WO2024053667A1

WO2024053667A1 - Steel sheet and plated steel sheet

Info

Publication number: WO2024053667A1
Application number: PCT/JP2023/032500
Authority: WO
Inventors: 卓哉光延; 卓史横山; 浩史竹林
Original assignee: 日本製鉄株式会社
Priority date: 2022-09-06
Filing date: 2023-09-06
Publication date: 2024-03-14

Abstract

The present invention addresses the problem of providing a steel sheet and a plated steel sheet that have high LME resistance and hydrogen desorption. This steel sheet and plated steel sheet are characterized: by having prescribed chemical components; in that the depth in the depth direction from the surface of the steel sheets at which the C concentration as measured by GDS is no more than 0.05% is at least 10 μm; in that the thickness in the depth direction from the surface of the steel sheets of a layer that has a ferrite phase area fraction of at least 90% is at least 20 μm; and in that the Ra surface roughness of the steel sheets is no more than 3.0 μm.

Description

Steel plate and plated steel plate

The present invention relates to a steel plate and a plated steel plate. More specifically, the present invention relates to a steel plate and a plated steel plate having high LME resistance and hydrogen desorption properties.

In recent years, efforts have been made to increase the strength of steel plates used in various fields such as automobiles, home appliances, and building materials. For example, in the automobile field, the use of high-strength steel plates is increasing in order to reduce the weight of vehicle bodies in order to improve fuel efficiency.

When welding zinc-plated steel sheets, especially high-strength steel sheets, there may be a problem of reduced weldability due to liquid metal embrittlement (LME) cracking, as described in Patent Document 1, for example. LME cracking is thought to occur when the surface layer of the steel plate transforms into austenite during welding, molten zinc intrudes into the grain boundaries and embrittles the steel plate, and furthermore, tensile stress is applied to the steel plate during welding.

Furthermore, as disclosed in Non-Patent Document 1, it is known that ferrite phase grain boundaries have lower LME susceptibility than austenite grain boundaries with respect to LME cracking.

Note that Patent Document 2 discloses that, as a steel sheet that suppresses LME cracking and improves weldability, Si oxide particles with a particle size of 20 nm or more are contained in a number density of 3000 to 6000 pieces/mm ² in the surface layer of the steel sheet. A steel sheet having a grain size distribution is disclosed.

International Publication No. 2019/116531 International Publication No. 2020/218575

In order to prevent LME cracking, it is effective to prevent Zn, etc. contained in the plating layer from penetrating into the austenite-transformed steel sheet. In this respect, there is room for improvement.

Furthermore, it is known that when high-strength steel sheets are exposed to an atmospheric corrosive environment where temperature and humidity vary greatly, hydrogen generated during the corrosion process penetrates into the steel. Hydrogen that has penetrated into steel segregates at martensite grain boundaries in the steel structure, embrittles the grain boundaries, and can cause cracks in the steel sheet. The phenomenon in which cracks occur due to penetrating hydrogen is called hydrogen embrittlement cracking (delayed fracture), and is often a problem when processing steel sheets. In order to prevent this, when hydrogen enters the steel sheet, it is effective to promote hydrogen desorption from the steel sheet into the atmosphere.

In view of these circumstances, it is an object of the present invention to provide a steel sheet and a plated steel sheet that have high LME resistance and hydrogen desorption performance.

The present inventors have diligently studied means for solving the above problems. As a result, the surface layer of the steel sheet is decarburized, the ferrite (α) phase is stabilized, and the surface layer of the steel sheet is It has been found that the material is covered with a ferrite phase having a low solid solution amount of C, and as a result, it is possible to suppress LME. Furthermore, it has been found that by covering the surface layer of the steel sheet with a ferrite phase in which the amount of solid solution of C is low, desorption of hydrogen from the steel sheet into the atmosphere is promoted even when hydrogen enters the steel sheet.

The present invention has been made based on the above findings and further studies, and the gist thereof is as follows.

(1) A steel plate having a tensile strength of 780 MPa or more, the chemical components of which are C: 0.05 to 0.40%, Si: 0.7 to 3.0%, Mn: 0. 1-5.0%, sol. Al: 0.7-2.0%, P: 0.0300% or less, S: 0.0300% or less, N: 0.0100% or less, B: 0-0.010%, Ti: 0-0. 150%, Nb: 0-0.150%, V: 0-0.150%, Cr: 0-2.00%, Ni: 0-2.00%, Cu: 0-2.00%, Mo: 0-1.00%, W: 0-1.00%, Ca: 0-0.100%, Mg: 0-0.100%, Zr: 0-0.100%, Hf: 0-0.100 %, REM: 0 to 0.100%, the remainder being Fe and impurities, Si and sol. The total value of Al content is 1.8% or more, and the depth at which the C concentration measured by GDS is 0.05% or less in the depth direction from the steel plate surface is 10 μm or more, and A steel plate characterized in that the thickness of the layer in which the area ratio of the ferrite phase is 90% or more is 20 μm or more in the depth direction, and the steel sheet has a surface roughness Ra of 3.0 μm or less.

(2) The steel plate according to (1) above, wherein the depth at which the C concentration measured by GDS is 0.05% or less in the depth direction from the surface of the steel plate is 20 μm or more.

(3) The steel sheet according to (1) above, wherein the layer having a ferrite phase area ratio of 90% or more has a thickness of 30 μm or more in the depth direction from the surface of the steel sheet.

(4) In the depth direction from the surface of the steel plate, the depth at which the C concentration measured by GDS is 0.05% or less is 20 μm or more, and in the depth direction from the surface of the steel plate, a ferrite phase is formed. The steel plate according to (1) above, wherein the thickness of the layer having an area ratio of 90% or more is 30 μm or more.

(5) An alloyed hot-dip galvanized layer is provided on at least a portion of the surface of the steel sheet according to any one of (1) to (4), and the plating layer contains 0 to 1.5% Al and An alloyed hot-dip galvanized steel sheet containing 3 to 15% Fe, with the remainder being Zn and impurities.

(6) The alloyed hot-dip galvanized steel sheet according to the above (5), wherein the roughness of the interface between the steel sheet and the alloyed hot-dip galvanized layer in the cross section of the alloyed hot-dip galvanized steel sheet is 2.0 μm or less in terms of Ra. steel plate.

According to the present invention, a steel plate and a plated steel plate having high LME resistance and hydrogen desorption properties can be obtained.

It is a figure showing the layered ferrite phase formed in the surface layer of the steel plate of the present invention. It is a figure explaining LME resistance evaluation in an example.

The present invention will be explained below. The present invention is not limited to the following embodiments. First, an outline of improving LME resistance in the steel sheet and plated steel sheet of the present invention will be described.

LME cracking occurs when the surface layer of a steel plate is heated during spot welding, the steel plate structure in the surface layer transforms into austenite, and the grain boundaries become brittle as hot-dip plating enters the steel plate structure along the austenite grain boundaries. It is caused by doing. It is thought that LME cracking occurs because tensile stress is applied to the steel plate during welding. The steel sheet of the present invention improves LME resistance due to the structure formed in the surface layer. Note that in this specification, the surface layer of a steel plate refers to the range from the outermost surface of the steel plate to a depth of 100 μm.

If the C element is contained in the surface layer of a steel sheet, LME cracking is likely to occur, so keeping the C concentration in the surface layer of the steel sheet low is effective in preventing LME cracking. Normally, when a steel plate is heated, such as during annealing, external oxidation occurs and oxides (scale) are formed on the surface of the steel plate, making it difficult for decarburization to proceed. Therefore, the C concentration in the surface layer of the steel sheet is difficult to decrease. On the other hand, in the steel plate of the present invention, in the depth direction from the steel plate surface, there is a region 10 μm or more from the steel plate surface where the C concentration measured by GDS is 0.05% or less. This means that the concentration of C, which is an element that tends to cause LME, is low in the surface layer of the steel sheet.

However, even if the C concentration in the surface layer of the steel sheet is low, if the structure of the surface layer contains a large amount of austenite (γ), etc., which is highly sensitive to LME, it is thought that this may lead to a decrease in LME resistance. Therefore, in the steel plate in this embodiment, the thickness of the region where the area ratio of the ferrite phase is 90% or more in the depth direction from the steel plate surface is 20 μm or more. Furthermore, the steel sheet of the present invention contains a large amount of Si, which is conventionally known to reduce LME resistance when contained in steel. As a result of the studies conducted by the present inventors, contrary to the conventional knowledge, Si and sol. This is because it has been found that LME resistance is improved by containing a large amount of Al in a steel plate.

In the present invention, a strong strain is applied to the surface layer of the steel plate without increasing the surface roughness, and the steel plate is annealed at a high dew point. This allows oxygen to diffuse into the steel sheet and form internal oxides, making it possible to suppress the formation of external oxides. As a result, the C concentration in the surface layer of the steel sheet is reduced, and Si and sol. This is thought to be due to the fact that the ferrite can be stabilized by the effect of the combined addition of Al.

That is, the steel plate of the present invention contains Si and sol. Due to the combined effect of high Al content, applying strain to the surface layer before annealing, and controlling the dew point during annealing, a layer with a low C concentration and a high area ratio of ferrite is formed on the surface layer of the steel sheet. This makes it possible to improve LME resistance.

Hereinafter, the present invention will be explained in detail.

First, the chemical composition of steel sheets will be explained. Hereinafter, "%" regarding chemical components means "% by mass". Furthermore, in numerical ranges for chemical components, a numerical range expressed using "~" means a range that includes the numerical values written before and after "~" as lower and upper limits.

(C: 0.05-0.40%)
C (carbon) is an element that ensures the strength of steel. In order to obtain a tensile strength of 780 MPa or more, which is the target of the present invention, the C content is set to 0.05 MPa in consideration of the balance with weldability and to prevent the C concentration in the surface layer of the steel sheet from becoming too high. ~0.40%. If the C content is too large, the C concentration in the surface layer will not be reduced even by high dew point annealing, which will be described later, and the ferrite fraction will not be high. The content of C may be 0.07% or more, 0.10% or more, or 0.12% or more. The content of C may be 0.35% or less, 0.30% or less, or 0.25% or less.

(Si: 0.7-3.0%, sol.Al: 0.7-2.0%, Si+sol.Al≧1.8%)
Si (silicon) is an element that promotes ferrite stabilization and decarburization when added in combination with Al (aluminum). In order to obtain the effect of improving LME resistance, Si: 0.7 to 3.0%, sol. Al: 0.7 to 2.0% is contained, and Si and sol. The total value of Al content is 1.8% or more. Si and sol. This is because when the Al content satisfies such a numerical range, decarburization of the surface layer of the steel sheet can be promoted and ferrite in the surface layer can be stabilized in the heat treatment of the manufacturing process of the steel sheet of this embodiment. sol. Al refers to acid-soluble Al that is not converted into oxides such as Al ₂ O ₃ and is soluble in acids, and was measured by subtracting the insoluble residue on the filter paper that is generated during the Al analysis process. Required as Al. The content of Si may be 0.8% or more, 0.9% or more, or 1.0% or more. The content of Si may be 2.8% or less, 2.5% or less, or 2.0% or less. sol. The Al content may be 0.8% or more, 0.9% or more, or 1.0% or more. sol. The Al content may be 1.8% or less, 1.6% or less, or 1.5% or less. Si and sol. The total content of Al may be 1.9% or more, or 2.0% or more.

(Mn: 0.1-5.0%)
Mn (manganese) is an effective element for improving the strength of steel by creating a hard structure. Considering the balance between the strength of the steel and the deterioration of workability due to Mn segregation, the Mn content is set to 0.1 to 5.0%. The Mn content may be 0.5% or more, 1.0% or more, or 1.5% or more. The Mn content may be 4.5% or less, 4.0% or less, or 3.5% or less.

(P: 0.0300% or less)
P (phosphorus) is an impurity generally contained in steel. If the P content exceeds 0.0300%, weldability may deteriorate. Therefore, the content of P is set to 0.0300% or less. The content of P may be 0.0200% or less, 0.0100% or less, or 0.0050% or less. It is preferable that P is not contained, and the lower limit of the P content is 0%. From the viewpoint of dephosphorization cost, the P content may be more than 0%, 0.0001% or more, or 0.0005% or more.

(S: 0.0300% or less)
S (sulfur) is an impurity generally contained in steel. If the S content exceeds 0.0300%, weldability will decrease, and furthermore, the amount of MnS precipitated may increase, leading to a possibility that workability such as bendability will decrease. Therefore, the S content is set to 0.0300% or less. The S content may be 0.0100% or less, 0.0050% or less, 0.0030% or less, 0.0020% or less, or 0.0010% or less. It is preferable that S is not contained, and the lower limit of the S content is 0%. From the viewpoint of desulfurization cost, the S content may be more than 0%, 0.0001% or more, or 0.0005% or more.

(N: 0.0100% or less)
N (nitrogen) is an impurity generally contained in steel. If the N content exceeds 0.0100%, weldability may deteriorate. Therefore, the N content is set to 0.0100% or less. The content of N may be 0.0080% or less, 0.0050% or less, 0.0030% or less, 0.0020% or less, or 0.0010% or less. It is preferable that N is not contained, and the lower limit of the N content is 0%. From the viewpoint of manufacturing cost, the N content may be more than 0%, 0.0001% or more, 0.0002% or more, 0.0003% or more, or 0.0005% or more.

(B: 0-0.010%)
B (boron) is an element that increases hardenability and contributes to improvement of strength, and also segregates at grain boundaries to strengthen grain boundaries and improve toughness, so it may be included as necessary. . Since B is not an essential element, the lower limit of the content of B is 0%. Although this effect can be obtained even when B is contained in a trace amount, it is preferable that the content of B is 0.0001% or more. The content of B may be 0.0002% or more, 0.0003% or more, 0.0005% or more, 0.0007% or more, or 0.0010% or more. On the other hand, from the viewpoint of ensuring sufficient toughness, the B content is set to 0.010% or less. The content of B may be 0.0080% or less, 0.0060% or less, 0.0050% or less, 0.0040% or less, or 0.0030% or less.

(Ti: 0-0.150%)
Ti (titanium) is an element that precipitates as TiC during cooling of steel and contributes to improving the strength, so it may be included as necessary. Since Ti is not an essential element, the lower limit of the content of Ti is 0%. Although this effect can be obtained even with a trace amount of Ti, the content of Ti is preferably 0.0001% or more. The content of Ti may be 0.0002% or more, 0.0003% or more, 0.0005% or more, 0.0007% or more, or 0.0010% or more. On the other hand, if excessively contained, coarse TiN may be generated and toughness may be impaired, so the content of Ti is set to 0.150% or less. The Ti content is 0.1000% or less, 0.0500% or less, 0.0300% or less, 0.0200% or less, 0.0100% or less, 0.0050% or less, or 0.0030% or less. good.

(Nb: 0-0.150%)
Since Nb (niobium) is an element that contributes to improving strength through improving hardenability, it may be included as necessary. Since Nb is not an essential element, the lower limit of the content of Nb is 0%. Although this effect can be obtained even with a trace amount of Nb, the content of Nb is preferably 0.001% or more. The Nb content may be 0.002% or more, 0.003% or more, 0.005% or more, or 0.008% or more. On the other hand, from the viewpoint of ensuring sufficient toughness, the Nb content is set to 0.150% or less. The Nb content may be 0.100% or less, 0.060% or less, 0.050% or less, 0.040% or less, or 0.030% or less.

(V: 0-0.150%)
Since V (vanadium) is an element that contributes to improving strength through improving hardenability, it may be contained as necessary. Since V is not an essential element, the lower limit of the content of V is 0%. Although this effect can be obtained even when V is contained in a trace amount, it is preferable that the content of V is 0.001% or more. The content of V may be 0.002% or more, 0.003% or more, or 0.005% or more. On the other hand, from the viewpoint of ensuring sufficient toughness, the V content is set to 0.150% or less. The V content may be 0.100% or less, 0.060% or less, 0.050% or less, 0.040% or less, or 0.030% or less.

(Cr: 0-2.00%)
Cr (chromium) is effective in improving the hardenability of steel and increasing the strength of steel, and therefore may be contained as necessary. Since Cr is not an essential element, the lower limit of the content of Cr is 0%. Although this effect can be obtained even with a trace amount of Cr, the content of Cr is preferably 0.001% or more. The content of Cr may be 0.01% or more, 0.02% or more, 0.03% or more, 0.05% or more, or 0.08% or more. On the other hand, if it is contained excessively, a large amount of Cr carbide will be formed, and the hardenability may be adversely affected, so the content of Cr is set to 2.00% or less. The content of Cr may be 1.80% or less, 1.50% or less, 1.20% or less, 1.00% or less, 0.70% or less, 0.50% or less, or 0.30% or less. .

(Ni: 0-2.00%)
Since Ni (nickel) is effective in improving the hardenability of steel and increasing the strength of steel, it may be contained as necessary. Since Ni is not an essential element, the lower limit of the Ni content is 0%. Although this effect can be obtained even with a trace amount of Ni, it is preferable that the Ni content is 0.001% or more. The Ni content may be 0.01% or more, 0.02% or more, 0.03% or more, 0.05% or more, or 0.07% or more. On the other hand, since excessive addition of Ni increases cost, the Ni content is set to 2.00% or less. The Ni content is 1.80% or less, 1.50% or less, 1.20% or less, 1.00% or less, 0.80% or less, 0.50% or less, 0.30% or less, or 0. It may be 20% or less.

(Cu: 0-2.00%)
Cu (copper) is effective in improving the hardenability of steel and increasing the strength of steel, and therefore may be contained as necessary. Since Cu is not an essential element, the lower limit of the content of Cu is 0%. Although this effect can be obtained even with a trace amount of Cu, the content of Cu is preferably 0.0001% or more. The content of Cu may be 0.0002% or more, 0.0003% or more, or 0.0005% or more. On the other hand, from the viewpoint of suppressing a decrease in toughness and cracking of the slab after casting, the content of Cu is set to 2.00% or less. The Cu content is 1.8000% or less, 1.5000% or less, 1.2000% or less, 1.0000% or less, 0.5000% or less, 0.1000% or less, 0.0500% or less, 0.0100 % or less, 0.0050% or less, 0.0030% or less, or 0.0020% or less.

(Mo: 0-1.00%)
Mo (molybdenum) is effective in improving the hardenability of steel and increasing the strength of steel, and therefore may be contained as necessary. Since Mo is not an essential element, the lower limit of the content of Mo is 0%. Although this effect can be obtained even with a trace amount of Mo, the content of Mo is preferably 0.001% or more. The Mo content may be 0.01% or more, 0.02% or more, 0.03% or more, 0.05% or more, or 0.08% or more. On the other hand, from the viewpoint of suppressing a decrease in toughness, the Mo content is set to 1.00% or less. The Mo content may be 0.90% or less, 0.70% or less, 0.50% or less, or 0.30% or less.

(W: 0-1.00%)
W (tungsten) is effective in improving the hardenability of steel and increasing the strength of steel, and therefore may be included as necessary. Since W is not an essential element, the lower limit of the content of W is 0%. Although this effect can be obtained even when a small amount of W is included, it is preferable that the content of W is 0.001% or more. The content of W may be 0.002% or more, 0.003% or more, or 0.004% or more. On the other hand, from the viewpoint of suppressing a decrease in toughness, the W content is set to 1.00% or less. The content of W is 0.900% or less, 0.700% or less, 0.500% or less, 0.300% or less, 0.100% or less, 0.050% or less, 0.030% or less, or 0. It may be 0.020% or less.

(Ca: 0-0.100%)
Ca (calcium) is an element that contributes to control of inclusions, particularly fine dispersion of inclusions, and has the effect of increasing toughness, and therefore may be contained as necessary. Since Ca is not an essential element, the lower limit of the content of Ca is 0%. Although this effect can be obtained even with a trace amount of Ca, the content of Ca is preferably 0.0001% or more. The Ca content may be 0.0002% or more, 0.0003% or more, or 0.0004% or more. On the other hand, since excessive Ca content may cause deterioration of surface properties, the Ca content is set to 0.100% or less. The Ca content is 0.0800% or less, 0.0500% or less, 0.0100% or less, 0.0050% or less, 0.0030% or less, 0.0020% or less, or 0.0010% or less. good.

(Mg: 0-0.100%)
Mg (magnesium) is an element that contributes to control of inclusions, particularly to fine dispersion of inclusions, and has the effect of increasing toughness, and therefore may be contained as necessary. Since Mg is not an essential element, the lower limit of the Mg content is 0%. Although this effect can be obtained even with a trace amount of Mg, it is preferable that the Mg content is 0.0001% or more. The content of Mg may be 0.0002% or more, 0.0003% or more, 0.0005% or more, or 0.0008% or more. On the other hand, since excessive Mg content may cause deterioration of surface properties, the content of Mg is set to 0.100% or less. The Mg content may be 0.090% or less, 0.080% or less, 0.050% or less, 0.010% or less, 0.005% or less, or 0.003% or less.

(Zr: 0-0.100%)
Zr (zirconium) is an element that contributes to control of inclusions, particularly fine dispersion of inclusions, and has the effect of increasing toughness, and therefore may be contained as necessary. Since Zr is not an essential element, the lower limit of the content of Zr is 0%. Although this effect can be obtained even when a small amount of Zr is contained, it is preferable that the content of Zr is 0.001% or more. The content of Zr may be 0.002% or more, 0.003% or more, 0.005% or more, or 0.010% or more. On the other hand, since excessive Zr content may cause deterioration of surface properties, the Zr content is set to 0.100% or less. The content of Zr may be 0.080% or less, 0.050% or less, 0.040% or less, or 0.030% or less.

(Hf: 0-0.100%)
Hf (hafnium) is an element that contributes to inclusion control, particularly fine dispersion of inclusions, and has the effect of increasing toughness, and therefore may be contained as necessary. Since it is not an essential element, the lower limit of the content of Hf is 0%. Although this effect can be obtained even with a trace amount of Hf, it is preferable that the Hf content is 0.0001% or more. The Hf content may be 0.0002% or more, 0.0003% or more, 0.0005% or more, or 0.0008% or more. On the other hand, since excessive Hf content may cause deterioration of surface properties, the content of Hf is set to 0.100% or less. The content of Hf is 0.050% or less, 0.030% or less, 0.010% or less, 0.005% or less, or 0.003% or less.

(REM: 0-0.100%)
REM (rare earth element) is an element that contributes to control of inclusions, particularly to fine dispersion of inclusions, and has the effect of increasing toughness, and therefore may be included as necessary. Since it is not an essential element, the lower limit of the content of REM is 0%. Although this effect can be obtained even with a trace amount of REM, the content of REM is preferably 0.0001% or more. The content of REM may be 0.0003% or more, 0.0005% or more, or 0.0007% or more. On the other hand, if excessively contained, deterioration of surface properties may become apparent, so the content of REM is set to 0.100% or less. The content of REM may be 0.0500% or less, 0.0300% or less, 0.0100% or less, 0.0050% or less, 0.0030% or less, or 0.0020% 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 remainder other than the above chemical components consists of Fe and impurities. Here, impurities are components that are mixed in due to various factors in the manufacturing process, including raw materials such as ore and scrap, when steel sheets are industrially manufactured. It means a substance that is contained within a range that does not adversely affect LME properties and hydrogen desorption properties, that is, it can provide the LME resistance and hydrogen desorption properties required for the steel sheet of the present invention.

The chemical components of the steel plate may be analyzed using elemental analysis methods known to those skilled in the art, such as inductively coupled plasma mass spectrometry (ICP-MS). However, C and S may be measured using the combustion-infrared absorption method, and N may be measured using the inert gas melting-thermal conductivity method. These analyzes may be performed using samples taken from steel plates using a method compliant with JIS G0417:1999.

Next, the surface layer portion of the steel plate will be explained.

[C concentration]
In the steel plate of the present invention, the depth at which the C concentration measured by GDS (glow discharge spectroscopy) is 0.05% or less is 10 μm or more in the depth direction from the steel plate surface.

Since LME sensitivity decreases as the C concentration decreases, LME resistance improves when the C concentration in the surface layer is low. Further, since C is an austenite stabilizing element, a small amount of C stabilizes the ferrite phase with low LME sensitivity, which will be described later. Furthermore, when there is less C in the surface layer, hydrogen that has entered the steel easily escapes, improving hydrogen desorption performance. This is presumed to be because the presence of less C, which is an interstitial element, in the ferrite phase makes it easier for hydrogen to pass through.

Such a surface structure can be obtained by changing the chemical composition of the steel sheet to a component containing a large amount of Si and Al, as described above, and by heat treatment described below.

If the depth at which the C concentration is 0.05% or less is 10 μm or more, the effect of improving LME resistance can be obtained, so the upper limit of the depth is not particularly limited. For example, it may be 50 μm or less, 40 μm or less, or 30 μm or less. The depth at which the C concentration is 0.05% or less is preferably 20 μm or more.

The GDS measurement is performed five times in the thickness direction, and the average value of these measurements is taken as the C concentration. The measurement conditions are as follows. The starting point of "depth" is the surface of the steel plate for an unplated steel plate, and the interface between the steel plate and the plating layer for a plated steel plate. The interface between the steel plate and the plating layer is located at a position where the Fe concentration measured by GDS measurement is 93% of the Fe concentration at a depth of 150 μm.

Equipment: High frequency glow discharge emission spectrometer (manufactured by LECO Japan LLC, model number “GDS850A”)
Ar gas pressure: 0.3MPa
Anode diameter: 4mmφ
RF output: 30W
Measurement time: 200-1500 seconds

[Ferrite phase]
In the steel sheet of the present invention, the thickness of the layer in which the area ratio of the ferrite phase is 90% or more is 20 μm or more in the depth direction from the steel sheet surface. FIG. 1 shows an example of a microstructure photograph taken by SEM near the surface layer of the steel sheet of the present invention. FIG. 1 is a cross section of a steel plate in the thickness direction, and the upper side of the drawing is the surface of the steel plate. The surface layer of the steel sheet in FIG. 1 has a low C concentration and includes a layer with a ferrite phase area ratio of 90% or more. The inside of the steel plate has a structure mainly composed of martensite and containing ferrite. In the steel plate of FIG. 1, a layer having a ferrite phase area ratio of 90% or more exists in the surface layer with a thickness of 40 μm.

It is known that ferrite phase grain boundaries have lower LME susceptibility than γ (austenite) grain boundaries (for example, Non-Patent Document 1). Therefore, by having a thick structure mainly composed of ferrite phase in the surface layer of the steel sheet, LME is less likely to occur even when the plating melts, and LME resistance can be improved. Such a surface structure can be obtained by changing the chemical composition of the steel sheet to a component containing a large amount of Si and Al, as described above, and by heat treatment described below.

If the thickness of the region where the area ratio of the ferrite phase is 90% or more is 20 μm or more, the effect of improving LME resistance can be obtained, so the upper limit of the thickness is not particularly limited. For example, it may be 100 μm or less, 80 μm or less, or 60 μm or less. The thickness of the region where the area ratio of the ferrite phase is 90% or more is preferably 30 μm or more.

The thickness of the region where the area ratio of the ferrite phase is 90% or more can be determined by nital etching the L cross section of the steel plate and observing it with SEM. Martensite, bainite, and ferrite can be distinguished from the structure morphology. Specifically, the cross section in the L direction is polished, and after mirror polishing, nital etching is used to expose the steel structure by corrosion. Thereafter, secondary electron images of five fields of view are photographed at equal intervals at a magnification of 1500 times in a range of 500 μm in the depth direction based on the steel surface. The area ratio of the ferrite phase is measured by a point counting method (based on ASTM E562), and the thickness of a region where the area ratio of the ferrite phase is 90% or more is measured for each photographic field of view.

Here, the area ratio of the ferrite phase refers to the area ratio determined by observing the L cross section. Even if there is a local part in the thickness direction where the area ratio of the ferrite phase is less than 90% when observing the C section, the area of the ferrite phase in the L section up to a depth of 20 μm If the rate is 90% or higher, there is no problem. More specific area ratios are as follows.

The ferrite area ratio is measured as follows. The steel plate is cut along an imaginary line on the surface of the steel plate perpendicular to the rolling direction of the steel plate, in the thickness direction, that is, perpendicular to the surface of the steel plate, and a test piece is cut out. Next, the cross section of the steel plate perpendicular to the rolling direction is polished to a mirror surface, the steel structure is exposed using a nital solution, and a secondary electron image is taken using a field emission scanning electron microscope. The field of view to be observed is from the surface of the steel sheet in the case of a non-plated steel sheet, and from the interface between the steel sheet and the plating layer in the case of a plated steel sheet to a depth of 500 μm, and five fields of view are observed at equal intervals. For the obtained tissue photographs, the fraction of each tissue is calculated by the point counting method. First, draw an equally spaced grid on the tissue photograph. Next, it is determined whether the structure at each lattice point corresponds to tempered martensite, pearlite, ferrite, fresh martensite, retained austenite, or bainite. By finding the number of lattice points corresponding to each tissue and dividing by the total number of lattice points, the fraction of each tissue can be measured. In the present invention, the grid spacing is 2 μm×2 μm, and the total number of grid points is 1500 points.

The criteria for determining pearlite, ferrite, martensite, and bainite are as follows. A region that has a substructure (lath boundary, block boundary) within the grain and in which carbides are precipitated in a plurality of variants is determined to be tempered martensite. In addition, a region where cementite is precipitated in a lamellar shape is determined to be pearlite. A region with low brightness and no underlying structure is determined to be ferrite. A region where the brightness is high and the underlying structure is not exposed by etching is determined to be fresh martensite or retained austenite. Areas that do not fall under any of the above are determined to be bainite. Simply speaking, the area ratio of the ferrite phase can be determined by distinguishing between ferrite and other structures.

[Surface roughness]
The steel plate of the present invention has a surface roughness of 3.0 μm or less in arithmetic mean height Ra defined by JIS B0601:2013. When the roughness increases, cracks are more likely to occur due to stress concentration, resulting in a decrease in LME resistance. The surface roughness may be 2.5 μm or less, or 2.0 μm or less.

[Tensile strength]
Since the present invention suppresses LME occurring in a high-strength steel plate, the steel plate according to the present invention has high strength. Specifically, it has a tensile strength of 780 MPa or more. The upper limit of the tensile strength is not particularly limited, but from the viewpoint of ensuring toughness, it may be, for example, 2000 MPa or less. The tensile strength may be measured in accordance with JIS Z 2241:2011 by taking a JIS No. 5 tensile test piece whose longitudinal direction is perpendicular to the rolling direction. The tensile strength may be 880 MPa or more, 980 MPa or more, 1080 MPa or more, or 1180 MPa or more. The tensile strength may be 1900 MPa or less, or 1800 MPa or less.

<Plated steel plate>
The plated steel sheet according to the present invention has an alloyed hot-dip galvanized layer on the above-described steel sheet according to the present invention. The plating layer is formed on at least a portion of the surface of the steel plate, and may be formed on one side or both sides of the steel plate.

[Chemical components of plating layer]
The chemical components of the alloyed hot-dip galvanizing in the present invention will be explained. "%" regarding the content of an element means "mass %" unless otherwise specified. In the numerical range of chemical components for the plating layer, the numerical range expressed using "~" means the range that includes the numbers written before and after "~" as the lower limit and upper limit, unless otherwise specified. do.

(Al: 0-1.5%)
Since Al is an element that improves the corrosion resistance of the plating layer by being included or alloyed with Zn, it may be included 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, and may be 0.1% or more. When Al in the plating layer is in the range of 0.3 to 1.5%, the effect of Al significantly reduces the rate at which Zn penetrates into the steel grain boundaries, making it possible to improve LME resistance. Therefore, from the viewpoint of improving LME resistance, the content of Al in the plating layer is preferably 0.3 to 1.5%.

(Fe: 3-15%)
Fe is contained in the plating layer by being diffused from the steel sheet when the plated steel sheet is heat treated after forming a plating layer containing Zn on the steel sheet. The Fe content may be 3.0% or more, and may be 4.0% or more or 5.0% or more. On the other hand, the Fe content may be 15.0% or less, for example, 12.0% or less, 10.0% or less, 8.0% or less, or 6.0% or less.

The remainder of the plating layer other than the above components consists of Zn and impurities. Impurities in the plating layer are components that are mixed into the plating layer due to various factors in the manufacturing process, including raw materials, and are not intentionally added to the plating layer. do. In the plating layer, trace amounts of elements other than the basic components and optionally added components described above may be included as impurities within a range that does not impede 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 acid solution containing an inhibitor may be, for example, a 10% by mass hydrochloric acid solution containing 0.06% by mass of an inhibitor (manufactured by Asahi Chemical Co., Ltd., Ivit).

The thickness of the plating layer may be, for example, 3 to 50 μm. Further, the amount of the plating layer deposited is not particularly limited, but may be, for example, 10 to 170 g/m ² per side. In the present invention, the amount of the plating layer deposited is determined by dissolving the plating layer in an acid solution containing an inhibitor that suppresses corrosion of the steel plate, and from the change in weight of the plating layer before and after pickling and stripping. The thickness of the plating layer may be 5 μm or more, 10 μm or more, 15 μm or more, or 20 μm or more. The thickness of the plating layer may be 40 μm or less, or 30 μm or less. The amount of the plating layer deposited on one side may be 20 g/m ² or more, 30 g/m ² or more, 40 g/m ² or more, or 50 g/m ² or more. The amount of the plating layer deposited per side may be 150 g/m ² or less, 130 g/m ² or less, 120 g/m ² or less, or 100 g/m ² or less.

The roughness of the interface between the steel plate and the plating layer corresponds to the roughness of the surface of the steel plate described above, so Ra is 3.0 μm or less. Considering the adhesion of plating, Ra may be 2.5 μm or less, or 2.0 μm or less. It may be the roughness of the interface between the steel plate and the plating layer, or the surface roughness of the steel plate measured by dissolving and removing the plating.

Note that the steel sheet of the present invention has the effect of improving LME resistance even if it is not galvanized. LME cracking does not occur when ungalvanized steel plates are welded together. However, even when welding is performed between galvanized steel plates on one side and non-galvanized steel plates on the other, molten galvanization occurs on the overlapping surfaces of the steel plates during welding. Therefore, there is a possibility that LME cracking may occur due to contact with the surface of the steel sheet that is not subjected to molten galvanization. Additionally, when welding an unplated steel plate using a welding electrode used to spot weld a galvanized steel plate, the zinc plating attached to the welding electrode melts and comes into contact with the surface of the steel plate, resulting in LME cracking. there is a possibility. If the steel sheet of the present invention is used as an unplated steel sheet, even in such a case, LME cracking can be suppressed even in the welding process because the C concentration in the surface layer is low and the surface layer is a ferrite phase. Can be done.

The thickness of the steel plate and plated steel plate of the present invention is not particularly limited. For example, it can be 0.6 to 3.2 mm. The plate thickness may be 0.8 mm or more, or 1.0 mm or more. The plate thickness may be 3.0 mm or less, 2.6 mm or less, 2.4 mm or less, 2.2 mm or less, 2.0 mm or less, or 1.8 mm or less.

Next, a method for manufacturing a steel plate according to the present invention will be explained.

The steel plate according to the present invention can be produced, for example, by a casting process in which molten steel with adjusted chemical components is cast to form a steel billet, a hot rolling process in which a hot rolled steel plate is obtained by hot rolling a steel billet, and a hot rolling process in which a hot rolled steel plate is wound. A winding process, a cold rolling process in which a cold-rolled steel plate is obtained by cold-rolling a coiled hot-rolled steel plate, a pre-treatment process in which a cold-rolled steel plate is brush-grinded, and an annealing process in which the pre-treated cold-rolled steel plate is annealed. It can be obtained by a manufacturing method comprising steps. Alternatively, the material may be pickled and then cold-rolled without being wound up after the hot-rolling process.

[Casting process]
The conditions of the casting process are not particularly limited. For example, following melting in a blast furnace, electric furnace, etc., various secondary smelting may be performed, and then casting may be performed by a method such as ordinary continuous casting or ingot casting.

[Hot rolling process]
A hot rolled steel plate can be obtained by hot rolling a steel piece obtained by casting. The hot rolling process is performed by directly or once cooling the cast steel billet, then reheating and hot rolling. When reheating is performed, the heating temperature of the steel piece may be, for example, 1100 to 1250°C. In the hot rolling process, rough rolling and finish rolling are usually performed. The temperature and reduction rate of each rolling may be changed as appropriate depending on the desired metal structure and plate thickness. For example, the end temperature of finish rolling may be 900 to 1050°C, and the reduction ratio of finish rolling may be 10 to 50%.

[Winding process]
Hot-rolled steel sheets can be rolled up at a predetermined temperature. The winding temperature may be changed as appropriate depending on the desired metal structure, etc., and may be, for example, 500 to 800°C. The hot-rolled steel sheet may be subjected to a predetermined heat treatment by unwinding the hot-rolled steel sheet before or after winding. Alternatively, it is also possible to perform cold rolling, which will be described later, by pickling after the hot rolling process without winding.

[Cold rolling process]
After pickling or the like is performed on the hot rolled steel sheet, the hot rolled steel sheet can be cold rolled to obtain a cold rolled steel sheet. The rolling reduction ratio of cold rolling may be changed as appropriate depending on the desired metallographic structure and plate thickness, and may be, for example, 20 to 80%. After the cold rolling process, the material may be cooled to room temperature by, for example, air cooling.

[Pre-treatment process]
As mentioned above, in the surface layer of the steel plate, the thickness of the layer in which the area ratio of the ferrite phase is 90% or more in the depth direction is 20 μm or more, and the C concentration in the depth direction is 0.05% as measured by GDS. In order to make the depth below 10 μm or more, it is necessary to perform a predetermined pretreatment and then perform annealing.

The pretreatment includes grinding the surface of the cold rolled steel plate with a grinding brush (brush grinding process). An example of a grinding brush that can be used is M-33 manufactured by Hotani Corporation. Thereby, strain can be introduced without increasing the surface roughness. When grinding, it is recommended to apply a 1.0 to 5.0% NaOH aqueous solution to the surface of the steel plate. It is preferable that the brush reduction amount is 0.5 to 10.0 mm and the rotation speed is 100 to 1000 rpm. By controlling the coating liquid conditions, brush reduction amount, and rotation speed to perform brush grinding, decarburization is promoted in the annealing process described later, and a stable ferrite structure is efficiently formed on the surface layer of the steel sheet. can be formed.

[Annealing process]
After the pretreatment step, the cold rolled steel sheet is annealed. Annealing is performed under a tension of 1 to 20 MPa, for example. Applying tension during annealing makes it possible to more effectively introduce strain into the steel sheet, promoting decarburization of the surface layer.

The holding temperature in the annealing step is 750 to 900°C. The holding temperature may be 770-870°C. By setting it within such a range, decarburization can be promoted, the C concentration in the surface layer can be reduced, and the ferrite phase can be stabilized. The heating rate up to the holding temperature is not particularly limited, but may be 1 to 10° C./sec.

The holding time at the holding temperature in the annealing step is 20 to 300 seconds. The holding time may be between 30 and 250 seconds. By setting it within such a range, decarburization can be promoted, the C concentration in the surface layer can be reduced, and the ferrite phase can be stabilized.

The atmosphere in the annealing step has a dew point of -30 to 20°C. The dew point may be -10 to 5°C. The atmosphere may be, for example, N ₂ -1 to 10 vol% H ₂ or N ₂ -2 to 4 vol% H ₂ . If the dew point is too high or too low, a phase containing oxides such as Si, Mn, and Al will be formed outside the steel sheet, and decarburization will not be promoted. Furthermore, mutual diffusion of plating components and steel components may be inhibited, resulting in insufficient plating properties.

By the manufacturing method including each of the steps described above, it is possible to obtain a steel plate in which decarburization is promoted and the ferrite phase is stabilized in the surface layer of the steel plate.

<Manufacturing method of plated steel sheet>
The plated steel sheet according to the present invention can be obtained by a plating process and an alloying process in which an alloyed hot-dip galvanized layer is formed on a steel plate manufactured as described above.

The plating process and the alloying process may be performed according to hot-dip plating methods and alloying processes known to those skilled in the art. The conditions for the plating process and the alloying process may be appropriately set in consideration of the desired chemical composition, thickness, adhesion amount, etc. of the plating layer. For example, it may be immersed in a hot-dip galvanizing bath at 420 to 480° C. with adjusted chemical components for 1 to 10 seconds, and then pulled out at 20 to 200 mm/sec after immersion, and the amount of plating deposited may be controlled by N ₂ wiping gas. The alloying treatment may be performed, for example, at 500 to 550° C. for 10 to 60 seconds.

The steel sheets and plated steel sheets according to the present invention have high strength, high LME resistance and hydrogen desorption properties, and therefore can be suitably used in a wide range of fields such as automobiles, home appliances, and building materials. It is particularly preferred to be used in the automotive field. Steel plates and plated steel plates used for automobiles are often spot welded, and in this case, LME cracking 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 LME resistance is suitably exhibited.

Hereinafter, the present invention will be explained in more detail with reference to Examples. The present invention is not limited to these examples.

《Example A》
First, preliminary experiments conducted by the inventors in obtaining the present invention will be described. Spot welding was performed on steel plates (1.2 mm thick) with varying Si and Al contents to a general alloyed hot-dip galvanized steel plate (1.6 mm thick) under the conditions shown in Table 1. Internal cracking in the LME was confirmed. Table 2 shows the results. As shown in Table 2, it was confirmed that LME was suppressed in the high Si-high Al steel.

《Example B》
<Example 1>
(Preparation of steel plate sample)
No. of Table 3 Molten steel adjusted to have the chemical composition described in 1 was melted in a blast furnace and cast by continuous casting to obtain a steel billet. The obtained steel piece was heated to 1200°C and hot rolled at a finish rolling end temperature of 950°C and a finish rolling reduction of 30% to obtain a hot rolled steel plate. The obtained hot-rolled steel sheet was wound up at a winding temperature of 650° C., pickled, and then cold-rolled at a rolling reduction of 50% to obtain a cold-rolled steel sheet. The thickness of the cold-rolled steel plate was 1.6 mm.

Next, a 2.0% NaOH aqueous solution was applied to the cold-rolled steel sheet, and a pretreatment of brush grinding was performed. Brush grinding was performed using Hotani M-33 as a grinding brush at a brush reduction amount of 2.0 mm and a rotation speed of 600 rpm (condition A in Table 4).

After the pretreatment step and before the annealing step, the surface roughness of the steel plate was measured in accordance with JIS B 0601:2013. That is, 10 locations are randomly selected on the surface of the surface layer side, the surface profile at each location is measured using a contact type surface roughness meter, and the arithmetic mean roughness Ra is obtained by arithmetic averaging of the surface roughness at those locations. , was evaluated as follows.

Evaluation AA: 2.0 μm or less Evaluation A: More than 2.0 μm, 3.0 μm or less Evaluation B: More than 3.0 μm

Thereafter, annealing was performed in a N ₂ -4% H ₂ gas atmosphere in a furnace with an oxygen concentration of 20 ppm or less at a dew point of 0° C., a holding temperature of 800° C., and a holding time of 100 seconds to prepare a steel plate sample. The temperature increase rate during annealing was 6.0°C/sec up to 500°C, and 2.0°C/sec from 500°C to the holding temperature. The annealing treatment was performed under a tension of 15 MPa.

<Examples 2 to 25, Comparative Examples 26 to 40>
A steel plate was produced in the same manner as in Example 1, except that the chemical components were as shown in Table 3, and the conditions for the pretreatment step and annealing step were as shown in Table 4. In addition, No. In No. 39, the pretreatment of brush grinding was omitted. Also, No. In No. 40, a grinding brush D-100 manufactured by Hotani Co., Ltd. was used (condition B in Table 4). D-100 is a brush with approximately twice the amount of grinding as M-33.

Following the annealing treatment, the steel sheets whose "Presence or absence of alloyed hot-dip galvanized layer" in Table 5 was "present" were subjected to plating treatment and alloying treatment to obtain alloyed hot-dip galvanized steel sheets. The plating treatment was performed by immersing the sample in a hot-dip galvanizing bath (Zn-0.14% Al) at 450°C for 3 seconds. After dipping, it was pulled out at 100 mm/sec, and the amount of plating deposited was controlled to 50 g/m ² using N ₂ wiping gas. Alloying treatment was performed at 520°C for 30 seconds. Steel plates with "no alloyed galvanized layer" in Table 5 were not subjected to plating or alloying.

(Surface tissue evaluation)
A sample cut to 30 mm x 30 mm was taken, and GDS measurements were performed five times in the thickness direction under the conditions described above to determine the depth at which the C concentration was 0.05% or less. .05% depth". Here, the starting point of "depth" is the surface of the steel plate for an unplated steel plate, and the interface between the plating layer and the steel plate for a plated steel plate.

In addition, a sample cut into 25 mm x 15 mm was taken, subjected to nital etching, and the thickness of the layer containing 90% or more of the ferrite phase was measured using the method described above. Indicated.

In addition, the surface of the steel plate for unplated steel sheets, and the plating for plated steel sheets were treated using a 10 mass% hydrochloric acid solution containing 0.06 mass% inhibitor (manufactured by Asahi Chemical Co., Ltd., Ivit). The surface roughness of the removed and exposed steel plate was measured in the same manner as before annealing, and is shown in "Steel plate surface or steel plate/plating interface roughness" in Table 5.

(Tensile strength evaluation)
For each steel plate, a JIS No. 5 tensile test piece with the longitudinal direction perpendicular to the rolling direction was taken, a tensile test was conducted in accordance with JIS Z 2241:2011, the tensile strength was determined, and it was evaluated as follows. .

Evaluation AAA: 1180MPa or more Evaluation AA: 980MPa or more, less than 1180MPa Evaluation A: 780MPa or more, less than 980MPa

(LME resistance evaluation)
Two samples cut to a size of 50 mm x 100 mm were taken from each steel plate, and these two samples were welded using a dome radius type welding electrode with a tip diameter of 8 mm at a welding angle of 2 degrees and a pressing force of 4. Spot welding was performed at 0 kN, energizing time 0.5 seconds, and energizing current 12 kA to produce a welded joint. Note that the steel sheet that has not been subjected to alloyed hot-dip galvanizing is the No. 1 steel sheet that has been subjected to hot-alloy galvanizing. Spot welding was performed in combination with 2.

Evaluation of LME resistance will be explained with reference to FIG. 2. The LME resistance was evaluated as follows by superimposing two steel plates 1 and performing spot welding, and based on the length of the crack 11 just outside the pressure welded part of the welded part 2. The term "directly outside the press-welded part of the welded part" refers to a position outside the press-welded part by spot welding on the mating surfaces of two steel plates, and in the vicinity of the press-welded part. In this example, if the evaluation was A or higher, it was determined that the LME resistance was excellent.

Evaluation AAA: 0μm
Evaluation AA: More than 0 μm and less than 60 μm Evaluation A: More than 60 μm and less than 120 μm Evaluation B: More than 120 μm

(Hydrogen desorption test)
A test piece having a size of 80 mm x 50 mm was cut out from each steel plate and electrochemically charged with hydrogen. Electricity was applied for 48 hours under constant current control (cathode current density 1 mA/cm ² ) in a 3% NaCl+3 g/L NH ₄ SCN aqueous solution. After hydrogen charging, the plated steel plate was left standing in the air at room temperature for 48 hours, and after a predetermined period of time, the amount of diffusible hydrogen contained in the plated steel plate was measured using the temperature-programmed desorption method, and the amount of diffusible hydrogen contained in the plated steel plate was measured as follows. evaluated. In the temperature programmed desorption method, the temperature was raised to 400°C at a heating rate of 100°C/h, and the total amount of hydrogen released from room temperature to 200°C was defined as the amount of diffusible hydrogen. In this example, if the evaluation was A or higher, it was determined that the hydrogen desorption property was excellent. When it was determined that both LME resistance and hydrogen desorption performance were excellent, it was determined that the problem to be solved by the present invention was solved.

Evaluation AAA: 10% or less of the initial hydrogen amount Evaluation AA: 20% or less of the initial hydrogen amount Evaluation A: Less than 50% of the initial hydrogen amount Evaluation B: 50% or more of the initial hydrogen amount

The results of each evaluation are shown in Table 5.

No. No. 26 is a comparative example in which the steel plate has a high C content. It is thought that because the steel sheet has a high C content, decarburization in the surface layer did not proceed even with high dew point annealing. Therefore, the depth where the C concentration is 0.05% or less and the thickness of the layer where the area ratio of the ferrite phase is 90% or more were not large. As a result, the LME resistance and hydrogen desorption properties were poor.

No. No. 27 is a comparative example in which the steel plate has a low Si content. It is thought that because the Si content of the steel sheet was low, decarburization did not progress in the surface layer even if high dew point annealing was performed, and ferrite was not stabilized. Therefore, the depth where the C concentration is 0.05% or less and the thickness of the layer where the area ratio of the ferrite phase is 90% or more were not large. As a result, the LME resistance and hydrogen desorption properties were poor.

No. 28 is the Si content of the steel plate, and the Si and sol. This is a comparative example in which the sum of Al contents is small. Si content of steel plate and Si and sol. It is thought that because the sum of the Al contents was small, decarburization did not proceed in the surface layer even if high dew point annealing was performed, and the ferrite was not stabilized. Therefore, the depth where the C concentration is 0.05% or less and the thickness of the layer where the area ratio of the ferrite phase is 90% or more were not large. As a result, the LME resistance and hydrogen desorption properties were poor.

No. No. 29 is a comparative example in which the steel plate has a high Si content. It is thought that because the Si content of the steel sheet was high, even if high dew point annealing was performed, external oxidation progressed and oxides (scale) were formed on the surface layer of the steel sheet, suppressing decarburization at the outermost surface. Therefore, the depth at which the C concentration was 0.05% or less was not large. As a result, the LME resistance and hydrogen desorption properties were poor.

No. 30 is the steel plate sol. This is a comparative example with a low Al content. Steel plate sol. It is thought that because the Al content was low, decarburization did not proceed in the surface layer even though high dew point annealing was performed, and the ferrite was not stabilized. Therefore, the depth where the C concentration is 0.05% or less and the thickness of the layer where the area ratio of the ferrite phase is 90% or more were not large. As a result, the LME resistance and hydrogen desorption properties were poor.

No. 31 is the steel plate sol. Al content, Si and sol. This is a comparative example in which the sum of the Al contents is small. Steel plate sol. Al content and sol. It is considered that because the sum of the Al contents was small, decarburization did not progress in the surface layer even if high dew point annealing was performed, and the ferrite was not stabilized. Therefore, the depth where the C concentration is 0.05% or less and the thickness of the layer where the area ratio of the ferrite phase is 90% or more were not large. As a result, the LME resistance and hydrogen desorption properties were poor.

No. 32 is the steel plate sol. This is a comparative example with a high content of Al. Steel plate sol. It is thought that because the Al content was high, even if high dew point annealing was performed, external oxidation progressed and oxides (scale) were formed on the surface layer of the steel sheet, suppressing decarburization at the outermost surface. Therefore, the depth at which the C concentration was 0.05% or less was not large. As a result, the LME resistance and hydrogen desorption properties were poor.

No. 33 is Si and sol. This is a comparative example in which the sum of the Al contents is small. Si and sol. of steel plate. It is thought that because the sum of the Al contents was small, decarburization did not proceed in the surface layer even if high dew point annealing was performed, and the ferrite was not stabilized. Therefore, the depth where the C concentration is 0.05% or less and the thickness of the layer where the area ratio of the ferrite phase is 90% or more were not large. As a result, the LME resistance and hydrogen desorption properties were poor.

No. In No. 34, the dew point during annealing was low, so a phase containing oxides such as Si, Mn, and Al was formed on the outside of the steel sheet during annealing, and mutual diffusion of plating components and steel components was inhibited during plating treatment. considered to be a thing. As a result, proper plating could not be obtained.

No. In No. 35, it is thought that because the dew point during annealing was low, a phase containing oxides such as Si, Mn, and Al was formed on the outside of the steel sheet during annealing, and decarburization was not promoted and ferrite was not stabilized. It will be done. Therefore, the depth where the C concentration is 0.05% or less and the thickness of the layer where the area ratio of the ferrite phase is 90% or more were not large.

No. In No. 36, because the dew point during annealing was high, a phase containing oxides such as Si, Mn, and Al was formed on the outside of the steel sheet during annealing, and decarburization was not promoted and ferrite was not stabilized. It will be done. Therefore, the depth where the C concentration is 0.05% or less and the thickness of the layer where the area ratio of the ferrite phase is 90% or more were not large.

No. In No. 37, because the dew point during annealing was high, a phase containing oxides such as Si, Mn, and Al was formed on the outside of the steel sheet during annealing, and mutual diffusion of plating components and steel components was inhibited during plating treatment. considered to be a thing. As a result, proper plating could not be obtained.

No. It is considered that in No. 38, decarburization was not sufficiently promoted during annealing because the holding temperature during annealing was low. Therefore, the depth where the C concentration is 0.05% or less and the thickness of the layer where the area ratio of the ferrite phase is 90% or more were not large. As a result, the LME resistance and hydrogen desorption properties were poor.

No. It is thought that in No. 39, decarburization was not sufficiently promoted during annealing because the holding temperature during annealing was high. Therefore, the depth where the C concentration is 0.05% or less and the thickness of the layer where the area ratio of the ferrite phase is 90% or more were not large. As a result, the LME resistance and hydrogen desorption properties were poor.

No. It is considered that in No. 40, decarburization was not sufficiently promoted during annealing because the holding time during annealing was short. Therefore, the depth where the C concentration is 0.05% or less and the thickness of the layer where the area ratio of the ferrite phase is 90% or more were not large. As a result, the LME resistance and hydrogen desorption properties were poor.

No. It is considered that No. 41 did not carry out the brush grinding in the pretreatment process, so no strain was introduced to the surface of the steel plate, and decarburization did not proceed during annealing. Therefore, the depth where the C concentration is 0.05% or less and the thickness of the layer where the area ratio of the ferrite phase is 90% or more were not large. As a result, the LME resistance and hydrogen desorption properties were poor.

No. No. 42 is considered to be because a brush with a large amount of grinding was used in the pretreatment process, which resulted in increased roughness of the steel plate surface and unstable ferrite phase. Therefore, the thickness of the layer in which the area ratio of the ferrite phase was 90% or more did not increase. As a result, the LME resistance and hydrogen desorption properties were poor.

On the other hand, No. Examples 1 to 25 of the present invention had high LME resistance and hydrogen desorption properties. It was confirmed that examples in which the depth of the C concentration is 0.05% or less and the thickness of the layer where the area ratio of the ferrite phase is 90% or more have particularly excellent LME resistance.

According to the present invention, it is possible to provide high-strength steel sheets and plated steel sheets that have high LME resistance and hydrogen desorption properties, and the steel sheets and plated steel sheets are used for automobiles, home appliances, building materials, etc., especially for automobiles. It can be suitably used for. Therefore, the present invention has extremely high industrial applicability.

1 Steel plate 2 Welded part 11 Cracks just outside the pressure welded part

Claims

A steel plate having a tensile strength of 780 MPa or more,
Chemical components are mass%,
C: 0.05-0.40%,
Si: 0.7-3.0%,
Mn: 0.1 to 5.0%,
sol. Al: 0.7-2.0%,
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: 0-2.00%,
Ni: 0-2.00%,
Cu: 0-2.00%,
Mo: 0-1.00%,
W: 0-1.00%,
Ca: 0-0.100%,
Mg: 0-0.100%,
Zr: 0 to 0.100%,
Hf: 0-0.100%,
REM: 0~0.100%
, the remainder being Fe and impurities,
Si and sol. The total value of Al content is 1.8% or more,
In the depth direction from the steel plate surface, the depth at which the C concentration measured by GDS is 0.05% or less is 10 μm or more,
In the depth direction from the steel plate surface, the thickness of the layer in which the area ratio of the ferrite phase is 90% or more is 20 μm or more,
A steel plate characterized in that the surface roughness of the steel plate is 3.0 μm or less in terms of Ra.
The steel plate according to claim 1, wherein the depth at which the C concentration measured by GDS is 0.05% or less in the depth direction from the surface of the steel plate is 20 μm or more.
The steel plate according to claim 1, wherein the layer in which the area ratio of the ferrite phase is 90% or more has a thickness of 30 μm or more in the depth direction from the surface of the steel plate.
In the depth direction from the surface of the steel plate, the depth at which the C concentration measured by GDS is 0.05% or less is 20 μm or more, and in the depth direction from the surface of the steel plate, the area ratio of the ferrite phase is The steel plate according to claim 1, wherein the thickness of the layer that is 90% or more is 30 μm or more.
The steel sheet according to any one of claims 1 to 4 is provided with an alloyed hot-dip galvanized layer on at least a part of the surface thereof, and the alloyed hot-dip galvanized layer has a content of 0 to 1.5% by mass. An alloyed hot-dip galvanized steel sheet containing Al and 3 to 15% Fe, with the balance being Zn and impurities.
The alloyed hot-dip galvanized steel sheet according to claim 5, wherein the roughness of the interface between the steel sheet and the alloyed hot-dip galvanized layer in the cross section of the alloyed hot-dip galvanized steel sheet is 2.0 μm or less in terms of Ra.