steel plate

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

Technical Field

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

Background Art

[0002] In recent years, weight reduction of automotive and machine parts has been promoted. By designing the part shape into an optimal shape to ensure rigidity, it is possible to reduce the weight of automotive and machine parts. Furthermore, in blank forming parts such as press formed parts, weight reduction can be achieved by reducing the plate thickness of the part material.

[0003] However, when trying to ensure the strength characteristics 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, for automotive underbody parts such as lower arms, trailing links, and knuckles, the application of steel sheets with a strength exceeding 780 MPa class has begun to be considered. These automotive underbody parts are manufactured by subjecting the steel sheet to bending forming or the like. Therefore, the steel sheet applied to these automotive underbody parts is required to have excellent formability, particularly bendability.

[0004] For example, Patent Document 1 discloses a precipitation hardening type martensitic steel having, in mass%, C: 0.02 to 0.08%, Al: not more than 0.05%, Cr: 8.0 to 13.0%, Ni: 2.0 to 8.0%, Co: 2.0 to 16.0%, Mo + 0.5W: 3.5 to 8.0% with Mo as an essential element, and the balance being Fe and impurities, and having both high tensile strength and Charpy absorption energy.

[0005] Furthermore, Patent Document 2 discloses a high-strength steel with high tensile strength and excellent ductility and bendability, wherein the microstructure has a ferrite area percentage of 5% or more and less than 50%, and the area percentage of a mixed structure of fresh martensite and retained austenite relative to the total structure is greater than 0% and less than or equal to 30%. Moreover, when analyzed with an electron probe microprobe analyzer, there is a region of 5% or more area where the Mn concentration is concentrated to 1.2 times or more the Mn concentration in the steel sheet, and when the fraction of the region where the Mn concentration is concentrated to 1.2 times or more the Mn concentration in the steel sheet is measured in 2 μm sections, the standard deviation when 100 sections are measured is 4.0% or more. [Prior art documents] [Patent Documents]

[0006] [Patent Document 1] Japanese Patent Publication No. 2014-208869 [Patent Document 2] Japanese Patent Publication No. 2015-193897 [Overview of the project] [Problems that the invention aims to solve]

[0007] However, Patent Document 1 does not consider bendability. Furthermore, the above-mentioned automobile undercarriage parts are manufactured by performing multiple forming processes on a steel sheet. Therefore, the steel sheet used for automobile undercarriage parts is required to have excellent formability even after being pre-strained to a certain extent in the previous process. When performing multiple forming processes, if the strain generated in the previous process is not sufficiently distributed, localized deformation may occur in the subsequent process, and the steel sheet may not be able to exhibit its original formability. Such localized deformation is particularly noticeable in bending processes. However, Patent Document 2 does not consider bendability after pre-straining.

[0008] This invention has been made in view of the above circumstances, and aims to provide a steel sheet that has high strength and excellent bendability after pre-straining. [Means for solving the problem]

[0009] Based on the above findings, the gist of this invention is as follows:

[0010] (1) The steel sheet according to one embodiment of the present invention has a chemical composition in 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%, Ti+Nb+(Mo / 2)+V:0.100~0.600%, The remainder consists of Fe and impurities. The metallic structure, in area percentage, Tempered martensite: 80.0-97.0% The total of perlite, ferrite, and bainite: 10.0% or less, and, The total of fresh martensite and retained austenite is 3.0-10.0%. The standard deviation of the Mn concentration in the fresh martensite and the retained austenite is 1.0 to 5.0%. The tensile strength is 1110 MPa or higher.

[0011] (2) The steel sheet described in (1) above may have a metallic structure in which retained austenite is 1.5% or more by area.

[0012] (3) The steel plate described in (1) or (2) above has a chemical composition in which, in addition to a portion of Fe, by mass%, Mo: 0.600% or less, and V: 0.300% or less, It may contain one or more selected from the following.

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

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

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

Advantages of the Invention

[0016] According to the present embodiment, it is possible to provide a steel sheet having high strength and excellent bending properties after pre-strain is applied.

Modes for Carrying Out the Invention

[0017] <> As a result of intensive studies on the method for obtaining the above steel sheet, the present inventors obtained the following findings. \n

[0018] By suppressing local deformation during bending forming after pre-strain is applied, cracking during forming in multiple steps can be prevented. For this purpose, it is important that the Mn distribution in fresh martensite and retained austenite is uniform, that is, the standard deviation of the Mn concentration in fresh martensite and retained austenite is small. [[ID=>

[0019] Furthermore, in order to suppress such localized deformation and to achieve a tensile strength of 1110 MPa or higher for the steel sheet, it is necessary to have tempered martensite as the main phase and to disperse fine precipitates.

[0020] 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 subsequent heat treatment conditions.

[0021] The steel plate 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 without departing from the spirit of the invention. The numerical limit ranges described below, separated by "~", include both a lower limit and an upper limit. Numerical values ​​indicated as "less than" or "greater than" do not include the numerical range. All "%" in relation to chemical composition refers to "mass%".

[0022] The steel sheet according to this embodiment has a chemical composition in mass percent of: 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%, Ti+Nb+(Mo / 2)+V: 0.100~0.600%, and the remainder: Fe and impurities. Each element will be described in detail below.

[0023] C: 0.08~0.17% Carbon (C) is an element necessary to obtain the desired tensile strength of steel sheets. If the C content is less than 0.08%, the desired tensile strength cannot be obtained. Therefore, the C content should be 0.08% or more. Preferably, the C content is 0.09% or more, 0.10% or more, or 0.11% or more.

[0024] On the other hand, if the carbon content exceeds 0.17%, weldability decreases. In addition, the amount of dissolved carbon becomes excessive, and austenite growth during heat treatment is carried out by the diffusion of carbon, resulting in a non-uniform Mn distribution in fresh martensite and retained austenite. For this reason, the carbon content should be 0.17% or less. Preferably, it is 0.15% or less or 0.14% or less. To lower the standard deviation of the Mn concentration in fresh martensite and retained austenite and to further improve the bendability after pre-straining, the carbon content should be 0.13% or less. The carbon 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: 0.03~1.40% 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 molding will decrease. Therefore, the Si content should be 0.03% or more. Preferably, the Si content is 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 properties of the steel sheet deteriorate. Therefore, the Si content should be 1.40% or less. Preferably, the Si content is 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: 1.60~3.00% Mn is an element necessary to improve the strength of steel sheets. If the Mn content is less than 1.60%, the area ratio of ferrite becomes too high, and the desired tensile strength cannot be obtained. Therefore, the Mn content should be 1.60% or more. Preferably, the Mn content is 1.80% or more. More preferably, the Mn content is 2.00% or more in order to lower the standard deviation of the Mn concentration in fresh martensite and retained austenite and to further improve the bendability after pre-straining.

[0028] On the other hand, if the Mn content exceeds 3.00%, cracking of the cast slab becomes more likely, and hot rolling may become difficult. Therefore, the Mn content should be 3.00% or less. Preferably, the Mn content is 2.70% or less, and more preferably 2.50% or less. Preferably, the Mn content is 1.80 to 2.70%, and more preferably 2.00 to 2.50%.

[0029] Al: 0.01~0.70% Al acts as a deoxidizing agent and improves the cleanliness of steel. If the Al content is less than 0.01%, a sufficient deoxidizing effect cannot be obtained, and a large amount of inclusions (oxides) are formed in the steel sheet. Such inclusions degrade the surface properties of the steel sheet. Therefore, the Al content should be 0.01% or more. Preferably, the Al content is 0.02% or more, more preferably 0.03% or more, and even more preferably 0.04% or more.

[0030] On the other hand, casting may become difficult if the Al content exceeds 0.70%. Therefore, the Al content should be 0.70% or less. Preferably, the Al content is 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 decreases. Therefore, the P content should be 0.080% or less. Preferably, the P content is 0.040% or less, and more preferably 0.020% or less.

[0032] A lower P content is preferable, and 0% is preferable; however, excessively reducing the P content significantly increases the cost of P removal. Therefore, the P content may be 0.0005% or higher.

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

[0034] A lower sulfur content is preferable, and 0% is preferable; however, excessively reducing the sulfur content significantly increases the cost of sulfur removal. Therefore, the sulfur content may be 0.0005% or higher.

[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 is more likely to occur, and hot rolling may become difficult. Therefore, the N content should be 0.0050% or less. Preferably, the N content is 0.0040% or less, and more preferably 0.0035% or less.

[0036] A lower N content is preferable, and 0% is preferable; however, excessively reducing the N content significantly increases the cost of nitrogen removal. Therefore, the N content may be 0.0005% or higher.

[0037] Ti: 0.020~0.180% Ti is an element that increases the strength of steel sheets by forming fine carbides and / or carbonitrides in the steel. If the Ti content is less than 0.020%, the desired tensile strength cannot be obtained. Therefore, the Ti content should be 0.020% or more. Preferably, the Ti content is 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 prone to cracking, which can make hot rolling difficult. Therefore, the Ti content should be 0.180% or less. Preferably, the Ti content is 0.160% or less, and more preferably 0.150% or less. Preferably, the Ti content is 0.050 to 0.160%, and more preferably 0.080 to 0.150%.

[0039] Nb: 0.010~0.050% Nb is an element that suppresses abnormal grain growth of austenite grains during hot rolling. Nb also increases the strength of steel sheets 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 should be 0.010% or more. Preferably, the Nb content is 0.013% or more, and more preferably 0.015% or more.

[0040] On the other hand, if the Nb content exceeds 0.050%, cracks are more likely to occur in the cast slab, making hot rolling difficult. Therefore, the Nb content should be 0.050% or less. Preferably, the Nb content is 0.040% or less, and more preferably 0.035% or less. Preferably, the Nb content is 0.013 to 0.040%, and more preferably 0.015 to 0.035%.

[0041] Ti+Nb+(Mo / 2)+V: 0.100~0.600% In this embodiment, the sum of the Ti content, Nb content, half the amount of Mo content (described later), and V content is controlled. That is, when the content of each element is expressed by its respective element symbol, Ti + Nb + (Mo / 2) + V is controlled. If the sum of these amounts is less than 0.100%, the effect of forming at least one of the 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 above sum is set to 0.100% or more.

[0042] Furthermore, by causing these elements to form carbides, the solid solution carbon in the steel is consumed. This makes it possible to control the growth of austenite through the diffusion of Mn. As a result, the Mn distribution in fresh martensite and retained austenite can be made uniform, which improves the bendability after pre-straining.

[0043] It is not necessary to include all of Ti, Nb, Mo, and V; the above effects can be obtained if the content of any one of Ti, Nb, or V, or half the amount of Mo, is 0.100% or more. The total amount is preferably 0.150% or more. To lower the standard deviation of Mn concentration in fresh martensite and retained austenite and to further improve the bendability after pre-straining, it is more preferably 0.200% or more, even more preferably 0.230% or more, and still more preferably 0.250% or more. If the total amount exceeds 0.230%, then at least one of Mo and V will be substantially essential.

[0044] On the other hand, if the above total amount exceeds 0.600%, the precipitates become coarse, and the desired tensile strength cannot be obtained. Therefore, the above total amount should be 0.600% or less. The above total amount is preferably 0.500% or less, 0.400% or less, or 0.300% or less. The above 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 remainder of the chemical composition of the steel sheet according to this embodiment may be Fe and impurities. In this embodiment, impurities refer to substances introduced from raw materials such as ore, scrap, or the manufacturing environment, or substances that are acceptable as long as they do not adversely affect the steel sheet according to this embodiment.

[0046] The steel sheet according to this embodiment may contain the following optional elements in place of a portion of Fe. The lower limit of the content when no optional elements are included is 0%. Each optional element is described below.

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

[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 reliably obtain 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%, cracking of the cast slab becomes more likely, and hot rolling may become difficult. Therefore, the V content should be 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 0.010~0.270%, 0.030~0.240%, or 0.050~0.200%.

[0049] B: 0.0030% or less B is an element that suppresses ferrite formation during the cooling process and increases the strength of the steel sheet. To reliably obtain 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, the above effect saturates if the B content exceeds 0.0030%. Therefore, the B content should be 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 effects similar to Mn. To reliably obtain the effect of increasing the strength of steel sheets due to Cr content, 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, the above effect saturates even if the Cr content exceeds 0.50%. Therefore, the Cr content should be 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 0.001-0.40%, 0.005-0.30%, or 0.010-0.20%.

[0051] Cu: 0.50% or less Cu has the effect of increasing the hardenability of steel sheets and increasing the strength of steel sheets by precipitation in the steel. To more reliably obtain the effects of 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%, grain boundary cracking of the slab may occur. Therefore, the Cu content should be 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 increasing the hardenability of steel plates and thereby increasing their strength. Furthermore, when Cu is included, Ni effectively suppresses grain boundary cracking in the slab caused by Cu. To more reliably obtain the effects of the above effects, it is preferable that the Ni content be 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 not economically desirable to include it in large quantities. Therefore, the Ni content should be 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 steel sheets by controlling the shape of inclusions to a desirable shape. Therefore, one or more of these elements may be included. To more reliably obtain the effects of the above action, it is preferable to have one or more of Ca, Mg, and REM in an amount of 0.0005% or more. It is more preferable that the content of Ca, Mg, and REM is 0.0010% or more, and even more preferable that it is 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, which may reduce the ductility of the steel sheet. Therefore, the Ca content and Mg content should be 0.020% or less, and the REM content should be 0.100% or less. The Ca and Mg content is preferably 0.015% or less, 0.010% or less, 0.0050% or less, or 0.0030% or less. The REM content is also preferably 0.050% or less, 0.030% or less, 0.010% or less, 0.0050% or less, or 0.0030% or less.

[0055] The calcium content is preferably 0.0005-0.015%, 0.0010-0.010%, 0.0015-0.0050%, or 0.0020-0.0030%. The magnesium content is preferably 0.0005-0.015%, 0.0010-0.010%, 0.0015-0.0050%, or 0.0020-0.0030%. The REM content is preferably 0.0001-0.050%, 0.0005-0.030%, 0.0010-0.010%, 0.0015-0.0050%, or 0.0020-0.0030%.

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

[0057] Bi:0.020% or less Bi has the effect of improving the formability of steel sheets by refining the solidification structure. To more reliably obtain the effect of this action, it is preferable to have a Bi content of 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 not economically desirable. Therefore, the Bi content should be 0.020% or less. The Bi content is preferably 0.010% or less. The Bi content is preferably 0.0005-0.016%, 0.0010-0.013%, or 0.0050-0.010%.

[0058] Sn: 0.05% or less Sn has the effect of improving the plating processability during the manufacture of plated steel sheets. To more reliably obtain this effect, it is preferable to have a Sn content of 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 should be 0.05% or less. The Sn content is preferably 0.04% or less. The Sn content is preferably between 0.01% and 0.04%, or between 0.02% and 0.03%.

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

[0060] The chemical composition of the steel sheet described above can be analyzed using a spark discharge emission spectrometer or similar device. The values ​​for C and S should be identified by combustion in an oxygen stream using a gas component analyzer and measured by infrared absorption. The value for N should be identified by melting a test piece taken from the steel sheet in a helium stream and measuring it by thermal conductivity. In cases where the steel sheet is plated, "chemical composition of the steel sheet" refers to the chemical composition of the base material excluding the plating layer.

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

[0062] The steel sheet according to this embodiment has a microstructure in which, by area percent, tempered martensite accounts for 80.0-97.0%, the total of pearlite, ferrite, and bainite accounts for 10.0% or less, and the total of fresh martensite and retained austenite accounts for 3.0-10.0%, with a standard deviation of Mn concentration of fresh martensite and retained austenite of 5.0% or less.

[0063] In this embodiment, the microstructure at the 1 / 4 position of the steel plate thickness is defined. This is because the microstructure at this position represents the typical microstructure of the steel plate. Here, in this specification, "1 / 4 position of plate thickness" means the region centered at the 1 / 4 position of plate thickness in the plate thickness direction. Furthermore, when the steel plate is plated, "plate thickness" refers to the plate thickness of the base material portion excluding the plating layer. In this invention, the thickness of the plating layer is determined by observation using an optical microscope.

[0064] Area ratio of tempered martensite: 80.0-97.0% Tempered martensite increases the strength of steel sheets. If the area ratio of tempered martensite is less than 80.0%, the desired tensile strength cannot be obtained. On the other hand, if the area ratio of tempered martensite exceeds 97.0%, the bendability after pre-straining decreases. Therefore, the area ratio of tempered martensite should be between 80.0% and 97.0%. Preferably, the area ratio of tempered martensite is between 85.0% and 95.0%.

[0065] Total area percentage of perlite, ferrite, and bainite: 10.0% or less If the combined area percentage of pearlite, ferrite, and bainite is high, the desired tensile strength cannot be obtained. Therefore, the combined area percentage of these structures should be 10.0% or less. Preferably, the combined area percentage of these structures is 7.0% or less, 5.0% or less, or 3.0% or less. A lower combined area percentage of these structures is preferable, so it may be 0.0%.

[0066] Furthermore, it is not necessary to include all three: perlite, ferrite, and bainite. It may include only one of them, provided its area ratio is within the above range, or it may include two or three of them, with the sum of their area ratios being within the above range.

[0067] Total area percentage of fresh martensite and retained austenite: 3.0-10.0% In this embodiment, if the total area ratio of fresh martensite and retained austenite is less than 3.0%, the deformation after pre-straining is uneven, and the bendability of the steel sheet after pre-straining decreases. The above effect cannot be obtained. Therefore, the total area ratio of fresh martensite and retained austenite should be 3.0% or more. Preferably, it should be 4.0% or more, or 5.0% or more.

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

[0069] Furthermore, it is not necessary to include both fresh martensite and retained austenite; it may include only one of them, and its area ratio may be within the range described above.

[0070] The combined area ratio of fresh martensite and retained austenite may be set to 3.0-10.0%, with the area ratio of retained austenite being 1.5% or more. By setting the area ratio of retained austenite in the second phase to 1.5% or more, the bendability after pre-strain can be further improved. It is more preferable that the area ratio of retained austenite be 2.0% or more, 3.0% or more, or 4.0% or more. There is no particular upper limit to the area ratio of retained austenite, but it may be 10.0% or less or 7.0% or less.

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

[0072] The following describes how to measure the area ratio of each tissue.

[0073] A test specimen is taken from the steel plate in a cross-section parallel to the rolling direction, so that the metallographic structure can be observed at a depth of 1 / 4 of the plate thickness from the surface and at the center in the width direction of the plate.

[0074] After polishing the cross-section of the above test specimen using silicon carbide paper ranging from #600 to #1500, it is finished to a mirror surface using a liquid in which diamond powder with a particle size of 1 to 6 μm is dispersed in a diluent such as alcohol or pure water. Next, the sample is polished for 30 minutes at room temperature using colloidal silica that does not contain alkaline solutions to remove the strain introduced into the surface of the sample. At any position in the longitudinal direction of the sample cross-section, the crystal orientation information is obtained by measuring a region with a length of 60 μm in the rolling direction and a length of 160 μm in the thickness direction, centered at a position 1 / 4 of the plate thickness from the surface, using electron backscatter diffraction at measurement intervals of 0.1 μm.

[0075] For the measurements, an EBSD system consisting of a field emission scanning electron microscope (FE-SEM: JEOL JSM-7001F) and an EBSD detector (TSL DVC5 detector) is used. The vacuum level inside the EBSD system is 9.6 × 10⁻⁶. -5The pressure is set to Pa or less, the acceleration voltage to 15kV, and the irradiation current level to 13. For the analysis, diffraction pattern databases for Iron Alpha (bcc structure) and Iron Gamma (fcc structure) are used. From the obtained crystal orientation information, the "Phase Map" function installed in the "TSL OIM Analysis®" software included with the EBSD analyzer is used to identify regions with an fcc crystal structure, and the area fraction of these regions is calculated. This gives the area fraction of retained austenite.

[0076] Next, materials with a bcc crystal structure are identified as bainite, ferrite, pearlite, fresh martensite, and tempered martensite. For these regions, the "Grain Orientation Spread" function included in the "TSL OIM Analysis®" software provided with the EBSD analyzer is used to extract regions where the "Grain Orientation Spread" is 1° or less, under the condition that 15° grain boundaries are considered crystal grain boundaries, as ferrite. The area ratio of the extracted ferrite is then calculated to obtain the area ratio of ferrite.

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

[0078] The extracted "perlite, fresh martensite, and tempered martensite" are distinguished from each other by the following method.

[0079] To observe a secondary electron image using FE-SEM in the same area as the EBSD measurement area, a Vickers indentation is imprinted near the observation position. Then, surface contaminants are polished off, leaving the microstructure of the observation surface, and then nital etching is performed. Next, the same field of view as the EBSD observation surface is observed at a magnification of 3000x using FE-SEM. Among the areas identified as "pearlite, fresh martensite, and tempered martensite" in the EBSD measurement, areas with a substructure within the grain and cementite precipitated with multiple variants are identified as tempered martensite. Areas where cementite precipitates in a lamellar pattern are identified as pearlite. Areas with a brightness greater than the surrounding microstructure and where the substructure is not revealed by etching are identified as fresh martensite. By calculating the area ratio of each, the area ratios of tempered martensite, pearlite, and fresh martensite are obtained.

[0080] To remove contaminants from the surface of the observation area as described above, buff polishing using alumina particles with a particle size of 0.1 μm or less, or methods such as Ar ion sputtering can be used.

[0081] Furthermore, if the metal structure does not contain ferrite, another test piece is taken from the steel plate to be measured, and this test piece is heat-treated to generate ferrite. Specifically, it is isothermally maintained at a temperature of 630 to 750°C for one 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 between other structures.

[0082] Standard deviation of Mn concentration in fresh martensite and retained austenite: 1.0–5.0% by mass By reducing the standard deviation of the Mn concentration in fresh martensite and retained austenite, the 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%, microscopic cracks are more likely to occur than in fresh martensite and retained austenite, and excellent bendability cannot be obtained after pre-straining. Therefore, the standard deviation of the Mn concentration in fresh martensite and retained austenite should be 5.0 mass% or less. Preferably, it is 4.0 mass% or less and 3.0 mass% or less.

[0083] A smaller standard deviation is preferable, but the practical 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 mass% by omitting the two heat treatments described later. However, steel sheets manufactured without the two heat treatments cannot have the metal structure defined in this invention, and therefore not only cannot obtain the desired strength, but the work hardening characteristics after pre-straining are reduced, and excellent bendability cannot be obtained after straining.

[0084] The Mn concentration of fresh martensite and residual austenite is measured by the following method.

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

[0086] Subsequently, a Vickers indentation is imprinted near the observation position in the same region as the FE-EPMA measurement area in order to observe a secondary electron image using FE-SEM. Then, surface contaminants are polished off, leaving the microstructure of the observation surface, and then Nital etching is performed. Next, the same region as the FE-EPMA measurement area is observed using FE-SEM at a magnification of 3000x. Then, only the measurement results from the regions identified as fresh martensite or retained austenite by FE-SEM are extracted from the FE-EPMA measurement results. This extracts only the Mn concentration of fresh martensite or retained austenite contained in the measurement area. The standard deviation of the extracted fresh martensite or retained austenite Mn concentration is then calculated and used as the standard deviation of the Mn concentration of fresh martensite and retained austenite.

[0087] Tensile strength: 1110 MPa or higher The steel sheet according to this embodiment has a tensile strength of 1110 MPa or higher. A tensile strength of 1110 MPa or higher allows for suitable application to various automobile undercarriage parts. The tensile strength may also be 1180 MPa or higher, or 1300 MPa or higher. While a higher tensile strength is preferable, it may be 1500 MPa or lower.

[0088] In this invention, the tensile strength will be measured in accordance with JIS Z 2241:2022. For the measurement, a JIS No. 5 test specimen (thickness: original thickness of the steel plate) as specified in JIS Z 2241:2022 will be used, with the longitudinal direction coinciding with the rolling direction of the steel plate. The gauge length will be 50 mm, and the tensile speed will be 3 mm / min using the crosshead displacement speed. In the case where the steel plate is plated, the tensile strength will be measured using a test specimen with the plated surface, but "thickness: original thickness of the steel plate" will be the thickness of the base material excluding the plating layer. That is, the original cross-sectional area of ​​the test specimen 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 a surface-treated steel sheet with a plating layer on its surface for the purpose of improving corrosion resistance, etc. The plating layer may be an electroplating layer or a hot-dip plating layer. Examples of electroplating layers include electroplated zinc and electroplated Zn-Ni alloy. Examples of hot-dip plating layers include hot-dip galvanizing, alloyed hot-dip galvanizing, 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 amount of plating applied is not particularly limited and may be the same as in the conventional method. Furthermore, it is possible to further enhance corrosion resistance by applying an appropriate chemical conversion treatment after plating (for example, application and drying of a silicate-based chromium-free chemical conversion treatment solution).

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

[0092] A preferred method for manufacturing steel plates according to this embodiment is: A slab heating step involves heating a slab having the above-mentioned chemical composition to a temperature range of 1200°C or higher and holding it at that temperature range for 30 minutes or more. A rough rolling process is performed in a temperature range of 1000 to 1300°C, such that the reduction ratio for each of the first to third passes is 10 to 30%, and the reduction ratio for each of the fourth pass and beyond is 15 to 50%. A finishing rolling process in which rolling is performed at least twice with a reduction ratio of 24% or more, the reduction ratio of the final pass is 24-60%, and the finishing rolling is completed at a temperature range of 960-1060°C, A cooling process in which the average cooling rate in the temperature range of 900-400°C is 30°C / s or more, A winding process that takes place in a temperature range of 200℃ or less, A heat treatment process in which the temperature range is 450-600°C and held for 10-200 seconds, The process involves sequentially performing a heat treatment step of holding the material at a temperature range of 650-750°C for 10-3010 seconds. In addition to the above-described process, a cold rolling process may be performed after the winding process and before the heat treatment process, in which the cumulative reduction ratio is 15% or less.

[0093] The following describes each step.

[0094] Slab heating process The heating temperature of the slab shall be 1200°C or higher. Furthermore, the holding time in the temperature range of 1200°C or higher shall be 30 minutes or more. If the heating temperature of the slab is less than 1200°C, or if the holding time in the temperature range of 1200°C or higher is less than 30 minutes, the coarse precipitates cannot be sufficiently dissolved, resulting in significant fluctuations in the tensile strength of the steel plate.

[0095] There are no particular limitations on the upper limit of the heating temperature or the upper limit of the holding time in the temperature range above 1200°C, but they may be set to 1300°C or less and 300 minutes or less, respectively. Furthermore, when holding in the temperature range above 1220°C, the steel plate temperature may be varied or kept constant.

[0096] Furthermore, the slab to be heated is not particularly limited except for having the chemical composition described above. For example, molten steel with the above chemical composition can be produced using a converter or electric furnace, and a slab manufactured by continuous casting can be used. Instead of continuous casting, methods such as ingot casting or thin slab casting may be employed.

[0097] Rough rolling process In the rough rolling process, rough rolling is performed in the temperature range of 1000 to 1300°C, with reduction ratios of 10 to 30% for the first three passes and 15 to 50% for the fourth pass and beyond. If the rough rolling temperature is below 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 the temperature range of 1000°C or higher. On the other hand, performing rough rolling above 1300°C increases fuel costs, so rough rolling is performed in the temperature range of 1300°C or lower.

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

[0099] Furthermore, if rolling is performed with a reduction ratio exceeding 30% in the first to third passes, or if rolling is performed with a reduction ratio exceeding 50% in the fourth pass or later, alloy carbides will precipitate, resulting in significant fluctuations in the tensile strength of the steel sheet. Therefore, the reduction ratio for each of the first to third passes should be 30% or less, and the reduction ratio for each of the fourth pass and later should be 50% or less. Preferably, the reduction ratio for each of the first to third passes should be 25% or less, and the reduction ratio for each of the fourth pass and later should be 40% or less.

[0100] The reduction ratio for each pass can be expressed as {1-(t1 / t0)}×100(%), where t0 is the inlet plate thickness of each pass and t1 is the outlet plate thickness of each pass.

[0101] Finishing rolling process In the finish rolling process, rolling is performed at least twice with a reduction ratio of 24% or more, so that the reduction ratio of the final pass is between 24% and 60%, and the finish rolling is completed at a temperature range of 960 to 1060°C.

[0102] In the finishing rolling process, if rolling to a reduction ratio of 24% or more is performed only once or less, a non-uniform strain distribution occurs within the metal microstructure after the finishing rolling process, causing some crystal grains to preferentially undergo reverse transformation into the austenite phase. As a result, the timing of austenite formation becomes non-uniform, and the standard deviation of Mn concentration in fresh martensite and retained austenite increases. Therefore, in the finishing rolling process, rolling to a reduction ratio of 24% or more should be performed at least twice.

[0103] The term "two times" here includes the final pass. In other words, in this embodiment, the reduction ratio of the final pass is set to 24% or more, and then rolling is performed at least once with a reduction ratio of 24% or more. There is no particular upper limit to the reduction ratio in the finishing rolling process, but the reduction ratio in each pass may be 60% or less.

[0104] The reduction ratio of the final pass shall be 24% or more. Preferably, it shall be 28% or more or 30% or more. Furthermore, from the viewpoint of suppressing an increase in equipment load, the reduction ratio of the final pass shall be 60% or less. Preferably, it shall be 50% or less or 40% or less.

[0105] If the finish rolling completion temperature is below 960°C, an austenite phase will form from the flattened grains, causing the grain size to coarse and resulting in uneven Mn concentration in the fresh martensite and retained austenite. Therefore, the finish rolling completion temperature should be 960°C or higher. Preferably, it should be 980°C or higher, or 1000°C or higher.

[0106] If the finish rolling completion temperature exceeds 1060°C, the average grain size of fresh martensite and retained austenite becomes coarser, reducing the toughness of the steel sheet. Therefore, the finish rolling completion temperature should be 1060°C or lower. Preferably, it should be 1040°C or lower. The finish rolling completion temperature refers to the exit temperature of the final pass of the finish rolling process.

[0107] cooling process In the cooling process, the average cooling rate in the 900-400°C temperature range is 30°C / s or higher. If the average cooling rate in the 900-400°C temperature range 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 process. Therefore, the average cooling rate in the 900-400°C temperature range should be 30°C / s or higher. Preferably, it should be 50°C / s or higher or 70°C / s or higher. There is no particular upper limit, but from the viewpoint of preventing an increase in cooling equipment, it may be 200°C / s or lower.

[0108] After cooling the temperature range of 900-400°C at the average cooling rate described above, the cooling process up to winding is not particularly limited. The average cooling rate here refers to the value obtained by dividing the temperature difference between the start and end points of the set range by the elapsed time from the start point to the end point.

[0109] Winding process In the winding process, winding is performed in a temperature range of 200°C or lower. If the winding temperature exceeds 200°C, bainite will form, the timing of austenite formation will become uneven, and the standard deviation of the Mn concentration in fresh martensite and retained austenite will increase. Therefore, the winding temperature should be 200°C or lower. Preferably, it should be 150°C or lower, or 100°C or lower.

[0110] Cold rolling process After unrolling the steel sheet, cold rolling may be performed to a cumulative reduction ratio of 15% or less. This cold rolling step is not mandatory and may be omitted. By performing cold rolling with a cumulative reduction ratio of 15% or less, fine precipitates are generated, which can further increase the strength of the steel sheet. The cumulative reduction ratio of cold rolling is preferably 10% or less. On the other hand, if the cumulative reduction ratio exceeds 15%, recrystallized ferrite is generated, and the standard deviation of the Mn concentration in fresh martensite and retained austenite increases. Pickling may be performed before cold rolling.

[0111] The cumulative reduction ratio in cold rolling can be expressed as (1-t / t0)×100(%), where t is the thickness of the sheet after cold rolling and t0 is the thickness of the sheet before cold rolling.

[0112] Heat treatment process After the winding process or the cold rolling process, two heat treatments are performed, each involving holding the material within a predetermined temperature range. The heat treatment process includes a first heat treatment, where the material is held at a temperature range of 450-600°C for 10-200 seconds, and a second heat treatment, where the material is held at a temperature range of 650-750°C for 10-3010 seconds.

[0113] If the heat treatment temperature for 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 in the second heat treatment, austenite growth will be significant, and the standard deviation of Mn concentration in fresh martensite and retained austenite will be large. Furthermore, if the heat treatment temperature for the first heat treatment is greater than 600°C or the heat treatment time is greater than 200 seconds, Mn enrichment in cementite will be significant, the timing of austenite formation will be uneven, and in the second heat treatment, austenite growth will be significant, and the standard deviation of Mn concentration in fresh martensite and retained austenite will be large. For this reason, the heat treatment temperature for the first heat treatment should be between 450°C and 600°C, and the heat treatment time should be 10 seconds or longer. Preferably, the heat treatment temperature should be 500°C or higher or 550°C or higher, and the heat treatment time should be 15 seconds or longer.

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

[0115] Steel plates heat-treated within the above temperature range may be allowed to cool to room temperature, or they may be gas-cooled or water-cooled. Alternatively, plating may be performed during the gas-cooling process.

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

[0117] Next, the effects of one aspect of the present invention will be described in more detail with reference to examples. However, the conditions in the examples are merely examples of conditions adopted to confirm the feasibility and effects of the present invention, and the present invention is not limited to these examples of conditions. The present invention can adopt various conditions as long as they do not depart from the spirit of the invention and achieve the objectives of the present invention.

[0118] Slabs with the chemical composition shown in Table 1 were manufactured by continuous casting. Using the obtained slabs, steel plates with a thickness of 3.0 mm were manufactured under the conditions shown in Tables 2 and 3. 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 spaces in Table 1 indicate that the element in question was intentionally omitted. In Table 3, the "average cooling rate" in the "cooling process" refers to the average cooling rate in the temperature range of 900 to 400°C.

[0119] [Table 1]

[0120] [Table 2]

[0121] [Table 3]

[0122] The obtained steel sheets were investigated using the method described above to determine the area ratio of each microstructure, the standard deviation of Mn concentration in fresh martensite and retained austenite, the tensile strength, and the bendability after applying a 2% tensile pre-strain. The results are shown in Table 4.

[0123] In Table 4, TM, P, etc., refer to the following: TM: Tempered Martensite P: Perlite F: Ferrite B: Baynight FM: Fresh Martensite γ: retained austenite Mn standard deviation: The standard deviation of Mn concentration in fresh martensite and residual austenite.

[0124] [Table 4]

[0125] If the tensile strength was 1110 MPa or higher, it was judged to have high strength and passed the test. If the tensile strength was less than 1110 MPa, it was judged to not have high strength and failed the test.

[0126] To evaluate the bending properties after pre-straining, the following procedure was followed. First, a JIS No. 5 test specimen (thickness: original thickness of the steel plate) as 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 this specimen, with a gauge length of 50 mm, a tensile speed of 3 mm / min (crosshead displacement rate), and other conditions conforming to JIS Z 2241:2022.

[0127] Next, both ends of the specimen, which had been subjected to tensile deformation, were cut, and a bending test specimen measuring 60 mm in length and 25 mm in width was prepared from the center of the parallel section of the specimen. Subsequently, a bending test was performed on the obtained bending test specimen using the V-block method specified in JIS Z 2248:2022, such that the bending axis passed through the center of the bending test specimen in the longitudinal direction and was parallel to the width direction of the bending test specimen. In this test, a V-shaped pressing device with a 90° angle and various tip radii R was used, and the angle of the tapered surface of the V-block was set to 90°. After the bending test, the surface of the bending test specimen was visually inspected, and the limit bending radius at which no cracks occurred was determined.

[0128] The critical bending deformation is the value obtained by dividing the tip radius R by the plate thickness t (R / t). If the critical bending deformation is 2.0 or less, it is judged to be a pass, indicating that it has excellent bendability after pre-straining. If the critical bending deformation exceeds 2.0, it is judged to be a fail, indicating that it does not have excellent bendability after pre-straining. Note that the bending test was performed using a plated test piece, but the "plate thickness t" mentioned above is the thickness of the base material portion excluding the plating layer.

[0129] As shown in Table 4, tests No. 1, 2, 18, 20, and 25-31, which fully satisfy the provisions of the present invention, exhibited strengths of 1110 MPa or higher and excellent bendability after pre-straining. In contrast, tests No. 3-5, 8, 9, 13, 15, 16, 22, 24, and 32 showed a decrease in bendability after pre-straining due to an excessive standard deviation of Mn concentration. In test No. 33, the desired metal structure could not be obtained because two heat treatments were not performed, resulting in decreased strength and a decrease in bendability after pre-straining despite an extremely low standard deviation of Mn concentration. Furthermore, in tests No. 6 and 10-12, the metal structure deviated from the provisions of the present invention, resulting in decreased bendability after pre-straining. In test No. 14, the metal structure deviated from the provisions of the present invention, resulting in decreased strength. In addition, in tests No. 7, 17, 19, and 21-23, the chemical composition deviated from the provisions of the present invention, resulting in decreased strength. [Industrial applicability]

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

Claims

1. The chemical composition is expressed in mass percent. 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 to 0.180%, Nb: 0.010-0.050%, Ti+Nb+(Mo / 2)+V: 0.100-0.600%, The remainder consists of Fe and impurities. The metallic structure, in area percentage, Tempered martensite: 80.0–97.0% The total amount of perlite, ferrite, and bainite: 10.0% or less, The total of fresh martensite and retained austenite is 3.0–10.0%, The standard deviation of the Mn concentration in the fresh martensite and the retained austenite is 1.0 to 5.0%. The tensile strength is 1110 MPa or more. steel plate.

2. The aforementioned metallographic structure, in area %, The retained austenite content is 1.5% or more. The steel plate according to claim 1.

3. The aforementioned chemical composition, in addition to a portion of Fe, is expressed in mass % as follows: Mo: 0.600% or less, V: 0.300% or less, Contains one or more selected from, The steel plate according to claim 1.

4. The chemical composition is, in place of a portion of Fe, by mass%, Mo: 0.600% or less, V: 0.300% or less, Contains one or more selected from, The steel plate according to claim 2.

5. The aforementioned chemical composition, in addition to a portion of Fe, is expressed in mass % as follows: B: 0.0030% or less, Cr: 0.50% or less, Cu: 0.50% or less, Ni: 2.0% or less, Contains one or more selected from, The steel plate according to claim 1.

6. The chemical composition is, in place of a portion of Fe, by mass%, B: 0.0030% or less, Cr: 0.50% or less, Cu: 0.50% or less, Ni: 2.0% or less, Contains one or more selected from, The steel plate according to claim 2.

7. The chemical composition is, in place of a portion of Fe, by mass%, B: 0.0030% or less, Cr: 0.50% or less, Cu: 0.50% or less, Ni: 2.0% or less, Contains one or more selected from, The steel plate according to claim 3.

8. The chemical composition is, in place of a portion of Fe, by mass%, B: 0.0030% or less, Cr: 0.50% or less, Cu: 0.50% or less, Ni: 2.0% or less, Contains one or more selected from, The steel plate according to claim 4.

9. The aforementioned chemical composition, in addition to a portion of Fe, is expressed in mass % as follows: Ca: 0.020% or less, Mg: 0.020% or less, REM: 0.100% or less, Bi: 0.020% or less, Contains one or more selected from, A steel plate according to any one of claims 1 to 8.

10. The aforementioned chemical composition, in addition to a portion of Fe, is expressed in mass % as follows: Contains Sn: 0.05% or less. A steel plate according to any one of claims 1 to 8.

11. The chemical composition is, in place of a portion of Fe, by mass%, Contains Sn: 0.05% or less. The steel plate according to claim 9.

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