Steel sheet and plated steel sheet

Abstract

The present application provides a kind of steel sheet and the plated steel sheet using it, the steel sheet contains C:0.05~0.40%, Si:0.2~3.0% and Mn:0.1~5.0% by mass, include granular type oxide in the surface layer of steel sheet, the average particle size of granular type oxide is 300nm or less, the number density of granular type oxide is 4.0 / μm 2 Above, the steel sheet includes Si-Mn deficient layer with the thickness of 3.0 μm or more from the surface of the steel sheet, the Si and Mn contents of the oxide-free region at the 1 / 2 position of the thickness of the Si-Mn deficient layer are respectively lower than 10% of the Si and Mn contents at the center portion of the plate thickness of the steel sheet.

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 coating 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, and Mn to enhance the steel's strength.

[0003] In the manufacture of high-strength steel plates, heat treatment such as annealing is generally performed after rolling. Furthermore, Si and Mn, which are easily oxidized elements typically found in high-strength steel plates, may 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 Si and Mn oxides 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 may 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] Associated with the internal oxide layer, Patent Documents 1 and 2 disclose a high-strength coated steel sheet having a zinc-based coating on a base steel sheet containing C, Si, and Mn, and an internal oxide layer containing oxides of Si and / or Mn in the surface layer of the base steel sheet, wherein the high-strength coated steel sheet has a tensile strength of 980 MPa or more.

[0006] Furthermore, Patent Document 3 proposes a method for manufacturing high-tensile hot-dip galvanized steel sheet containing high Si. In this method, when the steel contains high Si with a Si concentration of 0.3% or more, Si and other substances in the steel diffuse into the surface layer of the steel sheet in the form of oxides through heating of the steel sheet surface. These oxides hinder the wettability of the coating and make the coating adhesion worse. Therefore, the annealing conditions are appropriately controlled.

[0007] Existing technical documents

[0008] Patent documents

[0009] Patent Document 1: Japanese Patent Application Publication No. 2016-130357

[0010] Patent Document 2: Japanese Patent Application Publication No. 2018-193614

[0011] Patent Document 3: Japanese Patent Application Publication No. 4-202632 Summary of the Invention

[0012] The problem that the invention aims to solve

[0013] High-strength steel sheets used in automotive components and other applications may be 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 segregates at the martensite grain boundaries in the steel structure, causing cracking by embrittlement of these grain boundaries. This cracking phenomenon caused by hydrogen infiltration is called hydrogen embrittlement (delayed fracture), and it often becomes a problem during the processing of the steel sheet. Therefore, reducing the hydrogen accumulation in steel sheets used in corrosive environments is effective in preventing hydrogen embrittlement.

[0014] Furthermore, when hot stamping or welding is performed on coated steel sheets with Zn-based coatings or the like, the coated steel sheet may be processed in a molten state due to the high temperature (e.g., around 900°C). In this case, 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 the like from the coating into the steel sheet.

[0015] Patent documents 1 and 2 teach the following method: by oxidizing the oxide film using an oxidation zone at an air-fuel ratio of 0.9 to 1.4, followed by reducing the oxide film in a hydrogen atmosphere using a reduction zone, the average depth of the internal oxide layer is controlled to be more than 4 μm, allowing the internal oxide layer to function as a hydrogen trapping site, thereby preventing hydrogen intrusion and suppressing hydrogen embrittlement. Patent document 3 similarly discloses heating using an oxidation zone at an air ratio of 0.95 to 1.10. However, in none of these documents, any research was conducted on controlling the morphology of the oxides present in the internal oxide layer, leaving room for improvement in hydrogen embrittlement resistance. Furthermore, no research was conducted on improving LME resistance.

[0016] 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.

[0017] Methods for solving problems

[0018] The inventors of this invention have discovered that, in order to solve the aforementioned problems, the following is important: forming oxides on the surface of the steel plate, i.e., inside the steel plate, thereby controlling the morphology of the oxides present on the surface of the steel plate, and controlling the Si-Mn deficient layer formed on the surface of the steel plate due to the formation of such oxides to a range of specified thickness and composition. More specifically, the inventors of this invention have discovered that by forming internal oxides to ensure high plating performance, by forming fine and abundant granular oxides existing in the form of oxides 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 environments, but also as trapping sites for Zn that can penetrate into the steel during hot stamping or welding processes. Furthermore, by forming a Si-Mn deficient layer with a specified thickness and composition on the surface of the steel plate to promote hydrogen diffusion in the steel, thereby improving hydrogen expulsion from the steel, high LME performance and resistance to hydrogen embrittlement can be obtained.

[0019] This invention is based on the above-mentioned insights, and its main points are as follows.

[0020] (1) A steel plate having the following composition:

[0021] It contains, by mass%:

[0022] C: 0.05~0.40%

[0023] Si: 0.2-3.0%

[0024] Mn: 0.1–5.0%

[0025] sol.Al: 0% or more and less than 0.4000%

[0026] P: below 0.0300%

[0027] S: below 0.0300%

[0028] N: below 0.0100%

[0029] B: 0~0.010%

[0030] Ti: 0~0.150%

[0031] Nb: 0~0.150%

[0032] V: 0~0.150%

[0033] Cr: 0–2.00%

[0034] Ni: 0~2.00%

[0035] Cu: 0–2.00%

[0036] Mo: 0–1.00%

[0037] W: 0~1.00%

[0038] Ca: 0–0.100%

[0039] Mg: 0–0.100%

[0040] Zr: 0~0.100%

[0041] Hf: 0~0.100%, and

[0042] REM: 0–0.100%, the remainder consists of Fe and impurities.

[0043] The surface layer of the aforementioned steel plate contains granular oxides.

[0044] The average particle size of the above-mentioned granular oxides is less than 300 nm.

[0045] The number density of the above-mentioned granular oxides is 4.0 particles / μm. 2 above,

[0046] The steel plate includes a Si-Mn-deficient layer having a thickness of 3.0 μm or more extending from the surface of the steel plate.

[0047] The Si and Mn contents in the oxide-free region at half the thickness of the aforementioned Si-Mn deficient layer are 10% lower than the Si and Mn contents at the center of the thickness of the aforementioned steel plate.

[0048] (2) The steel plate according to (1), wherein the average particle size of the above-mentioned granular oxide is less than 200 nm.

[0049] (3) The steel plate according to (1) or (2), wherein the number density of the above-mentioned granular oxides is 10.0 particles / μm. 2 above.

[0050] (4) The steel plate according to any one of (1) to (3) further comprises grain boundary oxide in the surface layer of the steel plate.

[0051] (5) According to the steel plate described in (4), when the cross section of the surface layer of the steel plate is observed, the ratio A of the length of the grain boundary oxide projected onto the surface of the steel plate to the length of the surface of the steel plate is 50% or more.

[0052] (6) The steel plate according to (5), wherein the ratio A is 80% or more.

[0053] (7) A coated steel sheet having a coating containing Zn on any one of (1) to (6).

[0054] (8) The coated steel sheet according to (7), wherein the coating has a composition of Zn-(0.3-1.5)%Al.

[0055] Invention Effects

[0056] According to the present invention, the fine and abundant granular oxides present 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 greatly suppressed, significantly improving resistance to hydrogen embrittlement. Furthermore, these granular oxides also function as trapping sites for Zn intrusion into the steel during hot stamping or welding processes, significantly suppressing the amount of intruded Zn and significantly improving resistance to LME. Moreover, according to the present invention, by including a Si-Mn-deficient layer with a specified thickness and composition, hydrogen diffusion can be promoted, thereby improving hydrogen expulsion from the steel. As a result, intruded hydrogen can be released, reducing the amount of hydrogen accumulated in the steel and significantly improving resistance to hydrogen embrittlement. Furthermore, since the granular oxides and optionally grain boundary oxides are formed inside the steel sheet, sufficient interdiffusion between the steel composition and the composition of the coating occurs when a coating is formed, resulting in high coating quality. Therefore, through the present invention, high coating quality, LME resistance, and hydrogen embrittlement resistance can be obtained in high-strength steel sheets. Attached Figure Description

[0057] Figure 1 A schematic diagram showing a cross-section of a steel plate with an external oxide layer.

[0058] Figure 2 A schematic cross-sectional view of a steel plate according to one embodiment of the present invention.

[0059] Figure 3 Indicated for explanation Figure 2 A schematic diagram of the determination of the ratio A of the steel plates in the diagram.

[0060] Figure 4 A schematic cross-sectional view of a steel plate according to another embodiment of the present invention.

[0061] Figure 5 Indicated for explanation Figure 4 A schematic diagram of the determination of the ratio A of the steel plates in the diagram. Detailed Implementation

[0062] <steel plate>

[0063] The steel plate of the present invention is characterized by having the following composition: containing, by mass %:

[0064] C: 0.05~0.40%

[0065] Si: 0.2-3.0%

[0066] Mn: 0.1–5.0%

[0067] sol.Al: 0% or more and less than 0.4000%

[0068] P: below 0.0300%

[0069] S: below 0.0300%

[0070] N: below 0.0100%

[0071] B: 0~0.010%

[0072] Ti: 0~0.150%

[0073] Nb: 0~0.150%

[0074] V: 0~0.150%

[0075] Cr: 0–2.00%

[0076] Ni: 0~2.00%

[0077] Cu: 0–2.00%

[0078] Mo: 0–1.00%

[0079] W: 0~1.00%

[0080] Ca: 0–0.100%

[0081] Mg: 0–0.100%

[0082] Zr: 0~0.100%

[0083] Hf: 0~0.100%, and

[0084] REM: 0–0.100%, the remainder consists of Fe and impurities.

[0085] The surface layer of the aforementioned steel plate contains granular oxides.

[0086] The average particle size of the above-mentioned granular oxides is less than 300 nm.

[0087] The number density of the above-mentioned granular oxides is 4.0 particles / μm. 2 above,

[0088] The steel plate includes a Si-Mn-deficient layer having a thickness of 3.0 μm or more extending from the surface of the steel plate.

[0089] The Si and Mn contents in the oxide-free region at half the thickness of the aforementioned Si-Mn deficient layer are 10% lower than the Si and Mn contents at the center of the thickness of the aforementioned steel plate.

[0090] 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 oxidizable components in the steel plate (such as Si and Mn) combine with oxygen in the annealing atmosphere, thereby forming an oxide layer near the surface of the steel plate. For example, ... Figure 1 As 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), and therefore the adhesion between the steel and the coating cannot be adequately ensured, which may result in uncoated areas where no coating is formed.

[0091] In contrast, such as Figure 2 As illustrated in the illustration, the steel plate 11 of the present invention is not as... Figure 1Unlike 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 granular oxides 12 and optionally grain boundary oxides 13 present at the grain boundaries of the metal structure. Therefore, when a coating is formed on the surface of the steel plate 11, the steel plate 11 of the present invention, with granular oxides 12 and optionally grain boundary oxides 13 formed inside the base steel 14, achieves sufficient interdiffusion between the coating component and the steel component compared to the steel plate 1 having an external oxide layer 2, resulting in high coating performance. Thus, the inventors of the present invention have found that, from the viewpoint of obtaining high coating performance, it is effective to control the conditions during the annealing process to form oxides inside the steel plate. It should be noted that, when used with steel plates, the term "high coating performance" means that when a coating process is performed on the steel plate, a coating can be formed with little (e.g., less than 5.0 area%) or no uncoated portion (the portion without a coating). Furthermore, when used with galvanized steel sheets, the term "high coating quality" indicates that the uncoated portion is very small (e.g., less than 5.0% of the area) or completely non-coated.

[0092] 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 a steel plate, and more specifically, by making the oxides inside the steel plate into "granular oxides" with an average particle size and number density within a specified range, and further controlling the Si-Mn deficient layer formed by the reduction of the surrounding Si and Mn concentrations due to the formation of such internal oxides to within a specified thickness and composition range, the granular oxides function as trapping sites for hydrogen intrusion in the surface region of the steel plate under corrosive conditions. Furthermore, the Si-Mn deficient layer promotes the diffusion of intruded hydrogen, thereby improving hydrogen expulsion from the steel. As a result, not only is hydrogen intrusion suppressed, but the release of intruded hydrogen out of the system is also promoted, thereby reducing the amount of hydrogen accumulation 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 suppresses hydrogen embrittlement cracking.

[0093] 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, the surface of the base steel 14 contains fine and numerous granular oxides 12 dispersed in a granular manner, more specifically, the granular oxides have an average particle size of 300 nm or less and a number density of 4.0 particles / μm. 2The above-described methods are effective. While not bound by a specific theory, it is believed that the hydrogen-capturing function of oxides in steel plates is positively correlated with the surface area of ​​the oxides. That is, it is believed that by dispersing oxides finely and abundantly on the surface of the steel plate, the surface area of ​​the oxides on the surface of the steel plate increases, thereby enhancing the hydrogen-capturing function. Therefore, the inventors of this invention have found that, from the viewpoint of obtaining high resistance to hydrogen intrusion and thus high resistance to hydrogen embrittlement, it is important to control the conditions during the manufacture of the steel plate, especially during annealing, so that granular oxides, which function as hydrogen-capturing sites when placed in a corrosive environment, are present in finely and abundantly. It should be noted that since the metallic structure of the surface layer of the steel plate is typically composed of a metallic structure softer than that of the interior of the steel plate (e.g., at 1 / 8 or 1 / 4 of the plate thickness), hydrogen embrittlement cracking is not particularly problematic even if hydrogen is present in the surface layer of the steel plate.

[0094] Furthermore, the inventors of this invention have made contributions to the development of this invention. Figure 2 A detailed analysis was conducted on the relationship between the morphology of the Si-Mn deficient layer formed by the formation of internal oxides such as granular oxide 12, which reduces the surrounding Si and Mn concentration, and hydrogen expulsion. The results showed that controlling the Si-Mn deficient layer to a specified thickness and composition, more specifically, controlling it so that the thickness of the Si-Mn deficient layer is 3.0 μm or more from the surface of the steel plate, and the Si and Mn contents in the oxide-free region at half the thickness of the Si-Mn deficient layer are respectively less than 10% of the Si and Mn contents at the center of the steel plate thickness (hereinafter, these values ​​are also referred to as Si deficiency rate and Mn deficiency rate). While not bound by a specific theory, it is believed that in the case of steel containing a large amount of Si and / or Mn, the amount of Si and / or Mn dissolved in the steel also increases, and therefore these dissolved Si and / or Mn hinder hydrogen diffusion, resulting in a slower hydrogen diffusion rate in the steel. Figure 2As shown, if internal oxides such as granular oxide 12 and optionally grain boundary oxide 13 are formed in the surface layer of the steel plate, the Si and Mn dissolved in the steel are consumed through the formation of internal oxides. This results in the formation of internal oxides in the surface layer of the steel plate, while simultaneously generating a Si-Mn deficient layer with a relatively low concentration of Si and Mn in the surrounding area. Therefore, it is believed that by setting this Si-Mn deficient layer to be relatively thick, specifically controlling the thickness of the Si-Mn deficient layer to be 3.0 μm or more from the surface of the steel plate (or the interface between the coating and the steel plate if a coating is present on the surface), a sufficient hydrogen diffusion path can be ensured. Furthermore, by further reducing the Si and Mn content of the Si-Mn deficient layer sufficiently, specifically controlling it so that the Si and Mn deficiency rates are each less than 10%, the amount of dissolved Si and Mn that hinders hydrogen diffusion can be sufficiently reduced. Therefore, it is believed that by including a Si-Mn deficient layer with its thickness and composition controlled within the aforementioned range, hydrogen diffusion can be promoted, thereby significantly improving hydrogen removal from the steel. Therefore, by combining the aforementioned granular oxide with the Si-Mn-deficient layer, both resistance to hydrogen intrusion and hydrogen expulsion can be improved, thereby greatly enhancing the overall hydrogen embrittlement resistance of the steel plate.

[0095] Furthermore, it is known that hydrogen embrittlement cracking not only occurs when using the high-strength steel plates described above in atmospheric corrosive environments, but can also occur when hydrogen present in the annealing atmosphere during the annealing process of manufacturing the high-strength steel plate penetrates deeper into the surface layer of the base steel. In this invention, the inventors have discovered that the combination of the aforementioned granular oxide and Si-Mn-deficient layer is not only effective for use in corrosive environments, but also effective in suppressing hydrogen intrusion into the steel plate during the annealing process and in removing intruded hydrogen. As a result, high resistance to hydrogen embrittlement can be achieved in both the manufacturing and use of the steel plate.

[0096] 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 temperature during processing. If the Zn melts, it will penetrate 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 granular oxide has a desired average grain size and 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 granular oxide functions as a trapping site for Zn that wants to penetrate into the steel during high-temperature processing. Thus, for example, during hot stamping, the Zn that wants to penetrate into the steel is trapped by the granular oxide on the surface of the steel sheet, which can appropriately suppress the penetration of Zn into the grain boundaries. Therefore, it was found that, in order to improve not only the aforementioned resistance to hydrogen intrusion but also to improve resistance to LME, it is important that the granular oxides are present in fine and abundant quantities. It should be noted that the steel sheet of the present invention is not necessarily limited to the coated steel sheet described above, but also includes uncoated steel sheets. This is because, even with uncoated steel sheets, for example, when spot-welded to galvanized steel sheets, LME cracking may occur due to molten zinc in the galvanized steel sheet intruding into the uncoated steel sheet.

[0097] 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 may be, for example, 0.1 to 3.2 mm.

[0098] [Composition of steel plates]

[0099] The composition of the steel plate of the present invention will be described. Unless otherwise specified, the "%" of the element content refers to "mass %". In the numerical range of the composition, the numerical range indicated by "~" refers to the range including the values ​​before and after "~" as the lower limit and upper limit, unless otherwise specified.

[0100] (C: 0.05~0.40%)

[0101] Carbon (C) is an important element in ensuring the strength of steel. To ensure sufficient strength and thus obtain the desired morphology of internal oxides, the C content is set to 0.05% or more. The C content is preferably 0.07% or more, more preferably 0.10% or more, and even more preferably 0.12% or more. On the other hand, if the C content is excessive, weldability may decrease. Therefore, the C content is set to 0.40% or less. The C content can also be 0.38% or less, 0.35% or less, 0.32% or less, or 0.30% or less.

[0102] (Si: 0.2-3.0%)

[0103] Silicon (Si) is an effective element for improving the strength of steel. To ensure sufficient strength and thus adequate formation of the desired oxides, particularly granular oxides, within the steel sheet, the Si content is set to 0.2% or more. Preferably, the Si content is 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, excessive formation of external oxides may occur, even leading to deterioration of surface properties. Furthermore, it may also cause coarsening of the granular oxides. Therefore, the Si content is set to 3.0% or less. The Si content can also be 2.8% or less, 2.5% or less, 2.3% or less, or 2.0% or less.

[0104] (Mn: 0.1-5.0%)

[0105] Manganese (Mn) is an effective element for improving the strength of steel by obtaining a hard microstructure. To ensure sufficient strength and thus adequate formation of the desired oxides, particularly granular oxides, within 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, if the Mn content is excessive, excessive formation of external oxides may occur, or Mn segregation may lead to an uneven metal microstructure and reduced workability. Furthermore, it may also cause coarsening of the granular oxides. Therefore, the Mn content is set to 5.0% or less. The Mn content can also be 4.5% or less, 4.0% or less, 3.5% or less, or 3.0% or less.

[0106] (sol.Al: above 0% and below 0.4000%)

[0107] Al (aluminum) is an element that functions as a deoxidizer. The Al content can be 0%, but to achieve a sufficient deoxidation effect, the Al content is preferably 0.0010% or more. More preferably, the Al content is 0.0050% or more, even more preferably 0.0100% or more, and even more preferably 0.0150% or more. On the other hand, if the Al content is excessive, it may cause a decrease in processability and a deterioration in surface properties. Therefore, the Al content is set to be less than 0.4000%. The Al content can also 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).

[0108] (P: below 0.0300%)

[0109] Phosphorus (P) is generally an impurity contained in steel. Excessive P content can reduce weldability. Therefore, the P content is set to 0.0300% or less. Preferably, the P content is 0.0200% or less, more preferably 0.0100% or less, and even more preferably 0.0050% or less. The lower limit for P content is 0%, but from a manufacturing cost perspective, the P content can also be more than 0% or more than 0.0001%.

[0110] (S: below 0.0300%)

[0111] Sulfur (S) is generally an impurity contained in steel. Excessive S content can reduce weldability and increase MnS precipitation, leading to decreased workability such as flexibility. Therefore, the S content is set to 0.0300% or less. Preferably, the S content is 0.0100% or less, more preferably 0.0050% or less, and even more preferably 0.0020% or less. The lower limit for S content is 0%, but from the viewpoint of desulfurization cost, the S content can also be greater than 0% or greater than 0.0001%.

[0112] (N: below 0.0100%)

[0113] Nitrogen (N) is generally an impurity contained in steel. Excessive N content can reduce weldability. Therefore, the N content is set to 0.0100% or less. Preferably, the N content is 0.0080% or less, more preferably 0.0050% or less, and even more preferably 0.0030% or less. The lower limit for N content is 0%, but from a manufacturing cost perspective, the N content can also be more than 0% or more than 0.0010%.

[0114] The basic composition of the steel plate of the present invention is as described above. Furthermore, the steel plate may also contain, as needed, the following optional elements. The presence of these elements is not essential, and the lower limit of their content is 0%.

[0115] (B: 0~0.010%)

[0116] 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. The B content can be 0%, but it can be included as needed to achieve the aforementioned effects. The B content can also 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 can also be 0.008% or less, or 0.006% or less.

[0117] (Ti: 0~0.150%)

[0118] Titanium (Ti) is an element that precipitates as TiC during the cooling of steel and contributes to increased strength. The Ti content can be 0%, but it can be included as needed to achieve the aforementioned effect. The Ti content can also be 0.001% or more, 0.003% or more, 0.005% or more, or 0.010% or more. On the other hand, if Ti is present in excess, coarse TiN may form, impairing toughness. Therefore, the Ti content is preferably 0.150% or less, but can also be 0.100% or less, or 0.050% or less.

[0119] (Nb: 0~0.150%)

[0120] Niobium (Nb) is an element that contributes to increased strength by improving hardenability. The Nb content can be 0%, but it can be included as needed to achieve the aforementioned effect. The Nb content can also 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, but can also be 0.100% or less, or 0.060% or less.

[0121] (V: 0~0.150%)

[0122] Vanadium (V) is an element that contributes to increased strength by improving hardenability. The V content can be 0%, but it can be included as needed to achieve the aforementioned effect. The V content can also 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, but can also be 0.100% or less, or 0.060% or less.

[0123] (Cr: 0-2.00%)

[0124] Chromium (Cr) is effective in improving the hardenability of steel, thereby increasing its strength. The Cr content can be 0%, but it can be included as needed to achieve the aforementioned effect. The Cr content can also 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, if Cr is present in excess, a large amount of Cr carbides may form, which can impair hardenability. Therefore, the Cr content is preferably 2.00% or less, but can also be 1.80% or less or 1.50% or less.

[0125] (Ni: 0-2.00%)

[0126] Nickel (Ni) is an effective element for improving the hardenability of steel, thereby increasing its strength. The Ni content can be 0%, but it can be included as needed to achieve the aforementioned effects. The Ni content can also 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 leads to increased costs. Therefore, the Ni content is preferably 2.00% or less, but can also be 1.80% or less or 1.50% or less.

[0127] (Cu: 0~2.00%)

[0128] Cu (copper) is an effective element for improving the hardenability of steel, thereby increasing its strength. The Cu content can be 0%, but it can be included as needed to achieve the aforementioned effects. The Cu content can also be 0.001% or more, 0.005% or more, or 0.01% or more. On the other hand, from the viewpoint of suppressing reduced toughness, cracking of the cast slab, and reduced weldability, the Cu content is preferably 2.00% or less, but can also be 1.80% or less, 1.50% or less, or 1.00% or less.

[0129] (Mo: 0~1.00%)

[0130] Mo (Mo) is an effective element for improving the hardenability of steel, thereby increasing its strength. The Mo content can be 0%, but it can be included as needed to achieve the aforementioned effects. The Mo content can also 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 the reduction in toughness and weldability, the Mo content is preferably 1.00% or less, and can also be 0.90% or less, or 0.80% or less.

[0131] (W: 0~1.00%)

[0132] Tungsten (W) is an effective element for improving the hardenability of steel, thereby increasing its strength. The W content can be 0%, but it can be included as needed to achieve the aforementioned effects. The W content can also be 0.001% or more, 0.005% or more, or 0.01% or more. On the other hand, from the viewpoint of suppressing the reduction in toughness and weldability, the W content is preferably 1.00% or less, but can also be 0.90% or less, 0.80% or less, 0.50% or less, or 0.10% or less.

[0133] (Ca: 0~0.100%)

[0134] Ca (calcium) is an element that helps control inclusions, especially the fine dispersion of inclusions, and improves toughness. The Ca content can be 0%, but it can be included as needed to achieve the aforementioned effects. The Ca content can also be 0.0001% or more, 0.0005% or more, or 0.001% or more. On the other hand, if Ca is present in excess, the deterioration of surface properties may become more pronounced. Therefore, the Ca content is preferably 0.100% or less, but can also be 0.080% or less, 0.050% or less, 0.010% or less, or 0.005% or less.

[0135] (Mg: 0-0.100%)

[0136] Magnesium (Mg) is an element that helps control inclusions, particularly the fine dispersion of inclusions, and improves toughness. The Mg content can be 0%, but it can be included as needed to achieve the aforementioned effects. The Mg content can also be 0.0001% or more, 0.0005% or more, or 0.001% or more. On the other hand, if Mg is present in excess, the deterioration of surface properties may become more pronounced. Therefore, the Mg content is preferably 0.100% or less, but can also be 0.090% or less, 0.080% or less, 0.050% or less, or 0.010% or less.

[0137] (Zr: 0~0.100%)

[0138] Zirconium (Zr) is an element that helps control inclusions, particularly the fine dispersion of inclusions, and improves toughness. The Zr content can be 0%, but it can be included as needed to achieve the aforementioned effects. The Zr content can also be 0.001% or more, 0.005% or more, or 0.010% or more. On the other hand, if Zr is present in excess, the deterioration of surface properties may become more pronounced. Therefore, the Zr content is preferably 0.100% or less, but can also be 0.050% or less, 0.040% or less, or 0.030% or less.

[0139] (Hf: 0~0.100%)

[0140] Hafnium (Hf) is an element that helps control inclusions, particularly the fine dispersion of inclusions, and improves toughness. The Hf content can be 0%, but it can be included as needed to achieve the aforementioned effects. The Hf content can also be 0.0001% or more, 0.0005% or more, or 0.001% or more. On the other hand, if Hf is present in excessive amounts, the deterioration of surface properties may become more pronounced. Therefore, the Hf content is preferably 0.100% or less, but can also be 0.050% or less, 0.030% or less, or 0.010% or less.

[0141] (REM: 0~0.100%)

[0142] Rare earth elements (REMs) are elements that help control inclusions, especially the fine dispersion of inclusions, and improve toughness. The REM content can be 0%, but it can be included as needed to achieve the aforementioned effects. The REM content can also be 0.0001% or more, 0.0005% or more, or 0.001% or more. On the other hand, if the REM content is excessive, the deterioration of surface properties may become more pronounced. Therefore, the REM content is preferably 0.100% or less, but can also be 0.050% or less, 0.030% or less, or 0.010% or less. It should be noted that REM is an abbreviation for Rare Earth Metal, referring to elements belonging to the lanthanide series. REM is usually added as a mixed rare earth alloy.

[0143] 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.

[0144] 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.

[0145] [surface layer]

[0146] In this invention, the “surface” of the steel plate refers to the area extending 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) in the thickness direction to a specified depth, which is typically 50 μm or less.

[0147] like Figure 2 As illustrated, in the steel plate 11 of the present invention, granular oxide 12 is included in the surface layer of the steel plate 11. Preferably, the granular oxide 12 is present only in the surface layer of the steel plate 11. By having this granular oxide 12 present inside the base steel 14 (i.e., existing as an internal oxide), it interacts with... Figure 1Compared to the case where an external oxide layer 2 exists on the surface of the base steel 3, the steel sheet 11 becomes capable of high plating properties. It is believed that this is related to the formation of internal oxides, resulting in the absence or mere thinness of the external oxide layer that hinders 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, allowing sufficient interdiffusion of the plating components and the steel components. Therefore, the steel sheet and plated steel sheet of the present invention, which contain granular oxides on the surface of the steel sheet, i.e., inside the steel sheet, exhibit high plating properties.

[0148] In addition, such as Figure 2 As illustrated, in the steel plate 11 of the present invention, in addition to the aforementioned granular oxide 12, grain boundary oxide 13 may optionally be included in the surface layer of the steel plate 11. Since the grain boundary oxide 13 exists in the interior of the base steel 14 in the same way as the granular oxide 12, the steel plate and the plated steel plate containing both the granular oxide 12 and the grain boundary oxide 13 also have high plating properties.

[0149] [Granular Oxides]

[0150] In this invention, "granular oxide" refers to oxides dispersed in a granular manner within the grains or at the 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 largest line segment traversing the granular oxide (major axis) / the length of the largest line segment perpendicular to the major axis traversing the oxide (minor axis)). "Dispersed in a granular manner" 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 substantially 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 substantially circular. Figure 2 As an example, granular oxide 12 that appears to be round is shown.

[0151] (Average particle size)

[0152] In this invention, the average particle size of the granular oxide is 300 nm or less. By controlling the average particle size to such a range, the granular oxide can be finely dispersed in the surface layer of the steel plate. The granular oxide functions well as a hydrogen trapping site to inhibit hydrogen intrusion during annealing in corrosive environments and / or manufacturing processes, and further functions well as a Zn trapping site to prevent intrusion during hot stamping or welding of the coated steel plate. On the other hand, if the average particle size is too large, the granular oxide may not function sufficiently as a hydrogen trapping site and / or a Zn trapping site, resulting in poor resistance to hydrogen embrittlement and / or LME. The average particle size of the granular oxide is preferably 250 nm or less, more preferably 200 nm or less, and even more preferably 150 nm or less. The finer the granular oxide, the better. Therefore, the lower limit of the average particle size of the granular oxide is not particularly limited, but it can be, for example, 5 nm or more, 10 nm or more, or 50 nm or more.

[0153] (Number density)

[0154] In this invention, the number density of granular oxides is 4.0 particles / μm. 2 The above describes how, by controlling the number density within a certain range, granular oxides can be abundantly dispersed on the surface of the steel sheet. These granular oxides function effectively as hydrogen trapping sites to inhibit hydrogen intrusion during corrosive environments and / or annealing processes in manufacturing. Furthermore, they function effectively as trapping sites for Zn that may intrude during hot stamping or welding of the coated steel sheet. On the other hand, if the number density is below 4.0 particles / μm... 2 If the number density of hydrogen and / or Zn trapping sites is insufficient, the granular oxide may not function adequately as a hydrogen and / or Zn trapping site, 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. A higher quantity of granular oxides is preferred; therefore, there is no particular upper limit to the number density of granular oxides, but for example, it could be 100.0 particles / μm. 2 the following.

[0155] The average particle size and number density of the granular oxides were determined using a scanning electron microscope (SEM). The specific determination 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) excluding the grain boundary oxides described later were selected as the observation area. The observation position for each region was set as 1.0 μm in the depth direction (the direction perpendicular to the surface of the steel plate) from the surface of the steel plate to 1.5 μm, and 1.0 μm at any position in the aforementioned SEM image in the width direction (the direction parallel to the surface of the steel plate). Next, SEM images of each selected region were extracted, and binarization was performed to separate the oxide portion from the steel portion. The total area of ​​the granular oxide portion was calculated from each binarized image, and the number of granular oxides in each binarized image was further counted. The total area and number of granular oxides in the 10 regions obtained through this operation were used to determine the average particle size (nm) of the granular oxides as the equivalent circle diameter. Furthermore, the number density of the granular oxides (particles / μm) was also calculated. 2 The number of granular oxides counted from each binarized image is equal to the average number of granular oxides. It should be noted that if only a portion of the granular oxides is observed in the observation area, i.e., if the outline of the granular oxides is not entirely within the observation area, they are not counted. Furthermore, from the viewpoint of measurement accuracy, the lower limit for counting granular oxides is set to 5.0 nm or higher.

[0156] [Grain boundary oxides]

[0157] The steel plate of the present invention may further contain grain boundary oxides in its surface layer. In the present invention, "grain boundary oxides" refers to oxides existing along the grain boundaries of the steel, excluding oxides present within the grains of the steel. In fact, because the grain boundary oxides exist planarly along the grain boundaries in the surface layer of the steel plate, they are observed as lines when a cross-section of the surface layer of the steel plate is viewed. Figure 2 and Figure 3 As an example, a grain boundary oxide 13 that appears to be linear is shown. Furthermore, in... Figure 2 and Figure 3 In the example of steel plate 11, grain boundary oxide 13 is shown below granular oxide 12, but grain boundary oxide 13 may also be formed near the surface of the base steel 14.

[0158] (Ratio A)

[0159] When observing a cross-section of the surface layer of a steel plate, the ratio A of the length of the grain boundary oxide projected onto the surface of the steel plate to the length of the surface of the steel plate can be any value from 0 to 100%. In this invention, the term "ratio A" refers to... Figure 3 and 5 As shown, this refers to the ratio of "length of grain boundary oxides projected onto the surface of the steel plate: L (=L1+L2+L3+L4)" to "length of the steel plate surface: L0" in an observation image when a cross-section of the surface layer of the steel plate 11 is observed. In one embodiment of the present invention, the ratio A is 0% or more and less than 50%. In the steel plate of the present invention, since grain boundary oxides may not be present in the surface layer of the steel plate, the ratio A may also be 0%. The ratio A may, for example, be 1% or more, 3% or more, or 5% or more. In terms of manufacturing conditions that generate more grain boundary oxides, there is a tendency for the average particle size of granular oxides to become larger. Therefore, from the viewpoint of refining the average particle size of granular oxides, the ratio A is, for example, as follows: Figure 2 and 3 As shown, the ratio is preferably less than 50%, but it can also be less than 40%, less than 30%, less than 20%, less than 10%, or 0%. In another embodiment of the invention, the ratio A is 50% or more. By controlling the ratio A to such a range, a large amount of grain boundary oxides can be present in the surface layer of the steel sheet, allowing these grain boundary oxides to function well as an escape path for hydrogen invading the steel. Therefore, by not only having a Si-Mn deficient layer but also having a relatively large amount of grain boundary oxides, the hydrogen expulsion performance of the steel sheet of the present invention can be further improved. Therefore, from the viewpoint of further improving the hydrogen expulsion performance of the steel sheet, the ratio A is, for example, as shown in the figure. Figure 4 and 5 As shown, it is preferably 50% or more, but it can also be 60% or more, 70% or more, 80% or more, 90% or more, or 100%.

[0160] Ratio A Figure 3 and 5As 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. The observation position is set to a randomly selected location. 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 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 50 μm. Next, the position of the grain boundary oxide 13 is determined from the SEM image, and the determined grain boundary oxide 13 is projected onto the surface of the steel plate 11 (in the case of a plated steel plate, at the interface between the steel plate 11 and the plating layer), and the length L of the grain boundary oxide 13 in the field of view is calculated (=L1+L2+L3+L4). Based on L0 and L obtained by this operation, the ratio A (%) in this invention is calculated as 100×L / L0. It should be noted that... Figure 3 and 5 The diagram of granular oxide 12 has been omitted for illustrative purposes.

[0161] [Composition of Oxides]

[0162] In this invention, the granular oxide and optionally the grain boundary oxide (hereinafter also simply referred to as oxide) contain, in addition to oxygen, one or more of the elements contained in the aforementioned steel plate, typically having the following composition: containing Si, O, and Fe, and further containing Mn as appropriate. More specifically, the oxide typically contains Si: 5-25%, Mn: 0-10%, O: 40-65%, and Fe: 10-30%. In addition to the aforementioned elements, the oxide may also contain elements that can be contained in the aforementioned steel plate (e.g., Cr, etc.).

[0163] [Si-Mn-deficient layer]

[0164] The steel plate of the present invention comprises a Si-Mn deficient layer having a thickness of 3.0 μm or more from the surface of the steel plate. The Si and Mn contents in the oxide-free region at half the thickness of the Si-Mn deficient layer are each less than 10% of the Si and Mn contents at the center of the steel plate thickness. By setting the Si-Mn deficient layer formed on the surface of the steel plate due to the formation of granular oxides and optionally grain boundary oxides to a thickness of 3.0 μm or more, and controlling the Si and Mn deficient rates of this Si-Mn deficient layer to be less than 10%, the amount of dissolved Si and Mn that hinders hydrogen diffusion can be sufficiently reduced. As a result, hydrogen diffusion becomes more promoted, thereby significantly improving hydrogen expulsion from the steel. Since increasing the thickness of the Si-Mn deficient layer further promotes hydrogen diffusion from the steel, the thickness of the Si-Mn deficient 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 deficient layer, but for example, the thickness of the Si-Mn deficient layer can be below 50.0 μm.

[0165] Similarly, by further reducing the Si and Mn deficiency rates of the Si-Mn deficient layer, the amount of dissolved Si and Mn in the steel can be further reduced. Therefore, the Si deficiency rate of the Si-Mn deficient layer 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 not particularly limited, but it can also be 0%. Similarly, the Mn deficiency rate of the Si-Mn deficient layer 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 not particularly limited, but it can also be 0%. In this invention, the expression "oxide-free" means that it is free not only from the above-mentioned granular oxides and grain boundary oxides, but also from any other oxides. Such oxide-free regions can be determined by cross-sectional observation using SEM and energy-dispersive X-ray spectrometer (EDS). Furthermore, when the Si-Mn deficient layer of this invention simply forms internal oxides such as granular oxides, it is impossible to control the desired thickness and composition range. As explained in detail below, it becomes important to appropriately control the internal oxidation process during manufacturing.

[0166] The thickness of the Si-Mn deficient layer is as follows: Figure 5 As shown by D in the figure, it refers to the process from the surface of the steel plate 11 (in the case of clad steel plate, the interface between the steel plate and the coating) along the thickness direction of the steel plate 11 (the direction perpendicular to the surface of the steel plate) to the point where internal oxides exist. Figure 5The distance from the farthest point of the grain boundary oxide (13) is defined as follows. In the absence of grain boundary oxides, the thickness of the Si-Mn deficient layer refers to the distance from the surface of the steel plate (or the interface between the steel plate and the coating in the case of coated steel plate) along the thickness direction of the steel plate (perpendicular to the surface of the steel plate) to the farthest point where granular oxides exist. The thickness of the Si-Mn deficient layer can be determined from the same SEM image (the length L0 of the surface) used to measure the aforementioned ratio A. Furthermore, the Si and Mn content in the oxide-free region at half the thickness of the Si-Mn deficient layer is determined as follows: 10 points randomly selected at half the thickness of the Si-Mn deficient layer as determined by the aforementioned SEM image are analyzed using a transmission electron microscope (TEM-EDS) with an energy-dispersive X-ray spectrometer, and the obtained Si and Mn concentration measurements are arithmetically averaged. Furthermore, the Si and Mn contents at the center of the steel plate thickness were determined as follows: A cross-section of the center of the plate thickness was observed using a SEM. Ten randomly selected points at the center of the plate thickness from the SEM image were analyzed using a transmission electron microscope with an energy-dispersive X-ray spectrometer (TEM-EDS). The obtained Si and Mn concentration measurements were then arithmetically averaged. Finally, the Si and Mn contents at half the thickness of the Si-Mn deficient layer were divided by the Si and Mn contents at the center of the steel plate thickness, and the resulting values ​​were expressed as percentages. These percentages were then determined as the Si and Mn deficiency rates.

[0167] <Coated steel sheet>

[0168] 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, Zn-11%Al-3%Mg-0.2%Si, Zn-11%Ni, or Zn-15%Mg.

[0169] [Composition of the coating]

[0170] The composition of the Zn-containing coating in this invention will be described. Unless otherwise specified, the "%" for element content refers to "mass %". In the numerical ranges of the coating's composition, the range indicated by "~" refers to the range including the values ​​before and after "~" as both the lower and upper limits, unless otherwise specified.

[0171] (A1: 0-60.0%)

[0172] Al is an element that improves the corrosion resistance of a coating 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%. To form a coating containing both Zn and Al, the Al content is preferably 0.01% or more, for example, 0.1% or more, 0.5% or more, 1.0% or more, or 3.0% or more. On the other hand, even with excessive Al content, the effect of improving corrosion resistance becomes saturated, therefore the Al content is preferably 60.0% or less, 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. It should be noted that from the viewpoint of improving LME resistance, the Al content is preferably 0.4% to 1.5%.

[0173] (Mg: 0-15.0%)

[0174] 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%. To form a coating containing Zn, Al, and Mg, the Mg content is preferably 0.01% or more, for example, 0.1% or more, 0.5% or more, 1.0% or more, or 3.0% or more. On the other hand, if the Mg content is excessive, the Mg will not completely dissolve in the plating bath and will float as oxides. If zinc plating is performed in this bath, the oxides may adhere to the plating surface, causing poor appearance or creating unplated areas. Therefore, the Mg content is preferably 15.0% or less, for example, 10.0% or less, or 5.0% or less.

[0175] (Fe: 0–15.0%)

[0176] Fe is included in the coating by diffusion from the steel sheet when the coated steel sheet is heat-treated after forming a Zn-containing coating on the steel sheet. Therefore, in the untreated state, Fe is not included in the coating, and the Fe content can be 0%. Furthermore, 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, the Fe content is preferably 15.0% or less, and for example, it can also be 12.0% or less, 10.0% or less, 8.0% or less, or 6.0% or less.

[0177] (Si: 0-3.0%)

[0178] 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.

[0179] 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.

[0180] 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 those related to the raw materials. 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.

[0181] 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.

[0182] 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 adhered is determined by the weight change before and after pickling, after dissolving the coating in an acid solution containing an inhibitor that suppresses corrosion of the base metal.

[0183] [tensile strength]

[0184] 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, it can be 2000 MPa or less. The tensile strength can be determined by collecting a JIS 5 tensile test piece with the length direction perpendicular to the rolling direction as the length direction, and performing the test according to JIS Z2241 (2011).

[0185] The steel sheets and coated steel sheets of the present invention, due to their high strength and excellent plating properties, resistance to hydrogen embrittlement (LME), and resistance to hydrogen embrittlement, are suitable for use in a wide range of fields such as automobiles, home appliances, and building materials, but 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 sheets of the present invention are used as automotive steel sheets, the advantages of the present invention, such as high plating properties, 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 sheets and coated steel sheets of the present invention are used as automotive steel sheets, the advantages of the present invention, such as high resistance to hydrogen embrittlement and LME, can be appropriately utilized.

[0186] <Methods for manufacturing steel plates>

[0187] 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.

[0188] 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 grinding process in which dislocations are introduced onto the surface of the cold-rolled steel sheet; and an annealing process in which the ground cold-rolled steel sheet is annealed. Alternatively, after the hot rolling process, the coiling process can be skipped, and the cold rolling process can be performed directly after pickling.

[0189] [Casting Process]

[0190] There are no particular restrictions on the conditions for the casting process. For example, as long as the smelting is carried out in a blast furnace, electric furnace, or other similar facility, and various secondary refining processes are performed, casting can be carried out using conventional continuous casting or ingot casting methods.

[0191] [Hot rolling process]

[0192] Hot-rolled steel sheets can be obtained by hot rolling the cast steel billets as described above. The hot rolling process involves directly hot rolling the cast steel billets or temporarily cooling them and then reheating and hot rolling them. In the case of reheating, the heating temperature of the steel billets can be, for example, 1100℃ to 1250℃. The hot rolling process typically includes roughing and finishing rolling. The temperature and reduction rate of each rolling stage can be appropriately adjusted according to the desired metal structure and plate thickness. For example, the finishing rolling end temperature can be set to 900℃ to 1050℃, and the finishing rolling reduction rate can be set to 10% to 50%.

[0193] [Winding process]

[0194] Hot-rolled steel sheets can be coiled at a specified temperature. The coiling temperature can be adjusted appropriately 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 uncoiling and then subjected to a specified heat treatment. Alternatively, the coiling process can be omitted, and pickling followed by the cold rolling process described later can be performed.

[0195] [Cold rolling process]

[0196] After pickling and other processes, hot-rolled steel sheets can be cold-rolled to obtain cold-rolled steel sheets. The reduction rate during cold rolling can be adjusted appropriately according to the desired metal structure and sheet thickness, for example, from 20% to 80%. After the cold rolling process, the sheet can be cooled to room temperature, for example, by air cooling.

[0197] [Grinding process]

[0198] To obtain a fine and abundant amount of granular oxides on the surface of the final steel sheet, and subsequently, to obtain optional grain boundary oxides in a desired amount, and to form a Si-Mn deficient layer with the desired thickness and composition, it is effective to perform a grinding process before annealing the cold-rolled steel sheet. This grinding process introduces a large number of dislocations into 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 into the surface of the cold-rolled steel sheet creates numerous pathways, similar to the situation at grain boundaries. Therefore, during annealing, oxygen readily diffuses (intrudes) into the interior of the steel along these dislocations, and the diffusion rates of Si and Mn also increase. As a result, it becomes possible to promote the combination of oxygen with Si and / or Mn within the steel to form granular oxides, and subsequently, optional grain boundary oxides. Furthermore, the formation of these internal oxides promotes a decrease in the surrounding Si and Mn concentrations, thus also promoting the formation of a Si-Mn deficient layer with the desired thickness and composition. There are no particular limitations on the grinding process, but for example, a powerful grinding brush can be used to achieve a grinding depth of 10–200 g / m. 2 The grinding process is carried out under the following conditions: grinding is performed on the surface of cold-rolled steel sheet. The amount of grinding obtained using a high-powered grinding brush can be adjusted by any suitable method known to those skilled in the art. Although there are no particular limitations, adjustments can be made, for example, by appropriately selecting the number of high-powered grinding brushes, the rotational speed, the brush pressure, and the coating liquid used. By performing such a grinding process, the desired granular oxide and optionally grain boundary oxide can be formed in the annealing process described later, and a Si-Mn deficient layer with the desired thickness and composition (i.e., a thickness of 3.0 μm or more) and Si and Mn deficiency rates of less than 10% can be reliably and effectively formed on the surface of the steel sheet.

[0199] [Annealing process]

[0200] The cold-rolled steel sheet after the above-described grinding process is then annealed. Annealing is preferably performed with tension applied to the cold-rolled steel sheet along the rolling direction. In particular, in the region where the annealing temperature is 500°C or higher, it is preferable to increase the tension during annealing compared to other regions. Specifically, in the region where the annealing temperature is 500°C or higher, it is preferable to perform annealing with a tension of 3–150 MPa, particularly 15–150 MPa, applied to the cold-rolled steel sheet along the rolling direction. Applying tension during annealing allows for more effective introduction of a large number of dislocations onto the surface of the cold-rolled steel sheet. Therefore, oxygen diffuses (intrudes) more easily along these dislocations into the interior of the steel during annealing, and the diffusion rates of Si and Mn also increase, thus making it easier for oxides to form inside the steel sheet. As a result, it becomes advantageous for the increase in the number density of granular oxides and the refinement of the average grain size, the formation of grain boundary oxides at the desired ratio, and the formation of a Si-Mn deficient layer with the desired thickness and composition.

[0201] The holding temperature for the annealing process is preferably 700–870°C. From the viewpoint of generating fine and abundant granular oxides and suppressing the formation of grain boundary oxides within a range where the ratio A is less than 50%, the holding temperature for the annealing process is preferably 700–780°C, more preferably 720–760°C. If the holding temperature for the annealing process is less than 700°C, granular oxides may not be sufficiently generated, and resistance to hydrogen intrusion may become insufficient. On the other hand, from the viewpoint of generating fine and abundant granular oxides and generating abundant grain boundary oxides in a manner where the ratio A is 50% or more, the holding temperature for the annealing process is preferably more than 780°C and less than 870°C, more preferably 800–850°C. On the other hand, if the holding temperature for the annealing process exceeds 870°C, granular oxides may not be sufficiently generated, and resistance to hydrogen intrusion and thus resistance to hydrogen embrittlement may become insufficient, and consequently, resistance to LME may become insufficient. Furthermore, if the holding temperature of the annealing process exceeds 900°C, an external oxide layer may form on the steel plate surface, resulting in insufficient plating. The heating rate up to the aforementioned holding temperature is not particularly limited, but can be 1–10°C / second. Alternatively, the heating can be performed in two stages: a first heating rate of 1–10°C / second and a second heating rate of 1–10°C / second, different from the first heating rate.

[0202] The holding time at the aforementioned annealing holding temperature is preferably more than 50 seconds and less than 150 seconds, more preferably 80 to 120 seconds. If the holding time is less than 50 seconds, granular oxides and optionally grain boundary oxides may not be sufficiently formed, potentially resulting in insufficient resistance to hydrogen embrittlement and LME. On the other hand, if the holding time exceeds 150 seconds, the granular oxides may coarsen, potentially leading to insufficient resistance to hydrogen embrittlement and LME.

[0203] From the viewpoint of generating fine and abundant granular oxides, the dew point of the atmosphere in the annealing process is preferably -20 to 10°C, more preferably -10 to 5°C. If the dew point is too low, an external oxide layer may form on the surface of the steel plate without sufficient formation of internal oxides, potentially resulting in insufficient plating properties, resistance to hydrogen embrittlement, and resistance to LME. On the other hand, increasing the dew point can promote the formation of grain boundary oxides, but if the dew point is too high, Fe oxides may form as external oxides on the surface of the steel plate, resulting in insufficient plating properties. Furthermore, the granular oxides may coarsen, leading to insufficient resistance to hydrogen embrittlement and / or resistance to LME. In addition, the atmosphere in the annealing process can be a reducing atmosphere, more specifically a reducing atmosphere containing nitrogen and hydrogen, such as a reducing atmosphere with 1 to 10% hydrogen (e.g., 4% hydrogen and the balance nitrogen).

[0204] Furthermore, it is effective to remove the internal oxide layer (typically containing grain boundary oxides) of the steel sheet during the annealing process. Sometimes, an internal oxide layer forms on the surface of the steel sheet during the aforementioned rolling process, especially the hot rolling process. The internal oxide layer formed in such a rolling process may hinder the formation of granular oxides during the annealing process, therefore, this internal oxide layer is preferably removed 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, preferably 0.3 μm or less, more preferably 0.2 μm or less, and even more preferably 0.1 μm or less.

[0205] By performing the above-mentioned processes, it is possible to obtain a steel plate that contains sufficiently fine and abundant granular oxides in the surface layer of the steel plate, and includes a Si-Mn deficient layer with the desired thickness and composition.

[0206] It should be noted that when a process is set up as the pre-annealing stage, in which oxidation is performed using an oxidation zone at an air ratio or air-fuel ratio of 0.9 to 1.4, followed by reduction, the granular oxides grow excessively in the oxidation process, exceeding an average particle size of 300 nm. As a result, these granular oxides do not function adequately as hydrogen trapping sites and / or Zn trapping sites, making it difficult to obtain good resistance to hydrogen embrittlement and / or LME.

[0207] <Manufacturing Method of Coated Steel Sheet>

[0208] 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.

[0209] The coated steel sheet of the present invention can be obtained by a coating process in which a coating containing Zn is formed on a steel sheet manufactured as described above.

[0210] [Plating Process]

[0211] The plating process can be performed according to methods known to those skilled in the art. For example, the plating process can be performed by hot-dip plating or 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%, and Si: 0–3%, with the remainder consisting of Zn and impurities. More specifically, the conditions of the plating process can be appropriately set, for example, to form 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.

[0212] Example

[0213] 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.

[0214] In the following examples, steel sheets with a grain boundary oxide ratio A of 0% or more and less than 50% were manufactured in Example X, and steel sheets with a grain boundary oxide ratio A of 50% or more were manufactured in Example Y. The plating properties, hydrogen embrittlement resistance, and LME resistance of the steel sheets manufactured in each example were investigated.

[0215] (Example X)

[0216] (Preparation of steel plate samples)

[0217] Molten steel with adjusted composition was cast to form a billet, which 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 to the depth (μm) of the internal oxide layer 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.

[0218] Next, for each cold-rolled steel sheet, after applying a NaOH aqueous solution, a high-powered grinding brush is used at a speed of 10–200 g / m². 2 The surface of the cold-rolled steel sheet was ground with a grinding amount (sample No. 135 was not ground). Afterwards, each steel sheet sample was prepared by annealing at the dew point, holding temperature, and holding time (mainly holding temperature of 700–780°C and holding time exceeding 50 seconds but less than 150 seconds) as shown in Table 1 (annealing atmosphere: 4% hydrogen and balance nitrogen). 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, the cold-rolled steel sheet was annealed under a tension of 1 MPa or more along the rolling direction. In the region with an annealing temperature of 500°C or higher, annealing was performed under a higher tension along the rolling direction than in other regions, specifically 3–150 MPa (sample No. 134 did not use such tension). Table 1 shows the presence or absence of grinding using a high-powered grinding brush and the conditions of annealing treatment (the presence or absence of tension application of 3–150 MPa in the region with an annealing temperature above 500°C, dew point (°C), holding temperature (°C), and holding time (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, and tensile tests were conducted according to JIS Z2241 (2011). The results showed that for Nos. 116 and 118, the tensile strength was below 440 MPa, while for the other specimens, it was above 440 MPa.

[0219] (Analysis of the surface layer of the steel plate sample)

[0220] 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: in the depth direction (perpendicular to the surface of the steel plate), 1.0μm was defined as a distance of 0.2 to 1.2μm from the surface of the steel plate; in the width direction (perpendicular to the surface of the steel plate), 1.0μm was defined as any position within the SEM image. It should be noted that regions without grain boundary oxides were selected. Next, 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 images, and the number of granular oxides within the SEM images was further counted. The average grain size and number density of the granular oxides, as the equivalent circle diameter, were determined from the area and number of granular oxides in the 10 binarized images obtained through this operation. The average particle size (nm) and number density (particles / μm) of the granular oxides for each steel plate sample were calculated. 2 The values ​​are shown in Table 1. It should be noted that in Table 1, when there are no granular oxides in the SEM image (number density = 0), the average particle size is recorded as "-".

[0221] Furthermore, the ratio A for each steel plate specimen was determined by cross-sectional observation of the aforementioned embedded specimens. Specifically, the positions of grain boundary oxides were determined in a 150 μm wide (=L0) SEM image, and the determined grain boundary oxides were projected onto the surface of the steel plate to calculate the length L of the grain boundary oxides within the field of view. Based on L0 and L obtained in this manner, the ratio A(%) was calculated as 100×L / L0. The ratio A(%) of granular oxides for each steel plate specimen is shown in Table 1.

[0222] The thickness of the Si-Mn deficient layer was determined as follows: In a SEM image at measurement ratio A, the distance from the surface of the steel plate to the farthest point where grain boundary oxides (or granular oxides in the absence of grain boundary oxides) are measured, moving along the thickness direction of the steel plate (perpendicular to the surface of the steel plate). Furthermore, the Si and Mn content in the oxide-free region at half the thickness of the Si-Mn deficient layer was determined as follows: Ten randomly selected oxide-free points at half the thickness of the Si-Mn deficient layer determined by the above SEM image were analyzed using TEM-EDS, and the measured values ​​of Si and Mn concentrations were arithmetically averaged. Additionally, the Si and Mn content at the center of the steel plate thickness was determined as follows: A cross-section of the center of the plate thickness was observed using SEM, and TEM-EDS was analyzed at ten randomly selected points at the center of the plate thickness from the SEM image, and the measured values ​​of Si and Mn concentrations were arithmetically averaged. Finally, the Si and Mn contents at the halfway point of the Si-Mn deficient layer thickness were divided by the Si and Mn contents at the center of the steel plate thickness, and the resulting values ​​were expressed as percentages. These values ​​were then determined as the Si and Mn deficiency rates. Furthermore, the composition of granular oxides and grain boundary oxides was analyzed for each steel plate sample. The results showed that each oxide contained Si, O, and Fe, and most oxides also contained Mn. Therefore, the composition of each oxide consisted of Si: 5–25%, Mn: 0–10%, O: 40–65%, and Fe: 10–30%.

[0223] (Preparation of galvanized steel sheet samples)

[0224] After cutting each steel plate sample into 100mm × 200mm dimensions, coated steel plate samples were prepared by performing a coating process to form the coating types shown in Table 1. In Table 1, 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 recorded in the table)". In the hot-dip galvanizing process, the cut samples were immersed in a hot-dip galvanizing bath at 440°C for 3 seconds. After immersion, they were pulled out at 100mm / second, and the coating adhesion was controlled to 50g / m² using N2 wiping gas. 2 For plating type A, alloying treatment is then performed at 460°C.

[0225] (Compositional analysis of the coating)

[0226] The composition of the coating was determined as follows: a sample cut into 30 mm × 30 mm pieces was immersed in a 10% HCl aqueous solution containing an inhibitor (IBIT manufactured by Asahi Chemical Industry). After the coating was acid-washed and peeled off, the coating components dissolved in the aqueous solution were determined by ICP luminescence spectrophotometry.

[0227] (Coating performance evaluation)

[0228] For each coated steel sheet sample, the coating performance was evaluated by measuring the area ratio of the uncoated portion of the steel sheet surface. Specifically, a 1mm × 1mm area of ​​the surface of each coated steel sheet sample with a coating was observed using an optical microscope. The observed image was used to distinguish between the coated portion (coated portion) and the uncoated portion (uncoated portion). The area ratio of the uncoated portion (area of ​​uncoated portion / area of ​​the observed image) was calculated, and the coating performance was evaluated using the following criteria. The results are shown in Table 1. A indicates pass, and B indicates fail.

[0229] Rating A: Below 5.0%

[0230] Rating B: Over 5.0%

[0231] (LME resistance evaluation)

[0232] Each 100×100mm coated steel sheet sample was used for spot welding. Two coated steel sheets cut to 50mm×100mm were prepared. These two Zn-based coated steel sheet samples were spot welded using a dome-radius type welding electrode with a front diameter of 8mm, at an angle of 7°, a pressure of 3.0kN, an energizing time of 0.5 seconds, and a 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. AAA, AA, and A are considered acceptable, and B is considered unacceptable.

[0233] AAA rating: LME crack length exceeds 0 μm but is less than 150 μm

[0234] Evaluation AA: LME crack length exceeding 150μm but less than 300μm

[0235] Evaluation A: LME crack length exceeds 300μm but is less than 500μm

[0236] Rating B: LME crack length exceeds 500μm

[0237] (Evaluation of resistance to hydrogen embrittlement)

[0238] 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. Next, a composite cyclic corrosion test according to JASO (M609-91) was performed, and the amount of diffusible hydrogen after 120 cycles was determined by the temperature-release method. Specifically, the coated steel sheet sample was heated to 400°C in a furnace equipped with a gas chromatograph, and the total amount of hydrogen released until the temperature was lowered to 250°C was measured. Based on the measured amount of diffusible hydrogen, hydrogen embrittlement resistance (hydrogen accumulation in the sample) was evaluated according to the following criteria, and the results are shown in Table 1. AA and A are considered acceptable, and B is considered unacceptable.

[0239] Rating AA: Below 0.3 ppm

[0240] Evaluation A: 0.5–0.3 ppm

[0241] Rating B: Exceeds 0.5 ppm

[0242]

[0243] Samples No. 102–108 and 120–133 exhibit high plating properties, hydrogen embrittlement resistance, and LME resistance due to suitable steel composition, average particle size and number density of granular oxides, and thickness and composition of the Si-Mn deficient layer. On the other hand, samples No. 101 and 119, due to their thick internal oxide layer before annealing, were unable to form the desired granular oxides and also failed to form the desired Si-Mn deficient layer, thus failing to achieve high hydrogen embrittlement resistance and LME resistance. Sample No. 109 had a low dew point during annealing, resulting in the formation of an external oxide layer instead of an internal oxide layer, thus also failing to achieve high plating properties, hydrogen embrittlement resistance, and LME resistance. Sample No. 110 had a high dew point during annealing, resulting in the formation of an external oxide layer; furthermore, it was unable to refine the granular oxides, thus failing to achieve high plating properties, hydrogen embrittlement resistance, and LME resistance. The high holding temperature during annealing of sample No. 111 promoted the formation of grain boundary oxides, preventing the refinement of granular oxides and resulting in low resistance to hydrogen embrittlement and LME. Sample No. 112, due to its low holding temperature during annealing, did not sufficiently form internal oxides, nor did it form the desired Si-Mn deficiency layer, thus also failing to achieve high resistance to hydrogen embrittlement and LME. Sample No. 113, due to its short holding time during annealing, also failed to sufficiently form internal oxides and the desired Si-Mn deficiency layer, thus failing to achieve high resistance to hydrogen embrittlement and LME. Sample No. 114, due to its long holding time during annealing, promoted the formation of grain boundary oxides, preventing the refinement of granular oxides and resulting in low resistance to hydrogen embrittlement and LME. Samples No. 115 and 117, due to excessive Si and Mn content, experienced external oxide growth, leading to coarsening of the granular oxide layer. Furthermore, the desired Si-Mn deficient layer was not formed, resulting in poor plating performance, hydrogen embrittlement resistance, and LME resistance. Samples No. 116 and 118, with zero Si and Mn content, failed to form an internal oxide layer and the desired Si-Mn deficient layer, thus also failing to achieve high hydrogen embrittlement resistance and LME resistance. Sample No. 134, due to insufficient application of the specified tension during annealing, did not adequately form an internal oxide layer and the desired Si-Mn deficient layer. Consequently, high hydrogen embrittlement resistance and LME resistance were not achieved. Sample No. 135, due to the lack of pre-annealing grinding, did not adequately form an internal oxide layer and the desired Si-Mn deficient layer. Consequently, high hydrogen embrittlement resistance and LME resistance were not achieved.

[0244] (Example Y)

[0245] (Preparation of steel plate samples)

[0246] Except that the holding temperature during annealing was mainly set to be above 780°C and below 870°C, the steel plate specimens were prepared under the manufacturing conditions shown in Table 2, similar to Example X. 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, and tensile tests were performed according to JIS Z2241 (2011). The results showed that for Nos. 201, 216, and 218, the tensile strength was below 440 MPa, while for the other specimens it was above 440 MPa.

[0247] (Preparation of galvanized steel sheet samples)

[0248] After cutting each steel plate sample into 100mm × 200mm dimensions, coated steel plate samples were prepared by performing a coating process to form the coating types 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 recorded in the table)". In the hot-dip galvanizing process, the cut samples were immersed in a hot-dip galvanizing bath at 440°C for 3 seconds. After immersion, they were pulled out at 100mm / second, and the coating adhesion was controlled to 50g / m² using N2 wiping gas. 2 For plating type A, alloying treatment is then performed at 460°C.

[0249] The surface analysis, coating composition analysis, coating performance evaluation, LME resistance evaluation, and hydrogen embrittlement resistance evaluation of the steel plate specimens are related to Example X as described above.

[0250]

[0251] Samples No. 202–208 and 220–233 exhibit high plating properties, LME resistance, and hydrogen embrittlement resistance due to suitable steel plate composition, average particle size and number density of granular oxides, and thickness and composition of the Si-Mn deficient layer. Sample No. 201, due to insufficient carbon content, not only failed to achieve sufficient strength but also failed to form the desired granular oxides and the desired Si-Mn deficient layer, thus lacking high resistance to hydrogen embrittlement and LME. Sample No. 209, with its low dew point during annealing, formed an external oxide layer instead of an internal oxide layer, resulting in low plating properties, hydrogen embrittlement resistance, and LME resistance. Sample No. 210, with its high dew point during annealing, formed an external oxide layer but was unable to refine the granular oxides, also failing to achieve high plating properties, hydrogen embrittlement resistance, and LME resistance. Sample No. 211, due to its high holding temperature during annealing, generated external oxides but did not sufficiently form granular oxides, and also failed to form the desired Si-Mn deficiency layer. Therefore, it did not achieve high plating performance, hydrogen embrittlement resistance, or LME resistance. Sample No. 212, due to the failure to apply the specified tension during annealing, did not form the desired Si-Mn deficiency layer and therefore did not achieve high hydrogen embrittlement resistance. Sample No. 213, due to its short holding time during annealing, did not sufficiently form internal oxides, and also failed to form the desired Si-Mn deficiency layer. Therefore, it did not achieve high hydrogen embrittlement resistance or LME resistance. Samples No. 214 and 234, due to their long holding times during annealing, were unable to refine the granular oxides and also failed to form the desired Si-Mn deficiency layer. Therefore, they did not achieve high hydrogen embrittlement resistance or LME resistance. Samples No. 215 and 217, due to excessive Si and Mn content, experienced external oxide growth, leading to coarsening of the granular oxide layer. Furthermore, the desired Si-Mn deficient layer was not formed, resulting in poor plating performance, hydrogen embrittlement resistance, and LME resistance. Samples No. 216 and 218, with zero Si and Mn content, failed to form an internal oxide layer and the desired Si-Mn deficient layer, thus also failing to achieve high hydrogen embrittlement resistance and LME resistance. Sample No. 219, due to its thick internal oxide layer before annealing, failed to form the desired internal oxide layer after annealing and also failed to form the desired Si-Mn deficient layer, resulting in poor hydrogen embrittlement resistance and LME resistance. Sample No. 235, lacking pre-annealing grinding, did not sufficiently form an internal oxide layer and the desired Si-Mn deficient layer. Consequently, high hydrogen embrittlement resistance and LME resistance were not achieved.

[0252] Industrial availability

[0253] 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 crash safety and long service life. Therefore, the present invention can be considered an invention of extremely high industrial value.

[0254] Explanation of symbols

[0255] 1. Steel plate

[0256] 2. External oxide layer

[0257] 3. Base steel

[0258] 11 Steel Plate

[0259] 12 Granular oxides

[0260] 13 Grain boundary oxides

[0261] 14. Base steel

Claims

1. A steel sheet having the following composition consisting of, in mass %: Si: 0.2 to 3.0 %, C：0.05～0.40％、 Mn: 0.1 to 5.0 %, sol. Al: 0 % or more and less than 0.4000 %, P: 0.0300 % or less, S: 0.0300 % or less, N: 0.0100 % or less, Ti: 0 to 0.150 %, B：0～0.010％、 Nb: 0 to 0.150 %, Ni: 0 to 2.00 %, V：0～0.150％、 Cr：0～2.00％、 Cu: 0 to 2.00 %, Mo: 0 to 1.00 %, Ca: 0 to 0.100 %, W：0～1.00％、 Mg: 0 to 0.100 %, Hf: 0 to 0.100 %, and Zr：0～0.100％、 REM: 0 to 0.100 %, the remainder consisting of Fe and impurities, a granular type oxide is contained in a surface layer of the steel sheet, wherein an average particle diameter of the granular type oxide is 300 nm or less, the steel sheet contains a Si-Mn deficient layer having a thickness of 3.0 μm or more from a surface of the steel sheet, The number density of the granular oxide is 4.0 / μm 2 The above, Si and Mn contents of an oxide-free region at a position of 1 / 2 of the thickness of the Si-Mn deficient layer are each 10 % or less of Si and Mn contents at a center portion of the sheet thickness of the steel sheet, the surface layer of the steel sheet refers to a region up to 50 μm in a thickness direction from the surface of the steel sheet, and in the case of a plated steel sheet, refers to a region up to 50 μm in a thickness direction from an interface of the steel sheet and a plated layer, the granular type oxide refers to an oxide dispersed in a granular form within a grain or on a grain boundary of the steel, the granular form refers to a state of being present separately from each other within a steel matrix, and refers to a length-width ratio of 1.0 to 5.0, the thickness of the Si-Mn deficient layer refers to a distance from the surface of the steel sheet to a most distant position where an internal oxide exists in a case where a thickness direction of the steel sheet, that is, a direction perpendicular to the surface of the steel sheet is advanced from the surface of the steel sheet, and in the case of a plated steel sheet, refers to a distance from the surface of the steel sheet to a most distant position where an internal oxide exists in a case where a thickness direction of the steel sheet, that is, a direction perpendicular to the surface of the steel sheet is advanced from an interface of the steel sheet and a plated layer. the average particle diameter of the granular type oxide is 200 nm or less.

2. The steel sheet according to claim 1, wherein, 4. The steel sheet according to claim 1 or 2, further containing a grain boundary type oxide in a surface layer of the steel sheet, 3. The steel sheet according to claim 1 or 2, wherein, The number density of the granular oxide is 10.0 / μm 2 The above. the grain boundary type oxide refers to an oxide present along a grain boundary of the steel, and does not include an oxide present within a grain of the steel.

5. The steel sheet according to claim 4, in which a ratio A of a length of the grain boundary type oxide projected to the surface of the steel sheet with respect to a length of the surface of the steel sheet is 50 % or more in a case where a cross section of the surface layer of the steel sheet is observed. the ratio A is 80 % or more.

6. The steel sheet according to claim 5, wherein, 7. A plated steel sheet having a plated layer containing Zn on the steel sheet according to any one of claims 1 to 6. the plated layer has a composition consisting of Zn-(0.3 to 1.5) % Al.

8. The galvanized steel sheet according to claim 7, wherein, ​

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