Steel plate and its manufacturing method

Abstract

The present invention provides a steel sheet having a large maximum bending angle. The steel sheet has a predetermined chemical composition and metallographic structure, and in a cross section perpendicular to the thickness direction, the Mn concentration profile within a range of 50 μm from the surface in the thickness direction is determined by a coordinate system in which the direction perpendicular to the thickness direction is the y-axis, the direction in which the anisotropy of Mn segregation is strongest, where the Mn concentration profile function in the y direction is defined as f(y), and a new function f'(y) is defined as f'(y) = f(y) - [Mn] (where [Mn] is the Mn content in the base metal expressed in mass%), and further, f'(y) is subjected to a two-dimensional discrete Fourier transform to obtain the wave number k (unit :mm -1 ) is expressed as a function F(k), and the real part of F(k) at wave number k is a(k) and the imaginary part is b(k), When JPEG0007776799000017.jpg18169 is defined, what is the k that gives the maximum value among the wave number k and c(k) determined by the wave number k? For c(k), k>1.5, (maximum value of c(k))<-0.0044[Mn] 2 +0.06[Mn]+0.012 is satisfied.

Description

[Technical Field]

[0001] The present invention relates to a steel sheet. [Background technology]

[0002] Steel sheets used in automotive frame components such as side members must have enough strength to suppress bending deformation even when subjected to energy during a collision, in order to protect the lives of those inside the vehicle. At the same time, the need to reduce the weight of automotive components is driving the trend toward thinner steel sheets with even higher strength.

[0003] Patent Document 1 discloses a high-strength cold-rolled steel sheet with excellent formability. This high-strength cold-rolled steel sheet has an average Mn concentration in a surface layer portion from the steel sheet surface to a depth of 5 μm within a predetermined range, a tensile strength of 980 MPa or more, a yield strength of 690 MPa to 850 MPa, a total elongation of 12% or more, and a value obtained by dividing the minimum inner radius (mm) at which cracks do not occur by the sheet thickness (mm) in a bending test using the V-block method at a bending angle of 90° of 1.0 or less.

[0004] Patent Document 2 discloses a steel sheet in which the local concentration distribution of Mn is controlled. This steel sheet aims to improve bake hardenability, and the ratio C1 / C2 of the upper limit C1 of the Mn content to the lower limit C2 of the Mn content in mass% in the thickness direction cross section is 1.5 or less.

[0005] Patent Document 3 discloses a steel sheet that has excellent press formability and also has excellent toughness even after press forming and baking paint. As a technique for obtaining this steel sheet, the technique discloses a technique in which the microstructure before final annealing is mainly composed of lath-shaped bainite and martensite, thereby promoting the diffusion of Mn via dislocations during annealing heating and homogenizing the Mn concentration in austenite. [Prior art documents] [Patent documents]

[0006] [Patent Document 1] Japanese Patent Application Laid-Open No. 2014-051683 [Patent Document 2] International Publication No. 2019 / 093429 [Patent Document 3] International Publication No. 2022 / 080497 Summary of the Invention [Problem to be solved by the invention]

[0007] As the strength of steel increases, the risk of brittle fracture also increases. In particular, Mn, which is added in high concentrations to control the microstructure of automotive steel sheets to increase their strength, is known to embrittle the material.

[0008] The present invention focuses on Mn segregation and aims to provide a steel sheet having improved bending load and absorbed energy compared to conventional steel sheets. [Means for solving the problem]

[0009] The inventors discovered that simply increasing the strength of the base material results in the formation of regions with locally high Mn concentrations, which embrittle the material, reduce resistance to the propagation of microscopic initial cracks, and promote crack growth, thereby reducing the absorbed energy until the plate finally breaks. In particular, the decrease in absorbed energy was more pronounced when the Mn concentration distribution deviation was large in the surface layer, where the amount of strain applied during bending deformation is relatively large compared to the interior. Based on the above findings, the inventors discovered that large deformation during a collision can be suppressed by not only increasing the strength of the steel plate but also suppressing the deviation in the Mn concentration distribution in the surface layer, and investigated a method for suppressing the deviation in the Mn concentration distribution in the surface layer.

[0010] If Mn concentration deviations occur due to segregation during casting, areas with high Mn concentrations will become martensite and areas with low Mn concentrations will become ferrite during annealing, resulting in localized hardness differences and susceptibility to crack initiation during bending. Furthermore, while hard phases have low toughness, high Mn concentrations also embrittle grain boundaries, further promoting the propagation of microcracks. In particular, Mn concentration deviations in the in-plane direction in the surface region of steel sheets have a significant impact on bending properties, due in part to the concentration of strain in the surface region during bending deformation. Specifically, the greater the Mn concentration deviation, the more rapidly cracks open and propagate within the embrittled structure after reaching maximum load during bending. This propagates to the surrounding structure, resulting in a rapid load drop and brittle fracture. Therefore, in the manufacturing of martensite-based steel sheets, minimizing Mn concentration deviations in the in-plane direction in the surface region is important for improving bendability.

[0011] The present inventors have conducted extensive research into methods for reducing the deviation in Mn concentration in the surface layer, and have found that the Mn concentration distribution in the surface layer can be controlled to a form favorable for bending properties by controlling the temperature of the slab when it is inserted into a heating furnace for hot rolling after casting, the finishing temperature in hot rolling, and the time from the end of rolling to the start of cooling.

[0012] The present invention was made based on the above findings and through further investigation, and the gist of the present invention is as follows.

[0013] [1] The composition, in mass%, is: C: 0.14-0.35%, Si: 0.01-2.00%, Mn: 1.0-4.0%, Al: 0-0.30%, P: 0.100% or less, S: 0.0500% or less, N: 0.0100% or less, O: 0.0060% or less, Cu: 0-1.00%, Ni: 0-1.00%, Mo: 0-1.000%, Cr: 0-2.00%, Co: 0-0.50%, Ti: 0- 0.300%, Nb: 0-0.30%, V: 0-0.50%, Ta: 0-0.100%, W: 0-1.00%, Ca: 0-0.0400%, Mg: 0-0.040%, REM: 0-0.0400%, Zr: 0-0.050%, B: 0-0.0100%, Sn: 0-0.05%, Sb: 0-0.05%, and As: 0-0.050%, with the balance being Fe and impurities, The metal structure at a depth of 1 / 4 of the plate thickness of the steel plate has, in area ratios, a total of ferrite and bainite: 0 to 40%, a total of fresh martensite and tempered martensite: 50 to 100%, and a total of pearlite and retained austenite: 0 to 10%, In a cross section perpendicular to the thickness direction at a position 50 μm from the surface of the steel plate in the thickness direction, the direction in which the anisotropy of Mn segregation is strongest is the y-axis, The unit of y is mm, and the profile function of the Mn concentration (unit: mass%) in the y direction is set as f(y), and a new function f'(y) is defined as f'(y)=f(y)-[Mn] ([Mn] is the Mn content of the base material expressed in mass%.) and Furthermore, f'(y) is subjected to a two-dimensional discrete Fourier transform to obtain the wave number k (unit: mm -1 ) function F(k) Represents, Let a(k) be the real part of F(k) at wave number k and b(k) be the imaginary part.

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[0014] [2] The composition of the alloy is, in mass%, O: 0.0001 to 0.0060%, Cu: 0.01 to 1.00%, Ni: 0.01 to 1.00%, Mo: 0.001 to 1.000%, Cr: 0.01 to 2.00%, Co: 0.01 to 0.50%, Ti: 0.001 to 0.300%, Nb: 0.01 to 0.30%, V: 0.01 to 0.50%, Ta: 0.001 to 0.100%, W: 0.01 to 1.00 %, Ca: 0.0001 to 0.0400%, Mg: 0.001 to 0.040%, REM: 0.0001 to 0.0400%, Zr: 0.001 to 0.050%, B: 0.0001 to 0.0100%, Sn: 0.01 to 0.05%, Sb: 0.01 to 0.05%, and As: 0.001 to 0.050%.

[0015] [3] In a cross section parallel to the thickness direction of the steel plate, the metal structure within a range of 50 μm from the surface in the thickness direction is, in terms of area ratio, ferrite: 50% or more, martensite, bainite, pearlite, and a total of one or more of retained austenite: 0 to 50% The steel sheet according to [1] or [2], wherein the ratio Hs / Hc of the Vickers hardness (Hs) in the range of 50 μm from the surface in the sheet thickness direction to the Vickers hardness (Hc) at 1 / 4 of the sheet thickness satisfies Hs / Hc≦0.65.

[0016] [4] A method for producing the steel sheet according to [1] or [2] above, wherein the composition of the steel sheet is, in mass%, C: 0.14 to 0.35%, Si: 0.01 to 2.00%, Mn: 1.0 to 4.0%, Al: 0 to 0.30%, P: 0.100% or less, S: 0.0500% or less, N: 0.0100% or less, O: 0.0060% or less, Cu: 0 to 1.00%, Ni: 0 to 1.00%, Mo: 0 to 1.000%, Cr: 0 to 2.00%, Co: 0 to 0.50%, Ti: 0 to 0.300%, Nb: 0 to 0.30%, V: 0 a casting process in which molten steel containing 0.0400%, 0.0400%, 0.0400%, 0.050%, 0.0100%, 0.05 ... A hot rolling process in which the slab cast in the casting process is rolled at a finishing temperature FT of more than 750 ° C. without cooling to less than 500 ° C. and under conditions that satisfy the following formulas (3) and (4): a coiling step in which the hot-rolled steel sheet rolled in the hot-rolling step is cooled from a finishing temperature FT to 750°C at a cooling rate of 40°C / s or more, and then cooled to a coiling temperature at a cooling rate of 15°C / s or more, and then coiled; a cold-rolling step in which the coiled hot-rolled steel sheet is cold-rolled; and an annealing step in which the cold-rolled steel sheet rolled in the cold-rolling step is heated to a temperature of at least A3 point −30°C calculated by the following formula (5), and held at that temperature for 10 seconds or more.

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[0017] [5] A method for producing the steel sheet according to [3], wherein the composition of the steel sheet is, in mass%, C: 0.14 to 0.35%, Si: 0.01 to 2.00%, Mn: 1.0 to 4.0%, Al: 0 to 0.30%, P: 0.100% or less, S: 0.0500% or less, N: 0.0100% or less, O: 0.0060% or less, Cu: 0 to 1.00%, Ni: 0 to 1.00%, Mo: 0 to 1.000%, Cr: 0 to 2.00%, Co: 0 to 0.50%, Ti: 0 to 0.300%, Nb: 0 to 0.30%, V: 0 to 0. a casting process in which molten steel containing 0.50%, Ta: 0-0.100%, W: 0-1.00%, Ca: 0-0.0400%, Mg: 0-0.040%, REM: 0-0.0400%, Zr: 0-0.050%, B: 0-0.0100%, Sn: 0-0.05%, Sb: 0-0.05%, and As: 0-0.050%, with the balance being Fe and impurities, is cast under conditions in which the average cooling rate within the temperature range from the liquidus temperature to the solidus temperature at a depth of 10 mm from the surface of the slab is 10°C / sec or more; a hot rolling step in which the slab cast in the casting step is cooled to 500°C or higher, and then inserted into a hot rolling furnace and rolled under conditions that satisfy the following formulas (3) and (4); a coiling step in which the hot-rolled steel sheet rolled in the hot-rolling step is cooled from a finishing temperature FT to 750°C at a cooling rate of 40°C / s or more, and then cooled to a coiling temperature at a cooling rate of 15°C / s or more, and then coiled; a cold-rolling step in which the coiled hot-rolled steel sheet is cold-rolled; and an annealing step in which the cold-rolled steel sheet rolled in the cold-rolling step is heated to a temperature of at least A3 point −30°C calculated by the following formula (5) in an atmosphere having a dew point of at least −30°C and at most 20°C, and held at that temperature for at least 60 seconds.

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[0018] According to the present invention, it is possible to provide a steel sheet having a larger maximum bending angle than conventional steel sheets and suppressing a sudden drop in load after the maximum bending angle. Here, the "maximum bending angle" refers to the bending angle at the maximum load based on the VDA standard (VDA238-100) specified by the German Association of the Automotive Industry (hereinafter the same). [Brief explanation of the drawings]

[0019] [Figure 1] FIG. 2 is a diagram illustrating a procedure for determining a profile function, the maximum c(k), and the k that gives that c(k) in the steel sheet of the present invention. [Figure 2] FIG. 2 is a diagram illustrating measurement positions of the Mn concentration profile in the steel plate of the present invention. [Figure 3] 1 shows an example of the Mn concentration distribution in a steel sheet according to the present invention, where (a) is an EPMA image, (b) is a binarized image of the EPMA image (profile image of Mn concentration), and (c) is an image obtained by two-dimensional discrete Fourier transform of the binarized image. [Figure 4] FIG. 10 is a diagram illustrating the y-axis along which the profile function of the Mn concentration should be obtained. [Figure 5] FIG. 10 is a diagram showing an example of a profile function of Mn concentration. DETAILED DESCRIPTION OF THE INVENTION

[0020] The present invention will be described in detail below.

[0021] <Component composition> First, the chemical composition of the steel sheet of the present invention will be described. Hereinafter, "%" relating to the chemical composition means "% by mass."

[0022] (C: 0.14 to 0.35%) C is an element that improves the hardenability of steel sheet. C is also an element that has the effect of increasing strength when contained in a hard structure such as a martensite structure. C is also an element that has the effect of increasing bake hardenability. To effectively exert these effects, the C content is set to 0.14 to 0.35%. The lower limit of the C content may be 0.15%, 0.16%, 0.18%, or 0.20%. The upper limit of the C content may be 0.32%, 0.30%, 0.28%, or 0.25%.

[0023] (Si: 0.01 to 2.00%) Si suppresses the formation of carbides and improves the formability of steel sheets. The Si content is set to 0.01 to 2.00%. The lower limit of Si may be 0.05%, 0.10%, 0.15%, or 0.20%. Since Si is an element that deteriorates weldability and phosphatability, promotes Mn concentration in iron-based carbides, and increases the deviation in alloy concentration, the upper limit of Si content may be 2.00%, 1.80%, 1.50%, or 1.20%.

[0024] (Mn: 1.0 to 4.0%) Mn is an element that contributes to improving hardenability and is useful for increasing the strength of steel sheets. To effectively exert this effect, the Mn content is set to 1.0 to 4.0%. The lower limit of Mn may be 1.2%, 1.5%, 1.8%, or 3.0%. Mn is an element that is involved in the control of Mn concentration distribution, which is a feature of the present invention, and the higher the Mn content, the more effectively the effects of the present invention are exerted in the steel sheet. On the other hand, a high Mn content makes it easier for MnS to form and cracks to occur in the surface layer, so the upper limit of the Mn content may be 3.8%, 3.5%, 3.2%, or 3.0%.

[0025] (Al: 0 to 0.30%) Al is an element that is effective in deoxidizing and improving the yield of carbide-forming elements. Even when used for deoxidation, Al does not need to be contained in the final steel sheet product, and the lower limit of the Al content is 0. Since Al strongly promotes ferrite formation, the Al content is set to 0.001 to 0.30%. The lower limit of Al may be 0.01%, 0.05%, 0.01%, or 0.02%. The upper limit of the Al content may be 0.25% or 0.20%.

[0026] (P:0.100% or less) P is an element contained in steel as an impurity. The lower the P content, the better, and it can even be 0. From the viewpoint of weldability, the P content is set to 0.100% or less. Since reducing the P content is costly, the lower limit of the P content may be 0.0001%, 0.001%, or 0.005%. The upper limit of the P content may be 0.080%, 0.060%, or 0.050%.

[0027] (S:0.0500% or less) S is an element contained in steel as an impurity. The lower the S content, the better, and it can even be 0. The S content is set to 0.0500% or less to prevent a decrease in low-temperature toughness due to an increase in the amount of MnS precipitated and to prevent weldability. Since reducing the S content is costly, the lower limit of the S content may be 0.0001%, 0.0005%, or 0.0010%. The upper limit of the S content may be 0.0400%, 0.0300%, or 0.0200%.

[0028] (N:0.0100% or less) N is an element contained in steel as an impurity. The lower the N content, the better, and it can even be 0. From the viewpoint of weldability, the N content is set to 0.0100% or less. Since reducing the N content is costly, the lower limit of the N content may be 0.0001%, 0.0005%, or 0.0010%. The upper limit of the N content may be 0.0080%, 0.0060%, or 0.0050%.

[0029] (O:0.0060% or less) O is an element contained in steel as an impurity. The lower the O content, the better, and it may be 0. From the viewpoint of uniform elongation characteristics, the O content is set to 0.0060% or less. Since reducing the O content is costly, the lower limit of the O content may be 0.0001%, 0.0003%, or 0.0005%. The upper limit of the O content may be 0.0050%, 0.0040%, or 0.0030%.

[0030] (Cu:0~1.00%, Ni:0~1.00%, Mo:0~1.000%, Cr:0~2.00%, Co:0~0.50%) Cu, Ni, Mo, Cr, and Co are elements that contribute to improving strength. These elements may be contained in any combination of one or more as needed. The inclusion of these elements is not essential, and the content of these elements may be zero. To fully obtain the effects of their inclusion, the lower limits of the Cu and Ni contents may be 0.001%, 0.01%, 0.02%, or 0.05%, the lower limit of the Mo content may be 0.0001%, 0.001%, 0.005%, 0.10%, or 0.50%, the lower limit of the Cr content may be 0.001%, 0.01%, 0.05%, or 0.10%, and the lower limit of the Co content may be 0.001%, 0.01%, or 0.02%. Furthermore, the upper limits of the Cu and Ni contents may be 0.80%, 0.60%, 0.50%, or 0.40%, the upper limit of the Mo content may be 0.800%, 0.600%, 0.500%, 0.400%, or 0.300%, the upper limit of the Cr content may be 1.50%, 1.00%, 0.50%, or 0.30%, and the upper limit of the Co content may be 0.40%, 0.30%, 0.20%, or 0.10%.

[0031] (Ti:0~0.300%, Nb:0~0.30%, V:0~0.50%, Ta:0~0.100%) Ti, Nb, V, and Ta are elements that contribute to carbide morphology control and strength improvement. These elements may be contained in any combination of one or more as needed. The inclusion of these elements is not essential, and the content of these elements may be zero. To fully obtain the effects of their inclusion, the lower limit of the Ti content may be 0.0001%, 0.001%, 0.005%, or 0.010%, the lower limit of the Nb content may be 0.001%, 0.01%, 0.02%, 0.03%, or 0.05%, the lower limit of the V content may be 0.001%, 0.01%, 0.03%, or 0.05%, and the lower limit of the Ta content may be 0.0001%, 0.001%, or 0.002%. Furthermore, the upper limit of the Ti content may be 0.200%, 0.150%, 0.100%, or 0.050%, the upper limit of the Nb content may be 0.25%, 0.20%, or 0.15%, the upper limit of the V content may be 0.40%, 0.30%, or 0.20%, and the upper limit of the Ta content may be 0.080%, 0.050%, 0.030%, or 0.020%.

[0032] (W:0~1.00%, Ca:0~0.0400%, Mg:0~0.040%, REM:0~0.0400%, Zr:0~0.050%) W, Ca, Mg, REM, and Zr are elements that contribute to the fine dispersion of inclusions and increase toughness. Here, REM refers to a total of 17 elements, including Sc, Y, and lanthanides, and the REM content refers to the total content of these 17 elements. Any combination of one or more of these elements may be contained as needed. The inclusion of these elements is not essential, and the content of these elements may be zero. In order to fully obtain the effects of their inclusion, the lower limit of the W content may be 0.001%, 0.01%, or 0.02%, the lower limit of the Ca content may be 0.00001%, 0.0001%, 0.0005%, or 0.0010%, the lower limit of the Mg content may be 0.0001%, 0.001%, or 0.002%, the lower limit of the REM content may be 0.00001%, 0.0001%, or 0.005%, and the lower limit of the Zr content may be 0.0001%, 0.001%, or 0.002%. Furthermore, the upper limit of the W content may be 0.80%, 0.60%, 0.40%, or 0.20%, the upper limit of the Ca content may be 0.0200%, 0.0100%, 0.0080%, or 0.0050%, the upper limit of the Mg content may be 0.030%, 0.020%, or 0.010%, the upper limit of the REM content may be 0.0300%, 0.0200%, or 0.0100%, and the upper limit of the Zr content may be 0.040%, 0.020%, or 0.010%.

[0033] (B: 0 to 0.0100%) B is an element that improves hardenability and is useful for increasing the strength of steel sheets for bake hardening. The inclusion of B is not essential, and the B content may be 0. In order to fully obtain the effect of the inclusion, the lower limit of the B content may be 0.00001%, 0.0001%, or 0.0005%. The upper limit of the B content may be 0.0080%, 0.0050%, or 0.0030%.

[0034] (Sn:0~0.05%, Sb:0~0.05%, As:0~0.050%) Sn, Sb, and As are elements that can be contained in a steel sheet when scrap is used as a raw material. The lower the Sn, Sb, and As contents, the better, and the contents of these elements may be 0%. From the viewpoint of suppressing deterioration of cold formability, the Sn and Sb contents are preferably 0.05% or less, and the As content is preferably 0.050% or less. Since reducing the Sn, Sb, and As contents is costly, the lower limits of the Sn and Sb contents may be 0.001% and 0.01%, and the lower limit of the As content may be 0.0001% and 0.001%. Furthermore, the upper limits of the Sn and Sb contents may be 0.04% or less and 0.03% or less, and the upper limit of the As content may be 0.030%, 0.020%, and 0.010%.

[0035] In the steel sheet according to the embodiment of the present invention, the balance other than the above-mentioned elements consists of Fe and impurities. Impurities include components that are mixed in during the industrial production of steel sheets due to various factors in the manufacturing process, including raw materials such as ore and scrap, and include components that are not intentionally added to the steel sheet according to the embodiment of the present invention. Furthermore, impurities also include elements other than the above-described components that are present in the steel sheet at a level that does not affect the properties of the steel sheet according to the embodiment of the present invention in terms of the specific action and effect of the element.

[0036] Next, the structure of the steel sheet of the present invention will be described.

[0037] In high-strength steel sheets containing martensite, such as those targeted by the present invention, the properties may be adjusted by appropriate heat treatment (tempering) after the martensite is formed. During this process, significant structural and property changes occur in the martensite. The structures before and after this change are sometimes distinguished as "fresh martensite" and "tempered martensite," but such a distinction is unnecessary in the present invention. Hereinafter, the term "martensite" is used to include "fresh martensite" and "tempered martensite." For example, in terms of area fraction, "area fraction of martensite" means "the total area fraction of fresh martensite and tempered martensite."

[0038] <Metal structure at a depth of 1 / 4 of the plate thickness> In the present invention, the metallographic structure in the sheet thickness direction at a depth of 1 / 4 of the sheet thickness from the surface has an area ratio of 0 to 40% total of ferrite and bainite, 50 to 100% martensite, and 0 to 10% total of pearlite and retained austenite. By achieving such a structure, it is possible to obtain a steel sheet that achieves high strength by making the most of the effects of the present invention. Here, the term "surface" refers to the surface of the steel sheet excluding the coating layer (the surface of the steel sheet after the coating layer has been removed) when the steel sheet is a coated steel sheet having a coating layer, and to the outermost surface of the steel sheet when the steel sheet does not have a coating layer.

[0039] The area ratio of the metal structure at a depth of 1 / 4 of the plate thickness is measured by the following method.

[0040] First, we will explain the observation region for measuring the area fraction of the metallographic structure at a depth of 1 / 4 of the plate thickness. The observation region is a cross section parallel to the rolling direction and plate thickness direction, and is a 100 μm × 100 μm square region with sides measuring 100 μm in the plate thickness direction and 100 μm in the rolling direction, centered at a position 1 / 4 of the plate thickness from the surface of the steel plate. The rolling direction is determined by the method described below. A wider region including this square region may be observed, and a 100 μm × 100 μm square region may be cut out from that wider region. To reduce measurement error, five different fields of view on the same cross section are observed, and the area fraction of each metallographic structure is calculated using the method defined below, and the arithmetic mean value is used as each structure fraction.

[0041] To identify each metal structure and calculate the area ratio, first, for the above-mentioned specified observation area, the area to be identified as retained austenite is determined, and then, for the same area, ferrite, bainite, martensite, or pearlite is identified.

[0042] (Residual austenite) The area and area fraction of retained austenite are measured using the following method. Specifically, the observation surface of the specified observation area is polished with colloidal silica or electrolytic polishing, and diffracted electrons are measured at 0.2 μm intervals (lattice arrangement) using an EBSD attached to a scanning electron microscope. The resulting pseudo-Kikuchi pattern is analyzed to identify the crystal orientation and crystal system. The sample preparation conditions are within the range recommended by the Japan Society for Materials Science's standard "Crystal Orientation Measurement Standard for Material Evaluation Using Electron Backscatter Diffraction (EBSD) Method." The area detected as FCC phase from the measurement data is considered retained austenite, and its area and area fraction are measured.

[0043] The areas and area ratios of ferrite, bainite, martensite, and pearlite can be measured by the following method. That is, the same square area as that used for the observation of retained austenite above is etched with nital solution, and photographed (magnification: 5000 times) with a field emission scanning electron microscope (FE-SEM). The area ratio is determined by counting points at intervals of 2 μm (grid arrangement) from the obtained microstructure photograph.

[0044] Each organization will make the following determinations:

[0045] (ferrite and bainite) Ferrite has a granular or needle-like shape and does not contain iron-based carbides inside. Bainite has a lath-like shape (lath structure) and is a region in which iron-based carbides with a major axis of 20 nm or more exist within the lath structure and are elongated in the same direction.

[0046] (fresh martensite and tempered martensite) Fresh martensite has a lath structure and is a region that does not contain iron-based carbides with a major axis of 20 nm or more. Tempered martensite has a lath structure and is a region that contains iron-based carbides with a major axis of 20 nm or more, and the carbides are elongated in multiple different directions.

[0047] (Perlite) The region where ferrite and cementite are in a lamellar form is called pearlite.

[0048] When identifying ferrite, bainite, martensite, and pearlite, the above-mentioned visual identification is not performed on regions that have been determined to be retained austenite in the preceding identification of retained austenite. However, the area ratio is calculated as a ratio to the total area of ​​the observation range, including the retained austenite region.

[0049] These metal structure determinations are generally performed by those skilled in the art as part of their routine work, and can be easily made by those skilled in the art.

[0050] If the total area ratio of each texture obtained by the above evaluation method is different from 100%, the area ratio of each texture is multiplied by 100 / (total area ratio of each texture).

[0051] (Mn concentration profile at a position 50 μm from the surface) In the steel sheet of the present invention, in a cross section perpendicular to the sheet thickness direction, the profile function of the Mn concentration at a predetermined position 50 μm from the surface in the sheet thickness direction satisfies the following formulas (1) and (2). The profile function of the Mn concentration is a discrete function that indicates the change in the Mn concentration measured along a certain straight line, as will be explained below.

[0052] Specifically, in a cross section perpendicular to the thickness direction at a position 50 μm from the surface of the steel sheet, the direction in which the anisotropy of Mn segregation is strongest is the y-axis, and the unit of y is mm. The profile function of the Mn concentration (unit: mass%) in the y-direction is defined as f(y), and a new function f'(y) is defined as f'(y)=f(y)-[Mn] ([Mn] is the Mn content of the base material expressed in mass%.) It is stipulated that:

[0053] Furthermore, f'(y) is subjected to a two-dimensional discrete Fourier transform (2D-DFT, hereinafter simply referred to as "Fourier transform") to obtain the wave number k (unit: mm -1 ) is expressed as a function F(k), and the real part of F(k) at wave number k is Let a(k) be the imaginary part and b(k) be the imaginary part.

number

[0054] In this case, the maximum value of c(k) is determined by the wave number k, and k at that time is k>1.5 … (1) (Maximum value of c(k))<-0.0044[Mn] 2 +0.06[Mn]+0.012 … (2) Meet the following.

[0055] Here, the direction in which the anisotropy of Mn segregation is strongest, i.e., the direction of the y-axis (y direction), is defined as follows: In a cross section perpendicular to the thickness direction, a Mn concentration profile image showing the two-dimensional distribution of Mn concentration is obtained in a 2.0 mm × 2.0 mm region located 50 μm from the surface in the thickness direction. This image is then binarized and Fourier transformed, and the straight line with the highest brightness in the resulting 2D-DFT image is determined. In the Mn concentration measurement region, the direction parallel to this straight line is defined as the y direction. The y-direction profile function f(y) is a function that passes through the coordinates (measurement points) where the Mn concentration is maximum and minimum in the measurement region. Since there can be two or more points that give the maximum and minimum values ​​of Mn concentration, this function may be two or more. For these multiple profile functions, c(k) is calculated using the method described above. In the steel sheet of the present invention, the maximum c(k) and the k that gives that c(k) satisfy the above formulas (1) and (2). In equation (1), a large wave number k means that the period of f(y) is short. Furthermore, c(k) corresponds to the amplitude of f(y), and the maximum value of c(k) being smaller than a certain threshold value means that the amplitude, i.e., the maximum deviation of the Mn concentration, is smaller than a certain threshold value. The present invention was made based on the empirical finding by the inventors that a certain threshold value can be expressed as a function of the Mn concentration.

[0056] The profile function, the maximum c(k) and the k that gives that c(k) are determined according to the flowchart shown in Figure 1. Below, with reference to the drawings, we will explain in more detail how to determine the profile function, the maximum c(k) and the k that gives that c(k).

[0057] Figure 2 is a diagram illustrating measurement positions on a steel sheet. A 2.0 mm x 2.0 mm region 50 μm from the surface of steel sheet 11 in the sheet thickness direction (the direction of the x-axis in Figure 2) and perpendicular to the sheet thickness direction (parallel to the rolling and sheet width direction) is defined as Mn concentration profile image acquisition region 12. Even if the rolling direction of the steel sheet to be measured is unknown, the direction in which Mn segregation is greatest is determined as the y direction using a method described below. Therefore, it is not necessary to specify the y direction at this stage, and the orientation of the 2.0 mm x 2.0 mm region does not matter as long as it is perpendicular to the sheet thickness direction (parallel to the rolling and sheet width direction).

[0058] Next, the two-dimensional distribution of Mn concentration in the profile image acquisition area 12 is determined by area analysis using an EPMA (electron probe microanalyzer). Figure 3(a) shows an example of the obtained Mn concentration profile image (two-dimensional distribution of Mn concentration). This image is binarized and then Fourier transformed to determine the y-direction. Figure 3(b) shows the binarized EPMA image of Figure 3(a). Figure 3(c) shows the Fourier transformed image of Figure 3(b). In Figure 3(c), the extension direction of the white line 21 (the direction parallel to the line with the highest brightness) is defined as the y-direction. The y-direction can be either positive or negative. Note that the white line 21 is the direction of greatest Mn segregation. Since the y-direction is sufficient here, contrast adjustment before binarization may be performed to further emphasize the segregation. In EPMA area analysis, measurements are taken at a pitch of 10 μm or less. The y direction thus determined is generally the width direction of the steel plate, and the direction perpendicular to the width direction and thickness direction is the rolling direction.

[0059] There are no particular limitations on the method for obtaining a Fourier-transformed image from an EPMA image, as long as the direction of the white lines is known. For example, the image obtained by EPMA surface analysis can be converted to grayscale (8-bit display) using the image analysis software ImageJ. The image is then binarized to emphasize the density deviation. Since the direction of the white lines remains the same regardless of the binarization threshold setting, it can be adjusted appropriately to maximize the emphasis on the white lines. Next, the two-dimensional discrete Fourier transform (FFT) function of ImageJ can be used to obtain a Fourier-transformed image.

[0060] Next, the Mn concentration profile function f(y) is determined. f(y) is a function of a line drawn through the point with the highest Mn concentration in the Mn concentration distribution obtained by EPMA. Furthermore, a second f(y) is a function of the Mn concentration profile function of a line drawn through the point with the lowest Mn concentration in the Mn concentration distribution obtained by EPMA. If there are multiple points with the highest and lowest Mn concentrations, a line is drawn for each point to determine the Mn concentration profile function. In other words, at least two Mn concentration profile functions are obtained, but three or more may be obtained. Figure 4 shows an example in which two lines are drawn as the y-axis along which the Mn concentration profile function is to be obtained: line 31L passing through point 31 with the lowest Mn concentration, and line 32L passing through point 32 with the highest Mn concentration. Note that the range of y is the range that can be taken within the profile image acquisition area, and the position of the origin of y (y = 0) can be determined arbitrarily.

[0061] Next, as shown in FIG. 5, a new function f'(y) is calculated for each Mn concentration profile function f(y) as described above. Then, as described above, f'(y) is Fourier transformed to obtain the wave number k (unit: mm -1 ) function F(k) and find c(k). Here, f'(y) is Although there will be two or more c(k), the largest value among those obtained from each profile function is adopted as the maximum c(k) for the steel sheet. Then, this maximum c(k) and the k that gives this maximum c(k) are evaluated to see if they satisfy the above-mentioned formulas (1) and (2).

[0062] Here, k is the wave number of the long-period fluctuation of the Mn concentration, and c(k) is the amplitude at that time. The reason for limiting the period of the Mn concentration fluctuation to a long period is as follows.

[0063] Generally, changes in Mn concentration are known to occur due to partitioning in the temperature range where the liquid and solid phases coexist (enrichment in the liquid phase), and partitioning in the temperature range where the austenite and ferrite phases coexist (enrichment in the austenite phase). Concentration changes during solidification cause Mn enrichment in the central layer where solidification is delayed, and between dendritic structures during solidification. Concentration changes related to transformation also cause Mn enrichment in the martensite phase and retained austenite phase that existed as the austenite phase until the final stage of the heat treatment process, as well as in carbides that have a high affinity with Mn.

[0064] Of these, the concentration changes associated with solidification have a long period of fluctuation (the size of the structure that causes distribution) on the order of 0.1 mm to several tens of mm, due to the high temperature and little effect of processing. On the other hand, the concentration changes associated with transformation have a short period of fluctuation (the size of the structure that causes distribution) on the order of several μm to several tens of μm, due to the relatively low temperature range involved in transformation and the processing recrystallization that accompanies the application of rolling to ensure various properties of the steel sheet. Furthermore, the magnitude of the concentration changes associated with transformation is relatively small, because the steel sheet manufacturing process is a continuous process (continuous hot rolling, continuous annealing), and the time period during which element distribution occurs is short.

[0065] The present inventors have focused on and investigated the effect of such a change in Mn concentration on the deformation behavior of steel sheets during bending. As a result, it has been found that bending properties can be significantly improved by appropriately controlling the concentration change on the order of mm, rather than on the order of μm, i.e., by specifying the change in concentration within the ranges of the above formulas (1) and (2). The reason for this is not clear, but is thought to be as follows.

[0066] The important factor in the deformation behavior during the processing of steel sheets is thought to be the difference in behavior between hard martensite and soft ferrite. Since deformation is thought to be concentrated in the soft ferrite, it is thought that the key point is how to reduce the deformation concentration in the soft ferrite. Furthermore, in the bending process for applications in which the steel sheets of the present invention are used, the bending radius is on the order of mm, and it is necessary to consider the difference in deformation in the area of ​​mm order regarding the concentration of deformation.

[0067] In addition to these morphological size factors, as mentioned above, it is thought that the concentration changes on the order of μm, which are mainly related to transformation, are themselves small in magnitude and have little effect on bendability.

[0068] Furthermore, during bending, the amount of strain in the steel sheet surface layer increases, and the enrichment of Mn between the dendritic structures described above is likely to cause Mn segregation on the order of mm in the surface layer (slab surface layer) where solidification nuclei are formed intermittently.

[0069] The overlap of these factors is thought to be the reason why the definition of the long-period variation in Mn concentration in the surface layer region (50 μm from the surface in the sheet thickness direction) defined in the present invention is important. However, the inventors have discovered that the degree of the long-period variation determined by solidification can be suppressed by controlling the conditions of processes after solidification.

[0070] Furthermore, considering the manufacturing process, which includes normal hot rolling and cold rolling, the sheet is elongated at least 100 times in the rolling direction, so long-term deviations in the Mn concentration in the rolling direction do not affect the deformation behavior during bending. On the other hand, in the direction perpendicular to the rolling direction, i.e., the sheet width direction, the size change during rolling is small, so deviations in the Mn concentration reflecting solidification segregation are likely to remain. Therefore, the concentration distribution described above is roughly the distribution in the sheet width direction, and the y direction defined above corresponds to this direction. If it can be confirmed that the Mn concentration profile functions along the y direction, which pass through the coordinates giving the maximum Mn concentration and the coordinates giving the minimum Mn concentration, satisfy Equations (1) and (2), it can be confirmed that the conditions for obtaining good bendability are met.

[0071] Here, we will explain the direction perpendicular to the thickness direction (the y-axis direction) in which the above-mentioned concentration distribution should be formed. If we consider that the effect of the present invention is caused by the above-mentioned change in Mn concentration in the surface layer region during solidification, a concentration change on the order of mm should be observed in the width direction of the coil (direction perpendicular to the rolling direction) of the steel sheet (final product) that has undergone hot rolling and cold rolling. Because traces of concentration changes during solidification are stretched by more than 100 times in the rolling direction, even if concentration changes in the rolling direction can be observed in the final product, the change should be on the order of m (meters). It is not easy to detect such ultra-long-period concentration changes. Furthermore, even in steel sheets manufactured under conditions where the rolling direction can be reliably identified, a correlation between the concentration change in the width direction, which is perpendicular to the rolling direction, and the effect of the present invention has been confirmed. Taking this into consideration, in the present invention, in order to confirm the applicability of the technology of the present invention even in situations where the rolling direction cannot be confirmed in commercially available steel sheet pieces, the above-mentioned concentration distribution is satisfied in the direction perpendicular to the thickness direction, which is determined by the Mn concentration distribution in a cross section perpendicular to the thickness direction.

[0072] If the difference between the maximum and minimum Mn concentrations in the surface layer is large and the fluctuation period is long, the deviation in Mn concentration will affect the hardness during annealing after cold rolling. That is, areas with high Mn concentrations become hard phases, and areas with low Mn concentrations become soft phases, resulting in a deviation in hardness and making them more likely to become crack initiation points when the steel sheet is bent. By satisfying the above formulas (1) and (2), the deviation in Mn concentration is suppressed, resulting in a smaller deviation in hardness and good bendability.

[0073] (Metal structure within 50 μm from the surface) In the steel sheet of the present invention, the structure within the range of 50 μm from the surface is not particularly limited, and may be the same as the metal structure at a depth of 1 / 4 of the sheet thickness described above, for example.

[0074] The steel sheet of the present invention described above has excellent bendability due to the controlled Mn concentration distribution in the surface layer. To further enhance bendability, the metallographic structure in the region 50 μm from the surface may be controlled. Specifically, the metallographic structure in a 100 μm square region of a cross section parallel to the rolling direction and the sheet thickness direction within the region 50 μm from the surface may contain, in terms of area ratio, 50% or more of ferrite and 0 to 50% of one or more of martensite, bainite, pearlite, and retained austenite, and the ratio Hs / Hc of the Vickers hardness (Hs) in the region 50 μm from the surface to the Vickers hardness (Hc) at a position 1 / 4 of the sheet thickness may satisfy Hs / Hc≦0.65. By controlling the surface metallographic structure in this manner, it is possible to further improve the bendability of the steel sheet compared to steel sheets with similar tensile strength.

[0075] The metal structure within the range of 50 μm from the surface can be identified in the same way as the metal structure at a depth of 1 / 4 of the plate thickness described above.

[0076] <Tensile strength> The tensile strength and uniform elongation are measured by taking a JIS No. 5 tensile test piece described in JIS Z 2241:2011 from the steel sheet in a direction perpendicular to the rolling direction and conducting a tensile test in accordance with JIS Z 2241:2011.

[0077] <Bending load and absorbed energy> The bending load and absorbed energy of the steel plate of the present invention are evaluated by the tensile strength, the maximum bending angle based on the VDA standard (VDA238-100) specified by the German Association of the Automotive Industry, and the bending angle at which the load becomes half of the maximum load in the load reduction behavior after the maximum load.

[0078] The maximum bending angle is measured under the following conditions: In the present invention, the displacement at the maximum load obtained in the bending test is converted into an angle according to the VDA standard to determine the maximum bending angle α.

[0079] Test piece dimensions: 60mm x 60mm Bending ridge: Press with a punch so that the bending ridge is perpendicular to the rolling direction Test method: Roll support, punch indentation Roll diameter: φ30mm Punch shape: Tip R = 0.4 mm Distance between rolls: 2.0 x plate thickness (mm) + 0.5 mm Push-in speed: 20mm / min Testing machine: SIMADZU AUTOGRAPH 20kN

[0080] Furthermore, in the load reduction behavior after the maximum load, the bending angle is determined when the load becomes half of the maximum load. Whether the fracture is a brittle fracture or not can be determined based on the difference between the maximum bending angle and the bending angle at which the load becomes half of the maximum load. If this difference is small, it can be determined that the fracture is a brittle fracture, and that the maximum bending angle and the subsequent rapid load reduction have not been suppressed.

[0081] In the steel sheet of the present invention, the tensile strength is 1180 MPa or more, and the maximum bending angle is (7.3t 2 -37.2t+100)×1470×1700 / (TS×TS) (t: plate thickness ( It is preferable that the difference between the maximum bending angle and the subsequent bending angle at which the load is half the maximum load (mm), TS (tensile strength (MPa)), and the difference between the maximum bending angle and the subsequent bending angle at which the load is half the maximum load are 6° or more. If these are below the lower limit values, there is a risk of the part breaking earlier when subjected to bending deformation after processing.

[0082] The tensile strength may be 1200 MPa or more, 1300 MPa or more, 1400 MPa or more, or 1500 MPa or more. The difference between the maximum bending angle and the subsequent bending angle at which a load equal to half the maximum load is applied may be 7° or more, 8° or more, 9° or more, or 10° or more.

[0083] The thickness of the steel sheet of the present invention is 0.6 mm or more and less than 2.2 mm, and may be 0.8 mm or more, 0.9 mm or more, or 1.0 mm or more, or 2.0 mm or less, 1.9 mm or less, or 1.8 mm or less.

[0084] <Manufacturing method> Next, a method for producing a steel sheet according to the present invention will be described.

[0085] "Casting process" First, a steel slab is produced from molten steel having the same composition as that described above. From the viewpoint of manufacturability, the steel slab is preferably cast by a continuous casting method. During casting, the average cooling rate at a depth of 10 mm from the surface of the slab within the temperature range from the liquidus temperature to the solidus temperature is set to 10°C / s or more. This prevents long-period segregation of Mn in the surface layer. If the average cooling rate is less than 10°C / s, it becomes difficult to reduce the segregation of Mn in the surface layer of the final product even when the hot rolling conditions described below are applied. The average cooling rate is preferably 15°C / s or more, and more preferably 20°C / s or more.

[0086] Hot rolling is started without cooling the cast slab below 500°C. Because the slab is thick, the cooling rate is slow. During cooling, transformation proceeds at a transformation temperature corresponding to the alloy concentration at each location, causing alloy distribution and widening the deviation in alloy concentration, which is undesirable. Therefore, the cast slab is inserted into a reheating furnace without reheating while maintaining a temperature of 500°C or higher, or without lowering the temperature below 500°C, and subjected to the next step of hot rolling. The temperature is preferably 550°C or higher, more preferably 600°C or higher.

[0087] After casting, reheating before hot rolling is preferably at 1150°C or higher to fully dissolve borides, carbides, etc. Furthermore, it is preferably at 1350°C or lower to prevent excessive scale loss. Holding the slab at approximately 1250-1350°C for approximately 5-25 hours can also serve the purpose of homogenization. This promotes alloy diffusion, makes the distribution of Mn more uniform, and suppresses microsegregation. The heating holding time is preferably 15 hours or shorter, more preferably 10 hours or shorter, to prevent excessive scale loss.

[0088] "Hot rolling process"

[0089] [Rough rolling] In this method, the heated slab may be subjected to rough rolling before finish rolling in order to adjust the plate thickness or the like.

[0090] During rough rolling, rolling may be performed not only in the thickness direction but also in the width direction one or more times. Rolling by 3% or more in the width direction per time is preferable because lattice defects are introduced into the steel sheet due to plastic deformation, promoting the diffusion of Mn. Rolling by 3% or more in the width direction may be performed at any time before finish rolling. The rolling in the width direction per time is preferably 10% or more, more preferably 30% or more. On the other hand, if the deformation rate in the width direction per time exceeds 50%, slab cracks may occur, the slab shape may become non-uniform, and the dimensional accuracy of the hot-rolled steel sheet obtained by hot rolling may decrease. Therefore, the deformation rate in the width direction per time is set to 50% or less, preferably 40% or less. The temperature, reduction rate, and number of times of rough rolling are not limited. If multiple rough rollings are performed, the slab may be reheated between them.

[0091] [Finishing rolling] The finish rolling is carried out so that the final finish temperature FT exceeds 750°C and the following formula (3) is satisfied.

[0092]

number

[0093] where:

number

[0094] Strain from hot rolling is more likely to occur closer to the surface, and strain is more likely to occur when the shape ratio η (a function of the roll diameter and the plate thickness before and after rolling) defined by the above formula is large. Under conditions of large strain and low finishing temperatures, ferrite transformation is promoted, and Mn concentrates in the austenite due to ferrite transformation at high temperatures, making it more likely to cause concentration deviations. Therefore, it is important to control the final finishing temperature FT according to η so that it satisfies the following formula (3).

[0095] Specifically, by satisfying the above conditions, sufficient recrystallization is achieved while suppressing the transformation of γ (austenite) into ferrite in the surface layer, and the surface layer is rapidly cooled by ROT cooling immediately after being held in the temperature range above the A3 point (γ single-phase region) for a sufficient time. As a result, the Mn concentration deviation is reduced by the mechanism described below, and the above-mentioned formulas (1) and (2) are satisfied.

[0096] Here, point A3 is the temperature defined by the following formula: Each element symbol indicates the content (mass %) of the element.

[0097] A3=910-203√C+44.7Si-30Mn+700P-20Cu -15.2Ni-11Cr+31.5Mo+400Ti+104V+120Al

[0098] During hot rolling, approximately 10% of oil or fat may be added to the roll cooling water to perform lubricating rolling, which can further suppress strain in the surface layer and make the Mn distribution in the surface layer more uniform.

[0099] The left side of equation (4) represents the time (seconds) from the final stage of hot rolling to the start of cooling. Here, "cooling" refers to cooling at a cooling rate of 40°C / s or more in the temperature range from the finishing temperature described below to 750°C. If this time exceeds 1.3 seconds, the steel sheet will be slowly cooled during this time, which will promote alloy distribution and increase the deviation in Mn concentration.

[0100] The hot-rolled steel sheet is cooled from the finishing temperature to 750°C at a cooling rate of 40°C / s or more, and then cooled to the coiling temperature at a cooling rate of 15°C / s or more. These cooling rates are minimum cooling rates, meaning that cooling is always 40°C / s or more from the finishing temperature to 750°C, and always 15°C / s or more to the coiling temperature. By using a cooling rate of 40°C / s in the temperature range up to 750°C, the ferrite transformation start temperature can be lowered, alloy partitioning can be suppressed, and deviations in the Mn concentration can be suppressed. The higher the cooling rate, the lower the ferrite transformation start temperature can be, so the cooling rate is preferably 45°C / s or more, more preferably 50°C / s or more. On the other hand, a cooling rate exceeding 200°C / s makes control difficult, so the cooling rate is preferably 200°C / s or less.

[0101] By setting the cooling rate from 750°C to the coiling temperature to 15°C / s or more, the pearlite transformation start temperature can be lowered, alloy enrichment in cementite can be suppressed, and deviation in Mn concentration can be suppressed. The higher the cooling rate, the lower the pearlite transformation start temperature can be, so the cooling rate is preferably 20°C / s or more, and more preferably 30°C / s or more. On the other hand, if the cooling rate exceeds 200°C / s, control becomes difficult, so the cooling rate is preferably 200°C / s or less.

[0102] "Winding process" In the coiling step, the hot-rolled steel sheet obtained by hot rolling is coiled. The coiling temperature is preferably 650°C or lower. If the coiling temperature is too high, the surface property deteriorates due to surface oxidation, and the surface property of the final product is likely to deteriorate. The coiling temperature is more preferably 600°C or lower, and even more preferably 550°C or lower. On the other hand, if the coiling temperature is 500°C or lower, the hot-rolled steel sheet hardens, making cold rolling difficult, and therefore the coiling temperature is preferably 500°C or higher.

[0103] The coiled steel sheet may be subjected to pickling, if necessary.

[0104] "Cold rolling process" Thereafter, the steel sheet is subjected to cold rolling to obtain a cold-rolled steel sheet. The cold rolling conditions are not particularly limited and may be performed according to a conventional method. The rolling reduction may be, for example, 10 to 60%.

[0105] "Annealing process" In the annealing process, the steel sheet (cold-rolled steel sheet) after the cold rolling process is heated to a temperature range of at least A3 point minus 30°C and held at that temperature for 10 seconds or more for annealing. The temperature is preferably increased from 400°C to the A1 point at a heating rate of 15°C / s or more. This suppresses the concentration of alloys in cementite and allows for a uniform distribution of Mn. By holding the holding temperature within this temperature range, austenite is generated during annealing, making it easier to obtain a predetermined amount of martensite as the final structure. This makes it easier for the steel sheet to achieve the desired tensile strength. Holding temperatures above 950°C decrease productivity. Therefore, a holding temperature of 950°C or less is preferred, and 900°C or less is more preferred. Holding times of less than 10 seconds result in insufficient dissolution of cementite precipitated in the previous process and up to the holding temperature, resulting in localized distribution of Mn in the surface layer. The holding time may be 400 seconds or less. Following the heating in the annealing process, the cold-rolled steel sheet may be cooled, held, quenched, or tempered. Furthermore, hot-dip galvanizing or alloying may be performed.

[0106] Here, point A1 is the temperature defined by the following formula: Each element symbol indicates the content (mass %) of the element.

[0107] A1=723-10.7Mn-16.9Ni+29.1Si+16.9Cr +290As+6.38W

[0108] In the annealing step, the dew point of the annealing atmosphere is not limited. Following the heating in the annealing step, the cold-rolled steel sheet may be cooled, held, quenched, or tempered to form a desired structure at the quarter depth position. Furthermore, hot-dip galvanizing or alloying may also be performed.

[0109] When the dew point of the atmosphere during heating from the temperature rise in the annealing process is set to -30°C or more and 20°C or less, and the holding time in the temperature range of -30°C or more at the A3 point is set to 60 seconds or more, decarburization of the surface layer of the steel sheet during annealing can be promoted, and the formation of ferrite in the surface layer of the steel sheet can be promoted at the maximum heating temperature and during the subsequent cooling, and the area ratio of ferrite in the surface layer can be set to 50% or more, and the total area ratio of one or more of martensite, bainite, pearlite, and retained austenite can be set to 0 to 50%, which is preferable because it allows the ratio Hs / Hc of the Vickers hardness (Hs) of the soft surface portion to the Vickers hardness (Hc) at the 1 / 4 depth position to be 0.65 or less.

[0110] These structural control by heat treatments may be carried out within the scope of generally known techniques, and is a simple task for a person skilled in the art who controls the structure of steel materials as part of his or her regular work. Highly accurate structural control is possible by using data accumulated up to now and by conducting preliminary tests as necessary.

[0111] By the above method, the deviation of the Mn concentration in the surface layer is reduced, and as a result, a steel sheet can be obtained which has a large maximum bending angle and in which the maximum bending angle and subsequent rapid load reduction are suppressed. [Example]

[0112] The steel plates were prepared according to the following procedure.

[0113] First, steel slabs were produced by continuous casting from molten steel having the chemical composition shown in Table 1. In Table 2, the "cooling rate" under "casting" refers to the average cooling rate at a depth of 10 mm from the surface of the slab within the temperature range from the liquidus temperature to the solidus temperature during casting.

[0114] Next, some of the steel sheets were subjected to homogenization treatment under the conditions shown in Table 2.

[0115] Next, the cast slab or the slab that had been subjected to the homogenization treatment was inserted into a heating furnace at the temperature shown in Table 2 and maintained at the temperature shown in Table 2.

[0116] Next, some of the slabs were subjected to rough rolling in the width direction under the conditions shown in Table 2.

[0117] Next, finish rolling was performed under the conditions shown in Table 2 to obtain hot-rolled steel sheets. In Table 2, "x / v×60," "η average," and "right side of equation (3)" in "hot rolling" mean the same as those in the above-mentioned equations (3) and (4), and the explanation of each parameter. FT means the final finishing temperature, and "Equation (3) judgment" is marked with "○" when equation (3) is satisfied and "×" when it is not satisfied. In "hot rolling," "cooling rate to 750°C" means the cooling rate from the final finishing temperature FT to 750°C, and "cooling rate to coiling temperature" means the cooling rate from 750°C to the coiling temperature.

[0118] Next, the obtained hot-rolled steel sheet was coiled at the coiling temperature shown in Table 2. The coiling temperature is the temperature of the steel sheet surface. Thereafter, the coiled steel sheet was subjected to cold rolling at the rolling reduction shown in Table 2 to obtain a cold-rolled steel sheet.

[0119] Next, the obtained cold-rolled steel sheet was annealed under the conditions shown in Table 2. In Table 2, the "heating rate" of "annealing" means the average heating rate from 400°C to the A1 point. Furthermore, the "reference holding temperature" means the A3 point -30°C (reference holding temperature) or higher, and the "holding temperature judgment" was marked with "○" when the holding temperature was A3 point -30°C (reference holding temperature) or higher, and marked with "×" when the holding temperature was less than A3 point -30°C.

[0120] Next, some of the steel sheets were subjected to plating and alloying treatment.

[0121] [Table 1]

[0122] [Table 2]

[0123] Table 3 shows the properties of the obtained steel sheets.

[0124] In Table 3, "α + B," "P + γ," and "FM + TM" respectively represent the total area ratio of ferrite and bainite, the total area ratio of pearlite and retained austenite, and the total area ratio of fresh martensite and tempered martensite at a depth of 1 / 4 of the sheet thickness. Furthermore, "surface layer α" represents the area ratio of ferrite in the surface layer, and "surface layer M + B + P + γ" represents the total area ratio of martensite, bainite, pearlite, and retained austenite in the surface layer. The method for measuring the area ratios is as described above. The quantitative evaluation of the microstructure was performed in the sheet thickness direction on the metallographic structure at a depth of 1 / 4 of the sheet thickness from the surface. Here, "surface" refers to the surface of the steel sheet excluding the coating layer in the case of a plated steel sheet having a coating layer, and refers to the outermost surface of the steel sheet in the case of a steel sheet not having a coating layer. In the present examples, for steel sheets that had been plated and alloyed, the quantitative evaluation of the microstructure was performed after removing the coating layer.

[0125] In Table 3, "wave number k giving the maximum value of c(k)," "right side of equation (2)," and "maximum value of c(k)" refer to the Mn concentration profile in the range of 50 μm from the surface in the thickness direction in a cross section perpendicular to the thickness direction, and the details are as described above. The Mn concentration profile is measured by EPMA as described above. In addition, "Equation (2) judgment" is based on the equation (2): (maximum value of c(k))<-0.0044[Mn] 2 If the condition +0.06[Mn]+0.012 was met, it was marked as "○", and if it was not met, it was marked as "×".

[0126] In Table 3, "Difference in bending angle from 1 / 2 load" means the difference between the maximum bending angle and the subsequent bending angle at which the load is 1 / 2 of the maximum load.

[0127] <Tensile strength> The tensile strength and uniform elongation were measured by taking a JIS No. 5 tensile test piece described in JIS Z 2241:2011 from the steel sheet in a direction perpendicular to the rolling direction and conducting a tensile test in accordance with JIS Z 2241:2011.

[0128] The maximum bending angle was evaluated under the following measurement conditions based on the VDA standard (VDA238-100) specified by the German Association of the Automotive Industry. In the present invention, the displacement at the maximum load obtained in the bending test was converted to an angle according to the VDA standard to determine the maximum bending angle α.

[0129] Test piece dimensions: 60mm x 60mm Bending ridge: Press with a punch so that the bending ridge is perpendicular to the rolling direction Test method: Roll support, punch indentation Roll diameter: φ30mm Punch shape: Tip R = 0.4 mm Distance between rolls: 2.0 x plate thickness (mm) + 0.5 mm Push-in speed: 20mm / min Testing machine: SIMADZU AUTOGRAPH 20kN

[0130] Furthermore, in the load reduction behavior after the maximum load, the bending angle was determined when the load became half of the maximum load.

[0131] In this example, the tensile strength is 1180 MPa or more, and the maximum bending angle is (7.3 t 2 - 37.2t+100) × 1470 × 1700 / (TS × TS) (t: plate thickness (mm), TS: tensile strength (MPa)), and the difference between the maximum bending angle and the bending angle at which the load is half the maximum load thereafter is 6° or more, the properties are considered to be good and the problem of the present invention is solved. The "standard maximum bending angle" in Table 3 is (7.3t 2 -37.2t+100) × 1470 × 1700 / (TS × TS). In addition, the "maximum bending angle judgment" was marked as "○" when the maximum bending angle was greater than the standard maximum bending angle, and marked as "×" when it was equal to or less than the standard maximum bending angle.

[0132] [Table 3]

[0133] Good properties were obtained for invention examples Nos. 1 to 23, 36, and 37. In particular, Nos. 36 and 37 had annealing atmosphere dew points within a preferred range, and had larger maximum bending angles than steel sheets with comparable tensile strengths.

[0134] For No. 24, the average cooling rate in the temperature range from the liquidus temperature to the solidus temperature during casting was low. As a result, Mn segregation in the surface layer was not reduced. Specifically, the value of the wave number k that gives the maximum value of c(k) was below 1.5. This means that the period of the Mn concentration is large, i.e., the interval between solidification segregations is long. Furthermore, the above formula (2) was not satisfied, i.e., the maximum value of c(k) became large. This means that the deviation in Mn concentration became large. As a result, the difference between the maximum bending angle and the bending angle at which the load was 1 / 2 of the subsequent maximum load became small. In other words, it was determined to have been a brittle fracture.

[0135] For No. 25, the insertion temperature into the heating furnace during hot rolling was low. Therefore, the above formula (2) was not satisfied, i.e., the deviation in Mn concentration was large. As a result, the maximum bending angle was small. In addition, the difference between the maximum bending angle and the subsequent bending angle at which the load was half the maximum load was small. In other words, it was determined that the fracture was brittle.

[0136] No. 26 does not satisfy the above formula (4). In other words, the time from the final stage of hot rolling to the start of cooling is long. As a result, alloy distribution progressed, and the above formula (2) was not satisfied. In other words, the deviation in Mn concentration became large. As a result, the difference between the maximum bending angle and the bending angle at which the load was half the maximum load thereafter became small. In other words, it was determined that the fracture was brittle.

[0137] For No. 27, the hot rolling finishing temperature was low. As a result, the transformation of γ (austenite) to ferrite in the surface layer was not sufficiently suppressed, and the above formula (2) was not satisfied. In other words, the Mn concentration deviation increased. As a result, the maximum bending angle decreased. In addition, the difference between the maximum bending angle and the subsequent bending angle at which the load was 1 / 2 of the maximum load decreased. In other words, it was determined that the fracture was brittle.

[0138] For No. 28, the cooling rate from the hot rolling finishing temperature to 750°C was low. As a result, the ferrite transformation start temperature was high, and alloy distribution could not be sufficiently suppressed, so the above formula (2) was not satisfied. In other words, the deviation in Mn concentration could not be suppressed. As a result, the maximum bending angle was small. In addition, the difference between the maximum bending angle and the subsequent bending angle at which the load was 1 / 2 the maximum load was small. In other words, it was determined that the fracture was brittle.

[0139] In No. 29, the cooling rate from 750°C to the coiling temperature was low. As a result, the pearlite transformation start temperature increased, and alloy enrichment in cementite could not be suppressed. This did not satisfy the above formula (2), meaning that the deviation in Mn concentration was not suppressed. As a result, the difference between the maximum bending angle and the bending angle at which the load was half the maximum load thereafter became small. In other words, it was determined that the fracture was brittle.

[0140] No. 30 had a low holding temperature during annealing, which prevented it from obtaining the desired martensite, resulting in low tensile strength and a small maximum bending angle.

[0141] For No. 31, the holding time during annealing was short. As a result, the cementite that had precipitated in the previous process and up to the holding temperature did not dissolve sufficiently, and the above formula (2) was not satisfied. In other words, a deviation in the Mn concentration occurred in the surface layer. As a result, the maximum bending angle was small. In addition, the difference between the maximum bending angle and the bending angle at which the load was half the maximum load thereafter was small. In other words, it was determined that the fracture was brittle.

[0142] No. 32 had a high Si content. This promoted the enrichment of Mn in the iron-based carbides, which did not satisfy the above formula (2). In other words, the deviation in alloy concentration increased. As a result, the difference between the maximum bending angle and the subsequent bending angle at which the load was half the maximum load became smaller. In other words, it was determined that the fracture was brittle.

[0143] No. 33 had a high Al content. Therefore, ferrite formation was promoted in the surface layer before ROT cooling during hot rolling, and the above formula (2) was not satisfied. In other words, the Mn concentration deviation in the surface layer increased. As a result, the difference between the maximum bending angle and the bending angle at which the load was half the maximum load thereafter became smaller. In other words, it was determined that the fracture was brittle.

[0144] No. 34 had a high Mn content. This accelerated Mn enrichment in the iron-based carbides, which did not satisfy the above formula (2), i.e., the deviation in alloy concentration increased. Furthermore, the number of MnS particles in the surface layer was high, which accelerated crack initiation. As a result, the difference between the maximum bending angle and the subsequent bending angle at which the load was half the maximum load became smaller. In other words, it was determined that the fracture was brittle.

[0145] No. 35 has a low Mn content. As a result, the desired martensite fraction was not obtained due to insufficient quenching after annealing, resulting in low tensile strength. [Explanation of symbols]

[0146] 11 Steel plate 12 Profile image acquisition area 21 White Line 31 The point with the lowest Mn concentration 31L The line passing through the point with the lowest Mn concentration 32 Highest Mn concentration point 32L The line passing through the point with the highest Mn concentration

Claims

1. The component composition is, in mass%, C: 0.14-0.35%, Si: 0.01-2.00%, Mn: 1.0-4.0%, Al: 0-0.30%, P: 0.100% or less, S: 0.0500% or less, N: 0.0100% or less, O: 0.0060% or less, Cu: 0 to 1.00%, Ni: 0 to 1.00%, Mo: 0-1.000%, Cr: 0-2.00%, Co: 0 to 0.50%, Ti: 0-0.300%, Nb: 0 to 0.30%, V: 0 to 0.50%, Ta: 0-0.100%, W: 0-1.00%, Ca: 0-0.0400%, Mg: 0 to 0.040%, REM: 0-0.0400%, Zr: 0 to 0.050%, B: 0 to 0.0100%, Sn: 0 to 0.05%, Sb: 0 to 0.05%, and As: 0~0.050% and the balance being Fe and impurities, The metal structure at a depth of 1 / 4 of the plate thickness of the steel plate is, in terms of area ratio, Sum of ferrite and bainite: 0 to 40% Sum of fresh martensite and tempered martensite: 50 to 100% Sum of pearlite and retained austenite: 0 to 10% and In a cross section perpendicular to the thickness direction at a position 50 μm from the surface of the steel plate in the thickness direction, the direction in which the anisotropy of Mn segregation is strongest is the y-axis, The unit of y is mm, and the profile function of the Mn concentration (unit: mass%) in the y direction is set as f(y), and a new function f'(y) is defined as f'(y) = f(y) - [Mn] ([Mn] is the Mn content of the base material expressed in mass%.) and Furthermore, f'(y) is subjected to a two-dimensional discrete Fourier transform to obtain the wave number k (unit: mm -1 ) function F(k) Represents, Let a(k) be the real part of F(k) at wave number k and b(k) be the imaginary part. [Equation 1] When it is determined that The wave number k and the maximum value of c(k) determined by the wave number k, and then for k, k＞1.5 … (1) (Maximum value of c(k)) < -0.0044 [Mn] 2 +0.06[Mn]+0.012 … (2) fulfill A steel plate characterized by:

2. The component composition is, in mass%, O: 0.0001 to 0.0060%, Cu: 0.01 to 1.00%, Ni: 0.01-1.00%, Mo: 0.001 to 1.000%, Cr: 0.01-2.00%, Co: 0.01 to 0.50%, Ti: 0.001 to 0.300%, Nb: 0.01-0.30%, V: 0.01-0.50%, Ta: 0.001 to 0.100%, W: 0.01-1.00%, Ca: 0.0001-0.0400%, Mg: 0.001-0.040%, REM: 0.0001-0.0400%, Zr: 0.001 to 0.050%, B: 0.0001 to 0.0100%, Sn: 0.01-0.05%, Sb: 0.01 to 0.05%, and As: 0.001-0.050% The steel sheet according to claim 1, further comprising one or more selected from the group consisting of:

3. In a cross section parallel to the thickness direction of the steel plate, the metal structure in the range of 50 μm from the surface in the thickness direction is, in terms of area ratio, Ferrite: 50% or more, Sum of one or more of martensite, bainite, pearlite, and retained austenite: 0 to 50% Contains The ratio Hs / Hc of the Vickers hardness (Hs) within a range of 50 μm from the surface in the thickness direction to the Vickers hardness (Hc) at a position of 1 / 4 of the thickness of the plate satisfies Hs / Hc≦0.

65. The steel sheet according to claim 1 or 2.

4. A method for producing the steel sheet according to claim 1 or 2, The chemical composition, in mass%, is: C: 0.14-0.35%, Si: 0.01-2.00%, Mn: 1.0-4.0%, Al: 0-0.30%, P: 0.100% or less, S: 0.0500% or less, N: 0.0100% or less, O: 0.0060% or less, Cu: 0-1.00%, Ni: 0-1.00%, Mo: 0-1.000%, Cr: 0-2.00%, Co: 0-0.50%, Ti: 0-0.300%, Nb: 0-0.30%, V: 0-0.50%, Ta: 0-0. a casting step of casting molten steel containing 1.00% of the elements, W: 0-1.00%, Ca: 0-0.0400%, Mg: 0-0.040%, REM: 0-0.0400%, Zr: 0-0.050%, B: 0-0.0100%, Sn: 0-0.05%, Sb: 0-0.05%, and As: 0-0.050%, with the balance being Fe and impurities, under conditions in which the average cooling rate within a temperature range from the liquidus temperature to the solidus temperature at a depth of 10 mm from the surface of a slab is 10°C / sec or more; A hot rolling process in which the slab cast in the casting process is rolled at a finishing temperature FT of more than 750 ° C. without cooling to less than 500 ° C. and under conditions that satisfy the following formulas (3) and (4): a coiling step of cooling the hot-rolled steel sheet rolled in the hot rolling step from a finishing temperature FT to 750°C at a cooling rate of 40°C / s or more, cooling it to a coiling temperature at a cooling rate of 15°C / s or more, and then coiling it; a cold rolling step of cold rolling the hot-rolled steel sheet after coiling; an annealing step in which the cold-rolled steel sheet rolled in the cold rolling step is heated to a temperature of A3 point −30° C. or more calculated by the following formula (5) and held for 10 seconds or more; A method for manufacturing a steel sheet, comprising: [Equation 2] A3=910-203√C+44.7Si-30Mn+700P-20Cu -15.2Ni-11Cr+31.5Mo+400Ti+104V+120Al …（５） where: [Equation 3] and the element symbol in formula (5) represents the content (mass%) of the element.

5. A method for producing the steel sheet according to claim 3, The chemical composition, in mass%, is: C: 0.14-0.35%, Si: 0.01-2.00%, Mn: 1.0-4.0%, Al: 0-0.30%, P: 0.100% or less, S: 0.0500% or less, N: 0.0100% or less, O: 0.0060% or less, Cu: 0-1.00%, Ni: 0-1.00%, Mo: 0-1.000%, Cr: 0-2.00%, Co: 0-0.50%, Ti: 0-0.300%, Nb: 0-0.30%, V: 0-0.50%, Ta: 0-0. a casting step of casting molten steel containing 1.00% of the elements, W: 0-1.00%, Ca: 0-0.0400%, Mg: 0-0.040%, REM: 0-0.0400%, Zr: 0-0.050%, B: 0-0.0100%, Sn: 0-0.05%, Sb: 0-0.05%, and As: 0-0.050%, with the balance being Fe and impurities, under conditions in which the average cooling rate within a temperature range from the liquidus temperature to the solidus temperature at a depth of 10 mm from the surface of a slab is 10°C / sec or more; a hot rolling step in which the slab cast in the casting step is cooled to 500°C or higher, and then inserted into a hot rolling furnace and rolled under conditions that satisfy the following formulas (3) and (4); a coiling step of cooling the hot-rolled steel sheet rolled in the hot rolling step from a finishing temperature FT to 750°C at a cooling rate of 40°C / s or more, cooling it to a coiling temperature at a cooling rate of 15°C / s or more, and then coiling it; a cold rolling step of cold rolling the hot-rolled steel sheet after coiling; An annealing process in which the cold-rolled steel sheet rolled in the cold rolling process is heated to a temperature of A3 point −30° C. or more calculated by the following formula (5) in an atmosphere having a dew point of −30° C. or more and 20° C. or less, and held for 60 seconds or more; A method for manufacturing a steel sheet, comprising: [Equation 4] A3=910-203√C+44.7Si-30Mn+700P-20Cu -15.2Ni-11Cr+31.5Mo+400Ti+104V+120Al …（５） where: [Equation 5] and the element symbol in formula (5) represents the content (mass%) of the element.

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