Steel plates and coated steel plates

Abstract

To provide a high-strength steel sheet and a plated steel sheet having high plating property, liquid metal embrittlement (LME) resistance, and hydrogen embrittlement resistance. There is provided a steel sheet and a plated steel sheet using the same, the 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 having an internal oxide layer containing fine granular oxides, coarse granular oxides, and grain boundary oxides in the surface layer of the steel sheet. The number density of the fine granular oxides in the internal oxide layer is 4.0 particles / μm 2 or more, the number density of the coarse granular oxides in the internal oxide layer is 4.0 particles / 25 μm 2 or more. When observing the cross-section of the surface layer of the steel sheet, the ratio A of the length of the grain boundary oxides projected onto the surface of the steel sheet to the length of the surface of the steel sheet is 50% or more and 100% or less. The surface layer of the steel sheet contains a surface layer depletion layer, and the steel composition without oxides at a depth of 1 / 2 of the average depth of the internal oxide layer calculated from the cross-section SEM image of the steel sheet satisfies Si ≤ 0.6% and Al ≥ 0.05% by mass.

Description

Technical Field

[0001] The present invention relates to steel plates and coated steel plates. More specifically, the present invention relates to high-strength steel plates and coated steel plates having high coatability, LME resistance, and hydrogen embrittlement resistance. Background Art

[0002] In recent years, the strengthening of steel plates used in various fields such as automobiles, home appliances, and building materials has been promoted. For example, in the automotive field, in order to improve fuel efficiency and aim at vehicle body weight reduction, the use of high-strength steel plates is increasing. Such high-strength steel plates typically contain elements such as C, Si, Mn, and Al in order to increase the strength of the steel.

[0003] In the manufacture of high-strength steel plates, heat treatment such as annealing treatment is generally performed after rolling. In addition, among the elements typically contained in high-strength steel plates, oxidation-prone elements such as Si, Mn, or Al sometimes combine with oxygen in the atmosphere during the above heat treatment to form a layer containing oxides near the surface of the steel plate. As the form of such a layer, a form in which an oxide containing Si, Mn, or Al is formed in the form of a film on the outside (surface) of the steel plate (external oxide layer) and a form in which an oxide is formed inside the steel plate (surface layer) (internal oxide layer) can be cited.

[0004] When a plating layer (for example, a Zn-based plating layer) is formed on the surface of a steel plate having an external oxide layer, since the oxide exists in the form of a film on the surface of the steel plate, it sometimes hinders the mutual diffusion of the steel components (for example, Fe) and the plating components (for example, Zn), affects the adhesion between the steel and the plating layer, and the coatability becomes insufficient (for example, the unplated portion increases). Therefore, from the viewpoint of improving coatability, a steel plate having an internal oxide layer is more preferable than a steel plate having an external oxide layer.

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

[0006] Prior Art Documents

[0007] Patent Documents

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

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

[0010] Problems to be Solved by the Invention

[0011] High-strength steel sheets used in automotive components and the like are sometimes used in an atmospheric corrosion environment where the temperature and humidity change significantly. It is known that if a high-strength steel sheet is exposed to such an atmospheric corrosion environment, hydrogen generated during the corrosion process will penetrate into the steel. The hydrogen that penetrates into the steel can segregate at the martensite grain boundaries of the steel structure, embrittling the grain boundaries and causing cracking of the steel sheet. The phenomenon of cracking caused by the intrusion of this hydrogen is called hydrogen embrittlement cracking (delayed fracture), and it often becomes a problem during the processing of the steel sheet. Therefore, in order to prevent hydrogen embrittlement cracking, it is effective to reduce the hydrogen accumulation amount contained in the steel for steel sheets used in a corrosion environment.

[0012] In addition, in the case of hot stamping forming or welding of a plated steel sheet having a Zn-based coating or the like provided on a high-strength steel sheet, since the plated steel sheet is processed at a high temperature (for example, around 900 °C), it may be processed in a state where Zn contained in the coating is molten. In this case, sometimes the molten Zn penetrates into the steel and causes cracking inside the steel sheet. Such a phenomenon is called liquid metal embrittlement (LME), and it is known that the fatigue characteristics of the steel sheet are reduced due to this LME. Therefore, in order 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] In Patent Documents 1 and 2, it is taught that by controlling the average depth of the internal oxide layer to be relatively thick at 4 μm or more and making the internal oxide layer function as a hydrogen trapping site, the intrusion of hydrogen can be prevented and hydrogen embrittlement can be suppressed. However, no study has been conducted on the control of the morphology of the oxides present in the internal oxide layer, and there is room for improvement in terms of hydrogen embrittlement resistance. In addition, no study has been conducted on the improvement of LME resistance.

[0014] In view of such actual situations, the subject of the present invention is to provide a high-strength steel sheet and a plated steel sheet having high plating properties, LME resistance, and hydrogen embrittlement resistance.

[0015] Means for Solving the Subject

[0016] The present inventors have found that in order to solve the above-mentioned problems, it is important to form oxides on the surface layer of the steel plate, i.e., inside the steel plate, and further control the morphology of the oxides present in the surface layer of the steel plate. More specifically, the present inventors have found that by forming an internal oxide layer to ensure high coating properties, a large amount of fine granular oxides present in the grains of the metal structure in the form of oxides contained in the internal oxide layer are formed, so that the fine granular oxides not only function as capture sites for hydrogen that can penetrate into the steel under a corrosive environment, but also function as capture sites for Zn that can penetrate into the steel during hot stamping or welding, thereby achieving high LME resistance and hydrogen embrittlement resistance; and by also forming a large amount of coarse granular oxides, the coarse granular oxides not only function as capture sites for hydrogen that can penetrate into the steel under a corrosive environment, but also function as capture sites for Zn that can penetrate into the steel during hot stamping or welding. It functions as a capture site for hydrogen that can invade into the steel, and also functions as a capture site for Zn that can invade into the steel during hot stamping or welding, thereby achieving higher resistance to LME and hydrogen embrittlement. It also achieves higher resistance to hydrogen embrittlement by forming a large amount of grain boundary oxides present at the crystal grain boundaries of the metal structure, allowing the grain boundary oxides to function as escape paths for hydrogen that has invaded the steel. Furthermore, by forming a layered region (sometimes referred to as a surface deficiency layer) in which the composition of the metal structure at a depth of 1 / 2 of the internal oxide layer is low in Si and high in Al, it is possible to achieve higher resistance to LME.

[0017] The present invention has been made based on the above findings, and the gist of the present invention is as follows.

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

[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: 0.0300% or less,

[0024] S: 0.0300% or less,

[0025] N: 0.0100% or less,

[0026] B: 0~0.010%,

[0027] Ti: 0~0.150%,

[0028] Nb: 0-0.150%,

[0029] V: 0~0.150%

[0030] Cr: 0 to 2.00%,

[0031] Ni: 0 to 2.00%,

[0032] Cu: 0 to 2.00%,

[0033] Mo: 0 to 1.00%,

[0034] W: 0 to 1.00%,

[0035] Ca: 0 to 0.100%,

[0036] Mg: 0 to 0.100%,

[0037] Zr: 0 to 0.100%,

[0038] Hf: 0 to 0.100%, and

[0039] REM: 0 to 0.100%, the balance being composed of Fe and impurities.

[0040] The surface layer of the above steel plate has an internal oxide layer containing fine granular oxides, coarse granular oxides, and grain boundary oxides.

[0041] When observing the cross-section of the surface layer of the above steel plate,

[0042] The number density of the above fine granular oxides in the above internal oxide layer is 4.0 per μm 2 or more.

[0043] The number density of the above coarse granular oxides in the above internal oxide layer is 4.0 per 25 μm 2 or more.

[0044] The ratio A of the length of the above grain boundary oxides projected onto the surface of the above steel plate to the length of the surface of the above steel plate is 50% or more and 100% or less.

[0045] The surface layer of the above steel plate contains a surface layer depletion layer, and the steel composition that does not contain the above fine granular oxides, the above coarse granular oxides, and the above grain boundary oxides at a depth of 1 / 2 of the average depth of the above internal oxide layer satisfies Si ≤ 0.6% and Al ≥ 0.05% by mass.

[0046] (2) The steel plate according to (1), wherein

[0047] The number density of the above fine granular oxides is 4.0 to 30 per μm 2 .

[0048] (3) A plated steel sheet,

[0049] has a Zn-containing coating on the steel sheet described in (1) or (2).

[0050] (4) A plated steel sheet, characterized in that,

[0051] it is the Zn-plated steel sheet described in (3), and the Al contained in the coating is 0.3 to 1.5% by mass.

[0052] Advantages of the Invention

[0053] According to the present invention, fine granular oxides and coarse granular oxides abundantly present in the surface layer of the steel sheet can function as trapping sites for hydrogen invading in a corrosive environment. As a result, the amount of hydrogen invading in the corrosive environment can be significantly suppressed, and the hydrogen embrittlement resistance can be greatly improved. In addition, the fine granular oxides and coarse granular oxides also function as trapping sites for Zn invading into the steel during hot stamping forming or welding processing, and can significantly suppress the amount of invading Zn, and greatly improve the LME resistance. In addition, by forming a large amount of grain boundary oxides, the grain boundary oxides can function as a hydrogen escape path for hydrogen invading into the steel. As a result, the invading hydrogen can be released to reduce the amount of hydrogen accumulated in the steel, and the hydrogen embrittlement resistance can be greatly improved. Furthermore, according to the present invention, by forming a layered region (sometimes referred to as a "surface layer deficient layer") with a low Si and high Al composition at a depth of 1 / 2 of the internal oxide layer, Al can prevent Zn invading into the steel during hot stamping forming or welding processing from invading into the steel grain boundaries, and can further improve the LME resistance. Moreover, since the fine granular oxides, coarse granular oxides, grain boundary oxides and surface layer deficient layer are formed inside the steel sheet, when a coating is formed, the mutual diffusion between the steel components and the coating components proceeds sufficiently, and high plating properties can be obtained. Therefore, by the present invention, for high-strength steel sheets, high plating properties, LME resistance and hydrogen embrittlement resistance can be obtained. Brief Description of the Drawings

[0054] Figure 1 A schematic diagram showing a cross-section of a steel sheet having an external oxide layer.

[0055] Figure 2 A schematic diagram showing a cross-section of an exemplary steel sheet of the present invention.

[0056] Figure 3 A schematic diagram for explaining the measurement of ratio A in the present invention. Detailed Description of the Invention

[0057] <Steel Sheet>

[0058] The steel plate of the present invention is characterized in that it has the following composition: containing, by mass%,

[0059] C: 0.05 to 0.40%,

[0060] Si: 0.2 to 3.0%,

[0061] Mn: 0.1 to 5.0%,

[0062] sol.Al: 0.4 to 1.50%,

[0063] P: 0.0300% or less,

[0064] S: 0.0300% or less,

[0065] N: 0.0100% or less,

[0066] B: 0 to 0.010%,

[0067] Ti: 0 to 0.150%,

[0068] Nb: 0 to 0.150%,

[0069] V: 0 to 0.150%,

[0070] Cr: 0 to 2.00%,

[0071] Ni: 0 to 2.00%,

[0072] Cu: 0 to 2.00%,

[0073] Mo: 0 to 1.00%,

[0074] W: 0 to 1.00%,

[0075] Ca: 0 to 0.100%,

[0076] Mg: 0 to 0.100%,

[0077] Zr: 0 to 0.100%,

[0078] Hf: 0 to 0.100% and

[0079] REM: 0 to 0.100%, and the balance is composed of Fe and impurities.

[0080] An internal oxide layer containing fine granular oxides, coarse granular oxides and grain boundary oxides is present in the surface layer of the above steel plate.

[0081] When observing the cross-section of the surface layer of the above steel plate,

[0082] The number density of the above-mentioned fine granular oxides in the above-mentioned internal oxide layer is 4.0 pieces / μm 2 or more,

[0083] The number density of the above-mentioned coarse granular oxides in the above-mentioned internal oxide layer is 4.0 pieces / 25μm 2 or more,

[0084] The ratio A of the length of the above-mentioned grain boundary type oxides projected on the surface of the above-mentioned steel plate to the length of the surface of the above-mentioned steel plate is 50% or more and 100% or less,

[0085] The surface layer of the above-mentioned steel plate includes a surface layer depletion layer, and the steel composition that does not contain the above-mentioned fine granular oxides, the above-mentioned coarse granular oxides, and the above-mentioned grain boundary type oxides at a depth of 1 / 2 of the average depth of the above-mentioned internal oxide layer satisfies Si≤0.6% and Al≥0.05% by mass.

[0086] In the manufacture of high-strength steel plates, after rolling (typically hot rolling and cold rolling) a steel billet adjusted to a specified composition, annealing treatment is generally performed for the purpose of obtaining a desired structure and the like. In this annealing treatment, components in the steel plate that are relatively easy to oxidize (such as Si, Mn, Al) combine with oxygen in the annealing atmosphere, thereby forming a layer containing oxides near the surface of the steel plate. For example, as shown in Figure 1 Steel plate 1 shown, an external oxide layer 2 is formed in a film shape on the surface of the base metal steel 3 (that is, outside the base metal steel 3). If an external oxide layer 2 is formed in a film shape on the surface of the base metal steel 3, when a plating layer (such as a zinc-based plating layer) is formed, this external oxide layer 2 will hinder the mutual diffusion of the plating components (such as Zn, Al) and the steel components (such as Fe), so the adhesion between the steel and the plating layer cannot be fully ensured, and sometimes an unplated portion where the plating layer is not formed occurs.

[0087] In contrast, as exemplified in Figure 2 the steel plate 11 of the present invention is not like the steel plate 1 shown in Figure 1 where an external oxide layer 2 is formed on the surface of the base metal steel 3, but fine granular oxides 12, coarse granular oxides 15, and grain boundary type oxides 13 exist inside the base metal steel 14. Therefore, when a plating layer is formed on the surface of the steel plate 11, the steel plate 11 of the present invention in which oxides 12, coarse granular oxides 15, and grain boundary type oxides 13 are formed inside the base metal steel 14 and Figure 1Compared with the steel sheet 1 having the external oxide layer 2 as described above, sufficient interdiffusion between the plating component and the steel component occurs, and high plating property can be obtained. Therefore, the inventors of the present invention found that, from the viewpoint of obtaining high plating property, it is effective to control the conditions during the annealing treatment to form oxides inside the steel sheet. It should be noted that the term "high plating property" when used for a steel sheet means that when a plating treatment is carried out on the steel sheet, the plating layer can be formed in a state where the unplated part (the part where the plating layer is not formed) is small (for example, 5.0 area% or less) or completely absent. In addition, the term "high plating property" when used for a plated steel sheet means a plated steel sheet in a state where the unplated part is extremely small (for example, 5.0 area% or less) or completely absent. From the above viewpoint of plating property, although it is more preferable that the external oxide layer in the steel sheet 11 of the present embodiment is less, as long as it is within the range where high plating property can be obtained, it may have an external oxide layer.

[0088] In addition, high-strength steel sheets used in an atmospheric environment, particularly high-strength steel sheets for automobiles, are repeatedly exposed to various environments with different temperatures and humidities. Such an environment is called an atmospheric corrosion environment, and it is known that hydrogen is generated during the corrosion process in this atmospheric corrosion environment. Moreover, this hydrogen penetrates deeper than the surface layer region in the steel, segregates at the martensite grain boundaries in the steel sheet structure, and causes hydrogen embrittlement cracking (delayed fracture) in the steel sheet by embrittling the grain boundaries. Since martensite is a hard structure, it has a high hydrogen sensitivity and is prone to hydrogen embrittlement cracking. Such cracking can become a problem during the processing of the steel sheet. Therefore, in order to prevent hydrogen embrittlement cracking, in high-strength steel sheets used in an atmospheric corrosion environment, it is effective to reduce the hydrogen accumulation amount in the steel, more specifically, to reduce the hydrogen accumulation amount at a position deeper than the surface layer region of the steel sheet. The inventors of the present invention found that by controlling the morphology of the oxides present in the surface layer of the steel sheet, more specifically, by having "fine granular oxides" with a particle size and number density within a specified range as the oxides present, the fine granular oxides function as capture sites for hydrogen invading in the corrosion environment in the surface layer region of the steel sheet, and can reduce the hydrogen accumulation amount in the steel sheet used in the corrosion environment; in addition, there are also "coarse granular oxides" with a particle size and number density within a specified range as the oxides, and the coarse granular oxides function as capture sites for hydrogen invading in the corrosion environment in the surface layer region of the steel sheet, and can further reduce the hydrogen accumulation amount in the steel sheet used in the corrosion environment; furthermore, by coexisting with "grain boundary oxides" present at a specified ratio, the grain boundary oxides function as a path for the released hydrogen, and can reduce the hydrogen accumulation amount in the steel sheet used in the corrosion environment not only by suppressing the invasion of hydrogen but also by promoting the release of the invaded hydrogen to the outside of the system. It should be noted that the term "high hydrogen embrittlement resistance" means a state in which the amount of hydrogen accumulated in the steel sheet and the plated steel sheet is reduced in such a way that hydrogen embrittlement cracking can be sufficiently suppressed.

[0089] The inventors of the present invention have conducted a detailed analysis of the relationship between the morphology of oxides and their effectiveness as hydrogen capture sites, and found that: as shown in Figure 2 , it is effective to have a large number of finely granular oxides 12 dispersed in a granular form and separated from each other in the surface layer of the base steel 14. Moreover, it has been found that it is more effective to have a large number of coarsely granular oxides 15 dispersed in a granular form and separated from each other in the surface layer of the base steel 14. Although not bound by a specific theory, it is considered that the hydrogen capture function of the oxides in the steel plate with respect to the intruded hydrogen is positively correlated with the surface area of the oxides. That is, by having a large number of fine oxides discretely dispersed from each other in the surface layer of the steel plate, the surface area of the oxides in the surface layer of the steel plate increases, and the hydrogen capture function is improved. Furthermore, it is considered that in the case where hydrogen intrudes excessively and cannot be captured by the fine oxides, the capacity of the coarse oxides is relatively large and the amount of hydrogen that can be captured is also large, so that the excessively intruded hydrogen can also be captured, and the hydrogen capture function is further improved. Thus, the inventors of the present invention have found that from the viewpoint of obtaining high hydrogen embrittlement resistance, it is important to control the conditions during the manufacture of the steel plate, particularly during the annealing treatment, so that a large amount of finely granular oxides and coarsely granular oxides that function as hydrogen capture sites for the hydrogen intruded when placed in a corrosive environment are present. It should be noted that the metal structure of the surface layer of the steel plate typically consists of a softer metal structure than the inside of the steel plate (for example, at the 1 / 8 position or 1 / 4 position of the plate thickness). Therefore, even if hydrogen is present in the surface layer of the steel plate, hydrogen embrittlement cracking does not particularly become a problem.

[0090] In addition, the inventors of the present invention have conducted a detailed analysis of the relationship between the morphology of oxides and their effectiveness as hydrogen escape paths, and found that: as shown in Figure 2 , it is effective to have a large number of grain boundary oxides 13 present at the grain boundaries in the surface layer of the base steel 14. It has been found that by having a large number of grain boundary oxides 13 present, a path for hydrogen in the steel to the outside of the system can be ensured, and the hydrogen intruded into the steel can be effectively released to the outside of the system along the grain boundaries. In addition, it has also been found that if the grain boundary oxides exist in a deeper position of the steel plate, more hydrogen can be released from the inside of the steel plate to the outside of the system, and the hydrogen accumulation amount in the steel plate can be further reduced. Thus, by having the above-mentioned granular oxides coexist with the grain boundary oxides, the hydrogen embrittlement resistance can be greatly improved.

[0091] On the other hand, when hot stamping forming or welding is performed on a plated steel sheet having a Zn-containing coating on its surface, since it becomes high temperature during processing, Zn contained in the coating sometimes melts. If Zn melts, the molten Zn penetrates into the steel. If processing is performed in this state, liquid metal embrittlement (LME) cracking sometimes occurs inside the steel sheet, and the fatigue characteristics of the steel sheet are reduced due to this LME. The inventors of the present invention have also found that if the above-mentioned fine granular oxide and coarse granular oxide have a desired number density, not only the hydrogen embrittlement resistance is improved, but also it contributes to the improvement of LME resistance. More specifically, it has been found that the fine granular oxide and the coarse granular oxide function as capture sites for Zn that penetrates into the steel during processing at high temperature. Thus, for example, Zn that penetrates into the steel during hot stamping forming is captured by the fine granular oxide and the coarse granular oxide on the surface layer of the steel sheet, and the penetration of Zn into the crystal grain boundaries can be appropriately suppressed. Therefore, it has been found that it is important to have a large amount of fine granular oxide and coarse granular oxide not only for improving the above-mentioned hydrogen intrusion resistance but also for improving the LME resistance.

[0092] In addition, the fine granular oxide, the coarse granular oxide, and the grain boundary oxide are substances formed by oxidizing relatively easily oxidizable components (such as Si, Mn, Al) in the steel sheet. Therefore, the composition of the steel (in other words, the metal structure) around the oxide lacks these easily oxidizable component elements compared with the base material of the original steel sheet. The region where the elements of the steel composition are lacking compared with the original steel sheet base material is also called the "lacking region". The layered "lacking region" is also called the "lacking layer", and further, the lacking layer present in the surface layer of the steel sheet is also called the "surface layer lacking layer". In the lacking region, 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 can exist at a high concentration. The inventors of the present invention have also found that if a lacking region having a steel composition of low Si and high Al exists within a desired range, it also contributes to the improvement of LME resistance. More specifically, it has also been found that in addition to the granular oxide and the coarse granular oxide that function as Zn capture sites, and by having Al present in the composition of the steel around the granular oxide and the grain boundary oxide, this Al hinders the penetration of Zn that penetrates into the steel during processing at high temperature into the steel grain boundaries; in addition, the higher the concentration of Si in the steel composition, the more likely LME cracking occurs, and by setting Si to as low a concentration as possible, LME can be suppressed. Thus, for example, the penetration of Zn that penetrates into the steel during hot stamping forming into the steel grain boundaries is hindered by Al in the composition of the steel, and the penetration of Zn into the crystal grain boundaries can be appropriately suppressed. In addition, since Si that is likely to cause LME is at a low concentration, LME is not likely to occur. Therefore, it has been found that it is important to have a lacking region where Si exists at a low concentration and Al exists at a high concentration in order to improve the LME resistance.

[0093] The deficient region where Si is present at a low concentration and Al is present at a high concentration is a region that can overlap with the regions where the fine granular oxide, the coarse granular oxide, and the above-mentioned grain boundary oxide are distributed. That is, it is not formed like the external oxide layer 2 on the surface of the base metal steel 3 as shown in Figure 1 but can be formed inside the base metal steel. Therefore, when a plating layer is formed on the surface of the steel plate, the steel plate of the present invention in which a deficient region, more specifically, a surface deficient layer is formed inside the base metal steel Figure 1 compared with the steel plate 1 having the external oxide layer 2 as shown in

[0094] sufficient interdiffusion between the plating component and the steel component occurs, and high plating properties can be obtained.

[0095] [Composition of the steel plate]

[0096] The composition of the steel plate of the present invention will be described. "%" for the content of an element means "mass %" unless otherwise specified. Within the numerical range in the composition, for a numerical range represented by "~", unless otherwise specified, it means a range including the numerical values described before and after "~" as the lower limit value and the upper limit value.

[0097] (C: 0.05 to 0.40%)

[0098] C (carbon) is an element important for ensuring the strength of the steel. If the C content is insufficient, it may not be possible to ensure sufficient strength. Furthermore, sometimes the desired morphology of the internal oxide and / or the surface deficient layer cannot be obtained due to the insufficient C content. Therefore, the C content is 0.05% or more, preferably 0.07% or more, more preferably 0.10% or more, and further preferably 0.12% or more. On the other hand, if the C content is excessive, the weldability may decrease. Therefore, the C content is 0.40% or less, preferably 0.35% or less, and more preferably 0.30% or less.

[0099] (Si: 0.2 to 3.0%)

[0100] Si (Silicon) is an element effective in increasing the strength of steel. If the Si content is insufficient, it may not be possible to ensure sufficient strength. Furthermore, it may not be possible to sufficiently form the desired oxides, particularly fine granular oxides, coarse granular oxides, grain boundary oxides, and / or surface lack layers inside the steel sheet. Therefore, the Si content is 0.2% or more, preferably 0.3% or more, more preferably 0.5% or more, and further preferably 1.0% or more. On the other hand, if the Si content is excessive, it may cause deterioration of the surface properties. Furthermore, it may lead to coarsening of the granular oxides. Therefore, the Si content is 3.0% or less, preferably 2.5% or less, more preferably 2.0% or less.

[0101] (Mn: 0.1 - 5.0%)

[0102] Mn (Manganese) is an element effective in increasing the strength of steel by obtaining a hard structure. If the Mn content is insufficient, it may not be possible to ensure sufficient strength. Furthermore, it may not be possible to sufficiently form the desired oxides, particularly fine granular oxides, coarse granular oxides, grain boundary oxides, and / or surface lack layers inside the steel sheet. Therefore, the Mn content is 0.1% or more, preferably 0.5% or more, more preferably 1.0% or more, and further preferably 1.5% or more. On the other hand, if the Mn content is excessive, the metal structure may become non-uniform due to Mn segregation, and the workability may decrease. Furthermore, it may lead to coarsening of the granular oxides. Therefore, the Mn content is 5.0% or less, preferably 4.5% or less, more preferably 4.0% or less, and further more preferably 3.5% or less.

[0103] (sol.Al: 0.4 - 1.50%)

[0104] Al (Aluminum) is an element that acts as a deoxidizing element. If the Al content is insufficient, it may not be possible to ensure sufficient deoxidation effect. Furthermore, it may not be possible to sufficiently form the desired oxides, particularly fine granular oxides, coarse granular oxides, grain boundary oxides, and / or surface lack layers inside the steel sheet. The Al content can also be 0.4% or more, but in order to obtain sufficient desired effects, fine granular oxides, coarse granular oxides, grain boundary oxides, and surface lack layers, the Al content is preferably 0.5% or more, more preferably 0.6% or more, and even more preferably 0.7% or more. On the other hand, if the Al content is excessive, it may cause a decrease in workability and deterioration of the surface properties. Furthermore, it may lead to coarsening of the granular oxides. Therefore, the Al content is 1.50% or less, preferably 1.20% or less, more preferably 0.80% or less. The Al content refers to the so-called acid-soluble Al content (sol.Al).

[0105] (P: 0.0300% or less)

[0106] P (phosphorus) is generally an impurity contained in steel. When the P content exceeds 0.0300%, the weldability may decrease. Therefore, the P content is 0.0300% or less, preferably 0.0200% or less, more preferably 0.0100% or less, and further preferably 0.0050% or less. The lower limit of the P content is not particularly limited, and from the viewpoint of manufacturing cost, the P content may also exceed 0% or be 0.0001% or more.

[0107] (S: 0.0300% or less)

[0108] S (sulfur) is generally an impurity contained in steel. When the S content exceeds 0.0300%, the weldability may decrease, and furthermore, the precipitation amount of MnS increases and the workability such as bendability decreases. Therefore, the S content is 0.0300% or less, preferably 0.0100% or less, more preferably 0.0050% or less, and further preferably 0.0020% or less. The lower limit of the S content is not particularly limited, and from the viewpoint of desulfurization cost, the S content may also exceed 0% or be 0.0001% or more.

[0109] (N: 0.0100% or less)

[0110] N (nitrogen) is generally an impurity contained in steel. When the N content exceeds 0.0100%, the weldability may decrease. Therefore, the N content is 0.0100% or less, preferably 0.0080% or less, more preferably 0.0050% or less, and further preferably 0.0030% or less. The lower limit of the N content is not particularly limited, but from the viewpoint of manufacturing cost, the N content may also exceed 0% or be 0.0010% or more.

[0111] (B: 0 - 0.010%)

[0112] B (boron) is an element that improves hardenability and contributes to the improvement of strength. In addition, it segregates at grain boundaries to strengthen the grain boundaries and improve toughness, so it can also be contained as needed. Therefore, the B content is 0% or more, preferably 0.001% or more, more preferably 0.002% or more, and further 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.

[0113] (Ti: 0 - 0.150%)

[0114] Ti (titanium) is an element that precipitates as TiC during the cooling of steel and contributes to the improvement of strength, and it can also be contained as needed. Therefore, the Ti content is 0% or more, preferably 0.001% or more, more preferably 0.003% or more, further preferably 0.005% or more, and even more preferably 0.010% or more. On the other hand, if it is contained in excess, there is a possibility of forming coarse TiN and impairing toughness. Therefore, the Ti content is 0.150% or less, preferably 0.100% or less, more preferably 0.050% or less.

[0115] (Nb: 0 to 0.150%)

[0116] Nb (niobium) is an element that contributes to the improvement of strength by increasing hardenability, and thus can also be contained as needed. Therefore, the Nb content is 0% or more, preferably 0.010% or more, more preferably 0.020% or more, and further 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.

[0117] (V: 0 to 0.150%)

[0118] V (vanadium) is an element that contributes to the improvement of strength by increasing hardenability, and thus can also be contained as needed. Therefore, the V content is 0% or more, preferably 0.010% or more, more preferably 0.020% or more, and further 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.

[0119] (Cr: 0 to 2.00%)

[0120] Cr (chromium) is effective in increasing the hardenability of steel and thus increasing the strength of steel, and thus can also be contained as needed. Therefore, the Cr content is 0% or more, preferably 0.10% or more, more preferably 0.20% or more, further preferably 0.50% or more, and even more preferably 0.80% or more. On the other hand, if it is contained in excess, there is a possibility of forming a large amount of Cr carbides, which instead impairs hardenability. Therefore, the Cr content is 2.00% or less, preferably 1.80% or less, and more preferably 1.50% or less.

[0121] (Ni: 0 to 2.00%)

[0122] Ni (Nickel) is effective in increasing the hardenability and thus the strength of steel, and can therefore be contained as required. Therefore, the Ni content is 0% or more, preferably 0.10% or more, more preferably 0.20% or more, further preferably 0.50% or more, and even more preferably 0.80% or more. On the other hand, excessive addition of Ni will lead to an increase in cost, so the Ni content is 2.00% or less, preferably 1.80% or less, more preferably 1.50% or less.

[0123] (Cu: 0 - 2.00%)

[0124] Cu (Copper) is effective in increasing the hardenability and thus the strength of steel, and can therefore be contained as required. Therefore, the Cu content is 0% or more, preferably 0.10% or more, more preferably 0.20% or more, further preferably 0.50% or more, and even more preferably 0.80% or more. On the other hand, from the viewpoints of suppressing the reduction of toughness, cracking of the slab after casting, or reduction of weldability, the Cu content is 2.00% or less, preferably 1.80% or less, more preferably 1.50% or less.

[0125] (Mo: 0 - 1.00%)

[0126] Mo (Molybdenum) is effective in increasing the hardenability and thus the strength of steel, and can therefore be contained as required. Therefore, the Mo content is 0% or more, preferably 0.10% or more, more preferably 0.20% or more, further preferably 0.30% or more. On the other hand, from the viewpoints of suppressing the reduction of toughness and weldability, the Mo content is 1.00% or less, preferably 0.90% or less, more preferably 0.80% or less.

[0127] (W: 0 - 1.00%)

[0128] W (Tungsten) is effective in increasing the hardenability and thus the strength of steel, and can therefore be contained as required. Therefore, the W content is 0% or more, preferably 0.10% or more, more preferably 0.20% or more, further preferably 0.30% or more. On the other hand, from the viewpoints of suppressing the reduction of toughness and weldability, the W content is 1.00% or less, preferably 0.90% or less, more preferably 0.80% or less.

[0129] (Ca: 0 - 0.100%)

[0130] Ca (Calcium) is an element that helps in inclusion control, particularly in the fine dispersion of inclusions, and has the effect of improving toughness. Therefore, it can also be contained as needed. Thus, the Ca content is 0% or more, preferably 0.001% or more, more preferably 0.005% or more, further preferably 0.010% or more, and even more preferably 0.020% or more. On the other hand, if contained in excess, the deterioration of the surface properties may sometimes become apparent. Therefore, the Ca content is 0.100% or less, preferably 0.080% or less, and more preferably 0.050% or less.

[0131] (Mg: 0 to 0.100%)

[0132] Mg (Magnesium) is an element that helps in inclusion control, particularly in the fine dispersion of inclusions, and has the effect of improving toughness. Therefore, it can also be contained as needed. Thus, the Mg content is 0% or more, preferably 0.001% or more, more preferably 0.003% or more, and further preferably 0.010% or more. On the other hand, if contained in excess, the deterioration of the surface properties may sometimes become apparent. Therefore, the Mg content is 0.100% or less, preferably 0.090% or less, and more preferably 0.080% or less.

[0133] (Zr: 0 to 0.100%)

[0134] Zr (Zirconium) is an element that helps in inclusion control, particularly in the fine dispersion of inclusions, and has the effect of improving toughness. Therefore, it can also be contained as needed. Thus, the Zr content is 0% or more, preferably 0.001% or more, more preferably 0.005% or more, and further preferably 0.010% or more. On the other hand, if contained in excess, the deterioration of the surface properties may sometimes become apparent. Therefore, the Zr content is 0.100% or less, preferably 0.050% or less, and more preferably 0.030% or less.

[0135] (Hf: 0 to 0.100%)

[0136] Hf (Hafnium) is an element that helps in inclusion control, particularly in the fine dispersion of inclusions, and has the effect of improving toughness. Therefore, it can also be contained as needed. Thus, the Hf content is 0% or more, preferably 0.001% or more, more preferably 0.005% or more, and further preferably 0.010% or more. On the other hand, if contained in excess, the deterioration of the surface properties may sometimes become apparent. Therefore, the Hf content is 0.100% or less, preferably 0.050% or less, and more preferably 0.030% or less.

[0137] (REM: 0 to 0.100%)

[0138] REM (rare earth elements) are elements that contribute to inclusion control, especially fine dispersion of inclusions, and have the effect of improving toughness, so they can also be contained as needed. Therefore, 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 excessively contained, the deterioration of surface properties sometimes becomes apparent, so 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 the abbreviation of Rare Earth Metal, which refers to an element belonging to the lanthanide series. REM is usually added in the form of mixed rare earth metals.

[0139] In the steel sheet of the present invention, the remainder other than the above-mentioned composition is composed of Fe and impurities. Here, impurities are components that are introduced into raw materials such as ores and scrap during industrial steel sheet production due to various factors during the manufacturing process, and are permitted to be present within a range that does not adversely affect the properties of the steel sheet of the present invention.

[0140] In the present invention, the composition of the steel plate can be analyzed using elemental analysis methods known to those skilled in the art, such as inductively coupled plasma mass spectrometry (ICP-MS). Preferably, the combustion-infrared absorption method is used for C and S, and the inert gas fusion-thermal conductivity method is used for N. These analyses can be performed on samples collected from the steel plate using methods in accordance with JIS G0417:1999.

[0141] [surface layer]

[0142] In the present invention, the "surface layer" of a steel sheet refers to the region from the surface of the steel sheet (the interface between the steel sheet and the plating layer in the case of a plated steel sheet) to a predetermined depth in the sheet thickness direction. The "predetermined depth" is typically 50 μm or less.

[0143] like Figure 2 As shown in the example, in the steel plate 11 of the present invention, the surface layer of the steel plate 11 contains fine oxides 12, coarse granular oxides 15 and grain boundary oxides 13. It is preferred that the fine granular oxides 12, coarse granular oxides 15 and grain boundary oxides 13 exist only in the surface layer of the steel plate 11. The fine granular oxides 12, coarse granular oxides 15 and grain boundary oxides 13 exist inside the base steel 14 (that is, exist as internal oxides), and the fine granular oxides 12, coarse granular oxides 15 and grain boundary oxides 13 exist inside the base steel 14 (that is, exist as internal oxides). Figure 1Compared with the case where the external oxide layer 2 exists on the surface of the base metal steel 3 shown in [the figure], the steel sheet 11 can have high plating properties. It is considered that this is because when forming a plating layer (for example, a Zn-based plating layer) on the surface of the steel sheet, the oxide that can hinder the mutual diffusion of the plating components and the steel components is generated inside the steel sheet instead of outside the steel sheet. Therefore, the steel sheet and the plated steel sheet of the present invention, in which the surface layer of the steel sheet, that is, the inside of the steel sheet, contains granular oxides and grain boundary oxides, have high plating properties.

[0144] In addition, although not shown in Figure 2 in the steel sheet 11 of the present invention, in the surface layer of the steel sheet 11, in addition to the above-mentioned fine granular oxides 12, coarse granular oxides 15 and grain boundary oxides 13, a surface layer depletion layer is also included. The surface layer depletion layer is a region where the elemental ratio of the steel composition around the fine granular oxides 12, coarse granular oxides 15 and grain boundary oxides 13 is lacking compared to the original steel sheet base material, and exists in a manner repeating the regions where the fine granular oxides 12, coarse granular oxides 15 and grain boundary oxides 13 are distributed. That is, since the surface layer depletion layer exists inside the base metal steel 14 in the same way as the fine granular oxides 12, coarse granular oxides 15 and grain boundary oxides 13, the steel sheet and the plated steel sheet containing the fine granular oxides 12, coarse granular oxides 15, grain boundary oxides 13 and the surface layer depletion layer also have high plating properties.

[0145] [Fine granular oxides and coarse granular oxides]

[0146] In the present invention, the so-called "granular oxide" refers to an oxide dispersed in the form of granules in the grains or crystal grain boundaries of steel. In addition, the so-called "granular" means existing separately from each other in the steel matrix, for example, having an aspect ratio of 1.0 to 5.0 (the maximum line segment length (major axis) obtained by cutting the granular oxide transversely / the maximum line segment length (minor axis) obtained by cutting the oxide transversely perpendicular to the major axis). The so-called "dispersed in the form of granules" means that the positions of the respective particles of the oxide are not arranged according to a specific rule (for example, in a linear shape), but are randomly arranged. In fact, the granular oxides typically exist three-dimensionally in the form of spheres or substantially spheres in the surface layer of the steel sheet. Therefore, when observing the cross-section of the surface layer of the steel sheet, the granular oxides are typically observed in a circular or substantially circular shape. In Figure 2 as an example, the fine granular oxide 12 and the coarse granular oxide 15 that look substantially circular are shown. In Figure 2Among them, as a typical example of the steel plate 11, the coarse granular oxide 15 is shown below the fine granular oxide 12. It is considered that the larger the grain size of the granular oxide is more likely to grow in the interior of the steel plate. Near the surface of the steel plate, since the diffusion rate of oxygen diffusing from the atmosphere into the interior of the steel plate is fast, it is difficult for the granular oxide to coarsen. As the distance from the steel plate surface to the interior of the steel plate becomes farther, the diffusion rate of oxygen is slow, so the granular oxide becomes easy to coarsen. However, the coarse granular oxide 15 sometimes forms near the surface of the base metal steel 14.

[0147] (Particle size)

[0148] In the present invention, the particle size of the granular oxide is 20 nm or more and 600 nm or less. In this range, the particle size of the "fine" granular oxide is 20 nm or more and 100 nm or less, and the particle size of the "coarse" granular oxide is 150 nm or more and 600 nm or less. The upper limit of the particle size of the fine granular oxide (100 nm) and the lower limit of the particle size of the coarse granular oxide (150 nm) are set to avoid the difficulty in determining the fine granular oxide and the coarse granular oxide from the viewpoint of measurement accuracy. By controlling the particle size within such a range, the fine granular oxide and the coarse granular oxide can be dispersed in the surface layer of the steel plate. The fine granular oxide and the coarse granular oxide function well as hydrogen capture sites for suppressing hydrogen intrusion in a corrosive environment. Furthermore, they function well as capture sites for Zn that can penetrate during hot stamping forming or welding of a plated steel plate with a coating formed on the steel plate. On the other hand, if the particle size exceeds 600 nm, the number of granular oxides sometimes decreases, and it may not be possible to obtain the desired number density. The lower limit of the particle size of the granular oxide is 20 nm or more. The finer the granular oxide is, the higher the specific surface area becomes, and the higher the reactivity as a capture site is, but the amount of hydrogen and / or Zn that can be captured by each particle decreases, and it may not be possible to sufficiently capture hydrogen and / or Zn, and it may not function sufficiently as a hydrogen capture site and / or a Zn capture site.

[0149] (Number density of fine granular oxides)

[0150] In the present invention, the number density of the fine granular oxides is 4.0 pieces / μm 2 or more. By controlling the number density within such a range, a large amount of fine granular oxides can be dispersed in the surface layer of the steel plate. The fine granular oxides function well as hydrogen capture sites for suppressing hydrogen intrusion in a corrosive environment. Furthermore, they function well as capture sites for Zn that can penetrate during hot stamping forming or welding of a plated steel plate with a coating formed on the steel plate. On the other hand, if the number density is less than 4.0 pieces / μm2 Then, the number density of the hydrogen trapping site and / or the Zn trapping site is insufficient, and it is possible that the fine-grained oxide does not function sufficiently as the hydrogen trapping site and / or the Zn trapping site, and good hydrogen embrittlement resistance and / or LME resistance cannot be obtained. On the contrary, it is also possible that external oxidation is promoted and good plating properties cannot be obtained. The number density of the fine-grained oxide is preferably 6.0 pieces / μm 2 or more, more preferably 8.0 pieces / μm 2 or more, and still more preferably 10.0 pieces / μm 2 or more. From the viewpoint of functioning as the hydrogen trapping site and / or the Zn trapping site, the larger the amount of the fine-grained oxide present, the more preferable. However, the granular oxide sometimes becomes the starting point of LME cracking, and when it exceeds 100 pieces / μm 2 , the LME resistance may decrease. Therefore, the number density of the fine-grained oxide can also be 100 pieces / μm 2 or less, 90 pieces / μm 2 or less, 80 pieces / μm 2 or less, 70 pieces / μm 2 or less, 60 pieces / μm 2 or less, 50 pieces / μm 2 or less, 40 pieces / μm 2 or less, 30 pieces / μm 2 or less, 25 pieces / μm 2 or less, 20 pieces / μm 2 or less.

[0151] The particle size and number density of the fine granular oxide are measured by a scanning electron microscope (SEM). The specific measurement is as follows. The cross-section of the surface layer of the steel plate is observed by SEM to obtain an SEM image containing the fine granular oxide. As the observation area, a total of 10 areas of 1.0 μm (in the depth direction) × 1.0 μm (in the width direction) are selected from this SEM image. As the observation position of each area, in the depth direction (the direction perpendicular to the surface of the steel plate), it is set to 1.0 μm in the area from the surface of the steel plate to 1.5 μm, and in the width direction (the direction parallel to the surface of the steel plate), it is set to 1.0 μm at an arbitrary position in the above SEM image. Then, the SEM images of the respective areas selected as described above are extracted, binarized to separate the oxide part from the steel part, and the area of the granular oxide part is calculated from each binarized image. As the diameter of a circle having the same area as this area, that is, the equivalent circle diameter, the particle size (nm) of this granular oxide is obtained, and substances in the range of 20 nm or more and 100 nm or less in particle size are regarded as fine granular oxides. Furthermore, the number of fine granular oxides in each binarized image is counted. The average value of the total number of fine granular oxides in the 10 areas thus obtained is used as the number density of the fine granular oxides (pieces / μm 2 ). It should be noted that when only a part of the granular oxide is observed in the observation area, that is, when the entire contour of the granular oxide is not within the observation area, it is not counted as the number.

[0152] (Number density of coarse granular oxide)

[0153] In addition, the number density of the coarse granular oxide is 4.0 pieces / 25 μm 2 or more. By controlling the number density within such a range, a large amount of coarse fine granular oxides can be dispersed in the surface layer of the steel plate, and the coarse granular oxides function well as hydrogen capture sites for suppressing hydrogen intrusion in a corrosive environment. Furthermore, they function well as capture sites for Zn that can intrude during hot stamping forming or welding processing of a plated steel plate having a plating layer formed on the steel plate. On the other hand, if the number density is less than 4.0 pieces / 25 μm 2 , the number density as a hydrogen capture site and / or a Zn capture site is insufficient, and it is possible that the coarse granular oxides do not function sufficiently as a hydrogen capture site and / or a Zn capture site, and good hydrogen embrittlement resistance and / or LME resistance cannot be obtained. On the contrary, it is also possible that external oxidation is promoted and good plating properties cannot be obtained. The number density of the coarse granular oxides is preferably 6.0 pieces / 25 μm 2 or more, and more preferably 8.0 pieces / 25 μm 2More preferably, it is 10.0 pieces / 25 μm or more. 2 From the viewpoint of functioning as a hydrogen capture site and / or a Zn capture site, the larger the amount of the coarse granular oxide, the more preferable. However, the coarse granular oxide sometimes becomes a starting point for LME cracking. When it exceeds 50 pieces / 25 μm 2 there is a possibility that the LME resistance decreases. Therefore, the number density of the coarse granular oxide can also be 50 pieces / 25 μm 2 or less, 40 pieces / 25 μm 2 or less, 30 pieces / 25 μm 2 or less, 25 pieces / 25 μm 2 or less, 20 pieces / 25 μm 2 or less.

[0154] The particle size and number density of the coarse granular oxide are measured by a scanning electron microscope (SEM). The specific measurement is as follows. The cross-section of the surface layer of the steel plate is observed by SEM to obtain an SEM image containing the coarse granular oxide. As the observation area, a total of 10 areas of 5.0 μm (depth direction) × 5.0 μm (width direction) are selected from the SEM image. As the observation position of each area, in the depth direction (the direction perpendicular to the surface of the steel plate), it is set to 5.0 μm in the area from the surface of the steel plate to 8.0 μm, and in the width direction (the direction parallel to the surface of the steel plate), it is set to 5.0 μm at an arbitrary position of the above SEM image. Then, the SEM images of the respective areas selected as described above are extracted, binarized to separate the oxide part from the steel part, and the area of the granular oxide part is calculated from each binarized image. As the diameter of a circle having the same area as this area, that is, the equivalent circle diameter, the particle size (nm) of the granular oxide is obtained. The substance in the range where the particle size is 150 nm or more and 600 nm or less is regarded as the coarse granular oxide. Further, the number of coarse granular oxides in each binarized image is counted. The average value of the total number of coarse granular oxides in the 10 areas thus obtained is used as the number density of the coarse granular oxide (pieces / 25 μm 2 ). It should be noted that when only a part of the granular oxide is observed in the observation area, that is, when the entire contour of the granular oxide is not within the observation area, it is not counted as the number.

[0155] [Grain boundary oxide]

[0156] In the present invention, the so-called "grain boundary type oxide" refers to an oxide existing along the crystal grain boundaries of steel and does not include oxides existing within the grains of steel. In fact, since the grain boundary type oxide exists in a planar form along the crystal grain boundaries in the surface layer of the steel plate, when observing the cross-section of the surface layer of the steel plate, the grain boundary type oxide is observed as a line. In Figure 2 and Figure 3 , as an example, a grain boundary type oxide 13 that appears as a line is shown. In addition, in Figure 2 and Figure 3 , as a typical example of the steel plate 11, the grain boundary type oxide 13 is shown below the fine granular type oxide 12 and the coarse granular type oxide 15, but the grain boundary type oxide 13 may sometimes be formed near the surface of the base steel 14.

[0157] (Ratio A)

[0158] In the present invention, the so-called "Ratio A" is, as shown in Figure 3 , the ratio of "the length of the grain boundary type oxide projected onto the surface of the steel plate: L (= L1 + L2 + L3 + L4)" in the observation image to "the length of the surface of the steel plate: L0" when observing the cross-section of the surface layer of the steel plate 11. In the present invention, Ratio A is 50% or more and 100% or less. By controlling Ratio A within such a range, a large amount of the grain boundary type oxide 13 can exist in the surface layer of the steel plate, and the grain boundary type oxide 13 functions well as an escape path for hydrogen invading the steel. On the other hand, if Ratio A is less than 50%, there may not be a sufficient amount of the grain boundary type oxide 13 as an escape path for hydrogen, the hydrogen accumulation amount in the steel cannot be sufficiently reduced, and good hydrogen embrittlement resistance cannot be obtained. On the contrary, external oxidation may be promoted, and good plating properties cannot be obtained. Ratio A is preferably 60% or more, more preferably 70% or more, further preferably 80% or more, still further preferably 90% or more, and most preferably 100%.

[0159] Ratio A is as shown in Figure 3As shown in [reference], it is determined by observing the cross-section of the surface layer of the steel sheet 11. The specific measurement method is as described below. The cross-section of the surface layer of the steel sheet 11 is observed by SEM. The observation position is set at a randomly selected part. The length L0 of the surface (i.e., the width of the SEM image) is measured from the observed SEM image. The length L0 is set to be 100 μm or more (e.g., 100 μm, 150 μm, or 200 μm), and the measurement depth is set to the region from the surface of the steel sheet to 50 μm. Then, the position of the grain boundary type oxide 13 is determined from this SEM image, and the determined grain boundary type oxide 13 is projected onto the surface of the steel sheet 11 (in the case of a plated steel sheet, on the interface between the steel sheet 11 and the plating layer), and the length L (= L1 + L2 + L3 + L4) of the grain boundary type oxide 13 in the field of view is obtained. Based on L0 and L thus obtained, the ratio A (%) in the present invention is obtained as A (%) = 100 × L / L0. It should be noted that attention should be paid to Figure 3 is a figure in which the fine granular type oxide 12 and the coarse granular type oxide 15 are omitted for the purpose of illustration.

[0160] [Depth D]

[0161] In the present invention, the so-called "depth D" is as Figure 3 shown in [reference], and refers to the distance from the surface of the steel sheet 11 (in the case of a plated steel sheet, the interface between the steel sheet and the plating layer) along the thickness direction of the steel sheet 11 (the direction perpendicular to the surface of the steel sheet) to the farthest position where the grain boundary type oxide 13 exists. As described above, the grain boundary type oxide can function as a path for the escape of hydrogen invading the steel sheet. Therefore, if the depth D of this grain boundary type oxide is large, hydrogen can be released to the outside of the system from a deeper position of the steel sheet, and thus the above function can be more suitably exerted. In the steel sheet of the present invention, the depth D of the grain boundary type oxide is preferably 3.0 μm or more, more preferably 5.0 μm or more, and further preferably 7.0 μm or more. The upper limit of the depth D is not particularly limited, and the depth D is substantially 50.0 μm or less. The depth D can be obtained from the same image as the SEM image (the surface length L0) for measuring the above ratio A.

[0162] (Depth of the internal oxidation layer)

[0163] In the steel sheet of the present invention, the internal oxidation layer is a layer formed inside the steel sheet and contains the fine granular type oxide 12, the coarse granular type oxide 15, and the grain boundary type oxide 13. Therefore, the so-called "internal oxidation layer" is a layer formed by connecting the regions from the surface of the steel sheet to the farthest position where any one of the fine granular type oxide 12, the coarse granular type oxide 15, and the grain boundary type oxide 13 exists. Thus, the so-called "depth of the internal oxidation layer" is as Figure 2As shown as "Rn", it refers to the distance from the surface of the steel sheet 11 (the interface between the steel sheet and the coating in the case of a coated steel sheet) along the thickness direction of the steel sheet 11 (the direction perpendicular to the surface of the steel sheet) to the farthest position where any one of the fine granular oxides 12, coarse granular oxides 15 and grain boundary oxides 13 exists. However, since the actual surface of the steel sheet has irregularities, the positions of the fine granular oxides 12, coarse granular oxides 15 and grain boundary oxides 13 that are farthest from the surface of the steel sheet vary depending on which part (point) of the steel sheet surface is selected. Therefore, 10 observation regions are selected, and the average value of the results measured at these 10 positions is taken as the "average depth of the internal oxide layer" (sometimes also referred to as "R"). In Figure 2 As an example, the case where the grain boundary oxide 13 exists at the deepest position is shown. As described above, the fine granular oxide 12 and the coarse granular oxide 15 can function as trapping sites for hydrogen invading in a corrosive environment, and the grain boundary oxide 13 can function as a path for the hydrogen invading into the steel sheet to escape. Therefore, the larger the average depth R of the internal oxide layer, the more hydrogen can be trapped in the surface layer region of the steel sheet, and the more hydrogen can be discharged out of the system. In the steel sheet of the present invention, the lower limit of the average depth R of the internal oxide layer is not particularly limited. However, if it is too shallow, the fine granular oxides, the coarse granular oxides 15 and the grain boundary oxides 13 may not be sufficiently dispersed. Therefore, it is preferably 8 μm or more, more preferably 10 μm or more, still more preferably 15 μm or more, and further preferably 20 μm or more. The upper limit of the average depth R is not particularly limited, but is substantially 100 μm or less.

[0164] The depth R is as Figure 2It is determined by observing the cross-section of the surface layer of the steel sheet 11 as shown. The specific measurement method is as described below. The cross-section of the surface layer of the steel sheet 11 is observed by SEM. The observation positions are set at 10 randomly selected locations. The length L0 of the surface (i.e., the width of the SEM image) is measured from the observed SEM images. The length L0 is set to be 100 μm or more (e.g., 100 μm, 150 μm, or 200 μm), and the measurement depth is set to the region from the surface of the steel sheet up to 100 μm. Then, the positions of the fine granular oxides 12, the coarse granular oxides 15, and the grain boundary oxides 13 are determined from the SEM image, and any one of the fine granular oxides 12, the coarse granular oxides 15, and the grain boundary oxides 13 existing at the farthest position from the surface of the steel sheet is selected, and the distance from the surface of the steel sheet 11 to the farthest position where any one of the fine granular oxides 12, the coarse granular oxides 15, and the grain boundary oxides 13 exists is obtained as the depth Rn. The average value of Rn measured at 10 locations is obtained as the "average depth of the internal oxide layer" (sometimes also referred to as "R").

[0165] [Composition of Oxides]

[0166] In the present invention, the granular oxides and the grain boundary oxides (hereinafter, also simply referred to as oxides) are substances that contain one or more of the elements contained in the above steel sheet in addition to oxygen. Typically, they have a composition containing Si, O, and Fe, and optionally further contain Mn or Al. In addition to these elements, the oxides may contain elements that can be contained in the above steel sheet (e.g., Cr, etc.).

[0167] [Surface Layer Deficient Layer]

[0168] In the present invention, the fine granular oxide, the coarse granular oxide, and the grain boundary oxide are substances formed by the oxidation of components (such as Si, Mn, Al) that are relatively easy to oxidize in the steel sheet. Therefore, the composition of the steel (in other words, the metal structure) around the oxide lacks these easily oxidizable component elements compared to the base material of the original steel sheet. The region where the elements of the steel composition are lacking compared to the original steel sheet base material is also referred to as the "deficient region". The layered "deficient region" is also called the "deficient layer", and further, the deficient layer existing in the surface layer of the steel sheet is also called the "surface layer deficient layer". In the deficient region, since Si among the easily oxidizable elements is relatively easy to oxidize and Al is relatively difficult to oxidize, Si can exist at a low concentration and Al can exist at a high concentration. If a deficient region with a low Si and high Al steel composition exists within a desired range, it also contributes to an improvement in LME resistance. For this reason, although not wishing to be bound by a specific theory, it is considered that in addition to the granular oxide that functions as a Zn capture site, Al also exists in the steel composition around the granular oxide and the grain boundary oxide, thereby preventing Zn that is to invade the steel during hot processing at high temperatures from invading into the steel grain boundaries; in addition, the higher the concentration of Si in the steel composition, the easier it is to generate LME cracking. Therefore, by setting Si to as low a concentration as possible, LME can be suppressed. Thus, during hot stamping forming or welding processing, the invasion of Zn that is to invade the steel into the steel grain boundaries is hindered by Al in the steel composition, and the invasion of Zn into the crystal grain boundaries can be appropriately suppressed. In addition, since Si, which is prone to generating LME, is at a low concentration, LME is not easily generated, and the LME resistance can be improved.

[0169] In the present invention, the composition of the steel (i.e., the microstructure) that does not contain fine granular oxides, coarse granular oxides, and grain boundary oxides at a depth of 1 / 2 of the average depth of the internal oxide layer in the surface layer lacking layer with low Si and high Al satisfies Si ≤ 0.6% and Al ≥ 0.05% by mass. If Si exceeds 0.6%, LME cracking is likely to occur. Therefore, Si ≤ 0.6%. The lower limit of Si is not particularly limited, but it can also be 0% or more. In addition, Al hinders the intrusion of Zn, which is to penetrate into the steel during processing at high temperatures, into the grain boundaries of the steel. If Al is less than 0.05%, it may not be possible to sufficiently hinder the intrusion of Zn into the grain boundaries of the steel. Therefore, it is set to Al ≥ 0.05%. The more Al there is, the higher the function of hindering the intrusion of Zn into the grain boundaries of the steel becomes, and the more preferable it is. However, even if the Al concentration is too high, its effect saturates. Therefore, the upper limit of Al can also be set to 1.2% or less or 1.0% or less. In addition, the concentrations of Si and Al are the element concentrations in the steel composition of the internal oxide layer that does not contain fine granular oxides, coarse granular oxides, and grain boundary oxides, and are the element concentrations measured at a depth of 1 / 2 of the average depth R of the internal oxide layer. The base point of the average depth of the internal oxide layer is the steel plate surface (in the case of a plated steel plate, it is the interface between the steel plate and the plating layer). However, in the case where they have unevenness, the average line of the surface or interface at 10 points where the average depth of the internal oxide layer is obtained is set as the base point. The element concentration measurement here is carried out by EDS (Energy Dispersed Spectroscopy).

[0170] The surface layer lacking layer is a layer that can overlap with the region where fine granular oxides, coarse granular oxides, and grain boundary oxides are distributed, and is a layer present in the surface layer of the steel plate, that is, formed inside the base steel. Therefore, in the case where a plating layer is formed on the surface of the steel plate, compared with a steel plate having an external oxide layer, the mutual diffusion of the plating component and the steel component occurs sufficiently in the steel plate of the present invention in which a lacking region, more specifically, a surface layer lacking layer is formed inside the base steel, and high plating properties can be obtained.

[0171] <Plated steel plate>

[0172] The coated steel sheet of the present invention has a Zn-containing coating on the steel sheet of the present invention described above. This coating can be formed on one side of the steel sheet or on both sides. Examples of the Zn-containing coating include a hot-dip galvanized layer, an alloyed hot-dip galvanized layer, an electrogalvanized layer, an electroplated alloy zinc layer, etc. More specifically, as the coating type, for example, Zn-0.2% Al (GI), Zn-(0.3 to 1.5)% Al, Zn-4.5% Al, Zn-0.09% Al-10% Fe (GA), Zn-1.5% Al-1.5% Mg, or Zn-11% Al-3% Mg-0.2% Si, Zn-11% Ni, Zn-15% Mg, etc. can be used.

[0173] [Composition of the coating]

[0174] The composition of the components contained in the Zn-containing coating in the present invention will be described. "%" for the content of an element means "mass%" unless otherwise specified. In the numerical range regarding the composition of the coating components, the numerical range indicated by "~" means the range including the values described before and after "~" as the lower limit value and the upper limit value unless otherwise specified.

[0175] (Al: 0 to 60.0%)

[0176] Al is an element that improves the corrosion resistance of the coating by being contained or alloyed together with Zn, so it can also be contained as needed. Therefore, the Al content can also be 0%. In order to form a coating containing Zn and Al, it is preferably that the Al content is 0.01% or more, for example, 0.1% or more, 0.3% or more, 0.5% or more, 1.0% or more, or 3.0% or more. On the other hand, when it exceeds 60.0%, the effect of improving the corrosion resistance saturates, so the Al content is preferably 60.0% or less, for example, 55.0% or less, 50.0% or less, 40.0% or less, 30.0% or less, 20.0% or less, 10.0% or less, or 5.0% or less. The detailed mechanism is not clear, but when the Al in the coating is in the range of 0.3 to 1.5%, the intrusion rate of Zn into the steel grain boundaries is significantly reduced by the effect of Al, and the LME resistance can be improved. Therefore, from the viewpoint of improving the LME resistance, the Al in the coating is preferably 0.3 to 1.5%.

[0177] (Mg: 0 to 15.0%)

[0178] Mg is an element that improves the corrosion resistance of the coating by being included or alloyed with Zn and Al, so it can be contained as needed. Therefore, the Mg content can also be 0%. In order to form a coating containing Zn, Al and Mg, it is preferable that the Mg content is 0.01% or more, for example, 0.1% or more, 0.5% or more, 1.0% or more, or 3.0% or more. On the other hand, when it exceeds 15.0%, Mg is not completely dissolved in the plating bath and floats as oxides. If zinc plating is performed using this plating bath, the oxides may adhere to the plating surface and cause poor appearance or produce unplated parts. Therefore, it is preferable that the Mg content is 15.0% or less, for example, 10.0% or less, or 5.0% or less.

[0179] (Fe: 0-15.0%)

[0180] When Fe forms a Zn-containing coating on a steel sheet and then the plated steel sheet is heat-treated, Fe can be incorporated into the coating by diffusion from the steel sheet. Therefore, Fe is not incorporated into the coating without heat treatment, so the Fe content can be 0%. Alternatively, the Fe content can be 1.0% or more, 2.0% or more, 3.0% or more, 4.0% or more, or 5.0% or more. On the other hand, the Fe content is preferably 15.0% or less, and can be, for example, 12.0% or less, 10.0% or less, 8.0% or less, or 6.0% or less.

[0181] (Si: 0-3.0%)

[0182] Si is an element that further improves corrosion resistance when included in a Zn-containing plating layer, particularly a Zn-Al-Mg plating layer, and therefore may be included as needed. Therefore, the Si content may be 0%. From the perspective of improving corrosion resistance, the Si content may be, for example, 0.005% or more, 0.01% or more, 0.05% or more, 0.1% or more, or 0.5% or more. Furthermore, the Si content may be 3.0% or less, 2.5% or less, 2.0% or less, 1.5% or less, or 1.2% or less.

[0183] The basic composition of the plating layer is as described above. Furthermore, the plating layer may optionally contain one or more of Sb: 0-0.50%, Pb: 0-0.50%, Cu: 0-1.00%, Sn: 0-1.00%, Ti: 0-1.00%, Sr: 0-0.50%, Cr: 0-1.00%, Ni: 0-1.00%, and Mn: 0-1.00%. While not particularly limited, the total content of these optional additive elements is preferably 5.00% or less, and more preferably 2.00% or less, from the perspective of fully utilizing the effects and functions of the basic components constituting the plating layer.

[0184] The remaining portion of the coating other than the above components consists of Zn and impurities. The so-called impurities in the coating are components that are mixed in for various reasons during the manufacturing process, typically represented by raw materials, and are not components intentionally added to the coating. In the coating, as impurities, elements other than the basic components and optional additive components described above may be contained in trace amounts within a range that does not hinder the effects of the present invention.

[0185] Regarding the composition of the coating, it can be determined by dissolving the coating in an acid solution containing an inhibitor that suppresses the corrosion of the steel sheet and measuring the resulting solution by ICP (inductively coupled plasma) emission spectrometry.

[0186] The thickness of the coating is preferably, for example, 3 to 50 μm. In addition, the coating amount is not particularly limited, but for example, it is 10 to 170 g / m per side 2 is appropriate. In the present invention, for the coating amount of the coating, it is determined by the weight change before and after dissolution of the coating in an acid solution containing an inhibitor that suppresses the corrosion of the base metal.

[0187] [Tensile strength]

[0188] The steel sheet and the coated steel sheet of the present invention preferably have high strength. Specifically, they preferably have a tensile strength of 440 MPa or more. For example, the tensile strength can also be 500 MPa or more, 600 MPa or more, 700 MPa or more, or 800 MPa or more. The upper limit of the tensile strength is not particularly limited, but from the viewpoint of ensuring toughness, it is, for example, 2000 MPa or less. The tensile strength can be measured by collecting a JIS No. 5 tensile test piece with the length direction perpendicular to the rolling direction and performing the measurement according to JIS Z2241 (2011).

[0189] Since the steel sheet and the coated steel sheet of the present invention are of high strength and have high coatability, LME resistance, and hydrogen embrittlement resistance, they can be suitably used in a wide range of fields such as automobiles, household appliances, and building materials, but they are particularly preferably used in the automotive field. Steel sheets used for automotive applications are usually subjected to a coating treatment (typically a Zn-based coating treatment). Therefore, when the steel sheet of the present invention is used as a steel sheet for automobiles, the effects of the present invention, such as high coatability, can be suitably exerted. In addition, steel sheets and coated steel sheets used for automotive applications are mostly subjected to hot stamping forming. In this case, hydrogen embrittlement cracking and LME cracking can significantly become problems. Therefore, when the steel sheet and the coated steel sheet of the present invention are used as steel sheets for automobiles, the effects of the present invention, such as high hydrogen embrittlement resistance and LME resistance, can be suitably exerted.

[0190] <Manufacturing method of steel sheet>

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

[0192] The steel plate of the present invention can be obtained, for example, by performing the following processes: a casting process of casting molten steel with adjusted composition to form a steel slab; a hot rolling process of hot rolling the steel slab to obtain a hot rolled steel plate; a coiling process of coiling the hot rolled steel plate; a cold rolling process of cold rolling the coiled hot rolled steel plate to obtain a cold rolled steel plate; a pretreatment process of performing brush grinding treatment on the cold rolled steel plate, and an annealing process of annealing the pretreated cold rolled steel plate. Alternatively, it is also possible not to coil after the hot rolling process, perform pickling, and directly perform the cold rolling process.

[0193] [Casting Process]

[0194] The conditions of the casting process are not particularly limited. For example, as long as it is followed by melting using a blast furnace, an electric furnace, etc., various secondary refinings are carried out, and then, casting can be carried out by methods such as ordinary continuous casting and casting using the ingot method.

[0195] [Hot Rolling Process]

[0196] The steel slab cast as described above can be hot rolled to obtain a hot rolled steel plate. The hot rolling process is carried out by directly or temporarily cooling the cast steel slab and then reheating it for hot rolling. In the case of reheating, the heating temperature of the steel slab can be, for example, 1100°C to 1250°C. In the hot rolling process, rough rolling and finish rolling are usually carried out. The temperature and reduction ratio of each rolling can be appropriately changed according to the desired metal structure and plate thickness. For example, the finish rolling end temperature can also be set to 900 - 1050°C, and the finish rolling reduction ratio can be set to 10 - 50%.

[0197] [Coiling Process]

[0198] The hot rolled steel plate can be coiled at a specified temperature. The coiling temperature can be appropriately changed according to the desired metal structure, etc., and for example, it can be 500 - 800°C. It is also possible to uncoil before or after coiling and give the hot rolled steel plate a specified heat treatment. Alternatively, it is also possible not to perform the coiling process and perform pickling after the hot rolling process and then perform the cold rolling process described later.

[0199] [Cold Rolling Process]

[0200] After pickling the hot-rolled steel sheet or the like, the hot-rolled steel sheet can be cold-rolled to obtain a cold-rolled steel sheet. The reduction ratio of cold rolling can be appropriately changed according to the desired metal structure and sheet thickness. For example, it can be 20 to 80%. After the cold rolling process, for example, it can be air-cooled to room temperature.

[0201] [Pretreatment process]

[0202] In order to obtain a large amount of fine granular oxides, coarse granular oxides and grain boundary oxides in the surface layer of the finally obtained steel sheet, and further obtain a surface layer deficient layer, it is effective to perform a prescribed pretreatment process before annealing the cold-rolled steel sheet. This pretreatment process is a process of introducing a large amount of dislocations to the surface of the cold-rolled steel sheet. Since the diffusion of oxygen and the like is faster at the grain boundary than in the grain, by introducing a large amount of dislocations to the surface of the cold-rolled steel sheet, as many paths as in the case of the grain boundary can be formed. Therefore, during annealing, oxygen becomes easy to diffuse (invade) into the interior of the steel along these dislocations. In addition, since the diffusion rates of Si and Al are also increased, as a result, oxygen combines with Si and / or Al in the interior of the steel and becomes able to promote the formation of fine granular oxides, coarse granular oxides and grain boundary oxides. In addition, along with the promotion of the formation of such internal oxides, the reduction of the surrounding Si and Al concentrations is also promoted. Therefore, the formation of a surface layer deficient layer having a desired composition can also be promoted. Thus, in the case where such a pretreatment process is performed, the desired fine granular oxides, coarse granular oxides, grain boundary oxides and surface layer deficient layer are likely to be generated in the subsequent annealing process. This pretreatment process includes grinding the surface of the cold-rolled steel sheet with a high-strength grinding brush (brush grinding treatment). As the high-strength grinding brush, D-100 manufactured by HOTANI Co., Ltd. can also be used. It is preferable to coat an aqueous solution of 1.0 to 5.0% NaOH on the steel sheet surface during grinding. The brush pressing amount is 0.5 to 10.0 mm, more preferably 5.0 to 10.0 mm, and the rotation speed is preferably 100 to 1000 rpm. By controlling such coating solution conditions, brush pressing amount, and rotation speed to perform the brush grinding treatment, in the subsequent annealing process, fine granular oxides, coarse granular oxides, grain boundary oxides and surface layer deficient layer can be effectively formed in the surface layer of the steel sheet.

[0203] [Annealing process]

[0204] The cold-rolled steel sheet that has undergone the above-described pretreatment process is annealed. The annealing is preferably carried out, for example, in a state where a tension of 0.1 to 30.0 MPa is applied. When a tension is applied during annealing, strain can be more effectively introduced into the steel sheet. Through the strain, the dislocations in the metal structure of the steel sheet can be promoted, and oxygen becomes likely to invade the interior of the steel along the dislocations, so that oxides are likely to be formed inside the steel sheet. As a result, it is advantageous for increasing the number density of granular oxides, increasing the ratio of grain boundary oxides, and forming a surface depletion layer.

[0205] From the viewpoint of generating granular oxides in a desired size and in large quantities and generating grain boundary oxides in large quantities, the holding temperature in the annealing process is preferably 750°C to 900°C. If the holding temperature in the annealing process is lower than 750°C, it may not be possible to generate a sufficient amount of grain boundary oxides, and sometimes the hydrogen embrittlement resistance may be insufficient. On the other hand, if the holding temperature in the annealing process exceeds 900°C, the granular oxides may coarsen, and sometimes the desired granular oxides, grain boundary oxides, and / or surface depletion layer may not be obtained, and sometimes the hydrogen embrittlement resistance and / or the LME resistance may be insufficient. The heating rate up to the above holding temperature is not particularly limited, but it may be carried out at 1 to 10°C / second. In addition, the heating can also be carried out in two stages at a first heating rate of 1 to 10°C / second and a second heating rate of 1 to 10°C / second different from the first heating rate.

[0206] The holding time at the holding temperature in the above annealing process is preferably 50 to 300 seconds, more preferably 55 to 120 seconds. If the holding time is less than 50 seconds, it may not be possible to generate a sufficient amount of granular oxides and / or grain boundary oxides, and sometimes the LME resistance and / or the hydrogen embrittlement resistance may be insufficient. On the other hand, if the holding time exceeds 300 seconds, external oxidation may occur and internal oxidation may not occur, and sometimes the plating property, the hydrogen embrittlement resistance, and / or the LME resistance may be insufficient.

[0207] During the heating and holding (isothermal) in the annealing process, humidification is carried out from the viewpoint of generating the desired fine granular oxides, coarse granular oxides, grain boundary oxides, and surface depletion layer. The dew point of the atmosphere is preferably -20 to 10°C, more preferably -10 to 5°C, and the H2 content is 1 to 15 vol%. If the dew point is too low, an external oxide layer may be formed on the surface of the steel sheet, and an internal oxide layer may not be sufficiently formed, and sometimes the plating property, the hydrogen embrittlement resistance, and the LME resistance may be insufficient. 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 depletion layer may not be obtained.

[0208] It is preferable that the temperature at which humidification starts during heating is lower than 600°C. If humidification starts at a temperature exceeding 600°C, there is a possibility that an internal oxide layer and / or a surface depletion layer may not be sufficiently formed until the holding temperature is reached.

[0209] Furthermore, when performing the annealing process, it is effective to remove the internal oxide layer of the steel sheet in advance, particularly before the brush grinding treatment. Sometimes an internal oxide layer is formed in the surface layer of the steel sheet during the above rolling process, particularly the hot rolling process. The internal oxide layer formed in such a rolling process may hinder the formation of fine granular oxides, coarse granular oxides, grain boundary oxides, and / or surface depletion layers or promote the formation of an external oxide layer during the annealing process. Therefore, it is preferable to remove this internal oxide layer in advance by pickling treatment or the like before annealing. More specifically, when performing the annealing process, the depth of the internal oxide layer of the cold-rolled steel sheet is set to 0.5 μm or less, preferably 0.3 μm or less, more preferably 0.2 μm or less, and further preferably 0.1 μm or less.

[0210] By performing the above-described respective processes, a steel sheet can be obtained that sufficiently contains a large amount of granular oxides and grain boundary oxides in the surface layer of the steel sheet and has a surface depletion layer formed.

[0211] <Manufacturing method of plated steel sheet>

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

[0213] The plated steel sheet of the present invention can be obtained by a plating treatment process of forming a Zn-containing coating on the steel sheet manufactured as described above.

[0214] [Plating treatment process]

[0215] The plating treatment process may be carried out in accordance with methods well-known to those skilled in the art. For example, the plating treatment process can be carried out by hot dip plating or electroplating. Preferably, the plating treatment process is carried out by hot dip plating. The conditions of the plating treatment process may be appropriately set in consideration of the composition, thickness, and adhesion amount of the desired coating. After the plating treatment, an alloying treatment may also be carried out. Typically, the conditions of the plating treatment process are preferably set in such a way as to form a coating containing Al: 0 to 60.0%, Mg: 0 to 15.0%, Fe: 0 to 15%, Ni: 0 to 20%, and Si: 0 to 3%, with the balance being Zn and impurities. More specifically, the conditions of the plating treatment process are, for example, preferably set in such a way as to form Zn-0.2% Al (GI), Zn-0.8% Al, Zn-4.5% Al, Zn-0.09% Al-10% Fe (GA), Zn-1.5% Al-1.5% Mg, or Zn-11% Al-3% Mg-0.2% Si, Zn-11% Ni, Zn-15% Mg. From the viewpoint of improving the LME resistance, Al in the coating is preferably 0.3 to 1.5%.

[0216] Examples

[0217] Hereinafter, the present invention will be described in more detail by way of examples, but the present invention is not limited by any of these examples.

[0218] Example 1: Examples and Comparative Examples of Steel Sheets

[0219] (Production of Steel Sheet Specimens)

[0220] The molten steel with adjusted composition was cast to form a steel billet, the steel billet was hot-rolled, pickled, and then cold-rolled to obtain a cold-rolled steel sheet. Then, it was air-cooled to room temperature, and the cold-rolled steel sheet was subjected to pickling treatment to remove the internal oxide layer formed by rolling to the depth of the internal oxide layer before annealing (μm) shown in Table 1. Then, samples were collected from each cold-rolled steel sheet by the method according to JIS G0417:1999, and the composition of the steel sheet was analyzed by ICP-MS method or the like. The measured composition of the steel sheet is shown in Table 1. The thickness of all the steel sheets used was 1.6 mm.

[0221] Next, for a part of the cold-rolled steel sheets, a 2.0% aqueous solution of NaOH was coated, and a pretreatment of brush grinding was carried out using a strong grinding brush (D-100 manufactured by HOTANI Co., Ltd.) with a brush pressing amount of 2.0 mm and a rotation speed of 600 rpm. After that, annealing treatment was performed according to the dew point, holding temperature, and holding time shown in Table 1 to produce each steel sheet specimen. Among all the steel sheet specimens, regarding the heating rate during annealing, it was set to 6.0 °C / second until 500 °C, and from 500 °C to the holding temperature, it was set to 2.0 °C / second. In the above annealing treatment, for a part of the cold-rolled steel sheets, annealing treatment was carried out under a tension of 30.0 MPa, and for the other cold-rolled steel sheets, annealing treatment was carried out without applying tension. The presence or absence of the pretreatment and the conditions of the annealing treatment (presence or absence of tension, humidification zone, dew point (°C), hydrogen concentration (vol%), humidification start temperature (°C) in the heating process, holding temperature (°C), and holding time (seconds)) are shown in Table 1. It should be noted that for each steel sheet specimen, a JIS No. 5 tensile test piece with the direction perpendicular to the rolling direction as the length direction was collected, and a tensile test was carried out according to JIS Z 2241 (2011). As a result, for No. 1, the tensile strength was less than 440 MPa, and for the others, it was 440 MPa or more.

[0222] (Analysis of the surface layer of the steel sheet specimen: number density of fine granular oxides)

[0223] Each of the steel sheet specimens prepared as described above was cut into 25 mm × 15 mm, and the cut specimens were embedded in resin and mirror-polished. For the cross-section of each steel sheet specimen, 10 regions of 1.0 μm × 1.0 μm were observed by SEM. As the observation positions, for the depth direction (the direction perpendicular to the surface of the steel sheet), it was set to 1.0 μm from the surface of the steel sheet to 0.2 - 1.2 μm, and for the width direction (the direction parallel to the surface of the steel sheet), it was set to 1.0 μm at an arbitrary position of the above SEM image. The SEM images of each region of each steel sheet specimen obtained were binarized, and the area of the granular oxide part was calculated from the binarized image. As the diameter of a circle having the same area as this area, that is, the equivalent circle diameter, the particle size (nm) of this granular oxide was obtained. The substances in the particle size range of 20 - 100 nm were defined as fine granular oxides. Further, the number of fine granular oxides in the SEM image was counted. The average value of the number of fine granular oxides in the 10 binarized images thus obtained was used as the number density of fine granular oxides. The number density of fine granular oxides (number / μm 2 ) is shown in Table 1.

[0224] (Analysis of the surface layer of the steel sheet specimen: number density of coarse granular oxides)

[0225] Each steel plate specimen prepared as described above was cut into 25 mm × 15 mm, and the cut specimen was embedded in resin and mirror-polished. For the cross-section of each steel plate specimen, 10 regions of 5.0 μm × 5.0 μm were observed by SEM. As the observation positions, for the depth direction (the direction perpendicular to the surface of the steel plate), it was set to 5.0 μm from the steel plate surface to 0.2 to 8.0 μm, and for the width direction (the direction parallel to the surface of the steel plate), it was set to 5.0 μm at an arbitrary position in the above SEM image. The SEM images of each region of each steel plate specimen obtained were binarized, and the area of the granular oxide part was calculated from the binarized image. As the diameter of a circle having an area equal to this area, that is, the equivalent circle diameter, the particle size (nm) of this granular oxide was obtained, and substances in the particle size range of 150 to 600 nm were defined as coarse granular oxides. Further, the number of fine granular oxides in the SEM image was counted. The average value of the number of coarse granular oxides in the 10 binarized images thus obtained was used as the number density of coarse granular oxides. The number density (per 25 μm 2 ) of the coarse granular oxides for each steel plate specimen is shown in Table 1.

[0226] (Analysis of the surface layer of the steel plate specimen: Ratio A of grain boundary oxides)

[0227] In addition, the ratio A for each steel plate specimen was measured by observing the cross-section of the above-embedded specimen. Specifically, in the SEM image with a width of 150 μm (= L0), the positions of the grain boundary oxides were determined, and the determined grain boundary oxides were projected onto the surface of the steel plate, and the length L of the grain boundary oxides in the field of view was obtained. Based on L0 and L thus obtained, the ratio A (%) = 100 × L / L0 was calculated. The ratio A (%) of the granular oxides for each steel plate specimen is shown in Table 1. In addition, the depth D of the determined grain boundary oxides was measured from the same SEM image.

[0228] (Analysis of the surface layer of the steel plate specimen: Surface lack layer)

[0229] For each steel plate specimen, in order to evaluate the surface lack layer, at a depth of 1 / 2 of the average depth of the internal oxide layer calculated from the cross-section SEM image of the steel plate using TEM-EDS, the composition of the steel structure without oxides was analyzed. The case where Si ≤ 0.6% and Al ≥ 0.05% was set as "○", and the case where Si ≤ 0.6% and Al ≥ 0.05% was not satisfied was set as "×".

[0230] (Plating property evaluation)

[0231] For each steel plate specimen, plating was carried out, and the plating property was evaluated by measuring the area ratio of the unplated part on the surface of the plated steel plate. Specifically, hot-dip Zn-0.2% Al (bath temperature 450 - 470 °C) was carried out, and a 1 mm × 1 mm area on the surface of each plated steel plate specimen with a coating formed was observed with an optical microscope. The part with a coating formed (plated part) and the part without a coating formed (unplated part) were discriminated from the observed image, the area ratio of the unplated part (area of the unplated part / area of the observed image) was calculated, and the plating property was evaluated according to the following criteria. The results are shown in Table 1.

[0232] Evaluation A: The area ratio of the plated part is 95% or more (the area ratio of the unplated part is 5.0% or less)

[0233] Evaluation B: The area ratio of the plated part is less than 95% and 90% or more (the area ratio of the unplated part exceeds 5.0% and is 10% or less)

[0234] (Composition analysis of the coating)

[0235] The composition of the coating was determined by immersing a sample cut into 30 mm × 30 mm in a 10% hydrochloric acid aqueous solution containing an inhibitor (manufactured by Asahi Chemical Industry Co., Ltd., IBIT), pickling and peeling the coating, and then performing ICP analysis on the plating components dissolved in the aqueous solution.

[0236] (Evaluation of LME resistance)

[0237] Each 100 × 100 mm plated steel plate specimen was used for spot welding. Two plated steel plates cut into a size of 50 mm × 100 mm were prepared. For these two Zn-based plated steel plate specimens, a dome radius type welding electrode with a tip diameter of 8 mm was used for spot welding at a welding angle of 3°, a pressure of 3.5 kN, a clearance of 0.3 mm, a welding time of 0.5 seconds (20 cycles, power supply frequency 50 Hz), and a welding current of 7 kA to obtain a welded component. After the cross-section of the welded part was polished, it was observed with an optical microscope, and the length of LME cracking generated in the cross-section of the welded part was measured and evaluated as follows. The results are shown in Table 1. All the plating types in Table 1 were set as GA, and the plating types in Table 2 were set as described in Table 2.

[0238] Evaluation AAA: No LME cracking

[0239] Evaluation AA: The length of LME cracking exceeds 0 μm to 100 μm

[0240] Evaluation A: The length of LME cracking exceeds 100 μm to 200 μm

[0241] Evaluation B: The length of LME cracking exceeds 200 μm

[0242] (Evaluation of hydrogen embrittlement resistance)

[0243] Each 50 mm x 100 mm plated steel sheet sample was subjected to zinc phosphate treatment using a zinc phosphate chemical conversion treatment solution (SURFDINE SD5350 series: manufactured by Nipponpaint Industrial Coatings). A 20 μm electrolytic coating (PN1100 Powernix gray: manufactured by Nipponpaint Industrial Coatings) was then applied. The sample was baked at a baking temperature of 150°C for 20 minutes to form a coating film on the plated steel sheet sample. The sample was then cut into 30 × 100 mm pieces to expose the iron end face. The sample was then subjected to a combined cycle corrosion test in accordance with JASO (M609-91) using a bending jig in a state where the stress in the bent portion was 800 MPa. The hydrogen embrittlement resistance was evaluated using the following criteria, and the results are shown in Table 1.

[0244] Rating AA: No cracking up to 230 cycles

[0245] Evaluation A: Cracking occurs at 140 to less than 230 cycles

[0246] Evaluation B: Cracking occurs when the number of cycles is less than 140

[0247]

[0248] Example 2: Examples and Comparative Examples of Plated Steel Sheets

[0249] (Preparation of Plated Steel Sheet Samples)

[0250] After cutting each steel plate sample of Example 1 into a size of 100 mm × 200 mm, the plated steel plate sample of Example 2 was produced by performing a plating treatment for forming the plating types shown in Table 2. In Table 2, plating type a refers to "alloyed hot-dip galvanized steel plate (GA)", plating type b refers to "hot-dip Zn-0.2% Al steel plate (GI)", and plating type c refers to "hot-dip Zn-(0.3 to 1.5)% Al steel plate (the Al content is recorded in Table 2)". In the hot-dip galvanizing process, the cut sample was immersed in a hot-dip galvanizing bath at 440°C for 3 seconds. After dipping, it was pulled out at 100 mm / second, and the coating adhesion was controlled to 50 g / m by N2 wiping gas. 2 Regarding the plating type a, alloying treatment was then performed at 500°C.

[0251] For the plated steel sheet specimens obtained for Example 2, except for hydrogen embrittlement resistance, each evaluation item, namely the number density of fine granular oxides and coarse granular oxides, the ratio A of grain boundary oxides, the surface depletion layer, and plating properties, was evaluated by the same evaluation method as in Example 1, and it was confirmed that results equivalent to those of Example 1 could be obtained. Regarding the tensile strength, although it sometimes slightly decreases due to the plating treatment, as in Example 1, for No. 1, the tensile strength is less than 440 MPa, and for the others, it is 440 MPa or more. Regarding the LME resistance, when the plating type is c and the Al content is 0.3 to 1.5% by mass, the LME resistance is improved. The results are shown in Table 2.

[0252] (Evaluation of Hydrogen Embrittlement Resistance of Plated Steel Sheet)

[0253] Zinc phosphate treatment using a zinc phosphate-based chemical conversion treatment solution (SURFDINE SD5350 series: manufactured by Nipponpaint Industrial Coatings Co., Ltd.) was performed on each 50 mm × 100 mm plated steel sheet specimen. After that, an electrodeposition coating of 20 μm (PN1100 Powernix gray: manufactured by Nipponpaint Industrial Coatings Co., Ltd.) was formed, and baking was performed at a baking temperature of 150 °C for 20 minutes to form a coating film on the plated steel sheet specimen. Then, the specimen was cut into 30 × 100 mm to expose the iron end face. After that, it was subjected to a combined cyclic corrosion test according to JASO (M609-91) in a state where stress was applied using a bending jig so that the stress in the bent portion became 800 MPa. The hydrogen embrittlement resistance was evaluated based on the following criteria, and the results are shown in Table 2.

[0254] Evaluation AA: No cracking up to 180 cycles

[0255] Evaluation A: Cracking occurs at 90 to less than 180 cycles

[0256] Evaluation B: Cracking occurs at less than 90 cycles

[0257]

[0258] In this example, a high-strength steel sheet or a plated steel sheet having high plating properties, hydrogen embrittlement resistance, and LME resistance is evaluated when the tensile strength is 440 MPa or more, the plating property is evaluated as A, the hydrogen embrittlement resistance is evaluated as AA or A, and the LME resistance is evaluated as AAA, AA, or A. In Examples 1 and 2, regarding Specimens No. 2 to 8 and 23 to 36, since the component composition of the steel sheet, the number density of fine granular oxides and coarse granular oxides, the ratio A of grain boundary oxides, and the surface depletion layer satisfy the scope of the present invention, they have high plating properties, LME resistance, and hydrogen embrittlement resistance. For Specimen No. 1, due to insufficient C content, not only sufficient strength was not obtained, but also the desired fine granular oxides, coarse granular oxides, grain boundary oxides, and surface depletion layer were not obtained, so high plating properties, hydrogen embrittlement resistance, and LME resistance were not obtained. For Specimen No. 9, the dew point during annealing was low, and the internal oxide layer was not sufficiently formed, while the external oxide layer was formed, and high plating properties, hydrogen embrittlement resistance, and LME resistance were not obtained. For Specimen No. 10, the dew point during annealing was high, and the granular oxides were coarsened, and the desired granular oxides and grain boundary oxides were not obtained, and high plating properties, hydrogen embrittlement resistance, and LME resistance were not obtained. For Specimen No. 11, the holding temperature during annealing was high, and the granular oxides were coarsened, and the desired granular oxides and grain boundary oxides were not obtained, and high hydrogen embrittlement resistance and LME resistance were not obtained. For Specimen No. 12, the holding temperature during annealing was low, and the grain boundary oxide layer was not sufficiently formed, and high hydrogen embrittlement resistance was not obtained. For Specimen No. 13, the holding time during annealing was short, and the grain boundary oxide layer was not sufficiently formed, and high hydrogen embrittlement resistance was not obtained. For Specimen No. 14, the holding time during annealing was long, and the internal oxide layer was not sufficiently formed, while the external oxide layer was formed, and high plating properties, hydrogen embrittlement resistance, and LME resistance were not obtained. For Specimens No. 15 and 17, the Si and Mn contents were excessive respectively, and the desired internal oxide layer was not formed, while the external oxide layer was formed, and high plating properties, hydrogen embrittlement resistance, and LME resistance were not obtained. For Specimens No. 16 and 18, the Si and Mn contents were insufficient respectively, and the internal oxide layer was not sufficiently formed, while the external oxide layer was formed, and high plating properties, hydrogen embrittlement resistance, and LME resistance were not obtained. For Specimen No. 19, the Al content was excessive, and the internal oxide layer was not sufficiently formed, while the external oxide layer was formed, and high plating properties, hydrogen embrittlement resistance, and LME resistance were not obtained. For Specimen No. 20, the Al content was insufficient, and the surface depletion layer and the internal oxide layer were not sufficiently formed, and high hydrogen embrittlement resistance and LME resistance were not obtained. For Specimen No. 21, humidification was only performed during the annealing heating process, and the humidification time was short, and the coarse granular oxides and grain boundary oxide layer were not sufficiently formed, and high hydrogen embrittlement resistance and LME resistance were not obtained. For Specimen No. 22, the depth of the internal oxide layer before annealing was thick, and the internal oxide layer was not sufficiently formed after annealing, while the external oxide layer was formed, and high plating properties, hydrogen embrittlement resistance, and LME resistance were not obtained.Specimen No. 37 did not sufficiently form an internal oxide layer because no tension was applied to the steel sheet during annealing, and thus high hydrogen embrittlement resistance and LME resistance were not obtained. Specimen No. 38 did not sufficiently form an internal oxide layer because brush grinding treatment before annealing was not performed, and thus high hydrogen embrittlement resistance and LME resistance were not obtained. The humidification start temperature of Specimen No. 39 was 600 °C or higher, and an external oxide layer was formed instead of an internal oxide layer being sufficiently formed, and high plating property, hydrogen embrittlement resistance, and LME resistance were not obtained.

[0259] In the case of the inventive example, fine granular oxides, coarse granular oxides, and grain boundary oxide layers were confirmed at a prescribed number density or ratio, and a prescribed surface depletion layer was also confirmed by EDS. Therefore, high plating property, hydrogen embrittlement resistance, and LME resistance were obtained. On the other hand, in the case of the comparative example, an internal oxide layer and / or a surface depletion layer were not suitably formed near the surface of the steel sheet. Therefore, at least one of low plating property, a large amount of hydrogen intrusion, and poor LME resistance was confirmed.

[0260] Industrial Applicability

[0261] According to the present invention, it is possible to provide a high-strength steel sheet and a plated steel sheet having high plating property, LME resistance, and hydrogen embrittlement resistance, and the steel sheet and the plated steel sheet can be suitably used for applications such as automobiles, home appliances, building materials, etc., particularly for automobiles, and high collision safety and long life can be expected as an automobile steel sheet and an automobile plated steel sheet. Therefore, the present invention can be said to be an invention having extremely high industrial value.

[0262] Symbol Explanation

[0263] 1 Steel sheet

[0264] 2 External oxide layer

[0265] 3 Base steel

[0266] 11 Steel sheet

[0267] 12 Fine granular oxide

[0268] 13 Grain boundary oxide

[0269] 14 Base steel

[0270] 15 Coarse granular oxide

Claims

1. A steel plate having the following composition: containing by mass% C：0.05～0.40％、 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, B：0～0.010％、 Ti: 0 to 0.150%, Nb: 0 to 0.150%, V：0～0.150％、 Cr：0～2.00％、 Ni: 0 to 2.00%, Cu: 0 to 2.00%, Mo: 0 to 1.00%, W：0～1.00％、 Ca: 0 to 0.100%, Mg: 0 to 0.100%, Zr：0～0.100％、 Hf: 0 to 0.100% and REM: 0 to 0.100%, the balance being composed of Fe and impurities, An internal oxide layer containing fine granular oxides, coarse granular oxides and grain boundary oxides is present in the surface layer of the steel plate, When observing the cross-section of the surface layer of the steel plate, The number density of the fine particulate oxide in the internal oxide layer is 4.0 particles / μm 2 or more and 100 particles / μm 2 or less The number density of the coarse granular oxide in the internal oxide layer is 4.0 per 25 μm 2 or more and 50 per 25 μm 2 or less. The ratio A of the length of the grain boundary oxide projected on the surface of the steel plate to the length of the surface of the steel plate is 50% or more and 100% or less, The surface layer of the steel plate contains a surface layer deficient layer. The steel composition that does not contain the fine granular oxides, the coarse granular oxides and the grain boundary oxides at a depth of 1 / 2 of the average depth of the internal oxide layer satisfies Si ≤ 0.6% and Al ≥ 0.05% by mass%, Wherein, The granular oxide refers to an oxide dispersed in the form of particles in the steel grains or at the crystal grain boundaries, The particle size of the fine granular oxide is 20 nm or more and 100 nm or less, and the particle size of the coarse granular oxide is 150 nm or more and 600 nm or less, The grain boundary oxide refers to an oxide existing along the crystal grain boundaries of the steel, does not include oxides existing in the steel grains, and is observed in a linear form when observing the cross-section of the surface layer of the steel plate, The surface layer deficient layer refers to a region where the easily oxidizable component elements present in the surface layer of the steel plate are deficient compared to the base metal of the steel plate.

2. The steel plate according to claim 1, wherein, The number density of the fine particulate oxide is 4.0 to 30 particles / μm 2 .

3. A coated steel plate having a coating containing Zn on the steel plate according to claim 1 or 2.

4. A plated steel sheet, characterized in that, It is the coated steel plate having a coating containing Zn according to claim 3, and the Al contained in the coating is 0.3 to 1.5% by mass.

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