Steel sheet and component

Abstract

This steel sheet is characterized by having a desired chemical composition, wherein: at a position corresponding to 1 / 4 of the sheet thickness, the average value of the aspect ratios of prior austenite grains is 3.0-10.0, the area ratio of bainite is 70-95%, the total area ratio of martensite and retained austenite is 5-30%, the area ratio of pearlite is less than 5%, and the area ratio of ferrite is at most 15%; and in a surface layer region, the average value of the equivalent circle diameters of martensite and retained austenite is at most 3.0 μm, and the area ratio of ferrite is at least 10%. This component is manufactured using said 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. 2025-006165, 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, if high-strength steel plates are applied to the parts described above, necking or fracture may occur due to insufficient workability. For this reason, steel plates applied to the parts described above are required to have excellent workability, particularly ductility and hole-expanding properties.

[0004] Furthermore, as steel plates become stronger, their ductility after pre-straining decreases. As a result, the impact resistance of parts that have been processed and pre-strained deteriorates. Therefore, steel plates used in such parts are required to have excellent impact resistance after pre-straining.

[0005] For example, Patent Document 1 discloses a high-strength hot-rolled steel sheet having a structure in which the main phase is a bainite phase with an area ratio of 85% or more, the second phase is a martensite phase or a martensite-austenite mixed phase with an area ratio of 15% or less, the remainder being a ferrite phase, the average grain size of the second phase being 3.0 μm or less, the average aspect ratio of the prior austenite grains being 1.3 or more and 5.0 or less, the area ratio of recrystallized prior austenite grains to unrecrystallized prior austenite grains being 15% or less, and the precipitates with a diameter of less than 20 nm precipitated in the hot-rolled steel sheet being 0.10% or less by mass%, and the tensile strength TS being 980 MPa or more. Patent Document 1 discloses that with the above configuration, a high-strength hot-rolled steel sheet can be obtained with a tensile strength TS of 980 MPa or more and excellent punchability and hole-expanding properties.

[0006] International Publication No. 2017 / 017933

[0007] However, the hot-rolled steel sheet disclosed in Patent Document 1 requires further improvement in workability. Furthermore, Patent Document 1 does not consider the impact resistance characteristics after pre-straining.

[0008] This disclosure is made in view of the above circumstances. The purpose of this disclosure is to provide a steel sheet having high strength, excellent ductility and hole-expanding properties, and excellent impact resistance after pre-straining, as well as a part using this steel sheet.

[0009] The gist of this disclosure is as follows: [1] The chemical composition, in mass%, is: C: 0.045-0.120%, Si: 0-3.00%, Mn: 1.20-3.00%, Ti: 0.020-0.180%, 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, 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: 0-0.0010%, Ca: 0-0.0500%, 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: Fe and impurities, and 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 3.0-10.0, the area ratio of bainite is 70-95%, the sum of the area ratios of martensite and retained austenite is 5-30%, and the area ratio of pearlite is less than 5%. A steel sheet characterized in that the area ratio of ferrite is 15% or less, the average value of the equivalent circular diameter of martensite and retained austenite in the surface region, which is the region from the surface to a position 200 μm from the surface in the thickness direction of the sheet, is 3.0 μm or less, and the area ratio of ferrite is 10% or more.[2] The chemical composition is as follows, in mass%, 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 0.0010%, 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%, Zr: 0.001 to 0.500%, [1] A steel sheet characterized by containing one or more of the following: 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] A steel sheet according to [1] or [2], characterized in that, in the metal structure at a position 1 / 4 of the plate thickness from the surface in the plate thickness direction, the average value of the aspect ratio of the prior austenite grains is 3.0 to 6.0. [4] A steel sheet according to any one of items 1 to 3, characterized in that, at a position 1 / 4 of the plate thickness from the surface in the plate thickness direction, the area ratio of the ferrite is less than 10%. [5] A steel sheet according to any one of [1] to [4], characterized in that the sheet thickness is 0.4 to 8.0 mm. [6] A steel sheet according to any one of [1] to [5], characterized in that the chemical composition contains one or more of the following in mass%, N: 0.0050% or less, As: 0.050% or less, and Sn: 0.050% or less. [7] A steel sheet according to any one of [1] to [6], characterized in that the difference between the ferrite area ratio in the surface region and the ferrite area ratio at a position 1 / 4 of the sheet thickness from the surface in the direction of the sheet thickness is 2% or more. [8] A steel sheet according to any one of [1] to [7], characterized in that the value obtained by dividing the ferrite area ratio at a position 1 / 4 of the sheet thickness from the surface in the direction of the sheet thickness by the ferrite area ratio in the surface region is 0.86 or less. [9] A part containing a steel sheet according to any one of [1] to [8].

[0010] According to the above embodiments of this disclosure, it is possible to provide a steel sheet having high strength, excellent ductility and hole-expanding properties, and excellent impact resistance after pre-straining, as well as a part using this steel sheet.

[0011] This diagram illustrates a method for approximating the former austenite grains as ellipsoids. It also illustrates a drop weight test.

[0012] 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.

[0013] 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.

[0014] C: 0.045-0.120% C is an important element for improving the strength of steel sheets. To obtain the desired strength, the C content should be 0.045% or more. Preferably, the C content is 0.050% or more, 0.60% or more, 0.65% or more, or 0.70% or more. On the other hand, if the C content exceeds 0.120%, the ductility and hole-expanding properties of the steel sheet 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.

[0015] 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.

[0016] 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, or 1.80% or more. On the other hand, if the Mn content exceeds 3.00%, MnS, which adversely affects the ductility 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, 2.50% or less, 2.30% or less, or 2.10% or less.

[0017] Ti: 0.020 to 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. If the Ti content is less than 0.020%, the desired strength cannot be obtained. For this reason, the Ti content should be 0.020% or 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.

[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. If the Al content is less than 0.010%, the hole-expanding properties 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 0.002% 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 and 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] 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.

[0026] 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, degrading the ductility and / or hole-expanding properties of the steel sheet. 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.

[0027] 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.

[0028] Cr: 0 to 2.000% Cr is an element effective in improving the strength of the steel sheet. When this effect is to be surely obtained, it is preferable that the Cr content be 0.001% or more. The Cr content is more preferably 0.005% or more or 0.010% or more. On the other hand, when the Cr content becomes excessive, the ductility and hole expansion property of the steel sheet deteriorate. Therefore, the Cr content is made 2.000% or less. The Cr content is preferably 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.

[0029] Mo: 0 to 3.000% Mo is an element effective in precipitation strengthening of ferrite. When this effect is to be surely obtained, it is preferable that the Mo content be 0.001% or more. The Mo content is more preferably 0.005% or more or 0.010% or more. On the other hand, when the Mo content becomes excessive, the cracking susceptibility of the slab increases and handling of the slab becomes difficult. Therefore, the Mo content is made 3.000% or less. The Mo content is preferably 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.

[0030] Ni: 0 to 0.500% Ni has the effect of suppressing phase transformation at high temperature and increasing the strength of the steel sheet. When this effect is to be surely obtained, it is preferable that the Ni content be 0.001% or more. The Ni content is more preferably 0.005% or more, 0.010% or more. On the other hand, when the Ni content is excessive, the weldability of the steel sheet deteriorates. Therefore, the Ni content is made 0.500% or less. The Ni content is preferably 0.400% or less, 0.300% or less or 0.150% or less.

[0031] B: 0 to 0.0010% 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 0.0003% or more, or 0.0005% or more. On the other hand, if the B content is excessive, B precipitates will form and the strength of the steel sheet will decrease. For this reason, the B content should be 0.0010% or less. Preferably, the B content is 0.0008% or less, or 0.0006% or less.

[0032] 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.

[0033] 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.

[0034] REM: 0 to 0.1000% REM has the effect of increasing the yield ratio of the steel sheet by adjusting the shape of inclusions in the steel to a preferable shape. To surely obtain this effect, it is preferable that the REM content is 0.0001% or more. The REM content is more preferably 0.0005% or more or 0.0010% or more. On the other hand, when the REM content exceeds 0.1000%, inclusions are excessively generated in the steel, and the yield ratio of the steel sheet decreases. Therefore, the REM content is 0.1000% or less. The REM content is preferably 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 lanthanoids, and the content of the above REM refers to the total content of these elements. In the case of lanthanoids, it is industrially added in the form of mischmetal.

[0035] Bi: 0 to 0.100% Bi has the effect of increasing the yield ratio of the steel sheet by refining the solidification structure. To surely obtain this effect, it is preferable that the Bi content is 0.001% or more. The Bi content is more preferably 0.005% or more or 0.010% or more. On the other hand, when the Bi content exceeds 0.100%, the effect by the above action saturates, which is not economically preferable. Therefore, the Bi content is 0.100% or less. The Bi content is preferably 0.080% or less, 0.060% or less or 0.040% or less.

[0036] Ta: 0 to 0.100% Ta, similar to V, has the effect of increasing the strength of the steel sheet by forming fine carbides in the steel. To surely obtain this effect, it is preferable that the Ta content is 0.001% or more. The Ta content is more preferably 0.005% or more or 0.010% or more. On the other hand, when the Ta content exceeds 0.100%, the ductility and / or hole expansion property of the steel sheet deteriorates. Therefore, the Ta content is 0.100% or less. The Ta content is preferably 0.080% or less or 0.050% or less.

[0037] 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 ductility and / or 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.

[0038] Co: 0-3.000% Co has the effect of increasing the strength of the steel sheet 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 ductility and / or hole-expanding properties of the steel sheet 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.

[0039] Zn: 0 to 0.200% Zn has the effect of increasing the strength of the steel sheet 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 ductility and / or hole-expanding properties of the steel sheet 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.

[0040] W: 0-0.200% W has the effect of increasing the strength of the steel sheet 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 ductility and / or hole-expanding properties of the steel sheet 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.

[0041] Sb: 0-0.500% Sb has the effect of improving the ductility and 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.

[0042] 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.

[0043] Sn: 0-0.100% Sn has the effect of improving the ductility and 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.

[0044] 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.

[0045] Next, the metal structure (microstructure) of the steel sheet according to this embodiment will be described. In the steel sheet according to this embodiment, at a position 1 / 4 of the thickness from the surface in the thickness direction, the average aspect ratio of prior austenite grains is 3.0 to 10.0, the area ratio of bainite is 70 to 95%, the sum of the area ratios of martensite and retained austenite is 5 to 30%, the area ratio of pearlite is less than 5%, and the area ratio of ferrite is 15% or less. In the surface region, which is the area from the surface to a position 200 μm from the surface in the thickness direction, the average equivalent circle diameter of martensite and retained austenite is 3.0 μm or less, and the area ratio of ferrite is 10% or more.

[0046] In this embodiment, the 1 / 4 position of the plate thickness refers to the region from the 1 / 8 position of the plate thickness from the surface to the 3 / 8 position of the plate thickness from the surface. In other words, it refers to the region that starts at the 1 / 8 position of the plate thickness from the surface and ends at the 3 / 8 position of the plate thickness from the surface. When measuring the metal structure, etc., as described later, at the 1 / 4 position of the plate thickness, it is preferable to center the measurement at the 1 / 4 position of the plate thickness from the surface. Furthermore, the surface region refers to the region from the surface to a position 200 μm from the surface in the direction of the plate thickness. In other words, it refers to the region that starts at the surface and ends at a position 200 μm from the surface in the direction of the plate thickness from the surface. When the steel plate has a plating layer or coating on its surface, the surface referred to here refers to the interface between the steel plate and the plating layer or coating.

[0047] Average aspect ratio of prior austenite grains at the 1 / 4 position of plate thickness: 3.0 to 10.0 If the average aspect ratio of prior austenite grains at the 1 / 4 position of plate thickness is less than 3.0, the ductility of the steel plate deteriorates. Therefore, the average aspect ratio of prior austenite grains at the 1 / 4 position of plate thickness should be 3.0 or higher. Preferably, the average aspect ratio of prior austenite grains at the 1 / 4 position of plate thickness is 3.5 or higher, 4.0 or higher, or 4.2 or higher. On the other hand, if the average aspect ratio of prior austenite grains at the 1 / 4 position of plate thickness exceeds 10.0, the hole-expanding properties of the steel plate deteriorate. Therefore, the average aspect ratio of prior austenite grains at the 1 / 4 position of plate thickness should be 10.0 or lower. The average aspect ratio of the prior austenite grains at the 1 / 4 position of the plate thickness is preferably 8.0 or less, 7.0 or less, 6.0 or less, 5.5 or less, or 5.0 or less.

[0048] 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.

[0049] 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

[0050] 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.

[0051] 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 and 600 μm in the rolling direction), crystal orientation information is obtained by electron backscatter diffraction at measurement intervals of 0.1 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 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.

[0052] 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.

[0053] 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.

[0054] 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.

[0055] Area ratio of bainite: 70-95% Bainite is a structure consisting of fine crystal grains and carbides. If the area ratio of bainite is less than 70%, the desired ductility cannot be obtained in the steel sheet. Therefore, the area ratio of bainite should be 70% or more. Preferably, the area ratio of bainite is 75% or more or 80% or more. On the other hand, if the area ratio of bainite exceeds 95%, the desired strength cannot be obtained in the steel sheet. Therefore, the area ratio of bainite should be 95% or less. Preferably, the area ratio of bainite is 93% or less, 90% or less, 87% or less, 85% or less, 83% or less, or 81% or less.

[0056] Total area ratio of martensite and retained austenite: 5-30% If the total area ratio of martensite and retained austenite exceeds 30%, the desired amount of bainite cannot be secured, and the desired ductility cannot be obtained. Therefore, the total area ratio of martensite and retained austenite should be 30% or less. Preferably, the total area ratio of martensite and retained austenite is 25% or less, 20% or less, or 15% or less. Also, martensite is a structure that increases strength. If the total area ratio of martensite and retained austenite is less than 5%, the desired strength cannot be obtained. Therefore, the total area ratio of martensite and retained austenite should be 5% or more. Preferably, the total area ratio of martensite and retained austenite is 5% or more, or 10% or more. The area ratio of martensite may be 22% or more, 25% or more, 27% or more, 28% or more, 29% or more, or 29.5% or more. The area percentage of retained austenite may be 20% or less, 15% or less, 12% or less, 10% or less, 9% or less, or 8% or less.

[0057] Perlite area ratio: Less than 5% If the perlite area ratio is 5% or more, the desired strength cannot be obtained. Therefore, the perlite area ratio should be less than 5%. Preferably, the perlite area ratio is 3% or less or 1% or less. Since a lower perlite area ratio is preferable, the perlite area ratio may be 0%.

[0058] Ferrite area ratio: 15% or less. If the ferrite area ratio exceeds 15%, the desired strength cannot be obtained. Therefore, the ferrite area ratio should be 15% or less. Preferably, the ferrite area ratio should be 12% or less. By making the ferrite area ratio less than 10%, the strength of the steel plate can be further increased. Therefore, from the viewpoint of obtaining higher strength, it is preferable that the ferrite area ratio be less than 10%. More preferably, the ferrite area ratio is 9% or less, 7% or less, 5% or less, or 3% or less. Since a lower ferrite area ratio is preferable, the ferrite area ratio may be 0%.

[0059] The area ratio of each tissue 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) can be observed. The thickness cross section of the test piece is finished with mirror polishing and LePera etching. Then, an FE-SEM (thermal field emission scanning electron microscope, JEOL JSM-7001F) is used to observe a 200 μm (thickness direction) × 600 μm (perpendicular to the thickness direction) area at the 1 / 4 position of the plate thickness, and image analysis is performed.

[0060] In repera corrosion, martensite and retained austenite are not corroded. Therefore, the sum of the area percentages of martensite and retained austenite can be obtained by calculating the area percentage of the uncorroded region. In order to observe this region in the area percentage measurement described later (excluding X-ray diffraction), it is preferable to imprint Vickers indentations within 100 μm of each of the four corners of the observation area in FE-SEM, for example. By using these Vickers indentations as markers, the same region as the observation area in FE-SEM can be observed.

[0061] 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.

[0062] 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. If the calculated total area ratio of martensite is a negative value, the area ratio of martensite is set to 0%.

[0063] The area ratio of pearlite is obtained by the following method: In the same region (200 μm × 600 μm) used to determine the area ratios of martensite and retained austenite, only the eroded layer is removed by polishing and the surface is mirror-finished. Then, the region is etched with Nital solution, observed using FE-SEM, and image analysis is performed. Regions in which cementite and ferrite are arranged in a lamellar pattern are identified as pearlite, and the area ratio of that region is calculated to obtain the area ratio of pearlite.

[0064] The area fraction of ferrite is obtained by the following method. Furthermore, the following procedure is performed on areas other than those identified as pearlite by the above method. Colloidal polishing or electrolytic polishing is performed on the same area (200 μm × 600 μm) used when determining the area fractions of martensite and retained austenite, and then crystal orientation information is obtained by electron backscatter diffraction at measurement intervals of 0.2 μm. For the 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 should be below Pa, the acceleration voltage 25kV, and the irradiation current level 16.

[0065] 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. Crystal grains with a GAM value of 0.6° or less are considered ferrite, and their area fraction is calculated to obtain the ferrite area fraction.

[0066] The area ratio of bainite is obtained by subtracting the area ratios of martensite, retained austenite, pearlite, and ferrite obtained by the above method from 100%. If the calculated area ratio of bainite is a negative value, the area ratio of bainite is set to 0%. In this embodiment, since the area ratio of the metal 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".

[0067] The observation conditions for the FE-SEM are as follows: Electron gun type: Thermal emission type Current irradiation number: 9 Working distance (WD): 10 mm Acceleration voltage: 20 kV Objective aperture number: 4 Pixel count: 5120 x 3840

[0068] Average value of the equivalent circle diameter of martensite and retained austenite in the surface region: 3.0 μm or less. In the surface region, which is the area from the surface of the steel plate to a position 200 μm from the surface in the thickness direction, if the size of martensite and retained austenite is large, it is not possible to suppress the occurrence of cracks in the thickness direction, and the impact resistance after pre-strain cannot be improved. For this reason, the average value of the equivalent circle diameter of martensite and retained austenite in the surface region should be 3.0 μm or less. Preferably, the average value of the equivalent circle diameter of martensite and retained austenite in the surface region is 2.5 μm or less, 2.0 μm or less, or 1.5 μm or less. A smaller average value of the equivalent circle diameter of martensite and retained austenite in the surface region is preferable, but it may be 0.5 μm or more, 0.9 μm or more, or 1.0 μm or more.

[0069] The average equivalent diameter of martensite and retained austenite in the surface region is obtained by the following method: For the surface region, the microstructure is identified using the same method as at the 1 / 4 thickness position, and the equivalent diameter of martensite and retained austenite is determined to obtain the equivalent diameter of each microstructure. The average value of the obtained equivalent diameters is calculated to obtain the average value of the equivalent diameter of martensite and retained austenite. Note that martensite and retained austenite are identified as regions that have not been corroded by repera corrosion. The equivalent diameter of martensite and retained austenite is obtained by calculating the equivalent diameter of these uncorroded regions. Furthermore, equivalent diameters of 0.3 mm or less are excluded from the calculation of the average value.

[0070] Ferrite area ratio: 10% or more. If the ferrite area ratio in the surface region is less than 10%, the impact resistance after pre-straining cannot be improved. Therefore, the ferrite area ratio should be 10% or more. Preferably, the ferrite area ratio is 12% or more or 13% or more. If excessive ferrite is generated in the surface region, there is a concern that the planar bending fatigue characteristics of the steel sheet will decrease. Therefore, it is preferable that the ferrite area ratio is 40% or less. The ferrite area ratio may also be 30% or less or 20% or less, or 18% or less or 15% or less. The surface region may also contain the remaining structure other than ferrite as bainite: 70-95%, and one or more of martensite, retained austenite, and pearlite: 0-20%.

[0071] Here, it is preferable that the difference between the ferrite area ratio in the surface region and the ferrite area ratio at a position 1 / 4 of the plate thickness from the surface in the thickness direction (= "ferrite area ratio in the surface region" - "ferrite area ratio at a position 1 / 4 of the plate thickness from the surface in the thickness direction") is 2% or more. This further enhances the impact resistance after pre-straining. It is preferable that the difference (= "ferrite area ratio in the surface region" - "ferrite area ratio at a position 1 / 4 of the plate thickness from the surface in the thickness direction") be large, and may be 3% or more, 4% or more, 6% or more, 8% or more, or 10% or more.

[0072] Furthermore, it is preferable that the value obtained by dividing the ferrite area ratio at the 1 / 4 position of the plate thickness by the ferrite area ratio in the surface region is 0.86 or less. This further enhances the impact resistance after pre-straining. The ratio (= "ferrite area ratio at the 1 / 4 position of the plate thickness" / "ferrite area ratio in the surface region") is preferably small and may be 0.83 or less, 0.80 or less, 0.75 or less, 0.70 or less, 0.65 or less, 0.60 or less, 0.55 or less, 0.50 or less, or 0.45 or less.

[0073] The area ratio of ferrite in the surface region is measured using the same method as when measured at the 1 / 4 position of the plate thickness.

[0074] The thickness of the steel plate according to this embodiment is not particularly limited, but may be 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.

[0075] Strength: Tensile strength (TS) of 980 MPa or more The steel plate according to this embodiment preferably has a tensile strength of 980 MPa or more. By setting the tensile strength to 980 MPa or more, the effect of reducing the weight of the vehicle body can be greatly enhanced. The tensile strength is more preferably 990 MPa or more, 1000 MPa or more, 1030 MPa or more, or 1050 MPa or more. The upper limit of the tensile strength is preferably 1250 MPa or less, 1200 MPa or less, or 1150 MPa or less from the viewpoint of suppressing mold wear and ensuring the ductility of the steel plate.

[0076] Ductility: The total elongation (El) is preferably 10.0% or more. If the total elongation is 10.0% or more, it can be judged to have excellent ductility. Note that total elongation refers to "total elongation at break" as defined in JIS Z 2241:2022.

[0077] Tensile strength and total elongation are 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 small 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.

[0078] Hole expansion properties: A hole expansion ratio (λ) of 30% or more is preferred. If the hole expansion ratio is 30% or more, it can be judged that the material has excellent hole expansion properties. 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.

[0079] Impact Resistance After Pre-straining In this embodiment, the impact resistance after pre-straining is evaluated by performing a drop weight test on a specimen after a bending test using the V-block method in accordance with JIS Z 2248:2022. A specimen 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, a bending test is performed on the taken specimen using a press fitting and a V-block so that the bending ridge is perpendicular to the rolling direction. The radius R of the tip of the press fitting is set to 3.0 mm for the bending test. If it is not possible to take a specimen of the above size due to the small size of the steel plate or part, a small specimen measuring 20 mm in length perpendicular to the rolling direction and 30 mm in length in the rolling direction may be used.

[0080] Next, a 10 mm long x 50 mm long specimen is cut from the bending apex of the specimen after the bending test, perpendicular to the rolling direction, and a drop weight test is performed. If the bending test was performed using the small specimen described above, a 10 mm long x 30 mm long specimen is cut perpendicular to the rolling direction, and a drop weight test is performed. The specimen after the bending test is placed on a base with the outer bent side facing upwards, and a 15 kg weight is dropped free and hits the specimen at a speed of 15 km / h. After this drop weight test, the bent portion of the specimen is observed. If no fracture occurs in the bent portion of the specimen after the bending test, where the radius R at the tip of the clamp was 3.0 mm, it can be determined that it has excellent impact resistance after pre-straining. Also, even if fracture occurs in a similar specimen, if the fracture does not penetrate in the thickness direction, it can be determined that it has excellent impact resistance after pre-straining. Furthermore, after the drop weight test, the specimen is cut at the center of the bend (center in the rolling direction), and the presence or absence of fracture is determined by observing the cross-section of a 10 mm wide plate.

[0081] In the steel plate according to this embodiment, it is preferable that no fracture occurs in the bent portion of the test piece after a bending test and a drop weight test using a pressing tool with a tip radius R of 3.0 mm, or, if fracture occurs, that fracture does not penetrate in the thickness direction of the plate.

[0082] 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.

[0083] 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.

[0084] 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.

[0085] 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.

[0086] 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.

[0087] 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.

[0088] The steel sheet according to this embodiment has high strength, excellent ductility and hole-expanding properties, and excellent impact resistance after pre-straining. 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.

[0089] 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.

[0090] (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.

[0091] 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.

[0092] 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.

[0093] 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.

[0094] 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%.

[0095] (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.

[0096] 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.

[0097] 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.

[0098] 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.

[0099] 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%.

[0100] 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.

[0101] 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.

[0102] A preferred method for manufacturing a steel sheet according to this embodiment is: (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 30% or more and less than 65%, 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 rolling in the temperature range below 1000°C with a shape ratio of 3 to 10 is 20 to 50%; (2) After the completion of finish rolling, after 0.9 seconds and within 2.0 seconds, accelerated cooling is started with an average cooling rate of 30°C / s or more, and the sheet is cooled to a temperature range of 500 to 680°C, and the average cooling rate in the temperature range of 680 to 730°C is 41 to 70°C; (3) Slow cooling is performed in the temperature range of 500 to 680°C with an average cooling rate of 20°C / s or less for 2.0 to 6.0 seconds; (4) After slow cooling is complete, the process includes a step of accelerating cooling to 200°C at an average cooling rate of 31 to 67°C / s. Each step will be described below.

[0103] (1) In finish rolling in hot rolling, rough rolling and finish rolling are performed. In finish rolling, for the slab having the above-described chemical composition, the total reduction ratio in the temperature range below 1050°C is 30% or more and less than 65%, the number of rolling passes with a reduction ratio of 30% or more in the temperature range of 1000°C or higher is 3 or more, and the total reduction ratio of rolling with a shape ratio of 3 to 10 in the temperature range of 1000°C or lower is 20 to 50%. It is preferable to perform finish rolling. By setting the total reduction ratio in the temperature range below 1050°C to 30% or more and less than 60% and preferably controlling the total reduction ratio in the temperature range which is the non-recrystallization range, the average value of the aspect ratio of the prior austenite grains can be preferably controlled. Note that even if rolling with a reduction ratio of 30% or more and less than 60% is performed only in one pass in the temperature range below 1050°C, the average value of the aspect ratio of the prior austenite grains cannot be preferably controlled. Therefore, it is preferable to perform multi-stage rolling in this temperature range.

[0104] The total reduction ratio in the temperature range below 1050°C is such that the thickness of the slab at the inlet of the first rolling in the temperature range below 1050°C is t ０ and the thickness of the slab at the outlet of the last rolling in the temperature range below 1050°C is t １ When this is the case, it can be expressed by (1 - t １ / t ０ ) × 100 (%).

[0105] In finish rolling, the number of rolling passes with a reduction ratio of 30% or more in the temperature range of 1000°C or higher is preferably 3 or more. Further, the total reduction ratio of rolling with a shape ratio of 3 to 10 in the temperature range of 1000°C or lower is preferably 20 to 50%. By performing rolling with a reduction ratio of 30% or more in the temperature range of 1000°C or higher 3 or more times, the prior austenite grains in the surface layer region can be refined. Also, by setting the total reduction ratio of rolling with a shape ratio of 3 to 10 in the temperature range of 1000°C or lower to 20 to 50%, the prior austenite grains in the surface layer region can be flattened and the transformation to bainite can be promoted. By these and the cooling conditions described later, the sizes of martensite and retained austenite in the surface layer region can be reduced, and a desired amount of ferrite can be obtained in the surface layer region.

[0106] 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 (%).

[0107] The shape ratio can be expressed by the following formula (A): Shape ratio = l ｄ / h ｍ (A)

[0108] 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.

[0109] 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.

[0110] (2) After the completion of the accelerated cooling finish rolling, it is preferable to start accelerated cooling with an average cooling rate of 30°C / s or more after 0.9 seconds and within 2.0 seconds, and to cool to a temperature range of 500 to 680°C, and to cool so that the average cooling rate in the temperature range of 680 to 730°C is 41 to 70°C / s. By starting accelerated cooling with an average cooling rate of 30°C / s or more within 2.0 seconds after the completion of the finish rolling, grain growth of prior austenite grains can be suppressed. As a result, the size of martensite and retained austenite in the surface region can be reduced, and a desired amount of ferrite can be obtained in the surface region. It is preferable to carry out this accelerated cooling down to the temperature range of 500 to 680°C where slow cooling is performed. Furthermore, in the high temperature range after the completion of finish rolling, the cooling rate of accelerated cooling is suppressed from becoming too fast. Specifically, by cooling so that the average cooling rate in the temperature range of 680 to 730°C is 41 to 70°C / s, it is possible to make the prior austenite grains finer while securing the desired amount of ferrite in the surface region.

[0111] 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.

[0112] (3) Slow cooling After accelerating cooling to a temperature range of 500 to 680°C, it is preferable to slowly cool the material in this temperature range for 2.0 to 6.0 seconds at an average cooling rate of 20°C / s or less. Slow cooling is preferably carried out by air cooling, for example. This makes it possible to obtain the desired amount of bainite.

[0113] (4) Accelerated cooling after slow cooling After slow cooling is complete, it is preferable to accelerate cooling to 200°C at an average cooling rate of 31 to 67°C / s. By performing accelerated cooling under these conditions, the formation of pearlite can be suppressed and the formation of martensite can be promoted. After accelerated cooling, it is preferable to wind the material. It is more preferable to perform the accelerated cooling at an average cooling rate of 31 to 67°C / s down to a temperature range of 100°C or less.

[0114] By the method described above, the steel plates according to this embodiment can be manufactured stably.

[0115] 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.

[0116] 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.

[0117] 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 rolls above 1000°C: The number of rolls 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 rolls in the temperature range below 1000°C during finish rolling where the shape ratio is between 3 and 10. Time until accelerated cooling begins: The time from the completion of finish rolling until accelerated cooling with an average cooling rate of 30°C / s or more begins. Slow cooling time: The time spent slow cooling in the temperature range of 500 to 680°C with an average cooling rate of 20°C / s or less. Average cooling rate up to 200°C: The average cooling rate from the completion of slow cooling to 200°C (however, if the coiling temperature is above 200°C, it is the average cooling rate up to the coiling temperature). After the completion of finish rolling, accelerated cooling was performed until slow cooling began in the temperature range of 500 to 680°C. After the completion of slow cooling, accelerated cooling was performed up to the coiling temperature. In addition, in production No. 58, after the completion of finish rolling, cooling was initiated within 0.3 seconds, and the material was cooled to a temperature range of Ar3 or higher below 850°C at an average cooling rate of 200°C / s or more, and remained in that temperature range for 1.1 seconds. Then, it was cooled to a temperature range of 600°C or higher and below 750°C at an average cooling rate of 20°C / s or more, and remained in that temperature range for 1.2 seconds. However, the slow cooling time listed in the table is, as stated above, "the time spent slow cooling in the temperature range of 500 to 680°C at an average cooling rate of 20°C / s or less." The Ar3 point is the temperature at which ferrite is formed in the metal structure when the steel sheet is cooled, and it can be determined from the change in thermal expansion when the steel sheet is cooled at a cooling rate of 8°C / second.

[0118] For the obtained steel sheets, the average aspect ratio and microstructure of prior austenite grains at the 1 / 4 position of the sheet thickness, the average equivalent circle diameter of martensite and retained austenite in the surface region, the area ratio of ferrite, tensile strength, total elongation, hole expansion ratio, and impact resistance after pre-straining were evaluated using the method described above. The obtained measurement results are shown in Tables 4A and 4B.

[0119] The items in Tables 4A and 4B represent the following: B: Bainite M+γR: Martensite + retained austenite α: Ferrite P: Pearlite Aspect ratio: Average aspect ratio of prior austenite grains Equivalent circle diameter: Average equivalent circle diameter of martensite and retained austenite Δα: Difference between the ferrite area ratio of the surface region and the ferrite area ratio at 1 / 4 of the plate thickness (Δα = surface α - 1 / 4 position α) α ratio: Value obtained by dividing the ferrite area ratio at 1 / 4 of the plate thickness by the ferrite area ratio of the surface region (α ratio = 1 / 4 position α / surface α)

[0120] If the tensile strength was 980 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 980 MPa, it was judged to have insufficient strength and was deemed unacceptable.

[0121] If the total elongation was 10.0% or more, it was judged to have excellent ductility and was deemed acceptable. On the other hand, if the total elongation was less than 10.0%, it was judged to lack excellent ductility and was deemed unacceptable.

[0122] If the hole expansion ratio was 30% or more, it was judged to have excellent hole expansion properties and was deemed acceptable. On the other hand, if the hole expansion ratio was less than 30%, it was judged to have poor hole expansion properties and was deemed unacceptable.

[0123] In bending tests and drop weight tests using a punch with a punch tip radius R of 3.0 mm, if no fracture occurred in the bent portion of the test specimen, or if fracture occurred but did not penetrate in the thickness direction, the specimen was judged to have excellent impact resistance after pre-straining and was deemed to pass, and "Good" was written in the table. On the other hand, in bending tests and drop weight tests using a punch with a punch tip radius R of 3.0 mm, if fracture occurred in the bent portion of the test specimen and the fracture penetrated in the thickness direction, the specimen was judged to not have excellent impact resistance after pre-straining and was deemed to fail, and "Poor" was written in the table.

[0124]

[0125]

[0126]

[0127]

[0128]

[0129]

[0130] Tables 4A and 4B show that the steel sheet according to the present invention example has high strength, excellent ductility and hole-expanding properties, and excellent impact resistance after pre-straining. On the other hand, the steel sheet according to the comparative example is inferior in one or more of the above properties.

[0131] 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.

[0132] According to the above embodiments of this disclosure, it is possible to provide a steel sheet having high strength, excellent ductility and hole-expanding properties, and excellent impact resistance after pre-straining, as well as a part using this steel sheet.

Claims

1. The chemical composition, in mass%, is as follows: C: 0.045-0.120%, Si: 0-3.00%, Mn: 1.20-3.00%, Ti: 0.020-0.180%, 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, 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: 0-0.0010%, Ca: 0-0.0500%. 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: Fe and impurities, and 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 3.0-10.0, the area ratio of bainite is 70-95%, the sum of the area ratios of martensite and retained austenite is 5-30%, and the area ratio of pearlite is less than 5%. A steel sheet characterized in that the area ratio of ferrite is 15% or less, the average value of the equivalent circular diameter of martensite and retained austenite in the surface region, which is the region from the surface to a position 200 μm from the surface in the thickness direction of the sheet, is 3.0 μm or less, and the area ratio of ferrite is 10% or more.

2. The chemical composition is as follows, in mass%, 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: 0.0001-0.0010%, 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%, Zr: 0.001-0.500%, The steel sheet according to claim 1, characterized by containing one or more of the following: 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 steel sheet according to claim 1 or 2, characterized in that, at a position 1 / 4 of the thickness of the sheet from the surface in the thickness direction, the average value of the aspect ratio of the prior austenite grains is 3.0 to 6.

0.

4. The steel sheet according to any one of claims 1 to 3, characterized in that the area ratio of the ferrite is less than 10% at a position 1 / 4 of the thickness of the sheet in the thickness direction from the surface.

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

6. The steel sheet according to any one of claims 1 to 5, 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.

7. The steel sheet according to any one of claims 1 to 6, characterized in that the difference between the ferrite area ratio in the surface region and the ferrite area ratio at a position 1 / 4 of the plate thickness in the plate thickness direction from the surface is 2% or more.

8. The steel sheet according to any one of claims 1 to 7, characterized in that the value obtained by dividing the area ratio of the ferrite at a position 1 / 4 of the thickness of the sheet in the thickness direction from the surface by the area ratio of the ferrite in the surface region is 0.86 or less.

9. A component comprising a steel plate as described in any one of claims 1 to 8.

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