Steel sheet and plated steel sheet

Abstract

Provided is a steel sheet containing, in mass %, C: 0.05 to 0.40 %, Si: 0.2 to 3.0 %, and Mn: 0.1 to 5.0 %, and containing a grain boundary type oxide in a surface layer of the steel sheet, a ratio A of a length of the grain boundary type oxide projected on a surface of the steel sheet to a length of the surface of the steel sheet being 50 % or more and 100 % or less, and a number density of a granular type oxide being lower than 4.0 pieces / µm 2 , the surface layer of the steel sheet containing a Si-Mn deficient layer having a thickness of 3.0 µm or more from the surface of the steel sheet, Si and Mn contents of an oxide-free region at a 1 / 2 position of the thickness of the Si-Mn deficient layer being lower than 10 % of Si and Mn contents in a center portion of the plate thickness of the steel sheet, respectively.

Description

TECHNICAL FIELD

[0001] The present application relates to a steel sheet and plated steel sheet. More specifically, the present application relates to a high-strength steel sheet and plated steel sheet having high platability and hydrogen discharge property. BACKGROUND

[0002] In recent years, high-strength steel sheets used in various fields such as automobiles, home electric appliances, and construction materials are being promoted. For example, in the automobile field, in order to improve fuel efficiency, the use of high-strength steel sheets is increasing with the aim of weight reduction of the vehicle body. Such high-strength steel sheets typically contain elements such as C, Si, and Mn in order to increase the strength of the steel.

[0003] In the production of high-strength steel sheets, heat treatment such as annealing treatment is generally performed after rolling. Furthermore, among the elements typically contained in high-strength steel sheets, Si or Mn, which are easy-oxidizing elements, sometimes combine with oxygen in the atmosphere at the time of the above heat treatment, and form a layer containing oxides near the surface of the steel sheet. As the form of such a layer, there are a form in which oxides containing Si or Mn are formed in the form of a film on the outside (surface) of the steel sheet (external oxidation layer), and a form in which oxides are formed in the inside (surface layer) of the steel sheet (internal oxidation layer).

[0004] In the case where a plated layer (for example, a Zn-based plated layer) is formed on the surface of a steel sheet having an external oxidation layer, since the oxides exist in the form of a film on the surface of the steel sheet, the mutual diffusion of the steel components (for example, Fe) and the plated components (for example, Zn) is sometimes hindered, the adhesion of the steel and the plated layer is affected, and the platability becomes insufficient (for example, un-plated portions increase). Thus, from the viewpoint of improving platability, a steel sheet having an internal oxidation layer is preferable compared to a steel sheet having an external oxidation layer.

[0005] In connection with the internal oxidation layer, in Patent Documents 1 and 2, a high-strength plated steel sheet is described, which is a plated steel sheet having a zinc-based plated layer on a base steel sheet containing C, Si, and Mn, and has an internal oxidation layer containing Si and / or Mn in the surface layer of the base steel sheet, and the tensile strength is 980 MPa or more.

[0006] Furthermore, in Patent Document 3, a method for producing a high-strength hot-dip galvanized steel sheet of a high-Si-containing steel is proposed, and in the case of a high-Si-containing steel in which the Si concentration in the steel is 0.3% or more, the Si and the like in the steel diffuse to the surface layer of the steel sheet as oxides by heating of the surface of the steel sheet, and these oxides hinder the wettability of plating, and deteriorate the plating adhesion, and therefore the annealing conditions are appropriately controlled.

[0007] PRIOR ART DOCUMENTS

[0008] PATENT DOCUMENTS

[0009] Patent Literature 1: Japanese Patent Application Laid-Open (JP A) No. 2016-130357

[0010] Patent Literature 2: Japanese Patent Application Laid-Open (JP A) No. 2018-193614

[0011] Patent Literature 3: Japanese Patent Application Laid-Open (JP A) No. Hei 4-202632 SUMMARY

[0012] PROBLEMS TO BE SOLVED BY THE INVENTION

[0013] If annealing treatment is performed on a steel sheet in a manufacturing process of a high-strength steel sheet, hydrogen present in the atmosphere at the time of annealing sometimes intrudes into the steel sheet. The hydrogen that has intruded into the steel sheet can be segregated at the grain boundaries of the martensite structure of the steel sheet, and cause cracking in the steel sheet by embrittlement of the grain boundaries. The phenomenon of cracking due to the intruding hydrogen is called hydrogen embrittlement cracking (delayed fracture), and is often a problem at the time of working of the steel sheet. Therefore, in order to prevent hydrogen embrittlement cracking, it is effective to efficiently release the hydrogen that has intruded into the steel sheet from the inside of the steel sheet to the outside of the system at the time of annealing treatment.

[0014] In Patent Literatures 1 and 2, a method is taught in which oxidation is performed with an oxidation zone at an air ratio or air-fuel ratio of 0.9 to 1.4, and then the oxide film is reduced in a hydrogen atmosphere with a reduction zone, the average depth of the internal oxidation layer is controlled to be thick at 4 μm or more, the internal oxidation layer is made to function as a hydrogen trapping site, and thus the intrusion of hydrogen can be prevented and hydrogen embrittlement can be suppressed. In Patent Literature 3 as well, heating with an oxidation zone at an air ratio of 0.95 to 1.10 is specifically disclosed. However, in any of these documents, no study is made on the control of the form of the oxide present in the internal oxidation layer, and there is room for improvement in terms of hydrogen releasing property (hydrogen embrittlement resistance).

[0015] The present application was made in view of such actual circumstances, and the problem is to provide a high-strength steel sheet and plated steel sheet having high platability and hydrogen releasing property.

[0016] MEANS FOR SOLVING THE PROBLEMS

[0017] The present inventors have found that, in order to solve the above problems, it is important to form an oxide in the surface layer of a steel sheet, i.e., inside the steel sheet, further control the form of the oxide present in the surface layer of the steel sheet, and control the Si-Mn deficient layer generated in the surface layer of the steel sheet due to the formation of such an oxide to be within a prescribed thickness and composition range. More specifically, the present inventors have found that high platability is ensured by forming an internal oxide, and that a grain boundary type oxide present in the form of following the crystalline grain boundaries of the metal structure is formed in a large amount as the form of the oxide, this grain boundary type oxide functions as a path for the escape of hydrogen that has intruded into the steel at the time of annealing, and the emission of hydrogen from the steel to the outside of the system is promoted; further, hydrogen diffusion in the steel is promoted by forming a Si-Mn deficient layer having a prescribed thickness and composition in the surface layer of the steel sheet, whereby high hydrogen emission properties can be obtained. In addition, the present inventors have found that, on the basis of obtaining high hydrogen emission properties, it is effective to suppress the generation of a granular type oxide present in the crystalline grains.

[0018] The present application is based on the above knowledge, and the gist thereof is as described below.

[0019] (1) A steel sheet having the following composition: containing, in mass%,

[0020] C: 0.05 to 0.40%,

[0021] Si: 0.2 to 3.0%,

[0022] Mn: 0.1 to 5.0%,

[0023] sol. Al: 0 to less than 0.4000%,

[0024] P: 0.0300% or less,

[0025] S: 0.0300% or less,

[0026] N: 0.0100% or less,

[0027] B: 0 to 0.010%,

[0028] Ti: 0 to 0.150%,

[0029] Nb: 0 to 0.150%,

[0030] V: 0 to 0.150%,

[0031] Cr: 0 to 2.00%,

[0032] Ni: 0 to 2.00%,

[0033] Cu: 0 to 2.00%,

[0034] Mo: 0 to 1.00%,

[0035] W: 0 to 1.00%,

[0036] Ca: 0 to 0.100%,

[0037] Mg: 0 to 0.100%,

[0038] Zr: 0 to 0.100%,

[0039] Hf: 0 to 0.100%, and

[0040] REM: 0 to 0.100%,

[0041] the remainder consisting of Fe and impurities,

[0042] a grain boundary type oxide is contained in a surface layer of the steel sheet,

[0043] when a cross section of the surface layer of the steel sheet is observed, a ratio A of a length of the grain boundary type oxide projected on a surface of the steel sheet with respect to a length of the surface of the steel sheet is 50% or more and 100% or less,

[0044] the number density of the granular type oxide is lower than 4.0 pieces / μm 2 ,

[0045] the surface layer of the steel sheet contains a Si-Mn deficient layer having a thickness of 3.0 μm or more from the surface of the steel sheet,

[0046] the Si and Mn contents of an oxide-free region at a position of 1 / 2 of the thickness of the Si-Mn deficient layer are each lower than 10% of the Si and Mn contents in a center portion of the plate thickness of the steel sheet.

[0047] (2) The steel sheet according to (1), wherein

[0048] the ratio A is 80% or more.

[0049] (3) The steel sheet according to (1), wherein

[0050] the ratio A is 90% or more.

[0051] (4) The steel sheet according to any one of (1) to (3), wherein

[0052] the number density of the granular type oxide is lower than 2.0 pieces / μm 2 .

[0053] (5) A plated steel sheet,

[0054] having a plated layer containing Zn on the steel sheet according to any one of (1) to (4).

[0055] Inventive Effects

[0056] According to the present application, by making the grain boundary type oxide existing in the surface layer of the steel sheet in a manner along the crystal grain boundaries of the metal structure function as a path for the release of hydrogen intruding into the steel, the emission of hydrogen from the steel to the outside of the system is promoted; further, by including the Si-Mn deficient layer having a prescribed thickness and composition, the diffusion of hydrogen is promoted, whereby the hydrogen emission property can be greatly improved, as a result of which the amount of hydrogen accumulated in the steel can be greatly reduced. In addition, according to the present application, since the granular type oxide is formed in the interior of the steel sheet, in the case where a plated layer is formed, the interdiffusion of the steel components and the components of the plated layer is sufficiently performed, and a high plating property can be obtained. Thus, according to the present application, in a high-strength steel sheet, a high plating property and hydrogen emission property can be obtained. BRIEF DESCRIPTION OF DRAWINGS

[0057] Figure 1 shows a schematic view of a cross section with respect to a steel sheet having an outer oxide layer.

[0058] Figure 2 shows a schematic view of a cross section with respect to an exemplary steel sheet of the present application.

[0059] Figure 3 shows a schematic view for explaining the measurement of the ratio A in the present application. DETAILED DESCRIPTION

[0060] <Steel Sheet>

[0061] The steel sheet of the present application is characterized by having the following composition: containing, in mass%,

[0062] C: 0.05 to 0.40%,

[0063] Si: 0.2 to 3.0%,

[0064] Mn: 0.1 to 5.0%,

[0065] sol. Al: 0 to less than 0.4000%,

[0066] P: 0.0300% or less,

[0067] S: 0.0300% or less,

[0068] N: 0.0100% or less,

[0069] B: 0 to 0.010%,

[0070] Ti: 0 to 0.150%,

[0071] Nb: 0 to 0.150%,

[0072] V: 0 to 0.150%,

[0073] Cr: 0 to 2.00%,

[0074] Ni: 0 to 2.00%,

[0075] Cu: 0 to 2.00%,

[0076] Mo: 0 to 1.00%,

[0077] W: 0 to 1.00%,

[0078] Ca: 0 to 0.100%,

[0079] Mg: 0 to 0.100%,

[0080] Zr: 0 to 0.100%,

[0081] Hf: 0 to 0.100%, and

[0082] REM: 0 to 0.100%,

[0083] the remainder consisting of Fe and impurities,

[0084] containing grain boundary type oxides in a surface layer of the steel sheet,

[0085] when a cross section of the surface layer of the steel sheet is observed, a ratio A of a length of the grain boundary type oxides projected on the surface of the steel sheet with respect to a length of the surface of the steel sheet is 50% or more and 100% or less,

[0086] the number density of the granular type oxides is lower than 4.0 / μm 2 ,

[0087] the surface layer of the steel sheet contains a Si-Mn deficient layer having a thickness of 3.0 μm or more from the surface of the steel sheet,

[0088] the Si and Mn contents of an oxide-free region at a position of 1 / 2 of the thickness of the Si-Mn deficient layer are each lower than 10% of the Si and Mn contents in a center portion of the plate thickness of the steel sheet.

[0089] In the production of high-strength steel sheets, after a steel billet adjusted to a prescribed composition is subjected to rolling (typically hot rolling and cold rolling), an annealing treatment is generally performed for the purpose of obtaining a desired structure and the like. In this annealing treatment, a layer containing oxides is formed near the surface of the steel sheet by the combination of components (for example, Si, Mn) that are relatively easily oxidized in the steel sheet with oxygen in the annealing atmosphere. For example, as described in Japanese Patent Application Publication No. 2005- 328339 (JP 2005-328339 A), the oxides are formed in the vicinity of the surface of the steel sheet, and the oxides are present in the vicinity of the surface of the steel sheet. Figure 1As shown in the steel sheet 1 of the comparative example, the outer oxide layer 2 is formed in a film shape on the surface of the base steel 3 (i.e., outside of the base steel 3). If the outer oxide layer 2 is formed in a film shape on the surface of the base steel 3, in the case where a plated layer (e.g., zinc-based plated layer) is formed, the outer oxide layer 2 hinders interdiffusion of a plating component (e.g., Zn) and a steel component (e.g., Fe), and thus adhesion between the steel and the plated layer cannot be sufficiently ensured, and sometimes un-plated portions where the plated layer is not formed are generated.

[0090] On the contrary, as shown in the steel sheet 11 of the present application, Figure 2 As shown in the steel sheet 1 of the comparative example, the outer oxide layer 2 is formed in a film shape on the surface of the base steel 3 (i.e., outside of the base steel 3). If the outer oxide layer 2 is formed in a film shape on the surface of the base steel 3, in the case where a plated layer (e.g., zinc-based plated layer) is formed, the outer oxide layer 2 hinders interdiffusion of a plating component (e.g., Zn) and a steel component (e.g., Fe), and thus adhesion between the steel and the plated layer cannot be sufficiently ensured, and sometimes un-plated portions where the plated layer is not formed are generated. Figure 1 As shown in the steel sheet 1 of the comparative example, the outer oxide layer 2 is formed in a film shape on the surface of the base steel 3 (i.e., outside of the base steel 3). If the outer oxide layer 2 is formed in a film shape on the surface of the base steel 3, in the case where a plated layer (e.g., zinc-based plated layer) is formed, the outer oxide layer 2 hinders interdiffusion of a plating component (e.g., Zn) and a steel component (e.g., Fe), and thus adhesion between the steel and the plated layer cannot be sufficiently ensured, and sometimes un-plated portions where the plated layer is not formed are generated.

[0091] On the other hand, it is known that during annealing, hydrogen present in the annealing atmosphere penetrates the base steel, causing segregation at the martensitic grain boundaries, leading to grain boundary embrittlement and hydrogen embrittlement cracking. Therefore, to prevent hydrogen embrittlement cracking of the steel sheet, it is preferable to efficiently remove the hydrogen that has penetrated the steel from the system, i.e., to have high hydrogen removal capacity. The inventors have discovered that by controlling the morphology of the oxides present in the surface layer of the steel sheet, and by controlling the Si-Mn deficient layer formed in the surface layer of the steel sheet due to the formation of such oxides within a specified thickness and composition range, and more specifically, by making most or all of the oxides into grain boundary oxides connecting the interior and surface of the steel sheet, the oxides function to remove the hydrogen that has penetrated during the annealing process. Furthermore, by forming a Si-Mn deficient layer with a specified thickness and composition in the surface layer of the steel sheet, hydrogen diffusion in the steel is promoted, resulting in the efficient removal of hydrogen from the steel from the system. More specifically, the inventors conducted a detailed analysis of the relationship between the morphology of oxides and their effectiveness as pathways for hydrogen removal. The results showed that increasing the proportion (ratio A) of grain boundary oxides in the base steel 14, and more specifically, increasing ratio A to 50% or more, is effective. While not bound by a specific theory, it is believed that the hydrogen removal function of oxides in the steel sheet relative to intruded hydrogen is positively correlated with the ratio A of these oxides. That is, it is believed that the presence of grain boundary oxides at a high ratio A increases the hydrogen removal pathway for hydrogen intruded into the steel sheet, thereby improving the hydrogen removal function. Therefore, the inventors found that, from the viewpoint of obtaining high hydrogen removal performance, it is important to control the conditions during the manufacturing of the steel sheet, particularly during annealing, so that grain boundary oxides, which function as pathways for hydrogen intrusion during annealing, exist at a high ratio and preferably at deeper locations.

[0092] Furthermore, the present invention relates to the cause of Figure 2 A detailed analysis was conducted on the relationship between the morphology of the Si-Mn deficient layer formed by the formation of internal oxides such as oxides 12 and 13 as shown, and the decrease in the surrounding Si and Mn concentration, and the hydrogen expulsion capacity. The results showed that controlling the Si-Mn deficient layer to a specified thickness and composition, more specifically, controlling it so that the Si and Mn content of the oxide-free Si-Mn deficient layer at half the thickness of the steel plate (where the thickness is at least 3.0 μm from the surface and the Si and Mn content at the midpoint of this thickness is less than 10% of the Si and Mn content in the center of the steel plate) is effective (hereinafter, these values ​​are also referred to as Si deficiency rate and Mn deficiency rate). While not bound by a specific theory, it is believed that in the case of steel containing a large amount of Si and / or Mn, the amount of Si and / or Mn dissolved in the steel also increases, thus these dissolved Si and / or Mn hinder hydrogen diffusion, resulting in a slower hydrogen diffusion rate in the steel. Figure 2As shown in the middle, if internal oxides 12, 13 and the like are formed in the surface layer of the steel sheet, Si and Mn dissolved in the steel are consumed in the formation of the internal oxides, and thus the internal oxides are formed in the surface layer of the steel sheet, and a Si-Mn deficient layer in which the concentration of the surrounding Si and Mn is relatively low is generated. Therefore, it is considered that by setting the Si-Mn deficient layer to be relatively thick, specifically, by controlling the thickness of the Si-Mn deficient layer to be 3.0 μm or more from the surface of the steel sheet (in the case where a plated layer is present on the surface of the steel sheet, the interface between the plated layer and the steel sheet), the diffusion path of hydrogen is sufficiently secured, and by further controlling the Si and Mn contents of the Si-Mn deficient layer to be sufficiently low, specifically, by controlling the Si and Mn deficient rates to be less than 10% respectively, the amount of the solid-solved Si and Mn that hinders the diffusion of hydrogen can be sufficiently reduced. Therefore, it is considered that by including the Si-Mn deficient layer whose thickness and composition are controlled to be within the above ranges, the diffusion of hydrogen can be promoted, and the hydrogen emission property from the steel can be improved. Thus, by combining the above-described grain boundary type oxides with the Si-Mn deficient layer, the hydrogen emission property can be significantly improved.

[0093] Further, the present inventors have found that, in order to maximize the hydrogen emission function of the grain boundary type oxides, it is important to suppress the generation of oxides other than the grain boundary type oxides, i.e., the generation of the granular type oxides formed in the surface layer, as much as possible. The granular type oxides are selectively formed in the surface layer, and hinder the formation of the grain boundary type oxides in the surface layer. In addition, if the granular type oxides are excessively present, they sometimes function as a trapping site for hydrogen that is intended to be emitted to the outside of the system, and reduce the hydrogen emission function. Therefore, it has been found that, from the viewpoint of further improving the hydrogen emission function, in addition to the combination of the above-described grain boundary type oxides and the Si-Mn deficient layer, it is also important to suppress the generation of the granular type oxides, and more specifically, to control the number density (the number per unit area) of the granular type oxides.

[0094] Hereinafter, the steel sheet of the present application will be described in detail. Note that the thickness of the steel sheet of the present application is not particularly limited, but is preferably, for example, 0.1 to 3.2 mm.

[0095] [Composition of Steel Sheet]

[0096] The composition included in the steel sheet of the present application will be described. The "%" of the content of the elements means "mass%" unless otherwise specified. In the numerical range of the composition, the numerical range indicated using "~" means the range including the numerical values recited before and after the "~" as the lower limit value and the upper limit value, unless otherwise specified.

[0097] (C: 0.05 to 0.40%)

[0098] C (carbon) is an element important in ensuring the strength of the steel. To ensure sufficient strength, the C content is set to 0.05% or more. The C content is preferably 0.07% or more, more preferably 0.10% or more, and further preferably 0.12% or more. On the other hand, if the C content is excessive, there is a possibility that the weldability will decrease. Therefore, the C content is set to 0.40% or less. The C content can also be 0.38% or less, 0.35% or less, 0.32% or less, or 0.30% or less.

[0099] (Si: 0.2 to 3.0%)

[0100] Si (silicon) is an element effective for improving the strength of the steel. To ensure sufficient strength and further to cause the desired oxides, particularly intergranular oxides, to be generated sufficiently inside the steel sheet, the Si content is set to 0.2% or more. The Si content is preferably 0.3% or more, more preferably 0.5% or more, and further preferably 1.0% or more. On the other hand, if the Si content is excessive, there is a possibility that deterioration of the surface properties will occur, and there is also a possibility that the external oxidation growth will be promoted. Therefore, the Si content is set to 3.0% or less. The Si content can also be 2.8% or less, 2.5% or less, 2.3% or less, or 2.0% or less.

[0101] (Mn: 0.1 to 5.0%)

[0102] Mn (manganese) is an element effective for improving the strength of the steel by obtaining a hard structure. To ensure sufficient strength and further to cause the desired oxides, particularly intergranular oxides, to be generated sufficiently inside the steel sheet, the Mn content is set to 0.1% or more. The Mn content is preferably 0.5% or more, more preferably 1.0% or more, and further preferably 1.5% or more. On the other hand, if the Mn content is excessive, there is a possibility that the metal structure will become non-uniform by Mn segregation, the workability will decrease, and the external oxidation growth will be promoted. Therefore, the Mn content is set to 5.0% or less. The Mn content can also be 4.5% or less, 4.0% or less, 3.5% or less, or 3.0% or less.

[0103] (sol. Al: 0 to less than 0.4000%)

[0104] Al (aluminum) is an element that functions as a deoxidizing element. The Al content can also be 0%, but in order to obtain the effect of sufficient deoxidization, the Al content is preferably 0.0010% or more. The Al content is more preferably 0.0050% or more, further preferably 0.0100% or more, further more preferably 0.0150% or more. On the other hand, if the Al content is excessive, it can cause a decrease in workability, deterioration of surface properties. Therefore, the Al content is set to be less than 0.4000%. The Al content can also be 0.3900% or less, 0.3800% or less, 0.3700% or less, 0.3500% or less, 0.3400% or less, 0.3300% or less, 0.3000% or less, or 0.2000% or less. The Al content refers to the content of so-called acid-soluble Al (sol. Al).

[0105] (P: 0.0300% or less)

[0106] P (phosphorus) is generally an impurity contained in steel. If P is excessively contained, it can cause a decrease in weldability. Therefore, the P content is set to be 0.0300% or less. The P content is preferably 0.0200% or less, more preferably 0.0100% or less, further preferably 0.0050% or less. The lower limit of the P content is 0%, but from the viewpoint of manufacturing cost, the P content can be more than 0% or 0.0001% or more.

[0107] (S: 0.0300% or less)

[0108] S (sulfur) is generally an impurity contained in steel. If S is excessively contained, it can cause a decrease in weldability, and further, an increase in the amount of MnS precipitated, resulting in a decrease in workability such as bendability. Therefore, the S content is set to be 0.0300% or less. The S content is preferably 0.0100% or less, more preferably 0.0050% or less, further preferably 0.0020% or less. The lower limit of the S content is 0%, but from the viewpoint of desulfurization cost, the S content can be more than 0% or 0.0001% or more.

[0109] (N: 0.0100% or less)

[0110] N (nitrogen) is generally an impurity contained in steel. If N is excessively contained, it can cause a decrease in weldability. Therefore, the N content is set to be 0.0100% or less. The N content is preferably 0.0080% or less, more preferably 0.0050% or less, further preferably 0.0030% or less. The lower limit of the N content is 0%, but from the viewpoint of manufacturing cost, the N content can be more than 0% or 0.0010% or more.

[0111] The basic component composition of the steel sheet of the present application is as described above. Furthermore, the steel sheet can also contain the following optional elements as necessary. The inclusion of these elements is not necessary, and the lower limit of the content of these elements is 0%.

[0112] (B: 0 to 0.010%)

[0113] B (boron) is an element that contributes to the improvement of strength by improving hardenability, and also strengthens the grain boundaries to improve toughness by segregating at the grain boundaries. The B content is preferably 0%, but in order to obtain the above effects, it can also be contained as necessary. The B content can also be 0.0001% or more, 0.0005% or more, or 0.001% or more. On the other hand, from the viewpoint of ensuring sufficient toughness and weldability, the B content is preferably 0.010% or less, and can also be 0.008% or less or 0.006% or less.

[0114] (Ti: 0 to 0.150%)

[0115] Ti (titanium) is an element that contributes to the improvement of strength by precipitating as TiC during cooling of the steel. The Ti content is preferably 0%, but in order to obtain the above effects, it can also be contained as necessary. The Ti content can also be 0.001% or more, 0.003% or more, 0.005% or more, or 0.010% or more. On the other hand, if Ti is excessively contained, it can be possible to generate coarse TiN and damage toughness. Therefore, the Ti content is preferably 0.150% or less, and can also be 0.100% or less or 0.050% or less.

[0116] (Nb: 0 to 0.150%)

[0117] Nb (niobium) is an element that contributes to the improvement of strength by improving hardenability. The Nb content is preferably 0%, but in order to obtain the above effects, it can also be contained as necessary. The Nb content can also be 0.001% or more, 0.005% or more, 0.010% or more, or 0.015% or more. On the other hand, from the viewpoint of ensuring sufficient toughness and weldability, the Nb content is preferably 0.150% or less, and can also be 0.100% or less or 0.060% or less.

[0118] (V: 0 to 0.150%)

[0119] V (vanadium) is an element that contributes to the improvement of strength by improving hardenability. The V content is preferably 0%, but in order to obtain the above effects, it can also be contained as necessary. The V content can also be 0.001% or more, 0.010% or more, 0.020% or more, or 0.030% or more. On the other hand, from the viewpoint of ensuring sufficient toughness and weldability, the V content is preferably 0.150% or less, and can also be 0.100% or less or 0.060% or less.

[0120] (Cr: 0 to 2.00%)

[0121] Cr (chromium) is effective for improving the strength of the steel by improving the hardenability of the steel. It is desirable that the Cr content be 0%, but it can be contained as needed in order to obtain the above effect. The Cr content can also be 0.01% or more, 0.10% or more, 0.20% or more, 0.50% or more, or 0.80% or more. On the other hand, if Cr is contained in excess, it can be likely that Cr carbides are formed in a large amount, and the hardenability is impaired. Therefore, the Cr content is preferably 2.00% or less, and can be 1.80% or less or 1.50% or less.

[0122] (Ni: 0 to 2.00%)

[0123] Ni (nickel) is an element effective for improving the strength of the steel by improving the hardenability of the steel. It is desirable that the Ni content be 0%, but it can be contained as needed in order to obtain the above effect. The Ni content can also be 0.01% or more, 0.10% or more, 0.20% or more, 0.50% or more, or 0.80% or more. On the other hand, excessive addition of Ni can lead to an increase in cost. Therefore, the Ni content is preferably 2.00% or less, and can be 1.80% or less or 1.50% or less.

[0124] (Cu: 0 to 2.00%)

[0125] Cu (copper) is an element effective for improving the strength of the steel by improving the hardenability of the steel. It is desirable that the Cu content be 0%, but it can be contained as needed in order to obtain the above effect. The Cu content can also be 0.001% or more, 0.005% or more, or 0.01% or more. On the other hand, from the viewpoint of suppressing a decrease in toughness, cracking of a slab after casting, and a decrease in weldability, the Cu content is preferably 2.00% or less, and can be 1.80% or less, 1.50% or less, or 1.00% or less.

[0126] (Mo: 0 to 1.00%)

[0127] Mo (molybdenum) is an element effective for improving the strength of the steel by improving the hardenability of the steel. It is desirable that the Mo content be 0%, but it can be contained as needed in order to obtain the above effect. The Mo content can also be 0.01% or more, 0.10% or more, 0.20% or more, or 0.30% or more. On the other hand, from the viewpoint of suppressing a decrease in toughness and weldability, the Mo content is preferably 1.00% or less, and can be 0.90% or less or 0.80% or less.

[0128] (W: 0 to 1.00%)

[0129] W (tungsten) is an element effective for improving the hardenability of steel and thereby improving the strength of steel. It is desirable for the W content to be 0%, but it can be contained as needed in order to obtain the above effects. The W content can also be 0.001% or more, 0.005% or more, or 0.01% or more. On the other hand, from the viewpoint of suppressing a decrease in toughness and weldability, the W content is preferably 1.00% or less, and can be 0.90% or less, 0.80% or less, 0.50% or less, or 0.10% or less.

[0130] (Ca: 0 to 0.100%)

[0131] Ca (calcium) is an element that contributes to inclusion control, particularly fine dispersion of inclusions, and has the effect of improving toughness. It is desirable for the Ca content to be 0%, but it can be contained as needed in order to obtain the above effects. The Ca content can also be 0.0001% or more, 0.0005% or more, or 0.001% or more. On the other hand, if Ca is contained in excess, the deterioration of surface properties sometimes becomes apparent. Therefore, the Ca content is preferably 0.100% or less, and can be 0.080% or less, 0.050% or less, 0.010% or less, or 0.005% or less.

[0132] (Mg: 0 to 0.100%)

[0133] Mg (magnesium) is an element that contributes to inclusion control, particularly fine dispersion of inclusions, and has the effect of improving toughness. It is desirable for the Mg content to be 0%, but it can be contained as needed in order to obtain the above effects. The Mg content can also be 0.0001% or more, 0.0005% or more, or 0.001% or more. On the other hand, if Mg is contained in excess, the deterioration of surface properties sometimes becomes apparent. Therefore, the Mg content is preferably 0.100% or less, and can be 0.090% or less, 0.080% or less, 0.050% or less, or 0.010% or less.

[0134] (Zr: 0 to 0.100%)

[0135] Zr (zirconium) is an element that contributes to inclusion control, particularly fine dispersion of inclusions, and has the effect of improving toughness. It is desirable for the Zr content to be 0%, but it can be contained as needed in order to obtain the above effects. The Zr content can also be 0.001% or more, 0.005% or more, or 0.010% or more. On the other hand, if Zr is contained in excess, the deterioration of surface properties sometimes becomes apparent. Therefore, the Zr content is preferably 0.100% or less, and can be 0.050% or less, 0.040% or less, or 0.030% or less.

[0136] (Hf: 0 to 0.100%)

[0137] Hf (hafnium) is an element that contributes to inclusion control, particularly fine dispersion of inclusions, and has the effect of improving toughness. It is desirable for the Hf content to be 0%, but it can be contained as needed in order to obtain the above effects. The Hf content can also be 0.0001% or more, 0.0005% or more, or 0.001% or more. On the other hand, if Hf is contained in excess, the deterioration of surface properties can sometimes become apparent. Therefore, the Hf content is preferably 0.100% or less, and can be 0.050% or less, 0.030% or less, or 0.010% or less.

[0138] (REM: 0 to 0.100%)

[0139] REM (rare earth element) is an element that contributes to inclusion control, particularly fine dispersion of inclusions, and has the effect of improving toughness. It is desirable for the REM content to be 0%, but it can be contained as needed in order to obtain the above effects. The REM content can also be 0.0001% or more, 0.0005% or more, or 0.001% or more. On the other hand, if REM is contained in excess, the deterioration of surface properties can sometimes become apparent. Therefore, the REM content is preferably 0.100% or less, and can be 0.050% or less, 0.030% or less, or 0.010% or less. Note that REM is an abbreviation for Rare Earth Metal, and refers to an element belonging to the lanthanide series. REM is usually added as a mixed rare earth metal.

[0140] In the steel sheet of the present application, the remainder other than the above-described component composition is composed of Fe and impurities. Here, the so-called impurities are components and the like that are mixed due to various factors of the manufacturing process, represented by raw materials such as ore, scrap iron, and the like, when a steel sheet is manufactured industrially.

[0141] In the present application, the analysis of the component composition of the steel sheet can be performed using an elemental analysis method known to those skilled in the art, for example, by an inductively coupled plasma mass spectrometry method (ICP-MS method). Among these, it is desirable to measure C and S using a combustion-infrared absorption method, and N using an inert gas melting-thermal conductivity method. These analyses can be performed on a sample of the steel sheet that is collected by a method according to JIS G0417: 1999.

[0142] [Surface layer]

[0143] In the present application, the "surface layer" of the steel sheet refers to a region from the surface of the steel sheet (in the case of a plated steel sheet, the interface between the steel sheet and the plated layer) to a prescribed depth in the sheet thickness direction, and the "prescribed depth" is typically 50 μm or less.

[0144] As Figure 2As shown, in the steel plate 11 of the present invention, grain boundary oxides 13 are included in the surface layer of the steel plate 11. Preferably, only grain boundary oxides 13 are present. Because these grain boundary oxides 13 are present inside the base steel 14 (i.e., as internal oxides), they... Figure 1 Compared to the case where an external oxide layer 2 exists on the surface of the steel plate 1, the steel plate 11 becomes capable of high plating performance. This is believed to be a result of the fact that, in relation to the formation of internal oxides, since the external oxide layer, which hinders the interdiffusion of the plating components and the steel components when a coating (e.g., a Zn-based coating) is formed on the surface of the steel plate, is absent or exists only at a sufficiently thin thickness, the interdiffusion of the plating components and the steel components occurs sufficiently. Therefore, the steel plate and the plated steel plate of the present invention, which contain grain boundary oxides in the surface layer of the steel plate, i.e., the interior of the steel plate, have high plating performance.

[0145] In addition, such as Figure 2 As shown, in the steel plate 11 of the present invention, in addition to the aforementioned grain boundary oxide 13, granular oxide 12 may also be included in the surface layer of the steel plate 11. Since the granular oxide 12 exists within the base steel 14 in the same manner as the grain boundary oxide 13, the steel plate and the plated steel plate containing both granular oxide 12 and grain boundary oxide 13 also have high plating properties. On the other hand, since the granular oxide 12 exists independently as particles, it does not function as a hydrogen removal pathway during annealing. If it exists in excess, it may sometimes trap the hydrogen that wants to be removed, potentially reducing the hydrogen removal function of the grain boundary oxide 13. Furthermore, when granular oxide 12 is used in excess (for example, a number density of 4.0 particles / μm as described later), 2 When the steel sheet 11 is manufactured using the above-mentioned manufacturing conditions (especially annealing conditions), there is a tendency to promote the formation of granular oxides 12, insufficient formation of grain boundary oxides 13, and a tendency to reduce hydrogen removal capacity. Therefore, from the viewpoint of effectively increasing the hydrogen removal pathway during annealing, granular oxides 12 are preferably minimized. Therefore, more preferably, in the present invention, the surface layer of the steel sheet 11 may not contain granular oxides 12 (i.e., all oxides present in the surface layer of the steel sheet 11 may be grain boundary oxides 13).

[0146] [Granular Oxides]

[0147] In this invention, "granular oxide" refers to oxides dispersed in a granular form within the grains or at grain boundaries of steel. Furthermore, "granular" means existing separately within the steel matrix, for example, having an aspect ratio of 1.0 to 5.0 (the length of the largest segment across the granular oxide (major axis) / the length of the largest segment perpendicular to the major axis across the oxide (minor axis)). "Dispersed in a granular form" means that the positions of the oxide particles are not arranged according to a specific rule (e.g., in a straight line), but rather randomly. In practice, granular oxides typically exist in a three-dimensional, spherical or nearly spherical shape on the surface of a steel plate; therefore, when observing a cross-section of the steel plate's surface, the granular oxides are typically observed to be circular or nearly circular. Figure 2 As an example, a granular oxide 12 that appears to be spherical is shown. The average particle size of the granular oxide 12 is not particularly limited, but is preferably less than 300 nm, less than 200 nm, or less than 150 nm.

[0148] (Number density)

[0149] In this invention, the number density of granular oxides is less than 4.0 particles / μm. 2 In the steel plate of this invention, since the surface layer of the steel plate may not contain granular oxides, the number density can also be 0 particles / μm. 2 Granular oxides may hinder hydrogen removal from steel by functioning as hydrogen trapping sites. Therefore, especially from the viewpoint of improving hydrogen expulsion, a smaller number of granular oxides is preferred, with a number density set below 4.0 particles / μm. 2 From the viewpoint of further achieving good hydrogen expulsion performance, the number density is preferably below 3.0 atoms / μm. 2 More preferably, less than 2.0 particles / μm 2 More preferably, the number is less than 1.0 per μm. 2 .

[0150] The average particle diameter and number density of the granular oxide were measured by a scanning electron microscope (SEM). The measurement was performed as described below. The cross section of the surface layer of the steel sheet was observed by the SEM to obtain an SEM image including the granular oxide. From the SEM image, 10 regions of 1.0 μm (depth direction) x 1.0 μm (width direction) each, which did not include the grain boundary type oxide described later, were selected as observation regions. As the observation position of each region, 1.0 μm in the depth direction (direction perpendicular to the surface of the steel sheet) in the region from the surface of the steel sheet to 1.5 μm, and 1.0 μm in the width direction (direction parallel to the surface of the steel sheet) at an arbitrary position of the above SEM image were set. Note that in the case where the granular oxide was not observed in the observation region, the number density was 0 / μm 2 , and the average particle diameter was not present. Next, the SEM images of the regions selected as described above were extracted, and binarized in order to separate the oxide portion from the steel portion, the total area of the granular oxide portion was calculated from each binarized image, and the number of the granular oxides in each binarized image was counted. The total area and the number of the granular oxides of the 10 regions were summed up, and the average particle diameter (nm) of the granular oxide was calculated as the diameter of an equivalent circle. In addition, the number density (number / μm 2 ) of the granular oxide was equal to the average of the number of the granular oxides counted from each binarized image. Note that in the case where only a part of the granular oxide was observed in the observation region, that is, in the case where the outline of the granular oxide was not entirely within the observation region, the number was not counted. In addition, from the viewpoint of measurement accuracy, the lower limit of the number of the granular oxides counted was set to 5.0 nm or more.

[0151] [Grain boundary type oxide]

[0152] In the present application, the "grain boundary type oxide" refers to an oxide present along the crystal grain boundary of the steel, and does not include an oxide present within the crystal grain of the steel. In fact, the grain boundary type oxide exists in a planar shape along the crystal grain boundary in the surface layer of the steel sheet, and thus is observed in a linear shape when the cross section of the surface layer of the steel sheet is observed. In Figure 2 and Figure 3 , the grain boundary type oxide 13 is shown as an example in a linear shape. In addition, in Figure 2 and Figure 3 , the grain boundary type oxide 13 is shown in the lower portion of the granular oxide 12 as a typical example of the steel sheet 11, but the grain boundary type oxide 13 is also formed in the vicinity of the surface of the base steel 14. By forming the grain boundary type oxide 13 in the vicinity of the surface of the steel sheet, the grain boundary type oxide connects the surface of the steel sheet with the inside of the steel sheet, and sufficiently functions as a hydrogen discharge site.

[0153] (Ratio A)

[0154] In this invention, the so-called "ratio A" is as follows: Figure 3 As shown, the ratio A in the observed image refers to the ratio of "length of grain boundary oxide projected onto the surface of the steel plate: L(L1+L2+L3+L4)" to "length of the steel plate surface: L0" when observing a cross-section of the surface of the steel plate 11. In this invention, the ratio A is 50% or more and 100% or less. By controlling the ratio A within such a range, grain boundary oxides can be formed over a wide range inside the steel plate, and these grain boundary oxides function well as hydrogen removal pathways. On the other hand, if the ratio A becomes less than 50%, it may not function sufficiently as a hydrogen removal pathway, and good hydrogen removal performance may not be obtained. The ratio A is preferably 60% or more, more preferably 70% or more, further preferably 80% or more, and even more preferably 90% or more. The ratio A may also be 100%.

[0155] Ratio A, for example Figure 3 As shown, the determination is made by cross-sectional observation of the surface layer of the steel plate 11. The specific measurement method is as follows. The cross-section of the surface layer of the steel plate 11 is observed using SEM. The observation position is set to a randomly selected location. The length L0 of the surface (i.e., the width of the SEM image) is measured from the observed SEM image. The length L0 is set to 100 μm or more (e.g., 100 μm, 150 μm, or 200 μm), and the measurement depth is set to the area from the surface of the steel plate to 50 μm. Next, the position of the grain boundary oxide 13 is determined from the SEM image. The determined grain boundary oxide 13 is projected onto the surface of the steel plate 11 (in the case of a plated steel plate, at the interface between the steel plate 11 and the plating), and the length L of the grain boundary oxide 13 in the field of view is calculated (=L1+L2+L3+L4). Based on the L0 and L thus calculated, the ratio A (%) in this invention is calculated as 100×L / L0. It should be noted that... Figure 3 The diagram of granular oxide 12 has been omitted for illustrative purposes.

[0156] [Composition of Oxides]

[0157] In the present application, the granular-type oxide and the grain boundary-type oxide (hereinafter, also simply referred to as oxide) are substances containing one or two or more of the elements contained in the above-described steel sheet in addition to oxygen, and typically have a composition containing Si, O, and Fe, and further containing Mn depending on the case. More specifically, the oxide typically contains Si: 5 to 25%, Mn: 0 to 10%, O: 40 to 65%, and Fe: 10 to 30%. The oxide can contain the elements (e.g., Cr, etc.) that can be contained in the above-described steel sheet in addition to these elements.

[0158] [Si-Mn deficient layer]

[0159] The steel sheet of the present application contains a Si-Mn deficient layer having a thickness of 3.0 μm or more from the surface of the steel sheet, and the Si and Mn contents of the Si-Mn deficient layer not containing the oxide at a position of 1 / 2 of the thickness are each 10% or less of the Si and Mn contents in the center portion of the plate thickness of the steel sheet. By setting the Si-Mn deficient layer generated in the surface layer of the steel sheet due to the formation of the grain boundary-type oxide and / or the granular-type oxide to a thickness of 3.0 μm or more, and controlling the Si and Mn deficiency rates of the Si-Mn deficient layer to be each 10% or less, the amount of solid-solved Si and Mn that hinders the diffusion of hydrogen can be sufficiently reduced, and as a result, the diffusion of hydrogen can be promoted. Therefore, by combining the above-described grain boundary-type oxide with the Si-Mn deficient layer, the hydrogen releasability can be significantly improved. Further, by increasing the thickness of the Si-Mn deficient layer, the diffusion of hydrogen from the steel can be further promoted, and therefore the thickness of the Si-Mn deficient layer is preferably 4.0 μm or more, more preferably 5.0 μm or more, and most preferably 7.0 μm or more. The upper limit of the thickness of the Si-Mn deficient layer is not particularly limited, but for example, the thickness of the Si-Mn deficient layer is preferably 50.0 μm or less.

[0160] Similarly, by further reducing the Si and Mn deficiency rates of the Si-Mn deficient layer, the amount of dissolved Si and Mn in the steel can be further reduced. Therefore, the Si deficiency rate of the Si-Mn deficient layer is preferably 8% or less, more preferably 6% or less, and most preferably 4% or less. The lower limit of the Si deficiency rate is not particularly limited, but it can also be 0%. Similarly, the Mn deficiency rate of the Si-Mn deficient layer is preferably 8% or less, more preferably 6% or less, and most preferably 4% or less. The lower limit of the Mn deficiency rate is not particularly limited, but it can also be 0%. In this invention, the expression "oxide-free" means that it does not contain not only the above-mentioned grain boundary oxides and granular oxides, but also any other oxides. Such oxide-free regions can be determined by cross-sectional observation using SEM and energy-dispersive X-ray spectrometer (EDS). Furthermore, when the Si-Mn deficient layer of this invention simply forms internal oxides such as grain boundary oxides, it is impossible to control the desired thickness and composition range. As explained in detail below, it becomes important to appropriately control the internal oxidation process during manufacturing.

[0161] The thickness of the Si-Mn deficient layer is as follows: Figure 3 As shown by D, this refers to the distance from the surface of the steel plate 11 (or the interface between the steel plate and the coating in the case of a coated steel plate) along the thickness direction of the steel plate 11 (the direction perpendicular to the surface of the steel plate), to the farthest point where the grain boundary oxide 13 exists. The thickness of the Si-Mn deficient layer can be determined from the same SEM image (the length L0 of the surface) used to measure the ratio A mentioned above. Furthermore, the Si and Mn content in the oxide-free region at half the thickness of the Si-Mn deficient layer is determined as follows: 10 points randomly selected at half the thickness of the Si-Mn deficient layer as determined by the SEM image above are analyzed using a transmission electron microscope (TEM-EDS) with an energy-dispersive X-ray spectrometer, and the obtained Si and Mn concentration measurements are arithmetically averaged. Furthermore, the Si and Mn contents in the center of the steel plate were determined as follows: A cross-section of the center of the plate was observed using a SEM; ten randomly selected points in the center of the plate from the SEM image were analyzed using a transmission electron microscope with an energy-dispersive X-ray spectrometer (TEM-EDS); and the obtained Si and Mn concentration measurements were arithmetically averaged. Finally, the Si and Mn contents at half the thickness of the Si-Mn deficient layer were divided by the Si and Mn contents in the center of the steel plate, and the resulting values ​​expressed as percentages were determined as the Si and Mn deficiency rates.

[0162] <Coated steel sheet>

[0163] The plated steel sheet of the present application has a plated layer containing Zn on the above-described steel sheet of the present application. The plated layer can be formed on one side of the steel sheet or on both sides. As the plated layer containing Zn, for example, hot-dip galvanized layer, alloyed hot-dip galvanized layer, electroplated zinc layer, electroplated alloyed zinc layer, etc. can be listed. More specifically, as the plated species, for example, Zn-0.2% Al (GI), Zn-0.09% Al (GA), Zn-1.5% Al-1.5% Mg, or Zn-11% Al-3% Mg-0.2% Si, etc. can be used.

[0164] [Composition of plated layer]

[0165] The composition contained in the plated layer containing Zn in the present application is described. The "%" of the content of elements means "mass %" unless otherwise specified. In the numerical range of the composition of the plated layer, the numerical range indicated by "~" means the range including the numerical values recited before and after the "~" as the lower limit value and the upper limit value unless otherwise specified.

[0166] (Al: 0 to 60.0%)

[0167] Al is an element that improves the corrosion resistance of the plated layer by being contained or alloyed with Zn, and thus can be contained as needed. Therefore, the Al content can also be 0%. In order to form a plated layer containing Zn and Al, the Al content is preferably 0.01% or more, and for example, 0.1% or more, 0.5% or more, 1.0% or more, or 3.0% or more is preferable. On the other hand, even if Al is excessively contained, the effect of improving the corrosion resistance is saturated, and thus the Al content is preferably 60.0% or less, and 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 is preferable.

[0168] (Mg: 0 to 15.0%)

[0169] Mg is an element that improves the corrosion resistance of the plated layer by being contained or alloyed with Zn and Al, and thus can be contained as needed. Therefore, the Mg content can also be 0%. In order to form a plated layer containing Zn, Al, and Mg, the Mg content is preferably 0.01% or more, and for example, 0.1% or more, 0.5% or more, 1.0% or more, or 3.0% or more is preferable. On the other hand, if Mg is excessively contained, Mg does not completely dissolve in the plating bath and floats as an oxide, and if zinc plating is performed with this plating bath, the oxide can adhere to the plated surface layer to cause appearance defects, or un-plated portions can be generated. Therefore, the Mg content is preferably 15.0% or less, and for example, 10.0% or less, 5.0% or less is preferable.

[0170] (Fe: 0 to 15.0%)

[0171] Fe can be contained in the plated layer by diffusion from the steel sheet when the plated steel sheet is subjected to heat treatment after the formation of the plated layer containing Zn on the steel sheet. Therefore, in a state where heat treatment is not performed, Fe is not contained in the plated layer, and thus the Fe content can also be 0%. Furthermore, the Fe content can also be 1.0% or more, 2.0% or more, 3.0% or more, 4.0% or more, or 5.0% or more. On the other hand, the Fe content is preferably 15.0% or less, and can also be 12.0% or less, 10.0% or less, 8.0% or less, or 6.0% or less, for example.

[0172] (Si: 0 to 3.0%)

[0173] Si is an element that further improves corrosion resistance if contained in the plated layer containing Zn, particularly a Zn-Al-Mg plated layer, and thus can also be contained as needed. Therefore, the Si content can also be 0%. From the viewpoint of corrosion resistance improvement, the Si content can also be 0.005% or more, 0.01% or more, 0.05% or more, 0.1% or more, or 0.5% or more, for example. Furthermore, the Si content can also be 3.0% or less, 2.5% or less, 2.0% or less, 1.5% or less, or 1.2% or less.

[0174] The basic component composition of the plated layer is as described above. Furthermore, the plated layer can also arbitrarily contain one or two or more of Sb: 0 to 0.50%, Pb: 0 to 0.50%, Cu: 0 to 1.00%, Sn: 0 to 1.00%, Ti: 0 to 1.00%, Sr: 0 to 0.50%, Cr: 0 to 1.00%, Ni: 0 to 1.00%, and Mn: 0 to 1.00%. Although not particularly limited, from the viewpoint of sufficiently exerting the roles and functions of the above-described basic components constituting the plated layer, the total content of these arbitrary added elements is preferably set to 5.00% or less, and more preferably set to 2.00% or less.

[0175] The remaining portion other than the above-described components in the plated layer is constituted by Zn and impurities. The so-called impurities in the plated layer are components and the like that are mixed due to various factors of the manufacturing process, with raw materials as the representative, at the time of manufacturing the plated layer. In the plated layer, as impurities, elements other than the basic components and the arbitrary added components described above can also be contained in a trace amount within a range that does not hinder the effects of the present application.

[0176] The component composition of the plated layer can be determined by dissolving the plated layer in an acid solution to which an inhibitor that suppresses corrosion of the steel sheet is added, and measuring the obtained solution by ICP (high-frequency inductively coupled plasma) emission spectrometry.

[0177] The thickness of the plated layer is preferably 3 to 50 μm, for example. In addition, the amount of adhesion of the plated layer is not particularly limited, but is preferably 10 to 170 g / m2per side, for example. 2 In the present application, the amount of adhesion of the plated layer is determined from the change in weight before and after pickling by dissolving the plated layer in an acid solution to which an inhibitor for suppressing corrosion of the base metal is added.

[0178] [Tensile strength]

[0179] The steel sheet and plated steel sheet of the present application preferably have high strength, and specifically preferably have a tensile strength of 440 MPa or more. The tensile strength can be 500 MPa or more, 600 MPa or more, 700 MPa or more, or 800 MPa or more, for example. The upper limit of the tensile strength is not particularly limited, but can be 2000 MPa or less, for example, from the viewpoint of ensuring toughness. The tensile strength can be measured by taking a JIS No. 5 tensile test piece with the direction at right angles to the rolling direction as the length direction, and following JIS Z2241 (2011).

[0180] The steel sheet and plated steel sheet of the present application are high in strength, and have high plating properties and hydrogen discharge properties, and thus can be suitably used in a wide range of fields such as automobiles, home electric appliances, and building materials, but are particularly preferably used in the field of automobiles. Steel sheets used in the field of automobiles are generally subjected to plating treatment (typically Zn-based plating treatment), and thus the effect of having high plating properties of the present application can be suitably exhibited when the steel sheet of the present application is used as a steel sheet for automobiles. In addition, steel sheets and plated steel sheets used in the field of automobiles are mostly subjected to hot press forming, and in this case, hydrogen embrittlement cracking becomes a significant problem. Thus, the effect of having high hydrogen discharge properties of the present application can be suitably exhibited when the steel sheet and plated steel sheet of the present application are used as a steel sheet for automobiles.

[0181] [Method for manufacturing steel sheet]

[0182] Next, a preferred method for manufacturing the steel sheet of the present application will be described. The intention of the following description is to exemplify a characteristic method for manufacturing the steel sheet of the present application, and not to limit the steel sheet to one manufactured by the manufacturing method described below.

[0183] The steel sheet of the present application can be obtained, for example, by performing the following steps: a casting step of casting molten steel having an adjusted composition to form a steel slab; a hot rolling step of hot-rolling the steel slab to obtain a hot-rolled steel sheet; a coiling step of coiling the hot-rolled steel sheet; a cold rolling step of cold-rolling the hot-rolled steel sheet after coiling to obtain a cold-rolled steel sheet; a grinding step of introducing dislocations to the surface of the cold-rolled steel sheet; and an annealing step of annealing the ground cold-rolled steel sheet. Alternatively, the hot-rolling step can be followed by pickling without coiling and directly by the cold rolling step.

[0184] [casting step]

[0185] The conditions of the casting step are not particularly limited. For example, it is only necessary to perform various secondary refining after melting by a blast furnace or an electric furnace or the like, and then to perform casting by a usual continuous casting, casting by an ingot casting method, or the like.

[0186] [hot rolling step]

[0187] The steel slab casted as described above can be subjected to hot rolling to obtain a hot-rolled steel sheet. The hot rolling step is performed by hot-rolling the casted steel slab directly or after temporary cooling and reheating. In the case of performing reheating, the heating temperature of the steel slab is, for example, 1100°C to 1250°C. In the hot rolling step, usually, rough rolling and finish rolling are performed. The temperature of each rolling and the reduction rate can be appropriately changed depending on the desired metal structure and sheet thickness. For example, the finish rolling can be performed at a finish rolling end temperature of 900°C to 1050°C and a reduction rate of 10% to 50%.

[0188] [coiling step]

[0189] The hot-rolled steel sheet can be coiled at a prescribed temperature. The coiling temperature can be appropriately changed depending on the desired metal structure or the like, and is, for example, 500°C to 800°C. The hot-rolled steel sheet can be given a prescribed heat treatment by uncoiling before or after coiling. Alternatively, the coiling step can not be performed, and the hot-rolling step can be followed by pickling and a cold rolling step described later.

[0190] [cold rolling step]

[0191] After the hot-rolled steel sheet is subjected to pickling or the like, the hot-rolled steel sheet can be subjected to cold rolling to obtain a cold-rolled steel sheet. The reduction rate of cold rolling can be appropriately changed depending on the desired metal structure and sheet thickness, and is, for example, 20% to 80%. After the cold rolling step, for example, air cooling can be performed to cool to room temperature.

[0192] [grinding step]

[0193] In order to sufficiently form the grain boundary type oxide in the surface layer of the finally obtained steel sheet, and further form the Si-Mn deficient layer having the desired thickness and composition, it is effective to perform a grinding process before annealing the cold-rolled steel sheet. By this grinding process, a large number of dislocations can be introduced to the surface of the cold-rolled steel sheet. Diffusion of oxygen and the like is faster at the grain boundary than within the grain, and thus by introducing a large number of dislocations to the surface of the cold-rolled steel sheet, a large number of passages can be formed as in the case of the grain boundary. Therefore, at the time of annealing, oxygen becomes easy to diffuse (intrude) into the interior of the steel along these dislocations, and furthermore, the diffusion speed of Si and Mn is increased, and thus as a result, it becomes possible to promote the combination of oxygen with Si and / or Mn in the interior of the steel to form the grain boundary type oxide. Furthermore, in conjunction with the promotion of such interior oxide formation, the decrease in the concentration of Si and Mn in the surroundings is also promoted, and thus the formation of the Si-Mn deficient layer having the desired thickness and composition can also be promoted. The grinding process is not particularly limited, but for example, it can be performed by grinding the surface of the cold-rolled steel sheet using a power brush under conditions in which the grinding amount is 10 to 200 g / m 2 The grinding amount using the power brush can be adjusted by any appropriate method known to those skilled in the art, although it is not particularly limited, but for example, it can be adjusted by appropriately selecting the number of power brushes, the rotation speed, the brush pressure, and the coating liquid used, and the like. By performing such a grinding process, the desired grain boundary type oxide can be formed in the annealing process described later, and a Si-Mn deficient layer having the desired thickness and composition, that is, having a thickness of 3.0 pm or more and the Si and Mn deficiency rates each becoming lower than 10%, can be reliably and effectively formed in the surface layer of the steel sheet.

[0194] [Annealing Process]

[0195] The cold-rolled steel sheet after the above grinding process is annealed. The annealing is preferably performed in a state in which tension is applied to the cold-rolled steel sheet in the rolling direction. In particular, in a region in which the annealing temperature is 500°C or higher, it is preferable to perform the annealing with increased tension compared to other regions, and specifically, in a region in which the annealing temperature is 500°C or higher, it is preferable to perform the annealing in a state in which tension of 3 to 150 MPa, particularly 15 to 150 MPa, is applied to the cold-rolled steel sheet in the rolling direction. If tension is applied at the time of annealing, it becomes possible to more effectively introduce a large number of dislocations to the surface of the cold-rolled steel sheet. Therefore, at the time of annealing, oxygen becomes easy to diffuse (intrude) into the interior of the steel along these dislocations, and furthermore, the diffusion speed of Si and Mn is increased, and thus it becomes easy to generate oxides in the interior of the steel sheet. As a result, it becomes advantageous for forming the grain boundary type oxide having the desired rate, and forming the Si-Mn deficient layer having the desired thickness and composition.

[0196] From the viewpoint of forming grain boundary type oxides in the surface layer of the steel sheet and increasing the ratio A, the holding temperature of the annealing step is preferably from more than 780°C to 900°C, and more preferably from 800 to 850°C. If the holding temperature of the annealing step is 780°C or less, sometimes the formation of grain boundary type oxides becomes insufficient, and the hydrogen discharging function decreases. On the other hand, if the holding temperature of the annealing step exceeds 900°C, it is possible that an external oxide layer is formed on the surface of the steel sheet, and the plating property becomes insufficient. The temperature increasing rate up to the above-mentioned holding temperature is not particularly limited, but it is sufficient if it is from 1 to 10°C / sec. Furthermore, the temperature increasing can be performed in two stages by a first temperature increasing rate of from 1 to 10°C / sec and a second temperature increasing rate of from 1 to 10°C / sec which is different from the first temperature increasing rate.

[0197] The holding time at the above-mentioned holding temperature is preferably from 10 to 50 seconds, and more preferably from 30 to 50 seconds. If the holding time is less than 10 seconds, it is possible that grain boundary type oxides are not sufficiently formed, and sometimes the plating property and the hydrogen discharging property become insufficient. On the other hand, if the holding time exceeds 50 seconds, it is possible that granular type oxides are excessively formed, and sometimes the hydrogen discharging property becomes insufficient.

[0198] From the viewpoint of forming grain boundary type oxides in a wide range (at a high ratio A), the dew point of the atmosphere in the annealing step is preferably from -20 to 10°C, and more preferably from -10 to 5°C. If the dew point is too low, it is possible that an external oxide layer is formed on the surface of the steel sheet, and internal oxides are not sufficiently formed, and sometimes the plating property and the hydrogen discharging property become insufficient. On the other hand, if the dew point is too high, it is possible that Fe oxides are formed on the surface of the steel sheet as external oxides, and the plating property becomes insufficient. Furthermore, the atmosphere in the annealing step is preferably a reducing atmosphere, and more specifically a reducing atmosphere containing nitrogen and hydrogen, for example, a reducing atmosphere of from 1 to 10% of hydrogen (for example, 4% of hydrogen and nitrogen balance).

[0199] Furthermore, it is effective to remove the internal oxide layer of the steel sheet at the time of performing the annealing step in advance. Sometimes an internal oxide layer is formed in the surface layer of the steel sheet between the above-mentioned rolling steps, and particularly the hot rolling step. The internal oxide layer formed in such a rolling step can possibly hinder the formation of sufficient grain boundary type oxides in the annealing step, and therefore the internal oxide layer is preferably removed in advance before the annealing by pickling treatment or the like. More specifically, the depth of the internal oxide layer of the cold rolled steel sheet at the time of performing the annealing step is preferably set to 0.5 μm or less, more preferably 0.3 μm or less, and further more preferably 0.2 μm or less, and still further more preferably 0.1 μm or less.

[0200] By performing the above-mentioned respective steps, a steel sheet in which grain boundary type oxides are formed in the surface layer of the steel sheet in a wide range (at a high ratio A), the formation of granular type oxides can be sufficiently suppressed, and which contains a Si-Mn deficient layer having a desired thickness and composition can be obtained.

[0201] Note that, as a pre-stage of the annealing process, in the case where a process of performing oxidation using an oxidation zone at an air ratio or air-fuel ratio of 0.9 to 1.4, followed by reduction, is provided, the ratio A of the length of the grain boundary type oxide becomes lower than 50%, and thus the grain boundary type oxide does not sufficiently function as a hydrogen discharge path, and it becomes difficult to obtain good hydrogen discharge properties.

[0202] <Method for manufacturing plated steel sheet>

[0203] Hereinafter, a preferred method for manufacturing the plated steel sheet of the present application will be described. The intention of the following description is to exemplify a characteristic method for manufacturing the plated steel sheet of the present application, and not to limit the plated steel sheet to one manufactured by the manufacturing method as described below.

[0204] The plated steel sheet of the present application can be obtained by performing a plating treatment process of forming a plated layer containing Zn on the steel sheet manufactured as described above.

[0205] [Plating treatment process]

[0206] The plating treatment process can be performed as long as it is performed according to a method known to those skilled in the art. The plating treatment process can be performed, for example, by hot-dip plating, or by electroplating. It is preferred that the plating treatment process be performed by hot-dip plating. The conditions of the plating treatment process can be appropriately set in consideration of the composition, thickness, and adhesion amount, etc. of the desired plated layer. After the plating treatment, an alloying treatment can also be performed. Typically, the conditions of the plating treatment process are appropriately set in a manner to form a plated layer containing Al: 0 to 60.0%, Mg: 0 to 15.0%, Fe: 0 to 15%, and Si: 0 to 3%, with the remainder consisting of Zn and impurities. More specifically, the conditions of the plating treatment process can be appropriately set, for example, in a manner to form Zn-0.2% Al (GI), Zn-0.09% Al (GA), Zn-1.5% Al-1.5% Mg, or Zn-11% Al-3% Mg-0.2% Si.

[0207] Examples

[0208] Hereinafter, the present application will be described in more detail by way of examples, but the present application is not limited in any way by these examples.

[0209] (Production of steel sheet test specimens)

[0210] Molten steel with adjusted composition was cast to form a billet, which was then hot-rolled, pickled, and cold-rolled to obtain a cold-rolled steel sheet. Next, the sheet was air-cooled to room temperature, and then pickled to remove the internal oxide layer formed during rolling to the depth (μm) of the internal oxide layer before annealing as shown in Table 1. Samples were then collected from each cold-rolled steel sheet according to JIS G0417:1999, and the composition of the steel sheets was analyzed by ICP-MS and other methods. The measured composition of the steel sheets is shown in Table 1. All steel sheets used had a thickness of 1.6 mm.

[0211] Next, for each cold-rolled steel sheet, after applying a NaOH aqueous solution, a high-powered grinding brush is used at a speed of 10–200 g / m². 2 The surface of the cold-rolled steel sheet was ground with a grinding amount (sample No. 135 was not ground). Afterwards, annealing treatment (annealing atmosphere: 4% hydrogen and nitrogen balance) was performed using the dew point, holding temperature, and holding time shown in Table 1, and each steel sheet sample was prepared. For all steel sheet samples, the heating rate during annealing was set to 6.0 °C / second up to 500 °C, and 2.0 °C / second from 500 °C to the holding temperature. In the above annealing treatment, the cold-rolled steel sheet was annealed under a tension of 1 MPa or more along the rolling direction. In the region where the annealing temperature was 500 °C or higher, annealing was performed under a higher tension along the rolling direction than in other regions, specifically 3–150 MPa (such tension was not applicable to sample No. 134). Table 1 shows the presence or absence of grinding with a high-powered grinding brush and the conditions of annealing treatment (the applicability of tensions of 3–150 MPa in the region with annealing temperatures above 500°C, dew point (°C), holding temperature (°C), and holding time (seconds)). It should be noted that for each steel plate specimen, JIS No. 5 tensile test pieces were collected with the length direction perpendicular to the rolling direction as the reference direction, and tensile tests were conducted according to JIS Z 2241 (2011). The results showed that the tensile strength of No. 16 and No. 18 was below 440 MPa, while the others were above 440 MPa.

[0212] (Analysis of the surface layer of the steel plate sample)

[0213] Each steel sheet sample thus produced was cut into 25 mm x 15 mm, and the cut sample was embedded in a resin and mirror-polished. For the cross section of each steel sheet sample, 10 regions of 1.0 μm x 1.0 μm were observed by SEM. As the observation position, 1.0 μm from the surface of the steel sheet to 0.2 to 1.2 μm was set in the depth direction (direction perpendicular to the surface of the steel sheet), and 1.0 μm of an arbitrary position of the SEM image was set in the width direction (direction perpendicular to the surface of the steel sheet). Note that as the above regions, regions not containing grain boundary-type oxides were selected. Next, the SEM images of the regions of each steel sheet sample thus obtained were binarized, the area of the granular-type oxide portion was calculated from the binarized image, and the number of granular-type oxides in the SEM image was counted. From the area and the number of the granular-type oxides in the 10 binarized images thus calculated, the average particle diameter and the number density of the granular-type oxides were calculated as the equivalent circle diameter. The average particle diameter (nm) and the number density (number / μm2) of the granular-type oxides for each steel sheet sample are shown in Table 1. Note that in Table 1, in the case where no granular-type oxides were present in the SEM image (in the case where the number density = 0), the average particle diameter was recorded as "-". 2 ) shown in Table 1. Note that in Table 1, in the case where no granular-type oxides were present in the SEM image (in the case where the number density = 0), the average particle diameter was recorded as "-".

[0214] In addition, the ratio A of each steel sheet sample was determined from the above cross-sectional observation of the embedded sample. Specifically, in an SEM image of 150 μm width (= L0), the position of the grain boundary-type oxides was determined, the grain boundary-type oxides thus determined were projected onto the surface of the steel sheet, and the length L of the grain boundary-type oxides in the field of view was calculated. Based on L0 and L thus calculated, the ratio A (%) = 100 x L / L0 was calculated. The ratio A (%) of the granular-type oxides for each steel sheet sample is shown in Table 1.

[0215] For the thickness of the Si-Mn deficient layer, the distance from the surface of the steel sheet to the most distant position where the grain boundary type oxide exists was determined by advancing in the direction of the thickness of the steel sheet (the direction perpendicular to the surface of the steel sheet) from the surface of the steel sheet in the SEM image in which the ratio A was measured. Further, the Si and Mn contents of the oxide-free region at the 1 / 2 position of the thickness of the Si-Mn deficient layer were determined by analyzing 10 points randomly selected at the 1 / 2 position of the thickness of the Si-Mn deficient layer determined from the above SEM image using TEM-EDS, and arithmetically averaging the measured values of the obtained Si and Mn concentrations. Further, the Si and Mn contents in the center portion of the thickness of the steel sheet were determined by observing the cross section of the center portion of the thickness of the steel sheet using SEM, analyzing 10 points randomly selected in the center portion of the thickness from the SEM image using TEM-EDS, and arithmetically averaging the measured values of the obtained Si and Mn concentrations. Finally, the values obtained by dividing the Si and Mn contents at the 1 / 2 position of the thickness of the Si-Mn deficient layer by the Si and Mn contents in the center portion of the thickness of the steel sheet, respectively, were determined as the Si and Mn deficiency rates, expressed in percentage. Further, for each steel sheet sample, the composition of the granular type oxide and the grain boundary type oxide was analyzed, and as a result, either oxide contained Si, O, and Fe, and further contained Mn in most of the oxides, and thus the composition of either oxide contained Si: 5 to 25%, Mn: 0 to 10%, O: 40 to 65%, and Fe: 10 to 30%.

[0216] (Production of plated steel sheet samples)

[0217] After each steel sheet sample was cut into a size of 100 mm x 200 mm, a plated steel sheet sample was produced by performing plating treatment for forming the plated kind shown in Table 1. In Table 1, the plated kind A indicates "GA (galvannealed steel sheet)", the plated kind B indicates "GI (hot-dip galvanized steel sheet)", and the plated kind C indicates "Zn-1.5% Al-1.5% Mg". In the hot-dip galvanizing step, the cut sample was immersed in a hot-dip galvanizing bath at 440°C for 3 seconds. After the immersion, it was drawn at 100 mm / sec, and the plating adhesion amount was controlled to 50 g / m 2 For the plated kind A, an alloying treatment was then performed at 460°C.

[0218] (Evaluation of plating property)

[0219] For each plated steel sheet sample, the platability was evaluated by measuring the area ratio of the uncoated portion of the surface of the steel sheet. Specifically, a 1 mm x 1 mm region of the surface of each plated steel sheet sample on which a plated layer was formed was observed with an optical microscope, the portion on which a plated layer was formed (coated portion) and the portion on which no plated layer was formed (uncoated portion) were discriminated from the observed image, the area ratio of the uncoated portion (area of uncoated portion / area of observed image) was calculated, the platability was evaluated by the following criteria, and the results are shown in Table 1. O is pass, and X is fail.

[0220] Evaluation O: 5.0% or less

[0221] Evaluation X: more than 5.0%

[0222] (Evaluation of Hydrogen Discharge Property)

[0223] The edge portion of the plated steel sheet sample was masked, and electrochemical hydrogen charging was performed in the plated steel sheet sample. The hydrogen charging was performed by immersing each sample in a mixed solution of 0.1 M-H2SO4 (pH = 3) and 0.01 M-KSCN at room temperature and constant current (100 μA / mm 2 ) conditions. After that, for each plated steel sheet sample, the diffusible hydrogen amount was measured by the temperature deviating method. Specifically, the plated steel sheet sample was heated to 400°C in a heating furnace equipped with a gas chromatograph, and the total of the hydrogen amount discharged until it was reduced to 250°C was measured. Based on the measured diffusible hydrogen amount, the hydrogen discharge property was evaluated by the following criteria, and the results are shown in Table 1. O and O are pass, and X is fail.

[0224] Evaluation O: 0.2 ppm or less

[0225] Evaluation O: more than 0.2 ppm and 0.4 ppm or less

[0226] Evaluation X: more than 0.4 ppm

[0227]

[0228] The samples No. 2 to 8 and 20 to 33 have high plating property and hydrogen discharge property because the component composition, the ratio A of grain boundary type oxide, the number density of granular type oxide, and the thickness and composition of Si-Mn deficient layer are appropriate. On the other hand, the samples No. 1 and 19 do not have high hydrogen discharge property because the internal oxidation layer depth before annealing is thick, the grain boundary type oxide is not sufficiently formed, and the desired Si-Mn deficient layer is not formed. The sample No. 9 does not have high plating property and hydrogen discharge property because the dew point at the time of annealing is low, the external oxidation layer is formed, the internal oxide is not generated, and the desired Si-Mn deficient layer is not formed. The sample No. 10 does not have high plating property and hydrogen discharge property because the dew point at the time of annealing is high, the external oxide is generated, the internal oxide is not sufficiently formed, and the desired Si-Mn deficient layer is not formed. The sample No. 11 does not have high plating property and hydrogen discharge property because the holding temperature at the time of annealing is high, the external oxide grows, the internal oxide is not sufficiently formed, and the desired Si-Mn deficient layer is not formed. The sample No. 12 does not have high hydrogen discharge property because the holding temperature at the time of annealing is low, the generation of granular type oxide is promoted, the grain boundary type oxide layer is not sufficiently formed. The sample No. 13 does not have high plating property and hydrogen discharge property because the holding time at the time of annealing is short, the internal oxide is not formed, and the desired Si-Mn deficient layer is not formed. The sample No. 14 does not have high hydrogen discharge property because the holding time at the time of annealing is long, many granular type oxides are generated, and the desired Si-Mn deficient layer is not formed. The sample No. 15 does not have high plating property and hydrogen discharge property because the Si amount is excessive, the external oxide grows, the internal oxide is not sufficiently formed, and the desired Si-Mn deficient layer is not formed. The samples No. 16 and 18 do not have high hydrogen discharge property because the Si amount and Mn amount are 0 (zero), the internal type oxide layer is not formed, and the desired Si-Mn deficient layer is not formed. The sample No. 17 does not have high plating property and hydrogen discharge property because the Mn amount is excessive, the external oxide grows, the internal oxide is not sufficiently formed, and the desired Si-Mn deficient layer is not formed. The sample No. 34 does not have high hydrogen discharge property because the prescribed tension is not applied at the time of annealing, the grain boundary type oxide is not sufficiently formed, and the desired Si-Mn deficient layer is not formed. The sample No. 35 does not have high hydrogen discharge property because the grinding before annealing is not performed, the grain boundary type oxide is not sufficiently formed, and the desired Si-Mn deficient layer is not formed.

[0229] Industrial applicability

[0230] According to the present application, a high-strength steel sheet and plated steel sheet having high platability and hydrogen discharge properties can be provided, which can be suitably used for automobile, household electric appliance, building material, and the like, particularly for automobiles, and high collision safety and long life can be expected as an automobile steel sheet and plated steel sheet. Thus, the present application can be said to be an extremely valuable invention in industry.

[0231] Explanation of symbols

[0232] 1 Steel sheet

[0233] 2 External oxide layer

[0234] 3 Base steel

[0235] 11 Steel sheet

[0236] 12 Granular oxide

[0237] 13 Grain boundary oxide

[0238] 14 Base steel

Claims

1. A steel sheet having a composition consisting of, in mass %: C：0.05～0.40％、 Si: 0.2 to 3.0%, Mn: 0.1 to 5.0%, sol. Al: 0 to less than 0.4000%, P: 0.0300% or less, S: 0.0300% or less, N: 0.0100% or less, B：0～0.010％、 Ti: 0 to 0.150%, Nb: 0 to 0.150%, V：0～0.150％、 Cr：0～2.00％、 Ni: 0 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 remainder consisting of Fe and impurities, a grain boundary type oxide is contained in a surface layer of the steel sheet, a ratio A of a length of the grain boundary type oxide projected on a surface of the steel sheet with respect to a length of the surface of the steel sheet is 50% or more and 100% or less when a cross section of the surface layer of the steel sheet is observed, The number density of the granular type oxide is less than 4.0 / μm 2 , the surface layer of the steel sheet contains a Si-Mn deficient layer having a thickness of 3.0 μm or more from the surface of the steel sheet, Si and Mn contents of an oxide-free region at a position of 1 / 2 of the thickness of the Si-Mn deficient layer are each 10% or less of Si and Mn contents in a center portion of the sheet thickness of the steel sheet, wherein the surface layer of the steel sheet refers to a region from the surface of the steel sheet to a depth of 50 μm or less in the sheet thickness direction, and in the case of a plated steel sheet, refers to a region from an interface of the steel sheet and a plated layer to a depth of 50 μm, the grain boundary type oxide refers to an oxide present along a crystal grain boundary of steel, and does not include an oxide present within a crystal grain of steel, the granular type oxide refers to an oxide dispersed in a granular form within a crystal grain or on a crystal grain boundary of steel, and the granular form refers to a state of being present separately from each other within a steel matrix, and refers to a state of having an aspect ratio of 1.0 to 5.0, the thickness of the Si-Mn deficient layer refers to a distance from the surface of the steel sheet to a most distant position where the grain boundary type oxide exists in a case where a direction of the sheet thickness of the steel sheet, i.e., a direction perpendicular to the surface of the steel sheet, is advanced from the surface of the steel sheet, and in the case of a plated steel sheet, refers to a distance from the surface of the steel sheet to a most distant position where the grain boundary type oxide exists in a case where a direction of the sheet thickness of the steel sheet, i.e., a direction perpendicular to the surface of the steel sheet, is advanced from an interface of the steel sheet and a plated layer.

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

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

4. The steel sheet according to any one of claims 1 to 3, wherein, The number density of the granular type oxide is less than 2.0 / μm 2 .

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

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