Steel sheet

Abstract

A steel sheet according to an embodiment of the present invention has: a chemical composition of, by mass, C: 0.08-0.17%, Si: 0.03- 1.40%, Mn: 1.60-3.00%, Al: 0.01-0.70%, P: 0.080% or less, S: 0.0100% or less, N: 0.0050% or less, Ti: 0.020-0.180%, Nb: 0.010-0.050%, and Ti + Nb + (Mo / 2) + V: 0.100-0.600%, with the balance made up by Fe and impurities; and a metal structure comprising, by area, 80.0-97.0% of tempered martensite, a total of 10.0% or below of pearlite, ferrite and bainite, and a total of 3.0-10.0% of fresh martensite and retained austenite. The standard deviation of the Mn concentrations of fresh martensite and retained austenite is 1.0-5.0%, and the tensile strength is 1110 MPa or above.

Description

steel plate

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

[0002] In recent years, efforts have been made to reduce the weight of automobiles and machine parts. By ensuring rigidity through optimal part design, it is possible to reduce the weight of automobiles and machine parts. Furthermore, for blank-formed parts such as press-formed parts, weight can be reduced by reducing the thickness of the part material.

[0003] However, when trying to ensure the strength properties of parts, such as static fracture strength and yield strength, while reducing the plate thickness, it is necessary to use high-strength materials. In particular, the application of steel sheets of over 780 MPa class has begun to be considered for automobile suspension parts such as lower arms, trail links, and knuckles. These automobile suspension parts are manufactured by subjecting steel sheets to bending or other processes. Therefore, the steel sheets used for these automobile suspension parts are required to have excellent formability, particularly excellent bendability.

[0004] For example, Patent Document 1 discloses a precipitation-strengthened martensitic steel that contains, in mass %, C: 0.02 to 0.08%, Al: 0.05% or less, Cr: 8.0 to 13.0%, Ni: 2.0 to 8.0%, Co: 2.0 to 16.0%, Mo as an essential element, Mo+0.5W: 3.5 to 8.0%, and the balance being Fe and impurities, and that has both high tensile strength and Charpy absorbed energy.

[0005] Furthermore, Patent Document 2 discloses a high-strength steel having excellent tensile strength, ductility, and bendability, in which the microstructure has an area ratio of ferrite of 5% or more and less than 50%, and the area ratio of a mixed structure of fresh martensite and retained austenite to the total structure is more than 0% and 30% or less, and further, when analyzed with an electron beam microprobe analyzer, there are 5% or more areas of regions where the Mn concentration is 1.2 times or more the Mn concentration in the steel sheet, and the fraction of the regions where the Mn concentration is 1.2 times or more the Mn concentration in the steel sheet is measured in 2 μm sections, and the standard deviation when measuring 100 sections is 4.0% or more.

[0006] JP 2014-208869 A JP 2015-193897 A

[0007] However, Patent Document 1 does not take into consideration bendability. Furthermore, the above-mentioned automotive suspension parts are manufactured by subjecting steel sheets to multiple forming processes. Therefore, steel sheets used in automotive suspension parts are required to have excellent formability even after being subjected to a certain degree of pre-strain in a previous process. When performing multiple forming processes, if the strain generated in the previous process is not sufficiently dispersed, localized deformation may progress in a subsequent process, preventing the steel sheet from exhibiting its inherent formability. Such localized deformation is particularly pronounced in the case of bending. However, Patent Document 2 does not take into consideration bendability after being subjected to pre-strain.

[0008] The present invention has been made in view of the above circumstances, and has an object to provide a steel sheet having high strength and excellent bendability after pre-straining.

[0009] The gist of the present invention, which was made based on the above findings, is as follows.

[0010] (1) A steel sheet according to one embodiment of the present invention has a chemical composition, in mass%, of C: 0.08 to 0.17%, Si: 0.03 to 1.40%, Mn: 1.60 to 3.00%, Al: 0.01 to 0.70%, P: 0.080% or less, S: 0.0100% or less, N: 0.0050% or less, Ti: 0.020 to 0.180%, Nb: 0.010 to 0.050%, Ti + Nb + (Mo / 2) + V: 0.100 to 0.600%, and the balance: Fe and impurities; and a metallographic structure, in area%, of tempered martensite: 80.0 to 97.0%, and the sum of pearlite, ferrite, and bainite: 10.0% or less, The sum of fresh martensite and retained austenite: 3.0 to 10.0%, the standard deviation of the Mn concentration of the fresh martensite and the retained austenite is 1.0 to 5.0%, and the tensile strength is 1110 MPa or more.

[0011] (2) In the steel plate described in (1) above, the metal structure may have, in area percentage, 1.5% or more of the retained austenite.

[0012] (3) In the steel sheet according to (1) or (2) above, the chemical composition may contain, in mass %, one or more elements selected from Mo: 0.600% or less, and V: 0.300% or less, in place of a portion of Fe.

[0013] (4) In the steel sheet according to any one of (1) to (3) above, the chemical composition may contain, in mass %, one or more elements selected from the following, in place of a portion of Fe: B: 0.0030% or less, Cr: 0.50% or less, Cu: 0.50% or less, and Ni: 2.0% or less.

[0014] (5) In the steel sheet according to any one of (1) to (4) above, the chemical composition may contain, in mass %, one or more elements selected from the following, in place of a portion of Fe: Ca: 0.020% or less, Mg: 0.020% or less, REM: 0.100% or less, and Bi: 0.020% or less.

[0015] (6) In the steel sheet according to any one of (1) to (5) above, the chemical composition may contain, in mass %, Sn: 0.05% or less in place of a portion of Fe.

[0016] According to this embodiment, it is possible to provide a steel sheet having high strength and excellent bendability after pre-straining.

[0017] The present inventors have conducted extensive research into methods for obtaining the above steel sheet, and have come to the following findings.

[0018] By suppressing local deformation during bending after pre-straining, cracks can be prevented during multiple forming steps. To achieve this, it is important that the Mn distribution in fresh martensite and retained austenite is uniform, i.e., that the standard deviation of the Mn concentration in fresh martensite and retained austenite is small.

[0019] Furthermore, in order to suppress such local deformation and to make the tensile strength of the steel sheet 1110 MPa or more, it is necessary to make tempered martensite the main phase and to disperse fine precipitates.

[0020] In order to obtain the metal structure described above, it is effective to strictly control the rough rolling conditions, finish rolling conditions, cooling conditions after finish rolling, cold rolling conditions, and further the conditions for the subsequent heat treatment.

[0021] The steel sheet according to this embodiment will be described in detail below. However, the present invention is not limited to the configuration disclosed in this embodiment, and various modifications are possible within the scope of the present invention. The numerical ranges described below, separated by "to" include the lower and upper limits. Numerical values ​​indicated as "less than" and "greater than" do not include the numerical range. All "%" in chemical compositions refer to "mass %".

[0022] The steel sheet according to this embodiment has a chemical composition, in mass%, of 0.08 to 0.17% C, 0.03 to 1.40% Si, 1.60 to 3.00% Mn, 0.01 to 0.70% Al, 0.080% or less P, 0.0100% or less S, 0.0050% or less N, 0.020 to 0.180% Ti, 0.010 to 0.050% Nb, 0.100 to 0.600% Ti + Nb + (Mo / 2) + V, and the balance Fe and impurities. Each element will be described in detail below.

[0023] C: 0.08 to 0.17% C is an element necessary for obtaining the desired tensile strength of the steel sheet. If the C content is less than 0.08%, the desired tensile strength cannot be obtained. Therefore, the C content is set to 0.08% or more. The C content is preferably 0.09% or more, 0.10% or more, or 0.11% or more.

[0024] On the other hand, if the C content exceeds 0.17%, weldability is reduced. In addition, the amount of solute carbon becomes excessive, and austenite growth occurs due to the diffusion of C during heat treatment, resulting in non-uniform Mn distribution in fresh martensite and retained austenite. Therefore, the C content is set to 0.17% or less. Preferably, it is set to 0.15% or less or 0.14% or less. In order to reduce the standard deviation of the Mn concentration in fresh martensite and retained austenite and further improve bendability after pre-straining, the C content is more preferably set to 0.13% or less. The C content is preferably 0.09 to 0.15%, more preferably 0.10 to 0.14%, and even more preferably 0.11 to 0.13%.

[0025] Si: 0.03 to 1.40% Si is an element that improves the adhesion of zinc plating. If the Si content is less than 0.03%, the adhesion of zinc plating during forming decreases. Therefore, the Si content is set to 0.03% or more. The Si content is preferably 0.04% or more, and more preferably 0.05% or more.

[0026] On the other hand, if the Si content exceeds 1.40%, the surface quality of the steel sheet deteriorates. Therefore, the Si content is set to 1.40% or less. The Si content is preferably 1.10% or less, and more preferably 0.90% or less. The Si content is preferably 0.04 to 1.10%, and more preferably 0.05 to 0.90%.

[0027] Mn: 1.60 to 3.00% Mn is an element necessary for improving the strength of steel sheet. If the Mn content is less than 1.60%, the area ratio of ferrite becomes too high, making it impossible to obtain the desired tensile strength. Therefore, the Mn content is set to 1.60% or more. The Mn content is preferably 1.80% or more. In order to reduce the standard deviation of the Mn concentration in fresh martensite and retained austenite and further improve bendability after pre-straining, the Mn content is more preferably 2.00% or more.

[0028] On the other hand, if the Mn content exceeds 3.00%, the cast slab is more likely to crack, which may make hot rolling difficult. Therefore, the Mn content is set to 3.00% or less. The Mn content is preferably 2.70% or less, and more preferably 2.50% or less. The Mn content is preferably 1.80 to 2.70%, and more preferably 2.00 to 2.50%.

[0029] Al: 0.01 to 0.70% Al is an element that acts as a deoxidizer and improves the cleanliness of steel. If the Al content is less than 0.01%, a sufficient deoxidizing effect is not obtained, and a large amount of inclusions (oxides) is formed in the steel sheet. Such inclusions deteriorate the surface properties of the steel sheet. Therefore, the Al content is set to 0.01% or more. The Al content is preferably 0.02% or more, more preferably 0.03% or more, and even more preferably 0.04% or more.

[0030] On the other hand, if the Al content exceeds 0.70%, casting may become difficult. Therefore, the Al content is set to 0.70% or less. The Al content is preferably 0.60% or less, 0.30% or less, and more preferably 0.10% or less. The Al content is preferably 0.02 to 0.60%, more preferably 0.03 to 0.30%, and even more preferably 0.04 to 0.10%.

[0031] P: 0.080% or less P is an element that segregates in the center of the steel plate thickness. If the P content exceeds 0.080%, the weldability deteriorates. Therefore, the P content is set to 0.080% or less. The P content is preferably 0.040% or less, and more preferably 0.020% or less.

[0032] The lower the P content, the better, and 0% is preferable, but if the P content is reduced excessively, the cost of dephosphorization increases significantly, so the P content may be set to 0.0005% or more.

[0033] S: 0.0100% or less S is an element that exists as sulfide. If the S content exceeds 0.0100%, weldability decreases. Therefore, the S content is set to 0.0100% or less. The S content is preferably 0.0080% or less, more preferably 0.0050% or less, and even more preferably 0.0030% or less.

[0034] The lower the S content, the better, and 0% is preferable, but if the S content is reduced excessively, the desulfurization cost increases significantly, so the S content may be set to 0.0005% or more.

[0035] N: 0.0050% or less N is an element that forms coarse nitrides in steel. If the N content exceeds 0.0050%, slab cracking may occur, making hot rolling difficult. Therefore, the N content is set to 0.0050% or less. The N content is preferably 0.0040% or less, and more preferably 0.0035% or less.

[0036] The lower the N content, the better, and 0% is preferable, but if the N content is reduced excessively, the cost of denitrification increases significantly, so the N content may be set to 0.0005% or more.

[0037] Ti: 0.020 to 0.180% Ti is an element that increases the strength of steel sheet by forming fine carbides and / or carbonitrides in steel. If the Ti content is less than 0.020%, the desired tensile strength cannot be obtained. Therefore, the Ti content is set to 0.020% or more. The Ti content is preferably 0.050% or more, and more preferably 0.080% or more.

[0038] On the other hand, if the Ti content exceeds 0.180%, the cast slab is more likely to crack, which may make hot rolling difficult. Therefore, the Ti content is set to 0.180% or less. The Ti content is preferably 0.160% or less, and more preferably 0.150% or less. The Ti content is preferably 0.050 to 0.160%, and more preferably 0.080 to 0.150%.

[0039] Nb: 0.010 to 0.050% Nb is an element that suppresses abnormal grain growth of austenite grains during hot rolling. Nb is also an element that increases the strength of the steel sheet by forming fine carbides and / or nitrides. If the Nb content is less than 0.010%, the desired tensile strength cannot be obtained. Therefore, the Nb content is set to 0.010% or more. The Nb content is preferably 0.013% or more, and more preferably 0.015% or more.

[0040] On the other hand, if the Nb content exceeds 0.050%, cracks tend to occur in the cast slab, which may make hot rolling difficult. Therefore, the Nb content is set to 0.050% or less. The Nb content is preferably 0.040% or less, and more preferably 0.035% or less. The Nb content is preferably 0.013 to 0.040%, and more preferably 0.015 to 0.035%.

[0041] Ti + Nb + (Mo / 2) + V: 0.100 to 0.600% In this embodiment, the total amount of the above-mentioned Ti content, Nb content, half of the Mo content described below, and V content is controlled. That is, when the content of each element is expressed by the corresponding element symbol, Ti + Nb + (Mo / 2) + V is controlled. If the total amount of these elements is less than 0.100%, the effect of forming at least one of fine carbides, nitrides, and carbonitrides to increase the strength of the steel sheet is not sufficiently obtained, and the desired tensile strength cannot be obtained. Therefore, the total amount is set to 0.100% or more.

[0042] Furthermore, by forming carbides of these elements, the solute carbon in the steel is consumed, which makes it possible to control the growth of austenite through the diffusion of Mn. This makes it possible to homogenize the Mn distribution in fresh martensite and retained austenite, thereby improving the bendability after pre-straining.

[0043] It is not necessary to include all of Ti, Nb, Mo, and V, and the above effect can be obtained as long as the content of any one of Ti, Nb, and V, or half the Mo content, is 0.100% or more. The above total amount is preferably 0.150% or more. In order to reduce the standard deviation of the Mn concentration in fresh martensite and retained austenite and further improve bendability after pre-straining, the total amount is more preferably 0.200% or more, even more preferably 0.230% or more, and even more preferably 0.250% or more. Note that if the above total amount exceeds 0.230%, at least one of Mo and V will essentially be contained.

[0044] On the other hand, if the total amount exceeds 0.600%, the precipitates become coarse and the desired tensile strength cannot be obtained. Therefore, the total amount is set to 0.600% or less. The total amount is preferably 0.500% or less, 0.400% or less, or 0.300% or less. The total amount is preferably 0.200 to 0.500%, more preferably 0.230 to 0.400%, and even more preferably 0.250 to 0.300%.

[0045] The balance of the chemical composition of the steel sheet according to this embodiment may be Fe and impurities. In this embodiment, the impurities refer to substances that are mixed in from raw materials such as ore, scrap, or the manufacturing environment, or substances that are allowed to exist within a range that does not adversely affect the steel sheet according to this embodiment.

[0046] The steel sheet according to this embodiment may contain the following optional elements instead of part of Fe. When no optional elements are contained, the lower limit of the content is 0%. Each optional element will be described below.

[0047] Mo: 0.600% or less Mo is an element that increases the strength of steel sheet by forming fine carbides in the steel. To ensure this effect, the Mo content is preferably 0.001% or more, and more preferably 0.002% or more. On the other hand, even if the Mo content exceeds 0.600%, the above effect saturates. Therefore, the Mo content is set to 0.600% or less. The Mo content is preferably 0.500% or less, 0.400% or less, 0.300% or less, 0.200% or less, or 0.100% or less. The Mo content is preferably 0.001 to 0.500%, 0.002 to 0.400%, 0.003 to 0.300%, 0.004 to 0.200%, or 0.005 to 0.100%.

[0048] V: 0.300% or less V is an element that increases the strength of steel sheets by forming fine carbides and / or nitrides in the steel. To ensure this effect, the V content is preferably 0.010% or more, more preferably 0.050% or more, and even more preferably 0.100% or more. On the other hand, if the V content exceeds 0.300%, cracks may easily occur in the cast slab, making hot rolling difficult. Therefore, the V content is set to 0.300% or less. The V content is preferably 0.270% or less, 0.240% or less, or 0.200% or less. The V content is preferably 0.010 to 0.270%, 0.030 to 0.240%, or 0.050 to 0.200%.

[0049] B: 0.0030% or less B is an element that suppresses the formation of ferrite during the cooling process and increases the strength of the steel sheet. To ensure this effect, the B content is preferably 0.0001% or more, more preferably 0.0005% or more, and even more preferably 0.0010% or more. On the other hand, even if the B content exceeds 0.0030%, the above effect saturates. Therefore, the B content is set to 0.0030% or less. The B content is preferably 0.0025% or less, 0.0020% or less, or 0.0015% or less. The B content is preferably 0.0001 to 0.0025%, 0.0005 to 0.0020%, or 0.0010 to 0.0015%.

[0050] Cr: 0.50% or less Cr is an element that exhibits an effect similar to that of Mn. To ensure the effect of increasing the strength of the steel sheet due to the inclusion of Cr, the Cr content is preferably 0.001% or more, more preferably 0.005% or more, and even more preferably 0.010% or more. On the other hand, even if the Cr content exceeds 0.50%, the above effect saturates. Therefore, the Cr content is set to 0.50% or less. The Cr content is preferably 0.40% or less, 0.30% or less, or 0.20% or less. The Cr content is preferably 0.001 to 0.40%, 0.005 to 0.30%, or 0.010 to 0.20%.

[0051] Cu: 0.50% or less Cu has the effect of improving the hardenability of steel sheet and of precipitating in steel to increase the strength of the steel sheet. To more reliably obtain the above effects, the Cu content is preferably 0.01% or more, more preferably 0.05% or more, and even more preferably 0.10% or more. However, if the Cu content exceeds 0.50%, intergranular cracking of the slab may occur. Therefore, the Cu content is set to 0.50% or less. The Cu content is preferably 0.40% or less, 0.30% or less, or 0.20% or less. The Cu content is preferably 0.01 to 0.40%, 0.05 to 0.30%, or 0.10 to 0.20%.

[0052] Ni: 2.0% or less Ni has the effect of improving the hardenability of the steel sheet and increasing its strength. Furthermore, when Cu is contained, Ni has the effect of effectively suppressing intergranular cracking of the slab caused by Cu. To more reliably obtain the above-mentioned effect, the Ni content is preferably 0.02% or more, more preferably 0.10% or more, and even more preferably 0.30% or more. Since Ni is an expensive element, it is economically undesirable to include a large amount of Ni. Therefore, the Ni content is set to 2.0% or less. The Ni content is preferably 1.5% or less, 1.0% or less, or 0.50% or less. The Ni content is preferably 0.02 to 1.5% or less, 0.10 to 1.0% or less, or 0.30 to 0.50% or less.

[0053] Ca: 0.020% or less Mg: 0.020% or less REM: 0.100% or less Ca, Mg, and REM all have the effect of improving the formability of the steel sheet by controlling the shape of inclusions to a preferred shape. Therefore, one or more elements selected from these elements may be contained. To more reliably obtain the effects of the above-mentioned action, it is preferable that one or more of Ca, Mg, and REM be 0.0005% or more. The contents of Ca, Mg, and REM are each more preferably 0.0010% or more, and even more preferably 0.0020% or more.

[0054] However, if the Ca content and / or Mg content exceeds 0.020%, or if the REM content exceeds 0.100%, excessive inclusions may be formed in the steel, reducing the ductility of the steel sheet. Therefore, the Ca content and Mg content are each set to 0.020% or less, and the REM content to 0.100% or less. The Ca and Mg contents are preferably set to 0.015% or less, 0.010% or less, 0.0050% or less, or 0.0030% or less. Furthermore, the REM content is preferably set to 0.050% or less, 0.030% or less, 0.010% or less, 0.0050% or less, or 0.0030% or less.

[0055] The Ca content is preferably 0.0005 to 0.015%, 0.0010 to 0.010%, 0.0015 to 0.0050%, or 0.0020 to 0.0030%. The Mg content is preferably 0.0005 to 0.015%, 0.0010 to 0.010%, 0.0015 to 0.0050%, or 0.0020 to 0.0030%. The REM content is preferably 0.0001 to 0.050%, 0.0005 to 0.030%, 0.0010 to 0.010%, 0.0015 to 0.0050%, or 0.0020 to 0.0030%.

[0056] Here, REM refers to a total of 17 elements consisting of Sc, Y, and lanthanoids, and the content of REM refers to the total content of these elements. In the case of lanthanoids, they are industrially added in the form of misch metal.

[0057] Bi: 0.020% or less Bi has the effect of refining the solidification structure, thereby improving the formability of the steel sheet. To more reliably obtain the effect of this effect, the Bi content is preferably 0.0005% or more, more preferably 0.0010% or more, and even more preferably 0.0050% or more. However, even if the Bi content exceeds 0.020%, the effect of the above action saturates, which is economically undesirable. Therefore, the Bi content is set to 0.020% or less. The Bi content is preferably 0.010% or less. The Bi content is preferably 0.0005 to 0.016%, 0.0010 to 0.013%, or 0.0050 to 0.010%.

[0058] Sn: 0.05% or less Sn has the effect of improving plating processability during the production of plated steel sheets. To more reliably obtain this effect, the Sn content is preferably 0.01% or more, and more preferably 0.02% or more. However, if the Sn content exceeds 0.05%, defects may occur during hot rolling, so the Sn content is set to 0.05% or less. The Sn content is preferably 0.04% or less. The Sn content is preferably 0.01 to 0.04%, or 0.02 to 0.03%.

[0059] Furthermore, the inventors have confirmed that even if Zr, Co, Zn, and W are contained as impurities, the effects of the steel sheet according to this embodiment are not impaired as long as the total content is 1.00% or less. Therefore, one or more elements selected from Zr, Co, Zn, and W may be contained in a total content of 1.00% or less.

[0060] The chemical composition of the above-mentioned steel sheet may be analyzed using a spark discharge optical emission spectrometer or the like. Values ​​of C and S are determined by burning the steel sheet in an oxygen stream using a gas composition analyzer or the like and measuring by infrared absorption. Values ​​of N are determined by melting a test piece taken from the steel sheet in a helium stream and measuring by thermal conductivity. When the steel sheet is plated, the "chemical composition of the steel sheet" refers to the chemical composition of the base material excluding the plated layer.

[0061] Next, the metal structure of the steel sheet according to this embodiment will be described.

[0062] The steel plate according to this embodiment has a metal structure, in area %, of tempered martensite: 80.0 to 97.0%, pearlite, ferrite, and bainite in total: 10.0% or less, and fresh martensite and retained austenite in total: 3.0 to 10.0%, and the standard deviation of the Mn concentration of the fresh martensite and retained austenite is 5.0% or less.

[0063] In this embodiment, the metallographic structure at the 1 / 4 position of the steel sheet thickness is specified. The reason for this is that the metallographic structure at this position represents a typical metallographic structure of the steel sheet. Here, in this specification, the "1 / 4 position of the sheet thickness" refers to a region whose center in the sheet thickness direction is the 1 / 4 position of the sheet thickness. Furthermore, in cases where the steel sheet is plated, the "sheet thickness" refers to the sheet thickness of the base material portion excluding the plating layer. Note that in the present invention, the thickness of the plating layer is determined by observation using an optical microscope.

[0064] Area fraction of tempered martensite: 80.0 to 97.0% Tempered martensite increases the strength of the steel sheet. If the area fraction of tempered martensite is less than 80.0%, the desired tensile strength cannot be obtained. On the other hand, if the area fraction of tempered martensite exceeds 97.0%, the bendability after pre-straining decreases. Therefore, the area fraction of tempered martensite is set to 80.0 to 97.0%. The area fraction of tempered martensite is preferably 85.0% or more and 95.0% or less. The area fraction of tempered martensite is preferably 85.0 to 95.0%.

[0065] Sum of area fractions of pearlite, ferrite, and bainite: 10.0% or less If the sum of the area fractions of pearlite, ferrite, and bainite is high, the desired tensile strength cannot be obtained. Therefore, the sum of the area fractions of these structures is set to 10.0% or less. The sum of the area fractions of these structures is preferably 7.0% or less, 5.0% or less, or 3.0% or less. The lower the sum of the area fractions of these structures, the better, so it may be set to 0.0%.

[0066] It is not necessary for all of pearlite, ferrite, and bainite to be contained; only one of them may be contained, the area ratio of which falls within the above range, or two or three of them may be contained, the total area ratio of which falls within the above range.

[0067] Sum of area fractions of fresh martensite and retained austenite: 3.0 to 10.0% In the steel sheet according to this embodiment, if the sum of the area fractions of fresh martensite and retained austenite is less than 3.0%, deformation after pre-straining becomes non-uniform, and the bendability of the steel sheet after pre-straining deteriorates. The above-mentioned effect cannot be obtained. Therefore, the sum of the area fractions of fresh martensite and retained austenite is set to 3.0% or more. It is preferably 4.0% or more or 5.0% or more.

[0068] If the total area fraction of fresh martensite and retained austenite exceeds 10.0%, the bendability of the steel sheet after pre-straining is reduced. Therefore, the total area fraction of fresh martensite and retained austenite is set to 10.0% or less. Preferably, it is set to 9.0% or less or 8.0% or less. The total area fraction of fresh martensite and retained austenite is preferably 4.0 to 9.0%, and more preferably 5.0 to 8.0%.

[0069] It is not necessary to contain both fresh martensite and retained austenite, and only one of them may be contained, with the area ratio being within the above-mentioned range.

[0070] The area fraction of the retained austenite may be 1.5% or more, with the total area fraction of fresh martensite and retained austenite being 3.0 to 10.0%. By making the area fraction of the retained austenite in the second phase 1.5% or more, it is possible to further improve the bendability after pre-straining. The area fraction of the retained austenite is more preferably 2.0% or more, 3.0% or more, or 4.0% or more. There is no particular upper limit to the area fraction of the retained austenite, but it may be 10.0% or less or 7.0% or less.

[0071] In the metal structure of the steel sheet according to this embodiment, the total area ratio of the above-mentioned structures is preferably 100%. That is, it is preferable that, in area %, the total of pearlite, ferrite, and bainite is 10.0% or less, and the total of fresh martensite and retained austenite is 3.0 to 10.0%, with the remainder being tempered martensite.

[0072] The method for measuring the area ratio of each texture will be explained below.

[0073] A test piece is taken from the steel plate in a cross section parallel to the rolling direction so that the metal structure can be observed at a depth of ¼ of the plate thickness from the surface and at the center position in the plate width direction.

[0074] The cross section of the test piece was polished using #600 to #1500 silicon carbide paper, and then mirror-finished using a diluent such as alcohol or a liquid in which diamond powder with a grain size of 1 to 6 μm was dispersed in pure water. Next, the sample was polished for 30 minutes using colloidal silica without alkaline solution at room temperature to remove the strain introduced into the surface layer. At any position in the longitudinal direction of the sample cross section, a region 60 μm long in the rolling direction and 160 μm long in the thickness direction, centered at 1 / 4 of the thickness from the surface, was measured using electron backscatter diffraction at 0.1 μm measurement intervals to obtain crystal orientation information.

[0075] For the measurement, an EBSD device consisting of a field emission scanning electron microscope (FE-SEM: JSM-7001F manufactured by JEOL) and an EBSD detector (DVC5 type detector manufactured by TSL) is used. At this time, the degree of vacuum in the EBSD device is 9.6 × 10 －５ The pressure is set to 100 Pa or less, the acceleration voltage is 15 kV, and the probe current level is 13. For the analysis, a diffraction pattern database of Iron Alpha (bcc structure) and Iron Gamma (fcc structure) is used. From the obtained crystal orientation information, the "Phase Map" function installed in the software "TSL OIM Analysis (registered trademark)" attached to the EBSD analyzer is used to identify regions with an fcc crystal structure, and the area fraction of this region is calculated. This gives the area fraction of retained austenite.

[0076] Next, regions with a bcc crystal structure are determined to be bainite, ferrite, pearlite, fresh martensite, and tempered martensite. For these regions, the "Grain Orientation Spread" function installed in the software "TSL OIM Analysis (registered trademark)" attached to the EBSD analyzer is used to extract regions with a "Grain Orientation Spread" of 1° or less as ferrite under the condition that 15° grain boundaries are considered to be crystal grain boundaries. The area fraction of the extracted ferrite is calculated to obtain the area fraction of ferrite.

[0077] Next, within the remaining region (the region where "Grain Orientation Spread" exceeds 1°), grain boundaries with an orientation difference of 15° or more are considered to be crystal grain boundaries. When the maximum value of "Grain Average IQ" in the ferrite region is Iα, the region where Iα exceeds Iα / 2 is extracted as bainite, and the region where Iα / 2 or less is extracted as "pearlite, fresh martensite, and tempered martensite." The area fraction of bainite is obtained by calculating the area fraction of the extracted bainite.

[0078] The extracted "pearlite, fresh martensite, and tempered martensite" are distinguished into pearlite, fresh martensite, and tempered martensite by the following method.

[0079] In order to observe a secondary electron image using an FE-SEM in the same region as the EBSD measurement region, a Vickers indentation is made near the observation position. Subsequently, surface contamination is polished away, leaving the structure of the observation surface, and the specimen is then etched with nital. Next, the same field of view as the EBSD observation surface is observed using an FE-SEM at a magnification of 3000x. Of the regions identified as "pearlite, fresh martensite, and tempered martensite" in the EBSD measurement, regions with intragranular substructure and where cementite precipitates in multiple variants are determined to be tempered martensite. Regions where cementite precipitates in a lamellar form are determined to be pearlite. Regions with a brightness greater than that of the surrounding structure and where the substructure is not revealed by etching are determined to be fresh martensite. The area fractions of tempered martensite, pearlite, and fresh martensite are determined by calculating the area fractions of each.

[0080] Contamination on the surface of the observation surface can be removed by buffing using alumina particles with a particle size of 0.1 μm or less, or by Ar ion sputtering.

[0081] Furthermore, if the metal structure does not contain ferrite, another test piece is taken from the steel plate to be measured and subjected to heat treatment to generate ferrite. Specifically, the test piece is isothermally held in the range of 630 to 750°C for 1 hour, and then quenched under conditions where the average cooling rate from that temperature to 300°C is 50°C / s. The Iα of the generated ferrite is then measured, and this value is used to distinguish other structures.

[0082] Standard deviation of Mn concentration in fresh martensite and retained austenite: 1.0 to 5.0 mass% By reducing the standard deviation of the Mn concentration in fresh martensite and retained austenite, bendability after pre-straining can be improved. If the standard deviation of the Mn concentration in fresh martensite and retained austenite exceeds 5.0 mass%, microcracks are more likely to occur in the fresh martensite and retained austenite, and excellent bendability after pre-straining cannot be obtained. Therefore, the standard deviation of the Mn concentration in fresh martensite and retained austenite is set to 5.0 mass% or less. Preferably, it is 4.0 mass% or less, or 3.0 mass% or less.

[0083] The smaller this standard deviation, the better, but the substantial lower limit is 1.0%. It is considered possible to reduce the standard deviation of the Mn concentration in fresh martensite and retained austenite to less than 1.0% by mass by omitting the second heat treatment described below. However, a steel sheet manufactured without undergoing the second heat treatment cannot have the metal structure specified in the present invention, and therefore not only cannot obtain the desired strength, but also has reduced work hardening properties after pre-straining, making it impossible to obtain excellent bendability after pre-straining.

[0084] The Mn concentrations in fresh martensite and retained austenite are measured by the following method.

[0085] At a position 1 / 4 of the sheet thickness from the surface of the steel sheet, the Mn concentration in a region 100 μm in the sheet thickness direction and 200 μm in the rolling direction is measured using a field emission electron probe microanalyzer (FE-EPMA: JXA-8530F manufactured by JEOL). The measurement conditions are an acceleration voltage of 15 kV, and a distribution image of the Mn concentration is measured. More specifically, the measurement interval of the FE-EPMA is 0.2 μm, and the number of measurement points is 500,000.

[0086] Subsequently, a Vickers indentation is made near the observation position in the same region as the FE-EPMA measurement area to observe a secondary electron image using an FE-SEM. Surface contamination is then polished away, leaving the structure of the observation surface, and the specimen is then etched with nital. Next, the same region as the FE-EPMA measurement area is observed using an FE-SEM at a magnification of 3000x. Of the FE-EPMA measurement results, only those in the region identified by the FE-SEM as fresh martensite or retained austenite are extracted. This extracts only the Mn concentration of the fresh martensite or retained austenite contained in the measurement area. The standard deviation of the Mn concentration of the extracted fresh martensite or retained austenite is then calculated, and this is used as the standard deviation of the Mn concentrations of the fresh martensite and retained austenite.

[0087] Tensile strength: 1110 MPa or more The steel sheet according to this embodiment has a tensile strength of 1110 MPa or more. By making the tensile strength 1110 MPa or more, the steel sheet can be suitably applied to various automobile suspension parts. The tensile strength may be 1180 MPa or more, or 1300 MPa or more. The higher the tensile strength, the more preferable it is, but it may be 1500 MPa or less.

[0088] In the present invention, tensile strength is measured in accordance with JIS Z 2241:2022. For the measurement, a JIS No. 5 test piece (thickness: original thickness of steel sheet) specified in JIS Z 2241:2022, sampled so that the longitudinal direction coincides with the rolling direction of the steel sheet, is used. The gauge length is 50 mm, and the tensile speed is a crosshead displacement speed of 3 mm / min. In addition, when the steel sheet is plated, the tensile strength is measured using the test piece in the plated state, and the "thickness: original thickness of steel sheet" refers to the thickness of the base material excluding the plating layer. In other words, the original cross-sectional area of ​​the test piece used to calculate the tensile strength is the original cross-sectional area of ​​the base material excluding the plating layer.

[0089] The steel sheet according to this embodiment may be provided with a plating layer on the surface to improve corrosion resistance or the like, thereby forming a surface-treated steel sheet. The plating layer may be an electroplated layer or a hot-dip plated layer. Examples of electroplated layers include electrogalvanized plating and electrolytic Zn—Ni alloy plating. Examples of hot-dip plated layers include hot-dip galvannealed plating, hot-dip aluminum plating, hot-dip Zn—Al alloy plating, hot-dip Zn—Al—Mg alloy plating, and hot-dip Zn—Al—Mg—Si alloy plating.

[0090] The plating weight is not particularly limited and may be the same as in the past. In addition, it is also possible to further improve corrosion resistance by carrying out an appropriate chemical conversion treatment after plating (for example, by applying a silicate-based chromium-free chemical conversion treatment solution and drying it).

[0091] Next, a preferred method for producing the steel sheet according to this embodiment will be described. Note that the temperatures described below refer to the surface temperatures of the slab or steel sheet unless otherwise specified.

[0092] A preferred method for manufacturing the steel sheet according to this embodiment includes: a slab heating step of heating a slab having the above-described chemical composition to a temperature range of 1200°C or higher and holding the slab at that temperature range for 30 minutes or longer; a rough rolling step of performing rough rolling in a temperature range of 1000 to 1300°C, with each of the first to third passes having a reduction of 10 to 30% and each of the fourth and subsequent passes having a reduction of 15 to 50%; a finish rolling step of performing finish rolling at a reduction of 24% or higher twice or more, with the final pass having a reduction of 24 to 60% and with the finish rolling completion temperature being in the temperature range of 960 to 1060°C; a cooling step of cooling the slab at an average cooling rate of 30°C / s or higher in the temperature range of 900 to 400°C; a coiling step of coiling the slab at a temperature range of 200°C or lower; and a heat treatment step of holding the slab at a temperature range of 450 to 600°C for 10 to 200 seconds. and a heat treatment step of holding the steel sheet in a temperature range of 650 to 750° C. for 10 to 3,010 seconds. In addition to the steps described above, a cold rolling step of performing cold rolling with a cumulative reduction of 15% or less may be further performed after the coiling step and before the heat treatment step.

[0093] Each step will be described below.

[0094] Slab Heating Step The slab heating temperature is 1200°C or higher. The holding time in the temperature range of 1200°C or higher is 30 minutes or longer. If the slab heating temperature is lower than 1200°C or if the holding time in the temperature range of 1200°C or higher is shorter than 30 minutes, coarse precipitates cannot be sufficiently dissolved, resulting in significant fluctuations in the tensile strength of the steel sheet.

[0095] The upper limit of the heating temperature and the upper limit of the holding time in the temperature range of 1200°C or higher are not particularly limited, but may be 1300°C or lower and 300 minutes or shorter, respectively. Note that when holding in the temperature range of 1220°C or higher, the steel sheet temperature may be varied or may be constant.

[0096] The slab to be heated is not particularly limited except for the chemical composition described above. For example, a slab produced by melting molten steel having the above chemical composition using a converter or electric furnace, etc., and then subjecting the molten steel to continuous casting may be used. Instead of continuous casting, an ingot casting method, thin slab casting method, etc. may also be used.

[0097] Rough Rolling Process In the rough rolling process, rough rolling is performed in a temperature range of 1000 to 1300°C, with a rolling reduction of 10 to 30% for each of the first to third passes and 15 to 50% for each of the fourth and subsequent passes. If the temperature at which rough rolling is performed is less than 1000°C, precipitation of alloy carbides progresses, resulting in significant fluctuations in the tensile strength of the steel sheet. Therefore, rough rolling is performed in a temperature range of 1000°C or higher. On the other hand, rough rolling at a temperature of 1300°C or higher increases fuel costs, so rough rolling is performed in a temperature range of 1300°C or lower.

[0098] If rolling is performed with a reduction rate of less than 10% in the first to third passes, or if rolling is performed with a reduction rate of less than 15% in the fourth pass and thereafter, the crystal grains will become coarse, and the average grain size of the second phase will become coarse after the heat treatment process. Therefore, the reduction rates for each of the first to third passes are set to 10% or more, and for each of the fourth pass and thereafter, the reduction rates are set to 15% or more. Preferably, the reduction rates for each of the first to third passes are 15% or more or 20% or more, and for each of the fourth pass and thereafter, the reduction rates are 20% or more or 25% or more.

[0099] Furthermore, if rolling is performed at a reduction rate of more than 30% in the first to third passes, or at a reduction rate of more than 50% in the fourth pass or later, alloy carbides precipitate, resulting in significant fluctuations in the tensile strength of the steel sheet. Therefore, the reduction rates for each of the first to third passes are set to 30% or less, and for each of the fourth pass and later passes, the reduction rates are set to 50% or less. Preferably, the reduction rates for each of the first to third passes are set to 25% or less, and for each of the fourth pass and later passes, the reduction rates are set to 40% or less.

[0100] The reduction rate of each pass can be expressed as {1-(t1 / t0)}×100(%), where t0 is the thickness at the entrance of each pass and t1 is the thickness at the exit of each pass.

[0101] Finish Rolling Step In the finish rolling step, rolling is performed at least twice at a reduction rate of 24% or more, with the final pass reduction rate being 24 to 60% and the finish rolling completion temperature being in the temperature range of 960 to 1060°C.

[0102] If the finish rolling step involves rolling at a reduction rate of 24% or more less than once, a non-uniform strain distribution occurs in the metal structure after the finish rolling step, and some crystal grains preferentially reverse transform to the austenite phase. As a result, the timing of austenite generation becomes non-uniform, and the standard deviation of the Mn concentration in fresh martensite and retained austenite increases. Therefore, the finish rolling step involves rolling at a reduction rate of 24% or more more than twice.

[0103] The term "two passes" here includes the final pass. That is, in this embodiment, the reduction rate of the final pass is set to 24% or more, and rolling with a reduction rate of 24% or more is performed one or more times. There is no particular upper limit to the reduction rate in the finish rolling process, but the reduction rate in each pass may be 60% or less.

[0104] The rolling reduction in the final pass is set to 24% or more, preferably 28% or more or 30% or more. From the viewpoint of suppressing an increase in equipment load, the rolling reduction in the final pass is set to 60% or less, preferably 50% or less or 40% or less.

[0105] If the finish rolling completion temperature is less than 960°C, an austenite phase will be generated from flat grains, the grain size will become coarse, and the Mn concentration of the fresh martensite and the retained austenite will become non-uniform. Therefore, the finish rolling completion temperature is set to 960°C or higher, preferably 980°C or higher or 1000°C or higher.

[0106] If the finish rolling completion temperature exceeds 1060°C, the average grain sizes of fresh martensite and retained austenite become coarse, and the toughness of the steel sheet decreases. Therefore, the finish rolling completion temperature is set to 1060°C or less, and preferably 1040°C or less. The finish rolling completion temperature refers to the temperature at the outlet of the final pass of finish rolling.

[0107] Cooling Step In the cooling step, cooling is performed so that the average cooling rate in the temperature range of 900 to 400°C is 30°C / s or more. If the average cooling rate in the temperature range of 900 to 400°C is less than 30°C / s, a sufficient amount of martensite cannot be generated, and the desired amount of tempered martensite cannot be obtained after the heat treatment step. Therefore, the average cooling rate in the temperature range of 900 to 400°C is 30°C / s or more. It is preferably 50°C / s or more or 70°C / s or more. There is no particular upper limit, but it may be 200°C / s or less from the viewpoint of preventing an increase in cooling equipment.

[0108] There are no particular limitations on the cooling rate until coiling after cooling in the temperature range of 900 to 400° C. at the above average cooling rate. The average cooling rate here is the value obtained by dividing the temperature difference between the start point and the end point of the set range by the elapsed time from the start point to the end point.

[0109] Coiling process In the coiling process, the steel sheet is coiled at a temperature of 200°C or less. If the coiling temperature exceeds 200°C, bainite will form, the timing of austenite formation will become non-uniform, and the standard deviation of the Mn concentration in fresh martensite and retained austenite will increase. Therefore, the coiling temperature is set to 200°C or less. Preferably, the coiling temperature is 150°C or less or 100°C or less.

[0110] Cold Rolling Process After uncoiling the coiled steel sheet, cold rolling with a cumulative reduction of 15% or less may be performed. This cold rolling process is not an essential process and may not be performed. By performing cold rolling with a cumulative reduction of 15% or less, fine precipitates are formed, and the strength of the steel sheet can be further increased. The cumulative reduction of cold rolling is preferably 10% or less. On the other hand, if the cumulative reduction exceeds 15%, recrystallized ferrite is formed, and the standard deviation of the Mn concentration of fresh martensite and retained austenite becomes large. Note that pickling may be performed before cold rolling.

[0111] The cumulative reduction ratio of cold rolling is t, where t is the thickness after cold rolling and t is the thickness before cold rolling. ０ When this is the case, (1-t / t ０ ) × 100 (%).

[0112] After the coiling step or the cold rolling step, the steel sheet is subjected to two heat treatments, each held at a predetermined temperature range. The heat treatment includes a first heat treatment in a temperature range of 450 to 600°C for 10 to 200 seconds, and a second heat treatment in a temperature range of 650 to 750°C for 10 to 3,010 seconds.

[0113] If the heat treatment temperature of the first heat treatment is less than 450°C or the heat treatment time is less than 10 seconds, carbide precipitation will be insufficient, and the growth of austenite will be significant in the second heat treatment, resulting in a large standard deviation of the Mn concentrations of fresh martensite and retained austenite. Furthermore, if the heat treatment temperature of the first heat treatment is more than 600°C or more than 200 seconds, Mn enrichment in cementite will be significant, the timing of austenite formation will be uneven, and the growth of austenite will be significant in the second heat treatment, resulting in a large standard deviation of the Mn concentrations of fresh martensite and retained austenite. Therefore, the first heat treatment temperature is set to 450°C or more and 600°C or less, and the heat treatment time is set to 10 seconds or more. Preferably, the heat treatment temperature is set to 500°C or more or 550°C or more, and the heat treatment time is set to 15 seconds or more.

[0114] If the heat treatment temperature of the second heat treatment is less than 650°C or the heat treatment time is less than 10 seconds, the generation of fresh martensite and retained austenite will be insufficient. On the other hand, if the heat treatment temperature of the second heat treatment is more than 750°C or the heat treatment time is more than 3010 seconds, the generation of fresh martensite and retained austenite will be excessive, and the desired metal structure will not be obtained. Therefore, the second heat treatment temperature is 650°C or higher and 750°C or lower, and the heat treatment time is 10 seconds or longer. Preferably, the heat treatment temperature is 500°C or higher or 550°C or higher, and the heat treatment time is 15 seconds or higher and 3010 seconds or shorter. Preferably, the second heat treatment temperature is 680°C or higher and 720°C or lower, and the second heat treatment time is 100 seconds or shorter or 500 seconds or shorter.

[0115] The steel sheet heat-treated in the above-mentioned temperature range may be cooled to room temperature, or may be gas-cooled or water-cooled. Also, the steel sheet may be plated during gas cooling.

[0116] The steel sheet according to this embodiment can be manufactured by a manufacturing method including the steps described above.

[0117] Next, the effects of one embodiment of the present invention will be explained in more detail using examples, but the conditions in the examples are merely examples adopted to confirm the feasibility and effects of the present invention, and the present invention is not limited to these examples. Various conditions can be adopted in the present invention as long as they do not deviate from the gist of the present invention and the object of the present invention is achieved.

[0118] Slabs having the chemical compositions shown in Table 1 were produced by continuous casting. Steel plates having a thickness of 3.0 mm were produced using the obtained slabs under the conditions shown in Tables 2 and 3. Note that pickling was performed before cold rolling. After the second heat treatment, the plates were air-cooled to 500°C, plated, and then air-cooled to room temperature. Blank cells in Table 1 indicate that the element was not intentionally added. Furthermore, the "average cooling rate" in the "cooling process" in Table 3 refers to the average cooling rate in the temperature range of 900 to 400°C.

[0119]

[0120]

[0121]

[0122] The area fraction of each structure, the standard deviation of the Mn concentration of fresh martensite and retained austenite, the tensile strength, and the bendability after 2% pre-straining were investigated for the steel sheets obtained by the above-mentioned methods. The results are shown in Table 4.

[0123] In Table 4, TM, P, etc. respectively represent the following: TM: tempered martensite P: pearlite F: ferrite B: bainite FM: fresh martensite γ: retained austenite Mn standard deviation: standard deviation of Mn concentration in fresh martensite and retained austenite

[0124]

[0125] If the tensile strength was 1110 MPa or more, it was judged to have high strength and pass, and if the tensile strength was less than 1110 MPa, it was judged to not have high strength and fail.

[0126] To evaluate the bendability after pre-straining, a test was performed according to the following procedure. First, a JIS No. 5 test piece (thickness: original thickness of steel plate) specified in JIS Z 2241:2022 was taken so that the longitudinal direction coincided with the rolling direction of the steel plate. Then, a 2% tensile deformation was applied to the test piece, with a gauge length of 50 mm, a tensile speed of 3 mm / min in crosshead displacement rate, and other conditions in accordance with JIS Z 2241:2022.

[0127] Next, both ends of the test specimen after tensile deformation were cut, and a bending test specimen with a length of 60 mm and a width of 25 mm was prepared from the center of the parallel portion of the test specimen. Then, a bending test was performed on the obtained bending test specimen using the V-block method specified in JIS Z 2248:2022, so that the bending axis passed through the center of the length direction of the bending test specimen and was parallel to the width direction of the bending test specimen. In this case, V-shaped presser fittings with various tip radii R and a 90° angle were used, and the angle of the tapered surface of the V-block was 90°. The surface of the bending test specimen after the bending test was then visually inspected to determine the critical bending radius at which cracks did not occur.

[0128] The limiting bending deformation is the value (R / t) obtained by dividing the tip radius R by the plate thickness t. When the limiting bending deformation was 2.0 or less, the specimen was judged to have excellent bendability after pre-straining and to have passed the test, and when the limiting bending deformation was more than 2.0, the specimen was judged to have poor bendability after pre-straining and to have failed the test. Note that the bending test was performed using a test piece in a plated state, and the above "plate thickness t" refers to the thickness of the base material excluding the plated layer.

[0129] As shown in Table 4, Test Nos. 1, 2, 18, 20, and 25-31, which satisfied all of the requirements of the present invention, had strengths of 1110 MPa or more and exhibited excellent bendability after pre-straining. In contrast, Test Nos. 3-5, 8, 9, 13, 15, 16, 22, 24, and 32 had excessive standard deviations in Mn concentration, resulting in poor bendability after pre-straining. Test No. 33 did not undergo two heat treatments, failing to obtain the desired metallographic structure, resulting in poor strength. Despite the extremely low standard deviation in Mn concentration, poor bendability after pre-straining was observed. Furthermore, Test Nos. 6 and 10-12 had metallographic structures that did not meet the requirements of the present invention, resulting in poor bendability after pre-straining. Test No. 14 had metallographic structures that did not meet the requirements of the present invention, resulting in poor strength. Furthermore, Test No. 14 also had metallographic structures that did not meet the requirements of the present invention, resulting in poor strength. In Nos. 7, 17, 19 and 21 to 23, the chemical compositions were outside the scope of the present invention, resulting in a decrease in strength.

[0130] According to this embodiment, it is possible to provide a steel sheet having high strength and excellent bendability after pre-straining.

Claims

1. The chemical composition is in mass %, C: 0.08 to 0.17%, Si: 0.03 to 1.40%, Mn: 1.60 to 3.00%, Al: 0.01 to 0.70%, P: 0.080% or less, S: 0.0100% or less, N: 0.0050% or less, Ti: 0.020 to 0.180%, Nb: 0.010 to 0.050%, Ti + Nb + (Mo / 2) + V: 0.100 to 0.600%, the balance: Fe and impurities, the metallographic structure is in area %, tempered martensite: 80.0 to 97.0%, the total of pearlite, ferrite and bainite: 10.0% or less, and the total of fresh martensite and retained austenite: 3.0 to 10.0%, the standard deviation of the Mn concentration of the fresh martensite and the retained austenite is 1.0 to 5.0%, the tensile strength is 1110 MPa or more, a steel plate.

2. the metallographic structure is in area %, the retained austenite: 1.5% or more, the steel plate according to Claim 1.

3. the chemical composition contains, in mass %, instead of a part of Fe, Mo: 0.600% or less, and V: 0.300% or less, one or more selected therefrom, the steel plate according to Claim 1.

4. The chemical composition contains, in mass %, instead of a part of Fe, Mo: 0.600% or less, and V: 0.300% or less, one or more selected therefrom, the steel plate according to Claim 2.

5. the chemical composition contains, in mass %, instead of a part of Fe, B: 0.0030% or less, Cr: 0.50% or less, Cu: 0.50% or less, and Ni: 2.0% or less, one or more selected therefrom, the steel plate according to Claim 1.

6. The chemical composition contains, in mass %, instead of a part of Fe, B: 0.0030% or less, Cr: 0.50% or less, Cu: 0.50% or less, and Ni: 2.0% or less, one or more selected therefrom, the steel plate according to Claim 2.

7. The chemical composition contains, in mass %, instead of a part of Fe, B: 0.0030% or less, Cr: 0.50% or less, Cu: 0.50% or less, and Ni: 2.0% or less, one or more selected therefrom, the steel plate according to Claim 3.

8. The chemical composition contains, in mass %, instead of a part of Fe, B: 0.0030% or less, Cr: 0.50% or less, Cu: 0.50% or less, and Ni: 2.0% or less, one or more selected therefrom, The steel sheet according to claim 4.

9. The chemical composition contains, in mass %, instead of a part of Fe, Ca: 0.020% or less, Mg: 0.020% or less, REM: 0.100% or less, and Bi: 0.020% or less, one or more selected from the group consisting of The steel sheet according to any one of claims 1 to 8.

10. The chemical composition contains, in mass %, instead of a part of Fe, Sn: 0.05% or less, The steel sheet according to any one of claims 1 to 8.

11. The chemical composition contains, in mass %, instead of a part of Fe, Sn: 0.05% or less, The steel sheet according to claim 9.