Steel sheets and plated steel sheets

Abstract

To provide a high strength steel sheet and a plated steel sheet having high plating properties and hydrogen embrittlement resistance.SOLUTION: A steel sheet contains, by mass, 0.05 to 0.40% of C, 0.2 to 3.0% of Si and 0.1 to 5.0% of Mn. The steel sheet includes a dendrite type oxide on a surface layer thereof. The dendrite type oxide has an area ratio of 5.0% or more, and has a Si-Mn depletion layer with a thickness of 3.0 μm or more from a surface of the steel sheet. At a 1 / 2 location of the thickness, Si and Mn contents in the Si-Mn depletion layer not including an oxide are respectively below 10% of Si and Mn contents at a sheet thickness center part of the steel sheet. A plated steel sheet uses the steel sheet.SELECTED DRAWING: Figure 2

Description

[Technical Field]

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

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

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

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

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

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

[0007] High-strength steel sheets used in automotive components and other applications are often subjected to 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 into the steel. Hydrogen that penetrates into the steel segregates at martensite grain boundaries in 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). Therefore, to prevent hydrogen embrittlement cracking, it is effective to suppress hydrogen penetration into steel sheets used in corrosive environments and to expel the hydrogen that has penetrated into the steel sheet.

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

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

[0010] The present inventors have found that, 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 layer 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 by forming an internal oxide, high galvanizability can be ensured, and sufficient dendritic oxides present within the crystal grains of the metallographic structure can be formed as oxide morphologies, and the dendritic oxides can function as trap sites for hydrogen that can penetrate into the steel sheet in a corrosive environment, thereby suppressing hydrogen penetration into the steel. Furthermore, by forming an Si-Mn depleted layer of a predetermined thickness and composition in the surface layer of the steel sheet, hydrogen diffusion in the steel can be promoted, improving hydrogen desorption from the steel, thereby achieving high overall hydrogen embrittlement resistance.

[0011] The present invention was made based on the above findings, and the gist of the present invention is as follows. (1) In mass%, C: 0.05~0.40%, Si: 0.2 to 3.0% Mn: 0.1 to 5.0% sol.Al: 0 to less than 0.4000% P:0.0300% or less, S: 0.0300% or less, N: 0.0100% or less, B: 0~0.010%, Ti: 0 to 0.150% Nb: 0 to 0.150%, V: 0~0.150%, Cr: 0~2.00%, Ni: 0-2.00% Cu: 0-2.00% Mo: 0-1.00%, W: 0~1.00%, Ca: 0 to 0.100%, Mg: 0 to 0.100%, Zr: 0 to 0.100%, Hf: 0 to 0.100%, and A steel sheet having a composition containing REM: 0 to 0.100%, with the balance being Fe and impurities, The steel sheet contains dendritic oxides in a surface layer thereof, the area ratio of the dendritic oxide is 5.0% or more, 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 plate, respectively. (2) The steel sheet according to (1), wherein the area ratio of the dendritic oxide is 10.0% or more. (3) The steel sheet according to (1), wherein the area ratio of the dendritic oxide is 30.0% or more. (4) (1) The steel sheet according to (1), wherein the area ratio of the dendritic oxide is 50.0% or more. (5) A plated steel sheet having a plating layer containing Zn on the steel sheet according to any one of (1) to (4). (6) (5) The plated steel sheet according to (5), wherein the plated layer has a chemical composition of Zn-(0.3 to 1.5)% Al. [Effects of the Invention]

[0012] According to the present invention, a steel sheet includes a surface layer containing dendritic oxides at a predetermined area ratio and a Si-Mn-depleted layer having a predetermined thickness and composition. The dendritic oxides function as trap sites for hydrogen that penetrates into the steel sheet in a corrosive environment, and the Si-Mn-depleted layer promotes hydrogen diffusion, improving hydrogen desorption from the steel. As a result, the amount of hydrogen that penetrates into the steel sheet is significantly reduced, and the penetrated hydrogen is released, reducing the amount of hydrogen that accumulates in the steel, thereby significantly improving hydrogen embrittlement resistance. Furthermore, according to the present invention, because the dendritic oxides are formed inside the steel sheet, sufficient interdiffusion between the steel components and the coating components occurs when a coating layer is formed, resulting in high galvanizability. Therefore, the present invention enables high galvanizability and hydrogen embrittlement resistance to be achieved in a high-strength steel sheet. [Brief explanation of the drawings]

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

[0014] <Steel plate> The steel plate according to the present invention comprises, in mass%, C: 0.05~0.40%, Si: 0.2 to 3.0% Mn: 0.1 to 5.0% sol.Al: 0 to less than 0.4000% P:0.0300% or less, S: 0.0300% or less, N: 0.0100% or less, B: 0~0.010%, Ti: 0 to 0.150% Nb: 0 to 0.150%, V: 0~0.150%, Cr: 0~2.00%, Ni: 0-2.00% Cu: 0-2.00% Mo: 0-1.00%, W: 0~1.00%, Ca: 0 to 0.100%, Mg: 0 to 0.100%, Zr: 0 to 0.100%, Hf: 0 to 0.100%, and A steel sheet having a composition containing REM: 0 to 0.100%, with the balance being Fe and impurities, The steel sheet contains dendritic oxides in a surface layer thereof, the area ratio of the dendritic oxide is 5.0% or more, 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.

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

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

[0017] 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 humidity levels. 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 structure, 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 and prone to hydrogen embrittlement cracking. Therefore, in order to prevent hydrogen embrittlement cracking, it is effective to reduce the amount of hydrogen stored in the steel, more specifically, the amount of hydrogen stored deeper than the surface layer of the steel sheet, for high-strength steel sheets used in atmospheric corrosive environments. The inventors have found that by controlling the morphology of the oxides present in the surface layer of a steel sheet, more specifically, by forming the oxides into dendritic oxides with a predetermined area ratio, and further by controlling the Si-Mn depleted layer, which is formed as a result of a decrease in the surrounding Si and Mn concentrations due to the formation of dendritic oxides, to within a predetermined thickness and composition range, the dendritic oxides function to trap hydrogen that penetrates into the steel sheet in a corrosive environment, and the Si-Mn depleted layer promotes the diffusion of the penetrated hydrogen, thereby improving the ability to release hydrogen from the steel. As a result, the amount of hydrogen that accumulates inside the steel sheet is suppressed, and high resistance to hydrogen embrittlement can be achieved.

[0018] More specifically, the present inventors conducted a detailed analysis of the relationship between the morphology of oxides and their effectiveness as hydrogen trapping sites, and found that, as shown in FIG. 2 , it is effective to have dendritic oxides 12 present within the crystal grains of a base steel 13 at a certain area ratio or more, more specifically, at an area ratio of 5.0% or more. Without being bound by any particular theory, it is believed that the ability of oxides in a steel sheet to trap hydrogen is positively correlated with the surface area of the oxide. That is, the presence of an appropriate amount of oxide in the surface layer of the steel sheet is believed to increase the surface area of the oxide in the surface layer of the steel sheet, thereby improving the hydrogen trapping ability. Therefore, the present inventors found that, from the perspective of achieving high hydrogen penetration resistance, it is important to control the conditions during steel sheet manufacturing, particularly annealing, to ensure the presence of an appropriate amount of dendritic oxides that function as trapping sites for hydrogen that penetrates in a corrosive environment. The metal structure of the surface layer of a steel plate is typically softer than that of the interior of the steel plate (for example, at the 1 / 8 or 1 / 4 position of the plate thickness). Therefore, even if hydrogen is present in the surface layer of the steel plate, hydrogen embrittlement cracking does not pose a particular problem.

[0019] Furthermore, the present inventors conducted a detailed analysis of the relationship between hydrogen release performance and the 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 dendrite-type 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. As shown in Figure 2, when internal oxides such as dendrite-type oxides 12 form in the surface layer of a steel sheet, the Si and Mn dissolved in the steel are consumed in the formation of the internal oxides. This results in the formation of 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)—it is believed that sufficient hydrogen diffusion paths can be secured. 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% each—it is believed that the amounts of solute Si and Mn that inhibit hydrogen diffusion can be sufficiently reduced. Therefore, by including a Si-Mn-depleted zone whose thickness and composition are controlled within the above-mentioned ranges, it is believed that hydrogen diffusion can be promoted and hydrogen desorption from the steel can be significantly improved. Therefore, by combining the above-mentioned dendritic oxides with the Si-Mn depleted layer, it is possible to improve both the hydrogen penetration resistance and the hydrogen desorption resistance, thereby significantly improving the hydrogen embrittlement resistance of the steel sheet as a whole.

[0020] It is also known that hydrogen embrittlement cracking 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 dendritic oxide and Si-Mn depleted layer described above 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.

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

[0048] As illustrated in Fig. 2, a steel sheet 11 according to the present invention contains dendritic oxides 12 in the surface layer of the steel sheet 11. The presence of these dendritic oxides 12 inside the base steel 13 (i.e., as internal oxides) enables the steel sheet 11 to have high platability 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 dendritic oxides in the surface layer of the steel sheet, i.e., inside the steel sheet, have high platability.

[0049] [Dendrite-type oxide] In the present invention, "dendritic oxides" refer to oxides present in a dendrite shape within crystal grains inside steel. "Intragrain" as used herein refers to oxides with a crystal orientation misorientation of less than 10° as measured by electron backscatter diffraction (EBSD). Furthermore, "dendritic" refers to a dendritic shape formed by three-dimensional growth of multiple needle- or leaf-like branch portions (secondary arms) branching from a main branch (primary arm). For example, it refers to a primary arm having a length of 0.5 to 5.0 μm from which secondary arms having a length of 50 to 300 nm grow. The primary arm preferably has a length of 1.0 to 5.0 μm, more preferably 2.0 to 5.0 μm. The secondary arm preferably has a length of 70 to 250 nm, more preferably 100 to 250 nm. The lengths of the primary and secondary arms can be measured by observing the cross section of the steel sheet using a scanning electron microscope (SEM). In reality, dendritic oxides typically exist three-dimensionally in a dendritic form within steel sheet crystal grains. Therefore, when a cross section of the surface layer of a steel sheet is observed, the dendritic oxide is typically observed as a single primary arm with multiple secondary arms branching out on both sides, or as a dotted, approximately linear configuration of secondary arms at approximately equal intervals (e.g., the difference in spacing between adjacent points is 10% or less). Figure 2 shows, as an example, a dendritic oxide 12 observed as a single primary arm with multiple secondary arms branching out on both sides, and a dendritic oxide 12 observed as a dotted, approximately linear configuration of secondary arms. In certain embodiments, the dendritic oxide of the present invention may be present only in a region of the steel sheet, for example, from the surface to a depth of less than 8 μm, less than 7 μm, or less than 6 μm. In another specific embodiment, the dendritic oxides may be present only in a region of the steel sheet that is 1 μm or more and less than 8 μm, 2 μm or more and less than 8 μm, or 3 μm or more and less than 8 μm from the surface.

[0050] (area ratio) In the present invention, the area ratio of the dendritic oxide is 5.0% or more. By controlling the area ratio of the dendritic oxide within this range, a sufficient amount of dendritic oxide can be present within the crystal grains inside the steel sheet, and the dendritic oxide functions well as a hydrogen trapping site that suppresses hydrogen penetration in a corrosive environment. On the other hand, if the area ratio of the dendritic oxide is less than 5.0%, the amount is insufficient to function as a hydrogen trapping site, and hydrogen penetration in a corrosive environment cannot be sufficiently suppressed, which may result in poor hydrogen penetration resistance and therefore poor hydrogen embrittlement resistance. The area ratio of the dendritic oxide is preferably 10.0% or more or 20.0% or more, more preferably 30.0% or more, and even more preferably 50.0% or more. Since the presence of a large amount of dendritic oxide is preferable, the upper limit of the area ratio of the dendritic oxide is not particularly limited, but may be, for example, 70.0% or less or 60.0% or less.

[0051] The area ratio of dendritic oxides is measured using a scanning electron microscope (SEM). Specific measurements are as follows: A cross section of the surface layer of a steel sheet is observed using an SEM, and an SEM image containing dendritic oxides, such as that shown in Figure 2, is obtained. A total of 10 observation regions, each 1.0 μm (depth direction) × 1.0 μm (width direction), are selected from the SEM image. The observation position for each region is 1.0 μm in the depth direction (direction perpendicular to the steel sheet surface) within a region 0.5 μm to 5.0 μm deep from the steel sheet surface, and 1.0 μm at any position in the width direction (direction parallel to the steel sheet surface) within the SEM image. Next, SEM images of each region selected as above are extracted and binarized to separate the oxide portion from the steel portion, and the total area of the dendritic oxide portion is calculated from each binarized image. The total area of the dendritic oxides in the 10 regions thus obtained is then multiplied by the total area (10 μm) of the 10 regions. 2 ) to determine the "area ratio of dendritic structures" in the present invention. Note that the observation region selected above does not include any oxides that are not dendritic structures.

[0052] [Oxide composition] In the present invention, the dendritic oxide (hereinafter simply referred to as oxide) contains, in addition to oxygen, one or more of the elements contained in the steel sheet described above, and typically has a chemical composition containing Si, O, and Fe, and optionally further containing Mn. More specifically, the oxide typically contains 5-25% Si, 0-10% Mn, 40-65% O, and 10-30% Fe. In addition to these elements, the oxide may also contain elements that can be contained in the steel sheet described above (e.g., Cr, etc.).

[0053] [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 making the Si-Mn depleted layer formed at the surface of the steel sheet due to the formation of dendritic oxides 3.0 μm or more thick 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.

[0054] Similarly, by reducing the Si and Mn deficiency rates in the Si-Mn depleted zone, the amounts of solute Si and Mn in the steel can be further reduced. Therefore, the Si deficiency rate in 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 in 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 dendritic oxides but also any other oxides are not contained. Such oxide-free regions can be identified by cross-sectional observation using a SEM and an energy dispersive X-ray spectrometer (EDS). Furthermore, the Si-Mn depleted zone according to the present invention cannot be controlled to a desired thickness and composition range simply by forming internal oxides such as dendritic oxides. Therefore, as will be explained in detail later, it is important to appropriately control the progress of internal oxidation in the manufacturing process.

[0055] The thickness of the Si-Mn depleted zone, as shown by D in Figure 2, is the distance from the surface of the steel sheet 11 (or the interface between the steel sheet and the coating layer in the case of a plated steel sheet) to the farthest point on the steel sheet where dendritic oxides 12 are present, in the thickness direction of the steel sheet 11 (the direction perpendicular to the surface of the steel sheet). The thickness of the Si-Mn depleted zone can be determined from the same SEM image as the one used to measure the area ratio of dendritic oxides. 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 spectroscope (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 with a transmission electron microscope equipped with an energy dispersive X-ray spectrometer (TEM-EDS), and arithmetically averaging the measured values of Si and Mn concentrations. Finally, the Si and Mn deficiency rates are determined by dividing the Si and Mn contents at the half-thickness position 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.

[0056] <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.

[0057] [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.

[0058] (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.

[0059] (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.

[0060] (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.

[0061] (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.

[0062] 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.

[0063] 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.

[0064] 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) emission spectroscopy.

[0065] 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.

[0066] [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.

[0067] The steel sheet and plated steel sheet according to the present invention have high strength and excellent galvanizability 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 used in atmospheric corrosive environments, and in such environments, hydrogen embrittlement cracking due to the penetration of hydrogen generated in such environments can become a significant problem. Therefore, when the steel sheet and plated steel sheet according to the present invention are used as an automotive steel sheet, the effect of the present invention, i.e., high hydrogen embrittlement resistance, is suitably exhibited.

[0068] <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.

[0069] 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.

[0070] [Casting process] The conditions for the casting process are not particularly limited. For example, after melting in a blast furnace or electric furnace, various secondary smelting processes may be carried out, and then casting may be carried out by a conventional method such as continuous casting or ingot casting.

[0071] [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%.

[0072] [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.

[0073] [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.

[0074] [Grinding process] To ensure sufficient dendritic oxide formation 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 before annealing the cold-rolled steel sheet. 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. This ultimately promotes oxygen bonding with Si and / or Mn within the steel to form dendritic oxides. Furthermore, promoting the formation of such internal oxides also promotes a decrease in the surrounding Si and Mn concentrations, thereby promoting the formation of a Si-Mn depleted zone with the desired thickness and composition. The grinding process is not particularly limited, but for example, a grinding amount of 10 to 200 g / m is used with a heavy-duty grinding brush. 2 The grinding 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 reduction amount, the coating liquid to be used, etc. By performing such a grinding step, it becomes possible to form a desired dendritic oxide in the annealing step described below, and also to reliably and efficiently form a Si-Mn depleted layer with 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%, in the surface layer of the steel sheet.

[0075] [Annealing process] The cold-rolled steel sheet that has been subjected to the grinding step 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. This is advantageous for the formation of dendritic oxides with a desired area ratio and a Si-Mn depleted zone with a desired thickness and composition.

[0076] From the viewpoint of generating a sufficient amount of dendritic oxides, the holding temperature in the annealing step is preferably 700 to 870°C, and more preferably 740 to 840°C. If the holding temperature in the annealing step is less than 700°C, there is a risk that dendritic oxides will not be generated sufficiently, and hydrogen penetration resistance may become insufficient. The rate of temperature rise to the holding temperature is not particularly limited, but may be 1 to 10°C / second. Furthermore, the temperature may be raised in two stages, with a first heating rate of 1 to 10°C / second and a second heating rate of 1 to 10°C / second that is different from the first heating rate.

[0077] The holding time at the holding temperature is preferably more than 150 seconds to 300 seconds, and more preferably 200 to 280 seconds. If the holding time is 150 seconds or less, there is a risk that dendritic oxides will not be sufficiently formed. On the other hand, if the holding time is more than 300 seconds, there is a risk that the outer oxide will grow excessively, and dendritic oxides will not be sufficiently formed, which may result in insufficient plating properties and hydrogen penetration resistance.

[0078] The dew point of the atmosphere in the annealing step is preferably -20 to 10°C, more preferably -10 to 5°C, from the viewpoint of generating a sufficient amount of dendritic oxides. If the dew point is too low, an outer oxide layer may be formed on the surface of the steel sheet, and inner oxides may not be sufficiently formed, which may result in insufficient galvanization and hydrogen penetration resistance. On the other hand, if the dew point is too high, Fe oxide may be formed as the outer oxide on the surface of the steel sheet, and dendritic oxides may not be sufficiently formed, which may result in insufficient galvanization and hydrogen penetration resistance, and ultimately insufficient hydrogen embrittlement resistance. The atmosphere in the annealing step may also 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 being nitrogen).

[0079] Furthermore, it is effective to remove the internal oxide layer of the steel sheet before the annealing process. An internal oxide layer may be formed on the surface layer of the steel sheet during the above-mentioned rolling process, particularly the hot rolling process. Since such an internal oxide layer formed during the rolling process may hinder the formation of dendritic 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.

[0080] By carrying out the above-mentioned steps, it is possible to obtain a steel sheet that contains a sufficient amount of dendritic oxides in the surface layer of the steel sheet and also contains an Si-Mn depleted layer with a desired thickness and composition.

[0081] <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.

[0082] 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.

[0083] [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]

[0084] 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.

[0085] (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.

[0086] 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 / m2 The surface of the cold-rolled steel sheet was ground with a grinding amount of 0.01 mm (Sample No. 35 was not ground). Subsequently, annealing treatment (annealing atmosphere: 4% hydrogen and balance nitrogen) was performed at the dew point, holding temperature, and holding time shown in Table 1 to prepare each steel sheet sample. For all steel sheet samples, the heating rate during annealing was 6.0°C / s up to 500°C, and 2.0°C / s from 500°C to the holding temperature. In the annealing treatment, the cold-rolled steel sheet was annealed with a tension of 1 MPa or more applied in the rolling direction, and a higher tension, specifically a tension of 3 to 150 MPa, was applied in the rolling direction in the annealing temperature range of 500°C or higher than in other regions (Sample No. 34 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, Nos. 16 and 18 had a tensile strength of less than 440 MPa, while the others had a tensile strength of 440 MPa or higher.

[0087] (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. The cross sections of each steel sheet sample were observed using an SEM. A total of 10 1.0 μm × 1.0 μm regions were observed from the SEM images. The observation positions were 1.0 μm in the depth direction (direction perpendicular to the steel sheet surface) within a range of 2.0–3.0 μm from the steel sheet surface, and 1.0 μm at any position in the SEM image in the width direction (direction perpendicular to the steel sheet surface). Next, the SEM images of each region of each steel sheet sample were binarized, and the area of the dendritic oxide portion was calculated from the binarized images. The "dendritic oxide area ratio" for each steel sheet sample was calculated from the area of the dendritic oxide in the 10 binarized images obtained in this way. The dendritic oxide area ratio (%) for each steel sheet sample is shown in Table 1. In the SEM images, the lengths of the primary and secondary arms of the dendritic oxides observed as having multiple secondary arms branching out from the primary arm on both sides were measured. For samples Nos. 2 to 8 and 20 to 33, the primary arms were 0.5 to 5.0 μm and the secondary arms were 50 to 300 nm.

[0088] The thickness of the Si-Mn depleted zone was determined by measuring the distance from the surface of the steel sheet to the furthest point where dendritic oxides were present in the thickness direction (perpendicular to the surface of the steel sheet) in the SEM image where the area ratio of dendritic oxides was measured. 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 with an SEM, analyzing 10 randomly selected points in the center of the thickness from the SEM image 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, analysis of the dendritic oxide composition of each steel sheet revealed that all dendritic oxides contained Si, O, and Fe, and many oxides also contained Mn. Therefore, the composition of all oxides was Si: 5-25%, Mn: 0-10%, O: 40-65%, and Fe: 10-30%.

[0089] (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.

[0090] (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.

[0091] (Platability evaluation) For each plated steel sheet sample, the platability was evaluated by measuring the area ratio of unplated areas on the surface of the steel sheet. Specifically, a 1 mm x 1 mm area on the surface of each plated steel sheet sample on which a plating layer had been formed was observed with an optical microscope, and the areas on which a plating layer had been formed (plated areas) and areas on which no plating layer had been formed (unplated areas) were distinguished from the observed images. The area ratio of unplated areas (area of unplated areas / area of observed image) was calculated, and the platability was evaluated according to the following criteria. The results are shown in Table 1. ◯ indicates pass, and × indicates fail. Rating: 5.0% or less Rating ×: Over 5.0%

[0092] (Evaluation of hydrogen embrittlement resistance) Two parallel longitudinal cuts reaching the substrate were made in each plated steel sheet, and then the sheet was immersed in a 5% NaCl aqueous solution at 50°C for 480 hours to simulate an SDT (saltwater immersion test). The amount of diffusible hydrogen in each plated steel sheet sample was then measured using thermal desorption. Specifically, the plated steel sheet samples were heated to 400°C in a heating furnace equipped with gas chromatography, and the total amount of hydrogen released before the temperature dropped to 250°C was measured. Based on the measured amount of diffusible hydrogen, hydrogen embrittlement resistance (amount of accumulated hydrogen in the sample) was evaluated according to the following criteria, and the results are shown in Table 1. ◎ and ◯ indicate pass, and × indicates fail. ◎: 0.2 ppm or less Rating: Over 0.2 ppm and 0.4 ppm or less Rating ×: Over 0.4 ppm

[0093]

Table 1

[0094] Samples Nos. 2 to 8 and 20 to 33 had high galvanizability and hydrogen embrittlement resistance due to the appropriate chemical composition, dendritic oxide area ratio, and Si-Mn depleted layer thickness and composition. On the other hand, Samples Nos. 1 and 19 had a thick internal oxide layer before annealing, which prevented the formation of sufficient dendritic oxides and the desired Si-Mn depleted layer, resulting in poor hydrogen embrittlement resistance. Sample No. 9 had a low dew point during annealing, which resulted in the formation of an outer oxide layer, which prevented the formation of dendritic oxides and the desired Si-Mn depleted layer, resulting in poor galvanizability and hydrogen embrittlement resistance. Sample No. 10 had a high dew point during annealing, which resulted in the formation of an outer oxide, which prevented the formation of sufficient dendritic oxides and the desired Si-Mn depleted layer, resulting in poor galvanizability and hydrogen embrittlement resistance. Sample No. 11 was annealed at a high holding temperature, which resulted in the formation of an outer oxide and insufficient dendritic oxide formation, and the desired Si-Mn depleted layer was not formed. This resulted in poor galvanizability and hydrogen embrittlement resistance. Sample No. 12 was annealed at a low holding temperature, which resulted in poor dendritic oxide formation and the desired Si-Mn depleted layer, so it did not exhibit high hydrogen embrittlement resistance. Sample No. 13 was annealed at a short holding time, which resulted in poor dendritic oxide formation and the desired Si-Mn depleted layer, so it did not exhibit high hydrogen embrittlement resistance. Sample No. 14 was annealed at a long holding time, which resulted in the formation of an outer oxide and insufficient dendritic oxide formation and the desired Si-Mn depleted layer, so it did not exhibit high galvanizability and hydrogen embrittlement resistance. Sample No. 15 had an excessive amount of Si, which caused the outer oxide to grow, preventing the formation of sufficient dendritic oxides and the formation of the desired Si-Mn depleted layer, resulting in poor galvanizability and hydrogen embrittlement resistance.Samples No. 16 and No. 18 had zero Si and zero Mn contents, respectively, preventing the formation of dendritic oxides and the formation of the desired Si-Mn depleted layer, resulting in poor hydrogen embrittlement resistance.Sample No. 17 contained an excessive amount of Mn, which caused the outer oxide to grow, preventing the formation of sufficient dendritic oxides and the formation of the desired Si-Mn depleted layer. This resulted in poor galvanizability and hydrogen embrittlement resistance. Sample No. 34 did not receive the specified tension during annealing, preventing the formation of sufficient dendritic oxides and the formation of the desired Si-Mn depleted layer. As a result, it did not achieve high hydrogen embrittlement resistance. Sample No. 35 was not ground before annealing, preventing the formation of sufficient dendritic oxides and the formation of the desired Si-Mn depleted layer. As a result, it did not achieve high hydrogen embrittlement resistance. [Industrial Applicability]

[0095] According to the present invention, it is possible to provide a high-strength steel sheet and plated steel sheet having high galvanizability and hydrogen embrittlement resistance, and the steel sheet and plated steel sheet can be suitably used for applications such as automobiles, home appliances, and building materials, particularly for automobiles, and are expected to provide high collision safety and long life as automotive steel sheets and automotive plated steel sheets. Therefore, the present invention can be said to be an invention of extremely high industrial value. [Explanation of symbols]

[0096] 1 steel plate 2. Outer oxide layer 3 Base steel 11 Steel plate 12 Dendritic oxide 13 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 dendritic oxides in a surface layer thereof, the area ratio of the dendritic oxide is 5.0% or more, 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 plate, respectively; The area ratio of the dendritic oxides is determined by observing the cross section of the surface layer of the steel sheet using an SEM to obtain an SEM image containing dendritic oxides, then selecting a total of 10 regions of 1.0 μm (1.0 μm from the region from 0.5 μm to 5.0 μm in the depth direction from the surface of the steel sheet) x 1.0 μm (width direction), and dividing the total area of the dendritic oxides in the 10 regions by the total area of the 10 regions (10 μm2).

2. The steel sheet according to claim 1, wherein the area ratio of the dendritic oxide is 10.0% or more.

3. The steel sheet according to claim 1, wherein the area ratio of the dendritic oxide is 30.0% or more.

4. The steel sheet according to claim 1, wherein the area ratio of the dendritic oxides is 50.0% or more.

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

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

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