Steel sheet and parts

Abstract

The steel plate has a specified chemical composition, and in a region from a depth of 1 / 8 of the plate thickness to a depth of 3 / 8 of the plate thickness from the surface, it has the following specified metal structure: the average grain size of the original austenite grains is 25 μm or less, the area percentage of tempered martensite is 80.0% to 99.0%, and the area fraction of austenite (MA) is 1.0% to 20.0%. In the above region, microstructure photographs are obtained by continuously taking five fields of view along the rolling direction, each 100 μm in the rolling direction and 100 μm in the plate thickness direction. In the microstructure photographs, when at equal intervals of 10 μm or more line segments at a 45° angle relative to the plate thickness direction are drawn, the difference between the maximum and minimum number of austenite (MA) present on each 100 μm line segment in each microstructure photograph is calculated. When the average value of the difference between the maximum and minimum number of austenite (MA) in the microstructure photographs of the five fields of view is calculated, the average value is 8 to 16.

Description

Technical Field

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

[0002] This application claims priority based on Japanese Patent Application No. 2024-005797 filed on January 18, 2024, the contents of which are incorporated herein by reference. Background Technology

[0003] In recent years, efforts have been made to reduce CO2 emissions by lightweighting automobile bodies. Particularly for automotive running gear components such as lower arms, connecting rods, and steering knuckles, research has begun on using steel sheets with a strength exceeding 780 MPa. These automotive running gear components are manufactured by processing steel sheets through processes such as inner edge flanging, stretching flanges, and bending. Therefore, the steel sheets suitable for these automotive running gear components require formability, particularly a high yield ratio, as well as excellent ductility and hole-expanding properties.

[0004] For example, Patent Document 1 discloses a hot-dip galvanized steel sheet characterized by having the following steel structure: containing more than 40% and less than 95% ferrite by area, and one or more of one or more ultra-hard phase groups selected from martensite, retained austenite, and cementite, totaling more than 3% and less than 20%; wherein the average grain size of the ferrite is less than 2.5 μm, the average grain size of the ultra-hard phase group is less than 2.0 μm, the average value of the closest distance between the ultra-hard phase groups, i.e., the average spacing of the ultra-hard phases, is less than 2.0 μm, and the number density of Ti-B precipitates with an equivalent circle diameter of more than 1 μm is 500 precipitates / mm. 2 The hot-dip galvanized steel sheet has the following mechanical properties: tensile strength: 780 MPa or more, total elongation: 10% or more, hole expansion rate: 35% or more, and minimum inner radius without cracking in a bending test at a bending angle of 180°: less than 3.5 times the sheet thickness.

[0005] Patent document 2 discloses a hot-rolled steel sheet whose microstructure contains, by volume percentage, more than 70% martensite, tempered martensite, and bainite, and contains 5-20% retained austenite. In the surface region, from the surface to 1 / 10 of the sheet thickness, {211} <111> ~{111} <112> The average polar density of the orientation group formed is similar to {110} <001> The sum of the extreme densities of the crystal orientations is less than 6.0, the concentration of dissolved carbon in the retained austenite is more than 0.5% by mass, and the tensile strength of the hot-rolled steel plate is more than 980 MPa.

[0006] Existing technical documents Patent documents Patent Document 1: Japanese Patent No. 5910396 Patent Document 2: International Publication No. 2021 / 167079 Summary of the Invention

[0007] The problem that the invention aims to solve When manufacturing lightweight components using high-strength steel plates, it is necessary to compensate for the reduction in rigidity caused by thinning the component walls through the strengthening of the cross-sectional shape and raw materials. The inventors have discovered that, as... Figure 1 As shown, when high-strength steel plates are applied to, for example, the lower arm of an automotive running gear, cracks occur in the curved, bulging portion near the connecting rod. These cracks arise in the portion subjected to tensile deformation in a manner orthogonal to the crack after the bulging deformation.

[0008] The inventors have discovered that, as in the examples above, cracks occur when high-strength steel plates are applied to components with complex cross-sectional shapes to ensure high rigidity. The inventors have also discovered that the cracks occur in areas other than the flange portion and the inner edge flange end face, which have been previously the subject of research.

[0009] Analysis of the fracture site revealed that, at the fracture site, bending-bending recovery deformation occurs during stamping. Following this bending-bending recovery deformation, the material shrinks locally and fractures due to deformation in a direction orthogonal to the bending strain. The inventors found that, to suppress this fracture, excellent formability is achieved by imparting a pre-strain induced by the bending-bending recovery deformation, particularly requiring a high rate of reduction in the ultimate fracture thickness.

[0010] The ultimate fracture thickness reduction rate is a value calculated based on the minimum of the thickness of the tensile test specimen before fracture and the thickness of the tensile test specimen after fracture. A low ultimate fracture thickness reduction rate after pre-straining leads to premature fracture in subsequent processes, deterioration in durability and impact resistance when used as a component, and is therefore not preferred.

[0011] However, patent documents 1 and 2 do not consider the reduction rate of the ultimate fracture plate thickness after pre-straining.

[0012] This disclosure was made in view of the above-mentioned issues, and its object is to provide a steel plate having high strength and yield ratio, as well as excellent ductility and porosity, and having a high ultimate fracture thickness reduction rate after pre-straining, and components using the steel plate.

[0013] Methods for solving problems The main points of this invention are as follows.

[0014] [1] A steel plate, characterized in that its chemical composition, by mass%, contains: C: 0.090~0.210% Si: 0.20~1.00% Mn: 1.95~2.55%, P: below 0.060% S: Below 0.005% Al: 0.01~0.26% N: below 0.0070% O: 0~0.010% Ti: 0.10~0.18% Nb: 0.01~0.04% B: 0.0001~0.0030% Cr: 0~0.47% Mo: 0~0.12%, Cu: 0~0.40%, Ni: 0~0.30% V: 0~0.30%, Sn: 0~0.040% As: 0~0.100% Zr: 0~0.050%, Ca: 0~0.010%, Mg: 0~0.010%, Bi: 0~0.010% Co: 0~0.010%, W: 0~0.100% Zn: 0~0.010%, REM: 0~0.010%, Sb: 0~0.010%, and Ta: 0~0.010%, The remaining portion contains Fe and impurities. In the region where the depth from the surface is 1 / 8 of the plate thickness to 3 / 8 of the plate thickness, Metal structure, expressed as area %: Tempered martensite: 80.0~99.0% MA: 1.0~20.0% Ferrite and bainite: totaling less than 10.0%, and Pearlite: less than 2.0%, The average grain size of the original austenite grains is less than 25 μm. In the aforementioned region, microstructure photographs are obtained by continuously photographing five fields of view along the rolling direction, each 100 μm in the rolling direction and 100 μm in the plate thickness direction. In these microstructure photographs, at equal intervals of 10 μm or more, at a 45° angle relative to the plate thickness direction, [further details are needed]. When calculating the difference between the maximum and minimum number of MAs present on each 100μm line segment in each of the above tissue photographs, and calculating the average of the difference between the maximum and minimum number of MAs in the above tissue photographs in the above 5 fields of view, the average value is 8 to 16.

[0015] [2] The steel plate according to [1] is characterized in that the above chemical composition contains one or more of the following elements in mass percentage: Cr: 0.01~0.47% Mo: 0.01~0.12% Cu: 0.01~0.40%, Ni: 0.01~0.30% V: 0.01~0.30%, Sn: 0.001~0.040% As: 0.001~0.100% Zr: 0.001~0.050%, Ca: 0.001~0.010% Mg: 0.001~0.010% Bi: 0.001~0.010% Co: 0.001~0.010%, W: 0.001~0.100% Zn: 0.001~0.010%, REM: 0.001~0.010% Sb: 0.001~0.010%, and Ta: 0.001~0.010%.

[0016] [3] A component, characterized in that it comprises the steel plate described in [1] or [2] above.

[0017] Invention Effects According to the above-described solution of this disclosure, it is possible to provide a steel plate with high strength and yield ratio, excellent ductility and porosity, and a high ultimate fracture thickness reduction rate after pre-straining, as well as components using the steel plate. Attached Figure Description

[0018] Figure 1This is a diagram showing the cracked portion of the lower arm made of high-strength steel plate.

[0019] Figure 2 This is a diagram used to illustrate the method for determining the difference between the maximum and minimum number of MAs.

[0020] Figure 3 This is a graph showing the relationship between the average difference between the maximum and minimum number of MAs in the tissue photographs of the five fields of view in the embodiment and the reduction rate of the ultimate fracture plate thickness after pre-straining.

[0021] Figure 4 This is a diagram used to illustrate the manufacturing method of the cap component. Detailed Implementation

[0022] The inventors have discovered that, in order to achieve a high reduction rate in plate thickness at the ultimate fracture after pre-straining, while ensuring high strength and yield ratio, as well as excellent ductility and porosity through a desired metallic microstructure, it is important to preferably control the average grain size of the original austenite grains and, more preferably, the distribution of MA as a hard phase.

[0023] Furthermore, the inventors have discovered that, in order to obtain a steel sheet with the aforementioned metallic structure, it is effective to control the finishing rolling conditions while more strictly controlling the temperature change process from the end of roughing rolling to the start of finishing rolling.

[0024] A steel plate (hereinafter, sometimes referred to as the steel plate of this embodiment) according to one embodiment of the present disclosure will be described. However, the present disclosure is not limited to the configuration disclosed in this embodiment, and various modifications may be made without departing from the spirit of the present disclosure.

[0025] The various constituent elements of this disclosure will be described in detail below. First, the reasons for limiting the chemical composition of the steel plate of this embodiment will be explained.

[0026] In the numerical ranges enclosed in "~", the lower and upper limits are included. Values ​​expressed as "less than" or "more than" are not included in the range. In the following descriptions, percentages of chemical composition are by mass unless otherwise specified.

[0027] The chemical composition of the steel plate of this embodiment, by mass%, contains: C: 0.090~0.210%, Si: 0.20~1.00%, Mn: 1.95~2.55%, P: less than 0.060%, S: less than 0.005%, Al: 0.01~0.26%, N: less than 0.0070%, O: 0~0.010%, Ti: 0.10~0.18%, Nb: 0.01~0.04%, B: 0.0001~0.0030%, and the remainder: Fe and impurities. Each element is described in detail below.

[0028] C: 0.090~0.210% Carbon (C) is an element required to obtain the desired strength of the steel plate. If the C content is less than 0.090%, the desired strength cannot be obtained. Therefore, the C content is set to 0.090% or more. The preferred C content is 0.100% or more, 0.120% or more, or 0.150% or more.

[0029] On the other hand, if the carbon content exceeds 0.210%, the hole-expanding properties of the steel plate deteriorate. Therefore, the carbon content is set to 0.210% or less. Preferably, the carbon content is 0.200% or less or 0.180% or less.

[0030] Si: 0.20~1.00% Si is an element that improves the strength of steel plates through solid solution strengthening. If the Si content is less than 0.20%, the desired strength or porosity cannot be obtained. Therefore, the Si content is set to 0.20% or more. The Si content is preferably 0.40% or more or 0.50% or more.

[0031] On the other hand, if the Si content exceeds 1.00%, the porosity of the steel sheet deteriorates. Therefore, the Si content is set to 1.00% or less. The Si content is preferably 0.90%, 0.70%, or 0.60% or less.

[0032] Mn: 1.95~2.55% Mn is an element needed to improve the strength of steel plates. When the Mn content is less than 1.95%, the area fraction of ferrite becomes too high, making it impossible to obtain the desired strength and porosity. Therefore, the Mn content is set to 1.95% or more. The preferred Mn content is 2.00% or more or 2.10% or more.

[0033] On the other hand, when the Mn content exceeds 2.55%, the porosity of the steel plate deteriorates. Therefore, the Mn content is set to 2.55% or less. Preferably, the Mn content is 2.50% or less or 2.40% or less.

[0034] P: below 0.060% Phosphorus (P) is an element that segregates in the central part of the steel plate's thickness. P also causes embrittlement in welded sections. If the P content exceeds 0.060%, the steel plate's ability to expand its pores deteriorates. Therefore, the P content is set to 0.060% or less. Preferably, the P content is 0.030% or less, or 0.020% or less.

[0035] The lower the phosphorus (P) content, the better, with 0% being the preferred value. However, excessively reducing the P content significantly increases the cost of P removal. Therefore, the P content can be set to 0.001% or higher, or 0.005% or higher.

[0036] S: below 0.005% Sulfur (S) is an element that embrittles slabs by existing as sulfides. S also deteriorates the formability of steel sheets. If the S content exceeds 0.005%, the pore-expanding properties of the steel sheet deteriorate. Therefore, the S content is set to 0.005% or less. Preferably, the S content is 0.004% or less, or 0.003% or less.

[0037] The lower the sulfur content, the better; ideally, it should be 0%. However, excessively reducing the sulfur content significantly increases the cost of sulfur removal. Therefore, the sulfur content can be set to 0.0005% or higher, or 0.001% or higher.

[0038] Al: 0.01~0.26% Al is an element that improves the cleanliness of steel by acting as a deoxidizer. If the Al content is less than 0.01%, sufficient deoxidation cannot be achieved, resulting in a large number of inclusions (oxides) in the steel sheet. Such inclusions deteriorate the formability of the steel sheet. Therefore, the Al content is set to 0.01% or more. The preferred Al content is 0.02%, 0.03%, or 0.05% or more.

[0039] On the other hand, if the Al content exceeds 0.26%, the hole-expanding properties of the steel plate deteriorate. Therefore, the Al content is set to 0.26% or less. The Al content is preferably 0.25%, 0.20%, or 0.15% or less.

[0040] N: below 0.0070% Nitrogen (N) is an element that forms large nitrides in steel, deteriorating the porosity of the steel sheet. If the N content exceeds 0.0070%, the porosity of the steel sheet deteriorates. Therefore, the N content is set to 0.0070% or less. Preferably, the N content is 0.0060%, 0.0050%, 0.0040%, or 0.0030% or less.

[0041] The lower the nitrogen content, the better; 0% is preferred. However, excessively reducing the nitrogen content significantly increases the cost of nitrogen removal. Therefore, the nitrogen content can be set to 0.0005% or higher, or 0.0010% or higher.

[0042] O: 0~0.010% O is an element that forms large oxides that become fracture initiation points when present in large quantities in steel. If the O content exceeds 0.010%, the slab is prone to cracking. Therefore, the O content is set to 0.010% or less. The O content is preferably 0.005% or less or 0.001% or less.

[0043] Since it can be free of oxygen, the oxygen content can be 0%.

[0044] Ti: 0.10~0.18% 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 0.10%, the desired strength cannot be obtained. Therefore, the Ti content is set to 0.10% or more. The preferred Ti content is 0.12% or more or 0.13% or more.

[0045] On the other hand, if the Ti content exceeds 0.18%, the porosity of the steel plate deteriorates. Therefore, the Ti content is set to 0.18% or less. Preferably, the Ti content is 0.16% or less or 0.15% or less.

[0046] Nb: 0.01~0.04% Nitrogen (Nb) is an element that inhibits abnormal grain growth of austenite grains during hot rolling. Additionally, Nb increases the yield ratio of steel sheets by forming fine carbides. If the Nb content is less than 0.01%, the desired yield ratio cannot be obtained. Therefore, the Nb content is set to 0.01% or more. Preferably, the Nb content is 0.02% or more.

[0047] On the other hand, if the Nb content exceeds 0.04%, the hole-expanding properties of the steel plate deteriorate. Therefore, the Nb content is set to 0.04% or less. Preferably, the Nb content is 0.03% or less.

[0048] B: 0.0001~0.0030% Boron (B) is an element that inhibits the formation of ferrite during the cooling process and increases the strength of the steel plate. If the B content is less than 0.0001%, the desired strength cannot be obtained. Therefore, the B content is set to 0.0001% or more. The preferred B content is 0.0005% or more, 0.0010% or more, or 0.0015% or more.

[0049] On the other hand, if the boron content exceeds 0.0030%, the hole-expanding properties of the steel plate deteriorate. Therefore, the boron content is set to 0.0030% or less. Preferably, the boron content is 0.0025% or less or 0.0020% or less.

[0050] The remaining portion of the chemical composition of the steel plate in this embodiment contains Fe and impurities. In this embodiment, impurities refer to substances that have mixed in from the ore used as raw material, scrap iron, or the manufacturing environment.

[0051] The steel plate of this embodiment may also contain the following optional elements instead of a portion of the Fe. The lower limit of the content when the optional elements are not present is 0%. Hereinafter, each optional element will be described.

[0052] Cr: 0.01~0.47% Cr is an element that exhibits effects similar to Mn. To reliably obtain the strength-enhancing effect of Cr in steel plates, the Cr content is preferably set to 0.01% or more.

[0053] On the other hand, the above effect becomes saturated even when the Cr content exceeds 0.47%. Therefore, the Cr content is set to below 0.47%.

[0054] Mo: 0.01~0.12% Mo is an element that increases the strength of steel plates by forming fine carbides in the steel. To reliably achieve this effect, the Mo content is preferably set to 0.01% or more.

[0055] On the other hand, the above effect saturates even when the Mo content exceeds 0.12%. Therefore, the Mo content is set to below 0.12%.

[0056] Cu: 0.01~0.40% Cu improves the hardenability of steel plates and enhances their strength by precipitating as carbides at low temperatures. To reliably achieve these effects, the Cu content is preferably set to 0.01% or higher.

[0057] However, if the Cu content exceeds 0.40%, grain boundary fractures sometimes occur in the slab. Therefore, the Cu content is set below 0.40%.

[0058] Ni: 0.01~0.30% Ni improves the hardenability of steel sheets, thereby increasing their strength. Furthermore, Ni effectively suppresses grain boundary fractures in slabs containing Cu. To reliably obtain the effects described above, the Ni content is preferably set to 0.01% or higher.

[0059] Since Ni is a high-valence element, a high content is not economically desirable. Furthermore, even if the Ni content exceeds 0.30%, the aforementioned benefits become saturated. Therefore, the Ni content is set below 0.30%.

[0060] V: 0.01~0.30% V is an element that increases the strength of steel plates by forming fine carbides in the steel. To reliably achieve this effect, the V content is preferably set to 0.01% or more.

[0061] On the other hand, even if the vitamin C content exceeds 0.30%, the above effects become saturated. Therefore, the vitamin C content is set below 0.30%.

[0062] Sn: 0.001~0.040% Sn has the effect of improving the porosity of steel sheets by inhibiting the formation of oxides that become fracture initiation points. To reliably obtain this effect, the Sn content is preferably set to 0.001% or more.

[0063] On the other hand, the above effect saturates even when Sn exceeds 0.040%. Therefore, the Sn content is set to below 0.040%.

[0064] As: 0.001~0.100% As has the effect of improving the porosity of steel sheets by refining the original austenite grains through lowering the austenite single-phase conversion temperature. To reliably achieve this effect, the As content is preferably set to 0.001% or more.

[0065] On the other hand, if As is present in excess, slab cracks may occur. This effect becomes significant when the As content exceeds 0.100%, therefore the As content is set to be below 0.100%.

[0066] Zr: 0.001~0.050% Zr has the effect of improving porosity. To reliably obtain this effect, the Zr content is preferably set to 0.001% or more.

[0067] On the other hand, the above effect saturates even when the Zr content exceeds 0.050%. Therefore, the Zr content is set below 0.050%.

[0068] Ca: 0.001~0.010% Ca has the effect of dispersing a large amount of fine oxides during the deoxidation of molten steel, thereby refining the microstructure of the steel plate. In addition, Ca has the effect of fixing sulfur in the steel as spherical CaS and suppressing the formation of elongated inclusions such as MnS, thus improving the porosity of the steel plate. To reliably achieve this effect, the Ca content is preferably set to 0.001% or more.

[0069] On the other hand, the above effect saturates even when the Ca content exceeds 0.010%. Therefore, the Ca content is set below 0.010%.

[0070] Mg: 0.001~0.010% Mg has the effect of improving the porosity of steel sheets by adjusting the shape of inclusions in the steel to a preferred shape. To reliably obtain this effect, the Mg content is preferably set to 0.001% or more.

[0071] On the other hand, the above effect becomes saturated even when the Mg content exceeds 0.010%. Therefore, the Mg content is set to below 0.010%.

[0072] Bi: 0.001~0.010% Bi has the effect of improving the porosity of steel sheets by refining the solidification structure. To reliably obtain this effect, the Bi content is preferably set to 0.001% or more.

[0073] On the other hand, the above effect saturates even when the Bi content exceeds 0.010%. Therefore, the Bi content is set to below 0.010%.

[0074] Co: 0.001~0.010% Co has the effect of increasing the strength of steel plates through solid solution strengthening. To reliably obtain this effect, the Co content is preferably set to 0.001% or more.

[0075] On the other hand, the above effect becomes saturated even when the Co content exceeds 0.010%. Therefore, the Co content is set to be below 0.010%.

[0076] W: 0.001~0.100% W has the effect of increasing the strength of steel plates through solid solution strengthening. To reliably obtain this effect, the W content is preferably set to 0.001% or more.

[0077] On the other hand, the above effect saturates even when the W content exceeds 0.100%. Therefore, the W content is set to below 0.100%.

[0078] Zn: 0.001~0.010% Zn has the effect of improving the strength of steel plates through solid solution strengthening. To reliably obtain this effect, the Zn content is preferably set to 0.001% or more.

[0079] On the other hand, if the Zn content exceeds 0.010%, slab cracks may occur. Therefore, the Zn content is set to below 0.010%.

[0080] REM: 0.001~0.010% REM has the effect of increasing the yield ratio of steel plates by adjusting the shape of inclusions in the steel to a preferred shape. To reliably obtain this effect, the REM content is preferably set to 0.001% or more.

[0081] On the other hand, if the REM content exceeds 0.010%, slab cracks may occur. Therefore, the REM content is set to below 0.010%.

[0082] Here, REM refers to a total of 17 elements, including Sc, Y, and the lanthanides. The REM content mentioned above refers to the total content of these elements. In industry, lanthanides are added as a mixture of rare earth metals.

[0083] Sb: 0.001~0.010% Sb improves the ductility and porosity of steel sheets by inhibiting the formation of oxides that become fracture initiation points. To reliably achieve this effect, the Sb content is preferably set to 0.001% or higher.

[0084] On the other hand, even with a large amount of Sb, the above effect is saturated, so the Sb content is set to below 0.010%.

[0085] Ta: 0.001~0.010% Like V, Ta has the effect of increasing the strength of steel plates by forming fine carbides in the steel. To reliably obtain this effect, the Ta content is preferably set to 0.001% or more.

[0086] On the other hand, if the Ta content exceeds 0.010%, the ductility and porosity of the steel plate deteriorate. Therefore, the Ta content is set to below 0.010%.

[0087] The chemical composition of the aforementioned steel plates was analyzed using a spark discharge emission spectrometer. It should be noted that C and S were analyzed using a combustion-infrared absorption method, N was analyzed using an inert gas melting-thermal conductivity method, and O was analyzed using an inert gas melting-non-dispersive infrared absorption method.

[0088] When a steel plate has a coating or film on its surface, the coating or film may be removed by mechanical grinding or other means as needed, and then the chemical composition may be analyzed.

[0089] Next, the metal structure of the steel plate involved in this embodiment will be described.

[0090] For the steel plate of this embodiment, in the region from 1 / 8 of the plate thickness to 3 / 8 of the plate thickness from the surface, the microstructure, in terms of area percentage, is: tempered martensite: 80.0-99.0%, MA: 1.0-20.0%, ferrite and bainite: a total of 10.0% or less, and pearlite: 2.0% or less. The average grain size of the original austenite grains is 25 μm or less. In the above-mentioned region, five fields of view are continuously photographed along the rolling direction, with 10 [units of measurement missing]. Tissue photographs are obtained with a field of view of 0 μm × 100 μm in the plate thickness direction. In the aforementioned tissue photographs, when 10 or more line segments at equal intervals of 10 μm or more are drawn in a direction of 45° relative to the plate thickness direction, the difference between the maximum and minimum number of MAs existing on each 100 μm line segment in each of the aforementioned tissue photographs is calculated. When the average value of the difference between the maximum and minimum number of MAs in the aforementioned tissue photographs of the aforementioned 5 fields of view is calculated, the average value is 8 to 16.

[0091] It should be noted that the steel plate of this embodiment does not contain any structures other than those described above in its metallic microstructure. Therefore, in other words, in the region from 1 / 8 of the plate thickness to 3 / 8 of the plate thickness from the surface, the metallic microstructure of the steel plate of this embodiment consists only of tempered martensite (80.0-99.0%), martensite (MA) (1.0-20.0%), ferrite and bainite (total 10.0% or less), and pearlite (2.0% or less) in area percentage.

[0092] In this embodiment, the region extending from 1 / 8 of the plate thickness to 3 / 8 of the plate thickness from the surface refers to the area starting at 1 / 8 of the plate thickness and ending at 3 / 8 of the plate thickness. The reason for specifying the metal structure within this region is that it represents a representative metal structure of the steel plate.

[0093] When a 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.

[0094] The interface between the steel sheet and the coating or film is determined using a BSE image (or COMPO image) obtained by the following method: A test piece is cut out in a manner that allows observation of the steel sheet's thickness section. The cut test piece is then mechanically ground and mirror-finished. A scanning electron microscope is used to observe an area of ​​40,000 μm, for example, at 400x magnification. 2The above range applies. When observing a cross-section using a BSE image (or COMPO image), a clear difference in contrast can be observed between the plating or coating and the base metal (steel sheet). Therefore, the interface between the steel sheet and the plating or coating can be identified from the position where the contrast changes from the outermost surface. The same method is used to identify the interface when the component described later has a plating or coating.

[0095] Tempered martensite: 80.0~99.0% If the area ratio of tempered martensite is less than 80.0%, the yield ratio of the steel plate decreases. Therefore, the area ratio of tempered martensite is set to be 80.0% or higher. Preferably, the area ratio of tempered martensite is 83.0% or higher or 85.0% or higher.

[0096] On the other hand, if the area ratio of tempered martensite exceeds 99.0%, the ductility of the steel sheet deteriorates. Therefore, the area ratio of tempered martensite is set to 99.0% or less. Preferably, the area ratio of tempered martensite is 95.0% or less or 90.0% or less.

[0097] MA: 1.0~20.0% MA, or island martensite, refers to a mixed microstructure of primary martensite and retained austenite. If the area ratio of MA is less than 1.0%, the ductility of the steel sheet deteriorates. Therefore, the area ratio of MA is set to 1.0% or more. Preferably, the area ratio of MA is 3.0% or more, 5.0% or more, or 10.0% or more.

[0098] On the other hand, if the area ratio of tempered martensite (MA) exceeds 20.0%, the area ratio of tempered martensite decreases, and the yield ratio of the steel sheet decreases. Therefore, the area ratio of MA is set to 20.0% or less. The area ratio of MA is preferably 15.0% or less, 13.0% or less, or 10.0% or less.

[0099] Ferrite and bainite: totaling less than 10.0%. If the combined area ratio of ferrite and bainite exceeds 10.0%, the porosity of the steel sheet deteriorates. Therefore, the combined area ratio of ferrite and bainite is set to 10.0% or less. Preferably, the area ratio of ferrite and bainite is 5.0% or less, 3.0% or less, or 2.5% or less.

[0100] It can also be free of ferrite and bainite, so the total area ratio of ferrite and bainite can also be 0.0%.

[0101] Pearlite: less than 2.0% If the area fraction of pearlite exceeds 2.0%, the porosity of the steel sheet deteriorates. Therefore, the area fraction of pearlite is set to 2.0% or less. The area fraction of pearlite can be set to 1.5% or less, 1.0% or less, or 0.0%.

[0102] The area ratio of each tissue was determined by the following method.

[0103] First, the method for determining the area fraction of ferrite will be explained.

[0104] Test specimens were collected from a section of the steel plate at a position one-quarter of its width, allowing observation of a depth from one-eighth to three-eighths of the plate thickness from the surface. The section was periodically ground using #600 to #1500 silicon carbide paper, and then polished to a mirror finish using a liquid made by dispersing 1-6 μm diamond powder in an alcohol-based diluent or pure water. Next, the specimen was ground for 8 minutes at room temperature using colloidal silica with a particle size of 0.25 μm in a non-alkaline solution to remove strain introduced into the surface layer. In the region of the test specimen at a depth from one-eighth to three-eighths of the plate thickness from the surface, crystal orientation information was obtained by electron backscattering diffraction at measurement intervals of 0.1 μm, with a field of view of 200 μm in the rolling direction and 100 μm in the thickness direction centered at one-quarter of the plate thickness from the surface.

[0105] The measurements were performed using an EBSD analysis apparatus consisting of a thermal field emission scanning electron microscope (JEOL JSM-7001F) and an EBSD detector (TSL DVC5 detector). The vacuum level within the EBSD analysis apparatus was set to 9.6 × 10⁻⁶. -5 Below Pa, the accelerating voltage was set to 15 kV, the irradiation current level to 13, and the electron beam irradiation level to 62. Based on the obtained crystal orientation information, the "Phase Map" function in the "OIM Analysis (registered trademark)" software included with the EBSD analysis device was used to determine the regions with fcc crystal structures and the regions with bcc crystal structures.

[0106] For regions with a bcc crystal structure, the orientation difference (GAM value: Grain Average Misorientation) within the grain is calculated using the software "OIM Analysis (registered trademark)" included with the EBSD analysis device. Regions with a GAM value below 0.5° are classified as ferrite. The same operation is performed on five fields of view to calculate the area fraction of ferrite. Here, "GAM value" refers to the value obtained by averaging the orientation difference between adjacent pixels in a region surrounded by grain boundaries with an orientation difference of 15° or more.

[0107] Next, in order to observe the same field of view as the field of view used in the EBSD measurement, and to determine the observation position, Vickers indentations were made at three points at the four corners of the field of view used in the EBSD measurement, within a range of 100 μm from the corners. Then, the metallic structure of the observation surface was preserved, and surface contaminants were removed by grinding. By using the Vickers indentations as markers, the same field of view as the EBSD measurement field of view can be observed.

[0108] To remove contaminants, methods such as polishing and grinding with alumina particles smaller than 0.1 μm, grinding with colloidal silica that does not contain alkaline solutions at room temperature, or Ar ion sputtering can be used.

[0109] For the observation surface including the field of view where EBSD measurements were performed, nitric acid ethanol etching was used to expose the tissue. Then, using a scanning electron microscope, five fields of view identical to those used for EBSD measurements were photographed at 500x magnification. In the tissue photographs, lath-like regions were identified as "tempered martensite and MA". The area ratio of "tempered martensite and MA" in each tissue photograph was calculated, and their average value was calculated to obtain the area ratio of "tempered martensite and MA". The area ratio of tempered martensite was obtained by subtracting the area ratio of MA obtained by the method described later from this area ratio. Additionally, in each tissue photograph, regions with layered carbides were identified as pearlite, and their average area ratio was calculated to obtain the area ratio of pearlite.

[0110] Next, the method for determining the area ratio of MA will be explained.

[0111] For the observation surface including the field of view for EBSD measurements, the etched layer was removed by grinding, followed by mirror finishing and then LePera etching to reveal the tissue. Then, scanning electron microscopy was used to photograph five fields of view identical to those used for EBSD measurements at 500x magnification. In each tissue photograph, areas with white contrast were identified as MAs, and the average area ratio of these areas was calculated to obtain the area ratio of the MAs.

[0112] The area ratio of bainite is obtained by subtracting the area ratios of tempered martensite, MA, ferrite, and pearlite obtained using the above method from 100.0%. When the area ratio of bainite is negative, it is set to 0%.

[0113] When the area ratio of bainite is negative, meaning the sum of the area ratios of tempered martensite, MA, ferrite, and pearlite exceeds 100%, the area ratio of each microstructure is corrected by making their sum 100%. For example, if the sum of the area ratios of tempered martensite, MA, ferrite, and pearlite is 103.0%, the area ratio of each microstructure is corrected by multiplying the area ratio of each microstructure by "100.0 / 103.0".

[0114] It should be noted that the rolling direction of the steel plate is determined by the following method.

[0115] Test pieces were collected from any position at least 50 mm from the end of the steel plate, allowing for observation of the plate thickness section. After mirror polishing of the collected test pieces' plate thickness section, observations were performed using an optical microscope at magnifications of 100x, 200x, 500x, and 1000x. An appropriate magnification was selected based on the size of the inclusions to determine their dimensions. The observation range was set to a width of at least 500 μm and the full thickness of the plate, with darker areas identified as inclusions. Multiple fields of view could also be used during observation. Next, using the plate thickness section initially observed as a reference, cross-sectional observations were performed on surfaces parallel to a plane rotated in 5° increments within a range of 0° to 180° about the plate thickness direction, using the same method. 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 the inclusions was determined. The direction parallel to the major axis of the inclusions in this cross-section was identified as the rolling direction.

[0116] The same method is used to determine the rolling direction for components.

[0117] Average grain size of the original austenite grains: less than 25 μm The inventors have discovered that even when the distribution of MA is preferably controlled as described later, a high reduction rate of plate thickness at ultimate fracture cannot be obtained if the average grain size of the original austenite grains is too large. Therefore, in order to obtain a high reduction rate of plate thickness at ultimate fracture after pre-straining, it is important to control not only the distribution of MA but also the average grain size of the original austenite grains.

[0118] MA (mesh-like structures) are generated from the grain boundaries of the original austenite. If the average grain size of the original austenite grains is too large, the spacing between MA generated by the grain size increases, and the difference between areas with abundant and scarce MA becomes larger. As a result, shear bands locally exist in areas with scarce MA, leading to strain concentration. If the average grain size of the original austenite grains exceeds 25 μm, a high reduction rate in ultimate fracture thickness cannot be obtained after applying pre-strain. Therefore, the average grain size of the original austenite grains is set to be 25 μm or less. Preferably, the average grain size of the original austenite grains is 23 μm or less, 20 μm or less, or 15 μm or less.

[0119] There is no particular limitation on the lower limit of the average grain size of the original austenite grains. However, in order to make the average grain size of the original austenite grains less than 4 μm, the burden on the rolling equipment increases. Therefore, the average grain size of the original austenite grains is preferably set to 4 μm or more. The average grain size of the original austenite grains can be 5 μm or more or 10 μm or more.

[0120] The average grain size of the original austenite was determined by the following method.

[0121] Test specimens were collected at a position one-quarter of the width of the steel plate, allowing observation of a section of the plate thickness ranging from one-eighth to three-eighths of the plate thickness from the surface. The microstructure of the section was revealed by etching with a solution of sodium dodecylbenzenesulfonate in a saturated aqueous solution of picric acid. Using a scanning electron microscope, microstructure photographs were taken in the region of the test specimen at a depth of one-eighth to three-eighths of the plate thickness from the surface, with a field of view of 200 μm in the rolling direction and 100 μm in the thickness direction centered at the one-quarter depth from the surface. Five fields of view were captured for the microstructure photographs. The grain size of the proto-austenite grains was determined using the photographs. The equivalent circle diameter was calculated for one of the proto-austenite grains present in each field of view. Except for the austenite grains at the ends of the field of view, which are not included in the overall austenite grains within the field of view, the above operation is performed on all austenite grains included in each observation field of view to determine the equivalent circle diameter of all austenite grains in each field of view. The average grain size of the austenite grains is obtained by calculating the average of the equivalent circle diameters of the austenite grains obtained in each field of view.

[0122] Distribution of MA The inventors have discovered that, in order to achieve a high reduction rate of plate thickness at ultimate fracture after pre-straining, it is important to preferably control the average grain size of the original austenite grains and, more preferably, the distribution of the austenite matrix (MA) as a hard phase. The inventors investigated the relationship between the distribution of MA and the reduction rate of plate thickness at ultimate fracture after pre-straining, and obtained the following insights.

[0123] In the strain distribution of the portion subjected to bending-bending recovery deformation through stamping, the strain exhibits a banded locality along a 45° direction relative to the sheet thickness. This strain band (shear band) develops in a manner that avoids the maximum range (MA). Therefore, along the 45° direction relative to the sheet thickness, if the difference between areas with high and low MA is large, the shear band locally exists in areas with low MA, causing strain concentration. Therefore, it is important to reduce the difference between areas with high and low MA along the 45° direction relative to the sheet thickness.

[0124] In this embodiment, in order to reduce the difference between areas with more and less MA along a line at a 45° angle relative to the thickness direction of the steel plate, the following provisions are made.

[0125] In a region ranging from 1 / 8 of the plate thickness to 3 / 8 of the plate thickness from the surface, five consecutive fields of view (100 μm in the rolling direction × 100 μm in the plate thickness direction) are continuously photographed along the rolling direction to obtain tissue photographs. In these tissue photographs, at least 10 line segments at equal intervals of 10 μm or more are drawn at a 45° angle relative to the plate thickness direction. The difference between the maximum and minimum number of MAs present on each 100 μm line segment in each tissue photograph is calculated. The average value of the difference between the maximum and minimum number of MAs in the tissue photographs from the five fields of view is calculated, and the average value is 8 to 16. The following detailed description uses accompanying drawings.

[0126] First, test specimens were collected from a section of the steel plate at a point one-quarter of its width, allowing observation of a depth ranging from one-eighth to three-eighths of the plate thickness from the surface. After etching the observation surface with nitric acid and ethanol to expose the microstructure, tissue photographs were taken at 1000x magnification using a scanning electron microscope. Figure 2 This is a graph illustrating the method for determining the difference between the maximum and minimum number of MAs. Furthermore, in Figure 2 MA is not shown in the diagram. In the taking of organizational photographs, such as... Figure 2 As shown, using a scanning electron microscope, five fields of view were continuously photographed along the rolling direction, with a field of view of 100 μm in the rolling direction and a field of view of 100 μm in the thickness direction centered at a depth of 1 / 4 of the plate thickness from the surface of the steel plate, thus obtaining microstructure photographs.

[0127] Next, in a 100μm × 100μm photograph, draw line segment L1 along the diagonal at a 45° angle relative to the plate thickness direction. Relative to line segment L1, draw line segments L2, L3… and more than 10 other line segments at equal intervals of 10μm or more. Figure 2In the diagram, 11 line segments are drawn. Next, the number of 100μm length segments containing MAs is counted. Specifically, first, the number of MAs on the line segment from endpoint A of line segment L1 to point B (100μm) is counted. This count is taken as the number of MAs q1 for line segment AB. Then, the number of MAs on the line segment from point B to 100μm is counted again. If no line segment reaches 100μm, the number of MAs on the other line segments is counted with a total length of 100μm. Figure 2 In this case, the number of MAs existing on the line segment from point B to point C is counted. Then, the number of MAs existing on the line segment from point D to point E is counted, with the total length of the line segments being 100 μm. The resulting number of MAs is taken as the number of MAs q2 for line segment BCDE. For line segment L2, line segment L3... line segment L... 11 Perform the same operation on all of them to obtain the number of line segments q1, q2...q per 100μm. n .

[0128] Based on the obtained numbers q1, q2...q n To obtain the maximum number q Max The minimum number of q min By finding the maximum number q Max With the minimum number q min The difference between the maximum and minimum number of MAs in the first tissue photograph is obtained as D1. The same operation is performed on the remaining four fields of view consecutively along the rolling direction to obtain the differences between the maximum and minimum number of MAs as D2, D3, D4, and D5. The average of the differences D1 to D5 is then calculated to obtain the average difference between the maximum and minimum number of MAs in the tissue photographs of the five fields of view.

[0129] It should be noted that the line segments drawn from the tissue photograph should avoid being too thick, and should be set to a thickness within the common range for line segments.

[0130] Figure 3 This is a graph showing the relationship between the average difference between the maximum and minimum number of MAs in the tissue photographs from five fields of view in the embodiments described later, and the reduction rate of the ultimate fracture plate thickness after pre-straining. (See figure.) Figure 3 As shown, if the average difference between the maximum and minimum number of metamaterials (MAs) in the tissue photographs from the five fields of view is less than 8 or more than 16, a high reduction rate in ultimate fracture thickness cannot be obtained after applying pre-strain. Therefore, the average difference between the maximum and minimum number of MAs in the tissue photographs from the five fields of view is set to 8 to 16. Preferably, the average value is 10 or more, or 12 or more. Furthermore, the average value is preferably 15 or less, or 14 or less.

[0131] Tensile strength (TS): ≥1180MPa The tensile strength of the steel plate in this embodiment can be 1180 MPa or higher. More preferably, the tensile strength is 1200 MPa or higher or 1250 MPa or higher. By setting the tensile strength to 1180 MPa or higher, the applicable components are not limited, and the contribution to vehicle body lightweighting can be increased.

[0132] There is no need to specifically limit the upper limit of tensile strength. From the perspective of suppressing mold wear, it can be set to below 1500MPa or below 1400MPa.

[0133] Yield ratio (YR): 70% or higher The yield ratio of the steel plate in this embodiment can be 70% or more. Preferably, the yield ratio is 80% or more or 85% or more.

[0134] The yield ratio can be obtained by dividing the yield stress by the tensile strength and multiplying by 100 ({yield stress / tensile strength}×100).

[0135] Total elongation (E1): 4.0% or more Hole expansion ratio (λ): 45% or higher In this embodiment, the total elongation (total elongation at fracture) of the steel plate can be 4.0% or more, and the hole expansion rate can be 45% or more. The total elongation is preferably 5.0% or more or 6.0% or more. The hole expansion rate is preferably 50% or more or 60% or more.

[0136] Tensile strength and total elongation were evaluated by tensile testing according to JIS Z 2241:2022. The test piece was designated as test piece No. 5 according to JIS Z 2241:2022. The tensile test piece was collected from the portion 1 / 4 of the way from the end in the width direction of the plate, and the direction perpendicular to the rolling direction could be taken as the length direction.

[0137] It should be noted that when determining the tensile strength of a component, if it is impossible to collect a No. 5 test piece from the component due to its small size or complex shape, a long strip with a parallel section of arbitrary width can be collected for tensile testing. The tensile strength can then be calculated based on the maximum test force and the original cross-sectional area of ​​the parallel section.

[0138] Furthermore, the yield stress used to calculate the yield ratio is obtained by performing a tensile test using the method described above. When the steel plate yields discontinuously, the upper yield point is considered the yield stress; when yielding continuously, 0.2% of the yield strength is considered the yield stress.

[0139] The porosity was determined by porosity testing in accordance with JIS Z 2256:2020.

[0140] The reduction rate of the ultimate fracture thickness after pre-straining is greater than 0.25%. In this embodiment, using Figure 4 The method shown imparts pre-strain based on bending-bending recovery deformation by performing a tensile bending process on the steel plate. Figure 4 In the forming of the cap component based on tensile bending, the steel plate is subjected to bending deformation while contacting the punch during the formation of the longitudinal wall of the cap component. Tensile test specimens are collected from the longitudinal wall of the cap component with the height direction of the longitudinal wall becoming the length direction. Tensile tests are performed using the obtained specimens according to the above method. If the thickness of the specimen before the tensile test is set as t0, and the minimum thickness of the central portion of the specimen in the width direction (short side direction) after fracture is set as t1, the ultimate reduction rate of the fracture thickness is obtained by calculating the value of (t0-t1) / t0. Five tensile tests are performed, and the average of the three tests excluding the maximum and minimum values ​​of the ultimate reduction rate of the fracture thickness is calculated, thus obtaining the ultimate reduction rate of the fracture thickness after applying pre-strain.

[0141] If the reduction rate of the ultimate fracture thickness after applying pre-strain is 0.25 or more, it can be determined that the steel sheet still exhibits excellent formability even after applying pre-strain. Therefore, the reduction rate of the ultimate fracture thickness after applying pre-strain in the steel sheet of this embodiment can be 0.25 or more. The reduction rate of the ultimate fracture thickness after applying pre-strain is preferably 0.30 or more or 0.35 or more.

[0142] The thickness of the hot-rolled steel sheet involved in this embodiment is not particularly limited, but it can be set to 1.2 to 8.0 mm. When the thickness of the steel sheet is less than 1.2 mm, it is difficult to ensure the rolling end temperature, and the rolling load is too large, sometimes making hot rolling difficult. Therefore, the thickness of the steel sheet in this embodiment can also be set to 1.2 mm or more. Preferably, it is 1.4 mm or more.

[0143] On the other hand, when the plate thickness exceeds 8.0 mm, it is sometimes difficult to obtain the aforementioned metallic structure after hot rolling. Therefore, the plate thickness can also be set to 8.0 mm or less. Preferably, it is 6.0 mm or less.

[0144] For the hot-rolled steel sheet of this embodiment having the above-described chemical composition and metallic structure, a surface-treated steel sheet can also be manufactured by having a coating on the surface for the purpose of improving corrosion resistance, etc. The coating can be an electroplated coating or a hot-dip coating. Examples of electroplated coatings include electroplated zinc and electroplated Zn-Ni alloy. Examples of hot-dip 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. There are no particular limitations on the amount of coating applied, and it can be the same as in the past.

[0145] 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.

[0146] The steel sheet of this embodiment possesses high strength and yield ratio, excellent ductility and porosity, and exhibits a high reduction rate in ultimate fracture thickness after pre-straining. Therefore, it is considered to have excellent impact resistance properties and is suitable for use in components, particularly automotive components. In automotive components, it is applicable to running gear components such as lower arms, connecting rods, and steering knuckles. These automotive components can be constructed solely from the steel sheet of this embodiment, or they can be formed by joining the steel sheet of this embodiment with other steel sheets.

[0147] 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 coexist in 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, under heavy processing, they may not possess the aforementioned metallic structure. Therefore, when measuring the metallic structure of the component, the unprocessed portions are measured. In the case where there are no unprocessed portions, the portions that have not undergone heavy processing are measured. Unprocessed or unprocessed portions refer to, for example, flat portions of the component, and portions that avoid being subjected to punching, reaming, bending, etc. As an example, in the case of the above-described component, a test piece is collected near its center of gravity from the flat portion with the largest area for investigation.

[0148] Next, a preferred manufacturing method for the steel plate of this embodiment will be described. According to the manufacturing method described below, the steel plate of this embodiment can be manufactured stably. In the preferred manufacturing method of the steel plate of this embodiment, austenite grains are preferably formed by maintaining the plate within a desired temperature range after rough rolling, and by performing finish rolling under preferred conditions, the average grain size of the original austenite grains can be preferably controlled. Furthermore, the distribution of matrices (MA) generated from the grain boundaries of the original austenite grains during subsequent reheating can be preferably controlled.

[0149] 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, and are measured using a radiation thermometer. Additionally, Tf refers to the roughing rolling end temperature (°C).

[0150] The preferred method for manufacturing the steel plate in this embodiment includes: Rough rolling is performed with a total reduction rate of 60-85% and a rough rolling end temperature exceeding 1220℃. After rough rolling, the temperature is cooled to 1220°C within 1.00 second, and then held in the temperature range of 1100~1220°C for 3~25 seconds before finishing rolling begins. Finishing rolling is performed with a finishing temperature range of 940~1020℃, and the final reduction rate and the reduction rate in the rolling process preceding the final stage are 20~43% respectively. Cooling begins within 2 seconds after finishing rolling, and is carried out in a manner that reaches 25°C within 17 seconds after finishing rolling. Perform winding; Reheat to the temperature range of Ac1 to Ac3, and maintain this temperature range for more than 10 seconds; Temperature range from air-cooled to below 200℃.

[0151] The following is a description of each process.

[0152] Regarding the slabs supplied for rough rolling, there are no particular limitations other than having the aforementioned chemical composition. For example, slabs manufactured by continuously casting using steel with the aforementioned chemical composition smelted in a converter or electric furnace can be used. Alternatively, ingot casting or thin slab casting can be used instead of continuous casting. During the slab heating before rough rolling, the temperature should be maintained in the range of 1220~1300℃ for at least 40 minutes.

[0153] Rough rolling is preferably performed with a total reduction of 60% to 85%. By keeping the total reduction of rough rolling at 60% or more, it is possible to suppress the coarsening of the original austenite grains. In addition, by keeping the total reduction of rough rolling at 85% or less, it is possible to preferably control the distribution of austenite (MA).

[0154] It should be noted that when the initial thickness of the inlet plate in the roughing mill is set as t2 and the final thickness of the outlet plate in the roughing mill is set as t3, the total reduction rate of the roughing mill can be expressed as (1-t3 / t2)×100 (%).

[0155] The roughing end temperature (the exit temperature of the final section of the roughing roll) is preferably set to exceed 1220°C. By setting the roughing end temperature to exceed 1220°C, recrystallization can be promoted, and as a result, the average grain size of the original austenite grains can be preferably controlled.

[0156] After rough rolling, it is preferable to cool the grains to 1220°C within 1.00 second, hold them in the 1100~1220°C temperature range for 3~25 seconds, and then begin finish rolling. By setting the time until 1220°C is reached after rough rolling to within 1.00 second, coarsening of the original austenite grains can be prevented. Furthermore, by setting the holding temperature below 1220°C and the holding time below 25 seconds, coarsening of the original austenite grains can be prevented. By setting the holding time above 1100°C, flattening of the original austenite grains can be prevented, and as a result, the distribution of MA can be preferably controlled.

[0157] The finishing rolling is preferably carried out in a temperature range of 940~1020°C with a finishing rolling end temperature (the exit temperature of the final section), and with a final reduction rate and a reduction rate of 20~43% in the rolling of the section preceding the final section. By setting the finishing rolling end temperature above 940°C, the distribution of austenite (MA) can be preferably controlled. By setting the finishing rolling end temperature below 1020°C, the coarsening of the original austenite grains can be suppressed.

[0158] By ensuring that the final reduction rate and the reduction rate in the rolling process preceding the final stage are both 20% or more, it is possible to suppress the coarsening of the original austenite grains. By ensuring that the final reduction rate and the reduction rate in the rolling process preceding the final stage are both 43% or less, it is possible to preferably control the distribution of MA.

[0159] Cooling is preferably performed by initiating cooling within 2 seconds of the end of finishing rolling and reaching 25°C within 17 seconds. This method suppresses the excessive formation of bainite. After cooling, the steel sheet is coiled.

[0160] After winding, it is preferable to reheat to the temperature range of points Ac1 to Ac3 and hold in this temperature range for at least 10 seconds. By holding in this temperature range for at least 10 seconds, the desired amount of MA can be generated. In addition, the holding time in this temperature range is preferably set to 200 seconds or less. By keeping the holding time to 200 seconds or less, excessive generation of MA can be prevented.

[0161] It should be noted that points Ac1 and Ac3 can be obtained from the following formulas.

[0162] Ac1(℃)=727-32.7×C+14.9×Si+2×Mn-17×Cu-14.2×Ni+17.8×Cr+25.6×Mo Ac3(℃)=937.2-436.5×C+56×Si-19.7×Mn-16.3×Cu-26.6×Ni-4.9×Cr+38.1×Mo+124.8×V+136.3×Ti-19.1×Nb+198.4×Al+3315×B It should be noted that the element symbols in the above formula represent the content of each element in terms of mass %; if the element is not present, substitute 0.

[0163] After reheating and holding, it is preferable to air cool to a temperature range below 200°C. Air cooling refers to cooling with an average cooling rate of 10°C / s or less.

[0164] In addition, the average cooling rate mentioned in this embodiment 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.

[0165] Example Next, the effects of one solution of this disclosure will be described more specifically through embodiments. However, the conditions in the embodiments are examples of conditions adopted to confirm the feasibility and effects of this disclosure, and this disclosure is not limited to this one example of conditions. Various conditions can be adopted as long as they do not depart from the spirit of this disclosure and achieve the purpose of this disclosure.

[0166] Steels with the chemical compositions shown in Tables 1A to 2B are smelted and continuously cast into slabs with a thickness of 240–300 mm. It should be noted that empty columns in the tables indicate that the element is below the detection limit.

[0167] Using the obtained slabs, steel plates as shown in Tables 4A and 4B were obtained according to the manufacturing conditions shown in Tables 3A and 3B. The thickness of the obtained steel plates was 1.2~8.0 mm.

[0168] It should be noted that the slab is heated to a temperature range of 1220~1300℃, and the holding time in this temperature range is more than 40 minutes. In addition, after finishing rolling, cooling begins within 2.0 seconds, followed by reheating to the temperature range of Ac1~Ac3, and then air cooling to a temperature range below 200℃ (the average cooling rate is less than 10℃ / s).

[0169] It should be noted that in No. 5, the cooling amount after rough rolling was too large, so it could not be maintained in the preferred temperature range. In No. 6, the cooling amount after rough rolling was small, and the temperature rose due to reheating, so it could not be maintained in the preferred temperature range.

[0170] It should be noted that the items in the table are as follows.

[0171] Holding time: Holding time at the "Holding temperature" in the table. Reduction rate of the final section: Reduction rate of the final section of the finishing mill Reduction rate of the rolling process preceding the final stage: Reduction rate of the rolling process preceding the final stage of the finishing mill 25℃ arrival time: The elapsed time from the end of finishing rolling to reaching 25℃. Reheating time: the holding time in the temperature range from Ac1 to Ac3. For the obtained steel plate, the following parameters are obtained using the above method: the microstructure of the region from 1 / 8 of the plate thickness to 3 / 8 of the plate thickness from the surface, the average grain size of the original austenite grains, the average difference between the maximum and minimum number of MA grains, the yield ratio (YR), tensile strength (TS), total elongation (EL), porosity (λ), and the reduction rate of ultimate fracture thickness after pre-straining.

[0172] The obtained measurement results are shown in Tables 4A and 4B.

[0173] A yield ratio of 70% or higher is considered acceptable as a steel plate with a high yield ratio. Conversely, a yield ratio of less than 70% is considered unacceptable as a steel plate without a high yield ratio.

[0174] A tensile strength of 1180 MPa or higher is considered a high-strength steel plate and is deemed acceptable. Conversely, a tensile strength of less than 1180 MPa is considered a low-strength steel plate and is deemed unacceptable.

[0175] A total elongation of 4.0% or more is considered acceptable as a steel sheet with excellent ductility. Conversely, a total elongation of less than 4.0% is considered unacceptable as a steel sheet that does not possess excellent ductility.

[0176] A hole expansion ratio of 45% or higher is considered a qualified steel plate with excellent hole expansion properties. Conversely, a hole expansion ratio of less than 45% is considered an unqualified steel plate without excellent hole expansion properties.

[0177] If the reduction rate of the ultimate fracture thickness after pre-straining is 0.25 or more, it is considered high and exhibits excellent formability even after pre-straining, thus being deemed acceptable. Conversely, if the reduction rate of the ultimate fracture thickness after pre-straining is less than 0.25, it is considered unacceptable because it does not exhibit excellent formability after pre-straining.

[0178] As can be seen from Tables 1A to 4B, the steel plate of the present invention has high strength and yield ratio, as well as excellent ductility and pore-expanding properties, and has a high ultimate fracture thickness reduction rate after pre-straining.

[0179] Furthermore, in all embodiments, the lower arm (part) was manufactured by stamping. The flat portion of the lower arm was evaluated using the same method as described above. The measurement and evaluation results are the same as those shown in Tables 4A and 4B.

[0180] Industrial availability According to the above-described solution of this disclosure, it is possible to provide a steel plate with high strength and yield ratio, excellent ductility and porosity, and a high ultimate fracture thickness reduction rate after pre-straining, as well as components using the steel plate.

Claims

1. A steel sheet characterized by, Its chemical composition, by mass%, contains: C：0.090~0.210%、 Si: 0.20~1.00% Mn: 1.95~2.55%, P: below 0.060% S: Below 0.005% Al：0.01~0.26%、 N: below 0.0070% O：0~0.010%、 Ti: 0.10~0.18% Nb: 0.01~0.04% B：0.0001~0.0030%、 Cr：0~0.47%、 Mo: 0~0.12%, Cu: 0~0.40%, Ni: 0~0.30% V：0~0.30%、 Sn: 0~0.040% As: 0~0.100% Zr：0~0.050%、 Ca: 0~0.010%, Mg: 0~0.010%, Bi: 0~0.010% Co: 0~0.010%, W：0~0.100%、 Zn: 0~0.010%, REM: 0~0.010%, Sb: 0~0.010%, and Ta: 0~0.010%, The remaining portion contains Fe and impurities. In the region where the depth from the surface is 1 / 8 of the plate thickness to 3 / 8 of the plate thickness, Metal structure, expressed as area %: Tempered martensite: 80.0~99.0% MA: 1.0~20.0% Ferrite and bainite: totaling less than 10.0%, and Pearlite: less than 2.0%, The average grain size of the original austenite grains is less than 25 μm. In the region, microstructure photographs are obtained by continuously photographing five fields of view along the rolling direction, each 100 μm in the rolling direction and 100 μm in the plate thickness direction. In these microstructure photographs, at equal intervals of 10 μm or more, at a 45° angle relative to the plate thickness direction, [further details are needed]. When calculating the difference between the maximum and minimum number of MAs present on each 100μm line segment in each tissue photograph, and calculating the average value of the difference between the maximum and minimum number of MAs in the tissue photographs of the five fields of view, the average value is 8 to 16.

2. The steel sheet according to claim 1, characterized by The chemical composition, expressed as a percentage by mass, contains one or more of the following elements: Cr：0.01~0.47%、 Mo: 0.01~0.12% Cu: 0.01~0.40%, Ni: 0.01~0.30% V：0.01~0.30%、 Sn: 0.001~0.040% As: 0.001~0.100% Zr：0.001~0.050%、 Ca: 0.001~0.010% Mg: 0.001~0.010% Bi: 0.001~0.010% Co: 0.001~0.010%, W：0.001~0.100%、 Zn: 0.001~0.010%, REM: 0.001~0.010% Sb: 0.001~0.010%, and Ta: 0.001~0.010%.

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

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