Steel sheet and component

Abstract

The steel sheet has a prescribed chemical composition, and in the metallographic structure at a position 1 / 4 of the sheet thickness from the surface, the steel sheet contains, in area%, 60.0% or more but less than 90.0% of bainite, more than 10.0% but 40.0% or less of martensite, and 10.0% or less in total of ferrite, pearlite and retained austenite, with the balance being Fe and unavoidable impurities. The value obtained by dividing the standard deviation of the aspect ratio of the old austenite crystal grains by the average value of the aspect ratio of the old austenite crystal grains is 0.50 or less, and the average grain diameter of the old austenite crystal grains is 30 [mu] m or less.

Description

Technical Field

[0001] This disclosure relates to steel plates and components.

[0002] This application claims priority based on Japanese Patent Application No. 2023-193550, dated November 14, 2023, the contents of which are incorporated herein by reference. Background Technology

[0003] In recent years, efforts have been made to reduce CO2 emissions and achieve lightweighting of automobile bodies. In blank-formed components such as stamped parts, weight reduction can be achieved by reducing the thickness of the material. Particularly in automotive running gear components such as lower arms and trailing arms, research has begun on using steel plates with a strength exceeding 980 MPa to achieve lightweighting of the automobile body.

[0004] The aforementioned components have complex shapes. As steel sheets become stronger, their formability deteriorates. Therefore, if high-strength steel sheets are used in such components, necking sometimes occurs due to insufficient formability. Thus, steel sheets used in such components require excellent formability, particularly ductility and hole-expanding properties.

[0005] Furthermore, as steel sheets increase in strength, pre-strain embrittlement deteriorates. As a result, impact resistance deteriorates in components that have been pre-strained during processing. Therefore, for steel sheets used in such components, excellent impact resistance after pre-straining is required.

[0006] For example, Patent Document 1 discloses a high-tensile steel sheet characterized in that 90% or more of the steel microstructure is bainite, and the remaining portion is a mixed microstructure containing martensite and austenite (MA microstructure), ferrite, and quasi-polygonal ferrite. The average grain size of the bainite microstructure is 5-20 μm, and the average aspect ratio of the old austenite grains is 5.0 or higher. Patent Document 1 discloses that with the above-mentioned structure, weldability (HAZ toughness and resistance to weld cracking) and stable base material properties (tensile strength and toughness) can be achieved.

[0007] Patent Document 2 discloses a high-strength hot-rolled steel sheet having a microstructure containing more than 95% martensite phase by area, an average aspect ratio of more than 3.0 for the old austenite grains, a 5-minute relaxation stress value of less than 20 MPa when subjected to 400 MPa in a stress relaxation test, and a tensile strength of more than 1180 MPa. Patent Document 2 discloses that, with the above-described structure, it is possible to manufacture a high-strength hot-rolled steel sheet with high strength (TS: 1180 MPa or more), significantly improved resistance to delayed destructive forces, suitable as a raw material for automotive parts, and excellent resistance to delayed destructive forces.

[0008] Patent Document 3 discloses a method for manufacturing a high-strength hot-rolled steel sheet, characterized in that the average aspect ratio of the old austenite grains is 1.3 or more and 5.0 or less, and the area fraction of the bainite phase is 80% or more. Patent Document 3 discloses that a high-strength hot-rolled steel sheet with excellent porosity can be obtained through the above-described structure.

[0009] Existing technical documents Patent documents Patent Document 1: Japanese Patent Application Publication No. 2011-246768 Patent Document 2: International Publication No. 2021 / 193310 Patent Document 3: Japanese Patent Application Publication No. 2021-116476 Summary of the Invention

[0010] The problem that the invention aims to solve However, the impact resistance characteristics after pre-straining were not considered in patent documents 1-3.

[0011] This disclosure is made in view of the above-mentioned circumstances. The purpose of this disclosure is to provide a steel sheet having high strength, excellent ductility and porosity, and excellent impact resistance after being pre-stressed, and components using the steel sheet.

[0012] Methods for solving problems The main purpose of this disclosure is as follows.

[0013] [1] A steel plate, characterized in that, Chemical composition in mass% is C: 0.050~0.200% Si: 0.05~3.00% Mn: 1.00~3.00% sol.Al: 0.001~0.500% P: Below 0.100% S: Below 0.0300% N: below 0.1000% O: Below 0.0100% Nb: 0.001~1.000% Ti: 0.070~0.200% B: 0~0.0100% Cr: 0~2.00% Mo: 0~1.00% Cu: 0~2.00% Ni: 0~2.00% V: 0~0.50%, Sn: 0~0.050% As: 0~0.100% Zr: 0~1.000% Ca: 0~0.0200% Mg: 0~0.0200% Bi: 0~0.0200% Co: 0~1.000%, W: 0~1.000% Zn: 0~1.000%, REM: 0~0.1000% And the remainder: Fe and impurities, In the metal structure located at a position 1 / 4 of the plate thickness from the surface, In terms of area percentage, Bainite: 60.0% or more but less than 90.0% Martensite: exceeding 10.0% but below 40.0% Ferrite, pearlite, and retained austenite: totaling less than 10.0%. The standard deviation of the aspect ratio of the old austenite grains divided by the average aspect ratio of the old austenite grains is 0.50 or less. The average grain size of the aforementioned old austenite grains is less than 30 μm.

[0014] [2] According to the steel plate described in [1] above, the characteristic is that, The above chemical composition contains, by mass%, Selected from B: 0.0001~0.0100% Cr: 0.01~2.00% Mo: 0.01~1.00% Cu: 0.01~2.00% Ni: 0.01~2.00% V: 0.01~0.50%, Sn: 0.001~0.050% As: 0.001~0.100% Zr: 0.001~1.000% Ca: 0.0001~0.0200% Mg: 0.0001~0.0200% Bi: 0.0001~0.0200% Co: 0.001~1.000%, W: 0.001~1.000% Zn: 0.001~1.000% and REM: One or more of the following: 0.0001 to 0.1000%.

[0015] [3] A component, characterized in that it is made of the steel plate described in [1] or [2].

[0016] Invention Effects According to the above-described manner of this disclosure, it is possible to provide a steel plate with high strength, excellent ductility and porosity, and excellent impact resistance after being pre-stressed, and components using the steel plate. Detailed Implementation

[0017] The steel plate and components of this embodiment will now be described in detail. However, this disclosure is not limited to the configuration disclosed in this embodiment, and various modifications can be made without departing from the spirit of this disclosure.

[0018] Within the numerical range specified by the "~" symbol, both the lower and upper limits are included. Values ​​expressed as "less than" or "more than" are not included in the numerical range. All "%" values ​​related to chemical composition refer to "mass %".

[0019] The chemical composition of the steel plate in this embodiment, by mass%, contains: C: 0.050~0.200%, Si: 0.05~3.00%, Mn: 1.00~3.00%, sol.Al: 0.001~0.500%, P: 0.100% or less, S: 0.0300% or less, N: 0.1000% or less, O: 0.0100% or less, Nb: 0.001~1.000%, Ti: 0.070~0.200%, B: 0~0.0100%, Cr: 0~2.0%. 0%, Mo: 0~1.00%, Cu: 0~2.00%, Ni: 0~2.00%, V: 0~0.50%, Sn: 0~0.050%, As: 0~0.100%, Zr: 0~1.000%, Ca: 0~0.0200%, Mg: 0~0.0200%, Bi: 0~0.0200%, Co: 0~1.000%, W: 0~1.000%, Zn: 0~1.000%, REM: 0~0.1000%, and the remainder: Fe and impurities.

[0020] The following is a detailed explanation of each element.

[0021] C: 0.050~0.200% Carbon (C) is an element required to obtain the desired tensile strength in steel plates. When the C content is less than 0.050%, the desired tensile strength cannot be obtained in the steel plate. Therefore, the C content is 0.050% or more. Preferably, the C content is 0.060% or more, 0.070% or more, 0.080% or more, 0.090% or more, or 0.100% or more.

[0022] On the other hand, when the carbon content exceeds 0.200%, the strength of the steel plate becomes too high, and its ductility deteriorates. Therefore, the carbon content is 0.200% or less. Preferably, the carbon content is 0.190%, 0.170%, 0.150%, 0.130%, or 0.120% or less.

[0023] Si: 0.05~3.00% Si is an effective element for deoxidation. If the Si content is less than 0.05%, it is impossible to improve the quality of steel (suppress defects such as porosity in the steel) through deoxidation. Therefore, the Si content is 0.05% or more. The preferred Si content is 0.10% or more, 0.30% or more, 0.50% or more, 0.60% or more, or 0.70% or more.

[0024] On the other hand, when the Si content exceeds 3.00%, a striped pattern of Si oxide scale is formed on the surface of the steel plate, damaging the surface properties of the steel plate. Consequently, the chemical conversion treatmentability of the steel plate deteriorates. Therefore, the Si content is 3.00% or less. Preferably, the Si content is 2.50% or less, 2.00% or less, 1.50% or less, or 1.20% or less.

[0025] Mn: 1.00~3.00% Mn is an element required to improve the strength of steel plates. If the Mn content is less than 1.00%, the desired tensile strength cannot be obtained in the steel plate. Therefore, the Mn content is 1.00% or more. The preferred Mn content is 1.30% or more, 1.50% or more, 1.80% or more, or 2.00% or more.

[0026] On the other hand, if the Mn content exceeds 3.00%, the strength of the steel plate becomes too high, and the ductility of the steel plate deteriorates. Therefore, the Mn content is 3.00% or less. The preferred Mn content is 2.70% or less, 2.50% or less, 2.30% or less, or 2.20% or less.

[0027] sol.Al: 0.001~0.500% Sol.Al acts as a deoxidizer and is an element that improves the cleanliness of steel. To achieve this effect, the sol.Al content is 0.001% or more. Preferably, the sol.Al content is 0.005%, 0.010%, or 0.020% or more.

[0028] On the other hand, casting becomes difficult when the sol.Al content exceeds 0.500%. Therefore, the sol.Al content is 0.500% or less. Preferably, the sol.Al content is 0.400% or less, 0.300% or less, 0.200% or less, 0.100% or less, 0.050% or less, 0.040% or less, or 0.035% or less.

[0029] In addition, in this embodiment, sol.Al refers to acid-soluble Al, which means solid-solution Al that exists in the steel in a solid solution state.

[0030] P: below 0.100% Phosphorus (P) is an element that segregates in the central part of the steel plate. If the P content exceeds 0.100%, slab cracks are easily generated, making casting difficult. Therefore, the P content is 0.100% or less. The preferred P content is 0.080%, 0.050%, 0.040%, 0.030%, 0.020%, or 0.015% or less.

[0031] The lower the phosphorus (P) content, the better; therefore, 0% is preferred. However, if the P content is reduced excessively, the cost of removing P increases significantly; therefore, the P content can also be above 0.001% or above 0.005%.

[0032] S: Below 0.0300% Sulfur (S) is an element that embrittles slabs by existing as sulfides in steel. Additionally, S deteriorates the formability of steel sheets. If the S content exceeds 0.0300%, the hole-expanding properties of the steel sheet deteriorate. Therefore, the S content is 0.0300% or less. Preferably, the S content is 0.0200%, 0.0100%, 0.0080%, 0.0050%, 0.0040%, or 0.0030% or less.

[0033] The lower the sulfur content, the better; therefore, 0% is preferred. However, if the sulfur content is reduced excessively, the cost of sulfur removal will increase significantly. Therefore, the sulfur content can be 0.0001% or higher, 0.0005% or higher, or 0.0010% or higher.

[0034] N: below 0.1000% Nitrogen (N) forms coarse nitrides in steel, deteriorating the porosity of the steel sheet. If the N content exceeds 0.1000%, the porosity of the steel sheet deteriorates. Furthermore, a high N content increases the risk of slab cracking. Therefore, the N content is 0.1000% or less. Preferably, the N content is 0.0500%, 0.0100%, 0.0080%, 0.0060%, 0.0050%, 0.0040%, or 0.0030% or less.

[0035] The lower the nitrogen content, the better; therefore, 0% is preferred. However, if the nitrogen content is reduced excessively, the cost of nitrogen removal increases significantly. Therefore, the nitrogen content can be 0.0005% or higher, or 0.0010% or higher.

[0036] O: Below 0.0100% When oxygen (O) is present in large quantities in steel, it forms coarse oxides. If the O content exceeds 0.0100%, the porosity of the steel sheet deteriorates significantly. Therefore, the O content is 0.0100% or less. Preferably, the O content is 0.0080%, 0.0050%, 0.0040%, or 0.0030% or less.

[0037] The lower the oxygen content, the better; therefore, 0% is preferred. However, in order to disperse fine oxides in large quantities during the deoxidation of molten steel, the oxygen content can be 0.0005% or more, or 0.0010% or more.

[0038] Nb: 0.001~1.000% Nitrogen (Nb) is an element that suppresses abnormal grain growth of austenite grains during hot rolling. Additionally, Nb also increases the strength of steel sheets by forming fine carbides. If the Nb content is less than 0.001%, the old austenite grains cannot be reduced in the metal structure of the steel sheet, and the impact resistance deteriorates after pre-straining. Therefore, the Nb content is 0.001% or more. The preferred Nb content is 0.010% or more, 0.015% or more, or 0.020% or more.

[0039] On the other hand, if the Nb content exceeds 1.000%, the hole-expanding properties of the steel plate deteriorate. Therefore, the Nb content is 1.000% or less. The Nb content is preferably 0.900%, 0.500%, 0.100%, or 0.050% or less.

[0040] Ti: 0.070~0.200% Ti is an element that increases the strength of steel sheets by forming fine nitrides within the steel. If the Ti content is less than 0.070%, the strength of the steel sheet decreases. Therefore, the Ti content is 0.070% or more. Preferably, the Ti content is 0.080% or more, 0.090% or more, or 0.100% or more.

[0041] On the other hand, if the Ti content exceeds 0.200%, the porosity of the steel sheet deteriorates. Furthermore, a high Ti content increases the risk of slab cracking. Therefore, the Ti content is 0.200% or less. Preferably, the Ti content is 0.180%, 0.150%, 0.130%, or 0.120% or less.

[0042] The following elements are arbitrary elements, and their presence is not mandatory. The lower limit for the content of each arbitrary element is 0%. The following is an explanation of each arbitrary element.

[0043] B: 0~0.0100% Boron (B) is an element that inhibits phase transformation at high temperatures and increases the strength of steel plates. To achieve this effect more reliably, it is preferable to set the B content to 0.0001% or higher.

[0044] On the other hand, if the boron content is excessive, boron precipitates will form, reducing the strength of the steel plate. Therefore, the boron content is 0.0100% or less. Depending on the requirements, the boron content can also be 0.0080%, 0.0050%, or 0.0030% or less.

[0045] Cr: 0~2.00% Cr is an element that exhibits effects similar to Mn. To reliably obtain the effect of increasing the strength of steel plates by containing Cr, the Cr content is preferably 0.01% or more.

[0046] On the other hand, if the Cr content exceeds 2.00%, the martensite fraction increases, and the ductility of the steel plate deteriorates. Therefore, the Cr content is 2.00% or less. Depending on the requirements, the Cr content can also be 1.50% or less, 1.00% or less, or 0.60% or less.

[0047] Mo: 0~1.00% Mo is an element that increases the strength of steel plates by forming fine carbides in the steel. To reliably achieve this effect, it is preferable to have a Mo content of 0.01% or more.

[0048] On the other hand, if the Mo content exceeds 1.00%, the porosity of the steel plate deteriorates. Therefore, the Mo content is 1.00% or less. Depending on the requirements, the Mo content can also be 0.80%, 0.50%, or 0.25% or less.

[0049] Cu: 0~2.00% Cu improves the hardenability of steel sheets and enhances their strength by precipitating as carbides at low temperatures. To reliably achieve these effects, a Cu content of 0.01% or higher is preferred.

[0050] However, when the Cu content exceeds 2.00%, grain boundary cracks may sometimes occur in the slab. Therefore, the Cu content is kept below 2.00%. Depending on the requirements, the Cu content can also be kept below 1.50%, 1.00%, or 0.60%.

[0051] Ni: 0~2.00% Ni improves the hardenability of steel sheets, thereby increasing their strength. Furthermore, when Ni contains Cu, it effectively suppresses grain boundary cracks in the slab caused by Cu. To reliably obtain the effects described above, a Ni content of 0.01% or higher is preferred.

[0052] Ni is an expensive element, so a high content is not economically desirable. Therefore, the Ni content is 2.00% or less. Depending on the requirements, the Ni content can also be 1.50% or less, 1.00% or less, or 0.60% or less.

[0053] V: 0~0.50% V is an element that increases the strength of steel plates by forming fine carbides in the steel. To reliably achieve this effect, it is preferable to have a V content of 0.01% or more.

[0054] On the other hand, if the V content exceeds 0.50%, the hole-expanding properties of the steel plate deteriorate. Therefore, the V content is 0.50% or less. Depending on the requirements, the V content can also be set to 0.30% or less, 0.10% or less, or 0.06% or less.

[0055] Sn: 0~0.050% Sn is an element that improves the porosity of steel sheets by inhibiting the formation of oxides, which are the starting point for deterioration. To obtain this effect more reliably, it is preferable to have a Sn content of 0.001% or more.

[0056] On the other hand, even with a large amount of Sn, the above effect becomes saturated, therefore the Sn content should be 0.050% or less. Depending on the requirements, the Sn content can also be 0.030% or less, 0.010% or less, or 0.006% or less.

[0057] As: 0~0.100% As is an element that improves the porosity of steel sheets by refining the old austenite grains by lowering the austenite single-phase conversion temperature. To obtain this effect more reliably, it is preferable to set the As content to 0.001% or more.

[0058] On the other hand, even with a large amount of As, the above effect is saturated, so the As content is 0.100% or less. Depending on the needs, the As content can also be 0.050%, 0.010%, or 0.006% or less.

[0059] Zr: 0~1.000% Zr is an element that increases the strength of steel plates through solid solution strengthening. To obtain this effect more reliably, it is preferable to have a Zr content of 0.001% or more.

[0060] On the other hand, if the Zr content exceeds 1.000%, the porosity of the steel plate deteriorates. Therefore, the Zr content is 1.000% or less. Depending on the requirements, the Zr content can also be 0.050% or less, 0.010% or less, or 0.006% or less.

[0061] Ca: 0~0.0200% Ca is an element that improves the ductility and porosity of steel sheets by controlling the morphology of non-metallic inclusions that are the starting point of failure and cause deterioration of formability. To obtain this effect more reliably, it is preferable to have a Ca content of 0.0001% or more.

[0062] On the other hand, if the Ca content exceeds 0.1000%, excessive inclusions will form in the steel, deteriorating the ductility and porosity of the steel plate. Therefore, the Ca content is 0.1000% or less. Depending on the requirements, the Ca content can also be 0.0500%, 0.0100%, or 0.0060% or less.

[0063] Mg: 0~0.0200% Like Ca, Mg is an element that improves the ductility and porosity of steel sheets by controlling the morphology of non-metallic inclusions. To obtain this effect more reliably, it is preferable to have a Mg content of 0.0001% or higher.

[0064] On the other hand, if the Mg content exceeds 0.0200%, excessive inclusions will form in the steel, deteriorating the ductility and porosity of the steel plate. Therefore, the Mg content is 0.0200% or less. Depending on the requirements, the Mg content can also be 0.0100%, 0.0060%, or 0.0040% or less.

[0065] Bi: 0~0.0200% Bi (Bi) improves the formability of steel sheets by refining the solidification structure. To reliably achieve this effect, a Bi content of 0.0005% or higher is preferred. However, even when the Bi content exceeds 0.020%, the effect described above becomes saturated, making it economically undesirable. Therefore, a Bi content of 0.020% or less is preferred. A Bi content of 0.010% or less is preferred. Depending on the requirements, a Bi content of 0.0100% or less, 0.0060% or less, or 0.0040% or less may also be used.

[0066] Co: 0~1.000% Co is an element that increases the strength of steel plates through solid solution strengthening. To achieve this effect more reliably, it is preferable to have a Co content of 0.001% or more.

[0067] On the other hand, if the Co content exceeds 1.000%, the porosity of the steel plate deteriorates. Therefore, the Co content is 1.000% or less. Depending on the requirements, the Co content can also be 0.500%, 0.100%, or 0.060% or less.

[0068] W: 0~1.000% W is an element that increases the strength of steel plates through solid solution strengthening. To obtain this effect more reliably, it is preferable to have a W content of 0.001% or more.

[0069] On the other hand, if the W content exceeds 1.000%, the hole-expanding properties of the steel plate deteriorate. Therefore, the W content is 1.000% or less. Depending on the requirements, the W content can also be 0.500%, 0.100%, or 0.060% or less.

[0070] Zn: 0~1.000% Zn is an element that increases the strength of steel plates through solid solution strengthening. To achieve this effect more reliably, it is preferable to have a Zn content of 0.001% or more.

[0071] On the other hand, if the Zn content exceeds 1.000%, the porosity of the steel plate deteriorates. Therefore, the Zn content is 1.000% or less. Depending on the requirements, the Zn content can also be 0.500%, 0.100%, or 0.060% or less.

[0072] REM: 0~0.1000% Like Ca, REM is an element that improves the ductility and porosity of steel sheets by controlling the morphology of non-metallic inclusions. To obtain this effect more reliably, it is preferable to have a REM content of 0.0001% or higher.

[0073] On the other hand, if the REM content exceeds 0.1000%, excessive inclusions will form in the steel, deteriorating the ductility and porosity of the steel plate. Therefore, the REM content is 0.1000% or less. Depending on the requirements, the REM content can also be 0.0500%, 0.0100%, or 0.0060% or less.

[0074] Furthermore, REM refers to a total of 17 elements including Sc, Y, and the lanthanides, and the REM content mentioned above refers to the total content of these elements. In the case of lanthanides, they are added industrially as a mixture of rare earth metals.

[0075] The remaining chemical composition of the steel sheet in this embodiment consists of Fe and impurities. In this embodiment, impurities refer to substances that have been introduced from the ore used as raw material, waste, or the environment during manufacturing, or substances that are permissible within a range that do not adversely affect the properties of the steel sheet in this embodiment.

[0076] The chemical composition of the aforementioned steel plates can be analyzed using a spark discharge luminescence spectrophotometer or similar device. It should be noted that C and S are values ​​determined by combustion in an oxygen stream using a gas composition analyzer and infrared absorption method. N is determined by melting a test piece collected from the steel plate in a helium stream and measuring its thermal conductivity. O is determined using an inert gas melting-non-dispersive infrared absorption method. Sol and Al are determined, for example, according to JIS G 1257-10-2:2013.

[0077] When a steel plate has a coating or film on its surface, the coating or film can be removed by mechanical grinding or other means as needed, and then the chemical composition can be analyzed.

[0078] Next, the metal structure of the steel plate of this embodiment will be described.

[0079] In this embodiment, the metal microstructure of the steel plate at a position 1 / 4 of the plate thickness from the surface comprises, by area %, bainite: 60.0% or more and less than 90.0%, martensite: more than 10.0% and less than 40.0%, ferrite, pearlite, and retained austenite: a total of less than 10.0%, the standard deviation of the aspect ratio of the old austenite grains divided by the average aspect ratio of the old austenite grains is less than 0.50, and the average grain size of the old austenite grains is less than 30 μm.

[0080] The following is an explanation of each requirement.

[0081] Furthermore, in this embodiment, the metal structure is defined as being located at a position 1 / 4 of the plate thickness from the surface (a region ranging from 1 / 8 of the plate thickness to 3 / 8 of the plate thickness from the surface). In other words, this position is the region starting from 1 / 8 of the plate thickness from the surface and ending at 3 / 8 of the plate thickness from the surface.

[0082] The reason for specifying the metal structure at the location is that the metal structure at the location represents the representative metal structure of the steel plate.

[0083] It should be noted that when the steel plate has a coating or film on its surface, the surface referred to here is the interface between the steel plate and the coating or film.

[0084] Bainite: 60.0% or more but less than 90.0% Bainite is a microstructure that improves the strength, ductility, and porosity of steel sheets. When the area fraction of bainite is less than 60.0%, the desired ductility cannot be obtained in the steel sheet. Therefore, the area fraction of bainite is 60.0% or more. Preferably, the area fraction of bainite is 65.0% or more, 70.0% or more, 75.0% or more, or 80.0% or more.

[0085] On the other hand, when the bainite area ratio is above 90.0%, the desired amount of martensite cannot be obtained, and the strength of the steel plate decreases. Therefore, the bainite area ratio is less than 90.0%. The preferred bainite area ratio is 88.0% or less or 85.0% or less.

[0086] Martensite: exceeding 10.0% but less than 40.0% Martensite is a microstructure with higher strength than bainite, thus improving the strength of steel sheets. When the martensite area fraction is below 10.0%, the desired strength cannot be obtained in the steel sheet. Therefore, the martensite area fraction must exceed 10.0%. Preferably, the martensite area fraction is 12.0% or more, or 15.0% or more.

[0087] On the other hand, if the martensite area ratio exceeds 40.0%, the desired amount of bainite cannot be obtained, and the desired ductility cannot be achieved in the steel sheet. Therefore, the martensite area ratio is 40.0% or less. The martensite area ratio is preferably 35.0% or less, 30.0% or less, or 25.0% or less.

[0088] The sum of the area ratio of bainite and the area ratio of martensite is preferably 90.0% or more. This sum of area ratios is preferably 92.0% or more, 95.0% or more, or 97.0% or more, and may also be 98.5% or more, or 100.0%.

[0089] Ferrite, pearlite, and retained austenite: totaling less than 10.0%. In the microstructure of the steel sheet of this embodiment, ferrite, pearlite, and retained austenite may also be present as residual microstructures other than bainite and martensite. If the area fraction of these residual microstructures exceeds 10.0%, the desired strength cannot be obtained in the steel sheet. Therefore, the total area fraction of the residual microstructures is 10.0% or less. The area fraction of the residual microstructures is preferably 8.0% or less, 5.0% or less, 3.0% or less, 1.5% or less, or 0.0%.

[0090] The area ratio of each tissue was determined by the following method.

[0091] Test specimens were collected from a section of the steel plate with its thickness parallel to the rolling direction, in a manner that allowed observation of the metal microstructure at a position 1 / 4 of the plate thickness (the area from 1 / 8 to 3 / 8 of the plate thickness from the surface) and at the center of the plate width.

[0092] After grinding the cross-section of the test piece with #600 to #1500 silicon carbide sandpaper, it was then polished to a mirror finish using a liquid solution made by dispersing diamond powder with a particle size of 1-6 μm in a diluent such as alcohol or pure water. Next, it was ground for 8 minutes at room temperature using colloidal silica with a particle size of 0.25 μm without an alkaline solution to remove strain introduced into the surface layer of the test piece. At any position along the length of the test piece's cross-section, the crystal orientation information was obtained by electron backscatter diffraction at 400x magnification and measurement intervals of 0.1 μm, targeting a region 200 μm along the length of the test piece and extending from a depth of 1 / 8 to 3 / 8 of the plate thickness from the surface.

[0093] For the measurements, an EBSD apparatus consisting of a thermoelectric field radiation scanning electron microscope (JEOL JSM-7001F) and an EBSD detector (TSL DVC5 detector) is preferably used. In this case, the vacuum level within the EBSD apparatus is preferably 9.6 × 10⁻⁶. -5 Below Pa, the accelerating voltage is 15kV, the irradiation current level is 13, the electron beam irradiation level is 62, and other observation conditions are preferred as follows.

[0094] Electron gun types: Schottky WD (working distance): 15mm Objective aperture number: 4 Pixel count: 4096×5120 pix Based on the obtained crystal orientation information, the "Phase Map" function of the "OIM Analysis (registered trademark)" software included with the EBSD analysis device is used to calculate the area ratio of regions with a specific crystal structure of fcc. This yields the area ratio of retained austenite.

[0095] Next, the material with a bcc crystal structure was classified as "bainite, ferrite, pearlite, and martensite." For these regions, the "Grain Orientation Spread" function of the "OIM Analysis" software (registered trademark) included with the EBSD analysis device was used. Under the condition that the boundary with a crystal orientation difference of 15° is considered a grain boundary, the region with a "Grain Orientation Spread" of less than 1° was extracted as ferrite. The area fraction of the extracted ferrite was obtained by calculating the area fraction of the ferrite.

[0096] Next, in the remaining region (regions where "Grain Orientation Spread" exceeds 1°), under the condition that the boundary with a crystal orientation difference of 15° is considered a grain boundary, when the maximum value of "Grain Average IQ" of the ferrite region is set to Iα, the region exceeding Iα / 2 is extracted as bainite, and the region below Iα / 2 is extracted as "pearlite and martensite". The area ratio of bainite is obtained by calculating the area ratio of the extracted bainite.

[0097] If ferrite is not extracted from the field of view, the GAM "Grain Average Misorientation" function is used to extract regions with "Grain Average Misorientation" exceeding 0.50° but below 0.75° as bainite, and regions exceeding 0.75° as "pearlite and martensite," under the condition that boundaries with a crystal orientation difference of 5° are considered grain boundaries. The area ratio of bainite is obtained by calculating the area ratio of the extracted bainite.

[0098] In addition, the removal of contaminants from the surface of the observation area can be achieved by polishing or grinding with alumina particles smaller than 0.1 μm or by Ar ion sputtering.

[0099] In EBSD measurements, the following SEM observations were performed on regions identified as "pearlite and martensite".

[0100] To perform SEM observation of the same area as the EBSD measurement area, Vickers indentations were made near the observation location. The microstructure of the observation surface was preserved, surface contaminants were removed by grinding, and nitric acid ethanol etching was performed. The etched observation surface was then subjected to SEM observation in the same field of view as the EBSD observation area. The magnification was set, for example, to 170x. In the area identified as "pearlite and martensite" in the EBSD measurement, the layered structure of plate-like ferrite and Fe-based carbides was considered pearlite through SEM observation, and its area ratio was calculated to obtain the pearlite area ratio. The area ratio of retained austenite, ferrite, bainite, and pearlite obtained by the above method was subtracted from 100% to obtain the martensite area ratio.

[0101] It should be noted that the rolling direction of the steel plate is determined by the following method.

[0102] Test pieces were collected in a manner that allowed for observation of the steel plate's thickness section. After the collected test pieces were mirror-polished to refine the thickness section, they were observed using an optical microscope at magnifications of 100x, 200x, 500x, and 1000x. Based on the size of the inclusions, an appropriate magnification was selected to measure the inclusion size. The observation range was set to a width of 500 μm or more and the entire thickness of the plate, with darker areas identified as inclusions. Observation could also be performed in multiple fields of view. Next, using the thickness section initially observed as a reference, cross-sectional observations were performed on surfaces parallel to a plane that rotated in 5° increments within a 0° to 180° range along the thickness axis, using the same method. For each cross-section, the average length of the major axes of multiple inclusions in each cross-section was calculated. The cross-section with the largest average length of the major axes of specific inclusions was selected. The direction parallel to the major axis direction of the inclusions in this cross-section was identified as the rolling direction.

[0103] The value obtained by dividing the standard deviation of the aspect ratio of the old austenite grains by the average aspect ratio of the old austenite grains is 0.50 or less. If the standard deviation of the aspect ratio of the old austenite grains divided by the average aspect ratio of the old austenite grains (standard deviation / average value) exceeds 0.50, the uniformity of the old austenite grains is low, and the porosity in the steel sheet deteriorates. Therefore, the value obtained by dividing the standard deviation of the aspect ratio of the old austenite grains by the average aspect ratio of the old austenite grains is set to 0.50 or less. The value obtained by dividing the standard deviation of the aspect ratio of the old austenite grains by the average aspect ratio of the old austenite grains is preferably 0.45 or less, 0.40 or less, or 0.35 or less.

[0104] The lower limit of the value obtained by dividing the standard deviation of the aspect ratio of the old austenite grain by the average aspect ratio of the old austenite grain is not particularly limited, for example, it can be greater than 0.20 or greater than 0.25.

[0105] Average grain size of old austenite: less than 30 μm If the average grain size of the old austenite grains exceeds 30 μm, the impact resistance of the steel plate after pre-straining deteriorates. Therefore, the average grain size of the old austenite grains is 30 μm or less. Preferably, the average grain size of the old austenite grains is 28 μm or less, 25 μm or less, 23 μm or less, or 20 μm or less.

[0106] There is no particular lower limit to the average grain size of the old austenite grains, but if the average grain size of the old austenite grains is too small, the number of bainite nucleation sites increases, resulting in a greater amount of bainite and sometimes a decrease in the strength of the steel plate. Therefore, from the viewpoint of reliably improving the strength of the steel plate, the average grain size of the old austenite grains can be 5 μm or more, or 10 μm or more.

[0107] The aspect ratio and average grain size of the old austenite were determined by the following method.

[0108] Test pieces were collected from steel plates in a manner that allowed observation of the thickness section orthogonal to the rolling direction. The microstructure of the thickness section was visualized on the observation surface of the test piece using an etching solution. The etching solution was prepared by adding hydrochloric acid (0.5–2.0 cc), sodium dodecylbenzenesulfonate (0.3–1.0 g), ferric chloride (0.1–0.3 g), calcium chloride (0.1–0.3 g), and ethanol (0.5–2.0 cc) to 100 cc of picric acid saturated aqueous solution. The etching solution was heated to a temperature range of 40–60 °C before use. Secondary electron microscopy images of a 200 μm area in the rolling direction × 200 μm in the thickness direction were taken using a scanning electron microscope at 500x magnification in a region 1 / 4 of the thickness from the observation surface (1 / 8 to 3 / 8 of the thickness from the surface). The equivalent circle diameter and area of ​​the old austenite grains were determined using the obtained secondary electron microscopy images.

[0109] In addition, the scanning electron microscope is equipped with two electron detectors, at a resolution of 9.6 × 10⁻⁶. -5 In a vacuum below Pa, an electron beam is irradiated onto the observation surfaces with an accelerating voltage of 15 kV and an irradiation current level of 13, and secondary electron images are captured. As a scanning electron microscope, a setup consisting of a thermoelectric field emission scanning electron microscope (JEOL JSM-7001F) and an EBSD detector (TSL DVC5 detector) is preferred. The preferred observation conditions are as follows: At least 10 fields of view are captured. In the captured secondary electron images, the old austenite grain boundaries are imaged with bright contrast.

[0110] Electron gun types: Schottky WD (working distance): 15mm Objective aperture number: 4 Pixel count: 4096×5120 pix For one of the old austenite grains included in the field of view, calculate the equivalent circle diameter. Excluding old austenite grains at the ends of the field of view or those not entirely included in the field of view, perform the above operation on all old austenite grains included in the field of view, and determine the equivalent circle diameter of all old austenite grains in that field of view. Using the equivalent circle diameters of the old austenite grains obtained in each field of view, calculate the average grain size of the old austenite grains based on the following formula.

[0111] Average grain size of old austenite grains = Σ i (A) i ×d i ) / Σ i A i Here, d i A is the equivalent circle diameter of the i-th old austenite grain. i It is the area of ​​the i-th old austenite grain.

[0112] Furthermore, the equivalent circle diameter refers to the diameter of a circle whose area is the same as the area of ​​a specific grain.

[0113] Next, the major and minor axes of the old austenite grains with an equivalent circle diameter of 2 μm or more contained in the aforementioned secondary electron image were measured. The aspect ratio of each old austenite grain was obtained by calculating the ratio of its major to minor axis (major axis / minor axis). Using the obtained aspect ratio of the old austenite grains, the average aspect ratio of the old austenite grains was calculated based on the following formula.

[0114] The average aspect ratio of old austenite grains = Σ i (A) i ×E i ) / Σ i A i Here, E i A is the aspect ratio of the i-th old austenite grain. i It is the area of ​​the i-th old austenite grain.

[0115] In addition, the standard deviation of the aspect ratio of the old austenite grains is obtained by calculating the standard deviation of the aspect ratio of the old austenite grains based on the following formula.

[0116] The standard deviation of the aspect ratio of old austenite grains = √{Σa i ×(E i -E')} E' is the average aspect ratio of the old austenite grains obtained by the above method. i Similarly, is the aspect ratio of the i-th old austenite grain. i It can be done through formula (A)i / ΣA i Find (i.e., a) i =A i / ΣA i A i As mentioned above, this refers to the area of ​​the i-th old austenite grain.

[0117] It should be noted that the above measurements were performed excluding cases containing old austenite grains with an equivalent circle diameter of less than 2 μm. This is because it is believed that old austenite grains with an equivalent circle diameter of less than 2 μm will not affect the properties of the steel sheet of this embodiment.

[0118] Tensile strength (TS): 980~1200MPa In this embodiment, the tensile strength of the steel plate is preferably 980 MPa or higher. By setting the tensile strength to 980 MPa or higher, the effect of vehicle body lightweighting can be increased. More preferably, the tensile strength is 1000 MPa or higher, or 1050 MPa or higher.

[0119] From the viewpoint of suppressing mold wear and ensuring the ductility of the steel plate, the upper limit of tensile strength is set to 1200 MPa or less. More preferably, the tensile strength is 1150 MPa or more, or 1100 MPa or more.

[0120] Total elongation (E1): 12% or more Hole expansion ratio (λ): 50% or more The total elongation can be above 12%, and the porosity can be above 50%. If the total elongation is above 12%, it can be judged as having excellent ductility; if the porosity is above 50%, it can be judged as having excellent porosity.

[0121] Depending on the needs, the total elongation can be set to 13% or 14% or higher. There is no need to determine an upper limit for the total elongation, but it can also be set to below 20%, 18%, or 17%.

[0122] Depending on the requirements, the expansion rate can be above 53%, 55%, or 57%. There is no need to determine an upper limit for the expansion rate, but it can also be below 80%, 75%, or 70%.

[0123] It should be noted that total elongation refers to the "total elongation at break" as stated in JIS Z 2241:2022.

[0124] Tensile strength and total elongation can be obtained by collecting test piece No. 5 according to JIS Z 2241:2022 and conducting tensile tests according to JIS Z 2241:2022. The tensile test piece is collected from the end to 1 / 4 of the width of the plate, with the direction perpendicular to the rolling direction taken as the length direction.

[0125] The porosity was determined by porosity testing in accordance with JIS Z 2256:2020.

[0126] Impact resistance properties after pre-straining The impact resistance after pre-straining was evaluated by Charpy impact testing on specimens with a 5% pre-strain in the tensile test. Tensile testing was performed using the same method as described above, applying a 5% tensile pre-deformation to the specimens. Then, 2.5 mm V-notch specimens were collected from the pre-strained specimens according to JIS Z 2242:2023. Using these V-notch specimens, Charpy impact testing was performed according to JIS Z 2242:2023. The temperature at which the brittle fracture rate reached 50% was defined as the ductile-brittle transition temperature (DBTT). If the obtained DBTT was below -40°C, the impact resistance after pre-straining was considered excellent.

[0127] It should be noted that for steel plates with a thickness of less than 2.5 mm, the total thickness is used for measurement.

[0128] The thickness of the steel plate in this embodiment is not particularly limited and can be 1.2 to 8.0 mm. By making the steel plate thickness 1.2 mm or more, the rolling load becomes too large, making it difficult to suppress hot rolling. The plate thickness can be 1.6 mm or more, 2.0 mm or more, or 2.4 mm or more.

[0129] Furthermore, by making the plate thickness 8.0 mm or less, the aforementioned metal structure can be stably obtained after hot rolling. The plate thickness can also be set to 7.0 mm or less, 6.0 mm or less, or 4.8 mm or less.

[0130] The steel sheet of this embodiment can also be a surface-treated steel sheet with a coating for the purpose of improving surface corrosion resistance, etc. The coating can be an electroplated coating or a hot-dip galvanized coating. Examples of electroplated coatings include electroplated zinc and electroplated Zn-Ni alloy. Examples of hot-dip galvanized coatings include hot-dip galvanizing, alloyed hot-dip galvanizing, hot-dip aluminizing, hot-dip Zn-Al alloy plating, hot-dip Zn-Al-Mg alloy plating, and hot-dip Zn-Al-Mg-Si alloy plating. The amount of coating is not particularly limited and can be the same as in the past.

[0131] Alternatively, appropriate chemical conversion treatments (e.g., coating and drying of silicate-based chromium-free chemical conversion solutions) can be performed after plating to further improve corrosion resistance.

[0132] The steel plate of this embodiment is considered to have high strength, excellent ductility and porosity, and excellent impact resistance after pre-straining, thus making it suitable for components, especially automotive components. In automotive components, it can be applied to running gear such as lower arms and trailing arms.

[0133] The component manufactured using the steel sheet of this embodiment has the same chemical composition as the steel sheet described above. Furthermore, processed and unprocessed portions may be mixed within the component. The unprocessed portions have the same metallic structure as the steel sheet described above. The processed portions have essentially the same metallic structure as the steel sheet described above, but sometimes lack this metallic structure due to heavy processing. Therefore, when measuring the metallic structure of the component, the unprocessed portions are measured. In the absence of unprocessed portions, the portions that have not undergone heavy processing are measured. Unprocessed or lightly processed portions refer to, for example, portions that avoid planar areas of the component, as well as portions subjected to punching, reaming, bending, etc. As an example, in the case of the aforementioned component, the portion that is flat and has the largest area is measured by taking a test piece near its center of gravity.

[0134] For example, the lower arm can be manufactured by deep drawing, bending, and trimming the excess material from the steel plate of this embodiment, followed by punching and hole enlargement. Additionally, the longitudinal arm can be manufactured by flanging, bending, and cutting the steel plate of this embodiment.

[0135] Next, a preferred manufacturing method for the steel plate of this embodiment will be described. The steel plate of this embodiment can be stably manufactured according to the manufacturing method described below. The steel plate of this embodiment is manufactured by hot rolling a slab without subsequent cold rolling; therefore, it can also be called a hot-rolled steel plate.

[0136] It should be noted that the temperatures of the slab and the steel plate in this embodiment refer to the surface temperatures of the slab and the steel plate, respectively.

[0137] In the preferred manufacturing method of the steel plate in this embodiment, The slab is heated in a temperature range above 1200℃ for more than 3000 seconds. In hot rolling, Perform rolling at least twice with a reduction rate of over 40% in the temperature range of 1010~1180℃, and set the total interval time between passes in the 1010~1180℃ temperature range to be less than 20 seconds. The total reduction rate for the temperature range of 960~1010℃ is set to be above 0% and less than 5%. The total reduction rate for the temperature range below 960℃ is set to be above 20% and below 80%. After hot rolling, the temperature is cooled to a range of 500-650°C at an average cooling rate of 50°C / second or higher. Air cooling is performed for 3 to 10 seconds within the aforementioned temperature range of 500 to 650°C. After air cooling, the temperature is cooled to 100°C at an average cooling rate of 50°C / second or higher.

[0138] The following is a description of each process.

[0139] To ensure sufficient solid solution of carbides and achieve the desired strength in the steel sheet, the slab heating temperature is set to 1200°C or higher. From an energy cost perspective, the slab heating temperature is preferably below 1350°C.

[0140] It should be noted that, regarding the heated slab, there are no particular limitations other than having the aforementioned chemical composition. For example, slabs manufactured by continuously casting can be produced using steel with the aforementioned chemical composition smelted in a converter or electric furnace. Alternatively, ingot casting or thin slab casting can be used instead of continuous casting.

[0141] In hot rolling, rolling is performed twice or more at a reduction rate of 40% or more in the temperature range of 1010 to 1180°C, with a pass interval of 20 seconds or less. In the fully recrystallized region of 1010 to 1180°C, performing rolling twice or more at a high reduction rate and setting the pass interval to a desired time promotes recrystallization and refines the old austenite grains. Furthermore, the pass interval mentioned here is the total time of all pass intervals in the temperature range of 1010 to 1180°C. From the viewpoint of equipment limitations, the pass interval can be 3 seconds or more. To further reduce the value obtained by dividing the standard deviation of the aspect ratio of the old austenite grains by the average aspect ratio of the old austenite grains, it is preferable to further increase the reduction rate of rolling twice or more in the temperature range of 1010 to 1180°C.

[0142] It should be noted that the reduction rate mentioned here, when the plate thickness before rolling is set as t0 and the plate thickness after rolling is set as t1, can be expressed as (1-t1 / t0)×100 (%).

[0143] The total reduction rate in the temperature range of 960–1010°C is 0% or more and less than 5%, while the total reduction rate in the temperature range below 960°C is 20% or more and less than 80%. By reducing the reduction rate in the partially recrystallized region of 960–1010°C or by not rolling, the uniformity of the austenite grain morphology (aspect ratio) can be improved. As a result, it is preferable to control the value obtained by dividing the standard deviation of the aspect ratio of the old austenite grains by the average aspect ratio of the old austenite grains. To further reduce the value obtained by dividing the standard deviation of the aspect ratio of the old austenite grains by the average aspect ratio of the old austenite grains, it is preferable to set the total reduction rate in the temperature range of 960–1010°C to 3% or less. Alternatively, to reduce the reduction rate in the further expanded partially recrystallized region, it is preferable to set the total reduction rate in the temperature range of 940–1010°C to 0% or more and less than 5%.

[0144] The total reduction rate in the temperature region below 960°C is 20% or more and less than 80%. By ensuring that the total reduction rate in the non-recrystallized region below 960°C is 20% or more and less than 80%, dislocations can be preferentially introduced into austenite, resulting in the acquisition of desired amounts of bainite and martensite.

[0145] It should be noted that the total reduction rate refers to the ratio (1-t3 / t2)×100 (%) when the initial thickness of the plate before rolling in the set temperature zone is set as t2 and the final thickness of the plate after rolling in the set temperature zone is set as t3.

[0146] After hot rolling, the material is cooled to a temperature range of 500~650°C at an average cooling rate of 50°C / second or higher. By setting the average cooling rate to 50°C / second or higher, the desired amount of martensite can be obtained.

[0147] Furthermore, the average cooling rate mentioned 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 to the end point.

[0148] Air cooling is performed for 3 to 10 seconds within a temperature range of 500 to 650°C. By setting the air cooling temperature range above 500°C, the desired amount of bainite can be obtained. Furthermore, by setting the air cooling temperature range below 650°C, the area fraction of the remaining tissue can be reduced. Additionally, by setting the air cooling time to 3 to 10 seconds, the desired amount of bainite can be obtained.

[0149] In addition, in this embodiment, air cooling refers to cooling with an average cooling rate of less than 10°C / second.

[0150] After air cooling, the temperature is cooled to 100°C at an average cooling rate of 50°C / second or higher. By setting the average cooling rate up to 100°C to 50°C or higher, the desired amount of martensite can be obtained. For example, after air cooling, the temperature is cooled to a certain temperature at a high cooling rate, and even if the temperature is cooled to 100°C from that temperature at a low cooling rate, the average cooling rate from air cooling to 100°C should be 50°C / second or higher.

[0151] After cooling, the mixture can be rolled into a roll.

[0152] The steel plate of this embodiment can be manufactured stably using the manufacturing method described above.

[0153] Example Next, the effects of one aspect of this disclosure will be further explained in detail through embodiments. However, the conditions in the embodiments are examples adopted to confirm the feasibility and effects of implementing this disclosure, and this disclosure is not limited to this single example of conditions. This disclosure does not depart from its spirit, and various conditions can be adopted as long as the purpose of this disclosure is achieved.

[0154] By performing converter smelting and continuous casting, slabs with the chemical compositions shown in Tables 1A to 1D are obtained. Based on the conditions shown in Tables 2A and 2B, steel plates with thicknesses ranging from 2.1 to 6.8 mm are obtained from the obtained slabs.

[0155] It should be noted that the slab is heated to a temperature above 1250°C, and the holding time within this temperature range is 3500 seconds. Additionally, after hot rolling, the slab is cooled to the air-cooling start temperature at the average cooling rate recorded in Tables 2A and 2B, followed by 5 seconds of air cooling.

[0156] For the obtained steel plate, the microstructure, tensile strength, total elongation, porosity, and impact resistance after pre-straining are evaluated using the methods described above.

[0157] The results are shown in Tables 3A and 3B. It should be noted that the "Standard Deviation / Average" in Tables 3A and 3B represents the value obtained by dividing the standard deviation of the aspect ratio of the old austenite grains by the average aspect ratio of the old austenite grains. The "Retained Microstructure" in Tables 3A and 3B represents the sum of the area ratios of ferrite, pearlite, and retained austenite.

[0158] A tensile strength of 980 MPa or higher is considered high strength and thus deemed acceptable. Conversely, a tensile strength less than 980 MPa is considered insufficient and thus deemed unacceptable.

[0159] In addition, when the tensile strength exceeds 1200 MPa, it is considered that the strength is too high to ensure the desired ductility, and it is judged as unqualified.

[0160] When the total elongation rate is 12% or more, it is considered to have excellent ductility and judged as qualified. On the other hand, when the total elongation rate is less than 12%, it is considered not to have excellent ductility and judged as unqualified.

[0161] When the hole expansion rate is 50% or more, it is judged as qualified as having excellent hole expansion property. On the other hand, when the hole expansion rate is less than 50%, it is considered not to have excellent hole expansion property and judged as unqualified.

[0162] In the Charpy impact test after applying prestrain, when the ductile - brittle transition temperature (DBTT) is -40°C or lower, it is judged as qualified as having excellent collision resistance property after applying prestrain. On the other hand, when the ductile - brittle transition temperature exceeds -40°C, it is considered not to have excellent collision resistance property after applying prestrain and judged as unqualified.

[0163] From the observation of Table 3A and Table 3B, it can be seen that the steel plates of the present invention examples have high strength, excellent ductility and hole expansion property, and have excellent collision resistance property after applying prestrain.

[0164] On the other hand, it can be seen that one or more of the above properties of the steel plates of the comparative examples are poor.

[0165] In addition, for all the embodiments, the lower arm (component) is manufactured by stamping. For the flat part of the lower arm, the same evaluation as the above method is carried out. The measurement results and evaluation results are the same as those shown in Table 3A and Table 3B.

[0166] Industrial Applicability According to the above - mentioned aspect of the present disclosure, it is possible to provide a steel plate having high strength, excellent ductility and hole expansion property, and having excellent collision resistance property after applying prestrain, and a component using the steel plate.

Claims

1. A steel plate, characterized in that, Chemical composition in mass% is C：0.050~0.200%、 Si: 0.05~3.00% Mn: 1.00~3.00% sol.Al: 0.001~0.500% P: Below 0.100% S: Below 0.0300% N: below 0.1000% O: Below 0.0100% Nb: 0.001~1.000% Ti: 0.070~0.200% B：0~0.0100%、 Cr：0~2.00%、 Mo: 0~1.00% Cu: 0~2.00% Ni: 0~2.00% V：0~0.50%、 Sn: 0~0.050% As: 0~0.100% Zr：0~1.000%、 Ca: 0~0.0200% Mg: 0~0.0200% Bi: 0~0.0200% Co: 0~1.000%, W：0~1.000%、 Zn: 0~1.000%, REM: 0~0.1000%, and Remaining components: Fe and impurities. In the metal structure located at a position 1 / 4 of the plate thickness from the surface, In terms of area percentage, Bainite: 60.0% or more but less than 90.0% Martensite: exceeding 10.0% but below 40.0% Ferrite, pearlite, and retained austenite: totaling less than 10.0%. The standard deviation of the aspect ratio of the old austenite grains divided by the average aspect ratio of the old austenite grains yields a value of 0.50 or less. The average grain size of the old austenite grains is less than 30 μm.

2. The steel plate according to claim 1, characterized in that, The chemical composition contains, by mass%, Selected from B: 0.0001~0.0100% Cr：0.01~2.00%、 Mo: 0.01~1.00% Cu: 0.01~2.00% Ni: 0.01~2.00% V：0.01~0.50%、 Sn: 0.001~0.050% As: 0.001~0.100% Zr：0.001~1.000%、 Ca: 0.0001~0.0200% Mg: 0.0001~0.0200% Bi: 0.0001~0.0200% Co: 0.001~1.000%, W：0.001~1.000%、 Zn: 0.001~1.000% and REM: One or more of the following: 0.0001 to 0.1000%.

3. A component, characterized in that, It includes the steel plate as described in claim 1 or 2.

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