HIGH STRENGTH STEEL SHEET

Abstract

The present invention provides a high-strength steel sheet comprising a middle portion of the sheet thickness and a surface layer soft portion formed on one or both sides of the middle portion of the sheet thickness, wherein the middle portion of the sheet thickness has a predetermined chemical composition and has a microstructure comprising tempered martensite: 85% or more, the surface layer soft portion having a thickness of more than 10 μm to 5.0% or less of the sheet thickness, the microstructure comprising ferrite: 80% or more, and an internal oxide layer having a thickness of 3 μm or more from the surface, the average Vickers hardness (Hc) of the middle portion of the sheet thickness and the average Vickers hardness (Hs) of the surface layer soft portion satisfying Hs / Hc≤0.50, and the void area ratio in the area from the surface to the 10 μm depth position is 3.0% or less.

Description

Description HIGH STRENGTH STEEL SHEET Invention Engineering Field The present invention relates to a high strength steel sheet. Background of the Invention If steel sheet is made with high strength, its workability will decrease, and therefore, it is generally difficult to achieve both strength and workability of steel sheet simultaneously. For example, the arms of construction cranes have tended to become longer as the building levels have increased in recent years. Therefore, lighter weight and higher strength are increasingly required. Furthermore, when using steel sheet in components such as arms, bending operations are required, and therefore, the need for high-strength steel sheets with excellent bending capabilities is increasing. In the automotive industry, lighter car bodies are increasingly needed for fuel efficiency. To achieve both lighter body weight and crash safety, using high-strength steel sheets is an effective method. Work is underway to develop high-strength steel sheets based on this background. Generally, high-strength steel sheets have lower bending and forming capabilities than mild steel sheets. Forming methods used for mild steel sheets are sometimes inapplicable. Therefore, even in the automotive steel sheet industry, there is a high demand for high-strength steel sheets with excellent bending capabilities. PTL 1 describes a high strength steel sheet comprising a middle portion of the sheet thickness and a surface layer soft portion formed on one or both sides of the middle portion of the sheet thickness, wherein in the cross section of the high strength steel sheet, the metal structures of the middle portion of the sheet thickness include, based on the area ratio, tempered martensite: 85% or more, etc., the metal structures of the surface layer soft portion include, based on the area ratio, ferrite: 65% or more, pearlite: 5% or more and less than 20%, etc., the average distance between pearlite and pearlite in the surface layer soft portion is 3 pm or more, the Vickers hardness (Hc) of the middle portion of the sheet thickness and the Vickers hardness (Hs) of the surface layer soft portion meet 0.50 <Hs / Hc<0,75.Furthermore, PTL 1 explains that the bending load and bending ability of the steel sheet can be increased simultaneously by distributing pearlite as hard structures in the soft part of the surface layer. PTL 2 describes a high strength steel sheet having a tensile strength of 800 MPa or more that includes a center portion of the sheet thickness and a surface layer soft member arranged on one or both sides of the center portion of the sheet thickness, wherein each surface layer soft member has a thickness of more than 10 μm and 30% or less of the sheet thickness, the surface layer soft member has an average Vickers hardness of 0.60 times or less the average Vickers hardness of the 1 / 2 sheet thickness position, and the surface layer soft member has a nanohardness standard deviation of 0.8 or less. Furthermore, PTL 2 explains that the bending ability is significantly improved by suppressing the variations in the hardness of the surface layer soft member in addition to the presence of the surface layer soft member. PTL 3 describes a high strength hot rolled steel sheet having a predetermined chemical composition and having a microstructure in which 90% or more of the structure is martensite and the average aspect ratio of the initial austenite grains of the surface layer in the cross section in the rolling direction up to 1 / 8 of the sheet thickness is 3 or more and 20 or less. Furthermore, PTL 3 explains that, in accordance with the above composition, it is possible to produce a high strength hot rolled steel sheet having excellent bending ability and wear resistance and having a yield strength of 950 MPa or more. PTLs 4 to 10 respectively describe a high strength galvanized steel sheet having a hot dip galvanized layer or a hot dip galvanized layer on the surface of a base steel sheet, wherein said high strength galvanized steel sheet respectively has, from the interface of the base steel sheet and the galvanized layer, toward the base steel sheet, an internal oxide layer that includes at least one of the oxides selected from the group consisting only of Si and Mn; a soft layer that includes the internal oxide layer and has a Vickers hardness of 90% or less of the Vickers hardness in the t / 4 portion of the base steel sheet, wherein t is the sheet thickness of the base steel sheet; and a predetermined hard layer, wherein said high strength galvanized steel sheet satisfies an average depth D of the soft layer of 20 μm or greater;and an average depth d of the internal oxide layer of 4 μm or greater and less than D; and a tensile strength of 980 MPa or higher. Furthermore, PTL 4 to 10 explain that by controlling the average depth d of the internal oxide layer to a thickness of 4 μm or greater to utilize the internal oxide layer as a hydrogen trapping location, it is possible to effectively suppress hydrogen embrittlement, and that by precisely controlling the relationship between the average depth d of the internal oxide layer and the average depth D of the soft layer covering the internal oxide layer region, its bending capability will be greatly improved.; List of Citations Patent Literature PTL 1 WO2020 / 196060 PTL 2 WO2018 / 151331 PTL 3 Publication of Japanese Patent Application No. 2014-227583 PTL 4 WO2016 / 111271 PTL 5 WO2016 / 111272 PTL 6 WO2016 / 111273 PTL 7 WO2016 / 111274 PTL 8 WO2016 / 111275 PTL 9 WO2015 / 146692 PTL 10 WO2015 / 005191 Brief Description of the Invention Technical Issues As proposed in the previously disclosed technology, a soft layer can be provided on the surface of a steel sheet to improve its bending ability. On the other hand, by placing a soft layer on the surface of a steel sheet, its surface hardness is generally decreased, and therefore, its appearance is sometimes impaired due to the formation of defects, its wear resistance is decreased, etc. In this regard, PTL 3 explains that by making the average aspect ratio of the initial austenite grains of the surface layer to 1 / 8 of the sheet thickness of 3 or more and 20 or less, the surface hardness is increased and a steel sheet with excellent bending ability is obtained. However, PTL 3 has not yet fully studied the control of the microstructure of the surface layer other than the average aspect ratio of the initial austenite grains.Therefore, in the invention described in PTL 3, there is still room for improvement with regard to increasing its bending ability and surface hardness. Therefore, the present invention is aimed at providing high strength steel sheets having enhanced bending ability and capable of suppressing defect formation. Solution to the Problem The inventor of the present invention finds that in order to achieve the above-mentioned object, on a high-strength steel sheet having a tensile strength of 1,250 MPa or more, it is possible to provide a surface layer soft portion having an average Vickers hardness with a predetermined ratio to the average Vickers hardness of the middle portion of the sheet thickness so as to improve the bending ability and make it possible to form an internal oxide layer having a predetermined thickness on the outermost surface layer portion of the surface layer soft portion and further control the cavities formed near the surface layer to be within an appropriate range so as to increase its surface hardness and suppress the formation of defects on the surface of said steel sheet, and thereby accomplish the present invention. The present invention, which is capable of achieving the above-mentioned objectives, is as follows: (1) A high-strength steel sheet comprising a central portion of the sheet thickness and a soft surface layer formed on one or both sides of the central portion of the sheet thickness, wherein the central portion of the sheet thickness has a chemical composition that includes, on a mass % basis, C: 0.10 to 0.30%, Si: 0.01 to 2.50%, Mn: 0.10 to 10.00%, P: 0.100% or less, S: 0.0500% or less, Al: 0 to 1.50%, N: 0.0100% or less, O: 0.0060% or less, Cr: 0 to 2.00%, Mo: 0 to 1.00%, B: 0 to 0.0100%, Ti: 0 to 0.30%, Nb: 0 to 0.30%, V: 0 to 0.50%, Cu: 0 to 1.00%, Ni: 0 to 1.00%, Ca: 0 to 0.040%, Mg: 0 to 0.040%, REE: 0 to 0.040%, and the remainder including Fe and impurities, and meeting 1.50<[Si]+[Mn]+[Al]+[Cr]<20.00, where [Si], [Mn], [Al], and [Cr] are the contents (% by mass) of these elements, and having a microstructure that includes, based on the area ratio, tempered martensite: 85% or more, a soft portion of the surface layer having a thickness of more than 10 μm to 5.0% or less of the sheet thickness, a microstructure that includes, based on the area ratio, ferrite: 80% or more, and an internal oxide layer having a thickness of 3 μm or more of the surface of the high-strength steel sheet, the average Vickers hardness (Hc) of the center of the sheet thickness and the average Vickers hardness (Hs) of the soft portion of the surface layer meeting Hs / Hc<0.50, and the area ratio of voids in the area from the surface of the high-strength steel sheet to a depth position of 10 μm is 3.0% or less. (2) High strength steel sheet according to item (1) above, wherein the middle portion of the sheet thickness has a microstructure consisting solely of, based on area ratio: tempered martensite: 85% or more, at least one of ferrite, bainite, pearlite, and retained austenite: less than 15% in total, and as-quenched martensite (as obtained by sudden cooling): less than 5%. (3) High strength steel sheets according to item (1) or (2) above, wherein the soft portion of the surface layer has a microstructure consisting solely of, based on area ratio, ferrite: 80% or more, at least one of tempered martensite, bainite, and retained austenite: less than 20% in total, pearlite: less than 5%, and as-quenched martensite: less than 5%. (4) High strength steel sheets according to any one of items (1) to (3) above, wherein the high strength steel sheets further include a hot dip galvanized layer, a hot dip galvanized layer, or an electrogalvanized layer on the surface of the soft portion of the surface layer. Superior Effect of Invention According to the present invention, it is possible to provide a high-strength steel sheet having improved bending ability and being able to suppress the formation of defects. Such high-strength steel sheet has high resistance to the formation of defects and allows its performance properties to be maintained well, and therefore, for example, is very useful for use as frame components such as pillar components that require high strength as well as appearance and design sense - so-called pseudo-outer panel parts of automobiles. Furthermore, such high-strength steel sheet has high surface hardness and thus will have excellent wear resistance.Therefore, for example, this high-strength steel sheet is particularly suitable for applications such as crane arms for construction machinery that require not only high strength but also high bending and wear resistance. Complete Description of the Invention High Strength Steel Sheet A high strength steel sheet according to an embodiment of the present invention is characterized in that the high strength steel sheet includes a middle portion of the sheet thickness and a soft portion of a surface layer formed on one or both sides of the middle portion of the sheet thickness, wherein the middle portion of the sheet thickness has a chemical composition that includes, on a mass % basis, C: 0.10 to 0.30%, Si: 0.01 to 2.50%, Mn: 0.10 to 10.00%, P: 0.100% or less, S: 0.0500% or less, Al: 0 to 1.50%, N: 0.0100% or less, O: 0.0060% or less, Cr: 0 to 2.00%, Mo: 0 to 1.00%, B: 0 to 0.0100%, Ti: 0 to 0.30%, Nb: 0 to 0.30%, V: 0 to 0.50%, Cu: 0 to 1.00%, Ni: 0 to 1.00%, Ca: 0.00% to 0.04% Mg: 0.040%, REE: 0 to 0.040%, and the remainder including Fe and impurities, and meeting 1.50<[Si] + [Mn] + [Al] + [Cr]<20.00, where [Si], [Mn], [Al], and [Cr] are the contents (% by mass) of these elements, and having a microstructure that includes, based on the area ratio, tempered martensite: 85% or more, a soft portion of the surface layer having a thickness of more than 10 μm to 5.0% or less of the sheet thickness, a microstructure that includes, based on the area ratio, ferrite: 80% or more, and an internal oxide layer having a thickness of 3 μm or more of the surface of the high-strength steel sheet, the average Vickers hardness (Hc) of the center of the sheet thickness and the average Vickers hardness (Hs) of the soft portion of the surface layer meeting Hs / Hc<0.50, and the area ratio of voids in the area from the surface of the high-strength steel sheet to a depth position of 10 μm is 3.0% or less. As previously explained, by providing a soft layer on the surface of a steel sheet, it is possible to improve the bending ability, but on the other hand, due to the soft part of the surface layer, in general, the surface hardness decreases, and therefore, sometimes there is deterioration in appearance due to the formation of defects, a decrease in wear resistance, etc. Therefore, the inventor of the present invention studied a high-strength steel sheet having a tensile strength of 1,250 MPa or more with a focus not only on the soft part of the surface layer provided on one or both sides of the center of the sheet thickness, but also on the microstructure of the outermost surface layer or around the surface layer in the soft part of the surface layer.More specifically, the inventor of the present invention first discovered that by making the microstructure of the soft portion of the surface layer having a predetermined thickness contain, based on the area ratio, 80% or more of ferrite while controlling the average Vickers hardness (Hs) of the soft portion of the surface layer and the average Vickers hardness (Hc) of the middle portion of the sheet thickness so that it satisfies the formula Hs / Hcd0.50, it is possible to significantly improve the bending ability of the high strength steel sheet.Furthermore, the inventor of the present invention pays attention to and further studies the internal oxide layer formed on the outermost surface layer of the steel sheet due to relatively easily oxidized constituents (e.g., Si, Al, etc.) of the steel sheet bonding with oxygen in the annealing atmosphere during annealing treatments carried out after rolling (typically, hot rolling and cold rolling) and the cavities that sometimes form near the surface layer associated with other production conditions.As a result, the inventor of the present invention finds that by creating an internal oxide layer comprising oxides of Si, Al, etc., with a thickness of 3 μm or more on the surface of the steel sheet while controlling the area ratio of the voids formed around the surface of the layer, more specifically the area ratio of the voids in the area from the surface of the steel sheet to a depth position of 10 μm, to be 3.0% or less, it is possible to significantly increase the surface hardness of the steel sheet and also significantly suppress the formation of defects on the surface of the steel sheet. Without meaning to be tied to a particular theory, it is believed that the internal oxide particles present in the internal oxide layer act as barriers to dislocations in the steel, thus inhibiting the movement of dislocations and increasing the surface hardness of the steel sheet. More specifically, dislocations generally refer to streak-like crystal defects, but steel deformation generally occurs due to the repositioning of iron atoms near the dislocations contained in the steel due to external forces, etc., and thus the positions of the dislocations shift.Here, if an internal oxide layer having a predetermined thickness, specifically a thickness of 3 μm or more than the surface of the steel sheet (if there is a plating layer on the surface of the steel sheet, then the interface of the plating layer and the steel sheet), is formed on the surface layer of the steel sheet, because there will be many fine oxide particles dispersed in it, the internal oxide particles will act as barriers that hinder the movement of dislocations. As a result, it is believed that the surface hardness of the steel sheet is increased. On the other hand, if only the internal oxide layer is formed, the surface hardness is increased, but sometimes cracks or peeling or the formation of other defects cannot be reliably prevented. This time, the inventor of the present invention conducted further research and found that if there are a certain number or more cavities (holes) near the surface layer, if the steel sheet is subjected to some kind of external force, the cavities will sometimes become the starting points for peeling, cracks, or other defects and found that by controlling the area ratio of the cavities in the area from the surface of the steel sheet to the depth position of 10 μm at 3.0% or less, it is possible to reliably suppress the formation of such defects.Therefore, high strength steel sheets according to embodiments of the present invention can, for example, be conveniently used as high strength steel sheets for automotive purposes requiring excellent bending ability and high resistance to deformation, components for construction machinery, for example, crane arms, requiring excellent bending ability and wear resistance, and other applications. Below, high strength steel sheets according to embodiments of the present invention will be described in more detail. Chemical Composition of the Middle Part of the Sheet Thickness First, the chemical composition of the mid-thickness portion of the sheet will be explained. At the mid-thickness portion of the sheet, the chemical composition near the boundary with the soft portion of the surface layer will sometimes differ from that at a considerable distance from that boundary due to the diffusion of alloying elements with the soft portion of the surface layer. In such cases, below, the chemical composition of the mid-thickness portion of the sheet will mean the chemical composition measured near the 1 / 2-thickness position of the sheet. Furthermore, in the explanation below, the unit % element content means % by mass unless otherwise indicated. Furthermore, in this Description, the word up to indicating a range of numerical values, unless otherwise indicated, is used in a sense that includes the numerical values ​​before and after the word up to as its upper and lower limit values. C: 0.10 to 0.30% Carbon (C) is an effective element for securing a predetermined amount of tempered martensite and increasing the strength of steel sheets. To achieve these effects adequately, 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, the ductility and / or bending ability are sometimes reduced. 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. Si: 0.01 to 2.50% Silicon (Si) is an effective element for securing the ability of sudden cooling. Furthermore, Si is also an element that suppresses alloying with Al. To obtain these effects sufficiently, 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 middle part of the sheet thickness becomes brittle and sometimes the bending ability is reduced. 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. Mn: 0.10 to 10.00% Manganese (Mn) is an element that acts as a deoxidizer. Furthermore, Mn is an effective element for improving the ability of sudden cooling. To obtain these effects adequately, 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 are formed on the steel and sometimes the elongation of the steel sheet is reduced. 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. P: 0.100% or less Phosphorus (P) is an unavoidable element in the production process. The P content can also be as low as 0%. However, reducing the P content to less than 0.0001% requires time for purification and a decrease in productivity. Therefore, the P content can be 0.0001% or more, 0.0005% or more, 0.001% or more, or 0.005% or more. On the other hand, if the P content is excessive, sometimes the P segregates in the middle of the sheet thickness in the steel sheet, causing its toughness to decrease. 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. S: 0.0500% or less Sulfur (S) is an unavoidable element in the production process. The S content can also be as high as 0%. However, reducing the S content to less than 0.0001% requires time for purification and a decrease in productivity. Therefore, the S content can be 0.0001% or more, 0.0005% or more, or 0.010% or more. On the other hand, if the S content is excessive, it sometimes forms coarse MnS and the toughness of the steel sheet decreases. 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. Al: 0 to 1.50% Aluminum (Al) is an element that acts as a deoxidizer in steel and stabilizes ferrite. The Al content can also be as low as 0%, but to achieve such an effect, an Al content of 0.001% or more is preferred. The Al content can also be as high as 0.01% or more, 0.02% or more, or 0.03% or more. On the other hand, if the Al content is excessive, coarse Al oxide is formed and sometimes the elongation of the steel sheet is reduced and / or sometimes tempered martensite cannot be formed sufficiently. Therefore, the Al content is 1.50% or less. The Al content can also be as high as 1.40% or less, 1.30% or less, 1.00% or less, or 0.80% or less. N: 0.0100% or Less Nitrogen (N) is an unavoidable element in the production process. The N content can also be as low as 0%. However, reducing the N content to less than 0.0001% requires time for purification and a decrease in productivity. Therefore, the N content can be 0.0001% or more, 0.0005% or more, or 0.0010% or more. On the other hand, if the N content is excessive, coarse nitrides will form and sometimes the bending ability and / or toughness of the steel sheet will decrease. 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. O: 0.0060% or Less Oxygen (O2) is an unavoidable element in the production process. The O2 content can also be as low as 0%. However, to reduce the O2 content to less than 0.0001%, it takes time to refine and productivity decreases. Therefore, the O content can be 0.0001% or more, 0.0005% or more, or 0.0010% or more. On the other hand, if the O content is excessive, coarse inclusions will form and sometimes the toughness of the steel sheet is reduced. 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. The basic chemical composition of the mid-thickness portion of a sheet according to an embodiment of the present invention is as described above. Furthermore, the mid-thickness portion of the sheet, according to requirements, may contain at least one of the following optional elements instead of a portion of the remainder consisting of Fe. For example, the mid-thickness portion of the sheet may contain at least one selected from the group consisting only of Cr: 0 to 2.00%, Mo: 0 to 1.00%, and B: 0 to 0.0100%. Furthermore, the mid-thickness portion of the sheet may contain at least one selected from the group consisting only of Ti: 0 to 0.30%, Nb: 0 to 0.30%, and V: 0 to 0.50%. Furthermore, the mid-thickness portion of the sheet may contain at least one selected from the group consisting only of Cu: 0 to 1.00% and Ni: 0 to 1.00%.Furthermore, the middle portion of the sheet thickness may contain at least one selected from the group consisting only of Ca: 0 to 0.040%, Mg: 0 to 0.040%, and REE: 0 to 0.040%. Below, these optional elements will be described in detail. Cr: 0 to 2.00% Chromium (Cr) is an effective element for improving the hot-melt ability and producing high-strength steel sheets. The Cr content can also be as low as 0%, but to achieve such an effect, a Cr content of 0.001% or more is preferred. The Cr content can also be as high as 0.01% or more, 0.10% or more, or 0.20% or more. On the other hand, if the Cr content is excessive, the Cr segregates in the middle part of the sheet thickness in the steel sheet where coarse Cr carbides are formed and sometimes the elongation of the steel sheet is reduced. Therefore, a Cr content of 2.00% or less is preferred. The Cr content can also be as high as 1.80% or less, 1.00% or less, or 0.50% or less. Mo: 0 to 1.00% Molybdenum (Mo), like Cr, is an effective element for making steel sheets with high strength. The Mo content can also be as low as 0%, but to achieve such an effect, the Mo content is preferably 0.001% or more. The Mo content can also be 0.01% or more, 0.05% or more, or 0.10% or more. On the other hand, if the Mo content is excessive, coarse Mo carbides are formed and sometimes the cold workability of the steel sheet is reduced. 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. B: 0 to 0.0100% Boron (B) is an effective element for making steel sheets have high strength. The B content can also be as high as 0%, but to obtain such an effect, the B content is preferably 0.0001% or more. The B content can also be 0.0005% or more, 0.0010% or more, or 0.0015% or more. On the other hand, if the B content is excessive, the toughness and / or weldability are sometimes reduced. 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. Ti: 0 to 0.30% Titanium (Ti) is an effective element for controlling the formation of carbides and is also an element that promotes the increase in strength of ferrite. The Ti content can also be as low as 0%, but to achieve this effect, 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 too high, coarse oxides or nitridanitrides will form in the steel and sometimes the workability of the steel sheet is reduced. Therefore, the Ti content is 0.30% or less. The Ti content is 0.20% or less, 0.15% or less, or 0.10% or less. Nb: 0 to 0.30% Niobium (Nb), like Ti, is an effective element for controlling the shape of carbides and is also an element that refines the microstructure through the pinning effect (grain boundary retention effect) to contribute to the improvement of the toughness of the steel sheet. The Nb content can also be as low as 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, a large amount of fine hard Nb carbides will precipitate, the ductility decreases along with the increase in the strength of the steel sheet, and sometimes the workability of the steel sheet is reduced. 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. V: 0 to 0.50% Vanadium (V), like Ti and Nb, is an effective element for controlling the shape of carbides and is also an element that refines the microstructure through the pinning effect to contribute to the improvement of the toughness of the steel sheet. The V content can also be as low as 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 number of fine V carbides will precipitate, the ductility decreases along with the increase in the strength of the steel sheet, and sometimes the workability of the steel sheet is reduced. 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. Cu: 0 to 1.00% Copper (Cu) is an effective element for increasing the strength of steel sheets. The Cu content can also be as low as 0%, but to achieve such an effect, the Cu content is preferably 0.001% or more. The Cu content can also be 0.01% or more, 0.03% or more, or 0.05% or more. On the other hand, if the Cu content is excessive, red heat embrittlement will occur and sometimes the hot rolling productivity will decrease. 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. Ni: 0 to 1.00% Nickel (Ni), like Cu, is an effective element for increasing the strength of steel sheets. The Ni content can be as low as 0%, but to achieve this effect, a Ni content of 0.001% or more is preferred. The Ni content can also be as high as 0.01% or more, 0.03% or more, or 0.05% or more. On the other hand, if the Ni content is too high, the ductility decreases and sometimes the workability of the steel sheet is reduced. Therefore, a Ni content of 1.00% or less is preferred. The Ni content can also be as high as 0.80% or less, 0.60% or less, or 0.40% or less. Ca: 0 to 0.040% Calcium (Ca) is an element capable of controlling the formation of sulfides through the addition of trace amounts. The Ca content can also be as low as 0%, but to achieve such an effect, a Ca content of 0.0001% or more is preferred. The Ca content can also be as high as 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 oxide will form and sometimes the workability of the steel sheet will be reduced. Therefore, a Ca content of 0.040% or less is preferred. The Ca content can also be as high as 0.030% or less, 0.020% or less, or 0.015% or less. Mg: 0 to 0.040% Magnesium (Mg), like Ca, is an element that allows the formation of sulfides to be controlled by adding trace amounts. The Mg content can also be as large as 0%, but to achieve such an 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, coarse inclusions will form and sometimes the workability of the steel sheet will be reduced. 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. LTJ: 0 to 0.040% Rare earth metals (REEs), like Ca and Mg, are elements that allow the formation of sulfides to be controlled by adding trace amounts. The REE content can also be as low as 0%, but to achieve this effect, it is preferable to have a REE content of 0.0001% or more. It can also be as high as 0.0005% or more, 0.001% or more, or 0.005% or more. On the other hand, if the REE content is too high, coarse inclusions will form and sometimes the workability of the steel sheet will be reduced. Therefore, a REE content of 0.040% or less is preferable. It can also be as high as 0.030% or less, 0.020% or less, or 0.015% or less. LTJ in this Description is a general term for 17 elements with atomic number 21 scandium (Sc), atomic number 39 yttrium (Y), and lanthanides with atomic number 57 lanthanum (La) to atomic number 71 lutetium (Lu). This LTJ content is the total content of these elements. Etc Furthermore, the middle portion of the sheet thickness may intentionally or unavoidably contain the following elements. The effect of the present invention is not hindered by these elements. These elements are W: 0 to 0.10%, Ta: 0 to 0.10%, Co: 0 to 0.50%, Sn: 0 to 0.050%, Sb: 0 to 0.050%, As: 0 to 0.050%, and Zr: 0 to 0.050%. The content of these elements can also be 0.0001% or more or 0.001% or more, respectively. In the middle part of the thickness of the sheet according to the embodiment of the present invention, the remainder other than the above elements consists only of Fe and impurities. The impurities are the constituents, etc., which are inevitably introduced due to various factors in the production process, such as ore, scrap, and other such raw materials, when producing steel sheets or the middle part of the thickness of the sheet industrially. [1.50<[Si] + [Mn] + [Al] + [Cr]<20.00] The chemical composition of the middle thickness of the sheet according to the embodiment of the present invention must satisfy the following formula: 1.50<[Si] + [Mn] + [Al] + [Cr]<2 0, 00 where [Si], [Mn], [Al], and [Cr] are the contents (% mass) of these elements. As previously explained, in high strength steel sheets according to embodiments of the present invention, internal oxides formed in the outermost surface layer portion are very important in increasing the surface hardness of the steel sheet. The internal oxide layer is formed in the outermost surface layer portion of the steel sheet due to relatively easily oxidized constituents in the steel sheet, such as Si, Mn, Al, and Cr, which bind with oxygen in the annealing atmosphere especially during the annealing treatment after cold rolling. Therefore, in order to create an internal oxide layer with sufficient thickness to increase the surface hardness of the steel sheet, in particular to make it form with a thickness of up to 3 μη or more of the surface of the steel sheet, these elements must be contained in the steel in a certain amount or more in total.The chemical composition of the mid-thickness portion of the sheet according to an embodiment of the present invention is controlled to control the content of the alloying elements thereof to be within the previously described range with a total content of Si, Mn, Al, and Cr meeting 1.50% or more, that is, [Si] + [Mn] + [Al] + [Cr]>1.50. By appropriately combining the chemical composition of the mid-thickness portion of the sheet and in particular the annealing treatment conditions, etc., it is possible to reliably form an internal oxide layer having a thickness of 3 μη or more. As a result, high surface hardness is achieved and the formation of defects on the surface of the steel sheet is suppressed and it is possible to achieve excellent wear resistance. The total content of Si, Mn, Al, and Cr can 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 does not necessarily have a detrimental effect from the point of view of increasing the formation of internal oxides to make the surface hardness higher, the content of the individual alloying elements becomes too high, and therefore, sometimes the properties associated with them are reduced. Therefore, the total content of Si, Mn, Al, and Cr is 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. Microstructure of the Middle Thickness of the Sheet Temper Martensite: 85% or More The microstructure of the middle portion of the sheet thickness includes, based on the area ratio, 85% or more of tempered martensite. The tempered martensite forms high-strength and tough structures. In an embodiment according to the present invention, by making the predetermined chemical composition described above, in particular a C content of 0.10% or more, and including tempered martensite in the middle portion of the sheet thickness of 85% or more, high tensile strength, in particular a tensile strength of 1250 MPa or more, can be reliably achieved. The area ratio of the tempered martensite can also be 86% or more, 88% or more, or 90% or more. The upper limit of the area ratio of the tempered martensite is not particularly limited and can be 100%. For example, the area ratio of the tempered martensite can also be 98% or less, 96% or less, or 94% or less. At Least One of Ferrite, Bainite, Pearlite, and Remaining Austenite: Less than 15% in Total The mid-thickness microstructure of the sheet may contain any other structures provided that it meets the requirement of containing, by area ratio, 85% or more of tempered martensite. Although not specifically limited, for example, in the mid-thickness of the sheet, the total area ratio of at least one of ferrite, bainite, pearlite, and retained austenite is preferably less than 15%. Ferrite forms soft structures, and therefore, is easily deformed and contributes to improving the ductility of steel sheets. Therefore, from the perspective of improving the ductility of steel sheets, the microstructure of the middle thickness of the sheet can contain ferrite. However, because the interfaces of the hard structures of tempered martensite and the soft structures of ferrite can be the starting points of fracture, if ferrite is contained in excess, sometimes the expansion ability of the holes in the steel sheet is reduced. Furthermore, bainite is hard, so it contributes to improving the strength of steel sheets. Therefore, from the perspective of improving the strength of steel sheets, the microstructure of the middle thickness of the sheet can also contain bainite.However, if bainite is present in excess, the strength of the steel sheet will increase, but sometimes the microstructural uniformity will decrease and the hole expansion ability of the steel sheet will decrease. Bainite can be one of the upper bainite which has carbides between the blades, lower bainite which has carbides in the blades, bainitic ferrite which has no carbides, or granular bainitic ferrite where the bainite blade boundaries have recovered and become indistinct and can also be a mixture of these structures. Pearlite forms hard structures of soft ferrite and hard cementite arranged in layers and structures that contribute to increasing the strength of the steel sheet. Therefore, from the perspective of increasing the strength of the steel sheet, the microstructure of the middle part of the thickness of the sheet can also contain pearlite. However, because the interface between soft ferrite and hard cementite can be the initial points of fracture, if pearlite is contained in excess, sometimes the expansion ability of the hole in the steel sheet is reduced. Furthermore, retained austenite forms structures that contribute to increasing the ductility of the steel sheet through the work-induced transformation (TRIP) effect. Therefore, from the perspective of improving the ductility of the steel sheet, the microstructure of the middle part of the thickness of the sheet can contain retained austenite. On the other hand, because retained austenite transforms into as-quenched martensite through work-induced transformation, if retained austenite is contained in excess, sometimes the hole expansion ability of the steel sheet is reduced. By controlling the total area ratio of at least one of ferrite, bainite, pearlite, and retained austenite to less than 15%, it is possible to reliably avoid disadvantages resulting from excessive content of these structures, more particularly the reduction of hole expansion ability which is not related to the aim of the present invention and, on the other hand, to sufficiently show additional effects due to the presence of these structures. The total area ratio of at least one of ferrite, bainite, pearlite, and retained austenite can also be 0%, but, for example, it can also be 1% or more, 3% or more, 4% or more, or 5% or more. Furthermore, the total area ratio of at least one of ferrite, bainite, pearlite, and retained austenite can also be 14% or less, 12% or less, 11% or less, or 10% or less. As-quenched Martensite: Less than 5% As-quenched martensite means untempered martensite, that is, martensite that does not contain carbides. As-quenched martensite forms very hard structures. Therefore, the area ratio of as-quenched martensite can also be as high as 0%, but from the point of view of increasing strength, it can also be as high as 1% or more or 2% or more. On the other hand, because as-quenched martensite also forms brittle structures, from the point of view of securing higher toughness, the area ratio of as-quenched martensite is preferably less than 5%. The area ratio of as-quenched martensite can also be as high as 4% or less or 3% or less. Microstructure Identification and Calculation of Area Ratios in the Middle Thickness of the Sheet Temper Martensite and Bainite At the middle of the sheet thickness, the microstructure was identified and the area ratios were calculated as follows. First, a sample having a cross-section of the sheet thickness parallel to the rolling direction of the steel sheet was taken. This cross-section was used as the inspection surface. This inspection surface was corroded with Nital. On the corroded inspection surface, a region of 100 μm × 100 μm centered around the position of 1 / 4 of the sheet thickness from the steel sheet surface was used as the inspection region. This inspection region was examined using a field emission scanning electron microscope (FE-SEM) at 1000 to 50000X. Temper martensite and bainite were identified as follows from the position of cementite and the arrangement of cementite contained in the microstructure in this inspection region.In tempered martensite, cementite is present in the martensite laths, but there are two or more types of martensite laths and cementite crystal orientations. Cementite includes several variants, therefore tempered martensite can be identified. The area ratio of the tempered martensite that is then identified is calculated using the point counting method (based on ASTM E562). On the other hand, as a state of bainite, in some cases, cementite or retained austenite is present at the lath-shaped bainitic ferrite interfaces and in some cases, cementite is present in the interior of the lath-shaped bainitic ferrite. If cementite or retained austenite is present at the lath-shaped bainitic ferrite interfaces, since the bainitic ferrite interfaces can be determined, bainite can be identified.Furthermore, if cementite is present in the inner parts of the bainitic ferrite in the form of a blade, because there is one type of crystal orientation relationship between bainitic ferrite and cementite and there is one cementite variant, then bainite can be identified. The area ratio of identified bainite is calculated using the point counting method. Ferrite First, a sample having a cross-section of sheet thickness parallel to the rolling direction of the steel sheet was taken. The cross-section was used as the inspection surface. On the inspection surface, a region measuring 100 μm × 100 μm centered around the position of 1 / 4 of the sheet thickness from the surface of the steel sheet was used as the inspection area. This inspection area was examined using a scanning electron microscope at 1000 to 50000X to obtain an electron channel contrast image. Electron channel contrast image is a technique that detects the difference in crystal orientation within the crystal grain as a contrast difference. In the electron channel contrast image, the parts with uniform contrast are ferrite. The area ratio of the identified ferrite was calculated using the point counting method. Perlite The examined areas corroded with Nital described in relation to tempered martensite and bainite were examined using an optical microscope with 1000 to 50000X. In the examined images, dark contrast areas were identified as pearlite. The area ratio of the identified pearlite was calculated using the point counting method. Residual Austenite The volume ratio of retained austenite was measured using X-ray diffraction. First, on the sample taken as described above, a part of the steel sheet surface up to the position of 1 / 4 of the sheet thickness was removed by using mechanical polishing and chemical polishing to expose the surface at the position of 1 / 4 of the sheet thickness of the steel sheet surface. The exposed surface was irradiated with MoKa light to find the ratio of the integrated intensity of the diffraction peaks of the (200) face and (211) face of the bcc phase and the (200), (220) face, and (311) face of the fcc phase. From the ratio of the integrated intensity of the diffraction peaks, the volume ratio of retained austenite was calculated. As a method for this calculation, the general five-peak method was used. The calculated volume ratio of retained austenite was defined as the ratio of the area of ​​retained austenite. As-quenched Martensite First, the examined surface similar to the examined surface used for ferrite identification was etched with Lepera solution. The area similar to the area for ferrite identification was defined as the examined area. In corrosion using Lepera solution, martensite and retained austenite were not corroded. For this reason, the examined area corroded by Lepera solution was examined using FE-SEM and the uncorroded areas were identified as martensite and retained austenite. The total area ratio of identified martensite and retained austenite was calculated using the point counting method. Next, the area ratio of retained austenite determined above was subtracted from the total area ratio to determine the area ratio of as-quenched martensite. Soft Surface Layer The surface layer soft member formed on one side or both sides of the middle part of the sheet thickness has a thickness of more than 10 μm to 5.0% or less of the sheet thickness and has an average Vickers hardness (Hs) of 0.50 times or less of the average Vickers hardness (Hc) of the middle part of the sheet thickness (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 surface layer soft member on one side or both sides of the steel sheet can be reliably realized and, as a result, the bending ability of the steel sheet can be significantly improved. For example, to further enhance the bending ability improvement effect, the thickness of the surface layer soft member can be 15 μm or more, 20 μm or more, 25 μm or more, 30 μm or more, 35 μm or more, or 40 μm or more.Furthermore, the thickness of the soft portion of the surface layer may also be 4.5% or less, 4.0% or less, 3.5% or less, 3.0% or less, or 2.5% or less of the sheet thickness. If the soft portions of the surface layer are formed on both sides of the middle portion of the sheet thickness, then the thickness of the soft portion of the surface layer on one side and the thickness of the soft portion of the surface layer on the other side may be the same or may be different. Similarly, in order to further enhance the effect of improving the bending ability, the ratio (Hs / Hc) of the average Vickers hardness (Hs) of the soft portion of the surface layer and the average Vickers hardness (Hc) of the middle portion of the sheet thickness may be less than 0.50, 0.49 or less, 0.48 or less, 0.47 or less, 0.46 or less, or 0.45 or less. The lower limit of Hs / Hc is not specifically defined, but, for example, Hs / Hc can be 0.20 or more, 0.25 or more, or 0.30 or more.If the soft parts of the surface layer are formed on both sides of the middle part of the sheet thickness, then the Hs / Hc corresponding to the soft part of the surface layer on one side and the Hs / Hc corresponding to the soft part of the surface layer on the other side can be the same or can be different. In the present invention, the thickness of the soft part of the surface layer, the average Vickers hardness (Hc) of the middle part of the sheet thickness, and the average Vickers hardness (Hs) of the soft part of the surface layer are determined in the following manner. The Vickers hardness test is carried out according to JIS Z 2244-1:2020. First, the Vickers hardness at the 1 / 2 sheet thickness position of the steel sheet is measured using a compressive load of 10 g, then on a line from that position in a direction vertical to the sheet thickness and parallel to the rolling direction, likewise a total of three or more points, for example five points or 10 points, are measured for Vickers hardness using a compressive load of 10 g. The average value is determined as the average Vickers hardness (Hc) of the middle part of the sheet thickness. The distance between the measuring points is preferably 4 or more times the curvature.The distance of 4 or more times the indentation means the distance of 4 or more times the diagonal length of the rectangular opening of the indentation caused by the diamond indentation when measuring the Vickers hardness. Next, the light discharge optical emission spectroscope (GDS) was used to measure the C concentration in the depth direction from the surface. The region where the C concentration gradually increases from the surface to 1 / 2 of the average C concentration of the base phase (C content of the middle part of the sheet thickness) is defined as the soft part of the surface layer. The thickness (μη) of the soft part of the surface layer and its ratio to the sheet thickness (%) were determined. At the soft part of the surface layer thus determined, the Vickers hardness of 10 points was measured randomly using a compressive load of 10 g, and the average value was calculated to determine the average Vickers hardness (Hs) of the soft part of the surface layer.If the soft parts of the surface layer are formed on both sides of the middle part of the sheet thickness, measurements are made in the same way as described above to determine the thickness and average Vickers hardness (Hs) of the soft parts of the surface layer on the other side. Soft Part Microstructure of Ferrite Surface Layer: 80% or More The microstructure of the soft part of the surface layer contains, based on the area ratio, 80% or more ferrite. Ferrite forms structures that are soft and therefore easily deformed. For this reason, by containing 80% or more ferrite in the soft part of the surface layer, it is possible to achieve high bending capabilities. The ferrite area ratio can also be 82% or more, 85% or more, 87% or more, or 90% or more. The upper limit of the ferrite area ratio is not specifically specified, but can be as high as 100%. For example, the ferrite area ratio can also be 98% or less, 96% or less, or 94% or less. At Least One of Temper Martensite, Bainite, and Retained Austenite: Less than 20% in Total The microstructure of the soft part of the surface layer may contain any other structure provided that it meets the requirement of containing, by area ratio, 80% or more of ferrite. Although not specifically limited, for example, in the soft part of the surface layer, the total area ratio of at least one of tempered martensite, bainite, and retained austenite is preferably less than 20%. Tempered martensite and bainite form hard structures. Subsequently, retained austenite transforms into hard as-quenched martensite due to work-induced transformation. Therefore, from the viewpoint of further improving the bending ability of the steel sheet, for example, the total area ratio of at least one of tempered martensite, bainite, and retained austenite can be 18% or less, 16% or less, 14% or less, or 12% or less. The total area ratio of at least one of tempered martensite, bainite, and retained austenite can also be 0%, but, for example, it can also be 1% or more, 3% or more, 5% or more, 8% or more, or 10% or more. Perlite: Less than 5% As explained above, with a microstructure of the soft part of the surface layer containing, based on the area ratio, 80% or more of ferrite, a sufficiently high bending ability can be achieved, but from the viewpoint of further improving the bending ability of the steel sheet, the area ratio of the hard structure pearlite is preferably less than 5%. The area ratio of pearlite can also be 4.5% or less, 4% or less, or 3% or less. On the other hand, the lower limit of the area ratio of pearlite is not specifically specified and can also be 0%. For example, the area ratio of pearlite can also be 1% or more or 2% or more. As-quenched Martensite: Less than 5% In the same way as pearlite, from the viewpoint of further improving the bending ability of steel sheets, the as-quenched martensite area ratio of hard structures is preferably less than 5%. The as-quenched martensite area ratio can also be 4% or less or 3% or less. On the other hand, the lower limit of the as-quenched martensite area ratio is not specifically specified and can also be as high as 0%. For example, the as-quenched martensite area ratio can be 1% or more or 2% or more. Microstructure Identification and Calculation of Area Ratios in the Soft Parts of the Surface Layer In the soft part of the surface layer, the microstructure is identified and the area ratios are calculated in the following manner. First, a sample having a sheet thickness cross-section parallel to the rolling direction of the steel sheet is taken. This cross-section is used as the inspection surface. On this inspection surface, several inspection areas are randomly selected so that there is no bias in the sheet thickness direction within the range specified as the soft part of the surface layer. The total area of ​​this inspection area is 2.0x10-9m2 or more. The identification of the microstructure and the calculation of the area ratios other than the remaining austenite are the same as the identification of the microstructure and the calculation of the area ratios in the middle part of the sheet thickness except for the difference in the inspection areas. Residual Austenite The volume ratio of retained austenite in the soft part of the surface layer was found by obtaining information about the crystal orientations of the examined areas using electron backscatter diffraction (EBSD). Specifically, first, a sample having a sheet thickness cross-section parallel to the rolling direction of the steel sheet was taken. This cross-section served as the examined surface. Wet polishing using sandpaper, polishing using diamond abrasive having an average particle size of 1 μη, and chemical polishing were sequentially applied to the examined surface.Next, on the polished examined surface, some examined areas were randomly selected within the range defined as the soft part of the surface layer so that there was no concentration in the sheet thickness direction and the crystal orientations in the total area of ​​2.0x10-9m2 or more were obtained at intervals of 0.05 μη. As a software for obtaining data on crystal orientation, OIM Data Collection TM software (ver. 7) made by TSL Solutions was used. The obtained crystal orientation information was separated into bcc phase and fcc phase by OIM Analysis TM software (ver. 7) made by TSL Solutions. This fcc phase is retained austenite. The volume ratio of retained austenite obtained in this way was determined as the area ratio of retained austenite. Chemical Composition of the Soft Surface Layer In an embodiment of the present invention, the chemical composition of the soft portion of the surface layer is essentially the same as the chemical composition of the middle portion of the sheet thickness except that the carbon concentration near the surface is lower. From the definition of the soft portion of the surface layer previously described, the C content of the soft portion of the surface layer is 0.5 times or less than the C content of the middle portion of the sheet thickness. Internal Oxide Layer Thickness: 3 μm or More In an embodiment of the present invention, the soft portion of the surface layer comprises an internal oxide layer having a thickness of 3 μm or more than the surface of the steel sheet (if there is a plating layer on the surface of the steel sheet, then the interface of the plating layer and the steel sheet). Therefore, the movement of dislocations contained in the steel is restrained by a plurality of fine oxide particles contained in the internal oxide layer and, as a result, it is believed that it is possible to significantly increase the surface hardness of the steel sheet. 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. The upper limit of the thickness of the internal oxide layer is not specifically limited, but, for example, the thickness of the internal oxide layer can also be 30 μm or less, 25 μm or less, or 20 μm or less. The thickness of the internal oxide layer means the distance from the surface of the steel sheet to the furthest position where the internal oxides are located in the case starting from the surface of the steel sheet in the direction of the thickness of the steel sheet (the direction that is vertical to the surface of the steel sheet). The thickness of the internal oxide layer is determined by taking a sample that has a cross-section of the sheet thickness parallel to the rolling direction of the steel sheet and covering part of the surface layer of the steel sheet and examining the cross-section using SEM. The measured depth is the area up to 50 μm from the surface of the steel sheet. Void Area Ratio Near Surface Layer: 3.0% or Less In an embodiment of the present invention, the ratio of the void area in the area from the surface of the steel sheet (if there is a plating layer on the surface of the steel sheet, then the interface of the plating layer and the steel sheet) to a depth position of 10 μm is 3.0% or less. If there are a certain number or more voids (holes) near the surface layer, when the steel sheet is subjected to some kind of external force, for example, an external force from a bending operation, etc., sometimes these voids become the starting points for the formation of defects due to peeling, etc. According to an embodiment of the present invention, by controlling the void area ratio in the area from the surface of the steel sheet to a depth position of 10 μm at 3.0% or less, it becomes possible to reliably suppress the formation of such defects. The void area ratio can also be 2.0% or less, 1.5% or less, or 1.0% or less.The lower limit of the void area ratio is not specifically defined and can also be as high as 0%. For example, the void area ratio can also be as high as 0.1% or more or 0.5% or more. In the present invention, the void area ratio is determined in the following manner. First, the examined surface is polished to a mirror-like surface by rubbing. This surface is used as the examined sample. Next, the SEM is used to capture an image centered at a position of 5 μm from the examined sample surface or the interface between the plating layer and the base iron at a magnification of 9000X. Using a 10 μm × 10 μm area as a plane, reflected electron topographic images of 15 consecutive adjacent planes are obtained. The areas where the topographic parts are observed are analyzed using an energy dispersive X-ray spectrograph (EDS), distinguishing which are inclusions or cavities, only the simple hole parts are counted as cavities, and the ratio of the voids in the 10 μm × 150 μm area captured by the SEM is determined as the void area ratio. Sheet Thickness High strength steel sheets according to embodiments of the present invention generally have a sheet thickness of 0.6 to 6.0 mm. Although not specifically limited, the sheet thickness may 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.

[0069] Gilding The high-strength steel sheet according to the embodiment of the invention may further have a gilding layer on the surface of the soft part of the surface layer for the purpose of improving corrosion resistance, etc. The gilding layer may be a hot-dip galvanized layer and an electroplated layer. The hot-dip galvanized layer, for example, includes a hot-dip galvanized layer, a hot-dip galvanyl layer, a hot-dip aluminum coated layer, a hot-dip Zn-Al alloy coated layer, a hot-dip Zn-Al-Mg alloy coated layer, a hot-dip Zn-Al-Mg alloy coated layer, a hot-dip Zn-AlMg-Si alloy coated layer, etc. The electroplated layer, for example, includes an electrogalvanized layer, an electroplated Zn-Ni alloy gilding layer, etc. Preferably, the gilding layer is a hot-dip galvanized layer, a hot-dip galvanyl layer, or an electrogalvanized layer.The amount of gilding deposition is not specifically limited and can be as large as the general amount of deposition. Mechanical properties According to the high-strength steel sheet according to the embodiment of the present invention, excellent mechanical properties, for example, a tensile strength of 1,250 MPa or more, can be achieved. The tensile strength is preferably 1,300 MPa or more, more preferably 1,350 MPa or more. The upper limit is not specifically specified, but, for example, the tensile strength can also be 2,000 MPa or less, 1,800 MPa or less, or 1,650 MPa or less. Similarly, according to the high-strength steel sheet according to the embodiment of the present invention, high strength can be achieved. More specifically, an average Vickers hardness (Hc) of the middle portion of the sheet thickness (i.e., the average Vickers hardness at a position of 1 / 2 the sheet thickness) of more than 400 Hv can be achieved. The average Vickers hardness (Hc) of the middle portion of the sheet thickness is preferably 415 Hv or more, more preferably 430 Hv or more.Furthermore, according to the high strength steel sheet according to the embodiment of the present invention, it is possible to achieve excellent bending ability, more specifically, it is possible to achieve a total elongation of 10% or more. The total elongation is preferably 11% or more, more preferably 12% or more. The upper limit is not specifically specified, but, for example, the total elongation can also be 25% or less or 20% or less. The tensile strength and total elongation are measured by means of a tensile test in accordance with JIS Z2241:2011 based on a JIS No. 5 test specimen taken from the direction (C direction) parallel to the width direction of the steel sheet. High strength steel sheets according to embodiments of the present invention have improved bending ability and have high resistance to the formation of defects and maintain their appearance properties well, therefore, for example, they are very useful for use as components of automobile frames where the external appearance is highly required. Furthermore, high strength steel sheets have high surface hardness and therefore, also have excellent wear resistance, therefore, for example, they are very suitable in applications such as crane arms for construction machinery which require not only high strength, but also high bending ability and wear resistance. High Strength Steel Sheet Production Method Next, a preferred method for producing high-strength steel sheets according to embodiments of the present invention will be described. The following description is intended to illustrate characteristic methods for producing high-strength steel sheets according to embodiments of the present invention and is not intended to limit high-strength steel sheets to high-strength steel sheets produced using the production methods described below. A method for producing high strength steel sheets according to an embodiment of the present invention is characterized by including: a hot rolling step comprising heating a slab having the chemical composition described above with respect to the middle of the sheet thickness to a temperature of 1,100 to 1,250°C, then finishing rolling the slab and immediately cooling the finishing rolled steel sheet at an average cooling rate of 40°C / second or more, and rolling it at a temperature of 590°C or less, wherein the final finishing rolling temperature is 840 to 1,050°C, the maximum temperature of the hot rolled roll after rolling is controlled to 580°C or less, and the holding time in the temperature range from the maximum temperature to 500°C is limited to 4 hours or less, a step of giving a pickling treatment to the obtained hot rolled steel sheet, a cold rolling step of cold rolling the pickling-treated hot rolled steel sheet with a rolling reduction of 30 to 80%,annealing steps which include heating the cold-rolled steel sheet object in an atmosphere of logarithm logPO2 partial pressure of oxygen Po2 (atm) of -20 to -16 in the temperature range (Ac3-30)°C or more, cooling steps which include primary cooling of the cold-rolled steel sheet with an average cooling rate of 0.5 to 20°C / second to reduce it to a temperature of 680 to 780°C, then secondary cooling with an average cooling rate of more than 20°C / second to reduce it to a temperature of 25 to 600°C, and tempering steps which include leaving the cold-rolled steel sheet in the temperature range of 100 to 400°C for 150 to 1,000 seconds. Below, each of these steps will be explained in detail., Hot Rolled Slab Heating Steps First, a slab having the chemical composition described above with respect to the middle of the sheet thickness is heated. The slab used is preferably cast by continuous casting from a productivity standpoint, but it can also be produced using ingot-making or thin-slab casting methods. The slab used contains relatively large amounts of alloying elements to obtain a high-strength steel sheet. For this reason, it is necessary to heat the slab to dissolve the alloying elements in it before submitting it for hot rolling. If the heating temperature is less than 1,100°C, the alloying elements will not dissolve sufficiently in the slab and coarse alloying carbides will remain, sometimes causing embrittlement cracks during hot rolling. For this reason, a heating temperature of 1,100°C or higher is preferred.The upper limit of heating temperature is not specifically defined, but from the point of view of heating facility capacity and productivity, 1,250°C or less is preferred. Rough Rolling In this method, for example, the heated slab may be rough-rolled before finishing rolling to adjust the sheet thickness, etc. The rough-rolling need only be a rough rolling with which the desired sheet bar dimensions can be obtained. The conditions are not particularly restrictive. Completion planning Heated slabs or slabs that are also rough-rolled as needed are then solution-rolled. In the above way, the slab used contains alloy elements in a relatively large amount, so that during hot rolling, the rolling load must be made larger. For that reason, hot rolling is preferred to be done at high temperatures. In particular, the final temperature of solution rolling is important in the matter of controlling the metallographic structure of steel sheet. If the final solution rolling temperature is low, the metallographic structure becomes uneven and sometimes the forming ability decreases. Therefore, the final temperature of solution annealing is preferably 840°C or more. On the other hand, in order to keep the austenite from becoming rough, the final temperature of solution rolling is preferably 1,050°C or less. Winding Next, the finish-rolled steel sheet is immediately cooled at an average cooling rate of 40°C / s or more, for example, 40 to 100°C / s, and then rolled at a temperature of 590°C or less. If the time from after finish rolling to the start of cooling is long or the average cooling rate after finish rolling is slow or the rolling temperature is high, the formation of an internal oxide layer is promoted on the surface layer of the hot-rolled steel sheet. Since the formed internal oxide layer cannot be removed sufficiently even by subsequent pickling treatment, the cold rolling step is carried out under conditions that include the internal oxide layer. In this case, during cold rolling, voids are formed around the internal oxides. In the finally obtained steel sheet, sometimes a void area ratio of 3.0% or less cannot be achieved.To prevent such internal oxide layer from forming in the hot rolling step, the finish rolled steel sheet should be immediately cooled at an average cooling rate of 40°C / second or more, more specifically cooled at an average cooling rate of 40°C / second or more within 3 seconds after rolling is completed. For the same reason, the rolling temperature should be 590°C or less, preferably less than 550°C. The maximum temperature of the hot-rolled coil after rolling (hot-rolled steel sheet) is controlled to 580°C or less and the holding time in the temperature range from the maximum temperature of the hot-rolled coil to 500°C is limited to 4 hours or less. In order to prevent voids from forming around internal oxides during cold rolling, in addition to controlling the cooling and rolling temperature after finish rolling, it is also important to precisely control the heat history of the hot-rolled coil after rolling. For example, sometimes the hot-rolled coil after rolling is given a soaking treatment to secure the cold rolling ability, but if the soaking treatment has a high temperature and a long treatment time, sometimes the oxide scale of the hot-rolled coil and the internal oxide layer of the surface layer are formed thickly.In such a case, even with subsequent pickling treatment, it is not possible to remove it sufficiently. Uneven release occurs along the width direction and the length direction of the hot rolled coil and, as a result, voids are sometimes formed. The transformation of the microstructure of steel is an exothermic reaction, therefore depending on the speed of its transformation, even after rolling, the temperature will sometimes rise above the rolling temperature. Therefore, it is very important to monitor and control the heat history of the hot rolled coil precisely after rolling to suppress the formation of an excessive oxide scale or internal oxide layer. Preferably, the maximum temperature of the hot rolled coil after rolling is controlled to 570°C or less 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.The measurement method and location of temperature measurement are not specifically limited, but for example, the temperature at a position of about 25 m from the inner end of the hot-rolled coil to the outer end in the long direction of the hot-rolled coil can be measured using ThermoViewer from the outside or can be measured by inserting a thermocouple into the hot-rolled coil. Acidification Treatment Steps Next, the resulting hot-rolled steel sheet is subjected to an acid treatment to remove the oxide scale formed on the surface of the hot-rolled steel sheet. The acid treatment can be carried out under conditions suitable for oxide scale removal. This step can be performed once or divided into several steps to reliably remove the oxide scale. Cold Rolling Steps The hot-rolled steel sheet subjected to the pickling treatment is cold-rolled in the cold-rolling step with a rolling reduction of 30 to 80%. By making a rolling reduction of 30% or more in cold rolling, it is possible to maintain the shape of the cold-rolled steel sheet flat and maintain the ductility of the final product from the reduction. The rolling reduction in cold rolling is preferably 50% or more. On the other hand, by making the rolling reduction in cold rolling of 80% or less, it is possible to prevent the rolling load from being excessive and the rolling becoming difficult. The rolling reduction in cold rolling is preferably 70% or less. The number of rolling passes and the rolling reduction in each pass are not particularly limited and can be adjusted appropriately so that the rolling reduction of the cold rolling as a whole falls within the above range. Annealing Steps Logarithm of logPO2 Partial Pressure of Oxygen Po2 (atm) Atmospheric: -20 to -16 Annealing Temperature Range: (Ac3-30)°C or More The obtained cold-rolled steel sheet is annealed by heating, for example, in a heating furnace and a soaking furnace of a continuous annealing line in the temperature range (Ac3-30)°C or more while maintaining the logarithm of the partial pressure of oxygen PO2 (atm) of the internal furnace atmosphere at -20 to -16. Here, the Ac3 point can be calculated approximately according to the following formula: Ac3=937.2-436.5x[C]+56x[Si]-19.7x[Mn]-16.3x[Cu]—26.6x[Ni]4.9x[Cr]+38.1x[Mo] + 124.8x[V]+136.3x[Ti]-19.1x[Nb]+198.4x[Al]+3315x[B] where [C], [Si], [Mn], [Cu], [Ni], [Cr], [Mo], [V], [Ti], [Nb], [Al], and [B] are the contents (% mass) of the elements in the steel sheet. By annealing in the above-mentioned relatively oxidizing atmosphere and under high temperature conditions, it is possible to soften the surface layer of the steel sheet through decarburization to form a desired surface layer soft layer and to allow atmospheric oxygen to diffuse into the steel to form a desired internal oxide layer near the surface of the steel sheet. More specifically, by heating in a heating furnace and a soaking furnace in the temperature region of (Ac330)°C or above, decarburization occurs in the surface layer of the steel sheet and the amount of carbon in the surface layer is reduced. With the decrease in the amount of carbon in the surface layer, the sudden cooling ability of the surface layer is also reduced, so that it is possible to obtain an appropriate amount of ferrite in the surface layer.In order for such decarburization to occur, it is necessary to control the oxygen partial pressure P02 (atm) of the internal furnace atmosphere within a suitable range. If the logarithm of the atmospheric oxygen partial pressure logPO2 is -20 or more, the oxygen potential becomes sufficiently high and decarburization will occur. In addition, in such an oxidizing atmosphere, the diffusion of oxygen from the atmosphere into the interior of the steel is increased, internal oxidation of Si, Al, Mn, Cr, etc., present near the surface of the steel sheet takes place, and it is possible to form an internal oxide layer of sufficient thickness, more specifically, a thickness of 3 μm or more, near the surface of the steel sheet. The preferred logPO2 is -19 or more. On the other hand, by controlling logPO2 at -16 or less, it is possible to suppress excessive decarburization and internal oxidation resulting from too high an oxygen potential.For this reason, it is possible to reliably obtain the desired surface layer soft parts and internal oxide layers. Furthermore, oxidation of not only Si, Al, Mn, etc., but also the base steel sheet itself is suppressed and it is possible to obtain the desired surface conditions on the steel sheet more easily. LogP02 is preferably -17 or less. According to this method, the internal oxide layer is formed in the annealing step after the cold rolling step, therefore compared to the case where the internal oxide layer is formed in the hot rolling step, voids are not formed around the internal oxides during cold rolling and, in the steel sheet finally obtained, a void area ratio of 3.0% or less can be reliably achieved. In addition, in the annealing step, by heating in the temperature range (Ac3-30)°C or more, austenite is formed during annealing and it is possible to obtain a predetermined amount of tempered martensite as the final structure in the middle part of the sheet thickness. For this reason, it is possible to achieve the desired high strength in the steel sheet. On the other hand, if the annealing temperature range is too high, although there is no problem with the properties of the steel sheet, its productivity will decrease. For this reason, the heating temperature range in the annealing step is preferably 1,100°C or less, preferably 950°C or less.For example, when forming the soft part of the surface layer on only one side of the steel sheet, during the main annealing step, it is also possible to overlap two cold-rolled steel sheets and anneal under the conditions described above so as to decarburize and soften only one part of the surface layer of the steel sheets. Cooling Steps After the annealing step, to form the desired microstructure in the soft part of the surface layer and the middle part of the sheet thickness, the obtained cold-rolled steel sheet is subjected to primary cooling with an average cooling rate of 0.5 to 20°C / second to reduce to a temperature of 680 to 780°C, then subjected to secondary cooling with an average cooling rate of more than 20°C / second to reduce to a temperature of 25 to 600°C. Primary Cooling: Cooling at an Average Cooling Rate of 0.5 to 20°C / sec to Reduce to a Temperature of 680 to 780°C By making the average cooling rate of primary cooling 20°C / s or less, it is possible to promote the formation of ferrite in the soft part of the surface layer. Furthermore, an upper limit of the average cooling rate of primary cooling is determined so as to reliably obtain the effect of dividing the cooling step into two stages, namely primary cooling and secondary cooling. From that viewpoint, the average cooling rate of primary cooling is preferably 18°C / s or less, more preferably 16°C / s or less. By making the cooling step into two stages, for example, it is possible to prevent the formation of pearlite, etc., in the soft part of the surface layer or suppress the formation of pearlite, etc., while simultaneously achieving a higher ferrite area ratio.On the other hand, by making the average cooling rate of primary cooling 0.5°C / s or more, the development of excessive ferrite transformation and pearlite transformation not only in the soft part of the surface layer, but also in the middle part of the sheet thickness is suppressed, so that it is possible to easily obtain a predetermined amount of tempered martensite in the middle part of the sheet thickness. The average cooling rate of primary cooling is preferably 1°C / s or more, more preferably 2°C / s or more. Furthermore, by making the cooling stop temperature of primary cooling at 680°C or more, it is possible to keep a large amount of structures other than ferrite from forming in the soft part of the surface layer and the bending ability of the steel sheet from decreasing. The cooling stop temperature of primary cooling is preferably 700°C or more.On the other hand, by making the cooling stop temperature of the primary cooling 780°C or less, it is possible to increase the formation of ferrite in the soft part of the surface layer. Secondary Cooling: Cooling at an Average Cooling Rate of More Than 20°C / sec to Reduce to a Temperature of 25 to 600°C The average cooling rate and cooling stop temperature of secondary cooling are crucial in the formation of as-quenched martensite to obtain a predetermined amount of tempered martensite in the middle part of the sheet thickness. As-quenched martensite is formed by a transformation that uses, as its core, a number of fine dislocations present in the austenite grains before the transformation in the temperature range of 25 to 600°C. By making the average cooling rate from after primary cooling until reaching the temperature range of 25 to 600°C to be more than 20°C / s, it is possible to maintain the dislocations present in the austenite grains before the transformation from being exhausted. As a result, it is possible to reliably achieve 85% or more of tempered martensite in the final microstructure of the middle part of the sheet thickness. The average cooling rate of secondary cooling is preferably 23°C / s or more.Furthermore, the cooling stop temperature of secondary cooling is 25°C or more, but from the viewpoint of further increasing productivity, the cooling stop temperature is preferably 100°C or more. On the other hand, by making the cooling stop temperature 600°C or less, it is possible to suppress the formation of ferrite, bainite, and pearlite in the middle part of the sheet thickness while reliably forming martensite in a predetermined amount. The cooling stop temperature of secondary cooling is preferably 500°C or less. Tempering Steps After the cooling step, cold-rolled steel sheets contain mainly as-quenched martensite in the middle of the sheet thickness. Therefore, in the subsequent tempering step, this as-quenched martensite must be tempered into tempered martensite. More specifically, in the tempering step, the cold-rolled steel sheet is held at a temperature range of 100 to 400°C for 150 to 1,000 seconds, thereby tempering the as-quenched martensite in the middle of the sheet thickness into tempered martensite. Compared with when the middle of the sheet thickness contains mainly as-quenched martensite, it is possible to improve the workability of the steel sheet. By keeping the tempering temperature at 100°C or higher, it is possible to achieve a reliable tempering effect. On the other hand, by keeping the tempering temperature at 400°C or lower, it is possible to suppress excessive tempering and maintain the high strength of the steel sheet.Furthermore, by making the residence time 150 seconds or more, it is possible to reliably obtain the desired amount of tempered martensite. On the other hand, from a productivity standpoint, the preferred residence time is 1,000 seconds or less. Gilding and Surface Treatment When performing hot-dip galvanizing on a steel sheet as a plating treatment, for example, the steel sheet is heated or cooled to a temperature lower by 40°C than the galvanizing bath temperature or more and a temperature higher by 50°C than the galvanizing bath temperature or less, then the steel sheet is passed through the galvanizing bath. With such hot-dip galvanizing, a steel sheet equipped with a hot-dip galvanizing layer on its surface, i.e., a hot-dip galvanized steel sheet is obtained. The hot-dip galvanizing layer has a chemical composition composed of, for example, Fe: 7 to 15% by mass and the remainder: Zn, Al, and impurities. Furthermore, the hot-dip galvanizing layer can also be a zinc alloy. When treating steel sheets for alloying after hot-dip galvanizing, for example, hot-dip galvanized steel sheets are heated to 460°C or more and 600°C or less. If the heating temperature is less than 460°C, the alloying is sometimes insufficient. On the other hand, if the heating temperature is more than 600°C, the alloying becomes excessive and sometimes the corrosion resistance is deteriorated. Thanks to this alloying treatment, steel sheets equipped with a hot-dip galvanized layer on their surface, that is, hot-dip galvanized steel sheets, are obtained. Next, the steel sheet can be electroplated, vacuum-deposition plated, or plated in other ways. Furthermore, after electroplating, the steel sheet can be alloyed. Furthermore, organic coatings can be formed, films can be laminated, organic or inorganic salts can be used for treatment, chrome-free treatment can be performed, or other surface treatments can be applied to the steel sheet. Post-tempering process Finally, to adjust the strength, etc., of the steel sheet, the steel sheet can optionally be subjected to additional tempering. The tempering process is not specifically limited. For example, this tempering can be achieved by holding the steel sheet in a temperature range of 200 to 500°C for 2 seconds or more. Below, examples will be used to explain the present invention in more detail, but the present invention is not limited to these examples at all. Examples Examples A In each of these examples, first, a continuous cast slab with a sheet thickness of 20 mm having the chemical composition shown in Table 1 was heated to a predetermined temperature in the range of 1,100 to 1,250°C and hot rolled under conditions giving a final finish rolling temperature of 840 to 1,050°C. Within 3 seconds after finish rolling, the steel sheet was cooled at an average cooling rate of 40°C / second, and then rolled to the rolling temperature shown in Table 2. In the hot rolled coil after rolling, the maximum temperature was controlled to be 580°C or less. The holding time in the temperature range from the maximum temperature of the hot rolled coil to 500°C was 3.5 hours or less. The temperature of the hot rolled coil was measured by inserting a thermocouple at a position about 25 m from the inner end of the hot rolled coil toward the outer end in the length direction.Next, the obtained hot-rolled steel sheet is subjected to pickling treatment, and then cold-rolled with the rolling reduction shown in Table 2. Next, the obtained cold-rolled steel sheet is annealed under the conditions shown in Table 2 so as to decarburize and soften the surface layer of the steel sheet, and then cooled and tempered in a similar manner under the conditions shown in Table 2. In Table 3, the steel sheet equipped with a soft surface layer on only one side is decarburized and softened on only one surface layer of the steel sheet by stacking two cold-rolled steel sheets for annealing during the annealing step. Finally, according to the need, gilding and alloying as well as additional tempering treatments are carried out to obtain the product steel sheet.Samples taken from the obtained steel sheets were analyzed for the chemical composition of the part corresponding to the middle of the sheet thickness, where there was no change from the chemical composition shown in Table 1. Table 1 Turtle sun Middle part sheet thickness (%mass, rest: Fe and impurities) Ac3 C Si Mn PS Al NO Cr Mo B Ti Nb V Cu Ni Ca Mg LTJ Si + Mn+ Al+ Cr A 2.59 843 B 0.22 0.70 5.30 0.0 10 0.0 400 0.05 0.0 031 0.0 029 6.05 786 C 0.16 0. 35 1, 56 0.0 13 0.0 0.05 0.040 0.0 027 0.05 0.0 014 0.0 3 1. 95 875 D 0.15 0.56 9.20 0.0 12 0.0 029 0.06 0.0 0.0 031 0.0 031 0.05 9.8 0.023 E 0.083 2.60 0.0 11 0.00 50 0.02 0.0 015 0.0 014 3.42 830 F 0.22 0.17 2.80 0.0 12 0.0 039 0.04 0.0 0.0 0.0 0.3 0.3 0.3 G 0.77 1.50 0.0 10 0.0 050 0.06 0.0 043 0.0 015 0.0 015 2.33 911 H 0.15 0.60 2.50 0.0 11 0.0 058 0.00 0.0 0.02020 3.15 866 I 0.30 1.30 0.52 0.0 15 0.0 018 0.03 0.0 043 0.0 018 0.30 0.0 020 0.02 2.15 883 J 0,26 0,50 1,55 0,0 06 0,0 058 0,05 0,0 021 0,0 028 0,0 016 2,10 836 K 0,18 2,10 2,60 0,0 08 0,0 033 0,05 0,0 041 0,0 023 0,50 0,0 020 0, 02 5,25 942 5 L 0,20 0,36 2,76 0,0 15 0,0 200 0,03 0,0 044 0,0 018 0,0 10 3,15 822 M 0,29 0,05 2,43 0,008 0,0055 0,05 0,0016 0,0031 0,0 014 2,53 775 10 N 0,24 0,64 2,35 0,0 07 0,0 027 0,04 0,0 024 0,0 022 0,0 012 3,03 830 O 0,11 0,27 2,60 0,0 06 0,0 012 0,03 0,0 038 0,0 012 2,90 859 P 0,27 0,21 1,17 0,0 10 0,0 059 0,05 0,0 024 0,0 031 1,80 0,20 0,0 020 0, 30 3,23 815 15 Q 0,26 0,61 1,80 0,0 10 0,0 018 0,04 0,0 034 0,0 032 0, 03 0, 10 2,45 833 R 0,29 0,10 0,90 0,0 13 0,0 026 1,30 0,0 026 0,0 029 0,30 0,20 0,0 030 0, 03 0, 20 0, 10 2,60 1066 S 0,13 0,73 0,27 0,0 10 0,0 018 0,04 0,0 040 0,0 024 0,50 0,80 0,0 020 0, 02 0, 02 1,54 964 20 T 0,19 0,44 1,67 0,0 10 0,0 052 0,03 0,0 028 0,0 029 0, 06 2,14 859 U 0,12 0,53 2,18 0,0 06 0,0 021 0,02 0,0 037 0,0 018 0,20 0,10 0,0 020 0, 02 0, 02 0,020 2,93 887 V 0,06 1,60 2,50 0,0 20 0,0 056 0,13 0,0 021 0,0 011 4,23 977 W 0.45 1.20 2.00 0.0 20 0.0 042 0.13 0.0 035 0.0 035 3.33 794, X 0.25 2.70 2.60 0.0 20 0.0 048 0.06 0.0 037 010 1.80 0.0 036 AA 0.25 1.20 1.60 0.018 0.0010 0.02 0.0029 AB 0.28 0.20 1.00 0.0 10 0.0 090 0.02 0.0 024 0.0016 5.36 940 0.0 028 12.22 704 0.0 013 1.60 0.28 0.90 0.80 7.80 1140 0.0034 2.60 5.42 855 0.0 015 0.0 020 0.02 0.50 0.20 1.22 806 Bold underlines indicate values ​​outside the preferred range. Table 2 No. Assembly Hot welding step Cooling step Annealing step Cooling step Plating on the jute joint Setting temperature (°C) Setting time (seconds ) Slab heating temperature (°C) Final welding temperature (°C) Reduction rolling temperature (°C) thickness of the internal oxide layer (μη) log Po2 Ac330 (°C) Primary cooling and heating temperature (°C) Secondary cooling Temperature ting gal (°C) Wak tu ting gal (det ik) Cooling temperature - average temperature (k / h) cold (°C) Kecep at an average cold pen (°C / d etik) Tempe ratur lang kah pen cold (°C) 1 A 1233 859 491 35 0 -18 813 860 4 747 49 167 106 922 No Yes No 867 581 66 0 -18 756 893 2 753 62 161 203 570 No No No 3 C 1205 859 563 76 0 -17 845 889 2 717 61 350 183 83 No D No1238 920 435 45 0 -17 703 896 2 703 80 135 325 411 No No No 5 E 1228 859 530 40 0 ​​-20 800 866 2 681 28 250 246 468 Yes No No 6 F 1236 856 472 73 0 -19 773 895 10 728 52 280 241 155 Yes Yes No 7 G 1123 860 550 77 0 -17 881 932 2 721 42 146 203 327 No No No 8 H 1112 855 508 73 0 -19 836 880 2 746 44 133 169 328 No No Yes 300 300 9 I 1143 937 582 37 0 -20 853 890 3 756 87 268 272 817 No No No 10 J 1223 882 515 76 0 -18 806 884 2 709 23 139 347 156 No No No 11 K 1239 859 588 36 0 -18 912 948 2 766 96 290 382 519 Yes No No 12 L 1121 872 582 35 0 -19 792 891 16 695 89 300 133 740 Yes Yes Yes 380 600 13 M 1173 930 481 70 0 -19 745 854 4 735 36 147 162 272 No No Yes 320 500 14 N 1238 862 407 76 0 -19 800 893 6 763 77 135 312 579 No No No 15 O 1221 860 578 36 0 -17 829 889 14 707 78 131 143 700 Yes Yes Yes 320 600 16 P 1236 860 567 76 0 -16 785 854 2 773 67 320 295 785 No No No 17 Q 1243 901 543 36 0 -19 803 854 3 728 60 250 284 985 No No No 18 R 1238 859 576 76 0 -17 1036 1080 3 701 73 297 231 606 No No No 19 S 1235 920 452 34 0 -17 934 935 2 689 93 154 357 693 No No No 20 T 1135 862 577 37 0 -16 829 875 2 773 90 214 318 206 No No No 21 U 1119 857 435 54 0 -18 857 910 2 737 31 260 381 380 No No No 22 V 1237 900 411 39 0 -19 947 948 1 705 53 155 159 802 Yes No No 23 W 1236 953 556 74 0 -18 764 854 3 701 45 147 242 552 No No Yes 350 300 24 856 550 65 0 -19 674 891 1 733 42 100 361 706 Yes Yes No 26 Z 1220 1020 580 50 0 -18 1110950 2 750 32 250 350 500 No No No 27 AA 1204 880 570 60 0 -19 825 900 3 760 40 200 250 450 No No No 28 AB 1212 885 554 79 0 -20 776 890 3 720 42 300 377 650 No No No 29 H 1216 856 710 65 12 -19 836 891 1 733 42 280 361 706 Yes Yes No 30 H 1220 860 590 67 0 -18 836 860 2 710 25 620 380 200 No No No 31 H 1230 850 580 75 0 -17 836 880 32 732 35 320 254 600 Yes No No 32 H 1220 845 577 74 0 -21 836 840 5 710 41 300 280 450 Yes Yes No Bold underlines indicate values ​​outside the preferred range. The properties of the steel sheets obtained were measured and evaluated using the following methods: Thickness of the Surface Layer Soft Part, Average Vickers Hardness (Hc) of the Center of the Sheet Thickness, and Average Vickers Hardness (Hs) of the Surface Layer Soft Part The thickness of the surface layer soft part, the average Vickers Hardness (Hc) of the middle part of the sheet thickness”, and the average Vickers Hardness (Hs) of the surface layer soft part were determined in the following manner. The Vickers hardness test was carried out according to JIS Z 2244-1:2020. First, the Vickers hardness at the 1 / 2 sheet thickness position of the steel sheet was measured using a compressive load of 10 g, then, the Vickers hardness of a total of five points was measured using a compressive load of 10 g in the same manner on a line from that position in a direction vertical to the sheet thickness and parallel to the rolling direction, and the average value was determined as the average Vickers hardness (Hc) of the middle part of the sheet thickness. The distance between the measurement points was made 4 times or more indentation. Next, GDS was used to measure the C concentration in the depth direction from the surface.The area where the C concentration gradually increases from the surface until it reaches 1 / 2 of the average C concentration of the base phase is defined as the soft part of the surface layer. The thickness (%) of the soft part of the surface layer is determined. On the soft part of the surface layer that is then determined, the Vickers hardness of 10 points is measured randomly using a compressive load of 10 g and the average value is calculated to then determine the average Vickers hardness (Hs) of the soft part of the surface layer. Internal Oxide Layer Thickness The thickness of the internal oxide layer is determined by obtaining a sample that has a cross-section of the sheet thickness parallel to the rolling direction of the steel sheet and covers part of the surface layer of the steel sheet, examining the cross-section using SEM, and measuring the distance from the surface of the steel sheet to the furthest position where the internal oxides are located if moving from the surface of the steel sheet in the direction of the thickness of the steel sheet (the direction vertical to the surface of the steel sheet). The measurement depth is the area from the surface of the steel sheet to 50 μη. Cavity Area Ratio Near the Surface Layer The void area ratio near the surface layer was determined as follows: First, the examined surface was polished to a mirror finish by rubbing. The examined surface was used as the examined sample. Next, the SEM was used to capture images at 9000X magnification centered at 5 μη below the examined sample surface or the interface of the plating layer and the base iron. Using a 10 μη*10 μη area as a plane, the reflected electron topographic images of 15 consecutive adjacent planes were obtained. The areas where the topographic parts were observed were analyzed using EDS, distinguishing between inclusions and voids, only simple void parts were counted as voids, and the void ratio in the 10 μη*150 μη area captured by the SEM was determined as the void area ratio. Tensile Strength and Total Elongation The tensile strength TS and total elongation t-El were measured by conducting a tensile test in accordance with JIS Z2241:2011 based on the JIS No. 5 test specimen taken from the direction (C direction) parallel to the width direction of the steel sheet. Bending Ability Evaluation The bending ability was evaluated by measuring the bending angle α (°) with a bending test based on VDA (Verband der Automobilindustrie) 238-100: 2017-04. Defect Formation Evaluation The defect formation was evaluated based on whether microcracks with a length of 3 μη or more were formed around the indentations when pressed at 10 locations using a Vickers hardness testing machine (load of 100 g) to a depth of 5 μm from the surface of the steel sheet at room temperature (if there is a plating layer on the surface of the steel sheet, then the interface of the plating layer and the steel sheet). Specifically, cases where no microcracks were formed were evaluated as pass (OK) while cases where microcracks were formed were evaluated as fail (NG). Cases in which the tensile strength is 1250 MPa or more, the total elongation is 10% or more, the bending angle is 70° or more, and no microcracks are formed are evaluated as high-strength steel sheets that have improved bending ability and even suppress the formation of defects. The results are shown in Table 3. In Table 3, for steel sheets whose surface soft regions are formed on both sides of the middle part of the sheet thickness, only the values ​​corresponding to the surface soft regions and the internal oxide layer on one side are shown. However, each of these steel sheets is produced by subjecting both sides to the same treatment, therefore the values ​​corresponding to the surface soft regions and the internal oxide layers are essentially the same on both sides of the steel sheet.It has actually been confirmed on some steel sheets that these values ​​are the same on both sides. Table 3-1 No. Microstructure area ratio of the middle part of the sheet thickness Microstructure area ratio of the soft part of the surface layer Tempe r martensite ABCD Total A to D Martensite asquenc hed Ferri t EFG Total E to G Perlite Martensite asquenc ed Fer it Bai nit Perlite Y si sa Marte nsit tempe r Bai nit Bai nit 1 7 7 7 A Y 4 89 11 11 2 B 89 6 1 7 4 89 11 11 3 C 86 11 3 14 0 89 11 11 4 D 93 4 3 7 0 90 10 10 5 E 87 7 2 9 4 90 F 4 89 11 11 7 G 89 1 6 1 3 11 0 87 13 13 8 H 89 8 3 11 0 90 10 10 9 I 87 10 2 12 1 88 12 12 10 J 86 81 13 13 13 K 88 1 6 1 3 11 1 88 12 12 12 L 90 5 1 2 8 2 86 14 14 13 M 92 5 1 6 2 83 17 17 14 N 2 1 1 4 6 16 P 92 4 2 6 2 88 7 2 1 10 2 17 Q 95 4 1 5 0 89 11 11 18 R 90 5 3 8 2 89 11 11 19 S 4 4 8 2 89 11 11 21 U 90 5 2 7 3 85 15 15 22 V 82 6 4 10 8 85 9 9 6 23 W 93 5 2 7 87 13 13 24 X 94 2 2 7 87 Y 95 3 2 5 86 5 5 9 26 Z 30 26 0 26 44 83 9 9 8 27 AA 96 2 2 4 92 6 6 2 28 AB 91 6 3 9 86 10 10 4 29 H 85 8 4 12 3 85 10 10 5 30 H 40 48 7 5 60 88 5 5 7 31 H 87 4 7 2 13 55 8 8 2 35 32 H 95 5 5 92 4 4 4 Bold underlines indicate values ​​outside the preferred range. Table 3-2 No. Composition Sheet thickness (mm) Internal oxide layer thickness (pm) Position of the surface layer soft part Thickness of the surface layer soft part one side / sheet thickness (%) Hc Hs Hs / H c Cavity area ratio near the surface layer (%) Total tensile strength (MP) (%) Angle of bend α (°) Re Micr o Disclosure Description Average Vic kers hardness of the middle part of the sheet thickness (Hv) Average Vic kers hardness of the soft part of the per-surface layer (Hv) 1 A 1.3 5 Two-sided Example 2 B 1.8 12 Two-sided 2.1 479 211 0.44 0.8 1539 11 77 OK Example 3 C 2.2 5 Two-sided 1, 6 429 206 0.48 0.0 1417 14 99 4 D, OK 172 437 205 0.47 0.0 1400 14 83 OK Example 5 E 1.4 12 Two-sided 2.9 490 230 0.47 0.0 1578 12 84 OK Example 6 F 2.0 9 Two-sided 1474 13 85 OK Example 7 G 2.0 7 Two-sided 2.0 407 199 0.49 0.0 1289 12 80 OK Example 8 H 1.7 6 Two-sided 1. 9 439 211 0.48 0,0 1418 13 76 OK Example 9 I 2.2 8 Two-sided 1.7 536 257 0.48 0.0 1729 14 94 OK Example, 10 J 1.9 8 Two-sided 2.1 458 206 0.45 0.0 1501 13 72 OK Example 11 K 1.3 14 One-sided 4.8 430 211 0.49 0.0 1369 13 78 OK Example 12 L 2.2 10 Two-sided 1.8 448 215 0.48 0.6 1419 13 78 OK Example 13 M 2.2 6 Two-sided 1.4 472 203 0.43 0.0 1498 12 80 OK Example 14 N 1.7 6 Two-sided 1.8 478 220 0.46 0.0 1511 12 74 OK Example 15 O 2.1 5 Two-sided 1.5 408 200 0.49 0.0 1298 12 98 OK Example 16 P 2.1 11 Two-sided 2.0 461 212 0.46 0.0 1499 15 72 OK Example 17 Q 2.2 10 One-sided 1.8 473 199 0.42 0.0 1510 12 83 OK Example 18 R 2.2 8 Two-sided 2.0 492 231 0.47 0.0 1598 14 90 OK Example 19 S 2.2 3 Two-sided 1.8 432 199 0.46 0.0 1409 11 86 OK Example 20 T 1.8 6 Two-sided 1.9 435 209 0.48 0.0 1404 12 96 OK Example 21 U 1.4 9 Double-sided 2.8 457 215 0.47 0.0 1446 13 77 OK Example 22 V 2.3 10 Double-sided 1.7 270 175 0.65 0.0 868 15 85 OK Comparative example 23 W 1.7 11 Double-sided 2.6 584 286 0.49 0.0 1886 4 65 OK Comparative example 24 X 1.5 14 Double-sided 2.7 467 215 0.46 0.0 1521 8 68 OK Comparative example 25 Y 1.7 20 Two-sided 2,5 511 220 0,43 0, 0 1601 7 73 OK Comparative example 26 Z 1,6 11 Two-sided 2,2 420 197 0,47 0, 0 1378 4 60 OK Comparative example, 27 AA 1.2 9 Two-sided 3.0 502 236 0.47 0.0 1568 5 83 OK Comparison example 28 AB 2.2 1 Two-sided 1.4 445 200 0.45 0.0 1470 12 85 NG Comparison example 29 H 1.6 7 Two-sided 2.0 454 222 0.49 5.0 1478 13 88 NG Comparison example 30 H 1.6 8 Two-sided 2.2 296 193 0.65 0.0 955 4 86 OK Comparison example 31 H 1.8 8 Two-sided 2.0 434 290 0.67 0.0 1397 12 60 OK Example comparator 32 H 1,2 2 Two sides 3,1 434 213 0, 49 0, 0 1397 13 76 NG Examples of comparator Bold underlines indicate values ​​outside the preferred range. Referring to Table 3, in Comparative Example 22, the total area ratio of tempered martensite to as-quenched martensite is relatively high, but the C content is low, therefore the tensile strength decreases. In Comparative Example 23, the C content is high, therefore the tensile strength increases, but the bending ability decreases. In Comparative Example 24, the Si content is high, therefore the bending ability decreases. In Comparative Example 25, the Mn content is high, therefore the bending ability decreases. In Comparative Example 26, the Al content is high, therefore it is believed that coarse Al oxides are formed and, consequently, the bending ability decreases. In Comparative Example 27, the Cr content is high, therefore it is believed that coarse Cr carbides are formed and, consequently, the bending ability decreases.In Comparative Example 28, the total content of Si, Mn, Al, and Cr was low, therefore an internal oxide layer could not form sufficiently and, as a result, the surface hardness decreased and the formation of microcracks was observed. In Comparative Example 29, the rolling temperature was high, therefore during the hot rolling step, an internal oxide layer was formed. For this reason, it is believed that during cold rolling, voids were formed around the internal oxides and, as a result, in the final product steel sheet, the void area ratio near the surface layer could not be reduced sufficiently and the formation of microcracks was observed. In Comparative Example 30, the secondary cooling stop temperature was high, therefore in the middle part of the sheet thickness, the desired amount of tempered martensite was not formed and, as a result, the tensile strength decreased.In Comparative Example 31, the average cooling rate in the primary cooling was fast, therefore it was not possible to form sufficient ferrite in the soft part of the surface layer and, as a result, the Hs / Hc value was high and the bending ability decreased. In Comparative Example 32, the log PO2 oxygen partial pressure PO2 in the annealing step was low, therefore decarburization was not promoted and the internal oxide layer could not form sufficiently. As a result, the surface hardness decreased and the formation of microcracks was observed. In contrast to this, in each of Examples 1 to 21, by controlling the middle portion of the sheet thickness and the soft portion of the surface layer having a predetermined chemical composition and / or microstructure so that its average Vickers hardness satisfies Hs / Hc<0.50 and, further, by creating an internal oxide layer of 3 μm or more thick on the surface of the steel sheet while controlling the void area ratio near the surface layer at 3.0% or less, it is possible to improve its bending ability despite its high strength of 1,250 MPa or more and, furthermore, it is possible to suppress the formation of defects on the surface of the steel sheet significantly. Examples B In each of these examples, the influence of controlling the heat history after rolling on the properties of the obtained steel sheets was investigated. Specifically, using Example 16 of Table 3 as a reference (maximum temperature of hot rolled coil after rolling: 567°C and holding time in the temperature range from maximum temperature to 500°C: 3.5 hours), in each of Comparative Examples 33 and 34, the maximum temperature of hot rolled coil after rolling and the holding time in the temperature range from maximum temperature to 500°C were changed. Other production conditions in Comparative Examples 33 and 34 were the same as in Example 16. The results are shown in Table 4. Table 4 No. Component Maximum temperature after rolling (°C) Holding time from maximum temperature to 500°C (hours) Void area ratio near surface layer (%) Microcracks Remarks 16 P 567 3.5 0.0 OK Sample 33 P 643 3 3.9 NG Comparative sample 34 P 567 5.3 4.6 NG Comparative sample Bold underlines indicate values ​​outside the preferred range or outside the scope of this invention. Referring to Table 4, in Example 16 where the maximum temperature of the hot-rolled coil after rolling is 580°C or less and the holding time in the temperature range from the said maximum temperature to 500°C is 4 hours or less, as also shown in Table 3, the void area ratio near the surface layer of the finished product steel sheet is 0.0% and therefore the ratio can be adequately reduced to 3.0% or less. As a result, in Example 16, the formation of microcracks is not observed. On the other hand, in Comparative Example 33 where the maximum temperature of the hot-rolled coil after rolling is more than 580°C and in Comparative Example 34 where the holding time in the temperature range from the said maximum temperature to 500°C is more than 4 hours, the void area ratio near the surface layer cannot be controlled at 3.0% or less and the formation of microcracks is observed.This result is believed to occur due to the high maximum temperature of the hot-rolled coil after rolling or long holding time, therefore the internal oxide layer is finally formed during the hot rolling step and cavities are formed around the internal oxides during the subsequent cold rolling.

Claims

1. A high-strength steel sheet comprising a middle portion of the sheet thickness and a soft portion of a surface layer formed on one or both sides of the middle portion of the sheet thickness, wherein the middle portion of the sheet thickness has a chemical composition that includes, on a mass % basis, C: 0.10 to 0.30%, Si: 0.01 to 2.50%, Mn: 0.10 to 10.00%, P: 0.100% or less, S: 0.0500% or less, Al: 0 to 1.50%, N: 0.0100% or less, O: 0.0060% or less, Cr: 0 to 2.00%, Mo: 0 to 1.00%, B: 0 to 0.0100%, Ti: 0 to 0.30%, Nb: 0 to 0.30%, V: 0 to 0.50%, Cu: 0 to 1.00%, Ni: 0 to 1.00%, Ca: 0 to 0.040%, Mg: 0 to 0.040%, REE: 0 to 0.040%, and the remainder including Fe and impurities, and meeting 1.50<[Si] + [Mn] + [Al] + [Cr]<20.00, where [Si], [Mn], [Al], and [Cr] are the contents (%mass) of these elements, and having a microstructure including, by area ratio, tempered martensite: 85% or more,the soft part of the surface layer has a thickness of more than 10 μm to 5.0% or less of the sheet thickness, the microstructure of which includes, based on the area ratio, ferrite: 80% or more, and an internal oxide layer having a thickness of 3 μm or more of the surface of the high-strength steel sheet, the average Vickers hardness (Hc) of the middle part of the sheet thickness and the average Vickers hardness (Hs) of the soft part of the surface layer meet Hs / Hc<0.50, and the area ratio of the voids in the area from the surface of the high-strength steel sheet to the depth position of 10 μm is 3.0% or less., 2. A high strength steel sheet according to claim 1, wherein the middle portion of the thickness of the sheet has a microstructure consisting solely of, based on an area ratio, tempered martensite: 85% or more, at least one of ferrite, bainite, pearlite, and retained austenite: less than 15% in total, and as-quenched martensite: less than 5%.

3. A high strength steel sheet according to claim 1 or 2, wherein the soft portion of the surface layer has a microstructure consisting solely of, based on the area ratio, ferrite: 80% or more, at least one of tempered martensite, bainite, and retained austenite: less than 20% in total, pearlite: less than 5%, and as-quenched martensite: less than 5%.

4. A high strength steel sheet according to any one of claims 1 to 3, wherein the high strength steel sheet further includes a hot dip galvanized layer, a hot dip galvanized layer, or an electrogalvanized layer on the surface of the soft portion of the surface layer.