Steel sheet and component

Abstract

Provided is a steel sheet characterized by having a desired chemical composition, wherein at a position at 1 / 4 of the sheet thickness, the average value of the aspect ratio of prior austenite grains is 2.0-4.0, the martensite area ratio is not less than 90%, and when the area ratio of a region in which the GAM value in a surface layer region is not more than 1.4° is represented as GAMSL and the area ratio of a region in which the GAM value at the position at 1 / 4 of the sheet thickness is not more than 1.4° is represented as GAM1 / 4, GAMSL is not less than 25 area% and GAM1 / 4 is not more than 20 area%. Also provided is a component produced using the steel sheet.

Description

Steel plates and parts

[0001] This disclosure relates to steel sheets and components. This application claims priority under Japanese Patent Application No. 2025-006210, filed in Japan on January 16, 2025, the contents of which are incorporated herein by reference.

[0002] In recent years, CO ２ To reduce emissions, automobile bodies are being made lighter. In blank-formed parts such as press-formed parts, weight reduction is possible by reducing the thickness of the part material. In particular, for automobile suspension parts such as lower arms and trailing arms, the application of steel plates with a pressure of over 980 MPa is beginning to be considered in order to achieve weight reduction of the automobile body.

[0003] The parts described above have complex shapes. As steel plates become stronger, their workability decreases. Therefore, when high-strength steel plates are applied to such parts, necking or fracture may occur due to insufficient workability. For this reason, steel plates used in such parts are required to have excellent workability, especially in terms of hole-expanding ability.

[0004] Furthermore, when manufacturing the above-mentioned parts, bending high-strength steel plates can easily cause microscopic cracks (irregularities) to form within the bend. If microscopic cracks form within the bend, the impact resistance of the part deteriorates. Therefore, the steel plates used for the above-mentioned parts are required to be resistant to crack formation within the bend during processing, that is, to have excellent resistance to internal cracking during bending.

[0005] Furthermore, when cutting high-strength steel plates into blank shapes (performing punching), defects such as the sheared end surface delaminating into two layers are likely to occur. This delamination can progress into cracks perpendicular to the plate thickness direction. This delamination of the end surface that occurs during punching is called "peeling." Therefore, steel plates used in the above-mentioned parts are required to prevent peeling that occurs during punching, that is, to have excellent peel resistance.

[0006] For example, Patent Document 1 discloses a high-strength hot-rolled steel sheet having a structure containing a martensite phase with an area ratio of 95% or more at the 1 / 4 position of the sheet thickness of the steel sheet and an average aspect ratio of prior austenite grains of 3.0 or more, and having a 5-minute relaxation stress value of 20 MPa or less when 400 MPa is applied in a stress relaxation test and a tensile strength of 1180 MPa or more. Patent Document 1 discloses that it is possible to manufacture a high-strength hot-rolled steel sheet excellent in stress corrosion cracking resistance, which has a high strength of tensile strength TS: 1180 MPa or more and significantly improved stress corrosion cracking resistance and is suitable as a material for automotive parts.

[0007] Japanese Patent No. 7010418

[0008] However, in Patent Document 1, bend fracture resistance and peel resistance are not considered.

[0009] The present disclosure has been made in view of the above circumstances. An object of the present disclosure is to provide a steel sheet having high strength, excellent hole expansion property, bend fracture resistance and peel resistance, and a part using this steel sheet.

[0010] The gist of this disclosure is as follows: [1] The chemical composition, in mass%, is: C: 0.050-0.120%, Si: 0-3.00%, Mn: 1.20-3.00%, Al: 0.010-0.400%, P: 0.080% or less, S: 0.0100% or less, N: 0.0090% or less, O: 0.0100% or less, Ti: 0-0.180%, Nb: 0-0.100%, V: 0-1.000%, Cu: 0-1.000%, Cr: 0-2.000%, Mo: 0-3.000%, Ni: 0-0.500%, B: 0-0.0100%, Ca: 0-0.0500%. The composition is Mg: 0-0.050%, REM: 0-0.1000%, Bi: 0-0.100%, Ta: 0-0.100%, Zr: 0-0.500%, Co: 0-3.000%, Zn: 0-0.200%, W: 0-0.200%, Sb: 0-0.500%, As: 0-0.100%, Sn: 0-0.100%, and the remainder is Fe and impurities. At a position 1 / 4 of the plate thickness from the surface in the plate thickness direction, the average aspect ratio of prior austenite grains is 2.0-4.0, the area ratio of martensite is 90% or more, and the area ratio of the region in the surface layer region, which is the region from the surface to a position 200 μm from the surface in the plate thickness direction, where the GAM value is 1.4° or less is defined as GAM. ＳＬ The area ratio of the region where the GAM value is 1.4° or less at the 1 / 4 position of the plate thickness is determined by GAM １／４ In that case, the GAM ＳＬ The area percentage is 25% or more, and the GAM １／４A steel plate characterized by having an area percentage of 20% or less. [2] The chemical composition is as follows, in mass%, Ti: 0.001 to 0.180%, Nb: 0.001 to 0.100%, V: 0.001 to 1.000%, Cu: 0.001 to 1.000%, Cr: 0.001 to 2.000%, Mo: 0.001 to 3.000%, Ni: 0.001 to 0.500%, B: 0.0001 to 0.0100%, Ca: 0.0001 to 0.0500%, Mg: 0.001 to 0.050%, REM: 0.0001 to 0.1000%, Bi: 0.001 to 0.100%, Ta: 0.001 to 0.100%, The steel sheet according to [1], characterized by containing one or more of the following: Zr: 0.001 to 0.500%, Co: 0.001 to 3.000%, Zn: 0.001 to 0.200%, W: 0.001 to 0.200%, Sb: 0.001 to 0.500%, As: 0.001 to 0.100%, and Sn: 0.001 to 0.100%. [3] The GAM ＳＬ - The aforementioned GAM １／４ [1] or [2] steel plate characterized in that is 10 area percent or more. [4] Steel plate according to any one of [1] to [3], characterized in that the plate thickness is 0.4 to 8.0 mm. [5] Steel plate according to any one of [1] to [4], characterized in that the chemical composition contains one or more of the following by mass percent: N: 0.0050% or less, As: 0.050% or less, and Sn: 0.050% or less. [6] Parts containing the steel plate according to any one of [1] to [5].

[0011] According to the above embodiments of this disclosure, it is possible to provide a steel plate having high strength, as well as excellent hole-expanding properties, resistance to internal cracking during bending, and resistance to peeling, and a part using this steel plate.

[0012] This is a diagram illustrating a method for approximating the former austenite grains as ellipsoids. This is a diagram illustrating a bending test.

[0013] A steel plate and component according to one embodiment of this disclosure (hereinafter sometimes referred to as "steel plate and component according to this embodiment") will be described. However, this disclosure is not limited to the configuration disclosed in this embodiment, and various modifications are possible without departing from the spirit of this disclosure.

[0014] The individual constituent elements of this disclosure will be described in detail below. First, the reasons for limiting the chemical composition of the steel sheet according to this embodiment will be stated. The numerical limit ranges described below, separated by "~", include both a lower limit and an upper limit. Numerical values ​​indicated as "less than" or "greater than" do not include the numerical range. In the following description, percentages related to chemical composition are mass percentages unless otherwise specified. Each element will be described in detail below.

[0015] C: 0.050-0.120% C is an important element for improving the strength of steel plates. To obtain the desired strength, the C content should be 0.050% or more. Preferably, the C content is 0.60% or more, 0.70% or more, or 0.75% or more. On the other hand, if the C content exceeds 0.120%, the hole-expanding properties of the steel plate deteriorate. Therefore, the C content should be 0.120% or less. Preferably, the C content is 0.110% or less, 0.100% or less, or 0.90% or less.

[0016] Si: 0-3.00% Si is an element that suppresses the formation of carbides during ferrite transformation and improves the toughness of the steel sheet. Si is not required to be present, so the Si content may be 0%. To reliably obtain the above effect, it is preferable that the Si content be 0.10% or more. More preferably, the Si content is 0.20% or more, 0.50% or more, or 0.60% or more. On the other hand, if the Si content exceeds 3.00%, the crack susceptibility of the slab increases, making it difficult to handle the slab. Therefore, the Si content should be 3.00% or less. Preferably, the Si content is 2.50% or less, 2.30% or less, 2.00% or less, 1.80% or less, 1.50% or less, or 1.20% or less.

[0017] Mn: 1.20-3.00% Mn is an effective element for improving the strength of steel sheets by improving hardenability and solid solution strengthening. To obtain the desired strength, the Mn content should be 1.20% or more. Preferably, the Mn content is 1.30% or more, 1.50% or more, 1.70% or more, 1.80% or more, or 2.00% or more. On the other hand, if the Mn content exceeds 3.00%, MnS, which adversely affects the hole-expanding properties of the steel sheet, is easily generated. Therefore, the Mn content should be 3.00% or less. Preferably, the Mn content is 2.70% or less or 2.50% or less.

[0018] Al: 0.010 to 0.400% Al has the effect of sounding down steel through deoxidation and also has the effect of controlling ferrite transformation. Furthermore, if the Al content is less than 0.010%, the strength and hole-expandability of the steel sheet deteriorate. For this reason, the Al content should be 0.010% or more. Preferably, the Al content is 0.030% or more, 0.050% or more, or 0.070% or more. On the other hand, if the Al content exceeds 0.400%, alumina precipitates in a cluster-like manner, increasing the crack susceptibility of the slab and making it difficult to handle. For this reason, the Al content should be 0.400% or less. Preferably, the Al content is 0.300% or less, 0.250% or less, or 0.200% or less.

[0019] P: 0.080% or less. P is an element that affects the weldability of steel plates. In particular, if the P content exceeds 0.080%, the weldability of the steel plate deteriorates significantly. Furthermore, the crack susceptibility of the slab increases, making it difficult to handle. For this reason, the P content should be 0.080% or less. Preferably, the P content is 0.040% or less, 0.020% or less, or 0.015% or less. The P content may also be 0%. From the viewpoint of refining costs, the P content may be 0.001% or more, or 0.003% or more.

[0020] S: 0.0100% or less. S is an element that affects the hole-expanding properties of steel sheets. In particular, if the S content exceeds 0.0100%, a large amount of inclusions such as MnS, which are detrimental to the hole-expanding properties of steel sheets, are generated. For this reason, the S content should be 0.0100% or less. Preferably, the S content is 0.0080% or less, 0.0060% or less, 0.0050% or less, or 0.0045% or less. The S content may also be 0%. From the viewpoint of refining costs, the S content may be 0.0001% or more, or 0.0010% or more.

[0021] N: 0.0090% or less. N is an element that combines with Ti to form Ti nitrides. In particular, if the N content exceeds 0.0050%, the crack susceptibility of the slab increases, making it difficult to handle. Therefore, the N content should be 0.0090% or less. Preferably, the N content is 0.0085% or less, 0.0070% or less, 0.0050% or less, 0.0040% or less, 0.0035% or less, 0.0030% or less, or 0.0025% or less. The N content may be 0%. From the viewpoint of refining costs, the N content may be 0.0001% or more or 0.0010% or more.

[0022] O: 0.0100% or less. O is an element that, when present in large quantities in steel, forms coarse oxides that act as fracture initiation points, causing brittle fracture and hydrogen-induced cracking. If the O content exceeds 0.0100%, brittle fracture and hydrogen-induced cracking are more likely to occur. In addition, the hole-expanding properties of the steel sheet deteriorate. Therefore, the O content should be 0.0100% or less. Preferably, the O content is 0.0080% or less, 0.0060% or less, 0.0050% or less, 0.0040% or less, or 0.0035% or less. Since O does not need to be present, the O content may be 0%. In order to disperse a large number of fine oxides during the deoxidation of molten steel, the O content may be 0.0005% or more, or 0.0010% or more.

[0023] The steel sheet according to this embodiment may contain the above-mentioned chemical components, with the remainder consisting of Fe and impurities. In this embodiment, impurities refer to substances introduced from raw materials such as ore, scrap, or the manufacturing environment, and are acceptable within a range that does not adversely affect the properties of the steel sheet according to this embodiment.

[0024] To reduce manufacturing variations or further improve the strength of the steel sheet, the following optional elements may be included. However, the inclusion of these elements is not mandatory, so the lower limit of their content is 0%.

[0025] Ti: 0-0.180% Ti precipitates in steel as carbides or nitrides, and has the effect of refining the metal structure through a pinning effect, as well as increasing the strength and yield ratio of the steel sheet through precipitation strengthening. To reliably obtain this effect, it is preferable to have a Ti content of 0.020% or more. More preferably, the Ti content is 0.030% or more, 0.050% or more, or 0.100% or more. On the other hand, if the Ti content exceeds 0.180%, the hole-expanding properties of the steel sheet deteriorate due to excessive precipitation of TiC. For this reason, the Ti content should be 0.180% or less. Preferably, the Ti content is 0.160% or less, 0.150% or less, or 0.130% or less.

[0026] Nb: 0-0.100% Nb has the effect of increasing the strength of the steel sheet by refining the grain size of the steel sheet and strengthening it by NbC precipitation. To reliably obtain this effect, it is preferable to have an Nb content of 0.001% or more. More preferably, the Nb content is 0.005% or more or 0.010% or more. On the other hand, if the Nb content exceeds 0.100%, the above effect becomes saturated. Also, the hole-expanding properties of the steel sheet deteriorate. For this reason, even when Nb is included, the Nb content should be 0.100% or less. Preferably, the Nb content is 0.080% or less or 0.060% or less.

[0027] V: 0-1.000% V has the effect of increasing the strength of the steel sheet by strengthening through precipitates, strengthening through fine grain formation by suppressing the growth of ferrite crystal grains, and strengthening through dislocations by suppressing recrystallization. To reliably obtain these effects, it is preferable to have a V content of 0.001% or more. More preferably, the V content is 0.005% or more or 0.010% or more. On the other hand, if the V content is excessive, a large amount of carbonitride precipitates, and the hole-expanding properties of the steel sheet deteriorate. For this reason, the V content should be 1.000% or less. Preferably, the V content is 0.800% or less, 0.600% or less, 0.300% or less, or 0.100% or less.

[0028] Cu: 0 to 1.000% Cu exists in the steel in the form of fine particles and has the effect of increasing the strength of the steel sheet. To reliably obtain this effect, it is preferable to have a Cu content of 0.001% or more. More preferably, the Cu content is 0.005% or more or 0.010% or more. On the other hand, if the Cu content is excessive, the weldability of the steel sheet deteriorates. For this reason, the Cu content should be 1.000% or less. Preferably, the Cu content is 0.800% or less, 0.600% or less, 0.300% or less or 0.100% or less.

[0029] Cr: 0-2,000% Cr is an effective element for improving the strength of steel plates. To reliably obtain this effect, it is preferable to have a Cr content of 0.001% or more. More preferably, the Cr content is 0.005% or more or 0.010% or more. On the other hand, if the Cr content is excessive, the hole-expanding properties of the steel plate deteriorate. For this reason, the Cr content should be 2,000% or less. Preferably, the Cr content is 1,500% or less, 1,200% or less, 1,000% or less, 0,600% or less, 0,300% or less, or 0.100% or less.

[0030] Mo: 0-3.000% Mo is an element effective in strengthening ferrite precipitation. To reliably obtain this effect, it is preferable to have a Mo content of 0.001% or more. More preferably, the Mo content is 0.005% or more or 0.010% or more. On the other hand, if the Mo content is excessive, the slab becomes more susceptible to cracking and becomes difficult to handle. For this reason, the Mo content is kept below 3.000%. Preferably, the Mo content is 2.500% or less, 2.000% or less, 1.500% or less, 1.000% or less, 0.500% or less, or 0.150% or less.

[0031] Ni: 0 to 0.500% Ni has the effect of suppressing phase transformation at high temperatures and increasing the strength of the steel sheet. To reliably obtain this effect, it is preferable to have a Ni content of 0.001% or more. More preferably, the Ni content is 0.005% or more, or 0.010% or more. On the other hand, if the Ni content is excessive, the weldability of the steel sheet deteriorates. For this reason, the Ni content should be 0.500% or less. Preferably, the Ni content is 0.400% or less, 0.300% or less, or 0.150% or less.

[0032] B: 0 to 0.0100% B has the effect of suppressing phase transformation at high temperatures and increasing the strength of the steel sheet. To reliably obtain this effect, it is preferable to have a B content of 0.0001% or more. More preferably, the B content is 0.0005% or more, or 0.0010% or more. On the other hand, if the B content is excessive, B precipitates will be formed and the strength of the steel sheet will decrease. For this reason, the B content should be 0.0100% or less. Preferably, the B content is 0.0080% or less, 0.0060% or less, 0.0050% or less, or 0.0030% or less.

[0033] Ca: 0 to 0.0500% Ca has the effect of refining the structure of the steel sheet by dispersing a large number of fine oxides during the deoxidation of molten steel. In addition, Ca has the effect of fixing S in the steel as spherical CaS and suppressing the formation of stretched inclusions such as MnS, thereby improving the hole-expanding properties of the steel sheet. To reliably obtain these effects, it is preferable that the Ca content be 0.0001% or more. More preferably, the Ca content is 0.0005% or more or 0.0010% or more. On the other hand, the above effects saturate even if the Ca content exceeds 0.0500%. For this reason, the Ca content should be 0.0500% or less. Preferably, the Ca content is 0.0300% or less, 0.0200% or less or 0.0080% or less.

[0034] Mg: 0-0.050% Mg has the effect of increasing the yield ratio of steel sheets by adjusting the shape of inclusions in the steel to a desirable shape. In order to reliably obtain this effect, it is preferable that the Mg content be 0.001% or more. The Mg content is more preferably 0.005% or more or 0.010% or more. On the other hand, if the Mg content exceeds 0.050%, inclusions are excessively generated in the steel, and the yield ratio of the steel sheet decreases. For this reason, the Mg content should be 0.050% or less. The Mg content is preferably 0.040% or less or 0.030% or less.

[0035] REM: 0 to 0.1000% REM has the effect of increasing the yield ratio of steel sheets by adjusting the shape of inclusions in the steel to a desirable shape. To reliably obtain this effect, it is preferable to have a REM content of 0.0001% or more. More preferably, the REM content is 0.0005% or more or 0.0010% or more. On the other hand, if the REM content exceeds 0.1000%, inclusions are excessively generated in the steel, and the yield ratio of the steel sheet decreases. For this reason, the REM content should be 0.1000% or less. Preferably, the REM content is 0.0800% or less, 0.0600% or less, 0.0200% or less, or 0.0080% or less. Here, REM refers to a total of 17 elements consisting of Sc, Y, and lanthanides, and the above REM content refers to the total content of these elements. In the case of lanthanides, they are added industrially in the form of mischmetal.

[0036] Bi: 0-0.100% Bi has the effect of increasing the yield ratio of steel sheets by refining the solidification structure. To reliably obtain this effect, it is preferable to have a Bi content of 0.001% or more. More preferably, the Bi content is 0.005% or more or 0.010% or more. On the other hand, if the Bi content exceeds 0.100%, the effect due to the above action becomes saturated, which is economically undesirable. Therefore, the Bi content is 0.100% or less. Preferably, the Bi content is 0.080% or less, 0.060% or less, or 0.040% or less.

[0037] Ta: 0-0.100% Similar to V, Ta has the effect of increasing the strength of the steel sheet by forming fine carbides in the steel. To reliably obtain this effect, it is preferable to have a Ta content of 0.001% or more. More preferably, the Ta content is 0.005% or more or 0.010% or more. On the other hand, if the Ta content exceeds 0.100%, the hole-expanding properties of the steel sheet deteriorate. For this reason, the Ta content should be 0.100% or less. Preferably, the Ta content is 0.080% or less or 0.050% or less.

[0038] Zr: 0 to 0.500% Zr has the effect of increasing the strength of the steel sheet through solid solution strengthening. To reliably obtain this effect, it is preferable that the Zr content be 0.001% or more. More preferably, the Zr content is 0.005% or more or 0.010% or more. On the other hand, if the Zr content exceeds 0.500%, the hole-expanding properties of the steel sheet deteriorate. For this reason, the Zr content should be 0.500% or less. Preferably, the Zr content is 0.300% or less or 0.100% or less.

[0039] Co: 0-3.000% Co has the effect of increasing the strength of the steel plate through solid solution strengthening. To reliably obtain this effect, it is preferable to have a Co content of 0.001% or more. More preferably, the Co content is 0.005% or more or 0.010% or more. On the other hand, if the Co content exceeds 3.000%, the hole-expanding properties of the steel plate deteriorate. For this reason, the Co content should be 3.000% or less. Preferably, the Co content is 1.000% or less, 0.500% or less, 0.300% or less, or 0.100% or less.

[0040] Zn: 0 to 0.200% Zn has the effect of increasing the strength of steel plates through solid solution strengthening. To reliably obtain this effect, it is preferable to have a Zn content of 0.001% or more. More preferably, the Zn content is 0.005% or more or 0.010% or more. On the other hand, if the Zn content exceeds 0.200%, the hole-expanding properties of the steel plate deteriorate. For this reason, the Zn content should be 0.200% or less. Preferably, the Zn content is 0.150% or less, 0.100% or less, or 0.080% or less.

[0041] W: 0-0.200% W has the effect of increasing the strength of the steel plate through solid solution strengthening. To reliably obtain this effect, it is preferable to have a W content of 0.001% or more. More preferably, the W content is 0.005% or 0.010% or more. On the other hand, if the W content exceeds 0.200%, the hole-expanding properties of the steel plate deteriorate. For this reason, the W content should be 0.200% or less. Preferably, the W content is 0.150% or less, 0.100% or less, or 0.080% or less.

[0042] Sb: 0-0.500% Sb has the effect of improving the hole-expanding properties of steel sheets by suppressing the formation of oxides that serve as the starting point for fracture. To reliably obtain this effect, it is preferable that the Sb content be 0.001% or more. More preferably, the Sb content is 0.005% or more or 0.010% or more. On the other hand, even if a large amount of Sb is included, the above effect will saturate, so the Sb content should be 0.500% or less. Preferably, the Sb content is 0.300% or less, 0.100% or less or 0.060% or less.

[0043] As: 0-0.100% As has the effect of improving the hole-expanding properties of steel sheets by lowering the austenite single-phase temperature, thereby refining the prior austenite grains. To reliably obtain this effect, it is preferable to have an As content of 0.001% or more. More preferably, the As content is 0.005% or more or 0.010% or more. On the other hand, even if a large amount of As is included, the above effect will saturate, so the As content should be 0.100% or less. Preferably, the As content is 0.050% or less, 0.040% or less, 0.030% or less or 0.020% or less.

[0044] Sn: 0-0.100% Sn has the effect of improving the hole-expanding properties of steel sheets by suppressing the formation of oxides that serve as the starting point for fracture. To reliably obtain this effect, it is preferable that the Sn content be 0.001% or more. More preferably, the Sn content is 0.005% or more or 0.010% or more. On the other hand, even if a large amount of Sn is included, the above effect will saturate, so the Sn content should be 0.100% or less. Preferably, the Sn content is 0.050% or less, 0.040% or less, 0.030% or less or 0.020% or less.

[0045] The chemical composition of the steel sheet described above can be analyzed using a spark discharge emission spectrometer or similar device. For carbon (C) and sulfur (S), the values ​​identified by combustion in an oxygen stream and measurement by infrared absorption using a gas component analyzer or similar device should be adopted. For oxygen (O) and nitrogen (N), the values ​​identified by melting a test specimen taken from the steel sheet in a helium stream and measurement by thermal conductivity should be adopted. If the steel sheet has a plating layer or coating on its surface, the plating layer or coating should be removed by mechanical grinding or similar means as necessary before the chemical composition analysis. If molten steel analysis values, slab analysis values, or steel sheet analysis values ​​from other steel sheets manufactured from the same molten steel are available, the analysis of a test specimen taken from the steel sheet may be omitted, and those analysis values ​​may be considered as the chemical composition of the steel sheet.

[0046] Next, the metallographic structure (microstructure) of the steel sheet according to the present embodiment will be described. In the steel sheet according to the present embodiment, at the position of 1 / 4 of the sheet thickness in the sheet thickness direction from the surface, the average value of the aspect ratio of the prior austenite grains is 2.0 to 4.0, the area ratio of martensite is 90% or more, and in the surface layer region which is the region from the surface to the position 200 μm in the sheet thickness direction from the surface, the area ratio of the region where the GAM value is 1.4° or less is defined as GAM ＳＬ and the area ratio of the region where the GAM value is 1.4° or less at the position of 1 / 4 of the sheet thickness is defined as GAM １／４ When this is done, the GAM ＳＬ is 25 area% or more, and the GAM １／４ is 20 area% or less.

[0047] In the present embodiment, the position of 1 / 4 of the sheet thickness refers to the region from the position of 1 / 8 of the sheet thickness from the surface to the position of 3 / 8 of the sheet thickness from the surface. In other words, it refers to the region starting from the position of \\(1 / 8\\) of the sheet thickness from the surface and ending at the position of \\(3 / 8\\) of the sheet thickness from the surface. In the measurement of the metallographic structure described later, when measuring at the position of 1 / 4 of the sheet thickness, it is preferable to perform the measurement centered on the position of 1 / 4 of the sheet thickness from the surface. Further, the surface layer region refers to the region from the surface to the position 200 μm in the sheet thickness direction from the surface. In other words, it refers to the region starting from the surface and ending at the position 200 μm in the sheet thickness direction from the surface. When the steel sheet has a plating layer, a coating film, or the like on the surface, the surface referred to here is the interface between the steel sheet and the plating layer, the coating film, or the like.

[0048] Average aspect ratio of prior austenite grains at the 1 / 4 position of plate thickness: 2.0 to 4.0 If the average aspect ratio of prior austenite grains at the 1 / 4 position of plate thickness is less than 2.0, the strength of the steel plate will decrease. Therefore, the average aspect ratio of prior austenite grains at the 1 / 4 position of plate thickness should be 2.0 or higher. Preferably, the average aspect ratio of prior austenite grains at the 1 / 4 position of plate thickness is 2.1 or higher or 2.3 or higher. On the other hand, if the average aspect ratio of prior austenite grains at the 1 / 4 position of plate thickness exceeds 4.0, the hole-expanding properties of the steel plate will deteriorate. Therefore, the average aspect ratio of prior austenite grains at the 1 / 4 position of plate thickness should be 4.0 or lower. Preferably, the average aspect ratio of prior austenite grains at the 1 / 4 position of plate thickness is 3.5 or lower, 3.0 or lower, less than 3.0, or 2.8 or lower.

[0049] The aspect ratio of the old austenite grain is the value obtained by dividing the major axis diameter (long axis) of the old austenite grain by the minor axis diameter (short axis), and it takes a value of 1.00 or greater.

[0050] The average aspect ratio of the prior austenite grains is obtained by the following method: A sample is taken from the end face of the steel plate at a position 1 / 4 of the way from the plate width, so that the metallographic structure of the cross section (thickness direction × rolling direction cross section) with the plate width direction as the normal direction can be observed. The size of the sample depends on the measuring device, but for example, a rectangular parallelepiped with the total thickness in the thickness direction, 15 mm in the rolling direction, and 10 mm in the plate width direction can be used. Next, the observation surface is mirror-polished and then etched using the "Bechet-Beaujard method using a saturated picric acid solution" as specified in Annex JA. 2 of JIS G 0551:2020. The grains that appear black due to the etching are identified as prior austenite grains. The observation surface revealing the prior austenite grains is observed using an optical microscope, and eight fields of view are captured at a magnification of 1000x or higher, with a field of view of 200 μm in the thickness direction and 600 μm in the rolling direction, at a position of 1 / 4 of the plate thickness (the region from the surface to 1 / 8 of the plate thickness to 3 / 8 of the plate thickness). From the captured micrographs, each prior austenite grain is approximated as an ellipsoid using the method described below, and its major axis diameter (long axis) and minor axis diameter (short axis) are determined. The ratio of the major axis to the minor axis (aspect ratio) is calculated for all prior austenite grains in the micrograph, and the average value is calculated by weighting it by the area of ​​each prior austenite grain to obtain the average aspect ratio of the prior austenite grains. For example, if a former austenite grain G1 has a major axis / minor axis ratio of r1 and an area of ​​A1, and another former austenite grain G2 has a major axis / minor axis ratio of r2 and an area of ​​A2, the average of the aspect ratios of the two former austenite grains is calculated as (A1 × r1 + A2 × r2) / (A1 + A2). The general formula is as follows: Here, Ai is the area of ​​the i-th former austenite grain, and ri is the aspect ratio of the i-th former austenite grain. Average aspect ratio of former austenite grains = Σi(Ai × ri) / ΣiAi

[0051] If the above method does not sufficiently reveal the old austenite grains, the old austenite grains are identified by the reconstruction method described in "Kengo Hata, Masayuki Wakita, Kazuki Fujiwara, Kaori Kawano, Nippon Steel & Sumitomo Metal Technical Report, No. 114 (2017), pp. 26-31," and the average aspect ratio of the old austenite grains is calculated.

[0052] EBSD measurement data used in the reconstruction method is obtained by the following method. After performing colloidal polishing or electrolytic polishing on the observation field (field of view of 200 μm in the thickness direction × 600 μm in the rolling direction), crystal orientation information is obtained by electron backscatter diffraction at measurement intervals of 0.2 μm. For the measurement, an EBSD analysis system consisting of a thermal field emission scanning electron microscope (JEOL JSM-7200F) and an EBSD detector (EDAX Velocity® ultrafast operating EBSD detector) is used. At this time, the vacuum level inside the system is 9.6 × 10⁻⁶. －５ The pressure should be less than or equal to Pa, the acceleration voltage 25 kV, and the irradiation current level 16. For displaying the EBSD map, version 7 or later of OIM Analysis (registered trademark) from EDAX / TSL solution, Inc. should be used with the obtained crystal orientation information.

[0053] The prior austenite grains are approximated as ellipsoids by the following method. As shown in Figure 1, for the identified prior austenite grains G, the area S of the grain region not included in the ellipsoid is considered. ｏｕｔ And the area S of the region within the ellipsoid that is not a crystal grain. ｉｎ We approximate the ellipsoid g such that the sum of the two is minimized. By approximating it as an ellipsoid g in this way, we can find (x0, y0): the center of the ellipsoid g, a: the major axis of the ellipsoid g, and b: the minor axis of the ellipsoid g.

[0054] Furthermore, if the sample contains prior austenite grains with an equivalent diameter of less than 2 μm, these grains are excluded from the measurement described above. This is because prior austenite grains with an equivalent diameter of less than 2 μm do not adversely affect the properties of the steel sheet according to this embodiment.

[0055] Furthermore, the rolling direction of the steel plate is determined by the following method: A test piece is taken so that the thickness cross-section of the steel plate can be observed. The Z-direction is defined as the direction perpendicular to the plate surface, and a total of 12 test pieces are taken, rotated every 30° around this Z-direction axis. The thickness cross-section of the collected test pieces is polished, and the prior austenite grain boundaries are exposed using the aforementioned etching solution. The average aspect ratio of the prior austenite grains is calculated using the cutting method. The test piece with the largest average aspect ratio of the prior austenite grains is identified, and the direction in which that test piece was taken is determined to be the rolling direction of the steel plate. That is, the direction parallel to the thickness cross-section of the test piece and perpendicular to the thickness direction is determined to be the rolling direction of the steel plate. Note that if the rolling direction is known in advance, such as in mill-edge steel plates (so-called edged steel plates), the above determination of the rolling direction is unnecessary.

[0056] Martensite area ratio: 90% or more. Martensite is a structure that increases strength. If the martensite area ratio is less than 90%, the desired strength cannot be obtained. Therefore, the martensite area ratio should be 90% or more. Preferably, the martensite area ratio is 92% or more, 95% or more, or 97% or more. The martensite area ratio may also be 100%.

[0057] Residual structure: 0-10% The steel sheet according to this embodiment may contain bainite, ferrite, pearlite, and retained austenite as residual structures other than martensite. The area ratio of these residual structures is preferably 0-10% in relation to the area ratio of martensite. The area ratio of the residual structures is preferably 8% or less, 5% or less, or 3% or less. The area ratio of the residual structures may be 0%. The area ratio of bainite may be 10% or less, 8% or less, 6% or less, 4% or less, 3% or less, 2% or less, 1% or less, or 0%. The total area ratio of ferrite and retained austenite may be 3% or less, 2% or less, 1% or less, or 0%.

[0058] The area ratio of each microstructure is measured by the following method. A test piece is taken from the steel plate so that the metallographic structure at the 1 / 4 position of the plate thickness (from the surface in the thickness direction, in the range from the 1 / 8 position to the 3 / 8 position of the plate thickness) can be observed. The thickness cross section of the test piece is finished with mirror polishing and then etched with Nital. Using an optical microscope and a scanning electron microscope (SEM), eight fields of view are taken of a 200 μm (thickness direction) × 600 μm (perpendicular to the thickness direction) area at the 1 / 4 position of the plate thickness. By performing image analysis on the obtained microstructure images, the area ratios of ferrite, pearlite, and bainite are obtained. In order to observe the same area in the area ratio measurement (excluding X-ray diffraction) described later, it is preferable to imprint Vickers indentations within 100 μm from each of the four corners of the observation area, for example, at three of the four corners of the observation area. Using these Vickers indentations as markers, the same area as the observation area described above can be observed. Subsequently, the corroded surface was removed by polishing at the same observation position and given a mirror finish. Then, after LePera etching, eight fields of view were captured using an FE-SEM (thermal field emission scanning electron microscope, JEOL JSM-7001F) at a position 1 / 4 of the plate thickness, measuring 200 μm (in the plate thickness direction) × 600 μm (perpendicular to the plate thickness direction), and image analysis was performed. In LePera etching, martensite and retained austenite are not corroded. Therefore, the total area ratio of martensite and retained austenite is obtained by calculating the area ratio of the uncorroded area. The area ratio of martensite is obtained by subtracting the area ratio of retained austenite obtained by X-ray diffraction, described later, from the total area ratio of martensite and retained austenite. If the value obtained by subtracting the area ratio of retained austenite from the total area ratio of martensite and retained austenite is a negative value, the area ratio of martensite is considered to be 0%.

[0059] In the image analysis described above, each structure is identified by the following method: A structure consisting of massive crystal grains that does not contain substructures such as lath within the structure is considered ferrite. A structure in which plate-like ferrite and Fe-based carbides are layered is considered pearlite. The area percentage of bainite is obtained by subtracting the area percentages of martensite, retained austenite, ferrite, and pearlite from 100%. If the calculated area percentage of bainite is a negative value, the area percentage of bainite is considered to be 0%.

[0060] The area fraction of retained austenite is obtained by X-ray diffraction. A test specimen taken from a steel plate is ground down to the 1 / 4 position of the plate thickness (from the surface in the thickness direction, from the 1 / 8 position to the 3 / 8 position), and the exposed surface is used as the observation surface. This observation surface is mirror polished and then finished by electropolishing. Using a Rigaku RINT-2500, Mo-Kα, the integrated intensities of a total of five peaks, α(200), α(211), γ(200), γ(220), and γ(311), are determined on the observation surface, and the volume fraction of retained austenite is calculated using the intensity averaging method. This volume fraction of retained austenite is considered to be the area fraction of retained austenite.

[0061] The area ratio of martensite is obtained by subtracting the area ratio of retained austenite obtained by X-ray diffraction from the sum of the area ratios of martensite and retained austenite obtained by observation using the FE-SEM described above. The area ratio of the remaining structure is obtained by subtracting the area ratio of martensite from 100%. If the area ratio of the remaining structure becomes a negative value in the calculation, the area ratio of the remaining structure is set to 0%. In this embodiment, since the area ratio of the metallic structure is calculated by image analysis using FE-SEM, X-ray diffraction, and EBSD analysis, the sum of each structure may not equal 100%. In that case, the area ratio of each structure is corrected so that the sum equals 100%. For example, if the sum of the area ratios of each structure is 103%, the area ratio of each structure is corrected by multiplying it by "100 / 103".

[0062] GAM ＳＬ : Area ratio GAM of the area where the GAM value in the surface region is 1.4° or less in areas of 25% or more. ＳＬIf the area percentage is less than 25%, the bending crack resistance of the steel plate deteriorates. GAM ＳＬ By setting the area percentage to 25% or more, work hardening ability is improved, and irregularities within the bend can be suppressed. As a result, the resistance of the steel sheet to internal cracking during bending can be increased. Therefore, GAM ＳＬ This shall be 25% or more of the area. GAM ＳＬ Preferably, the area percentage shall be 28% or more, 30% or more, 32% or more, or 35% or more. GAM ＳＬ This may be 50% or less, 45% or less, or 40% or less.

[0063] GAM １／４ : Area ratio GAM of the region where the GAM value at the 1 / 4 position of the plate thickness is 1.4° or less (20% area % or less) １／４ If the area percentage exceeds 20%, the peel resistance of the steel plate deteriorates. １／４ By keeping the area percentage below 20%, the instability of the crack propagation direction can be suppressed, and the occurrence of delamination at the shear end face can be suppressed. As a result, the delamination resistance of the steel plate can be improved. Therefore, GAM １／４ It shall be 20% or less of the area. GAM １／４ Preferably, it is 18% or less, 15% or less, 13% or less, or 11% or less. GAM １／４ The lower limit may be 0 area %.

[0064] GAM ＳＬ - GAM １／４ : Area ratio GAM of the area where the GAM value in the surface region is 1.4° or less in areas of 10% or more of the surface region. ＳＬ And, the area ratio GAM of the region where the GAM value at the 1 / 4 position of the plate thickness is 1.4° or less. １／４ The difference (GAM ＳＬ - GAM １／４ By setting a high value for ), the composition ratio of the shear end face can be controlled to an appropriate range, thereby further improving peel resistance. Therefore, GAM ＳＬ - GAM １／４ It is preferable that the area percentage be 10% or more. GAM ＳＬ - GAM １／４ Preferably, it is 15% or more, 20% or more, or 25% or more. GAMＳＬ - GAM １／４ There is no particular upper limit, but for example, it may be set at 50% or less of the area, or 40% or less of the area.

[0065] GAM ＳＬ and GAM １／４ The following method is used for measurement. A test piece is taken from the steel plate so that the metallographic structure in the surface region (from the surface to 200 μm in the thickness direction from the surface) and the 1 / 4 position of the plate thickness (from 1 / 8 of the plate thickness to 3 / 8 of the plate thickness in the thickness direction from the surface) can be observed. After colloidal polishing or electrolytic polishing is performed on the observation surface of the test piece, crystal orientation information is obtained by electron backscatter diffraction at measurement intervals of 0.2 μm. For measurement, an EBSD analyzer consisting of a thermal field emission scanning electron microscope (JEOL JSM-7200F) and an EBSD detector (EDAX Velocity® ultrafast EBSD detector) is used. At this time, the vacuum level inside the apparatus is 9.6 × 10⁻⁶. －５ The pressure (Pa) will be less than or equal to 25 kV, the acceleration voltage 25 kV, and the irradiation current level 16. There will be eight measurement fields for each setting. At the 1 / 4 position of the plate thickness, a region of 200 μm (in the direction of plate thickness) × 600 μm (perpendicular to the direction of plate thickness) will be imaged.

[0066] Using the obtained crystal orientation information, the following analysis is performed using version 7 or later of OIM Analysis (registered trademark) from EDAX / TSL solution. Measurement points with a crystal orientation difference of 15° or more are considered grain boundaries, and the region enclosed by these grain boundaries is considered a crystal grain. Next, the difference in crystal orientation between all measurement points within the crystal grain is calculated, and the average value of this difference is calculated to obtain the GAM value (Grain Average Misorientation value) of that crystal grain. For the surface region and the 1 / 4 position of the plate thickness, the area ratio of crystal grains with a GAM value of 1.4° or less is calculated, respectively, to obtain the GAM value. ＳＬ and GAM １／４ To obtain.

[0067] The thickness of the steel plate according to this embodiment is not particularly limited, but is preferably 0.4 to 8.0 mm. From the viewpoint of further improving impact resistance, the plate thickness may be 0.5 mm or more, 0.8 mm or more, or 1.0 mm or more. Alternatively, the plate thickness may be 5.0 mm or less, 4.0 mm or less, 3.6 mm or less, 3.2 mm or less, or 2.8 mm or less.

[0068] Strength: Tensile strength (TS) of 1180 MPa or more. The steel sheet according to this embodiment preferably has a tensile strength of 1180 MPa or more. By setting the tensile strength to 1180 MPa or more, the effect of reducing the weight of the vehicle body can be greatly enhanced. The tensile strength is more preferably 1200 MPa or more or 1250 MPa or more. The upper limit of the tensile strength is preferably 1400 MPa or less or 1350 MPa or less from the viewpoint of suppressing mold wear and ensuring the ductility of the steel sheet.

[0069] Tensile strength is obtained by taking a No. 5 test specimen in accordance with JIS Z 2241:2022 and performing a tensile test in accordance with JIS Z 2241:2022. The tensile test specimen is preferably taken from the 1 / 4 portion from the end in the width direction of the steel plate, with the longitudinal direction perpendicular to the rolling direction. The tensile test is performed twice, and the average value is used as the representative value. If a No. 5 test specimen cannot be taken due to the small size or complex shape of the steel plate or part, the tensile strength and total elongation may be determined by taking a strip-shaped piece with parallel sections of any width and performing a tensile test on that piece. The longitudinal direction of the strip-shaped piece should be perpendicular to the rolling direction.

[0070] Hole expansion properties: The hole expansion ratio (λ) should be 50% or more. Preferably, the hole expansion ratio should be 30% or more. If the hole expansion ratio is 50% or more, it can be judged that the hole expansion properties are excellent. The hole expansion ratio is obtained by performing a hole expansion test in accordance with JIS Z 2256:2020. The sampling position for the hole expansion test specimen is preferably 1 / 4 of the way from the edge in the width direction of the steel plate, the same as for the tensile test specimen. The test is performed three times, and the average value is used as the representative value.

[0071] Bending crack resistance is evaluated by the maximum length of cracks generated in the bent cross-section after a bending test using the V-block method in accordance with JIS Z 2248:2022. A test piece measuring 50 mm in length perpendicular to the rolling direction and 100 mm in length in the rolling direction is taken from the steel plate. As shown in Figure 2, the taken test piece is subjected to a bending test using a presser and V-block so that the bending ridge is perpendicular to the rolling direction. The radius R of the tip of the presser is set to 3.0 mm. If it is not possible to take a test piece of the above size due to the small size of the steel plate or part, a small test piece measuring 20 mm in length perpendicular to the rolling direction and 30 mm in length in the rolling direction may be used.

[0072] Next, the thickness cross section perpendicular to the bending ridge is observed at the center of the width of the specimen after the bending test. The length of the crack in the thickness cross section within the bend is measured. If the maximum value (maximum length) of the measured crack length is 50 μm or less, it can be determined that cracks are unlikely to occur within the bend, i.e., that the material has excellent resistance to internal cracking during bending.

[0073] Peeling resistance is evaluated by the rate of peeling at the end face of the punched hole after the hole expansion test. Three punched holes are created by performing a hole expansion test on a steel plate with a hole diameter of φ of 20 mm, a clearance of 20%, and other conditions conforming to JIS Z 2256:2020. Next, the angle of the area where peeling (damage resembling stripes) is observed by visual inspection is measured around the entire circumference of the end face of the punched hole. The peeling rate (%) for one punched hole is calculated by dividing the total angle of the area where peeling occurs around the entire circumference of the punched hole by 360° and multiplying by 100. For example, if a 0° point (and a 360° point) is defined around the entire circumference of one punched hole, and peeling occurs between 60° and 63°, and between 95° and 110°, the damage rate is (3 + 15) / 360 × 100 = 5.0%. The delamination rate is obtained by calculating the average delamination rate of three punched holes. If the delamination rate is 15.0% or less, it can be determined that delamination is prevented during punching, i.e., that the material has excellent delamination resistance.

[0074] The steel sheet according to this embodiment may have a coating on part or all of its surface. The coating may be an Al-based coating (a coating mainly composed of an Fe-Al alloy), a Zn-based coating (a coating mainly composed of an Fe-Zn alloy), or may contain an epoxy resin applied by electrodeposition coating. The coating is also called a film, an alloyed plating layer, or an intermetallic compound layer. Having a coating can improve corrosion resistance. The thickness of the coating is preferably 5 to 100 μm.

[0075] Al-based coatings (coatings mainly composed of Fe-Al alloys) are coatings that contain a total of 70% or more by mass of Fe and Al, and whose Al / (Al+Fe) ratio exceeds 20% by mass. Zn-based coatings (coatings mainly composed of Fe-Zn alloys) are coatings that contain a total of 70% or more by mass of Fe and Zn, and whose Zn / (Zn+Fe) ratio exceeds 20% by mass. If a coating satisfies both definitions, it is considered both an Al-based coating and a Zn-based coating.

[0076] Al-based coatings (coatings mainly composed of Fe-Al alloys) may contain, in addition to Fe and Al, one or more of the following: Si, Mg, Ca, Sr, Ni, Cu, Mo, Mn, Cr, C, Nb, Ti, B, V, Sn, W, Sb, Zn, Co, In, Bi, Zr, Se, As, and REM, with the remainder being impurities. Zn-based coatings (coatings mainly composed of Fe-Zn alloys) may contain, in addition to Fe and Zn, one or more of the following: Si, Mg, Ca, Sr, Ni, Cu, Mo, Mn, Cr, C, Nb, Ti, B, V, Sn, W, Sb, Al, Co, In, Bi, Zr, Se, As, and REM, with the remainder being impurities.

[0077] The chemical composition and thickness of the coating can be determined by cross-sectional observation using a scanning electron microscope. A sample is cut from any position at least 10 mm away from the end face. The cross-section of the cut sample is mechanically polished, and then mirror-finished. The observation range with the scanning electron microscope is, for example, 400x magnification, covering an area of ​​40,000 μm. ２ The above range applies.

[0078] Cross-sectional observation using BSE COMPO imaging reveals a clear contrast difference between the coating and the underlying steel plate. Therefore, the thickness of the coating can be measured by measuring the thickness from the outermost surface to the point where the contrast changes. Measurements are taken at 20 equally spaced points within the observation photograph, with a distance of 6.5 μm between each measurement point. In addition, five fields of view are observed using the above procedure, and the average value is used to determine the thickness of the coating.

[0079] The chemical composition of the coating can be determined by performing elemental analysis of spots (beam diameter 1 μm or less) using an electron probe microanalyzer (EPMA) within the same observation range as described above, thereby determining the concentrations of Fe, Al, and Zn contained in the coating. Ten points are analyzed in any 10 fields of view of the coating, and the average value is taken as the concentration of Fe, Al, and Zn contained in the coating. The same method is used to determine the concentrations of elements other than Fe, Al, and Zn that are present.

[0080] The steel sheet according to this embodiment has high strength, as well as excellent hole-expanding properties, resistance to internal cracking during bending, and resistance to peeling. Therefore, it can be suitably used for parts, especially automotive parts. Among automotive parts, it can be suitably used for automotive suspension parts such as lower arms and trailing arms. These automotive parts may consist solely of the steel sheet according to this embodiment, or they may be formed by joining the steel sheet according to this embodiment with other steel sheets.

[0081] Parts manufactured using the steel sheet according to this embodiment have the same chemical composition as the steel sheet described above. Furthermore, parts may contain a mixture of processed and unprocessed portions. Unprocessed portions have the same metallic structure as the steel sheet described above. Processed portions basically have the same metallic structure as the steel sheet described above, but in areas that have undergone heavy processing or welding, at the edges of parts, or where red rust has occurred, the metallic structure described above may not be present, or it may be difficult to determine. Therefore, when measuring the metallic structure of a part, these areas should be avoided, and measurements should be taken only in unprocessed portions. If there are no unprocessed portions, measurements should be taken in portions that have not undergone heavy processing. Unprocessed or heavily processed portions refer to, for example, flat portions of a part, portions where the increase or decrease in plate thickness due to processing is small, and portions that have not undergone punching, hole widening, or bending. As an example, in the case of the above part, it is the flat portion with the largest surface area, and a test piece is taken from near its center of gravity and investigated. Specific examples of areas to avoid measuring include the following (i) to (iv). Furthermore, if the part has a coating or chemical treatment coating on its surface, the part will be measured after going through the following coating removal process and chemical treatment coating removal process: (i) Welded parts: within 20 mm from the toe of spot welds and within 20 mm from the toe of arc / laser welds. (ii) Machined parts: machined parts with a radius of curvature of less than 15 mm and within 5 mm from the said machined parts. (iii) Ends: ends within 5 mm from the cut end face of the part. (iv) Red rust: within 5 mm from areas where red rust is visible.

[0082] (Coating removal process) Apply a coating remover (Neoriver #160, manufactured by Sansai Chemical Co., Ltd.) to the surface of a sample cut from a part at room temperature and let it stand for 5 minutes. Then, rub the surface of the sample to which the coating remover has been applied using a hard sponge (for example, "Kanefeel", manufactured by AION Co., Ltd.) to remove the coating from the surface of the sample.

[0083] Next, the surface of the sample after the coating has been removed is washed with water and dried. At this time, the remaining state of the coating is confirmed by SEM-EPMA measurement of the surface of the sample (100 μm square, 5 fields of view) after washing with water and drying.

[0084] In the elemental distribution image obtained by EPMA, regions with a C concentration of 10% by mass or more are identified, and if the area ratio of these regions is 5% or more, it is determined that the coating has not been sufficiently removed.

[0085] To measure the area percentage of regions where the C concentration is 10% by mass or more, first, an elemental distribution image of C is obtained using EPMA with the C concentration range set to 10-30%. The specific measurement conditions for EPMA are as follows: Apparatus: JXA-8230 electron probe microanalyzer manufactured by JEOL Ltd. Acceleration voltage: 15kV Irradiation current: 0.05μA Surface analysis: WDS Analysis interval: 300μm or more Area percentage: Average value of 5 fields Next, the area percentage is measured by image processing of the obtained elemental distribution image of C. The image analysis software "ImageJ" is used for image processing. Specifically, after loading the above elemental distribution image of C into ImageJ, the image is binarized using "Make Binary" in "Binary" under "Process" so that regions where the C concentration is 10% by mass or more are displayed in black and regions where the C concentration is less than 10% by mass are displayed in white. After binarization, use the "Measure" function in "Analyze" to read the "Area fraction" value in "Results". This value is determined as the area fraction of the region where the C concentration is 10% by mass or more.

[0086] If the paint film is not sufficiently removed, the removal of the paint film is repeated until the area ratio of the region where the C concentration is 10% by mass or more is less than 5%.

[0087] (Removal of chemical conversion coating) The chemical conversion coating is removed from the surface of the sample, which has been cut out from the part and had the coating removed, by following the method in accordance with JIS K 3151:1996. Specifically, the chemical conversion coating is removed from the surface of the sample by immersing the sample after the coating has been removed in a 5% by mass aqueous solution of chromic acid heated to 75°C for 15 minutes.

[0088] Next, the surface of the sample after the chemical conversion coating has been removed is washed with water and dried. At this time, the remaining state of the chemical conversion crystals is confirmed by SEM-EPMA measurement of the surface of the sample (100 μm square, 5 fields of view) after washing with water and drying.

[0089] In the elemental distribution image obtained by EPMA, regions with a P concentration of 5% by mass or more are identified, and if the area ratio of these regions is 5% or more, it is determined that the chemical conversion coating has not been sufficiently removed.

[0090] To measure the area fraction of regions where the P concentration is 5 mass% or higher, first, an elemental distribution image of P is obtained in an EPMA with a P concentration range of 5-10%. Next, the area fraction is measured by image processing of the obtained elemental distribution image of P. The image analysis software "ImageJ" is used for image processing. Specifically, after loading the above elemental distribution image of P into ImageJ, the image is binarized using "Make Binary" in "Binary" under "Process" so that regions where the P concentration is 5 mass% or higher are displayed in black and regions where it is less than 5 mass% are displayed in white. After binarization, the value of "Area fraction" in "Results" is read using "Measure" under "Analyze". This read value is determined as the area fraction of regions where the P concentration is 5 mass% or higher.

[0091] If the chemical conversion coating is not sufficiently removed, the removal of the chemical conversion coating is repeated until the area ratio of the region where the P concentration is 5% by mass or more is less than 5%.

[0092] For example, the lower arm can be manufactured by drawing, bending, and trimming excess material from the steel plate according to this embodiment, followed by punching and hole widening. The trailing arm can also be manufactured by burring, bending, and cutting the steel plate according to this embodiment.

[0093] Next, a preferred manufacturing method for the steel sheet according to this embodiment will be described. According to the manufacturing method described below, the steel sheet according to this embodiment can be manufactured stably. Since the steel sheet according to this embodiment is manufactured by hot rolling of a slab, it can also be called a hot-rolled steel sheet. In this embodiment, the temperature of the slab and the temperature of the steel sheet refer to the surface temperature of the slab and the surface temperature of the steel sheet.

[0094] A preferred method for manufacturing a steel sheet according to this embodiment includes: (1) a step of performing finish rolling on a slab having the above-described chemical composition such that the total reduction ratio in the temperature range below 1050°C is 20 to 45%, the number of rollings in the temperature range of 1000°C or higher with a reduction ratio of 30% or more is 3 or more, and the total reduction ratio of rolling in the temperature range of 1000°C or lower with a shape ratio of 3 to 10 is 11 to 29%; (2) a step of cooling after the completion of finish rolling such that the average cooling rate in the temperature range of 750 to 200°C is 30 to 90°C / s, and the average cooling rate in the temperature range of 400 to 500°C is 60 to 88°C / s; and (3) a step of winding in the temperature range of 185°C or lower. Each step will be described below.

[0095] (1) Finish Rolling In hot rolling, rough rolling and finish rolling are performed. In finish rolling, it is preferable to perform finish rolling on a slab having the above-mentioned chemical composition such that the total reduction ratio in the temperature range below 1050°C is 20 to 45%, the number of rollings in the temperature range above 1000°C with a reduction ratio of 30% or more is 3 or more, and the total reduction ratio of rollings in the temperature range below 1000°C with a shape ratio of 3 to 10 is 11 to 29%. By setting the total reduction ratio in the temperature range below 1050°C to 20 to 45%, the average aspect ratio of the prior austenite grains can be preferably controlled. Note that if only one pass of rolling with a reduction ratio of 20 to 45% is performed in the temperature range below 1050°C, even if recrystallization occurs, it may not be possible to preferably control the average aspect ratio of the prior austenite grains, so it is preferable to perform multiple stages of rolling in this temperature range.

[0096] The total reduction ratio in the temperature range below 1050°C is calculated by multiplying the entry plate thickness of the first roll in the temperature range below 1050°C by t ０ The exit plate thickness of the final rolling in a temperature range below 1050°C is t １ When this is the case, (1-t １ / t ０ It can be expressed as ) × 100 (%).

[0097] In finish rolling, it is preferable to perform rolling at a temperature of 1000°C or higher with a reduction ratio of 30% or more three times or more. Furthermore, it is preferable to perform rolling at a temperature of 1000°C or lower with a total reduction ratio of 11% to 29% with a shape ratio of 3 to 10. By performing rolling at a temperature of 1000°C or higher with a reduction ratio of 30% or more three times or more, the prior austenite grains in the surface region can be refined. In addition, by setting the total reduction ratio of rolling at a temperature of 1000°C or lower with a shape ratio of 3 to 10 to 11% to 29%, the transformation rate can be favorably controlled in the surface region and at the 1 / 4 position of the plate thickness, in combination with the aforementioned refinement. As a result, GAM ＳＬ - GAM １／４ This allows for desirable control.

[0098] The reduction ratio is calculated by multiplying the thickness of the entry plate by t. ２ The thickness of the exit plate is set to t ３ When this is the case, (1-t ３ / t ２ It can be expressed as ) × 100 (%).

[0099] The shape ratio can be expressed by the following formula (A): Shape ratio = l ｄ / h ｍ (A)

[0100] l in the above formula (A) ｄ is the projected contact arc length, h ｍ l is the average plate thickness. ｄ and h ｍ These can be expressed by the following equations (B) and (C). ｄ =√{Dr / 2 × (h ｉｎ +h ｏｕｔ )} (B) h ｍ = (h ｉｎ +2 × h ｏｕｔ) / 3 (C) Here, Dr is the roll radius, and h ｉｎ is the thickness of the entry side plate, h ｏｕｔ This is the thickness of the outer plate.

[0101] Furthermore, the slab is not particularly limited except for having the chemical composition described above. For example, a slab produced by continuous casting using molten steel with the above chemical composition, produced using a converter or electric furnace, can be used. Instead of continuous casting, ingot casting, thin slab casting, etc., may be employed. For heating the slab before hot rolling, the heating temperature should be in the temperature range of 1200 to 1300°C. Also, the conditions for rough rolling in hot rolling are not particularly limited.

[0102] (2) After the completion of the cooling finish rolling, it is preferable to cool the material so that the average cooling rate in the temperature range of 750 to 200°C is 30 to 90°C / s. By combining cooling to an average cooling rate of 30 to 90°C / s in the temperature range of 750 to 200°C with the above-mentioned process, GAM ＳＬ - GAM １／４ This can be controlled to a favorable degree. Furthermore, in the above cooling, it is preferable to increase the cooling rate in the temperature range of 400 to 500°C, specifically, to cool so that the average cooling rate in the temperature range of 400 to 500°C is 60 to 88°C / s. By setting the average cooling rate in the temperature range of 400 to 500°C to 60 to 88°C / s, GAM １／４ This allows for desirable control.

[0103] In this embodiment, the average cooling rate is the value obtained by dividing the temperature difference between the start and end points of the set range by the elapsed time from the start point to the end point.

[0104] (3) Winding The winding temperature is preferably in the temperature range of 200°C or lower. That is, cooling with an average cooling rate of 30 to 90°C / s is preferably carried out up to a temperature range of 200°C or lower. By setting the winding temperature to a temperature range of 185°C or lower and combining it with the above-mentioned process, GAM ＳＬ - GAM １／４ This allows for desirable control.

[0105] By the method described above, the steel plates according to this embodiment can be manufactured stably.

[0106] Next, the effects of one aspect of this disclosure will be described in more detail by reference to examples. However, the conditions in the examples are merely examples of conditions adopted to confirm the feasibility and effectiveness of this disclosure, and this disclosure is not limited to these examples of conditions. This disclosure may adopt various conditions as long as they do not depart from the gist of this disclosure and achieve the objectives of this disclosure.

[0107] Steel having the chemical compositions shown in Tables 1 and 2 was melted, and slabs with a thickness of 240 to 300 mm were manufactured by continuous casting. Using the obtained slabs, steel plates shown in Tables 4A and 4B were obtained under the manufacturing conditions shown in Tables 3A and 3B. The thickness of the obtained steel plates was 0.4 to 8.0 mm. Underlined text in the tables indicates that the information is outside the scope of this disclosure, the manufacturing conditions are undesirable, or the characteristic values ​​are undesirable.

[0108] The items in Tables 3A and 3B are as follows: Total reduction ratio below 1050°C: The total reduction ratio in the temperature range below 1050°C during finish rolling. Number of rolling operations above 1000°C: The number of rolling operations in the temperature range above 1000°C during finish rolling where the reduction ratio is 30% or more. Total reduction ratio below 1000°C: The total reduction ratio of rolling operations in the temperature range below 1000°C during finish rolling where the shape ratio is between 3 and 10. Average cooling rate: The average cooling rate in the temperature range of 750 to 200°C (however, if the coiling temperature is above 200°C, it is the average cooling rate up to the coiling temperature).

[0109] For the obtained steel sheet, the average aspect ratio of prior austenite grains at the 1 / 4 position of the sheet thickness, the metal structure, and GAM are determined by the method described above. ＳＬ GAM １／４ GAM ＳＬ - GAM １／４The tensile strength (TS), hole expansion ratio (λ), the maximum length of cracks generated in the inner cross-section after bending after the bending test, and the delamination rate at the end face of the punched hole after the hole expansion test were evaluated. The obtained measurement results are shown in Tables 4A and 4B. In Tables 4A and 4B, the average value of the aspect ratio of prior austenite grains at the 1 / 4 position of the plate thickness is simply referred to as "aspect ratio," the maximum length of cracks generated in the inner cross-section after bending after the bending test is simply referred to as "maximum length," and the delamination rate at the end face of the punched hole after the hole expansion test is simply referred to as "occurrence rate."

[0110] If the tensile strength was 1180 MPa or higher, it was judged to have high strength and was deemed acceptable. On the other hand, if the tensile strength was less than 1180 MPa, it was judged to have insufficient strength and was deemed unacceptable.

[0111] If the hole expansion ratio was 50% or more, it was judged to have excellent hole expansion properties and was deemed to pass. On the other hand, if the hole expansion ratio was less than 50%, it was judged to have poor hole expansion properties and was deemed to fail.

[0112] If the maximum length of cracks that occurred in the internal cross-section after bending was 50 μm or less, the material was judged to have excellent resistance to internal bending cracking and was deemed acceptable. On the other hand, if the maximum length of cracks exceeded 50 μm, the material was judged to have poor resistance to internal bending cracking and was deemed unacceptable.

[0113] If the peeling rate at the end face of the punched hole after the hole expansion test was 15.0% or less, it was judged to have excellent peel resistance and was deemed acceptable. On the other hand, if the peeling rate exceeded 15.0%, it was judged to have poor peel resistance and was deemed unacceptable. Furthermore, if the peeling rate was 10.0% or less, it was judged to have even better peel resistance.

[0114]

[0115]

[0116]

[0117]

[0118]

[0119]

[0120] Tables 4A and 4B show that the steel sheet according to the present invention example has high strength, as well as excellent hole-expanding properties, resistance to internal cracking during bending, and resistance to peeling. On the other hand, the steel sheet according to the comparative example is inferior in one or more of the above properties.

[0121] Furthermore, in all embodiments, the lower arms (parts) were manufactured by press working. The flat portion of the lower arms was evaluated in the same manner as described above. The measurement and evaluation results were the same as those shown in Tables 4A and 4B.

[0122] According to the above embodiments of this disclosure, it is possible to provide a steel plate having high strength, as well as excellent hole-expanding properties, resistance to internal cracking during bending, and resistance to peeling, and a part using this steel plate.

Claims

1. The chemical composition, in mass%, is as follows: C: 0.050-0.120%, Si: 0-3.00%, Mn: 1.20-3.00%, Al: 0.010-0.400%, P: 0.080% or less, S: 0.0100% or less, N: 0.0050% or less, O: 0.0100% or less, Ti: 0-0.180%, Nb: 0-0.100%, V: 0-1.000%, Cu: 0-1.000%, Cr: 0-2.000%, Mo: 0-3.000%, Ni: 0-0.500%, B: 0-0.0100%, Ca: 0-0.0500%. The composition is Mg: 0-0.050%, REM: 0-0.1000%, Bi: 0-0.100%, Ta: 0-0.100%, Zr: 0-0.500%, Co: 0-3.000%, Zn: 0-0.200%, W: 0-0.200%, Sb: 0-0.500%, As: 0-0.100%, Sn: 0-0.100%, and the remainder is Fe and impurities. At a position 1 / 4 of the plate thickness from the surface in the plate thickness direction, the average aspect ratio of prior austenite grains is 2.0-4.0, the area ratio of martensite is 90% or more, and the area ratio of the region in the surface layer region, which is the region from the surface to a position 200 μm from the surface in the plate thickness direction, where the GAM value is 1.4° or less is defined as GAM. ＳＬ The area ratio of the region where the GAM value is 1.4° or less at the 1 / 4 position of the plate thickness is determined by GAM １／４ In that case, the GAM ＳＬ The area percentage is 25% or more, and the GAM １／４ A steel plate characterized by having an area percentage of 20% or less.

2. The chemical composition is as follows, in mass%, Ti: 0.001-0.180%, Nb: 0.001-0.100%, V: 0.001-1.000%, Cu: 0.001-1.000%, Cr: 0.001-2.000%, Mo: 0.001-3.000%, Ni: 0.001-0.500%, B: 0.0001-0.0100%, Ca: 0.0001-0.0500%, Mg: 0.001-0.050%, REM: 0.0001-0.1000%, Bi: 0.001-0.100%, Ta: 0.001-0.100%, The steel sheet according to claim 1, characterized by containing one or more of the following: Zr: 0.001 to 0.500%, Co: 0.001 to 3.000%, Zn: 0.001 to 0.200%, W: 0.001 to 0.200%, Sb: 0.001 to 0.500%, As: 0.001 to 0.100%, and Sn: 0.001 to 0.100%.

3. The aforementioned GAM ＳＬ - The aforementioned GAM １／４ The steel plate according to claim 1 or 2, characterized in that the area percentage is 10% or more.

4. A steel plate according to any one of claims 1 to 3, characterized in that the plate thickness is 0.4 to 8.0 mm.

5. The steel sheet according to any one of claims 1 to 4, characterized in that the chemical composition contains one or more of the following in mass percent: N: 0.0050% or less, As: 0.050% or less, and Sn: 0.050% or less.

6. A component comprising a steel plate as described in any one of claims 1 to 5.

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