Steel sheets and plated steel sheets

Abstract

To provide a high strength steel sheet and a plated steel sheet having high plating properties and hydrogen embrittlement resistance.SOLUTION: A steel sheet contains, by mass, 0.05 to 0.40% of C, 0.2 to 3.0% of Si and 0.1 to 5.0% of Mn. The steel sheet includes an internal oxidation layer containing a granular type oxide, a grain boundary type oxide and a dendrite type oxide, on a surface layer thereof. The granular type oxide has an average grain diameter of 350nm or less. A number density of the granular type oxide in the internal oxidation layer is 4.0 pcs. / μm2 or more. A ratio A of a length of the grain boundary type oxide projected onto the surface of the steel sheet to a length of the surface of the steel sheet is 50% to 100%, inclusive. An area ratio of the dendrite type oxide is 5.0% or more. A depth of the internal oxidation layer is 8 μm or more. At a 1 / 2 location of the depth, Si and Mn contents in the steel not including an oxide are respectively below 10% of Si and Mn contents at a sheet thickness center part of the steel sheet. A plated steel sheet uses the steel sheet.SELECTED DRAWING: Figure 2

Description

[Technical Field]

[0001] The present invention relates to a steel sheet and a plated steel sheet, and more specifically to a high-strength steel sheet and a plated steel sheet having high galvanizability and hydrogen embrittlement resistance. [Background technology]

[0002] In recent years, efforts have been made to increase the strength of steel sheets used in various fields such as automobiles, home appliances, and building materials. For example, in the automobile field, the use of high-strength steel sheets has been increasing in order to reduce the weight of vehicle bodies in order to improve fuel efficiency. Such high-strength steel sheets typically contain elements such as C, Si, and Mn to improve the strength of the steel.

[0003] In the production of high-strength steel sheets, heat treatments such as annealing are generally performed after rolling. Among the elements typically contained in high-strength steel sheets, Si and Mn, which are easily oxidized elements, may combine with oxygen in the atmosphere during the heat treatment to form a layer containing oxides near the surface of the steel sheet. Examples of the form of such a layer include a film of oxides containing Si and Mn formed on the exterior (surface) of the steel sheet (external oxide layer), and an internal oxide layer formed inside (surface layer) of the steel sheet.

[0004] When a plating layer (e.g., a Zn-based plating layer) is formed on the surface of a steel sheet with an outer oxide layer, the oxide exists as a film on the surface of the steel sheet, which inhibits interdiffusion between the steel components (e.g., Fe) and the plating components (e.g., Zn), affecting the adhesion between the steel and the plating and resulting in insufficient plating (e.g., an increase in unplated areas). Therefore, from the perspective of improving plating ability, a steel sheet with an inner oxide layer is preferable to a steel sheet with an outer oxide layer.

[0005] In relation to the internal oxidation layer, Patent Documents 1 and 2 describe a high-strength plated steel sheet having a tensile strength of 980 MPa or more, which is a plated steel sheet having a zinc-based plating layer on a base steel sheet containing C, Si, Mn, etc., and in which the surface layer of the base steel sheet has an internal oxidation layer containing oxides of Si and / or Mn. [Prior art documents] [Patent documents]

[0006] [Patent Document 1] Japanese Patent Application Laid-Open No. 2016-130357 [Patent Document 2] Japanese Patent Application Publication No. 2018-193614 Summary of the Invention [Problem to be solved by the invention]

[0007] High-strength steel sheets, particularly those used for automotive components, are typically electrocoated to impart desired properties (e.g., corrosion resistance) after a zinc-based coating or other plating layer is formed on the surface. It is known that during this electrocoating process, hydrogen is generated through the electrolysis of water by the application of a voltage. The hydrogen generated during the electrocoating penetrates the steel sheet and reaches a depth deeper than the surface layer of the steel sheet. As a result, it segregates at the martensite grain boundaries of the steel structure, embrittling the grain boundaries and potentially causing cracks in the steel sheet. This phenomenon of cracks caused by absorbed hydrogen is called hydrogen embrittlement cracking (delayed fracture). Therefore, to prevent hydrogen embrittlement cracking, it is effective to suppress hydrogen penetration into the steel sheet during the electrocoating process and, moreover, to expel the hydrogen that has penetrated into the steel sheet.

[0008] Patent Documents 1 and 2 teach that controlling the average depth of the internal oxide layer to 4 μm or more and making the internal oxide layer function as a hydrogen trapping site can prevent hydrogen penetration and suppress hydrogen embrittlement. However, no consideration has been given to controlling the morphology of the oxides present in the internal oxide layer, and there is room for improvement in hydrogen embrittlement resistance.

[0009] In view of the above circumstances, an object of the present invention is to provide a high-strength steel sheet and a plated steel sheet having high platability and hydrogen embrittlement resistance. [Means for solving the problem]

[0010] The present inventors have found that, to achieve the above-mentioned object, it is important to form oxides in the surface layer of a steel sheet, i.e., the interior of the steel sheet, and further to control the morphology of the oxides present in the surface layer of the steel sheet. More specifically, the present inventors have found that forming an internal oxide layer ensures high galvanizability, and that forming a large amount of fine granular oxides present within the crystal grains of the metallographic structure and sufficient dendritic oxides present within the crystal grains allows the dendritic oxides to function as trap sites for hydrogen that can penetrate into the steel sheet during electrodeposition coating, and that forming a large amount of grain boundary oxides along the grain boundaries allows the grain boundary oxides to function as escape routes for hydrogen that has penetrated into the steel, thereby suppressing hydrogen penetration into the steel and promoting hydrogen release from the steel to the outside, thereby achieving high hydrogen embrittlement resistance. The present inventors have also found that controlling the Si and Mn contents of the oxide-free portion of the internal oxide layer within predetermined ranges promotes hydrogen diffusion in the steel and further improves hydrogen release from the steel. In addition, the inventors have found that by allowing these granular oxides, grain boundary oxides and dendritic oxides to exist at positions deeper from the surface of the steel sheet, i.e., by increasing the depth of the internal oxide layer, the above-mentioned hydrogen trapping function and hydrogen discharge function are more effectively exhibited, and hydrogen embrittlement resistance is significantly improved.

[0011] The present invention was made based on the above findings, and the gist of the present invention is as follows. (1) In mass%, C: 0.05~0.40%, Si: 0.2 to 3.0% Mn: 0.1 to 5.0% sol.Al: 0 to less than 0.4000% P:0.0300% or less, S: 0.0300% or less, N: 0.0100% or less, B: 0~0.010%, Ti: 0 to 0.150% Nb: 0 to 0.150%, V: 0~0.150%, Cr: 0~2.00%, Ni: 0-2.00% Cu: 0-2.00% Mo: 0-1.00%, W: 0~1.00%, Ca: 0 to 0.100%, Mg: 0 to 0.100%, Zr: 0 to 0.100%, Hf: 0 to 0.100%, and A steel sheet having a composition containing REM: 0 to 0.100%, with the balance being Fe and impurities, The steel sheet has an internal oxide layer on the surface thereof, the internal oxide layer including granular oxides, grain boundary oxides, and dendritic oxides, The average particle size of the granular oxide is 350 nm or less, The number density of the granular oxide in the internal oxide layer is 4.0 particles / μm 2 That's all, When a cross section of a surface layer of the steel sheet is observed, a ratio A of a length of grain boundary oxide projected onto the surface of the steel sheet to a length of the surface of the steel sheet is 50% or more and 100% or less, the area ratio of the dendritic oxide is 5.0% or more, The depth of the internal oxide layer is 8 μm or more, A steel plate, wherein the Si and Mn contents in the oxide-free steel at the half depth position are less than 10% of the Si and Mn contents at the center of the thickness of the steel plate, respectively. (2) The number density of the granular oxide in the internal oxide layer is 5.0 particles / μm 2 The steel sheet according to (1) above. (3) The steel sheet according to (1) or (2), wherein the depth of the internal oxide layer is 15 μm or more. (4) The steel sheet according to any one of (1) to (3), wherein the ratio A is 80% or more. (5) A plated steel sheet having a plating layer containing Zn on the steel sheet according to any one of (1) to (4). (6) (5) The plated steel sheet according to (5), wherein the plated layer has a chemical composition of Zn-(0.3 to 1.5)% Al. [Effects of the Invention]

[0012] According to the present invention, it is possible to make the granular oxides and dendritic oxides function as trap sites for hydrogen that penetrates the steel sheet during electrodeposition coating, and to make the grain boundary oxides function as escape routes for hydrogen that has penetrated the steel sheet. As a result, the amount of hydrogen that penetrates can be reduced and the amount of hydrogen that is released can be increased, thereby significantly improving hydrogen embrittlement resistance. Furthermore, by forming an internal oxide layer containing these oxides deeper from the surface of the steel sheet and further controlling the Si and Mn contents of the Si-Mn depleted region (the oxide-free portion of the internal oxide layer) that forms in the internal oxide layer due to the formation of these oxides within a predetermined range, it is possible to further significantly improve hydrogen embrittlement resistance. Furthermore, according to the present invention, since the oxides are formed inside the steel sheet, sufficient interdiffusion between the steel components and the coating components occurs when a coating layer is formed, resulting in high galvanizability. Therefore, the present invention makes it possible to achieve high galvanizability and hydrogen embrittlement resistance in high-strength steel sheets. [Brief explanation of the drawings]

[0013] [Figure 1] 1 shows a schematic diagram of a cross section of a steel plate having an outer oxide layer. [Figure 2] 1 shows a schematic diagram of a cross section of an exemplary steel plate according to the present invention. [Figure 3] 1 shows a schematic diagram for explaining the measurement of ratio A in the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0014] <Steel plate> The steel plate according to the present invention comprises, in mass%, C: 0.05~0.40%, Si: 0.2 to 3.0% Mn: 0.1 to 5.0% sol.Al: 0 to less than 0.4000% P:0.0300% or less, S: 0.0300% or less, N: 0.0100% or less, B: 0~0.010%, Ti: 0 to 0.150% Nb: 0 to 0.150%, V: 0~0.150%, Cr: 0~2.00%, Ni: 0-2.00% Cu: 0-2.00% Mo: 0-1.00%, W: 0~1.00%, Ca: 0 to 0.100%, Mg: 0 to 0.100%, Zr: 0 to 0.100%, Hf: 0 to 0.100%, and A steel sheet having a composition containing REM: 0 to 0.100%, with the balance being Fe and impurities, The steel sheet has an internal oxide layer on the surface thereof, the internal oxide layer including granular oxides, grain boundary oxides, and dendritic oxides, The average particle size of the granular oxide is 350 nm or less, The number density of the granular oxide in the internal oxide layer is 4.0 particles / μm 2 That's all, When a cross section of a surface layer of the steel sheet is observed, a ratio A of a length of grain boundary oxide projected onto the surface of the steel sheet to a length of the surface of the steel sheet is 50% or more and 100% or less, the area ratio of the dendritic oxide is 5.0% or more, The depth of the internal oxide layer is 8 μm or more, The oxide-free steel is characterized in that the Si and Mn contents at the half depth position are less than 10% of the Si and Mn contents at the center of the thickness of the steel plate.

[0015] In the production of high-strength steel sheets, steel billets adjusted to a predetermined chemical composition are rolled (typically hot-rolled and cold-rolled) and then annealed to obtain a desired microstructure. During this annealing process, relatively easily oxidizable components (e.g., Si, Mn) in the steel sheet combine with oxygen in the annealing atmosphere, forming an oxide-containing layer near the surface of the steel sheet. For example, as shown in FIG. 1 , an outer oxide layer 2 is formed in the form of a film on the surface of the base steel 3 (i.e., outside the base steel 3). When the outer oxide layer 2 is formed in the form of a film on the surface of the base steel 3, if a coating layer (e.g., a zinc-based coating layer) is formed, the outer oxide layer 2 inhibits the interdiffusion of the coating components (e.g., Zn) and the steel components (e.g., Fe). This can result in insufficient adhesion between the steel and the coating, resulting in uncoated areas where no coating layer is formed.

[0016] In contrast, as illustrated in FIG. 2 , a steel sheet 11 according to the present invention does not form an outer oxide layer 2 on the surface of a base steel 3 as in the steel sheet 1 shown in FIG. 1 , but has oxides 12, 13, and 14 present inside a base steel 15. Therefore, when a coating layer is formed on the surface of the steel sheet 11, the steel sheet 11 according to the present invention, in which oxides 12, 13, and 14 are formed inside the base steel 15, allows for sufficient interdiffusion between the coating components and the steel components compared to the steel sheet 1 having an outer oxide layer 2, thereby enabling high coating ability to be achieved. Therefore, the present inventors have discovered that, from the perspective of achieving high coating ability, it is effective to control the annealing conditions to form oxides inside the steel sheet. Note that the term "high coating ability," when used in reference to a steel sheet, indicates that a coating layer can be formed on the steel sheet with little (e.g., 5.0 area % or less) or no uncoated areas (areas on which a coating layer is not formed) when the steel sheet is subjected to a coating treatment. Furthermore, the term "high platability," when used in reference to plated steel sheets, refers to plated steel sheets with very little (for example, 5.0% or less) or no unplated areas.

[0017] On the other hand, during electrodeposition coating, which is commonly performed in the manufacture of high-strength steel sheets, particularly for automobiles, hydrogen is generated by the electrolysis of water. It is known that this hydrogen penetrates deeper than the surface layer of the base steel, segregating at the martensite grain boundaries of the base steel and embrittling the grain boundaries, causing hydrogen embrittlement cracking. Because martensite is a hard structure, it is highly sensitive to hydrogen and prone to hydrogen embrittlement cracking. Therefore, in order to prevent hydrogen embrittlement cracking of steel sheets, it is preferable to suppress the penetration of hydrogen into the steel sheet during electrodeposition coating and promote the release of the penetrated hydrogen, i.e., to have high hydrogen embrittlement resistance. The present inventors have found that by controlling the morphology of oxides present in the surface layer of a steel sheet, more specifically, by forming granular, grain boundary, and dendritic oxides on the surface layer of the steel sheet, the granular and dendritic oxides function to trap hydrogen that enters during electrodeposition coating, and the grain boundary oxide functions to release hydrogen from the inside of the steel sheet to the outside, thereby suppressing the amount of hydrogen that accumulates inside the steel sheet and achieving high hydrogen embrittlement resistance.In addition, the present inventors have found that the formation of granular, grain boundary, and dendritic oxides reduces the surrounding Si and Mn concentrations, and that controlling this decrease in Si and Mn concentrations within a predetermined range can promote hydrogen diffusion in the steel and further improve hydrogen release from the steel. The inventors also found that in order to further improve the hydrogen trapping function and hydrogen discharge function, it is effective to make the internal oxide layer containing these oxides exist at a deeper position, specifically to a depth of 8 μm from the surface of the steel sheet (or, in the case where a plating layer is present on the surface of the steel sheet, from the interface between the plating layer and the steel sheet).

[0018] Furthermore, the present inventors conducted a detailed analysis of the relationship between the morphology of oxides and their effectiveness as hydrogen trapping sites. As a result, they found that, as shown in Figure 2, it is effective to have a large amount of fine, spaced-apart granular oxides 12 in the surface layer of a base steel 15, and to have a certain area ratio, more specifically, 5.0% or more, of dendritic oxides 14 in a dendrite shape. Without being bound by any particular theory, it is believed that the hydrogen trapping ability of oxides in a steel sheet is positively correlated with the surface area of the oxides. In other words, it is believed that when a large amount of granular oxides are finely and discretely dispersed in the surface layer of a steel sheet and an appropriate amount of dendritic oxides are present, the surface area of the oxides in the surface layer of the steel sheet increases, significantly improving the hydrogen trapping ability. Therefore, the inventors have discovered that, from the viewpoint of achieving high hydrogen embrittlement resistance, it is important to control the conditions during steel sheet production, particularly annealing, to ensure the presence of a large amount of fine granular oxides, which function as trap sites for hydrogen that penetrates during electrodeposition coating, and an appropriate amount of dendritic oxides. Furthermore, the inventors have conducted a detailed analysis of the relationship between the morphology of oxides and their effectiveness as hydrogen escape routes. As shown in Figure 2, the inventors have found that it is effective to have a large amount of grain boundary oxides 13 present at grain boundaries in the surface layer of the base steel 15. The presence of a large amount of grain boundary oxides 13 ensures a pathway for hydrogen in the steel to escape from the system, enabling hydrogen that has penetrated into the steel to be efficiently released along the grain boundaries. Therefore, as illustrated in Figure 2, the coexistence of granular oxides 12, grain boundary oxides 13, and dendritic oxides 14 in the surface layer of the steel sheet 11, i.e., the base steel 15, can significantly improve hydrogen embrittlement resistance. The metal structure of the surface layer of a steel plate is typically softer than that of the interior of the steel plate (for example, at the 1 / 8 or 1 / 4 position of the plate thickness). Therefore, even if hydrogen is present in the surface layer of the steel plate, hydrogen embrittlement cracking does not pose a particular problem.

[0019] Furthermore, the present inventors conducted a detailed analysis of the relationship between hydrogen release performance and the Si-Mn depletion zone (the oxide-free portion of the internal oxide layer) that forms in the internal oxide layer due to the decrease in the surrounding Si and Mn concentrations caused by the formation of internal oxides such as granular oxide 12, grain boundary oxide 13, and dendritic oxide 14, as shown in Fig. 2 . As a result, they found that it is effective to control the composition of the Si-Mn depletion zone within a predetermined range, more specifically, to control the Si and Mn contents of the oxide-free zone at half the depth of the internal oxide layer so that they are less than 10% of the Si and Mn contents at the center of the steel sheet thickness (hereinafter, these values are also referred to as the Si depletion rate and the Mn depletion rate). Without being bound by any particular theory, it is believed that when steel contains a large amount of Si and / or Mn, the amount of Si and / or Mn dissolved in the steel also increases, and this dissolved Si and / or Mn inhibits hydrogen diffusion, resulting in a slower hydrogen diffusion rate in the steel. As shown in Figure 2, when internal oxides such as granular oxides 12, grain boundary oxides 13, and dendritic oxides 14 are formed in the surface layer of a steel sheet, the Si and Mn dissolved in the steel are consumed in the formation of the internal oxides. This results in the formation of Si-Mn-depleted regions in the surface layer of the steel sheet, where the Si and Mn concentrations are relatively low. By setting the depth of the internal oxide layer to 8 μm or more as described above, the thickness of the Si-Mn-depleted regions naturally becomes similar, which is believed to ensure sufficient hydrogen diffusion paths. Furthermore, by sufficiently reducing the Si and Mn contents in the Si-Mn-depleted regions—specifically, by controlling the Si and Mn deficiency rates to be less than 10% each—the amounts of solute Si and Mn that inhibit hydrogen diffusion are believed to be sufficiently reduced. Therefore, the inclusion of such a controlled Si-Mn-depleted region in the internal oxide layer is believed to promote hydrogen diffusion and significantly improve hydrogen desorption from the steel. Therefore, by combining the above-mentioned granular oxides, grain boundary oxides, and dendritic oxides with the Si-Mn depleted regions, it is possible to improve both the hydrogen penetration resistance and the hydrogen desorption resistance, thereby significantly improving the hydrogen embrittlement resistance of the steel sheet as a whole.

[0020] It is also known that hydrogen embrittlement cracking can occur not only due to the penetration of hydrogen into steel sheets during electrodeposition coating as described above, but also due to the penetration of hydrogen present in the annealing atmosphere deeper than the surface layer region of the base steel during the annealing treatment in the production of high-strength steel sheets. The present inventors have now discovered that the combination of the morphology of the internal oxide layer and the characteristics of the Si-Mn depleted region in the internal oxide layer described above is effective in suppressing hydrogen penetration into steel sheets and in expelling the penetrated hydrogen not only during electrodeposition coating but also during annealing treatment.

[0021] The steel plate according to the present invention will be described in detail below. The thickness of the steel plate according to the present invention is not particularly limited, but may be, for example, 0.1 to 3.2 mm.

[0022] [Steel plate composition] The composition of the elements contained in the steel sheet according to the present invention will be described. Unless otherwise specified, "%" regarding the content of an element means "% by mass." In the numerical range of the element composition, a numerical range expressed using "to" means a range that includes the numerical values written before and after "to" as the lower and upper limits, unless otherwise specified.

[0023] (C: 0.05 to 0.40%) C (carbon) is an important element for ensuring the strength of steel. To ensure sufficient strength, the C content is set to 0.05% or more. The C content is preferably 0.07% or more, more preferably 0.10% or more, and even more preferably 0.12% or more. On the other hand, if the C content is excessive, there is a risk of reduced weldability. Therefore, the C content is set to 0.40% or less. The C content may be 0.38% or less, 0.35% or less, 0.32% or less, or 0.30% or less.

[0024] (Si: 0.2 to 3.0%) Silicon (Si) is an element effective in improving the strength of steel. To ensure sufficient strength and to allow the desired oxides to be sufficiently formed inside the steel sheet, the Si content is set to 0.2% or more. The Si content is preferably 0.3% or more, more preferably 0.5% or more, and even more preferably 1.0% or more. On the other hand, an excessive Si content may result in excessive formation of outer oxides, which may in turn cause deterioration of the surface properties. Therefore, the Si content is set to 3.0% or less. The Si content may be 2.8% or less, 2.5% or less, 2.3% or less, or 2.0% or less.

[0025] (Mn: 0.1 to 5.0%) Manganese (Mn) is an element effective in improving the strength of steel by obtaining a hard structure. To ensure sufficient strength and further ensure that the desired oxides are sufficiently formed inside the steel sheet, the Mn content is set to 0.1% or more. The Mn content is preferably 0.5% or more, more preferably 1.0% or more, and even more preferably 1.5% or more. On the other hand, excessive Mn content may result in excessive formation of outer oxides, or Mn segregation may cause the metal structure to become non-uniform, resulting in reduced workability. Therefore, the Mn content is set to 5.0% or less. The Mn content may be 4.5% or less, 4.0% or less, 3.5% or less, or 3.0% or less.

[0026] (sol.Al: 0 to less than 0.4000%) Al (aluminum) is an element that acts as a deoxidizing element. The Al content may be 0%, but to obtain a sufficient deoxidizing effect, the Al content is preferably 0.0010% or more. The Al content is more preferably 0.0050% or more, even more preferably 0.0100% or more, and even more preferably 0.0150% or more. On the other hand, an excessive Al content may cause a decrease in workability and deterioration of surface properties. Therefore, the Al content is set to less than 0.4000%. The Al content may be 0.3900% or less, 0.3800% or less, 0.3700% or less, 0.3500% or less, 0.3400% or less, 0.3300% or less, 0.3000% or less, or 0.2000% or less. The Al content refers to the content of so-called acid-soluble Al (sol. Al).

[0027] (P:0.0300% or less) P (phosphorus) is an impurity generally contained in steel. Excessive P content may reduce weldability. Therefore, the P content is set to 0.0300% or less. The P content is preferably 0.0200% or less, more preferably 0.0100% or less, and even more preferably 0.0050% or less. The lower limit of the P content is 0%, but from the viewpoint of production costs, the P content may be more than 0% or 0.0001% or more.

[0028] (S:0.0300% or less) S (sulfur) is an impurity generally contained in steel. Excessive S content may reduce weldability and, further, increase the amount of MnS precipitation, which may reduce workability such as bendability. Therefore, the S content is set to 0.0300% or less. The S content is preferably 0.0100% or less, more preferably 0.0050% or less, and even more preferably 0.0020% or less. The lower limit of the S content is 0%, but from the viewpoint of desulfurization costs, the S content may be more than 0% or 0.0001% or more.

[0029] (N:0.0100% or less) N (nitrogen) is an impurity generally contained in steel. Excessive N content may reduce weldability. Therefore, the N content is set to 0.0100% or less. The N content is preferably 0.0080% or less, more preferably 0.0050% or less, and even more preferably 0.0030% or less. The lower limit of the N content is 0%, but from the viewpoint of production costs, the N content may be more than 0% or 0.0010% or more.

[0030] The basic chemical composition of the steel sheet according to the present invention is as described above. Furthermore, the steel sheet may contain the following optional elements as needed. The inclusion of these elements is not essential, and the lower limit of the content of these elements is 0%.

[0031] (B: 0 to 0.010%) Boron (B) is an element that contributes to improving strength by improving hardenability and segregates at grain boundaries to strengthen the grain boundaries and improve toughness. The B content may be 0%, but may be contained as necessary to obtain the above effects. The B content may be 0.0001% or more, 0.0005% or more, or 0.001% or more. On the other hand, from the viewpoint of ensuring sufficient toughness and weldability, the B content is preferably 0.010% or less, and may be 0.008% or less, or 0.006% or less.

[0032] (Ti: 0 to 0.150%) Ti (titanium) is an element that precipitates as TiC during cooling of steel and contributes to improving strength. The Ti content may be 0%, but may be contained as necessary to obtain the above-mentioned effects. The Ti content may be 0.001% or more, 0.003% or more, 0.005% or more, or 0.010% or more. On the other hand, excessive Ti content may generate coarse TiN, which may impair toughness. Therefore, the Ti content is preferably 0.150% or less, and may be 0.100% or less, or 0.050% or less.

[0033] (Nb: 0 to 0.150%) Nb (niobium) is an element that contributes to improving strength by improving hardenability. The Nb content may be 0%, but may be contained as necessary to obtain the above effects. The Nb content may be 0.001% or more, 0.005% or more, 0.010% or more, or 0.015% or more. On the other hand, from the viewpoint of ensuring sufficient toughness and weldability, the Nb content is preferably 0.150% or less, and may be 0.100% or less, or 0.060% or less.

[0034] (V:0~0.150%) V (vanadium) is an element that contributes to improving strength by improving hardenability. The V content may be 0%, but may be contained as necessary to obtain the above effects. The V content may be 0.001% or more, 0.010% or more, 0.020% or more, or 0.030% or more. On the other hand, from the viewpoint of ensuring sufficient toughness and weldability, the V content is preferably 0.150% or less, and may be 0.100% or less, or 0.060% or less.

[0035] (Cr: 0 to 2.00%) Cr (chromium) is effective in improving the hardenability of steel and increasing its strength. The Cr content may be 0%, but may be contained as necessary to obtain the above effect. The Cr content may be 0.01% or more, 0.10% or more, 0.20% or more, 0.50% or more, or 0.80% or more. On the other hand, excessive Cr content may form a large amount of Cr carbide, which may adversely affect hardenability. Therefore, the Cr content is preferably 2.00% or less, and may be 1.80% or less or 1.50% or less.

[0036] (Ni: 0 to 2.00%) Ni (nickel) is an element that is effective in improving the hardenability of steel and increasing its strength. The Ni content may be 0%, but may be contained as necessary to obtain the above effects. The Ni content may be 0.01% or more, 0.10% or more, 0.20% or more, 0.50% or more, or 0.80% or more. On the other hand, excessive addition of Ni increases costs. For this reason, the Ni content is preferably 2.00% or less, and may be 1.80% or less or 1.50% or less.

[0037] (Cu: 0 to 2.00%) Cu (copper) is an element effective in improving the hardenability of steel and increasing its strength. The Cu content may be 0%, but may be contained as necessary to obtain the above effects. The Cu content may be 0.001% or more, 0.005% or more, or 0.01% or more. On the other hand, from the viewpoint of suppressing a decrease in toughness, cracking of the slab after casting, and a decrease in weldability, the Cu content is preferably 2.00% or less, and may be 1.80% or less, 1.50% or less, or 1.00% or less.

[0038] (Mo: 0-1.00%) Mo (molybdenum) is an element effective in improving the hardenability of steel and increasing its strength. The Mo content may be 0%, but may be contained as necessary to obtain the above effects. The Mo content may be 0.01% or more, 0.10% or more, 0.20% or more, or 0.30% or more. On the other hand, from the viewpoint of suppressing deterioration of toughness and weldability, the Mo content is preferably 1.00% or less, and may be 0.90% or less or 0.80% or less.

[0039] (W:0~1.00%) W (tungsten) is an element effective in improving the hardenability of steel and increasing its strength. The W content may be 0%, but may be contained as necessary to obtain the above effects. The W content may be 0.001% or more, 0.005% or more, or 0.01% or more. On the other hand, from the viewpoint of suppressing deterioration in toughness and weldability, the W content is preferably 1.00% or less, and may be 0.90% or less, 0.80% or less, 0.50% or less, or 0.10% or less.

[0040] (Ca: 0 to 0.100%) Ca (calcium) is an element that contributes to inclusion control, particularly to the fine dispersion of inclusions, and has the effect of increasing toughness. The Ca content may be 0%, but may be contained as necessary to obtain the above effects. The Ca content may be 0.0001% or more, 0.0005% or more, or 0.001% or more. On the other hand, excessive Ca content may cause noticeable deterioration in surface properties. For this reason, the Ca content is preferably 0.100% or less, and may be 0.080% or less, 0.050% or less, 0.010% or less, or 0.005% or less.

[0041] (Mg: 0 to 0.100%) Magnesium (Mg) is an element that contributes to inclusion control, particularly to the fine dispersion of inclusions, and has the effect of increasing toughness. The Mg content may be 0%, but may be contained as necessary to obtain the above effects. The Mg content may be 0.0001% or more, 0.0005% or more, or 0.001% or more. On the other hand, excessive Mg content may cause noticeable deterioration in surface properties. For this reason, the Mg content is preferably 0.100% or less, and may be 0.090% or less, 0.080% or less, 0.050% or less, or 0.010% or less.

[0042] (Zr: 0 to 0.100%) Zr (zirconium) is an element that contributes to inclusion control, particularly to the fine dispersion of inclusions, and has the effect of increasing toughness. The Zr content may be 0%, but may be contained as necessary to obtain the above effects. The Zr content may be 0.001% or more, 0.005% or more, or 0.010% or more. On the other hand, excessive Zr content may cause noticeable deterioration in surface properties. For this reason, the Zr content is preferably 0.100% or less, and may be 0.050% or less, 0.040% or less, or 0.030% or less.

[0043] (Hf: 0 to 0.100%) Hf (hafnium) is an element that contributes to inclusion control, particularly to the fine dispersion of inclusions, and has the effect of increasing toughness. The Hf content may be 0%, but may be contained as necessary to obtain the above effects. The Hf content may be 0.0001% or more, 0.0005% or more, or 0.001% or more. On the other hand, excessive Hf content may cause noticeable deterioration in surface properties. Therefore, the Hf content is preferably 0.100% or less, and may be 0.050% or less, 0.030% or less, or 0.010% or less.

[0044] (REM: 0 to 0.100%) REM (rare earth elements) are elements that contribute to inclusion control, particularly to the fine dispersion of inclusions, and have the effect of increasing toughness. The REM content may be 0%, but may be added as necessary to achieve the above-mentioned effects. The REM content may be 0.0001% or more, 0.0005% or more, or 0.001% or more. On the other hand, excessive REM content may cause noticeable deterioration in surface properties. For this reason, the REM content is preferably 0.100% or less, and may be 0.050% or less, 0.030% or less, or 0.010% or less. REM is an abbreviation for Rare Earth Metal and refers to elements belonging to the lanthanide series. REM is usually added as misch metal.

[0045] The balance of the steel sheet according to the present invention other than the above-mentioned chemical composition is Fe and impurities, which are components that are mixed in due to various factors in the manufacturing process, including raw materials such as ore and scrap, during industrial manufacturing of the steel sheet.

[0046] In the present invention, the chemical composition of the steel sheet may be analyzed by an elemental analysis method known to those skilled in the art, for example, inductively coupled plasma mass spectrometry (ICP-MS). However, C and S may be measured using a combustion-infrared absorption method, and N may be measured using an inert gas fusion-thermal conductivity method. These analyses may be performed on samples taken from the steel sheet in accordance with JIS G0417:1999.

[0047] [surface] In the present invention, the "surface layer" of a steel sheet means a region extending from the surface of the steel sheet (the interface between the steel sheet and the plating layer in the case of a plated steel sheet) to a predetermined depth in the sheet thickness direction, and the "predetermined depth" is typically 50 μm or less.

[0048] As illustrated in Fig. 2, a steel sheet 11 according to the present invention has an internal oxide layer at the surface thereof, which includes a granular oxide 12, a grain boundary oxide 13, and a dendritic oxide 14. As will be described later, the granular oxide 12 and the dendritic oxide 14 are present within the crystal grains of the metallographic structure of the steel sheet 11, and the grain boundary oxide 13 is present along the grain boundaries of the metallographic structure. The presence of these oxides 12, 13, and 14 inside the base steel 15 (i.e., as internal oxides) enables the steel sheet 11 to have higher platability than the case in which an outer oxide layer 2 exists on the surface of the base steel 3 shown in Fig. 1. This is thought to be the result of sufficient interdiffusion between the plating components and the steel components when a plating (e.g., a Zn-based plating) is formed on the surface of the steel sheet, since the outer oxide layer that inhibits interdiffusion between the plating components and the steel components is absent or present only in a sufficiently thin thickness. Therefore, the steel sheet and plated steel sheet according to the present invention, which contain oxides 12, 13, 14 in the surface layer of the steel sheet, i.e., inside the steel sheet, have high platability.

[0049] [Granular oxide] In the present invention, "granular oxides" refer to oxides dispersed in a granular shape within crystal grains or on grain boundaries in the steel surface layer. Furthermore, "granular" refers to oxides present at intervals within the steel matrix, e.g., having an aspect ratio (maximum line segment length intersecting the granular oxide (major axis) / maximum line segment length intersecting the oxide perpendicular to the major axis (minor axis)) of 1.0 to 5.0. "Granularly dispersed" refers to the position of each particle of the granular oxide not being arranged according to a specific rule (e.g., linearly or approximately linearly) but being randomly arranged. In reality, granular oxides are typically present three-dimensionally in spherical or approximately spherical shapes in the surface layer of the steel sheet. Therefore, when a cross section of the surface layer of the steel sheet is observed, the granular oxides are typically observed in a circular or approximately circular shape. FIG. 2 shows a granular oxide 12 that appears circular as an example. 2, the granular oxide 12 is typically located closer to the surface than the dendritic oxide 14. However, the granular oxide 12 and the dendritic oxide 14 may coexist within the crystal grain.

[0050] (Average particle size) In the present invention, the average particle size of the granular oxide is 350 nm or less. By controlling the average particle size within this range, the granular oxide can be finely dispersed in the surface layer of the steel sheet, and the granular oxide functions well as a hydrogen trapping site that suppresses hydrogen penetration during electrodeposition coating and / or annealing in the manufacturing process. On the other hand, if the average particle size exceeds 350 nm, the granular oxide does not function sufficiently as a hydrogen trapping site, and good hydrogen embrittlement resistance may not be obtained. The average particle size of the granular oxide is preferably 330 nm or less, more preferably 320 nm or less, and even more preferably 310 nm or less. Since the finer the granular oxide, the better, the average particle size of the granular oxide is not particularly limited, but may be, for example, 5 nm or more, 10 nm or more, 50 nm or more, or 100 nm or more.

[0051] (number density) In the present invention, the number density of the granular oxide in the internal oxide layer is 4.0 particles / μm 2 By controlling the number density within this range, it is possible to disperse a large amount of granular oxides in the surface layer of the steel sheet, and the granular oxides function well as hydrogen trap sites that suppress hydrogen penetration during electrodeposition coating and / or annealing treatment in the manufacturing process. On the other hand, when the number density is 4.0 particles / μm 2 If the number density of the granular oxides is less than 4.5 / μm, the number density of hydrogen trapping sites is insufficient, and hydrogen penetration during electrodeposition coating and / or annealing in the manufacturing process cannot be sufficiently suppressed, and good hydrogen embrittlement resistance may not be obtained. 2 More preferably, 5.0 particles / μm 2 More preferably, 6.0 particles / μm 2 The more granular oxide there is, the better. Therefore, the upper limit of the number density of the granular oxide is not particularly limited, but it is not particularly limited to, for example, 100.0 particles / μm 2 It may be the following:

[0052] The average particle size and number density of granular oxides are measured using a scanning electron microscope (SEM). The specific measurement procedure is as follows: A cross section of the surface layer of a steel sheet is observed using an SEM to obtain an SEM image containing granular oxides. Ten observation regions, each measuring 1.0 μm (depth direction) × 1.0 μm (width direction), are selected from the SEM image, excluding the grain boundary oxides and dendritic oxides described below. The observation position for each region is 1.0 μm in the depth direction (direction perpendicular to the steel sheet surface) within a region extending 1.5 μm from the steel sheet surface, and 1.0 μm at any position in the width direction (direction parallel to the steel sheet surface). Next, SEM images of each of the selected regions are extracted and binarized to separate the oxide and steel regions. The total area of the granular oxide regions is calculated from each binarized image, and the number of granular oxides in each binarized image is counted. From the total area and number of granular oxides in the 10 regions thus obtained, the average particle size (nm) of the granular oxides is calculated as the circle equivalent diameter. 2) is equal to the average number of granular oxides counted from each binarized image. Note that if only a portion of the granular oxide is observed in the observation area, that is, if the entire outline of the granular oxide is not within the observation area, it is not counted. In addition, from the viewpoint of measurement accuracy, the lower limit for counting the number of granular oxides is 5.0 nm or more.

[0053] [Grain boundary oxide] In the present invention, the term "grain boundary oxide" refers to oxides present along the grain boundaries of steel, and does not include oxides present within the grains of steel. In reality, grain boundary oxides are present in a planar form along the grain boundaries in the surface layer of a steel sheet, and therefore, when a cross section of the surface layer of the steel sheet is observed, the grain boundary oxides are observed in a linear form. As an example, FIGS. 2 and 3 show a linear grain boundary oxide 13. Also, in FIG. 2, as a typical example of steel sheet 11, grain boundary oxide 13 is shown below granular oxide 12, but grain boundary oxide 13 also forms near the surface of base steel 15. When grain boundary oxide 13 is formed near the surface of the steel sheet, it connects the steel sheet surface with the interior of the steel sheet, thereby fully demonstrating the hydrogen release function.

[0054] (Ratio A) In the present invention, the "ratio A" refers to the ratio of the "length of the grain boundary oxide projected onto the surface of the steel sheet: L (L1 + L2 + L3 + L4)" to the "length of the surface of the steel sheet: L0" in the observed image when the cross section of the surface layer of the steel sheet 11 is observed, as shown in FIG. 3 . In the present invention, the ratio A is 50% or more and 100% or less. By controlling the ratio A within this range, the grain boundary oxide can be formed over a wide area inside the steel sheet, and the grain boundary oxide functions well as an escape path for hydrogen. On the other hand, if the ratio A is less than 50%, the grain boundary oxide may not function sufficiently as an escape path for hydrogen, and good hydrogen embrittlement resistance may not be obtained. The ratio A is preferably 60% or more, more preferably 70% or more, even more preferably 80% or more, and even more preferably 90% or more. The ratio A may be 100%.

[0055] The ratio A is determined by observing a cross section of the surface layer of the steel sheet 11, as shown in FIG. 3 . The specific measurement method is as follows: The cross section of the surface layer of the steel sheet 11 is observed using an SEM. The observation position is selected at random. The surface length L0 (i.e., the width of the SEM image) is measured from the observed SEM image. The length L0 is 100 μm or more (e.g., 100 μm, 150 μm, or 200 μm), and the measurement depth is a region from the surface of the steel sheet to 50 μm. Next, the position of the grain boundary oxide 13 is identified from the SEM image, and the identified grain boundary oxide 13 is projected onto the surface of the steel sheet 11 (on the interface between the steel sheet 11 and the coating layer in the case of a plated steel sheet), and the length L (= L1 + L2 + L3 + L4) of the grain boundary oxide 13 within the field of view is determined. Based on the thus determined L0 and L, the ratio A (%) = 100 × L / L0 in the present invention is calculated. It should be noted that for the sake of explanation, the granular oxide 12 and the dendritic oxide 14 are omitted from FIG.

[0056] [Dendrite-type oxide] In the present invention, "dendritic oxides" refer to oxides present in a dendrite shape within crystal grains inside steel. "Intragrain" as used herein refers to oxides with a crystal orientation misorientation of less than 10° as measured by electron backscatter diffraction (EBSD). Furthermore, "dendritic" refers to a dendritic shape formed by three-dimensional growth of multiple needle- or leaf-like branch portions (secondary arms) branching from a main branch (primary arm). For example, this refers to a primary arm having a length of 0.5 to 5.0 μm from which secondary arms having a length of 50 to 300 nm grow. The primary arm preferably has a length of 1.0 to 5.0 μm, more preferably 2.0 to 5.0 μm. The secondary arm preferably has a length of 70 to 250 nm, more preferably 100 to 250 nm. The lengths of the primary and secondary arms can be measured by observing the cross section of the steel sheet using a scanning electron microscope (SEM). In reality, dendritic oxides typically exist three-dimensionally in a dendritic pattern within steel sheet crystal grains. Therefore, when a cross section of the surface layer of a steel sheet is observed, the dendritic oxide is typically observed as a single primary arm with multiple secondary arms branching out on both sides, or as a dotted, approximately linear pattern of secondary arms at approximately equal intervals (e.g., the difference in spacing between adjacent points is 10% or less). Figure 2 shows, as an example, a dendritic oxide 14 observed as a single primary arm with multiple secondary arms branching out on both sides, and a dendritic oxide 14 observed as a dotted, approximately linear pattern of secondary arms. In certain embodiments, the dendritic oxide may not be present in a region up to 1 μm, 2 μm, 3 μm, or 5 μm from the surface of the steel sheet.

[0057] (area ratio) In the present invention, the area ratio of the dendritic oxide is 5.0% or more. By controlling the area ratio of the dendritic oxide within this range, a sufficient amount of dendritic oxide can be present within the crystal grains inside the steel sheet, and the dendritic oxide functions well as a hydrogen trapping site that suppresses hydrogen penetration during electrodeposition coating. On the other hand, if the area ratio of the dendritic oxide is less than 5.0%, the amount is insufficient to function as a hydrogen trapping site, and hydrogen penetration during electrodeposition coating cannot be sufficiently suppressed, which may result in poor hydrogen penetration resistance and therefore poor hydrogen embrittlement resistance. The area ratio of the dendritic oxide is preferably 10.0% or more or 20.0% or more, more preferably 30.0% or more, and even more preferably 50.0% or more. Since the presence of a large amount of dendritic oxide is preferable, the upper limit of the area ratio of the dendritic oxide is not particularly limited, but may be, for example, 70.0% or less or 60.0% or less.

[0058] The area ratio of dendritic oxides is measured using a scanning electron microscope (SEM). The specific measurement procedure is as follows: A cross section of the surface layer of a steel sheet is observed using an SEM to obtain an SEM image containing dendritic oxides, such as that shown in Figure 2. Ten observation regions, each measuring 1.0 μm (depth direction) × 1.0 μm (width direction) and containing dendritic oxides, are selected from the SEM image. The observation regions are selected to be regions free of the aforementioned granular oxides and intergranular oxides. Regarding the distinction between granular oxides and dendritic oxides (especially dendritic oxides observed as having only secondary arms arranged in a roughly linear dotted pattern), a dendritic oxide can be considered to exist when five or more oxide dots are arranged in a roughly linear pattern in the SEM image. The observation position for each region was 1.0 μm in the depth direction (direction perpendicular to the surface of the steel sheet) within a region from the surface of the steel sheet to a depth of 5.0 μm to 10.0 μm, and 1.0 μm at any position on the SEM image in the width direction (direction parallel to the surface of the steel sheet). Next, SEM images of each region selected as above were extracted and binarized to separate the oxide portion from the steel portion, and the total area of the dendritic oxide portion was calculated from each binarized image. The total area of the dendritic oxide in the 10 regions thus obtained was multiplied by the total area of the 10 regions (10 μm 2 ) to determine the "area ratio of dendritic oxides" in the present invention.

[0059] (depth of internal oxide layer) In the steel sheet according to the present invention, the internal oxide layer is a layer formed inside the steel sheet and includes a granular oxide 12, a grain boundary oxide 13, and a dendritic oxide 14. Therefore, the "internal oxide layer" refers to the region from the surface of the steel sheet to the furthest position where any of the oxides 12, 13, and 14 is present. Therefore, the "depth of the internal oxide layer" refers to the distance from the surface of the steel sheet 11 (the interface between the steel sheet and the coating layer in the case of a plated steel sheet) to the furthest position where the granular oxide 12, the grain boundary oxide 13, and the dendritic oxide 14 are present, as indicated by "R" in FIG. 2 , when moving from the surface of the steel sheet 11 (the interface between the steel sheet and the coating layer in the case of a plated steel sheet) in the thickness direction of the steel sheet 11 (the direction perpendicular to the surface of the steel sheet). FIG. 2 shows, as an example, a case where the dendritic oxide 14 is present at the deepest position. As described above, the granular oxide 12 and the dendritic oxide 14 function as trap sites for hydrogen that penetrates during electrodeposition coating, and the grain boundary oxide 13 can function as an escape route for hydrogen that penetrates into the steel sheet. Therefore, the greater the depth R of the internal oxide layer, the more hydrogen can be trapped in the surface region of the steel sheet, and the more hydrogen can be released to the outside of the system. In the steel sheet according to the present invention, the depth R of the internal oxide layer is 8 μm or more, preferably 10 μm or more, more preferably 12 μm or more, and most preferably 15 μm or more. There is no particular upper limit to the depth R, but it is substantially 100 μm or less. The depth R may be determined from the same SEM image (surface length L0) as the one used to measure the ratio A described above.

[0060] (Si-Mn depleted region) In the steel sheet according to the present invention, the Si and Mn contents in the oxide-free region at half the depth of the internal oxide layer are less than 10% of the Si and Mn contents in the center of the steel sheet thickness. The Si-Mn depleted region (the oxide-free portion of the internal oxide layer) formed in the surface layer of the steel sheet due to the formation of granular, intergranular, and dendritic oxides is made thick enough to promote hydrogen diffusion by setting the depth of the internal oxide layer to 8 μm or more. Furthermore, by controlling the Si and Mn deficiency rates of the Si-Mn depleted region to less than 10%, the amounts of solute Si and Mn that inhibit hydrogen diffusion can be sufficiently reduced. As a result, hydrogen diffusion can be promoted, significantly improving hydrogen desorption from the steel sheet.

[0061] Furthermore, by reducing the Si and Mn deficiency rates in the Si-Mn depleted region, the amount of solute Si and Mn in the steel can be further reduced. Therefore, the Si deficiency rate in the Si-Mn depleted region is preferably 8% or less, more preferably 6% or less, and most preferably 4% or less. The lower limit of the Si deficiency rate is, but may be, 0%. Similarly, the Mn deficiency rate in the Si-Mn depleted region is preferably 8% or less, more preferably 6% or less, and most preferably 4% or less. The lower limit of the Mn deficiency rate is, but may be, 0%. In the present invention, the expression "oxide-free" means that the Si-Mn depleted region does not contain any oxides other than the granular oxides, grain boundary oxides, and dendritic oxides described above. Such oxide-free regions can be identified by cross-sectional observation using a SEM and an energy dispersive X-ray spectrometer (EDS). Furthermore, the Si-Mn depleted region according to the present invention cannot be controlled to a desired composition range simply by forming internal oxides such as granular oxides. Therefore, as will be explained in detail later, it is important to appropriately control the progress of internal oxidation in the manufacturing process.

[0062] In the present invention, the Si and Mn contents of the oxide-free region at half the depth of the internal oxide layer are determined by analyzing ten randomly selected oxide-free points at half the depth of the internal oxide layer determined from the SEM image where the ratio A was measured, using a transmission electron microscope with an energy dispersive X-ray spectrometer (TEM-EDS), and arithmetically averaging the measured values of Si and Mn concentrations. The Si and Mn contents at the center of the thickness of the steel sheet are determined by observing a cross section of the center of the thickness with an SEM, analyzing ten randomly selected points at the center of the thickness from the SEM image, using a transmission electron microscope with an energy dispersive X-ray spectrometer (TEM-EDS), and arithmetically averaging the measured values of Si and Mn concentrations. Finally, the Si and Mn deficiency rates are determined by dividing the Si and Mn contents at half the depth of the internal oxide layer by the Si and Mn contents at the center of the thickness of the steel sheet, respectively, and expressing the results as percentages.

[0063] [Oxide composition] In the present invention, the granular oxide, grain boundary oxide, and dendritic oxide (hereinafter simply referred to as oxide) contain, in addition to oxygen, one or more of the elements contained in the steel sheet described above, and typically have a chemical composition containing Si, O, and Fe, and optionally further containing Mn. More specifically, the oxide typically contains 5-25% Si, 0-10% Mn, 40-65% O, and 10-30% Fe. In addition to these elements, the oxide may also contain elements that can be contained in the steel sheet described above (e.g., Cr, etc.).

[0064] <Plated steel sheet> The plated steel sheet according to the present invention has a Zn-containing coating layer on the above-described steel sheet according to the present invention. This coating layer may be formed on one or both sides of the steel sheet. Examples of Zn-containing coating layers include hot-dip galvanized layers, alloyed hot-dip galvanized layers, electrogalvanized layers, and electroalloy galvanized layers. More specifically, examples of coating types that can be used include Zn-0.2%Al (GI), Zn-(0.3-1.5)%Al, Zn-4.5%Al, Zn-0.09%Al-10%Fe (GA), Zn-1.5%Al-1.5%Mg, Zn-11%Al-3%Mg-0.2%Si, Zn-11%Ni, and Zn-15%Mg.

[0065] [Composition of plating layer] The composition of the elements contained in the Zn-containing plating layer of the present invention will be described. Unless otherwise specified, "%" regarding the content of an element means "% by mass." In the numerical range of the composition of the plating layer, a numerical range expressed using "to" means a range that includes the numerical values before and after "to" as the lower and upper limits, unless otherwise specified.

[0066] (Al: 0 to 60.0%) Al is an element that improves the corrosion resistance of a coating layer when contained together with Zn or alloyed with Zn, and therefore may be contained as needed. Therefore, the Al content may be 0%. To form a coating layer containing Zn and Al, the Al content is preferably 0.01% or more, and may be, for example, 0.1% or more, 0.5% or more, 1.0% or more, or 3.0% or more. On the other hand, since the effect of improving corrosion resistance saturates even when excessive Al is contained, the Al content is preferably 60.0% or less, and may be, for example, 55.0% or less, 50.0% or less, 40.0% or less, 30.0% or less, 20.0% or less, 10.0% or less, or 5.0% or less.

[0067] (Mg: 0-15.0%) Mg is an element that improves the corrosion resistance of the plating layer when included or alloyed with Zn and Al, and therefore may be included as needed. Therefore, the Mg content may be 0%. To form a plating layer containing Zn, Al, and Mg, the Mg content is preferably 0.01% or more, and may be, for example, 0.1% or more, 0.5% or more, 1.0% or more, or 3.0% or more. On the other hand, if Mg is included in an excessive amount, the Mg will not completely dissolve in the plating bath and will float as an oxide. When zinc plating is performed using this plating bath, the oxide will adhere to the plating surface, causing poor appearance or the occurrence of unplated areas. Therefore, the Mg content is preferably 15.0% or less, and may be, for example, 10.0% or less or 5.0% or less.

[0068] (Fe: 0 to 15.0%) When a Zn-containing coating layer is formed on a steel sheet and then the plated steel sheet is heat-treated, Fe can be contained in the coating layer by diffusing from the steel sheet. Therefore, in a state where heat treatment has not been performed, Fe is not contained in the coating layer, and the Fe content may be 0%. The Fe content may also be 1.0% or more, 2.0% or more, 3.0% or more, 4.0% or more, or 5.0% or more. On the other hand, the Fe content is preferably 15.0% or less, and may be, for example, 12.0% or less, 10.0% or less, 8.0% or less, or 6.0% or less.

[0069] (Si: 0 to 3.0%) Since Si is an element that further improves corrosion resistance when contained in a Zn-containing coating layer, particularly a Zn-Al-Mg coating layer, it may be contained as needed. Therefore, the Si content may be 0%. From the viewpoint of improving corrosion resistance, the Si content may be, for example, 0.005% or more, 0.01% or more, 0.05% or more, 0.1% or more, or 0.5% or more. Alternatively, the Si content may be 3.0% or less, 2.5% or less, 2.0% or less, 1.5% or less, or 1.2% or less.

[0070] The basic component composition of the plating layer is as described above. Furthermore, the plating layer may optionally contain one or more of Sb: 0-0.50%, Pb: 0-0.50%, Cu: 0-1.00%, Sn: 0-1.00%, Ti: 0-1.00%, Sr: 0-0.50%, Cr: 0-1.00%, Ni: 0-1.00%, and Mn: 0-1.00%. While not particularly limited, in order to fully exert the effects and functions of the above-described basic components constituting the plating layer, the total content of these optional additional elements is preferably 5.00% or less, and more preferably 2.00% or less.

[0071] The remainder of the plating layer other than the above components consists of Zn and impurities. The impurities in the plating layer are components that are mixed in due to various factors in the manufacturing process, including raw materials, when the plating layer is produced. The plating layer may contain trace amounts of elements other than the basic components and optional additional components described above as impurities, within a range that does not impair the effects of the present invention.

[0072] The component composition of the plating layer can be determined by dissolving the plating layer in an acid solution to which an inhibitor that suppresses corrosion of the steel sheet has been added, and measuring the resulting solution by ICP (inductively coupled plasma) atomic emission spectroscopy.

[0073] The thickness of the plating layer may be, for example, 3 to 50 μm. The coating weight of the plating layer is not particularly limited, but may be, for example, 10 to 170 g / m per side. 2 In the present invention, the coating weight of the plating layer is determined by dissolving the plating layer in an acid solution to which an inhibitor that suppresses corrosion of the base steel has been added, and measuring the change in weight before and after pickling.

[0074] [Tensile strength] The steel sheet and plated steel sheet according to the present invention preferably have high strength, specifically, a tensile strength of 440 MPa or more. For example, the tensile strength may be 500 MPa or more, 600 MPa or more, 700 MPa or more, or 800 MPa or more. The upper limit of the tensile strength is not particularly limited, but from the viewpoint of ensuring toughness, it may be, for example, 2000 MPa or less. The tensile strength may be measured in accordance with JIS Z 2241 (2011) using a JIS No. 5 tensile test piece with the longitudinal direction perpendicular to the rolling direction.

[0075] The steel sheet and plated steel sheet according to the present invention have high strength and excellent platability and hydrogen embrittlement resistance, and therefore can be suitably used in a wide range of fields, such as automobiles, home appliances, and building materials, but are particularly preferably used in the automobile field. Steel sheets used for automobiles are usually plated (typically Zn-based plating), and when the steel sheet according to the present invention is used as an automotive steel sheet, the effect of the present invention, i.e., high platability, is suitably exhibited. Furthermore, steel sheets and plated steel sheets used for automobiles are often electrocoated, and in this case, hydrogen embrittlement cracking due to hydrogen penetration during electrocoating can become a significant problem. Therefore, when the steel sheet and plated steel sheet according to the present invention are used as an automotive steel sheet, the effect of the present invention, i.e., high hydrogen embrittlement resistance, is suitably exhibited.

[0076] <Steel sheet manufacturing method> A preferred method for manufacturing a steel plate according to the present invention will be described below. The following description is intended to exemplify a characteristic method for manufacturing a steel plate according to the present invention, but is not intended to limit the steel plate to one manufactured by the manufacturing method described below.

[0077] The steel sheet according to the present invention can be obtained by, for example, carrying out a casting step in which molten steel having an adjusted composition is cast to form a steel billet, a hot rolling step in which the steel billet is hot rolled to obtain a hot rolled steel sheet, a coiling step in which the hot rolled steel sheet is coiled, a cold rolling step in which the coiled hot rolled steel sheet is cold rolled to obtain a cold rolled steel sheet, a grinding step in which dislocations are introduced into the surface of the cold rolled steel sheet, and an annealing step in which the ground cold rolled steel sheet is annealed. Alternatively, the steel sheet may be pickled after the hot rolling step and then directly subjected to the cold rolling step without being coiled.

[0078] [Casting process] The conditions for the casting process are not particularly limited. For example, after melting in a blast furnace or electric furnace, various secondary smelting processes may be carried out, and then casting may be carried out by a conventional method such as continuous casting or ingot casting.

[0079] [Hot rolling process] The cast steel slabs can be hot-rolled to obtain hot-rolled steel sheets. The hot-rolling step is carried out by hot-rolling the cast steel slabs directly or after cooling them once and then reheating them. When reheating is carried out, the heating temperature of the steel slabs may be, for example, 1100°C to 1250°C. In the hot-rolling step, rough rolling and finish rolling are usually carried out. The temperature and reduction ratio of each rolling step may be appropriately changed depending on the desired metal structure and plate thickness. For example, the finishing temperature of finish rolling may be 900 to 1050°C, and the reduction ratio of finish rolling may be 10 to 50%.

[0080] [Winding process] The hot-rolled steel sheet can be coiled at a predetermined temperature. The coiling temperature may be changed as appropriate depending on the desired metal structure, etc., and may be, for example, 500 to 800°C. The hot-rolled steel sheet may be recoiled before or after coiling and subjected to a predetermined heat treatment. Alternatively, the hot-rolled steel sheet may be pickled after the hot rolling step and then subjected to the cold rolling step described below without performing the coiling step.

[0081] [Cold rolling process] After subjecting the hot-rolled steel sheet to pickling or the like, the hot-rolled steel sheet is cold-rolled to obtain a cold-rolled steel sheet. The reduction ratio of the cold rolling may be appropriately changed depending on the desired metal structure and sheet thickness, and may be, for example, 20 to 80%. After the cold-rolling step, the sheet may be cooled to room temperature, for example, by air cooling.

[0082] [Grinding process] To obtain a desired internal oxide layer in the surface layer of the final steel sheet and further form a Si-Mn depleted region with the desired composition within the internal oxide layer, it is effective to perform a predetermined grinding process on the cold-rolled steel sheet before annealing. This grinding process can introduce a large number of dislocations into the surface of the cold-rolled steel sheet. Because oxygen and other elements diffuse faster at grain boundaries than within grains, introducing a large number of dislocations into the surface of the cold-rolled steel sheet can form many paths, similar to those at grain boundaries. This facilitates oxygen diffusion (penetration) into the steel along these dislocations during annealing, and also increases the diffusion rate of Si and Mn. As a result, oxygen can combine with Si and / or Mn within the steel to form granular oxides, grain boundary oxides, and dendritic oxides. Furthermore, promoting the formation of such internal oxides also promotes a decrease in the surrounding Si and Mn concentrations, thereby facilitating the formation of Si-Mn depleted regions with the desired composition. The grinding process is not particularly limited, but for example, a grinding amount of 10 to 200 g / m is used with a heavy-duty grinding brush. 2 The grinding step can be carried out by grinding the surface of the cold-rolled steel sheet under the following conditions. The amount of grinding by the heavy abrasive brush can be adjusted by any appropriate method known to those skilled in the art, and is not particularly limited. For example, it can be adjusted by appropriately selecting the number of heavy abrasive brushes, the rotation speed, the brush pressure, and the coating liquid used. By carrying out such a grinding step, it is possible to form an internal oxide layer containing desired oxides in the annealing step described below, and to ensure that the Si and Mn deficiencies in the internal oxide layer are each less than 10%.

[0083] [Annealing process] The cold-rolled steel sheet that has undergone the grinding process is then annealed. Annealing is preferably performed with tension applied to the cold-rolled steel sheet in the rolling direction. In particular, in the annealing temperature range of 500°C or higher, annealing is preferably performed with a higher tension than in other regions. Specifically, in the annealing temperature range of 500°C or higher, annealing is preferably performed with a tension of 3 to 150 MPa, particularly 15 to 150 MPa, applied to the cold-rolled steel sheet in the rolling direction. Applying tension during annealing allows for more effective introduction of a large number of dislocations into the cold-rolled steel sheet. Therefore, oxygen is more likely to diffuse (penetrate) into the steel along these dislocations during annealing, and the diffusion rates of Si and Mn are also improved, facilitating the formation of oxides within the steel sheet. As a result, this is advantageous for increasing the number density of granular oxides, refining the average grain size, forming grain boundary oxides in a desired ratio, forming dendritic oxides in a desired area ratio, and forming Si-Mn-depleted regions with a desired composition.

[0084] From the viewpoint of efficiently generating granular oxides, grain boundary oxides, and dendritic oxides, the holding temperature in the annealing step is preferably above 870°C and up to 900°C, more preferably 880 to 890°C. If the holding temperature in the annealing step is 870°C or lower, there is a risk that the depth of the internal oxide layer will not be sufficiently ensured. On the other hand, if the holding temperature in the annealing step is above 900°C, an external oxide layer will form on the steel sheet surface, making it impossible to obtain the desired oxide, and there is a risk that the galvanization ability and hydrogen embrittlement resistance will be insufficient. The rate of temperature rise to the above holding temperature is not particularly limited, but may be 1 to 10°C / second. In addition, the temperature may be raised in two stages, with a first heating rate of 1 to 10°C / second and a second heating rate of 1 to 10°C / second that is different from the first heating rate.

[0085] The holding time at the annealing temperature is preferably more than 150 seconds to 300 seconds, and more preferably 200 to 280 seconds. If the holding time is 150 seconds or less, there is a risk that dendritic oxides are not sufficiently generated, resulting in insufficient hydrogen embrittlement resistance. On the other hand, if the holding time is more than 300 seconds, the outer oxide grows excessively, making it impossible to obtain the desired inner oxide, resulting in insufficient platability and hydrogen embrittlement resistance.

[0086] The dew point of the atmosphere in the annealing step is preferably -20 to 10°C, more preferably -10 to 5°C, from the viewpoint of favorably producing the desired oxides. If the dew point is too low, an outer oxide layer may be formed on the surface of the steel sheet, and an inner oxide layer may not be sufficiently formed, which may result in insufficient galvanization and hydrogen embrittlement resistance. On the other hand, if the dew point is too high, Fe oxide may be formed as the outer oxide on the surface of the steel sheet, and the desired inner oxide layer may not be obtained, which may result in insufficient galvanization and hydrogen embrittlement resistance. The atmosphere in the annealing step may be a reducing atmosphere, more specifically a reducing atmosphere containing nitrogen and hydrogen, for example, a reducing atmosphere of 1 to 10% hydrogen (for example, 4% hydrogen and the balance nitrogen).

[0087] Furthermore, it is effective to remove the internal oxide layer of the steel sheet before the annealing process. An internal oxide layer may be formed on the surface layer of the steel sheet during the above-mentioned rolling process, particularly the hot rolling process. Since such an internal oxide layer formed during the rolling process may hinder the formation of the desired oxide according to the present invention in the annealing process, it is preferable to remove the internal oxide layer formed during the rolling process by pickling or the like before annealing. More specifically, the depth of the internal oxide layer of the cold-rolled steel sheet before the annealing process should be 0.5 μm or less, preferably 0.3 μm or less, more preferably 0.2 μm or less, and even more preferably 0.1 μm or less.

[0088] By carrying out the above-mentioned steps, it is possible to obtain a steel sheet having an internal oxide layer in the surface layer of the steel sheet, which contains granular oxides, grain boundary oxides, and dendritic oxides in a desired form, and which also contains a desired Si-Mn depleted region.

[0089] <Method of manufacturing plated steel sheets> A preferred method for producing a plated steel sheet according to the present invention will be described below. The following description is intended to exemplify a characteristic method for producing a plated steel sheet according to the present invention, but is not intended to limit the plated steel sheet to one produced by the production method described below.

[0090] The plated steel sheet according to the present invention can be obtained by carrying out a plating treatment step of forming a plating layer containing Zn on the steel sheet produced as described above.

[0091] [Plating process] The plating process may be performed according to a method known to those skilled in the art. The plating process may be performed, for example, by hot-dip galvanization or electroplating. Preferably, the plating process is performed by hot-dip galvanization. The plating process conditions may be appropriately set taking into account the desired component composition, thickness, and coating weight of the plating layer. After the plating process, an alloying process may be performed. Typically, the plating process conditions are set to form a plating layer containing 0-60.0% Al, 0-15.0% Mg, 0-15% Fe, and 0-3% Si, with the balance being Zn and impurities. More specifically, the plating process conditions may be appropriately set to form, for example, Zn-0.2%Al (GI), Zn-0.09%Al (GA), Zn-1.5%Al-1.5%Mg, or Zn-11%Al-3%Mg-0.2%Si. [Example]

[0092] The present invention will be described in more detail below with reference to examples, but the present invention is not limited to these examples in any way.

[0093] (Preparation of steel plate samples) The molten steel with the adjusted composition was cast to form a steel billet, which was then hot-rolled, pickled, and cold-rolled to obtain a cold-rolled steel sheet. The cold-rolled steel sheet was then air-cooled to room temperature and pickled to remove the internal oxide layer formed by rolling to the depth (μm) of the internal oxide layer before annealing shown in Table 1. Samples were then taken from each cold-rolled steel sheet according to JIS G0417:1999, and the composition of the steel sheet was analyzed by ICP-MS or other methods. The measured composition of the steel sheet is shown in Table 1. The thickness of all steel sheets used was 1.6 mm.

[0094] Next, each cold-rolled steel sheet was coated with an aqueous NaOH solution, and then polished with a heavy abrasive brush at a rate of 10 to 200 g / m 2 The surface of the cold-rolled steel sheet was ground with a grinding amount of 0.01 mm (No. 33 was not ground). Subsequently, annealing treatment (annealing atmosphere: 4% hydrogen and balance nitrogen) was performed at the dew point, holding temperature, and holding time shown in Table 1 to prepare each steel sheet sample. For all steel sheet samples, the heating rate during annealing was 6.0°C / s up to 500°C, and 2.0°C / s from 500°C to the holding temperature. In the above annealing treatment, the cold-rolled steel sheet was annealed with a tension of 1 MPa or more applied in the rolling direction, and annealing was performed under a higher tension in the rolling direction in the region where the annealing temperature was 500°C or higher than in other regions, specifically a tension of 3 to 150 MPa (no such tension was applied to Sample No. 32). The presence or absence of grinding with a heavy abrasive brush, and the annealing conditions (presence or absence of application of a tension of 3 to 150 MPa in the annealing temperature range of 500°C or higher, dew point (°C), holding temperature (°C), and holding time (seconds)) are shown in Table 1. JIS No. 5 tensile test pieces were taken from each steel plate sample, with the longitudinal direction perpendicular to the rolling direction, and tensile tests were conducted in accordance with JIS Z 2241 (2011). As a result, Nos. 14 and 16 had a tensile strength of less than 440 MPa, while the others had a tensile strength of 440 MPa or higher.

[0095] (Analysis of the surface layer of steel plate samples: granular oxides) Each steel sheet sample prepared as described above was cut into 25 mm × 15 mm pieces. The cut samples were embedded in resin and mirror-polished. The cross sections of each steel sheet sample were observed using an SEM. A total of 10 1.0 μm × 1.0 μm regions were observed from the SEM images. The observation positions were 1.0 μm from 0.2 to 1.2 μm from the steel sheet surface in the depth direction (direction perpendicular to the steel sheet surface), and 1.0 μm at any position in the SEM image in the width direction (direction perpendicular to the steel sheet surface). Note that each of the above regions was selected to be free of grain boundary oxides and dendritic oxides (SEM images in which five or more oxide dots are arranged in a roughly linear fashion). Next, the SEM images of each region of each steel sheet sample were binarized. The area of the granular oxide portion was calculated from the binarized image, and the number of granular oxides in the SEM images was counted. From the area and number of granular oxides in the 10 binarized images thus obtained, the average particle size and number density of granular oxides were calculated as the circle equivalent diameter. The average particle size (nm) and number density (particles / μm) of granular oxides for each steel plate sample were calculated. 2 ) are shown in Table 1. In Table 1, when no granular oxide was present in the SEM image (when the number density was 0), the average particle size was recorded as "-".

[0096] (Analysis of the surface layer of steel plate samples: dendritic oxides) Each steel sheet sample prepared as described above was cut into 25 mm × 15 mm pieces. The cut samples were embedded in resin and mirror-polished. The cross sections of each steel sheet sample were observed using an SEM, and a total of 10 1.0 μm × 1.0 μm regions were observed from the SEM images. The observation positions were 1.0 μm in the depth direction (direction perpendicular to the steel sheet surface) within a range of 5.5 to 6.5 μm from the steel sheet surface, and 1.0 μm at any position in the SEM image in the width direction (direction perpendicular to the steel sheet surface). Note that each of the above regions was selected to be free of granular oxides and grain boundary oxides. Next, the SEM images of each region for each steel sheet sample were binarized, and the area of the dendritic oxide portion was calculated from the binarized images. The "area ratio of dendritic oxide" for each steel sheet sample was calculated from the area of dendritic oxide in the 10 binarized images obtained in this way. The area ratio (%) of dendritic oxides for each steel sheet sample is shown in Table 1. In the SEM images, the dendritic oxides were observed as having multiple secondary arms branching off from the primary arm on both sides. When the lengths of the primary and secondary arms of the dendritic oxides were measured, the results for Samples Nos. 2 to 6 and 18 to 31 were primary arms: 0.5 to 5.0 μm and secondary arms: 50 to 300 nm.

[0097] (Analysis of the surface layer of steel plate samples: grain boundary oxide) The ratio A of each steel sheet sample was measured by observing the cross section of the embedded sample. Specifically, the position of the grain boundary oxide in the region from the surface to 50 μm was identified in a 150 μm-wide (= L0) SEM image, and the identified grain boundary oxide was projected onto the surface of the steel sheet to determine the length L of the grain boundary oxide within the field of view. Based on the thus determined L0 and L, the ratio A (%) = 100 × L / L0 was calculated. The depth R of the identified internal oxide layer was also measured from the same SEM image. Table 1 shows the ratio A (%) of the granular oxide and the depth (μm) of the internal oxide layer for each steel sheet sample.

[0098] The Si and Mn contents of the oxide-free region at half the depth of the internal oxide layer were determined by analyzing 10 randomly selected oxide-free points at half the depth R of the internal oxide layer from an SEM image of the internal oxide layer using TEM-EDS, and then arithmetically averaging the measured Si and Mn concentrations. The Si and Mn contents at the center of the thickness of the steel sheet were determined by observing the cross section of the center of the thickness with an SEM, analyzing 10 randomly selected points at the center of the thickness from the SEM image using TEM-EDS, and then arithmetically averaging the measured Si and Mn concentrations. Finally, the Si and Mn depletion rates were determined by dividing the Si and Mn contents at half the depth of the internal oxide layer by the Si and Mn contents at the center of the thickness of the steel sheet, respectively, and expressing the results as percentages. Furthermore, the composition of the granular oxide, grain boundary oxide, and dendritic oxide of each steel sheet sample was analyzed. All oxides contained Si, O, and Fe, and many oxides also contained Mn. Therefore, the composition of all oxides was Si: 5-25%, Mn: 0-10%, O: 40-65%, and Fe: 10-30%.

[0099] (Preparation of plated steel sheet samples) Each steel sheet sample was cut into a size of 100 mm x 200 mm, and then subjected to a plating process to form the coating types shown in Table 1, thereby producing plated steel sheet samples. In Table 1, coating type A refers to "galvannealed steel sheet (GA)," coating type B refers to "hot-dip Zn-0.2% Al coated steel sheet (GI)," and coating type C refers to "hot-dip Zn-(0.3-1.5)% Al coated steel sheet (the amount of Al is shown in the table)." In the hot-dip galvanizing process, the cut samples were immersed in a hot-dip galvanizing bath at 440°C for 3 seconds. After immersion, they were withdrawn at 100 mm / s, and N2 wiping gas was used to reduce the coating weight to 50 g / m. 2 For plating type A, alloying treatment was then carried out at 460°C.

[0100] (Analysis of plating layer composition) The composition of the plating layer was determined by immersing a sample cut to 30 mm x 30 mm in a 10% HCl aqueous solution containing an inhibitor (Ibit, manufactured by Asahi Chemical Industry Co., Ltd.), pickling the plating layer to remove it, and then measuring the plating components dissolved in the aqueous solution using ICP emission spectroscopy.

[0101] (Platability evaluation) For each plated steel sheet sample, the platability was evaluated by measuring the area ratio of unplated areas on the surface of the steel sheet. Specifically, a 1 mm x 1 mm area on the surface of each plated steel sheet sample on which a plating layer had been formed was observed with an optical microscope, and the areas on which a plating layer had been formed (plated areas) and areas on which no plating layer had been formed (unplated areas) were distinguished from the observed images. The area ratio of unplated areas (area of unplated areas / area of observed image) was calculated, and the platability was evaluated according to the following criteria. The results are shown in Table 1. ◯ indicates pass, and × indicates fail. Rating: 5.0% or less Rating ×: Over 5.0%

[0102] (Evaluation of hydrogen embrittlement resistance) Each plated steel sheet sample had a coating weight of 2.3 g / m 3 After chemical conversion treatment using a zinc phosphate-based chemical conversion treatment solution (Surfdyne SD5350 series, manufactured by Nippon Paint Industrial Coating Co., Ltd.) to achieve a coating thickness of 17 μm, electrodeposition coating was performed using an electrodeposition coating reagent (PN110 Powernics Gray, manufactured by Nippon Paint Industrial Coating Co., Ltd.). The coating was then baked at 170°C for 25 minutes. Furthermore, the amount of diffusible hydrogen was measured for each plated steel sheet sample using the thermal desorption method. Specifically, the plated steel sheet samples were heated to 400°C in a heating furnace equipped with a gas chromatograph, and the total amount of hydrogen released before the temperature cooled to 250°C was measured. Based on the measured amount of diffusible hydrogen, hydrogen embrittlement resistance was evaluated according to the following criteria, and the results are shown in Table 1. ◎ and ◯ indicate pass, and × indicates fail. ◎: 0.2 ppm or less Rating: Over 0.2 ppm and 0.4 ppm or less Rating ×: Over 0.4 ppm

[0103]

Table 1

[0104] Samples Nos. 2 to 6 and 18 to 31 had high galvanizability and hydrogen embrittlement resistance because the chemical composition, morphology of the internal oxide layer (requirements for granular oxide, grain boundary oxide, and dendritic oxide), and depth and composition of the internal oxide layer satisfied the specified ranges. On the other hand, Samples Nos. 1 and 17 had a thick internal oxide layer before annealing, which prevented the desired internal oxide layer from forming during annealing, and therefore did not achieve high hydrogen embrittlement resistance. Sample No. 7 had a low dew point during annealing, which resulted in the formation of an outer oxide layer and the failure to form the desired internal oxide layer, and therefore did not achieve high galvanizability and hydrogen embrittlement resistance. Sample No. 8 had a high dew point during annealing, which resulted in the formation of an outer oxide layer and the failure to form the desired internal oxide layer, and therefore did not achieve high galvanizability and hydrogen embrittlement resistance. Sample No. 9 had a high holding temperature during annealing, which resulted in the formation of an outer oxide layer and the failure to form the desired internal oxide layer, and therefore did not achieve high galvanizability and hydrogen embrittlement resistance. Sample No. 10 had a low annealing temperature, which prevented the formation of the desired internal oxide layer, and thus did not achieve high hydrogen embrittlement resistance. Sample No. 11 had a short annealing time, which prevented the formation of the desired internal oxide layer, and thus did not achieve high hydrogen embrittlement resistance. Sample No. 12 had a long annealing time, which resulted in the formation of an outer oxide layer and prevented the formation of the desired internal oxide layer, and thus did not achieve high galvanizability and hydrogen embrittlement resistance. Sample No. 13 had an excessive Si content, which caused the growth of an outer oxide layer and prevented the formation of the desired internal oxide layer, and thus did not achieve high galvanizability and hydrogen embrittlement resistance. Samples No. 14 and No. 16 had zero Si and zero Mn contents, respectively, which prevented the formation of the desired internal oxide layer and therefore did not achieve high hydrogen embrittlement resistance. Sample No. 15 had an excessive Mn content, which caused the growth of an outer oxide layer and prevented the formation of the desired internal oxide layer, and thus did not achieve high galvanizability and hydrogen embrittlement resistance. For sample No. 32, the specified tension was not applied during annealing, so the desired internal oxide layer did not form, and high hydrogen embrittlement resistance was not achieved. For sample No. 33, grinding before annealing was not performed, so the desired internal oxide layer did not form, and high hydrogen embrittlement resistance was not achieved. For sample No. 34, the holding temperature during annealing was low, so the internal oxide layer did not form deep, and high hydrogen embrittlement resistance was not achieved. [Industrial Applicability]

[0105] According to the present invention, it is possible to provide a high-strength steel sheet and plated steel sheet having high galvanizability and hydrogen embrittlement resistance, and the steel sheet and plated steel sheet can be suitably used for applications such as automobiles, home appliances, and building materials, particularly for automobiles, and are expected to provide high collision safety and long life as automotive steel sheets and automotive plated steel sheets. Therefore, the present invention can be said to be an invention of extremely high industrial value. [Explanation of symbols]

[0106] 1 steel plate 2. Outer oxide layer 3 Base steel 11 Steel plate 12 Granular oxide 13 Grain boundary oxide 14 Dendritic oxide 15 Base steel

Claims

1. In mass%, C: 0.05-0.40%, Si: 0.2-3.0%, Mn: 0.1 to 5.0%, sol. Al: 0 to less than 0.4000%; P: 0.0300% or less, S: 0.0300% or less, N: 0.0100% or less, B: 0 to 0.010%, Ti: 0 to 0.150%, Nb: 0 to 0.150%, V: 0 to 0.150%, Cr: 0-2.00%, Ni: 0-2.00%, Cu: 0-2.00%, Mo: 0-1.00%, W: 0-1.00%, Ca: 0-0.100%, Mg: 0-0.100%, Zr: 0 to 0.100%, Hf: 0 to 0.100%, and A steel sheet having a chemical composition containing REM: 0 to 0.100%, with the balance being Fe and impurities, The steel sheet has an internal oxide layer on the surface thereof, the internal oxide layer including granular oxides, grain boundary oxides, and dendritic oxides, The average particle size of the granular oxide is 350 nm or less, The number density of the granular oxide in the internal oxide layer is 4.0 particles / μm 2 That's all, When a cross section of a surface layer of the steel sheet is observed, a ratio A of a length of grain boundary oxide projected onto the surface of the steel sheet to a length of the surface of the steel sheet is 50% or more and 100% or less, the area ratio of the dendritic oxide is 5.0% or more, The depth of the internal oxide layer is 8 μm or more, the Si and Mn contents in the oxide-free steel at the half depth position are less than 10% of the Si and Mn contents at the center of the thickness of the steel plate, respectively; The number density of the particulate oxides in the internal oxide layer was determined by observing a cross section of the surface layer of the steel sheet by SEM to obtain an SEM image containing particulate oxides, then selecting a total of 10 regions of 1.0 μm (1.0 μm from the region extending from the steel sheet surface to 1.5 μm in the depth direction) × 1.0 μm (width direction) that did not contain grain boundary oxides or dendritic oxides, and counting the total number of particulate oxides in the 10 regions; The area ratio of the dendritic oxides is determined by observing the cross section of the surface layer of the steel sheet using an SEM to obtain an SEM image containing dendritic oxides, then selecting a total of 10 regions of 1.0 μm (1.0 μm from the region from 5.0 μm to 10.0 μm in the depth direction from the surface of the steel sheet) x 1.0 μm (width direction), and dividing the total area of the dendritic oxides in the 10 regions by the total area of the 10 regions (10 μm2).

2. The number density of the granular oxide in the internal oxide layer is 5.0 particles / μm 2 The steel sheet according to claim 1 .

3. The steel sheet according to claim 1 or 2, wherein the depth of the internal oxide layer is 15 μm or more.

4. The steel plate according to any one of claims 1 to 3, wherein the ratio A is 80% or more.

5. A plated steel sheet having a plating layer containing Zn on the steel sheet according to any one of claims 1 to 4.

6. The plated steel sheet according to claim 5, wherein the plating layer has a chemical composition of Zn-(0.3 to 1.5)% Al.

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