Hot-rolled steel sheet

Abstract

The hot-rolled steel sheet has a prescribed chemical composition, a metal structure in which the total of martensite and tempered martensite exceeds 92.0% and is 100.0% or less in area%, retained austenite is less than 3.0%, ferrite is less than 5.0%, an entropy value obtained by analyzing an SEM image of the above metal structure using a gray level co-occurrence matrix method is 11.0 or more, a reverse difference normalized value is less than 1.020, a cluster shade value is -8.0 x 10 5 ~ 8.0 x 10 5 , a standard deviation of Mn concentration is 0.60 mass% or less, and the hot-rolled steel sheet has a tensile strength of 980 MPa or more.

Description

Technical Field

[0001] This invention relates to hot-rolled steel sheets. More specifically, it relates to hot-rolled steel sheets that can be formed into various shapes by pressing or other processes, particularly hot-rolled steel sheets that have high strength and a high reduction rate in sheet thickness at the ultimate breaking point, and excellent hole-expanding and shearing properties.

[0002] This application claims priority based on Japanese Patent Application No. 2021-166960 filed on October 11, 2021, the contents of which are incorporated herein by reference. Background Technology

[0003] In recent years, from the perspective of protecting the Earth's environment, efforts have been made in many fields to reduce carbon dioxide emissions. Automakers are also actively developing technologies for lightweight vehicle bodies aimed at reducing fuel consumption. However, to ensure passenger safety, the focus is also on improving crashworthiness, making vehicle lightweighting a challenging task.

[0004] To balance lightweight vehicle body and crashworthiness, the use of high-strength steel sheets to thin the walls of components has been investigated. Therefore, steel sheets possessing both high strength and excellent formability are highly desirable, and several technologies have been proposed to address these requirements. Since there are various processing methods for automotive components, the required formability varies depending on the component being applied, but the reduction rate of sheet thickness at ultimate fracture and the ability to expand the hole have been identified as important indicators of formability. The reduction rate of sheet thickness at ultimate fracture is the value calculated from the minimum of the sheet thickness of the tensile test specimen before fracture and the sheet thickness of the tensile test specimen after fracture. A low reduction rate of sheet thickness at ultimate fracture makes the material more prone to early fracture when subjected to tensile strain during compression forming, and is therefore undesirable.

[0005] Automotive components are formed through compression molding, but the blanks produced by compression molding are mostly manufactured through high-productivity shearing processes. For blanks manufactured through shearing, excellent end-face precision is required after shearing.

[0006] For example, if the straightness of the fracture surface and the boundary of the shear surface at the shear end face is low, the accuracy of the shear end face will be significantly degraded.

[0007] For example, Patent Document 1 discloses a hot-rolled steel sheet with excellent surface properties after pressing, which can be used as a raw material for cold-rolled steel sheets. It controls the Mn segregation and P segregation in the central part of the sheet thickness.

[0008] However, Patent Document 1 does not take into account the ultimate fracture thickness reduction rate and shear workability of hot-rolled steel sheets.

[0009] Existing technical documents

[0010] Patent documents

[0011] Patent Document 1: International Publication No. 2020 / 044445

[0012] Non-patent literature

[0013] Non-patent literature 1: J. Webel, J. Gola, D. Britz, F. Mucklich, Materials Characterization 144(2018)584-596

[0014] Non-Patent Literature 2: DLNaik, HUSajid, R. Kiran, Metals 2019, 9, 546

[0015] Non-Patent Literature 3: K. Zuiderveld, Contrast Limited Adaptive Histogram Equalization, Chapter VIII.5, Graphics Gems IV. P.S. Heckbert (Eds.), Cambridge, MA, Academic Press, 1994, pp. 474-485. Summary of the Invention

[0016] The problem that the invention aims to solve

[0017] The present invention was made in view of the above-mentioned facts, and its object is to provide a hot-rolled steel sheet with high strength and ultimate fracture thickness reduction rate, and excellent hole expansion and shearing workability.

[0018] Methods for solving problems

[0019] The main points of this invention are as follows.

[0020] (1) The chemical composition of the hot-rolled steel sheet according to one embodiment of the present invention, expressed in mass percent, is as follows:

[0021] C: 0.040~0.250%

[0022] Si: 0.05~3.00%

[0023] Mn: 1.00~4.00%

[0024] sol.Al: 0.001~0.500%

[0025] P: below 0.100%

[0026] S: below 0.0300%

[0027] N: below 0.1000%

[0028] O: Below 0.0100%

[0029] Ti: 0~0.300%

[0030] Nb: 0~0.100%

[0031] V: 0~0.500%

[0032] Cu: 0–2.00%

[0033] Cr: 0–2.00%

[0034] Mo: 0–1.00%

[0035] Ni: 0~2.00%

[0036] B: 0~0.0100%

[0037] Ca: 0~0.0200%

[0038] Mg: 0~0.0200%

[0039] REM: 0~0.1000%

[0040] Bi: 0~0.0200%

[0041] As: 0~0.100%

[0042] Zr: 0~1.00%

[0043] Co: 0-1.00%

[0044] Zn: 0~1.00%

[0045] W: 0~1.00%

[0046] Sn: 0–0.05%, and

[0047] Remaining components: Fe and impurities.

[0048] Satisfy the following equation (A),

[0049] The microstructure is as follows: in area %:

[0050] The total amount of martensite and tempered martensite exceeds 92.0% but is below 100.0%;

[0051] Residual austenite content is less than 3.0%;

[0052] Ferrite content is less than 5.0%;

[0053] The entropy value obtained by analyzing the SEM image of the above-mentioned metal structure using the gray-level co-occurrence matrix method is greater than 11.0, as expressed by the following equation (1).

[0054] The inverse difference normalized value represented by the following equation (2) is less than 1.020;

[0055] The cluster shade value represented by equation (3) is -8.0 × 10⁻⁶. 5 ~8.0×10 5 ;

[0056] The standard deviation of Mn concentration is below 0.60% by mass.

[0057] The tensile strength of the hot-rolled steel plate is above 980 MPa.

[0058] Zr + Co + Zn + W ≤ 1.00% (A)

[0059] In the above formula (A), each element symbol represents the content of the element in terms of mass % and is substituted with 0% if the element is not present.

[0060] In equations (1) to (5), P(i,j) is the gray-level co-occurrence matrix; in equation (2), L is the number of gray-level scale levels obtainable from the SEM image; in equations (2) and (3), i and j are natural numbers from 1 to L; and in equation (3), μ... x and μ y They are represented by the following formulas (4) and (5), respectively.

[0061] Entropy = -∑ i ∑ j P(i,j).log(P(i,j)) (1)

[0062]

[0063] Cluster shadow = ∑ i ∑ j (i+j-μ x -μ y ) 3 P(i,j) (3)

[0064] μ x =∑ i ∑ j i(P(i,j)) (4)

[0065] μ y =∑ i ∑j j(P(i,j)) (5)

[0066] (2) According to the hot-rolled steel plate described in (1) above, the average crystal grain size of its surface layer can also be less than 3.0 μm.

[0067] (3) The hot-rolled steel sheet according to (1) or (2) above may also contain, in mass percent, one or more elements selected from the following:

[0068] Ti: 0.001~0.300%

[0069] Nb: 0.001~0.100%

[0070] V: 0.001~0.500%

[0071] Cu: 0.01–2.00%

[0072] Cr: 0.01~2.00%

[0073] Mo: 0.01~1.00%

[0074] Ni: 0.01~2.00%

[0075] B: 0.0001~0.0100%

[0076] Ca: 0.0001~0.0200%

[0077] Mg: 0.0001~0.0200%

[0078] REM: 0.0001~0.1000%

[0079] Bi: 0.0001~0.0200%

[0080] As: 0.001~0.100%

[0081] Zr: 0.01~1.00%

[0082] Co: 0.01~1.00%

[0083] Zn: 0.01~1.00%

[0084] W: 0.01–1.00%, and

[0085] Sn: 0.01–0.05%.

[0086] Invention Effects

[0087] According to the above-described embodiments of the present invention, hot-rolled steel sheets with high strength and ultimate fracture thickness reduction rate, as well as excellent hole expansion and shearing workability, can be obtained. Furthermore, according to the preferred embodiments of the present invention, hot-rolled steel sheets that, in addition to possessing the aforementioned characteristics, also suppress the generation of bending internal cracks, i.e., exhibit excellent resistance to bending internal cracks, can be obtained.

[0088] The hot-rolled steel sheet of the above-described solution of the present invention is suitable as an industrial raw material for use in automotive components, mechanical structural components, and even building components. Attached Figure Description

[0089] Figure 1 This is a diagram used to illustrate the method for determining the straightness of the boundary between the fracture surface and the shear surface at the end face after shearing. Detailed Implementation

[0090] The chemical composition and microstructure of the hot-rolled steel sheet of this embodiment will be described in more detail below. However, the present invention is not limited to the structure disclosed in this embodiment, and various modifications can be made without departing from the spirit of the invention.

[0091] The lower and upper limits of the numerical ranges indicated by “~” are included within these ranges. Values ​​expressed as “less than” or “more than” are not included within these ranges. In the following descriptions, “%” for chemical composition refers to “mass%” unless otherwise specified.

[0092] Chemical composition

[0093] The chemical composition of the hot-rolled steel sheet of this embodiment will be described in detail below.

[0094] C: 0.040~0.250%

[0095] C increases the area fraction of the hard phase. Furthermore, C increases the strength of martensite by combining with precipitation strengthening elements such as Ti, Nb, and V. When the C content is below 0.040%, it becomes difficult to obtain the desired strength. Additionally, when the C content is below 0.040%, the ferrite fraction increases, and the I value also increases due to the flat ferrite structure. Therefore, the C content is set to 0.040% or more. The C content is preferably 0.060% or more, more preferably 0.070% or more, or 0.080% or more.

[0096] On the other hand, when the carbon content exceeds 0.250%, the strength of the hot-rolled steel sheet decreases due to the promotion of low-strength pearlite formation and a reduction in the area ratio of martensite and tempered martensite. Furthermore, when the carbon content exceeds 0.250%, the flat cementite structure increases, and the E value decreases due to the influence of regions where carbides with small brightness differences are formed. Therefore, the carbon content is set to 0.250% or less or 0.220% or less. Preferably, the carbon content is 0.200% or less, 0.170% or less, 0.150% or less, or 0.120% or less.

[0097] Si: 0.05~3.00%

[0098] Si (silicon) delays cementite precipitation. This effect increases the area ratio of martensite and tempered martensite, and strengthens the hot-rolled steel sheet through solid solution strengthening. Furthermore, Si improves the steel structure through deoxidation (suppressing defects such as porosity). When the Si content is below 0.05%, the aforementioned effects are not achieved. Moreover, when the Si content is below 0.05%, the flat cementite structure increases, and the I value also increases due to the influence of regions with small brightness differences in carbide formation. Therefore, the Si content is set to 0.05% or more. Preferably, the Si content is 0.50% or more, 0.80% or more, or 1.00% or more.

[0099] On the other hand, when the Si content exceeds 3.00%, the surface properties, chemical conversion treatmentability, and consequently weldability of the steel sheet deteriorate significantly, and the A3 phase transformation point increases significantly. This makes stable hot rolling difficult. Furthermore, when the Si content exceeds 3.00%, the ferrite area ratio increases, and the E value decreases due to the flat ferrite structure. Therefore, the Si content is set to 3.00% or less. The Si content is preferably 2.70% or less, more preferably 2.50% or less, 2.20% or less, 2.00% or less, 1.80% or less, or 1.50% or less.

[0100] Mn: 1.00~4.00%

[0101] Mn plays a role in inhibiting ferrite phase transformation, thereby improving the strength of hot-rolled steel sheets. When the Mn content is below 1.00%, the desired strength cannot be obtained. Therefore, the Mn content is set to 1.00% or more. Preferably, the Mn content is 1.50% or more, 2.00% or more, or 2.30% or more.

[0102] On the other hand, when the Mn content exceeds 4.00%, due to Mn segregation, the crystal orientation difference of the grains in the hard phase becomes uneven, and the straightness of the fracture surface and the boundary of the shear surface at the end face after shearing decreases. Furthermore, when the Mn content exceeds 4.00%, the area ratio of retained austenite increases, and the I value also increases due to the effect of the flat retained austenite structure. Therefore, the Mn content is set to 4.00% or less. The Mn content is preferably 3.70% or less, 3.50% or less, 3.20% or less, or 2.90% or less.

[0103] sol.Al: 0.001~0.500%

[0104] Like Si, Al also improves steel through deoxidation and increases the surface area ratio of martensite and tempered martensite by inhibiting the precipitation of cementite from austenite. When the sol.Al content is below 0.001%, the effects described above 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, 0.030% or more, or 0.050% or more, and more preferably 0.080% or more, 0.100% or more, or 0.150% or more.

[0105] On the other hand, when the sol.Al content exceeds 0.500%, the above-mentioned effects saturate, and it is not economically preferable. Therefore, the sol.Al content is set to 0.500% or less. The sol.Al content is preferably 0.400% or less, more preferably 0.300% or less, or 0.250% or less.

[0106] It should be noted that in this embodiment, sol.Al refers to acid-soluble Al, which means solid-solution Al that exists in the steel in a solid solution state.

[0107] P: below 0.100%

[0108] Phosphorus (P) is generally present as an impurity, but it also enhances strength through solid solution strengthening. Therefore, P can be actively included. However, P is prone to segregation, and if the P content exceeds 0.100%, the reduction in the ultimate fracture thickness due to grain boundary segregation becomes significant. Therefore, the P content is set to 0.100% or less. Preferably, the P content is 0.050%, 0.030%, 0.020%, or 0.015% or less. A specific lower limit for the P content is not necessary, but a lower limit of 0% is acceptable. From the perspective of refining cost, it can also be set to 0.001%, 0.003%, or 0.005%.

[0109] S: below 0.0300%

[0110] Sulfur (S) is an element present as an impurity, forming sulfide inclusions in steel and reducing the porosity and reduction in thickness at the ultimate fracture point of hot-rolled steel sheets. If the S content exceeds 0.0300%, the porosity and reduction in thickness at the ultimate fracture point of the hot-rolled steel sheet are significantly reduced. Therefore, the S content is set to 0.0300% or less. Preferably, the S content is 0.0100%, 0.0070%, or 0.0050% or less. A specific lower limit for the S content is not necessary, but the lower limit is 0%. From the perspective of refining costs, the lower limit for the S content can also be set to 0.0001%, 0.0005%, 0.0010%, or 0.0020%.

[0111] N: below 0.1000%

[0112] Nitrogen (N) is an element contained in steel as an impurity, which reduces the porosity and thickness reduction rate at ultimate fracture of hot-rolled steel sheets. When the N content exceeds 0.1000%, the porosity and thickness reduction rate at ultimate fracture of hot-rolled steel sheets decrease significantly. 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, or 0.0500% or less.

[0113] There is no need to specifically specify the lower limit of the N content, but the lower limit of the N content is 0%. Alternatively, the lower limit of the N content can be set to 0.0001%. As described below, when the metal structure is refined by containing one or more of Ti, Nb, and V, the N content is preferably set to 0.0010% or more, more preferably 0.0020% or more, 0.0080% or more, or 0.0150% or more, in order to promote the precipitation of carbonitrides.

[0114] O: Below 0.0100%

[0115] If oxygen (O) is present in large quantities in steel, it forms coarse oxides that become fracture initiation points, leading to brittle fracture and hydrogen-induced cracking. Therefore, the O content is set to 0.0100% or less. Preferably, the O content is set to 0.0080%, 0.0050%, or 0.0030% or less. The lower limit for O content is 0%, but to ensure a large dispersion of fine oxides during deoxidation of the molten steel, the O content can also be set to 0.0005% or more, or 0.0010% or more.

[0116] In addition to the elements described above, the hot-rolled steel sheet of this embodiment may also contain the following elements as optional elements. The lower limit of the content of any optional element is 0%. The optional elements will be described in detail below.

[0117] Ti: 0.001~0.300%

[0118] Nb: 0.001~0.100%

[0119] V: 0.001~0.500%

[0120] Ti, Nb, and V all precipitate as carbides or nitrides in steel and refine the metal structure through a pinning effect; therefore, one or more of these elements may be included. To more reliably obtain the effects described above, it is preferable to set the Ti content to 0.001% or more, the Nb content to 0.001% or more, or the V content to 0.001% or more. That is, even if it is only one of Ti, Nb, or V, it is preferable to set its content to 0.001% or more.

[0121] However, even with excessive amounts of these elements, the effects described above become saturated, making it economically undesirable. Therefore, the Ti content is set to 0.300% or less, the Nb content to 0.100% or less, and the V content to 0.500% or less.

[0122] Cu: 0.01–2.00%

[0123] Cr: 0.01~2.00%

[0124] Mo: 0.01~1.00%

[0125] Ni: 0.01~2.00%

[0126] B: 0.0001~0.0100%

[0127] Cu, Cr, Mo, Ni, and B all improve the hardenability of hot-rolled steel sheets. Furthermore, Cu and Mo precipitate as carbides in steel at low temperatures, increasing strength. Moreover, Ni, when containing Cu, effectively inhibits grain boundary cracking in the slab caused by Cu. Therefore, one or more of these elements may be included.

[0128] As described above, Cu improves the hardenability of hot-rolled steel sheets and, at low temperatures, precipitates as carbides in the steel, thus increasing the strength of the hot-rolled steel sheets. To reliably obtain the effects resulting from these effects, the Cu content is preferably set to 0.01% or more, more preferably 0.05% or more. However, when the Cu content exceeds 2.00%, grain boundary cracking in 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, 0.70% or less, or 0.50% or less.

[0129] As described above, Cr improves the hardenability of hot-rolled steel sheets and enhances strength by precipitating as carbides in the steel at low temperatures. To reliably obtain the effects resulting from these effects, it is preferable to set the Cr content to 0.01% or more, more preferably 0.05% or more. However, when the Cr content exceeds 2.00%, the chemical conversion treatability of the hot-rolled steel sheet decreases significantly. Therefore, the Cr content is set to 2.00% or less. The Cr content is preferably 1.50% or less, more preferably 1.00% or less, 0.70% or less, or 0.50% or less.

[0130] As described above, Mo has the effect of improving the hardenability of hot-rolled steel sheets and, by precipitating as a carbide in the steel, improving the strength of the hot-rolled steel sheets. To more reliably obtain the effects resulting from these effects, it is preferable to set the Mo content to 0.01% or more, more preferably 0.02% or more. However, even if the Mo content is set to more than 1.00%, the effects resulting from these effects become saturated, which is not economically desirable. 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, or 0.10% or less.

[0131] As described above, Ni improves the hardenability of hot-rolled steel sheets. Furthermore, when Ni contains Cu, it effectively suppresses grain boundary cracking in the slab caused by Cu. To reliably obtain the effects described above, it is preferable to set the Ni content to 0.01% or more. Ni is an expensive element, so a large content is not economically desirable. Therefore, the Ni content is set to 2.00% or less. The Ni content is preferably 1.50% or less, more preferably 1.00% or less, 0.70% or less, or 0.50% or less.

[0132] As described above, B has the effect of improving the hardenability of hot-rolled steel sheets. To more reliably obtain the effect resulting from this property, it is preferable to set the B content to 0.0001% or more, more preferably 0.0002% or more. However, when the B content exceeds 0.0100%, the porosity of the hot-rolled steel sheet decreases significantly; therefore, the B content is set to 0.0100% or less. The B content is preferably set to 0.0050% or less, or 0.0025% or less.

[0133] Ca: 0.0001~0.0200%

[0134] Mg: 0.0001~0.0200%

[0135] REM: 0.0001~0.1000%

[0136] Bi: 0.0001~0.0200%

[0137] As: 0.001~0.100%

[0138] Ca, Mg, and REM all improve the porosity of hot-rolled steel sheets by adjusting the shape of inclusions to a preferred shape. Furthermore, Bi improves the formability of hot-rolled steel sheets by refining the solidification structure. Therefore, one or more of these elements may be included. To reliably obtain the effects of the above-mentioned effects, it is preferable to set the content of any one or more of Ca, Mg, REM, and Bi to 0.0001% or more. However, if the Ca or Mg content exceeds 0.0200% or the REM content exceeds 0.1000%, excessive inclusions will be generated in the steel, which may actually reduce the porosity of the hot-rolled steel sheet. Furthermore, even if the Bi content is set to exceed 0.0200%, the effects of the above-mentioned effects saturate, which is not economically preferable. Therefore, the Ca and Mg contents 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 Ca, Mg, and Bi contents are preferably 0.0100% or less, more preferably 0.0070% or less or 0.0040% or less. The REM content is preferably 0.0070% or less or 0.0040% or less. As helps to improve the ductility of hot-rolled steel sheets by lowering the austenite single-phase temperature, thereby refining the original austenite grains. To reliably obtain this effect, it is preferable to set the As content to 0.001% or more. On the other hand, even with a large amount of As, the above effect is saturated, so the As content is set to 0.100% or less.

[0139] REM refers to a total of 17 elements, including Sc, Y, and the lanthanides. The REM content mentioned above refers to the total content of these elements. In the case of lanthanides, they are added industrially in the form of mixed rare earth alloys.

[0140] Zr: 0.01~1.00%, Co: 0.01~1.00%, Zn: 0.01~1.00%, W: 0.01~1.00%

[0141] Zr + Co + Zn + W ≤ 1.00% (A)

[0142] In the above formula (A), each element symbol represents the content of the element in terms of mass % and is substituted with 0% if the element is not present.

[0143] Sn: 0–0.05%

[0144] Regarding Zr, Co, Zn, and W, the inventors of this invention have confirmed that even if these elements are present in a total of 1.00% or less, the effect of the hot-rolled steel sheet of this embodiment is not impaired. Therefore, one or more of Zr, Co, Zn, and W may be present in a total of 1.00% or less. That is, the value on the left side of the above formula (A) may be set to 1.00% or less, or it may be set to 0.50%, 0.10%, or 0.05% or less. The contents of Zr, Co, Zn, W, and Sn may also be set to 0.50%, 0.10%, or 0.05% or less, respectively. Zr, Co, Zn, and W may also be absent, and therefore their contents may be 0%. To improve the strength of the steel sheet through solid solution strengthening, the contents of Zr, Co, Zn, and W may also be 0.01% or more, respectively.

[0145] Furthermore, the inventors of this invention have confirmed that even a small amount of Sn does not impair the effectiveness of the hot-rolled steel sheet of this embodiment. However, if a large amount of Sn is present, defects may occur during hot rolling; therefore, the Sn content is set to 0.05% or less. Sn may also be absent, so the Sn content may be 0%. To improve the corrosion resistance of the hot-rolled steel sheet, the Sn content may also be set to 0.01% or more.

[0146] The remaining portion of the chemical composition of the hot-rolled steel sheet in this embodiment may also contain Fe and impurities. In this embodiment, impurities are substances mixed in from the ore, waste, or manufacturing environment used as raw materials, and are substances that are permissible within a range that does not adversely affect the hot-rolled steel sheet of this embodiment.

[0147] The chemical composition of the hot-rolled steel sheet described above can be determined using general analytical methods. For example, ICP-AES (Inductively Coupled Plasma-Atomic Emission Spectrometry) can be used. It should be noted that sol.Al can be determined by ICP-AES using the filtrate obtained after heating and decomposing the sample with acid. C and S can be determined using the combustion-infrared absorption method, N can be determined using the inert gas melting-thermal conductivity method, and O can be determined using the inert gas melting-non-dispersive infrared absorption method.

[0148] When hot-rolled steel sheets have a coating on their surface, the coating can be removed by mechanical grinding or other means as needed, and then the chemical composition can be analyzed.

[0149] Metallographic structure of hot-rolled steel sheet

[0150] Next, the metal structure of the hot-rolled steel sheet of this embodiment will be described.

[0151] The microstructure of the hot-rolled steel sheet in this embodiment is as follows: the total amount of martensite and tempered martensite, expressed as an area percentage, is more than 92.0% and less than 100.0%, the amount of retained austenite is less than 3.0%, and the amount of ferrite is less than 5.0%. The entropy value obtained by analyzing the SEM image of the above microstructure using the gray-scale co-occurrence matrix method is greater than 11.0, the inverse normalized value expressed as an equation (2) is less than 1.020, and the cluster shadow value expressed as an equation (3) is -8.0 × 10⁻⁶. 5 ~8.0×10 5 The standard deviation of Mn concentration is less than 0.60% by mass.

[0152] Therefore, the hot-rolled steel sheet of this embodiment has high strength and a high reduction rate in sheet thickness at the ultimate breaking point, and also exhibits excellent hole-expanding and shearing workability. It should be noted that in this embodiment, the microstructure is defined at the center of the section parallel to the rolling direction, at a depth of 1 / 4 of the sheet thickness from the surface (the region from 1 / 8 to 3 / 8 of the sheet thickness from the surface) and in the width direction. This is because the microstructure at this location represents the representative microstructure of the steel sheet.

[0153] It should be noted that the "surface" referred to here, when hot-rolled steel sheets have a coating, refers to the interface between the coating and the steel sheet.

[0154] Area ratio of retained austenite: less than 3.0%

[0155] Retained austenite is a metallic microstructure that exists in a face-centered cubic lattice even at room temperature. Retained austenite improves the porosity of hot-rolled steel sheets through transformation plasticity (TRIP). On the other hand, since retained austenite transforms into high-carbon martensite during shearing, it hinders the formation of stable cracks, thus reducing the straightness of the fracture surface boundary with the sheared surface at the sheared end face. When the area fraction of retained austenite is 3.0% or more, the above effect becomes more pronounced, and the straightness of the fracture surface boundary with the sheared surface at the sheared end face decreases. Therefore, the area fraction of retained austenite is set to be less than 3.0%. The area fraction of retained austenite is preferably 1.5% or less, more preferably less than 1.0%. Less retained austenite is more preferred, so the area fraction of retained austenite can also be 0%. However, setting the area fraction of retained austenite to 0% is not easy; its lower limit can also be set to 0.5% or 1.0%.

[0156] Ferrite area fraction: less than 5.0%

[0157] Ferrite is generally a soft metallic structure. If the ferrite content exceeds a certain amount, the desired strength may not be obtained, and the shear surface area at the end face after shearing may increase. If the shear surface area at the end face after shearing increases, the straightness of the fracture surface and the boundary of the shear surface at the end face after shearing decreases, which is undesirable. When the ferrite area fraction is 5.0% or more, the above effects become more pronounced. Therefore, the ferrite area fraction is set to be less than 5.0%. The ferrite area fraction is preferably 3.0% or less, more preferably 2.0% or less, and even more preferably less than 1.0%. The less ferrite, the better; therefore, the ferrite area fraction can also be 0%. However, setting the ferrite area fraction to 0% is not easy, and its lower limit can also be set to 0.5%, 1.0%, or 1.5%.

[0158] Known methods for determining the area ratio of retained austenite include X-ray diffraction, EBSP (electron backscattering diffraction pattern) analysis, and magnetic measurement methods. In this embodiment, the area ratio of retained austenite is determined using X-ray diffraction, which is less affected by grinding (if grinding occurs, the retained austenite may transform into other phases such as martensite, thus making it impossible to determine the true area ratio), provides relatively simple and accurate measurement results, and is less affected by grinding.

[0159] In the determination of the retained austenite area ratio using X-ray diffraction in this embodiment, firstly, in a section of the hot-rolled steel sheet parallel to the rolling direction at a depth of 1 / 4 of the sheet thickness (a region from 1 / 8 to 3 / 8 of the sheet thickness from the surface) and at the center of the sheet width, Co-Kα rays are used to determine the integrated intensity of a total of six peaks: α(110), α(200), α(211), γ(111), γ(200), and γ(220). The volume fraction of retained austenite is then calculated using the intensity averaging method. The obtained volume fraction of retained austenite is considered as the area fraction of retained austenite.

[0160] The area fraction of ferrite was determined by the following method.

[0161] The plate section parallel to the rolling direction was precision machined to a mirror finish and then ground for 8 minutes at room temperature using colloidal silica without an alkaline solution to remove the strain introduced into the sample surface. Crystal orientation information was obtained by electron backscatter diffraction at arbitrary locations along the length of the sample section, in regions with a length of 50 μm and a depth from 1 / 8 to 3 / 8 of the plate thickness from the surface, at measurement intervals of 0.1 μm. For the measurements, an EBSD analysis apparatus consisting of a thermal field emission scanning electron microscope (JEOL JSM-7001F) and an EBSD detector (TSL DVC5 detector) was used. The vacuum level within the EBSD analysis apparatus was set to 9.6 × 10⁻⁶. -5 Below Pa, the accelerating voltage is set to 15 kV, the irradiation current level is set to 13, and the electron beam irradiation level is set to 62. The observation area is set to 40000 μm. 2 .

[0162] Next, a reflected electron image is captured in the same field of view. The reflected electron image is used to determine the ferrite and cementite grains that precipitate in layers, and the area ratio of the grains is calculated, thereby obtaining the area ratio of pearlite.

[0163] Subsequently, for grains identified as body-centered cubic (BCC) other than those identified as pearlite, the "Grain Average Misorientation" function within the "OIM Analysis" software (registered trademark) included in the EBSD analysis device was used to determine the region with a Grain Average Misorientation value of 1.0° or less as ferrite. At this point, the Grain Tolerance Angle was set to 15°, and the area ratio of the ferrite was obtained by calculating the area ratio of the regions identified as ferrite.

[0164] Next, when the maximum value of the "Grain Average IQ" of the ferrite region in the region other than the region identified as pearlite or ferrite is set to Iα, the region exceeding Iα / 2 will be extracted (determined) as bainite. The area ratio of bainite is obtained by calculating the area ratio of the region extracted (determined) as bainite.

[0165] The total area ratio of martensite and tempered martensite: exceeding 92.0% but less than 100.0%.

[0166] If the combined area ratio of martensite and tempered martensite is less than 92.0%, the desired strength cannot be obtained. Therefore, the combined area ratio of martensite and tempered martensite is set to be greater than 92.0%. Preferably, it is 93.0% or more, 95.0% or more, 97.0% or more, or 99.0% or more. The higher the combined area ratio of martensite and tempered martensite, the better; therefore, it can also be set to 100.0%.

[0167] The following explanation outlines the methods for determining the area ratio of martensite and tempered martensite.

[0168] First, to observe the same area as the EBSD measurement area where the ferrite area fraction was measured using SEM, Vickers indentations were made near the observation location. Then, the microstructure of the observation surface was preserved, surface contaminants were removed by grinding, and nitric acid ethanol etching was performed. Next, the same field of view as the EBSD observation surface was observed using SEM at 3000x magnification.

[0169] In EBSD measurements, regions identified as non-ferrite structures containing intragranular substructure and where cementite precipitates in multiple variants are classified as tempered martensite. Regions with high brightness and where no substructure appears after etching are classified as "martensite and retained austenite". The area ratios of tempered martensite and "martensite and retained austenite" are calculated. The area ratio of martensite is obtained by subtracting the area ratio of retained austenite obtained through X-ray diffraction from the area ratio of "martensite and retained austenite". The total area ratio of martensite and tempered martensite is obtained by summing the area ratios of martensite and tempered martensite.

[0170] It should be noted that for removing contaminants from the surface of the observation surface, polishing with alumina particles smaller than 0.1 μm or Ar ion sputtering is sufficient.

[0171] In the hot-rolled steel sheet of this embodiment, the residual microstructure may also include one or two types of bainite and pearlite, with a total area fraction of 0% or more and less than 8.0%. The upper limit of the area fraction of the residual microstructure may also be set to 6.0%, 5.0%, 4.0%, 3.0%, or 2.5%.

[0172] In this embodiment, since the area fraction of each tissue is measured using X-ray diffraction, EBSD analysis, and SEM observation, the sum of the measured area fractions of each tissue may not be 100.0%. If the sum of the area fractions of each tissue obtained by the above method is not 100.0%, the area fraction of each tissue is converted to a sum that is 100.0%. For example, if the sum of the area fractions of each tissue is 103.0%, the area fraction of each tissue is multiplied by "100.0 / 103.0" to obtain the area fraction of each tissue.

[0173] Entropy value: above 11.0; Normalized inverse value: below 1.020

[0174] To improve the straightness of the boundary between the fracture surface and the shear surface at the end face after shearing, it is important to reduce the periodicity and homogeneity of the metal microstructure. In this embodiment, the straightness of the boundary between the fracture surface and the shear surface at the end face after shearing is improved by controlling the entropy value (E value) representing the periodicity of the metal microstructure and the inverse normalized value (I value) representing the homogeneity of the metal microstructure.

[0175] The E value represents the periodicity of the metal structure. When the periodicity of the metal structure is high due to the formation of banded structures, etc., resulting in a periodic arrangement of brightness, the E value decreases. In this embodiment, since a metal structure with low periodicity is required, the E value needs to be increased. If the E value is below 11.0, the straightness of the boundary between the fracture surface and the sheared surface at the end face after shearing becomes easier to reduce. In a metal structure with high periodicity (i.e., a low E value), cracks are generated along multiple banded structures near the starting point, forming a fracture surface. Therefore, it is presumed that the straightness of the boundary between the fracture surface and the sheared surface at the end face after shearing becomes easier to reduce. Therefore, the E value is set to 11.0 or higher. Preferably, it is 11.1 or higher, more preferably 11.2 or higher. A higher E value is preferred; there is no specific upper limit, but it can also be set to 13.5 or lower, 13.0 or lower, 12.5 or lower, or 12.0 or lower.

[0176] The I value represents the uniformity of the metal structure, and increases with the width of the area with a certain brightness. A high I value indicates high uniformity of the metal structure. For the hot-rolled steel sheet of this embodiment, which has a metal structure with a combined area ratio of martensite and tempered martensite of 92.0% or less, it is necessary to produce a metal structure dominated by martensite with low brightness uniformity. Therefore, in this embodiment, it is necessary to reduce the I value. If the uniformity of the metal structure is high, i.e., the I value is high, cracks are more likely to occur at the tip of the shearing tool due to the influence of precipitates and elemental concentration differences within the grains, as well as hardness differences caused by the soft ferrite phase. As a result, the straightness of the fracture surface and the boundary of the sheared surface at the end face after shearing becomes more likely to decrease. That is, it is presumed that if the I value is 1.020 or higher, the straightness of the fracture surface and the boundary of the sheared surface at the end face after shearing cannot be improved. Therefore, the I value is set to be lower than 1.020. Preferably, it is 1.015 or lower, more preferably 1.010 or lower. There is no specific lower limit for the I value, but it can be set to above 0.900, above 0.950, or above 1.000.

[0177] Cluster shadow value: -8.0×10 5 ~8.0×10 5

[0178] Cluster shadow value (CS value) represents the strain of the metal structure. Regarding the CS value, it is a positive value if there are more points with brightness exceeding the average value in the image obtained from photographing the metal structure, and a negative value if there are more points with brightness below the average value.

[0179] In the two electron images obtained from a scanning electron microscope, the brightness increases in areas with greater surface unevenness and decreases in areas with less unevenness. The surface unevenness of the object is significantly affected by the grain size and intensity distribution within the metal structure. Regarding the CS value in this embodiment, it increases if the intensity non-uniformity of the metal structure is large or the structure units are small, and decreases if the intensity non-uniformity is small or the structure units are large.

[0180] In this embodiment, it is important to keep the CS value within the desired range, close to 0. If the CS value is below -8.0 × 10⁻⁶, it will cause problems. 5 This reduces the reduction rate of the ultimate fracture thickness of hot-rolled steel sheets. It is presumed that this is because large grains exist in the metal microstructure, and these grains preferentially fracture during ultimate deformation. Therefore, the CS value is set to -8.0 × 10⁻⁶. 5 The above. Preferably -7.5 × 10⁻⁶. 5 The above is further preferred to be -7.0×10 5 above.

[0181] On the other hand, if the CS value exceeds 8.0 × 10 5 The reduction rate of the ultimate fracture thickness of hot-rolled steel sheet decreases. This is presumably due to the large inhomogeneity of micro-strength in the metal microstructure, causing strain to concentrate locally during ultimate deformation, making it more prone to fracture. Therefore, the CS value is set to 8.0 × 10⁻⁶. 5 The following is preferred: 7.5 × 10⁻⁶ 5 The following is a further preferred value: 7.0 × 10 5 the following.

[0182] The E value, I value, and CS value can be obtained using the following methods.

[0183] In this embodiment, the SEM image (secondary electron image from a scanning electron microscope) taken to calculate the E, I, and CS values ​​is located at a depth of 1 / 4 of the plate thickness from the surface (a region from 1 / 8 to 3 / 8 of the plate thickness from the surface) in a cross-section parallel to the rolling direction, and at the center of the plate width. For SEM image capture, a Hitachi High-Technologies Corporation SU-6600 Schottky electron gun is used, with the emitter set to tungsten and the accelerating voltage set to 1.5 kV. Based on these settings, the SEM image is output at 1000x magnification with a grayscale scale of 256 gray levels.

[0184] Next, the SEM image obtained by cropping an 880×880 pixel region (the observation area is 160μm×160μm in actual size) was subjected to smoothing processing as described in Non-Patent Document 3, with a contrast enhancement limiting factor set to 2.0 and a tile grid size of 8×8. Except for 90 degrees, the smoothed SEM image was rotated counterclockwise in 1-degree increments from 0 to 179 degrees, and images were created for each 1-degree increment, resulting in a total of 179 images. Then, for each of these 179 images, the GLCM method described in Non-Patent Document 1 was used to collect the frequency values ​​of brightness between adjacent pixels in a row-column format.

[0185] For the 179 frequency values ​​collected using the above method, k is set as the rotation angle relative to the original image, expressed as p. k (k = 0…89, 91, …179). For each image, the generated p k After summing all the k (k = 0…89, 91…179), calculate the 256×256 row and column P that is normalized so that the sum of each component is 1. Then, use the following formulas (1) to (5) described in Non-Patent Document 2 to calculate the E value, I value and CS value respectively.

[0186] In equations (1) to (5) below, P(i,j) is the gray-level co-occurrence matrix, and the value of the i-th row and j-th column of row and column P is recorded as P(i,j). It should be noted that since 256×256 rows and columns P are used to calculate as described above, if you want to emphasize this point, the following equations (1) to (5) can be modified to the following equations (1') to (5').

[0187] Here, L in equation (2) below represents the number of grayscale levels (quantization levels of grayscale) achievable in the SEM image. In this embodiment, since the SEM image is output with a grayscale of 256 levels as described above, L is 256. i and j in equations (2) and (3) below are natural numbers from 1 to L, and μ in equation (3) below... x and μ y They are represented by the following formulas (4) and (5), respectively.

[0188] In the following equations (1') to (5'), the value of the i-th row and j-th column of row and column P is recorded as P. ij .

[0189] Entropy = -∑ i ∑ j P(i,j).log(P(i,j)) (1)

[0190]

[0191] Cluster shadow = ∑ i ∑ j (i+j-μ x -μ y ) 3 P(i,j) (3)

[0192] μ x =∑ i ∑ j i(P(i,j)) (4)

[0193] μ y =∑ i ∑ j j(P(i,j)) (5)

[0194]

[0195]

[0196]

[0197]

[0198]

[0199] Standard deviation of Mn concentration: below 0.60% by mass

[0200] In this embodiment, the standard deviation of the Mn concentration at a depth of 1 / 4 of the plate thickness from the surface (the region from 1 / 8 to 3 / 8 of the plate thickness from the surface) and at the center of the plate width direction in the hot-rolled steel sheet is 0.60% by mass or less. This allows for uniform dispersion of the hard phase and prevents a decrease in the straightness of the fracture surface and the sheared surface boundary at the end face after shearing. The standard deviation of the Mn concentration is preferably 0.55% by mass or less, or 0.50% by mass or less, and more preferably 0.47% by mass or less. From the viewpoint of suppressing excessive flywing, a smaller lower limit for the Mn concentration is preferred; however, due to manufacturing process constraints, a practically acceptable lower limit is 0.10% by mass. If necessary, this lower limit can also be set to 0.20% by mass or 0.28% by mass.

[0201] After mirror polishing a section of hot-rolled steel sheet parallel to the rolling direction, the Mn concentration at a depth of 1 / 4 of the sheet thickness (ranging from 1 / 8 to 3 / 8 of the sheet thickness) and at the center of the sheet width was measured using an electron probe microanalyzer (EPMA). The standard deviation of the Mn concentration was determined. The measurement conditions were: accelerating voltage set to 15 kV, magnification set to 5000x, and Mn concentration distribution images within a 20 μm range in both the rolling and thickness directions were measured. More specifically, the measurement interval was set to 0.1 μm, and the Mn concentration was measured at over 40,000 locations. The standard deviation of the Mn concentration was then calculated based on the Mn concentration obtained from all measurement points.

[0202] Average crystal grain size of the surface layer: less than 3.0 μm

[0203] By refining the grain size of the surface crystals, bending cracking in hot-rolled steel sheets can be suppressed. The higher the strength of the hot-rolled steel sheet, the easier it becomes to generate cracks from the inside of the bend during bending (hereinafter referred to as bending cracking). The mechanism of bending cracking is presumed as follows: During bending, compressive stress is generated on the inside of the bend. Initially, the processing progresses while deformation occurs uniformly throughout the inside of the bend, but if the amount of processing increases, uniform deformation alone becomes insufficient, and deformation progresses due to strain concentration in localized areas (generation of shear bands). These shear bands further grow, resulting in cracks that develop along the shear bands on the surface inside the bend. It is presumed that bending cracking becomes more prevalent with increasing strength because: the reduced work hardening capacity associated with increasing strength makes uniform deformation difficult, making it easier to generate uneven deformation, thus generating shear bands in the early stages of processing (or under lenient processing conditions).

[0204] Through their research, the inventors of this invention have discovered that bending cracking becomes significant in steel sheets with a tensile strength of 980 MPa or higher. Furthermore, the inventors have found that the finer the grain size of the surface layer of the hot-rolled steel sheet, the more localized strain concentration is suppressed, making bending cracking less likely. To achieve this effect, the average grain size of the surface layer of the hot-rolled steel sheet is preferably set to less than 3.0 μm. More preferably, it is 2.7 μm or less, or 2.5 μm or less. The lower limit of the average grain size of the surface region is not particularly specified, but it can also be set to 0.5 μm or 1.0 μm.

[0205] It should be noted that, in this embodiment, the term "surface layer" refers to the area from the surface of the hot-rolled steel sheet to a depth of 50 μm. As mentioned above, the term "surface" here, when the hot-rolled steel sheet has a coating, refers to the interface between the coating and the steel sheet.

[0206] The surface crystal grain size was determined using EBSP-OIM (Electron Backscatter Diffraction Pattern-Orientation Image Microscopy). EBSP-OIM is performed using a combination of a scanning electron microscope and an EBSP analysis device, specifically an OIM Analysis instrument manufactured by AMETEK (a registered trademark). The analyzable area for EBSP-OIM is the area observable by SEM. While the resolution varies depending on the SEM, EBSP-OIM can achieve a minimum resolution of 20 nm.

[0207] In a section of hot-rolled steel sheet parallel to the rolling direction, at a depth of 50 μm from the surface and at the center of the sheet width, analytical measurements were performed at 1200x magnification over a 40 μm × 30 μm area in at least five fields of view. Grain boundaries were defined as locations where the angular difference between adjacent measurement points was greater than 5°, and the area-average grain size was calculated. This area-average grain size was then used as the average grain size of the surface layer.

[0208] It should be noted that retained austenite is not a structure formed through phase transformation below 600℃ and does not have the effect of dislocation accumulation. Therefore, in this measurement method (method for determining the average grain size of the surface layer), retained austenite is not considered as an object of analysis. In cases where the area fraction of retained austenite is 0%, it is not necessary to exclude it from the analysis. However, in cases where it may affect the determination of the average grain size of the surface layer, retained austenite with an fcc crystal structure is excluded from the analysis in the EBSP-OIM method.

[0209] Tensile properties

[0210] The tensile strength (TS) of the hot-rolled steel sheet in this embodiment is 980 MPa or higher. If the tensile strength is lower than 980 MPa, the applicable parts are limited, and the contribution to vehicle body lightweighting is small. There is no need to specifically limit the upper limit, but from the viewpoint of suppressing mold wear, it can also be set to 1780 MPa.

[0211] Tensile strength was measured using test piece No. 5 according to JIS Z 2241:2011. The tensile test piece was collected at a distance of 1 / 4 of the distance from the end in the width direction of the plate, with the direction perpendicular to the rolling direction taken as the length direction.

[0212] Hole enlargement characteristics

[0213] In this embodiment, the expansion ratio (λ) of the hot-rolled steel sheet is preferably 55% or more. If the expansion ratio (λ) is 55% or more, the applicable components are not limited, and a hot-rolled steel sheet that significantly contributes to vehicle body lightweighting can be obtained. There is no need to specifically limit the upper limit. While it is not necessary to specify an upper limit for the expansion ratio λ, it can be set to 85% or 80%.

[0214] The hole expansion ratio (λ) was determined using test piece No. 5 according to JIS Z 2241:2011 and in accordance with JIS Z 2256:2010. The sampling position of the hole expansion test piece was set to 1 / 4 of the distance from the end of the hot-rolled steel plate in the width direction.

[0215] Plate thickness

[0216] The thickness of the hot-rolled steel sheet in this embodiment is not particularly limited, but can be set to 0.5 to 8.0 mm. By setting the thickness of the hot-rolled steel sheet to 0.5 mm or more, it becomes easier to ensure the rolling completion temperature, while simultaneously reducing the rolling load and facilitating hot rolling. Therefore, the thickness of the hot-rolled steel sheet in this embodiment can also be set to 0.5 mm or more. Preferably, it is 1.2 mm or more, 1.4 mm or more, or 1.8 mm or more. Furthermore, by setting the thickness to 8.0 mm or less, it becomes easier to refine the metal structure, and the aforementioned metal structure can be easily ensured. Therefore, the thickness can also be set to 8.0 mm or less. Preferably, it is 6.0 mm or less, 5.0 mm or less, or 4.0 mm or less.

[0217] Coating

[0218] The hot-rolled steel sheet of this embodiment, having the aforementioned chemical composition and metallic structure, can also be surface-treated by applying a coating to its surface for the purpose of improving corrosion resistance, etc. The coating can be an electroplated layer or a hot-dip galvanized layer. Examples of electroplated layers include electroplated zinc layers and electroplated Zn-Ni alloy layers. Examples of hot-dip galvanized layers include hot-dip galvanized layers, alloyed hot-dip galvanized layers, hot-dip aluminized layers, hot-dip Zn-Al alloy layers, hot-dip Zn-Al-Mg alloy layers, and hot-dip Zn-Al-Mg-Si alloy layers. The coating amount is not particularly limited and can be set as before. Furthermore, by performing appropriate chemical conversion treatment after coating (e.g., coating and drying with a silicate-based chromium-free chemical conversion solution), corrosion resistance can be further improved.

[0219] Manufacturing conditions

[0220] A preferred manufacturing method for the hot-rolled steel sheet of this embodiment having the above-described chemical composition and metallic structure is described below.

[0221] In the preferred manufacturing method of the hot-rolled steel sheet of this embodiment, the following steps (1) to (9) are performed sequentially. It should be noted that the temperature of the slab and the temperature of the steel sheet in this embodiment refer to the surface temperature of the slab and the surface temperature of the steel sheet. In addition, stress refers to the tension applied along the rolling direction of the steel sheet.

[0222] (1) After holding the slab in a temperature range of 700 to 850°C for more than 900 seconds, further heat it and hold it in a temperature range of 1100°C or higher for more than 6000 seconds.

[0223] (2) Hot rolling is carried out in a temperature range of 850 to 1100°C, such that the total thickness is reduced by more than 90%.

[0224] (3) The steel plate is subjected to a stress of more than 170 kPa after the rolling of the first section before the final section of hot rolling and before the final section.

[0225] (4) Set the reduction rate in the final section of hot rolling to 8% or more, and complete the hot rolling in a manner that makes the rolling completion temperature Tf above 900°C and below 960°C.

[0226] (5) The stress applied to the steel plate after the final stage of hot rolling and before the steel plate is cooled to 800°C is set to be less than 200 kPa.

[0227] (6) After cooling to a temperature range below the hot-rolling completion temperature Tf-50°C within 1 second after hot rolling, accelerated cooling is performed with an average cooling rate of 50°C / second or more up to 600°C. Among these, cooling to a temperature range below the hot-rolling completion temperature Tf-50°C within 1 second after hot rolling is a more preferred cooling condition.

[0228] (7) Cooling is carried out in a manner where the average cooling rate in the temperature range of 450 to 600°C is more than 30°C / second and less than 50°C / second.

[0229] (8) Cooling is performed in such a way that the average cooling rate in the temperature range of winding temperature ~450℃ is 50℃ / second or higher.

[0230] (9) Winding is performed in a temperature range below 350°C.

[0231] By employing the above manufacturing method, hot-rolled steel sheets with high strength, high reduction rate at ultimate fracture, and excellent hole-expanding and shearing workability can be stably manufactured. Specifically, by appropriately controlling the slab heating and hot-rolling conditions, the reduction of Mn segregation and the equiaxing of austenite before phase transformation can be achieved. This, combined with the cooling conditions after hot rolling (described later), enables the stable manufacture of hot-rolled steel sheets with the desired microstructure.

[0232] (1) Slab, slab temperature and holding time during hot rolling

[0233] The slabs supplied for hot rolling can be slabs obtained by continuous casting, slabs obtained by casting and billet cutting, etc. In addition, slabs obtained by hot working or cold working as needed can be used.

[0234] Preferably, the hot-rolled slab is heated to a temperature range of 700°C to 850°C for at least 900 seconds, and then further heated to a temperature range of 1100°C or higher for at least 6000 seconds. Furthermore, when holding the slab in the 700°C to 850°C temperature range, the slab temperature can be varied within that range or set to a constant temperature. Similarly, when holding the slab in the 1100°C or higher temperature range, the slab temperature can be varied within that range or set to a constant temperature.

[0235] During the austenitic phase transformation at 700℃ to 850℃, Mn is distributed between ferrite and austenite. By extending the transformation time, Mn can diffuse within the ferrite region. This eliminates the unevenly distributed Mn microsegregation in the slab and significantly reduces the standard deviation of Mn concentration. Reducing the standard deviation of Mn concentration improves the straightness of the fracture surface and the sheared surface at the sheared end face. Furthermore, the I value can be set to the desired value.

[0236] Furthermore, in order to reduce the standard deviation of Mn concentration and set the I value to the desired value, the holding time in the temperature range above 1100°C is preferably set to 6000 seconds or more. In order to obtain the desired amount of martensite and tempered martensite, the temperature for holding for 6000 seconds or more is preferably set to 1100°C or more.

[0237] Hot rolling is preferably performed as a multi-pass rolling process using a reversible mill or a tandem mill. In particular, from the viewpoint of industrial productivity, it is more preferable that at least the final few stages are set to hot rolling using a tandem mill.

[0238] (2) Hot rolling reduction rate: a reduction of more than 90% in plate thickness in the temperature range of 850 to 1100℃.

[0239] By performing hot rolling at a temperature range of 850–1100°C to achieve a total thickness reduction of 90% or more, the refinement of recrystallized austenite grains can be primarily achieved. Furthermore, the accumulation of strain energy within the non-recrystallized austenite grains can be promoted, thus facilitating austenite recrystallization and Mn atomic diffusion, thereby reducing the standard deviation of Mn concentration. Additionally, the I value can be set to a desired value. Therefore, hot rolling at a temperature range of 850–1100°C to achieve a total thickness reduction of 90% or more is preferable.

[0240] It should be noted that the reduction in plate thickness in the so-called temperature range of 850 to 1100°C can be expressed as (t0-t1) / t0×100 (%) when the inlet plate thickness before the first pass in the rolling process of this temperature range is set as t0 and the outlet plate thickness after the last pass in the rolling process of this temperature range is set as t1.

[0241] (3) Stress above 170 kPa after the rolling of the section preceding the final section and before the final section of hot rolling.

[0242] Preferably, the stress applied to the hot-rolled steel sheet after the rolling of the section preceding the final stage and before the final stage is set to 170 kPa or more. This reduces the {110} precipitate in the recrystallized austenite after the rolling of the section preceding the final stage. <001> The number of grains with the desired crystal orientation. {110} <001> Because of its difficult-to-recrystallize crystal orientation, suppressing its formation can effectively promote recrystallization through the final stage of pressing. As a result, the banded structure of the hot-rolled steel sheet is improved, the periodicity of the metal structure is reduced, and the E value increases. When the stress applied to the steel sheet is below 170 kPa, it may be impossible to set the desired E value. A stress applied to the steel sheet is more preferably 190 kPa or higher. The stress applied to the steel sheet can be controlled by adjusting the roll rotation speed in tandem rolling, and can be calculated by dividing the load measured in the rolling direction in the rolling stand by the cross-sectional area of ​​the steel sheet being rolled.

[0243] (4) Reduction rate in the final section of hot rolling: 8% or more; hot rolling completion temperature Tf: 900℃ or more and below 960℃

[0244] Preferably, the reduction rate in the final stage of hot rolling is set to 8% or more, and the hot rolling completion temperature Tf is set to 900°C or more. By setting the reduction rate in the final stage of hot rolling to 8% or more, recrystallization induced by the reduction in the final stage can be promoted. As a result, the banded structure of the hot-rolled steel sheet is improved, the periodicity of the metal structure is reduced, and the E value increases. By setting the hot rolling completion temperature Tf to 900°C or more, the excessive increase in the number of ferrite nucleation sites in austenite can be suppressed. As a result, the formation of ferrite in the final structure (the metal structure of the manufactured hot-rolled steel sheet) can be suppressed, resulting in a high-strength hot-rolled steel sheet. Furthermore, by setting the hot rolling completion temperature Tf below 960°C, the coarsening of austenite grain size can be suppressed, the periodicity of the metal structure can be reduced, and the E value can be set to the desired value.

[0245] (5) Stress after the final stage of hot rolling and before the steel plate is cooled to 800°C: less than 200 kPa

[0246] The stress applied to the steel sheet after the final stage of hot rolling and before it is cooled to 800°C is preferably set to less than 200 kPa. By setting the stress (tension) applied along the rolling direction of the steel sheet to less than 200 kPa, the recrystallization of austenite proceeds preferentially along the rolling direction, thus suppressing the periodic increase of the metal structure. As a result, the E value can be set to the desired value. The stress applied to the steel sheet is more preferably less than 180 MPa. It should be noted that the stress applied along the rolling direction of the steel sheet can be controlled by adjusting the rotational speed of the rolling mill stand and the coiling device, and can be obtained by dividing the measured load in the rolling direction by the cross-sectional area of ​​the steel sheet being rolled.

[0247] (6) Accelerated cooling is carried out in such a manner that the average cooling rate from the completion of hot rolling to 600°C is 50°C / second or higher.

[0248] By accelerating cooling at an average cooling rate of 50°C / second or higher from the completion of hot rolling to 600°C, ferrite, bainite, and / or pearlite phase transformations within the steel sheet can be suppressed, resulting in the desired strength. Furthermore, the I value can be set to the desired value. If air cooling or similar methods are performed during accelerated cooling to 600°C after hot rolling, the ferrite content increases, and the I value cannot be set to the desired value; therefore, this is not preferable.

[0249] There is no specific upper limit for the average cooling rate, but increasing the cooling rate leads to larger-scale cooling equipment and higher costs. Therefore, considering equipment costs, an average cooling rate of 300°C / second or less is preferable for accelerated cooling.

[0250] It should be noted that the average cooling rate referred to here is the following value: the temperature drop of the steel plate from the start of accelerated cooling (when the steel plate is introduced into the cooling equipment) to 600°C divided by the time required from the start of accelerated cooling to when the steel plate temperature reaches 600°C.

[0251] In the cooling process after hot rolling, it is more preferable to cool to a temperature range of Tf-50°C within one second after hot rolling. That is, it is more preferable to set the cooling rate of 50°C or more within one second after hot rolling. This is because it can suppress the growth of austenite grains that have become finer during hot rolling. In order to cool to a temperature range below Tf-50°C within one second after hot rolling, it is sufficient to perform cooling with a high average cooling rate immediately after hot rolling, such as spraying cooling water onto the surface of the steel plate. By cooling to a temperature range below Tf-50°C within one second after hot rolling, the grain size of the surface layer can be refined, thereby improving the resistance of the hot-rolled steel plate to internal bending cracking.

[0252] After the hot rolling is completed, the temperature is cooled to the hot rolling completion temperature Tf-50℃ within 1 second. Then, the cooling is accelerated as described above, with an average cooling rate of 50℃ / second or more up to 600℃.

[0253] (7) Cooling is carried out in a manner where the average cooling rate in the temperature range of 450-600℃ is more than 30℃ / second but less than 50℃ / second.

[0254] After the aforementioned accelerated cooling is completed, it is preferable to perform cooling at an average cooling rate of 30°C / second or higher but less than 50°C / second within the temperature range of 450–600°C. By setting the average cooling rate within this temperature range to 30°C / second or higher but less than 50°C / second, the CS value can be set to the desired value. When the average cooling rate exceeds 50°C / second, coarse grains are easily formed in the metal structure, and the CS value becomes lower than -8.0 × 10⁻⁶. 5 When the average cooling rate is below 30°C / s, the strength of the hard tissue increases, widening the strength difference with the soft tissue, thus the CS value exceeds 8.0 × 10⁻⁶. 5 .

[0255] It should be noted that the so-called average cooling rate here refers to the following value: the temperature drop of the steel plate from the cooling stop temperature of accelerated cooling with an average cooling rate of 50℃ / second or higher to the cooling stop temperature of cooling with an average cooling rate of 30℃ / second or higher but lower than 50℃ / second, divided by the time required from the end of accelerated cooling with an average cooling rate of 50℃ / second or higher to the end of cooling with an average cooling rate of 30℃ / second or higher but lower than 50℃ / second.

[0256] (8) Average cooling rate in the temperature range of winding temperature ~450℃: 50℃ / second or higher

[0257] To suppress the area ratio of pearlite and obtain the desired strength, it is preferable to set the average cooling rate in the temperature range of winding temperature to 450°C to 50°C / second or higher. This allows the parent phase structure to become harder.

[0258] It should be noted that the so-called average cooling rate here refers to the following value: the temperature drop of the steel plate from the cooling stop temperature at an average cooling rate of 30℃ / second or higher but lower than 50℃ / second to the coiling temperature, divided by the time required from the end of cooling at an average cooling rate of 30℃ / second or higher but lower than 50℃ / second to the coiling.

[0259] (9) Winding temperature: below 350℃

[0260] The winding temperature is preferably set to 350°C or below. By setting the winding temperature to 350°C or below, the driving force for the phase transformation from austenite to bcc can be increased, and the deformation strength of austenite can also be increased. Therefore, the hard phase is more uniformly distributed during the martensitic transformation from austenite, reducing inhomogeneity. As a result, the I value can be reduced, and the straightness of the fracture surface and the boundary of the sheared surface at the end face after shearing can be improved. Therefore, the winding temperature is preferably set to 350°C or below.

[0261] Example

[0262] Next, the effects of one aspect of the present invention will be described in more detail through embodiments. However, the conditions in the embodiments are examples adopted to confirm the feasibility and effects of the present invention, and the present invention is not limited to these one example. Various conditions can be adopted by the present invention as long as they do not depart from the spirit of the present invention and achieve the purpose of the present invention.

[0263] Steels with the chemical compositions shown in Tables 1 and 2 are smelted and continuously cast to produce slabs with a thickness of 240–300 mm. Using the obtained slabs, hot-rolled steel sheets as shown in Tables 5 and 6 are obtained under the manufacturing conditions shown in Tables 3 and 4.

[0264] It should be noted that Manufacturing No. 11 was air-cooled for 5.0 seconds after being cooled to 791°C from hot rolling. The average cooling rate during air cooling was less than 5.0°C / second.

[0265] For the obtained hot-rolled steel sheet, the area ratio, E value, I value, CS value, standard deviation of Mn concentration, average grain size of the surface layer, tensile strength (TS), and porosity (λ) of the metal structure were obtained using the above method. The measured results are shown in Tables 5 and 6.

[0266] It should be noted that the remaining tissue consists of one or two types of bainite and pearlite.

[0267] Evaluation methods for the properties of hot-rolled steel sheets

[0268] tensile strength

[0269] If the tensile strength (TS) is 980 MPa or higher, it is deemed to have high strength and is therefore acceptable. On the other hand, if the tensile strength (TS) is lower than 980 MPa, it is deemed to lack high strength and is therefore unacceptable.

[0270] Pore ​​expansion rate

[0271] When the porosity (λ) is 55% or higher, it is considered excellent and deemed acceptable. On the other hand, when the porosity is less than 55%, it is considered poor and deemed unacceptable.

[0272] Limiting reduction rate of plate thickness at fracture

[0273] The ultimate reduction rate of thickness at break of hot-rolled steel sheet is evaluated by tensile testing.

[0274] Tensile tests were conducted using the same method as when evaluating tensile properties. With the plate thickness before the tensile test set as t1 and the minimum plate thickness at the center of the fractured tensile specimen in the width direction set as t2, the ultimate reduction rate of plate thickness at fracture was obtained by calculating (t1-t2)×100 / t1. Five tensile tests were performed, and the ultimate reduction rate of plate thickness at fracture was obtained by calculating the average of the three tests excluding the maximum and minimum values.

[0275] If the reduction rate of plate thickness at the ultimate fracture is 75.0% or higher, the hot-rolled steel sheet is deemed acceptable as having a high reduction rate of plate thickness at the ultimate fracture. On the other hand, if the reduction rate of plate thickness at the ultimate fracture is less than 75.0%, the hot-rolled steel sheet is deemed unacceptable as not having a high reduction rate of plate thickness at the ultimate fracture.

[0276] Shear workability (evaluation of the straightness of the boundary between the fracture surface and the shear surface)

[0277] The straightness of the boundary between the fracture surface and the shear surface in the shear workability of hot-rolled steel sheets is evaluated by conducting punching tests to determine the straightness at the boundary between the fracture surface and the shear surface.

[0278] Five punching holes were made at the center of the width of a hot-rolled steel sheet, with a diameter of 10 mm, a clearance of 15%, and a punching speed of 3 m / s. Next, for each of the five punching holes, ten end faces (two locations parallel to the rolling direction) were photographed using an optical microscope. The resulting photographs allow observation of... Figure 1 An end face as shown in (a). Figure 1 As shown in (a) and (b), burrs, shear surfaces, fracture surfaces, and fins are observed at the punched end face. It should be noted that... Figure 1 (a) is a schematic view of the end face of the punch hole parallel to the rolling direction. Figure 1 (b) is a schematic view of the side of the punched hole. The so-called collapsed edge is an R-shaped smooth surface, the so-called shear surface is the punched end face separated by shear deformation, the so-called fracture surface is the punched end face separated by cracks generated near the cutting edge after shear deformation, and the so-called wing is a surface with a protrusion extending from the lower surface of the hot-rolled steel plate.

[0279] In the observation photographs of 10 end faces obtained from 5 end faces, the straightness at the boundary between the fracture surface and the shear is determined by the method described later, and the maximum value of the obtained straightness is calculated.

[0280] It should be noted that the straightness at the boundary between the fracture surface and the shear is obtained by the following method.

[0281] like Figure 1 As shown in (b), the point that determines the boundary between the shear plane and the fracture plane relative to the end face ( Figure 1 (b) Points A and B). Measure the distance x connecting these points A and B with a straight line. Next, measure the length y of the curve along the boundary between the fracture surface and the shear surface. Divide the obtained y by x, and use the resulting value as the straightness at the boundary between the fracture surface and the shear surface.

[0282] If the maximum straightness value obtained in the punching test is less than 1.045, the hot-rolled steel sheet is deemed qualified as having excellent shear workability.

[0283] On the other hand, if the maximum straightness value obtained is 1.045 or higher, the hot-rolled steel sheet is judged as unqualified because it does not have excellent shearing workability.

[0284] Flexural internal cracking resistance

[0285] The bending test piece is a strip of 100mm × 30mm cut from the 1 / 2 position of the width of the hot-rolled steel plate. The following bending test is used to evaluate the resistance to internal cracking during bending.

[0286] For both bending (L-axis bending) where the bending edge is parallel to the rolling direction (L-direction) and bending (C-axis bending) where the bending edge is parallel to the direction perpendicular to the rolling direction (C-direction), tests were conducted using the V-block method (bending angle θ = 90°) according to JIS Z 2248:2006. The minimum bending radius without cracking was then determined. Resistance to internal bending cracking was investigated. The average of the minimum bending radii along the L-axis and C-axis, divided by the plate thickness, was set as the ultimate bending radius R / t, serving as an indicator of resistance to internal bending cracking. Hot-rolled steel plates with R / t below 3.0 were considered to have excellent resistance to internal bending cracking.

[0287] Regarding the presence or absence of cracks, the cross-section obtained by cutting the test piece after the test with a surface parallel to the bending direction and perpendicular to the plate surface is mirror-polished, and the cracks are observed with an optical microscope. If the length of the crack observed on the inner side of the bending of the test piece exceeds 30μm, it is judged to have cracks.

[0288] [Table 1]

[0289]

[0290] [Table 2]

[0291]

[0292] [Table 3]

[0293]

[0294] An underline indicates that the manufacturing conditions are not optimal.

[0295] [Table 4]

[0296]

[0297] An underline indicates that the manufacturing conditions are not optimal.

[0298] [Table 5]

[0299]

[0300] Underlined features indicate features that are outside the scope of the invention or are not preferred.

[0301] [Table 6]

[0302]

[0303] Underlined features indicate features that are outside the scope of the invention or are not preferred.

[0304] As can be seen from Tables 5 and 6, the hot-rolled steel sheet of this invention exhibits high strength and a high reduction rate in thickness at the ultimate fracture point, as well as excellent hole-expanding and shearing workability. Furthermore, it can be seen that the hot-rolled steel sheet of this invention, with an average surface grain size of less than 3.0 μm, possesses not only the aforementioned characteristics but also excellent resistance to internal bending cracking.

[0305] On the other hand, it can be seen that the hot-rolled steel sheet of the comparative example has deteriorated in one or more of the following: strength, reduction rate of ultimate fracture thickness, hole expansion capability, and shearing processability.

[0306] Industrial availability

[0307] According to the above-described embodiments of the present invention, hot-rolled steel sheets with high strength and ultimate fracture thickness reduction rate, as well as excellent hole-expanding and shearing workability, can be provided. Furthermore, according to the preferred embodiments of the present invention, hot-rolled steel sheets that, in addition to possessing the aforementioned characteristics, also suppress the generation of bending internal cracks, i.e., exhibit excellent resistance to bending internal cracks, can be obtained.

[0308] The hot-rolled steel sheet of the present invention is suitable as an industrial raw material for use in automotive components, mechanical structural components, and even building components.

Claims

1. A hot-rolled steel plate, characterized in that, The chemical composition, expressed as a percentage by mass, is as follows: C：0.040～0.250%、 Si: 0.05~3.00% Mn: 1.00~4.00% sol.Al: 0.001~0.500%, P: Below 0.100% S: Below 0.0300% N: below 0.1000% O: Below 0.0100% Ti: 0~0.300%, Nb: 0~0.100%, V：0～0.500%、 Cu: 0~2.00%, Cr：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: 0~0.100% Zr：0～1.00%、 Co: 0~1.00%, Zn: 0~1.00%, W：0～1.00%、 Sn: 0–0.05%, and Remaining components: Fe and impurities. It satisfies the following equation (A). The metal structure is as follows: (in area %) The total amount of martensite and tempered martensite exceeds 92.0% but is below 100.0%; Residual austenite is less than 3.0%; Ferrite content is less than 5.0%; The entropy value obtained by analyzing the SEM image of the metal structure using the gray-level co-occurrence matrix method is greater than 11.0, as expressed by the following equation (1). The normalized value of the inverse, as expressed by the following equation (2), is less than 1.020; The cluster shadow value represented by the following equation (3) is -8.0 × 10 5 ~8.0×10 5 , The standard deviation of Mn concentration is below 0.60% by mass. The hot-rolled steel plate has a tensile strength of 980 MPa or higher. Zr + Co + Zn + W ≤ 1.00% (A) In formula (A), each element symbol represents the content of that element in terms of mass percentage. If the element is not present, 0% is used as the input. In equations (1) to (5), P(i,j) is the gray-level co-occurrence matrix; in equation (2), L is the number of gray-level scale levels obtainable in the SEM image; in equations (2) and (3), i and j are natural numbers from 1 to L; and in equation (3), μ... x and μ y Represented by the following formulas (4) and (5), respectively, ， The standard deviation of the Mn concentration was obtained as follows: Electron probe microanalyzer was used to measure the Mn concentration at a depth of 1 / 4 of the plate thickness and at the center of the plate width direction from the surface of the hot-rolled steel plate. The measurement conditions were: accelerating voltage set to 15 kV, magnification set to 5000x, and distribution images within a range of 20 μm in the rolling direction and 20 μm in the plate thickness direction were measured. The measurement interval was set to 0.1 μm, and the Mn concentration was measured at more than 40,000 locations. The standard deviation was then calculated based on the Mn concentration obtained from all measurement points.

2. The hot-rolled steel plate according to claim 1, characterized in that, The average crystal grain size of the surface layer is less than 3.0 μm.

3. The hot-rolled steel plate according to claim 1 or 2, characterized in that, The chemical composition, expressed as a percentage by mass, contains one or more elements selected from the following: Ti: 0.001~0.300% Nb: 0.001~0.100% V：0.001～0.500%、 Cu: 0.01~2.00%, Cr：0.01～2.00%、 Mo: 0.01~1.00%, Ni: 0.01~2.00% B：0.0001～0.0100%、 Ca: 0.0001~0.0200% Mg: 0.0001~0.0200%, REM: 0.0001~0.1000% Bi: 0.0001~0.0200% As: 0.001~0.100% Zr：0.01～1.00%、 Co: 0.01~1.00%, Zn: 0.01~1.00%, W: 0.01~1.00%, and Sn: 0.01~0.05%.

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