Alloyed hot-dip galvanized steel sheet

Abstract

Provided is a high-strength plated steel sheet having high hydrogen embrittlement resistance. Provided is an alloyed hot-dip galvanized steel sheet including a steel sheet containing C: 0.05 to 0.40%, Si: 0.2 to 3.0%, Mn: 0.1 to 5.0%, and sol. Al: 0.4 to 1.50%, and a zinc alloy layer containing Fe: 5.0 to 15.0% and Al: 0.01 to 1.0% attached to at least one surface of the steel sheet, wherein an internal oxidation layer containing grain boundary type oxides is present in a surface layer of the steel sheet, and a ratio A of a length of the grain boundary type oxides projected on an interface between the steel sheet and the zinc alloy layer to a length of the interface is 50% or more and 100% or less when a cross section of the surface layer of the steel sheet is observed, and the surface layer of the steel sheet contains a surface layer deficiency layer composed of a steel free of oxides at a depth of 1 / 2 of an average depth of the internal oxidation layer calculated from a cross-sectional SEM image of the steel sheet, and satisfies Si ≤ 0.6% and Al ≥ 0.05% in terms of mass%. 2 An alloyed hot-dip galvanized steel sheet in which an alloyed hot-dip galvanized layer containing Fe: 5.0 to 15.0% and Al: 0.01 to 1.0% is attached to at least one surface of the steel sheet, wherein an internal oxidation layer containing grain boundary type oxides is present in a surface layer of the steel sheet, and a ratio A of a length of the grain boundary type oxides projected on an interface between the steel sheet and the alloyed hot-dip galvanized layer to a length of the interface is 50% or more and 100% or less when a cross section of the surface layer of the steel sheet is observed, and the surface layer of the steel sheet contains a surface layer deficiency layer composed of a steel free of oxides at a depth of 1 / 2 of an average depth of the internal oxidation layer calculated from a cross-sectional SEM image of the steel sheet, and satisfies Si ≤ 0.6% and Al ≥ 0.05% in terms of mass%.

Description

Technical Field

[0001] This invention relates to alloyed hot-dip galvanized steel sheets. More specifically, this invention relates to high-strength alloyed hot-dip galvanized steel sheets with high resistance to LME and hydrogen embrittlement. Background Technology

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

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

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

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

[0006] Existing technical documents

[0007] Patent documents

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

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

[0010] The problem that the invention aims to solve

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

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

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

[0014] In view of the actual situation, the present invention aims to provide a high-strength alloyed hot-dip galvanized steel sheet with high resistance to LME and hydrogen embrittlement.

[0015] Methods for solving problems

[0016] The inventors have discovered that, in order to solve the above-mentioned problems, it is important to form oxides in the surface layer of the steel plate, i.e., inside the steel plate, and to further control the morphology of the oxides present in the surface layer of the steel plate. More specifically, the inventors have discovered that, as the morphology of the oxides contained in the internal oxide layer, by forming a large amount of grain boundary oxides present at the crystal boundaries of the metal structure, these grain boundary oxides function as a pathway for the removal of hydrogen penetrating into the steel, thereby achieving high resistance to hydrogen embrittlement; furthermore, by forming a layered region (sometimes called a surface-deficient layer) with a low Si and high Al composition in the metal structure at half the depth of the internal oxide layer, high LME performance can be obtained.

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

[0018] (1) An alloyed hot-dip galvanized steel sheet, comprising a steel sheet and an alloyed hot-dip galvanized layer, wherein the steel sheet has the following composition: containing, by mass %

[0019] C: 0.05~0.40%

[0020] Si: 0.2-3.0%

[0021] Mn: 0.1–5.0%

[0022] sol.Al: 0.4–1.50%

[0023] P: below 0.0300%

[0024] S: below 0.0300%

[0025] N: below 0.0100%

[0026] B: 0~0.010%

[0027] Ti: 0~0.150%

[0028] Nb: 0~0.150%

[0029] V: 0~0.150%

[0030] Cr: 0–2.00%

[0031] Ni: 0~2.00%

[0032] Cu: 0–2.00%

[0033] Mo: 0–1.00%

[0034] W: 0~1.00%

[0035] Ca: 0–0.100%

[0036] Mg: 0–0.100%

[0037] Zr: 0~0.100%

[0038] Hf: 0~0.100% and

[0039] REM: 0–0.100%, the remainder consists of Fe and impurities.

[0040] The alloyed hot-dip galvanized layer has a concentration of 10-100 g / m 2 Attached to at least one surface of the aforementioned steel plate,

[0041] It contains, by mass percent, 5.0–15.0% Fe and 0.01–1.0% Al, with the remainder consisting of Zn and impurities.

[0042] The surface of the aforementioned steel plate contains an internal oxide layer comprising grain boundary oxides.

[0043] When observing the cross-section of the surface layer of the aforementioned steel plate, the ratio A of the length of the grain boundary oxide projected onto the interface between the steel plate and the alloyed hot-dip galvanized layer to the length of the interface is 50% or more and 100% or less.

[0044] The steel plate has a surface layer lacking a content of Si ≤ 0.6% and Al ≥ 0.05% by mass, which is located at a depth of 1 / 2 the average depth of the internal oxide layer.

[0045] (2) The alloyed hot-dip galvanized steel sheet according to (1), wherein,

[0046] The aforementioned ratio A is over 90%.

[0047] (3) The alloyed hot-dip galvanized steel sheet according to (1) or (2), wherein,

[0048] The alloyed hot-dip galvanized layer contains oxides with a particle size of 0.1 to 1.5 μm at a number density of 1 to 10 per (5 μm × 5 μm).

[0049] Invention Effects

[0050] According to the present invention, by forming a large amount of grain boundary oxides in the surface layer of the steel sheet, these grain boundary oxides can function as a pathway for the removal of hydrogen intruding into the steel. As a result, the intruding hydrogen can be released, reducing the amount of hydrogen accumulated in the steel and significantly improving resistance to hydrogen embrittlement. Furthermore, according to the present invention, by forming a layered region with low Si and high Al composition (sometimes referred to as a "surface-deficient layer") at half the depth of the internal oxide layer, Al also functions as a trapping site for Zn intruding into the steel during hot stamping or welding, significantly suppressing the amount of intruding Zn and further improving resistance to LME. Therefore, through the present invention, high resistance to LME and hydrogen embrittlement can be obtained for high-strength alloyed hot-dip galvanized steel sheets. Attached Figure Description

[0051] Figure 1 A schematic diagram showing a cross-section of a steel plate with an external oxide layer.

[0052] Figure 2 A schematic diagram showing a cross-section of a steel plate as illustrated in this embodiment.

[0053] Figure 3 This is a schematic diagram used to illustrate the determination of ratio A in this embodiment.

[0054] Figure 4 An exemplary schematic diagram showing the alloyed hot-dip galvanized steel sheet of this embodiment. Detailed Implementation

[0055] <steel plate>

[0056] The alloyed hot-dip galvanized steel sheet of the present invention is characterized in that it comprises a steel sheet and an alloyed hot-dip galvanized layer, said steel sheet having the following composition: containing, by mass %

[0057] C: 0.05~0.40%

[0058] Si: 0.2-3.0%

[0059] Mn: 0.1–5.0%

[0060] sol.Al: 0.4–1.50%

[0061] P: below 0.0300%

[0062] S: below 0.0300%

[0063] N: below 0.0100%

[0064] B: 0~0.010%

[0065] Ti: 0~0.150%

[0066] Nb: 0~0.150%

[0067] V: 0~0.150%

[0068] Cr: 0–2.00%

[0069] Ni: 0~2.00%

[0070] Cu: 0–2.00%

[0071] Mo: 0–1.00%

[0072] W: 0~1.00%

[0073] Ca: 0–0.100%

[0074] Mg: 0–0.100%

[0075] Zr: 0~0.100%

[0076] Hf: 0~0.100% and

[0077] REM: 0–0.100%, the remainder consists of Fe and impurities.

[0078] The alloyed hot-dip galvanized layer has a concentration of 10-100 g / m 2 Attached to at least one surface of the aforementioned steel plate,

[0079] It contains, by mass percent, 5-15% Fe and 0.01-1% Al, with the remainder consisting of Zn and impurities.

[0080] The surface of the aforementioned steel plate contains an internal oxide layer comprising grain boundary oxides.

[0081] When observing the cross-section of the surface layer of the aforementioned steel plate, the ratio A of the length of the grain boundary oxide projected onto the interface between the steel plate and the alloyed hot-dip galvanized layer to the length of the interface is 50% or more and 100% or less.

[0082] The steel plate has a surface layer lacking a content of Si ≤ 0.6% and Al ≥ 0.05% by mass, which is located at a depth of 1 / 2 the average depth of the internal oxide layer.

[0083] Alloyed hot-dip galvanized steel sheets are obtained by performing an alloying treatment after hot-dip galvanizing. First, we will explain the steel sheets, particularly high-strength steel sheets, sometimes used in alloyed hot-dip galvanized steel sheets. In the manufacture of steel sheets, especially high-strength steel sheets, after rolling (typically hot-rolled and cold-rolled) a steel billet with a specified composition, annealing is generally performed to obtain the desired microstructure, etc. In this annealing process, easily oxidized components in the steel sheet (e.g., Si, Mn, Al) combine with oxygen in the annealing atmosphere, thereby forming a layer containing oxides near the surface of the steel sheet. 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), thus failing to adequately ensure the adhesion between the steel and the coating, and sometimes resulting in uncoated areas where no coating is formed.

[0084] In contrast, such as Figure 2 As illustrated, the steel sheet 11 included in the alloyed hot-dip galvanized steel sheet of the present invention is preferably 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 fine granular oxides 12, coarse granular oxides 15, and grain boundary oxides 13 within the base steel 14. Therefore, in the case where a coating is formed on the surface of the steel plate 11, the steel plate 11 of this embodiment, with oxides 12, coarse granular oxides 15, and grain boundary oxides 13 formed within the base steel 14, is similar to... Figure 1 Compared to a steel sheet 1 with an external oxide layer 2, the interdiffusion of the coating component and the steel component is sufficiently generated, resulting in high plating performance. Therefore, the inventors have found that, from the viewpoint of obtaining high plating performance, it is effective to control the conditions during the annealing process to form oxides inside the steel sheet. It should be noted that, when used with steel sheets, the term "high plating performance" means that when a plating process is performed on the steel sheet, a coating can be formed with minimal (e.g., 5.0% or less) or no uncoated portions (areas without a coating). Furthermore, when used with coated steel sheets, the term "high plating performance" means a coated steel sheet with extremely minimal (e.g., 5.0% or less) or no uncoated portions. From the above viewpoint of plating performance, the less external oxide layer in the steel sheet 11 of this embodiment, the better; however, an external oxide layer is permissible as long as high plating performance is achieved.

[0085] 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 known to be generated during corrosion. Moreover, this hydrogen penetrates deeper than the surface layer of the steel, segregating at the martensite grain boundaries in the steel structure, causing hydrogen embrittlement (delayed fracture) by embrittlement of the grain boundaries. Because martensite is a hard structure, it is highly sensitive to hydrogen and prone to hydrogen embrittlement. Such cracking can become a problem during the processing of the steel sheet. Therefore, to prevent hydrogen embrittlement, in high-strength steel sheets used in atmospheric corrosion environments, reducing the amount of hydrogen accumulated in the steel, and more specifically, reducing the amount of hydrogen accumulated deeper than the surface layer of the steel sheet, is effective. The inventors have discovered that by controlling the morphology of oxides present on the surface of a steel plate, and more specifically, by including "fine-grained oxides" with a specified range of particle size and number density as oxides, these fine-grained oxides function as trapping sites for hydrogen intrusion under corrosive conditions in the surface region of the steel plate, thereby reducing the amount of hydrogen accumulation in the steel plate used in corrosive environments. Furthermore, by including "coarse-grained oxides" with a specified range of particle size and number density as oxides, these coarse-grained oxides function as trapping sites for hydrogen intrusion under corrosive conditions in the surface region of the steel plate, further reducing the amount of hydrogen accumulation in the steel plate used in corrosive environments. Moreover, by coexisting with "grain boundary oxides" present in a specified ratio, these grain boundary oxides function as pathways for the release of intruded hydrogen, thereby reducing the amount of hydrogen accumulation in the steel plate used in corrosive environments not only by inhibiting hydrogen intrusion but also by promoting the release of intruded hydrogen out of the system. 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 can sufficiently suppress hydrogen embrittlement cracking.

[0086] The inventors conducted a detailed analysis of the relationship between the morphology of oxides and their effectiveness as hydrogen trapping sites, and found that: Figure 2As shown, it is effective to have a large quantity of fine granular oxides 12 dispersed in a granular manner on the surface of the base steel 14. Furthermore, it has been found that having a large quantity of coarse granular oxides 15 dispersed in a granular manner on the surface of the base steel 14 is even more effective. While not bound by a specific theory, it is believed that the hydrogen capture function of oxides in the steel sheet relative to intruding hydrogen is positively correlated with the surface area of ​​the oxides. That is, by having fine oxides dispersed in a large quantity and discretely on the surface of the steel sheet, the surface area of ​​the oxides in the surface of the steel sheet increases, thereby improving the hydrogen capture function. Furthermore, it is believed that in cases where excessive hydrogen intrusion prevents the capture of fine oxides, coarse oxides have a relatively large capacity and can capture a larger amount of hydrogen, thus also capturing excessive hydrogen, further improving the hydrogen capture function. Therefore, the inventors have discovered that, from the viewpoint of obtaining high resistance to hydrogen embrittlement, it is important to control the conditions during the manufacture of the steel sheet, particularly during annealing, so that a large amount of fine-grained and coarse-grained oxides are present, functioning as hydrogen trapping sites when placed in a corrosive environment. It has been found that when a hot-dip galvanized layer is applied to the surface of the base steel and alloying treatment is performed, at least a portion of the fine-grained and coarse-grained oxides present in the surface layer of the base steel remain in the alloyed hot-dip galvanized layer and can function as hydrogen trapping sites. It should be noted that the metallic structure of the surface layer of the steel sheet typically consists of a softer metallic structure than the interior of the steel sheet (e.g., at 1 / 8 or 1 / 4 of the sheet thickness), therefore, even if hydrogen is present in the surface layer of the steel sheet, hydrogen embrittlement cracking is not particularly problematic. In addition, alloyed hot-dip galvanized layers are typically composed of a metal structure that is softer than the interior of the steel sheet (e.g., at 1 / 8 or 1 / 4 of the sheet thickness), so hydrogen embrittlement is not particularly a problem even if hydrogen is present in the alloyed hot-dip galvanized layer.

[0087] Furthermore, the inventors conducted a detailed analysis of the relationship between the morphology of oxides and their effectiveness as hydrogen removal pathways, and found that: Figure 2As shown, it is effective to have a large amount of grain boundary oxides 13 present at the grain boundaries in the surface layer of the base steel 14. It was found that the large presence of grain boundary oxides 13 ensures a pathway for hydrogen in the steel to escape from the system, effectively releasing hydrogen infiltrating the steel along the grain boundaries. Furthermore, it was found that if the grain boundary oxides are present deeper into the steel plate, more hydrogen can be released from the interior of the steel plate to the outside of the system, further reducing the amount of hydrogen accumulated in the steel plate. Therefore, by having the aforementioned granular oxides coexist with the grain boundary oxides, hydrogen embrittlement resistance can be greatly improved. It was found that even when a hot-dip galvanized layer is applied to the surface of the base steel and alloying treatment is performed, at least a portion of the grain boundary oxides present in the surface layer of the base steel remains in the base steel further below the alloyed hot-dip galvanized layer, functioning as a hydrogen escape pathway.

[0088] On the other hand, if a coated steel sheet with a Zn-containing coating on its surface is subjected to hot stamping or welding, the Zn contained in the coating may melt due to the high temperatures during processing. If the Zn melts, it penetrates into the steel. If processing continues in this state, liquid metal embrittlement (LME) cracking may occur inside the steel sheet, leading to a decrease in the fatigue properties of the steel sheet. The inventors have also discovered that if the aforementioned fine-grained oxides and coarse-grained oxides have a desired number density, not only is the resistance to hydrogen embrittlement improved, but the resistance to LME is also enhanced. More specifically, the fine-grained oxides and coarse-grained oxides have been found to function as trapping sites for Zn that intends to penetrate the steel during high-temperature processing. Thus, for example, during hot stamping, the Zn that intends to penetrate the steel is trapped by the fine-grained oxides and coarse-grained oxides on the surface of the steel sheet, which appropriately suppresses the penetration of Zn into the grain boundaries. Therefore, it was found that the presence of both fine and coarse granular oxides is important not only for improving resistance to hydrogen intrusion but also for improving resistance to LME. It was discovered that when a hot-dip galvanized layer is applied to the surface of the base steel and alloying treatment is performed, at least a portion of the fine and coarse granular oxides present in the surface layer of the base steel remain in the alloyed hot-dip galvanized layer, and can function as trapping sites for Zn to penetrate the steel during high-temperature processing.

[0089] Furthermore, fine-grained oxides, coarse-grained oxides, and grain boundary oxides are substances formed by the oxidation of easily oxidizable components (such as Si, Mn, and Al) in the steel sheet. Therefore, the composition of the steel (in other words, the metal structure) surrounding these oxides is deficient in these easily oxidizable elements compared to the original steel sheet base material. The region where the elements of this steel composition are deficient compared to the original steel sheet base material is also called a "deficient region." Layered "deficient regions" are also called "deficient layers," and further, the deficient layers existing on the surface of the steel sheet are also called "surface deficient layers." In the deficient regions, since Si is relatively easily oxidized and Al is relatively difficult to oxidize among the easily oxidizable elements, Si can exist at a low concentration and Al at a high concentration. The inventors have also discovered that if the steel composition is a low-Si and high-Al deficient region within a desired range, it also contributes to improved LME resistance. More specifically, it was found that, in addition to granular oxides and coarse granular oxides functioning as Zn trapping sites, Al is also present in the steel composition surrounding these granular oxides and grain boundary oxides. This Al acts as a trapping site for Zn to penetrate the steel during high-temperature processing. Furthermore, higher Si concentrations in the steel composition increase the likelihood of LME cracking; by keeping the Si concentration as low as possible, LME can be suppressed. Thus, for example, during hot stamping, Zn attempting to penetrate the steel is trapped by Al in the steel composition, appropriately suppressing Zn intrusion into grain boundaries. Moreover, since the Si concentration, which readily forms LME, is low, LME formation is less likely. Therefore, it was found that the presence of regions lacking low Si concentrations and high Al concentrations is important for improving LME resistance. Findings: Even when a hot-dip galvanized layer is applied to the surface of the base steel and alloying treatment is performed, at least a portion of the low-Si and high-Al deficient region of the steel remains in the base steel below the alloyed hot-dip galvanized layer. Al exists in a high concentration in this deficient region, thereby functioning as a trapping site for Zn to penetrate into the steel during high-temperature processing. In addition, the presence of low-Si concentration in this deficient region can suppress LME.

[0090] The region lacking Si at low concentrations and Al at high concentrations is a region that can overlap with the regions distributed in fine-grained oxides, coarse-grained oxides, and the aforementioned grain boundary oxides; that is, it is not like... Figure 1 The outer oxide layer 2 is formed on the surface of the base steel 3, and can be formed inside the base steel. Therefore, when a coating is formed on the surface of the steel sheet, a lack area is formed inside the base steel. More specifically, a surface lack layer is formed. The alloyed hot-dip galvanized steel sheet of the present invention and... Figure 1Compared to steel plates 1 with an external oxide layer 2, the interdiffusion between the coating components and the steel components is more fully achieved, resulting in high coating performance.

[0091] The steel plate of this embodiment will now be described in detail. It should be noted that the thickness of the steel plate in this embodiment is not particularly limited, but for example, 0.1 to 3.2 mm is acceptable.

[0092] [Composition of steel plates]

[0093] The composition of the steel plate in this embodiment will be described. Unless otherwise specified, the "%" for element content refers to "mass %". Within the numerical range of the composition, the range indicated by "~" refers to the range of values ​​before and after "~" as the lower and upper limits, unless otherwise specified.

[0094] (C: 0.05~0.40%)

[0095] Carbon (C) is an important element in ensuring the strength of steel. Insufficient C content may prevent the attainment of adequate strength. Furthermore, insufficient C content may sometimes prevent the desired morphology of internal oxides and / or a lack of surface layers. Therefore, the C content is 0.05% or more, preferably 0.07% or more, more preferably 0.10% or more, and even more preferably 0.12% or more. On the other hand, excessive C content may reduce weldability. Therefore, the C content is 0.40% or less, preferably 0.35% or less, and more preferably 0.30% or less.

[0096] (Si: 0.2-3.0%)

[0097] Silicon (Si) is an effective element for improving the strength of steel. If the Si content is insufficient, adequate strength may not be ensured. Furthermore, the desired oxides, particularly fine-grained oxides, coarse-grained oxides, grain boundary oxides, and / or a lack of surface layer, may not be sufficiently formed within the steel sheet. Therefore, the Si content is 0.2% or more, preferably 0.3% or more, more preferably 0.5% or more, and even more preferably 1.0% or more. On the other hand, if the Si content is excessive, it may cause deterioration of surface properties. Furthermore, it may lead to coarsening of granular oxides. Therefore, the Si content is 3.0% or less, preferably 2.5% or less, and more preferably 2.0% or less.

[0098] (Mn: 0.1-5.0%)

[0099] Manganese (Mn) is an effective element for increasing the strength of steel by obtaining a hard microstructure. If the Mn content is insufficient, adequate strength may not be ensured. Furthermore, the desired oxides, particularly fine-grained oxides, coarse-grained oxides, grain boundary oxides, and / or a lack of surface layer, may not be sufficiently formed within the steel sheet. Therefore, the Mn content is 0.1% or more, preferably 0.5% or more, more preferably 1.0% or more, and even more preferably 1.5% or more. On the other hand, if the Mn content is excessive, the metal microstructure may become uneven due to Mn segregation, reducing workability. Furthermore, it may lead to coarsening of granular oxides. Therefore, the Mn content is 5.0% or less, preferably 4.5% or less, more preferably 4.0% or less, and even more preferably 3.5% or less.

[0100] (sol.Al: 0.4-1.50%)

[0101] Al (aluminum) is an element that functions as a deoxidizer. If the Al content is insufficient, adequate deoxidation may not be achieved. Consequently, the desired oxides, particularly fine-grained oxides, coarse-grained oxides, grain boundary oxides, and / or a lack of surface layer, may not be sufficiently formed inside the steel sheet. While the Al content can be 0.4% or more, to obtain sufficient desired effects, fine-grained oxides, coarse-grained oxides, grain boundary oxides, and a lack of surface layer, an Al content of 0.5% or more, preferably 0.6% or more, and more preferably 0.7% or more is preferable. On the other hand, excessive Al content may lead to reduced processability and deterioration of surface properties. Furthermore, it may cause coarsening of granular oxides. Therefore, the Al content is 1.50% or less, preferably 1.20% or less, and more preferably 0.80% or less. Al content refers to the content of so-called acid-soluble Al (sol.Al).

[0102] (P: below 0.0300%)

[0103] Phosphorus (P) is generally an impurity found in steel. When the P content exceeds 0.0300%, weldability may decrease. Therefore, the P content is preferably 0.0300% or less, more preferably 0.0200% or less, more preferably 0.0100% or less, and even more preferably 0.0050% or less. There is no particular limitation on the lower limit of the P content; from a manufacturing cost perspective, the P content can also exceed 0% or be 0.0001% or more.

[0104] (S: below 0.0300%)

[0105] Sulfur (S) is generally an impurity found in steel. When the S content exceeds 0.0300%, weldability may decrease, leading to increased MnS precipitation and decreased workability such as bendability. Therefore, the S content is preferably 0.0300% or less, more preferably 0.0100% or less, more preferably 0.0050% or less, and even more preferably 0.0020% or less. There is no particular limitation on the lower limit of the S content; from the viewpoint of desulfurization cost, the S content can also exceed 0% or be more than 0.0001%.

[0106] (N: below 0.0100%)

[0107] Nitrogen (N) is generally an impurity found in steel. When the N content exceeds 0.0100%, weldability may decrease. Therefore, the N content is preferably 0.0100% or less, more preferably 0.0080% or less, more preferably 0.0050% or less, and even more preferably 0.0030% or less. There is no particular limitation on the lower limit of the N content, but from the viewpoint of manufacturing cost, the N content can also exceed 0% or be more than 0.0010%.

[0108] (B: 0~0.010%)

[0109] Boron (B) is an element that improves hardenability, thus contributing to increased strength. Furthermore, it segregates at grain boundaries, strengthening them and improving toughness; therefore, it may be included as needed. Thus, the B content is 0% or more, preferably 0.001% or more, more preferably 0.002% or more, and even more preferably 0.003% or more. On the other hand, from the viewpoint of ensuring sufficient toughness and weldability, the B content is 0.010% or less, preferably 0.008% or less, and more preferably 0.006% or less.

[0110] (Ti: 0~0.150%)

[0111] Titanium (Ti) is an element that contributes to strength improvement by precipitating TiC during the cooling of steel, and may be included as needed. Therefore, the Ti content is 0% or more, preferably 0.001% or more, more preferably 0.003% or more, even more preferably 0.005% or more, and even more preferably 0.010% or more. On the other hand, if it is contained in excess, coarse TiN may be formed, which may impair toughness. Therefore, the Ti content is 0.150% or less, preferably 0.100% or less, and even more preferably 0.050% or less.

[0112] (Nb: 0~0.150%)

[0113] Niobium (Nb) is an element that contributes to increased strength by improving hardenability, and therefore may be included as needed. Thus, the Nb content is 0% or more, preferably 0.010% or more, more preferably 0.020% or more, and even more preferably 0.030% or more. On the other hand, from the viewpoint of ensuring sufficient toughness and weldability, the Nb content is 0.150% or less, preferably 0.100% or less, and more preferably 0.060% or less.

[0114] (V: 0~0.150%)

[0115] Vanadium (V) is an element that contributes to increased strength by improving hardenability, and therefore may be included as needed. Therefore, the V content is 0% or more, preferably 0.010% or more, more preferably 0.020% or more, and even more preferably 0.030% or more. On the other hand, from the viewpoint of ensuring sufficient toughness and weldability, the V content is 0.150% or less, preferably 0.100% or less, and more preferably 0.060% or less.

[0116] (Cr: 0-2.00%)

[0117] Chromium (Cr) is effective in improving the hardenability of steel and thus its strength, and therefore may be included as needed. Therefore, the Cr content is 0% or more, preferably 0.10% or more, more preferably 0.20% or more, even more preferably 0.50% or more, and even more preferably 0.80% or more. On the other hand, if it is contained in excess, a large amount of Cr carbides may form, which conversely impairs hardenability; therefore, the Cr content is 2.00% or less, preferably 1.80% or less, and even more preferably 1.50% or less.

[0118] (Ni: 0-2.00%)

[0119] Nickel (Ni) is effective in improving the hardenability of steel and thus its strength, and therefore may be included as needed. Therefore, the Ni content is 0% or more, preferably 0.10% or more, more preferably 0.20% or more, even more preferably 0.50% or more, and even more preferably 0.80% or more. On the other hand, excessive addition of Ni leads to increased costs, therefore the Ni content is 2.00% or less, preferably 1.80% or less, and even more preferably 1.50% or less.

[0120] (Cu: 0~2.00%)

[0121] Cu (copper) is effective in improving the hardenability of steel and thus its strength, and therefore may be included as needed. Therefore, the Cu content is 0% or more, preferably 0.10% or more, more preferably 0.20% or more, even more preferably 0.50% or more, and even more preferably 0.80% or more. On the other hand, from the viewpoint of suppressing a decrease in toughness, cracking of the cast slab, or a decrease in weldability, the Cu content is 2.00% or less, preferably 1.80% or less, and even more preferably 1.50% or less.

[0122] (Mo: 0~1.00%)

[0123] Mo (molybdenum) is effective in improving the hardenability of steel and thus its strength, and therefore may be included as needed. Therefore, the Mo content is 0% or more, preferably 0.10% or more, more preferably 0.20% or more, and even more preferably 0.30% or more. On the other hand, from the viewpoint of suppressing the reduction in toughness and weldability, the Mo content is 1.00% or less, preferably 0.90% or less, and more preferably 0.80% or less.

[0124] (W: 0~1.00%)

[0125] Tungsten (W) is effective in improving the hardenability of steel and thus its strength, and therefore may be included as needed. Therefore, the W content is 0% or more, preferably 0.10% or more, more preferably 0.20% or more, and even more preferably 0.30% or more. On the other hand, from the viewpoint of suppressing the reduction in toughness and weldability, the W content is 1.00% or less, preferably 0.90% or less, and more preferably 0.80% or less.

[0126] (Ca: 0~0.100%)

[0127] Ca (calcium) is an element that helps control inclusions, especially the fine dispersion of inclusions, and improves toughness; therefore, it may be included as needed. Thus, the Ca content is 0% or more, preferably 0.001% or more, more preferably 0.005% or more, even more preferably 0.010% or more, and even more preferably 0.020% or more. On the other hand, if it is contained in excess, the deterioration of surface properties may become more pronounced; therefore, the Ca content is 0.100% or less, preferably 0.080% or less, and even more preferably 0.050% or less.

[0128] (Mg: 0-0.100%)

[0129] Magnesium (Mg) is an element that helps control inclusions, especially the fine dispersion of inclusions, and improves toughness; therefore, it may be included as needed. Thus, the Mg content is 0% or more, preferably 0.001% or more, more preferably 0.003% or more, and even more preferably 0.010% or more. On the other hand, if it is contained in excess, the deterioration of surface properties may become more pronounced; therefore, the Mg content is 0.100% or less, preferably 0.090% or less, and more preferably 0.080% or less.

[0130] (Zr: 0~0.100%)

[0131] Zirconium (Zr) is an element that helps control inclusions, particularly the fine dispersion of inclusions, and improves toughness; therefore, it may be included as needed. Thus, the Zr content is 0% or more, preferably 0.001% or more, more preferably 0.005% or more, and even more preferably 0.010% or more. On the other hand, if it is contained in excess, the deterioration of surface properties may become more pronounced; therefore, the Zr content is 0.100% or less, preferably 0.050% or less, and more preferably 0.030% or less.

[0132] (Hf: 0~0.100%)

[0133] Hafnium (Hf) is an element that helps control inclusions, particularly the fine dispersion of inclusions, and improves toughness; therefore, it may be included as needed. Thus, the Hf content is 0% or more, preferably 0.001% or more, more preferably 0.005% or more, and even more preferably 0.010% or more. On the other hand, if it is contained in excess, the deterioration of surface properties may become more pronounced; therefore, the Hf content is 0.100% or less, preferably 0.050% or less, and more preferably 0.030% or less.

[0134] (REM: 0~0.100%)

[0135] Rare earth elements (REMs) are elements that help control inclusions, especially the fine dispersion of inclusions, and improve toughness; therefore, they may be included as needed. Thus, the REM content is 0% or more, preferably 0.001% or more, more preferably 0.005% or more, and even more preferably 0.010% or more. On the other hand, if it is contained in excess, the deterioration of surface properties may become more pronounced; therefore, the REM content is 0.100% or less, preferably 0.050% or less, and more preferably 0.030% or less. It should be noted that REM is an abbreviation for Rare Earth Metal, referring to elements belonging to the lanthanide series. REMs are usually added as a mixed rare earth metal.

[0136] In the steel sheet of this embodiment, 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 reasons in the manufacturing process, such as raw materials like ores and waste. They are components that are permissible to be present within a range that does not adversely affect the properties of the steel sheet of this embodiment.

[0137] In this embodiment, the composition 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 analysis (ICP-MS). Specifically, it is preferable to use combustion-infrared absorption for C and S, and inert gas melting-thermal conductivity for N. These analyses can be performed on samples of the steel plate collected according to the method of JIS G0417:1999.

[0138] [surface layer]

[0139] In this embodiment, the “surface layer” of the steel plate refers to the area from the surface of the steel plate (or the interface between the steel plate and the coating in the case of a plated steel plate) to a specified depth in the thickness direction, and the “specified depth” is typically 50 μm or less.

[0140] like Figure 2 As illustrated in the example, it is preferable that the steel plate 11 in this embodiment contains fine oxides 12, coarse granular oxides 15, and grain boundary oxides 13 on its surface. More preferably, the fine granular oxides 12, coarse granular oxides 15, and grain boundary oxides 13 are present only on the surface of the steel plate 11. By having these fine granular oxides 12, coarse granular oxides 15, and grain boundary oxides 13 present inside the base steel 14 (i.e., as internal oxides), and... Figure 1 Compared to the case where an external oxide layer 2 exists on the surface of the base steel 3, the steel sheet 11 exhibits high plating performance. This is believed to be because oxides that hinder the interdiffusion of plating components and steel components when forming a coating (e.g., a Zn-based coating) on ​​the surface of the steel sheet are formed internally rather than externally. Therefore, the steel sheet and plated steel sheet of this embodiment, which contain granular oxides and grain boundary oxides in the surface layer, i.e., the interior of the steel sheet, exhibit high plating performance.

[0141] In addition, although Figure 2Not shown in the figure, but in the steel plate 11 of this embodiment, the surface layer of the steel plate 11 includes a surface deficiency layer in addition to the aforementioned fine granular oxides 12, coarse granular oxides 15, and grain boundary oxides 13. This surface deficiency layer is a region where the elements of the steel composition surrounding the fine granular oxides 12, coarse granular oxides 15, and grain boundary oxides 13 are less abundant than those in the original steel base material, and exists in a manner that repeats the distribution of the fine granular oxides 12, coarse granular oxides 15, and grain boundary oxides 13. That is, since the surface deficiency layer exists within the base steel 14 in the same way as the fine granular oxides 12, coarse granular oxides 15, and grain boundary oxides 13, the steel plate and the plated steel plate containing the fine granular oxides 12, coarse granular oxides 15, grain boundary oxides 13, and the surface deficiency layer also have high plating properties.

[0142] [Fine-grained oxides and coarse-grained oxides]

[0143] In this embodiment, "granular oxide" refers to oxides dispersed in a granular form within the grains or at grain boundaries of the 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 segment crossing the granular oxide (major axis) / the length of the largest segment crossing the oxide perpendicular to the major axis (minor axis)). "Dispersed in a granular form" means that the positions of the oxide particles are not arranged according to a specific rule (e.g., in a straight line), but rather randomly. In practice, granular oxides typically exist in a three-dimensional, spherical or nearly spherical shape on the surface of the steel sheet; therefore, when observing a cross-section of the steel sheet's surface, the granular oxides are typically observed as circular or nearly circular. Figure 2 As examples, fine-grained oxide 12 and coarse-grained oxide 15, which appear to be roughly spherical, are shown. Figure 2 In the example of steel plate 11, coarse granular oxides 15 are shown below fine granular oxides 12. This is believed to be because the particle size of the granular oxides tends to grow larger towards the interior of the steel plate. It is thought that near the surface of the steel plate, the diffusion rate of oxygen from the atmosphere to the interior of the steel plate is fast, making it difficult for the granular oxides to coarsen. As the direction becomes farther from the surface of the steel plate towards the interior, the diffusion rate of oxygen is slower, and therefore the granular oxides become easier to coarsen. However, coarse granular oxides 15 sometimes also form near the surface of the base steel 14.

[0144] (particle size)

[0145] In this embodiment, the particle size of the granular oxide is preferably 20 nm or more and 600 nm or less. Within this range, the particle size of "fine" granular oxide is 20 nm or more and 100 nm or less, and the particle size of "coarse" granular oxide is 150 nm or more and 600 nm or less. Setting the upper limit (100 nm) for the particle size of fine granular oxide and the lower limit (150 nm) for the particle size of coarse granular oxide is to avoid difficulties in determining the fine and coarse granular oxides from the viewpoint of measurement accuracy. By controlling the particle size within such a range, both fine and coarse granular oxides can be dispersed in the surface layer of the steel plate. Both fine and coarse granular oxides function well as hydrogen capture sites to inhibit hydrogen intrusion in corrosive environments, and further function well as Zn capture sites that can intrude during hot stamping or welding of coated steel plates with coatings formed on them. On the other hand, if the particle size exceeds 600 nm, the number of granular oxides may decrease, potentially failing to achieve the desired number density. The lower limit for the particle size of granular oxides is above 20 nm. The finer the granular oxide, the higher the specific surface area, and the more reactive it becomes as a trapping site. However, the amount of hydrogen and / or Zn that can be trapped per particle decreases, potentially preventing it from sufficiently trapping hydrogen and / or Zn and thus failing to function adequately as a hydrogen and / or Zn trapping site.

[0146] (Number density of fine-grained oxides)

[0147] In this embodiment, the preferred number density of the fine granular oxide is 4.0 particles / μm. 2 The above describes how, by controlling the number density within a certain range, fine particulate oxides can be dispersed in large quantities on the surface of the steel plate. These fine particulate oxides function effectively as hydrogen trapping sites to inhibit hydrogen intrusion under corrosive environments, and further function effectively as trapping sites for Zn that can intrude during hot stamping or welding of coated steel plates. 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 fine-grained oxide may not function adequately as hydrogen and / or Zn trapping sites, resulting in poor resistance to hydrogen embrittlement and / or LME. Conversely, external oxidation may be promoted, leading to poor plating properties. The preferred number density of the fine-grained 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. 2The above. From the viewpoint that fine particulate oxides function as hydrogen and / or Zn trapping sites, the greater the quantity present, the better. However, particulate oxides sometimes become the initiation point for LME cracking, exceeding 100 per μm. 2 At times, resistance to LME and fatigue properties may decrease; therefore, the number density of fine-grained oxides can also be 100 particles / μm. 2 Below, 90 / μm 2 Below, 80 / μm 2 Below, 70 / μm 2 Below, 60 / μm 2 Below, 50 / μm 2 Below, 40 / μm 2 Below, 30 / μm 2 Below, 25 / μm 2 Below, 20 / μm 2 the following.

[0148] The particle size and number density of the fine granular oxides were determined using scanning electron microscopy (SEM). The specific determination process is as follows: A cross-section of the surface layer of the steel plate was observed using SEM to obtain an SEM image containing the fine granular oxides. Ten regions of 1.0 μm (depth direction) × 1.0 μm (width direction) were selected from the SEM image as the observation area. For the depth direction (the direction perpendicular to the surface of the steel plate), the observation position was set to 1.0 μm within the region extending from the surface of the steel plate to 1.5 μm. For the width direction (the direction parallel to the surface of the steel plate), the observation position was set to 1.0 μm at any location within the aforementioned SEM image. Next, SEM images of the selected regions as described above were extracted. To separate the oxide portion from the steel portion, binarization was performed. The area of ​​the granular oxide portion was calculated from each binarized image. The diameter of the equivalent circle (circle diameter) with an area equal to this area was used to determine the particle size (nm) of the granular oxide. Substances with a particle size between 20nm and 100nm were defined as fine granular oxides. The number of fine granular oxides in each binarized image was then counted. The average number of fine granular oxides in the 10 regions thus determined was taken as the number density of fine granular oxides (numbers / μm). 2 It should be noted that if only a portion of the granular oxide is observed in the observation area, that is, if the entire outline of the granular oxide is not within the observation area, it is not counted as a number.

[0149] (Number density of coarse-grained oxides)

[0150] Furthermore, the preferred number density of coarse-grained oxides is 4.0 particles / 25 μm. 2 The above describes how, by controlling the number density within a certain range, a large amount of coarse-grained oxides can be dispersed on the surface of the steel plate. These coarse-grained oxides function well as hydrogen trapping sites to inhibit hydrogen intrusion under corrosive conditions, and further function well as trapping sites for Zn that can intrude during hot stamping or welding of coated steel plates. On the other hand, if the number density is lower than 4.0 particles / 25μm... 2 If the number density of hydrogen and / or Zn trapping sites is insufficient, the coarse-grained oxide may not function adequately as hydrogen and / or Zn trapping sites, resulting in poor resistance to hydrogen embrittlement and / or LME. Conversely, external oxidation may be promoted, leading to poor plating properties. The preferred number density of coarse-grained oxide is 6.0 per 25 μm. 2 The above, more preferably 8.0 per 25μm 2 The above is further preferred to be 10.0 particles / 25μm. 2 The above. From the viewpoint that coarse-grained oxides function as hydrogen and / or Zn trapping sites, the greater the quantity present, the better. However, coarse-grained oxides sometimes become the initiation point for LME cracking, exceeding 50 per 25 μm. 2 At times, LME resistance and fatigue properties may decrease, therefore the number density of coarse-grained oxides can also be 50 particles / 25μm. 2 Below, 40 per 25μm 2 Below, 30 per 25μm 2 Below, 25 per 25μm 2 Below, 20 per 25μm 2 the following.

[0151] The particle size and number density of the coarse granular oxides were determined using scanning electron microscopy (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 coarse granular oxides. Ten regions of 5.0 μm (depth direction) × 5.0 μm (width direction) were selected from the SEM image as the observation area. For the depth direction (the direction perpendicular to the surface of the steel plate), the observation position was set to 5.0 μm within the region extending from the surface of the steel plate to 8.0 μm. For the width direction (the direction parallel to the surface of the steel plate), the observation position was set to 5.0 μm at any location within the aforementioned SEM image. Next, SEM images of the selected regions as described above were extracted. To separate the oxide and steel portions, binarization was performed. The area of ​​the granular oxide portion was calculated from each binarized image. This area was used as the diameter of a circle with an area equal to this diameter, i.e., the equivalent circle diameter. The particle size (nm) of the granular oxide was then determined. Materials with a particle size between 150nm and 600nm were classified as coarse granular oxides. The number of coarse granular oxides in each binarized image was further counted. The average number of coarse granular oxides in the 10 regions thus determined was taken as the number density of fine granular oxides (numbers / 25μm). 2 It should be noted that if only a portion of the granular oxide is observed in the observation area, that is, if the entire outline of the granular oxide is not within the observation area, it is not counted as a number.

[0152] [Grain boundary oxides]

[0153] In this embodiment, "grain boundary oxides" refer to oxides existing along the grain boundaries of the steel, excluding oxides present within the grains of the steel. In reality, because 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 steel plate's surface 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 fine granular oxide 12 and coarse granular oxide 15, but grain boundary oxide 13 is sometimes also formed near the surface of the base steel 14.

[0154] (Ratio A)

[0155] In this embodiment, the so-called "ratio A" is as follows: Figure 3As shown, the ratio A 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 the observed image when observing a cross-section of the surface of the steel plate 11. In this embodiment, the ratio A is 50% or more and 100% or less. By controlling the ratio A to such a range, a large amount of grain boundary oxides 13 can be present in the surface of the steel plate, and the grain boundary oxides 13 function well as an escape pathway for hydrogen penetrating into the steel. On the other hand, if the ratio A is less than 50%, there may not be a sufficient amount of grain boundary oxides 13 as an escape pathway for hydrogen, and the amount of hydrogen accumulation in the steel cannot be sufficiently reduced, resulting in poor resistance to hydrogen embrittlement. Conversely, external oxidation may be promoted, resulting in poor plating properties. The ratio A is preferably 60% or more, more preferably 70% or more, further preferably 80% or more, even more preferably 90% or more, and most preferably 100%.

[0156] Ratio A Figure 3 As shown, the determination is made by cross-sectional observation of the surface layer of the steel plate 11. The specific measurement method is as follows. The cross-section of the surface layer of the steel plate 11 is observed using SEM. 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 the L0 and L thus calculated, the ratio A (%) in this embodiment is calculated as 100×L / L0. It should be noted that... Figure 3 The diagrams of fine granular oxide 12 and coarse granular oxide 15 have been omitted for illustrative purposes.

[0157] [Depth D]

[0158] In this embodiment, the so-called "depth D" is as follows: Figure 3As shown, this refers to the distance from the surface of the steel plate 11 (or the interface between the steel plate and the coating in the case of a plated steel plate) along the thickness direction of the steel plate 11 (the direction perpendicular to the surface of the steel plate), to the farthest point where the grain boundary oxide 13 exists. As described above, the grain boundary oxide can function as a pathway for hydrogen to escape from the steel plate. Therefore, if the depth D of the grain boundary oxide is large, hydrogen can be released from a deeper location in the steel plate, thus more effectively fulfilling the aforementioned function. In the steel plate of this embodiment, the depth D of the grain boundary oxide is preferably 3.0 μm or more, more preferably 5.0 μm or more, and even more preferably 7.0 μm or more. There is no particular upper limit to the depth D; the depth D is substantially 50.0 μm or less. The depth D can be determined simply from an image identical to the SEM image (the length L0 of the surface) used to measure the aforementioned ratio A.

[0159] The alloyed hot-dip galvanized steel sheet 17 of this embodiment is typically obtained by hot-dip galvanizing the steel sheet 11 of this embodiment described above and then alloying it, as exemplified in... Figure 4 In the schematic diagram, as shown, at least a portion of the grain boundary oxide 13 contained in the steel plate 11 of this embodiment remains in the base steel 14 below the alloyed hot-dip galvanized layer 16 even after a hot-dip galvanized layer is applied to the surface of the steel plate and alloying treatment is performed. The grain boundary oxide 13 remaining in the base material 14 below the alloyed hot-dip galvanized layer 16 is a substance derived from the grain boundary oxide 13 contained in the steel plate of this embodiment. When observing the cross-section of the alloyed hot-dip galvanized steel plate 17, the ratio A of the length of the grain boundary oxide 13 projected onto the interface between the base steel 14 and the alloyed hot-dip galvanized layer 16 to the length of the interface is 50% or more and 100% or less. Here, "ratio A" is determined by the same method as the determination of "ratio A" of the grain boundary oxide contained in the steel plate 11 of this embodiment. By controlling the ratio A of grain boundary oxides 13 remaining in the base steel 14 below the alloyed hot-dip galvanized layer 16 to a range that allows a large amount of grain boundary oxides 13 to remain in the surface layer of the base steel 14, enabling them to function effectively as a pathway for hydrogen removal from the steel. On the other hand, if the ratio A is less than 50%, a sufficient amount of grain boundary oxides 13 as a hydrogen removal pathway may not remain, failing to adequately reduce hydrogen accumulation in the steel and resulting in poor resistance to hydrogen embrittlement. The ratio A is preferably 60% or more, more preferably 70% or more, further preferably 80% or more, even more preferably 90% or more, and most preferably 100%.

[0160] (Depth of the internal oxide layer)

[0161] In the steel sheet of this embodiment, the internal oxide layer is a layer formed inside the steel sheet, comprising fine-grained oxide 12, coarse-grained oxide 15, and grain boundary oxide 13. Therefore, the term "internal oxide layer" refers to a layer extending from the surface of the steel sheet to the furthest point where any of the fine-grained oxide 12, coarse-grained oxide 15, and grain boundary oxide 13 exists. Thus, the term "depth of the internal oxide layer" is as follows: Figure 2 As indicated by "Rn", this refers to the distance from the surface of the steel plate 11 (or the interface between the steel plate and the coating in the case of a coated steel plate) along the thickness direction of the steel plate 11 (the direction perpendicular to the surface of the steel plate), to the farthest point where any of the fine-grained oxides 12, coarse-grained oxides 15, and grain boundary oxides 13 are present. However, since the surface of an actual steel plate is uneven, the positions of the fine-grained oxides 12, coarse-grained oxides 15, and grain boundary oxides 13 farthest from the surface of the steel plate vary depending on which part (point) of the steel plate surface is selected. Therefore, an observation area of ​​10 points is selected, and the average value of the results measured at these 10 points is taken as the "average depth of the internal oxide layer" (sometimes also called "R"). Figure 2 In the example shown, the grain boundary oxide 13 is present at the deepest location. As described above, the fine-grained oxide 12 and the coarse-grained oxide 15 can function as trapping sites for hydrogen that penetrates during electrodeposition coating, and the grain boundary oxide 13 can function as an escape path for hydrogen that has penetrated into the steel plate. Therefore, the greater the average depth R of the internal oxide layer, the more hydrogen can be trapped in the surface area of ​​the steel plate, and the more hydrogen can be discharged outside the system. In the steel plate of this embodiment, the lower limit of the average depth R of the internal oxide layer is not particularly limited, but if it is too shallow, the fine-grained oxide, the coarse-grained oxide 15, and the grain boundary oxide 13 may not be sufficiently dispersed. Therefore, it is preferably 8 μm or more, more preferably 10 μm or more, more preferably 15 μm or more, and even more preferably 20 μm or more. The upper limit of the average depth R is not particularly limited, but it is substantially 100 μm or less.

[0162] Depth R such Figure 2As shown, the determination is made by cross-sectional observation of the surface layer of the steel plate 11. The specific measurement method is as follows: The cross-section of the surface layer of the steel plate 11 is observed using SEM. Ten observation locations are randomly selected. The length L0 of the surface (i.e., the width of the SEM image) is measured from the observed SEM image. The length L0 is set to 100 μm or more (e.g., 100 μm, 150 μm, or 200 μm), and the measurement depth is set to the area from the surface of the steel plate to 100 μm. Next, the positions of fine-grained oxide 12, coarse-grained oxide 15, and grain boundary oxide 13 are determined from the SEM image. From the determined fine-grained oxide 12, coarse-grained oxide 15, and grain boundary oxide 13, the one located furthest from the surface of the steel plate is selected. The distance from the surface of the steel plate 11 to the furthest location containing any of these three oxides is calculated as the depth Rn. The average value of Rn measured at 10 locations is calculated as the "average depth of the internal oxide layer" (sometimes also called "R").

[0163] [Composition of Oxides]

[0164] In this embodiment, the granular oxide and grain boundary oxide (hereinafter also simply referred to as oxide) are substances that contain one or more of the elements contained in the steel plate mentioned above, in addition to oxygen. Typically, they have a composition containing Si, O, and Fe, and, depending on the situation, further containing Mn or Al. In addition to these elements, the oxide may also contain elements that can be contained in the steel plate mentioned above (e.g., Cr).

[0165] [Surface lacking layer]

[0166] In this embodiment, fine-grained oxides, coarse-grained oxides, and grain boundary oxides are substances formed by the oxidation of easily oxidizable components (such as Si, Mn, and Al) in the steel sheet. Therefore, the composition of the steel (in other words, the metal structure) surrounding these oxides is deficient in these easily oxidizable elements compared to the original steel sheet base material. The region where the elements of this steel composition are deficient compared to the original steel sheet base material is also called a "deficient region." The layered "deficient region" is also called a "deficient layer," and further, the deficient layer existing on the surface of the steel sheet is also called a "surface deficient layer." In the deficient region, since Si is relatively easily oxidized and Al is relatively difficult to oxidize, Si can exist at a low concentration and Al at a high concentration. If the steel composition has a low-Si and high-Al deficient region within the desired range, it also contributes to improved LME resistance. The rationale is that, while not adhering to a specific theory, it is believed that Al exists in the steel composition surrounding the granular oxides and grain boundary oxides, in addition to functioning as Zn trapping sites. This Al acts as a trapping site for Zn to penetrate the steel during high-temperature processing. Furthermore, higher Si concentrations in the steel composition increase the likelihood of LME cracking; therefore, by keeping the Si concentration as low as possible, LME can be suppressed. Thus, during hot stamping or welding, Zn attempting to penetrate the steel is trapped by Al in the steel composition, appropriately suppressing Zn intrusion into grain boundaries. Moreover, since the Si concentration is low, which readily forms LME, LME formation is less likely, improving LME resistance.

[0167] In this embodiment, the composition of the steel (in other words, the metal microstructure) with a low Si and high Al surface layer lacking a layer at a depth of half the average depth of the internal oxide layer, and free from fine-grained oxides, coarse-grained oxides, and grain boundary oxides, satisfies, by mass%, Si ≤ 0.6% and Al ≥ 0.05%. If Si exceeds 0.6%, LME cracking becomes more likely. Therefore, Si ≤ 0.6%. The lower limit of Si is not particularly limited, but it can also be 0% or more. Furthermore, Al functions as a trapping site for Zn to penetrate into the steel during high-temperature processing. If Al is below 0.05%, it may not function sufficiently as a Zn trapping site. Therefore, Al ≥ 0.05% is set. The higher the Al content, the better the function as a trapping site becomes, which is more preferred, but even if the Al concentration is too high, its effect will saturate, so the upper limit of Al can also be set to 1.2% or less or 1.0% or less. Furthermore, the concentrations of Si and Al are elemental concentrations in the steel composition of the internal oxide layer, free from fine-grained oxides, coarse-grained oxides, and grain boundary oxides. These concentrations are measured at a depth of half the average depth R of the internal oxide layer. The baseline for the average depth of the internal oxide layer is the steel plate surface (or the interface between the steel plate and the coating in the case of coated steel). However, in cases where the surface is uneven, the average line of the surface or interface at 10 points from which the average depth of the internal oxide layer is determined is set as the baseline. The elemental concentrations here are measured using EDS (Energy Dispersed Spectroscopy).

[0168] The surface deficiency layer is a layer that repeats the distribution areas of fine-grained oxides, coarse-grained oxides, and grain boundary oxides. It is a layer existing on the surface of the steel plate, that is, formed inside the base steel. Therefore, when a coating is formed on the surface of the steel plate, a deficiency area is formed inside the base steel. More specifically, the steel plate of the present invention, which has a surface deficiency layer, allows for sufficient interdiffusion of the coating component and the steel component compared to a steel plate with an external oxide layer, resulting in high plating performance.

[0169] The alloyed hot-dip galvanized layer 16 of this embodiment is typically obtained by alloying the steel plate 11 of this embodiment after hot-dip galvanizing, as illustrated in the example shown below. Figure 4In the schematic diagram. Although not shown, even after hot-dip galvanizing and alloying treatment of the surface of the base steel 14, at least a portion of the surface-deficient layer of this embodiment remains in the base steel 14 below the alloyed hot-dip galvanized layer 16. The high concentration of Al in this deficient region functions as a trapping site for Zn to penetrate the steel during high-temperature processing; furthermore, the low concentration of Si in this deficient region suppresses LME. The surface-deficient layer remaining in the base steel 14 below the alloyed hot-dip galvanized layer 16 is derived from the surface-deficient layer contained in the steel plate 11 of this embodiment. The composition of the steel (in other words, the metal structure) at a depth of half the average depth of the internal oxide layer, free of oxides, especially free of grain boundary oxides, satisfies Si ≤ 0.6% and Al ≥ 0.05% by mass. If Si exceeds 0.6%, LME cracking becomes more likely. Therefore, Si ≤ 0.6%. The lower limit for Si is not particularly limited, but it can be 0% or higher. Furthermore, Al functions as a trapping site for Zn to penetrate the steel during high-temperature processing. If Al is below 0.05%, it may not function sufficiently as a Zn trapping site. Therefore, Al ≥ 0.05% is set. The higher the Al content, the better its function as a trapping site, which is preferred; however, even if the Al concentration is too high, its effect saturates, so the upper limit for Al can be set to 1.2% or less or 1.0% or less. The average depth and elemental concentration of the internal oxide layer are measured using the same method as the method used to measure the lack of a surface layer in the steel plate 11 of this embodiment.

[0170] Alloyed hot-dip galvanized steel sheet

[0171] The alloyed hot-dip galvanized steel sheet of the present invention has an alloyed hot-dip galvanized layer containing Zn on the steel sheet of the present embodiment described above. This alloyed hot-dip galvanized layer may be formed on one side or both sides of the steel sheet.

[0172] [Composition of Alloyed Hot-Dip Galvanized Coating]

[0173] The composition of the alloyed hot-dip galvanized layer in this embodiment will be described. Unless otherwise specified, the "%" for element content refers to "mass %". In the numerical range of the composition of the coating, the range indicated by "~" unless otherwise specified, refers to the range of values ​​before and after "~" as the lower limit and upper limit.

[0174] (Al: 0.01~1.0%)

[0175] Al is an element that improves the corrosion resistance of the coating by being included or alloyed with Zn. Since the steel sheet of this embodiment contains a predetermined amount of Al, the alloyed hot-dip galvanized layer of this embodiment contains 0.01% or more Al. Depending on the desired corrosion resistance, the Al content can also be 0.01% or more, for example, 0.1% or more, or 0.13% or more. On the other hand, if excessive Al is added, it can sometimes hinder the Zn-Fe alloying reaction, making alloying heat treatment difficult and increasing costs; therefore, the upper limit of the Al content is set to 1.0%. Furthermore, from the viewpoint of stable alloying, the Al content can be set to 0.2% or less, preferably 0.15% or less. The Al concentration in the plating bath can also be adjusted to obtain the desired characteristics.

[0176] (Fe: 5.0–15.0%)

[0177] When a Zn-containing coating is formed on a steel sheet and the coated steel sheet is heat-treated, Fe can diffuse into the coating through diffusion from the steel sheet. Therefore, in the alloyed hot-dip galvanized layer of this embodiment, the heat treatment for alloying is performed, and the Fe content is 5.0% or more. Depending on the degree of alloying, the Fe content may also be 6.0% or more, 7.0% or more, 8.0% or more, 9.0% or more, or 10.0% or more. On the other hand, from the viewpoint of the sliding properties of the coated steel sheet, the Fe content may also be 15.0% or less, 12.0% or less, 10.0% or less, 8.0% or less, or 6.0% or less.

[0178] (Mg: 0-15.0%)

[0179] Mg is an element that improves the corrosion resistance of alloyed hot-dip galvanized layers by inclusion or alloying with Zn and Al, and therefore can be included as needed. Thus, the Mg content can also be 0%. For forming an alloyed hot-dip galvanized layer containing Zn, Al, and Mg, a Mg content of 0.01% or more is preferred, for example, 0.1% or more, 0.5% or more, 1.0% or more, or 3.0% or more. On the other hand, when the Mg content exceeds 15.0%, Mg is not completely dissolved in the plating bath and floats as oxides. If galvanizing is performed using this bath, oxides may adhere to the plating surface, causing poor appearance or creating unplated areas. Therefore, a Mg content of 15.0% or less is preferred, for example, 10.0% or less, or 5.0% or less.

[0180] (Si: 0-3.0%)

[0181] Si is an element that further improves corrosion resistance when included in Zn-containing coatings, especially Zn-Al-Mg-containing coatings, and therefore may 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.

[0182] The basic composition of the alloyed hot-dip galvanized layer is as described above. Furthermore, the alloyed hot-dip galvanized layer 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 role and function of the aforementioned basic components constituting the alloyed hot-dip galvanized layer, the total content of these arbitrarily added elements is preferably set to 5.00% or less, more preferably 2.00% or less.

[0183] The remainder of the alloyed hot-dip galvanized layer, other than the components described above, consists of Zn and impurities. Impurities in the alloyed hot-dip galvanized layer refer to components introduced during the manufacturing process, represented by the raw materials, due to various factors in the manufacturing steps. These are components that are not intentionally added to the alloyed hot-dip galvanized layer. As impurities, the alloyed hot-dip galvanized layer may also contain trace amounts of the basic components described above and elements other than those arbitrarily added, without impairing the effects of the present invention.

[0184] The composition of the coating can be determined by dissolving the coating in an acid solution containing an inhibitor that inhibits corrosion of the steel plate, and then measuring the resulting solution using ICP (inductively coupled plasma) emission spectroscopy.

[0185] Figure 4The alloyed hot-dip galvanized layer 16 of this embodiment, as illustrated in the illustration, is typically obtained by alloying the steel sheet 11 of this embodiment after hot-dip galvanizing. Therefore, although not illustrated, at least a portion of the fine granular oxides 12 and coarse granular oxides 15 present in the surface layer of the steel sheet 11 remain in the alloyed hot-dip galvanized layer 16, which can function as hydrogen trapping sites. Preferably, the alloyed hot-dip galvanized layer 16 contains oxides with a particle size of 0.1 to 1.5 μm at a number density of 1 to 10 per (5 μm × 5 μm). If the particle size of the oxide is less than 0.1 μm, and / or if the number density is less than 1 per (5 μm × 5 μm), it may not function sufficiently as a hydrogen trapping site. When the particle size of the oxide exceeds 1.5 μm and / or the number density exceeds 10 particles / (5 μm × 5 μm), the homogeneity of the alloyed hot-dip galvanized layer may be reduced. The particle size and number density of the oxide are determined by the same method as for the determination of fine-grained oxides and / or coarse-grained oxides in the steel sheet of this embodiment.

[0186] The coating thickness is preferably 3–50 μm. Furthermore, the coating adhesion amount per side is 10–100 g / m. 2 In this invention, the amount of coating adhesion is determined by the weight change before and after dissolution in an acidic solution containing an inhibitor that suppresses corrosion of the base metal.

[0187] [tensile strength]

[0188] The hot-dip galvanized steel sheet of the present invention preferably has 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 is acceptable as long as it is below 2000 MPa. The tensile strength is determined by collecting JIS 5 tensile test specimens with the length direction set perpendicular to the rolling direction, and performing the test according to JIS Z2241 (2011).

[0189] The hot-dip galvanized steel sheet of the present invention, due to its high strength and high resistance to hydrogen embrittlement (LME) and hydrogen embrittlement, is suitable for use in a wide range of fields such as automobiles, home appliances, and building materials, but is particularly preferred for use in the automotive field. Most hot-dip galvanized steel sheets used in automotive applications are hot-stamped, in which case hydrogen embrittlement and LME cracking can become significant problems. Therefore, when the hot-dip galvanized steel sheet of the present invention is used as an automotive steel sheet, the advantages of the present invention, such as high resistance to hydrogen embrittlement and LME, can be effectively utilized.

[0190] <Methods for manufacturing steel plates>

[0191] Hereinafter, a preferred method for manufacturing the steel plate of the present invention will be described. The purpose of the following description is to illustrate a characteristic method for manufacturing the steel plate of the present invention, and it is not intended to limit the steel plate to a steel plate manufactured by the method described below.

[0192] The steel sheet of the present invention can be obtained, for example, by performing the following steps: a casting process in which molten steel with adjusted composition is cast to form a steel billet; a hot rolling process in which the steel billet is hot rolled to obtain a hot-rolled steel sheet; a coiling process in which the hot-rolled steel sheet is coiled; a cold rolling process in which the coiled hot-rolled steel sheet is cold-rolled to obtain a cold-rolled steel sheet; a pretreatment process in which the cold-rolled steel sheet is subjected to electrobrushing; and an annealing process in which the pretreated cold-rolled steel sheet is annealed. Alternatively, after the hot rolling process, pickling can be performed without coiling, and the cold rolling process can be performed directly.

[0193] [Casting Process]

[0194] 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, ingot casting, or other methods.

[0195] [Hot rolling process]

[0196] Hot-rolled steel sheets can be obtained by hot rolling the cast steel billet as described above. The hot rolling process is performed by directly or temporarily cooling and then reheating the cast steel billet. In the case of reheating, the heating temperature of the steel billet can be, for example, only 1100℃ to 1250℃. The hot rolling process typically includes roughing and finishing rolling. The temperature and reduction rate of each rolling pass can be appropriately varied 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%.

[0197] [Winding process]

[0198] 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 is sufficient. Alternatively, the hot-rolled steel sheet can be uncoiled before or after coiling and 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.

[0199] [Cold rolling process]

[0200] 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, 20% to 80%. After the cold rolling process, cooling to room temperature via air is sufficient.

[0201] [Pre-treatment process]

[0202] To obtain a large amount of fine-grained oxides, coarse-grained oxides, and grain boundary oxides on the surface of the final steel sheet, and thus a surface-deficient layer, a pretreatment process prior to annealing the cold-rolled steel sheet is effective. This pretreatment process introduces a large number of dislocations onto the surface of the cold-rolled steel sheet. Since the diffusion of oxygen and the like is faster at grain boundaries than within grains, introducing a large number of dislocations onto the surface of the cold-rolled steel sheet creates as many pathways as at grain boundaries. Therefore, during annealing, oxygen becomes more readily diffused (intruded) into the interior of the steel along these dislocations. Furthermore, since the diffusion rates of Si and Al are also increased, the combination of oxygen with Si and / or Al within the steel promotes the formation of fine-grained oxides, coarse-grained oxides, and grain boundary oxides. In addition, the reduction in the surrounding Si and Al concentration is also promoted along with the promotion of such internal oxide formation, thus also promoting the formation of a surface-deficient layer with the desired composition. Therefore, with such a pretreatment process, the desired fine-grained oxides, coarse-grained oxides, grain boundary oxides, and surface-deficient layers are easily formed in the annealing process described later. This pretreatment process includes grinding the surface of the cold-rolled steel sheet using a high-power grinding brush (brush grinding). HOTANI's D-100 brush can also be used as the high-power grinding brush. It is preferable to apply a 1.0–5.0% aqueous solution of NaOH to the steel sheet surface during grinding. The brush reduction is preferably 0.5–10.0 mm, more preferably 5.0–10.0 mm, and the rotation speed is preferably 100–1000 rpm. By controlling the coating solution conditions, brush reduction, and rotation speed during brush grinding, fine-grained oxides, coarse-grained oxides, grain boundary oxides, and surface-deficient layers can be effectively formed on the surface of the steel sheet in the annealing process described later.

[0203] [Annealing process]

[0204] The cold-rolled steel sheet that has undergone the above pretreatment process is then annealed. Annealing is preferably performed, for example, under a tension of 0.1 to 30.0 MPa. Applying tension during annealing allows for more effective introduction of strain into the steel sheet. This strain promotes the formation of dislocations in the metal structure of the steel sheet, making it easier for oxygen to penetrate into the interior of the steel along these dislocations, thereby facilitating the formation of oxides within the steel sheet. As a result, it becomes advantageous to increase the number density of granular oxides, increase the proportion of grain boundary oxides, and form a surface-deficient layer.

[0205] From the viewpoint of generating a large quantity of granular oxides and grain boundary oxides at the desired size, the holding temperature of the annealing process is preferably 750°C to 900°C, and more preferably 830°C to 880°C. If the holding temperature of the annealing process is below 750°C, the grain boundary oxides may not be generated sufficiently, and sometimes the resistance to hydrogen embrittlement becomes insufficient. On the other hand, if the holding temperature of the annealing process exceeds 900°C, the granular oxides may coarsen, and sometimes the desired granular oxides, grain boundary oxides, and / or surface layer deficiency may not be obtained, and sometimes the resistance to hydrogen embrittlement and / or LME resistance becomes insufficient. The heating rate up to the aforementioned holding temperature is not particularly limited, but it is acceptable as long as it is 1 to 10°C / second. Alternatively, the heating can be carried out in two stages: a first heating rate of 1 to 10°C / second and a second heating rate of 1 to 10°C / second, which is different from the first heating rate.

[0206] The holding time at the holding temperature in the above-mentioned annealing process is preferably 50 to 300 seconds, and more preferably 150 to 250 seconds. If the holding time is less than 50 seconds, granular oxides and / or grain boundary oxides may not be sufficiently generated, and sometimes the resistance to LME and / or hydrogen embrittlement becomes insufficient. On the other hand, if the holding time exceeds 300 seconds, external oxidation may occur without internal oxidation, and sometimes the plating properties, resistance to hydrogen embrittlement, and / or resistance to LME become insufficient.

[0207] During the heating and holding (isothermal) phases of the annealing process, humidification is performed to generate the desired fine-grained oxides, coarse-grained oxides, grain boundary oxides, and surface-deficient layers. The dew point of the atmosphere is preferably -20 to 10°C, more preferably -10 to 5°C, and the concentration is 1 to 15 vol% H2. If the dew point is too low, an external oxide layer may form on the surface of the steel plate, and an internal oxide layer may not form sufficiently, sometimes resulting in insufficient plating properties, resistance to hydrogen embrittlement, and resistance to LME. On the other hand, if the dew point is too high, the granular oxides may coarsen, and sometimes the desired granular oxides, grain boundary oxides, and / or surface-deficient layers cannot be obtained.

[0208] It is advisable to begin humidification at a temperature below 600°C during the heating process. If humidification begins above 600°C, it is possible that the internal oxide layer and / or the surface layer will not be sufficiently formed until the holding temperature is reached.

[0209] Furthermore, it is effective to remove the internal oxide layer of the steel sheet before the annealing process, especially before the brush grinding treatment. Sometimes, an internal oxide layer forms on the surface of the steel sheet between the aforementioned rolling processes, particularly hot rolling processes. Such an internal oxide layer formed during rolling processes may hinder the formation of fine granular oxides, coarse granular oxides, grain boundary oxides, and / or a lack of surface layer during the annealing process, or promote the formation of an external oxide layer. Therefore, it is preferable to remove this internal oxide layer before annealing by pickling or the like. More specifically, the depth of the internal oxide layer of the cold-rolled steel sheet during the annealing process is preferably set to 0.5 μm or less, more preferably 0.3 μm or less, more preferably 0.2 μm or less, and even more preferably 0.1 μm or less.

[0210] By performing the above-mentioned processes, it is possible to obtain a steel plate that contains a sufficient amount of granular oxides and grain boundary oxides in the surface layer of the steel plate, and that has a surface layer lacking.

[0211] <Manufacturing Method of Coated Steel Sheet>

[0212] Hereinafter, a preferred method for manufacturing the coated steel sheet of the present invention will be described. The purpose of the following description is to illustrate a characteristic method for manufacturing the coated steel sheet of the present invention, and it is not intended to limit the coated steel sheet to the coated steel sheet manufactured by the method described below.

[0213] 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.

[0214] [Plating Process]

[0215] 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 by electroplating. Hot-dip plating is preferred. The conditions for the plating process can be appropriately set considering the desired coating composition, thickness, and adhesion amount.

[0216] [Alloying process]

[0217] After plating, an alloying process is performed. The alloying process can be carried out according to methods known to those skilled in the art. Alloying is performed by heating to the temperature required to alloy the plating. Typically, although this varies depending on the amount of plating, alloying is performed, for example, by heating in a temperature range of 480°C to 580°C for a time of 1 second to 50 seconds.

[0218] Example

[0219] 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.

[0220] Example 1: Examples and comparative examples of alloyed hot-dip galvanized steel sheets

[0221] (Preparation of steel plate samples)

[0222] Molten steel with adjusted composition was cast to form a billet. The billet was then hot-rolled, pickled, and cold-rolled to obtain a cold-rolled steel sheet. Next, the sheet was air-cooled to room temperature, and then pickled to remove the internal oxide layer formed during rolling, down to the internal oxide layer depth (μm) before annealing as shown in Table 1. Samples were then collected from each cold-rolled steel sheet according to JIS G0417:1999, and the composition of the steel sheets was analyzed by ICP-MS and other methods. The measured composition of the steel sheets is shown in Table 1. All steel sheets used had a thickness of 1.6 mm.

[0223] Next, for a portion of the cold-rolled steel sheets, a 2.0% NaOH aqueous solution was applied, followed by pretreatment using a high-power grinding brush (HOTANI D-100) with a brush reduction of 2.0 mm and a rotation speed of 600 rpm. Afterward, annealing was performed according to the dew point, holding temperature, and holding time shown in Table 1, resulting in various steel sheet samples. 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. During the annealing process, some cold-rolled steel sheets were annealed under a tension of 30.0 MPa, while other cold-rolled steel sheets were annealed without tension. The presence or absence of pretreatment and the annealing conditions (tension, humidification zone, dew point (°C), hydrogen concentration (vol%), humidification start temperature (°C), holding temperature (°C), and holding time (seconds) during the heating process) are shown in Table 1. It should be noted that for each steel plate sample, JIS No. 5 tensile test pieces were collected with the direction perpendicular to the rolling direction set as the length direction, and tensile tests were conducted according to JIS Z 2241 (2011). The results were all above 440 MPa.

[0224] (Preparation of alloyed hot-dip galvanized steel sheet samples)

[0225] After cutting the aforementioned steel plate samples into 100mm × 200mm dimensions, coated steel plate samples were prepared by plating. In Table 1, the plating type is "GA (Alloyed Hot-Dip Galvanized Steel Sheet)". During the hot-dip galvanizing process, the cut samples were immersed in a 440℃ hot-dip galvanizing bath 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 The alloyed hot-dip galvanized steel sheet samples were then obtained by heating at 500℃ for a period of more than 1 second but less than 50 seconds, typically around 20 seconds. The composition of the alloyed hot-dip galvanized layer was analyzed by ICP-MS and other methods, confirming that it contained 5.0–15.0% Fe and 0.01–1.0% Al, with the remainder consisting of Zn and impurities.

[0226] (Analysis of the surface layer of alloyed hot-dip galvanized steel sheet samples: the ratio of grain boundary oxides A)

[0227] Each alloyed hot-dip galvanized steel sheet specimen, prepared as described above, was cut into 25mm × 15mm pieces. The cut specimens were then embedded in resin and mirror-polished to create embedded specimens. The ratio A of grain boundary oxides was measured for each steel sheet specimen by observing the cross-section of the embedded specimen. Specifically, the location of the grain boundary oxides was determined in a 150μm width (=L0) SEM image. The determined grain boundary oxides were projected onto the interface between the steel sheet substrate and the coating, and the length L of the grain boundary oxides within the field of view was calculated. Based on the calculated L0 and L, the ratio A(%) = 100 × L / L0 was calculated. The ratio A(%) of granular oxides for each steel sheet specimen is shown in Table 1. Furthermore, the depth D of the determined grain boundary oxides was measured from the same SEM image.

[0228] (Analysis of the surface layer of alloyed hot-dip galvanized steel sheet samples: number density of oxides inside the coating)

[0229] Based on the cross-sectional observations of the aforementioned embedded samples, for each steel plate sample, 10 5.0 μm × 5.0 μm regions were observed using SEM. The observation locations were set as follows: for the depth direction (the direction perpendicular to the interface between the steel plate substrate and the coating), 5.0 μm was defined as the distance from the surface of the coating to the aforementioned interface; for the width direction (the direction parallel to the surface of the steel plate), 5.0 μm was defined as any location within the aforementioned SEM image. The SEM images of each region of each steel plate sample were binarized. The area of ​​the oxide portion was calculated from the binarized image. Using this area as the diameter of a circle with an area equal to this diameter (i.e., the equivalent circle diameter), the particle size (μm) of the oxide was determined, and the number of oxide particles within the particle size range of 0.1 to 1.5 μm was counted. The average number of oxide particles in the 10 binarized images obtained in this way was taken as the number density of fine-grained oxides. The number density of oxides for each steel plate sample was set as 1 to 10 particles / 5 × 5 μm. 2 The case of ) is set as "○", and the other cases are set as "×", as shown in Table 1.

[0230] (Analysis of the surface layer of alloyed hot-dip galvanized steel sheet samples: lack of surface layer)

[0231] For each steel plate sample, to evaluate the lack of a surface layer, the composition of the oxide-free steel microstructure was analyzed at a depth of half the average depth of the internal oxide layer calculated from the cross-sectional SEM image of the steel plate using TEM-EDS. Cases satisfying Si ≤ 0.6% and Al ≥ 0.05% were marked as "○", while cases not satisfying Si ≤ 0.6% and Al ≥ 0.05% were marked as "×".

[0232] (LME resistance evaluation)

[0233] Resistance to LME was evaluated by a hot tensile test. Strips of 130 mm × 30 mm × 1.6 mm were used to test the steel plate specimens.

[0234] Heating to 800℃ at a heating rate of 100℃ / s and immediately conducting a hot tensile test at 800℃ with a tensile speed of 10mm / s until fracture, the tensile strength is determined.

[0235] The tensile strength of the coated sample is compared with that of the uncoated sample, and evaluated as follows.

[0236] A: The tensile strength of the coated sample / the tensile strength of the uncoated sample ≥ 85%

[0237] B: Tensile strength of coated sample / Tensile strength of uncoated sample < 85%

[0238] (Evaluation of resistance to hydrogen embrittlement)

[0239] Each 50mm × 100mm coated steel sheet sample was treated with zinc phosphate using a zinc phosphate-based chemical conversion treatment solution (SURFDINE SD5350 series: manufactured by Nipponpaint Industrial Coatings). Following this, a 20μm electrodeposition coating (PN1100 Powernix gray: manufactured by Nipponpaint Industrial Coatings) was applied, and the sample was baked at 150°C for 20 minutes to form a coating film on the coated steel sheet sample. The sample was then cut into 30 × 100mm pieces, exposing the iron end face. Subsequently, a salt spray test (SST, JIS Z2371) was conducted under stress using a bending fixture, with the stress at the bending section set to 800MPa. Hydrogen embrittlement resistance was evaluated using the following criteria, and the results are shown in Table 1.

[0240] Rating AA: No cracking after 180 cycles.

[0241] Rating A: Cracking occurs at 90 to below 180 cycles.

[0242] Rating B: Cracking occurs below 90 cycles.

[0243]

[0244] In this example, a high-strength steel-coated sheet with a tensile strength of 440 MPa or higher, a hydrogen embrittlement resistance rating of AA or A, and a LME resistance rating of A is evaluated as having high hydrogen embrittlement resistance and LME resistance. Regarding samples No. 2-8 and 23-36, since the steel composition, the ratio of grain boundary oxides (A), and the lack of a surface layer meet the scope of this invention, they exhibit high LME resistance and hydrogen embrittlement resistance. Sample No. 1, due to insufficient carbon content, not only failed to obtain sufficient strength but also failed to obtain the desired grain boundary oxides and surface layer deficiency, thus failing to achieve high hydrogen embrittlement resistance and LME resistance. Sample No. 9, with a low dew point during annealing, did not sufficiently form an internal oxide layer, resulting in an external oxide layer, and therefore did not achieve high hydrogen embrittlement resistance and LME resistance. Sample No. 10, with a high dew point during annealing, exhibited coarsened granular oxides, forming an external oxide layer, but failed to obtain the desired grain boundary oxides, and therefore did not achieve high hydrogen embrittlement resistance. Sample No. 11 had a high holding temperature during annealing, resulting in coarsened granular oxide and failure to obtain the desired grain boundary oxide, thus failing to achieve high resistance to hydrogen embrittlement. Sample No. 12 had a low holding temperature during annealing, resulting in insufficient formation of a grain boundary oxide layer, also failing to achieve high resistance to hydrogen embrittlement. Sample No. 13 had a short holding time during annealing, resulting in insufficient formation of a grain boundary oxide layer, also failing to achieve high resistance to hydrogen embrittlement. Sample No. 14 had a long holding time during annealing, resulting in insufficient formation of an internal oxide layer and the formation of an external oxide layer, also failing to achieve high resistance to hydrogen embrittlement. Samples No. 15 and 17 had excessive Si and Mn content, respectively, resulting in insufficient formation of an internal oxide layer and the formation of an external oxide layer, also failing to achieve high resistance to hydrogen embrittlement. Samples No. 16 and 18 had insufficient Si and Mn content, respectively, resulting in insufficient formation of an internal oxide layer, and failing to achieve high resistance to hydrogen embrittlement and LME resistance. Sample No. 19 had an excessive Al content, resulting in insufficient formation of an internal oxide layer and the formation of an external oxide layer, thus failing to achieve high resistance to hydrogen embrittlement. Sample No. 20 had insufficient Al content, resulting in insufficient formation of both a surface deficient layer and an internal oxide layer, thus failing to achieve high resistance to hydrogen embrittlement and LME. Sample No. 21 was only humidified during the annealing heating process, resulting in a shortened humidification time and insufficient formation of a grain boundary oxide layer, thus failing to achieve high resistance to hydrogen embrittlement. Sample No. 22 had a thick internal oxide layer before annealing, but after annealing, it failed to form an internal oxide layer, resulting in the formation of an external oxide layer, thus failing to achieve high resistance to hydrogen embrittlement. Sample No. 37 failed to form an internal oxide layer due to the lack of tension applied to the steel plate during annealing, thus failing to achieve high resistance to hydrogen embrittlement. Sample No. 38 failed to form an internal oxide layer due to the lack of pre-annealing electrobrushing treatment, thus failing to achieve high resistance to hydrogen embrittlement. The humidification start temperature of sample No. 39 was above 600℃, which did not form an internal oxide layer sufficiently, and thus did not achieve high resistance to hydrogen embrittlement and LME.

[0245] In the example of the invention, a grain boundary oxide layer was identified in the base steel beneath the coating at a predetermined ratio, and a predetermined surface lack layer was also identified by EDS. Therefore, high resistance to hydrogen embrittlement and LME was obtained. On the other hand, in the comparative example, an internal oxide layer containing grain boundary oxides and / or a surface lack layer were not suitably formed near the surface of the base steel. Therefore, at least one of the following was identified: significant hydrogen intrusion and poor LME resistance.

[0246] Industrial availability

[0247] According to the present invention, a high-strength hot-dip galvanized steel sheet with high resistance to LME and hydrogen embrittlement can be provided. This hot-dip galvanized steel sheet is suitable for use in automobiles, home appliances, building materials, etc., especially in automobiles. As a coated steel sheet for automobiles, it is expected to have high collision safety and long service life. Therefore, the present invention can be said to be an invention of extremely high industrial value.

[0248] Symbol Explanation

[0249] 1. Steel plate

[0250] 2. External oxide layer

[0251] 3. Base steel

[0252] 11 Steel Plate

[0253] 12 Fine-grained oxides

[0254] 13 Grain boundary oxides

[0255] 14. Base steel

[0256] 15. Coarse-grained oxides

[0257] 16 Alloyed hot-dip galvanized layer

[0258] 17 Alloyed hot-dip galvanized steel sheet

Claims

1. An alloyed hot-dip galvanized steel sheet comprising a steel sheet and an alloyed hot-dip galvanized layer, the steel sheet having a composition consisting of, in mass %: Si: 0.2 to 3.0%, Mn: 0.1 to 5.0%, sol. Al: 0.4 to 1.50%, P: 0.0300% or less, S: 0.0300% or less, N: 0.0100% or less, Ti: 0 to 0.150%, Nb: 0 to 0.150%, Ni: 0 to 2.00%, Cu: 0 to 2.00%, Mo: 0 to 1.00%, Ca: 0 to 0.100%, Mg: 0 to 0.100%, Hf: 0 to 0.100%, and REM: 0 to 0.100%, the remainder consisting of Fe and impurities, and having a composition consisting of, in mass %: Fe: 5.0 to 15.0%, and Al: 0.01 to 1.0%, the remainder consisting of Zn and impurities, an internal oxidation layer containing a grain boundary type oxide, which is an oxide present along a crystal grain boundary of the steel, not including an oxide present within a crystal grain of the steel, and observed in a linear shape when a cross section of a surface layer of the steel sheet is observed, the grain boundary type oxide, when the cross section of the surface layer of the steel sheet is observed, has a ratio A of a length of the grain boundary type oxide projected on an interface of the steel sheet and the alloyed hot-dip galvanized layer with respect to a length of the interface of 50% or more and 100% or less, the surface layer of the steel sheet contains, at a depth of 1 / 2 of an average depth of the internal oxidation layer, a surface layer deficient layer free of a fine granular type oxide, a coarse granular type oxide, and the grain boundary type oxide, the steel composition of the surface layer deficient layer satisfies Si ≤ 0.6% and Al ≥ 0.05% in mass %, the surface layer deficient layer refers to a region in which an easily oxidizable component element present in the surface layer of the steel sheet is deficient compared to a base material of the steel sheet, the granular type oxide refers to an oxide dispersed in a granular shape within a crystal grain or on a crystal grain boundary of the steel, the fine granular type oxide has a particle diameter of 20 nm or more and 100 nm or less, and the coarse granular type oxide has a particle diameter of 150 nm or more and 600 nm or less. C：0.05～0.40%、 The ratio A is 90% or more. The alloyed hot-dip galvanized layer contains an oxide having a particle diameter of 0.1 to 1.5 pm at a number density of 1 to 10 pieces / (5 pm x 5 pm). ​ ​ ​ ​ B：0～0.010%、 ​ ​ V：0～0.150%、 Cr：0～2.00%、 ​ ​ ​ W：0～1.00%、 ​ ​ Zr：0～0.100%、 ​ ​ The alloyed hot-dip galvanizing layer is 10-100 g / m 2 is attached to at least one side of the steel sheet, ​ ​ ​ ​ ​ ​ 2. The galvannealed steel sheet according to claim 1, wherein, ​ 3. Galvannealed steel sheet according to claim 1 or 2, wherein, ​

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