Hot-rolled steel sheet and component

Abstract

This hot-rolled steel sheet has a desired chemical composition, and in the internal region, the average aspect ratio of prior austenite grains is 2.00 or more but less than 4.00, the area ratio of martensite is 90% or more, and the area ratio of martensite is 90% or more. The value obtained by dividing the average aspect ratio of prior austenite grains in the surface layer region by the average aspect ratio of the prior austenite grains in the inner region is less than 0.950.

Description

Technical Field

[0001] This disclosure relates to hot-rolled steel sheets and components. Specifically, it relates to hot-rolled steel sheets having high strength and excellent hole-expanding properties, bending properties, and crack propagation resistance in the thickness direction, as well as components manufactured using the hot-rolled steel sheets.

[0002] This application claims priority based on Japanese Patent Application No. 2023-178181, filed on October 16, 2023, the contents of which are incorporated herein by reference. Background Technology

[0003] In recent years, lightweighting of automotive and mechanical components has been progressing. Lightweighting of automotive and mechanical components can be achieved by designing the component shape to ensure rigidity. Additionally, in blank-formed components such as stamped parts, lightweighting can be achieved by reducing the thickness of the component material.

[0004] However, high-strength materials are required to reduce sheet thickness while maintaining the strength properties of components, such as static fracture strength and yield strength. In particular, research is underway to use higher-strength steel sheets for automotive running gear components such as lower arms, connecting rods, and steering knuckles. These automotive running gear components can be manufactured by flanging, stretching flanges, and bending of the steel sheets. Therefore, the steel sheets used in these automotive running gear components require excellent formability, especially excellent hole-filling and bending properties.

[0005] Furthermore, in the aforementioned automotive running gear components, if the steel sheet is made with high strength, cracks are prone to occur during forming or in the early stages of deformation during a collision. Therefore, the steel sheets used in these automotive running gear components are also required to have excellent impact resistance.

[0006] For example, Patent Document 1 discloses a high-strength hot-rolled steel sheet, wherein at a position of 1 / 4 of the sheet thickness, it has a microstructure containing a martensitic phase of more than 95% by area and an average aspect ratio of the original austenite grains of more than 3.0, and the 5-minute relaxation stress value when 400 MPa is applied in a stress relaxation test is less than 20 MPa, and the tensile strength of the high-strength hot-rolled steel sheet is more than 1180 MPa.

[0007] Existing technical documents Patent documents Patent Document 1: Japanese Patent No. 7010418 Summary of the Invention

[0008] The problem that the invention aims to solve However, Patent Document 1 does not consider impact resistance. The inventors believe that by increasing the crack propagation resistance in the thickness direction of the plate, an improvement in impact resistance can be expected.

[0009] This disclosure was made in view of the above-mentioned issues, and its object is to provide a hot-rolled steel sheet with high strength and excellent hole expansion, bending and crack propagation resistance in the thickness direction, and components manufactured using the hot-rolled steel sheet.

[0010] Methods for solving problems The inventors have discovered that by controlling the average aspect ratio of the original austenite grains in the internal region, high strength, excellent hole expansion, and bending properties can be obtained in hot-rolled steel sheets.

[0011] Furthermore, the inventors have discovered that by making the average aspect ratio of the original austenite grains in the surface region smaller than that in the internal region, excellent bending properties and crack propagation resistance in the thickness direction can be obtained without compromising strength.

[0012] The subject of this disclosure, which is based on the above insights, is as follows.

[0013] (1) A hot-rolled steel plate, characterized in that its chemical composition, by mass%, comprises: C: 0.050~0.120% Si: 0~3.00% Mn: 1.20~3.00% Al: 0.010~0.400% P: 0~0.080%, S: 0~0.0100% N: 0~0.0050% O: 0~0.0100% 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%, 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: 0~0.050%, and Sn: 0~0.050%, The remaining portion contains Fe and impurities. The inner region is the area from 1 / 8 of the plate thickness to 3 / 8 of the plate thickness from the surface. The average aspect ratio of the original austenite grains was greater than 2.00 and less than 4.00. The martensite area ratio is over 90%. The value obtained by dividing the average aspect ratio of the original austenite grains in the surface region (i.e., the region at a depth of 1 / 15 of the plate thickness) by the average aspect ratio of the original austenite grains in the inner region is less than 0.950.

[0014] (2) The hot-rolled steel plate according to (1) above, characterized in that the above chemical composition, by mass%, contains one or more elements selected from the group consisting of: 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% Zr: 0.001~0.500%, Co: 0.001~3.000%, Zn: 0.001~0.200% W: 0.001~0.200% Sb: 0.001~0.500% As: 0.001~0.050%, and Sn: 0.001~0.050%.

[0015] (3) The hot-rolled steel sheet according to (1) or (2) above, characterized in that the value obtained by dividing the average aspect ratio of the original austenite grains in the surface region by the average aspect ratio of the original austenite grains in the internal region is 0.900 or less.

[0016] (4) The hot-rolled steel sheet according to any one of (1) to (3) above, characterized in that the average aspect ratio of the original austenite grains in the internal region is less than 3.00.

[0017] (5) A component comprising any one of (1) to (4) above.

[0018] Invention Effects According to the above-described solution of this disclosure, it is possible to provide a hot-rolled steel sheet with high strength and excellent hole expansion, bending and crack propagation resistance in the thickness direction, and components manufactured using the hot-rolled steel sheet. Detailed Implementation

[0019] Hereinafter, a hot-rolled steel sheet and component according to an embodiment of the present disclosure (hereinafter sometimes referred to as the hot-rolled steel sheet and component of this embodiment) will be described. However, the present disclosure is not limited to the configuration disclosed in this embodiment, and various modifications can be made without departing from the spirit of the present disclosure.

[0020] The various constituent elements of this disclosure will now be described in detail. First, the reasons for limiting the chemical composition of the hot-rolled steel sheet in this embodiment will be explained.

[0021] In the numerical ranges enclosed in "~", the lower and upper limits are included. Values ​​expressed as "below" or "above" are not included in the range. In the following descriptions, percentages of chemical composition are by mass unless otherwise specified.

[0022] The hot-rolled steel sheet of this embodiment contains, by mass%, the following: C: 0.050~0.120%, Si: 0~3.00%, Mn: 1.20~3.00%, Al: 0.010~0.400%, P: 0~0.080%, S: 0~0.0100%, N: 0~0.0050%, O: 0~0.0100%, and the remainder being Fe and impurities. Each element is described in detail below.

[0023] C: 0.050~0.120% Carbon (C) is an important element for improving the strength of hot-rolled steel sheets. To obtain the desired strength, the C content is set to 0.050% or more. Preferably, the C content is 0.055% or more, 0.060% or more, or 0.070% or more.

[0024] On the other hand, when the carbon content exceeds 0.120%, the hole-expanding and bending properties of hot-rolled steel sheets deteriorate. Therefore, the carbon content is set to 0.120% or less. Preferably, the carbon content is 0.110% or less or 0.100% or less.

[0025] Si: 0~3.00% Si is an element that inhibits the formation of carbides during ferrite phase transformation and improves the toughness of hot-rolled steel sheets. Since it can be free of Si, the Si content can be 0%. To reliably achieve the above effects, the Si content is preferably 0.10% or more. More preferably, the Si content is 0.50% or more, or 0.70% or more.

[0026] On the other hand, when the Si content exceeds 3.00%, the slab's crack susceptibility increases, making slab processing more difficult. Therefore, the Si content is set to 3.00% or less. Preferably, the Si content is 2.00% or less, 1.50% or less, or 1.20% or less.

[0027] Mn: 1.20~3.00% Mn is an effective element for improving the strength of hot-rolled steel sheets by enhancing hardenability and solid solution strengthening. To obtain the desired strength, the Mn content is set to 1.20% or more. Preferably, the Mn content is 1.30% or more, or 1.50% or more.

[0028] On the other hand, when the Mn content exceeds 3.00%, MnS is easily generated, which adversely affects the hole expansion and bending properties of hot-rolled steel sheets. Therefore, the Mn content is set to 3.00% or less. The preferred Mn content is 2.70% or less, 2.50% or less, or 2.20% or less.

[0029] Al: 0.010~0.400% Al plays a role in improving the quality of steel through deoxidation and also controls the ferrite phase transformation. Furthermore, when the Al content is below 0.010%, the hole-expanding properties, bending properties, and crack propagation resistance in the thickness direction of hot-rolled steel sheets deteriorate. Therefore, the Al content is set to 0.010% or more. Preferably, the Al content is 0.015% or more, or 0.020% or more.

[0030] On the other hand, when the Al content exceeds 0.400%, alumina is formed in clusters, increasing the slab's susceptibility to cracking and making slab processing more difficult. Therefore, the Al content is set to 0.400% or less. Preferably, the Al content is 0.300% or less, 0.250% or less, or 0.200% or less.

[0031] P: 0~0.080% Phosphorus (P) is an element that affects the weldability of hot-rolled steel sheets. In particular, when the P content exceeds 0.080%, the weldability of hot-rolled steel sheets deteriorates significantly. Furthermore, the slab's susceptibility to cracking increases, making slab processing more difficult. Therefore, the P content is set below 0.080%. Preferably, the P content is below 0.040%, 0.020%, or 0.010%.

[0032] The phosphorus (P) content can also be 0%. From the perspective of refining costs, the P content can be set to 0.001% or higher.

[0033] S: 0~0.0100% Sulfur (S) is an element that affects the porosity and bendability of hot-rolled steel sheets. In particular, when the S content exceeds 0.0100%, a large amount of inclusions such as MnS, which are detrimental to the porosity and bendability of hot-rolled steel sheets, are generated. Therefore, the S content is set to 0.0100% or less. The S content is preferably 0.0080% or less and 0.0060% or less.

[0034] The sulfur content can also be 0%. From the perspective of refining costs, the sulfur content can be set to 0.0001% or higher.

[0035] N: 0~0.0050% Nitrogen (N) is an element that combines with Ti to form Ti nitrides. In particular, when the N content exceeds 0.0050%, the slab's crack susceptibility increases, making slab processing more difficult. Therefore, the N content is set to 0.0050% or less. Preferably, the N content is 0.0040% or less, or 0.0030% or less.

[0036] The nitrogen content can also be 0%. From the perspective of refining costs, the nitrogen content can also be set to 0.0001% or higher.

[0037] O: 0~0.0100% O (oxide) 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. When the O content exceeds 0.0100%, brittle fracture and hydrogen-induced cracking are more likely to occur. Furthermore, the expansion capacity, bending properties, and crack propagation resistance in the thickness direction of hot-rolled steel sheets deteriorate. Therefore, the O content is set to 0.0100% or less. Preferably, the O content is 0.0080% or less, 0.0060% or less, 0.0040% or less, or 0.0035% or less.

[0038] O can be omitted, so the O content can be 0%. In order to disperse a large amount of fine oxides during the deoxidation of molten steel, the O content can be set to 0.0005% or more, or 0.0010% or more.

[0039] The hot-rolled steel sheet of this embodiment may contain the aforementioned chemical composition, with the remainder comprising Fe and impurities. In this embodiment, impurities refer to substances mixed in from the ore, scrap iron, or manufacturing environment used as raw materials, and / or substances that are permissible within a range that will not adversely affect the properties of the hot-rolled steel sheet of this embodiment.

[0040] While not essential for achieving the desired properties, the following optional elements may be included to reduce manufacturing deviations or further improve the strength of hot-rolled steel sheets. However, the inclusion of these elements is not mandatory, therefore the lower limit for their content is 0%.

[0041] Ti: 0.001~0.180% Ti precipitates in steel as carbides or nitrides, which can refine the metal structure through pinning effect and improve the strength and yield ratio of hot-rolled steel sheets through precipitation strengthening. To reliably achieve these effects, the Ti content is preferably 0.001% or more. More preferably, the Ti content is 0.005% or more, or 0.010% or more.

[0042] On the other hand, when the Ti content exceeds 0.180%, the expansion and bending properties of the hot-rolled steel sheet deteriorate due to the excessive precipitation of TiC. Therefore, the Ti content is set to 0.180% or less. The Ti content is preferably 0.160% or less or 0.150% or less.

[0043] Nb: 0.001~0.100% Nb has the effect of improving the strength of hot-rolled steel sheets by refining the grain size of the crystals and by strengthening the precipitation of NbC. To reliably achieve this effect, the Nb content is preferably 0.001% or more. More preferably, the Nb content is 0.005% or more, or 0.010% or more.

[0044] On the other hand, the aforementioned effects saturate when the Nb content exceeds 0.100%. Furthermore, the hole-expanding properties, bending properties, and crack propagation resistance in the thickness direction of hot-rolled steel sheets deteriorate. Therefore, even when Nb is present, the Nb content is set to 0.100% or less. The Nb content is preferably 0.080% or less, or 0.060% or less.

[0045] V: 0.001~1.000% V has the effect of improving the strength of hot-rolled steel sheets through strengthening by precipitates, grain refinement strengthening due to inhibition of ferrite grain growth, and dislocation strengthening due to inhibition of recrystallization. To reliably achieve these effects, the V content is preferably set to 0.001% or more. More preferably, the V content is 0.005% or more, or 0.010% or more.

[0046] On the other hand, when the V content is excessive, a large amount of carbonitrides precipitate, which deteriorates the hole-expanding and bending properties of the hot-rolled steel sheet. Therefore, the V content is set to 1.000% or less. The V content is preferably 0.800% or less or 0.600% or less.

[0047] Cu: 0.001~1.000% Cu exists in steel in the form of fine particles, which improves the strength of hot-rolled steel sheets. To reliably achieve this effect, the Cu content is preferably 0.001% or more. More preferably, the Cu content is 0.005% or more, or 0.010% or more.

[0048] On the other hand, when the Cu content is excessive, the weldability of hot-rolled steel sheets deteriorates. Therefore, the Cu content is set to 1.000% or less. The Cu content is preferably 0.800% or less or 0.600% or less.

[0049] Cr: 0.001~2.000% Cr is an effective element for improving the strength of hot-rolled steel sheets. To reliably achieve this effect, the Cr content is preferably set to 0.001% or more. More preferably, the Cr content is 0.005% or more, or 0.010% or more.

[0050] On the other hand, when the Cr content is excessive, the expansion and bending properties of hot-rolled steel sheets deteriorate. Therefore, the Cr content is set to 2.000% or less. The Cr content is preferably 1.500%, 1.200%, or 1.000% or less.

[0051] Mo: 0.001~3.000% Mo is an effective element for enhancing the precipitation of ferrite. To reliably achieve this effect, the Mo content is preferably 0.001% or more. More preferably, the Mo content is 0.005% or more or 0.010% or more.

[0052] On the other hand, when the Mo content is excessive, the slab's susceptibility to cracking increases, making slab processing more difficult. Therefore, the Mo content is set to 3.000% or less. Preferably, the Mo content is 2.500% or less, 2.000% or less, or 1.500% or less.

[0053] Ni: 0.001~0.500% Ni has the effect of suppressing phase transformation at high temperatures and improving the strength of hot-rolled steel sheets. To reliably achieve this effect, the Ni content is preferably 0.001% or more. More preferably, the Ni content is 0.005% or more, or 0.010% or more.

[0054] On the other hand, when the Ni content is excessive, the weldability of hot-rolled steel sheets deteriorates. Therefore, the Ni content is set to 0.500% or less. The Ni content is preferably 0.300% or less or 0.150% or less.

[0055] B: 0.0001~0.0100% Boron (B) has the effect of suppressing phase transformation at high temperatures and improving the strength of hot-rolled steel sheets. To reliably achieve this effect, the B content is preferably set to 0.0001% or more. More preferably, the B content is 0.0005% or more, or 0.0010% or more.

[0056] On the other hand, when the boron content is excessive, boron precipitates are formed, which reduces the strength of the hot-rolled steel sheet. Therefore, the boron content is set to 0.0100% or less. The boron content is preferably 0.0080% or less or 0.0050% or less.

[0057] Ca: 0.0001~0.0500% Ca has the effect of dispersing a large amount of fine oxides during the deoxidation of molten steel, thereby refining the microstructure of hot-rolled steel sheets. Furthermore, Ca has the effect of fixing sulfur in steel as spherical CaS and suppressing the formation of extended inclusions such as MnS, thus improving the porosity of hot-rolled steel sheets. To reliably achieve these effects, the Ca content is preferably set to 0.0001% or more. More preferably, the Ca content is 0.0005% or more, or 0.0010% or more.

[0058] On the other hand, the above effect will saturate even if the Ca content exceeds 0.0500%. Therefore, the Ca content is set to 0.0500% or less. The Ca content is preferably 0.0300% or less or 0.0200% or less.

[0059] Mg: 0.001~0.050% Mg has the effect of increasing the yield ratio of hot-rolled steel sheets by adjusting the shape of inclusions in the steel to a preferred shape. To reliably obtain this effect, the Mg content is preferably set to 0.001% or more. More preferably, the Mg content is 0.005% or more or 0.010% or more.

[0060] On the other hand, when the Mg content exceeds 0.050%, excessive inclusions are formed in the steel, reducing the yield ratio of the hot-rolled steel sheet. Therefore, the Mg content is set below 0.050%. The Mg content is preferably below 0.040% or 0.030%.

[0061] REM: 0.0001~0.1000% REM (reinforced organic matter) has the effect of increasing the yield ratio of hot-rolled steel sheets by adjusting the shape of inclusions in the steel to a preferred shape. To reliably obtain this effect, the REM content is preferably set to 0.0001% or more. The REM content is more preferably 0.0005% or more or 0.0010% or more.

[0062] On the other hand, when the REM content exceeds 0.1000%, excessive inclusions are formed in the steel, reducing the yield ratio of the hot-rolled steel sheet. Therefore, the REM content is set to 0.1000% or less. The REM content is preferably 0.0800% or less or 0.0600% or less.

[0063] Here, REM refers to a total of 17 elements, including Sc, Y, and the lanthanides. The REM content mentioned above refers to the total content of these elements. In industry, lanthanides are added as a mixture of rare earth metals.

[0064] Bi: 0.001~0.100% Bi has the effect of improving the yield ratio of hot-rolled steel sheets by refining the solidification structure. To reliably achieve this effect, the Bi content is preferably set to 0.001% or more. More preferably, the Bi content is 0.005% or more or 0.010% or more.

[0065] On the other hand, when the Bi content exceeds 0.100%, the effects described above become saturated, which is not economically preferable. Therefore, the Bi content is set to 0.100% or less. The preferred Bi content is 0.080% or less, 0.060% or less, or 0.040% or less.

[0066] Ta: 0.001~0.100% Like V, Ta has the effect of increasing the strength of hot-rolled steel sheets by forming fine carbides in the steel. To reliably obtain this effect, the Ta content is preferably 0.001% or more. More preferably, the Ta content is 0.005% or more or 0.010% or more.

[0067] On the other hand, when the Ta content exceeds 0.100%, the hole-expanding and bending properties of hot-rolled steel sheets deteriorate. Therefore, the Ta content is set to 0.100% or less. The Ta content is preferably 0.080% or less and 0.050% or less.

[0068] Zr: 0.001~0.500% Zr has the effect of increasing the strength of hot-rolled steel sheets through solid solution strengthening. To reliably obtain this effect, the Zr content is preferably set to 0.001% or more. More preferably, the Zr content is 0.005% or more or 0.010% or more.

[0069] On the other hand, when the Zr content exceeds 0.500%, the hole-expanding and bending properties of hot-rolled steel sheets deteriorate. Therefore, the Zr content is set to 0.500% or less. The Zr content is preferably 0.300% or less and 0.100% or less.

[0070] Co: 0.001~3.000% Co has the effect of increasing the strength of hot-rolled steel sheets through solid solution strengthening. To reliably obtain this effect, the Co content is preferably set to 0.001% or more. More preferably, the Co content is 0.005% or more or 0.010% or more.

[0071] On the other hand, when the Co content exceeds 3.000%, the expansion and bending properties of hot-rolled steel sheets deteriorate. Therefore, the Co content is set to 3.000% or less. The Co content is preferably 1.000% or less and 0.500% or less.

[0072] Zn: 0.001~0.200% Zn has the effect of increasing the strength of hot-rolled steel sheets through solid solution strengthening. To reliably obtain this effect, the Zn content is preferably set to 0.001% or more. More preferably, the Zn content is 0.005% or more or 0.010% or more.

[0073] On the other hand, when the Zn content exceeds 0.200%, the expansion and bending properties of hot-rolled steel sheets deteriorate. Therefore, the Zn content is set to 0.200% or less. The Zn content is preferably 0.150% or less and 0.100% or less.

[0074] W: 0.001~0.200% W has the effect of increasing the strength of hot-rolled steel sheets through solid solution strengthening. To reliably obtain this effect, the W content is preferably set to 0.001% or more. More preferably, the W content is 0.005% or 0.010% or more.

[0075] On the other hand, when the W content exceeds 0.200%, the hole-expanding and bending properties of hot-rolled steel sheets deteriorate. Therefore, the W content is set to 0.200% or less. The W content is preferably 0.150% or less and 0.100% or less.

[0076] Sb: 0.001~0.500% Sb has the effect of improving the porosity of hot-rolled steel sheets by suppressing the formation of oxides that become the initiation point of fracture. To reliably obtain this effect, the Sb content is preferably set to 0.001% or more. More preferably, the Sb content is 0.005% or more or 0.010% or more.

[0077] On the other hand, since the above-mentioned effects are saturated even with a large amount of Sb, the Sb content is set to 0.500% or less. The Sb content is preferably 0.300% or less or 0.100% or less.

[0078] As: 0.001~0.050% As has the effect of improving the porosity of hot-rolled steel sheets by refining the original austenite grains through lowering the austenite single-phase conversion temperature. To reliably achieve this effect, the As content is preferably 0.001% or more. More preferably, the As content is 0.005% or more, or 0.010% or more.

[0079] On the other hand, even with a large amount of As, the above effect becomes saturated; therefore, the As content is set to 0.050% or less. The As content is preferably 0.040% or less or 0.030% or less.

[0080] Sn: 0.001~0.050% Sn has the effect of improving the porosity of hot-rolled steel sheets by suppressing the formation of oxides that become the initiation point of fracture. To reliably achieve this effect, the Sn content is preferably 0.001% or more. More preferably, the Sn content is 0.005% or more, or 0.010% or more.

[0081] On the other hand, since the above-mentioned effect is saturated even with a large amount of Sn, the Sn content is set to 0.050% or less. The Sn content is preferably 0.040% or less or 0.030% or less.

[0082] The chemical composition of the hot-rolled steel sheet can be analyzed using a spark discharge emission spectrometer or similar device. Furthermore, C and S are determined by combustion in an oxygen stream using a gas composition analyzer and measured using infrared absorption. O and N are determined by melting a test piece collected from the steel sheet in a helium stream and measuring its thermal conductivity.

[0083] When hot-rolled steel sheets have coatings or films on their surface, the chemical composition is analyzed after removing the coatings or films by mechanical grinding or other means as needed.

[0084] Next, the metal structure of the hot-rolled steel sheet of this embodiment will be described.

[0085] For the hot-rolled steel sheet of this embodiment, in the inner region, which is the area from 1 / 8 of the sheet thickness to 3 / 8 of the sheet thickness from the surface, the average aspect ratio of the original austenite grains is 2.00 or more and less than 4.00, and the martensite area ratio is 90% or more. The value obtained by dividing the average aspect ratio of the original austenite grains in the surface region (from the surface to 1 / 15 of the sheet thickness from the surface) by the average aspect ratio of the original austenite grains in the inner region is less than 0.950.

[0086] As described above, in this embodiment, the so-called internal region refers to the region with a depth of 1 / 8 of the plate thickness from the surface to 3 / 8 of the plate thickness. In other words, it refers to the region starting from a depth of 1 / 8 of the plate thickness from the surface and ending at a depth of 3 / 8 of the plate thickness from the surface.

[0087] Furthermore, the so-called surface region refers to the area extending from the surface to a depth of 1 / 15 of the plate thickness from the surface. In other words, it refers to the region starting from the surface and ending at a depth of 1 / 15 of the plate thickness from the surface.

[0088] When hot-rolled steel sheets have coatings or platings, the term "surface" as used here refers to the interface between the steel sheet and the coatings or platings.

[0089] internal area The average aspect ratio of the original austenite grains was greater than 2.00 and less than 4.00. If the average aspect ratio of the original austenite grains in the internal region is less than 2.00, the strength of the hot-rolled steel sheet decreases, and the crack propagation resistance in the thickness direction deteriorates. Therefore, the average aspect ratio of the original austenite grains in the internal region is set to 2.00 or higher. Preferably, the average aspect ratio of the original austenite grains in the internal region is 2.20 or higher, 2.30 or higher, 2.40 or higher, or 2.60 or higher.

[0090] On the other hand, when the average aspect ratio of the original austenite grains in the internal region is 4.00 or higher, the hole-expanding and bending properties of the hot-rolled steel sheet deteriorate. Due to the deterioration in bending properties, the impact resistance also deteriorates. Therefore, the average aspect ratio of the original austenite grains in the internal region is set to be less than 4.00. Preferably, the average aspect ratio of the original austenite grains in the internal region is 3.80 or less, 3.60 or less, or 3.40 or less.

[0091] From the viewpoint of obtaining better hole expansion and bending properties in hot-rolled steel sheets, the average aspect ratio of the original austenite grains in the internal region is preferably set to be less than 3.00.

[0092] In addition, the aspect ratio of the original austenite grain refers to the value obtained by dividing the major axis of the original austenite grain by the minor axis, and is a value of 1.00 or higher.

[0093] The average aspect ratio of the original austenite grains was determined by the following method.

[0094] A sample is collected at a position one-quarter of the way from the end face in the width direction of the hot-rolled steel sheet, in a manner that allows observation of the microstructure of the cross-section (thickness direction × rolling direction) with the width direction as the normal. The sample size varies depending on the measuring device; for example, it can be a cuboid with the total thickness in the thickness direction, 15 mm in the rolling direction, and 10 mm in the width direction. Next, after mirror polishing of the observation surface, etching is performed using an etching solution (saturated aqueous solution of picric acid, aqueous solution containing surfactant and oxalic acid) according to JIS G 0551:2020, employing the Bechet-Beaujard method. Grains that appear black through etching are identified as proto-austenite grains. The observation surface revealing the proto-austenite grains was observed using an optical microscope. For the internal region (a depth from 1 / 8 of the plate thickness to 3 / 8 of the plate thickness from the surface), eight fields of view were captured at a magnification of 1000x or higher, covering a thickness of 200 μm in the plate thickness direction and a rolling direction of 600 μm. Based on the captured microstructure photographs, the ratio of the major axis to the minor axis measured for each proto-austenite grain was calculated, and the average value was calculated by weighting the areas of each proto-austenite grain, thus obtaining the average aspect ratio of the proto-austenite grains. For example, when the major axis / minor axis ratio of a proto-austenite grain G1 is r1 and the area is A1, and the major axis / minor axis ratio of another proto-austenite grain G2 is r2 and the area is A2, the average aspect ratio of the two proto-austenite grains is calculated as (A1×r1+A2×r2) / (A1+A2).

[0095] When the original austenite grains cannot be fully revealed by the above methods, the original austenite grains are determined by the reconstruction method described in "A Study on the High-Precision Reconstruction Method of Austenite Structure of Steel" (Kengo Hata, Masayuki Wakita, Tomoya Fujiwara, Kaori Kono, Nippon Steel & Sumitomo Metal Technical Report No. 404 (2016), pp. 24-30), and the average aspect ratio of the original austenite grains is obtained.

[0096] The EBSD measurement data used for the reconstruction method were obtained through the following methods.

[0097] After performing colloidal grinding or electrolytic grinding on the above-mentioned field of view (200 μm in the thickness direction and 600 μm in the rolling direction), the crystal orientation information was obtained by electron backscatter diffraction at a measurement interval of 0.1 μm.

[0098] The measurements were performed using an EBSD analysis apparatus consisting of a thermal field emission scanning electron microscope (JEOL JSM-7200F) and an EBSD detector (EDAX Velocity (registered trademark) ultra-high-speed action EBSD detector). The vacuum level within the apparatus was set to 9.6 × 10⁻⁶. -5 Below Pa, the accelerating voltage is set to 25kV and the irradiation current level is set to 16.

[0099] In the display of EBSD spectra, the obtained crystal orientation information is obtained using OIM Analysis (registered trademark) version 7 or higher manufactured by EDAX / TSL solution.

[0100] Furthermore, when the sample contains pre-austenite grains with an equivalent circle diameter of less than 2 μm, these grains are excluded from the above-described measurements. This is because pre-austenite grains with an equivalent circle diameter of less than 2 μm do not adversely affect the properties of the hot-rolled steel sheet according to this embodiment.

[0101] In addition, the rolling direction of hot-rolled steel sheets is determined by the following method.

[0102] Test pieces were collected in a manner that allowed observation of the thickness section of the hot-rolled steel sheet. The test piece was rotated 30 degrees at a time, with the direction perpendicular to the sheet surface as the Z-axis, for a total of 12 pieces. The thickness section of the collected test pieces was ground, and the aforementioned etching solution was used to expose the original austenite grain boundaries. The average aspect ratio of the original austenite grains was calculated using a cut-off method. The test piece with the largest average aspect ratio of the original austenite grains was identified, and the direction in which this test piece was collected was determined as the rolling direction of the hot-rolled steel sheet. That is, the direction of the test piece, parallel to the thickness section and perpendicular to the thickness direction, was determined as the rolling direction of the hot-rolled steel sheet.

[0103] Martensite area ratio: over 90% Martensite is the microstructure that improves the strength of hot-rolled steel sheets. If the martensite area ratio is less than 90%, the desired strength cannot be obtained. Therefore, the martensite area ratio is set to 90% or more. Preferably, the martensite area ratio is 92% or more, 94% or more, or 96% or more. The martensite area ratio can also be 100%.

[0104] Remaining tissue: less than 10% In the internal metal structure of the hot-rolled steel sheet of this embodiment, in addition to martensite, bainite, ferrite, pearlite, and retained austenite may also be included as residual microstructure. Considering the relationship with the area ratio of martensite, the area ratio of the residual microstructure can also be 10% or less. Furthermore, considering the relationship with the area ratio of martensite, the area ratio of the residual microstructure can also be set to 8% or less, 6% or less, or 4% or less. Since residual microstructure may not be included, the area ratio of the residual microstructure can also be 0%.

[0105] The area ratio of martensite and remaining tissue was determined by the following method.

[0106] Test specimens were collected from hot-rolled steel sheets to allow observation of the microstructure at a location 1 / 4 of the sheet thickness (ranging from 1 / 8 to 3 / 8 of the sheet thickness in the thickness direction from the surface). The sheet thickness section was finished by mirror grinding, then etched using LePera, and the 200 μm (thickness direction) × 600 μm (perpendicular to the thickness direction) region at the 1 / 4 thickness location was observed and analyzed using a FE-SEM (thermal field emission scanning electron microscope, JEOL JSM-7001F).

[0107] In LePera corrosion, 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 region.

[0108] The area ratio of the retained austenite was obtained by X-ray diffraction.

[0109] For test pieces collected from hot-rolled steel plates, they were ground to a position 1 / 4 of the plate thickness from the surface (within the range of 1 / 8 to 3 / 8 of the plate thickness from the surface in the thickness direction), with the exposed surface serving as the observation surface. This observation surface was then mirror-polished and further finished by electrolytic polishing. The integrated intensities of five peaks (α(200), α(211), γ(200), γ(220), and γ(311)) were calculated on the observation surface using Rigaku RINT-2500 and Mo-Kα, and the volume fraction of retained austenite was calculated using the intensity averaging method. This volume fraction of retained austenite was considered as the area fraction of retained austenite.

[0110] The total martensite area ratio is obtained by subtracting the area ratio of retained austenite obtained by X-ray diffraction from the total area ratio of martensite and retained austenite obtained by observation using the above Fe-SEM. In the calculation, when the total martensite area ratio is negative, it is set to 0%.

[0111] The area ratio of pearlite was obtained by the following method.

[0112] For the same region (200μm×600μm) as when calculating the area ratio of martensite and retained austenite, after removing only the corrosion layer by grinding and performing mirror finishing, etching was performed using nitric acid ethanol solution, and the images were observed and analyzed using FE-SEM.

[0113] The area ratio of pearlite is obtained by identifying regions where cementite and ferrite are arranged in layers as pearlite and calculating the area ratio of these regions.

[0114] The area ratio of ferrite was obtained using the following method. Additionally, for regions other than those identified as pearlite using the above method, the following operation was performed.

[0115] For the same region (200 μm × 600 μm) used to determine the area ratio of martensite and retained austenite, colloidal or electrolytic polishing was performed, and crystal orientation information was obtained by electron backscatter diffraction at measurement intervals of 0.2 μm. An EBSD analysis apparatus consisting of a thermal field emission scanning electron microscope (JEOL JSM-7200F) and an EBSD detector (EDAX Velocity (registered trademark) ultra-high-speed action EBSD detector) was used. The vacuum level within the apparatus was set to 9.6 × 10⁻⁶. -5 Below Pa, the accelerating voltage is set to 25kV and the irradiation current level is set to 16.

[0116] The obtained crystal orientation information was analyzed using OIM Analysis (registered trademark) version 7 or later manufactured by EDAX / TSL Solution. At this point, measurement points with a crystal orientation difference of 15 degrees or more were considered grain boundaries, and the regions enclosed by these grain boundaries were considered grains. Next, the differences in crystal orientation between all measurement points within a grain were calculated, and the average of these differences was calculated to obtain the grain's GAM (Grain Average Misorientation) value. Grains with a GAM value of 0.5° or less were considered ferrite, and their area fraction was calculated to obtain the ferrite area fraction.

[0117] The area ratio of bainite is obtained by subtracting the area ratios of martensite, retained austenite, pearlite, and ferrite obtained above from 100%. In the calculation, when the area ratio of bainite is negative, the area ratio of bainite is set to 0%.

[0118] In this embodiment, since the area fraction of the metal structure is calculated through FE-SEM image analysis, X-ray diffraction, and EBSD analysis, the sum of the individual structures is sometimes not equal to 100%. In this case, the area fraction of each structure is corrected to make the sum 100%. For example, if the sum of the area fractions of each structure is 103%, the area fraction of each structure is corrected by multiplying the area fraction of each structure by "100 / 103".

[0119] In addition, the observation conditions for FE-SEM are as follows.

[0120] Electron gun types: thermal emission type Current Irradiation Number: 9 WD (working distance): 10mm Accelerating voltage: 20kV Objective aperture number: 4 Pixel count: 5120×3840 The value obtained by dividing the average aspect ratio of the original austenite grains in the surface region by the average aspect ratio of the original austenite grains in the internal region is less than 0.950. When the value obtained by dividing the average aspect ratio of the original austenite grains in the surface region by the average aspect ratio of the original austenite grains in the internal region is 0.950 or higher, the desired bending properties and crack propagation resistance in the thickness direction cannot be obtained in hot-rolled steel sheets. Therefore, the value obtained by dividing the average aspect ratio of the original austenite grains in the surface region by the average aspect ratio of the original austenite grains in the internal region is set to be lower than 0.950. The value obtained by dividing the average aspect ratio of the original austenite grains in the surface region by the average aspect ratio of the original austenite grains in the internal region is preferably 0.930 or lower and 0.900 or lower.

[0121] The lower limit of the value obtained by dividing the average aspect ratio of the original austenite grains in the surface region by the average aspect ratio of the original austenite grains in the internal region is not particularly limited, and can be set to above 0.800 or above 0.850.

[0122] The average aspect ratio of the original austenite grains in the surface region (the region from the surface to a depth of 1 / 15 of the plate thickness from the aforementioned surface) was measured using the same method as when measuring the average aspect ratio of the original austenite grains in the internal region.

[0123] Tensile strength (TS): ≥1180MPa The tensile strength of the hot-rolled steel sheet in this embodiment can be 1180 MPa or higher. More preferably, the tensile strength is 1200 MPa or higher. By setting the tensile strength to 1180 MPa or higher, the application components are not limited, and this greatly contributes to vehicle body lightweighting.

[0124] There is no need to specifically limit the upper limit of tensile strength, but from the perspective of mold wear suppression, it can be set to below 1500MPa or below 1300MPa.

[0125] Hole expansion ratio (λ): 50% or more The hole expansion ratio of the hot-rolled steel sheet in this embodiment can be 50% or more. Preferably, the hole expansion ratio is 55% or more, or 60% or more.

[0126] Tensile strength was evaluated by tensile testing according to JIS Z 2241:2022. The test piece was designated as test piece No. 5 according to JIS Z 2241:2022. The tensile test piece was collected from the end of the plate in the width direction at a distance of 1 / 4, and the direction perpendicular to the rolling direction could be taken as the length direction.

[0127] When it is impossible to collect a No. 5 test piece due to the small size or complex shape of the hot-rolled steel plate or component, a small strip with a parallel portion of arbitrary width can be collected and used to conduct a tensile test to determine the tensile strength. The orientation of the small strip is defined as the length direction perpendicular to the rolling direction.

[0128] The porosity was determined by porosity testing in accordance with JIS Z 2256:2020.

[0129] Maximum bending angle: 55° or more In the hot-rolled steel sheet of this embodiment, the maximum bending angle obtained by bending test based on the following VDA standard can be 55 degrees or more. The maximum bending angle is preferably 60 degrees or more.

[0130] The test piece used for the bending test is a 60mm (rolling direction) × 30mm (width direction) test piece taken from a hot-rolled steel plate. Using this test piece, the bending test is performed according to the VDA standard (VDA238-100:2017-04) specified by the German Association of the Automotive Industry, under the following conditions.

[0131] In addition, when the thickness of the test piece exceeds 2.5 mm, the surface on the punch side is ground to reduce the thickness to 2.5 mm before a bending test is performed.

[0132] Furthermore, when the thickness of the test piece is less than 2.5 mm, the maximum bending angle obtained from the following formula is used. However, in the following formula, α t The value represents the maximum bending angle obtained through the bending test, t represents the plate thickness, and uEL represents the uniform elongation (total elongation at maximum test force). The uniform elongation is a value obtained by performing a tensile test using the method described above.

[0133] The maximum bending angle when the plate thickness is less than 2.5mm is α. t -13.852×(1-t / 2.5)×(uEL+0.22) 0.292 Test piece dimensions: 60mm (rolling direction) × 30mm (width direction) Curved edge: a direction parallel to the width direction Test method: roller support, punch pressing Roller diameter: 30mm Punch shape: Front end R=0.4mm Roller spacing: 2.0 × plate thickness (mm) + 0.5mm Pressing speed: 20mm / min Testing machine: SHIMADZU AUTOGRAPH 20kN Impact resistance characteristics: W > 5.0 × 10 5 (°•N) In this embodiment, the impact resistance of the hot-rolled steel sheet is evaluated by the crack propagation resistance in the thickness direction. The crack propagation resistance in the thickness direction is obtained by calculating the energy W from the bending angle-load curve during the VDA bending test. In this embodiment, W of the hot-rolled steel sheet can exceed 5.0 × 10⁻⁶. 5 (°•N).

[0134] Here, W is the value obtained by integrating the load with respect to the bending angle (∫Fdα). F is the load, and α is the bending angle obtained from the bending test based on the VDA standard described above. Furthermore, the bending test was conducted with a test piece thickness of 2.5 mm, and other conditions as described above. When the test piece thickness exceeded 2.5 mm, the punch side surface was ground to a thickness of 2.5 mm before the bending test was performed.

[0135] Furthermore, when the thickness of the test piece is 2.5 mm or less, W is obtained from the following formula. Wherein, in the following formula, W... t This represents the energy obtained from the bending angle-load curve during the VDA bending test, and t represents the plate thickness.

[0136] When the plate thickness is less than 2.5mm, W = W t × (2.5 / t) 2.02 The thickness of the hot-rolled steel sheet in this embodiment is not particularly limited, but can be set to 1.2 to 8.0 mm. If the thickness of the hot-rolled steel sheet is less than 1.2 mm, it is difficult to ensure the rolling end temperature, and the rolling load becomes too large, making hot rolling sometimes difficult. Therefore, the thickness of the hot-rolled steel sheet in this embodiment can be set to 1.2 mm or more. Preferably, it is 1.4 mm or more.

[0137] On the other hand, when the plate thickness exceeds 8.0 mm, it is sometimes difficult to obtain the aforementioned metallic structure after hot rolling. Therefore, the plate thickness can also be set to 8.0 mm or less. Preferably, it is 6.0 mm or less.

[0138] For the purpose of improving corrosion resistance, the hot-rolled steel sheet of this embodiment, having the above-mentioned chemical composition and metallic structure, can also be coated on its surface to form a surface-treated steel sheet. The coating can be an electroplated coating or a hot-dip coating. Examples of electroplated coatings include electroplating with zinc and electroplating with Zn-Ni alloys. Examples of hot-dip coatings include hot-dip galvanizing, alloyed hot-dip galvanizing, hot-dip aluminizing, hot-dip Zn-Al alloying, hot-dip Zn-Al-Mg alloying, and hot-dip Zn-Al-Mg-Si alloying. There are no particular limitations on the amount of coating applied, and it can be the same as in the past.

[0139] Alternatively, appropriate chemical conversion treatments (such as coating and drying with silicate-based chromium-free chemical conversion solutions) can be performed after plating to further improve corrosion resistance.

[0140] Because the hot-rolled steel sheet of this embodiment has high strength and excellent hole-expanding properties, bending properties, and crack propagation resistance in the thickness direction, it is suitable for use in components, especially automotive components. In automotive components, it is also suitable for use in automotive running gear components such as lower arms, connecting rods, and steering knuckles. These automotive components can be made solely of the hot-rolled steel sheet of this embodiment, or they can be formed by joining the hot-rolled steel sheet of this embodiment with other steel sheets.

[0141] The component manufactured using the hot-rolled steel sheet according to this embodiment has the same chemical composition as the steel sheet described above. Furthermore, the component may contain a mixture of processed and unprocessed portions. The unprocessed portions have the same metallic structure as the steel sheet described above. The processed portions have a metallic structure substantially the same as the steel sheet described above, but sometimes, due to heavy processing or at the ends of the component, they may lack the aforementioned metallic structure, or it may be difficult to determine. Therefore, when measuring the metallic structure of the component, the ends are avoided, and the unprocessed portions are measured. When no unprocessed portions exist, the portions that have not undergone heavy processing are measured. Unprocessed or unprocessed portions refer to, for example, the flat portions of the component, portions where the sheet thickness increases or decreases with processing, and portions that avoid punching, reaming, and bending processes. As an example, in the case of the aforementioned component, a test piece is collected from near the center of gravity of the flat portion with the largest area, and the analysis is conducted.

[0142] Next, a preferred manufacturing method for the hot-rolled steel sheet of this embodiment will be described. According to the following manufacturing method, the hot-rolled steel sheet of this embodiment can be manufactured stably. Furthermore, in this embodiment, the temperatures of the slab and the steel sheet refer to the surface temperatures of the slab and the steel sheet, respectively.

[0143] The preferred manufacturing method of the hot-rolled steel sheet in this embodiment includes the following steps: (1) A process of subjecting a slab with the above chemical composition to one or more strains with a total strain of 3 to 15% in the width direction before rough rolling; (2) A finishing rolling process for a slab subjected to strain, wherein the total reduction rate in a temperature range below 1040°C exceeds 30% but is less than 46%, and the reduction rate in a temperature range of 1020~1040°C is 20~30%; and (3) After finishing rolling, the process of accelerating cooling to 200°C at an average cooling rate of 30°C / s or higher.

[0144] The following is a description of each process.

[0145] (1) Strain imparted before rough rolling Before rough rolling, it is preferable to apply one or more strains to the slab having the above chemical composition, with a total strain in the width direction of 3 to 15%. This allows for the preferred control of the ratio of the average aspect ratio of the original austenite grains in the internal region to that in the surface region. The strain can be applied after the slab has been heated for rough rolling.

[0146] In addition, when the width direction length of the slab before the first strain is applied is set as w0, and the width direction length of the slab after the last strain is applied is set as w1, the total strain applied to the width direction of the slab can be expressed as (1-w1 / w0)×100 (%).

[0147] As a method of applying strain in the width direction of the slab, one example that can be listed is using a roller that is set so that its axis of rotation is perpendicular to the surface of the slab to apply strain.

[0148] Furthermore, there are no particular restrictions on the slabs subjected to strain, except for having the aforementioned chemical composition. For example, slabs produced by continuously casting can be manufactured using steel with the aforementioned chemical composition smelted in a converter or electric furnace. Alternatively, ingot casting or thin slab casting can be used instead of continuous casting. During the slab heating before rough rolling, the heating temperature can be set in the range of 1100–1300°C.

[0149] (2) Finish rolling For finish rolling, it is preferable to roll the strained slab with a total reduction of more than 30% and less than 46% in a temperature range below 1040°C, and a reduction of 20% to 30% in a temperature range of 1020 to 1040°C. By ensuring that the total reduction in the temperature range below 1040°C is more than 30% and less than 46%, it is possible to preferably control the average aspect ratio of the original austenite grains in the internal regions.

[0150] For the total reduction rate in the temperature region below 1040°C, when the initial rolling thickness of the inlet plate in the temperature region below 1040°C is set as t0 and the final rolling thickness of the outlet plate in the temperature region below 1040°C is set as t1, it can be expressed as (1-t1 / t0)×100 (%).

[0151] Furthermore, in finish rolling, by performing rolling with a reduction rate of 20-30% in a temperature range of 1020-1040°C, it is possible to preferably control the ratio of the average aspect ratio of the original austenite grains in the internal region to the average aspect ratio of the original austenite grains in the surface region.

[0152] In addition, when the thickness of the inlet side plate is set to t2 and the thickness of the outlet side plate is set to t3, the reduction rate can be expressed as (1-t3 / t2)×100 (%).

[0153] (3) Accelerate cooling to 200℃ After finishing rolling, it is preferable to accelerate cooling to 200°C at an average cooling rate of 30°C / s or higher. By accelerating cooling under these conditions, the desired amount of martensite can be obtained.

[0154] Furthermore, the so-called average cooling rate in this embodiment refers to the value obtained by dividing the temperature difference between the start and end points of a set range by the elapsed time from the start to the end point.

[0155] Example Next, the effects of one solution of this disclosure will be further illustrated through embodiments. However, the conditions in the embodiments are examples of conditions used to confirm the feasibility and effects of this disclosure, and this disclosure is not limited to these examples. Various conditions can be used to achieve the purpose of this disclosure as long as they do not depart from its main theme.

[0156] Steels with the chemical compositions shown in Tables 1 and 2 were smelted and continuously cast to produce slabs with a thickness of 240–300 mm. Using the obtained slabs, hot-rolled steel sheets as shown in Table 4 were obtained under the manufacturing conditions shown in Table 3. The thickness of the obtained hot-rolled steel sheets was 1.2–8.0 mm.

[0157] For the obtained hot-rolled steel sheet, the average aspect ratio and microstructure of the original austenite grains in the internal region, the average aspect ratio of the original austenite grains in the surface region, tensile strength, porosity, maximum bending angle, and impact resistance (W) are determined by the above method.

[0158] Furthermore, for the maximum bending angle and W, when the thickness of the hot-rolled steel plate is less than 2.5 mm, the values ​​corrected by the above formula are used.

[0159] In the internal metal structure of hot-rolled steel plates, in addition to martensite, there are also examples of bainite, ferrite, pearlite and retained austenite as residual structures.

[0160] The results of the measurements are shown in Table 4.

[0161] Evaluation methods for the properties of hot-rolled steel sheets When the tensile strength reaches 1180 MPa or higher, it is judged as a high-strength hot-rolled steel sheet and is deemed acceptable. On the other hand, when the tensile strength is lower than 1180 MPa, it is judged as a non-high-strength hot-rolled steel sheet and is deemed unacceptable.

[0162] When the expansion rate is 50% or higher, the hot-rolled steel sheet is deemed acceptable as having excellent expansion properties. Conversely, when the expansion rate is less than 50%, the hot-rolled steel sheet is deemed unacceptable as not having excellent expansion properties.

[0163] When the maximum bending angle reaches 55% or more, the hot-rolled steel sheet is deemed acceptable as having excellent bending properties. On the other hand, when the maximum bending angle is less than 55%, the hot-rolled steel sheet is deemed unacceptable as not having excellent bending properties.

[0164] When W exceeds 5.0 × 10 5 At (°•N), it is judged as qualified as a hot-rolled steel sheet with excellent crack propagation resistance in the thickness direction and excellent impact resistance. On the other hand, when W is 5.0×10 5 In cases where the temperature is below (°•N), the hot-rolled steel sheet is deemed unqualified as it does not possess excellent impact resistance properties.

[0165] As shown in Table 4, the hot-rolled steel sheet of the present invention has high strength and excellent hole expansion, bending and crack propagation resistance in the thickness direction.

[0166] On the other hand, it can be seen that any one or more of the above-mentioned characteristics of the hot-rolled steel sheet of the comparative example have deteriorated.

[0167] In addition, for all embodiments, the lower arm (part) was manufactured by stamping. The flat portion of the lower arm was evaluated using the same method as described above. The measurement and evaluation results are the same as those shown in Table 4.

[0168] Industrial availability According to the above-described solution of this disclosure, it is possible to provide a hot-rolled steel sheet with high strength and excellent hole expansion, bending and crack propagation resistance in the thickness direction, as well as components manufactured using the hot-rolled steel sheet.

Claims

1. A hot-rolled steel plate, characterized in that, Its chemical composition, expressed as a percentage by mass, includes: C：0.050~0.120%、 Si: 0~3.00% Mn: 1.20~3.00% Al：0.010~0.400%、 P：0~0.080%、 S：0~0.0100%、 N：0~0.0050%、 O：0~0.0100%、 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%, 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: 0~0.050%, and Sn: 0~0.050%, The remaining portion contains Fe and impurities. The inner region is defined as the area from a depth of 1 / 8 of the plate thickness to a depth of 3 / 8 of the plate thickness from the surface. The average aspect ratio of the original austenite grains was greater than 2.00 and less than 4.

00. The martensite area ratio is over 90%. The value obtained by dividing the average aspect ratio of the original austenite grains in the surface region (i.e., the region at a depth of 1 / 15 of the plate thickness) by the average aspect ratio of the original austenite grains in the inner region is less than 0.

950.

2. The hot-rolled steel plate according to claim 1, characterized in that, The chemical composition, expressed as a percentage by mass, contains one or more elements selected from the group consisting of: 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% Zr：0.001~0.500%、 Co: 0.001~3.000%, Zn: 0.001~0.200% W：0.001~0.200%、 Sb: 0.001~0.500% As: 0.001~0.050%, and Sn: 0.001~0.050%.

3. The hot-rolled steel plate according to claim 1 or 2, characterized in that, The value obtained by dividing the average aspect ratio of the original austenite grains in the surface region by the average aspect ratio of the original austenite grains in the inner region is 0.900 or less.

4. The hot-rolled steel plate according to any one of claims 1 to 3, characterized in that, The average aspect ratio of the original austenite grains in the internal region is less than 3.

00.

5. A component comprising the hot-rolled steel sheet according to any one of claims 1 to 4.

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