Steel plate

Abstract

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

Description

Technical Field

[0001] The present invention relates to steel plates. Background Art

[0002] In recent years, lightweighting of automotive and mechanical components has been a growing trend. This can be achieved by optimizing component shapes to ensure rigidity. Furthermore, in blanked parts, such as stamped parts, lightweighting can be achieved by reducing the thickness of the component material.

[0003] However, to reduce plate thickness while maintaining component strength characteristics such as static load breaking strength and yield strength, high-strength materials are necessary. In particular, research is underway to utilize steel plates with strengths exceeding 780 MPa in automotive chassis components, such as lower arms, trailing arms, and steering knuckles. These automotive chassis components are manufactured by bending steel plates, among other processes. Therefore, the steel plates used in these chassis components must exhibit excellent formability, particularly bendability.

[0004] For example, Patent Document 1 discloses a precipitation-hardened martensitic steel having both high tensile strength and Charpy absorption energy, which comprises, 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+0.5W with Mo as an essential element: 3.5 to 8.0%, and the balance being Fe and impurities.

[0005] Patent Document 2 discloses a high-strength steel having high tensile strength, excellent ductility, and bendability, wherein the microstructure comprises ferrite of 5% or more and less than 50% by area, and a mixed structure of primary martensite and retained austenite of more than 0% and less than 30% by area relative to the total structure. Furthermore, when analyzed with an electron beam microprobe analyzer, 5% or more by area of the steel sheet has a Mn concentration concentrated to 1.2 times or more of the Mn concentration in the steel sheet, and the standard deviation of the fraction of the region having a Mn concentration concentrated to 1.2 times or more of the Mn concentration in the steel sheet measured in 2 μm sections for 100 sections is 4.0% or more.

[0006] Prior art literature

[0007] Patent Literature

[0008] Patent Document 1: Japanese Patent Application Laid-Open No. 2014-208869

[0009] Patent Document 2: Japanese Patent Application Laid-Open No. 2015-193897 Summary of the Invention

[0010] Problems to be solved by the invention

[0011] However, Patent Document 1 does not consider bendability. In addition, automobile running parts such as those described above are manufactured by subjecting steel sheets to forming processes in multiple steps. Therefore, steel sheets used for automobile running parts are required to have excellent formability even after being subjected to a certain degree of pre-strain in the previous steps. In the case of forming processes that involve multiple steps, if the strain generated in the previous steps cannot be fully dispersed, local deformation will occur in the subsequent steps, and sometimes the original formability of the steel sheet cannot be exerted. Such local deformation is significant in the case of bending forming. However, Patent Document 2 does not consider bendability after being subjected to pre-strain.

[0012] The present invention has been made in view of the above-mentioned actual situation, and an object of the present invention is to provide a steel plate having high strength and excellent bendability after being given prestrain.

[0013] Solutions for solving problems

[0014] The gist of the present invention, which has been accomplished based on the above findings, is as follows.

[0015] (1) The chemical composition of the steel sheet according to one embodiment of the present invention is as follows in terms of mass %.

[0016] C: 0.08~0.17%,

[0017] Si: 0.03-1.40%,

[0018] Mn: 1.60-3.00%,

[0019] Al: 0.01~0.70%,

[0020] P: 0.080% or less,

[0021] S: 0.0100% or less,

[0022] N: 0.0050% or less,

[0023] Ti: 0.020~0.180%,

[0024] Nb: 0.010~0.050%,

[0025] Ti+Nb+(Mo / 2)+V: 0.100~0.600%,

[0026] Balance: Fe and impurities,

[0027] The metallographic structure of the steel plate is calculated in area %.

[0028] Tempered martensite: 80.0~97.0%,

[0029] Total of pearlite, ferrite and bainite: less than 10.0%, and

[0030] The total of primary martensite and retained austenite: 3.0-10.0%,

[0031] 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 of the steel sheet is 1110 MPa or higher.

[0032] (2) The steel sheet according to (1) above, wherein the metallographic structure may be such that the retained austenite is 1.5% or more in terms of area%.

[0033] (3) The steel sheet according to (1) or (2), wherein the chemical composition may contain, in mass %, a selected

[0034] Mo: 0.600% or less, and

[0035] V: 0.300% or less

[0036] One or more of the following may be used to replace part of Fe.

[0037] (4) The steel sheet according to any one of (1) to (3) above, wherein the chemical composition may contain, in mass %, a member selected from

[0038] B: 0.0030% or less,

[0039] Cr: 0.50% or less,

[0040] Cu: 0.50% or less, and

[0041] Ni: 2.0% or less

[0042] One or more of the following may be used to replace part of Fe.

[0043] (5) The steel sheet according to any one of (1) to (4) above, wherein the chemical composition may contain, in mass %, a member selected from

[0044] Ca: 0.020% or less,

[0045] Mg: 0.020% or less,

[0046] REM: 0.100% or less, and

[0047] Bi: 0.020% or less

[0048] One or more of the following may be used to replace part of Fe.

[0049] (6) The steel sheet according to any one of (1) to (5) above, wherein the chemical composition may contain, in mass %,

[0050] Sn: 0.05% or less

[0051] To replace part of Fe.

[0052] Effects of the Invention

[0053] According to this embodiment, it is possible to provide a steel sheet having high strength and excellent bendability after imparting prestrain. DETAILED DESCRIPTION

[0054] The present inventors have conducted intensive studies on a method for obtaining the above-mentioned steel sheet, and as a result, have obtained the following findings.

[0055] By suppressing local deformation during bending after applying prestrain, cracks can be prevented during multiple forming steps. To this end, it is important that the Mn distribution in the primary martensite and retained austenite is uniform, that is, the standard deviation of the Mn concentration in the primary martensite and retained austenite is small.

[0056] Furthermore, in order to achieve a tensile strength of the steel sheet of 1110 MPa or more while suppressing such local deformation, it is necessary to disperse fine precipitates with tempered martensite as the main phase.

[0057] In order to obtain the above-mentioned metallographic structure, it is effective to strictly control the rough rolling conditions, the finish rolling conditions, the cooling conditions after the finish rolling, the cold rolling conditions, and the subsequent heat treatment conditions.

[0058] The steel sheet of this embodiment is described in detail below. However, the present invention is not limited to the configuration disclosed in this embodiment, and various modifications can be made without departing from the spirit of the present invention. In the numerical ranges described below, the lower and upper limits are included in the range. Numerical values expressed as "less than" or "exceed" are not included in the numerical range. All "%" in chemical composition refers to "mass %."

[0059] The steel sheet of this embodiment has a chemical composition, in mass%, comprising: 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 balance: Fe and impurities. Each element will be described in detail below.

[0060] C: 0.08~0.17%

[0061] C is an element necessary to achieve the desired tensile strength of the steel sheet. If the C content is less than 0.08%, the desired tensile strength cannot be achieved. Therefore, the C content is set to 0.08% or higher. The C content is preferably 0.09% or higher, 0.10% or higher, or 0.11% or higher.

[0062] On the other hand, when the C content exceeds 0.17%, weldability decreases. Furthermore, the amount of dissolved carbon becomes excessive, and during heat treatment, austenite growth proceeds through the diffusion of C, resulting in uneven Mn distribution in the fresh martensite and retained austenite. Therefore, the C content is set to 0.17% or less. It is preferably 0.15% or less or 0.14% or less. To reduce the standard deviation of the Mn concentration in the fresh martensite and retained austenite and further improve bendability after prestraining, the C content is more preferably 0.13% or less. The C content is preferably 0.09-0.15%, more preferably 0.10-0.14%, and even more preferably 0.11-0.13%.

[0063] Si: 0.03-1.40%

[0064] Si is an element that improves the adhesion of galvanizing. If the Si content is less than 0.03%, the adhesion of galvanizing during forming is reduced. 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.

[0065] On the other hand, if the Si content exceeds 1.40%, the surface properties of the steel sheet deteriorate. 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%.

[0066] Mn: 1.60~3.00%

[0067] 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 achieved. Therefore, the Mn content is set to 1.60% or higher. The Mn content is preferably 1.80% or higher. To reduce the standard deviation of the Mn concentrations in primary martensite and retained austenite and further improve bendability after prestraining, the Mn content is more preferably 2.00% or higher.

[0068] On the other hand, if the Mn content exceeds 3.00%, cracks in the cast slab are more likely to occur, making 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%.

[0069] Al: 0.01~0.70%

[0070] Al acts as a deoxidizer, improving the purity of steel. If the Al content is less than 0.01%, sufficient deoxidation is not achieved, resulting in the formation of numerous inclusions (oxides) in the steel sheet. These inclusions degrade the surface properties of the steel sheet. Therefore, the Al content is set to 0.01% or higher. The Al content is preferably 0.02% or higher, more preferably 0.03% or higher, and even more preferably 0.04% or higher.

[0071] 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%.

[0072] P: 0.080% or less

[0073] P is an element that segregates in the center of the steel plate thickness. A P content exceeding 0.080% reduces weldability. 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.

[0074] The lower the P content, the better, and 0% is preferred. However, if the P content is too low, the P removal cost increases significantly. Therefore, the P content may be 0.0005% or more.

[0075] S: 0.0100% or less

[0076] S is an element that exists as a sulfide. When the S content exceeds 0.0100%, weldability deteriorates. 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.

[0077] The lower the S content, the better, and 0% is preferred. However, if the S content is too low, the cost of removing the S will increase significantly. Therefore, the S content may be 0.0005% or more.

[0078] N: 0.0050% or less

[0079] Nitrogen is an element that forms coarse nitrides in steel. When the N content exceeds 0.0050%, slab cracking is more likely to 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.

[0080] The lower the N content, the better, and 0% is preferred. However, if the N content is too low, the cost of removing N increases significantly. Therefore, the N content may be 0.0005% or more.

[0081] Ti: 0.020~0.180%

[0082] Ti is an element that increases the strength of steel sheets by forming fine carbides and / or carbonitrides in steel. A Ti content of less than 0.020% does not achieve the desired tensile strength. Therefore, the Ti content is set to 0.020% or higher. The Ti content is preferably 0.050% or higher, and more preferably 0.080% or higher.

[0083] On the other hand, if the Ti content exceeds 0.180%, cracks in the cast slab are more likely to occur, making 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 0.050-0.160%, and more preferably 0.080-0.150%.

[0084] Nb: 0.010~0.050%

[0085] Nb is an element that suppresses abnormal grain growth of austenite grains during hot rolling. It 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 achieved. 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.

[0086] On the other hand, if the Nb content exceeds 0.050%, cracks in the cast slab are more likely to occur, making 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%.

[0087] Ti+Nb+(Mo / 2)+V: 0.100~0.600%

[0088] In this embodiment, the total amount of the aforementioned Ti content, Nb content, half of the Mo content (described later), and V content is controlled. Specifically, when expressing the content of each element using its symbol, the total amount is controlled as follows: Ti + Nb + (Mo / 2) + V. If the total amount is less than 0.100%, the effect of increasing the strength of the steel sheet by forming at least one of fine carbides, nitrides, and carbonitrides cannot be fully achieved, and the desired tensile strength cannot be achieved. Therefore, the total amount is set to 0.100% or more.

[0089] Furthermore, by causing these elements to form carbides, the dissolved carbon in the steel is consumed. This allows the growth of austenite to be controlled through the diffusion of Mn. This allows for a uniform distribution of Mn in the primary martensite and retained austenite, thereby improving the bendability after prestraining.

[0090] The above effects can be achieved without the need to contain all of Ti, Nb, Mo, and V. The above effects can be achieved as long as the content of any one of Ti, Nb, or V, or half of the Mo content, is 0.100% or more. The total amount is preferably 0.150% or more. To reduce the standard deviation of the Mn concentration in fresh martensite and retained austenite and further improve bendability after prestraining, the content is more preferably 0.200% or more, even more preferably 0.230% or more, and even more preferably 0.250% or more. It should be noted that when the total amount exceeds 0.230%, at least one of Mo and V must be substantially contained.

[0091] On the other hand, if the total amount exceeds 0.600%, the precipitates coarsen and the desired tensile strength cannot be achieved. 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 0.200-0.500%, more preferably 0.230-0.400%, and even more preferably 0.250-0.300%.

[0092] The balance of the chemical composition of the steel sheet of this embodiment may be Fe and impurities. In this embodiment, impurities refer to substances that are mixed from raw materials such as ore, scrap, or the manufacturing environment, or substances that are allowed within a range that does not adversely affect the steel sheet of this embodiment.

[0093] The steel sheet of this embodiment may contain the following optional elements in place of a portion of Fe. When no optional elements are contained, the lower limit of the content is 0%. Each optional element will be described below.

[0094] Mo: 0.600% or less

[0095] Mo is an element that increases the strength of steel sheets by forming fine carbides in steel. To reliably achieve this effect, the Mo content is preferably 0.001% or more, more preferably 0.002% or more. On the other hand, even if the Mo content exceeds 0.600%, the above effect is saturated. 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 0.001-0.500%, 0.002-0.400%, 0.003-0.300%, 0.004-0.200%, or 0.005-0.100%.

[0096] V: 0.300% or less

[0097] V is an element that increases the strength of steel sheets by forming fine carbides and / or nitrides in steel. To reliably achieve 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 are likely to form in the cast slab, and hot rolling may become 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 0.010-0.270%, 0.030-0.240%, or 0.050-0.200%.

[0098] B: 0.0030% or less

[0099] B is an element that suppresses the formation of ferrite during the cooling process and improves the strength of the steel sheet. To reliably achieve 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 B exceeds 0.0030%, the above effect is saturated. 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 0.0001-0.0025%, 0.0005-0.0020%, or 0.0010-0.0015%.

[0100] Cr: 0.50% or less

[0101] Cr is an element that exhibits similar effects to Mn. To reliably achieve the effect of increasing the strength of the steel sheet due to the inclusion of Cr, the Cr content is preferably set to 0.001% or more, more preferably to 0.005% or more, and even more preferably to 0.010% or more. On the other hand, even if the Cr content exceeds 0.50%, the above effect is saturated. 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 0.001-0.40%, 0.005-0.30%, or 0.010-0.20%.

[0102] Cu: 0.50% or less

[0103] Cu has the effect of improving the hardenability of the steel sheet and the effect of precipitating in the steel to increase the strength of the steel sheet. In order to more reliably obtain the effects brought about by these effects, the Cu content is preferably set to 0.01% or more, more preferably to 0.05% or more, and even more preferably to 0.10% or more. However, when the Cu content exceeds 0.50%, grain boundary cracks in 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 0.01-0.40%, 0.05-0.30%, or 0.10-0.20%.

[0104] Ni: 2.0% or less

[0105] Ni has the effect of improving the hardenability of the steel sheet and thus increasing the strength of the steel sheet. In addition, when Ni contains Cu, it has the effect of effectively suppressing the grain boundary cracks of the slab caused by Cu. In order to more reliably obtain the effect brought about by 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. Ni is an expensive element, so containing a large amount is not economically preferred. 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 0.02-1.5% or less, 0.10-1.0% or less, or 0.30-0.50% or less.

[0106] Ca: 0.020% or less

[0107] Mg: 0.020% or less

[0108] REM: 0.100% or less

[0109] Ca, Mg, and REM all have the effect of improving the formability of steel sheets by controlling the shape of inclusions to a preferred form. Therefore, one or more selected from these elements may be contained. To more reliably achieve the effects of these effects, it is preferred that the content of at least one of Ca, Mg, and REM be 0.0005% or more. The content of each of Ca, Mg, and REM is more preferably 0.0010% or more, and even more preferably 0.0020% or more.

[0110] However, if the Ca content and / or Mg content exceeds 0.020%, or the REM content exceeds 0.100%, excessive inclusions may form in the steel, sometimes 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 is set to 0.100% or less. The Ca and Mg contents are preferably 0.015% or less, 0.010% or less, 0.0050% or less, or 0.0030% or less. Furthermore, the REM content is preferably 0.050% or less, 0.030% or less, 0.010% or less, 0.0050% or less, or 0.0030% or less.

[0111] The Ca content is preferably 0.0005-0.015%, 0.0010-0.010%, 0.0015-0.0050%, or 0.0020-0.0030%. The Mg 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%.

[0112] Here, REM refers to a total of 17 elements consisting of Sc, Y, and lanthanoid elements, and the above REM content refers to the total content of these elements. Lanthanoid elements are added in the form of misch metals in industry.

[0113] Bi: 0.020% or less

[0114] Bi improves the formability of steel sheets by refining the solidified structure. To more reliably achieve 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 this effect is saturated, making it economically undesirable. Therefore, the Bi content is kept below 0.020%. 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%.

[0115] Sn: 0.05% or less

[0116] Sn improves the plating processability during the production of plated steel sheets. To more reliably achieve this effect, the Sn content is preferably 0.01% or higher, more preferably 0.02% or higher. However, a Sn content exceeding 0.05% may cause defects during hot rolling, so the Sn content is set to 0.05% or lower. The Sn content is preferably 0.04% or lower. The Sn content is preferably 0.01-0.04% or 0.02-0.03%.

[0117] Furthermore, the present inventors have confirmed that even if Zr, Co, Zn, and W are contained as impurities, the effects of the steel sheet of this embodiment are not impaired as long as the total content is 1.00% or less. Therefore, a total content of 1.00% or less of one or more selected from Zr, Co, Zn, and W may be contained.

[0118] The chemical composition of the steel sheet can be analyzed using a spark discharge emission spectrometer or the like. Note that C and S are values determined by burning in an oxygen stream using a gas composition analyzer or the like and measuring by infrared absorption. Furthermore, N is determined by melting a test piece taken from the steel sheet in a helium stream and measuring by the thermal conductivity method. When the steel sheet is plated, the "chemical composition of the steel sheet" refers to the chemical composition of the base material excluding the plating layer.

[0119] Next, the metallographic structure of the steel plate according to this embodiment will be described.

[0120] The metallographic structure of the steel plate of this embodiment is, in terms of area %, 80.0% to 97.0% of tempered martensite, 10.0% or less of the total of pearlite, ferrite, and bainite, and 3.0% to 10.0% of the total of fresh martensite and retained austenite. The standard deviation of the Mn concentration in the fresh martensite and retained austenite is 5.0% or less.

[0121] In this embodiment, the metallographic structure at the 1 / 4 position of the steel plate thickness is specified. The reason for this is that the metallographic structure at this position represents the representative metallographic structure of the steel plate. Here, in this application specification, "1 / 4 position of the plate thickness" refers to the area with the 1 / 4 position of the plate thickness as the center in the plate thickness direction. In addition, when the steel plate is plated, the "plate thickness" refers to the plate thickness of the base material portion excluding the plated layer. It should be noted that in the present invention, the thickness of the plated layer is specified by observation using an optical microscope.

[0122] Area ratio of tempered martensite: 80.0-97.0%

[0123] 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 achieved. On the other hand, if the area fraction of tempered martensite exceeds 97.0%, the bendability after prestraining decreases. Therefore, the area fraction of tempered martensite is 80.0% to 97.0%. The area fraction of tempered martensite is preferably 85.0% to 95.0%. The area fraction of tempered martensite is preferably 85.0% to 95.0%.

[0124] Total area ratio of pearlite, ferrite and bainite: 10.0% or less

[0125] If the total area ratio of pearlite, ferrite, and bainite is high, the desired tensile strength cannot be achieved. Therefore, the total area ratio of these structures is set to 10.0% or less. The total area ratio of these structures is preferably 7.0% or less, 5.0% or less, or 3.0% or less. The lower the total area ratio of these structures, the better, and therefore it can be 0.0%.

[0126] It should be noted that it is not necessary to contain all of pearlite, ferrite, and bainite, and only one of them may be contained with its area ratio within the above range, or two or three of them may be contained with their total area ratio within the above range.

[0127] Total area ratio of fresh martensite and retained austenite: 3.0-10.0%

[0128] In the steel sheet of this embodiment, if the total area ratio of fresh martensite and retained austenite is less than 3.0%, deformation after prestraining becomes uneven, and the bendability of the steel sheet after prestraining decreases. This prevents the aforementioned effects from being achieved. Therefore, the total area ratio of fresh martensite and retained austenite is set to 3.0% or greater. Preferably, it is 4.0% or greater, or 5.0% or greater.

[0129] If the total area ratio of fresh martensite and retained austenite exceeds 10.0%, the bendability of the steel sheet after prestraining decreases. Therefore, the total area ratio of fresh martensite and retained austenite is set to 10.0% or less. It is preferably 9.0% or less or 8.0% or less. The total area ratio of fresh martensite and retained austenite is preferably 4.0 to 9.0%, and more preferably 5.0 to 8.0%.

[0130] It should be noted that it is not necessary to contain both fresh martensite and retained austenite, and only one of them may be contained, and the area ratio thereof may be within the above-mentioned range.

[0131] The area fraction of retained austenite can also be set to 1.5% or greater, while the total area fraction of fresh martensite and retained austenite is set to 3.0% to 10.0%. By setting the area fraction of retained austenite in the second phase to 1.5% or greater, bendability after prestraining can be further improved. The area fraction of retained austenite is more preferably set to 2.0% or greater, 3.0% or greater, or 4.0% or greater. The upper limit of the area fraction of retained austenite is not particularly limited and may be 10.0% or less or 7.0% or less.

[0132] In the metallographic structure of the steel sheet of this embodiment, the total area ratio of the above-mentioned structures is preferably 100%. That is, in terms of area %, the total of pearlite, ferrite, and bainite is preferably 10.0% or less, and the total of fresh martensite and retained austenite is preferably 3.0 to 10.0%, with the remainder being tempered martensite.

[0133] The following describes a method for measuring the area ratio of each structure.

[0134] A test piece was collected from the steel plate so that the metallographic structure at a depth of 1 / 4 of the plate thickness from the surface and at the center in the plate width direction could be observed in a cross section parallel to the rolling direction.

[0135] The cross-section of the test piece was polished using #600 to #1500 silicon carbide paper and then finished to a mirror finish using a solution consisting of 1-6 μm diamond powder dispersed in a diluent such as alcohol or pure water. The sample was then polished for 30 minutes at room temperature using colloidal silica without an alkaline solution to remove strain introduced into the sample surface. Crystal orientation information was obtained by measuring electron backscatter diffraction at 0.1 μm intervals at any location along the length of the sample cross-section, over an area 60 μm in the rolling direction and 160 μm in the thickness direction, centered at a position 1 / 4 of the thickness from the surface.

[0136] The measurement was performed using an EBSD apparatus consisting of a field emission scanning electron microscope (FE-SEM: JEOL JSM-7001F) and an EBSD detector (TSL DVC5 detector). The vacuum level in the EBSD apparatus was 9.6 × 10 -5 Pa or less, with an accelerating voltage of 15 kV and an irradiation current level of 13. The analysis used a database of diffraction patterns for Iron Alpha (bcc structure) and Iron Gamma (fcc structure). Based on the resulting crystal orientation information, the "Phase Map" function included in the "TSL OIM Analysis (registered trademark)" software bundled with the EBSD analyzer was used to identify regions with fcc crystal structures and calculate the area fraction of these regions. This yielded the area fraction of retained austenite.

[0137] Next, regions with a bcc crystal structure were identified as bainite, ferrite, pearlite, fresh martensite, and tempered martensite. For these regions, the "Grain Orientation Spread" function included in the "TSL OIM Analysis (registered trademark)" software included with the EBSD analyzer was used to extract regions with a "Grain Orientation Spread" of 1° or less as ferrite, treating 15° grain boundaries as grain boundaries. The area ratio of the extracted ferrite was calculated to obtain the ferrite area ratio.

[0138] Next, in the remainder region (the region where the "Grain Orientation Spread" exceeds 1°), assuming grain boundaries with an orientation difference of 15° or greater are considered grain boundaries, and assuming the maximum value of the "Grain Average IQ" in the ferrite region is Iα, the region exceeding Iα / 2 is extracted as bainite, and the region below Iα / 2 is extracted as "pearlite, primary martensite, and tempered martensite." The area ratio of the extracted bainite is calculated to obtain the area ratio of bainite.

[0139] The extracted “pearlite, fresh martensite, and tempered martensite” were distinguished by the following method.

[0140] In order to observe the secondary electron image of the same area as the EBSD measurement area using FE-SEM, a Vickers indentation is engraved near the observation position. Then, the structure of the remaining observation surface is ground to remove the contaminants on the surface and etched with nitric acid. Next, the same field of view as the EBSD observation surface is observed using FE-SEM at a magnification of 3000 times. In the EBSD measurement, in the area determined as "pearlite, primary martensite and tempered martensite", the area with a lower structure in the grain and cementite having multiple variants and precipitated is judged to be tempered martensite. The area where cementite precipitates in layers is judged to be pearlite. The area where the brightness is greater than the surrounding structure and the lower structure does not appear by etching is judged to be primary martensite. By calculating the respective area ratios, the area ratios of tempered martensite, pearlite and primary martensite are obtained.

[0141] It should be noted that the contaminants on the surface of the observation surface can be removed by lapping with aluminum oxide particles having a particle size of 0.1 μm or less, or by Ar ion sputtering.

[0142] If the metallographic structure does not contain ferrite, a separate test piece is collected from the steel plate being measured and heat treated to generate ferrite. Specifically, the steel is isothermally held at 630-750°C for one hour and then quenched at an average cooling rate of 50°C / s from this temperature to 300°C. The Iα of the generated ferrite is then measured and used to distinguish other structures.

[0143] Standard deviation of Mn concentration in primary martensite and retained austenite: 1.0 to 5.0 mass%

[0144] By reducing the standard deviation of the Mn concentration in the fresh martensite and retained austenite, bendability after prestraining can be improved. If the standard deviation of the Mn concentration in the fresh martensite and retained austenite exceeds 5.0 mass%, microcracks are likely to form in the fresh martensite and retained austenite, and excellent bendability cannot be achieved after prestraining. Therefore, the standard deviation of the Mn concentration in the 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.

[0145] The smaller the standard deviation, the better, but the actual lower limit is 1.0%. It should be noted that if the two heat treatments described below are omitted, it is believed that the standard deviation of the Mn concentration in the primary martensite and retained austenite can be reduced to less than 1.0 mass%. However, in steel sheets produced without the two heat treatments, the metallographic structure specified in the present invention cannot be achieved, and therefore, not only can the desired strength not be achieved, but the work hardening characteristics after pre-straining are also reduced, and excellent bendability cannot be achieved after straining.

[0146] The Mn concentrations of fresh martensite and retained austenite were measured by the following method.

[0147] The Mn concentration in an area 100 μm in the thickness direction and 200 μm in the rolling direction was measured using a field emission electron probe microanalyzer (FE-EPMA: JEOL JXA-8530F) at a distance of 1 / 4 the thickness from the steel plate surface. The measurement conditions were an accelerating voltage of 15 kV, and the Mn concentration distribution profile was obtained. More specifically, the FE-EPMA measurement interval was 0.2 μm, and the number of measurement points was 500,000.

[0148] Afterwards, in order to observe the secondary electron image of the same area as the FE-EPMA measurement area using FE-SEM, a Vickers indentation is engraved near the observation position. Then, the structure of the remaining observation surface is ground to remove the contaminants on the surface, and etching is performed with nitric acid. Next, the same area as the FE-EPMA measurement area is observed using FE-SEM at a magnification of 3000 times. Then, from the measurement results obtained using the above-mentioned FE-EPMA, only the measurement results in the area specified by FE-SEM as primary martensite or retained austenite are extracted. Thus, only the Mn concentration of the primary martensite or retained austenite contained in the measurement area is extracted. Then, the standard deviation of the Mn concentration of the extracted primary martensite or retained austenite is calculated as the standard deviation of the Mn concentration of the primary martensite and retained austenite.

[0149] Tensile strength: 1110MPa or more

[0150] The steel sheet of this embodiment has a tensile strength of 1110 MPa or greater. By setting the tensile strength to 1110 MPa or greater, it can be suitably used in various automotive chassis components. The tensile strength can be 1180 MPa or greater, or 1300 MPa or greater. A higher tensile strength is preferred, but it can also be 1500 MPa or less.

[0151] In the present invention, the tensile strength is measured in accordance with JIS Z 2241: 2022. During the measurement, a JIS No. 5 test piece (thickness: original thickness of the steel plate) specified in JIS Z 2241: 2022, which is collected in a manner consistent with the rolling direction of the steel plate, is used. The gauge length is 50 mm and the tensile speed is 3 mm / min in terms of the crosshead displacement velocity. It should be noted that when the steel plate is plated, the tensile strength is measured using the plated test piece, but "thickness: original thickness of the steel plate" is set to the thickness of the base material portion excluding the coating. That is, the original cross-sectional area of the test piece used for the calculation of the tensile strength is the original cross-sectional area of the base material portion excluding the coating.

[0152] The steel sheet of this embodiment may also be provided with a coating on the surface for the purpose of improving corrosion resistance, etc., to form a surface-treated steel sheet. The coating may be an electroplated layer or a hot-dip coated layer. Examples of the electroplated layer include electrogalvanizing and electroplating of Zn-Ni alloys. Examples of the hot-dip coated layer include hot-dip galvanizing, hot-dip alloyed zinc, hot-dip aluminum, hot-dip Zn-Al alloy, hot-dip Zn-Al-Mg alloy, and hot-dip Zn-Al-Mg-Si alloy.

[0153] The coating adhesion amount is not particularly limited and can be the same as in the past. In addition, it is also possible to implement appropriate chemical conversion treatment (for example, coating and drying of a chromium-free chemical conversion treatment liquid of a silicate system) after plating to further improve corrosion resistance.

[0154] Next, a preferred method for producing the steel plate according to the present embodiment will be described. It should be noted that the temperatures described below refer to the surface temperatures of the slab or steel plate unless otherwise specified.

[0155] The preferred method for manufacturing the steel plate of this embodiment sequentially performs the following steps:

[0156] a slab heating step of heating the slab having the above chemical composition to a temperature range of 1200° C. or higher and maintaining the temperature in the temperature range for more than 30 minutes;

[0157] The rough rolling process is performed in a temperature range of 1000 to 1300° C. with a reduction rate of 10 to 30% for each of the first to third passes and a reduction rate of 15 to 50% for each of the fourth and subsequent passes.

[0158] The finishing rolling process is performed by performing rolling with a reduction ratio of 24% or more for two or more times, with the final pass reduction ratio being 24 to 60% and the finishing temperature being in the temperature range of 960 to 1060°C.

[0159] The cooling step is performed at an average cooling rate of 30°C / s or more in a temperature range of 900°C to 400°C.

[0160] The coiling process is carried out in a temperature range below 200°C;

[0161] A heat treatment step of maintaining the temperature in the range of 450 to 600°C for 10 to 200 seconds; and

[0162] The heat treatment step is to maintain the temperature in the range of 650 to 750°C for 10 to 3010 seconds.

[0163] In addition to the above steps, a cold rolling step may be further performed after the coiling step and before the heat treatment step, wherein the cold rolling is performed at a cumulative reduction ratio of 15% or less.

[0164] Hereinafter, each step will be described.

[0165] Slab heating process

[0166] The slab heating temperature is 1200°C or higher. Furthermore, the holding time in the temperature range of 1200°C or higher is set to 30 minutes or longer. If the slab heating temperature is lower than 1200°C, or the holding time in the temperature range of 1200°C or higher is less than 30 minutes, coarse precipitates cannot be fully dissolved, resulting in significant fluctuations in the tensile strength of the steel sheet.

[0167] The upper limits of the heating temperature and the holding time in the temperature range of 1200°C or higher are not particularly limited, and may be 1300°C or lower and 300 minutes or lower, respectively. It should be noted that during holding in the temperature range of 1220°C or higher, the steel sheet temperature may be varied or kept constant.

[0168] It should be noted that the heated slab is not particularly limited except for having the above-mentioned chemical composition. For example, a slab produced by continuous casting by melting molten steel having the above-mentioned chemical composition in a converter or electric furnace can be used. Alternatively, an ingot casting method or thin slab casting method can be used instead of continuous casting.

[0169] Rough rolling process

[0170] During the rough rolling process, rough rolling is performed in the temperature range of 1000-1300°C, with a reduction ratio of 10-30% for the first to third passes and a reduction ratio of 15-50% for the fourth and subsequent passes. Rough rolling at temperatures below 1000°C causes the precipitation of alloy carbides, 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 above. On the other hand, rough rolling at temperatures above 1300°C increases fuel costs, so rough rolling is performed in the temperature range below 1300°C.

[0171] If rolling is performed with a reduction ratio of less than 10% in the first to third passes, or with a reduction ratio of less than 15% in the fourth and subsequent passes, the grains coarsen, and the average grain size of the second phase coarsens after the heat treatment process. Therefore, the reduction ratio for each of the first to third passes is set to 10% or more, and the reduction ratio for each of the fourth and subsequent passes is set to 15% or more. Preferably, the reduction ratio for each of the first to third passes is 15% or more or 20%, and the reduction ratio for each of the fourth and subsequent passes is 20% or more or 25% or more.

[0172] Furthermore, if rolling is performed at a reduction ratio exceeding 30% in the first through third passes, or at a reduction ratio exceeding 50% in the fourth and subsequent passes, alloy carbides precipitate, resulting in significant fluctuations in the tensile strength of the steel sheet. Therefore, the reduction ratio for each of the first through third passes is 30% or less, and the reduction ratio for the fourth and subsequent passes is 50% or less. Preferably, the reduction ratio for each of the first through third passes is 25% or less, and the reduction ratio for the fourth and subsequent passes is 40% or less.

[0173] It should be noted that the reduction ratio of each pass can be expressed as {1-(t1 / t0)}×100(%) when the entry plate thickness t0 of each pass and the exit plate thickness t1 of each pass are used.

[0174] Finishing process

[0175] In the finishing rolling process, rolling is performed at a reduction ratio of 24% or more twice or more, with the final pass reduction ratio being 24-60% and the finishing rolling completion temperature being in the temperature range of 960-1060°C.

[0176] If rolling with a reduction of 24% or more is performed once or less during the finish rolling process, uneven strain distribution occurs within the metallographic structure after the finish rolling process, causing some grains to preferentially reverse-transform to the austenite phase. This results in uneven austenite formation timing and increases the standard deviation of the Mn concentration in primary martensite and retained austenite. Therefore, the finish rolling process should be performed two or more times with a reduction of 24% or more.

[0177] The term "two times" here also includes the final pass. That is, in this embodiment, rolling is performed at a reduction ratio of 24% or higher once, after achieving a final pass reduction ratio of 24% or higher. The upper limit of the reduction ratio in the finishing rolling process is not particularly limited; the reduction ratio in each pass can be set to 60% or lower.

[0178] The final pass reduction is 24% or greater, preferably 28% or greater or 30% or greater. Furthermore, to minimize equipment load increases, the final pass reduction is 60% or less, preferably 50% or less or 40% or less.

[0179] If the finish rolling temperature is lower than 960°C, flat grains will form an austenite phase, causing the grain size to coarsen and the Mn concentration in the primary martensite and retained austenite to become uneven. Therefore, the finish rolling temperature should be set to 960°C or higher, preferably 980°C or higher or 1000°C or higher.

[0180] If the finish rolling 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 temperature is set to 1060°C or lower, preferably 1040°C or lower. The finish rolling temperature refers to the exit temperature of the final pass of the finish rolling process.

[0181] Cooling process

[0182] In the cooling process, cooling is performed in a manner such 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 process. Therefore, the average cooling rate in the temperature range of 900 to 400°C is set to 30°C / s or more. Preferably, it is 50°C / s or more or 70°C / s or more. The upper limit is not particularly limited, but from the viewpoint of preventing the increase in cooling equipment, it can be set to 200°C / s or less.

[0183] The cooling process from cooling at the above average cooling rate in the temperature range of 900 to 400°C to coiling is not particularly limited. The average cooling rate referred to herein is the value obtained by dividing the temperature difference between the start and end points of the set range by the time elapsed from the start to the end point.

[0184] Coiling process

[0185] During the coiling process, coiling is performed in a temperature range of 200°C or lower. Coiling temperatures exceeding 200°C cause bainite formation, resulting in uneven austenite formation timing and a larger standard deviation of the Mn concentration in primary martensite and retained austenite. Therefore, the coiling temperature is set to 200°C or lower. Preferably, it is set to 150°C or lower, or 100°C or lower.

[0186] Cold rolling process

[0187] After uncoiling the coiled steel sheet, it can be cold rolled with a cumulative reduction of 15% or less. This cold rolling process is not essential and therefore can be omitted. By cold rolling with a cumulative reduction of 15% or less, fine precipitates are generated, further improving the strength of the steel sheet. 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 in the primary martensite and retained austenite increases. It should be noted that pickling can also be performed before cold rolling.

[0188] When the plate thickness after cold rolling is t and the plate thickness before cold rolling is t0, the cumulative reduction ratio of cold rolling can be expressed as (1-t / t0)×100(%).

[0189] Heat treatment process

[0190] After the coiling process or the cold rolling process, two heat treatments are performed in a predetermined temperature range. The heat treatment process includes a first heat treatment at a temperature range of 450-600°C for 10-200 seconds and a second heat treatment at a temperature range of 650-750°C for 10-3010 seconds.

[0191] If the heat treatment temperature of the first heat treatment is lower than 450°C or the heat treatment time is less than 10 seconds, the precipitation of carbides becomes insufficient, and in the second heat treatment, the growth of austenite is significant, and the standard deviation of the Mn concentration of primary martensite and retained austenite becomes larger. Furthermore, if the heat treatment temperature of the first heat treatment exceeds 600°C or exceeds 200 seconds, the enrichment of Mn in cementite becomes significant, the timing of austenite formation becomes uneven, and in the second heat treatment, the growth of austenite is significant, and the standard deviation of the Mn concentration of primary martensite and retained austenite becomes larger. Therefore, the first heat treatment temperature is set to above 450°C and below 600°C, and the heat treatment time is set to above 10 seconds. Preferably, the heat treatment temperature is above 500°C or above 550°C, and the heat treatment time is above 15 seconds.

[0192] If the heat treatment temperature of the second heat treatment is lower than 650°C or the heat treatment time is less than 10 seconds, the formation of primary martensite and retained austenite becomes insufficient. On the other hand, if the heat treatment temperature of the second heat treatment exceeds 750°C or the heat treatment temperature exceeds 3010 seconds, the formation of primary martensite and retained austenite becomes excessive, and the desired metallographic structure cannot be obtained. Therefore, the second heat treatment temperature is set to be greater than 650°C and less than 750°C, and the heat treatment time is set to be greater than 10 seconds. Preferably, the heat treatment temperature is greater than 500°C or greater than 550°C, and the heat treatment time is greater than 15 seconds and less than 3010 seconds. Preferably, the second heat treatment temperature is greater than 680°C and less than 720°C, and the second heat treatment time is less than 100 seconds or less than 500 seconds.

[0193] The steel sheet heat-treated in the above temperature range may be cooled naturally to room temperature, or may be gas-cooled or water-cooled. In addition, the plating treatment may be performed during gas cooling.

[0194] The steel sheet of this embodiment can be manufactured by the manufacturing method including the steps described above.

[0195] Example

[0196] Next, the effects of one embodiment of the present invention will be described in more detail using examples. However, the conditions in the examples are merely examples of conditions employed to confirm the feasibility and effects of the present invention, and the present invention is not limited to this example. Various conditions can be employed as long as the present invention achieves its objectives without departing from its spirit.

[0197] Slabs having the chemical composition shown in Table 1 were manufactured by continuous casting. The obtained slabs were used to manufacture steel plates with a thickness of 3.0 mm according to the conditions shown in Tables 2 to 3. It should be noted that pickling was performed before cold rolling. In addition, after the second heat treatment, the steel was air-cooled to 500°C, and then plated, and air-cooled to room temperature. The blank columns in Table 1 indicate that the element was not intentionally contained. In addition, the "average cooling rate" in the "cooling process" of Table 3 refers to the average cooling rate in the temperature range of 900 to 400°C.

[0198] [Table 1]

[0199]

[0200] [Table 2]

[0201] Table 2

[0202]

[0203] [Table 3]

[0204] Table 3

[0205]

[0206] The obtained steel sheets were examined by the above-mentioned methods for the area ratio of each structure, the standard deviation of the Mn concentration of fresh martensite and retained austenite, the tensile strength, and the bendability after applying a 2% prestrain to the tensile deformation.

[0207] It should be noted that TM, P, etc. in Table 4 are as follows.

[0208] TM: Tempered Martensite

[0209] P: Pearlite

[0210] F: Ferrite

[0211] B: Bainite

[0212] FM: fresh martensite

[0213] γ: Retained austenite

[0214] Mn standard deviation: Standard deviation of Mn concentration in primary martensite and retained austenite

[0215] [Table 4]

[0216] Table 4

[0217]

[0218] When the tensile strength was 1110 MPa or more, the sample was judged as having high strength and passed, and when the tensile strength was less than 1110 MPa, the sample was judged as not having high strength and failed.

[0219] To evaluate the bendability after applying prestrain, the following test procedure was performed. First, a JIS No. 5 test piece (thickness: the original thickness of the steel sheet) specified in JIS Z 2241:2022 was collected, with its longitudinal direction aligned with the rolling direction of the steel sheet. This test piece was then subjected to a 2% tensile strain, with a gauge length of 50 mm and a tensile speed of 3 mm / min using a crosshead displacement velocimeter. Other conditions were in accordance with JIS Z 2241:2022.

[0220] Next, the ends of the tensile-deformed test piece were cut off, and a 60mm long and 25mm wide bending test piece was prepared from the center of the parallel portion of the test piece. The resulting bending test piece was then subjected to a bending test using the V-block method specified in JIS Z 2248:2022, with the bending axis passing through the center of the length direction of the bending test piece and parallel to the width direction of the bending test piece. At this time, V-shaped fasteners 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 piece after the bending test was then visually inspected to determine the limit bending radius at which no cracks would occur.

[0221] The ultimate bending strain is the value obtained by dividing the tip radius R by the plate thickness t (R / t). A maximum bending strain of 2.0 or less indicates excellent bendability after prestraining and is considered acceptable. A maximum bending strain exceeding 2.0 indicates poor bendability after prestraining and is considered unacceptable. It should be noted that the bending test is conducted using plated test pieces, and the "plate thickness t" mentioned above refers to the thickness of the base material excluding the plated layer.

[0222] As shown in Table 4, Tests Nos. 1, 2, 18, 20, and 25-31, all of which met the requirements of the present invention, had strengths exceeding 1110 MPa and exhibited excellent bendability after prestraining. In contrast, the standard deviation of the Mn concentration in Tests Nos. 3-5, 8, 9, 13, 15, 16, 22, 24, and 32 was excessive, resulting in reduced bendability after prestraining. Test No. 33, lacking a double heat treatment, failed to achieve the desired metallographic structure, resulting in reduced strength. Furthermore, despite having an extremely low standard deviation of the Mn concentration, bendability after prestraining was reduced. Furthermore, the metallographic structures of Tests Nos. 6 and 10-12 deviated from the requirements of the present invention, resulting in reduced bendability after prestraining. Test No. 14 deviated from the requirements of the present invention, resulting in reduced strength. Furthermore, the chemical compositions of Tests Nos. 7, 17, 19, and 21-23 deviated from the requirements of the present invention, resulting in reduced strength.

[0223] Industrial applicability

[0224] According to this embodiment, it is possible to provide a steel sheet having high strength and excellent bendability after imparting prestrain.

Claims

1. A steel plate having a chemical composition of C: 0.08-0.17% by mass, 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%, Balance: Fe and impurities, The metallographic structure of the steel plate is calculated in area %. Tempered martensite: 80.0~97.0%, Total of pearlite, ferrite and bainite: less than 10.0%, and The total of primary martensite and retained austenite: 3.0-10.0%, The standard deviation of the Mn concentration of the primary martensite and the retained austenite is 1.0-5.0%, The tensile strength of the steel plate is greater than 1110 MPa.

2. The steel plate according to claim 1, wherein The metallographic structure is calculated as the retained austenite in area %: 1.5% or more.

3. The steel plate according to claim 1 or claim 2, wherein: The chemical composition contains, in mass%, Mo: 0.600% or less, and V: 0.300% or less One or more of the following may be used to replace part of Fe.

4. The steel plate according to any one of claims 1 to 3, wherein The chemical composition contains, in mass%, B: 0.0030% or less, Cr: 0.50% or less, Cu: 0.50% or less, and Ni: 2.0% or less One or more of the following may be used to replace part of Fe.

5. The steel plate according to any one of claims 1 to 4, wherein The chemical composition contains, in mass%, Ca: 0.020% or less, Mg: 0.020% or less, REM: 0.100% or less, and Bi: 0.020% or less One or more of the following may be used to replace part of Fe.

6. The steel plate according to any one of claims 1 to 5, wherein The chemical composition contains in mass % Sn: 0.05% or less To replace part of Fe.

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