Steel sheet and plated steel sheet

Abstract

The present invention provides a high-strength steel sheet having high platability, LME resistance, and hydrogen embrittlement resistance, and a plated steel sheet. A steel sheet and a plated steel sheet using the same are provided, the steel sheet including C: 0.05 to 0.40%, Si: 0.2 to 3.0%, Mn: 0.1 to 5.0%, and sol. Al: 0.4 to 1.50%, wherein an internal oxidation layer including fine-grained oxides is present in a surface layer of the steel sheet, the number density of the fine-grained oxides in the internal oxidation layer is 4.0 pieces / µm 2 In the above, the surface layer of the steel sheet includes a surface layer deficiency layer, the surface layer deficiency layer consisting of an oxide-free steel at a depth of 1 / 2 of the average depth of the internal oxidation layer calculated from a cross-sectional SEM image of the steel sheet, the oxide-free steel satisfying, in mass%, Si ≤ 0.6% and Al ≥ 0.05%.

Description

Technical Field

[0001] This invention relates to steel sheets and coated steel sheets. More specifically, this invention relates to high-strength steel sheets and coated steel sheets having high plating properties, LME resistance, and hydrogen embrittlement resistance. Background Technology

[0002] In recent years, the use of high-strength steel sheets in various fields such as automobiles, home appliances, and building materials has been trending towards higher strength. For example, in the automotive industry, the use of high-strength steel sheets is increasing to improve fuel efficiency and reduce vehicle weight. Such high-strength steel sheets typically contain elements such as C, Si, Mn, and Al to enhance steel strength.

[0003] In the manufacture of high-strength steel plates, heat treatment such as annealing is generally performed after rolling. Furthermore, easily oxidizable elements typically found in high-strength steel plates, namely Si, Mn, and Al, sometimes combine with oxygen in the atmosphere during the aforementioned heat treatment, forming an oxide layer near the surface of the steel plate. Examples of such layers include: an external oxide layer (where oxides of Si, Mn, and Al form as a film on the outer surface of the steel plate); and an internal oxide layer (where oxides form on the inner surface of the steel plate).

[0004] When a coating (e.g., a Zn-based coating) is formed on the surface of a steel sheet with an external oxide layer, the oxide exists on the surface of the steel sheet in the form of a film. This can sometimes hinder the interdiffusion of the steel component (e.g., Fe) and the coating component (e.g., Zn), affecting the adhesion between the steel and the coating, and resulting in insufficient coating performance (e.g., an increase in uncoated areas). Therefore, from the viewpoint of improving coating performance, a steel sheet with an internal oxide layer is preferred over a steel sheet with an external oxide layer.

[0005] Related to the internal oxide layer, Patent Documents 1 and 2 disclose a high-strength coated steel sheet with a tensile strength of 980 MPa or more, which is a coated steel sheet having a zinc-based coating on a base steel sheet containing C, Si, Mn and Al, etc., and having an internal oxide layer containing oxides of Si and / or Mn on the surface of the base steel sheet.

[0006] Existing technical documents

[0007] Patent documents

[0008] Patent Document 1: Japanese Patent Application Publication No. 2016-130357

[0009] Patent Document 2: Japanese Patent Application Publication No. 2018-193614 Summary of the Invention

[0010] The problem that the invention aims to solve

[0011] High-strength steel sheets used in automotive components and other applications are sometimes used in atmospheric corrosive environments with significant variations in temperature and humidity. It is known that if high-strength steel sheets are exposed to such environments, hydrogen generated during corrosion can penetrate the steel. This hydrogen can segregate at the martensite grain boundaries in the steel structure, causing cracking by embrittlement of these grain boundaries. This cracking phenomenon caused by hydrogen intrusion is called hydrogen embrittlement (delayed fracture), and it often becomes a problem during the processing of the steel sheets. Therefore, to prevent hydrogen embrittlement, reducing the amount of hydrogen stored in the steel is effective for steel sheets used in corrosive environments.

[0012] Furthermore, when hot stamping or welding is performed on coated steel sheets with Zn-based coatings on high-strength steel sheets, the coated steel sheets may be processed at high temperatures (e.g., around 900°C), and the Zn contained in the coating may be molten. In this case, the molten Zn may penetrate into the steel, causing internal cracking. This phenomenon is called liquid metal embrittlement (LME), and it is known to reduce the fatigue properties of the steel sheet. Therefore, to prevent LME cracking, it is effective to suppress the penetration of Zn and other substances contained in the coating into the steel sheet.

[0013] Patent documents 1 and 2 teach that by controlling the average depth of the internal oxide layer to be greater than 4 μm and enabling this internal oxide layer to function as a hydrogen trapping site, hydrogen intrusion can be prevented to suppress hydrogen embrittlement. However, no research has been conducted on controlling the morphology of the oxides present in the aforementioned internal oxide layer, leaving room for improvement in hydrogen embrittlement resistance. Furthermore, no research has been conducted on improving LME resistance.

[0014] In view of the actual situation, the present invention aims to provide high-strength steel sheets and coated steel sheets with high plating properties, LME resistance and hydrogen embrittlement resistance.

[0015] Methods for solving problems

[0016] The inventors of this invention have discovered that, in order to solve the above-mentioned problems, it is important to form oxides on the surface of the steel sheet, i.e., inside the steel sheet, and to further control the morphology of the oxides present on the surface of the steel sheet. More specifically, the inventors of this invention have discovered that, by forming an internal oxide layer to ensure high plating performance, and by forming a large amount of fine granular oxides present within the grains of the metal structure, these granular oxides function not only as trapping sites for hydrogen that can penetrate into the steel under corrosive conditions, but also as trapping sites for Zn that can penetrate into the steel during hot stamping or welding processes. This results in high LME performance and resistance to hydrogen embrittlement. In addition, by forming a layered region (sometimes called a surface-deficient layer) with low Si and high Al composition in the metal structure at half the depth of the internal oxide layer, even higher LME performance can be obtained.

[0017] This invention is based on the above-mentioned insights, and its main points are as follows.

[0018] (1) A steel plate having the following composition:

[0019] It contains, by mass%:

[0020] C: 0.05~0.40%

[0021] Si: 0.2-3.0%

[0022] Mn: 0.1–5.0%

[0023] sol.Al: 0.4–1.50%

[0024] P: below 0.0300%

[0025] S: below 0.0300%

[0026] N: below 0.0100%

[0027] B: 0~0.010%

[0028] Ti: 0~0.150%

[0029] Nb: 0~0.150%

[0030] V: 0~0.150%

[0031] Cr: 0–2.00%

[0032] Ni: 0~2.00%

[0033] Cu: 0–2.00%

[0034] Mo: 0–1.00%

[0035] W: 0~1.00%

[0036] Ca: 0–0.100%

[0037] Mg: 0–0.100%

[0038] Zr: 0~0.100%

[0039] Hf: 0~0.100%, and

[0040] REM: 0–0.100%, the remainder consists of Fe and impurities.

[0041] The steel plate has an internal oxide layer on its surface containing fine granular oxides.

[0042] When observing the cross-section of the surface layer of the aforementioned steel plate, the number density of the aforementioned fine granular oxides in the aforementioned inner oxide layer is 4.0 particles / μm. 2 above,

[0043] The surface of the steel plate includes a surface deficiency layer, which is located at a depth of 1 / 2 the average depth of the internal oxide layer. The steel composition, free of the fine granular oxides, satisfies Si ≤ 0.6% and Al ≥ 0.05% by mass.

[0044] (2) The steel plate according to (1), wherein the number density of the above-mentioned fine granular oxide is 8 particles / μm. 2 above.

[0045] (3) A coated steel sheet having a coating containing Zn on the steel sheet described in (1) or (2).

[0046] (4) A coated steel sheet, characterized in that it is the Zn-coated steel sheet described in (3), wherein the coating contains 0.3 to 1.5% Al by mass.

[0047] Invention Effects

[0048] According to the present invention, the fine granular oxides present in large quantities on the surface of the steel sheet can function as trapping sites for hydrogen intrusion under corrosive conditions. As a result, the amount of hydrogen intrusion under corrosive conditions can be significantly suppressed, and the resistance to hydrogen embrittlement can be significantly improved. Furthermore, these fine granular oxides also function as trapping sites for Zn intrusion into the steel during hot stamping or welding, significantly suppressing the amount of Zn intrusion and significantly improving resistance to LME. Moreover, according to the present invention, by forming a layered region (sometimes referred to as a "surface-deficient layer") with a low Si and high Al composition at half the depth of the internal oxide layer, Al also functions as a trapping site for Zn intrusion into the steel during hot stamping or welding, significantly suppressing the amount of Zn intrusion and further improving resistance to LME. Furthermore, since the fine granular oxides and the surface-deficient layer are formed inside the steel sheet, sufficient interdiffusion between the steel composition and the plating composition can be achieved when a coating is formed, resulting in high plating performance. Therefore, this invention enables the achievement of high plating properties, LME resistance, and hydrogen embrittlement resistance in high-strength steel plates. Attached Figure Description

[0049] Figure 1 A schematic diagram showing a cross-section of a steel plate with an external oxide layer.

[0050] Figure 2 A schematic diagram showing a cross-section of a steel plate exemplified in relation to the present invention. Detailed Implementation

[0051] <steel plate>

[0052] The steel plate of the present invention is characterized in that it has the following composition: containing, by mass %:

[0053] C: 0.05~0.40%

[0054] Si: 0.2-3.0%

[0055] Mn: 0.1–5.0%

[0056] sol.Al: 0.4–1.50%

[0057] P: below 0.0300%

[0058] S: below 0.0300%

[0059] N: below 0.0100%

[0060] B: 0~0.010%

[0061] Ti: 0~0.150%

[0062] Nb: 0~0.150%

[0063] V: 0~0.150%

[0064] Cr: 0–2.00%

[0065] Ni: 0~2.00%

[0066] Cu: 0–2.00%

[0067] Mo: 0–1.00%

[0068] W: 0~1.00%

[0069] Ca: 0–0.100%

[0070] Mg: 0–0.100%

[0071] Zr: 0~0.100%

[0072] Hf: 0~0.100%, and

[0073] REM: 0–0.100%, the remainder consists of Fe and impurities.

[0074] The steel plate has an internal oxide layer on its surface containing fine granular oxides.

[0075] The number density of the aforementioned fine granular oxides in the aforementioned inner oxide layer is 4.0 particles / μm. 2 above,

[0076] The surface of the steel plate includes a surface deficiency layer, which is located at a depth of 1 / 2 the average depth of the internal oxide layer. The steel composition, free of the fine granular oxides, satisfies Si ≤ 0.6% and Al ≥ 0.05% by mass.

[0077] In the manufacture of high-strength steel plates, after rolling a steel billet with a specified composition (typically hot rolling and cold rolling), annealing is generally performed to obtain the desired microstructure. During this annealing process, easily oxidized components in the steel plate (such as Si, Mn, and Al) combine with oxygen in the annealing atmosphere, thereby forming an oxide layer near the surface of the steel plate. For example, ... Figure 1As shown in the steel plate 1, an external oxide layer 2 is formed in a film on the surface of the base steel 3 (i.e., the outside of the base steel 3). If the external oxide layer 2 is formed in a film on the surface of the base steel 3, then when a coating (e.g., a zinc-based coating) is formed, the external oxide layer 2 will hinder the interdiffusion of the coating components (e.g., Zn, Al) and the steel components (e.g., Fe), thus failing to adequately ensure the adhesion between the steel and the coating, and sometimes resulting in uncoated areas where no coating is formed.

[0078] In contrast, such as Figure 2 As illustrated in the illustration, the steel plate 11 of the present invention is not as... Figure 1 Unlike the steel plate 1 shown, which has an external oxide layer 2 formed on the surface of the base steel 3, the steel plate 14 contains fine granular oxides 12. Therefore, when a coating is formed on the surface of the steel plate 11, the steel plate 11 of the present invention, with oxides 12 formed inside the base steel 14, achieves more sufficient interdiffusion between the coating component and the steel component compared to the steel plate 1 with an external oxide layer 2, resulting in higher plating performance. Thus, the inventors of the present invention have found that, from the viewpoint of obtaining high plating performance, it is effective to control the conditions during annealing to form oxides inside the steel plate. It should be noted that, when the term "high plating performance" is used with steel plates, it means that when plating is performed on the steel plate, a coating can be formed with minimal (e.g., less than 5.0% of the area) or no uncoated portions (areas without a coating). Furthermore, when the term "high coating quality" is used with coated steel sheets, it indicates that the uncoated portion is very small (e.g., less than 5.0% of the area) or completely non-coated.

[0079] Furthermore, high-strength steel sheets used in atmospheric environments, especially automotive high-strength steel sheets, are repeatedly exposed to various environments with different temperatures and humidity levels. Such environments are known as atmospheric corrosion environments, where hydrogen is generated during corrosion. This hydrogen then penetrates deeper into the steel beyond the surface layer, segregating at the martensite grain boundaries and causing hydrogen embrittlement (delayed fracture) by embrittlement of these grain boundaries. Because martensite is a hard structure, it is highly sensitive to hydrogen and prone to hydrogen embrittlement. This cracking can become a problem during steel sheet processing. Therefore, to prevent hydrogen embrittlement in high-strength steel sheets used in atmospheric corrosion environments, reducing the amount of hydrogen stored in the steel, and more specifically, reducing the amount of hydrogen stored deeper than the surface layer, is effective. The inventors of this invention have discovered that by controlling the morphology of oxides present on the surface of steel plates, and more specifically, by using "fine-grained oxides" with a specified range of particle size and number density, these fine-grained oxides function as trapping sites for hydrogen intrusion in corrosive environments on the surface of the steel plate, thereby reducing the amount of hydrogen accumulated in steel plates used in corrosive environments. It should be noted that the term "high resistance to hydrogen embrittlement" refers to a state in which the amount of hydrogen accumulated in steel plates and coated steel plates is reduced in a manner that sufficiently inhibits hydrogen embrittlement cracking.

[0080] The inventors of this invention conducted a detailed analysis of the relationship between the morphology of oxides and their effectiveness as hydrogen-capturing sites, and found that: Figure 2 As shown, it is effective to ensure that a large quantity of finely granular oxides 12, dispersed in a granular manner, are present separately from each other in the surface layer of the base steel 14. While not bound by a specific theory, it is believed that the hydrogen-capturing function of oxides in the steel sheet is positively correlated with the surface area of ​​the oxides. That is, it is believed that by dispersing fine oxides in a large quantity and discretely from each other in the surface layer of the steel sheet, the surface area of ​​the oxides in the surface layer of the steel sheet increases, thereby improving the hydrogen-capturing function. Therefore, the inventors of the present invention have found that, from the viewpoint of obtaining high resistance to hydrogen embrittlement, it is important to control the conditions during the manufacture of the steel sheet, especially during the annealing process, so that a large quantity of finely granular oxides, which function as hydrogen-capturing sites when placed in a corrosive environment, are present. It should be noted that the metallic structure of the surface layer of the steel sheet typically consists of a metallic structure softer than that of the interior of the steel sheet (e.g., at 1 / 8 or 1 / 4 of the sheet thickness), so even if hydrogen is present in the surface layer of the steel sheet, hydrogen embrittlement cracking is not particularly a problem.

[0081] On the other hand, if a coated steel sheet with a Zn-containing coating on its surface is subjected to hot stamping or welding, the Zn contained in the coating may melt due to the high temperatures during processing. If the Zn melts, it penetrates into the steel. If processing continues in this state, liquid metal embrittlement (LME) cracking may occur inside the steel sheet, leading to a decrease in the fatigue properties of the steel sheet. The inventors of this invention have also discovered that if the aforementioned fine-grained oxide has a desired number density, not only is the resistance to hydrogen embrittlement improved, but it also contributes to the improvement of resistance to LME. More specifically, it has been found that the fine-grained oxide functions as a trapping site for Zn that wants to penetrate into the steel during high-temperature processing. Thus, for example, Zn that wants to penetrate into the steel during hot stamping is trapped by the fine-grained oxide on the surface of the steel sheet, which appropriately suppresses the penetration of Zn into the grain boundaries. Therefore, it was found that it is important to have a large amount of fine particulate oxides, not only to improve the above-mentioned resistance to hydrogen intrusion, but also to improve the resistance to LME.

[0082] Furthermore, the fine granular oxides are formed by the oxidation of easily oxidizable components (such as Si, Mn, and Al) in the steel sheet. Therefore, the composition of the steel (in other words, the metal structure) surrounding these granular oxides is deficient in these easily oxidizable elements compared to the original steel sheet base material. This region, lacking elements in the steel composition compared to the original steel sheet base material, is also called a "deficient region." The layered "deficient region" is also called a "deficient layer," and the deficient layer existing on the surface of the steel sheet is also called a "surface deficient layer." In the deficient region, since Si is relatively easily oxidized and Al is relatively difficult to oxidize, Si can exist at a low concentration and Al at a high concentration. The inventors of this invention have also discovered that if the steel composition described above, with a low-Si and high-Al deficient region existing within a desired range, also contributes to improved LME resistance. More specifically, it was found that, in addition to the presence of granular oxides that function as Zn trapping sites, the presence of Al in the steel composition surrounding these granular oxides also functions as trapping sites for Zn seeking to penetrate the steel during high-temperature processing. Furthermore, the higher the Si concentration in the steel composition, the more prone LME cracking is to occur; by setting the Si concentration as low as possible, LME can be suppressed. Thus, for example, during hot stamping processes, Zn seeking to penetrate the steel is trapped by Al in the steel composition, appropriately suppressing Zn intrusion into grain boundaries. Moreover, since the Si concentration, which is prone to LME formation, is low, LME formation is less likely. Therefore, it was found that the presence of regions lacking low Si concentrations and high Al concentrations is important for improving LME resistance.

[0083] The regions lacking Si at low concentrations and Al at high concentrations can overlap with the regions distributed by fine-grained oxides; that is, they are not like... Figure 1 The outer oxide layer 2 is formed on the surface of the base steel 3, and can be formed inside the base steel. Therefore, when a coating is formed on the surface of the steel plate, the steel plate of the present invention, compared with the steel plate 1 having an outer oxide layer 2, can sufficiently generate mutual diffusion between the coating component and the steel component, and obtain high coating performance.

[0084] The steel plate of the present invention will now be described in detail. It should be noted that the thickness of the steel plate of the present invention is not particularly limited, but for example, it can be 0.1 to 3.2 mm.

[0085] [Composition of steel plates]

[0086] The composition of the steel plate of the present invention will be described. Unless otherwise specified, the "%" of element content refers to "mass %". The numerical ranges in the composition, indicated by "~", unless otherwise specified, refer to the range including the values ​​before and after "~" as the lower and upper limits.

[0087] (C: 0.05~0.40%)

[0088] Carbon (C) is a crucial element in ensuring the strength of steel. Insufficient C content may compromise adequate strength. Furthermore, insufficient C content can sometimes result in undesirable internal oxide and / or surface layer morphology. Therefore, the C content is 0.05% or more, preferably 0.07% or more, more preferably 0.10% or more, and even more preferably 0.12% or more. On the other hand, excessive C content may reduce weldability. Therefore, the C content is 0.40% or less, preferably 0.35% or less, and more preferably 0.30% or less.

[0089] (Si: 0.2-3.0%)

[0090] Silicon (Si) is an effective element for improving the strength of steel. If the Si content is insufficient, adequate strength may not be ensured. Furthermore, the desired oxides, particularly fine-grained oxides, and / or a lack of surface layer may not be sufficiently formed within the steel sheet. Therefore, the Si content is 0.2% or more, preferably 0.3% or more, more preferably 0.5% or more, and even more preferably 1.0% or more. On the other hand, if the Si content is excessive, it may cause deterioration of surface properties. Furthermore, it may lead to coarsening of granular oxides. Therefore, the Si content is 3.0% or less, preferably 2.5% or less, and more preferably 2.0% or less.

[0091] (Mn: 0.1-5.0%)

[0092] Manganese (Mn) is an effective element for improving the strength of steel by obtaining a hard microstructure. If the Mn content is insufficient, adequate strength may not be ensured. Furthermore, the desired oxides, particularly fine-grained oxides, and / or a lack of surface layer may not be sufficiently formed within the steel sheet. Therefore, the Mn content is 0.1% or more, preferably 0.5% or more, more preferably 1.0% or more, and even more preferably 1.5% or more. On the other hand, if the Mn content is excessive, Mn segregation may lead to an uneven metal microstructure and reduced workability. Furthermore, it may cause coarsening of the granular oxides. Therefore, the Mn content is 5.0% or less, preferably 4.5% or less, more preferably 4.0% or less, and even more preferably 3.5% or less.

[0093] (sol.Al: 0.4-1.50%)

[0094] Al (aluminum) is an element that functions as a deoxidizer. If the Al content is insufficient, adequate deoxidation may not be achieved. Consequently, the desired oxides, particularly fine-grained oxides and / or a lack of surface layer, may not be sufficiently formed inside the steel sheet. The Al content can be 0.4% or more, but to obtain sufficient desired effects, fine-grained oxides, and a lack of surface layer, an Al content of 0.5% or more is preferable, preferably 0.6% or more, and more preferably 0.7% or more. On the other hand, excessive Al content may lead to reduced processability and deterioration of surface properties. Furthermore, it may cause coarsening of granular oxides. Therefore, the Al content is 1.50% or less, preferably 1.20% or less, and more preferably 0.80% or less. Al content refers to the content of so-called acid-soluble Al (sol.Al).

[0095] (P: below 0.0300%)

[0096] Phosphorus (P) is generally an impurity contained in steel. When the P content exceeds 0.0300%, weldability may decrease. Therefore, the P content is preferably 0.0300% or less, more preferably 0.0200% or less, more preferably 0.0100% or less, and even more preferably 0.0050% or less. There is no particular limitation on the lower limit of the P content, but from the viewpoint of manufacturing cost, the P content can also exceed 0% or be more than 0.0001%.

[0097] (S: below 0.0300%)

[0098] Sulfur (S) is generally an impurity contained in steel. When the S content exceeds 0.0300%, weldability may decrease, and the precipitation of MnS may increase, thereby reducing workability such as flexibility. Therefore, the S content is preferably 0.0300% or less, more preferably 0.0100% or less, more preferably 0.0050% or less, and even more preferably 0.0020% or less. There is no particular limitation on the lower limit of the S content, but from the viewpoint of desulfurization cost, the S content may also exceed 0% or be more than 0.0001%.

[0099] (N: below 0.0100%)

[0100] Nitrogen (N) is generally an impurity contained in steel. When the N content exceeds 0.0100%, weldability may decrease. Therefore, the N content is preferably 0.0100% or less, more preferably 0.0080% or less, more preferably 0.0050% or less, and even more preferably 0.0030% or less. There is no particular limitation on the lower limit of the N content, but from the viewpoint of manufacturing cost, the N content can also exceed 0% or be more than 0.0010%.

[0101] (B: 0~0.010%)

[0102] Boron (B) is an element that improves hardenability, thus contributing to increased strength, and also strengthens grain boundaries through segregation at grain boundaries, thereby improving toughness. Therefore, it can be included as needed. Thus, the B content is 0% or more, preferably 0.001% or more, more preferably 0.002% or more, and even more preferably 0.003% or more. On the other hand, from the viewpoint of ensuring sufficient toughness and weldability, the B content is 0.010% or less, preferably 0.008% or less, and more preferably 0.006% or less.

[0103] (Ti: 0~0.150%)

[0104] Titanium (Ti) is an element that contributes to strength improvement by precipitating TiC during the cooling of steel, and can be included as needed. Therefore, the Ti content is 0% or more, preferably 0.001% or more, more preferably 0.003% or more, even more preferably 0.005% or more, and even more preferably 0.010% or more. On the other hand, if it is contained in excess, coarse TiN may be formed, which may impair toughness. Therefore, the Ti content is 0.150% or less, preferably 0.100% or less, and even more preferably 0.050% or less.

[0105] (Nb: 0~0.150%)

[0106] Niobium (Nb) is an element that contributes to increased strength by improving hardenability, and therefore can be included as needed. Thus, the Nb content is 0% or more, preferably 0.010% or more, more preferably 0.020% or more, and even more preferably 0.030% or more. On the other hand, from the viewpoint of ensuring sufficient toughness and weldability, the Nb content is 0.150% or less, preferably 0.100% or less, and more preferably 0.060% or less.

[0107] (V: 0~0.150%)

[0108] Vanadium (V) is an element that contributes to increased strength by improving hardenability, and therefore can be included as needed. Thus, the V content is 0% or more, preferably 0.010% or more, more preferably 0.020% or more, and even more preferably 0.030% or more. On the other hand, from the viewpoint of ensuring sufficient toughness and weldability, the V content is 0.150% or less, preferably 0.100% or less, and more preferably 0.060% or less.

[0109] (Cr: 0-2.00%)

[0110] Chromium (Cr) is effective in improving the hardenability of steel, thereby increasing its strength, and can therefore be included as needed. Thus, the Cr content is 0% or more, preferably 0.10% or more, more preferably 0.20% or more, even more preferably 0.50% or more, and even more preferably 0.80% or more. On the other hand, if it is contained in excess, a large amount of Cr carbides may form, which may impair hardenability. Therefore, the Cr content is 2.00% or less, preferably 1.80% or less, and even more preferably 1.50% or less.

[0111] (Ni: 0-2.00%)

[0112] Nickel (Ni) is effective in improving the hardenability of steel, thereby increasing its strength, and can therefore be included as needed. Thus, the Ni content is 0% or more, preferably 0.10% or more, more preferably 0.20% or more, even more preferably 0.50% or more, and even more preferably 0.80% or more. On the other hand, excessive addition of Ni leads to increased costs; therefore, the Ni content is 2.00% or less, preferably 1.80% or less, and even more preferably 1.50% or less.

[0113] (Cu: 0~2.00%)

[0114] Copper (Cu) is effective in improving the hardenability of steel, thereby increasing its strength, and can therefore be included as needed. Thus, the Cu content is 0% or more, preferably 0.10% or more, more preferably 0.20% or more, even more preferably 0.50% or more, and even more preferably 0.80% or more. On the other hand, from the viewpoint of suppressing reduced toughness, cracking of the cast slab, or reduced weldability, the Cu content is 2.00% or less, preferably 1.80% or less, and even more preferably 1.50% or less.

[0115] (Mo: 0~1.00%)

[0116] Mo (molybdenum) is effective in improving the hardenability of steel, thereby increasing its strength, and therefore can be included as needed. Therefore, the Mo content is 0% or more, preferably 0.10% or more, more preferably 0.20% or more, and even more preferably 0.30% or more. On the other hand, from the viewpoint of suppressing the reduction in toughness and weldability, the Mo content is 1.00% or less, preferably 0.90% or less, and more preferably 0.80% or less.

[0117] (W: 0~1.00%)

[0118] Tungsten (W) is effective in improving the hardenability of steel, thereby increasing its strength, and therefore can be included as needed. Thus, the W content is 0% or more, preferably 0.10% or more, more preferably 0.20% or more, and even more preferably 0.30% or more. On the other hand, from the viewpoint of suppressing the reduction in toughness and weldability, the W content is 1.00% or less, preferably 0.90% or less, and more preferably 0.80% or less.

[0119] (Ca: 0~0.100%)

[0120] Ca (calcium) is an element that helps control inclusions, especially the fine dispersion of inclusions, and improves toughness; therefore, it can be included as needed. Thus, the Ca content is 0% or more, preferably 0.001% or more, more preferably 0.005% or more, even more preferably 0.010% or more, and even more preferably 0.020% or more. On the other hand, if it is contained in excess, the deterioration of surface properties may sometimes become more pronounced; therefore, the Ca content is 0.100% or less, preferably 0.080% or less, and even more preferably 0.050% or less.

[0121] (Mg: 0-0.100%)

[0122] Magnesium (Mg) is an element that helps control inclusions, especially the fine dispersion of inclusions, and improves toughness; therefore, it can be included as needed. Thus, the Mg content is 0% or more, preferably 0.001% or more, more preferably 0.003% or more, and even more preferably 0.010% or more. On the other hand, if it is contained in excess, the deterioration of surface properties may sometimes become more pronounced; therefore, the Mg content is 0.100% or less, preferably 0.090% or less, and more preferably 0.080% or less.

[0123] (Zr: 0~0.100%)

[0124] Zirconium (Zr) is an element that helps control inclusions, particularly the fine dispersion of inclusions, and improves toughness; therefore, it can be included as needed. Thus, the Zr content is 0% or more, preferably 0.001% or more, more preferably 0.005% or more, and even more preferably 0.010% or more. On the other hand, if it is contained in excess, the deterioration of surface properties may become more pronounced; therefore, the Zr content is 0.100% or less, preferably 0.050% or less, and more preferably 0.030% or less.

[0125] (Hf: 0~0.100%)

[0126] Hafnium (Hf) is an element that helps control inclusions, especially the fine dispersion of inclusions, and improves toughness; therefore, it can be included as needed. Thus, the Hf content is 0% or more, preferably 0.001% or more, more preferably 0.005% or more, and even more preferably 0.010% or more. On the other hand, if it is contained in excess, the deterioration of surface properties may become more pronounced; therefore, the Hf content is 0.100% or less, preferably 0.050% or less, and more preferably 0.030% or less.

[0127] (REM: 0~0.100%)

[0128] Rare earth elements (REMs) are elements that help control inclusions, especially the fine dispersion of inclusions, and improve toughness; therefore, they can be included as needed. Thus, the REM content is 0% or more, preferably 0.001% or more, more preferably 0.005% or more, and even more preferably 0.010% or more. On the other hand, if it is contained in excess, the deterioration of surface properties may become more pronounced; therefore, the REM content is 0.100% or less, preferably 0.050% or less, and more preferably 0.030% or less. It should be noted that REM is an abbreviation for Rare Earth Metal, referring to elements belonging to the lanthanide series. REMs are usually added in the form of mixed rare earth alloys.

[0129] In the steel sheet of the present invention, the remaining portion other than the above-mentioned components consists of Fe and impurities. Here, impurities refer to components that are mixed in during the industrial manufacturing of steel sheets due to various factors in the manufacturing process, such as raw materials like ores and waste, and are components that are permissible to be present within a range that does not adversely affect the properties of the steel sheet of the present invention.

[0130] In this invention, the compositional analysis of the steel plate can be performed using elemental analysis methods known to those skilled in the art, such as inductively coupled plasma mass spectrometry (ICP-MS). Specifically, it is preferable to use the combustion-infrared absorption method to determine C and S, and the inert gas melting-thermal conductivity method to determine N. These analyses can be performed on samples of the steel plate collected according to the method of JIS G0417:1999.

[0131] [surface layer]

[0132] In this invention, the “surface” of the steel plate refers to the area from the surface of the steel plate (or the interface between the steel plate and the coating in the case of a plated steel plate) to a predetermined depth in the thickness direction, the “predetermined depth” typically being 50 μm or less.

[0133] like Figure 2 As illustrated, in the steel plate 11 of the present invention, fine oxides 12 are included in the surface layer of the steel plate 11. Preferably, the fine granular oxides 12 are present only in the surface layer of the steel plate 11. By having these fine granular oxides 12 present inside the base steel 14 (i.e., existing as internal oxides), they are combined with... Figure 1 Compared to the case where an external oxide layer 2 exists on the surface of the base steel 3, the steel sheet 11 exhibits high plating properties. This is believed to be due to the fact that oxides that hinder the interdiffusion of the plating components and the steel components when forming a coating (e.g., a Zn-based coating) on ​​the surface of the steel sheet are formed internally rather than externally. Therefore, the steel sheet and plated steel sheet of the present invention, which contain granular oxides in the surface layer (i.e., the interior of the steel sheet), exhibit high plating properties.

[0134] In addition, although Figure 2 Not shown in the figures, but in the steel plate 11 of the present invention, in addition to the aforementioned fine granular oxide 12, the surface layer of the steel plate 11 also includes a surface deficiency layer. This surface deficiency layer is a region where the elements of the surrounding steel composition are lacking compared to the original steel base material, accompanying the formation of the fine granular oxide 12, and exists in a manner that repeats the region where the fine granular oxide 12 is distributed. That is, since the surface deficiency layer exists in the interior of the base steel 14 in the same way as the fine granular oxide 12, the steel plate and the plated steel plate containing both the fine granular oxide 12 and the surface deficiency layer also have high plating properties.

[0135] [Fine-grained oxides]

[0136] In this invention, "granular oxide" refers to oxides dispersed in a granular form within the grains or at grain boundaries of steel. Furthermore, "granular" means existing separately within the steel matrix, for example, having an aspect ratio of 1.0 to 5.0 (the length of the longest line segment traversing the granular oxide (major axis) / the length of the longest line segment perpendicular to the major axis traversing the oxide (minor axis)). "Dispersed in a granular form" means that the positions of the oxide particles are not arranged according to a specific rule (e.g., in a straight line), but rather randomly. In practice, granular oxides typically exist in a three-dimensional, spherical or nearly spherical shape on the surface of a steel plate; therefore, when observing a cross-section of the steel plate's surface, the granular oxides are typically observed to be circular or nearly circular. Figure 2 As an example, granular oxide 12 that appears to be roughly spherical is shown.

[0137] (particle size)

[0138] In this invention, the particle size of the "fine" granular oxide is 20 nm to 100 nm. By controlling the particle size within this range, the fine granular oxide can be dispersed in the surface layer of the steel plate. The fine granular oxide functions well as a hydrogen trapping site to inhibit hydrogen intrusion in corrosive environments, and further functions well as a Zn trapping site that can intrude during hot stamping or welding of the coated steel plate. On the other hand, if the particle size exceeds 100 nm, the number of granular oxides may decrease, and the desired number density may not be obtained. The lower limit of the particle size of the granular oxide is 20 nm or more. The finer the granular oxide, the higher the specific surface area, and the higher the reactivity as a trapping site. However, the amount of hydrogen and / or Zn that can be trapped by each particle decreases, and it may not be able to sufficiently trap hydrogen and / or Zn, thus failing to function adequately as a hydrogen trapping site and / or a Zn trapping site.

[0139] (Number density)

[0140] In this invention, the number density of the fine granular oxide is 4.0 particles / μm. 2 The above describes how, by controlling the number density within a certain range, fine particulate oxides can be abundantly dispersed on the surface of the steel sheet. These fine particulate oxides function effectively as hydrogen trapping sites to inhibit hydrogen intrusion under corrosive conditions, and further function effectively as trapping sites for Zn that can intrude during hot stamping or welding of coated steel sheets. On the other hand, if the number density is below 4.0 particles / μm... 2If the number density of hydrogen and / or Zn trapping sites is insufficient, the granular oxide may not function adequately as hydrogen and / or Zn trapping sites, resulting in poor resistance to hydrogen embrittlement and / or LME. The preferred number density of the granular oxide is 6.0 sites / μm. 2 The above, more preferably 8.0 particles / μm 2 The above is further preferred to be 10.0 particles / μm. 2 The above. From the viewpoint of functioning as hydrogen and / or Zn trapping sites, the greater the presence of particulate oxides, the better. However, particulate oxides can sometimes become the initiation point for LME cracking, exceeding 30 per μm. 2 The resistance to LME may decrease at times, therefore the number density of granular oxides can also be 30 particles / μm. 2 Below, 25 / μm 2 Below, 20 / μm 2 the following.

[0141] The particle size and number density of the granular oxides were determined using scanning electron microscopy (SEM). The specific determination process is as follows: A cross-section of the surface layer of the steel plate was observed using SEM to obtain an SEM image containing the granular oxides. From this SEM image, a total of 10 regions of 1.0 μm (depth direction) × 1.0 μm (width direction) were selected as the observation area. For the depth direction (the direction perpendicular to the surface of the steel plate), the observation position was set to 1.0 μm within the region extending from the surface of the steel plate to 1.5 μm. For the width direction (the direction parallel to the surface of the steel plate), the observation position was set to 1.0 μm at any location within the aforementioned SEM image. Next, the SEM images of the selected regions as described above are extracted. To separate the oxide portion from the steel portion, binarization is performed. The area of ​​the granular oxide portion is calculated from each binarized image. The diameter of the equivalent circle (circle diameter) with an area equal to this area is used to determine the particle size (nm) of the granular oxide. Particles with a size range of 20nm to 100nm are defined as fine granular oxides. The number of fine granular oxides in each binarized image is then counted. The average number of fine granular oxides in the 10 regions obtained in this way is taken as the number density of fine granular oxides (particles / μm). 2 It should be noted that when only a portion of the granular oxide is observed in the observation area, i.e., when the entire outline of the granular oxide is not within the observation area, it is not counted as a number.

[0142] (Depth of the internal oxide layer)

[0143] In the steel sheet of the present invention, the internal oxide layer is a layer formed inside the steel sheet, comprising fine granular oxides 12. Therefore, the term "internal oxide layer" refers to a layer extending from the surface of the steel sheet to the furthest point where the fine granular oxides 12 exist. Thus, the term "depth of the internal oxide layer" refers to... Figure 2 As indicated by "Rn", this distance is the distance from the surface of the steel plate 11 (or the interface between the steel plate and the coating in the case of a coated steel plate) along the thickness direction of the steel plate 11 (the direction perpendicular to the surface of the steel plate) to the farthest point where the fine particulate oxide 12 exists. However, since the surface of an actual steel plate is uneven, the position of the fine particulate oxide 12 farthest from the surface of the steel plate varies depending on which part (point) of the steel plate surface is selected. Therefore, 10 observation areas are selected, and the average value of the results measured at these 10 locations is taken as the "average depth of the internal oxide layer" (sometimes also called "R"). As described above, the fine particulate oxide 12 can function as a capture site for hydrogen that penetrates during processes such as electrodeposition coating. Therefore, the larger the average depth R of the internal oxide layer, the more hydrogen can be captured in the surface area of ​​the steel plate. In the steel plate of the present invention, the lower limit of the average depth R of the internal oxide layer is not particularly limited, but if it is too shallow, the fine granular oxides may not be sufficiently dispersed. Therefore, it is preferably 1 μm or more, more preferably 2 μm or more, even more preferably 3 μm or more, more preferably 8 μm or more, even more preferably 10 μm or more, more preferably 15 μm or more, and even more preferably 20 μm or more. The upper limit of the average depth R is not particularly limited, but is substantially 100 μm or less.

[0144] Depth R such Figure 2 As shown, the determination is made by cross-sectional observation of the surface layer of the steel plate 11. The specific measurement method is as follows. The cross-section of the surface layer of the steel plate 11 is observed using SEM. Ten observation locations are randomly selected. The length L0 of the surface (i.e., the width of the SEM image) is measured from the observed SEM image. The length L0 is set to be 100 μm or more (e.g., 100 μm, 150 μm, or 200 μm), and the measurement depth is set to the area from the surface of the steel plate to 100 μm. Next, the location of the fine granular oxide 12 is determined from the SEM image. From the determined fine granular oxide 12, the fine granular oxide 12 present at the position farthest from the surface of the steel plate is selected, and the distance from the surface of the steel plate 11 to the farthest position where the fine granular oxide 12 exists is calculated as the depth Rn. The average value of Rn measured at the 10 locations is calculated as the "average depth of the internal oxide layer" (sometimes also called "R").

[0145] [Composition of Oxides]

[0146] In this invention, the granular oxide (hereinafter also simply referred to as oxide) contains one or more of the elements contained in the steel plate mentioned above, in addition to oxygen. Typically, it has the following composition: containing Si, O, and Fe, and, depending on the case, further containing Mn and Al. In addition to the elements mentioned above, the oxide may also contain elements that may be contained in the steel plate mentioned above (e.g., Cr).

[0147] [Surface lacking layer]

[0148] In this invention, the fine granular oxide is formed by the oxidation of easily oxidizable components (e.g., Si, Mn, Al) in the steel sheet. Therefore, the composition of the steel (in other words, the metal structure) surrounding this granular oxide is deficient in these easily oxidizable elements compared to the original steel sheet base material. This region, lacking elements in the steel composition compared to the original steel sheet base material, is also called a "deficient region." The layered "deficient region" is also called a "deficient layer," and further, the deficient layer present on the surface of the steel sheet is also called a "surface deficient layer." In the deficient region, since Si is relatively easily oxidized and Al is relatively difficult to oxidize, Si can exist at a low concentration and Al at a high concentration. If the steel composition exhibits a low-Si, high-Al deficient region within the desired range, it also contributes to improved LME resistance. The rationale is not to adhere to a specific theory, but it is believed that: in addition to the presence of granular oxides that function as Zn trapping sites, the presence of Al in the steel composition surrounding these granular oxides also functions as trapping sites for Zn that wants to penetrate the steel during high-temperature processing; moreover, the higher the Si concentration in the steel composition, the easier it is for LME cracking to occur, so by setting the Si concentration as low as possible, LME can be suppressed. Thus, during hot stamping or welding, Zn that wants to penetrate the steel is trapped by Al in the steel composition, which can appropriately suppress the invasion of Zn into the crystal boundaries, and since the Si concentration that easily forms LME is low, LME is less likely to occur, thus improving LME resistance.

[0149] In this invention, the composition of the steel (in other words, the metallic microstructure) lacking a surface layer with low Si and high Al, and free of fine-grained oxides at a depth of half the average depth R of the internal oxide layer, satisfies the following mass percentage: Si ≤ 0.6% and Al ≥ 0.05%. If Si exceeds 0.6%, LME cracking becomes more likely. Therefore, Si ≤ 0.6% is preferred. The lower limit of Si is not particularly limited and can be 0% or more. Furthermore, Al functions as a trapping site for Zn that wants to penetrate the steel during high-temperature processing. If Al is below 0.05%, it may not function sufficiently as a Zn trapping site. Therefore, Al ≥ 0.05% is set. The higher the Al content, the better the function as a trapping site, which is more preferred, but even if the Al concentration is too high, its effect will saturate. Therefore, the upper limit of Al can be set to 1.2% or less or 1.0% or less. In addition, the concentrations of Si and Al are the elemental concentrations in the steel composition of the internal oxide layer that lacks fine-grained oxides, and are the elemental concentrations measured at a depth of half the average depth R of the internal oxide layer. The baseline for the average depth of the internal oxide layer is the steel plate surface (or the interface between the steel plate and the coating in the case of coated steel plates). However, in cases where the surface or interface has unevenness, the average line at 10 points of the average depth of the internal oxide layer is used as the baseline. Elemental concentrations are determined using EDS (Energy Dispersed Spectroscopy).

[0150] The surface absence layer can overlap with the area where fine granular oxides are distributed and exists within the surface of the steel plate, i.e., it is formed inside the base steel. Therefore, when a coating is formed on the surface of the steel plate, the steel plate of the present invention, which has a surface absence layer, can achieve sufficient interdiffusion between the coating component and the steel component compared to a steel plate with an external oxide layer, thus obtaining high plating performance.

[0151] <Coated steel sheet>

[0152] The coated steel sheet of the present invention has a coating containing Zn on the steel sheet described above. This coating can be formed on one side or both sides of the steel sheet. Examples of Zn-containing coatings include hot-dip galvanized layers, alloyed hot-dip galvanized layers, electroplated zinc layers, and electroplated alloy zinc layers. More specifically, as coating types, examples 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, or Zn-11%Al-3%Mg-0.2%Si, Zn-11%Ni, and Zn-15%Mg.

[0153] [Composition of the coating]

[0154] The composition of the Zn-containing coating in this invention will be described. Unless otherwise specified, the "%" for elemental content refers to "mass %". Regarding the numerical ranges in the composition of the coating, the range indicated by "~" unless otherwise specified, refers to the range including the values ​​before and after "~" as both the lower and upper limits.

[0155] (A1: 0-60.0%)

[0156] Al is an element that improves the corrosion resistance of coatings by being included together with Zn or alloyed with it, and therefore can be included as needed. Thus, the Al content can also be 0%. For forming a coating containing Zn and Al, an Al content of 0.01% or more is preferred, for example, 0.1% or more, 0.3% or more, 0.5% or more, 1.0% or more, or 3.0% or more. On the other hand, the effect of improving corrosion resistance saturates when it exceeds 60.0%, therefore, an Al content of 60.0% or less is preferred, 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. The detailed mechanism is unclear, but when the Al content in the coating is in the range of 0.3% to 1.5%, the effect of Al significantly reduces the rate of Zn intrusion into the steel grain boundaries, thereby improving LME resistance. Therefore, from the viewpoint of improving LME resistance, the Al content in the coating is preferably 0.3% to 1.5%.

[0157] (Mg: 0-15.0%)

[0158] Mg is an element that improves the corrosion resistance of the coating by being included or alloyed with Zn and Al, and can therefore be included as needed. Thus, the Mg content can also be 0%. For forming a coating containing Zn, Al, and Mg, a Mg content of 0.01% or more is preferred, for example, 0.1% or more, 0.5% or more, 1.0% or more, or 3.0% or more. On the other hand, when the Mg content exceeds 15.0%, Mg will not completely dissolve in the plating bath and will float as oxides. If zinc plating is performed in such a bath, oxides may adhere to the plating surface, causing poor appearance or creating unplated areas. Therefore, a Mg content of 15.0% or less is preferred, for example, 10.0% or less, or 5.0% or less.

[0159] (Fe: 0–15.0%)

[0160] When a Zn-containing coating is formed on a steel sheet and the coated steel sheet is then heat-treated, Fe can be incorporated into the coating through diffusion from the steel sheet. Therefore, in the untreated state, Fe is not incorporated into the coating, and the Fe content can be 0%. Alternatively, the Fe content can 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, an Fe content of 15.0% or less is preferable; for example, it can also be 12.0% or less, 10.0% or less, 8.0% or less, or 6.0% or less.

[0161] (Si: 0-3.0%)

[0162] Si is an element that further improves corrosion resistance when included in Zn-containing coatings, especially Zn-Al-Mg coatings, and therefore can be included as needed. Thus, the Si content can also be 0%. From the viewpoint of improving corrosion resistance, the Si content can, for example, be 0.005% or more, 0.01% or more, 0.05% or more, 0.1% or more, or 0.5% or more. Furthermore, the Si content can also be 3.0% or less, 2.5% or less, 2.0% or less, 1.5% or less, or 1.2% or less.

[0163] The basic composition of the coating is as described above. Furthermore, the coating may optionally contain one or more of the following: 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, from the viewpoint of fully utilizing the functions of the aforementioned basic components constituting the coating, the total content of these optionally added elements is preferably set to 5.00% or less, more preferably 2.00% or less.

[0164] The remaining portion of the coating, besides the components described above, consists of Zn and impurities. Impurities in the coating refer to components introduced during the manufacturing process, such as the raw materials, and are not intentionally added to the coating. As impurities, the coating may also contain trace amounts of elements other than the basic components and optional additives described above, without impairing the effects of the present invention.

[0165] The composition of the coating can be determined by dissolving the coating in an acid solution containing an inhibitor that inhibits steel corrosion, and then measuring the resulting solution using ICP (inductively coupled plasma) luminescence spectrophotometry.

[0166] The thickness of the coating can be, for example, 3–50 μm. Furthermore, the coating amount is not particularly limited, but can be, for example, 10–170 g / m² per single side. 2 In this invention, the amount of coating adhesion is determined by the weight change of the coating before and after acid pickling and peeling, after dissolving the coating in an acid solution containing an inhibitor that suppresses corrosion of the base metal.

[0167] [tensile strength]

[0168] The steel sheet and coated steel sheet of the present invention preferably have high strength, specifically preferably a tensile strength of 440 MPa or more. For example, the tensile strength may also be 500 MPa or more, 600 MPa or more, 700 MPa or more, or 800 MPa or more. There is no particular upper limit to the tensile strength, but from the viewpoint of ensuring toughness, for example, 2000 MPa or less is acceptable. Regarding the determination of tensile strength, a JIS 5 tensile test piece with the length direction perpendicular to the rolling direction is collected and the test is performed according to JIS Z2241 (2011).

[0169] The steel sheet and coated steel sheet of the present invention are suitable for use in a wide range of fields such as automobiles, home appliances, and building materials due to their high strength, high plating properties, LME resistance, and hydrogen embrittlement resistance. However, they are particularly preferred for use in the automotive field. Steel sheets used in automotive applications are typically coated (typically Zn-based coatings). Therefore, when the steel sheet of the present invention is used as an automotive steel sheet, the high plating properties of the present invention can be appropriately utilized. Furthermore, steel sheets and coated steel sheets used in automotive applications are mostly hot-stamped, in which case hydrogen embrittlement and LME cracking become significant problems. Therefore, when the steel sheet and coated steel sheet of the present invention are used as automotive steel sheets, the high hydrogen embrittlement resistance and LME resistance of the present invention can be appropriately utilized.

[0170] <Methods for manufacturing steel plates>

[0171] Hereinafter, a preferred method for manufacturing the steel plate of the present invention will be described. The following description intentionally illustrates a characteristic method for manufacturing the steel plate of the present invention, and is not intended to limit the steel plate to being manufactured by the method described below.

[0172] The steel sheet of the present invention can be obtained, for example, by performing the following steps: a casting process in which molten steel with adjusted composition is cast to form a steel billet; a hot rolling process in which the steel billet is hot rolled to obtain a hot-rolled steel sheet; a coiling process in which the hot-rolled steel sheet is coiled; a cold rolling process in which the coiled hot-rolled steel sheet is cold-rolled to obtain a cold-rolled steel sheet; a pretreatment process in which the cold-rolled steel sheet is brush-ground; and an annealing process in which the pretreated cold-rolled steel sheet is annealed. Alternatively, after the hot rolling process, coiling may be omitted, and the cold rolling process may be performed directly after pickling.

[0173] [Casting Process]

[0174] There are no particular restrictions on the conditions of the casting process. For example, smelting can be carried out using a blast furnace or electric furnace, followed by various secondary refining processes, and then casting can be performed using conventional continuous casting or ingot casting methods.

[0175] [Hot rolling process]

[0176] Hot-rolled steel sheets can be obtained by hot rolling the cast steel billet as described above. The hot rolling process is carried out by directly heating the cast steel billet or by temporarily cooling it and then reheating it. In the case of reheating, the heating temperature of the steel billet can be, for example, 1100℃~1250℃. In the hot rolling process, rough rolling and finish rolling are usually performed. The temperature and reduction rate of each rolling process can be appropriately changed according to the desired metal structure and plate thickness. For example, the finishing temperature can be set to 900~1050℃, and the finishing reduction rate can be set to 10~50%.

[0177] [Winding process]

[0178] Hot-rolled steel sheets can be coiled at a specified temperature. The coiling temperature can be appropriately varied according to the desired metal structure, for example, 500–800°C. Alternatively, the hot-rolled steel sheet can be subjected to a specified heat treatment before coiling or after rewinding. Alternatively, the coiling process can be omitted, and the cold rolling process (described later) can be performed after pickling following the hot rolling process.

[0179] [Cold rolling process]

[0180] After hot-rolled steel sheets undergo pickling and other processes, they can be cold-rolled to obtain cold-rolled steel sheets. The reduction rate during cold rolling can be appropriately varied according to the desired metal structure and sheet thickness, for example, from 20% to 80%. After the cold rolling process, the sheets can be cooled to room temperature, for example, by air cooling.

[0181] [Pre-treatment process]

[0182] To obtain a large amount of fine-grained oxides and thus a surface-deficient layer in the final steel sheet, a pretreatment process prior to annealing the cold-rolled steel sheet is effective. This pretreatment process introduces a large number of dislocations onto the surface of the cold-rolled steel sheet. Since the diffusion of oxygen and the like is faster at grain boundaries than within the grains, introducing a large number of dislocations onto the surface of the cold-rolled steel sheet creates as many pathways as at grain boundaries. Therefore, during annealing, oxygen readily diffuses (intrudes) into the interior of the steel along these dislocations. Furthermore, since the diffusion rates of Si and Al are also increased, oxygen can combine with Si and / or Al within the steel, promoting the formation of fine-grained oxides. In addition, the reduction in the surrounding Si and Al concentration is also promoted along with the formation of such internal oxides, thus promoting the formation of a surface-deficient layer with the desired composition. Therefore, with such a pretreatment process, the desired fine-grained oxides and surface-deficient layer are easily generated during the annealing process described later. This pretreatment process includes grinding the surface of the cold-rolled steel sheet using a high-powered grinding brush (brush grinding treatment). HOTANI's D-100 brush can also be used as the high-powered grinding brush. It is preferable to apply a 1.0–5.0% NaOH aqueous solution to the steel sheet surface during grinding. The brush deflection should be 0.5–10.0 mm, more preferably 5.0–10.0 mm, and the rotation speed should be 100–1000 rpm. By controlling the coating conditions, brush deflection, and rotation speed during brush grinding treatment, fine-grained oxides and a surface-deficiency layer can be effectively formed on the surface of the steel sheet during the annealing process described later.

[0183] [Annealing process]

[0184] The cold-rolled steel sheet, after undergoing the aforementioned pretreatment process, is then annealed. Annealing is preferably performed, for example, under a tension of 0.1 to 30.0 MPa. Applying tension during annealing allows for more effective introduction of strain into the steel sheet. This strain promotes the formation of dislocations in the steel's microstructure, making it easier for oxygen to penetrate the interior of the steel along these dislocations, thereby facilitating the formation of oxides within the steel sheet. As a result, it becomes more favorable to increase the number density of granular oxides, refine their grain size, and form a surface-deficient layer.

[0185] To ensure proper formation of the internal oxide layer, the holding temperature during the annealing process is preferably between 700°C and 900°C, and more preferably between 720°C and 870°C. Setting the temperature within this range helps suppress the formation of the external oxide layer, allowing oxides to form internally within the steel sheet. If the holding temperature is below 700°C, the desired internal oxide layer and / or a lack of surface layer may not form sufficiently during annealing. If the holding temperature exceeds 900°C, an external oxide layer may form during annealing, resulting in insufficient formation of the internal oxide layer and / or a lack of surface layer. The heating rate up to the holding temperature is not particularly limited, but a rate of 1 to 10°C / second is acceptable. Alternatively, the heating can be performed in two stages: a first heating rate of 1 to 10°C / second and a second heating rate of 1 to 10°C / second, different from the first heating rate.

[0186] The holding time at the holding temperature in the above-mentioned annealing process is preferably 10 to 300 seconds, and more preferably 30 to 250 seconds. By setting it within this range, the formation of an external oxide layer can be suppressed, and oxides can be formed inside the steel plate. If the holding time is less than 10 seconds, the desired internal oxide may not be sufficiently formed during annealing. If the holding time exceeds 300 seconds, an external oxide layer may form during annealing.

[0187] During the heating process of annealing, humidification is performed from the viewpoint of generating a large amount of fine granular oxides and forming a surface-deficient layer. The dew point of the atmosphere is preferably -20 to 10°C, and more preferably -10 to 5°C, with a concentration of 1 to 15 vol% H2. If the dew point is too low, an external oxide layer may form on the surface of the steel plate, but an internal oxide layer and / or a surface-deficient layer may not form sufficiently, potentially resulting in insufficient plating properties, resistance to hydrogen embrittlement, and resistance to LME. On the other hand, if the dew point is too high, the granular oxides may coarsen, potentially failing to meet the desired number density, and / or the desired surface-deficient layer may not be obtained. From the viewpoint of maintaining the internal oxide layer formed during heating, the dew point of the atmosphere during the holding (isothermal) phase of the annealing process is preferably below -20°C. If the dew point is above -20°C, the granular oxides may coarsen, reducing the desired number density of fine granular oxides, which may result in insufficient resistance to hydrogen embrittlement and / or LME.

[0188] It is advisable to begin humidification at a temperature below 600°C during the heating process. If humidification begins above 600°C, the internal oxide layer and / or the surface layer may not form sufficiently before reaching the holding temperature.

[0189] Furthermore, it is effective to remove the internal oxide layer of the steel sheet during the annealing process, especially before brush grinding. Sometimes, an internal oxide layer forms on the surface of the steel sheet during the aforementioned rolling process, particularly hot rolling. This internal oxide layer formed during such rolling may hinder the formation of fine-grained oxides and / or a lack of surface layer during the annealing process; therefore, it is preferable to remove this 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 during the annealing process is preferably set to 0.5 μm or less, more preferably 0.3 μm or less, more preferably 0.2 μm or less, and even more preferably 0.1 μm or less.

[0190] By performing the above-mentioned processes, it is possible to obtain a steel plate that contains a large amount of fine granular oxides in the surface layer and has a surface-deficient layer.

[0191] <Manufacturing Method of Coated Steel Sheet>

[0192] Hereinafter, a preferred method for manufacturing the coated steel sheet of the present invention will be described. The following description intentionally illustrates a characteristic method for manufacturing the coated steel sheet of the present invention, and is not intended to limit the coated steel sheet to be manufactured by the method described below.

[0193] The plated steel sheet of the present invention can be obtained by performing the following plating process: forming a coating containing Zn on a steel sheet manufactured as described above.

[0194] [Plating Process]

[0195] The plating process can be performed according to methods known to those skilled in the art. The plating process can be performed, for example, by hot-dip plating or by electroplating. Hot-dip plating is preferred. The conditions of the plating process can be appropriately set considering the desired coating composition, thickness, and adhesion amount. Alloying can also be performed after plating. Typically, the conditions of the plating process are preferably set to form a coating comprising: Al: 0–60.0%, Mg: 0–15.0%, Fe: 0–15%, Ni: 0–20%, and Si: 0–3%, with the remainder consisting of Zn and impurities. More specifically, the conditions for the plating process can be appropriately set, for example, to form 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, or Zn-11%Al-3%Mg-0.2%Si, Zn-11%Ni, Zn-15%Mg. From the viewpoint of improving LME resistance, the Al content in the coating is preferably 0.3-1.5%.

[0196] Example

[0197] The present invention will now be described in more detail by way of examples, but the present invention is not limited to these examples in any way.

[0198] Example 1: Examples and comparative examples of steel plates

[0199] (Preparation of steel plate samples)

[0200] Molten steel with adjusted composition was cast to form a billet. The billet was then hot-rolled, pickled, and cold-rolled to obtain a cold-rolled steel sheet. Next, the sheet was air-cooled to room temperature, and then pickled to remove the internal oxide layer formed during rolling, down to the internal oxide layer depth (μm) before annealing as shown in Table 1. Samples were then collected from each cold-rolled steel sheet according to JIS G0417:1999, and the composition of the steel sheets was analyzed by ICP-MS and other methods. The measured composition of the steel sheets is shown in Table 1. All steel sheets used had a thickness of 1.6 mm.

[0201] Next, for a portion of the cold-rolled steel sheets, a 2.0% NaOH aqueous solution was applied, followed by the following pretreatment: Brush grinding was performed using a high-powered grinding brush (HOTANI D-100) with a brush reduction of 2.0 mm and a rotation speed of 600 rpm. Afterwards, annealing was performed at the dew point and holding temperature shown in Table 1, and individual steel sheet samples were prepared. For all steel sheet samples, the heating rate during annealing was set to 6.0 °C / second up to 500 °C, and 2.0 °C / second from 500 °C to the holding temperature. In the above annealing process, a portion of the cold-rolled steel sheets were annealed under a tension of 30.0 MPa, while the other cold-rolled steel sheets were annealed without tension. Table 1 shows the presence or absence of pretreatment and the conditions for annealing (tension, humidification zone, dew point (°C), hydrogen concentration (vol%), humidification start temperature (°C) during the heating process, and heating completion temperature (holding temperature) (°C). The holding time was set to 10–300 seconds. It should be noted that for each steel plate specimen, JIS No. 5 tensile test pieces were collected with the length direction perpendicular to the rolling direction as the length direction, and tensile tests were conducted according to JIS Z 2241 (2011). The results showed that for No. 1, the tensile strength was less than 440 MPa, and for all other specimens, it was greater than 440 MPa.

[0202] (Analysis of the surface layer of the steel plate sample: number density of fine granular oxides)

[0203] Each steel plate sample prepared as described above was cut into 25mm × 15mm pieces. The cut samples were then embedded in resin and mirror-polished. For the cross-section of each steel plate sample, 10 1.0μm × 1.0μm regions were observed using SEM. The observation positions were set as follows: for the depth direction (perpendicular to the surface of the steel plate), 1.0μm was defined as extending from the surface of the steel plate to 0.2–1.2μm; for the width direction (parallel to the surface of the steel plate), 1.0μm was defined as any position in the aforementioned SEM image. The SEM images of each region of the obtained steel plate sample were binarized. The area of ​​the granular oxide portion was calculated from the binarized image. The diameter of the equivalent circle, with an area equal to this area, was used to determine the particle size (nm) of the granular oxide. Particles with a size range of 20–100nm were considered as fine granular oxides. The number of fine granular oxides in the SEM image was then counted. The average number of fine granular oxides in the 10 binarized images obtained in this way is taken as the number density of fine granular oxides. The number density (numbers / μm) of fine granular oxides for each steel plate sample is then calculated. 2 The results are shown in Table 1.

[0204] (Analysis of the surface layer of the steel plate sample: lack of a surface layer)

[0205] For each steel plate specimen, to evaluate the lack of a surface layer, the composition of the oxide-free steel microstructure was analyzed at half the average depth of the internal oxide layer calculated from the cross-sectional SEM image of the steel plate using TEM-EDS. Cases satisfying Si ≤ 0.6% and Al ≥ 0.05% were marked as "○", while cases not satisfying Si ≤ 0.6% and Al ≥ 0.05% were marked as "×".

[0206] (Coating performance evaluation)

[0207] For each steel plate sample, plating was performed, and the plating performance was evaluated by measuring the area ratio of the unplated portion of the surface of the plated steel plate. Specifically, hot-dip plating with Zn-0.2%Al (bath temperature 450-470℃) was performed. A 1mm × 1mm area on the surface of each plated steel plate sample with the plating was observed using an optical microscope. The areas with the plating (plated area) and the areas without the plating (unplated area) were identified from the observed images. The area ratio of the unplated area (area of ​​unplated area / area of ​​the observed image) was calculated, and the plating performance was evaluated using the following criteria. The results are shown in Table 1.

[0208] Evaluation A: The area ratio of the plated part is over 95% (the area ratio of the unplated part is less than 5.0%).

[0209] Evaluation B: The area ratio of the plated part is less than 95% but more than 90% (the area ratio of the unplated part exceeds 5.0% but is less than 10%).

[0210] (Composition analysis of the coating)

[0211] The composition of the coating was determined as follows: a sample cut into 30mm×30mm pieces was immersed in a 10% hydrochloric acid aqueous solution containing an inhibitor (Asahi Chemical, IBIT). After the coating was acid-washed and peeled off, the coating components dissolved in the aqueous solution were analyzed by ICP.

[0212] (LME resistance evaluation)

[0213] Each 100×100mm coated steel sheet sample was used for spot welding. Two coated steel sheets cut to 50mm×100mm were prepared. For these two Zn-based coated steel sheet samples, spot welding was performed using a dome-radius type welding electrode with a front diameter of 8mm, at an inclination angle of 7°, a pressure of 4.0kN, an energizing time of 0.5 seconds, and an energizing current of 7kA to obtain welded components. After grinding the weld section, it was observed using an optical microscope, and the length of LME cracks generated in the weld section was measured and evaluated as follows. The results are shown in Table 1. The coating type in Table 1 is set as GA, and the coating type in Table 2 is set as described in Table 2.

[0214] AAA rating: No LME cracking

[0215] Evaluation AA: LME crack length exceeds 0 μm but is less than 100 μm

[0216] Evaluation A: LME crack length exceeds 100μm but is less than 200μm

[0217] Rating B: LME crack length exceeds 200μm

[0218] (Evaluation of resistance to hydrogen embrittlement)

[0219] Each 50mm × 100mm coated steel sheet sample was treated with zinc phosphate using a zinc phosphate-based chemical conversion treatment solution (SURFDINESD5350 series: manufactured by Nippon Paint Industrial Coatings), followed by a 20μm electrodeposition coating (PN110POWERNICS gray: manufactured by Nippon Paint Industrial Coatings). The sample was then baked at 150°C for 20 minutes to form a coating film. The diffusivity of the electrodeposited coating was then evaluated by measuring the diffusivity using a temperature-release method. Specifically, in the diffusivity determination, the test piece was heated to 400°C in a gas chromatograph-equipped furnace, and the total amount of hydrogen released until the temperature dropped to 250°C was measured. Based on the measured diffusivity, hydrogen embrittlement resistance (hydrogen accumulation in the sample) was evaluated according to the following criteria, and the results are shown in Table 1.

[0220] Evaluation AA: Diffuse hydrogen content is below 0.2 ppm.

[0221] Evaluation A: Diffuse hydrogen content exceeds 0.2 ppm but is below 0.4 ppm

[0222] Rating B: Diffuse hydrogen content exceeds 0.4 ppm

[0223]

[0224] Example 2: Examples and comparative examples of galvanized steel sheets

[0225] (Preparation of galvanized steel sheet samples)

[0226] After cutting the steel plate samples from Example 1 into 100mm × 200mm dimensions, various coatings were applied as shown in Table 2. In Table 2, coating type a refers to "alloyed hot-dip galvanized steel sheet (GA)", coating type b refers to "hot-dip Zn-0.2% Al steel sheet (GI)", and coating type c refers to "hot-dip Zn-(0.3~1.5)% Al steel sheet (Al content is listed in Table 2)". 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 100mm / second, and the coating adhesion was controlled to 50g / m² using N2 wiping gas. 2 Regarding plating type a, alloying treatment is then carried out at 500℃.

[0227] For the coated steel sheet samples obtained for Example 2, the evaluation items—namely, the number density of fine granular oxides, the lack of a surface layer, the coating properties, and the resistance to hydrogen embrittlement—were evaluated using the same evaluation method as in Example 1, confirming that results equivalent to those of Example 1 could be obtained. Regarding tensile strength, although there were slight variations due to the coating treatment, similar to Example 1, the tensile strength for No. 1 was below 440 MPa, while for the other samples it was above 440 MPa. Regarding LME resistance, the resistance to LME improved when using coating type c and the Al content was 0.3–1.5% by mass. The results are shown in Table 2.

[0228]

[0229] In this example, a high-strength steel sheet or coated steel sheet with a tensile strength of 440 MPa or above, a coating performance rating of A, a hydrogen embrittlement resistance rating of AA or A, and a LME resistance rating of AAA, AA, or A is evaluated as having high coating performance, hydrogen embrittlement resistance, and LME resistance. In Examples 1 and 2, for samples No. 2 to 8 and 21 to 34, since the steel sheet composition, the number density of fine granular oxides, and the lack of a surface layer meet the scope of the present invention, it has high coating performance, LME resistance, and hydrogen embrittlement resistance. Sample No. 1, due to insufficient carbon content, not only did not obtain sufficient strength, but also failed to obtain the desired internal oxide layer, fine granular oxides, and lack of a surface layer, thus failing to obtain high coating performance, hydrogen embrittlement resistance, and LME resistance. Sample No. 9 had a low dew point during annealing heating, and failed to form the desired internal oxide layer and lack of a surface layer, thus failing to obtain high coating performance, hydrogen embrittlement resistance, and LME resistance. Sample No. 10 had a high dew point during annealing, resulting in the coarsening of the granular internal oxide layer instead of the desired fine granular structure, and the absence of a surface layer deficiency, thus failing to achieve high plating performance, hydrogen embrittlement resistance, and LME resistance. Sample No. 11 had a high holding temperature during annealing, resulting in insufficient formation of the internal oxide layer, and thus failing to achieve high plating performance, hydrogen embrittlement resistance, and LME resistance. Sample No. 12 had a low holding temperature during annealing, resulting in insufficient internal oxidation and failing to achieve high plating performance, hydrogen embrittlement resistance, and LME resistance. Samples No. 13 and 15 had excessive Si and Mn content, respectively, leading to coarsening of the granular oxide layer and failing to achieve high hydrogen embrittlement resistance and LME resistance. Samples No. 14 and 16 had insufficient Si and Mn content, respectively, failing to form the desired internal oxide layer and surface layer deficiency, and thus failing to achieve high plating performance, hydrogen embrittlement resistance, and LME resistance. Sample No. 17 had excessive Al content, resulting in coarsening of the granular oxide layer and failing to achieve high resistance to hydrogen embrittlement and LME. Sample No. 18 had insufficient Al content, failing to form the desired surface deficiencies and also failing to achieve high resistance to hydrogen embrittlement and LME. Sample No. 19 had its dew point set to 0°C during both the annealing heating and holding periods, resulting in prolonged humidification time, coarsening of the granular oxide layer, and failing to achieve high resistance to hydrogen embrittlement and LME. Sample No. 20 had a thick internal oxide layer before annealing, which prevented the formation of the desired internal oxide layer and surface deficiencies after annealing, resulting in low plating quality, resistance to hydrogen embrittlement, and LME. Sample No. 35 did not have sufficient internal oxide layer formation due to the lack of tension applied to the steel sheet during annealing, thus failing to achieve high resistance to hydrogen embrittlement and LME. Sample No. 36 did not undergo brush grinding before annealing, therefore the internal oxide layer was not sufficiently formed, resulting in low resistance to hydrogen embrittlement and LME. Sample No. 37 had a humidification start temperature above 600℃, which also resulted in insufficient formation of the internal oxide layer, leading to low plating performance, low resistance to hydrogen embrittlement, and low resistance to LME.

[0230] In the example of the invention, a large amount of fine granular oxide was identified, and EDS analysis confirmed the presence of a defined surface-deficient layer. Therefore, high plating performance, resistance to hydrogen embrittlement, and resistance to LME were achieved. On the other hand, in the comparative example, an internal oxide layer and / or a surface-deficient layer were not adequately formed near the surface of the steel plate. Therefore, at least one of the following was identified: low plating performance, significant hydrogen intrusion, and poor resistance to LME.

[0231] Industrial availability

[0232] According to the present invention, high-strength steel sheets and coated steel sheets with high plating properties, LME resistance, and hydrogen embrittlement resistance can be provided. These steel sheets and coated steel sheets are suitable for use in automobiles, home appliances, building materials, and other applications, especially in automobiles. As automotive steel sheets and coated steel sheets, they are expected to offer high collision safety and long service life. Therefore, the present invention can be considered an invention of extremely high industrial value.

[0233] Explanation of symbols

[0234] 1 steel plate

[0235] 2. External oxide layer

[0236] 3. Base steel

[0237] 11 steel plate

[0238] 12 Fine-grained oxides

[0239] 14. Base steel

Claims

1. A steel plate having the following composition: It contains, by mass%: C：0.05～0.40％、 Si: 0.2-3.0% Mn: 0.1–5.0% sol.Al: 0.4–1.50% P: below 0.0300% S: below 0.0300% N: below 0.0100% B：0～0.010％、 Ti: 0~0.150% Nb: 0~0.150% 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: 0–0.100% Mg: 0–0.100% Zr：0～0.100％、 Hf: 0~0.100%, and REM: 0–0.100%, the remainder consists of Fe and impurities. The steel plate has an internal oxide layer on its surface containing fine granular oxides. The fine granular oxide is an oxide with an aspect ratio of 1.0 to 5.0 and a particle size of 20 nm to 100 nm. When observing the cross-section of the surface layer of the steel plate, the number density of the fine granular oxides in the inner oxide layer is 4.0 particles / μm. 2 above, The surface of the steel plate includes a surface-deficient layer, wherein the surface-deficient layer is located at a depth of 1 / 2 the average depth of the internal oxide layer. The steel composition, by mass percent, is free of the fine-grained oxides and satisfies Si ≤ 0.6% and Al ≥ 0.05%. Among them, granular oxides refer to oxides that are dispersed in granular form within the grains or at the grain boundaries of steel. The internal oxide layer is a layer formed by connecting the surface of the steel plate to the farthest point where the fine granular oxides exist. The surface deficiency layer refers to the area on the surface of the steel plate where easily oxidizable constituent elements are lacking compared to the base material of the steel plate.

2. The steel plate according to claim 1, wherein, The number density of the fine granular oxide is 8 particles / μm. 2 above.

3. A coated steel sheet having a coating containing Zn on the steel sheet of claim 1 or 2.

4. A galvanized steel sheet, characterized in that, It is the coated steel sheet as described in claim 3, wherein the Al content in the coating is 0.3 to 1.5% by mass.

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