Steel sheets and plated steel sheets

Abstract

Provided are: a steel sheet that contains 0.05-0.40% of C, 0.2-3.0% of Si, and 0.1-5.0% of Mn, that contains a granular oxide at a surface layer of the steel sheet, and in which the average grain size of the granular oxide is at most 300 nm, and the number density of the granular oxide is at least 4.0 / μm2, a Si-Mn depletion layer is included that has a thickness of at least 3.0 μm from the surface of the steel sheet, and the contained amounts of Si and Mn in the Si-Mn depletion layer excluding oxides at a position 1 / 2 of the thickness are each less than 10% of the contained amounts of Si and Mn, respectively, at a center portion of the thickness of the steel sheet; and a plated steel sheet using the steel sheet.

Description

[Technical Field]

[0001] The present invention relates to a steel sheet and a plated steel sheet. More specifically, the present invention relates to a high-strength steel sheet and a plated steel sheet having high plateability, LME resistance, 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.

[0006] Furthermore, Patent Document 3 proposes a method for producing high-tensile hot-dip galvanized steel sheet made of high-Si steel by appropriately controlling annealing conditions, because in the case of high-Si steel with an Si concentration of 0.3% or more, Si and other elements in the steel are diffused to the surface layer of the steel sheet as oxides when the surface of the steel sheet is heated, and these oxides inhibit the wettability of the coating and deteriorate the coating adhesion. [Prior art documents] [Patent documents]

[0007] [Patent Document 1] Japanese Patent Application Laid-Open No. 2016-130357 [Patent Document 2] Japanese Patent Application Publication No. 2018-193614 [Patent Document 3] Japanese Patent Application Publication No. 4-202632 Summary of the Invention [Problem to be solved by the invention]

[0008] High-strength steel sheets used in automotive components and other applications are often used in atmospheric corrosive environments where temperature and humidity fluctuate greatly. It is known that when high-strength steel sheets are exposed to such atmospheric corrosive environments, hydrogen produced during the corrosion process penetrates the steel. Hydrogen that penetrates the steel 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 cracking due to absorbed hydrogen is called hydrogen embrittlement cracking (delayed fracture), and it often becomes a problem during steel sheet processing. Therefore, in order to prevent hydrogen embrittlement cracking, it is effective to reduce the amount of accumulated hydrogen in steel sheets used in corrosive environments.

[0009] Furthermore, when hot stamping or welding a coated steel sheet with a Zn-based coating layer on a high-strength steel sheet, the coated steel sheet is processed at high temperatures (e.g., approximately 900°C), and the Zn contained in the coating layer may be in a molten state during processing. In this case, the molten Zn may penetrate into the steel and cause cracks inside the steel sheet. This phenomenon is called liquid metal embrittlement (LME), and it is known that LME reduces the fatigue properties of steel sheets. Therefore, to prevent LME cracking, it is effective to prevent the penetration of Zn and other elements contained in the coating layer into the steel sheet.

[0010] Patent Documents 1 and 2 teach that oxidation is performed in an oxidation zone at an air ratio or air-fuel ratio of 0.9 to 1.4, and then the oxide film is reduced in a reduction zone in a hydrogen atmosphere, thereby controlling the average depth of the internal oxide layer to 4 μm or more, and allowing the internal oxide layer to function as a hydrogen trapping site, thereby preventing hydrogen penetration and suppressing hydrogen embrittlement. Patent Document 3 also specifically discloses heating in the oxidation zone at an air ratio of 0.95 to 1.10. However, neither of these documents discusses control of the morphology of the oxides present in the internal oxide layer, leaving room for improvement in hydrogen embrittlement resistance. Furthermore, no discussion is given on improving LME resistance.

[0011] 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 that have high platability, LME resistance, and hydrogen embrittlement resistance. [Means for solving the problem]

[0012] The present inventors have found that, in order to solve the above problems, it is important to form oxides in the surface layer of a steel sheet, i.e., inside the steel sheet, and further to control the morphology of the oxides present in the surface layer of the steel sheet and to control the Si-Mn depleted zone formed in the surface layer of the steel sheet due to the formation of such oxides within a predetermined thickness and composition range. More specifically, the present inventors have found that forming an internal oxide ensures high galvanizability, and forming a large amount of fine granular oxides present within the crystal grains of the metallographic structure as the oxide form allows the granular oxides to function as trap sites for hydrogen that can penetrate into the steel in a corrosive environment and also for Zn that can penetrate into the steel during hot stamping and welding, and that forming an Si-Mn depleted zone of a predetermined thickness and composition in the surface layer of the steel sheet promotes hydrogen diffusion in the steel and improves hydrogen desorption from the steel, thereby achieving high LME resistance and hydrogen embrittlement resistance.

[0013] 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 contains granular oxides in a surface layer thereof, The average particle size of the granular oxide is 300 nm or less, The number density of the granular oxide is 4.0 particles / μm 2 That's all, The steel plate includes a Si-Mn depleted layer having a thickness of 3.0 μm or more from the surface thereof, The Si and Mn contents of the oxide-free Si-Mn depleted layer at the half-thickness position are less than 10% of the Si and Mn contents at the center of the thickness of the steel sheet, respectively. (2) The steel sheet according to (1), wherein the average particle size of the granular oxide is 200 nm or less. (3) The number density of the granular oxide is 10.0 particles / μm 2 The steel sheet according to (1) or (2), (4) The steel sheet according to any one of (1) to (3), further comprising grain boundary oxides in a surface layer of the steel sheet. (5) (4) The steel sheet according to (4), wherein, when a cross section of a surface layer of the steel sheet is observed, a ratio A of the length of grain boundary oxides projected onto the surface of the steel sheet to the length of the surface of the steel sheet is 50% or more. (6) The steel sheet according to (5), wherein the ratio A is 80% or more. (7) A plated steel sheet having a Zn-containing plating layer on the steel sheet according to any one of (1) to (6). (8) The plated steel sheet according to (7), wherein the plated layer has a chemical composition of Zn-(0.3 to 1.5)% Al. [Effects of the Invention]

[0014] According to the present invention, it is possible to make the fine and abundant granular oxides present in the surface layer of a steel sheet function as trap sites for hydrogen that penetrates in a corrosive environment. As a result, the amount of hydrogen that penetrates in a corrosive environment can be significantly reduced, significantly improving hydrogen embrittlement resistance. Furthermore, the granular oxides also function as trap sites for Zn that penetrates into the steel during hot stamping or welding, significantly reducing the amount of Zn that penetrates, significantly improving LME resistance. Furthermore, according to the present invention, the inclusion of a Si-Mn-depleted layer having a predetermined thickness and composition promotes hydrogen diffusion and improves hydrogen desorption from the steel. As a result, the amount of hydrogen that penetrates can be released, reducing the amount of hydrogen that accumulates in the steel, and significantly improving hydrogen embrittlement resistance. Furthermore, because the granular oxides and optional grain boundary oxides are formed inside the steel sheet, when a coating layer is formed, sufficient interdiffusion between the steel components and the coating components is achieved, resulting in high coatability. Therefore, the present invention makes it possible to achieve high coatability, LME resistance, and hydrogen embrittlement resistance in a high-strength steel sheet. [Brief explanation of the drawings]

[0015] [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 cross-sectional schematic view of a steel plate according to one embodiment of the present invention. [Figure 3] FIG. 2 shows a schematic diagram for explaining the measurement of the ratio A of the steel plate. [Figure 4] 2 shows a cross-sectional schematic view of a steel plate according to another embodiment of the present invention. [Figure 5] FIG. 4 shows a schematic diagram for explaining the measurement of the ratio A of the steel plate. DETAILED DESCRIPTION OF THE INVENTION

[0016] <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 contains granular oxides in a surface layer thereof, The average particle size of the granular oxide is 300 nm or less, The number density of the granular oxide is 4.0 particles / μm 2 That's all, The steel plate includes a Si-Mn depleted layer having a thickness of 3.0 μm or more from the surface thereof, The Si and Mn contents of the oxide-free Si-Mn depleted zone at the half-thickness position are less than 10% of the Si and Mn contents at the center of the thickness of the steel plate, respectively.

[0017] 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 oxidized 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., the outside of 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, Al) and the steel components (e.g., Fe). This can result in insufficient adhesion between the steel and the coating, resulting in the formation of uncoated areas where no coating layer is formed.

[0018] 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 instead has a granular oxide 12 inside the base steel 14 and, optionally, a grain boundary oxide 13 at the grain boundaries of the metal structure. Therefore, when a coating layer is formed on the surface of the steel sheet 11, the steel sheet 11 according to the present invention, which has the granular oxide 12 and the optional grain boundary oxide 13 formed inside the base steel 14, allows for sufficient interdiffusion between the coating components and the steel components compared to the steel sheet 1 having the outer oxide layer 2, thereby enabling high coating ability. Therefore, the 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. 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 where no coating layer is 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% by area or less) or no unplated areas at all.

[0019] Furthermore, high-strength steel sheets used in atmospheric environments, particularly high-strength steel sheets for automobiles, are repeatedly exposed to various environments with different temperatures and humidities. Such environments are called atmospheric corrosive environments, and it is known that hydrogen is generated during the corrosion process in these atmospheric corrosive environments. This hydrogen then penetrates deeper into the steel than the surface layer and segregates at the martensite grain boundaries of the steel sheet, embrittling the grain boundaries and causing hydrogen embrittlement cracking (delayed fracture) in the steel sheet. Because martensite is a hard structure, it is highly susceptible to hydrogen embrittlement cracking. This type of cracking can be problematic during steel sheet processing. Therefore, in order to prevent hydrogen embrittlement cracking, it is effective to reduce the amount of hydrogen stored in the steel, specifically at positions deeper than the surface layer of the steel sheet, for high-strength steel sheets used in atmospheric corrosive environments. The inventors have discovered that by controlling the morphology of oxides present in the surface layer of a steel sheet, more specifically, by forming oxides in the interior of the steel sheet into "granular oxides" with a predetermined average particle size and number density, and by controlling the Si-Mn depleted zone, which is formed due to the reduction in the surrounding Si and Mn concentrations caused by the formation of such internal oxides, to within a predetermined thickness and composition range, the granular oxides function as trap sites for hydrogen that penetrates into the surface layer of the steel sheet in a corrosive environment, and the Si-Mn depleted zone promotes the diffusion of the penetrated hydrogen, thereby improving hydrogen desorption from the steel. As a result, not only is hydrogen penetration suppressed, but also the release of the penetrated hydrogen is promoted, thereby reducing the amount of hydrogen accumulation in the steel sheet used in a corrosive environment. The term "high hydrogen embrittlement resistance" refers to a state in which the amount of hydrogen accumulated in the steel sheet and plated steel sheet is reduced so that hydrogen embrittlement cracking can be sufficiently suppressed.

[0020] The present inventors conducted a detailed analysis of the relationship between the morphology of oxides and their effectiveness as hydrogen trapping sites, and as a result, as shown in FIG. 2, it was found that the oxides 12 dispersed in a granular form on the surface layer of the base steel 14 are present in a large amount and finely spaced apart from each other, and more specifically, the oxides 12 have an average particle size of 300 nm or less and a number density of 4.0 particles / μm 2It has been found that the presence of granular oxides in such a manner as described above is effective. 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. That is, it is believed that when oxides are dispersed in a fine, large amount and discrete manner in the surface layer of the steel sheet, the surface area of ​​the oxides in the surface layer of the steel sheet increases, thereby improving the hydrogen-trapping ability. Therefore, the present inventors have found that, from the viewpoint of obtaining high hydrogen penetration resistance and thus high hydrogen embrittlement resistance, it is important to control the conditions during the production of the steel sheet, particularly during the annealing treatment, so that fine, large amounts of granular oxides are present, which function as trap sites for hydrogen that penetrates when placed in a corrosive environment. Note that the metal structure of the surface layer of a steel sheet is typically softer than that of the interior of the steel sheet (e.g., at a 1 / 8 or 1 / 4 position of the sheet thickness), so that hydrogen embrittlement cracking does not pose a particular problem even if hydrogen is present in the surface layer of the steel sheet.

[0021] Furthermore, the present inventors conducted a detailed analysis of the relationship between hydrogen release performance and the morphology of a Si-Mn depleted zone, which is formed when the surrounding Si and Mn concentrations decrease due to the formation of internal oxides such as particulate oxides 12 as shown in Fig. 2 . As a result, they found that it is effective to control the Si-Mn depleted zone within a predetermined thickness and composition range. More specifically, the thickness of the Si-Mn depleted zone is 3.0 µm or more from the surface of the steel sheet, and the Si and Mn contents of the oxide-free Si-Mn depleted zone at a position halfway through the thickness are less than 10% of the Si and Mn contents at the center of the steel sheet (hereinafter, these values ​​are also referred to as the Si deficiency rate and the Mn deficiency 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 is also large, and this dissolved Si and / or Mn inhibits hydrogen diffusion, resulting in a slower hydrogen diffusion rate in the steel. 2, when internal oxides such as granular oxides 12 and optional grain boundary oxides 13 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, resulting in the formation of a Si-Mn-depleted zone in the surface layer of the steel sheet, where the surrounding Si and Mn concentrations are relatively low. Therefore, by making the Si-Mn-depleted zone relatively thick, specifically by controlling the thickness of the Si-Mn-depleted zone to 3.0 μm or more from the surface of the steel sheet (or, if a coating layer is present on the surface of the steel sheet, the interface between the coating layer and the steel sheet), sufficient hydrogen diffusion paths can be ensured. Furthermore, by sufficiently reducing the Si and Mn contents in the Si-Mn-depleted zone, specifically by controlling the Si and Mn deficiency rates to be less than 10%, the amounts of solute Si and Mn that inhibit hydrogen diffusion can be sufficiently reduced. Therefore, it is believed that the inclusion of a Si-Mn depleted layer whose thickness and composition are controlled within the above ranges can promote hydrogen diffusion and significantly improve the hydrogen desorption properties from the steel. Therefore, by combining the above-mentioned granular oxide with the Si-Mn depleted layer, both the hydrogen penetration resistance and the hydrogen desorption resistance can be improved, thereby significantly improving the hydrogen embrittlement resistance of the steel sheet as a whole.

[0022] It is also known that hydrogen embrittlement cracking may occur not only when high-strength steel sheets as described above are used in atmospheric corrosive environments, but also when hydrogen present in the annealing atmosphere penetrates deeper than the surface layer of the base steel during the annealing treatment in the manufacture of the high-strength steel sheet. The present inventors have now discovered that the combination of the granular oxide and Si-Mn depleted layer is effective not only for use in corrosive environments, but also for suppressing hydrogen penetration into the steel sheet and for expelling the penetrated hydrogen during the annealing treatment in the manufacturing process, thereby achieving high hydrogen embrittlement resistance both during the manufacture and use of the steel sheet.

[0023] On the other hand, when hot stamping or welding is performed on a coated steel sheet having a Zn-containing coating layer on its surface, the high temperatures generated during processing can cause the Zn contained in the coating layer to melt. When Zn melts, it penetrates into the steel. If processing is performed in this state, liquid metal embrittlement (LME) cracking occurs inside the steel sheet, and the LME can degrade the fatigue properties of the steel sheet. The inventors have discovered that when the above-mentioned granular oxide has a desired average particle size and number density, it contributes not only to improved hydrogen embrittlement resistance but also to improved LME resistance. More specifically, they have discovered that the granular oxide functions as a trap site for Zn that attempts to penetrate into the steel during high-temperature processing. As a result, Zn that attempts to penetrate into the steel during, for example, hot stamping is captured by the granular oxide on the surface of the steel sheet, effectively suppressing the penetration of Zn into the grain boundaries. Therefore, they have discovered that the presence of a large amount of fine granular oxide is important not only for improving the above-mentioned hydrogen penetration resistance but also for improving LME resistance. The steel sheet according to the present invention is not necessarily limited to such plated steel sheets, but also includes unplated steel sheets, because even when an unplated steel sheet is spot-welded to, for example, a zinc-plated steel sheet, LME cracking may occur due to the penetration of molten zinc in the zinc-plated steel sheet into the unplated steel sheet.

[0024] 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.

[0025] [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.

[0026] (C: 0.05 to 0.40%) C (carbon) is an important element for ensuring the strength of steel. To ensure sufficient strength and obtain the desired internal oxide morphology, 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, weldability may be reduced. 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.

[0027] (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, particularly granular 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 surface properties. Furthermore, it may also cause the granular oxides to become coarse. 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.

[0028] (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 desired oxides, particularly granular 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. Furthermore, it may also cause the granular oxides to become coarse. 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.

[0029] (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).

[0030] (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.

[0031] (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.

[0032] (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.

[0033] 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%.

[0034] (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.

[0035] (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.

[0036] (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.

[0037] (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.

[0038] (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.

[0039] (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.

[0040] (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.

[0041] (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.

[0042] (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.

[0043] (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.

[0044] (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.

[0045] (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.

[0046] (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.

[0047] (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.

[0048] 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.

[0049] 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.

[0050] [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.

[0051] As illustrated in FIG. 2 , a steel sheet 11 according to the present invention contains a granular oxide 12 in the surface layer of the steel sheet 11. Preferably, the granular oxide 12 is present only in the surface layer of the steel sheet 11. The presence of this granular oxide 12 inside the base steel 14 (i.e., as an internal oxide) enables the steel sheet 11 to have high galvanizability compared to the case in which an outer oxide layer 2 is present on the surface of the base steel 3 shown in FIG. 1 . This is thought to be the result of the formation of the internal oxide, which is related to the absence or presence of only a sufficiently thin outer oxide layer that inhibits 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, thereby allowing sufficient interdiffusion between the plating components and the steel components. Therefore, the steel sheet and plated steel sheet according to the present invention, which contain granular oxide in the surface layer of the steel sheet, i.e., inside the steel sheet, have high galvanizability.

[0052] 2, the steel sheet 11 according to the present invention may optionally contain grain boundary oxides 13 in the surface layer of the steel sheet 11 in addition to the granular oxides 12. Since the grain boundary oxides 13 are present inside the base steel 14, like the granular oxides 12, steel sheets and plated steel sheets that contain both the granular oxides 12 and the grain boundary oxides 13 also have high galvanizability.

[0053] [Granular oxide] In the present invention, "granular oxides" refer to oxides dispersed in a granular shape within steel crystal grains or on grain boundaries. 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 oxide particles not being arranged according to a specific rule (e.g., linearly) but being randomly arranged. In reality, granular oxides are typically present three-dimensionally in spherical or nearly spherical shapes in the surface layer of a 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 nearly circular shape. Figure 2 shows a granular oxide 12 that appears circular as an example.

[0054] (Average particle size) In the present invention, the average particle size of the granular oxides is 300 nm or less. By controlling the average particle size within this range, the granular oxides can be finely dispersed in the surface layer of the steel sheet, and the granular oxides function well as hydrogen trapping sites that suppress hydrogen penetration in a corrosive environment and / or during annealing in the manufacturing process. Furthermore, the granular oxides function well as Zn trapping sites that may penetrate when a plated steel sheet having a plating layer formed thereon is subjected to hot stamping or welding. On the other hand, if the average particle size is too large, the granular oxides may not function sufficiently as hydrogen trapping sites and / or Zn trapping sites, and good hydrogen embrittlement resistance and / or LME resistance may not be obtained. The average particle size of the granular oxides is preferably 250 nm or less, more preferably 200 nm or less, and even more preferably 150 nm or less. Since the finer the granular oxides, the better, the lower limit of the average particle size of the granular oxides is not particularly limited, but may be, for example, 5 nm or more, 10 nm or more, or 50 nm or more.

[0055] (number density) In the present invention, the number density of the granular oxide is 4.0 pieces / μm 2 That is all. 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 trapping sites that suppress hydrogen penetration in a corrosive environment and / or during annealing treatment in the manufacturing process, and further function well as Zn trapping sites that can penetrate when a plated steel sheet having a plating layer formed thereon is subjected to hot stamping or welding. 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 6.0 particles / μm, the number density of the granular oxides as hydrogen trapping sites and / or Zn trapping sites will be insufficient, and the granular oxides will not function satisfactorily as hydrogen trapping sites and / or Zn trapping sites, which may result in failure to obtain good hydrogen embrittlement resistance and / or LME resistance. 2 More preferably, 8.0 particles / μm 2More preferably, 10.0 particles / μm 2 The more granular oxides there are, the better. Therefore, the upper limit of the number density of the granular oxides is not particularly limited, but it is not particularly limited to, for example, 100.0 particles / μm 2 It may be the following:

[0056] 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) and not including the grain boundary oxide described below, are selected from the SEM image. The observation position for each region is 1.0 μm from the steel sheet surface to 1.5 μm 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 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 portions. The total area of ​​the granular oxide portions 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.

[0057] [Grain boundary oxide] The steel sheet according to the present invention may further contain grain boundary oxides in the surface layer of the steel sheet. In the present invention, "grain boundary oxides" refers to oxides present along the grain boundaries of the steel, and does not include oxides present within the grains of the steel. In reality, grain boundary oxides are present in a planar form along the grain boundaries in the surface layer of the 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. In FIGS. 2 and 3, a grain boundary oxide 13 that appears linear is shown as an example. Furthermore, in FIGS. 2 and 3, as a typical example of steel sheet 11, grain boundary oxide 13 is shown below granular oxide 12, but grain boundary oxide 13 may also be formed near the surface of base steel 14.

[0058] (Ratio A) When a cross section of the surface layer of a steel sheet is observed, the ratio A of the length of the grain boundary oxide projected onto the surface of the steel sheet to the surface length of the steel sheet may be any value between 0 and 100%. 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 a cross section of the surface layer of the steel sheet 11 is observed, as shown in FIGS. 3 and 5. In one embodiment of the present invention, the ratio A is 0% or more and less than 50%. Since the steel sheet according to the present invention does not need to contain grain boundary oxide in the surface layer of the steel sheet, the ratio A may be 0%. The ratio A may be, for example, 1% or more, 3% or more, or 5% or more. Under production conditions under which a relatively large amount of grain boundary oxide is produced, the average particle size of the granular oxide tends to be larger. Therefore, from the viewpoint of refining the average particle size of the granular oxides, the ratio A is preferably less than 50%, as shown in FIGS. 2 and 3, and may be 40% or less, 30% or less, 20% or less, 10% or less, or 0%. In another embodiment of the present invention, the ratio A is 50% or more. By controlling the ratio A within this range, a large amount of grain boundary oxides can be present in the surface layer of the steel sheet, and the grain boundary oxides can function effectively as escape paths for hydrogen that has penetrated into the steel. Therefore, by having a relatively large amount of grain boundary oxides in addition to the Si-Mn depleted zone, the hydrogen desorption performance of the steel sheet according to the present invention can be further improved. Therefore, from the viewpoint of further improving the hydrogen desorption performance of the steel sheet, the ratio A is preferably 50% or more, as shown in FIGS. 4 and 5, and may be 60% or more, 70% or more, 80% or more, 90% or more, or 100%.

[0059] The ratio A is determined by observing a cross section of the surface layer of the steel sheet 11, as shown in FIGS. 3 and 5 . 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 the granular oxide 12 is omitted from FIGS. 3 and 5 for the sake of explanation.

[0060] [Oxide composition] In the present invention, the granular oxides and optional grain boundary oxides (hereinafter simply referred to as oxides) 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 oxides typically contain 5-25% Si, 0-10% Mn, 40-65% O, and 10-30% Fe. In addition to these elements, the oxides may also contain elements that can be contained in the steel sheet described above (e.g., Cr, etc.).

[0061] [Si-Mn depleted layer] The steel sheet according to the present invention includes a Si-Mn depleted layer having a thickness of 3.0 μm or more from the surface of the steel sheet, and the Si and Mn contents of the oxide-free Si-Mn depleted layer at a position halfway along the thickness are less than 10% of the Si and Mn contents at the center of the steel sheet. By controlling the thickness of the Si-Mn depleted layer formed at the surface of the steel sheet due to the formation of particulate oxides and optional grain boundary oxides to 3.0 μm or more and controlling the Si and Mn depletion rates of the Si-Mn depleted layer to less than 10%, respectively, the amounts of solute Si and Mn that inhibit hydrogen diffusion can be sufficiently reduced, thereby promoting hydrogen diffusion and significantly improving hydrogen desorption from the steel. Since increasing the thickness of the Si-Mn depleted layer can further promote hydrogen diffusion from the steel, the thickness of the Si-Mn depleted layer is preferably 4.0 μm or more, more preferably 5.0 μm or more, and most preferably 7.0 μm or more. There is no particular upper limit to the thickness of the Si--Mn depleted layer, but the thickness of the Si--Mn depleted layer may be, for example, 50.0 μm or less.

[0062] Similarly, by reducing the Si and Mn deficiency rates of the Si-Mn depleted zone, the amounts of solute Si and Mn in the steel can be further reduced. Therefore, the Si deficiency rate of the Si-Mn depleted zone 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 of the Si-Mn depleted zone 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 not only the above-mentioned granular oxides and grain boundary oxides but also any other oxides are not contained. Such oxide-free regions can be identified by cross-sectional observation using a SEM and energy dispersive X-ray spectroscopy (EDS). Furthermore, the Si-Mn depleted zone according to the present invention cannot be controlled to a desired thickness and 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.

[0063] The thickness of the Si-Mn depleted zone, as shown by D in Figure 5, 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 coated steel sheet) in the thickness direction of the steel sheet 11 (the direction perpendicular to the surface of the steel sheet), to the farthest position where an internal oxide (grain boundary oxide 13 in Figure 5) exists. When no grain boundary oxide exists, the thickness of the Si-Mn depleted zone refers to the distance from the surface of the steel sheet (the interface between the steel sheet and the coating layer in the case of a coated steel sheet) in the thickness direction of the steel sheet (the direction perpendicular to the surface of the steel sheet) to the farthest position where a granular oxide exists. The thickness of the Si-Mn depleted zone can be determined from the same SEM image (surface length L0) as the one used to measure the ratio A described above. The Si and Mn contents of the oxide-free region at half the thickness of the Si-Mn depleted zone are determined by analyzing 10 randomly selected, oxide-free points at half the thickness of the Si-Mn depleted zone determined from the SEM image using a transmission electron microscope with an energy dispersive X-ray spectrometer (TEM-EDS) and arithmetically averaging the measured 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 10 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 Si and Mn concentrations. Finally, the Si and Mn deficiency rates are determined by dividing the Si and Mn contents at half the thickness of the Si-Mn depleted zone by the Si and Mn contents at the center of the thickness of the steel sheet, respectively, and expressing the results as percentages.

[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. From the viewpoint of improving LME resistance, the Al content is preferably 0.4 to 1.5%.

[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 galvanizability, LME resistance, 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 galvanizability, is suitably exhibited. Furthermore, steel sheets and plated steel sheets used for automobiles are often hot stamped, in which case hydrogen embrittlement cracking and LME cracking can become significant problems. 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 and LME 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 continuous casting method, an ingot casting method, or the like.

[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 fine and abundant granular oxide and a desired amount of optional grain boundary oxide in the surface layer of the final steel sheet, and to form a Si-Mn depleted zone with the desired thickness and composition, it is effective to perform a 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 in the steel to form granular oxides and optional grain boundary oxides. Furthermore, promoting the formation of these internal oxides also promotes a decrease in the surrounding Si and Mn concentrations, thereby facilitating the formation of a Si-Mn depleted zone with the desired thickness and composition. The grinding step is not particularly limited, but for example, a grinding amount of 10 to 200 g / m is used using a heavy grinding brush. 2 The grinding amount 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 to be used. By performing such a grinding step, it becomes possible to form the desired granular oxide and optional grain boundary oxide in the annealing step described below, and to reliably and efficiently form a Si-Mn depleted layer in the surface layer of the steel sheet having the desired thickness and composition, i.e., a thickness of 3.0 μm or more and with Si and Mn deficiency rates of less than 10%, respectively.

[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 surface of the cold-rolled steel sheet. Therefore, oxygen is more likely to diffuse (penetrate) into the interior of 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 with a desired ratio, and forming a Si-Mn depleted zone with a desired thickness and composition.

[0084] The holding temperature in the annealing step is preferably 700 to 870°C. From the viewpoint of generating a large amount of fine granular oxides while suppressing the generation of grain boundary oxides so that the ratio A is less than 50%, the holding temperature in the annealing step is preferably 700 to 780°C, and more preferably 720 to 760°C. If the holding temperature in the annealing step is less than 700°C, there is a risk that the granular oxides will not be generated sufficiently, resulting in insufficient hydrogen penetration resistance. On the other hand, from the viewpoint of generating a large amount of fine granular oxides and a large amount of grain boundary oxides so that the ratio A is 50% or more, the holding temperature in the annealing step is preferably greater than 780°C to 870°C, and more preferably 800 to 850°C. On the other hand, if the holding temperature in the annealing step is greater than 870°C, there is a risk that the granular oxides will not be generated sufficiently, resulting in insufficient hydrogen penetration resistance, hydrogen embrittlement resistance, and further insufficient LME resistance. Furthermore, if the holding temperature in the annealing step exceeds 900°C, an outer oxide layer may form on the surface of the steel sheet, resulting in insufficient galvanizability. The rate at which the temperature is raised to the holding temperature is not particularly limited, but may be 1 to 10°C / sec. The temperature may be raised in two stages: a first rate of 1 to 10°C / sec and a second rate of 1 to 10°C / sec that is different from the first rate.

[0085] The holding time at the annealing temperature is preferably more than 50 seconds to 150 seconds, more preferably 80 to 120 seconds. If the holding time is 50 seconds or less, there is a risk that the granular oxides and optional grain boundary oxides are not sufficiently generated, and the hydrogen embrittlement resistance and LME resistance may be insufficient. On the other hand, if the holding time is more than 150 seconds, there is a risk that the granular oxides may become coarse, and the hydrogen embrittlement resistance and LME resistance may be insufficient.

[0086] The dew point of the atmosphere used in the annealing step is preferably −20 to 10°C, more preferably −10 to 5°C, from the viewpoint of forming a large amount of fine particulate oxides. If the dew point is too low, an outer oxide layer may be formed on the surface of the steel sheet, and the inner oxide may not be sufficiently formed, which may result in insufficient galvanization, hydrogen embrittlement resistance, and LME resistance. On the other hand, while increasing the dew point can promote the formation of grain boundary oxides, if the dew point is too high, Fe oxide may be formed as the outer oxide on the steel sheet surface, which may result in insufficient galvanization, and the particulate oxide may become coarse, resulting in insufficient hydrogen embrittlement resistance and / or LME resistance. The atmosphere used in the annealing step may be a reducing atmosphere, more specifically a reducing atmosphere containing nitrogen and hydrogen, for example, a reducing atmosphere containing 1 to 10% hydrogen (for example, 4% hydrogen and the balance nitrogen).

[0087] Furthermore, it is effective to remove the internal oxide layer (typically including grain boundary oxides) 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 granular oxides in the annealing process, it is preferable to remove the internal oxide layer before annealing by pickling or the like. 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 a Si-Mn depleted layer with a desired thickness and composition, in which a large amount of fine granular oxides are contained in the surface layer of the steel sheet.

[0089] If a process of oxidation at an air ratio or air-fuel ratio of 0.9 to 1.4 in an oxidation zone and then reduction is performed as a pre-stage of the annealing process, the particulate oxides grow excessively in the oxidation process to an average particle size exceeding 300 nm, and the particulate oxides do not function adequately as hydrogen trapping sites and / or Zn trapping sites, making it difficult to obtain good hydrogen embrittlement resistance and / or LME resistance.

[0090] <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.

[0091] 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.

[0092] [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]

[0093] 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.

[0094] In the following examples, in Example X, a steel sheet having a ratio A of grain boundary oxides of 0% or more and less than 50% was produced, and in Example Y, a steel sheet having a ratio A of grain boundary oxides of 50% or more was produced, and the galvanizability, hydrogen embrittlement resistance, and LME resistance of the steel sheets produced in each example were investigated.

[0095] (Example X) (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.

[0096] 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 2The surface of the cold-rolled steel sheet was ground with a grinding amount of 0.01 mm (Sample No. 135 was not ground). Subsequently, each steel sheet sample was prepared by annealing (annealing atmosphere: 4% hydrogen and balance nitrogen) at the dew point, holding temperature, and holding time shown in Table 1 (mainly holding temperature 700 to 780°C and holding time 50 seconds to 150 seconds). 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 annealing process, the cold-rolled steel sheet was subjected to a tension of 1 MPa or more in the rolling direction. In the region where the annealing temperature was 500°C or higher, a higher tension, specifically a tension of 3 to 150 MPa, was applied in the rolling direction compared to the other regions (Sample No. 134 was not subjected to such tension). 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, the tensile strength of Nos. 116 and 118 was less than 440 MPa, while the other samples were 440 MPa or higher.

[0097] (Analysis of the surface of steel plate samples) 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. Ten 1.0 μm × 1.0 μm regions were observed on the cross section of each steel sheet sample using an SEM. 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 on 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 a region that did not contain grain boundary oxides. Next, the SEM image of each region for each steel sheet sample obtained was binarized, and the area of ​​the granular oxide portion was calculated from the binarized image. Furthermore, the number of granular oxides in the SEM image 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 the granular oxides were calculated as the circle equivalent diameter. The average particle size (nm) and number density (particles / μm 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 "-".

[0098] Furthermore, 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 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 ratio A (%) of the granular oxide for each steel sheet sample is shown in Table 1.

[0099] The thickness of the Si-Mn depleted zone was determined by measuring the distance from the surface of the steel sheet in the thickness direction (perpendicular to the surface of the steel sheet) of the steel sheet, from the surface of the steel sheet where the ratio A was measured, to the furthest point where intergranular oxides existed (or granular oxides existed if no intergranular oxides existed). The Si and Mn contents of the oxide-free region at half the thickness of the Si-Mn depleted zone were determined by analyzing 10 randomly selected oxide-free points at half the thickness of the Si-Mn depleted zone determined from the SEM image using TEM-EDS and 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 of the steel sheet with an SEM, analyzing 10 randomly selected points at the center of the thickness of the steel sheet using TEM-EDS, and arithmetically averaging the measured Si and Mn concentrations. Finally, the Si and Mn depletion rates were determined as percentages by dividing the Si and Mn contents at half the thickness of the Si-Mn depleted zone by the Si and Mn contents at the center of the steel sheet. Furthermore, the composition of the granular and grain boundary oxides in each steel sheet was analyzed. All oxides contained Si, O, and Fe, and most oxides also contained Mn. Therefore, the composition of all oxides was Si: 5-25%, Mn: 0-10%, O: 40-65%, and Fe: 10-30%.

[0100] (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. 2For plating type A, alloying treatment was then carried out at 460°C.

[0101] (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.

[0102] (Platability evaluation) For each plated steel sheet sample, platability was evaluated by measuring the area ratio of unplated areas on the steel sheet surface. 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 platability was evaluated according to the following criteria, the results of which are shown in Table 1. A is pass, and B is fail. Rating A: 5.0% or less Rating B: Over 5.0%

[0103] (LME resistance evaluation) Each 100 x 100 mm plated steel sheet sample was subjected to spot welding. Two 50 x 100 mm pieces were prepared and spot welded to the two Zn-based plated steel sheet samples using a dome radius welding electrode with a tip diameter of 8 mm at a 7° impact angle, 3.0 kN pressure, 0.5 second welding time, and 7 kA current to obtain welded components. The cross-sections of the welds were polished and then observed with an optical microscope. The length of the LME cracks that had occurred on the cross-section of the welds was measured and evaluated as follows. The results are shown in Table 1. AAA, AA, and A are acceptable, and B is unacceptable. Rating AAA: LME crack length over 0 μm to 150 μm Rating AA: LME crack length over 150 μm to 300 μm Rating A: LME crack length over 300 μm to 500 μm Rating B: LME crack length over 500 μm

[0104] (Evaluation of hydrogen embrittlement resistance) Each 50mm x 100mm plated steel specimen was treated with a zinc phosphate treatment using a zinc phosphate conversion treatment solution (Surfdyne SD5350 series, manufactured by Nippon Paint Industrial Coating Co., Ltd.). A 20μm thick electrocoat (PN110 Powernics Gray, manufactured by Nippon Paint Industrial Coating Co., Ltd.) was then applied and baked at 150°C for 20 minutes to form a coating on the plated steel specimen. The specimens were then subjected to a cyclic corrosion test in accordance with JASO (M609-91). The amount of diffusible hydrogen after 120 cycles was measured using the thermal desorption method. Specifically, the plated steel specimens were heated to 400°C in a heating furnace equipped with a gas chromatograph, and the total amount of hydrogen released until the temperature cooled to 250°C was measured. Based on the measured amount of diffusible hydrogen, hydrogen embrittlement resistance (amount of hydrogen accumulated in the specimen) was evaluated according to the following criteria. The results are shown in Table 1. AA and A indicate pass, and B indicates fail. Rating AA: Less than 0.3 ppm Rating A: 0.5 to 0.3 ppm or less Rating B: Over 0.5 ppm

[0105] [Table 1]

[0106] Samples Nos. 102 to 108 and 120 to 133 had high galvanizability, hydrogen embrittlement resistance, and LME resistance due to the appropriate steel composition, average particle size and number density of the granular oxides, and thickness and composition of the Si-Mn depleted layer. On the other hand, samples Nos. 101 and 119 had a thick internal oxide layer before annealing, preventing the desired granular oxide from forming. The desired Si-Mn depleted layer also did not form, resulting in poor hydrogen embrittlement and LME resistance. Sample No. 109 had a low dew point during annealing, resulting in the formation of an outer oxide layer rather than an inner oxide layer, resulting in poor galvanizability, hydrogen embrittlement resistance, and LME resistance. Sample No. 110 had a high dew point during annealing, resulting in the formation of an outer oxide layer and the inability to refine the granular oxide layer, resulting in poor galvanizability, hydrogen embrittlement resistance, and LME resistance. Sample No. 111 was annealed at a high holding temperature, which promoted the formation of grain boundary oxides and prevented the granular oxides from being refined, resulting in poor hydrogen embrittlement and LME resistance. Sample No. 112 was annealed at a low holding temperature, which prevented the formation of sufficient internal oxides and the desired Si-Mn depleted layer. This resulted in poor hydrogen embrittlement and LME resistance. Sample No. 113 was annealed at a short holding time, which prevented the formation of sufficient internal oxides and the desired Si-Mn depleted layer. This resulted in poor hydrogen embrittlement and LME resistance. Sample No. 114 was annealed at a long holding time, which promoted the formation of grain boundary oxides and prevented the granular oxides from being refined. This resulted in poor hydrogen embrittlement and LME resistance. Samples No. 115 and No. 117 contained excessive amounts of Si and Mn, respectively. This resulted in the growth of an outer oxide, the coarsening of granular oxides, and the failure to form the desired Si-Mn depleted layer. Consequently, they did not achieve high galvanizability, hydrogen embrittlement resistance, or LME resistance. Samples No. 116 and No. 118 contained zero Si and zero Mn, respectively. This resulted in the failure to form an inner oxide layer or the desired Si-Mn depleted layer. Therefore, they did not achieve high hydrogen embrittlement or LME resistance. Sample No. 134 did not receive the specified tension during annealing, resulting in the failure to form a sufficient inner oxide layer or the desired Si-Mn depleted layer. As a result, they did not achieve high hydrogen embrittlement or LME resistance.Since sample No. 135 was not ground before annealing, the internal oxide was not sufficiently formed, and the desired Si-Mn depleted layer was not formed either, resulting in poor hydrogen embrittlement resistance and LME resistance.

[0107] (Example Y) (Preparation of steel plate samples) Steel plate samples were prepared under the manufacturing conditions shown in Table 2 in the same manner as in Example X, except that the holding temperature in the annealing treatment was mainly set to above 780°C to 870°C. For each steel plate sample, a JIS No. 5 tensile test piece was taken with the longitudinal direction perpendicular to the rolling direction, and a tensile test was conducted in accordance with JIS Z 2241 (2011). As a result, Nos. 201, 216, and 218 had a tensile strength of less than 440 MPa, while the others had a tensile strength of 440 MPa or more.

[0108] (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 2, thereby producing plated steel sheet samples. In Table 2, 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 pulled out 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.

[0109] The analysis of the surface layer of the steel sheet sample, the composition analysis of the plating layer, the evaluation of plating property, the evaluation of LME resistance, and the evaluation of hydrogen embrittlement resistance were as described above in relation to Example X.

[0110] [Table 2]

[0111] Samples Nos. 202 to 208 and 220 to 233 had high galvanizability, LME resistance, and hydrogen embrittlement resistance due to the appropriate steel sheet composition, average particle size and number density of granular oxides, and thickness and composition of the Si-Mn depleted layer. Sample No. 201 not only failed to obtain sufficient strength due to an insufficient C content, but also failed to form the desired granular oxides and Si-Mn depleted layer, resulting in poor hydrogen embrittlement and LME resistance. Sample No. 209 had a low dew point during annealing, resulting in the formation of an outer oxide layer rather than an inner oxide, and therefore poor galvanizability, hydrogen embrittlement, and LME resistance. Sample No. 210 had a high dew point during annealing, resulting in the formation of an outer oxide layer and the inability to refine the granular oxides, resulting in poor galvanizability, hydrogen embrittlement, and LME resistance. Sample No. 211 was annealed at a high holding temperature, which resulted in the formation of an outer oxide, insufficient granular oxide, and the desired Si-Mn depleted layer. This resulted in poor galvanizability, hydrogen embrittlement resistance, and LME resistance. Sample No. 212 was annealed without applying the required tension, which resulted in the desired Si-Mn depleted layer not being formed. This resulted in poor hydrogen embrittlement resistance. Sample No. 213 was annealed for a short holding time, which resulted in the insufficient formation of an inner oxide and the desired Si-Mn depleted layer not being formed. This resulted in poor hydrogen embrittlement resistance and LME resistance. Samples No. 214 and 234 were annealed for a long holding time, which resulted in the inability to refine the granular oxide and the desired Si-Mn depleted layer not being formed. This resulted in poor hydrogen embrittlement resistance and LME resistance. Samples No. 215 and No. 217 contained excessive amounts of Si and Mn, respectively, which resulted in the growth of an outer oxide, the coarsening of granular oxides, and the failure to form the desired Si-Mn depleted layer. Therefore, they did not achieve high galvanizability, hydrogen embrittlement resistance, or LME resistance. Samples No. 216 and No. 218 contained zero Si and zero Mn, respectively, which resulted in the failure to form an inner oxide layer or the desired Si-Mn depleted layer. Therefore, they did not achieve high hydrogen embrittlement or LME resistance. Sample No. 219 had a thick inner oxide layer before annealing, which prevented the desired inner oxide from forming after annealing, and the desired Si-Mn depleted layer also did not form. Therefore, they did not achieve high hydrogen embrittlement or LME resistance.Since sample No. 235 was not ground before annealing, the internal oxide was not sufficiently formed, and the desired Si-Mn depleted layer was not formed either, resulting in poor hydrogen embrittlement resistance and LME resistance. [Industrial Applicability]

[0112] According to the present invention, it is possible to provide high-strength steel sheets and plated steel sheets having high galvanizability, LME resistance, and hydrogen embrittlement resistance, and the steel sheets and plated steel sheets 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]

[0113] 1 steel plate 2. Outer oxide layer 3 Base steel 11 Steel plate 12 Granular oxide 13 Grain boundary oxide 14 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 contains granular oxides in a region extending from the surface to 50 μm in the sheet thickness direction, The average particle size of the granular oxide is 300 nm or less, The number density of the granular oxide is 4.0 particles / μm 2 That's all, a Si-Mn depleted layer having a thickness of 3.0 μm or more from the surface of the steel plate, the Si and Mn contents of the oxide-free Si-Mn depleted layer at the half-thickness position are less than 10% of the Si and Mn contents at the center of the thickness of the steel sheet, respectively; and the average particle size and number density of the granular oxides are determined by observing a cross section of the surface layer of the steel sheet with an SEM to obtain an SEM image containing granular oxides, and then selecting 10 regions of 1.0 μm (1.0 μm from the region from the steel sheet surface to 1.5 μm in the depth direction) × 1.0 μm (in the width direction) that do not contain grain boundary oxides, and then calculating the total area and number of granular oxides in the 10 regions.

2. The steel sheet according to claim 1, wherein the average particle size of the particulate oxide is 200 nm or less.

3. The number density of the granular oxide is 10.0 particles / μm 2 The steel sheet according to claim 1 or 2.

4. The steel sheet according to any one of claims 1 to 3, further comprising grain boundary oxides in a region extending from the surface of the steel sheet to 50 µm in the sheet thickness direction.

5. The steel sheet according to claim 4, wherein, when a cross section of the region 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.

6. The steel plate according to claim 5, wherein the ratio A is 80% or more.

7. A plated steel sheet having a plating layer containing Zn on the steel sheet according to any one of claims 1 to 6.

8. The plated steel sheet according to claim 7, wherein the plating layer has a chemical composition of Zn-(0.3 to 1.5)% Al.

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