High-strength steel sheet

Abstract

Provided is a high-strength steel sheet including a plate-thickness center portion having a prescribed chemical composition and a microstructure including tempered martensite: 85% or more, and a surface layer soft portion formed on one side or both sides of the plate-thickness center portion, the surface layer soft portion having a thickness of more than 10 μm and 5% or less of the plate thickness, and having a microstructure including ferrite: 80% or more, and including an internal oxidation layer having a thickness of 3 μm or more from the surface, the average Vickers hardness (Hc) of the plate-thickness center portion and the average Vickers hardness (Hs) of the surface layer soft portion satisfying Hs / Hc ≤ 0.50, and the void area ratio in a region from the surface to a depth position of 10 μm being 3.0% or less.

Description

Technical Field

[0001] This invention relates to high-strength steel plates. Background Technology

[0002] Increasing the strength of steel plates reduces their machinability, making it generally difficult to achieve a balance between strength and machinability in steel plates. For example, the booms of cranes used in construction machinery tend to be longer and thinner due to the increasing height of buildings in recent years, thus requiring higher strength as a means of weight reduction. Furthermore, when steel plates are used in components such as booms, bending processes are performed, increasing the demand for high-strength steel plates with excellent bending machinability.

[0003] In the automotive industry, there is a demand for lightweight vehicle bodies from the perspective of improving fuel efficiency. To balance lightweighting and crash safety, using high-strength steel sheets is one effective method, leading to the development of high-strength steel sheets. Generally speaking, high-strength steel sheets have lower formability, such as bending workability, compared to mild steel sheets, and sometimes cannot be formed using the same methods as mild steel sheets. Therefore, in the field of automotive steel sheets, there is a high demand for high-strength steel sheets with excellent bending workability.

[0004] Patent Document 1 describes a high-strength steel sheet having a thick central portion and a soft surface portion formed on one or both sides of the thick central portion. In the cross-section of the high-strength steel sheet, the microstructure of the thick central portion, by area ratio, contains at least 85% tempered martensite, and the microstructure of the soft surface portion, by area ratio, contains at least 65% ferrite and at least 5% pearlite and less than 20%, etc. The average spacing between the pearlite particles in the soft surface portion is at least 3 μm, and the Vickers hardness (Hc) of the thick central portion and the Vickers hardness (Hs) of the soft surface portion satisfy 0.50 ≤ Hs / Hc ≤ 0.75. Furthermore, Patent Document 1 describes how distributing pearlite as a hard microstructure in the soft surface portion simultaneously improves the bending load and bending resistance of the steel sheet.

[0005] Patent Document 2 discloses a high-strength steel plate characterized by having a tensile strength of 800 MPa or more, comprising a central portion of the plate thickness and surface softening portions disposed on one or both sides of the central portion of the plate thickness. Each surface softening portion has a thickness of more than 10 μm and less than 30% of the plate thickness. The average Vickers hardness of the surface softening portions is less than 0.60 times the average Vickers hardness at half the plate thickness, and the standard deviation of the nanohardness of the surface softening portions is less than 0.8. Furthermore, Patent Document 2 teaches that by suppressing the hardness unevenness of the surface softening portions in addition to having surface softening portions, the bending performance is significantly improved.

[0006] Patent Document 3 discloses a high-strength hot-rolled steel sheet characterized by having a prescribed chemical composition, with over 90% of its microstructure being martensite, and an average aspect ratio of 3 or more and 20 or less for the original austenite grains in the cross-section along the rolling direction, extending from the surface to 1 / 8 of the sheet thickness. Furthermore, Patent Document 3 describes a high-strength hot-rolled steel sheet with a yield strength of 950 MPa or more, exhibiting excellent bending workability and wear resistance, based on the aforementioned structure.

[0007] Patent documents 4-10 disclose a high-strength galvanized steel sheet, which is a galvanized steel sheet having a hot-dip galvanized layer or an alloyed hot-dip galvanized layer on the surface of a base steel sheet. From the interface between the base steel sheet and the galvanized layer toward the base steel sheet, the sheet sequentially comprises: an internal oxide layer containing at least one oxide selected from the group consisting of Si and Mn; a soft layer containing the internal oxide layer, wherein when the thickness of the base steel sheet is set to t, the Vickers hardness satisfies 90% or less of the Vickers hardness at t / 4 of the base steel sheet; and a specified hard layer, wherein the average depth D of the soft layer is 20 μm or more, and the average depth d of the internal oxide layer is 4 μm or more and less than D, and the tensile strength is 980 MPa or more. Furthermore, patent documents 4-10 teach that by controlling the average depth d of the internal oxide layer to be more than 4 μm and utilizing the internal oxide layer as a hydrogen trapping site, hydrogen embrittlement can be effectively suppressed. In particular, by appropriately controlling the relationship between the average depth d of the internal oxide layer and the average depth D of the soft layer in the region containing the internal oxide layer, flexibility is improved.

[0008] Existing technical documents

[0009] Patent documents

[0010] Patent Document 1: International Publication No. 2020 / 196060

[0011] Patent Document 2: International Publication No. 2018 / 151331

[0012] Patent Document 3: Japanese Patent Application Publication No. 2014-227583

[0013] Patent Document 4: International Publication No. 2016 / 111271

[0014] Patent Document 5: International Publication No. 2016 / 111272

[0015] Patent Document 6: International Publication No. 2016 / 111273

[0016] Patent Document 7: International Publication No. 2016 / 111274

[0017] Patent Document 8: International Publication No. 2016 / 111275

[0018] Patent Document 9: International Publication No. 2015 / 146692

[0019] Patent Document 10: International Publication No. 2015 / 005191 Summary of the Invention

[0020] The problem that the invention aims to solve

[0021] As previously proposed, a soft layer can be applied to the surface of a steel sheet to improve its bending workability. However, applying a soft layer to the surface of a steel sheet generally reduces surface hardness, sometimes leading to deterioration in appearance due to defects and reduced wear resistance. Relatedly, Patent Document 3 teaches that by setting the average aspect ratio of the original austenite grains from the surface to 1 / 8 of the sheet thickness to 3 or more and 20 or less, a steel sheet with improved surface hardness and excellent bending workability can be obtained. However, Patent Document 3 may not have adequately studied the microstructure control in the surface portion other than the average aspect ratio of the original austenite grains; therefore, in the invention described in Patent Document 3, there is still room for improvement in terms of bending workability and surface hardness.

[0022] Therefore, the object of the present invention is to provide a high-strength steel sheet with improved bending workability and suppression of defect generation.

[0023] Methods for solving problems

[0024] The inventors discovered that, in order to achieve the above-mentioned objective, in a high-strength steel plate with a tensile strength of 1250 MPa or more, by providing a soft surface layer with an average Vickers hardness that is in a predetermined proportion relative to the average Vickers hardness of the center of the plate thickness, the bending workability is improved. Furthermore, an internal oxide layer with a predetermined thickness is formed in the outermost part of this soft surface layer, thereby controlling the voids formed near the surface layer within a suitable range. This improves the surface hardness and suppresses the generation of defects on the surface of the steel plate, thus completing the present invention.

[0025] The present invention, which achieves the above objectives, is described below.

[0026] (1) A high-strength steel plate comprising a central portion of plate thickness and a soft surface portion formed on one or both sides of the central portion of plate thickness.

[0027] The central portion of the aforementioned plate thickness has the following chemical composition, expressed as a percentage by mass:

[0028] C: 0.10~0.30%

[0029] Si: 0.01~2.50%

[0030] Mn: 0.10~10.00%

[0031] P: below 0.100%

[0032] S: Below 0.0500%

[0033] Al: 0–1.50%

[0034] N: below 0.0100%

[0035] O: Below 0.0060%

[0036] Cr: 0–2.00%

[0037] Mo: 0–1.00%

[0038] B: 0~0.0100%

[0039] Ti: 0-0.30%

[0040] Nb: 0–0.30%

[0041] V: 0~0.50%

[0042] Cu: 0–1.00%

[0043] Ni: 0~1.00%

[0044] Ca: 0–0.040%

[0045] Mg: 0–0.040%

[0046] REM: 0–0.040%, and

[0047] The remaining portion consists of Fe and impurities.

[0048] The formula satisfies 1.50 ≤ [Si] + [Mn] + [Al] + [Cr] ≤ 20.00, where [Si], [Mn], [Al] and [Cr] are the contents (mass%) of each element.

[0049] It also has a microstructure containing more than 85% tempered martensite by area ratio.

[0050] The aforementioned soft surface layer has a thickness exceeding 10 μm and less than 5.0% of the plate thickness.

[0051] It also has a microstructure containing more than 80% ferrite by area ratio.

[0052] It also includes an internal oxide layer with a thickness of 3 μm or more extending from the surface of the aforementioned high-strength steel plate.

[0053] The average Vickers hardness (Hc) of the central part of the aforementioned plate thickness and the average Vickers hardness (Hs) of the aforementioned soft surface part satisfy Hs / Hc ≤ 0.50.

[0054] The porosity of the region from the surface of the high-strength steel plate to a depth of 10 μm is less than 3.0%.

[0055] (2) The high-strength steel plate according to (1) above, wherein the central portion of the plate thickness has a microstructure composed of the following in terms of area ratio:

[0056] Tempered martensite: 85% or more

[0057] At least one of ferrite, bainite, pearlite, and retained austenite: totaling less than 15%, and

[0058] Martensite in quenched state: less than 5%.

[0059] (3) The high-strength steel plate according to (1) or (2) above, wherein the soft surface portion has a microstructure composed of the following in terms of area ratio:

[0060] Ferrite: 80% or more

[0061] At least one of tempered martensite, bainite, and retained austenite: total less than 20%,

[0062] Pearlite: less than 5%, and

[0063] Martensite in quenched state: less than 5%.

[0064] (4) The high-strength steel plate according to any one of (1) to (3) above, wherein the surface of the soft part of the surface layer further includes a hot-dip galvanized layer, an alloyed hot-dip galvanized layer or an electro-galvanized layer.

[0065] Invention Effects

[0066] According to the present invention, a high-strength steel sheet with improved bending workability and suppressed defect formation can be provided. Because of its high resistance to defect formation, such a high-strength steel sheet can maintain its appearance well, making it very useful for applications such as skeletal components in automobiles, particularly those referred to as quasi-outer panel parts, requiring high strength, design, and aesthetics. Furthermore, because of its high surface hardness, such a high-strength steel sheet also exhibits excellent wear resistance, making it ideal for applications such as crane booms for construction machinery, where high bending workability and wear resistance are required in addition to high strength. Detailed Implementation

[0067] High-strength steel plate

[0068] The high-strength steel plate according to embodiments of the present invention is characterized in that it includes a central portion of plate thickness and a soft surface portion formed on one or both sides of the central portion of plate thickness.

[0069] The central portion of the aforementioned plate thickness has the following chemical composition, expressed as a percentage by mass:

[0070] C: 0.10~0.30%

[0071] Si: 0.01~2.50%

[0072] Mn: 0.10~10.00%

[0073] P: below 0.100%

[0074] S: Below 0.0500%

[0075] Al: 0–1.50%

[0076] N: below 0.0100%

[0077] O: Below 0.0060%

[0078] Cr: 0–2.00%

[0079] Mo: 0–1.00%

[0080] B: 0~0.0100%

[0081] Ti: 0-0.30%

[0082] Nb: 0–0.30%

[0083] V: 0~0.50%

[0084] Cu: 0–1.00%

[0085] Ni: 0~1.00%

[0086] Ca: 0–0.040%

[0087] Mg: 0–0.040%

[0088] REM: 0–0.040%, and

[0089] The remaining portion consists of Fe and impurities.

[0090] The formula satisfies 1.50 ≤ [Si] + [Mn] + [Al] + [Cr] ≤ 20.00, where [Si], [Mn], [Al] and [Cr] are the contents (mass%) of each element.

[0091] It also has a microstructure containing more than 85% tempered martensite by area ratio.

[0092] The aforementioned soft surface layer has a thickness exceeding 10 μm and less than 5.0% of the plate thickness.

[0093] It also has a microstructure containing more than 80% ferrite by area ratio.

[0094] It also includes an internal oxide layer with a thickness of 3 μm or more extending from the surface of the aforementioned high-strength steel plate.

[0095] The average Vickers hardness (Hc) of the central part of the aforementioned plate thickness and the average Vickers hardness (Hs) of the aforementioned soft surface part satisfy Hs / Hc ≤ 0.50.

[0096] The porosity of the region from the surface of the high-strength steel plate to a depth of 10 μm is less than 3.0%.

[0097] As previously described, while the bending workability can be improved by depositing a soft layer on the surface of the steel sheet, the surface hardness is generally reduced due to this soft layer, which sometimes leads to deterioration in appearance and reduced wear resistance caused by defects. Therefore, the inventors have studied the microstructure of the outermost layer and the area near the surface of the soft layer in high-strength steel sheets with a tensile strength of 1250 MPa or more, in addition to the soft layer on one or both sides of the center of the sheet thickness. More specifically, the inventors first discovered that by making the microstructure of the soft layer of a specified thickness contain ferrite at a surface area ratio of 80% or more, and by controlling the average Vickers hardness (Hs) of the soft layer and the average Vickers hardness (Hc) of the center of the sheet thickness in a manner that satisfies the formula Hs / Hc ≤ 0.50, the bending workability of the high-strength steel sheet can be significantly improved. Furthermore, the inventors have focused on the internal oxide layer formed on the outermost layer of the steel sheet during annealing after rolling (typically hot rolling and cold rolling), which is formed by the combination of easily oxidizable components (e.g., Si, Al, etc.) in the steel sheet with oxygen in the annealing atmosphere, and the voids that sometimes form near the surface in conjunction with other manufacturing conditions. As a result, the inventors discovered that by setting the internal oxide layer containing oxides such as Si or Al to a thickness of 3 μm or more from the surface of the steel sheet, and by controlling the area ratio of voids formed near the surface, more specifically, the area ratio of voids in the region from the surface of the steel sheet to a depth of 10 μm, to 3.0% or less, in addition to significantly increasing the surface hardness of the steel sheet, the generation of defects on the surface of the steel sheet can be significantly suppressed.

[0098] While not intentionally bound by any particular theory, it is believed that the internal oxide particles present in the internal oxide layer act as obstacles to dislocations in the steel, thereby pinning dislocation movement and increasing the surface hardness of the steel sheet. To explain in more detail, a dislocation generally refers to a linear crystal defect, but deformation of steel generally occurs when iron atoms near a dislocation within the steel are rearranged by external forces, causing the dislocation to move. Here, it is believed that if an internal oxide layer with a specified thickness, specifically more than 3 μm thick from the surface of the steel sheet (at the interface between the coating and the steel sheet if a coating is present), is formed in the surface layer of the steel sheet, a large number of fine oxide particles are dispersed within it. These internal oxide particles act as obstacles, hindering dislocation movement, resulting in increased surface hardness of the steel sheet. On the other hand, while the surface hardness increases simply by forming an internal oxide layer, it is sometimes impossible to reliably prevent defects such as cracking and peeling.

[0099] In this further study, the inventors discovered that when a certain amount of voids exist near the surface, these voids can sometimes become the starting point for defects such as peeling and cracking when the steel plate is subjected to certain external forces. By controlling the void area ratio in the region from the steel plate surface to a depth of 10 μm to 3.0% or less, the generation of such defects can be reliably suppressed. Therefore, the high-strength steel plate according to the embodiments of the present invention can be used well, for example, in applications requiring excellent bending workability and high resistance to defects, such as high-strength steel plates for automobiles, and further, in construction machinery components requiring excellent bending workability and wear resistance, such as crane booms. Hereinafter, the high-strength steel plate according to the embodiments of the present invention will be described in more detail.

[0100] [Chemical composition of the center of the plate]

[0101] First, the chemical composition of the center of the plate thickness will be explained. Near the boundary between the center of the plate thickness and the surface soft layer, the chemical composition may sometimes differ from that of locations sufficiently far from the boundary due to diffusion of alloying elements from the surface soft layer. In such cases, the chemical composition of the center of the plate thickness described below refers to the chemical composition measured near the 1 / 2 position of the plate thickness. Furthermore, in the following description, the unit of content for each element, "%", refers to "mass %" unless otherwise specified. Additionally, in this specification, the "~" indicating a numerical range is used to mean the lower and upper limits, including the values ​​listed before and after it, unless otherwise specified.

[0102] [C: 0.10~0.30%]

[0103] Carbon (C) is an effective element for ensuring a specified amount of tempered martensite and improving the strength of steel sheets. To fully achieve these effects, the C content is 0.10% or more. The C content can also be 0.12% or more, 0.14% or more, 0.16% or more, or 0.18% or more. On the other hand, if the C content is excessive, ductility and / or bending workability may decrease. Therefore, the C content is 0.30% or less. The C content can also be 0.28% or less, 0.26% or less, 0.24% or less, or 0.22% or less.

[0104] [Si: 0.01~2.50%]

[0105] Silicon (Si) is an effective element for ensuring hardenability. Furthermore, Si also inhibits alloying with Al. To fully achieve these effects, the Si content is 0.01% or more. The Si content can also be 0.05% or more, 0.10% or more, 0.15% or more, or 0.30% or more. On the other hand, if the Si content is excessive, the center of the sheet may become brittle, reducing bending workability. Therefore, the Si content is 2.50%. The Si content can also be 2.20% or less, 2.10% or less, 2.00% or less, 1.80% or less, or 1.50% or less.

[0106] [Mn: 0.10~10.00%]

[0107] Manganese (Mn) is an element that acts as a deoxidizer. Furthermore, Mn is also effective in improving hardenability. To fully obtain these effects, the Mn content is 0.10% or more. The Mn content can also be 0.20% or more, 0.50% or more, 0.80% or more, or 1.00% or more. On the other hand, if the Mn content is excessive, coarse Mn oxides can sometimes form in the steel, reducing the tensile strength of the steel sheet. Therefore, the Mn content is 10.00% or less. The Mn content can also be 9.00% or less, 8.00% or less, 6.00% or less, or 5.00% or less.

[0108] [P: below 0.100%]

[0109] Phosphorus (P) is an element introduced during the manufacturing process. The P content can also be 0%. However, refining to reduce the P content to below 0.0001% requires time, leading to reduced productivity. Therefore, the P content can also be 0.0001% or more, 0.0005% or more, 0.001% or more, or 0.005% or more. On the other hand, excessive P content can sometimes segregate in the center of the steel plate, reducing toughness. Therefore, the P content is 0.100% or less. The P content can also be 0.080% or less, 0.060% or less, 0.040% or less, or 0.020% or less.

[0110] [S: below 0.0500%]

[0111] Sulfur (S) is an element introduced during the manufacturing process. The S content can also be 0%. However, refining to reduce the S content to below 0.0001% requires time, leading to reduced productivity. Therefore, the S content can also be 0.0001% or more, 0.0005% or more, or 0.0010% or more. On the other hand, excessive S content can sometimes form coarse MnS, reducing the toughness of the steel plate. Therefore, the S content is 0.0500% or less. The S content can also be 0.0400% or less, 0.0300% or less, 0.0200% or less, or 0.0100% or less.

[0112] [A1: 0-1.50%]

[0113] Aluminum (Al) acts as a deoxidizer in steel, stabilizing ferrite. The Al content can be 0%, but to achieve this effect, an Al content of 0.001% or more is preferred. The Al content can also be 0.01% or more, 0.02% or more, or 0.03% or more. On the other hand, excessive Al content can lead to the formation of coarse Al oxides, resulting in a decrease in the tensile strength of the steel sheet, and / or insufficient formation of tempered martensite. Therefore, the Al content is 1.50% or less. The Al content can also be 1.40% or less, 1.30% or less, 1.00% or less, or 0.80% or less.

[0114] [N: below 0.0100%]

[0115] Nitrogen (N) is an element introduced during the manufacturing process. The N content can also be 0%. However, refining to reduce the N content to below 0.0001% requires time, leading to reduced productivity. Therefore, the N content can also be 0.0001% or more, 0.0005% or more, or 0.0010% or more. On the other hand, excessive N content can sometimes form coarse nitrides, reducing the bending workability and / or toughness of the steel sheet. Therefore, the N content is 0.0100% or less. The N content can also be 0.0080% or less, 0.0060% or less, or 0.0050% or less.

[0116] [O: below 0.0060%]

[0117] Oxygen (O) is an element introduced during the manufacturing process. The O content can also be 0%. However, refining to reduce the O content to below 0.0001% requires time, leading to reduced productivity. Therefore, the O content can also be 0.0001% or more, 0.0005% or more, or 0.0010% or more. On the other hand, excessive O content can sometimes form large inclusions, reducing the toughness of the steel plate. Therefore, the O content is 0.0060% or less. The O content can also be 0.0050% or less, 0.0045% or less, or 0.0040% or less.

[0118] The basic chemical composition of the plate thickness center portion according to the embodiments of the present invention is as described above. Furthermore, the plate thickness center portion may, as needed, contain at least one of the following optional elements to replace a portion of the remaining Fe. For example, the plate thickness center portion may contain at least one element selected from the group consisting of Cr: 0–2.00%, Mo: 0–1.00%, and B: 0–0.0100%. Additionally, the plate thickness center portion may contain at least one element selected from the group consisting of Ti: 0–0.30%, Nb: 0–0.30%, and V: 0–0.50%. Furthermore, the plate thickness center portion may contain at least one element selected from the group consisting of Cu: 0–1.00% and Ni: 0–1.00%. Furthermore, the plate thickness center portion may contain at least one element selected from the group consisting of Ca: 0–0.040%, Mg: 0–0.040%, and REM: 0–0.040%. These optional elements will be described in detail below.

[0119] [Cr: 0~2.00%]

[0120] Chromium (Cr) is an effective element for improving hardenability and thus increasing the strength of steel sheets. The Cr content can be 0%, but to achieve this effect, a Cr content of 0.001% or more is preferred. The Cr content can also be 0.01% or more, 0.10% or more, or 0.20% or more. On the other hand, if the Cr content is excessive, Cr may segregate in the center of the steel sheet thickness, forming coarse Cr carbides, which reduces the tensile strength of the steel sheet. Therefore, the Cr content is preferably 2.00% or less. The Cr content can also be 1.80% or less, 1.00% or less, or 0.50% or less.

[0121] [Mo: 0~1.00%]

[0122] Molybdenum (Mo), like chromium (Cr), is an effective element for increasing the strength of steel sheets. The Mo content can be 0%, but to achieve this effect, a Mo content of 0.001% or more is preferred. The Mo content can also be 0.01% or more, 0.05% or more, or 0.10% or more. On the other hand, excessive Mo content can sometimes lead to the formation of coarse Mo carbides, reducing the cold workability of the steel sheet. Therefore, the Mo content is preferably 1.00% or less. The Mo content can also be 0.90% or less, 0.80% or less, or 0.60% or less.

[0123] [B: 0~0.0100%]

[0124] Boron (B) is an effective element for increasing the strength of steel sheets. The B content can be 0%, but to achieve this effect, a B content of 0.0001% or more is preferred. The B content can also be 0.0005% or more, 0.0010% or more, or 0.0015% or more. On the other hand, excessive B content can sometimes reduce toughness and / or weldability. Therefore, the B content is preferably 0.0100% or less. The B content can also be 0.0080% or less, 0.0060% or less, or 0.0040% or less.

[0125] [Ti: 0-0.30%]

[0126] Titanium (Ti) is an effective element for controlling the morphology of carbides and also promotes the increase of ferrite strength. The Ti content can be 0%, but to achieve these effects, the Ti content is preferably 0.001% or more. The Ti content can also be 0.005% or more, 0.01% or more, or 0.02% or more. On the other hand, if the Ti content is excessive, coarse oxides or nitrides can sometimes form in the steel, reducing the workability of the steel sheet. Therefore, the Ti content is preferably 0.30% or less. The Ti content can also be 0.20% or less, 0.15% or less, or 0.10% or less.

[0127] [Nb: 0~0.30%]

[0128] Niobium (Nb), like Ti, is an element effective in controlling the morphology of carbides and contributes to improving the toughness of steel plates by refining the microstructure through a pinning effect. The Nb content can be 0%, but to achieve these effects, the Nb content is preferably 0.001% or more. The Nb content can also be 0.005% or more, 0.01% or more, or 0.02% or more. On the other hand, if the Nb content is excessive, fine and hard Nb carbides may precipitate in large quantities, leading to decreased ductility as the strength of the steel plate increases, thus reducing the workability of the steel plate. Therefore, the Nb content is preferably 0.30% or less. The Nb content can also be 0.20% or less, 0.15% or less, or 0.10% or less.

[0129] [V: 0~0.50%]

[0130] Vanadium (V), like Ti and Nb, is an effective element for controlling the morphology of carbides and contributes to improving the toughness of steel sheets by refining the microstructure through a pinning effect. The V content can be 0%, but to achieve these effects, the V content is preferably 0.001% or more. The V content can also be 0.005% or more, 0.01% or more, or 0.02% or more. On the other hand, if the V content is excessive, a large amount of finely precipitated V carbides may occur, leading to decreased ductility as the strength of the steel sheet increases, thus reducing the workability of the steel sheet. Therefore, the V content is preferably 0.50% or less. The V content can also be 0.30% or less, 0.20% or less, or 0.10% or less.

[0131] [Cu: 0~1.00%]

[0132] Copper (Cu) is an effective element for improving the strength of steel sheets. The Cu content can be 0%, but to achieve this effect, a Cu content of 0.001% or more is preferred. The Cu content can also be 0.01% or more, 0.03% or more, or 0.05% or more. On the other hand, excessive Cu content can sometimes lead to red-hot brittleness and reduce productivity in hot rolling. Therefore, the Cu content is preferably 1.00% or less. The Cu content can also be 0.80% or less, 0.60% or less, or 0.40% or less.

[0133] [Ni: 0~1.00%]

[0134] Nickel (Ni), like Cu, is an effective element for improving the strength of steel sheets. The Ni content can be 0%, but to achieve this effect, a Ni content of 0.001% or more is preferred. The Ni content can also be 0.01% or more, 0.03% or more, or 0.05% or more. On the other hand, excessive Ni content can sometimes reduce ductility and thus decrease the workability of the steel sheet. Therefore, the Ni content is preferably 1.00% or less. The Ni content can also be 0.80% or less, 0.60% or less, or 0.40% or less.

[0135] [Ca: 0~0.040%]

[0136] Calcium (Ca) is an element whose sulfide form can be controlled by adding it in trace amounts. The Ca content can be 0%, but to achieve this effect, the Ca content is preferably 0.0001% or more. The Ca content can also be 0.0005% or more, 0.001% or more, or 0.005% or more. On the other hand, if the Ca content is excessive, coarse Ca oxides can sometimes be formed, reducing the workability of the steel sheet. Therefore, the Ca content is preferably 0.040% or less. The Ca content can also be 0.030% or less, 0.020% or less, or 0.015% or less.

[0137] [Mg: 0-0.040%]

[0138] Magnesium (Mg), like calcium (Ca), is an element whose sulfide morphology can be controlled by adding trace amounts. The Mg content can be 0%, but to achieve this effect, the Mg content is preferably 0.0001% or more. The Mg content can also be 0.0005% or more, 0.001% or more, or 0.005% or more. On the other hand, if the Mg content is excessive, large inclusions may sometimes form, reducing the workability of the steel sheet. Therefore, the Mg content is preferably 0.040% or less. The Mg content can also be 0.030% or less, 0.020% or less, or 0.015% or less.

[0139] [REM: 0~0.040%]

[0140] Rare earth metals (REMs), like Ca and Mg, are elements whose sulfide morphology can be controlled by adding them in trace amounts. The REM content can be 0%, but to achieve this effect, the REM content is preferably 0.0001% or more. The REM content can also be 0.0005% or more, 0.001% or more, or 0.005% or more. On the other hand, excessive REM content can sometimes generate large inclusions, reducing the workability of the steel sheet. Therefore, the REM content is preferably 0.040% or less. The REM content can also be 0.030% or less, 0.020% or less, or 0.015% or less. In this specification, REM refers to the total content of these 17 elements: scandium (Sc) (atomic number 21), yttrium (Y) (atomic number 39), and the lanthanides (La, atomic number 57) to lutetium (Lu, atomic number 71).

[0141] (other)

[0142] Furthermore, the central portion of the plate thickness may intentionally or unavoidably contain the following elements, which will not hinder the effects of the present invention. These elements are W: 0-0.10%, Ta: 0-0.10%, Co: 0-0.50%, Sn: 0-0.050%, Sb: 0-0.050%, As: 0-0.050%, and Zr: 0-0.050%. The content of these elements may also be 0.0001% or more, or 0.001% or more, respectively.

[0143] In the central portion of the plate thickness in the embodiment of the present invention, the remaining portion other than the aforementioned elements is composed of Fe and impurities. Impurities are components that are mixed in during the industrial manufacturing of steel plates or their central portions due to various factors in the manufacturing process, such as raw materials like ore or scrap iron.

[0144] [1.50≤[Si]+[Mn]+[Al]+[Cr]≤20.00]

[0145] The chemical composition of the central portion of the plate thickness in the embodiments of the present invention must satisfy the following formula.

[0146] 1.50≤[Si]+[Mn]+[Al]+[Cr]≤20.00

[0147] In the formula, [Si], [Mn], [Al], and [Cr] represent the content (mass%) of each element. As previously explained, in the high-strength steel sheet of the embodiments of the present invention, the internal oxide layer formed in the outermost layer is extremely important in improving the surface hardness of the steel sheet. This internal oxide layer is mainly formed in the outermost layer of the steel sheet by the combination of easily oxidized components in the steel sheet, such as Si, Mn, Al, and Cr, with oxygen in the annealing atmosphere during the annealing process after cold rolling. Therefore, in order to form an internal oxide layer of sufficient thickness for improving the surface hardness of the steel sheet, specifically from the surface of the steel sheet to a thickness of 3 μm or more, these elements must be present in a certain total amount in the steel. For the chemical composition of the center thickness of the sheet in the embodiments of the present invention, the content of each alloying element is controlled within the range previously explained, and is controlled such that the total content of Si, Mn, Al, and Cr is ≥1.50%, i.e., [Si]+[Mn]+[Al]+[Cr]≥1.50. By appropriately combining the chemical composition of the central portion of such a thick plate with, in particular, the conditions of the annealing treatment, it is possible to reliably form an internal oxide layer with a thickness of 3 μm or more. As a result, high surface hardness can be achieved, suppressing the formation of defects on the steel plate surface, and excellent wear resistance can be achieved.

[0148] The total content of Si, Mn, Al, and Cr can also be 1.60% or more, 1.70% or more, 1.80% or more, 1.90% or more, 2.00% or more, 2.20% or more, or 2.50% or more. On the other hand, if the total content of Si, Mn, Al, and Cr is too high, although it may not have an adverse effect from the viewpoint of promoting the formation of internal oxides and thus increasing surface hardness, the properties associated with each alloying element may sometimes decrease due to the excessively high content of each alloying element. Therefore, the total content of Si, Mn, Al, and Cr is set to 20.00% or less. For example, the total content of Si, Mn, Al, and Cr can also be 15.00% or less, 12.00% or less, 10.00% or less, 9.00% or less, 8.00% or less, or 7.00% or less.

[0149] [Microstructure of the center of the plate thickness]

[0150] [Tempered martensite: 85% or higher]

[0151] The microstructure of the central portion of the plate thickness contains at least 85% tempered martensite by area. Tempered martensite is a high-strength and tough microstructure. In embodiments of the present invention, by having the previously specified chemical composition, particularly a C content of 0.10% or more, and containing at least 85% tempered martensite in the central portion of the plate thickness, high tensile strength, specifically 1250 MPa or more, can be reliably achieved. The area ratio of tempered martensite can also be 86% or more, 88% or more, or 90% or more. There is no particular upper limit to the area ratio of tempered martensite, and it can also be 100%. For example, the area ratio of tempered martensite can also be 98% or less, 96% or less, or 94% or less.

[0152] [At least one of ferrite, bainite, pearlite, and retained austenite: total less than 15%]

[0153] The microstructure of the central portion of the plate thickness may contain any other microstructure as long as it meets the requirement of containing at least 85% tempered martensite in terms of area ratio. Although not particularly limited, for example, in the central portion of the plate thickness, the total area ratio of at least one of ferrite, bainite, pearlite and retained austenite is preferably set to be less than 15%.

[0154] Ferrite, being a soft microstructure, is easily deformable, contributing to improved ductility of steel sheets. Therefore, from the viewpoint of improving ductility, the microstructure in the center of the sheet thickness can also contain ferrite. However, the interface between the hard structure of tempered martensite and the soft structure of ferrite can become a fracture initiation point, thus, excessive ferrite inclusion can sometimes reduce the porosity of the steel sheet. Furthermore, bainite, being hard, contributes to improved strength of steel sheets. Therefore, from the viewpoint of improving strength, the microstructure in the center of the sheet thickness can also contain bainite. However, while excessive bainite inclusion increases the strength of the steel sheet, it can sometimes reduce the uniformity of the microstructure, thereby reducing the porosity of the steel sheet. Bainite can be any of the following: upper bainite with carbides between laths, lower bainite with carbides within laths, bainitic ferrite without carbides, or granular bainitic ferrite where the lath boundaries of bainite become indistinct, or a mixture of these.

[0155] Pearlite is a hard microstructure consisting of soft ferrite and hard cementite arranged in layers, contributing to increased strength in steel sheets. Therefore, from the viewpoint of improving steel sheet strength, the microstructure in the center of the sheet thickness can also include pearlite. However, the interface between soft ferrite and hard cementite can become a fracture initiation point, thus excessive pearlite content can sometimes reduce the porosity of the steel sheet. Furthermore, retained austenite contributes to increased ductility in steel sheets through the process-induced transformation (TRIP) effect. Therefore, from the viewpoint of improving steel sheet ductility, the microstructure in the center of the sheet thickness can also include retained austenite. On the other hand, retained austenite transforms into quenched martensite through process-induced transformation, thus excessive retained austenite content can sometimes reduce the porosity of the steel sheet.

[0156] By controlling the total area fraction of at least one of ferrite, bainite, pearlite, and retained austenite to less than 15%, the disadvantages of excessive inclusion of these structures, and more specifically, the reduction in porosity unrelated to the purpose of the present invention, can be reliably avoided. On the other hand, the additional effects caused by these structures can be fully manifested. The total area fraction of at least one of ferrite, bainite, pearlite, and retained austenite may also be 0%, but may be, for example, 1% or more, 3% or more, 4% or more, or 5% or more. Furthermore, the total area fraction of at least one of ferrite, bainite, pearlite, and retained austenite may also be 14% or less, 12% or less, 11% or less, or 10% or less.

[0157] [Quenched martensite content: less than 5%]

[0158] Quenched martensite refers to untempered martensite, i.e., martensite without carbides. Quenched martensite is a very hard microstructure. Therefore, the area fraction of quenched martensite can be 0%, but from the viewpoint of increasing strength, it can be 1% or more, or 2% or more. On the other hand, quenched martensite is also a brittle microstructure, so from the viewpoint of ensuring higher toughness, the area fraction of quenched martensite is preferably set to less than 5%. The area fraction of quenched martensite can also be 4% or less, or 3% or less.

[0159] [Identification of microstructure and calculation of area ratio in the center of the plate thickness]

[0160] [Tempered martensite and bainite]

[0161] The identification of the microstructure and calculation of the area ratio in the center of the plate thickness are performed as follows. First, a sample with a plate thickness section parallel to the rolling direction of the steel plate is collected, and this section is used as the observation surface. This observation surface is etched with nitric acid ethanol reagent, and a 100 μm × 100 μm area centered at 1 / 4 of the plate thickness from the steel plate surface is defined as the observation area. This observation area is observed using a field emission scanning electron microscope (FE-SEM) at magnifications of 1000–50000. The tempered martensite and bainite are identified based on the position and arrangement of cementite contained within the microstructure in this observation area, as follows. Regarding tempered martensite, cementite is present within the martensite laths, but there are more than two crystal orientations for the martensite laths and cementite, and cementite has multiple variants; therefore, tempered martensite can be identified. The area ratio of the tempered martensite identified in this way is calculated using the point counting method (according to ASTM E562). On the other hand, the presence of bainite can occur in two states: cementite or retained austenite exists at the interface of lath-shaped bainitic ferrite, and cementite exists within the lath-shaped bainitic ferrite. When cementite or retained austenite exists at the interface of lath-shaped bainitic ferrite, the interface is clear, thus bainite can be identified. Furthermore, when cementite exists within the lath-shaped bainitic ferrite, since the crystal orientation relationship between bainitic ferrite and cementite is the same, and the cementite has the same variant, bainite can be identified. The area ratio of bainite identified in this way is calculated using a point counting method.

[0162] [Ferrite]

[0163] First, a sample with a cross-section parallel to the rolling direction of the steel plate is collected and used as the observation surface. A 100μm × 100μm region centered at a point one-quarter of the plate thickness from the surface of the steel plate is defined as the observation area. This observation area is then observed using a scanning electron microscope at magnifications ranging from 1000 to 50000 to obtain electron channel contrast images. Electron channel contrast imaging is a method of detecting contrast differences by using the difference in crystal orientation within the grains. The portion with uniform contrast in this electron channel contrast image is ferrite. The area fraction of the ferrite identified in this way is calculated using a point counting method.

[0164] [Pearlite]

[0165] The areas etched with nitric acid ethanol reagent, as described in relation to tempered martensite and bainite, were observed using an optical microscope at magnifications ranging from 1000 to 50000. Areas with dark contrast in the observed images were identified as pearlite. The area fraction of the identified pearlite was calculated using a point counting method.

[0166] [Retained austenite]

[0167] The volume fraction of retained austenite was determined by X-ray diffraction. First, the sample collected as described above was removed from the surface of the steel plate down to one-quarter of the plate thickness by mechanical and chemical grinding, exposing the surface at one-quarter of the plate thickness from the surface. The exposed surface was irradiated with MoKα rays, and the integral intensity ratios of the diffraction peaks of the (200) and (211) planes of the bcc phase, and the (200), (220), and (311) planes of the fcc phase were determined. The volume fraction of retained austenite was calculated from these integral intensity ratios. The standard five-peak method was used for this calculation. The calculated volume fraction of retained austenite was then defined as the area fraction of retained austenite.

[0168] [Quenched martensite]

[0169] First, the same observation surface used in the ferrite identification was etched with Lepera solution, and the same area as the ferrite identification area was designated as the observation area. During Lepera etching, martensite and retained austenite were not etched. Therefore, the observation area etched with Lepera solution was observed using FE-SEM, and the un-etched areas were identified as martensite and retained austenite. The total area ratio of the identified martensite and retained austenite was calculated using a point counting method. Next, the area ratio of the quenched martensite was determined by subtracting the previously determined area ratio of retained austenite from this total area ratio.

[0170] [Soft surface layer]

[0171] The soft surface layer formed on one or both sides of the aforementioned plate thickness center has a thickness of more than 10 μm and less than 5.0% of the plate thickness, and has an average Vickers hardness (Hs) less than 0.50 times the average Vickers hardness (Hc) of the plate thickness center (i.e., Hs / Hc ≤ 0.50). By having a thickness of more than 10 μm and satisfying Hs / Hc ≤ 0.50, the effect of providing a soft surface layer on one or both sides of the steel plate can be reliably achieved, resulting in a significant improvement in the bending workability of the steel plate. For example, to further improve the bending workability, the thickness of the soft surface layer can also be more than 15 μm, more than 20 μm, more than 25 μm, more than 30 μm, more than 35 μm, or more than 40 μm. Furthermore, the thickness of the soft surface layer can also be less than 4.5%, less than 4.0%, less than 3.5%, less than 3.0%, or less than 2.5% of the plate thickness. When a soft surface layer is formed on both sides of the center portion of the sheet thickness, the thickness of the soft surface layer on one side can be the same as or different from the thickness of the soft surface layer on the other side. Similarly, to further improve the bending workability, the ratio (Hs / Hc) of the average Vickers hardness (Hs) of the soft surface layer to the average Vickers hardness (Hc) of the center portion of the sheet thickness can be less than 0.50, 0.49, 0.48, 0.47, 0.46, or 0.45. There is no particular limitation on the lower limit of Hs / Hc, but for example, Hs / Hc can be more than 0.20, 0.25, or 0.30. When a soft surface layer is formed on both sides of the center portion of the sheet thickness, the Hs / Hc of the soft surface layer on one side can be the same as or different from the Hs / Hc of the soft surface layer on the other side.

[0172] In this invention, the "further increase in the thickness of the soft surface layer," the "average Vickers hardness (Hc) of the center of the plate thickness," and the "average Vickers hardness (Hs) of the soft surface layer" are determined as follows. For the Vickers hardness test, it is performed according to JIS Z2244-1:2020. First, the Vickers hardness at the position of 1 / 2 of the plate thickness is measured with an indentation load of 10g. Then, from that position, on a line perpendicular to the plate thickness and parallel to the rolling direction, a total of 3 or more, for example 5 or 10 points, are measured with an indentation load of 10g. The average value of these measurements is determined as the average Vickers hardness (Hc) of the center of the plate thickness. The interval between each measurement point is preferably set to a distance of 4 times or more of the indentation. A distance of 4 times or more of the indentation means a distance of 4 times or more of the length of the diagonal of the rectangular opening of the indentation produced by the diamond indenter during the Vickers hardness measurement. Next, surface analysis is performed using glow discharge. The apparatus (GDS) measures the carbon concentration along the depth direction from the surface. The region from the surface to half the average carbon concentration (carbon content in the center of the plate thickness) of the parent phase, where the carbon concentration gradually increases, is defined as the surface soft layer. The thickness (μm) of the surface soft layer and its proportion (%) in the plate thickness are determined. Within this defined surface soft layer, 10 Vickers hardness points are randomly measured with an indentation load of 10g, and the average Vickers hardness (Hs) of the surface soft layer is determined by calculating their average value. In the case where surface soft layers are formed on both sides of the center of the plate thickness, the thickness and average Vickers hardness (Hs) of the surface soft layer on the other side are determined by performing the same measurements as described above.

[0173] [Microstructure of the soft surface layer]

[0174] [Ferrite content: 80% or higher]

[0175] The microstructure of the soft surface layer contains ferrite of 80% or more by area. Ferrite, being a soft material, is easily deformable. Therefore, by including 80% or more ferrite in the soft surface layer, high bending workability can be achieved. The area fraction of ferrite can also be 82% or more, 85% or more, 87% or more, or 90% or more. There is no particular upper limit to the area fraction of ferrite, and it can also be 100%. For example, the area fraction of ferrite can also be 98% or less, 96% or less, or 94% or less.

[0176] [At least one of tempered martensite, bainite, and retained austenite: total less than 20%]

[0177] The microstructure of the surface soft layer may contain any other microstructure as long as it meets the requirement of containing ferrite of 80% or more in terms of area ratio. Although not particularly limited, for example, in the surface soft layer, the total area ratio of at least one of tempered martensite, bainite, and retained austenite is preferably set to be less than 20%.

[0178] Tempered martensite and bainite are hard microstructures. Furthermore, retained austenite undergoes a phase transformation induced by processing, transforming into hard, quenched martensite. Therefore, from the viewpoint of further improving the bending workability of the steel sheet, the total area percentage of at least one of tempered martensite, bainite, and retained austenite can be, for example, 18% or less, 16% or less, 14% or less, or 12% or less. The total area percentage of at least one of tempered martensite, bainite, and retained austenite can also be 0%, but can be, for example, 1% or more, 3% or more, 5% or more, 8% or more, or 10% or more.

[0179] [Pearlite: less than 5%]

[0180] As described above, the microstructure of the soft surface layer, by containing ferrite of 80% or more in terms of area ratio, achieves sufficiently high bending workability. However, from the viewpoint of further improving the bending workability of the steel sheet, the area ratio of the hard structure, namely pearlite, is preferably set to less than 5%. The area ratio of pearlite may also be 4.5% or less, 4% or less, or 3% or less. On the other hand, there is no particular limitation on the lower limit of the area ratio of pearlite, and it may also be 0%. For example, the area ratio of pearlite may also be 1% or more, or 2% or more.

[0181] [Quenched martensite content: less than 5%]

[0182] Similar to the case of pearlite, from the viewpoint of further improving the bending workability of the steel sheet, the area ratio of the hard structure, i.e., the quenched martensite, is preferably set to be less than 5%. The area ratio of the quenched martensite can also be less than 4% or less, or less than 3%. On the other hand, there is no particular limitation on the lower limit of the area ratio of the quenched martensite, and it can also be 0%. For example, the area ratio of the quenched martensite can also be more than 1% or more, or more than 2%.

[0183] [Identification of microstructures and calculation of area ratio in the soft surface layer]

[0184] The identification of the microstructure and the calculation of the area ratio in the soft surface region were performed as follows. First, a specimen with a thickness section parallel to the rolling direction of the steel plate was collected, and this section was used as the observation surface. Within this observation surface, defined as the soft surface region, multiple observation areas were randomly selected in a manner consistent with the thickness direction. The total area of ​​these observation areas was set to 2.0 × 10⁻⁶. -9 m 2The identification of microstructures other than retained austenite and the calculation of area ratio are the same as those for the identification of microstructures in the center of the plate thickness, except for the difference in the observation area.

[0185] [Retained austenite]

[0186] The volume fraction of retained austenite in the soft surface region was determined using electron backscatter diffraction (EBSD) to obtain crystal orientation information of the observation area. Specifically, first, a sample with a thickness section parallel to the rolling direction of the steel plate was collected. This section was used as the observation surface, and the surface was sequentially subjected to wet polishing with sandpaper, polishing with diamond abrasive grains having an average particle size of 1 μm, and chemical polishing. Next, within the area defined as the soft surface region of the polished observation surface, multiple observation areas were randomly selected without deviation in the thickness direction, and a total of 2.0 × 10⁻⁶ observation areas were obtained at 0.05 μm intervals. -9 m 2 The crystal orientation of the above regions was determined. The software used for acquiring the crystal orientation data was "OIM Data Collection TM (ver.7)" manufactured by TSL Solutions, Inc. The acquired crystal orientation information was separated into bcc and fcc phases using "OIM Analysis TM (ver.7)" software manufactured by TSL Solutions, Inc. The fcc phase is retained austenite. The volume fraction of the retained austenite obtained in this way was determined as the area fraction of the retained austenite.

[0187] [Chemical composition of the soft outer layer]

[0188] In embodiments of the present invention, the chemical composition of the surface soft layer is essentially the same as that of the center layer of the plate thickness, except that the carbon concentration near the surface is lower. As defined previously, the carbon content of the surface soft layer is 0.5 times or less than that of the center layer of the plate thickness.

[0189] [Thickness of internal oxide layer: 3μm or more]

[0190] In embodiments of the present invention, the surface soft layer includes an internal oxide layer having a thickness of 3 μm or more from the surface of the steel plate (or, in the case of a coating on the surface of the steel plate, the interface between the coating and the steel plate). It is believed that by including an internal oxide layer with a thickness of 3 μm or more, the movement of dislocations contained in the steel is pinned by a large number of fine oxide particles present in this internal oxide layer, resulting in a significant increase in the surface hardness of the steel plate. The thickness of the internal oxide layer can also be 4 μm or more, 5 μm or more, 6 μm or more, 8 μm or more, or 10 μm or more. There is no particular upper limit to the thickness of the internal oxide layer, but for example, the thickness can also be 30 μm or less, 25 μm or less, or 20 μm or less.

[0191] The thickness of the internal oxide layer refers to the distance from the surface of the steel plate to the furthest point where internal oxides exist, taken from the surface along the thickness direction of the steel plate (perpendicular to the surface). The thickness of the internal oxide layer is determined by collecting a sample with a thickness section parallel to the rolling direction of the steel plate and including the surface portion of the steel plate, and observing this section using SEM. The depth to be measured is set to the area from the surface of the steel plate to 50 μm.

[0192] [Porosity near the surface: below 3.0%]

[0193] In embodiments of the present invention, the void area ratio in the region from the surface of the steel plate (or the interface between the coating and the steel plate if a coating exists on the surface) to a depth of 10 μm is 3.0% or less. When a certain amount of voids exists near the surface layer, these voids can sometimes become the starting point for defects such as peeling when the steel plate is subjected to an external force, such as bending. According to embodiments of the present invention, by controlling the void area ratio in the region from the surface of the steel plate to a depth of 10 μm to 3.0% or less, the generation of such defects can be reliably suppressed. This void area ratio can also be 2.0% or less, 1.5% or less, or 1.0% or less. The lower limit of this void area ratio is not particularly limited and can also be 0%. For example, the void area ratio can also be 0.1% or more, or 0.5% or more.

[0194] In this invention, the porosity is determined as follows: First, a sample obtained by mirror finishing of the observation surface through polishing and grinding is used as the observation sample. Next, using a SEM, images are taken at 9000x magnification, centered 5 μm below the surface of the observation sample or the interface between the coating and the substrate metal. A 10 μm × 10 μm area is taken as one field of view, and 15 consecutive fields of view are obtained to obtain reflected electron concavity images. The areas where concavity is observed are analyzed using an energy-dispersive X-ray spectrometer (EDS) to distinguish between inclusions and voids. Purely void portions are counted as voids, and the proportion of voids in the 10 μm × 150 μm area captured by the SEM is determined as the porosity.

[0195] [Plate thickness]

[0196] The high-strength steel plate in the embodiments of the present invention generally has a thickness of 0.6 to 6.0 mm. Although not particularly limited, the plate thickness may also be 1.0 mm or more, 1.2 mm or more, or 1.4 mm or more, and / or may be 5.0 mm or less, 4.0 mm or less, 3.0 mm or less, or 2.5 mm or less.

[0197] [Platinum coating]

[0198] The high-strength steel sheet of the embodiments of the present invention can also be further coated with a plating layer on the surface of the soft portion for the purpose of improving corrosion resistance, etc. The plating layer can be any of hot-dip galvanized coatings and electroplated coatings. Examples of hot-dip galvanized coatings include hot-dip zinc plating, alloyed hot-dip zinc plating, hot-dip aluminum plating, hot-dip Zn-Al alloy plating, hot-dip Zn-Al-Mg alloy plating, and hot-dip Zn-Al-Mg-Si alloy plating. Examples of electroplated coatings include electroplated zinc plating and electroplated Zn-Ni alloy plating. Preferably, the coating is a hot-dip zinc plating, an alloyed hot-dip zinc plating, or an electroplated zinc plating. There is no particular limitation on the amount of coating applied; a general amount is preferable.

[0199] [Mechanical Properties]

[0200] The high-strength steel sheet according to embodiments of the present invention achieves excellent mechanical properties, such as a tensile strength of 1250 MPa or more. The tensile strength is preferably 1300 MPa or more, more preferably 1350 MPa or more. There is no particular upper limit, but for example, the tensile strength may be 2000 MPa or less, 1800 MPa or less, or 1650 MPa or less. Similarly, the high-strength steel sheet according to embodiments of the present invention achieves high hardness, more specifically, an average Vickers hardness (Hc) at the center of the sheet thickness exceeding 400 Hv (i.e., the average Vickers hardness at half the sheet thickness). The average Vickers hardness (Hc) at the center of the sheet thickness is preferably 415 Hv or more, more preferably 430 Hv or more. Furthermore, the high-strength steel sheet according to embodiments of the present invention achieves excellent bending workability, more specifically, a total elongation of 10% or more. The total elongation is preferably 11% or more, more preferably 12% or more. There is no particular upper limit, but for example, the total elongation may be 25% or less, or 20% or less. Tensile strength and total elongation were determined by tensile testing according to JIS Z2241:2011 using JIS No. 5 test specimens collected from a direction parallel to the width of the steel plate (C direction).

[0201] The high-strength steel sheet of the present invention has improved bending workability and high resistance to defects, thus maintaining good appearance. Therefore, it is very useful for applications such as automotive frame components, where aesthetics are also important. Furthermore, due to its high surface hardness, this high-strength steel sheet also exhibits excellent wear resistance, making it ideal for applications such as crane booms for construction machinery where high strength, bending workability, and wear resistance are required in addition to high strength.

[0202] <Manufacturing Methods of High-Strength Steel Plates>

[0203] Next, a preferred method for manufacturing the high-strength steel plate according to an embodiment of the present invention will be described. The following description is intended to illustrate a characteristic method for manufacturing the high-strength steel plate according to an embodiment of the present invention, and is not intended to limit the high-strength steel plate to the high-strength steel plate manufactured by the manufacturing method described below.

[0204] The method for manufacturing high-strength steel plates according to embodiments of the present invention is characterized by comprising the following steps:

[0205] The hot rolling process includes heating a slab having a chemical composition described above in relation to the center of the plate thickness to a temperature of 1100 to 1250°C, followed by finish rolling, immediately cooling the finished steel plate at an average cooling rate of 40°C / second or higher and coiling it at a temperature of 590°C or lower, wherein the end temperature of the finish rolling is 840 to 1050°C, the maximum temperature of the coiled hot-rolled coil is controlled to be 580°C or lower, and the holding time in the temperature range from the maximum temperature to 500°C is limited to 4 hours or less;

[0206] The process of pickling the obtained hot-rolled steel sheet;

[0207] The cold rolling process involves cold rolling pickled hot-rolled steel sheets at a reduction rate of 30-80%.

[0208] The annealing process includes annealing the obtained cold-rolled steel sheet under an oxygen partial pressure P. O2 log P (atm) O2 Heating is performed in an atmosphere ranging from -20°C to -16°C within a temperature range above (Ac3-30)°C.

[0209] The cooling process includes cooling the cold-rolled steel sheet once to a temperature of 680-780°C at an average cooling rate of 0.5-20°C / second, and then cooling it twice to a temperature of 25-600°C at an average cooling rate exceeding 20°C / second; and

[0210] The tempering process includes holding the cold-rolled steel sheet in a temperature range of 100 to 400°C for 150 to 1000 seconds.

[0211] The following is a detailed description of each process.

[0212] [Hot rolling process]

[0213] [Heating of slab]

[0214] First, a slab having the chemical composition described above, related to the center of the plate thickness, is heated. From a productivity standpoint, the slab is preferably cast using a continuous casting method, but it can also be manufactured using an ingot casting method or a thin slab casting method. The slab contains a relatively high amount of alloying elements to obtain high-strength steel. Therefore, it is necessary to heat the slab before hot rolling to dissolve the alloying elements in it. If the heating temperature is below 1100°C, the alloying elements are not sufficiently dissolved in the slab, leaving behind coarse alloy carbides, which can sometimes cause embrittlement and cracking during hot rolling. Therefore, a heating temperature of 1100°C or higher is preferred. There is no particular upper limit to the heating temperature, but from the viewpoint of heating equipment capacity and productivity, 1250°C or lower is preferred.

[0215] [Rough rolling]

[0216] In this method, for example, for heated slabs, rough rolling can be performed before finish rolling for purposes such as thickness adjustment. Rough rolling is not particularly limited as long as the desired slab size can be ensured.

[0217] [Precision rolling]

[0218] The heated slab, or, if necessary, the rough-rolled slab, is then subjected to finish rolling. Since the slab used as described above contains a relatively high amount of alloying elements, it is necessary to increase the rolling load during hot rolling. Therefore, hot rolling is preferably performed at a high temperature. In particular, the finish rolling temperature is important for controlling the microstructure of the steel sheet. If the finish rolling temperature is low, the microstructure becomes inhomogeneous, and sometimes the formability decreases. Therefore, the finish rolling temperature is preferably 840°C or higher. On the other hand, to suppress the coarsening of austenite, the finish rolling temperature is preferably 1050°C or lower.

[0219] [Roll in]

[0220] Next, the finished steel sheet is immediately cooled at an average cooling rate of 40°C / second or higher, for example, 40 to 100°C / second, and then coiled at a temperature of 590°C or lower. If the time from finishing rolling to the start of cooling is long, or the average cooling rate after finishing rolling is slow, or the coiling temperature is high, the formation of an internal oxide layer will be promoted in the surface of the hot-rolled steel sheet. The formed internal oxide layer cannot be sufficiently removed even by subsequent pickling, and thus the cold rolling process is carried out in a state containing the internal oxide layer. In this case, voids are formed around the internal oxide during cold rolling, and sometimes a void area ratio of less than 3.0% cannot be achieved in the final steel sheet. In order to reliably suppress the formation of such an internal oxide layer in the hot rolling process, the finished steel sheet must be cooled immediately at an average cooling rate of 40°C / second or higher, more specifically, at an average cooling rate of 40°C / second or higher within 3 seconds after finishing rolling. For the same reason, the coiling temperature must be set to 590°C or lower, preferably below 550°C.

[0221] The maximum temperature of the hot-rolled coil (hot-rolled steel sheet) after coiling is controlled to below 580°C, and the holding time in the temperature range from the maximum temperature of the hot-rolled coil to 500°C is limited to less than 4 hours. To suppress the formation of voids around the internal oxides during cold rolling, in addition to controlling the cooling after finishing and the coiling temperature, it is also important to appropriately control the thermal history of the hot-rolled coil after coiling. For example, heat preservation treatment is sometimes performed on the hot-rolled coil after coiling to ensure cold rollability. However, if such heat preservation treatment is at a high temperature and for a long time, a thick oxide scale or internal oxide layer on the surface of the hot-rolled coil may sometimes form. In such cases, even subsequent pickling cannot completely remove them, and uneven removal may occur along the width or length direction of the hot-rolled coil, resulting in voids. Since the phase transformation of the steel structure is an exothermic reaction, depending on the phase transformation rate, the temperature after coiling may sometimes be higher than the coiling temperature. Therefore, it is extremely important to properly monitor and control the thermal history of the hot-rolled coil after winding, and to suppress the formation of excessive oxide scale and internal oxide layer. Preferably, the maximum temperature of the hot-rolled coil after winding is controlled to be below 570°C, and the holding time in the temperature range from the maximum temperature of the hot-rolled coil to 500°C is limited to 3.5 hours or less. There are no particular limitations on the method and location for temperature measurement, but for example, the temperature can be measured externally using a thermal imager at a position approximately 25m from the inner end of the hot-rolled coil towards the outer end along its length, or it can be measured by inserting a thermocouple into the hot-rolled coil.

[0222] [Pickling process]

[0223] Next, the obtained hot-rolled steel sheet is pickled to remove the oxide scale formed on its surface. Pickling can be carried out under conditions suitable for removing the oxide scale, and can be done in one go or in multiple stages to reliably remove the oxide scale.

[0224] [Cold rolling process]

[0225] After pickling, hot-rolled steel sheets are cold-rolled at a reduction rate of 30% to 80% in the cold rolling process. By setting the cold rolling reduction rate to 30% or more, the shape of the cold-rolled steel sheet can be kept flat, and the reduction in ductility in the final product can be suppressed. The cold rolling reduction rate is preferably 50% or more. On the other hand, by setting the cold rolling reduction rate to 80% or less, it is possible to prevent the rolling load from becoming too large and making rolling difficult. The cold rolling reduction rate is preferably 70% or less. There are no particular limitations on the number of rolling passes and the reduction rate of each pass, as long as they are appropriately set in a way that the overall cold rolling reduction rate falls within the above range.

[0226] [Annealing process]

[0227] Oxygen partial pressure P in the atmosphereO2 log P (atm) O2 [-20 to -16]

[0228] Annealing temperature range: (Ac3-30)℃ and above

[0229] The resulting cold-rolled steel sheet, for example, is subjected to a process in the heating furnace and soaking furnace of a continuous annealing production line, where the oxygen partial pressure P in the furnace atmosphere is reduced. O2 log P (atm) O2 Annealing is performed by heating in a temperature range of -20 to -16 °C and above (Ac3-30) °C. Here, the Ac3 point can be approximately calculated based on the following formula.

[0230] Ac3=937.2-436.5×[C]+56×[Si]-19.7×[Mn]-16.3×[Cu]-26.6×[Ni]-4.9×[ Cr]+38.1×[Mo]+124.8×[V]+136.3×[Ti]-19.1×[Nb]+198.4×[Al]+3315×[B]

[0231] In the formula, [C], [Si], [Mn], [Cu], [Ni], [Cr], [Mo], [V], [Ti], [Nb], [Al] and [B] are the contents (mass%) of each element in the steel plate.

[0232] By annealing in a relatively oxidizing atmosphere at high temperatures, as described above, the surface layer of the steel plate can be softened through decarburization to form a desired soft surface layer, and oxygen from the atmosphere diffuses into the steel to form a desired internal oxide layer near the surface of the steel plate. More specifically, decarburization of the surface layer of the steel plate is performed by heating in a heating furnace and soaking furnace at a temperature range above (Ac3-30)°C, thus reducing the carbon content of the surface layer. The reduced carbon content of the surface layer decreases its hardenability, thereby allowing the formation of a suitable amount of ferrite in the surface layer. To promote such decarburization, the oxygen partial pressure P in the furnace atmosphere must be increased. O2 (atm) is controlled within a suitable range. If the oxygen partial pressure P of the atmosphere... O2 log P O2 At temperatures above -20°C, the oxygen potential becomes sufficiently high for decarburization. Furthermore, such an oxidizing atmosphere promotes oxygen diffusion from the atmosphere into the steel, facilitating the internal oxidation of Si, Al, Mn, and Cr present near the steel plate surface, resulting in an internal oxide layer of sufficient thickness, specifically 3 μm or more, near the steel plate surface. O2 Preferably -19 or higher. On the other hand, by using logP O2By controlling the oxygen potential to below -16, excessive decarburization and internal oxidation caused by excessively high oxygen potential can be suppressed. Therefore, the desired soft surface layer and internal oxide layer can be reliably obtained. Furthermore, oxidation of not only Si, Al, and Mn, but also the base steel itself can be suppressed, making it easier to obtain the desired surface condition in the steel sheet. O2 Preferably, the temperature is -17 or lower. According to this method, an internal oxide layer is formed in the annealing process after the cold rolling process. Therefore, compared with the case where an internal oxide layer is formed in the hot rolling process, no voids are formed around the internal oxide during cold rolling, and a void area ratio of less than 3.0% can be reliably achieved in the final steel sheet.

[0233] Furthermore, by heating in a temperature range of (Ac3-30)°C or higher during the annealing process, austenite can be generated during annealing, and as the final microstructure in the center of the sheet thickness, a specified amount of tempered martensite can be easily obtained. Therefore, the desired high strength in the steel sheet can be achieved. On the other hand, if the annealing temperature range is too high, although there is no problem with the properties of the steel sheet, the productivity will decrease. Therefore, the heating temperature range of the annealing process is preferably 1100°C or lower, and more preferably 950°C or lower. For example, if only a soft surface layer is formed on one side of the steel sheet, two cold-rolled steel sheets can be overlapped during this annealing process, and the surface layer on only one side of the steel sheet can be decarburized and softened by performing annealing under the conditions described above.

[0234] [Cooling Process]

[0235] Following the annealing process, in order to form the desired microstructure in the soft surface layer and the center of the plate thickness, the obtained cold-rolled steel sheet is cooled once to a temperature of 680-780°C at an average cooling rate of 0.5-20°C / second, and then cooled twice to a temperature of 25-600°C at an average cooling rate of 20°C / second.

[0236] [First cooling cycle: Cooling to a temperature of 680–780°C at an average cooling rate of 0.5–20°C / second]

[0237] By setting the average cooling rate of the first cooling step to 20°C / second or less, the formation of ferrite in the soft surface layer can be promoted. Furthermore, the upper limit of the average cooling rate in the first cooling step is specified to reliably achieve the effect of dividing the cooling process into two stages: first cooling and second cooling. From this viewpoint, the average cooling rate of the first cooling step is preferably 18°C / second or less, more preferably 16°C / second or less. By setting the cooling process into such two stages, for example, the formation of pearlite or the like can be prevented or suppressed in the soft surface layer, and a higher ferrite area ratio can be achieved. On the other hand, by setting the average cooling rate of the first cooling step to 0.5°C / second or more, the excessive occurrence of ferrite and pearlite phase transformations not only in the soft surface layer but also in the center of the plate thickness can be suppressed, thus making it easier to obtain a specified amount of tempered martensite in the center of the plate thickness. The average cooling rate of the first cooling step is preferably 1°C / second or more, more preferably 2°C / second or more. Furthermore, by setting the cooling stop temperature for the first cooling cycle to 680°C or higher, it is possible to suppress the formation of a large amount of non-ferrite microstructure in the soft surface layer, thus preventing a decrease in the bending workability of the steel sheet. The cooling stop temperature for the first cooling cycle is preferably 700°C or higher. On the other hand, by setting the cooling stop temperature for the first cooling cycle to 780°C or lower, it is possible to promote the formation of ferrite in the soft surface layer.

[0238] [Two-stage cooling: Cooling to temperatures of 25–600°C at an average cooling rate exceeding 20°C / second]

[0239] The average cooling rate and cooling stop temperature of the two cooling cycles are particularly important for forming quenched martensite in the center of the plate thickness to obtain a specified amount of tempered martensite. Quenched martensite is generated by a phase transformation in a temperature range of 25–600°C using trace dislocations present in the austenite grains before the phase transformation as nuclei. After the first cooling cycle, by setting the average cooling rate up to the temperature range of 25–600°C to more than 20°C / second, the disappearance of dislocations contained in the austenite grains before the phase transformation can be suppressed. As a result, more than 85% tempered martensite can be reliably achieved in the final microstructure of the center of the plate thickness. The average cooling rate of the two cooling cycles is preferably 23°C / second or higher. Furthermore, the cooling stop temperature of the two cooling cycles is 25°C or higher, but from the viewpoint of further improving productivity, it is preferably 100°C or higher. On the other hand, by setting the cooling stop temperature to 600°C or lower, the formation of ferrite, bainite, and pearlite in the center of the plate thickness can be suppressed, and a specified amount of martensite can be reliably generated. The cooling stop temperature for the second cooling cycle is preferably below 500°C.

[0240] [Tempering process]

[0241] After the cooling process, cold-rolled steel sheets mainly contain quenched martensite in the center of the sheet thickness. Therefore, this quenched martensite must be tempered into tempered martensite in the next tempering process. More specifically, in the tempering process, by holding the cold-rolled steel sheet in a temperature range of 100–400°C for 150–1000 seconds, the quenched martensite in the center of the sheet thickness is tempered into tempered martensite, which improves the workability of the steel sheet compared to the case where the center of the sheet thickness mainly contains quenched martensite. By setting the holding temperature to 100°C or higher, the tempering effect can be reliably obtained. On the other hand, by setting the holding temperature to 400°C or lower, excessive tempering can be suppressed, and the strength of the steel sheet can be maintained at a high level. Furthermore, by setting the holding time to 150 seconds or higher, a specified amount of tempered martensite can be reliably obtained. On the other hand, from a productivity point of view, the holding time is preferably set to 1000 seconds or lower.

[0242] [Platering and Surface Treatment]

[0243] When hot-dip galvanizing is performed on steel sheets as a coating process, the steel sheet is heated or cooled to a temperature at least 40°C lower than the zinc bath temperature and at least 50°C higher than the zinc bath temperature, and then passed through a zinc bath. Through this hot-dip galvanizing process, a steel sheet with a hot-dip galvanized layer on its surface, i.e., a hot-dip galvanized steel sheet, is obtained. The hot-dip galvanized layer has, for example, a chemical composition of Fe: 7-15% by mass, and the remainder consisting of Zn, Al, and impurities. Furthermore, the hot-dip galvanized layer can also be a zinc alloy.

[0244] When alloying is performed after hot-dip galvanizing, for example, the hot-dip galvanized steel sheet is heated to a temperature between 460°C and 600°C. When the heating temperature is below 460°C, alloying is sometimes insufficient. On the other hand, when the heating temperature exceeds 600°C, alloying sometimes becomes excessive, leading to a deterioration in corrosion resistance. Through such alloying treatment, a steel sheet with an alloyed hot-dip galvanized layer on its surface can be obtained, i.e., an alloyed hot-dip galvanized steel sheet.

[0245] In addition, electroplating, vapor deposition, and other plating treatments can be applied to the steel sheet, and alloying can be further performed after electroplating. Furthermore, surface treatments such as organic film formation, film lamination, organic or inorganic salt treatment, and chromium-free treatment can also be applied to the steel sheet.

[0246] [Tempering in subsequent processes]

[0247] Finally, additional tempering can be optionally performed on the steel plate to adjust its strength, etc. There are no particular limitations to this tempering; for example, it can be performed by holding the steel plate in a temperature range of 200–500°C for more than 2 seconds.

[0248] The present invention will now be described in more detail by way of examples, but the present invention is not limited to these examples in any way.

[0249] Example

[0250] [Example A]

[0251] In this example, firstly, a continuously cast slab with a thickness of 20 mm and the chemical composition shown in Table 1 is heated to a specified temperature within the range of 1100–1250°C. Hot rolling is then performed under conditions where the finishing rolling end temperature is 840–1050°C. After finishing rolling, the slab is cooled at an average cooling rate of 40°C / second for up to 3 seconds, and then coiled at the coiling temperature shown in Table 2. For the coiled hot-rolled coil, the maximum temperature is controlled below 580°C, and the holding time in the temperature range from the maximum temperature of the hot-rolled coil to 500°C is set to 3.5 hours or less. The temperature of the hot-rolled coil is measured by inserting a thermocouple approximately 25 m from the inner end of the hot-rolled coil towards the outer end in the length direction. Next, the resulting hot-rolled steel sheet is pickled, and then cold-rolled at the reduction rate shown in Table 2. Next, the surface layer of the cold-rolled steel sheet was decarburized and softened by annealing under the conditions shown in Table 2, followed by cooling and tempering under the same conditions. In Table 3, the steel sheet with a soft surface layer on only one side was obtained by annealing two cold-rolled steel sheets overlapping each other during the annealing process, thereby decarburizing and softening only one side of the surface layer. Finally, as needed, plating, alloying, and additional tempering were performed to obtain the finished steel sheet. The chemical composition of the portion corresponding to the center of the sheet thickness was analyzed from samples taken from the obtained steel sheet, and the results showed no change from the chemical composition shown in Table 1.

[0252]

[0253]

[0254] The properties of the obtained steel plate were measured and evaluated using the following methods.

[0255] [Thickness of the soft surface layer, average Vickers hardness (Hc) at the center of the plate, and average Vickers hardness (Hs) of the soft surface layer]

[0256] The thickness of the soft surface layer, the average Vickers hardness (Hc) at the center of the plate thickness, and the average Vickers hardness (Hs) of the soft surface layer are determined as follows. For the Vickers hardness test, it is performed according to JIS Z 2244-1:2020. First, the Vickers hardness at half the plate thickness is measured with an indentation load of 10g. Then, from this position, along a line perpendicular to the plate thickness and parallel to the rolling direction, a total of five points are measured with an indentation load of 10g. The average value of these measurements is determined as the average Vickers hardness (Hc) at the center of the plate thickness. The interval between each measurement point is set to at least four times the distance of the indentation. Next, the carbon concentration is measured from the surface along the depth direction using a GDS. The region from the surface to half the average carbon concentration of the parent phase, where the carbon concentration gradually increases, is defined as the soft surface layer. The thickness (%) of the soft surface layer is then determined. Within a defined soft surface layer, 10 Vickers hardness points are randomly measured with an indentation load of 10g. The average Vickers hardness (Hs) of the soft surface layer is determined by calculating their average value.

[0257] [Internal oxide layer thickness]

[0258] The thickness of the internal oxide layer was determined by collecting a sample with a thickness section parallel to the rolling direction of the steel plate and including the surface portion of the steel plate. This section was then observed using SEM, and the distance from the surface of the steel plate to the furthest point where the internal oxide layer exists was measured, while advancing along the thickness direction of the steel plate (perpendicular to the surface of the steel plate). The measurement depth was set to the area from the surface of the steel plate to 50 μm.

[0259] [Porosity near the surface]

[0260] The porosity near the surface was determined as follows. First, a sample with a mirror-finished surface obtained through polishing and grinding was used as the observation sample. Next, a SEM was used to capture images at 9000x magnification, centered 5 μm below the surface of the observation sample or the interface between the coating and the substrate metal. A 10 μm × 10 μm area was used as one field of view, resulting in 15 consecutive fields of view of reflected electron concavity / concavity images. The areas with observed concavity / concavity were analyzed using EDS to distinguish between inclusions and voids. Purely void portions were counted as voids, and the proportion of voids in the 10 μm × 150 μm area captured by the SEM was determined as the porosity.

[0261] [Tensile strength and total elongation]

[0262] The tensile strength TS and total elongation t-El were determined by tensile testing according to JIS Z2241:2011 based on JIS No. 5 test specimens collected from a direction parallel to the width of the steel plate (C direction).

[0263] [Evaluation of bending workability]

[0264] Bending workability is evaluated by measuring the bending angle α (°) using a bending test according to VDA (German Association of the Automotive Industry) standard 238-100:2017-04.

[0265] [Evaluation of Defects]

[0266] The occurrence of defects is evaluated by testing at room temperature using a Vickers hardness tester (100g load) at 10 locations, extending 5μm from the surface of the steel plate (or, in the case of a coating, the interface between the coating and the steel plate), and observing whether microcracks with a length of 3μm or more appear around the indentation. Specifically, cases without microcracks are evaluated as acceptable (OK), while cases with microcracks are evaluated as unacceptable (NG).

[0267] High-strength steel sheets with a tensile strength of 1250 MPa or higher, a total elongation of 10% or higher, a bending angle of 70° or higher, and no microcracks were evaluated as having improved bending workability and suppressing the formation of defects. The results are shown in Table 3. In Table 3, for steel sheets with a soft surface layer formed on both sides of the center of the sheet thickness, only the values ​​for the soft surface layer and the internal oxide layer on one side are shown. However, since these steel sheets are manufactured with the same treatment on both sides, the values ​​for the soft surface layer and the internal oxide layer are substantially the same on both sides of the steel sheet; in fact, these values ​​were confirmed to be the same on both sides of the steel sheet in several steel sheets.

[0268]

[0269]

[0270] Referring to Table 3, in Comparative Example 22, although the total area ratio of tempered martensite and quenched martensite was relatively high, the tensile strength decreased due to the low C content. In Comparative Example 23, the tensile strength increased due to the high C content, but the bending workability decreased. In Comparative Example 24, the bending workability decreased due to the high Si content. In Comparative Example 25, the bending workability decreased due to the high Mn content. In Comparative Example 26, it is believed that the high Al content resulted in the formation of coarse Al oxides, resulting in decreased bending workability. In Comparative Example 27, it is believed that the high Cr content resulted in the formation of coarse Cr carbides, resulting in decreased bending workability. In Comparative Example 28, the low total content of Si, Mn, Al, and Cr prevented the sufficient formation of an internal oxide layer, resulting in decreased surface hardness and the observation of microcracks. In Comparative Example 29, the high coiling temperature resulted in the formation of an internal oxide layer during the hot rolling process. Therefore, it is believed that voids are formed around the internal oxides during subsequent cold rolling, resulting in insufficient reduction of the void area ratio near the surface in the final steel sheet, and the formation of microcracks is observed. In Comparative Example 30, due to the high stopping temperature of the second cooling, the desired amount of tempered martensite was not formed in the center of the sheet thickness, resulting in reduced tensile strength. In Comparative Example 31, due to the fast average cooling rate of the first cooling, ferrite could not be sufficiently formed in the soft surface layer, resulting in a higher Hs / Hc value and reduced bending workability. In Comparative Example 32, due to the high oxygen partial pressure P during the annealing process... O2 log P O2 The surface hardness is low, thus failing to promote decarburization and preventing the adequate formation of an internal oxide layer. This results in reduced surface hardness and the observation of microcracks.

[0271] In contrast, in Examples 1 to 21, by controlling the central portion and the soft portion of the plate thickness with a specified chemical composition and / or microstructure in such a way that their average Vickers hardness satisfies Hs / Hc≤0.50, the internal oxide layer is set to a thickness of 3 μm or more from the surface of the steel plate, and the porosity near the surface is controlled to be 3.0% or less. Thus, despite having a high strength of 1250 MPa or more, bending workability can be improved, and the generation of defects on the surface of the steel plate can be significantly suppressed.

[0272] [Example B]

[0273] In this example, the effect of controlling the thermal process after coiling on the properties of the resulting steel sheet was investigated. Specifically, using Example 16 in Table 3 as a baseline (maximum temperature of the hot-rolled coil after coiling: 567°C and holding time in the temperature range from that maximum temperature to 500°C: 3.5 hours), the maximum temperature of the hot-rolled coil after coiling and the holding time in the temperature range from that maximum temperature to 500°C were varied in Comparative Examples 33 and 34. Other manufacturing conditions in Comparative Examples 33 and 34 were the same as in Example 16. The results are shown in Table 4.

[0274] [Table 4]

[0275]

[0276] Bold text with an underline indicates text that is outside the scope of preferred options or outside the scope of this invention.

[0277] Referring to Table 4, in Example 16, where the maximum temperature of the hot-rolled coil after coiling is below 580°C and the holding time in the temperature range from the maximum temperature to 500°C is below 4 hours, as already shown in Table 3, the porosity near the surface layer in the final product steel sheet is 0.0%, thus sufficiently reduced to below 3.0%. As a result, no microcracks were observed in Example 16. On the other hand, in Comparative Example 33, where the maximum temperature of the hot-rolled coil after coiling exceeds 580°C, and in Comparative Example 34, where the holding time in the temperature range from the maximum temperature to 500°C exceeds 4 hours, it was not possible to control the porosity near the surface layer to below 3.0%, and microcracks were observed. This result is believed to be due to the fact that because the maximum temperature of the hot-rolled coil after coiling is high or the holding time is long, an internal oxide layer is formed during the hot rolling process, and voids are formed around the internal oxide layer during the subsequent cold rolling.

Claims

1. A high-strength steel plate comprising a central portion of plate thickness and a soft surface portion formed on one or both sides of the central portion of plate thickness. in, The central portion of the plate thickness has the following chemical composition, expressed as a percentage by mass: C：0.10～0.30%、 Si: 0.01~2.50%, Mn: 0.10~10.00% P: Below 0.100% S: Below 0.0500% Al：0～1.50%、 N: below 0.0100% O: Below 0.0060% Cr：0～2.00%、 Mo: 0~1.00%, B：0～0.0100%、 Ti: 0~0.30%, Nb: 0~0.30%, V：0～0.50%、 Cu: 0~1.00%, Ni: 0~1.00%, Ca: 0–0.040% Mg: 0–0.040% REM: 0–0.040%, and The remaining portion consists of Fe and impurities. The formula satisfies 1.50≤[Si]+[Mn]+[Al]+[Cr]≤20.00, where [Si], [Mn], [Al] and [Cr] are the contents of each element in terms of mass%. It also has a microstructure containing more than 85% tempered martensite by area ratio. The soft surface layer has a thickness exceeding 10 μm and less than 5.0% of the plate thickness. It also has a microstructure containing more than 80% ferrite by area ratio. It also includes an internal oxide layer with a thickness of more than 3 μm extending from the surface of the high-strength steel plate. The average Vickers hardness Hc of the central part of the plate thickness and the average Vickers hardness Hs of the soft surface part satisfy Hs / Hc≤0.

50. The porosity of the region from the surface of the high-strength steel plate to a depth of 10 μm is less than 3.0%.

2. The high-strength steel plate according to claim 1, wherein, The central portion of the plate has a microstructure composed of the following, in terms of area ratio: Tempered martensite: 85% or more At least one of ferrite, bainite, pearlite, and retained austenite: totaling less than 15%, and Martensite in quenched state: less than 5%.

3. The high-strength steel plate according to claim 1 or 2, wherein, The soft surface portion has a microstructure composed of the following, in terms of area ratio: Ferrite: 80% or more At least one of tempered martensite, bainite, and retained austenite: totaling less than 20%. Pearlite: less than 5%, and Martensite in quenched state: less than 5%.

4. The high-strength steel plate according to claim 1 or 2, wherein, The surface of the soft outer layer further includes a hot-dip galvanized layer or an electro-galvanized layer.

5. The high-strength steel plate according to claim 1 or 2, wherein, The surface of the soft outer layer further comprises an alloyed hot-dip galvanized layer.

6. The high-strength steel plate according to claim 3, wherein, The surface of the soft portion further includes a hot-dip galvanized layer or an electro-galvanized layer.

7. The high-strength steel plate according to claim 3, wherein, The surface of the soft outer layer further comprises an alloyed hot-dip galvanized layer.

Images