Hot-rolled thin steel sheets

Abstract

Invention details;

Description

hot rolled steel plate

[0001] The present invention relates to a hot-rolled steel sheet. This application claims priority to Japanese Patent Application No. 2022-208520, filed on December 26, 2022, the contents of which are incorporated herein by reference.

[0002] In recent years, efforts to reduce carbon dioxide emissions have been made in many fields from the perspective of protecting the global environment. Automobile manufacturers are also actively developing technologies to reduce the weight of vehicles in order to improve fuel efficiency. However, reducing the weight of vehicles is not easy, as emphasis is also placed on improving crashworthiness to ensure the safety of passengers.

[0003] To achieve both lightweight vehicle bodies and crashworthiness, the use of high-strength steel sheets to reduce the thickness of components has been considered. Therefore, steel sheets that combine high strength with excellent formability are highly desired, and several technologies have been proposed to meet these requirements. Because there are various processing methods for automotive components, the required formability varies depending on the component to be used. Among these, the critical thickness reduction rate at fracture and hole expandability are considered important formability indicators.

[0004] The critical thickness reduction rate at fracture is a value calculated from the thickness of the tensile test piece before fracture and the minimum thickness of the tensile test piece after fracture. A higher critical thickness reduction rate at fracture is preferable because it makes the test piece less likely to fracture early when tensile strain is applied during press forming.

[0005] Automotive components are formed by press molding, and the blanks used for this press molding are often produced by shearing, which has high productivity. Blanks produced by shearing require excellent edge accuracy after shearing.

[0006] For example, if the boundary between the fracture surface and the shear surface at the sheared edge has low linearity, the accuracy of the sheared edge will be significantly degraded.

[0007] Furthermore, steel sheets used in automobile components are required to have better corrosion resistance.

[0008] For example, Patent Document 1 discloses a hot-rolled steel sheet in which the Mn segregation degree and the P segregation degree in the central part of the sheet thickness are controlled, and which serves as a raw material for a cold-rolled steel sheet having excellent surface properties after press working.

[0009] International Publication No. 2020 / 044445

[0010] J. Webel, J. Gola, D. Britz, F. Mucklich, Materials Characterization 144 (2018) 584-596D. L. Naik, H. U. Sajid, R. Kiran, Metals 2019, 9, 546K. Zuiderveld, Contrast Limited Adaptive Histogram Equalization, Chapter VIII. 5, Graphics Gems IV. P. S. Heckbert (Eds.), Cambridge, MA, Academic Press, 1994, pp. 474-485

[0011] However, Patent Document 1 does not take into consideration the critical thickness reduction rate at fracture, shear workability, and corrosion resistance of the hot-rolled steel sheet.

[0012] The present invention has been made in consideration of the above-mentioned circumstances, and aims to provide a hot-rolled steel sheet having high strength and a critical fracture thickness reduction rate, as well as excellent hole expandability, shear workability, and corrosion resistance.

[0013] The gist of the present invention is as follows: (1) A hot-rolled steel sheet according to one aspect of the present invention has a chemical composition, in mass %, of C: 0.040 to 0.250%, Si: 0.40 to 1.00%, Mn: 1.00 to 4.00%, sol. Al: 0.100-0.500%, Cr: 0.50-2.00%, P: 0.100% or less, S: 0.0300% or less, N: 0.1000% or less, O: 0.0100% or less, Ti: 0-0.300%, Nb: 0-0.100%, V : 0-0.500%, Cu: 0-2.00%, Mo: 0-1.00%, Ni: 0-2.00%, B: 0-0.0100%, Ca: 0-0.0200%, Mg: 0-0.0200%, REM: 0-0.1000%, Bi: 0-0.0200%, As: 0 to 0.100%, The steel sheet contains Zr: 0-1.00%, Co: 0-1.00%, Zn: 0-1.00%, W: 0-1.00%, and Sn: 0-0.05%, with the balance being Fe and impurities, and satisfies the following formula (A), and the metallographic structure at a quarter position from the surface in the sheet thickness direction has, in area %, a total of more than 92.0% and 100.0% or less of martensite and tempered martensite, less than 3.0% retained austenite, and less than 5.0% ferrite, and the entropy value, which is obtained by analyzing an SEM image of the metallographic structure by a gray level co-occurrence matrix method and is represented by the following formula (1), is 11.0 or more, and the inverse difference normalized value, which is represented by the following formula (2), is less than 1.020, The Cluster Shade value shown by the following formula (3) is −8.0×10 ５ ~8.0 x 10 ５ the standard deviation of the Mn concentration is 0.60 mass% or less, and the number density of Si—Al—Cr oxide particles having a circle-equivalent radius of 0.500 μm or more on the surface is 2.0×10 ３ pieces / cm ２ The number density of Si—Al—Cr oxide particles having a circle-equivalent radius of 0.005 to 0.050 μm is 1.0×10 ５ pieces / cm ２or more. Zr+Co+Zn+W≦1.00 ... (A) However, each element symbol in the formula (A) indicates the content of the element in mass %, and 0 is substituted when the element is not contained. Here, P(i, j) in the following formulas (1) to (5) is a gray level co-occurrence matrix, L in the following formula (2) is the number of gray scale levels that the SEM image can take, i and j in the following formulas (2) and (3) are natural numbers from 1 to L, and μ in the following formula (3) is ｘ and μ ｙ are respectively represented by the following formulas (4) and (5). (2) The hot-rolled steel sheet according to (1) above, wherein the chemical composition is, in mass%, Ti: 0.001 to 0.300%, Nb: 0.001 to 0.100%, V: 0.001 to 0.500%, Cu: 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%, As: 0.001 to 0.100%, It may contain one or more selected from the group consisting of Zr: 0.01 to 1.00%, Co: 0.01 to 1.00%, Zn: 0.01 to 1.00%, W: 0.01 to 1.00%, and Sn: 0.01 to 0.05%.

[0014] According to the above aspect of the present invention, it is possible to obtain a hot-rolled steel sheet having high strength, a critical thickness reduction rate at fracture, and excellent hole expandability, shear workability, and corrosion resistance. The hot-rolled steel sheet according to the above aspect of the present invention is suitable as an industrial material used for automotive components, machine structural components, and even building components.

[0015] FIG. 10 is a diagram for explaining a method for measuring the linearity of the boundary between the fracture surface and the shear surface on the end surface after shearing.

[0016] The chemical composition and metal structure of the hot-rolled steel sheet according to this embodiment will be described in more detail below. However, the present invention is not limited to the configuration disclosed in this embodiment, and various modifications are possible within the scope of the present invention.

[0017] Below, numerical ranges defined by "to" include the lower and upper limits. Numerical values ​​indicated as "less than" or "greater than" are not included in the numerical range. In the following description, percentages relating to chemical compositions are mass % unless otherwise specified.

[0018] Chemical Composition The chemical composition of the hot-rolled steel sheet according to this embodiment includes, in mass%, C: 0.040 to 0.250%, Si: 0.40 to 1.00%, Mn: 1.00 to 4.00%, sol. Al: 0.100 to 0.500%, Cr: 0.50 to 2.00%, P: 0.100% or less, S: 0.0300% or less, N: 0.1000% or less, O: 0.0100% or less, and the balance: Fe and impurities. Each element will be described in detail below.

[0019] C: 0.040 to 0.250% C increases the area ratio of the hard phase. If the C content is less than 0.040%, the area ratio of ferrite and the remaining structure in the hot-rolled steel sheet increases, making it impossible to obtain the desired strength and hole expandability. Therefore, the C content is set to 0.040% or more. The C content is preferably 0.045% or more, and more preferably 0.050% or more. On the other hand, if the C content exceeds 0.250%, the flat cementite structure increases, and the E value decreases due to the influence of the formation of carbide regions with small brightness differences. Therefore, the C content is set to 0.250% or less. The C content is preferably 0.200% or less, 0.150% or less.

[0020] Si: 0.40 to 1.00% Si promotes the formation of ferrite to improve the hole expandability of hot-rolled steel sheets, and solid-solution strengthens ferrite to increase the strength of hot-rolled steel sheets. Si also improves the soundness of steel by deoxidizing (suppressing the occurrence of defects such as blowholes in steel). If the Si content is less than 0.40%, the above effects cannot be achieved. Therefore, the Si content is set to 0.40% or more. The Si content is preferably 0.50% or more, and more preferably 0.60% or more. However, if the Si content exceeds 1.00%, the number density of oxides on the steel sheet surface increases, deteriorating chemical treatability. Furthermore, excessive ferrite is more likely to form, resulting in a decrease in strength. Therefore, the Si content is set to 1.00% or less. The Si content is preferably 0.90% or less, and more preferably 0.80% or less.

[0021] Mn: 1.00 to 4.00% Mn has the effect of suppressing ferrite transformation and increasing the strength of the hot-rolled steel sheet. If the Mn content is less than 1.00%, the desired strength cannot be obtained in the hot-rolled steel sheet. Therefore, the Mn content is set to 1.00% or more. The Mn content is preferably 1.10% or more, and more preferably 1.20% or more. On the other hand, if the Mn content exceeds 4.00%, the standard deviation of the Mn concentration cannot be reduced. Therefore, the Mn content is set to 4.00% or less. The Mn content is preferably 3.50% or less, 3.00% or less, and more preferably 2.50% or less, 2.00% or less.

[0022] Sol. Al: 0.100 to 0.500% Like Si, Al has the effect of deoxidizing steel to improve its soundness, as well as promoting the formation of ferrite and improving the hole expandability of hot-rolled steel sheets. If the sol. Al content is less than 0.100%, the above-mentioned effects cannot be obtained. Therefore, the sol. Al content is set to 0.100% or more. The sol. Al content is preferably 0.200% or more. On the other hand, if the sol. Al content exceeds 0.500%, the above-mentioned effects saturate and it is economically undesirable. Therefore, the sol. Al content is set to 0.500% or less. The sol. Al content is preferably 0.450% or less, more preferably 0.400% or less, and even more preferably 0.350% or less. Note that sol. Al refers to acid-soluble Al, which refers to solute Al present in the steel in a solid solution state.

[0023] Cr: 0.50 to 2.00% Cr has the effect of improving the hardenability of hot-rolled steel sheets. Cr also forms oxides with Si and Al. If the Cr content is less than 0.50%, the effect of improving hardenability cannot be obtained. Furthermore, Si-Al-Cr oxides cannot be favorably formed on the surface. Therefore, the Cr content is set to 0.50% or more. The Cr content is preferably 0.60% or more, more preferably 0.70% or more, and even more preferably 0.80% or more. On the other hand, if the Cr content exceeds 2.00%, the chemical conversion treatability of the hot-rolled steel sheet is significantly reduced. Therefore, the Cr content is set to 2.00% or less. The Cr content is preferably 1.80% or less, and more preferably 1.60% or less.

[0024] P: 0.100% or less P is an element that has the effect of increasing the strength of a hot-rolled steel sheet through solid solution strengthening. Therefore, P may be actively contained. However, P is an element that easily segregates, and if the P content exceeds 0.100%, the hole expandability and critical fracture thickness reduction rate of the hot-rolled steel sheet due to grain boundary segregation will be significantly reduced. Therefore, the P content is set to 0.100% or less. The P content is preferably set to 0.030% or less. There is no need to particularly specify a lower limit for the P content, and it may be 0%, but from the viewpoint of refining costs, it is preferably set to 0.001% or more.

[0025] S: 0.0300% or less S forms sulfide-based inclusions in steel, reducing the hole expandability and critical fracture thickness reduction rate of the hot-rolled steel sheet. If the S content exceeds 0.0300%, the hole expandability and critical fracture thickness reduction rate of the hot-rolled steel sheet will be significantly reduced. Therefore, the S content is set to 0.0300% or less. The S content is preferably 0.0050% or less. There is no need to particularly specify the lower limit of the S content, and it may be 0%, but from the viewpoint of refining costs, it is preferably set to 0.0001% or more.

[0026] N: 0.1000% or less N has the effect of reducing the hole expandability and critical fracture thickness reduction rate of the hot-rolled steel sheet. If the N content exceeds 0.1000%, the hole expandability and critical fracture thickness reduction rate of the hot-rolled steel sheet will be significantly reduced. 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 even more preferably 0.0100% or less. There is no need to particularly specify the lower limit of the N content, and it may be 0%. However, when one or more of Ti, Nb, and V are added to further refine the metal structure, the N content is preferably 0.0010% or more, more preferably 0.0020% or more, in order to promote the precipitation of carbonitrides.

[0027] O: 0.0100% or less If a large amount of O is contained in steel, it forms coarse oxides that become the starting points of fracture, causing brittle fracture and hydrogen-induced cracking. Therefore, the O content is set to 0.0100% or less. The O content is preferably 0.0080% or less, and more preferably 0.0050% or less. The O content may be 0%, but in order to disperse a large number of fine oxides during deoxidation of molten steel, the O content may be set to 0.0005% or more, or 0.0010% or more.

[0028] The balance of the chemical composition of the heat-rolled steel sheet according to this embodiment may be Fe and impurities. In this embodiment, the impurities refer to substances mixed in from raw materials such as ore, scrap, or the manufacturing environment, and / or substances that are allowed to the extent that they do not adversely affect the heat-rolled steel sheet according to this embodiment.

[0029] The hot-rolled steel sheet according to this embodiment may contain the following elements as optional elements in place of a portion of Fe. When these optional elements are not contained, the lower limit of the content is 0%. The optional elements will be described in detail below.

[0030] Ti: 0.001 to 0.300% Nb: 0.001 to 0.100% V: 0.001 to 0.500% Ti, Nb, and V are elements that finely precipitate in steel as carbides and nitrides, improving the strength of the steel through precipitation strengthening. To more reliably obtain the effects of the above-mentioned actions, it is preferable that the content of at least one of Ti, Nb, and V is 0.001% or more. The contents of Ti, Nb, and V are each preferably 0.010% or more. On the other hand, if the Ti content exceeds 0.300%, if the Nb content exceeds 0.100%, or if the V content exceeds 0.500%, the workability of the hot-rolled steel sheet deteriorates. Furthermore, excessive inclusion of these elements is economically undesirable. Therefore, the Ti content is set to 0.300% or less, the Nb content is set to 0.100% or less, and the V content is set to 0.500% or less.

[0031] Cu: 0.01 to 2.00% Mo: 0.01 to 1.00% Ni: 0.02 to 2.00% B: 0.0001 to 0.0100% Cu, Mo, Ni, and B all have the effect of improving the hardenability of hot-rolled steel sheet. Cu and Mo also precipitate as carbides in the steel, thereby increasing the strength of the hot-rolled steel sheet. Furthermore, when Cu is included, Ni has the effect of effectively suppressing intergranular cracking of the slab caused by Cu. Therefore, one or more of these elements may be included.

[0032] As described above, Cu has the effect of improving the hardenability of hot-rolled steel sheets and precipitating as carbides in the steel at low temperatures to increase the strength of the hot-rolled steel sheets. To more reliably obtain the effects of these functions, the Cu content is preferably 0.01% or more, and more preferably 0.05% or more. However, if the Cu content exceeds 2.00%, grain boundary cracking of the slab may occur. Therefore, the Cu content is set to 2.00% or less. The Cu content is preferably 1.50% or less, more preferably 1.00% or less.

[0033] As described above, Mo has the effect of improving the hardenability of hot-rolled steel sheets and of precipitating as carbides in the steel to increase the strength of the hot-rolled steel sheets. To ensure the above effects, the Mo content is preferably 0.01% or more, and more preferably 0.02% or more. However, even if the Mo content exceeds 1.00%, the above effects are saturated and it is not economically preferable. Therefore, the Mo content is set to 1.00% or less. The Mo content is preferably 0.50% or less, more preferably 0.20% or less.

[0034] As described above, Ni has the effect of improving the hardenability of hot-rolled steel sheets. Furthermore, when Cu is contained, Ni has the effect of effectively suppressing grain boundary cracking of slabs caused by Cu. To more reliably obtain the above-mentioned effects, the Ni content is preferably 0.02% or more. Since Ni is an expensive element, it is economically undesirable to include a large amount of Ni. Therefore, the Ni content is set to 2.00% or less.

[0035] As described above, B has the effect of improving the hardenability of the hot-rolled steel sheet. To more reliably obtain this effect, the B content is preferably 0.0001% or more, and more preferably 0.0002% or more. However, if the B content exceeds 0.0100%, the formability of the hot-rolled steel sheet is significantly reduced, so the B content is set to 0.0100% or less. The B content is preferably 0.0050% or less.

[0036] Ca: 0.0005 to 0.0200% Mg: 0.0005 to 0.0200% REM: 0.0005 to 0.1000% Bi: 0.0005 to 0.0200% Ca, Mg, and REM all have the effect of improving the hole expandability of hot-rolled steel sheets by adjusting the shape of inclusions in the steel to a preferred shape. Bi also has the effect of improving the hole expandability of hot-rolled steel sheets by refining the solidification structure. Therefore, one or more of these elements may be contained. To more reliably obtain the effects of the above actions, it is preferable that the content of one or more of Ca, Mg, REM, and Bi be 0.0005% or more. However, if the Ca content or Mg content exceeds 0.0200%, or if the REM content exceeds 0.1000%, excessive inclusions may be formed in the steel, which may actually reduce the hole expandability of the hot-rolled steel sheet. Furthermore, even if the Bi content exceeds 0.0200%, the effects of the above-mentioned action saturate, making it economically undesirable. Therefore, the Ca content and Mg content are set to 0.0200% or less, the REM content to 0.1000% or less, and the Bi content to 0.0200% or less. The Bi content is preferably 0.0100% or less. Here, REM refers to a total of 17 elements consisting of Sc, Y, and lanthanides, and the REM content refers to the total content of these elements. Lanthanides are industrially added in the form of misch metal.

[0037] As: 0.001 to 0.100% As reduces the austenite single-phase temperature, thereby refining prior austenite grains and contributing to improving the hole expandability of hot-rolled steel sheets. To ensure this effect, the As content is preferably 0.001% or more. However, even if a large amount of As is added, the above effect saturates, so the As content is set to 0.100% or less.

[0038] Zr: 0.01 to 1.00% Co: 0.01 to 1.00% Zn: 0.01 to 1.00% W: 0.01 to 1.00% Zr + Co + Zn + W ≦ 1.00% (A) where each element symbol in formula (A) indicates the content of the element in mass%, and 0 is substituted if the element is not contained. The inventors have confirmed that the effects of the hot-rolled steel sheet according to this embodiment are not impaired even if the total content of Zr, Co, Zn, and W is 1.00% or less. Therefore, one or more of Zr, Co, Zn, and W may be contained in a total content of 1.00% or less. That is, the value of the left side of formula (A) may be 1.00% or less. Since Zr, Co, Zn, and W are not necessarily contained, their respective contents may be 0%. In order to improve the strength of the hot-rolled steel sheet by solid solution strengthening, the content of each of Zr, Co, Zn and W may be 0.01% or more.

[0039] Sn: 0 to 0.05% The inventors have confirmed that the effects of the hot-rolled steel sheet according to this embodiment are not impaired even if a small amount of Sn is added. However, if a large amount of Sn is added, defects may occur during hot rolling, so the Sn content is set to 0.05% or less. Since Sn does not need to be added, the Sn content may be 0%. In order to improve the corrosion resistance of the hot-rolled steel sheet, the Sn content may be set to 0.01% or more.

[0040] The chemical composition of the above-mentioned hot-rolled steel sheet may be measured by a common analytical method. For example, it may be measured using ICP-AES (Inductively Coupled Plasma-Atomic Emission Spectrometry). Sol. Al may be measured by ICP-AES using the filtrate obtained by thermally decomposing a sample with acid. C and S may be measured using a combustion-infrared absorption method, N may be measured using an inert gas fusion-thermal conductivity method, and O may be measured using an inert gas fusion-non-dispersive infrared absorption method. If the hot-rolled steel sheet has a plating layer on its surface, the plating layer may be removed by mechanical grinding or the like, as necessary, before analyzing the chemical composition.

[0041] Next, the metallographic structure of the hot-rolled steel sheet according to this embodiment will be described. The hot-rolled steel sheet according to this embodiment has a metallographic structure at a quarter position from the surface in the sheet thickness direction, in terms of area %, in which martensite and tempered martensite account for more than 92.0% and not more than 100.0%, retained austenite is less than 3.0%, and ferrite is less than 5.0%, and the Entropy value shown in the following formula (1) obtained by analyzing an SEM image of the metallographic structure by the gray level co-occurrence matrix method is 11.0 or more, the Inverse difference normalized value shown in the following formula (2) is less than 1.020, and the Cluster Shade value shown in the following formula (3) is −8.0 × 10 ５ ~8.0 x 10 ５ the standard deviation of the Mn concentration is 0.60 mass% or less, and the number density of Si—Al—Cr oxide particles having a circle-equivalent radius of 0.500 μm or more on the surface is 2.0×10 ３ pieces / cm ２ The number density of Si—Al—Cr oxide particles having a circle-equivalent radius of 0.005 to 0.050 μm is 1.0×10 ５ pieces / cm ２ That's all.

[0042] Therefore, the hot-rolled steel sheet according to this embodiment can achieve high strength and critical fracture thickness reduction rate, as well as excellent hole expandability, shear workability, and corrosion resistance. In this embodiment, the structure fraction, Entropy value, inverse difference normalized value, Cluster Shade value, and standard deviation of Mn concentration of the metal structure in a region located 1 / 4 of the way from the surface in the thickness direction are specified. This is because the metal structure at this position represents a representative metal structure of the steel sheet. Furthermore, the term "surface" as used herein refers to the interface between the coating layer and the steel sheet when the hot-rolled steel sheet has a coating layer.

[0043] Area Fraction of Retained Austenite: Less than 3.0% Retained austenite is a metal structure that exists as a face-centered cubic lattice even at room temperature. Retained austenite enhances the hole expandability of hot-rolled steel sheets through transformation-induced plasticity (TRIP). On the other hand, retained austenite transforms into high-carbon martensite during shearing, inhibiting stable crack initiation and reducing the linearity of the boundary between the fracture surface and the shear surface at the end face after shearing. When the area fraction of retained austenite is 3.0% or more, the above effects become apparent, reducing the linearity of the boundary between the fracture surface and the shear surface at the end face after shearing. Furthermore, the desired strength cannot be achieved in the hot-rolled steel sheet. Therefore, the area fraction of retained austenite is set to less than 3.0%. The area fraction of retained austenite is preferably less than 1.5%, more preferably less than 1.0%. Since the smaller the amount of retained austenite, the more preferable it is, the area fraction of retained austenite may be 0%.

[0044] Methods for measuring the area fraction of retained austenite include X-ray diffraction, EBSP (Electron Backscattering Diffraction Pattern) analysis, magnetic measurement, etc. In this embodiment, the area fraction of retained austenite is measured by X-ray diffraction.

[0045] In this embodiment, the measurement of the area fraction of retained austenite by X-ray diffraction involves first collecting a sample from the cross section of the hot-rolled steel sheet at a quarter-way position from the surface in the thickness direction so that the metal structure can be observed in a region of 1 mm or more at any position in the longitudinal direction and 1 mm or more from the center of the quarter-way position from the end face in the width direction. The sample is then analyzed using Co-Kα radiation to determine the integrated intensities of a total of six peaks: α(110), α(200), α(211), γ(111), γ(200), and γ(220). Next, the volume fraction of retained austenite is calculated from the integrated intensities using the intensity averaging method. The volume fraction of retained austenite thus obtained is considered to be the area fraction of retained austenite.

[0046] Ferrite area ratio: less than 5.0% Ferrite is generally a soft metal structure. If the ferrite contains more than a certain amount, the desired strength may not be obtained and the area of ​​the sheared surface at the end surface after shearing may increase. If the area of ​​the sheared surface at the end surface after shearing increases, the linearity of the boundary between the fracture surface and the sheared surface at the end surface after shearing decreases, which is undesirable. If the ferrite area ratio is 5.0% or more, the above-mentioned effect becomes apparent. Therefore, the ferrite area ratio is less than 5.0%. The ferrite area ratio is preferably 3.0% or less, more preferably 2.0% or less, and even more preferably less than 1.0%. Since the less ferrite, the better, the ferrite area ratio may be 0%.

[0047] Sum of area ratios of martensite and tempered martensite: more than 92.0% and 100.0% or less If the sum of the area ratios of martensite and tempered martensite is 92.0% or less, the desired strength cannot be obtained in the hot-rolled steel sheet. Therefore, the sum of the area ratios of martensite and tempered martensite is set to more than 92.0%. Preferably, it is 93.0% or more, 95.0% or more, 97.0% or more, or 99.0% or more. Since the higher the sum of the area ratios of martensite and tempered martensite, the better, it may be set to 100.0%.

[0048] The hot-rolled steel sheet according to this embodiment may contain one or both of bainite and pearlite as a remaining structure, with a total area ratio of 0% or more and less than 8.0%. The upper limit of the area ratio of the remaining structure may be 6.0%, 5.0%, 4.0%, 3.0%, or 1.5%.

[0049] The area ratio of each structure is measured using the following method. A thickness cross section parallel to the rolling direction is mirror-finished and polished for 8 minutes at room temperature using colloidal silica with a particle size of 0.25 μm that does not contain an alkaline solution to remove strain introduced into the surface layer of the sample. Crystal orientation information is obtained from the sample cross section at any position in the longitudinal direction, in a region extending 50 mm or more from the center of the quarter position from the end face in the thickness direction. The crystal orientation information is obtained by measuring using electron backscatter diffraction at a measurement interval of 0.1 μm. For the measurement, an EBSD analyzer consisting 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. The degree of vacuum in the EBSD analyzer is 9.6 × 10 －５ Pa or less, acceleration voltage 15 kV, irradiation current level 13, and electron beam irradiation level 62. The observation area is 40,000 μm ２ Let's say.

[0050] Next, a backscattered electron image is taken in the same field of view. From the backscattered electron image, crystal grains in which ferrite and cementite are precipitated in layers are identified, and the area ratio of these crystal grains is calculated to obtain the area ratio of pearlite.

[0051] Thereafter, for the crystal grains determined to have a body-centered cubic structure among the crystal grains excluding those determined to be pearlite, the obtained crystal orientation information is used to determine the region where the grain average misorientation value is 1.0° or less as ferrite using the "Grain Average Misorientation" function installed in the software "OIM Analysis (registered trademark)" attached to the EBSD analyzer. At this time, the grain tolerance angle is set to 15°, and the area ratio of the region determined to be ferrite is obtained by calculating the area ratio of the ferrite.

[0052] Next, the method for measuring the area fractions of martensite and tempered martensite will be described below. First, to observe the same area as the EBSD measurement area using an SEM, a Vickers indentation is made near the observation position. Then, surface contamination is polished away, leaving the structure of the observation surface, and the specimen is etched with nital. Next, the same field of view as the EBSD observation surface is observed using an SEM at a magnification of 3000x.

[0053] In the EBSD measurement, among the regions determined to have a structure other than ferrite, regions that have a substructure within the grains and in which cementite precipitates with multiple variants are determined to be tempered martensite. Regions with high brightness and in which the substructure is not revealed by etching are determined to be "martensite and retained austenite." The area fractions of tempered martensite and "martensite and retained austenite" are obtained by calculating the area fractions of each. The area fraction of martensite is obtained by subtracting the area fraction of retained austenite obtained by the X-ray diffraction described above from the area fraction of "martensite and retained austenite" obtained. The total area fraction of martensite and tempered martensite is obtained by calculating the sum of the area fractions of martensite and tempered martensite.

[0054] Contamination on the surface of the observation surface can be removed by buffing using alumina particles with a particle size of 0.1 μm or less, or by Ar ion sputtering.

[0055] In this embodiment, the area ratios of the respective structures are measured by X-ray diffraction, EBSD analysis, and SEM observation, and therefore the sum of the area ratios of the respective structures obtained by the measurements may not be 100.0%. When the sum of the area ratios of the respective structures obtained by the above-described method is not 100.0%, the area ratios of the respective structures are converted so that the sum of the area ratios of the respective structures becomes 100.0%. For example, when the sum of the area ratios of the respective structures is 103.0%, the area ratios of the respective structures are multiplied by "100.0 / 103.0" to obtain the area ratio of each structure.

[0056] Entropy value: 11.0 or more Inverse difference normalized value: less than 1.020 In order to improve the linearity of the boundary between the fracture surface and the shear surface at the end face after shearing, it is important to reduce the periodicity of the metal structure and reduce the uniformity of the metal structure. In this embodiment, the Entropy value (E value), which indicates the periodicity of the metal structure, and the Inverse difference normalized value (I value), which indicates the uniformity of the metal structure, are controlled to improve the linearity of the boundary between the fracture surface and the shear surface at the end face after shearing.

[0057] The E value represents the periodicity of the metal structure. When the brightness is periodically arranged due to the influence of the formation of a band-shaped structure, i.e., when the periodicity of the metal structure is high, the E value decreases. In this embodiment, a metal structure with low periodicity is required, so the E value needs to be increased. If the E value is less than 11.0, the linearity of the boundary between the fracture surface and the shear surface at the end face after shearing tends to decrease. In a metal structure with high periodicity, i.e., a low E value, cracks are generated starting from the periodically arranged structure and traveling through multiple band-shaped structures present near the starting point to form a fracture surface. This is presumably to cause the linearity of the boundary between the fracture surface and the shear surface at the end face after shearing to decrease. Therefore, the E value is set to 11.0 or more. It is preferably 11.1 or more, more preferably 11.2 or more. The higher the E value, the better. There is no particular upper limit specified, but it may be 13.5 or less, 13.0 or less, 12.5 or less, or 12.0 or less.

[0058] The I value represents the uniformity of the metal structure, and increases as the area of ​​the region with a constant brightness increases. A high I value indicates high uniformity of the metal structure. In this embodiment, a hot-rolled steel sheet having a metal structure in which the total area ratio of martensite and tempered martensite exceeds 92.0% needs to have a metal structure mainly composed of martensite with low brightness uniformity. For this reason, in this embodiment, the I value needs to be reduced. When the uniformity of the metal structure is high, i.e., when the I value is high, cracks are likely to occur from the tip of the shearing tool due to the influence of precipitates and element concentration differences within the crystal grains and hardness differences caused by the soft ferrite phase. As a result, the linearity of the boundary between the fracture surface and the shear surface at the end surface after shearing tends to decrease. In other words, if the I value is 1.020 or more, it is estimated that the linearity of the boundary between the fracture surface and the shear surface at the end surface after shearing cannot be improved. Therefore, the I value is set to be less than 1.020. It is preferably 1.015 or less, and more preferably 1.010 or less. The lower limit of the I value is not particularly specified, but it may be 0.900 or more, 0.950 or more, or 1.000 or more.

[0059] Cluster Shade value: -8.0 x 10 ５ ~8.0 x 10 ５ The Cluster Shade (CS) value indicates the degree of distortion of the metallographic structure. The CS value is positive when many points have a brightness higher than the average brightness in an image obtained by photographing the metallographic structure, and negative when many points have a brightness lower than the average brightness.

[0060] In a secondary electron image of an electron microscope, brightness increases where the surface roughness of the object being observed is large, and decreases where the roughness is small. The surface roughness of the object being observed is greatly affected by the grain size and strength distribution within the metal structure. In this embodiment, the CS value increases when the strength variation of the metal structure is large or the structural units are small, and decreases when the strength variation is small or the structural units are large.

[0061] In this embodiment, it is important to keep the CS value in a desired range close to 0. ５If the CS value is less than -8.0 × 10, the critical thickness reduction rate at fracture of the hot-rolled steel sheet decreases. This is presumably because large grains exist in the metal structure, and these grains are preferentially fractured during ultimate deformation. Therefore, the CS value is -8.0 × 10 ５ Preferably, it is -7.5 × 10 ５ or more, and more preferably −7.0×10 ５ On the other hand, the CS value is 8.0 × 10 ５ If the CS value is more than 8.0 × 10, the critical reduction in thickness at fracture of the hot-rolled steel sheet decreases. This is presumably because the microscopic strength in the metal structure varies greatly, and strain during extreme deformation is concentrated locally, making the sheet more susceptible to fracture. Therefore, the CS value is 8.0 × 10 ５ Preferably, it should be 7.5 x 10 ５ or less, and even more preferably 7.0 × 10 ５ The following is the result.

[0062] The E value, I value, and CS value can be obtained by the following method. In this embodiment, the photographed area of ​​the SEM image taken to calculate the E value, I value, and CS value is 160 μm × 160 μm, centered on a 1 / 4 position from the end face in the sheet width direction, in a cross section parallel to the rolling direction, from the surface in the sheet thickness direction, and the number of observation fields is 5. To photograph the SEM image, a SU-6600 Schottky electron gun manufactured by Hitachi High-Technologies Corporation is used, the emitter is tungsten, and the acceleration voltage is 1.5 kV. Under the above settings, the SEM image is output at a magnification of 1000 times and a gray scale of 256 gradations.

[0063] Next, the obtained SEM image is cut into an 880 x 880 pixel region (the observation region is 160 μm x 160 μm in actual size), and the image is subjected to smoothing processing with a tile grid size of 8 x 8 and a contrast enhancement limiting magnification of 2.0, as described in Non-Patent Document 3. The smoothed SEM image is rotated counterclockwise in 1-degree increments from 0 to 179 degrees, excluding 90 degrees, and an image is created for each degree, resulting in a total of 179 images. Next, for each of these 179 images, the GLCM method described in Non-Patent Document 1 is used to extract the frequency values ​​of brightness between adjacent pixels in the form of a matrix.

[0064] The matrix of 179 frequency values ​​obtained by the above method is expressed as p ｋ (k=0...89,91...179). For each image, the generated p ｋ are summed for all k (k=0...89, 91...179), and then a 256 x 256 matrix P is calculated, normalized so that the sum of each component is 1. Furthermore, the E value, I value, and CS value are calculated using the following formulas (1) to (5) described in Non-Patent Document 2. The average values ​​obtained by measuring the entire field of view are calculated.

[0065] In the following formulas (1) to (5), P(i, j) is a gray level co-occurrence matrix, and the value in the i-th row and j-th column of the matrix P is expressed as P(i, j). As described above, calculations are performed using a 256 x 256 matrix P, so if this point needs to be emphasized, the following formulas (1) to (5) can be modified to the following formulas (1') to (5'). Here, L in the following formula (2) is the number of grayscale levels that the SEM image can take (quantization levels of grayscale), and in this embodiment, L is 256 because the SEM image is output in 256 grayscale levels as described above. The dot symbol "." in the following formula (1) represents a product. In the following formulas (2) and (3), i and j are natural numbers from 1 to the aforementioned L, and μ in the following formula (3) is ｘ and μ ｙ are respectively expressed by the following formulas (4) and (5). In the following formulas (1') to (5'), the value in the i-th row and j-th column of the matrix P is expressed as P ｉｊ It is written as follows.

[0066]

[0067]

[0068]

[0069]

[0070]

[0071]

[0072]

[0073]

[0074]

[0075]

[0076] Standard deviation of Mn concentration: 0.60 mass% or less. The standard deviation of the Mn concentration at a depth of 1 / 4 of the sheet thickness from the surface of the hot-rolled steel sheet according to this embodiment (a region from 1 / 8 of the sheet thickness from the surface to 3 / 8 of the sheet thickness from the surface) and at the center position in the sheet width direction is 0.60 mass% or less. This allows the hard phase to be uniformly dispersed, preventing a decrease in the linearity of the boundary between the fracture surface and the sheared surface at the end face after shearing. The standard deviation of the Mn concentration is preferably 0.55 mass% or 0.50 mass% or less, and more preferably 0.47 mass% or less. From the viewpoint of suppressing excessive burrs, a smaller lower limit of the standard deviation of the Mn concentration is desirable. However, due to constraints of the manufacturing process, the substantial lower limit is 0.10 mass%. If necessary, the lower limit may be set to 0.20 mass% or 0.28 mass%.

[0077] The standard deviation of the Mn concentration is obtained by the following method. After mirror-polishing a cross section of the thickness of a hot-rolled steel sheet parallel to the rolling direction, the standard deviation of the Mn concentration is measured using an electron probe microanalyzer (EPMA) at a depth of 1 / 4 of the thickness from the surface (a region from 1 / 8 of the thickness from the surface to 3 / 8 of the thickness from the surface) and at the center position in the width direction. The measurement conditions are an acceleration voltage of 15 kV and a magnification of 5000x, and a distribution image is measured over a range of 20 μm in the rolling direction and 20 μm in the thickness direction of the sample. More specifically, the measurement interval is 0.1 μm, and the Mn concentration is measured at 40,000 or more locations. Next, the standard deviation is calculated based on the Mn concentrations obtained from all measurement points to obtain the standard deviation of the Mn concentration.

[0078] Number density of Si—Al—Cr oxide particles having a circle-equivalent radius of 0.500 μm or more on the surface: 2.0 × 10 ３ pieces / cm ２ Number density of Si—Al—Cr oxide particles having a circle-equivalent radius of 0.005 to 0.050 μm on the surface: 1.0 × 10５ pieces / cm ２ The present inventors have found that the corrosion resistance of a hot-rolled steel sheet can be improved by controlling the number densities of Si—Al—Cr oxides formed on the surface, which have an equivalent circle radius of 0.500 μm or more, and those which have an equivalent circle radius of 0.005 to 0.050 μm, within desired ranges.

[0079] On the surface, the number density of Si-Al-Cr oxides having a circle equivalent radius of 0.500 μm or more is 2.0 × 10 ３ pieces / cm ２ If the concentration exceeds 2.0×10, the electrochemical reaction between the solution and the steel sheet is locally hindered during chemical conversion treatment, the zinc phosphate film does not adhere to the steel sheet, and the corrosion resistance of the hot-rolled steel sheet deteriorates. ３ pieces / cm ２ The number density of Si—Al—Cr oxide particles having a circle-equivalent radius of 0.500 μm or more on the surface is preferably 1.8×10 ３ pieces / cm ２ or less, more preferably 1.5 × 10 ３ pieces / cm ２ The number density of Si—Al—Cr oxides having a circle-equivalent radius of 0.500 μm or more is 0.1×10 ３ pieces / cm ２ It may be more than that.

[0080] On the surface, the number density of Si-Al-Cr oxides with a circle equivalent radius of 0.005 to 0.050 μm is 1.0 × 10 ５ pieces / cm ２ If the surface area is less than 1.0×10, the corrosion resistance of the hot-rolled steel sheet will deteriorate. Although the mechanism is unclear, it is thought that minute Si-Al-Cr oxides have an anchoring effect due to the increased surface area, which makes it easier for a zinc phosphate film to adhere, rather than having an effect of inhibiting the electrochemical reaction. Therefore, the number density of Si-Al-Cr oxides on the surface with a circle-equivalent radius of 0.005 to 0.050 μm is 1.0×10 ５ pieces / cm ２The number density of Si—Al—Cr oxide particles having a circle-equivalent radius of 0.005 to 0.050 μm on the surface is preferably 3.0×10 ５ pieces / cm ２ More preferably, 5.0 × 10 ５ pieces / cm ２ The number density of Si-Al-Cr oxides with a circle-equivalent radius of 0.005 to 0.050 μm on the surface is 100.0 × 10 ５ pieces / cm ２ Below, 50.0 x 10 ５ pieces / cm ２ Below, 30.0 x 10 ５ pieces / cm ２ Below, 15.0 x 10 ５ pieces / cm ２ The following may also be used.

[0081] In this embodiment, it is not necessary to limit the number density of Si—Al—Cr oxides having an equivalent circle radius of less than 0.005 μm and the number density of Si—Al—Cr oxides having an equivalent circle radius of more than 0.050 μm and less than 0.500 μm on the surface. This is because, in a heat-rolled steel sheet manufactured by the chemical composition according to this embodiment and the manufacturing method described below, Si—Al—Cr oxides having an equivalent circle radius of less than 0.005 μm and Si—Al—Cr oxides having an equivalent circle radius of more than 0.050 μm and less than 0.500 μm are not formed in amounts that would adversely affect the properties of the heat-rolled steel sheet according to this embodiment.

[0082] The number density of Si—Al—Cr oxides on the surface is measured using the following method. A sample is cut from a hot-rolled steel sheet so that the surface in the thickness direction (sheet surface) serves as the observation surface. The observation surface is degreased at 60°C for 60 seconds using a Nihon Parkerizing FC-E6403, then immersed in acetone and ultrasonically cleaned for 90 seconds. Ten or more fields of view are then observed at a magnification of 3000x. The composition of the precipitates can be measured using an energy dispersive X-ray spectrometer (EDS). Among the precipitates containing Si, Al, Cr, and O, those with an equivalent circle radius of 0.500 μm or more and those with an equivalent circle radius of 0.005 to 0.050 μm are selected, counted, and divided by the measured area to obtain the number density of Si—Al—Cr oxides with an equivalent circle radius of 0.500 μm or more and the number density of Si—Al—Cr oxides with an equivalent circle radius of 0.005 to 0.050 μm.

[0083] When EDS analysis of a precipitate detects 10 atomic percent or more of Si, Cr, and O, respectively, and 5 atomic percent or more of Al, the precipitate is regarded as Si-Al-Cr oxide. When a precipitate is found to have a similar grain shape or electron beam intensity contrast to a precipitate identified as Si-Al-Cr oxide by EDS analysis, the precipitate can be regarded as Si-Al-Cr oxide without EDS analysis.

[0084] If the hot-rolled steel sheet has scale on its surface, the sample is pickled under the following conditions before being subjected to the above-mentioned surface treatment. The pickling treatment may be performed by a conventional method, for example, by immersing the sample in hydrochloric acid having a hydrochloric acid concentration of 3 to 10% by volume at a temperature of 85 to 98°C for 20 to 300 seconds. The pickling may be performed once or multiple times as necessary. The above pickling time (20 to 300 seconds) refers to the time of the pickling if only one pickling is performed, and refers to the total time of the pickling if multiple picklings are performed. A pickling temperature of 85°C or higher is preferable because it allows sufficient removal of oxides from the surface layer. The upper limit of the pickling temperature is not particularly limited, but in practice it is around 98°C. A pickling time of more than 300 seconds results in excessively rough surface roughness, deteriorating surface properties. Furthermore, the unevenness remaining after cold rolling may cause a notch-like effect, which may deteriorate the bendability of the hot-rolled steel sheet. The upper limit of the pickling time is preferably 200 seconds.

[0085] When a hot-rolled steel sheet has a surface treatment film, such as a plating layer or paint, on its surface, the surface treatment film is removed, followed by the aforementioned pickling treatment on the resulting steel substrate surface, and then the surface treatment is performed. The method for removing the surface treatment film can be appropriately selected depending on the type of surface treatment film, as long as it does not affect the surface roughness of the steel substrate. For example, when the surface treatment film is a zinc coating layer, such as electrogalvanized, electrogalvanized Zn—Ni alloy coating, hot-dip galvanized, galvannealed, hot-dip Zn—Al alloy coating, hot-dip Zn—Al—Mg alloy coating, or hot-dip Zn—Al—Mg—Si alloy coating, the zinc coating layer can be dissolved using dilute hydrochloric acid containing an inhibitor. This allows only the zinc coating layer to be stripped from the steel sheet. The inhibitor is an additive used to prevent excessive dissolution of the steel substrate and suppress changes in roughness. For example, a corrosion inhibitor for hydrochloric acid pickling, "IBIT No. 700BK" manufactured by Asahi Chemical Industry Co., Ltd., can be added to 5% by volume of hydrochloric acid diluted to a concentration of 0.6 g / L. Furthermore, when the surface treatment film is an aluminum plating layer such as hot-dip aluminum plating, the aluminum plating is dissolved by sequentially immersing the sample in a sodium hydroxide aqueous solution and a dilute hydrochloric acid aqueous solution containing hexamethylenetetramine in accordance with JIS G 3314:2019 until foaming due to dissolution of the plating subsides. Furthermore, when the surface treatment film is an electrodeposition coating, the electrodeposition coating is stripped using a stripper (Neo River SP-751, manufactured by Sansai Kako Co., Ltd.).

[0086] Tensile Strength The tensile strength of the hot-rolled steel sheet is evaluated in accordance with JIS Z 2241:2011. The test piece is a No. 5 test piece of JIS Z 2241:2011. The test piece is taken from a quarter position from the end face in the sheet width direction, and the sheet width direction is set as the longitudinal direction of the test piece.

[0087] The hot-rolled steel sheet according to this embodiment preferably has a tensile strength of 980 MPa or more. The tensile strength is more preferably 1000 MPa or more. By setting the tensile strength to 980 MPa or more, the applicable parts are not limited, and the contribution to vehicle body weight reduction can be increased. There is no particular need to set an upper limit to the tensile strength, but it may be set to 1780 MPa from the viewpoint of suppressing die wear.

[0088] Hole Expandability The hot-rolled steel sheet according to this embodiment preferably has a hole expansion ratio of 55% or more. A hole expansion ratio of 55% or more allows for a hot-rolled steel sheet that contributes greatly to reducing the weight of a vehicle body without being limited to specific parts. The upper limit of the hole expansion ratio does not need to be particularly limited, but it may be 85% or less or 80% or less. The hole expansion ratio (λ) is measured in accordance with JIS Z 2256:2010 using a No. 5 test piece of JIS Z 2241:2011. The hole expansion test piece may be taken from a quarter portion of the hot-rolled steel sheet from the end in the sheet width direction.

[0089] Plate Thickness The plate thickness of the hot-rolled steel plate according to this embodiment is not particularly limited, but may be 0.5 to 8.0 mm. If the plate thickness of the hot-rolled steel plate is less than 0.5 mm, it may be difficult to ensure the rolling completion temperature and the rolling load may become excessive, making hot rolling difficult. Therefore, the plate thickness of the hot-rolled steel plate according to this embodiment may be 0.5 mm or more. Preferably, it is 1.2 mm or more or 1.4 mm or more. On the other hand, if the plate thickness exceeds 8.0 mm, it may be difficult to refine the metal structure, making it difficult to obtain the above-mentioned metal structure. Therefore, the plate thickness may be 8.0 mm or less. Preferably, it is 6.0 mm or less.

[0090] Coating Layer The hot-rolled steel sheet according to this embodiment, having the above-described chemical composition and metallographic structure, may be provided with a coating layer on its surface to provide a surface-treated steel sheet for the purpose of improving corrosion resistance, etc. The coating layer may be an electroplated layer or a hot-dip coated layer. Examples of electroplated layers include electrogalvanized coating and electrolytic Zn—Ni alloy coating. Examples of hot-dip coated layers include hot-dip galvanized coating, alloyed hot-dip galvanized coating, hot-dip aluminum coating, hot-dip Zn—Al alloy coating, hot-dip Zn—Al—Mg alloy coating, and hot-dip Zn—Al—Mg—Si alloy coating. The coating weight is not particularly limited and may be the same as conventional coatings. Furthermore, corrosion resistance can be further improved by performing an appropriate chemical conversion treatment after coating (e.g., applying a silicate-based chromium-free chemical conversion treatment solution and drying it).

[0091] Manufacturing Conditions A suitable method for manufacturing the hot-rolled steel sheet according to this embodiment having the above-described chemical composition and metallographic structure is as follows.

[0092] In a preferred method for manufacturing a hot-rolled steel sheet according to this embodiment, the following steps (1) to (11) are performed in sequence. Note that the slab temperature and the steel sheet temperature in this embodiment refer to the surface temperatures of the slab and the steel sheet, respectively. Furthermore, stress refers to the tension applied to the steel sheet in the rolling direction.

[0093] (1) The slab is held in a temperature range of 700 to 850°C for 900 seconds or more, then further heated and held in a temperature range of 1100°C or higher for 6000 seconds or more. (2) Before rough rolling, descaling is performed at least once in a temperature range of 1200°C or higher, and during rough rolling, descaling is performed at least twice in a temperature range of 1150°C or higher, and the maximum total reduction during descaling in a temperature range of 1150°C or higher is 40% or more. (3) During rough rolling, the maximum temperature from the final stage of rolling, where the reduction exceeds 20%, to descaling after completion of rough rolling is 1130°C or lower. (4) Hot rolling is performed so that the total reduction in the temperature range of 850 to 1100°C is 90% or higher. (5) A stress of 170 kPa or more is applied to the steel sheet from the completion of the second-to-last stage of hot rolling until the start of the final stage of rolling. (6) The reduction rate in the final stage of hot rolling is set to 8% or more, and hot rolling is completed so that the rolling completion temperature Tf is 900°C or more and less than 1010°C. (7) A stress of less than 200 kPa is applied to the steel sheet from the completion of rolling in the final stage of hot rolling until the steel sheet is cooled to 800°C. (8) Accelerated cooling is performed from the completion of hot rolling to 600°C at an average cooling rate of 50°C / s or more. (9) Cooling is performed so that the average cooling rate in the temperature range of 450 to 600°C is 30°C / s or more and less than 50°C / s. (10) Accelerated cooling is performed so that the average cooling rate in the temperature range from the coiling temperature to 450°C is 50°C / s or more. (11) Coil in a temperature range of 350°C or less.

[0094] By employing the above manufacturing method, it is possible to stably manufacture a hot-rolled steel sheet having high strength and a critical thickness reduction rate at fracture, as well as excellent hole expandability, shear workability, and corrosion resistance.

[0095] (1) Slab, slab temperature and holding time when subjected to hot rolling The slab to be subjected to hot rolling may be a slab obtained by continuous casting or a slab obtained by casting and blooming. If necessary, these may be subjected to hot working or cold working.

[0096] During slab heating, the slab to be subjected to hot rolling is preferably held in a temperature range of 700 to 850°C for 900 seconds or more, and then further heated and held in a temperature range of 1100°C or higher for 6000 seconds or more. When held in the temperature range of 700 to 850°C, the steel sheet temperature may be varied within this temperature range or may be constant. When held at 1100°C or higher, the steel sheet temperature may be varied within a temperature range of 1100°C or higher or may be constant.

[0097] During austenite transformation in the temperature range of 700 to 850°C, Mn distributes between ferrite and austenite, and by extending the transformation time, Mn can diffuse within the ferrite region. This eliminates Mn microsegregation unevenly distributed in the slab and significantly reduces the standard deviation of Mn concentration. Furthermore, by holding the slab at a temperature of 1100°C or higher for 6000 seconds or longer, the standard deviation of Mn concentration can be significantly reduced.

[0098] The hot rolling is preferably performed using a reverse mill or a tandem mill as multi-pass rolling. In particular, from the viewpoints of industrial productivity and stress load on the steel sheet during rolling, it is more preferable to perform hot rolling using a tandem mill for at least the final two stages.

[0099] (2) Descaling Conditions Before Rough Rolling, Descaling and Rolling Conditions During Rough Rolling Before rough rolling, it is preferable to perform descaling at least once in a temperature range of 1200°C or higher. By performing descaling at least once in a temperature range of 1200°C or higher before rough rolling, primary scale formed in the heating furnace can be removed, and the occurrence of subsequent descaling defects can be suppressed. As a result, the number density of Si-Al-Cr oxides having a circle-equivalent radius of 0.500 μm or more on the surface of the hot-rolled steel sheet can be preferably controlled. There is no particular upper limit on the number of descalings in a temperature range of 1200°C or higher, but it may be six or less.

[0100] In the rough rolling, rolling and descaling are performed multiple times. During the rough rolling, descaling is performed between rolling passes or after multiple rolling passes. In this embodiment, it is preferable that descaling is performed two or more times in a temperature range of 1150°C or higher during the rough rolling, and that the maximum value of the total reduction between descaling passes in the temperature range of 1150°C or higher is 40% or higher. By performing descaling two or more times in a temperature range of 1150°C or higher, the thickness of the scale formed in the first stage of rough rolling is reduced or the scale is removed, thereby making it possible to preferably control the number density of Si—Al—Cr oxides having a circle-equivalent radius of 0.500 μm or higher on the surface of the hot-rolled steel sheet. Furthermore, by making the maximum value of the total reduction between descaling passes in the temperature range of 1150°C or higher 40% or higher, the effect of descaling can be enhanced, and the number density of Si—Al—Cr oxides having a circle-equivalent radius of 0.500 μm or higher on the surface of the hot-rolled steel sheet can be preferably controlled.

[0101] In order to make the maximum total reduction between descalings in the temperature range of 1150°C or higher 40% or more, for example, one rolling step with a reduction of 40% or more may be performed during a certain descaling step in the temperature range of 1150°C or higher, or multiple rolling steps may be performed so that the total reduction is 40% or more.

[0102] The total reduction rate between descalings in the temperature range of 1150°C or higher is t ０ The outlet thickness after the n+1th descaling in the temperature range of 1150°C or higher is t １ Then, {(t ０ -t １ ) / t ０}×100(%). Between the nth descaling and the (n+1)th descaling, only one rolling may be performed, or multiple rolling may be performed.

[0103] (3) Temperature Conditions During and After Completion of Rough Rolling During rough rolling, the maximum temperature from the final stage of rolling where the reduction rate exceeds 20% to descaling after completion of rough rolling is preferably 1130°C or less. After rough rolling, descaling is performed to remove scale generated during rough rolling. Furthermore, after rolling in the final stage of rough rolling, the temperature of the steel sheet rises due to reheating from inside the steel sheet. In this embodiment, during rough rolling, from the final stage of rolling where the reduction rate exceeds 20% to descaling performed after completion of rough rolling, the maximum temperature of the steel sheet temperature increased by reheating is preferably controlled to 1130°C or less. By setting the maximum temperature during rough rolling from the final stage of rolling where the reduction rate exceeds 20% to descaling performed after completion of rough rolling to 1130°C or less, the number density of Si—Al—Cr oxides having a circle equivalent radius of 0.005 to 0.050 μm can be preferably controlled.

[0104] In addition, even when the steel sheet temperature is increased by heating between the final stage of rough rolling where the reduction rate exceeds 20% and descaling performed after completion of rough rolling for the purpose of soaking the steel sheet, it is preferable to control the maximum temperature to 1130°C or less. Furthermore, the "final stage of rolling where the reduction rate exceeds 20%" here does not necessarily mean the final rolling of finish rolling. For example, if the reduction rate of the final rolling of finish rolling is 20% or less and the reduction rate of the rolling before the final stage exceeds 20%, this rolling before the final stage is the "final stage of rolling where the reduction rate exceeds 20%."

[0105] (4) Total reduction rate of hot rolling: 90% or more in the temperature range of 850 to 1100°C Hot rolling with a total reduction rate of 90% or more in the temperature range of 850 to 1100°C mainly refines the recrystallized austenite grains and promotes the accumulation of strain energy in the unrecrystallized austenite grains. This promotes the recrystallization of austenite and the atomic diffusion of Mn, thereby reducing the standard deviation of the Mn concentration. Therefore, it is preferable to perform hot rolling with a total reduction rate of 90% or more in the temperature range of 850 to 1100°C. Note that the hot rolling referred to here includes rough rolling and finish rolling.

[0106] The reduction ratio in the temperature range of 850 to 1100 ° C. is the initial thickness before rolling in this temperature range. ０ The outlet thickness after the final stage of rolling in this temperature range is t １ Then, {(t ０ -t １ ) / t ０}×100(%).

[0107] (5) Stress applied to steel sheet after completion of the last-to-last hot rolling stage until start of final-to-last rolling stage: 170 kPa or more. It is preferable to apply a stress of 170 kPa or more to the steel sheet after completion of the last-to-last hot rolling stage until start of final-to-last rolling stage. This reduces the number of crystal grains having the {110}<001> crystal orientation in the recrystallized austenite after completion of the last-to-last hot rolling stage. Because {110}<001> is a crystal orientation that is difficult to recrystallize, suppressing the formation of this crystal orientation effectively promotes recrystallization during the final-to-last rolling stage. As a result, the band-shaped structure of the hot-rolled steel sheet is improved, the periodicity of the metallographic structure is reduced, and the E value is increased.

[0108] The rolling one stage before the final stage of hot rolling referred to here means the rolling one stage before the final stage of finish rolling. For example, if finish rolling is performed in passes F1, F2, ... F6, and F7, this refers to the pass F6.

[0109] If the stress applied to the steel sheet is less than 170 kPa, it may not be possible to achieve the desired E value in some cases. The stress applied to the steel sheet is more preferably 190 kPa or more. The stress applied to the steel sheet refers to the tension applied in the longitudinal direction of the steel sheet, which can be controlled by adjusting the roll rotation speed during tandem rolling, and can be determined by dividing the load in the rolling direction measured in the rolling stand by the cross-sectional area of ​​the steel sheet being passed through.

[0110] (6) Reduction ratio in the final stage of hot rolling: 8% or more, hot rolling completion temperature Tf: 900°C or more, less than 1010°C. It is preferable that the reduction ratio in the final stage of hot rolling is 8% or more, and the hot rolling completion temperature Tf is 900°C or more. By setting the reduction ratio in the final stage of hot rolling to 8% or more, recrystallization due to the reduction in the final stage can be promoted. As a result, the band-shaped structure of the hot-rolled steel sheet is improved, the periodicity of the metallographic structure is reduced, and the E value is increased. By setting the hot-rolling completion temperature Tf to 900°C or more, an excessive increase in the number of ferrite nucleation sites in austenite can be suppressed. As a result, the generation of ferrite in the final structure (the metallographic structure of the hot-rolled steel sheet after production) can be suppressed, and a high-strength hot-rolled steel sheet can be obtained. Furthermore, by setting Tf to less than 1010°C, coarsening of the austenite grain size can be suppressed, the periodicity of the metallographic structure can be reduced, and the E value can be set to a desired value.

[0111] (7) Stress applied to the steel sheet from the completion of the final stage of hot rolling until the steel sheet is cooled to 800°C: less than 200 kPa It is preferable to apply a stress of less than 200 kPa to the steel sheet from the completion of the final stage of hot rolling until the steel sheet is cooled to 800°C. By applying a stress of less than 200 kPa to the steel sheet, austenite recrystallization proceeds preferentially in the rolling direction, and an increase in the periodicity of the metal structure can be suppressed. As a result, the E value can be set to a desired value. The stress applied to the steel sheet is more preferably 180 kPa or less.

[0112] (8) Average cooling rate from completion of hot rolling to 600°C: 50°C / s or more By performing accelerated cooling so that the average cooling rate from completion of hot rolling to 600°C is 50°C / s or more, ferrite transformation, bainite transformation, and / or pearlite transformation within the steel sheet can be suppressed, and the desired strength can be obtained in the hot-rolled steel sheet. In addition, the I value can be set to a desired value. If air cooling or the like is performed during accelerated cooling to 600°C after completion of hot rolling, the amount of ferrite may increase, which is not preferable.

[0113] Although the upper limit of the average cooling rate is not particularly specified, increasing the cooling rate requires a large-scale cooling facility, which increases the facility cost. Therefore, in consideration of the facility cost, the average cooling rate of the accelerated cooling is preferably 300°C / s or less.

[0114] The average cooling rate referred to here means the value obtained by dividing the temperature drop of the steel plate from the start of accelerated cooling (the time when the steel plate is introduced into the cooling equipment) to 600°C by the time required from the start of accelerated cooling until the temperature of the steel plate reaches 600°C.

[0115] (9) Average cooling rate in the temperature range of 450 to 600°C: 30°C / s or more and less than 50°C / s After the accelerated cooling is completed, it is preferable to cool the material so that the average cooling rate in the temperature range of 450 to 600°C is 30°C / s or more and less than 50°C / s. By setting the average cooling rate in the temperature range to 30°C / s or more and less than 50°C / s, the CS value can be set to a desired value. If the average cooling rate exceeds 50°C / s, coarse crystal grains are likely to be generated in the metal structure, and the CS value will be -8.0 × 10 ５ When the average cooling rate is less than 30°C / s, the strength of the hard structure increases, and the difference in strength with the soft structure increases, resulting in a CS value of 8.0 × 10 ５ It becomes super.

[0116] The average cooling rate referred to here means the value obtained by dividing the temperature drop of the steel plate from the cooling stop temperature of accelerated cooling at an average cooling rate of 50°C / s or more to the cooling stop temperature of cooling at an average cooling rate of 30°C / s or more and less than 50°C / s by the time required from the stop of accelerated cooling at an average cooling rate of 50°C / s or more to the stop of cooling at an average cooling rate of 30°C / s or more and less than 50°C / s.

[0117] (10) Average cooling rate in the temperature range from the coiling temperature to 450°C: 50°C / s or more In order to suppress the area ratio of pearlite and obtain the desired strength in the hot-rolled steel sheet, it is preferable to set the average cooling rate in the temperature range from the coiling temperature to 450°C to 50°C / s or more. This makes it possible to harden the matrix structure.

[0118] The average cooling rate referred to here means the value obtained by dividing the temperature drop width of the steel sheet from the cooling stop temperature to the coiling temperature in cooling where the average cooling rate is 30°C / s or more and less than 50°C / s, by the time required from the stop of cooling to coiling in cooling where the average cooling rate is 30°C / s or more and less than 50°C / s.

[0119] (11) Coiling temperature: 350°C or less The coiling temperature is preferably 350°C or less. By setting the coiling temperature to 350°C or less, the driving force for the transformation from austenite to bcc can be increased, and the deformation strength of austenite can be increased. Therefore, when austenite transforms into martensite, the hard phase is uniformly distributed, and the variation can be improved. As a result, the I value can be reduced, and the linearity of the boundary between the fracture surface and the shear surface on the end surface after shearing can be improved. Therefore, the coiling temperature is preferably 350°C or less.

[0120] Next, the effects of one embodiment of the present invention will be explained in more detail using examples, but the conditions in the examples are merely examples adopted to confirm the feasibility and effects of the present invention, and the present invention is not limited to these examples. Various conditions can be adopted in the present invention as long as they do not deviate from the gist of the present invention and the object of the present invention is achieved.

[0121] Steels having the chemical compositions shown in Tables 1 and 2 were melted and continuously cast into slabs with thicknesses of 240 to 300 mm. Using the resulting slabs, hot-rolled steel sheets shown in Tables 5A to 6B were obtained under the production conditions shown in Tables 3A to 4B. Note that the coiling temperature shown in Tables 4A and 4B has a measurement lower limit of 50°C, so the actual coiling temperature in the examples shown as 50°C was 50°C or lower. Furthermore, Production No. 15 was cooled to 721°C after completion of hot rolling, and then air-cooled for 6.0 seconds. The average cooling rate during air-cooling was less than 5.0°C / s.

[0122] The area ratio of the metallographic structure, the E value, the I value, the CS value, the standard deviation of the Mn concentration, the number density of Si-Al-Cr oxides having an equivalent circle radius of 0.500 μm or more, the number density of Si-Al-Cr oxides having an equivalent circle radius of 0.005 to 0.050 μm on the surface, the tensile strength (TS), and the hole expansion ratio (λ) were determined for the obtained hot-rolled steel sheets by the above-mentioned methods. The obtained measurement results are shown in Tables 5A to 6B.

[0123] Evaluation method of properties of hot-rolled steel sheets Tensile strength If the tensile strength (TS) was 980 MPa or more, the steel sheet was judged to have high strength and pass. On the other hand, if the tensile strength (TS) was less than 980 MPa, the steel sheet was judged to not have high strength and fail.

[0124] Hole expansion ratio: When the hole expansion ratio (λ) was 55% or more, the hole was judged to be excellent and pass. On the other hand, when the hole expansion ratio (λ) was less than 55%, the hole was judged to be poor and fail.

[0125] The critical fracture thickness reduction rate of the hot-rolled steel sheet was evaluated by a tensile test. The tensile test was performed using the same method as when evaluating the tensile properties. The critical fracture thickness reduction rate was obtained by calculating the value of (t1-t2) x 100 / t1, where t1 is the thickness before the tensile test and t2 is the minimum thickness at the center of the width direction of the tensile test piece after fracture. The tensile test was performed five times, and the critical fracture thickness reduction rate was obtained by calculating the average value of three tests excluding the maximum and minimum values ​​of the critical fracture thickness reduction rate.

[0126] When the critical fracture thickness reduction rate was 75.0% or more, the hot-rolled steel sheet was judged to have a high critical fracture thickness reduction rate and to have passed the test. On the other hand, when the critical fracture thickness reduction rate was less than 75.0%, the hot-rolled steel sheet was judged to have a high critical fracture thickness reduction rate and to have failed the test.

[0127] Shear workability (evaluation of the linearity of the boundary between the fracture surface and the shear surface) The linearity of the boundary between the fracture surface and the shear surface, among the shear workability of the hot-rolled steel sheet, was evaluated by performing a punching test and determining the linearity at the boundary between the fracture surface and the shear. Five punched holes were created at the center of the plate width of the hot-rolled steel sheet with a hole diameter of 10 mm, a clearance of 15%, and a punching speed of 3 m / s. Next, for the five punched holes, the appearance of 10 end faces parallel to the rolling direction (two end faces per punched hole) was photographed using an optical microscope. In the obtained observation photograph, the end faces as shown in FIG. 1(a) can be observed. As shown in FIGS. 1(a) and 1(b), sagging, shear surfaces, fracture surfaces, and burrs are observed on the end faces after punching. Note that FIG. 1(a) is a schematic diagram of the end face parallel to the rolling direction of the punched hole, and FIG. 1(b) is a schematic diagram of the side of the punched hole. A sag is a smooth, rounded surface, a shear surface is a punched end surface that has separated due to shear deformation, a fracture surface is a punched end surface that has separated due to a crack that has developed near the cutting edge after shear deformation has ended, and a burr is a surface with a protrusion that protrudes from the underside of the hot-rolled steel plate.

[0128] In the observation photographs of 10 end faces obtained from the five end faces, the linearity at the boundary between the fracture surface and the shear was measured by the method described below, and the maximum linearity obtained was calculated.

[0129] The linearity at the boundary between the fracture surface and the shear plane was obtained as follows. As shown in Figure 1(b), the boundary points between the shear plane and the fracture surface (points A and B in Figure 1(b)) were determined for the end face. The length of the distance x connecting points A and B with a straight line was measured. Next, the length y of the curve along the boundary between the fracture surface and the shear plane was measured. The value obtained by dividing the obtained y by x was taken as the linearity at the boundary between the fracture surface and the shear plane.

[0130] When the maximum straightness value obtained in the punching test was less than 1.045, the hot-rolled steel sheet was judged to have excellent shear workability and to have passed the test. On the other hand, when the maximum straightness value obtained was 1.045 or more, the hot-rolled steel sheet was judged to have poor shear workability and to have failed the test.

[0131] Corrosion Resistance A 70 mm x 40 mm sample was taken from the hot-rolled steel sheet after pickling and subjected to zinc phosphate treatment (SD5350 system: standard manufactured by Nippon Paint Industrial Coating Co., Ltd.). Three locations (center and both ends) along the length of the test piece were observed at 1000x magnification using a scanning electron microscope (SEM). The pickling conditions were the same as those for the pickling treatment described above. If the coverage of the chemical conversion crystals was 98% or more of the surface area, the hot-rolled steel sheet was judged to have excellent corrosion resistance and was judged to have passed. On the other hand, if the coverage of the chemical conversion crystals was less than 98% of the surface area, the hot-rolled steel sheet was judged not to have excellent corrosion resistance and was judged to have failed.

[0132]

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[0140]

[0141] From Tables 5A to 6, it can be seen that the hot-rolled steel sheets according to the examples of the present invention have high strength and critical thickness reduction at fracture, as well as excellent hole expandability, shear workability, and corrosion resistance. On the other hand, it can be seen that the hot-rolled steel sheets according to the comparative examples are degraded in one or more of the properties.

[0142] According to the above aspects of the present invention, it is possible to provide a hot-rolled steel sheet having high strength, a critical thickness reduction rate at fracture, and excellent hole expandability, shear workability, and corrosion resistance. The hot-rolled steel sheet according to the present invention is suitable as an industrial material used for automotive components, machine structural components, and even building components.

Claims

DEPCT6918 / 09 / 25681. Hot-rolled steel sheet containing the following chemical elements in percentage by mass: C: 0.040% to 0.250%; Si: 0.40% to 1.00%; Mn: 1.00% to 4.00%; Sol.Al: 0.100% to 0.500%; Cr: 0.50% to 2.00%; P: 0.100% or less; S: 0.0300% or less; N: 0.1000% or less; O: 0.0100% or less; Ti: 0% to 0.300%; Nb: 0%. Percentage to 0.100 percent; V: 0 percent to 0.500 percent; Cu: 0 percent to 2.00 percent; Mo: 0 percent to 1.00 percent; Ni: 0 percent to 2.00 percent; B: 0 percent to 0.0100 percent; Ca: 0 percent to 0.0200 percent; Mg: 0 percent to 0.0200 percent; REM: 0 percent to 0.1000 percent; Bi: 0 percent to 0.0200 percent; As: 0 percent to 0.100 percent; Zr: 0 percent to 1.00 percent; Co: 0 percent to 1.00 percent; Zn: 0 percent to 1.00 percent; W: 0 percent to 1.00 percent.00 percent; Sn: 0 percent to 0.05 percent; the remainder, which consists of Fe and impurities, and corresponds to the following formula (A), where in the microstructure at the position 1 / 4 from the surface in the sheet thickness direction, in percentage by area of ​​the total combined martensite and temper martensite, is greater than 92.0 percent and 100.0 percent or less, the residual austenite is less than 3.0 percent, ferrite is less than 5.0 percent, and the entropy value expressed by the following formula (1) is 11.

0. Or more, the normalized inverse difference value shown by the following formula(2) is less than 1.020, and the group darkening value shown by the following formula(3) is -8.0x10⁵ to 8.0x10⁵ obtained by SEM image analysis of the microstructure with the grayscale co-occurrence matrix method, the standard deviation of the Mn concentration is 0.60 percent by mass or less, at the surface, the number density of Si-Al-Cr oxide with an equivalent radius of a circle of 0.500 μm or more is 2.0x10³ pcs / cm² or less, and the number density of Si-Al-Cr oxides with an equivalent radius of a circle of 0.005 µm to 0.050 µm is 1.0x10⁵ pcs / cm² or more, Zr+Co+Zn+W less than or equal to 1.00 percent...(A), here each elemental symbol in formula(A) represents the amount of the element in percentage by mass, and 0 percent is replaced when no element is present, p(i,j) in formulas(1) to(5) below are the co-occurrence matrix of the gray scale, L in formula(2) below are the possible count divisions of the gray scale. The grayscales of the SEM images, i and j in the following formulas (2) and (3) are natural numbers from 1 to L, mu x and mu y in the following formula (3) are represented by the following formulas (4) and (5): [Formula 1] Entropy = (Formula)•••(1) [Formula 2] Normalized Inverse Difference = (Formula)•••(2) [Formula 3] Group Darkness Level = (Formula)•••(3) [Formula 4] mu x = (Formula)•••(4) [Formula 5] mu y = (Formula)•••(5) 2. Hot-rolled thin steel sheets according to claim 1 in which the chemical composition includes (in percentage by mass) one or two or more elements selected from a group consisting of: Ti: 0.001 percent to 0.300 percent; Nb: 0.001 percent to 0.100 percent; V: 0.001 percent to 0.500 percent; Cu: 0.01 percent to 2.00 percent; Mo: 0.01 percent to 1.00 percent; Ni: 0.02 percent to 2.00 percent; B: 0.0001 percent to 0.0100 percent; Ca: 0.0005 percent to 0.0200 percent; Mg: 0.0005 percent to 0.0200 percent Percentage; REM: 0.0005% to 0.1000%; Bi: 0.0005% to 0.0200%; As: 0.001% to 0.100%; Zr: 0.01% to 1.00%; Co: 0.01% to 1.00%; Zn: 0.01% to 1.00%; W: 0.01% to 1.00%; and Sn: 0.01% to 0.05%.

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