Steel sheets and plated steel sheets

Abstract

Provided is a steel sheet comprising 0.05-0.40% C, 0.2-3.0% Si, and 0.1-5.0% Mn, in mass%, wherein: the surface layer of the steel sheet contains a grain-boundary-type oxide; the ratio A of the length of the grain-boundary-type oxide projected on the surface of the steel sheet to the length of the surface of the steel sheet is 50-100%; the number density of grain-type oxide is less than 4.0 / μm2; a Si-Mn deficient layer having a thickness of not less than 3.0 μm from the surface of the steel sheet is included; and the Si content and the Mn content in the Si-Mn deficient layer not including an oxide at position at half the thickness are less than 10% of the Si content and the Mn content, respectively, at the sheet-thickness-wise center part of the steel sheet. Also provided is 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 galvanizability and hydrogen desorption properties. [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] When steel sheets are annealed during the manufacturing process of high-strength steel sheets, hydrogen present in the atmosphere during annealing can penetrate into the steel sheet. Hydrogen that penetrates into the steel sheet segregates at the martensite grain boundaries of the steel sheet structure, embrittling the grain boundaries and potentially causing cracks in the steel sheet. This phenomenon of cracks caused by absorbed hydrogen is called hydrogen embrittlement cracking (delayed fracture) and is often a problem during steel sheet processing. Therefore, to prevent hydrogen embrittlement cracking, it is effective to efficiently release the hydrogen that penetrated into the steel sheet during annealing from the inside of the steel sheet to the outside of the system.

[0009] 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 controlling the morphology of the oxides present in the internal oxide layer, and there is room for improvement in hydrogen release properties (hydrogen embrittlement resistance).

[0010] 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 and hydrogen desorption properties. [Means for solving the problem]

[0011] The present inventors have found that, to achieve the above-mentioned object, it is important to form oxides in the surface layer of a steel sheet, i.e., in the interior of the steel sheet, control the morphology of the oxides present in the surface layer of the steel sheet, and 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 that forming a large amount of grain boundary oxide along the grain boundaries of the metallographic structure as an oxide form allows the grain boundary oxide to function as an escape route for hydrogen that has penetrated into the steel during annealing, thereby promoting the release of hydrogen from the steel to the outside. Furthermore, the present inventors have found 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, thereby achieving high hydrogen desorption performance. Furthermore, the present inventors have found that suppressing the formation of particulate oxides present within grains is effective in achieving high hydrogen desorption performance.

[0012] 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 grain boundary oxides in a surface layer thereof, When a cross section of a surface layer of the steel sheet is observed, a ratio A of a length of grain boundary oxide projected onto the surface of the steel sheet to a length of the surface of the steel sheet is 50% or more and 100% or less, The number density of granular oxide is 4.0 pieces / μm 2 is less than 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 ratio A is 80% or more. (3) The steel sheet according to (1), wherein the ratio A is 90% or more. (4) The number density of the granular oxide is 2.0 particles / μm 2 The steel sheet according to any one of (1) to (3), wherein the tensile strength is less than 1000 MPa. (5) A plated steel sheet having a plating layer containing Zn on the steel sheet according to any one of (1) to (4). [Effects of the Invention]

[0013] According to the present invention, grain boundary oxides present along the grain boundaries of the metallographic structure in the surface layer of a steel sheet function as escape routes for hydrogen that has penetrated into the steel, facilitating the release of hydrogen from the steel to the outside. Furthermore, the inclusion of a Si-Mn depleted layer with a predetermined thickness and composition promotes hydrogen diffusion, significantly improving hydrogen desorption and, as a result, significantly reducing the amount of hydrogen that accumulates in the steel. Furthermore, according to the present invention, because the granular oxides are formed inside the steel sheet, when a coating layer is formed, sufficient interdiffusion between the steel components and the coating components occurs, resulting in high coating performance. Therefore, according to the present invention, high coating performance and hydrogen desorption performance can be achieved in a high-strength steel sheet. [Brief explanation of the drawings]

[0014] [Figure 1] 1 shows a schematic diagram of a cross section of a steel plate having an outer oxide layer. [Figure 2] 1 shows a schematic diagram of a cross section of an exemplary steel plate according to the present invention. [Figure 3] 1 shows a schematic diagram for explaining the measurement of ratio A in the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0015] <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 grain boundary oxides in a surface layer thereof, When a cross section of a surface layer of the steel sheet is observed, a ratio A of a length of grain boundary oxide projected onto the surface of the steel sheet to a length of the surface of the steel sheet is 50% or more and 100% or less, The number density of granular oxide is 4.0 pieces / μm 2 is less than 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.

[0016] 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.

[0017] In contrast, as illustrated in FIG. 2 , a steel sheet 11 according to the present invention does not form an outer oxide layer 2 on the surface of a base steel 3 as in the steel sheet 1 shown in FIG. 1 , but has oxides 12 and 13 (preferably only oxide 13) present inside the base steel 14. Therefore, when a coating layer is formed on the surface of the steel sheet 11, the steel sheet 11 according to the present invention, in which oxides are formed inside the base steel, 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 present inventors have discovered that, from the perspective of achieving high coating ability, it is effective to control the annealing conditions to form oxides inside the steel sheet. Note that the term “high coating ability,” when used with respect to a steel sheet, indicates that a coating layer can be formed with little (e.g., 5.0 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 area % or less) or no uncoated areas.

[0018] On the other hand, it is known that during annealing, hydrogen present in the annealing atmosphere penetrates into a base steel material, where the penetrated hydrogen segregates at martensite grain boundaries in the base steel material, embrittling the grain boundaries and causing hydrogen embrittlement cracking. Therefore, in order to prevent hydrogen embrittlement cracking of steel sheets, it is preferable to efficiently discharge hydrogen that has penetrated into the steel material, i.e., to have high hydrogen dischargeability. The inventors have discovered that by controlling the morphology of oxides present in the surface layer of a steel sheet and controlling the Si-Mn depleted zone formed in the surface layer of the steel sheet due to the formation of such oxides to within a predetermined thickness and composition range, more specifically, by converting most or all of the oxides into grain boundary oxides connecting the interior of the steel sheet with the steel sheet surface, the oxides can discharge hydrogen that penetrates during annealing. Furthermore, by forming a Si-Mn depleted zone having a predetermined thickness and composition in the surface layer of the steel sheet, hydrogen diffusion in the steel can be promoted, thereby enabling efficient discharge of hydrogen in the steel material. More specifically, the present inventors conducted a detailed analysis of the relationship between the morphology of oxides and their effectiveness as escape paths for hydrogen, and found that increasing the abundance ratio (ratio A) of grain boundary oxides in the base steel 14, more specifically, increasing ratio A to 50% or more, is effective. Without being bound by a particular theory, it is believed that the release function of oxides in a steel sheet against absorbed hydrogen is positively correlated with the ratio A of the oxides. That is, the presence of grain boundary oxides at a high ratio A is thought to increase the number of escape paths for discharging hydrogen that has penetrated into the steel sheet, thereby improving the hydrogen release function. Therefore, the present inventors found that, from the perspective of obtaining high hydrogen release properties, it is important to control the conditions during steel sheet production, particularly during annealing treatment, so that grain boundary oxides that function as escape paths for hydrogen that penetrates during annealing are present in a high ratio, preferably at a deeper position.

[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 oxides 12 and 13 shown in Figure 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 oxides 12 and 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. 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 improved. Therefore, by combining the above-mentioned grain boundary oxide with the Si--Mn depleted layer, it becomes possible to significantly improve the hydrogen discharge property.

[0020] Furthermore, the inventors have found that in order to maximize the hydrogen-exhausting function of grain boundary oxides, it is important to suppress as much as possible the formation of granular oxides, which are oxides in a different form from the grain boundary oxides and form in the surface layer. Granular oxides are selectively formed in the surface layer and inhibit the formation of grain boundary oxides in the surface layer. Furthermore, if granular oxides are present in excess, they may function as trap sites for hydrogen attempting to escape from the system, thereby reducing the hydrogen-exhausting function. Therefore, in order to further improve the hydrogen-exhausting function, in addition to the combination of grain boundary oxides and Si-Mn depleted layers described above, it has also been found that it is important to suppress the formation of granular oxides, more specifically, to control the number density (number per unit area) of granular oxides.

[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 grain boundary 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 cause deterioration of the surface properties and may also promote the growth of external oxides. 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 to allow the desired oxides, particularly grain boundary oxides, to be 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 cause Mn segregation to cause the metal structure to become non-uniform, which may reduce workability and promote the growth of external oxides. 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 shown in FIG. 2, a steel sheet 11 according to the present invention contains grain boundary oxides 13 in the surface layer of the steel sheet 11. Preferably, only grain boundary oxides 13 are present. The presence of this grain boundary oxide 13 inside the base steel 14 (i.e., as internal oxides) enables the steel sheet 11 to have high platability compared to the steel sheet 1 shown in FIG. 1 in which an outer oxide layer 2 is present on the surface. This is thought to be the result of the formation of internal oxides, 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 grain boundary oxides in the surface layer of the steel sheet, i.e., inside the steel sheet, have high platability.

[0049] Furthermore, as shown in FIG. 2, the steel sheet 11 according to the present invention may contain granular oxides 12 in the surface layer of the steel sheet 11 in addition to the grain boundary oxides 13. Since the granular oxides 12 are present inside the base steel 14, like the grain boundary oxides 13, steel sheets and plated steel sheets containing both the granular oxides 12 and the grain boundary oxides 13 also have high galvanic properties. On the other hand, since the granular oxides 12 exist as independent particles, they do not function as an escape route for hydrogen during annealing. If they are present in excess, they may trap hydrogen that is trying to escape, thereby reducing the hydrogen discharge function of the grain boundary oxides 13. Furthermore, if the granular oxides 12 are present in excess (for example, when the number density is 4.0 particles / μm as described later), 2 When the steel sheet 11 is manufactured under manufacturing conditions (particularly annealing conditions) that result in the formation of the above-mentioned oxides, the formation of the particulate oxides 12 is promoted, and the grain boundary oxides 13 are not sufficiently formed, which tends to reduce the hydrogen discharge function. Therefore, from the viewpoint of effectively increasing the escape routes of hydrogen during annealing, it is preferable that the amount of the particulate oxides 12 is as small as possible. Therefore, more preferably, in the present invention, the surface layer of the steel sheet 11 does not need to contain the particulate oxides 12 (i.e., the oxides present in the surface layer of the steel sheet 11 may all be the grain boundary oxides 13).

[0050] [Granular oxide] In the present invention, "granular oxides" refer to oxides dispersed in a granular shape within steel crystal grains or on grain boundaries. The term "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. The term "granularly dispersed" refers to oxide particles not being arranged according to a specific rule (e.g., linearly) but being randomly arranged. In practice, 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. FIG. 2 shows a granular oxide 12 that appears circular as an example. The average particle size of the granular oxide 12 is not particularly limited and may be, for example, 300 nm or less, 200 nm or less, or 150 nm or less.

[0051] (number density) In the present invention, the number density of the granular oxide is 4.0 pieces / μm 2 In the steel sheet of the present invention, the surface layer of the steel sheet does not need to contain granular oxides, so the number density is 0 particles / μm 2 Particulate oxides may function as hydrogen trap sites and hinder the escape of hydrogen from the steel. Therefore, from the viewpoint of improving hydrogen desorption, it is preferable that the particulate oxides are small, and the number density is 4.0 particles / μm 2 From the viewpoint of obtaining even better hydrogen discharging properties, the number density is preferably less than 3.0 particles / μm 2 Less than 2.0 particles / μm 2 less than 1.0 particles / μm, more preferably 1.0 particles / μm 2 is less than.

[0052] The average particle size and number density of granular oxides are measured using a scanning electron microscope (SEM). The specific measurement procedure is as follows: A cross section of the surface layer of a steel sheet is observed using an SEM to obtain an SEM image containing granular oxides. A total of 10 regions of 1.0 μm (depth direction) × 1.0 μm (width direction) that do not contain the grain boundary oxides described below are selected from the SEM image as observation regions. The observation position for each region is 1.0 μm in the depth direction (direction perpendicular to the steel sheet surface) within a region extending 1.5 μm from the steel sheet surface, and 1.0 μm at any position in the width direction (direction parallel to the steel sheet surface) of the SEM image. Note that if no granular oxides are observed in the observation region, the number density is 0 particles / μm. 2 In this case, there is no average grain size. Next, the SEM image of each region selected as above is extracted and binarized to separate the oxide portion from the steel portion. The total area of the granular oxide portion 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 calculated, the average grain size (nm) of the granular oxides is calculated as the circle equivalent diameter. In addition, the number density (number / μm 2 ) is equal to the average number of granular oxides counted from each binarized image. Note that if only a portion of the granular oxide is observed in the observation area, that is, if the entire outline of the granular oxide is not within the observation area, it is not counted. In addition, from the viewpoint of measurement accuracy, the lower limit for counting the number of granular oxides is 5.0 nm or more.

[0053] [Grain boundary oxide] In the present invention, the term "grain boundary oxide" refers to oxides present along the grain boundaries of steel, and does not include oxides present within the grains of steel. In reality, grain boundary oxides are present in a planar form along the grain boundaries in the surface layer of a steel sheet, and therefore, when a cross section of the surface layer of the steel sheet is observed, the grain boundary oxides are observed in a linear form. In FIGS. 2 and 3, a linear grain boundary oxide 13 is shown as an example. In addition, 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 also forms near the surface of base steel 14. The formation of grain boundary oxide 13 near the surface of the steel sheet connects the steel sheet surface and the interior of the steel sheet, thereby fully demonstrating the hydrogen release function.

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

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

[0056] [Oxide composition] In the present invention, the granular oxides and 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.).

[0057] [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 in the surface layer of the steel sheet due to the formation of grain boundary oxides and / or granular oxides 3.0 μm or more thick and controlling the Si and Mn deficiency 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. Therefore, combining the grain boundary oxides and the Si-Mn depleted layer can significantly improve hydrogen desorption. Furthermore, 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.

[0058] 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 the zone does not contain not only the grain boundary oxides and granular oxides described above, but also any other oxides. 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 grain boundary oxides. Therefore, as will be explained in detail later, it is important to appropriately control the progress of internal oxidation in the manufacturing process.

[0059] The thickness of the Si-Mn depleted zone, as shown by D in Figure 3, is 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) to the farthest point on the steel sheet 11 where grain boundary oxides 13 are present, when moving 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 (surface length L0) used to measure the ratio A. 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.

[0060] <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-0.09%Al (GA), Zn-1.5%Al-1.5%Mg, and Zn-11%Al-3%Mg-0.2%Si.

[0061] [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.

[0062] (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.

[0063] (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.

[0064] (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.

[0065] (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.

[0066] 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.

[0067] 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.

[0068] 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.

[0069] 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.

[0070] [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.

[0071] The steel sheet and plated steel sheet according to the present invention have high strength and excellent galvanizability and hydrogen desorption properties, 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., excellent galvanizability, is suitably exhibited. Furthermore, steel sheets and plated steel sheets used for automobiles are often hot stamped, in which case hydrogen embrittlement cracking 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., excellent hydrogen desorption properties, is suitably exhibited.

[0072] <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.

[0073] 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.

[0074] [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.

[0075] [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%.

[0076] [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.

[0077] [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.

[0078] [Grinding process] To ensure sufficient grain boundary 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 grain boundary 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, and the coating liquid to be used. By performing such a grinding step, it becomes possible to form a desired 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 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%, respectively.

[0079] [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. This is advantageous for the formation of grain boundary oxides with the desired ratio and a Si-Mn depleted zone with the desired thickness and composition.

[0080] From the viewpoint of forming grain boundary oxides on the surface layer of the steel sheet and increasing the ratio A, the holding temperature in the annealing step is preferably above 780°C and up to 900°C, and more preferably 800 to 850°C. If the holding temperature in the annealing step is 780°C or lower, the generation of grain boundary oxides may be insufficient, and the hydrogen discharge function may be reduced. On the other hand, if the holding temperature in the annealing step is above 900°C, an outer oxide layer may be formed on the surface of the steel sheet, and the galvanization property may be 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.

[0081] The holding time at the above holding temperature is preferably 10 to 50 seconds, more preferably 30 to 50 seconds. If the holding time is less than 10 seconds, there is a risk that grain boundary oxides are not sufficiently generated, and plating properties and hydrogen desorption properties may be insufficient. On the other hand, if the holding time is more than 50 seconds, there is a risk that particulate oxides are excessively generated, and hydrogen desorption properties may be insufficient.

[0082] 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 grain boundary oxides over a wide area (at a high ratio A). 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, resulting in insufficient galvanization and hydrogen desorption. On the other hand, if the dew point is too high, Fe oxide may be formed as the outer oxide on the steel sheet surface, resulting in insufficient galvanization. The atmosphere in the annealing step may be a reducing atmosphere, more specifically a reducing atmosphere containing nitrogen and hydrogen, for example, a reducing atmosphere containing 1 to 10% hydrogen (for example, 4% hydrogen and the balance being nitrogen).

[0083] Furthermore, it is effective to remove the internal oxide layer of the steel sheet before performing 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 sufficient grain boundary 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 performing 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.

[0084] By carrying out the above-mentioned steps, grain boundary oxides are formed over a wide area (at a high ratio A) in the surface layer of the steel sheet, the formation of particulate oxides is sufficiently suppressed, and a steel sheet including an Si-Mn depleted layer with a desired thickness and composition can be obtained.

[0085] 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 length ratio A of the grain boundary oxides will be less than 50%, and therefore the grain boundary oxides will not function sufficiently as an escape route for hydrogen, making it difficult to obtain good hydrogen discharge properties.

[0086] <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.

[0087] 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.

[0088] [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]

[0089] 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.

[0090] (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.

[0091] Next, each cold-rolled steel sheet was coated with an aqueous NaOH solution, and then polished with a heavy abrasive brush at a rate of 10 to 200 g / m 2 The surface of the cold-rolled steel sheet was ground with a grinding amount of 0.01 mm (Sample No. 135 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. 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. 16 and 18 was less than 440 MPa, while the other samples were 440 MPa or higher.

[0092] (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 "-".

[0093] 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.

[0094] 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 grain boundary oxides were present in the thickness direction (perpendicular to the surface of the steel sheet) of the steel sheet in the SEM image where the ratio A 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, 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%.

[0095] (Preparation of plated steel sheet samples) Each steel sheet sample was cut to 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 indicates "GA (galvannealed hot-dip galvanized steel sheet)," coating type B indicates "GI (hot-dip galvanized steel sheet)," and coating type C indicates "Zn-1.5%Al-1.5%Mg." 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.

[0096] (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%

[0097] (Evaluation of hydrogen release) The edges of the plated steel sheet samples were masked, and hydrogen was electrochemically charged into the plated steel sheet samples. Hydrogen charging was performed by immersing each sample in a mixed solution of 0.1 M H2SO4 (pH = 3) and 0.01 M KSCN at room temperature and applying a constant current (100 μA / mm 2 ) conditions. Thereafter, the amount of diffusible hydrogen was measured for each plated steel sheet sample by thermal desorption. Specifically, the plated steel sheet sample was heated to 400°C in a heating furnace equipped with gas chromatography, and the total amount of hydrogen released until the temperature dropped to 250°C was measured. Based on the measured amount of diffusible hydrogen, hydrogen desorption performance 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

[0098] [Table 1]

[0099] Samples Nos. 2 to 8 and 20 to 33 had high galvanizability and hydrogen desorption properties because they had appropriate chemical compositions, the ratio A of grain boundary oxides, the number density of granular oxides, and the thickness and composition of the Si-Mn depleted layer. On the other hand, Samples Nos. 1 and 19 had a thick internal oxide layer before annealing, which prevented the formation of sufficient grain boundary oxides and the desired Si-Mn depleted layer. Therefore, high hydrogen desorption properties were not achieved. Sample No. 9 had a low dew point during annealing, which resulted in the formation of an outer oxide layer, no internal oxide, and no desired Si-Mn depleted layer. Therefore, high galvanizability and hydrogen desorption properties were not achieved. Sample No. 10 had a high dew point during annealing, which resulted in the formation of an outer oxide, no internal oxide, and no desired Si-Mn depleted layer. Sample No. 11 was annealed at a high holding temperature, which resulted in the growth of an outer oxide, the insufficient formation of an inner oxide, and the formation of a desired Si-Mn depleted layer. This resulted in poor galvanizability and hydrogen desorption. Sample No. 12 was annealed at a low holding temperature, which promoted the formation of granular oxides and prevented the formation of a sufficient intergranular oxide layer. This resulted in poor hydrogen desorption. Sample No. 13 was annealed at a short holding time, which prevented the formation of an inner oxide and the formation of a desired Si-Mn depleted layer. This resulted in poor galvanizability and hydrogen desorption. Sample No. 14 was annealed at a long holding time, which prevented the formation of a large amount of granular oxides and the formation of a desired Si-Mn depleted layer. This resulted in poor hydrogen desorption. Sample No. 15 contained an excessive amount of Si, which resulted in the growth of an outer oxide, the insufficient formation of an inner oxide, and the formation of a desired Si-Mn depleted layer. This resulted in poor galvanizability and hydrogen desorption. Samples Nos. 16 and 18 had zero Si and Mn contents, respectively, and therefore neither an internal oxide layer nor the desired Si-Mn depleted layer was formed, resulting in poor hydrogen desorption.Sample No. 17 had an excessive Mn content, which resulted in the growth of an external oxide, the formation of a sufficient internal oxide, and the formation of a desired Si-Mn depleted layer, resulting in poor plating and hydrogen desorption.For sample No. 34, the specified tension was not applied during annealing, so that grain boundary oxides were not sufficiently formed and the desired Si-Mn depleted layer was not formed, and therefore high hydrogen desorption performance was not achieved.For sample No. 35, grinding before annealing was not performed, so that grain boundary oxides were not sufficiently formed and the desired Si-Mn depleted layer was not formed, and therefore high hydrogen desorption performance was not achieved. [Industrial Applicability]

[0100] According to the present invention, it is possible to provide high-strength steel sheets and plated steel sheets having high galvanizability and hydrogen desorption properties, 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]

[0101] 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 grain boundary oxides in a region up to 50 μm from the surface in the sheet thickness direction, 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 and 100% or less, The number density of granular oxide is 4.0 pieces / μm 2 is less than 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 number density of the granular oxides is determined by observing a cross section of the surface layer of a steel sheet with an SEM to obtain an SEM image containing granular oxides, then selecting a total of 10 regions of 1.0 μm (1.0 μm from the region extending from the surface of the steel sheet to 1.5 μm in the depth direction) × 1.0 μm (in the width direction) that do not contain grain boundary oxides, and counting the total number of granular oxides in the 10 regions.

2. The steel plate according to claim 1 , wherein the ratio A is 80% or more.

3. The steel plate according to claim 1, wherein the ratio A is 90% or more.

4. The number density of the granular oxide is 2.0 particles / μm 2 The steel plate according to any one of claims 1 to 3, wherein the tensile strength is less than 1000 kJ / cm.

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

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