JP2022169341A - 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 a dendrite type oxide on a surface layer thereof. The dendrite type oxide has an area ratio of 5.0% or more, and has a Si-Mn depletion layer with a thickness of 3.0 μm or more from a surface of the steel sheet. At a 1/2 location of the thickness, Si and Mn contents in the Si-Mn depletion layer 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 used for automotive parts and the like are sometimes used in an atmospheric corrosive environment where temperature and humidity fluctuate greatly. It is known that when high-strength steel sheets are exposed to such an atmospheric corrosion environment, hydrogen generated during the corrosion process penetrates into the steel. Hydrogen that has penetrated into the steel segregates at martensite grain boundaries in the steel structure, embrittles the grain boundaries, and can cause cracks in the steel sheet. 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, in steel sheets used in corrosive environments, it is necessary to suppress the penetration of hydrogen into the inside of the steel sheet under the corrosive environment, It is effective to discharge to

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 formed an oxide in the surface layer of the steel sheet, that is, in the interior of the steel sheet, and furthermore controlled the form of the oxide present in the surface layer of the steel sheet. It has been found that it is important to control the Si--Mn depleted layer formed on the surface layer of the steel sheet due to the formation of oxides within a predetermined range of thickness and composition. More specifically, the present inventors ensure high plating properties by forming internal oxides, and sufficiently form dendrite-type oxides present in the crystal grains of the metal structure in the form of oxides. Then, the dendrite-type oxide functions as a trap site for hydrogen that can penetrate into the steel sheet in a corrosive environment to suppress hydrogen penetration into the steel, and furthermore, the surface layer of the steel sheet has a predetermined thickness and composition. It was found that by forming a Si—Mn depleted layer, diffusion of hydrogen in the steel is promoted and the ability to remove hydrogen from the steel is improved, thereby obtaining high resistance to hydrogen embrittlement as a whole.

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 contains a dendrite-type oxide,
The dendrite-type oxide has an area ratio of 5.0% or more,
including a Si—Mn depleted layer having a thickness of 3.0 μm or more from the surface of the steel sheet,
A steel sheet, wherein the Si and Mn contents of the Si—Mn depleted layer containing no oxides at the ½ position of the thickness are respectively less than 10% of the Si and Mn contents at the central portion of the thickness of the steel sheet.
(2)
The steel sheet according to (1), wherein the dendrite-type oxide has an area ratio of 10.0% or more.
(3)
The steel sheet according to (1), wherein the dendrite-type oxide has an area ratio of 30.0% or more.
(4)
The steel sheet according to (1), wherein the dendrite-type oxide has an area ratio of 50.0% 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 surface layer of the steel sheet contains a dendritic oxide with a predetermined area ratio, and further includes a Si—Mn depleted layer having a predetermined thickness and composition, so that the dendritic oxide can be generated in a corrosive environment. functions as a trap site for hydrogen penetrating into the inside of the steel sheet, and the Si—Mn depleted layer promotes the diffusion of hydrogen to improve the ability to remove hydrogen from the steel, and as a result, the inside of the steel sheet In addition, the amount of hydrogen that has penetrated can be released to reduce the amount of hydrogen that accumulates in the steel, and the resistance to hydrogen embrittlement can be greatly improved. In addition, according to the present invention, since the dendrite-type 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 possible. 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.

<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 contains a dendrite-type oxide,
The dendrite-type oxide has an area ratio of 5.0% or more,
including a Si—Mn depleted layer having a thickness of 3.0 μm or more from the surface of the steel sheet,
The Si and Mn contents of the Si—Mn depleted layer not containing oxides at the 1/2 position of the thickness are respectively less than 10% of the Si and Mn contents at the center of the plate thickness of the steel plate. and

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 oxide 12 is present inside the base steel 13 . Therefore, when a coating layer is formed on the surface of the steel sheet 11, the steel sheet 11 according to the present invention in which the oxide 12 is formed inside the base material steel 13 has a higher coating thickness than the steel sheet 1 having the outer oxide layer 2. The interdiffusion of the 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.

Further, high-strength steel sheets used in an atmospheric environment, particularly high-strength steel sheets for automobiles, are repeatedly exposed to various environments with different temperatures and humidity. Such an environment is called an atmospheric corrosion environment, and it is known that hydrogen is generated in the corrosion process under the atmospheric corrosion environment. Then, this hydrogen penetrates deeper than the surface layer region in the steel, segregates at the martensite grain boundary of the steel sheet structure, and embrittles the grain boundary, thereby causing hydrogen embrittlement cracking (delayed fracture) in the steel sheet. 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, in high-strength steel sheets used in an atmospheric corrosion environment, the amount of hydrogen accumulated in the steel, more specifically, the amount of hydrogen accumulated at a position deeper than the surface layer region of the steel sheet It is effective to reduce The present inventors have attempted to control the morphology of oxides present in the surface layer of a steel sheet, more specifically, to convert oxides into dendrite-type oxides having a predetermined area ratio, By controlling the Si-Mn depleted layer generated by the decrease in the surrounding Si and Mn concentrations due to the formation of the dendrite oxide in a corrosive environment It exhibits the function of trapping hydrogen that penetrates inside the steel sheet, and promotes the diffusion of hydrogen that has penetrated into the Si-Mn depleted layer to improve the ability to remove hydrogen from the steel, and as a result, accumulates inside the steel sheet. The inventors have found that the amount of hydrogen used is suppressed, and high resistance to hydrogen embrittlement can be obtained.

More specifically, the present inventors analyzed in detail the relationship between the morphology of oxides and their effectiveness as trap sites for hydrogen. It has been found that it is effective to allow the dendrite-type oxide 12 to exist in the inside at a certain area ratio or more, more specifically at an area ratio of 5.0% or more. 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, it is thought that the existence of an appropriate amount of oxides on the surface layer of the steel sheet increases the surface area of the oxides on the surface layer of the steel sheet, thereby improving the hydrogen trapping function. Therefore, from the viewpoint of obtaining high hydrogen penetration resistance, the present inventors controlled the conditions during steel sheet production, particularly during annealing treatment, to obtain dendritic oxide that functions as a trap site for hydrogen that penetrates in a corrosive environment. We have found that it is important to have things present in the right amount. 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 have investigated the morphology of the Si—Mn depletion layer produced by the reduction in the surrounding Si and Mn concentrations due to the formation of internal oxides such as dendrite-type oxides 12 as shown in FIG. As a result of detailed analysis of the relationship between and hydrogen discharge property, the Si-Mn depleted layer is within a predetermined thickness and composition range, more specifically, the thickness of the Si-Mn depleted layer is the surface of the steel sheet The Si and Mn contents of the Si—Mn depleted layer that is 3.0 μm or more and does not contain oxides at the 1/2 position of the thickness are 10% of the Si and Mn contents at the center of the thickness of the steel sheet, respectively. It was found that it is effective to control the content to be less than (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 dendrite-type oxides 12 are formed on the surface layer of the steel sheet, Si and Mn dissolved in the steel are consumed by the formation of the internal oxides. Therefore, in the surface layer of the steel sheet, along with the formation of internal oxides, a surrounding Si—Mn depleted layer with a relatively low concentration of Si and Mn is formed. Therefore, the Si—Mn depleted layer is made relatively thick, specifically, the thickness of the Si—Mn depleted layer is set to the surface of the steel sheet (if there is a coating layer on the surface of the steel sheet, the coating layer and the steel sheet The Si and Mn content of the Si—Mn depleted layer is sufficiently low while sufficiently securing the diffusion path of hydrogen by controlling the distance from the interface of the ) to 3.0 μm or more, specifically Si and Mn depleted By controlling the ratios to be less than 10%, it is believed that the amounts of solid solution Si and Mn that inhibit hydrogen diffusion can be sufficiently reduced. Therefore, by including a Si—Mn depleted layer whose thickness and composition are controlled within the above ranges, it is believed that it will be possible to promote the diffusion of hydrogen and significantly improve the ability to remove hydrogen from the steel. be done. Therefore, by combining the above-described dendrite-type oxide and the Si—Mn depleted layer, both the resistance to hydrogen penetration and the resistance to hydrogen discharge are improved, thereby significantly improving the resistance to hydrogen embrittlement of the steel sheet as a whole. becomes possible.

In addition, hydrogen embrittlement cracking occurs not only when the high-strength steel sheet described above is used in an atmospheric corrosive environment, but also when hydrogen present in the annealing atmosphere during the annealing process for manufacturing the high-strength steel sheet. It is also known that it can occur by penetrating deeper than the surface layer of the base steel. The present inventors have now found that the above combination of dendrite-type oxides and Si—Mn depleted layers is not only suitable for use in corrosive environments, but also for hydrogen transfer into steel sheets during annealing in the manufacturing process. It has been found that it works effectively for suppressing penetration and discharging the penetrated hydrogen, and as a result, high resistance to hydrogen embrittlement can be achieved both during production and during use of the steel sheet.

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, particularly dendrite-type 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, particularly dendrite-type 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 effects. 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 effect. 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 effects. 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 effect. 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 effect. 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 obvious. 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 effect. 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 effect. 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 illustrated in FIG. 2 , the steel sheet 11 according to the present invention includes dendrite-type oxides 12 in the surface layer of the steel sheet 11 . Since the dendritic oxide 12 exists inside the base steel 13 (that is, as an internal oxide), when the external oxide layer 2 exists on the surface of the base steel 3 shown in FIG. In comparison, the steel plate 11 can have high plateability. 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 dendrite-type oxides in the surface layer of the steel sheet, that is, in the interior of the steel sheet, have high plateability.

[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 means that the difference in crystal orientation is 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 12 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 exist in a dotted shape and substantially straight lines. A dendritic oxide 12 observed as a shape is shown. In certain embodiments, the dendritic oxide in the present invention may be present, for example, only in a region of less than 8 μm, less than 7 μm, or less than 6 μm from the surface of the steel sheet. In another specific embodiment, the dendritic oxide may be present only in a region 1 μm or more and less than 8 μm, 2 μm or more and less than 8 μm, or 3 μm or more and less than 8 μm from the surface of the steel sheet.

(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 can be used in a corrosive environment. 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 penetration under a corrosive environment 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) are selected as observation regions from the SEM image. 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 0.5 μm to 5.0 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 of the 10 regions thus obtained by the total area (10 μm ² ) of the 10 regions, the “dendritic area ratio” in the present invention is obtained. As the observation region, a region containing non-dendritic oxide is not selected.

[Component composition of oxide]
In the present invention, the dendrite-type oxide (hereinafter also simply referred to as oxide) contains one or more of the elements contained in the steel sheet described above in addition to oxygen, and typically includes: It has a component composition containing Si, O and Fe, and optionally further containing 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.

[Si—Mn depleted layer]
The steel sheet according to the present invention includes a Si—Mn depleted layer having a thickness of 3.0 μm or more from the surface of the steel sheet, and does not contain oxides at 1/2 positions of the thickness Si of the Si—Mn depleted layer and Mn contents are each less than 10% of the Si and Mn contents in the thickness center of the steel sheet. The Si—Mn depleted layer formed on the surface layer of the steel sheet due to the formation of dendrite-type oxides has a thickness of 3.0 μm or more, and the Si and Mn depleted layers of the Si—Mn depleted layer are each controlled to less than 10%. By doing so, the amount of solid solution Si and Mn that inhibits the diffusion of hydrogen can be sufficiently reduced, and as a result, the diffusion of hydrogen can be promoted and the ability to discharge hydrogen from steel can be significantly improved. It becomes possible. By increasing the thickness of the Si—Mn depleted layer, the diffusion of hydrogen from the steel can be promoted, so the thickness of the Si—Mn depleted layer is preferably 4.0 μm or more, more preferably 5 0 μm or more, most preferably 7.0 μm or more. Although the upper limit of the thickness of the Si--Mn depleted layer is not particularly limited, the thickness of the Si--Mn depleted layer may be, for example, 50.0 μm or less.

Similarly, by making the Si and Mn depletion rates of the Si—Mn depleted layer smaller, 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 layer 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 layer is preferably 8% or less, more preferably 6% or less, and most preferably 4% or less. The lower limit of the Mn deficiency rate is not particularly limited, but may be 0%. In the present invention, the expression "free of oxides" means not only the dendritic oxides described above, but also free of any other oxides. The region can be identified by cross-sectional observation by SEM and energy dispersive X-ray spectroscopy (EDS). In addition, the Si—Mn depleted layer according to the present invention cannot be controlled within the desired thickness and composition ranges by simply forming an internal oxide such as a dendritic oxide. Furthermore, it is important to appropriately control the progress of internal oxidation in the manufacturing process.

The thickness of the Si—Mn depleted layer is, as indicated by D in FIG. vertical direction) from the surface of the steel sheet 11 to the furthest position where the dendrite-type oxide 12 exists. The thickness of the Si--Mn depleted layer can be obtained from the same SEM image as the above-mentioned SEM image for measuring the area ratio of the dendrite-type oxide. In addition, the Si and Mn contents of the oxide-free region at the 1/2 position of the thickness of the Si—Mn depleted layer are determined from the SEM image, and the 1/2 position of the thickness of the Si—Mn depleted layer Measured values of Si and Mn concentrations obtained by analyzing 10 randomly selected points that do not contain oxides using a transmission electron microscope with an energy dispersive X-ray spectrometer (TEM-EDS) is determined by arithmetically averaging 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 values obtained by dividing the Si and Mn contents at the 1/2 position of the thickness of the Si—Mn depleted layer by the Si and Mn contents at the center of the thickness of the steel sheet, respectively, are expressed as percentages. Si and Mn determined as the deficiency rate.

<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. In addition, steel sheets and plated steel sheets used for automobiles are often used in an atmospheric corrosive environment, and in that case, hydrogen embrittlement cracking due to the intrusion of hydrogen generated in the environment 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 sufficiently form dendrite-type oxides in the surface layer of the finally obtained steel sheet and to form a Si—Mn depleted layer having a desired thickness and composition, a grinding step is performed before annealing the cold-rolled steel sheet. It is effective to 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. It is possible to promote the formation of a dendritic oxide by combining with 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 layer having a desired thickness and 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, the desired dendritic oxide is formed in the annealing step described later, and the desired thickness and composition, that is, the thickness of 3.0 μm or more, and the Si and Mn depletion rate of less than 10% each can be reliably and efficiently formed on the surface layer of the steel sheet.

[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 to the surface of 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, it is advantageous for forming a dendrite-type oxide with a desired area ratio and forming a Si--Mn depleted layer having a desired thickness and composition.

From the viewpoint of generating a sufficient amount of dendrite-type oxide, the holding temperature in the annealing step is preferably 700 to 870°C, more preferably 740 to 840°C. If the holding temperature in the annealing step is less than 700° C., the dendrite-type oxide may not be sufficiently formed, and the resistance to hydrogen penetration may become 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 above 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 dendrite-type oxide may not be generated sufficiently. On the other hand, if the holding time is longer than 300 seconds, the external oxide may grow excessively, the dendritic oxide may not be sufficiently formed, and the plating properties and resistance to hydrogen penetration 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 generating a sufficient amount of dendrite-type oxide. If the dew point is too low, an external oxide layer may be formed on the surface of the steel sheet, and internal oxide may not be sufficiently formed, resulting in insufficient plating properties and resistance to hydrogen penetration. 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, and there is a risk that the dendritic oxide may not be formed sufficiently, resulting in poor plating properties, hydrogen penetration resistance, and hydrogen embrittlement resistance. may be sufficient. 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 a dendrite-type oxide in the annealing process, the internal oxide layer is removed by pickling or the like before annealing. is preferred. 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 carrying out the steps described above, it is possible to obtain a steel sheet containing a sufficient amount of dendritic oxide in the surface layer of the steel sheet and containing a Si—Mn depleted layer having a desired thickness and composition.

<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, after applying an aqueous NaOH solution to each cold-rolled steel sheet, the surface of the cold-rolled steel sheet was ground with a heavy grinding brush at a grinding amount of 10 to 200 g/m ² (no grinding for sample No. 35). 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. 34). 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). 16 and 18 had a tensile strength of less than 440 MPa, and the others had a tensile strength of 440 MPa or more.

(Analysis of surface layer of steel plate sample)
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 2.0 to 3.0 μm from the steel plate surface for the depth direction (direction perpendicular to the surface of the steel plate), and for the width direction (direction perpendicular to the surface of the steel plate). is 1.0 μm at an arbitrary position in the SEM image. 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" for each steel plate sample was obtained. 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 2-8 and 20-33, primary arm: 0.5-5.0 μm and secondary arm: 50-300 nm.

The thickness of the Si—Mn depleted layer is the surface of the steel sheet when proceeding in the thickness direction of the steel sheet (direction perpendicular to the surface of the steel sheet) from the surface of the steel sheet in the SEM image in which the area ratio of the dendritic oxide is measured. to the furthest position where the dendritic oxide is present. In addition, the Si and Mn contents of the oxide-free region at the 1/2 position of the thickness of the Si—Mn depleted layer are determined from the SEM image, and the 1/2 position of the thickness of the Si—Mn depleted layer Ten randomly selected oxide-free points 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 contents obtained by dividing the Si and Mn contents at the 1/2 position of the thickness of the Si—Mn depleted layer by the Si and Mn contents at the center of the thickness of the steel sheet, respectively, are expressed as percentages. determined as the deficiency rate. In addition, when the chemical composition of the dendrite-type oxide was analyzed for each steel plate sample, all dendrite oxides contained Si, O and Fe, and many oxides further contained Mn. The component composition also 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 was immersed in a 5% NaCl aqueous solution at 50° C. for 480 hours to simulate an SDT (salt water immersion test) after making two vertical parallel cuts reaching the substrate. Thereafter, 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, resistance to hydrogen embrittlement (amount of accumulated hydrogen in the sample) was evaluated according to the following criteria. ⊚ 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. Nos. 2 to 8 and 20 to 33 had high platability and hydrogen embrittlement resistance because the component composition, the area ratio of the dendrite-type oxide, and the thickness and composition of the Si—Mn depleted layer were appropriate. rice field. On the other hand, sample no. In Nos. 1 and 19, the depth of the internal oxide layer before annealing was large, and the dendritic oxide could not be sufficiently formed, and the desired Si—Mn depleted layer was not formed, so high hydrogen embrittlement resistance could not be obtained. I didn't. Sample no. No. 9 has a low dew point during annealing, forms an external oxide layer, does not form a dendritic oxide, and does not form the desired Si—Mn depleted layer, so that high plating properties and resistance to hydrogen embrittlement are obtained. I couldn't. Sample no. In No. 10, the dew point during annealing was high, an external oxide was formed, a dendritic oxide was not sufficiently formed, and the desired Si—Mn depleted layer was not formed, so high plating properties and hydrogen embrittlement resistance were obtained. did not get Sample no. In No. 11, the holding temperature during annealing was high, and an external oxide was formed, a dendritic oxide was not sufficiently formed, and the desired Si—Mn depleted layer was not formed, resulting in high plating properties and resistance to hydrogen embrittlement. I didn't get the sex. Sample no. In No. 12, the holding temperature during annealing was low, and the dendritic oxide was not sufficiently formed, and the desired Si--Mn depleted layer was not formed, so high hydrogen embrittlement resistance could not be obtained. Sample no. In No. 13, the holding time during annealing was short, the dendritic oxide was not sufficiently formed, and the desired Si—Mn depleted layer was not formed, so high hydrogen embrittlement resistance could not be obtained. Sample no. In No. 14, the holding time during annealing was long, an external oxide was generated, a sufficient dendritic oxide was not formed, and the desired Si—Mn depleted layer was not formed, so that the plateability and hydrogen embrittlement resistance were high. chemistries could not be obtained. Sample no. In No. 15, the amount of Si is excessive, the outer oxide grows, the dendritic oxide is not sufficiently formed, and the desired Si-Mn depleted layer is not formed, so high plating properties and hydrogen embrittlement resistance did not get Sample no. In Nos. 16 and 18, the amount of Si and the amount of Mn were respectively 0 (zero), no dendritic oxide was formed, and the desired Si—Mn depleted layer was not formed, so high hydrogen embrittlement resistance was not obtained. I didn't. Sample no. In No. 17, the amount of Mn is excessive, the outer oxide grows, the dendritic oxide is not sufficiently formed, and the desired Si-Mn depleted layer is not formed, so high plating properties and hydrogen embrittlement resistance did not get Sample no. In No. 34, since a predetermined tension was not applied during annealing, the dendritic oxide was not sufficiently formed, and the desired Si--Mn depleted layer was not formed. As a result, high hydrogen embrittlement resistance could not be obtained. Sample no. In No. 35, since no grinding was performed before annealing, a sufficient dendritic oxide was not formed, and the desired Si--Mn depleted layer was not formed. As a result, high hydrogen embrittlement resistance could not be 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 dendritic oxide 13 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 contains a dendrite-type oxide,
The dendrite-type oxide has an area ratio of 5.0% or more,
including a Si—Mn depleted layer having a thickness of 3.0 μm or more from the surface of the steel sheet,
A steel sheet, wherein the Si and Mn contents of the Si—Mn depleted layer containing no oxides at the ½ position of the thickness are respectively less than 10% of the Si and Mn contents at the central portion of the thickness of the steel sheet. The steel sheet according to claim 1, wherein the dendrite-type oxide has an area ratio of 10.0% or more. The steel sheet according to claim 1, wherein the dendrite-type oxide has an area ratio of 30.0% or more. The steel sheet according to claim 1, wherein the dendrite-type oxide has an area ratio of 50.0% 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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