNo.° C.s° C. / s° C.Note 16613.811250Present Invention Example 26663.411850Present Invention Example 36724.010250Comparative Example 46433.210250Comparative Example 56733.914850Comparative Example 66663.212050Comparative Example 76583.916350Comparative Example 86442.111250Comparative Example 96253.2 9450Comparative Example106284.8 7950Present Invention Example116671.310250Comparative Example126444.512950Comparative Example136974.4 8350Comparative Example146704.4 4550Comparative Example156403.4 750Comparative Example166623.914750Present Invention Example176784.515050Present Invention Example186024.310250Present Invention Example196384.114450Present Invention Example206254.5 8250Present Invention Example216273.413750Present Invention Example226472.314250Present Invention Example236653.0 8250Present Invention Example246674.410450Present Invention Example256263.210350Present Invention ExampleUnderlines indicate that values are outside the range of the present invention or are not preferable manufacturing conditions.TABLE 4BSheet thickness reductionLoaded stress afterLoaded stress afterof light rolling atrolling at final standrolling at final stand ofAveragetemperature range ofof hot rolling andlight rolling until steelcooling rate of840° C. or higher andbefore first rolling ofsheet being cooled toacceleratedManufacturingSteellower than 900° C.light rolling800° C.coolingNo.No.%kPakPa° C. / s26M611016110627N612818012528O612117712029P6 92 2610430Q61121438331R6106 9810032S612510410933T610115011434U6 531509235U6 97 8810336U6109 8510337U6115 4511838U6113 4710639U6127 575540U611320510041U9107 5311442U6 8117112043U6 98 3010944U610018712645V6204 7511746V610210412047V610512511648W6 9710410949X6102 9811450G6127 88103Cooling stopSlow cooling time inAveragetemperature oftemperature range ofcooling rateaccelerated600° C. or higher anduntil coilingCoilingManufacturingcoolinglower than 680° C.temperaturetemperatureNo.° C.s° C. / s° C.Note266733.19150Present Invention Example276722.013150Present Invention Example286333.314350Present Invention Example296143.112250Comparative Example306754.012550Comparative Example316463.513350Comparative Example326644.49950Comparative Example336253.113350Comparative Example346283.511450Comparative Example356323.2130210Present Invention Example366523.115250Present Invention Example376653.710050Comparative Example386733.810850Present Invention Example396613.610150Present Invention Example406204.913650Comparative Example416554.812050Comparative Example426491.98750Comparative Example436684.012150Present Invention Example446223.8135140Present Invention Example456753.160320Comparative Example466304.011450Present Invention Example475953.710550Comparative Example486774.16150Present Invention Example496738.213350Present Invention Example506653.19950Present Invention ExampleUnderlines indicate that values are outside the range of the present invention or are not preferable manufacturing conditions.TABLE 5AAverage sphereequivalentAverage numberMnRemainderradius of alloydensity of alloystandardManu-Residualin micro-carbides incarbides inEIdevi-facturingSteelausteniteFerritePearlitestructureferriteferritevaluevalueationNo.No.Area %Area %Area %Area %nm×1016 pieces / cm3——Mass %Note 1A0.059.60.040.49.11.3911.41.0500.43Present Invention Example 2B0.026.80.073.26.20.6210.81.0210.46Present Invention Example 3B0.031.40.068.64.60.8311.21.0240.65Comparative Example 4B0.030.10.069.97.40.4910.81.0230.62Comparative Example 5B0.026.00.074.09.10.6311.01.0230.65Comparative Example 6B0.023.30.076.77.20.1410.51.0200.45Comparative Example 7B0.031.80.068.23.01.1910.31.0370.43Comparative Example 8B0.026.50.073.57.80.1210.51.0320.45Comparative Example 9B0.020.40.079.60.41.2710.71.0330.44Comparative Example10B0.027.90.072.19.40.6311.21.0430.49Present Invention Example11B0.0 8.30.091.76.80.0811.01.0210.45Comparative Example12B0.057.05.937.18.11.5510.61.0660.49Comparative Example13B0.059.50.040.55.11.5310.51.0960.40Comparative Example14B0.044.75.150.20.40.0910.81.0530.41Comparative Example15B5.559.69.225.711.1 0.0711.11.0870.40Comparative Example16C1.416.00.082.67.60.5711.11.0390.47Present Invention Example17D0.017.20.082.85.71.0011.21.0310.51Present Invention Example18E2.251.30.046.56.41.3711.11.0690.42Present Invention Example19F0.042.90.057.15.20.5810.71.0400.50Present Invention Example20G0.015.30.084.72.71.2411.11.0230.58Present Invention Example21H0.025.30.074.75.20.5610.71.0220.47Present Invention Example22I0.015.20.084.80.71.1310.91.0220.50Present Invention Example23J0.028.10.071.98.70.1511.11.0210.42Present Invention Example24K0.032.50.067.51.61.2611.21.0330.51Present Invention Example25L0.031.20.068.87.30.3610.91.0210.43Present Invention ExampleUnderlines indicate that values are outside the range of the present invention or properties are not preferable.TABLE 5BAverage sphereequivalentAverage numberMnRemainderradius of alloydensity of alloystandardManu-Residualin micro-carbides incarbides inEIdevi-facturingSteelausteniteFerritePearlitestructureferriteferritevaluevalueationNo.No.Area %Area %Area %Area %nm×1016 pieces / cm3——Mass %Note26M0.034.90.065.15.20.9611.01.0300.50Present Invention Example27N0.018.70.081.39.80.1511.11.0230.40Present Invention Example28O0.030.20.069.87.70.4011.11.0250.39Present Invention Example29P0.082.60.017.40.30.1210.91.0920.49Comparative Example30Q2.1 2.00.095.99.90.2410.91.0170.45Comparative Example31R2.668.10.029.39.01.4410.91.0550.48Comparative Example32S0.074.00.026.06.80.8311.21.0850.46Comparative Example33T0.027.30.072.78.00.0911.01.0320.45Comparative Example34U0.057.80.042.28.11.4111.11.0310.62Comparative Example35U0.051.70.048.38.40.4810.81.0250.41Present Invention Example36U0.036.80.063.29.20.2610.91.0250.45Present Invention Example37U0.054.60.045.45.20.3610.81.0330.61Comparative Example38U0.053.90.046.18.10.3911.31.0400.51Present Invention Example39U0.038.10.061.98.51.3911.01.0350.45Present Invention Example40U0.059.10.040.96.10.3110.61.0290.46Comparative Example41U0.041.60.058.410.3 0.0810.71.0270.47Comparative Example42U0.011.50.088.53.90.0611.21.0360.43Comparative Example43U0.056.10.043.95.01.1011.11.0230.53Present Invention Example44U0.059.30.040.78.90.4411.11.0330.55Present Invention Example45V0.036.20.063.80.60.1110.51.1070.45Comparative Example46V0.043.10.056.94.11.4411.21.0280.48Present Invention Example47V0.011.00.089.00.30.0911.01.0310.44Comparative Example48W2.416.24.776.75.30.8811.11.0250.39Present Invention Example49X2.117.63.077.39.20.5810.81.0350.51Present Invention Example50G0.036.80.063.23.01.2411.21.0270.43Present Invention ExampleUnderlines indicate that values are outside the range of the present invention or properties are not preferable.TABLE 6AFatigue propertyShearing propertyTensileTotal(occurrence or(presence orSheetstrengthelongationTS ×non-occurrenceabsence of secondaryManufacturingSteelthicknessTSElElof repeated hardening)sheared surface)No.No.mmMPa%MPa · %——Note 1A2.6 99117.317144GoodGoodPresent Invention Example 2B2.6102015.615912GoodGoodPresent Invention Example 3B2.6101415.816021GoodNGComparative Example 4B2.6100115.115115GoodNGComparative Example 5B2.6103215.11.5583GoodNGComparative Example 6B2.6103314.414875GoodNGComparative Example 7B2.6102613.914261GoodNGComparative Example 8B2.6100815.315422GoodNGComparative Example 9B2.6108412.413442NGGoodComparative Example10B2.6104315.516167GoodGoodPresent Invention Example11B2.61086 9.610426NGGoodComparative Example12B2.6 966 9.4 9080NGNGComparative Example13B2.6 99516.015920NGNGComparative Example14B2.6 965 8.9 8589NGGoodComparative Example15B2.6 915 7.2 6588NGNGComparative Example16C6.0159113.020683GoodGoodPresent Invention Example17D2.6 99714.614556GoodGoodPresent Invention Example18E2.6113913.114921GoodGoodPresent Invention Example19F1.6 99816.816766GoodGoodPresent Invention Example20G2.6130610.413582GoodGoodPresent Invention Example21H2.6 98316.416121GoodGoodPresent Invention Example22I2.6102613.313646GoodGoodPresent Invention Example23J2.6100313.813841GoodGoodPresent Invention Example24K2.6104516.216929GoodGoodPresent Invention Example25L2.6 99215.315178GoodGoodPresent Invention ExampleUnderlines indicate that corresponding values are outside the range of the present invention or not preferable properties.TABLE 6BFatigue propertyShearing propertyTensileTotal(occurrence or(presence orSheetstrengthelongationTS ×non-occurrenceabsence of secondaryManufacturingSteelthicknessTSElElof repeated hardening)sheared surface)No.No.mmMPa%MPa · %——Note26M2.6103415.115613GoodGoodPresent Invention Example27N2.6101313.213372GoodGoodPresent Invention Example28O2.6 99215.615475GoodGoodPresent Invention Example29P2.6 90216.014432NGGoodComparative Example30Q2.61895 9.217434GoodNGComparative Example31R2.61020 9.6 9792GoodGoodComparative Example32S2.6 97717.817391GoodGoodComparative Example33T2.6 93915.914930NGGoodComparative Example34U3.2104415.516182GoodNGComparative Example35U3.2102614.414774GoodGoodPresent Invention Example36U3.2105714.515327GoodGoodPresent Invention Example37U3.2103114.214640GoodNGComparative Example38U3.2107115.516601GoodGoodPresent Invention Example39U3.2105714.915749GoodGoodPresent Invention Example40U3.2105515.316142GoodNGComparative Example41U3.2104416.116808NGNGComparative Example42U3.2109211.412449NGGoodComparative Example43U3.2102615.515903GoodGoodPresent Invention Example44U3.2103815.015570GoodGoodPresent Invention Example45V2.9103218.619195GoodNGComparative Example46V2.9104915.616364GoodGoodPresent Invention Example47V2.91093 9.710602NGGoodComparative Example48W2.6107614.215279GoodGoodPresent Invention Example49X2.6112713.314989GoodGoodPresent Invention Example50G2.6103114.915362GoodGoodPresent Invention ExampleUnderlines indicate that corresponding values are outside the range of the present invention or not preferable properties.From Table 5A to Table 6B, 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.

Examples

examples

[0201]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.

[0202]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 5A to Table 6B under the manufacturing conditions shown in Table 3A to Table 4B.

[0203]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 4A and Table 4B is 50° C....

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. 2022-135960, filed on Aug. 29, 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 1 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. H10-168544Non-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 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] Q: 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 0.0200%,

[0036] As: 0% to 0.100%,

[0037] one or two or more of Zr, Co, Zn, and W: 0% to 1.00% in total,

[0038] Sn: 0% to 0.05%, and

[0039] a remainder comprising Fe and impurities,

[0040] in which, in a microstructure,

[0041] in terms of area %,

[0042] residual austenite is less than 3.0%,

[0043] ferrite is 15.0% or more and less than 60.0%, and

[0044] pearlite is less than 5.0%,

[0045] an average sphere equivalent radius of alloy carbides in the ferrite is 0.5 nm or more and less than 10.0 nm,

[0046] an average number density of the alloy carbides in the ferrite is 0.10×1016 pieces / cm3 or more and less than 1.45×1016 pieces / cm3,

[0047] an E value that indicates periodicity of the microstructure is 10.7 or more,

[0048] an I value that indicates uniformity of the microstructure is 1.020 or more, and

[0049] a standard deviation of Mn concentration is 0.60 mass % or less.

[0050] (2) The hot-rolled steel sheet according to (1), in which the chemical composition may comprise, in terms of mass %, one or two or more selected from the group consisting of:

[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%,

[0059] Bi: 0.0005% to 0.0200%, and

[0060] As: 0.001% to 0.100%.Effects of the Invention

[0061] 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.

[0062] 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

[0063] FIG. 1 is an example of a sheared end surface of a hot-rolled steel sheet according to a present invention example.

[0064] 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

[0065] 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.

[0066] 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

[0067] 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%

[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 more than 0.070%, 0.75% or more or 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.Si: 0.05% to 3.00%

[0070] 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.

[0071] 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%

[0072] 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.

[0073] 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

[0074] 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, more preferably 0.100% or more, and still more preferably 0.120% or more.

[0075] 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%

[0076] 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.

[0077] 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 0.400% or less, more preferably 0.300% or less, and still more preferably 0.250% or less.

[0078] 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

[0079] P 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 the P content is preferably set to 0.001% from the viewpoint of the refining cost.S: 0.0300% or less

[0080] S 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 the S content is preferably set to 0.0001% from the viewpoint of the refining cost.N: 0.1000% or less

[0081] N 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 0.0050% or less. Although the lower limit of the N content does not need to be particularly specified, but 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 in a case where one or two or more of Ti, Nb, and V are contained to further refine the microstructure.O: 0.0100% or Less

[0082] 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, and still more preferably 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.

[0083] 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.

[0084] 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.

[0085] Cu: 0.01% to 2.00%

[0086] Cr: 0.01% to 2.00%

[0087] Mo: 0.01% to 1.00%

[0088] Ni: 0.02% to 2.00%

[0089] B: 0.0001% to 0.0100%

[0090] 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.

[0091] 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.

[0092] 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.

[0093] 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.

[0094] 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.

[0095] 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.

[0096] Ca: 0.0005% to 0.0200%

[0097] Mg: 0.0005% to 0.0200%

[0098] REM: 0.0005% to 0.1000%

[0099] Bi: 0.0005% to 0.0200%

[0100] 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 0.0200%, 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 0.0200% or less. The Bi content is preferably 0.0100% or less.

[0101] 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.As: 0.001% to 0.100%

[0102] As lowers an austenitizing temperature and thus refines the prior austenite grains, thereby contributing for an improvement of ductility of the hot-rolled steel sheet. In order to reliably obtain the effects, the As content is preferably set to 0.001% or more.

[0103] On the other hand, since the above effects are saturated even in a case where a large amount of As is contained, the As content is set to 0.100% or less.One or Two or More of Zr, Co, Zn, or W: 0% to 1.00% in TotalSn: 0% to 0.05%

[0104] 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.

[0105] 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.

[0106] 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.

[0107] When a plating layer or a coating film is provided on the surface of the hot-rolled steel sheet, the chemical composition is analyzed after the plating layer or the coating film is removed by mechanical grinding or the like as necessary.Microstructure of Hot-Rolled Steel Sheet

[0108] Next, the microstructure of the hot-rolled steel sheet according to the present embodiment will be described.

[0109] 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 10.0 nm, an average number density of the alloy carbides in the ferrite is 0.10×1016 pieces / cm3 or more and less than 1.45×1016 pieces / cm3, 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.

[0110] 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.

[0111] 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 region of a position of ¼ from the surface in a sheet thickness direction of the hot-rolled steel sheet and a position of ¼ from an end surface in a direction (a sheet width direction) perpendicular to a rolling direction and the sheet thickness direction (the rolling direction is an arbitrary position) are specified. The reason therefor is that the microstructure at this position indicates a typical microstructure of the steel sheet. When the hot-rolled steel sheet has the plating layer or the coating film, the “surface” refers to the interface of the plating layer or the coating film and the steel sheet.Area Ratio of Residual Austenite: Less than 3.0%

[0112] 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%.

[0113] 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.

[0114] In the measurement of the area ratio of the residual austenite by X-ray diffraction in the present embodiment, first, in a cross section at a position of ¼ from the surface in the sheet thickness direction of the hot-rolled steel sheet, a sample is collected so that the microstructure at a region of 1 mm or more in an arbitrary position of the rolling direction and 1 mm or more centered in a position of ¼ from the end surface in the sheet width direction can be observed. For the sample, 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 using Co-Kαrays. Next, the volume ratio of the residual austenite is obtained using the strength averaging method from the integrated intensities, and the obtained volume ratio is regarded as an area ratio of the residual austenite.Area Ratio of Ferrite: 15.0% or More and Less than 60.0%

[0115] 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.

[0116] 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%

[0117] 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. Furthermore, in the interface of ferrite and cementite that are contained in pearlite, micro voids which deteriorates ductility generate in early stage, and therefore a desired ductility and fatigue property cannot be obtained when the area ratio of pearlite is 5.0% or more. 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 still more preferably 0%.

[0118] 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.

[0119] Measurement of the area ratios of the microstructure other than residual austenite is conducted by the following method. First, in a position of ¼ from the end surface in the sheet width direction of the hot-rolled steel sheet, a sample is collected so that the microstructure in a region of a position of ¼ from the surface in the sheet thickness direction in a cross section parallel to the rolling direction can be observed. The size of the sample depends on a measurement device, but is set to a size that can be observed by about 10 mm in the rolling direction. Next, after mirror finishing the observed cross section of the sample by polishing, the observed cross section of the sample is polished at room temperature with colloidal silica not containing an alkaline solution for 8 minutes, thereby removing strain introduced into the surface layer of the sample. For a region of 200 μm or more at an arbitrary position in the rolling direction and 200 μm or more centered in a position of ¼ from the surface in the sheet thickness direction of the observed cross section, crystal orientation information is obtained by a measurement using electron backscatter diffraction at a measurement interval of 0.1 μm. 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.

[0120] 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.

[0121] The area ratio of a remainder of the microstructure is obtained by subtracting the area ratios of residual austenite, pearlite, and ferrite from 100%. Note that the remainder of the microstructure can be estimated to be a hard structure consisting of one or two or more of bainite, martensite, and tempered martensite from the chemical composition of the hot-rolled steel sheet and the manufacturing conditions.

[0122] Note that the rolling direction of the hot-rolled steel sheet is determined by the following method.

[0123] First, a test piece is collected so that a sheet thickness cross section of the hot-rolled steel sheet can be observed. The sheet thickness cross section of the collected test piece is observed using an optical microscope after mirror polishing and corrosion with a picric acid-saturated aqueous solution. An observation range is set to an entire thickness of the sheet thickness, and an extending direction of grains is determined. Here, an angle difference between the extending direction and the sheet thickness direction is set to θ. Furthermore, a surface at a position of ¼ of the sheet thickness that is perpendicular to the sheet thickness direction and parallel to the above-mentioned extending direction is polished similar to the sheet thickness cross section, and the extending direction of grains is determined. Here, an angle difference between the sheet thickness cross section and the extending direction is set to φ. In a spherical coordinate system in accordance with ISO 3785 with the sheet thickness direction as the Z axis, the direction with θ and φ, which are obtained from the extending direction of the grains in the above-mentioned two cross sections, as deflection angles is determined as the rolling direction. To determine the extending direction θ and φ, the “Analyze Particles” function of the image analysis software “ImageJ” can be used to obtain the extending direction of the grains by setting the circularity to 0.7 or less.Average Sphere Equivalent Radius of Alloy Carbides in Ferrite: 0.5 nm or More and Less than 10.0 nm

[0124] In the hot-rolled steel sheet according to the present embodiment, 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 nm, the strength against repeated deformation of ferrite cannot be increased and a desired fatigue property cannot be obtained.

[0125] Therefore, the average sphere equivalent radius of alloy carbides in the ferrite is set to 0.5 nm or more. The average sphere equivalent radius of alloy carbides in the ferrite is preferably 1.0 nm or more.

[0126] On the other hand, when the average sphere equivalent radius of alloy carbides in the ferrite is 10.0 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 10.0 nm. The average sphere equivalent radius of alloy carbides in the ferrite is preferably 8.0 nm or less, 6.0 nm or less or 4.0 nm or less, and more preferably less than 2.0 nm.Average Number Density of Alloy Carbides in Ferrite: 0.10×1016 Pieces / cm3 or More and Less than 1.45×1016 Pieces / cm3

[0127] When the average number density of the alloy carbides in the ferrite is less than 0.10×1016 pieces / cm3 or 1.45×1016 pieces / cm3 or more, 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 0.10×1016 pieces / cm3 or more and less than 1.45×1016 pieces / cm3. The average number density of the alloy carbides in the ferrite is preferably 0.50×1016 pieces / cm3 or more, and more preferably 1.00×1016 pieces / cm3 or more. In addition, the average number density of the alloy carbides in the ferrite is preferably 1.40×1016 pieces / cm3 or less, more preferably 1.20×1016 pieces / cm3 or less, and still more preferably 1.10×1016 pieces / cm3 or less.

[0128] In the present embodiment, the alloy carbides refer to carbides containing one or two or more of Ti, Nb, Mo, and V.

[0129] 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.

[0130] 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 1 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.

[0131] 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, and 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.

[0132] 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 1D Concentration Profile function of the data analysis software IVAS 3.6.14 (manufactured by CAMECA Instruments Inc.).

[0133] 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.

[0134] E Value: 10.7 or more

[0135] I Value: 1.020 or more

[0136] 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.

[0137] 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.

[0138] 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.

[0139] In the present embodiment, the photographing region of a SEM image photographed for calculating the E value and the I value is set to, in a cross section parallel to the rolling direction at a position of ¼ from the end surface in the sheet width direction, 200 μm×200 μm centered in a position of ¼ from the surface in the sheet thickness direction, and the number of the observation fields is set to 5. 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.

[0140] 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 gray level co-occurrence matrices method (the GLCM method) described in Non-Patent Document 1.

[0141] 179 matrices 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 matrices P standardized so 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. Note that the average value obtained by measuring the entire visual fields is calculated.

[0142] P(i,j) in the following formula (1) and formula (2) is the gray level co-occurrence matrix, the value at the ith row and jth column of the matrix P is expressed as P(i,j). As described above, since the calculation is performed using the 256×256 matrices P, when this point is emphasized, the following formula (1) can be modified to the following formula (1′), and the following formula (2) can be modified to the following formula (2′). In the following formula (1′) and formula (2′), the value at the ith row and the jth column of the matrix P is expressed as Pij.[Formula⁢ 1]E=-∑ i?p⁡(i,j)·log⁢ (p⁡(i,j))(1)[Formula⁢ 2]I=?∑jp⁡(i,j)1+<semantics definitionURL="">❘<annotation encoding="Mathematica">"\[LeftBracketingBar]"< / annotation>< / semantics>i-j<semantics definitionURL="">❘<annotation encoding="Mathematica">"\[RightBracketingBar]"< / annotation>< / semantics>L(2)[Formula⁢ 3]E=-?⁢Pij⁢ log⁢ Pij(1′)[Formula⁢ 4]I=?Pij / (1+<semantics definitionURL="">❘<annotation encoding="Mathematica">"\[LeftBracketingBar]"< / annotation>< / semantics>i-j<semantics definitionURL="">❘<annotation encoding="Mathematica">"\[RightBracketingBar]"< / annotation>< / semantics> / 256(2′)?indicates text missing or illegible when filedStandard Deviation of Mn Concentration: 0.60 Mass % or Less

[0143] The standard deviation of the Mn concentration of the hot-rolled steel sheet according to the present embodiment 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.The Standard Deviation of the Mn Concentration can be Obtained by the Following Method.

[0144] First, in a position of ¼ from the end surface in the sheet width direction, a sample is collected so that a region of ¼ from the surface in the sheet thickness direction in a cross section parallel to the rolling direction can be observed. The size of the sample depends on a measurement device, but is set to a size that can be observed by about 10 mm in the rolling direction. Next, after mirror polishing the sample, the standard deviation of the Mn concentration is measured using an electron probe micro analyzer (EPMA). As the measurement conditions, the acceleration voltage is set to 15 kV and the magnification is set to 5000 times, and the distribution image of 40000 or more points in a range that is 20 μm in the rolling direction and 20 μm in the sheet thickness direction are measured with the measurement interval of to 0.1 μm. 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

[0145] 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 test piece may be set to a position of ¼ from the end surface in the sheet width direction of the hot-rolled steel sheet, and the sheet width direction may be set to the longitudinal direction.

[0146] In the hot-rolled steel sheet according to the present embodiment, the tensile (maximum) strength is preferably 980 MPa or more. The tensile strength is more preferably 1000 MPa or more. By setting the tensile strength to 980 MPa or more, 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.

[0147] 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.

[0148] 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

[0149] When hardening of materials occurs during repeated deformation, since an amount of plastic deformation during repeated deformation become small, fatigue life may be longer. Therefore, it is preferable that hardening of materials occurs during repeated deformation. Whether or not hardening occurs during repeated deformation can be determined by the following method.

[0150] In a position of ¼ from the end surface in the sheet width direction of the hot-rolled steel sheet, in accordance with JIS Z 2275-1978, the test piece of the symbol “1-15” is collected so that the sheet width direction is the longitudinal direction of the test piece. Using this test piece, a plane bending fatigue test is performed in accordance with JIS Z 2275-1978 with the repeated stress so that the fracture repeated counts becomes 3 million or more and less than 10 million 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 repeated stress at 100 repeated counts is defined as standard stress, in a range of repeated counts of 100 thousand to 1 million, when the repeated stress becomes 5% or more higher than the standard stress, the hot-rolled steel sheet can be determined that repeated hardening occurs and has excellent fatigue property.Sheet Thickness

[0151] 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.

[0152] 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

[0153] 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

[0154] 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.

[0155] 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.

[0156] 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 tension that is loaded in the rolling direction of the steel sheet. The stress can be controlled by adjusting the rotation speed of the rolling stand and the coiling device, and can be determined by dividing the measured load in the rolling direction by the cross-sectional area of the passing sheet.

[0157] (1) The slab is retained in a temperature range of 700° C. to 850° C. for 900 seconds or longer, then, further heated, and retained in a temperature range of 1100° C. or higher for 6000 seconds or longer.

[0158] (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.

[0159] (3) Rolling 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.

[0160] (4) The rolling reduction 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.

[0161] (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.

[0162] (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.

[0163] (7) After the light rolling, the steel sheet is rapidly cooled to a temperature range of 600° C. or higher and lower than 680° C. at an average cooling rate of 50° C. / s or faster.

[0164] (8) Slow cooling at an average cooling rate of slower than 5° C. / s is performed in a temperature range of 600° C. or higher and lower than 680° C. for 2.0 seconds or longer.

[0165] (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.

[0166] (10) The steel sheet is coiled in a temperature range of 350° C. or lower.

[0167] 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.

[0168] (1) Slab, Slab Temperature and Retention Time on Hot Rolling

[0169] 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 retained in a temperature range of 700° C. to 850° C. for 900 seconds or longer during slab heating, then, further heated, and retained in a temperature range of 1100° C. or higher for 6000 seconds or longer.

[0170] Note that 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.

[0171] 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.

[0172] 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.

[0173] (2) Rolling Reduction of Hot Rolling: Sheet Thickness Reduction of 90% or more in Total in Temperature Range of 850° C. to 1100° C.

[0174] 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.

[0175] 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 t0 and an outlet sheet thickness after the rolling of the final stand in this temperature range is t1.

[0176] (3) Rolling Temperature at One Stand before Final Stand: 900° C. or Higher and Lower Than 1010° C., Stress After Rolling at One Stand before Final Stand of Hot Rolling and Before Rolling at Final Stand: 170 kPa or More

[0177] 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. Note that the stress that is loaded to the steel sheet refers to tension in the longitudinal direction of the steel sheet, and can be controlled by adjusting the roll rotation speed during tandem rolling.

[0178] The upper limit of the stress that is loaded to the steel sheet is not particularly limited, but may be 350 kPa or lower.

[0179] (4) Rolling Reduction at Final Stand of Hot Rolling: 8% or more, Finishing Temperature Tf: 900° C. or higher and lower than 1010° C.

[0180] It is preferable that the rolling reduction 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 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. 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. Note that the upper limit of the rolling reduction at the final stand of the hot rolling is not particularly limited, but can be 30% or lower, 20% or lower, and preferably set to 15% or lower.

[0181] (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.

[0182] 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.

[0183] Light rolling may be performed, for example, at the final stand of the finishing mill, or by introducing new rolling equipment between the finishing mill and the cooling bed.

[0184] 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 t0 and an outlet sheet thickness after the rolling of the final stand in the light rolling.

[0185] (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

[0186] 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.

[0187] (7) After Light Rolling. Accelerated Cooling to Temperature Range of 600° C. or Higher and Lower than 680° C. at Average Cooling Rate of 50° C. / s 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 lower than 680° C. at the average cooling rate of 50° C. / s or faster after the light rolling. By performing the accelerated cooling to the temperature range of lower than 680° C., the average number density of the alloy carbides in the ferrite can be set to less than 1.45×1016 pieces / cm3. In addition, by setting the average cooling rate in the temperature range of 600° C. or higher and lower than 680° C. to 50° C. / s or faster, excessive generation of pearlite can be suppressed.

[0189] Note that 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. / s or slower. In addition, in order to set the area ratio of ferrite to 15.0% or more, the average number density of the alloy carbides in the ferrite to 0.10×1016 pieces / cm3 or more, and the average sphere equivalent radius of alloy carbides in the ferrite to 0.5 nm or more, the cooling stop temperature of the accelerated cooling is preferably set to 600° 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.

[0192] (8) Slow Cooling at Average Cooling Rate of Slower Than 5° C. / s Being Performed in Temperature Range of 600° C. or Higher and Lower than 680° C. for 2.0 Seconds or Longer

[0193] When slow cooling at an average cooling rate of slower than 5° C. / s is performed in a temperature range of 600° C. or higher and lower than 680° C. for 2.0 seconds or longer, a desired amount of ferrite can be obtained. In addition, the average number density of the alloy carbides in the ferrite can be controlled in a desired amount.

[0194] Note that 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.

[0195] 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.

[0196] (9) Average Cooling Rate to Coiling Temperature: 50° C. / s or Faster

[0197] 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. / s or faster. In such a case, the primary phase structure can be made full hard, and the average sphere equivalent radius and the average number density of the alloy carbides in the ferrite can be controlled in a desired amount.

[0198] Note that 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.

[0199] (10) Coiling Temperature: 350° C. or Lower

[0200] 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.Examples

[0201] 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.

[0202] 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 5A to Table 6B under the manufacturing conditions shown in Table 3A to Table 4B.

[0203] 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 4A and Table 4B is 50° C. the actual coiling temperatures of the examples with a value of 50° C. are 50° C. or lower. In addition, 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.

[0204] 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 5A to Table 6B.Evaluation Method of Properties of Hot-Rolled Steel SheetsTensile Properties

[0205] 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

[0206] By performing the plane bending fatigue test by the above-described method. in a range of repeated counts of 100 thousand to 1 million, when the repeated stress became 5% or more higher than the standard stress, it was determined that repeated hardening occurred, and as being the hot-rolled steel sheet having excellent fatigue property and being successful.

[0207] On the other hand, in a range of repeated counts of 100 thousand to 1 million, when the repeated stress did not become 5% or more higher than the standard stress, it was determined that repeated hardening did not occur, and as not being the hot-rolled steel sheet having excellent fatigue property and not being successful.

[0208] The examples that were determined as being successful were mentioned with “Good” in the column of fatigue property in Tables, and the examples that were determined as not being successful were mentioned with “NG” in Tables.Shearing Property (Evaluation of Secondary Sheared Surface)

[0209] The shearing property of the hot-rolled steel sheet was evaluated by a punching test.

[0210] 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.

[0211] 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. 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. The examples that were determined as being successful were mentioned with “Good” in the column of shearing property in Tables, and the examples that were determined as not being successful were mentioned with “NG” in Tables.TABLE 1SteelMass %, remainder is Fe and impuritiesNo.CSiMnTiNbVTi + Nb + Vsol. AlPSNONoteA0.0611.451.920.0870.0870.0290.0130.00050.00300.0024Present Invention ExampleB0.0981.271.600.0930.0930.0260.0020.00250.00150.0028Present Invention ExampleC0.1591.341.910.1260.1260.0710.0190.00460.00270.0015Present Invention ExampleD0.1100.511.660.0860.0860.0390.0240.00420.00350.0019Present Invention ExampleB0.0832.801.960.1110.1110.0350.0260.00250.00290.0031Present Invention ExampleF0.1020.981.350.0980.0980.0630.0180.00490.00300.0013Present Invention ExampleG0.1031.223.630.1030.0390.1420.0260.0230.00060.00300.0046Present Invention ExampleH0.1020.821.870.0600.0600.0570.0190.00280.00350.0028Present Invention ExampleI0.1041.191.850.1400.1400.0330.0070.00530.00230.0030Present Invention ExampleJ0.0961.131.870.0300.0330.0750.1380.0360.0210.00520.00280.0042Present Invention ExampleK0.0960.941.920.1240.1240.0500.0220.00410.00190.0036Present Invention ExampleL0.0820.901.820.1090.1090.0570.0310.00310.00470.0029Present Invention ExampleM0.0901.201.700.1070.0610.1680.0510.0310.00300.00370.0046Present Invention ExampleN0.0851.131.590.1020.1020.0430.0190.00450.00490.0027Present Invention ExampleO0.1061.131.860.2030.2030.0540.0150.00260.00310.0031Present Invention ExampleP0.0441.021.890.1230.1230.0220.0010.00320.00470.0027Comparative ExampleQ0.2620.951.810.1180.1180.0620.0130.00390.00490.0060Comparative ExampleR0.0953.191.750.1000.1000.0280.0210.00210.00340.0038Comparative ExampleS0.0960.930.730.1290.1290.0570.0100.00060.00620.0004Comparative ExampleT0.1151.181.890.0370.0110.0480.0400.0150.00530.00250.0051Comparative ExampleU0.0781.482.100.1050.1050.3090.0110.00260.00420.0024Present Invention ExampleV0.0651.882.400.0860.0860.3840.0230.00130.00270.0024Present Invention ExampleW0.2371.071.880.0700.0700.0630.0140.00300.00260.0025Present Invention ExampleX0.2151.351.520.1320.1300.1100.3720.0520.0210.00240.00320.0024Present Invention ExampleUnderlines indicate that values are outside the range of the present invention.TABLE 2SteelMass %, remainder is Fe and impuritiesNo.CuCrMoNiBCaMgREMBiA0.00360.0014BC0.0035D0.380.180.39E0.0050FGHIJK0.22L0.27M0.15N0.36O0.0041UVWXSteelMass %, remainder is Fe and impuritiesNo.AsZrCoZnWSnNoteAPresent Invention ExampleBPresent Invention ExampleCPresent Invention ExampleDPresent Invention ExampleE0.27Present Invention ExampleFPresent Invention ExampleGPresent Invention ExampleH0.03Present Invention ExampleIPresent Invention ExampleJPresent Invention ExampleK0.13Present Invention ExampleLPresent Invention ExampleM0.03Present Invention ExampleNPresent Invention ExampleO0.02Present Invention ExamplePComparative ExampleQComparative ExampleRComparative ExampleSComparative ExampleTComparative ExampleU0.095Present Invention ExampleVPresent Invention ExampleWPresent Invention ExampleXPresent Invention ExampleTABLE 3ASheetLoaded stressRetentionRetentionthicknessafter rollingtime intime inreduction inat one standtemperaturetemperaturetemperaturebefore finalRollingrange ofrange ofrange ofstand andFinishingreductionManu-700° C. toHeating1100° C. or850° C. tobefore rollingtemperatureat finalfacturingSteel850° C.temperaturehigher1100° C.at final standTfstandNo.No.s° C.s%kPa° C.%Note 1A15561219932997200943 9Present Invention Example 2B14261219910194212973 9Present Invention Example 3B 800127289259122393110Comparative Example 4B1096122292718921295112Comparative Example 5B1232120756109520195911Comparative Example 6B1402120895609616298612Comparative Example 7B153512098943932221035  8Comparative Example 8B14201255806591197986 6Comparative Example 9B1283125091329723194710Comparative Example10B1276128795679522995112Present Invention Example11B1315122689509322996210Comparative Example12B1467120595269620494110Comparative Example13B1119125189849721197011Comparative Example14B15011286932594227932 8Comparative Example15B14051280903196203935 9Comparative Example16C13531289912394214946 8Present Invention Example17D15921233974195202987 8Present Invention Example18E1596119792939322694811Present Invention Example19F15921188934090201941 8Present Invention Example20G141012039296922141002 28Present Invention Example21H15401272950294176957 9Present Invention Example22I1458121490189421092111Present Invention Example23J1547128095289621196111Present Invention Example24K1356129394229921794611Present Invention Example25L1434127593199721393812Present Invention ExampleUnderlines indicate that values are outside the range of the present invention or are not preferable manufacturing conditions.TABLE 3BSheetLoaded stressRetentionRetentionthicknessafter rollingtime intime inreduction inat one standtemperaturetemperaturetemperaturebefore finalRollingrange ofrange ofrange ofstand andFinishingreductionManu-700° C. toHeating1100° C. or850° C. tobefore rollingtemperatureat finalfacturingSteel850° C.temperaturehigher1100° C.at final standTfstandNo.No.s° C.s%kPa° C.%Note26M129412688920952239679Present Invention Example27N1166120295029721293611Present Invention Example28O1201124195719623196510Present Invention Example29P1450127492029521796912Comparative Example30Q121112928882952059338Comparative Example31R1582129890989622793711Comparative Example32S1398128594239521298011Comparative Example33T1424127095539722195912Comparative Example34U 887124490609623794912Comparative Example35U2227123794469721791412Present Invention Example36U1472124311930 9520494813Present Invention Example37U123312305840942089449Comparative Example38U1357120494269320293112Present Invention Example39U1495126590799723297812Present Invention Example40U1319121688649622094010Comparative Example41U1196125793939422093411Comparative Example42U131112698948922299349Comparative Example43U 908126791689421494111Present Invention Example44U1335124066639621996813Present Invention Example45V1313125591249620397410Comparative Example46V126212159569932329509Present Invention Example47V1255123790539424294312Comparative Example48W1501128791239021293711Present Invention Example49X1495128595539522795911Present Invention Example50G14051203920297208100812Present Invention ExampleUnderlines indicate that values are outside the range of the present invention or are not preferable manufacturing conditions.TABLE 4ASheet thickness reductionLoaded stress afterLoaded stress afterof light rolling atrolling at final standrolling at final stand ofAveragetemperature range ofof hot rolling andlight rolling until steelcooling rate of840° C. or higher andbefore first rolling ofsheet being cooled toacceleratedManufacturingSteellower than 900° C.light rolling800° C.coolingNo.No.%kPakPa° C. / s 1A6 9647104 2B6100152  97 3B6 8150 80 4B6117182 110 5B6102137 102 6B6119180 114 7B6 90114  89 8B6 6394 59 9B4 8092 7810B6 8878 9311B6126184 12712B620292 4113B6108201 10614B6118178 11915B6 95133  9916C6 8251 9017D6 968510418E6107127 11619F6 8953 9720G61137511521H6117148 11522I61038210123J610218 9824K6112103 11825L611249106Cooling stopSlow cooling time inAveragetemperature oftemperature range ofcooling rateaccelerated600° C. or higher anduntil coilingCoilingManufacturingcoolinglower than 680° C.temperaturetemperatureNo.° C.s° C. / s° C.Note 16613.811250Present Invention Example 26663.411850Present Invention Example 36724.010250Comparative Example 46433.210250Comparative Example 56733.914850Comparative Example 66663.212050Comparative Example 76583.916350Comparative Example 86442.111250Comparative Example 96253.2 9450Comparative Example106284.8 7950Present Invention Example116671.310250Comparative Example126444.512950Comparative Example136974.4 8350Comparative Example146704.4 4550Comparative Example156403.4 750Comparative Example166623.914750Present Invention Example176784.515050Present Invention Example186024.310250Present Invention Example196384.114450Present Invention Example206254.5 8250Present Invention Example216273.413750Present Invention Example226472.314250Present Invention Example236653.0 8250Present Invention Example246674.410450Present Invention Example256263.210350Present Invention ExampleUnderlines indicate that values are outside the range of the present invention or are not preferable manufacturing conditions.TABLE 4BSheet thickness reductionLoaded stress afterLoaded stress afterof light rolling atrolling at final standrolling at final stand ofAveragetemperature range ofof hot rolling andlight rolling until steelcooling rate of840° C. or higher andbefore first rolling ofsheet being cooled toacceleratedManufacturingSteellower than 900° C.light rolling800° C.coolingNo.No.%kPakPa° C. / s26M611016110627N612818012528O612117712029P6 92 2610430Q61121438331R6106 9810032S612510410933T610115011434U6 531509235U6 97 8810336U6109 8510337U6115 4511838U6113 4710639U6127 575540U611320510041U9107 5311442U6 8117112043U6 98 3010944U610018712645V6204 7511746V610210412047V610512511648W6 9710410949X6102 9811450G6127 88103Cooling stopSlow cooling time inAveragetemperature oftemperature range ofcooling rateaccelerated600° C. or higher anduntil coilingCoilingManufacturingcoolinglower than 680° C.temperaturetemperatureNo.° C.s° C. / s° C.Note266733.19150Present Invention Example276722.013150Present Invention Example286333.314350Present Invention Example296143.112250Comparative Example306754.012550Comparative Example316463.513350Comparative Example326644.49950Comparative Example336253.113350Comparative Example346283.511450Comparative Example356323.2130210Present Invention Example366523.115250Present Invention Example376653.710050Comparative Example386733.810850Present Invention Example396613.610150Present Invention Example406204.913650Comparative Example416554.812050Comparative Example426491.98750Comparative Example436684.012150Present Invention Example446223.8135140Present Invention Example456753.160320Comparative Example466304.011450Present Invention Example475953.710550Comparative Example486774.16150Present Invention Example496738.213350Present Invention Example506653.19950Present Invention ExampleUnderlines indicate that values are outside the range of the present invention or are not preferable manufacturing conditions.TABLE 5AAverage sphereequivalentAverage numberMnRemainderradius of alloydensity of alloystandardManu-Residualin micro-carbides incarbides inEIdevi-facturingSteelausteniteFerritePearlitestructureferriteferritevaluevalueationNo.No.Area %Area %Area %Area %nm×1016 pieces / cm3——Mass %Note 1A0.059.60.040.49.11.3911.41.0500.43Present Invention Example 2B0.026.80.073.26.20.6210.81.0210.46Present Invention Example 3B0.031.40.068.64.60.8311.21.0240.65Comparative Example 4B0.030.10.069.97.40.4910.81.0230.62Comparative Example 5B0.026.00.074.09.10.6311.01.0230.65Comparative Example 6B0.023.30.076.77.20.1410.51.0200.45Comparative Example 7B0.031.80.068.23.01.1910.31.0370.43Comparative Example 8B0.026.50.073.57.80.1210.51.0320.45Comparative Example 9B0.020.40.079.60.41.2710.71.0330.44Comparative Example10B0.027.90.072.19.40.6311.21.0430.49Present Invention Example11B0.0 8.30.091.76.80.0811.01.0210.45Comparative Example12B0.057.05.937.18.11.5510.61.0660.49Comparative Example13B0.059.50.040.55.11.5310.51.0960.40Comparative Example14B0.044.75.150.20.40.0910.81.0530.41Comparative Example15B5.559.69.225.711.1 0.0711.11.0870.40Comparative Example16C1.416.00.082.67.60.5711.11.0390.47Present Invention Example17D0.017.20.082.85.71.0011.21.0310.51Present Invention Example18E2.251.30.046.56.41.3711.11.0690.42Present Invention Example19F0.042.90.057.15.20.5810.71.0400.50Present Invention Example20G0.015.30.084.72.71.2411.11.0230.58Present Invention Example21H0.025.30.074.75.20.5610.71.0220.47Present Invention Example22I0.015.20.084.80.71.1310.91.0220.50Present Invention Example23J0.028.10.071.98.70.1511.11.0210.42Present Invention Example24K0.032.50.067.51.61.2611.21.0330.51Present Invention Example25L0.031.20.068.87.30.3610.91.0210.43Present Invention ExampleUnderlines indicate that values are outside the range of the present invention or properties are not preferable.TABLE 5BAverage sphereequivalentAverage numberMnRemainderradius of alloydensity of alloystandardManu-Residualin micro-carbides incarbides inEIdevi-facturingSteelausteniteFerritePearlitestructureferriteferritevaluevalueationNo.No.Area %Area %Area %Area %nm×1016 pieces / cm3——Mass %Note26M0.034.90.065.15.20.9611.01.0300.50Present Invention Example27N0.018.70.081.39.80.1511.11.0230.40Present Invention Example28O0.030.20.069.87.70.4011.11.0250.39Present Invention Example29P0.082.60.017.40.30.1210.91.0920.49Comparative Example30Q2.1 2.00.095.99.90.2410.91.0170.45Comparative Example31R2.668.10.029.39.01.4410.91.0550.48Comparative Example32S0.074.00.026.06.80.8311.21.0850.46Comparative Example33T0.027.30.072.78.00.0911.01.0320.45Comparative Example34U0.057.80.042.28.11.4111.11.0310.62Comparative Example35U0.051.70.048.38.40.4810.81.0250.41Present Invention Example36U0.036.80.063.29.20.2610.91.0250.45Present Invention Example37U0.054.60.045.45.20.3610.81.0330.61Comparative Example38U0.053.90.046.18.10.3911.31.0400.51Present Invention Example39U0.038.10.061.98.51.3911.01.0350.45Present Invention Example40U0.059.10.040.96.10.3110.61.0290.46Comparative Example41U0.041.60.058.410.3 0.0810.71.0270.47Comparative Example42U0.011.50.088.53.90.0611.21.0360.43Comparative Example43U0.056.10.043.95.01.1011.11.0230.53Present Invention Example44U0.059.30.040.78.90.4411.11.0330.55Present Invention Example45V0.036.20.063.80.60.1110.51.1070.45Comparative Example46V0.043.10.056.94.11.4411.21.0280.48Present Invention Example47V0.011.00.089.00.30.0911.01.0310.44Comparative Example48W2.416.24.776.75.30.8811.11.0250.39Present Invention Example49X2.117.63.077.39.20.5810.81.0350.51Present Invention Example50G0.036.80.063.23.01.2411.21.0270.43Present Invention ExampleUnderlines indicate that values are outside the range of the present invention or properties are not preferable.TABLE 6AFatigue propertyShearing propertyTensileTotal(occurrence or(presence orSheetstrengthelongationTS ×non-occurrenceabsence of secondaryManufacturingSteelthicknessTSElElof repeated hardening)sheared surface)No.No.mmMPa%MPa · %——Note 1A2.6 99117.317144GoodGoodPresent Invention Example 2B2.6102015.615912GoodGoodPresent Invention Example 3B2.6101415.816021GoodNGComparative Example 4B2.6100115.115115GoodNGComparative Example 5B2.6103215.11.5583GoodNGComparative Example 6B2.6103314.414875GoodNGComparative Example 7B2.6102613.914261GoodNGComparative Example 8B2.6100815.315422GoodNGComparative Example 9B2.6108412.413442NGGoodComparative Example10B2.6104315.516167GoodGoodPresent Invention Example11B2.61086 9.610426NGGoodComparative Example12B2.6 966 9.4 9080NGNGComparative Example13B2.6 99516.015920NGNGComparative Example14B2.6 965 8.9 8589NGGoodComparative Example15B2.6 915 7.2 6588NGNGComparative Example16C6.0159113.020683GoodGoodPresent Invention Example17D2.6 99714.614556GoodGoodPresent Invention Example18E2.6113913.114921GoodGoodPresent Invention Example19F1.6 99816.816766GoodGoodPresent Invention Example20G2.6130610.413582GoodGoodPresent Invention Example21H2.6 98316.416121GoodGoodPresent Invention Example22I2.6102613.313646GoodGoodPresent Invention Example23J2.6100313.813841GoodGoodPresent Invention Example24K2.6104516.216929GoodGoodPresent Invention Example25L2.6 99215.315178GoodGoodPresent Invention ExampleUnderlines indicate that corresponding values are outside the range of the present invention or not preferable properties.TABLE 6BFatigue propertyShearing propertyTensileTotal(occurrence or(presence orSheetstrengthelongationTS ×non-occurrenceabsence of secondaryManufacturingSteelthicknessTSElElof repeated hardening)sheared surface)No.No.mmMPa%MPa · %——Note26M2.6103415.115613GoodGoodPresent Invention Example27N2.6101313.213372GoodGoodPresent Invention Example28O2.6 99215.615475GoodGoodPresent Invention Example29P2.6 90216.014432NGGoodComparative Example30Q2.61895 9.217434GoodNGComparative Example31R2.61020 9.6 9792GoodGoodComparative Example32S2.6 97717.817391GoodGoodComparative Example33T2.6 93915.914930NGGoodComparative Example34U3.2104415.516182GoodNGComparative Example35U3.2102614.414774GoodGoodPresent Invention Example36U3.2105714.515327GoodGoodPresent Invention Example37U3.2103114.214640GoodNGComparative Example38U3.2107115.516601GoodGoodPresent Invention Example39U3.2105714.915749GoodGoodPresent Invention Example40U3.2105515.316142GoodNGComparative Example41U3.2104416.116808NGNGComparative Example42U3.2109211.412449NGGoodComparative Example43U3.2102615.515903GoodGoodPresent Invention Example44U3.2103815.015570GoodGoodPresent Invention Example45V2.9103218.619195GoodNGComparative Example46V2.9104915.616364GoodGoodPresent Invention Example47V2.91093 9.710602NGGoodComparative Example48W2.6107614.215279GoodGoodPresent Invention Example49X2.6112713.314989GoodGoodPresent Invention Example50G2.6103114.915362GoodGoodPresent Invention ExampleUnderlines indicate that corresponding values are outside the range of the present invention or not preferable properties.From Table 5A to Table 6B, 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.

Examples

examples

[0201]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.

[0202]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 5A to Table 6B under the manufacturing conditions shown in Table 3A to Table 4B.

[0203]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 4A and Table 4B is 50° C....

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 two er-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 0.0200%;As: 0% to 0.100%;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 has, 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 is 0.5 nm or more and less than 10.0 nm,an average number density of the alloy carbides in the ferrite is 0.10×1016 pieces / cm3 or more and less than 1.45×1016 pieces / cm3,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, anda standard deviation of Mn concentration is 0.60 mass % or less.

2. The hot-rolled steel sheet according to claim 1, wherein the chemical composition comprises, in terms of mass %, one or more of: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%;Bi: 0.0005% to 0.0200%; andAs: 0.001% to 0.100%.

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