steel sheet

Abstract

The steel sheet has a prescribed chemical composition, and when the sheet thickness is set to t, the metal structure at a position of t / 4 from the surface in the sheet thickness direction section includes, in terms of volume fraction: martensite: 70% or more, residual austenite: 10% or more, the maximum particle diameter of the residual austenite is less than 5.0 μm, in a square region of the sheet thickness direction section having the position of t / 4 as the center and a length of t / 4 on one side, when the Mn concentration is measured at a plurality of measurement points at intervals of 1 μm, the proportion of measurement points where the Mn concentration is 1.1 times or more the average of the Mn concentrations of all the plurality of measurement points is less than 10.0%, and the tensile strength of the steel sheet is 1470 MPa or more.

Description

TECHNICAL FIELD

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

[0002] This application claims priority from Japanese Patent Application No. 2021-051257 filed on March 25, 2021, the contents of which are incorporated herein by reference. BACKGROUND

[0003] In recent years, from the viewpoint of the restriction of the amount of greenhouse gas emissions in line with measures against global warming, there is a demand for improvement in the fuel efficiency of automobiles, and in order to ensure the lightness of the vehicle body and the safety in the event of a collision, the application of high-strength steel sheets is gradually expanding.

[0004] For high-strength steel sheets for automobile parts, not only strength but also characteristics required for part forming such as press formability are required. There is generally a trade-off relationship between strength and press formability, but as a steel sheet excellent in both strength and press formability, a TRIP steel sheet (TRansformation Induced Plasticity) utilizing transformation induced plasticity of retained austenite is known.

[0005] For example, in Patent Documents 1 and 2, a technology relating to a high-strength TRIP steel sheet is disclosed, which improves elongation and hole expansion ratio by controlling the volume fraction of the structure to a prescribed range.

[0006] PRIOR ART DOCUMENTS

[0007] PATENT DOCUMENTS

[0008] Patent Document 1: International Publication No. 2013 / 051238

[0009] Patent Document 2: Japanese Patent No. 4445365 SUMMARY

[0010] PROBLEMS TO BE SOLVED BY THE INVENTION

[0011] In the field of automobiles, when high-strength steel sheets are applied, in particular, recently, there is an increasing demand for high-strength steel sheets having a tensile strength of 1470 MPa or more and excellent formability. However, the following problem has been a problem for TRIP steels having a tensile strength of 1470 MPa or more: the strength of the welded joint when welding is performed is lowered.

[0012] With regard to such a problem, although Patent Documents 1 and 2 relate to high-strength TRIP steel sheets, they do not show that a tensile strength of 1470 MPa or more can be obtained, and there is no consideration of the joint strength in a TRIP steel sheet having a tensile strength of 1470 MPa or more.

[0013] Thus, there has been no proposal for improving the strength of a welded joint of a TRIP steel having a tensile strength of 1470 MPa or more.

[0014] The present application has an object to provide a steel sheet having a tensile strength of 1470 MPa or more and excellent formability, and having a sufficient strength of a welded joint.

[0015] Means for solving the problem

[0016] The present inventors have studied the reason why a sufficient strength of a welded joint cannot be obtained in a TRIP steel sheet having a tensile strength of 1470 MPa or more.

[0017] As a result, it was found that if coarse residual austenite or primary martensite exists in a weld heat affected portion, cracking easily occurs from them. The present inventors have further studied, and as a result, it was recognized that in order to suppress such cracking, the refinement of residual austenite is effective, and for this purpose, the suppression of Mn segregation is effective.

[0018] The present application has been made in view of the above problem. The gist of the present application is as follows.

[0019] [1] A steel sheet according to one aspect of the present application has the following chemical composition:

[0020] C: 0.20% to 0.45%, Si: 0.50% to 2.50%, Mn: 1.50% to 3.50%, Al: 0.005% to 1.500%, P: 0% to 0.040%, S: 0% to 0.010%, N: 0% to 0.0100%, O: 0% to 0.0060%, Cr: 0% to 0.50%, Ni: 0% to 1.00%, Cu: 0% to 1.00%, Mo: 0% to 0.50%, Ti: 0% to 0.200%, Nb: 0% to 0.200%, V: 0% to 0.500%, B: 0% to 0.0100%, W: 0% to 0.1000%, Ta: 0% to 0.1000%, Sn: 0% to 0.0500%, Co: 0% to 0.5000%, Sb: 0% to 0.0500%, As: 0% to 0.0500%, Mg: 0% to 0.0500%, Ca: 0% to 0.0400%, Y: 0% to 0.0500%, La: 0% to 0.0500%, Ce: 0% to 0.0500%, Zr: 0% to 0.0500%, and the remainder: containing Fe and impurities, when a plate thickness is set to t, a metal structure at a position of t / 4 from a surface in a plate thickness direction section, that is, a t / 4 position, contains, in terms of a volume rate, martensite: 70% or more, residual austenite: 10% or more, the residual austenite has a maximum particle diameter of less than 5.0 μm, when a Mn concentration is measured at a plurality of measurement points at intervals of 1 μm in a square region of the plate thickness direction section having the t / 4 position as a center and a length of one side of t / 4, a proportion of measurement points where the Mn concentration is 1.1 times or more of an average value of the Mn concentration of all of the plurality of measurement points is less than 10.0%, and the steel plate has a tensile strength of 1470 MPa or more.

[0021] [2] The steel sheet according to the above-mentioned [1], wherein the chemical composition can also contain one or more elements selected from the group consisting of Cr: 0.01 to 0.50%, Ni: 0.01 to 1.00%, Cu: 0.01 to 1.00%, Mo: 0.01 to 0.50%, Ti: 0.001 to 0.200%, Nb: 0.001 to 0.200%, V: 0.001 to 0.500%, B: 0.0001 to 0.0100%, W: 0.0005 to 0.1000%, Ta: 0.0005 to 0.1000%, Sn: 0.0010 to 0.0500%, Co: 0.0010 to 0.5000%, Sb: 0.0010 to 0.0500%, As: 0.0010 to 0.0500%, Mg: 0.0001 to 0.0500%, Ca: 0.0001 to 0.0400%, Y: 0.0001 to 0.0500%, La: 0.0001 to 0.0500%, Ce: 0.0001 to 0.0500%, and Zr: 0.0001 to 0.0500% in mass%.

[0022] [3] The steel sheet according to the above-mentioned [1] or [2], which can also have a hot-dip galvanized layer on the surface.

[0023] [4] The steel sheet according to the above-mentioned [3], wherein the hot-dip galvanized layer can also be an alloyed hot-dip galvanized layer.

[0024] Effects of Invention

[0025] According to the above-mentioned aspect of the present application, a steel sheet having a tensile strength of 1470 MPa or more, excellent formability, and sufficient strength of a welded joint can be provided. BRIEF DESCRIPTION OF DRAWINGS

[0026] Figure 1 is a view illustrating an observation area of a structure of a cross section in a sheet thickness direction and a measurement area of a Mn concentration. DETAILED DESCRIPTION

[0027] As for the steel sheet of one embodiment of the present application (steel sheet of this embodiment), (a) has a prescribed chemical composition, (b) when the sheet thickness is set to t, the metal structure at a position at a distance of t / 4 from the surface in the sheet thickness direction cross section (t / 4 position) contains, in terms of volume fraction, martensite: 70% or more, and retained austenite: 10% or more, (c) the maximum particle diameter of the retained austenite is less than 5.0 μm, (d) in a square region of the above-described sheet thickness direction cross section having the above-described t / 4 position as the center and a length of t / 4 on one side, when the Mn concentration is measured at a plurality of measurement points at intervals of 1 μm, the proportion of measurement points where the Mn concentration is 1.1 times or more the average value of the Mn concentrations of all the above-described plurality of measurement points is less than 10.0%, and (e) the tensile strength of the steel sheet is 1470 MPa or more.

[0028] Hereinafter, each will be described.

[0029] <Chemical composition>

[0030] The chemical composition of the steel sheet of this embodiment will be described. The "%" of the content of each element indicates "mass %" unless otherwise specified.

[0031] C: 0.20% to 0.45%

[0032] C (carbon) is an essential element for securing the strength of the steel sheet. By setting the C content to 0.20% or more, a desired high strength can be obtained. The C content can be 0.21% or more or 0.22% or more.

[0033] On the other hand, in order to secure the workability and weldability, the C content is set to 0.45% or less. The C content can be 0.42% or less, 0.40% or less, or 0.38% or less.

[0034] Si: 0.50% to 2.50%

[0035] Si (silicon) is an element useful for industrially producing a TRIP steel sheet. By containing Si, the generation of iron carbide in austenite after the increase in C concentration can be suppressed, and stable retained austenite can be obtained even at room temperature. In order to obtain this effect, the Si content is set to 0.50% or more.

[0036] On the other hand, in order to secure the weldability of the steel sheet, the Si content is set to 2.50% or less. The Si content can be 2.40% or less, 2.20% or less, or 2.00% or less.

[0037] Mn: 1.50% to 3.50%

[0038] Mn (manganese) is a strong austenite stabilizing element and is an effective element for high strength of the steel sheet. In order to obtain these effects, the Mn content is set to 1.50% or more. The Mn content can be 1.60% or more or 1.70% or more. In addition, in order to ensure weldability, low-temperature toughness, the Mn content is set to 3.50% or less. The Mn content can be 3.40% or less, 3.20% or less, or 3.00% or less.

[0039] Al: 0.005% to 1.500%

[0040] Al (aluminum) is an element for deoxidization of the steel and, like Si, is an effective element for suppressing generation of iron carbide and for leaving residual austenite. Therefore, the Al content is set to 0.005% or more.

[0041] On the other hand, even if Al is contained in excess, the effect is saturated, not only the cost increases, but also the phase transformation temperature of the steel increases and the load at hot rolling increases. Therefore, the Al content is set to 1.500% or less. The Al content is preferably 1.200% or less, 1.000% or less, or 0.800% or less.

[0042] P: 0% to 0.040%

[0043] P (phosphorus) is a solid solution strengthening element and is an effective element for high strength of the steel sheet, but excessive content deteriorates weldability and toughness. Therefore, the P content is set to 0.040% or less. The P content is preferably 0.035% or less, 0.030% or less, or 0.020% or less. The P content can also be 0%, but in order to extremely reduce the P content, the dephosphorization cost becomes high. Therefore, from the viewpoint of economy, the P content can be set to 0.001% or more.

[0044] S: 0% to 0.010%

[0045] S (sulfur) is an element contained as an impurity and is an element that deteriorates toughness and hole expandability by forming MnS in the steel. Therefore, as a range in which deterioration of toughness and hole expandability is not significant, the S content is set to 0.010% or less. The S content is preferably 0.005% or less, 0.004% or less, or 0.003% or less. The S content can also be 0%, but in order to extremely reduce the S content, the desulfurization cost becomes high. Therefore, from the viewpoint of economy, the S content can be set to 0.0001% or more or 0.001% or more.

[0046] N: 0% to 0.0100%

[0047] N (nitrogen) is an element contained as an impurity, and is an element that, if its content exceeds 0.0100%, forms coarse nitrides in the steel, degrading bendability and hole expandability. Therefore, the N content is set to 0.0100% or less. The N content is preferably 0.0080% or less, 0.0060% or less, or 0.0050% or less. The N content can also be 0%, but in order to extremely reduce the N content, the N removal cost becomes high. Therefore, from the viewpoint of economy, the N content can be set to 0.0001% or more.

[0048] O: 0% to 0.0060%

[0049] O (oxygen) is an element contained as an impurity, and is an element that, if its content exceeds 0.0060%, forms coarse oxides in the steel, degrading bendability and hole expandability. Therefore, the O content is set to 0.0060% or less. The O content is preferably 0.0050% or less, or 0.0040% or less. The O content can also be 0%, but from the viewpoint of manufacturing cost, the O content can be set to 0.0001% or more.

[0050] The basic chemical composition of the steel sheet of the present embodiment contains the above-described elements (basic elements), and the remainder contains Fe and impurities. Here, the "impurities" refer to components that are mixed due to various reasons of the raw materials such as ores, waste materials, and manufacturing processes, and can be allowed within a range that does not adversely affect the present application.

[0051] However, the steel sheet can also contain the following elements (optional elements) instead of a part of Fe, as needed. These elements can also not necessarily be contained, and thus the lower limit is 0%. In addition, the following elements are sometimes mixed from the waste materials of the raw materials and the like, but as long as the content is the upper limit value described later or less, they can also be contained as impurities.

[0052] Cr: 0% to 0.50%

[0053] Ni: 0% to 1.00%

[0054] Cu: 0% to 1.00%

[0055] Cr (chromium), Ni (nickel), and Cu (copper) are all elements that contribute to an increase in strength. Therefore, one or more selected from among these elements can be contained as needed. In the case where the above-described effects are desired, the content of one or more selected from among Cr, Ni, and Cu is preferably 0.01% or more, and more preferably 0.10% or more.

[0056] On the other hand, Cr content exceeding 0.50%, Ni content exceeding 1.00%, or Cu content exceeding 1.00% can possibly reduce the pickling property, weldability, and hot workability. Therefore, the Cr content is set to 0.50% or less, the Ni content is set to 1.00% or less, and the Cu content is set to 1.00% or less. The Cr content can be 0.40% or less, 0.30% or less, or 0.10% or less. The Ni content can be 0.80% or less, 0.60% or less, or 0.20% or less. The Cu content can be 0.80% or less, 0.60% or less, or 0.20% or less.

[0057] Mo: 0% to 0.50%

[0058] Mo (molybdenum) is an element that, like Mn, improves the hardenability of the steel and contributes to an increase in strength. Therefore, Mo can be contained as needed. In the case where the above-described effects are desired, the Mo content is preferably 0.01% or more, and more preferably 0.10% or more.

[0059] On the other hand, if the Mo content exceeds 0.50%, the hot workability can possibly be reduced, and the productivity can possibly be reduced. Therefore, the Mo content is set to 0.50% or less. The Mo content is preferably 0.40% or less, 0.30% or less, or 0.10% or less.

[0060] Ti: 0% to 0.200%

[0061] Nb: 0% to 0.200%

[0062] V: 0% to 0.500%

[0063] Ti (titanium), Nb (nickel), and V (vanadium) are all elements that contribute to an increase in the strength of the steel sheet through precipitation strengthening, fine-grain strengthening due to growth inhibition of grains, and dislocation strengthening due to inhibition of recrystallization. Therefore, one or more selected from among these elements can be contained as needed. In the case where the above-described effects are desired, it is preferable to contain one or more selected from among 0.001% or more of Ti, 0.0001% or more of Nb, and 0.001% or more of V in the steel sheet.

[0064] On the other hand, Ti exceeding 0.200%, Nb exceeding 0.200% or V exceeding 0.500% can possibly cause precipitation of coarse carbonitride, resulting in deterioration of formability. Therefore, the Ti content is set to 0.200% or less, the Nb content is set to 0.200% or less and the V content is set to 0.500% or less. The Ti content can be set to 0.180% or less, 0.150% or less or 0.100% or less. The Nb content can be set to 0.180% or less, 0.150% or less or 0.100% or less. The V content can be set to 0.400% or less, 0.300% or less or 0.100% or less.

[0065] B: 0% to 0.0100%

[0066] B (boron) is an element that segregates at the grain boundaries of austenite at the time of welding, strengthens the grain boundaries and contributes to improvement of resistance to brittle cracking of molten metal. Therefore, B can be contained as necessary. In the case where the above-mentioned effects are desired, the B content is preferably 0.0001% or more, more preferably 0.0005% or more or 0.0008% or more.

[0067] On the other hand, if the B content exceeds 0.0100%, carbides and nitrides are generated, the above-mentioned effects are saturated and hot workability is deteriorated. Therefore, the B content is set to 0.0100% or less. The B content is preferably 0.0080% or less, 0.0050% or less or 0.0030% or less.

[0068] W: 0% to 0.1000%

[0069] Ta: 0% to 0.1000%

[0070] Sn: 0% to 0.0500%

[0071] Co: 0% to 0.5000%

[0072] As: 0% to 0.0500%

[0073] W (tungsten), Ta (tantalum), Sn (tin), Co (cobalt), and As (arsenic) are elements that contribute to an increase in the strength of the steel sheet by precipitation strengthening and suppression of grain coarsening. Therefore, these elements can be contained. In the case where an effect is to be obtained, the W content can be set to 0.0005% or more, 0.0010% or more, 0.0050% or more, or 0.0100% or more. The Ta content can be set to 0.0005% or more, 0.0010% or more, 0.0050% or more, or 0.0100% or more. The Sn content can be set to 0.0010% or more, 0.0020% or more, or 0.0050% or more. The Co content can be set to 0.0010% or more, 0.0100% or more, or 0.0300% or more. The As content can be set to 0.0010% or more, 0.0020% or more, or 0.0050% or more.

[0074] On the other hand, if the above elements are contained in a large amount, it is possible that various properties of the steel sheet will be impaired. Therefore, the W content is set to 0.1000% or less, the Ta content is set to 0.1000% or less, the Sn content is set to 0.0500% or less, the Co content is set to 0.5000% or less, and the As content is set to 0.0500% or less. The W content can be set to 0.0800% or less, 0.0500% or less, or 0.0300% or less. The Ta content can be set to 0.0800% or less, 0.0500% or less, or 0.0300% or less. The Sn content can be set to 0.0400% or less, 0.0300% or less, or 0.0100% or less. The Co content can be set to 0.4000% or less, 0.3000% or less, or 0.1000% or less. The As content can be set to 0.0400% or less, 0.0300% or less, or 0.0100% or less.

[0075] Mg: 0% to 0.0500%

[0076] Ca: 0% to 0.0400%

[0077] Y: 0% to 0.0500%

[0078] La: 0% to 0.0500%

[0079] Ce: 0% to 0.0500%

[0080] Zr: 0% to 0.0500%

[0081] Sb: 0% to 0.0500%

[0082] Ca (calcium), Mg (magnesium), Y (yttrium), La (lanthanum), Ce (cerium), and Zr (zirconium), and Sb (antimony) are elements that contribute to improvement in formability. Therefore, one or more selected from among these elements can be contained as needed. In the case where the above-described effects are intended to be obtained, the content of one or more selected from among Mg, Ca, Y, La, Ce, Zr, and Sb is preferably 0.0001% or more or 0.0010% or more. The content of Sb is more preferably 0.0020% or more or 0.0050% or more.

[0083] On the other hand, Mg, Y, La, Ce, Zr, or Sb in a content exceeding 0.0500% or Ca in a content exceeding 0.0400% can possibly reduce pickling property, weldability, and hot workability. Therefore, the content of Mg, Y, La, Ce, Zr, and Sb is set to 0.0500% or less, and the content of Ca is set to 0.0400% or less. The content of each of Mg, Ca, Y, La, Ce, Zr, and Sb is preferably 0.0350% or less, 0.0300% or less, or 0.0100% or less.

[0084] As described above, the chemical composition of the steel sheet of the present embodiment contains essential elements, and the remainder contains Fe and impurities, or contains essential elements and further contains one or more of optional elements, and the remainder contains Fe and impurities.

[0085] Distance of the metal structure at a position of t / 4 from the surface in a cross section in a sheet thickness direction when the sheet thickness is set to t

[0086] [Martensite: 70% or more by volume]

[0087] In the steel sheet of the present embodiment, in order to secure a tensile strength of 1470 MPa or more, the volume fraction of martensite is set to 70% or more. If the volume fraction of martensite is less than 70%, sufficient tensile strength cannot be secured. If the volume fraction of martensite exceeds 90%, sufficient volume fraction of residual austenite cannot be secured, and therefore the volume fraction of martensite is 90% or less.

[0088] In the steel sheet of the present embodiment, the martensite includes so-called primary martensite and tempered martensite.

[0089] [Residual Austenite: 10% or more by volume]

[0090] Residual austenite is a structure that improves the elongation of the steel sheet through a TRIP effect in the deformation of the steel sheet, the TRIP effect being a phase change into martensite through a processing-induced phase change. Therefore, the volume fraction of residual austenite is set to 10% or more.

[0091] The higher the volume fraction of retained austenite, the greater the elongation of the steel sheet. However, to obtain a large amount of retained austenite, a large amount of alloying elements such as carbon is required. Therefore, the volume fraction of retained austenite is set to be below 30%.

[0092] [Remaining portion: selected from one or more of ferrite, pearlite, and bainite]

[0093] The remaining portion, excluding martensite and retained austenite, may contain one or more types selected from ferrite, pearlite, and bainite. The volume fraction of the remaining portion may, for example, be 10% or less or 5% or less. The volume fraction of the remaining portion may also be 0%.

[0094] The volume fraction of martensite at position t / 4 is determined by the following method.

[0095] The observation surface of the sample was etched using a nitric acid-ethanol solution, such as... Figure 1 As shown in Figure A, within a range of 1 / 8 to 3 / 8 of the plate thickness, centered at a position on the cross-section along the thickness direction at a distance of 1 / 4 from the surface, a 100 μm × 100 μm area was observed using FE-SEM at a magnification of 3000x. In nitric acid-ethanol corrosion, martensite and retained austenite are not corroded; therefore, the area ratio of the uncorroded region is the combined area ratio of martensite and retained austenite. Furthermore, in this embodiment, the combined area ratio of martensite and retained austenite is considered their combined volume ratio. The volume ratio of martensite is calculated by subtracting the volume ratio of retained austenite, determined by the method described later, from the area ratio (i.e., volume ratio) of the uncorroded region.

[0096] The volume fraction of retained austenite can be calculated using X-ray diffraction. In X-ray diffraction, the area from the surface (rolled surface) of the sample to a depth of 1 / 4 of the plate thickness is first removed by mechanical and chemical grinding. Then, in the area at a depth of 1 / 4 of the plate thickness t, MoKα rays are used as characteristic X-rays to determine the integral intensity ratios of the diffraction peaks (200), (211) of the bcc phase and (200), (220), (311) of the fcc phase. Based on these integral intensity ratios, the volume fraction of retained austenite is calculated.

[0097] The volume fractions of ferrite, bainite, and pearlite at position t / 4 were determined using the following steps. The observation surface of the sample was etched with a nitric acid-ethanol solution, as shown in the image. Figure 1 As shown in Figure A, within a range of 1 / 8 to 3 / 8 of the plate thickness, centered at a position 1 / 4 of the plate thickness from the surface along the thickness direction, a 100 μm × 100 μm area was observed using FE-SEM at a magnification of 3000x.

[0098] The area in the crystal not containing cementite is judged to be ferrite, the area in the crystal containing cementite and the cementite being arranged in layers is judged to be pearlite, and the area in the crystal containing cementite and the cementite having a plurality of variants is judged to be bainite, and the area ratio is found by the point counting method (based on ASTM E562). The area ratio is equivalent to the volume ratio, and the area ratio of each structure is taken as the volume ratio.

[0099] [Maximum particle diameter of residual austenite in the metal structure at the t / 4 position: less than 5.0 μm]

[0100] If coarse residual austenite or primary martensite is present in the weld heat affected zone, cracking easily occurs from them as a starting point. In order to suppress cracking from residual austenite (γ) or primary martensite in the heat affected zone as a starting point for the purpose of high strength of the welded joint, the maximum particle diameter of residual γ at the t / 4 position of the final product (steel sheet) is less than 5.0 μm.

[0101] The lower limit of the maximum particle diameter is not limited, but it is not easy to set it to less than 0.1 μm, and thus the substantial lower limit is 0.1 μm.

[0102] The maximum particle diameter of residual austenite is found by the following method. In the observation of the structure, crystal orientation analysis (SEM-EBSD) using a scanning electron microscope (SEM) and backscattered electrons is used.

[0103] First, the observation surface of the sample is wet-polished using emery paper, and further polished to a mirror surface using diamond abrasive grains having an average particle diameter of 1 μm. Next, the strain introduced to the polished surface is removed by the mechanical polishing described above, and colloidal silica polishing is performed using a suspension with alcohol as a solvent. In the colloidal silica polishing, if the load during polishing is increased, strain is sometimes further introduced, and thus it is important to suppress the load during polishing. Therefore, in the polishing using colloidal silica, VibroMet 2 manufactured by BUEHLER Co. can be used, and automatic polishing is performed for 1 hour with an output of 40% set.

[0104] The samples, after being adjusted according to the above steps, were observed using SEM-EBSD within a range of 1 / 8 to 3 / 8 of the plate thickness, centered at the t / 4 position. The magnification was selected from 1000 to 9000x based on the number of retained austenite grains in the microstructure (containing more than 10 grains), for example, 3000x. The crystal orientation data of FCC-iron was determined using SEM-EBSD. The measurement interval (STEP) was set to 0.01–0.10 μm, and 0.05 μm could be selected. In the FCC-iron crystal orientation map (MAP) data obtained under these measurement conditions, boundaries with a crystal orientation difference of 15 degrees or more were considered grain boundaries, and the maximum grain size of retained austenite was determined.

[0105] <In a square region centered at position t / 4 and with one side length t / 4 in the thickness direction of the plate, when Mn concentration is measured at multiple measurement points at 1 μm intervals, the proportion of measurement points with Mn concentration greater than 1.1 times the average Mn concentration of all measurement points is less than 10.0%.>

[0106] As mentioned above, if coarse residual austenite or primary martensite is present in the heat-affected zone of the weld joint, it is prone to become the initiation point for cracking and thus prone to cracking.

[0107] To suppress such cracking, refining the retained austenite in the steel plate before welding is effective. Coarse retained austenite forms in the Mn segregation regions, therefore, refining the retained austenite effectively suppresses Mn segregation.

[0108] Specifically, when in such Figure 1 In a square region centered at position t / 4 and with one side length t / 4 in the thickness direction section of the plate as shown in Figure B, when measuring the Mn concentration at multiple measurement points using an EPMA (Electron Probe Micro Analyzer) at 1 μm intervals, the proportion (number ratio) of measurement points with an Mn concentration 1.1 times or more (1.1 or more when the average value is set to 1.0) relative to the average Mn concentration of all measurement points needs to be less than 10.0%. That is, when defining the segregation as "concentration at each measurement point / average concentration of all measurement points in the measurement region," the proportion of measurement points with a segregation of 1.1 or more needs to be less than 10.0%.

[0109] <Mechanical Properties>

[0110] [Tensile strength: ≥1470MPa]

[0111] The tensile strength of the steel sheet of the present embodiment is preferably 1470 MPa or more in view of contributing to the light weight of the automobile body.

[0112] In addition, in the steel sheet of the present embodiment, the tensile strength x total elongation (TS x tEl) is preferably 18000 MPa% or more.

[0113] The tensile strength (TS) and the total elongation (tEl) are obtained by collecting JIS No. 5 tensile test pieces from the steel sheet in a direction perpendicular to the rolling direction, and performing a tensile test in accordance with JIS Z 2241:2011.

[0114] [Plating layer]

[0115] The steel sheet of the present embodiment described above can have a hot-dip galvanized layer on the surface. By having a hot-dip galvanized layer on the surface, the corrosion resistance is improved.

[0116] For example, when use in an environment in which a steel sheet is corroded is considered, there are concerns about openings and the like, and thus sometimes even if high-strength is achieved, it is not possible to thin the sheet to a certain thickness or less. One of the purposes of high-strength of a steel sheet is to achieve light weight by thinning, and thus even if a high-strength steel sheet is developed, if the corrosion resistance is low, the application site is limited. Therefore, plating such as hot-dip galvanizing, which has high corrosion resistance, is considered to be applied to the steel sheet. The plating layer is, for example, a zinc plating layer such as a hot-dip galvanized layer or an electro-galvanized layer. In addition, the zinc plating layer can be a plating layer containing Si, Al, and / or Mg in addition to Zn.

[0117] In addition, the hot-dip galvanized layer can be an alloyed hot-dip galvanized layer that is alloyed. In the alloyed hot-dip galvanized layer, Fe is incorporated into the hot-dip galvanized layer by alloying treatment, and thus excellent weldability and paintability are obtained.

[0118] In addition, for the purpose of improving the paintability and weldability, upper layer plating can also be applied on the zinc plating layer. In addition, with respect to the cold-rolled steel sheet of the present embodiment, various treatments such as chromate treatment, phosphate treatment, lubricity improvement treatment, weldability improvement treatment, and the like can be applied on the hot-dip galvanized layer.

[0119] [Joint strength]

[0120] In consideration of the weldability in the assembly of an automobile body, the steel sheet of the present embodiment is preferably made to have a joint strength of 6.0 kN or more when a joint is made.

[0121] The joint strength was obtained by taking test pieces described in JIS Z3137 (1999) from the steel sheet in a direction perpendicular to the rolling direction, performing welding using a servo motor pressurized single-phase AC spot welder (power source frequency: 50 Hz), and then performing a cross tensile force test in accordance with JIS Z3137 (1999).

[0122] <Manufacturing Method>

[0123] The steel sheet of the present embodiment can be manufactured by a manufacturing method including the following steps.

[0124] (I) a first Mn segregation reduction step of keeping a slab obtained by continuous casting or the like at 1300°C or higher for 5.0 hours or more, and cooling to 200°C or lower at an average cooling rate of 20°C / hour to 80°C / hour;

[0125] (II) a second Mn segregation reduction step of heating the above-mentioned slab and keeping it at 1200°C or higher for 1.0 hours or more;

[0126] (III) a hot rolling step of performing hot rolling on the above-mentioned slab after the above-mentioned second Mn segregation reduction step to manufacture a hot rolled steel sheet;

[0127] (IV) a coiling step of coiling the above-mentioned hot rolled steel sheet;

[0128] (V) a cold rolling step of performing cold rolling on the above-mentioned hot rolled steel sheet after the above-mentioned coiling step to manufacture a cold rolled steel sheet; and

[0129] (VI) an annealing step of performing annealing on the above-mentioned cold rolled steel sheet.

[0130] Hereinafter, each step will be described.

[0131] [First Mn Segregation Reduction Step]

[0132] In the first Mn segregation reduction step, a slab obtained by continuous casting or the like is kept at 1300°C or higher for 5.0 hours or more before the hot rolling step, and is cooled to 200°C or lower at an average cooling rate of 20°C / hour to 80°C / hour.

[0133] By keeping the slab at a high temperature of 1300°C or higher for 5.0 hours or more, the diffusion speed of Mn is increased, and the segregation of Mn is reduced. However, the reduction of Mn segregation is not sufficient by only this keeping. Further cooling to 200°C or lower at an average cooling speed of 20°C / hour or more is required. By cooling to 200°C or lower at an average cooling speed of 20°C / hour or more, dislocations generated by the difference in thermal shrinkage are introduced. The dislocations become a high-speed diffusion path of Mn at the time of heating in the next process of the second Mn segregation process, and thus Mn can be diffused efficiently, and the degree of Mn segregation can be reduced.

[0134] The faster the average cooling speed, the more dislocations can be introduced, but if the cooling speed is too fast, the difference in thermal shrinkage becomes excessive, and the risk of cracking of the slab becomes high, and thus the average cooling speed is set to 80°C / hour or lower.

[0135] If the heating temperature is excessively increased, the manufacturing cost increases, and if the heating time is lengthened, the productivity deteriorates. From these viewpoints, the heating temperature of the slab is set to 1400°C or lower, and the keeping time at 1300°C or higher can also be set to 50.0 hours or lower.

[0136] [Second Mn segregation reduction process]

[0137] In the second Mn segregation reduction process, the slab after the first Mn segregation reduction process is heated to 1200°C or higher in a heating furnace, and kept at this temperature range for 1.0 hour or more.

[0138] By keeping at 1200°C or higher for 1.0 hour or more on the basis of the first Mn segregation process, the dislocations introduced into the slab can be used as a high-speed diffusion path to diffuse Mn. Thus, the Mn segregation can be further reduced.

[0139] If the heating temperature is excessively increased, the manufacturing cost increases, and if the heating time is lengthened, the productivity deteriorates. From these viewpoints, the heating temperature of the slab is set to 1300°C or lower, and the keeping time at 1200°C or higher can also be set to 5.0 hours or lower.

[0140] This second Mn segregation reduction process can also be performed in a hot-rolling heating furnace as heating for hot rolling.

[0141] [Hot-rolling process]

[0142] In the hot-rolling process, the slab heated to 1200°C or higher in the heating furnace and kept at this temperature range for 1.0 hour or more in the second Mn segregation reduction process is subjected to hot rolling, and a hot-rolled steel sheet is obtained.

[0143] The hot rolling conditions are not particularly limited. For example, the finish rolling can be ended at 800°C to 980°C, and then cooled to a temperature of 600°C to 750°C at an average cooling rate of 2.5°C / sec or more, and cooled to a coiling temperature of 600°C or less.

[0144] [Coiling process]

[0145] [Cold rolling process]

[0146] The hot-rolled steel sheet after the hot rolling process is coiled under known conditions to produce a hot-rolled coil, and then cold-rolled under known conditions to become a cold-rolled steel sheet. For example, the total reduction can be set to 20% to 85%.

[0147] [Annealing process]

[0148] In the annealing process, in order to make the cold-rolled steel sheet have a structure containing 70% by volume or more of martensite and 10% by volume or more of residual austenite after the annealing process, the cold-rolled steel sheet is heated to a soaking temperature of Ac3°C or more and less than 900°C, held at the soaking temperature for 5 seconds or more, then cooled to a temperature range of (Ms point - 100) °C to Bs point at an average cooling rate of 10°C / sec to 50°C / sec, and further held for 10 seconds to 600 seconds in the temperature range of (Ms point - 100) °C to Bs point.

[0149] In order to sufficiently perform austenitization, the steel sheet is heated to Ac3 point (°C) or more, and a soaking treatment is performed at this temperature (maximum heating temperature). However, if the heating temperature is excessively increased, not only will the toughness be deteriorated due to the coarsening of the austenite grain diameter, but also the annealing equipment will be damaged. Therefore, the maximum heating temperature is set to 950°C or less, and preferably set to 900°C or less.

[0150] If the soaking time is short, the austenitization will not be sufficiently performed. Therefore, the soaking time is set to 5 seconds or more. It is preferable to be 30 seconds or more or 60 seconds or more. On the other hand, if the soaking time is excessively long, the productivity will be hindered. Therefore, the soaking time is preferably set to 600 seconds or less, and more preferably set to 500 seconds or less. In the soaking, it is not necessarily required to keep the steel sheet at a constant temperature, and the temperature can be varied within a range satisfying the above conditions.

[0151] The Ac3 point is calculated by the following method.

[0152] Ac3 (°C) = 910 - 203 x V[C] + 44.7 x [Si] - 30 x [Mn] + 700 x [P] - 20 x [Cu] - 15.2 x [Ni] - 11 x [Cr] + 31.5 x [Mo] + 400 x [Ti] + 104 x [V] + 120 x [Al]

[0153] [C], [Si], [Mn], [P], [Cu], [Ni], [Cr], [Mo], [Ti], [V], and [Al] are contents (mass %) of each element contained in the slab.

[0154] Next, the steel sheet is cooled at an average cooling rate of 10 to 50°C / sec to a temperature range of (Ms point - 100)°C to Bs point (°C), and then the temperature of the steel sheet is maintained in this temperature range. The holding time of the steel sheet in the temperature range of (Ms point - 100)°C to Bs point is set to 10 to 600 seconds.

[0155] The Ms point refers to a temperature at which martensite starts to be generated in cooling after quenching. In the manufacturing method of the present embodiment, a value calculated from the following mathematical formula is regarded as the Ms point (°C).

[0156] Ms (°C) = 541 - 474 x [C] / (1 - Sα / 100) - 15 x [Si] - 35 x [Mn] - 17 x [Cr] - 17 x [Ni] + 19 x [Al]

[0157] The Bs point refers to a temperature (°C) at which a bainite transformation starts in cooling after quenching. In the manufacturing method of the present embodiment, a value calculated from the following mathematical formula is regarded as the Bs point.

[0158] Bs (°C) = 820 - 290 x [C] / (1 - Sα / 100) - 37 x [Si] - 90 x [Mn] - 65 x [Cr] - 50 x [Ni] + 70 x [Al]

[0159] [Element symbol] contained in the Ms calculation formula and the Bs calculation formula indicates a content (mass %) of each element contained in the steel sheet. The symbol Sα contained in the formula is a ferrite fraction (volume %) of the steel sheet at the time when heating for quenching is finished.

[0160] It is difficult to find the area fraction of ferrite of the steel sheet in manufacturing. Therefore, a steel sheet which has undergone the same temperature history as the actual manufacturing process of the steel sheet is prepared in advance, the area fraction of ferrite of the center portion of the steel sheet is found, and this area fraction of ferrite is used for the calculation of Ms and Bs. The ferrite fraction of the steel sheet basically depends on the heating temperature for quenching. Therefore, when the cooling conditions are investigated, the manufacturing conditions of the process before cooling are first determined, the steel sheet is manufactured under the manufacturing conditions, the ferrite fraction thereof is measured, and thus Sα can be determined. In addition, in the case where the cooling speed for quenching is fast (is a cooling speed at which no ferrite transformation occurs), the ferrite fraction after quenching can also be regarded as the ferrite fraction at the time when heating for quenching is finished.

[0161] The average cooling rate is a value obtained by dividing the difference between the surface temperature of the steel sheet at the time of starting cooling and the surface temperature of the steel sheet at the time of ending cooling (i.e., the cooling stop temperature) by the cooling time. For example, in the case where annealing and the temperature holding described later are performed using a furnace, the time of starting cooling refers to the time of taking the steel sheet out of the furnace for annealing, and the time of ending cooling refers to the time of loading the steel sheet into the furnace for temperature holding.

[0162] The holding time in the temperature range of (Ms point - 100) °C to Bs point (°C) refers to the time during which the surface temperature of the steel sheet is in the temperature range. In the temperature range, the temperature of the steel sheet can fluctuate.

[0163] By setting the average cooling rate of the steel sheet up to (Ms point - 100) °C to Bs point (°C) to 10 to 50 °C / sec, a sufficient amount of martensite and / or bainite can be generated in the steel sheet. By setting the cooling stop temperature of the steel sheet to be in the temperature range of (Ms point - 100) °C to Bs point (°C), a sufficient amount of residual austenite can be generated in the subsequent temperature holding. In addition, by setting the holding time of the steel sheet in the temperature range of (Ms point - 100) °C to Bs point (°C) to 10 to 600 seconds, a sufficient amount of residual austenite can be generated, and a decrease in the tensile strength of the steel sheet can be prevented.

[0164] [Hot dip galvanizing step]

[0165] [Alloying step]

[0166] The cold-rolled steel sheet after annealing can also be dipped in a hot dip galvanizing bath to produce a hot dip galvanized steel sheet having a hot dip galvanized layer on the surface. In addition, the hot dip galvanized steel sheet can be subjected to an alloying treatment to produce an alloyed hot dip galvanized steel sheet. In this case, the temperature holding of the steel sheet described above can be performed using the heat applied to the steel sheet at the time of hot dip galvanizing and alloying. Any conditions can be applied to the known conditions.

[0167] Examples

[0168] Slabs (Steel Nos. A to Z) having the chemical compositions shown in Table 1-1 and Table 1-2 (unit: mass%, the remainder being Fe and impurities) were produced by continuous casting.

[0169] The slabs were heated, held, and cooled to 200°C or lower as shown in Table 2-1 and Table 2-2.

[0170] Then, the slab was further heated, held as shown in Tables 2-1 and 2-2, and then hot-rolled in such a manner that finish rolling ends at 800 to 980°C, and then cooled to a coiling temperature of 600°C or lower at an average cooling rate of 2.5°C / sec or more until the temperature reaches 600 to 750°C, and coiled at 600°C or lower, thereby obtaining a hot-rolled steel sheet of 2.0 to 4.0 mm.

[0171] In addition, the hot-rolled steel sheets were cold-rolled at a reduction ratio of 20 to 85% to obtain cold-rolled steel sheets of 0.8 to 2.0 mm.

[0172] The cold-rolled steel sheets were annealed under the conditions shown in Tables 3-1 and 3-2 (however, the example in which the slab cracked was not subjected to the process after hot-rolling).

[0173] In addition, a portion of the cold-rolled steel sheets were subjected to hot-dip galvanizing as shown in Tables 3-1 and 3-2, and further, a portion of the cold-rolled steel sheets were subjected to alloying treatment.

[0174] From the obtained cold-rolled steel sheets (including plated steel sheets), samples were collected in accordance with the above-mentioned protocol, and observation of the microstructure was performed, and the volume fraction of martensite, residual austenite, and other phases, and the maximum particle diameter of the residual austenite were calculated.

[0175] In addition, the Mn concentration was measured using EPMA in accordance with the above-mentioned protocol, and the proportion of measurement points at which the measured point concentration / the average concentration of all measurement points in the measurement area (degree of segregation) was 1.1 or more was calculated.

[0176] The results are shown in Tables 4-1 and 4-2.

[0177] In addition, JIS No. 5 tensile test pieces were collected from the cold-rolled steel sheets after annealing in a direction perpendicular to the rolling direction, and a tensile test was performed in accordance with JIS Z 2241:2011, and the tensile strength and total elongation were calculated.

[0178] If the tensile strength (TS) is 1470 MPa or more, and the tensile strength x total elongation (TS x tEl) is 18000 MPa% or more, it was judged to be high-strength and excellent formability.

[0179] The results are shown in Tables 5-1 and 5-2.

[0180] In addition, from the obtained cold-rolled steel sheet, a test piece described in JIS Z 3137 (1999) was taken in a direction perpendicular to the rolling direction, a servo motor pressurized single-phase AC spot welder (power source frequency: 50 Hz) was used, the diameter of the electrode was set to 6 mm, the pressure at the time of welding was set to 4 kN, the welding current was set to 6.0 kA to 9.0 kA, the current time was set to 0.4 seconds, the holding time was set to 0.1 seconds, welding was performed in such a manner that the nugget diameter became 5Vt (t: sheet thickness), and then a cross tensile test was performed in accordance with JIS Z 3137 (1999), whereby the joint strength was found.

[0181] If the joint strength exceeds 6.0 kN, it is judged that the welded joint strength is excellent.

[0182] The results are shown in Table 5-1, Table 5-2.

[0183] [Table 1-1]

[0184]

[0185] [Table 1-2]

[0186]

[0187] [Table 2-1]

[0188]

[0189] [Table 2-2]

[0190]

[0191] [Table 3-1]

[0192]

[0193] [Table 3-2]

[0194]

[0195] [Table 4-1]

[0196]

[0197] [Table 4-2]

[0198]

[0199] [Table 5-1]

[0200]

[0201] [Table 5-2]

[0202]

[0203] As is apparent from Tables 1-1 to 5-2, the steel sheets of the examples according to the present application are all excellent in formability and have a tensile strength of 1470 MPa or more, and sufficient joint strength is obtained.

[0204] On the other hand, the comparative examples in which at least one of the chemical composition, the volume fraction of each phase of the microstructure, the maximum particle diameter of the residual austenite, and the proportion of the measurement points at which the degree of segregation is 1.1 or more does not satisfy the range of the present application have one or more of the tensile strength, the formability, and the joint strength not satisfying the target value.

[0205] Explanation of symbols

[0206] Observation area of A structure (100 μm x 100 μm area in the range of t / 8 to 3t / 8 centered on the t / 4 position)

[0207] Measurement area of B Mn concentration (square area of t / 4 length on one side centered on the t / 4 position)

[0208] t Sheet thickness

[0209] Industrial applicability

[0210] According to the present application, a steel sheet excellent in formability and having a tensile strength of 1470 MPa or more, and from which sufficient joint strength is obtained, can be provided. Therefore, the industrial applicability is high.

Claims

1. A steel sheet having the following chemical composition: in mass %: C: 0.20 to 0.45%, Si: 0.50 to 2.50%, Mn: 1.50 to 3.50%, Al：0.005％～1.500％、 P：0％～0.040％、 S：0％～0.010％、 N：0％～0.0100％、 O：0％～0.0060％、 Cr：0％～0.50％、 Ni: 0 to 1.00%, Cu: 0 to 1.00%, Mo: 0 to 0.50%, Ti: 0 to 0.200%, Nb: 0 to 0.200%, V：0％～0.500％、 B：0％～0.0100％、 W：0％～0.1000％、 Ta: 0 to 0.1000%, Sn: 0 to 0.0500%, Co: 0 to 0.5000%, Sb: 0 to 0.0500%, As: 0 to 0.0500%, Mg: 0 to 0.0500%, Ca: 0 to 0.0400%, Y：0％～0.0500％、 La: 0 to 0.0500%, Ce: 0 to 0.0500%, Zr: 0 to 0.0500%, and the remainder: Fe and impurities, when the sheet thickness is set to t, the metal structure at a position of t / 4 from the surface in the sheet thickness direction section, that is, at a t / 4 position, contains, in volume fraction: martensite: 82 to 90%, retained austenite: 10% or more, the maximum particle diameter of the retained austenite is less than 5.0 μm, when the Mn concentration is measured at a plurality of measurement points at intervals of 1 μm in a square region of t / 4 in length on one side centered on the t / 4 position in the sheet thickness direction section, the proportion of measurement points where the Mn concentration is 1.1 times or more the average value of the Mn concentrations of all the plurality of measurement points is less than 10.0%, the tensile strength of the steel sheet is 1470 MPa or more.

2. The steel sheet according to claim 1, wherein, the chemical composition contains, in mass %, one or more selected from the group consisting of: Cr：0.01％～0.50％、 Ni: 0.01 to 1.00%, Cu: 0.01 to 1.00%, Mo: 0.01 to 0.50%, Ti: 0.001 to 0.200%, Nb: 0.001 to 0.200%, V：0.001％～0.500％、 B：0.0001％～0.0100％、 W：0.0005％～0.1000％、 Ta: 0.0005 to 0.1000%, Sn: 0.0010 to 0.0500%, Co: 0.0010 to 0.5000%, Sb: 0.0010 to 0.0500%, As: 0.0010 to 0.0500%, Mg: 0.0001 to 0.0500%, Ca: 0.0001 to 0.0400%, Y：0.0001％～0.0500％、 La: 0.0001 to 0.0500%, Ce: 0.0001 to 0.0500%, and Zr：0.0001％～0.0500％。 3. The steel sheet according to claim 1 or 2, having a hot-dip galvanized layer on the surface.

4. The steel sheet according to claim 3, wherein, the hot-dip galvanized layer is an alloyed hot-dip galvanized layer. the hot-dip galvanized layer is an alloyed hot-dip galvanized layer.

Images