JP2022169169A - Steel sheet and plated steel sheet - Google Patents

Abstract

To provide a high strength steel sheet and a plated steel sheet having high plating properties and hydrogen embrittlement resistance.SOLUTION: A steel sheet contains, by mass, 0.05 to 0.40% of C, 0.2 to 3.0% of Si and 0.1 to 5.0% of Mn. The steel sheet includes an internal oxidation layer containing a granular type oxide, a grain boundary type oxide and a dendrite type oxide, on a surface layer thereof. The granular type oxide has an average grain diameter of 350nm or less. A number density of the granular type oxide in the internal oxidation layer is 4.0 pcs./μm2 or more. A ratio A of a length of the grain boundary type oxide projected onto the surface of the steel sheet to a length of the surface of the steel sheet is 50% to 100%, inclusive. An area ratio of the dendrite type oxide is 5.0% or more. A depth of the internal oxidation layer is 8 μm or more. At a 1/2 location of the depth, Si and Mn contents in the steel not including an oxide are respectively below 10% of Si and Mn contents at a sheet thickness center part of the steel sheet. A plated steel sheet uses the steel sheet.SELECTED DRAWING: Figure 2

Description

The present invention relates to steel sheets and plated steel sheets. More specifically, the present invention relates to high-strength steel sheets and plated steel sheets having high plateability and hydrogen embrittlement resistance.

BACKGROUND ART In recent years, steel sheets used in various fields such as automobiles, home electric appliances, and building materials are being strengthened. For example, in the field of automobiles, the use of high-strength steel sheets is increasing for the purpose of reducing the weight of automobile bodies in order to improve fuel efficiency. Such high strength steel sheets typically contain elements such as C, Si and Mn to improve the strength of the steel.

In the manufacture of high-strength steel sheets, heat treatment such as annealing is generally performed after rolling. In addition, among the elements typically contained in high-strength steel sheets, Si and Mn, which are easily oxidizable elements, combine with oxygen in the atmosphere during the heat treatment, and can form a layer containing oxides near the surface of the steel sheet. be. The form of such a layer includes a form in which an oxide containing Si or Mn is formed as a film on the outside (surface) of the steel sheet (external oxide layer), and an form in which an oxide is formed inside (surface layer) of the steel sheet. morphology (internal oxide layer).

When forming a plating layer (for example, a Zn-based plating layer) on the surface of a steel sheet on which an external oxide layer is formed, the oxide exists as a film on the surface of the steel sheet, so the steel composition (for example, Fe) and the plating Interdiffusion with components (for example, Zn) is hindered, and the adhesion between steel and plating may be affected, resulting in insufficient plateability (for example, increased unplated areas). Therefore, from the viewpoint of improving plateability, a steel sheet having an internal oxide layer is more preferable than a steel sheet having an external oxide layer.

In relation to the internal oxide layer, Patent Documents 1 and 2 disclose a plated steel sheet having a zinc-based plating layer on a base steel sheet containing C, Si, Mn, etc., wherein Si and/or Mn A high-strength plated steel sheet having a tensile strength of 980 MPa or more is described, which has an internal oxide layer containing oxides of

JP 2016-130357 A JP 2018-193614 A

High-strength steel sheets, especially high-strength steel sheets used for automotive parts, are generally coated with an electroplating layer after a plating layer such as a zinc-based plating layer is formed on the surface to impart desired properties (for example, corrosion resistance). A coating is applied. It is known that in this electrodeposition coating process, water is electrolyzed by voltage application to generate hydrogen. Hydrogen generated during the electrodeposition coating penetrates into the steel sheet and reaches a position deeper than the surface layer region of the steel sheet. can give rise to A phenomenon in which cracking occurs due to this penetrating hydrogen is called hydrogen embrittlement cracking (delayed fracture). Therefore, in order to prevent hydrogen embrittlement cracking, it is effective to suppress the entry of hydrogen into the inside of the steel sheet during electrodeposition coating, and to discharge the hydrogen that has entered the inside of the steel sheet to the outside of the system.

Patent Documents 1 and 2 teach that by controlling the average depth of the internal oxide layer to a thickness of 4 μm or more and allowing the internal oxide layer to function as a hydrogen trap site, hydrogen penetration can be prevented and hydrogen embrittlement can be suppressed. It is However, control of the form of oxides present in the internal oxide layer has not been studied at all, and there is room for improvement in resistance to hydrogen embrittlement.

In view of such circumstances, an object of the present invention is to provide a high-strength steel sheet and a plated steel sheet having high plateability and hydrogen embrittlement resistance.

In order to solve the above problems, the present inventors have found that it is important to form oxides in the surface layer of the steel sheet, that is, in the interior of the steel sheet, and to control the form of the oxides present in the surface layer of the steel sheet. I found out. More specifically, the present inventors have found that high plating properties are ensured by forming an internal oxide layer, and that fine and large amounts of granular type oxide present in the crystal grains of the metal structure as the form of oxide. and sufficiently form the dendritic oxide present in the crystal grains, the dendritic oxide functions as a trap site for hydrogen that can enter the steel sheet during electrodeposition coating, and the grain boundary A large amount of grain boundary type oxide is formed along the , and the grain boundary type oxide functions as an escape route for hydrogen that has entered the steel, suppressing the penetration of hydrogen into the steel, and from the inside of the steel to the outside of the system. It was found that high hydrogen embrittlement resistance can be obtained by promoting the discharge of hydrogen to. In addition, the present inventors have found that by controlling the Si and Mn contents of the oxide-free portion of the internal oxide layer within a predetermined range, hydrogen diffusion in the steel is promoted and hydrogen is discharged from the steel. It has been found that the performance can be further improved. In addition, the present inventors found that these granular type oxides, grain boundary type oxides, and dendrite type oxides exist from the surface of the steel sheet to a deeper position, that is, to increase the depth of the internal oxide layer. As a result, the above-described hydrogen trapping function and hydrogen discharging function are more effectively exhibited, and the resistance to hydrogen embrittlement is significantly improved.

The present invention was made based on the above findings, and the gist thereof is as follows.
(1)
in % by mass,
C: 0.05 to 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 to 0.010%,
Ti: 0 to 0.150%,
Nb: 0 to 0.150%,
V: 0 to 0.150%,
Cr: 0 to 2.00%,
Ni: 0 to 2.00%,
Cu: 0 to 2.00%,
Mo: 0 to 1.00%,
W: 0 to 1.00%,
Ca: 0-0.100%,
Mg: 0-0.100%,
Zr: 0 to 0.100%,
A steel sheet containing Hf: 0 to 0.100% and REM: 0 to 0.100%, with the balance being Fe and impurities,
The surface layer of the steel sheet has an internal oxide layer containing a granular type oxide, a grain boundary type oxide and a dendrite type oxide,
The average particle size of the particulate type oxide is 350 nm or less,
The number density in the internal oxide layer of the particulate type oxide is 4.0 pieces/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 type oxide projected on 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 dendrite-type oxide has an area ratio of 5.0% or more,
The depth of the internal oxide layer is 8 μm or more,
A steel sheet, wherein the Si and Mn contents in the oxide-free steel at the half depth position are respectively less than 10% of the Si and Mn contents at the thickness center of the steel sheet.
(2)
The steel sheet according to (1), wherein the number density in the internal oxide layer of the particulate type oxide is 5.0 particles/μm ² or more.
(3)
The steel sheet according to (1) or (2), wherein the internal oxide layer has a depth of 15 μm or more.
(4)
The steel sheet according to any one of (1) to (3), wherein the ratio A is 80% or more.
(5)
A plated steel sheet having a plating layer containing Zn on the steel sheet according to any one of (1) to (4).
(6)
The plated steel sheet according to (5), wherein the plated layer has a composition of Zn-(0.3 to 1.5)% Al.

According to the present invention, the granular type oxide and the dendrite type oxide serve as trap sites for hydrogen that enters the steel sheet during electrodeposition coating, and the grain boundary type oxide serves as an escape route for hydrogen that has entered the steel sheet. As a result, the hydrogen embrittlement resistance can be greatly improved by suppressing the amount of hydrogen entering and increasing the amount of hydrogen discharged. Furthermore, an internal oxide layer containing these oxides is formed from the surface of the steel sheet to a deeper position, and a Si—Mn depleted region ( By controlling the Si and Mn contents of the oxide-free portion of the internal oxide layer within a predetermined range, hydrogen embrittlement resistance can be further improved. In addition, according to the present invention, since the oxide is formed inside the steel sheet, when forming the coating layer, the interdiffusion of the steel components and the coating components is sufficiently performed, and high coating properties can be obtained. becomes. Therefore, according to the present invention, it is possible to obtain high plateability and hydrogen embrittlement resistance in a high-strength steel sheet.

1 shows a schematic view of a cross-section of a steel sheet with an external oxide layer; FIG. 1 shows a schematic view of a cross-section of an exemplary steel plate according to the invention; FIG. 1 shows a schematic diagram for explaining the measurement of ratio A in the present invention. FIG.

<Steel plate>
The steel sheet according to the present invention is mass%,
C: 0.05 to 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 to 0.010%,
Ti: 0 to 0.150%,
Nb: 0 to 0.150%,
V: 0 to 0.150%,
Cr: 0 to 2.00%,
Ni: 0 to 2.00%,
Cu: 0 to 2.00%,
Mo: 0 to 1.00%,
W: 0 to 1.00%,
Ca: 0-0.100%,
Mg: 0-0.100%,
Zr: 0 to 0.100%,
A steel sheet containing Hf: 0 to 0.100% and REM: 0 to 0.100%, with the balance being Fe and impurities,
The surface layer of the steel sheet has an internal oxide layer containing a granular type oxide, a grain boundary type oxide and a dendrite type oxide,
The average particle size of the particulate type oxide is 350 nm or less,
The number density in the internal oxide layer of the particulate type oxide is 4.0 pieces/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 type oxide projected on 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 dendrite-type oxide has an area ratio of 5.0% or more,
The depth of the internal oxide layer is 8 μm or more,
The Si and Mn contents in the oxide-free steel at the half depth are less than 10% of the Si and Mn contents at the thickness center of the steel sheet, respectively.

In the manufacture of high-strength steel sheets, after rolling (typically hot rolling and cold rolling) steel billets adjusted to a predetermined chemical composition, they are generally annealed for the purpose of obtaining a desired structure. processing takes place. In this annealing treatment, a layer containing oxides is formed in the vicinity of the surface of the steel sheet by combining relatively easily oxidizable components (eg, Si, Mn) in the steel sheet with oxygen in the annealing atmosphere. For example, like the steel plate 1 shown in FIG. 1, an external oxide layer 2 is formed in a film on the surface of the base steel 3 (that is, on the outside of the base steel 3). When the external oxide layer 2 is formed in the form of a film on the surface of the base material steel 3, when a plating layer (for example, a zinc-based plating layer) is formed, the external oxide layer 2 becomes a plating component (for example, Zn). Since it inhibits interdiffusion with steel components (for example, Fe), sufficient adhesion between the steel and the plating cannot be ensured, and unplated areas where no plating layer is formed may occur.

On the other hand, as illustrated in FIG. 2, the steel plate 11 according to the present invention does not form an external oxide layer 2 on the surface of the base steel 3 like the steel plate 1 shown in FIG. , the oxides 12 , 13 , 14 are present inside the base steel 15 . Therefore, when a plating layer is formed on the surface of the steel sheet 11, the steel sheet 11 according to the present invention in which the oxides 12, 13, and 14 are formed inside the base steel 15 is different from the steel sheet 1 having the outer oxide layer 2. In contrast, mutual diffusion between the plating components and the steel components occurs sufficiently, making it possible to obtain high plating properties. Therefore, the present inventors have found that it is effective to control the conditions during annealing to form oxides inside the steel sheet from the viewpoint of obtaining high plateability. In addition, when the term "high plateability" is used for a steel plate, the non-plated portion (portion where the plated layer is not formed) is small when the steel plate is plated (for example, 5.0 area% or less). Or, it shows that the plating layer can be formed in the absence of any. Also, the term "highly plated" when used for a plated steel sheet indicates a plated steel sheet with very little (eg, 5.0% or less) or no non-plated portion.

On the other hand, in electrodeposition coating, which is generally performed especially in the production of high-strength steel sheets for automobiles, hydrogen is generated by electrolysis of water. When this hydrogen penetrates deeper than the surface layer region of the base steel, the penetrated hydrogen segregates in the martensitic grain boundaries of the base steel, embrittles the grain boundaries, and causes hydrogen embrittlement cracking. It has been known. Since martensite is a hard structure, it is highly sensitive to hydrogen and prone to hydrogen embrittlement cracking. Therefore, in order to prevent hydrogen embrittlement cracking of a steel sheet, it is necessary to suppress the penetration of hydrogen into the steel sheet during electrodeposition coating and to promote the discharge of the penetrated hydrogen, that is, high hydrogen embrittlement resistance It is preferred to have The present inventors have found that by controlling the morphology of oxides present in the surface layer of the steel sheet, more specifically, granular type oxides, grain boundary type oxides, and dendrite type oxides are formed in the surface layer of the steel sheet. By doing so, the granular type oxide and the dendrite type oxide exhibit the function of trapping hydrogen that enters during electrodeposition coating, and the grain boundary type oxide has the function of discharging hydrogen from the inside of the steel sheet to the outside of the system. As a result, the amount of hydrogen accumulated inside the steel sheet is suppressed, and high hydrogen embrittlement resistance is obtained. In addition, the inventors have found that the surrounding Si and Mn concentrations decrease due to the formation of granular-type oxides, grain-boundary-type oxides, and dendrite-type oxides, and that such Si and Mn concentrations It has been found that by controlling the decrease in the σ within a predetermined range, the diffusion of hydrogen in the steel can be promoted to further improve the ability to expel hydrogen from the steel. In addition, in order to further improve the hydrogen trapping function and the hydrogen discharging function, the present inventors have found that the internal oxide layer in which these oxides are present should be removed to a deeper position, specifically, the surface of the steel sheet (the surface of the steel sheet). It has been found that, when a plating layer is present, it is effective to allow it to exist to a depth of 8 μm from the interface between the plating layer and the steel sheet.

In addition, the present inventors have analyzed the relationship between the morphology of oxides and their effectiveness as hydrogen trap sites in detail. As a result, as shown in FIG. The dispersed granular type oxide 12 is present in a fine and large amount spaced apart from each other, and the dendrite type oxide 14 existing in the form of dendrites has a certain area ratio or more, more specifically, an area of 5.0% or more. It has been found that it is effective to make the ratio exist. Although not bound by any particular theory, it is believed that the trapping function of the oxides in the steel sheet for penetrating hydrogen has a positive correlation with the surface area of the oxides. That is, the particulate type oxide is finely dispersed in a large amount on the surface layer of the steel sheet, and the presence of the dendrite type oxide in an appropriate amount increases the surface area of the oxide on the surface layer of the steel sheet, It is considered that the hydrogen trapping function is greatly improved. Therefore, from the viewpoint of obtaining high resistance to hydrogen embrittlement, the present inventors controlled the conditions during the production of the steel sheet, particularly during the annealing treatment, to obtain a granular type that functions as a trap site for hydrogen that penetrates during electrodeposition coating. It has been found that it is important to allow the oxide to exist finely and in a large amount, and to allow the dendritic oxide to exist in an appropriate amount. Furthermore, the present inventors analyzed in detail the relationship between the morphology of oxides and their effectiveness as escape routes for hydrogen. As a result, as shown in FIG. It has been found that it is effective to allow a large amount of the grain boundary type oxide 13 existing in the . The presence of a large amount of the grain boundary type oxide 13 ensures a path for hydrogen in the steel to exit the system, and efficiently releases the hydrogen that has penetrated into the steel to the outside of the system along the grain boundaries. found that it is possible. Therefore, as exemplified in FIG. 2, the surface layer of the steel plate 11, that is, the base material steel 15, is made to coexist with the granular type oxide 12, the grain boundary type oxide 13, and the dendrite type oxide 14, so that hydrogen embrittlement resistance It is possible to greatly improve the chemical property. In addition, the metal structure of the surface layer of the steel plate is typically composed of a softer metal structure than the inside of the steel plate (e.g., 1/8 position or 1/4 position of the plate thickness), so hydrogen is present in the surface layer of the steel plate. Hydrogen embrittlement cracking does not pose a particular problem even if it is used.

In addition, the present inventors found that the surrounding Si and Mn concentrations were reduced due to the formation of internal oxides such as granular type oxides 12, grain boundary type oxides 13 and dendrite type oxides 14 as shown in FIG. As a result of detailed analysis of the relationship between the Si—Mn depleted region (the portion of the internal oxide layer that does not contain oxide) generated in the internal oxide layer due to the decrease and the hydrogen discharge property, the Si—Mn depleted region The composition of is within a predetermined range, more specifically, the Si and Mn content in the region not containing oxides at the 1/2 position of the depth of the internal oxide layer is the Si and Mn content at the center of the thickness of the steel sheet, respectively. It has been found that it is effective to control the amount to be less than 10% (hereinafter these values are also referred to as Si depletion rate and Mn depletion rate). Although not bound by a particular theory, 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 is also increased. It is believed that Mn inhibits the diffusion of hydrogen, resulting in a slow hydrogen diffusion rate in steel. As shown in FIG. 2, when internal oxides such as granular type oxides 12, grain boundary type oxides 13 and dendrite type oxides 14 are formed on the surface layer of the steel sheet, Si and Since Mn is consumed in the formation of internal oxides, a Si—Mn depleted region in which the surrounding Si and Mn concentrations are relatively low is generated on the surface layer of the steel sheet along with the formation of the internal oxides. Become. By setting the depth of the internal oxide layer to 8 μm or more as described above, naturally the thickness of the Si—Mn depleted region will also have a similar thickness. It is thought that it is possible to secure Furthermore, by sufficiently reducing the Si and Mn contents of the Si—Mn depleted region, specifically by controlling the Si and Mn depletion rates to be less than 10%, the solid solution Si that inhibits the diffusion of hydrogen and the amount of Mn can be sufficiently reduced. Therefore, it is considered that the inclusion of such a controlled Si—Mn deficient region in the internal oxide layer promotes the diffusion of hydrogen and significantly improves the ability to remove hydrogen from the steel. . Therefore, by combining the above-described granular type oxide, grain boundary type oxide, and dendrite type oxide with the Si—Mn depleted region, both the hydrogen penetration resistance and the hydrogen discharge property are improved, and the steel sheet as a whole It is possible to greatly improve the hydrogen embrittlement resistance of

Hydrogen embrittlement cracking is caused not only by the entry of hydrogen into the steel sheet during electrodeposition coating as described above, but also by the presence of hydrogen present in the annealing atmosphere during the annealing process when manufacturing high-strength steel sheets. It is also known that it can occur by penetrating deeper than the surface layer of the material steel. This time, the present inventors have found that the combination of the above-described morphology of the internal oxide layer and the characteristics of the Si—Mn depleted region in the internal oxide layer is effective not only during electrodeposition coating but also during annealing treatment. It has been found that it also effectively suppresses the intrusion of hydrogen and discharges infiltrated hydrogen.

Hereinafter, the steel sheet according to the present invention will be described in detail. The thickness of the steel sheet according to the present invention is not particularly limited, but may be, for example, 0.1 to 3.2 mm.

[Component composition of steel plate]
The chemical composition contained in the steel sheet according to the present invention will be described. "%" regarding the content of an element means "% by mass" unless otherwise specified. In the numerical range in the component composition, unless otherwise specified, the numerical range represented using "to" means the range including the numerical values before and after "to" as the lower and upper limits.

(C: 0.05-0.40%)
C (carbon) is an important element for ensuring the strength of steel. In order to ensure sufficient strength, the C content should be 0.05% or more. The C content is preferably 0.07% or more, more preferably 0.10% or more, still more preferably 0.12% or more. On the other hand, if the C content is excessive, weldability may deteriorate. Therefore, the C content should be 0.40% or less. The C content may be 0.38% or less, 0.35% or less, 0.32% or less, or 0.30% or less.

(Si: 0.2 to 3.0%)
Si (silicon) is an effective element for improving the strength of steel. The Si content is set to 0.2% or more to ensure sufficient strength and to sufficiently generate desired oxides inside the steel sheet. The Si content is preferably 0.3% or more, more preferably 0.5% or more, and still more preferably 1.0% or more. On the other hand, if the Si content is excessive, an excessive amount of external oxides may be generated, which may lead to deterioration of the surface properties. Therefore, the Si content should be 3.0% or less. The Si content may be 2.8% or less, 2.5% or less, 2.3% or less, or 2.0% or less.

(Mn: 0.1-5.0%)
Mn (manganese) is an element effective in improving the strength of steel by obtaining a hard structure. The Mn content is set to 0.1% or more in order to ensure sufficient strength and to sufficiently generate desired oxides inside the steel sheet. The Mn content is preferably 0.5% or more, more preferably 1.0% or more, still more preferably 1.5% or more. On the other hand, if the Mn content is excessive, an excessive amount of external oxides may be generated, or the metal structure may become non-uniform due to Mn segregation, resulting in a decrease in workability. Therefore, the Mn content should be 5.0% or less. The Mn content may be 4.5% or less, 4.0% or less, 3.5% or less, or 3.0% or less.

(sol. Al: 0 to less than 0.4000%)
Al (aluminum) is an element that acts as a deoxidizing element. Although the Al content may be 0%, the Al content is preferably 0.0010% or more in order to obtain a sufficient deoxidizing effect. The Al content is more preferably 0.0050% or more, still more preferably 0.0100% or more, and even more preferably 0.0150% or more. On the other hand, if the Al content is excessive, there is a risk of causing deterioration in workability and surface properties. Therefore, the Al content should be less than 0.4000%. Al content is 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 means the so-called acid-soluble Al content (sol. Al).

(P: 0.0300% or less)
P (phosphorus) is an impurity generally contained in steel. Excessive P content may reduce weldability. Therefore, the P content should be 0.0300% or less. The P content is preferably 0.0200% or less, more preferably 0.0100% or less, still more preferably 0.0050% or less. The lower limit of the P content is 0%, but from the viewpoint of manufacturing costs, the P content may be more than 0% or 0.0001% or more.

(S: 0.0300% or less)
S (sulfur) is an impurity generally contained in steel. If the S content is excessive, the weldability is lowered, and furthermore, the amount of precipitation of MnS increases, which may lead to a decrease in workability such as bendability. Therefore, the S content should be 0.0300% or less. The S content is preferably 0.0100% or less, more preferably 0.0050% or less, still more preferably 0.0020% or less. The lower limit of the S content is 0%, but from the viewpoint of desulfurization cost, the S content may be more than 0% or 0.0001% or more.

(N: 0.0100% or less)
N (nitrogen) is an impurity generally contained in steel. If N is contained excessively, weldability may deteriorate. Therefore, the N content should be 0.0100% or less. The N content is preferably 0.0080% or less, more preferably 0.0050% or less, still more preferably 0.0030% or less. Although the lower limit of the N content is 0%, the N content may be more than 0% or 0.0010% or more from the viewpoint of manufacturing cost.

The basic chemical composition of the steel sheet according to the present invention is as described above. Further, the steel sheet may contain the following arbitrary elements as necessary. The content of these elements is not essential, and the lower limit of the content of these elements is 0%.

(B: 0 to 0.010%)
B (boron) is an element that increases hardenability and contributes to strength improvement, and segregates at grain boundaries to strengthen the grain boundaries and improve toughness. The B content may be 0%, but may be contained as necessary in order to obtain the above effects. The B content may 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 may be 0.008% or less or 0.006% or less.

(Ti: 0 to 0.150%)
Ti (titanium) is an element that precipitates as TiC during cooling of steel and contributes to an improvement in strength. Although the Ti content may be 0%, it may be contained as necessary in order to obtain the above effect. The Ti content may 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 contained excessively, coarse TiN may be generated and the toughness may be impaired. Therefore, the Ti content is preferably 0.150% or less, and may be 0.100% or less or 0.050% or less.

(Nb: 0 to 0.150%)
Nb (niobium) is an element that contributes to strength improvement through improvement of hardenability. Although the Nb content may be 0%, it may be contained as necessary in order to obtain the above effect. The Nb content may 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 may be 0.100% or less or 0.060% or less.

(V: 0-0.150%)
V (vanadium) is an element that contributes to strength improvement through improvement of hardenability. Although the V content may be 0%, it may be contained as necessary in order to obtain the above effects. The V content may 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 may be 0.100% or less or 0.060% or less.

(Cr: 0 to 2.00%)
Cr (chromium) is effective in increasing the hardenability of steel and increasing the strength of steel. Although the Cr content may be 0%, it may be contained as necessary in order to obtain the above effects. The Cr content may 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 excessively, a large amount of Cr carbide is formed, which may adversely impair the hardenability. Therefore, the Cr content is preferably 2.00% or less, and may be 1.80% or less or 1.50% or less.

(Ni: 0 to 2.00%)
Ni (nickel) is an element effective in increasing the hardenability of steel and increasing the strength of steel. Although the Ni content may be 0%, it may be contained as necessary in order to obtain the above effect. The Ni content may 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 causes an increase in cost. Therefore, the Ni content is preferably 2.00% or less, and may be 1.80% or less or 1.50% or less.

(Cu: 0 to 2.00%)
Cu (copper) is an element effective in increasing the hardenability of steel and increasing the strength of steel. Although the content of Cu may be 0%, it may be contained as necessary in order to obtain the above effects. The Cu content may be 0.001% or more, 0.005% or more, or 0.01% or more. On the other hand, from the viewpoint of suppressing deterioration of toughness, cracking of the slab after casting, and deterioration of weldability, the Cu content is preferably 2.00% or less, 1.80% or less, 1.50% or less, or 1 00% or less.

(Mo: 0-1.00%)
Mo (molybdenum) is an element effective in increasing the hardenability of steel and increasing the strength of steel. The Mo content may be 0%, but may be contained as necessary in order to obtain the above effects. Mo content may 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 deterioration of toughness and weldability, the Mo content is preferably 1.00% or less, and may be 0.90% or less or 0.80% or less.

(W: 0-1.00%)
W (tungsten) is an element effective in increasing the hardenability of steel and increasing the strength of steel. Although the W content may be 0%, it may be contained as necessary in order to obtain the above effects. The W content may be 0.001% or more, 0.005% or more, or 0.01% or more. On the other hand, from the viewpoint of suppressing deterioration of toughness and weldability, the W content is preferably 1.00% or less, 0.90% or less, 0.80% or less, 0.50% or less, or 0.10%. % or less.

(Ca: 0 to 0.100%)
Ca (calcium) is an element that contributes to inclusion control, particularly fine dispersion of inclusions, and has an effect of increasing toughness. Although the Ca content may be 0%, it may be contained as necessary in order to obtain the above effects. The Ca content may be 0.0001% or more, 0.0005% or more, or 0.001% or more. On the other hand, if Ca is contained excessively, deterioration of surface properties may become apparent. Therefore, the Ca content is preferably 0.100% or less, and may be 0.080% or less, 0.050% or less, 0.010% or less, or 0.005% or less.

(Mg: 0 to 0.100%)
Mg (magnesium) is an element that contributes to inclusion control, particularly fine dispersion of inclusions, and has an effect of increasing toughness. Although the Mg content may be 0%, it may be contained as necessary in order to obtain the above effects. The Mg content may be 0.0001% or more, 0.0005% or more, or 0.001% or more. On the other hand, when Mg is contained excessively, deterioration of the surface properties may become apparent. Therefore, the Mg content is preferably 0.100% or less, and may be 0.090% or less, 0.080% or less, 0.050% or less, or 0.010% or less.

(Zr: 0 to 0.100%)
Zr (zirconium) is an element that contributes to inclusion control, particularly fine dispersion of inclusions, and has the effect of increasing toughness. Although the Zr content may be 0%, it may be contained as necessary in order to obtain the above effects. The Zr content may be 0.001% or more, 0.005% or more, or 0.010% or more. On the other hand, if Zr is contained excessively, deterioration of the surface properties may become apparent. Therefore, the Zr content is preferably 0.100% or less, and may be 0.050% or less, 0.040% or less, or 0.030% or less.

(Hf: 0 to 0.100%)
Hf (hafnium) is an element that contributes to inclusion control, particularly fine dispersion of inclusions, and has an effect of increasing toughness. Although the Hf content may be 0%, it may be contained as necessary in order to obtain the above effects. The Hf content may be 0.0001% or more, 0.0005% or more, or 0.001% or more. On the other hand, when Hf is excessively contained, deterioration of the surface properties may become apparent. Therefore, the Hf content is preferably 0.100% or less, and may be 0.050% or less, 0.030% or less, or 0.010% or less.

(REM: 0-0.100%)
REM (rare earth element) is an element that contributes to inclusion control, particularly fine dispersion of inclusions, and has an effect of increasing toughness. Although the REM content may be 0%, it may be contained as necessary in order to obtain the above effects. The REM content may be 0.0001% or greater, 0.0005% or greater, or 0.001% or greater. On the other hand, if the REM content is excessive, deterioration of the surface properties may become apparent. Therefore, the REM content is preferably 0.100% or less, and may 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 misch metal.

In the steel sheet according to the present invention, the balance other than the above composition consists of Fe and impurities. The term "impurities" as used herein refers to components and the like that are mixed due to various factors in the manufacturing process, including raw materials such as ores and scraps, when steel sheets are manufactured industrially.

In the present invention, the chemical composition of the steel sheet may be analyzed using an elemental analysis method known to those skilled in the art, such as inductively coupled plasma mass spectrometry (ICP-MS method). However, C and S should be measured using the combustion-infrared absorption method, and N should be measured using the inert gas fusion-thermal conductivity method. These analyzes may be performed on samples obtained from steel sheets by a method conforming to JIS G0417:1999.

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

As exemplified in FIG. 2 , the steel sheet 11 according to the present invention has an internal oxide layer containing granular type oxides 12 , grain boundary type oxides 13 and dendrite type oxides 14 on the surface layer of the steel sheet 11 . As will be described later, the granular type oxide 12 and the dendrite type oxide 14 exist within the grains of the metal structure of the steel sheet 11, and the grain boundary type oxide 13 exists along the grain boundaries of the metal structure. The presence of these oxides 12, 13, 14 inside the base steel 15 (that is, as internal oxides) results in the presence of an external oxide layer 2 on the surface of the base steel 3 shown in FIG. The steel plate 11 can have high plateability compared to the case where it is applied. In relation to the formation of internal oxides, this is because there is no external oxide layer that inhibits interdiffusion between plating components and steel components when coating (for example, Zn-based coating) is formed on the surface of the steel sheet. This is considered to be the result of sufficient interdiffusion between the coating components and the steel components, since they exist only in a sufficiently thin thickness. Therefore, the steel sheet and the plated steel sheet according to the present invention containing the oxides 12, 13, 14 in the surface layer of the steel sheet, that is, in the interior of the steel sheet, have high plateability.

[Particulate oxide]
In the present invention, the term "particulate type oxide" refers to an oxide that is dispersed in the form of particles within grains or on grain boundaries in the steel surface layer. In addition, "granular" refers to being separated from each other in the steel matrix, for example, an aspect ratio of 1.0 to 5.0 (maximum line segment length across the granular type oxide ( long axis)/maximum line segment length (minor axis) crossing the oxide perpendicular to the long axis). The term “granularly dispersed” means that the positions of the particles of the particulate oxide are not arranged according to a specific rule (for example, linearly or substantially linearly) but are randomly arranged. In fact, the granular oxide typically exists three-dimensionally in a spherical or nearly spherical shape on the surface layer of the steel sheet. It is generally observed to be circular or approximately circular. In FIG. 2, as an example, a granular type oxide 12 that looks like a circle is shown. Further, typically, as illustrated in FIG. 2, the granular type oxide 12 is often present closer to the surface than the dendrite type oxide 14 is. However, the granular type oxide 12 and the dendrite type oxide 14 may coexist within the crystal grains.

(Average particle size)
In the present invention, the average grain size of the particulate type oxide is 350 nm or less. By controlling the average particle diameter within such a range, the particulate oxide can be finely dispersed in the surface layer of the steel sheet, and the particulate oxide is dispersed during electrodeposition coating and/or during annealing in the manufacturing process. It functions well as a hydrogen trap site that suppresses hydrogen penetration. On the other hand, if the average grain size exceeds 350 nm, the particulate oxide may not sufficiently function as a hydrogen trap site, and good resistance to hydrogen embrittlement may not be obtained. The average particle size of the particulate oxide is preferably 330 nm or less, more preferably 320 nm or less, and even more preferably 310 nm or less. Since the finer the granular oxide, the better, the lower limit of the average particle diameter of the granular oxide is not particularly limited, but may be, for example, 5 nm or more, 10 nm or more, 50 nm or more, or 100 nm or more.

(number density)
In the present invention, the number density in the inner oxide layer of the particulate type oxide is 4.0 pieces/μm ² or more. By controlling the number density within such a range, a large amount of particulate oxide can be dispersed on the surface layer of the steel sheet, and the particulate oxide can disperse hydrogen during electrodeposition coating and/or annealing treatment in the manufacturing process. It functions well as a hydrogen trap site that suppresses penetration. On the other hand, when the number density is less than 4.0 sites/μm ² , the number density of hydrogen trap sites is not sufficient, and hydrogen penetration during electrodeposition coating and/or annealing in the manufacturing process cannot be sufficiently suppressed. Therefore, there is a possibility that good hydrogen embrittlement resistance cannot be obtained. The number density of the particulate oxide is preferably 4.5 pieces/μm ² or more, more preferably 5.0 pieces/μm ² or more, and still more preferably 6.0 pieces/μm ² or more. Since a large amount of the particulate oxide is preferable, the upper limit of the number density of the particulate oxide is not particularly limited, but may be, for example, 100.0/μm ² or less.

The average particle size and number density of particulate type oxides are measured by scanning electron microscopy (SEM). Specific measurements are as follows. A cross-section of the surface layer of the steel sheet is observed by SEM to obtain an SEM image containing particulate type oxides. From the SEM image, a total of 10 regions of 1.0 μm (depth direction)×1.0 μm (width direction) that do not contain grain boundary type oxides and dendrite type oxides, which will be described later, are selected as observation regions. As the observation position of each region, the depth direction (direction perpendicular to the surface of the steel plate) is set to 1.0 µm in the region from the steel plate surface to 1.5 µm, and the width direction (direction parallel to the surface of the steel plate) ) is 1.0 μm at an arbitrary position in the SEM image. Then extract an SEM image of each region selected as above, binarize to separate the oxide portion and the steel portion, calculate the total area of the granular type oxide portion from each binarized image, In addition, the number of granular-type oxides in each binarized image is counted. From the total area and the number of the granular oxides in the 10 regions obtained in this way, the average grain size (nm) of the granular oxides is obtained as the equivalent circle diameter. Further, the number density (pieces/μm ² ) of the granular oxide is equal to the average number of the granular oxides counted from each binarized image. If only part of the granular oxide is observed in the observation area, that is, if the entire outline of the granular oxide is not within the observation area, the number is not counted. Also, from the viewpoint of measurement accuracy, the lower limit of the number of particulate oxides to be counted is set to 5.0 nm or more.

[Grain boundary type oxide]
In the present invention, the term "grain boundary type oxide" refers to oxides present along grain boundaries of steel, and does not include oxides present within grains of steel. In fact, since the grain boundary oxide exists in a plane along the grain boundary in the surface layer of the steel sheet, when observing the cross section of the surface layer of the steel sheet, the grain boundary oxide is linear Observed. In FIGS. 2 and 3, as an example, a grain boundary type oxide 13 that looks linear is shown. In FIG. 2, as a typical example of the steel plate 11, the grain boundary type oxide 13 is shown below the grain type oxide 12, but the grain boundary type oxide 13 is near the surface of the base steel 15. is also formed in By forming the grain boundary type oxide 13 near the surface of the steel sheet, the grain boundary type oxide connects the steel sheet surface and the inside of the steel sheet, and the hydrogen discharging function is sufficiently exhibited.

(ratio A)
In the present invention, as shown in FIG. 3, the “ratio A” means the “surface of the steel sheet” with respect to the “length of the surface of the steel sheet: L ₀ ” in the observed image when the cross section of the surface layer of the steel sheet 11 is observed. The length of the grain boundary type oxide projected on : L(L ₁ +L ₂ +L ₃ +L ₄ )” ratio. In the present invention, the ratio A is 50% or more and 100% or less. By controlling the ratio A to such a range, the grain boundary type oxide can be formed in a wide range inside the steel sheet, and the grain boundary type oxide functions well as an escape route for hydrogen. On the other hand, if the ratio A is less than 50%, it may not function sufficiently as an escape route for hydrogen, and good resistance to hydrogen embrittlement may not be obtained. The ratio A is preferably 60% or more, more preferably 70% or more, even more preferably 80% or more, and even more preferably 90% or more. The ratio A may be 100%.

The ratio A is determined by cross-sectionally observing the surface layer of the steel plate 11, as shown in FIG. A specific measuring method is as follows. A cross section of the surface layer of the steel plate 11 is observed by SEM. The observation position is selected at random. Measure the surface length L ₀ (ie, the width of the SEM image) from the observed SEM image. The length L ₀ is set to 100 μm or more (for example, 100 μm, 150 μm or 200 μm), and the depth to be measured is a region from the surface of the steel sheet to 50 μm. Next, the position of the grain boundary type oxide 13 is identified from the SEM image, and the identified grain boundary type oxide 13 is projected onto the surface of the steel sheet 11 (on the interface between the steel sheet 11 and the plating layer in the case of a plated steel sheet). , the length L (=L ₁ +L ₂ +L ₃ +L ₄ ) of the grain boundary type oxide 13 within the field of view is obtained. Based on L ₀ and L obtained in this manner, the ratio A (%) in the present invention=100×L/L ₀ is obtained. It should be noted that FIG. 3 omits the granular-type oxide 12 and the dendrite-type oxide 14 for the sake of explanation.

[Dendrite-type oxide]
In the present invention, the term "dendritic oxide" means an oxide that exists in the form of dendrites in crystal grains inside steel. The term "inside the crystal grain" as used herein refers to a crystal orientation difference of less than 10° in electron beam backscatter diffraction (EBSD) measurement. In addition, the term "dendritic" refers to a dendritic shape in which branch portions (secondary arms) branch from a main branch (primary arm) into a plurality of needle-like or leaf-like branches and grow three-dimensionally. , that the secondary arm with a length of 50-300 nm grows from the primary arm with a length of 0.5-5.0 μm. The primary arm is preferably 1.0-5.0 μm long, more preferably 2.0-5.0 μm long. The secondary arm is preferably 70-250 nm long, more preferably 100-250 nm long. The lengths of the primary arm and secondary arm can be measured by observing the cross section of the steel plate with a scanning electron microscope (SEM). In fact, the dendritic oxide typically exists three-dimensionally in a dendritic form within the grains of the steel sheet. Specifically, a shape in which a plurality of secondary arms are branched on both sides from a single primary arm, or only the secondary arms are dotted with approximately equal intervals (for example, the difference in intervals between adjacent points is 10% or less) ) is observed as a substantially linear shape. In FIG. 2, as an example, the dendritic oxide 14 observed as a shape in which a plurality of secondary arms are branched on both sides from a single primary arm, and only the secondary arms are present in a dotted shape and substantially linearly. A dendrite-type oxide 14 observed as a shape is shown. In certain embodiments, dendritic oxides may be absent from the surface of the steel sheet up to 1 μm, 2 μm, 3 μm, or 5 μm.

(area ratio)
In the present invention, the area ratio of the dendrite-type oxide is 5.0% or more. By controlling the area ratio of the dendrite-type oxide to such a range, a sufficient amount of the dendrite-type oxide can be present in the crystal grains inside the steel sheet, and the dendrite-type oxide is reduced during electrodeposition coating. It functions well as a hydrogen trap site that suppresses hydrogen penetration. On the other hand, if the area ratio of the dendritic oxide is less than 5.0%, the amount is not sufficient to function as a hydrogen trap site, and hydrogen entry during electrodeposition coating cannot be sufficiently suppressed, resulting in a favorable There is a possibility that good resistance to hydrogen penetration and, in turn, resistance to hydrogen embrittlement cannot be obtained. The area ratio of the dendrite-type oxide is preferably 10.0% or more or 20.0% or more, more preferably 30.0% or more, and still more preferably 50.0% or more. Since a large amount of the dendrite-type oxide is preferable, the area ratio of the dendrite-type oxide is not particularly limited, but may be, for example, 70.0% or less or 60.0% or less.

The area ratio of dendrite-type oxides is measured with a scanning electron microscope (SEM). Specific measurements are as follows. A cross-section of the surface layer of the steel sheet is observed with an SEM, and an SEM image containing dendrite-type oxides such as that shown in FIG. 2 is obtained. A total of 10 regions of 1.0 μm (depth direction)×1.0 μm (width direction) containing dendritic oxides are selected from the SEM image as observation regions. As an observation region, a region in which the above-mentioned granular type oxide and grain boundary type oxide do not exist is selected. In addition, regarding the discrimination between the granular type oxide and the dendritic type oxide (especially, the dendritic type oxide observed as a shape in which only the secondary arms are present in a dotted shape and substantially linearly), 5 points or more in the SEM image If the points of the oxide are arranged in a substantially straight line, it can be regarded as a dendrite-type oxide. As for the observation position of each region, the depth direction (the direction perpendicular to the surface of the steel plate) is 1.0 μm in the depth region from the steel plate surface to 5.0 μm to 10.0 μm, and the width direction ( The direction parallel to the surface of the steel sheet) is 1.0 μm at an arbitrary position in the SEM image. An SEM image of each selected region as described above is then extracted and binarized to separate the oxide and steel portions, and the total area of the dendritic oxide portion is calculated from each binarized image. By dividing the total area of the dendritic oxide in the 10 regions thus obtained by the total area (10 μm ² ) of the 10 regions, the “area ratio of the dendritic oxide” in the present invention is obtained.

(depth of internal oxide layer)
In the steel sheet according to the present invention, the internal oxide layer is a layer formed inside the steel sheet and includes granular type oxides 12, grain boundary type oxides 13 and dendrite type oxides . Therefore, the "internal oxide layer" is the region from the surface of the steel sheet to the furthest position where any of the oxides 12, 13, 14 are present. Therefore, as indicated by "R" in FIG. It is the distance from the surface of the steel sheet 11 to the farthest position where the granular oxide 12, the grain boundary oxide 13 and the dendrite oxide 14 exist when the surface of the steel sheet 11 extends in the direction perpendicular to the surface of the steel sheet. FIG. 2 shows, as an example, the case where the dendrite-type oxide 14 exists at the deepest position. As described above, the granular type oxide 12 and the dendrite type oxide 14 function as trap sites for hydrogen that penetrates during electrodeposition coating, and the grain boundary type oxide 13 functions as an escape route for hydrogen that has penetrated into the steel sheet. can do. Therefore, the greater the 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 to the outside of the system. In the steel sheet according to the present invention, the depth R of the internal oxide layer is 8 µm or more, preferably 10 µm or more, more preferably 12 µm or more, and most preferably 15 µm or more. Although the upper limit of the depth R is not particularly limited, it is substantially 100 μm or less. The depth R can be obtained from the same SEM image (surface length L ₀ ) used to measure the ratio A described above.

(Si—Mn depleted region)
In the steel sheet according to the present invention, the Si and Mn contents of the oxide-free region at the half depth of the internal oxide layer are less than 10% of the Si and Mn contents at the center of the thickness of the steel sheet, respectively. is. The Si—Mn depleted region (the portion containing no oxide in the internal oxide layer) generated on the surface layer of the steel sheet due to the formation of granular type oxides, grain boundary type oxides, and dendrite type oxides is referred to as the above. By setting the depth of the internal oxide layer to 8 μm or more, the Si and Mn depletion rates of the Si—Mn depleted region are each controlled to be less than 10% while making the thickness sufficient to promote the diffusion of hydrogen. can sufficiently reduce the amounts of solid solution Si and Mn that inhibit the diffusion of hydrogen. As a result, it becomes possible to promote the diffusion of hydrogen and remarkably improve the ability to remove hydrogen from the steel.

Further, by further reducing the Si and Mn depletion rate of the Si—Mn depleted region, the amounts of solid solution Si and Mn in the steel can be further reduced. Therefore, the Si depletion rate of the Si—Mn depleted region is preferably 8% or less, more preferably 6% or less, and most preferably 4% or less. The lower limit of the Si depletion rate is not particularly limited, but may be 0%. Similarly, the Mn depletion rate of the Si—Mn depleted region 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 may be 0%. In the present invention, the expression "free of oxides" means not only the above-mentioned granular-type oxides, grain-boundary-type oxides and dendrite-type oxides, but also any other oxides. and such an oxide-free region can be identified by cross-sectional observation by SEM and energy dispersive X-ray spectroscopy (EDS). In addition, the Si—Mn depleted region according to the present invention cannot be controlled within a desired composition range by simply forming an internal oxide such as a granular oxide. It is important to appropriately control the progress of internal oxidation in the process.

In the present invention, the Si and Mn contents of the oxide-free region at the 1/2 position of the depth of the internal oxide layer are the depth of the internal oxide layer determined from the SEM image for measuring the ratio A described above. 10 randomly selected oxide-free points at the 1/2 position were analyzed using a transmission electron microscope with an energy dispersive X-ray spectrometer (TEM-EDS), and the obtained Si and Mn Determined by arithmetic averaging the concentration measurements. In addition, the Si and Mn contents in the center of the thickness of the steel plate are obtained by observing the cross section of the center of the thickness with an SEM, and from the SEM image, 10 randomly selected points in the center of the thickness are energy It is determined by arithmetically averaging the Si and Mn concentration measurements obtained using a transmission electron microscope with dispersive X-ray spectroscopy (TEM-EDS). Finally, the Si and Mn depletion rate is the percentage obtained by dividing the Si and Mn contents at the 1/2 position of the depth of the internal oxide layer by the Si and Mn contents at the thickness center of the steel sheet. is determined as

[Component composition of oxide]
In the present invention, the granular type oxide, the grain boundary type oxide and the dendrite type oxide (hereinafter also simply referred to as oxide) are, in addition to oxygen, one or more of the elements contained in the steel sheet described above. and typically has a component composition that includes Si, O and Fe, and optionally Mn. More specifically, the oxide typically contains Si: 5-25%, Mn: 0-10%, O: 40-65%, and Fe: 10-30%. The oxide may contain an element (for example, Cr) that may be contained in the steel sheet described above, in addition to these elements.

<Plated steel sheet>
The plated steel sheet according to the present invention has a plating layer containing Zn on the steel sheet according to the present invention described above. This plating layer may be formed on one side of the steel sheet, or may be formed on both sides. The plating layer containing Zn includes, for example, a hot-dip galvanized layer, an alloyed hot-dip galvanized layer, an electro-galvanized layer, an electro-alloyed galvanized layer, and the like. More specifically, plating types include, 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, Zn-11% Al-3% Mg-0.2% Si, Zn-11% Ni, or Zn- 15% Mg or the like can be used.

[Component composition of plating layer]
The component composition contained in the plating layer containing Zn in the present invention will be described. "%" regarding the content of an element means "% by mass" unless otherwise specified. In the numerical range of the component composition of the plating layer, unless otherwise specified, the numerical range represented using "~" means the range including the numerical values before and after "~" as the lower and upper limits. do.

(Al: 0-60.0%)
Al is an element that improves the corrosion resistance of the plating layer by being contained together with Zn or being alloyed with it, so it may be contained as necessary. Therefore, the Al content may be 0%. In order to form a plating layer containing Zn and Al, the Al content is preferably 0.01% or more, for example, 0.1% or more, 0.5% or more, 1.0% or more, or It may be 3.0% or more. On the other hand, even if Al is contained excessively, the effect of improving the corrosion resistance is saturated, so the Al content is preferably 60.0% or less, for example, 55.0% or less, 50.0% or less, It may be 40.0% or less, 30.0% or less, 20.0% or less, 10.0% or less, or 5.0% or less.

(Mg: 0-15.0%)
Mg is an element that improves the corrosion resistance of the plating layer by being contained together with Zn and Al or being alloyed with it, so it may be contained as necessary. Therefore, the Mg content may be 0%. In order to form a plating layer containing Zn, Al, and Mg, the Mg content is preferably 0.01% or more, for example, 0.1% or more, 0.5% or more, 1.0% or more. , or 3.0% or more. On the other hand, when Mg is contained excessively, Mg cannot be completely dissolved in the plating bath and floats as an oxide, and when zinc is plated in this plating bath, the oxide adheres to the plating surface layer, causing poor appearance or non-plating. part may occur. Therefore, the Mg content is preferably 15.0% or less, and may be, for example, 10.0% or less, or 5.0% or less.

(Fe: 0 to 15.0%)
Fe can be contained in the coating layer by diffusing from the steel sheet when the coating layer containing Zn is formed on the steel sheet and then heat-treated. Therefore, the Fe content may be 0% since Fe is not contained in the plated layer when the heat treatment is not performed. Also, the Fe content may 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 may be, for example, 12.0% or less, 10.0% or less, 8.0% or less, or 6.0% or less.

(Si: 0 to 3.0%)
Si is an element that further improves corrosion resistance when contained in a Zn-containing plating layer, particularly a Zn--Al--Mg plating layer, and thus may be contained as necessary. Therefore, the Si content may be 0%. From the viewpoint 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. Also, 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.

The basic component composition of the plating layer is as described above. Furthermore, the plating layer is optionally 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%, one or more may contain. Although not particularly limited, the total content of these optional additive elements is preferably 5.00% or less, and 2.00%, from the viewpoint of sufficiently exhibiting the actions and functions of the basic components that constitute the plating layer. More preferably:

In the plated layer, the balance other than the above components consists of Zn and impurities. Impurities in the plating layer are components and the like that are mixed due to various factors in the manufacturing process, including raw materials, when manufacturing the plating layer. The plating layer may contain, as impurities, a trace amount of elements other than the above-described basic components and optional additive components within a range that does not interfere with the effects of the present invention.

The chemical composition of the plating layer can be determined by dissolving the plating layer in an acid solution containing an inhibitor that suppresses corrosion of the steel sheet, and measuring the resulting solution by ICP (inductively coupled plasma) emission spectroscopy. can.

The thickness of the plating layer may be, for example, 3-50 μm. Also, the amount of the plated layer deposited is not particularly limited, but may be, for example, 10 to 170 g/m ² per side. In the present invention, the adhesion amount of the plating layer is determined by dissolving the plating layer in an acid solution to which an inhibitor for suppressing corrosion of the base iron is added, and from the change in weight before and after pickling.

[Tensile strength]
The steel sheet and plated steel sheet according to the present invention preferably have high strength, and specifically preferably have a tensile strength of 440 MPa or more. For example, the tensile strength may be 500 MPa or greater, 600 MPa or greater, 700 MPa or greater, or 800 MPa or greater. Although the upper limit of the tensile strength is not particularly limited, it may be, for example, 2000 MPa or less from the viewpoint of ensuring toughness. The tensile strength may be measured according to JIS Z 2241 (2011) by taking a JIS No. 5 tensile test piece whose longitudinal direction is perpendicular to the rolling direction.

The steel sheet and plated steel sheet according to the present invention have high strength, high platability, and high resistance to hydrogen embrittlement. It is preferably used in the field. Steel sheets used for automobiles are usually subjected to plating treatment (typically Zn-based plating treatment). The effect is exhibited suitably. Further, steel sheets and plated steel sheets used for automobiles are often subjected to electrodeposition coating, and in such cases, hydrogen embrittlement cracking due to penetration of hydrogen during electrodeposition coating can become a significant problem. Therefore, when the steel sheet and plated steel sheet according to the present invention are used as steel sheets for automobiles, the effect of the present invention of having high resistance to hydrogen embrittlement is suitably exhibited.

<Manufacturing method of steel plate>
A preferred method for manufacturing a steel sheet according to the present invention will be described below. The following description is intended to exemplify the characteristic method for manufacturing the steel sheet according to the present invention, and the steel sheet is limited to those manufactured by the manufacturing method described below. not intended.

The steel sheet according to the present invention includes, for example, a casting process in which molten steel having an adjusted chemical composition is cast to form a steel slab, a hot rolling process in which the steel slab is hot rolled to obtain a hot-rolled steel sheet, and a hot-rolled steel sheet is coiled. A coiling process, a cold rolling process of cold-rolling the coiled hot-rolled steel sheet to obtain a cold-rolled steel sheet, a grinding process of introducing dislocations into the surface of the cold-rolled steel sheet, and an annealing process of annealing the ground cold-rolled steel sheet. You can get it by doing. Alternatively, the cold rolling process may be performed as it is after pickling without winding after the hot rolling process.

[Casting process]
Conditions for the casting process are not particularly limited. For example, following smelting by a blast furnace or an electric furnace, various secondary smelting may be performed, and then casting may be performed by a method such as ordinary continuous casting or casting by an ingot method.

[Hot rolling process]
A hot-rolled steel sheet can be obtained by hot-rolling the steel slab cast as described above. The hot-rolling process is performed by hot-rolling a cast steel slab directly or by reheating it after cooling it once. When reheating is performed, the heating temperature of the steel slab may be, for example, 1100.degree. C. to 1250.degree. Rough rolling and finish rolling are usually performed in the hot rolling process. The temperature and rolling reduction for each rolling may be appropriately changed according to the desired metal structure and plate thickness. For example, the finishing temperature of finish rolling may be 900 to 1050° C., and the rolling reduction of finish rolling may be 10 to 50%.

[Winding process]
A hot-rolled steel sheet can be coiled at a predetermined temperature. The coiling temperature may be appropriately changed according to the desired metal structure and the like, and may be, for example, 500 to 800°C. The hot-rolled steel sheet may be subjected to a predetermined heat treatment by unwinding before or after winding. Alternatively, the coiling process may not be performed, and after the hot rolling process, pickling may be performed and the cold rolling process described below may be performed.

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

[Grinding process]
In order to obtain a desired internal oxide layer in the surface layer of the finally obtained steel sheet and to form a Si—Mn deficient region having a desired composition in the internal oxide layer, a predetermined It is effective to perform the grinding process of This grinding process can introduce a large amount of dislocations into the surface of the cold-rolled steel sheet. Since the diffusion of oxygen and the like is faster in grain boundaries than in grains, introducing a large amount of dislocations on the surface of the cold-rolled steel sheet enables the formation of many paths as in the case of grain boundaries. For this reason, oxygen tends to diffuse (penetrate) into the steel along these dislocations during annealing, and the diffusion rate of Si and Mn also increases. can promote the formation of granular-type oxides, grain-boundary-type oxides and dendrite-type oxides. In addition, as the formation of such an internal oxide is promoted, the concentration of Si and Mn in the surrounding area is also reduced, thereby promoting the formation of a Si--Mn depleted region having a desired composition. The grinding step is not particularly limited, but can be carried out, for example, by grinding the surface of the cold-rolled steel sheet with a heavy grinding brush at a grinding amount of 10 to 200 g/m ² . The amount of grinding by the heavy grinding brush can be adjusted by any appropriate method known to those skilled in the art, and is not particularly limited. can be adjusted by appropriately selecting By performing such a grinding step, in the annealing step described later, an internal oxide layer containing a desired oxide is formed, and the Si and Mn depletion rates in the internal oxide layer are each less than 10%. It is possible to make sure.

[Annealing process]
Annealing is performed on the cold-rolled steel sheet that has been subjected to the grinding process. 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 the region where the annealing temperature is 500 ° C. or higher, it is preferable to perform annealing with a higher tension than in other regions. Specifically, in the region where the annealing temperature is 500 ° C. or higher, the cold rolled steel sheet is Annealing is preferably performed with a tension of 3 to 150 MPa, particularly 15 to 150 MPa, applied in the rolling direction. When tension is applied during annealing, a large amount of dislocations can be effectively introduced into the cold-rolled steel sheet. Therefore, during annealing, oxygen tends to diffuse (penetrate) into the steel along these dislocations, and the diffusion rate of Si and Mn also increases, so oxides tend to form inside the steel sheet. As a result, the number density of granular type oxides is increased, the average grain size is refined, grain boundary type oxides are formed in desired ratios, dendrite type oxides are formed in desired area ratios, and Si- It is advantageous for the formation of Mn-depleted regions.

From the viewpoint of efficiently forming granular type oxides, grain boundary type oxides, and dendrite type oxides, the holding temperature in the annealing step is preferably above 870°C to 900°C, and is 880 to 890°C. is more preferred. If the holding temperature in the annealing step is 870° C. or less, there is a possibility that a sufficient depth of the internal oxide layer cannot be ensured. On the other hand, if the holding temperature in the annealing step exceeds 900°C, an external oxide layer is formed on the surface of the steel sheet, the desired oxide cannot be obtained, and there is a risk that the plating properties and hydrogen embrittlement resistance will be insufficient. . The rate of temperature increase to the holding temperature is not particularly limited, but may be 1 to 10° C./sec. Also, the temperature rise may be performed in two steps, with a first temperature rise rate of 1 to 10° C./sec and a second temperature rise rate of 1 to 10° C./sec different from the first temperature rise rate. good.

The holding time at the sintering holding temperature is preferably more than 150 seconds to 300 seconds, more preferably 200 to 280 seconds. If the holding time is 150 seconds or less, the dendritic oxide may not be sufficiently formed, and the hydrogen embrittlement resistance may become insufficient. On the other hand, when the holding time exceeds 300 seconds, the outer oxide grows excessively, the desired inner oxide cannot be obtained, and the plating properties and hydrogen embrittlement resistance may become insufficient.

The dew point of the atmosphere in the annealing step is preferably -20 to 10°C, more preferably -10 to 5°C, from the viewpoint of favorable formation of the desired oxide. If the dew point is too low, an outer oxide layer may be formed on the surface of the steel sheet, and an inner oxide layer may not be sufficiently formed, resulting in insufficient plateability and hydrogen embrittlement resistance. On the other hand, if the dew point is too high, Fe oxide is formed as an external oxide on the surface of the steel sheet, a desired internal oxide layer cannot be obtained, and the plating properties and hydrogen embrittlement resistance may become insufficient. Also, the atmosphere in the annealing step may be a reducing atmosphere, more specifically a reducing atmosphere containing nitrogen and hydrogen, such as a reducing atmosphere containing 1-10% hydrogen (eg, 4% hydrogen and nitrogen balance).

Furthermore, it is effective to remove the internal oxide layer of the steel sheet when performing the annealing process. An internal oxide layer may be formed on the surface layer of the steel sheet during the above-described rolling process, particularly during the hot rolling process. Since the internal oxide layer formed in such a rolling process may hinder the formation of the desired oxide according to the present invention in the annealing process, the internal oxide layer formed in the rolling process is pickled. It is preferable to remove it before annealing. More specifically, the depth of the internal oxide layer of the cold-rolled steel sheet during the annealing process is 0.5 μm or less, preferably 0.3 μm or less, more preferably 0.2 μm or less, and still more preferably 0.1 μm. You should do the following.

By performing each of the steps described above, an internal oxide layer containing a desired form of a granular type oxide, a grain boundary type oxide, and a dendrite type oxide on the surface layer of the steel sheet, which includes a desired Si—Mn depleted region. A steel sheet having an internal oxide layer can be obtained.

<Manufacturing method of plated steel sheet>
A preferred method for producing a plated steel sheet according to the present invention will be described below. The following description is intended to exemplify the characteristic method for manufacturing the plated steel sheet according to the present invention, and the plated steel sheet is limited to those manufactured by the manufacturing method described below. is not intended to be

The plated steel sheet according to the present invention can be obtained by performing a plating treatment step of forming a plating layer containing Zn on the steel sheet manufactured as described above.

[Plating process]
The plating process may be performed according to a method known to those skilled in the art. The plating treatment step may be performed by, for example, hot dip plating or electroplating. Preferably, the plating step is performed by hot dip plating. The conditions of the plating process may be appropriately set in consideration of the composition, thickness, adhesion amount, etc. of the desired plating layer. An alloying treatment may be performed after the plating treatment. Typically, the conditions for the plating process include Al: 0-60.0%, Mg: 0-15.0%, Fe: 0-15%, and Si: 0-3%, with the balance being Zn. and impurities to form a plating layer. More specifically, the conditions of the plating process are, 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.

EXAMPLES The present invention will be described in more detail below with reference to Examples, but the present invention is not limited to these Examples.

(Preparation of steel plate sample)
Molten steel having an adjusted chemical composition was cast to form a steel slab, and the steel slab was hot-rolled, pickled, and then cold-rolled to obtain a cold-rolled steel sheet. Next, the cold-rolled steel sheet was air-cooled to room temperature, and the cold-rolled steel sheet was pickled to remove the internal oxide layer formed by rolling to the internal oxide layer depth (μm) before annealing shown in Table 1. Next, a sample was taken from each cold-rolled steel sheet by a method conforming to JIS G0417:1999, and the chemical composition of the steel sheet was analyzed by the ICP-MS method or the like. Table 1 shows the chemical compositions of the measured steel sheets. All of the steel plates used had a plate thickness of 1.6 mm.

Next, each cold-rolled steel sheet was coated with an aqueous NaOH solution, and then ground with a heavy grinding brush at a grinding amount of 10 to 200 g/m ² (no grinding for No. 33). After that, annealing treatment (annealing atmosphere: 4% hydrogen and nitrogen balance) was performed at the dew point, holding temperature and holding time shown in Table 1 to prepare each steel plate sample. In all the steel plate samples, the heating rate during annealing was 6.0°C/sec up to 500°C, and 2.0°C/sec from 500°C to the holding temperature. In the above annealing treatment, the cold-rolled steel sheet is annealed with a tension of 1 MPa or more in the rolling direction, and the annealing temperature is higher in the region where the annealing temperature is 500 ° C. or higher in the rolling direction than in other regions, Specifically, annealing was performed with a tension of 3 to 150 MPa applied (no such tension was applied to sample No. 32). Presence or absence of grinding with a heavy grinding brush, and annealing treatment conditions (presence or absence of application of tension of 3 to 150 MPa in the annealing temperature range of 500 ° C. or higher, dew point (° C.), holding temperature (° C.), and holding time (seconds)) are shown in Table 1. For each steel plate sample, a JIS No. 5 tensile test piece having a longitudinal direction perpendicular to the rolling direction was collected, and a tensile test was performed according to JIS Z 2241 (2011). The tensile strength of Nos. 14 and 16 was less than 440 MPa, and the tensile strength of the others was 440 MPa or more.

(Analysis of surface layer of steel plate sample: granular type oxide)
Each steel plate sample prepared as described above was cut into 25 mm × 15 mm, the cut sample was embedded in resin and mirror-polished, and the cross section of each steel plate sample was observed by SEM. A total of 10 areas of 1.0 μm were observed. The observation position is 1.0 μm from 0.2 to 1.2 μm from the steel plate surface in the depth direction (direction perpendicular to the surface of the steel plate), and in the width direction (direction perpendicular to the surface of the steel plate). , 1.0 μm at an arbitrary position in the above SEM image. As each of the above regions, a region was selected that did not contain grain boundary type oxides and dendrite type oxides (five or more points of oxides aligned substantially linearly in the SEM image). Next, the obtained SEM image of each region of each steel plate sample was binarized, the area of the granular oxide portion was calculated from the binarized image, and the number of granular oxides in the SEM image was counted. . From the areas and numbers of the granular oxides in the 10 binarized images obtained in this way, the average particle size and number density of the granular oxides were obtained as circle equivalent diameters. Table 1 shows the average grain size (nm) and number density (pieces/μm ² ) of the particulate oxide for each steel plate sample. In addition, in Table 1, when no particulate type oxide exists in the SEM image (number density = 0), the average particle size is indicated as "-".

(Analysis of surface layer of steel plate sample: dendritic oxide)
Each steel plate sample prepared as described above was cut into 25 mm × 15 mm, the cut sample was embedded in resin and mirror-polished, and the cross section of each steel plate sample was observed by SEM. A total of 10 areas of 1.0 μm were observed. The observation position is 1.0 μm in the range of 5.5 to 6.5 μm from the steel plate surface in the depth direction (direction perpendicular to the surface of the steel plate), and in the width direction (direction perpendicular to the surface of the steel plate). is 1.0 μm at an arbitrary position in the SEM image. For each of the above regions, a region that does not contain the granular type oxide and the grain boundary type oxide was selected. Next, the SEM image of each region of each steel plate sample obtained was binarized, and the area of the dendrite-type oxide portion was calculated from the binarized image. From the areas of the dendritic oxides in the 10 binarized images obtained in this manner, the "area ratio of the dendritic oxides" was obtained for each steel plate sample. Table 1 shows the area ratio (%) of the dendrite-type oxide for each steel plate sample. In the above SEM image, when the lengths of the primary arm and the secondary arm of the dendritic oxide observed as a shape in which a plurality of secondary arms branched on both sides from the primary arm were measured, the sample No. For 2-6 and 18-31, primary arm: 0.5-5.0 μm and secondary arm: 50-300 nm.

(Analysis of surface layer of steel plate sample: Grain boundary type oxide)
In addition, the ratio A for each steel plate sample was measured by observing the cross section of the embedded sample. Specifically, in a SEM image of 150 μm width (=L ₀ ), the position of the grain boundary type oxide is specified in the region from the surface to 50 μm, and the specified grain boundary type oxide is projected onto the surface of the steel sheet. , the length L of the grain boundary oxide within the field of view was obtained. Based on L ₀ and L thus determined, the ratio A (%)=100×L/L ₀ was determined. Also, the depth R of the identified internal oxide layer was measured from the same SEM image. Table 1 shows the ratio A (%) of the granular type oxide and the depth (μm) of the internal oxide layer for each steel plate sample.

In addition, the Si and Mn contents of the oxide-free region at the position of 1/2 the depth of the internal oxide layer were obtained from the SEM image in which the depth R of the internal oxide layer was measured. Ten randomly selected oxide-free points at 1/2 positions were analyzed using TEM-EDS and the Si and Mn concentration measurements obtained were determined by arithmetic averaging. In addition, the Si and Mn contents in the center of the plate thickness of the steel plate are obtained by observing the cross section of the center of the plate thickness with an SEM, and 10 randomly selected points in the center of the plate thickness from the SEM image. - analyzed using EDS and determined by arithmetic averaging of the resulting Si and Mn concentration measurements. Finally, the Si and Mn content at the position of 1/2 of the depth of the internal oxide layer is divided by the Si and Mn content at the center of the plate thickness of the steel plate, respectively, and the value expressed as a percentage is the Si and Mn deficiency rate. decided as In addition, when each steel plate sample was analyzed for the component composition of granular type oxides, grain boundary type oxides and dendrite type oxides, all oxides contained Si, O and Fe, and many oxides further contained Mn. Therefore, the component composition of any oxide contained Si: 5-25%, Mn: 0-10%, O: 40-65%, and Fe: 10-30%.

(Preparation of plated steel sheet sample)
After each steel plate sample was cut into a size of 100 mm×200 mm, a plated steel plate sample was prepared by performing a plating treatment for forming the types of plating shown in Table 1. In Table 1, plating type A is "alloyed hot-dip galvanized steel sheet (GA)", plating type B is "hot-dip Zn-0.2% Al-plated steel sheet (GI)", and plating type C is "hot-dip Zn-(0 .3 to 1.5)% Al-plated steel sheet (the amount of Al is described in the table)”. In the hot dip galvanizing step, the cut sample was immersed in a 440° C. hot dip galvanizing bath for 3 seconds. After immersion, it was pulled out at 100 mm/sec, and the coating weight was controlled to 50 g/m ² with N ₂ wiping gas. For the plating type A, alloying treatment was performed at 460°C after that.

(Component composition analysis of plating layer)
The component composition of the plating layer was obtained by immersing a sample cut to 30 mm x 30 mm in a 10% HCl aqueous solution containing an inhibitor (Ibit, manufactured by Asahi Chemical Industry Co., Ltd.), pickling and peeling the plating layer, and removing the plating components dissolved in the aqueous solution. Determined by measuring by ICP emission spectroscopy.

(Plating evaluation)
For each plated steel sheet sample, the plating property was evaluated by measuring the area ratio of the unplated portion on the surface of the steel sheet. Specifically, an area of 1 mm × 1 mm on the surface of each plated steel sheet sample on which the plating layer was formed was observed with an optical microscope, and from the observed image, the part where the plating layer was formed (plating part) and the plating layer were formed. The area ratio of the non-plated portion (area of the non-plated portion/area of the observed image) is calculated, the plating property is evaluated according to the following criteria, and the results are shown. 1. 〇 is a pass, and × is a fail.
Evaluation ○: 5.0% or less Evaluation ×: More than 5.0%

(Evaluation of hydrogen embrittlement resistance)
Each plated steel sheet sample was chemically treated with a zinc phosphate-based chemical conversion treatment solution (Surfdyne SD5350 series: manufactured by Nippon Paint Industrial Coating Co., Ltd.) so that the coating amount of the chemical conversion film was 2.3 g/m ³ . Using an electrodeposition coating reagent (PN110 Powernics Gray: manufactured by Nippon Paint Industrial Coating Co., Ltd.), electrodeposition coating was applied to a coating thickness of 17 μm. After that, baking was performed at 170° C. for 25 minutes. Furthermore, the amount of diffusible hydrogen was measured for each plated steel sheet sample by the temperature programmed desorption method. Specifically, a plated steel sheet sample was heated to 400°C in a heating furnace equipped with gas chromatography, and the total amount of hydrogen released until the temperature dropped to 250°C was measured. Based on the measured amount of diffusible hydrogen, hydrogen embrittlement resistance was evaluated according to the following criteria, and the results are shown in Table 1. ⊚ and 0 are acceptable, and × is unacceptable.
Evaluation ◎: 0.2 ppm or less Evaluation ◯: More than 0.2 ppm and less than or equal to 0.4 ppm Evaluation ×: More than 0.4 ppm

Sample no. 2 to 6 and 18 to 31, the component composition, the form of the internal oxide layer (requirements for granular type oxide, grain boundary type oxide and dendrite type oxide), and the depth and composition of the internal oxide layer are within a predetermined range , it had high plating properties and hydrogen embrittlement resistance. On the other hand, sample no. In Nos. 1 and 17, the depth of the internal oxide layer before annealing was large, a desired internal oxide layer could not be formed during annealing, and high hydrogen embrittlement resistance could not be obtained. Sample no. In No. 7, the dew point during annealing was low, an outer oxide layer was formed, a desired inner oxide layer was not formed, and high plating properties and hydrogen embrittlement resistance could not be obtained. Sample no. In No. 8, the dew point during annealing was high, an external oxide was formed, a desired internal oxide layer was not formed, and high platability and hydrogen embrittlement resistance could not be obtained. Sample no. In No. 9, the holding temperature during annealing was high, an outer oxide layer was formed, a desired inner oxide layer was not formed, and high plating properties and hydrogen embrittlement resistance could not be obtained. Sample no. In No. 10, the holding temperature during annealing was low, a desired internal oxide layer was not formed, and high hydrogen embrittlement resistance was not obtained. Sample no. In No. 11, the holding time during annealing was short, the desired internal oxide layer was not formed, and high hydrogen embrittlement resistance was not obtained. Sample no. In No. 12, the holding time during annealing was long, an external oxide was formed, a desired internal oxide layer was not formed, and high platability and hydrogen embrittlement resistance could not be obtained. Sample no. In No. 13, the amount of Si was excessive, the outer oxide grew, the desired inner oxide layer was not formed, and high platability and hydrogen embrittlement resistance could not be obtained. Sample no. In Nos. 14 and 16, the amount of Si and the amount of Mn were respectively 0 (zero), the desired internal oxide layer was not formed, and high resistance to hydrogen embrittlement was not obtained. Sample no. In No. 15, the amount of Mn was excessive, the outer oxide grew, the desired inner oxide layer was not formed, and high platability and hydrogen embrittlement resistance could not be obtained. Sample no. In No. 32, since a predetermined tension was not applied during annealing, a desired internal oxide layer was not formed and high hydrogen embrittlement resistance could not be obtained. Sample no. In No. 33, since no grinding was performed before annealing, a desired internal oxide layer was not formed and high resistance to hydrogen embrittlement could not be obtained. Sample no. In No. 34, the holding temperature during annealing was low, the internal oxide layer was not formed to a deep position, and high hydrogen embrittlement resistance was not obtained.

According to the present invention, it is possible to provide a high-strength steel sheet and a plated steel sheet having high plateability and hydrogen embrittlement resistance, and the steel sheet and the plated steel sheet are used for automobiles, home appliances, building materials, etc., especially for automobiles. It can be suitably used for steel sheets for automobiles and plated steel sheets for automobiles, and high collision safety and long life are expected. Therefore, the present invention can be said to be an invention of extremely high industrial value.

REFERENCE SIGNS LIST 1 steel plate 2 external oxide layer 3 base steel 11 steel plate 12 granular type oxide 13 grain boundary type oxide 14 dendrite type oxide 15 base steel

Claims (6)

in % by mass,
C: 0.05 to 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 to 0.010%,
Ti: 0 to 0.150%,
Nb: 0 to 0.150%,
V: 0 to 0.150%,
Cr: 0 to 2.00%,
Ni: 0 to 2.00%,
Cu: 0 to 2.00%,
Mo: 0 to 1.00%,
W: 0 to 1.00%,
Ca: 0-0.100%,
Mg: 0-0.100%,
Zr: 0 to 0.100%,
A steel sheet containing Hf: 0 to 0.100% and REM: 0 to 0.100%, with the balance being Fe and impurities,
The surface layer of the steel sheet has an internal oxide layer containing a granular type oxide, a grain boundary type oxide and a dendrite type oxide,
The average particle size of the particulate type oxide is 350 nm or less,
The number density in the internal oxide layer of the particulate type oxide is 4.0 pieces/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 type oxide projected on 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 dendrite-type oxide has an area ratio of 5.0% or more,
The depth of the internal oxide layer is 8 μm or more,
A steel sheet, wherein the Si and Mn contents in the oxide-free steel at the half depth position are respectively less than 10% of the Si and Mn contents at the thickness center of the steel sheet. The steel sheet according to claim 1, wherein the number density in the internal oxide layer of the particulate type oxide is 5.0 pieces/μm ² or more. The steel sheet according to claim 1 or 2, wherein the depth of said internal oxide layer is 15 µm or more. The steel sheet according to any one of claims 1 to 3, wherein said ratio A is 80% or more. A plated steel sheet having a plating layer containing Zn on the steel sheet according to any one of claims 1 to 4. The plated steel sheet according to claim 5, wherein the plated layer has a chemical composition of Zn-(0.3 to 1.5)% Al.

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