Steel sheet and method for manufacturing same

Abstract

The objective of the present invention is to provide a steel sheet having a large maximum bending angle. A steel sheet according to the present invention comprises a predetermined component composition and metal structure. In a cross section perpendicular to the sheet thickness direction, with respect to the Mn concentration profile in a range of 50 μm from the surface in the sheet thickness direction, the y-axis is set along the direction perpendicular to the sheet thickness direction in which the anisotropy of Mn segregation is strongest in a coordinate system. Let f(y) be the profile function of the Mn concentration in the y direction, and define a new function f'(y) as f'(y) = f(y) - [Mn] (where [Mn] is the Mn content of the base material expressed as mass%). Further, the function f'(y) is subjected to a two-dimensional discrete Fourier transform and expressed as a function F(k) of wavenumber k (unit: mm-1), where the real and imaginary parts of F(k) at wave number k are denoted by a(k) and b(k), respectively. Of the wave number k and c(k) determined by the wavenumber k, the k that gives a maximum value and the c(k) at that time satisfy k > 1.5 and (maximum value of c(k)) < -0.0044[Mn]2 + 0.06[Mn] + 0.012.

Description

Steel plate and its manufacturing method

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

[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, which has an average Mn concentration in a surface layer portion extending to a depth of 5 μm from the surface of the steel sheet 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 a V-block method with 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 a cross section in the thickness direction 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 or martensite, thereby promoting the diffusion of Mn via dislocations during annealing heating and homogenizing the Mn concentration in austenite.

[0006] JP 2014-051683 A International Publication No. 2019 / 093429 International Publication No. 2022 / 080497

[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.

[0009] The inventors have found that simply increasing the strength of the base material results in the presence of locally high Mn concentration regions, which embrittle the material, reduce resistance to the propagation of microscopic initial cracks, and promote the growth of the cracks, thereby reducing the absorbed energy until the plate finally breaks. In particular, the decrease in absorbed energy was more pronounced when the deviation in Mn concentration distribution was large in the surface layer, where the amount of strain applied during bending deformation is relatively large compared to the interior. From the above, it was found 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 Mn concentration distribution in the surface layer, and a method for suppressing the deviation in Mn concentration distribution in the surface layer was investigated.

[0010] If Mn concentration deviations occur due to segregation during casting, etc., 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 becoming crack initiation sites during bending. Furthermore, while hard phases have low toughness, a high Mn concentration also embrittles grain boundaries, making the propagation of microcracks more likely. In particular, Mn concentration deviations in the in-plane direction in the surface region of a steel sheet, coupled with the concentration of strain in the surface region during bending, have a significant impact on bending properties. 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 production 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 component composition 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: 0-0.30%, 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%, and in a coordinate system in which the y-axis is the direction in which the anisotropy of Mn segregation is strongest in a cross section perpendicular to the plate thickness direction at a position 50 μm from the surface of the steel plate in the plate thickness direction, 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 a value expressed as mass% of the Mn content of the base material), and further, f'(y) is subjected to a two-dimensional discrete Fourier transform to obtain a 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 the wave number k and the maximum value of c(k) determined by the wave number k are defined, and k at that time, k>1.5 … (1) (maximum value of c(k))<-0.0044[Mn] 2 +0.06[Mn]+0.012 … (2).

[0014] [2] The component composition 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] The steel plate according to [1] or [2], wherein in a cross section of the steel plate parallel to the plate thickness direction, the metal structure in a range of 50 μm from the surface in the plate thickness direction contains, in terms of area ratio, ferrite: 50% or more, and one or more of martensite, bainite, pearlite, and retained austenite: 0 to 50% in total, and the ratio Hs / Hc of the Vickers hardness (Hs) in the range of 50 μm from the surface in the plate thickness direction to the Vickers hardness (Hc) at a position of 1 / 4 of the plate 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 step of casting molten steel containing Cr: 0.050%, 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, 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 the slab is 10°C / sec or more; a hot rolling step of rolling the slab cast in the casting step to a finishing temperature FT of more than 750°C without cooling it to less than 500°C, 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 the finishing temperature FT to 750°C at a cooling rate of 40°C / s or more, and then 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; and an annealing step of heating the cold-rolled steel sheet rolled in the cold rolling step to a temperature of at least A3 point −30°C, which is calculated by the following formula (5), and holding the temperature for 10 seconds or more. A3 = 910 - 203√C + 44.7Si - 30Mn + 700P - 20Cu - 15.2Ni - 11Cr + 31.5Mo + 400Ti + 104V + 120Al ... (5) where, and the element symbol in formula (5) represents the content (mass%) of the element.

[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 heating 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 20°C, and held at that temperature for at least 60 seconds. A3 = 910 - 203√C + 44.7Si - 30Mn + 700P - 20Cu - 15.2Ni - 11Cr + 31.5Mo + 400Ti + 104V + 120Al ... (5) where, and the element symbol in formula (5) represents the content (mass%) of the element.

[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).

[0019] FIG. 1 is a diagram illustrating a procedure for determining a profile function, the maximum c(k), and the k that gives that c(k) in a steel sheet of the present invention. FIG. 2 is a diagram illustrating measurement positions for a Mn concentration profile in a steel sheet of the present invention. FIG. 3 is an example of a Mn concentration distribution in a steel sheet of 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. FIG. 4 is a diagram illustrating the y-axis along which the Mn concentration profile function should be obtained. FIG. 5 is a diagram illustrating an example of a Mn concentration profile function.

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

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

[0022] (C: 0.14 to 0.35%) C is an element that improves the hardenability of steel plate. 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 improving bake hardenability. In order to effectively exert the above 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 sheet. The Si content is 0.01 to 2.00%. The lower limit of Si may be 0.05%, 0.10%, 0.15%, or 0.20%. Si is an element that not only deteriorates weldability and chemical conversion treatability, but also promotes Mn concentration in iron-based carbides and increases the deviation in alloy concentration, so the upper limit of the 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 sheet. 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 related to 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 on the steel sheet. On the other hand, if the Mn content is high, MnS is more likely to be generated, making cracks more likely 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 steel sheet that is the final 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 as an impurity in steel. The lower the P content, the better, and it may 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 as an impurity in steel. The lower the S content, the better, and it may be 0. From the viewpoint of weldability and a decrease in low-temperature toughness due to an increase in the amount of MnS precipitated, the S content is set to 0.0500% or less. 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 as an impurity in steel. The lower the N content, the better, and it may 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 as an impurity in steel. 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. 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 0. In order 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 controlling the morphology of carbides and improving strength. 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 0. In order to fully obtain the effect 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 finely dispersing inclusions and enhance toughness. Here, REM refers to Sc, Y, and lanthanoids, a total of 17 elements, 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 necessary. The inclusion of these elements is not essential, and the content of these elements may be 0. In order to fully obtain the effect 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 to 0.05%, Sb: 0 to 0.05%, As: 0 to 0.050%) Sn, Sb, and As are elements that can be contained in 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 a decrease in cold formability, the contents of Sn and Sb are preferably 0.05% or less, and the As content is preferably 0.050% or less. Since reducing the contents of Sn, Sb, and As is costly, the lower limits of the Sn and Sb contents may be 0.001% and 0.01%, respectively, and the lower limit of the As content may be 0.0001% and 0.001%. The upper limits of the Sn and Sb contents may be 0.04% or less, and 0.03% or less, and the upper limits 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.

[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, "martensite" is used as a term that includes "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 Sheet Thickness> In the present invention, the metal structure at a depth of 1 / 4 of the sheet thickness from the surface in the sheet thickness direction has, in area percentages, a total of ferrite and bainite: 0 to 40%, martensite: 50 to 100%, and a total of pearlite and retained austenite: 0 to 10%. 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) in the case of a plated steel sheet having no coating layer, and to the outermost surface of the steel sheet in the case of a steel sheet having no 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, the observation region for measuring the area fraction of the metallographic structure at a depth of 1 / 4 of the plate thickness will be described. The observation region is a cross section parallel to the rolling direction and the plate thickness direction, and is a 100 μm × 100 μm square region with sides of 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 this 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 by the method defined below, and the arithmetic mean value thereof 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] (Retained Austenite) The area and area fraction of retained austenite are measured by the following method. That is, for the above-mentioned specified observation region, the observation surface is finished by colloidal silica polishing or electrolytic polishing, and diffracted electrons are measured at intervals of 0.2 μm (lattice arrangement) using an EBSD attached to a scanning electron microscope. The obtained pseudo-Kikuchi pattern is analyzed to identify the crystal orientation and crystal system. Note that the sample preparation conditions are within the range of conditions recommended in the Japan Society for Materials Science standard "Crystal Orientation Misalignment Measurement Standard for Material Evaluation by Electron Backscatter Diffraction (EBSD) Method." The region detected as an FCC phase from the measurement data is considered to be 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: the same square region as that used for the observation of retained austenite above is etched with a nital solution, photographed (magnification: 5000x) with a field emission scanning electron microscope (FE-SEM), and the ratio of each structure obtained from the obtained structural photograph by a point counting method at intervals of 2 μm (grid arrangement) is taken as the area ratio.

[0044] Each organization will make the following determinations:

[0045] (Ferrite and Bainite) Ferrite has a granular or acicular 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 are present inside the lath structure and the carbides are elongated in the same direction.

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

[0047] (Pearlite) 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] When the total area ratio of each structure obtained by the above evaluation method is different from 100%, the area ratio of each structure is multiplied by 100 / (total area ratio of each structure), and the value obtained is used as the area ratio of each structure.

[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, a 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 Mn concentration profile function is a discrete function that indicates the change in Mn concentration measured along a certain straight line, as will be described below.

[0052] Specifically, in a cross section perpendicular to the thickness direction at a position 50 μm from the surface of the steel plate, the y-axis is set to the direction in which the anisotropy of Mn segregation is strongest, 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] (where [Mn] is the Mn content of the base material expressed in mass%).

[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 a(k) and the imaginary part is b(k), It is stipulated that:

[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) is satisfied.

[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 brightest line in the resulting 2D-DFT image is determined. In the Mn concentration measurement region, the direction parallel to this 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 inventors have made this invention based on their empirical discovery 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] 2 is a diagram illustrating measurement positions on a steel plate. A 2.0 mm × 2.0 mm region located 50 μm from the surface of the steel plate 11 in the plate thickness direction (the direction of the x-axis in FIG. 2 ) and perpendicular to the plate thickness direction (parallel to the rolling and plate width direction) is defined as the Mn concentration profile image acquisition region 12. Even if the rolling direction of the steel plate to be measured is unknown, the direction in which Mn segregation is greatest is determined as the y direction by the method described below. Therefore, it is not necessary to specify the y direction at this stage. The orientation of the 2.0 mm × 2.0 mm region does not matter as long as it is perpendicular to the plate thickness direction (parallel to the rolling and plate width direction).

[0058] Next, the two-dimensional distribution of Mn concentration in the profile image acquisition region 12 is determined by area analysis using an EPMA (electron probe microanalyzer). Figure 3(a) shows an example of the determined 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 (shape) 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 in which Mn segregation is greatest. Since the y-direction is sufficient here, contrast adjustment may be performed before binarization to further emphasize the segregation. In EPMA area analysis, measurements are performed 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] The method for obtaining a Fourier-transformed image from an EPMA image is not particularly limited, as long as the direction of the white lines can be determined. For example, the image obtained by EPMA surface analysis is converted to grayscale (8-bit display) using image analysis software ImageJ. The image is then binarized so that the concentration deviation is emphasized. Since the direction of the white lines does not change regardless of how the binarization threshold is set, it can be adjusted appropriately so that the white lines are most emphasized. 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 a line drawn through the point with the lowest Mn concentration in the Mn concentration distribution obtained by EPMA, and this line is used as the Mn concentration profile function. 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. That is, at least two Mn concentration profile functions are obtained, and 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) may 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 ) is expressed as a function F(k), and c(k) is found. Here, there will be two or more values ​​of f'(y), but the largest value of c(k) found 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 Mn enrichment 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 change associated with solidification is hardly affected by high temperatures or processing, and therefore the fluctuation period (the size of the structure that causes the distribution) is long, on the order of 0.1 mm to several tens of mm. On the other hand, the concentration change associated with transformation is short, on the order of several μm to several tens of μm, due to the relatively low temperature range associated with 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 change associated with transformation is relatively small because the steel sheet manufacturing process is a continuous process (continuous hot rolling, continuous annealing), and the time over which element distribution occurs is short.

[0065] The present inventors have focused on and investigated the influence 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 defining 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 factors of morphological size, as mentioned above, it is thought that the concentration change on the order of μm, which is mainly related to transformation, is small in magnitude itself, and therefore has little effect on bendability.

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

[0069] It is believed that the combination of these factors makes it important to define the long-period variation in Mn concentration in the surface layer region (50 μm from the surface in the sheet thickness direction) as defined in the present invention. 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 including normal hot rolling and cold rolling, the sheet is elongated by at least about 100 times in the rolling direction, so long-period deviations in 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 Mn concentration reflecting solidification segregation are likely to remain. Therefore, the concentration distribution is generally the distribution in the sheet width direction, and the y direction defined as above corresponds to this direction. If it can be confirmed that the Mn concentration profile functions on the line passing through the coordinates giving the maximum Mn concentration along the y direction and the line passing through the coordinates giving the minimum Mn concentration satisfy Equation (1) and Equation (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. Considering 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 (direction perpendicular to the rolling direction) of the coil in the steel sheet (final product) that has undergone hot rolling and cold rolling. Because traces of concentration changes during solidification are stretched in the rolling direction by more than 100 times, even if the concentration change 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 a long-period concentration change. 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 order to confirm the applicability of the present technology even in situations where the rolling direction cannot be confirmed in commercially available steel sheet pieces, in the present invention, 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 will become hard phases, and areas with low Mn concentrations will become soft phases, resulting in a deviation in hardness and making these areas 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 small deviation in hardness and good bendability.

[0073] (Metal structure in a range of 50 μm from the surface) In the steel sheet of the present invention, the structure in a range of 50 μm from the surface is not particularly limited. For example, it may be the same as the metal structure at a depth of ¼ of the sheet thickness described above.

[0074] The steel sheet of the present invention described above has excellent bendability, which is achieved by controlling the Mn concentration distribution in the surface layer. However, to further improve bendability, the metallographic structure in a region 50 μm from the surface may be controlled. Specifically, within the region 50 μm from the surface, the metallographic structure in a 100 μm square region of a cross section parallel to the rolling direction and the sheet thickness direction may contain, in terms of area ratio, ferrite: 50% or more and one or more of martensite, bainite, pearlite, and retained austenite: 0 to 50% in total. 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 achieving such a surface metallographic structure, 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 a 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 a 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 sheet 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: 60 mm x 60 mm Bending ridge: Pressed with a punch so that the bending ridge was perpendicular to the rolling direction Test method: Roll support, punch pressing Roll diameter: φ30 mm Punch shape: Tip R = 0.4 mm Distance between rolls: 2.0 x plate thickness (mm) + 0.5 mm Pressing speed: 20 mm / min Testing machine: SIMADZU AUTOGRAPH 20 kN

[0080] Furthermore, in the load reduction behavior after the maximum load, the bending angle when the load becomes half of the maximum load is determined. 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 it can be determined that the maximum bending angle and the subsequent rapid load reduction have not been suppressed.

[0081] In the steel plate of the present invention, the tensile strength is 1180 MPa or more, the maximum bending angle is (7.3t 2 It is preferable that 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. 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 plate 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 manufacturing the steel sheet of 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. The cooling rate of the slab is slow due to its thick plate thickness. During cooling, transformation proceeds at a transformation temperature corresponding to the alloy concentration at each location, resulting in 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 to 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, to suppress excessive scale loss, it is preferable to set the temperature at 1350°C or lower. Furthermore, holding the slab at approximately 1250-1350°C for approximately 5-25 hours can also serve the purpose of homogenization treatment. This promotes alloy diffusion, makes the distribution of Mn more uniform, and suppresses microsegregation. The heating holding time is preferably 15 hours or less, more preferably 10 hours or less, to suppress excessive scale loss.

[0088] "Hot rolling process"

[0089] [Rough Rolling] In the present 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, or the shape of the slab may become non-uniform, resulting in a decrease in the dimensional accuracy of the hot-rolled steel sheet obtained by hot rolling. 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] [Finish Rolling] Finish rolling is performed so that the final finishing temperature FT exceeds 750°C and the following formula (3) is satisfied.

[0092]

[0093] where: is.

[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 temperature, ferrite transformation is promoted, and Mn concentrates in 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 immediately after holding the material in a temperature range equal to or higher than the A3 point (γ single-phase region) for a sufficient time, the surface layer is rapidly cooled by ROT cooling. 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 a 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 deviation in 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 at 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, 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] "Coiling step" 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 oxidation of the surface layer, 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 Step" The steel sheet is then cold rolled to obtain a cold rolled steel sheet. The cold rolling conditions are not particularly limited and may be any of the usual methods. 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 -30°C and annealed at that temperature for at least 10 seconds. The temperature is preferably increased from 400°C to the A1 point at a heating rate of at least 15°C / s. This suppresses the concentration of alloys in cementite and allows for a uniform distribution of Mn. By setting 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 do not sufficiently dissolve the cementite precipitated in the previous process and up to the holding temperature, resulting in a localized distribution of Mn in the surface layer. The holding time may be 400 seconds or less. Following the heating in the annealing step, the cold-rolled steel sheet may be cooled, held, quenched, or tempered. It may also be hot-dip galvanized or alloyed.

[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 step 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 from the A3 point, is set to 60 seconds or more, decarburization of the surface layer of the steel sheet during annealing can be advanced, and the formation of ferrite in the surface layer of the steel sheet can be promoted at the maximum heating temperature and during subsequent cooling, and the area ratio of ferrite in the surface layer portion 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 a 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.

[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] The cast slab or the homogenized slab was then 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 formula (3)" in "hot rolling" mean the terms described in the above formulas (3) and (4) and the explanation of each parameter. FT means the final finishing temperature, and "formula (3) judgment" is marked with "○" when formula (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 a 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 a rolling reduction shown in Table 2 to obtain a cold-rolled steel sheet.

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

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

[0121]

[0122]

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

[0124] In Table 3, "α + B," "P + γ," and "FM + TM" respectively indicate 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" indicates the area ratio of ferrite in the surface layer, and "surface layer M + B + P + γ" indicates 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 structure was performed on the metal structure in the sheet thickness direction 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 where the steel sheet is a plated steel sheet having a coating layer, and refers to the outermost surface of the steel sheet in the case where the steel sheet does not have a coating layer. In the examples, for steel sheets that had been plated and alloyed, the quantitative evaluation of the structure was performed after removing the coating layer.

[0125] In Table 3, "wave number k giving the maximum value of c(k)," "right side of formula (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 formula (2): (maximum value of c(k))<-0.0044[Mn] 2 If the value of +0.06[Mn]+0.012 was satisfied, it was marked as "○", and if it was not satisfied, 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 a load equals 1 / 2 of the maximum load is reached.

[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 into an angle according to the VDA standard to determine the maximum bending angle α.

[0129] Test piece dimensions: 60 mm x 60 mm Bending ridge: Pressed with a punch so that the bending ridge was perpendicular to the rolling direction Test method: Roll support, punch pressing Roll diameter: φ30 mm Punch shape: Tip R = 0.4 mm Distance between rolls: 2.0 x plate thickness (mm) + 0.5 mm Pressing speed: 20 mm / min Testing machine: SIMADZU AUTOGRAPH 20 kN

[0130] Furthermore, in the load reduction behavior after the maximum load, the bending angle was determined when the load became half 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. 2 -37.2t + 100) × 1470 × 1700 / (TS × TS). In addition, the "maximum bending angle judgment" was rated as "○" when the maximum bending angle was greater than the standard maximum bending angle, and "×" when it was equal to or less than the standard maximum bending angle.

[0132]

[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] No. 24 had a low average cooling rate within the temperature range from the liquidus temperature to the solidus temperature during casting. As a result, Mn segregation in the surface layer was not reduced. Specifically, the wave number k, which gives the maximum value of c(k), was below 1.5. This means that the Mn concentration cycle was large, i.e., the interval between solidification segregations was 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] In No. 25, the insertion temperature into the heating furnace during hot rolling was low. Therefore, the above formula (2) was not satisfied, that is, the deviation of the Mn concentration was large. 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.

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

[0137] In No. 27, the hot rolling finishing temperature was low. Therefore, 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 was large. 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.

[0138] In 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 elevated, and alloy partitioning could not be sufficiently suppressed, failing to satisfy the above formula (2). In other words, the deviation in Mn concentration could not be suppressed. As a result, the maximum bending angle was small. Furthermore, the difference between the maximum bending angle and the bending angle at which the load was 1 / 2 the maximum load thereafter 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. Therefore, the pearlite transformation start temperature was high, and alloy enrichment in cementite could not be suppressed, so the above formula (2) was not satisfied. In other words, 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 1 / 2 the maximum load thereafter became small. In other words, it was determined that the fracture was brittle.

[0140] In No. 30, the holding temperature during annealing was low, so the desired martensite was not obtained, resulting in low tensile strength and a small maximum bending angle.

[0141] In No. 31, the holding time during annealing was short. Therefore, the cementite precipitated in the previous process and up to the holding temperature did not dissolve sufficiently, and the above formula (2) was not satisfied. That is, 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. Therefore, Mn enrichment in the iron-based carbides was promoted, and the above formula (2) was not satisfied. In other words, the deviation in alloy concentration 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.

[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. Therefore, Mn enrichment in the iron-based carbides was promoted, and the above formula (2) was not satisfied. In other words, the deviation in alloy concentration was increased. In addition, the number of MnS in the surface layer was large, which promoted crack initiation. 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.

[0145] No. 35 had a low Mn content, which resulted in insufficient quenching after annealing, making it impossible to obtain the desired martensite fraction, resulting in low tensile strength.

[0146] 11 Steel plate 12 Profile image acquisition area 21 White line 31 Point of lowest Mn concentration 31L Straight line passing through the point of lowest Mn concentration 32 Point of highest Mn concentration 32L Straight line passing through the point of highest Mn concentration

Claims

1. The chemical composition, in mass%, is: 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 0.50%, Ta: 0 to 0.100%, W: 0 to 1.00%, A steel plate containing Ca: 0 to 0.0400%, Mg: 0 to 0.040%, REM: 0 to 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 to 0.050%, with the balance being Fe and impurities, wherein a metallographic 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%, and wherein a coordinate system in which the direction in which the anisotropy of Mn segregation is strongest is the y-axis in a cross section perpendicular to the plate thickness direction at a position 50 μm from the surface of the steel plate in the plate thickness direction, The unit of y is mm, and the profile function of the Mn concentration (unit: mass%) in the y direction is defined as f(y). 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%). Furthermore, 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 the wave number k and the maximum value of c(k) determined by the wave number k are defined, and k is then k>1.5 … (1) (maximum value of c(k))<-0.0044[Mn] 2 +0.06[Mn]+0.012 … (2).

2. The chemical composition 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%, The steel plate according to claim 1, characterized in that it contains one or more selected from the group consisting of 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%.

3. The steel plate according to claim 1 or 2, characterized in that in a cross section of the steel plate parallel to the thickness direction, the metal structure within a range of 50 μm from the surface in the thickness direction contains, in area ratios, ferrite: 50% or more, and one or more of martensite, bainite, pearlite, and retained austenite: 0 to 50% in total, and 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 1 / 4 of the thickness satisfies Hs / Hc≦0.

65.

4. A method for producing the steel sheet according to claim 1 or 2, wherein the chemical 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: 0-1.00%, 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 of rolling the slab cast in the casting step to a finishing temperature FT of more than 750°C without cooling it to less than 500°C, 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 the finishing temperature FT to 750°C at a cooling rate of 40°C / s or more, and then 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; and an annealing step of heating the cold-rolled steel sheet rolled in the cold rolling step to a temperature of at least A3 point −30°C, which is calculated by the following formula (5), and holding the temperature for 10 seconds or more. A3 = 910 - 203√C + 44.7Si - 30Mn + 700P - 20Cu - 15.2Ni - 11Cr + 31.5Mo + 400Ti + 104V + 120Al ... (5) where, 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, wherein the chemical 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: 0-1.00%, 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 of cooling the slab cast in the casting step to 500°C or higher, and then inserting it into a hot rolling heating furnace, and rolling it 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, and then 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 coiled hot-rolled steel sheet; and an annealing step of heating the cold-rolled steel sheet rolled in the cold rolling step 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 holding the temperature for 60 seconds or more. A3 = 910 - 203√C + 44.7Si - 30Mn + 700P - 20Cu - 15.2Ni - 11Cr + 31.5Mo + 400Ti + 104V + 120Al ... (5) where, and the element symbol in formula (5) represents the content (mass%) of the element.

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