EP4650479A1

EP4650479A1 - Steel sheet and plated steel sheet

Info

Publication number: EP4650479A1
Application number: EP24741594.6A
Authority: EP
Inventors: Takuya MITSUNOBU; Noriyuki Furukawa; Takashi OKE; Hiroshi Takebayashi
Original assignee: Nippon Steel Corp
Current assignee: Nippon Steel Corp
Priority date: 2023-01-13
Filing date: 2024-01-12
Publication date: 2025-11-19
Also published as: CN120530218A; KR20250117811A; EP4650479A4; WO2024150822A1; MX2025008116A; JPWO2024150822A1

Abstract

The present invention has as its object the provision of steel sheet and plated steel sheet having a high LME resistance. The steel sheet and plated steel sheet of the present invention are characterized by having a predetermined chemical composition, having a depth with a C concentration measured by GDS of 0.01% or less is 3 µm or more in a depth direction from the steel sheet surface, having a thickness of a layer with an area ratio of cementite of 10% or less is 5 µm or more in a depth direction from the steel sheet surface, and having a surface roughness of the steel sheet of an arithmetic average roughness Ra of 3.0 µm or less.

Description

FIELD

The present invention relates to steel sheet and plated steel sheet. More specifically, the present invention relates to steel sheet and plated steel sheet having high LME resistance.

BACKGROUND

In recent years, the steel sheet used in automobiles, household electric appliances, building materials, and various other fields has been made increasingly higher in strength. For example, in the automotive field, high strength steel sheet has been increasingly used for the purpose of lightening the weight of car bodies so as to improve fuel economy.
In welding steel sheet giving a Zn-based plating, in particular high strength steel sheet, for example as described in PTL 1, sometimes the drop in weldability due to liquid metal embrittlement (LME) cracking becomes a problem. LME cracking is believed to occur due to the surface layer part of steel sheet transforming to austenite at the time of welding, the molten zinc penetrating the grain boundaries causing the steel sheet to become brittle, and further tensile stress acting on the steel sheet at the time of welding.
Note that, PTL 2 discloses, as steel sheet suppressed in LME cracking and improved in weldability, steel sheet having at its surface layer part particle size 20 nm or more Si oxide particles in a 3000 to 6000/mm² number density present in a suitable particle size distribution.

[CITATION LIST]

[PATENT LITERATURE]

[PTL 1] WO2019/116531
[PTL 2] WO2020/218575

SUMMARY

[TECHNICAL PROBLEM]

To prevent LME cracking, it is effective to keep the Zn etc. contained in the plating layer from penetrating the steel sheet transformed to austenite. There is room for improvement on this point.
The present invention, in consideration of such a situation, has as its object the provision of steel sheet and plated steel sheet having a high LME resistance.

[SOLUTION TO PROBLEM]

The inventors engaged in intensive studies on means for solving the above problem. As a result, they discovered that by using an abrasive material to impart strain at the steel sheet before annealing under suitable conditions to render it a suitable surface state and performing high dew point annealing, the steel sheet surface layer is decarburized and further a layer with a low cementite fraction is formed and as a result, it becomes possible to suppress LME.
The present invention is a result of further studies conducted based on the above findings and has as its gist the following:

(1) A steel sheet with a tensile strength of 780 MPa or more, wherein a chemical composition comprises, by mass%, C: 0.08 to 0.40%, Si: 0.4 to 2.0%, Mn: 0.1 to 5.0%, sol.Al: 0 to 2.0%, P: 0.0300% or less, S: 0.0300% or less, N: 0.0100% or less, B: 0 to 0.0100%, Ti: 0 to 0.1500%, Nb: 0 to 0.150%, V: 0 to 0.150%, Cr: 0 to 2.0%, Ni: 0 to 2.00%, Cu: 0 to 2.0000%, Mo: 0 to 1.00%, W: 0 to 1.000%, Ca: 0 to 0.1000%, Mg: 0 to 0.100%, Zr: 0 to 0.100%, Hf: 0 to 0.100%, an REM: 0 to 0.1000% and a balance of Fe and impurities, in a depth direction from the steel sheet surface, a depth with a C concentration measured by GDS of 0.01% or less is 3 µm or more, in a depth direction from the steel sheet surface, a thickness of a layer with an area ratio of cementite of 10% or less is 5 µm or more, and a surface roughness of the steel sheet is an arithmetic average roughness Ra of 3.0 µm or less.
(2) The steel sheet according to (1), wherein a total value of the contents of Si and sol. Al is less than 1.8%.
(3) The steel sheet according to (1), wherein in a depth direction from the steel sheet surface, a depth with a C concentration measured by GDS of 0.01% or less is 5 µm or more.
(4) The steel sheet according to (1), wherein in a depth direction from the steel sheet surface, a depth with a C concentration measured by GDS of 0.01% or less is 7 µm or more.
(5) The steel sheet according to (1), wherein a surface roughness of the steel sheet is an arithmetic average roughness Ra of 2.0 µm or less.
(6) A plated steel sheet comprising the steel sheet according to any one of (1) to (5) and a plating layer containing Zn at least at part of the surface of the steel sheet.

[ADVANTAGEOUS EFFECTS OF INVENTION]

According to the present invention, it is possible to obtain steel sheet and plated steel sheet having a high LME resistance.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a view showing a layer formed at a surface layer of the steel sheet of the present invention.
FIG. 2 is a view explaining evaluation of the LME resistance in the examples.

DESCRIPTION OF EMBODIMENTS

Below, the present invention will be explained. The present invention is not limited to the following embodiments. First, in the steel sheet of the present invention, the configuration for improving the LME resistance will be explained in outline.
If spot welding plated steel sheet, the plating melts and the surface layer part of the steel sheet is heated whereby the steel sheet surface transforms to austenite. At that time, the molten plating penetrates the steel sheet structure along the grain boundaries of the austenite and the crystal grain boundaries become brittle. For this reason, if stress is applied to the steel sheet at the time of welding, LME cracking easily occurs at the crystal grain boundaries. In particular, if tensile stress is applied to the steel sheet at the time of welding, LME is believed to easily occur. The steel sheet of the present invention is improved in LME resistance by the structure formed at the surface layer of the steel sheet. Note that, in this Description, the "surface layer of the steel sheet" means the range from the surface of the steel sheet to a depth of 100 µm.
If the C element is contained in the steel sheet surface layer, LME cracking easily occurs, therefore keeping the C concentration of the steel sheet surface layer low is effective for preventing LME cracking. Normally, if heating steel sheet like with annealing, external oxidation occurs and oxides (scale) are formed at the steel sheet surface, therefore decarburization becomes difficult. For this reason, the C concentration at the steel sheet surface layer is difficult to reduce. On the other hand, in the steel sheet of the present invention, in the depth direction from the steel sheet surface, the depth at which the C concentration measured by GDS is 0.01% or less is 3 µm or more. This means that the concentration of C, which is an element causing easy occurrence of LME, at the surface layer of the steel sheet is low.
However, even if the C concentration at the steel sheet surface layer is low, if C remains as carbides (cementite), the molten plating penetrates the inside of the steel sheet along the grain boundaries of cementite and is liable to become the starting point of LME cracking. For this reason, if the C at the steel sheet surface layer is present as cementite, it is believed that this is liable to lead to a drop in the LME resistance. Therefore, in the steel sheet of the present invention, the thickness of the layer with an area ratio of cementite of 10% or less in the depth direction from the steel sheet surface is 5 µm or more. That is, the steel sheet of the present invention improves the LME resistance by controlling the form of the C at the surface layer.
In this way, the fact that to lower the area ratio of the cementite at the surface layer, control of the dew point at the time of annealing in addition to imparting strong strain to the surface layer of the steel sheet for annealing is important was discovered by the inventors. In the present invention, by imparting strong strain to the surface layer of the steel sheet without increasing surface roughness, it is possible to promote diffusion of oxygen to the inside of the steel sheet and lower the C concentration at the surface layer of the steel sheet. Further, the fact that by making the C concentration of the surface layer of the steel sheet and the area ratio of the cementite as explained, the LME resistance is improved was discovered by the inventors and led to the completion of the present invention.
Below, the present invention will be explained in detail.
First, the chemical composition of the steel sheet will be explained. Below, the "%" relating to the chemical composition shall mean "mass%". Further, in the numerical ranges in the chemical constituents, numerical ranges expressed using "to" mean ranges including the numerical values described before and after the "to" as lower limit values and upper limit values.

(C: 0.08 to 0.40%)

C (carbon) is an element securing the strength of steel. To obtain the 780 MPa or more tensile strength covered by the present invention, considering the balance with the weldability and so that the concentration of C at the surface layer of the steel sheet does not become too high, the content of C is 0.08 to 0.40%. If the content of C is too large, the C concentration of the surface layer and the cementite fraction will no longer become lower even by the later explained high dew point annealing. The content of C may also be 0.10% or more, 0.12% or more, or 0.15% or more. The content of C may also be 0.35% or less, 0.30% or less, or 0.25% or less.

(Si: 0.4 to 2.0%)

Si (silicon) is an element promoting ferrite stabilization and decarburization. By Si being contained, due to the later explained pretreatment and heat treatment, decarburization at the surface layer part proceeds and ferrite at the surface layer part stabilizes, whereby the LME resistance is improved. To obtain this effect, the content of Si is 0.4 to 2.0%. If the content of Si is too great, even if performing high dew point annealing, external oxidation proceeds, oxides (scale) are formed at the surface layer of the steel sheet, conversely decarburization at the surface is suppressed, and the effect of improvement of the LME resistance becomes smaller. The content of Si may also be 0.5% or more, 0.6% or more, 0.7% or more, or 0.8% or more. The content of Si may also be 1.8% or less, 1.6% or less, 1.4% or less, or 1.2% or less.

(Sol. Al: 0 to 2.0%)

Al (aluminum) is an element forming a solid solution in the steel and thereby, like Si, promoting ferrite stabilization and decarburization. "sol. Al" means acid soluble Al not forming Al₂ O₃ or other oxides and able to dissolve in acid and is found as the Al measured in the process of analysis of Al when excluding insoluble residue on the filter paper. In the steel sheet of the present invention, the role of sol. Al can also be obtained by inclusion of Si, therefore sol. Al is not essential. The lower limit of the content of sol. Al is 0%. If the content of sol. Al is too great, even if performing high dew point annealing, external oxidation proceeds, oxides (scale) are formed at the surface layer of the steel sheet, conversely decarburization at the surface part is suppressed, and the effect of improvement of the LME resistance becomes smaller, so the content of sol. Al is 2.0% or less. The content of sol. Al may also be 0.1% or more, 0.2% or more, or 0.3% or more. The content of sol. Al may also be 1.5% or less, 1.2% or less, or 1.0% or less.
Note that, Si and Al are also elements causing a drop in the LME resistance, therefore the total value of contents of Si and sol. Al is preferably less than 1.8%. The total of the contents of Si and sol. Al may also be less than 1.7% or less than 1.6%.

(Mn: 0.1 to 5.0%)

Mn (manganese) is an element forming hard structures and therefore is effective for improving the strength of the steel. Considering the balance of the strength of the steel and the drop in formability due to Mn segregation, the content of Mn is 0.1 to 5.0%. The content of Mn may also be 0.5% or more, 1.0% or more, or 1.5% or more. The content of Mn may also 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 content of P is more than 0.0300%, the weldability is liable to fall. Therefore, the content of P is 0.0300% or less. The content of P may also be 0.0200% or less, 0.0100% or less, or 0.0050% or less. P is preferably not contained. The lower limit of the content of P is 0%. From the viewpoint of the dephosphorization costs, the content of P may also 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 content of S is more than 0.0300%, the weldability is liable to fall and, further, the amount of precipitation of MnS is liable to increase and the bendability and other formability are liable to fall. Therefore, the content of S is 0.0300% or less. The content of S may also be 0.0100% or less, 0.0050% or less, or 0.0020% or less. S is preferably not contained. The lower limit of the content of S is 0%. From the viewpoint of the desulfurization costs, the content of S may also 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 content of N is more than 0.0100%, the weldability is liable to fall. Therefore, the content of N is 0.0100% or less. The content of N may also be 0.0080% or less, 0.0050% or less, or 0.0030% or less. N is preferably not contained. The lower limit of the content of N is 0%. From the viewpoint of production costs, the content of N may also be more than 0%, 0.0001% or more, 0.0005% or more, or 0.0010% or more.

(B: 0 to 0.0100%)

B (boron) is an element raising the hardenability and contributing to improvement of the strength and segregating at the grain boundaries to strengthen the grain boundaries and improve toughness, therefore may be included in accordance with need. It is not an essential element, therefore the lower limit of the content of B is 0%. The effect is obtained even with inclusion in a trace amount. The content of B if included is preferably 0.0001% or more. The content of B may also be 0.0002% or more or 0.0003% or more. On the other hand, from the viewpoint of securing sufficient toughness, the content of B is 0.0100% or less. The content of B may also be 0.0090% or less, 0.0080% or less, 0.0060% or less, 0.0040% or less, 0.0030% or less, or 0.0020% or less.

(Ti: 0 to 0.1500%)

Ti (titanium) is an element precipitating as TiC during cooling of steel and contributing to improvement of strength, therefore may be included in accordance with need. It is not an essential element, therefore the lower limit of the content of Ti is 0%. The effect is obtained even with inclusion in a trace amount. The content of Ti if included is preferably 0.0001% or more. The content of Ti may also be 0.0002% or more or 0.0003% or more. On the other hand, if excessively contained, coarse TiN is liable to be formed and the toughness is liable to be reduced, therefore the content of Ti is 0.1500% or less. The content of Ti may also be 0.1350% or less, 0.1200% or less, 0.0900% or less, 0.0600% or less, 0.0450% or less, 0.0300% or less, 0.0150% or less, 0.0050% or less, 0.0030% or less, or 0.0020% or less.

(Nb: 0 to 0.150%)

Nb (niobium) is an element contributing to improvement of strength through improvement of hardenability therefore may be included in accordance with need. It is not an essential element, therefore the lower limit of the content of Nb is 0%. The effect is obtained even with inclusion in a trace amount. The content of Nb if included is preferably 0.001% or more. The content of Nb may also be 0.002% or more, 0.004% or more, 0.006% or more, or 0.007% or more. On the other hand, from the viewpoint of securing sufficient toughness, the content of Nb is 0.150% or less. The content of Nb may also be 0.135% or less, 0.120% or less, 0.095% or less, 0.065% or less, 0.050% or less, 0.030% or less, or 0.020% or less.

(V: 0 to 0.150%)

V (vanadium) is an element contributing to improvement of strength through improvement of hardenability therefore may be included in accordance with need. It is not an essential element, therefore the lower limit of the content of V is 0%. The effect is obtained even with inclusion in a trace amount. The content of V if included is preferably 0.001% or more. The content of V may also be 0.003% or more, 0.005% or more, or 0.006% or more. On the other hand, from the viewpoint of securing sufficient toughness, the content of V is 0.150% or less. The content of V may also be 0.135% or less, 0.120% or less, 0.095% or less, 0.065% or less, 0.050% or less, 0.045% or less, 0.025% or less, or 0.020% or less.

(Cr: 0 to 2.00%)

Cr (chromium) is effective for raising the hardenability to raise the strength of the steel, therefore may be included in accordance with need. It is not an essential element, therefore the lower limit of the content of Cr is 0%. The effect is obtained even with inclusion in a trace amount. The content of Cr if included is preferably 0.001% or more. The content of Cr may also be 0.01% or more, 0.02% or more, 0.04% or more, 0.06% or more, or 0.07% or more. On the other hand, if excessively contained, Cr carbides are formed in large amounts and conversely the hardenability is liable to be impaired, therefore the content of Cr is 2.00% or less. The content of Cr may also be 1.80% or less, 1.60% or less, 1.25% or less, 0.85% or less, 0.65% or less, 0.50% or less, 0.30% or less, or 0.20% or less.

(Ni: 0 to 2.00%)

Ni (nickel) is effective for raising the hardenability to raise the strength of the steel, therefore may be included in accordance with need. It is not an essential element, therefore the lower limit of the content of Ni is 0%. The effect is obtained even with inclusion in a trace amount. The content of Ni if included is preferably 0.001% or more. The content of Ni may also be 0.01% or more, 0.03% or more, 0.04% or more, or 0.05% or more. On the other hand, excessive addition of Ni causes a rise in cost, therefore the content of Ni is 2.00% or less. The content of Ni may also be 1.80% or less, 1.60% or less, 1.25% or less, 0.85% or less, 0.65% or less, 0.40% or less, 0.25% or less, or 0.15% or less.

(Cu: 0 to 2.0000%)

Cu (copper) is effective for raising the hardenability to raise the strength of the steel, therefore may be included in accordance with need. It is not an essential element, therefore the lower limit of the content of Cu is 0%. The effect is obtained even with inclusion in a trace amount. The content of Cu if included is preferably 0.0001% or more. The content of Cu may also be 0.0002% or more or 0.0004% or more. On the other hand, from the viewpoint of suppressing a drop of toughness or cracking of a slab after casting or a drop in weldability, the content of Cu is 2.0000% or less. The content of Cu may also be 1.8000% or less, 1.6000% or less, 1.2000% or less, 0.8000% or less, 0.6000% or less, 0.4000% or less, 0.2000% or less, 0.1000% or less, 0.0070% or less, 0.0050% or less, 0.0035% or less, 0.0020% or less, or 0.0015% or less.

(Mo: 0 to 1.00%)

Mo (molybdenum) is effective for raising the hardenability to raise the strength of the steel, therefore may be included in accordance with need. It is not an essential element, therefore the lower limit of the content of Mo is 0%. The effect is obtained even with inclusion in a trace amount. The content of Mo if included is preferably 0.001% or more. The content of Mo may also be 0.01% or more, 0.03% or more, 0.05% or more, or 0.06% or more. On the other hand, from the viewpoint of suppressing a drop in toughness, the content of Mo is 1.00% or less. The content of Mo may also be 0.90% or less, 0.80% or less, 0.65% or less, 0.45% or less, 0.35% or less, 0.30% or less, or 0.20% or less.

(W: 0 to 1.000%)

W (tungsten) is effective for raising the hardenability to raise the strength of the steel, therefore may be included in accordance with need. It is not an essential element, therefore the lower limit of the content of W is 0%. The effect is obtained even with inclusion in a trace amount. The content of W if included is preferably 0.001% or more. The content of W may also be 0.002% or more or 0.003% or more. On the other hand, from the viewpoint of suppression of a drop in toughness, the content of W is 1.000% or less. The content of W may also be 0.900% or less, 0.800% or less, 0.600% or less, 0.400% or less, 0.300% or less, 0.200% or less, 0.100% or less, 0.050% or less, 0.025% or less, 0.015% or less, or 0.010% or less.

(Ca: 0 to 0.1000%)

Ca (calcium) is an element contributing to control of inclusions, in particular the fine dispersion of inclusions and having the action of raising the toughness, therefore may be included in accordance with need. It is not an essential element, therefore the lower limit of the content of Ca is 0%. The effect is obtained even with inclusion in a trace amount. The content of Ca if included is preferably 0.0001% or more. The content of Ca may also be 0.0002% or more or 0.0003% or more. On the other hand, if excessively contained, deterioration of the surface properties sometimes surfaces, therefore the content of Ca is 0.1000% or less. The content of Ca may also be 0.0900% or less, 0.0800% or less, 0.0600% or less, 0.0400% or less, 0.0300% or less, 0.0200% or less, 0.0100% or less, 0.0050% or less, 0.0025% or less, 0.0015% or less, or 0.0010% or less.

(Mg: 0 to 0.100%)

Mg (magnesium) is an element having the action of contributing to control of inclusions, in particular fine dispersion of the inclusions and raising the toughness, therefore may be included in accordance with need. It is not an essential element, therefore the lower limit of the content of Mg is 0%. The effect is obtained even with inclusion in a trace amount, but the content of Mg if included is preferably 0.0001% or more. The content of Mg may also be 0.0002% or more, 0.0003% or more, or 0.0005% or more. On the other hand, if excessively contained, deterioration of the surface properties sometimes surfaces, therefore the content of Mg is 0.100% or less. The content of Mg may also be 0.090% or less, 0.080% or less, 0.060% or less, 0.040% or less, 0.030% or less, 0.020% or less, 0.010% or less, 0.005% or less, 0.003% or less, or 0.002% or less.

(Zr: 0 to 0.100%)

Zr (zirconium) is an element having the action of contributing to control of inclusions, in particular fine dispersion of the inclusions and raising the toughness, therefore may be included in accordance with need. It is not an essential element, therefore the lower limit of the content of Zr is 0%. The effect is obtained even with inclusion in a trace amount. The content of Zr if included is preferably 0.001% or more. The content of Zr may also be 0.003% or more, 0.006% or more, 0.009% or more, or 0.010% or more. On the other hand, if excessively contained, deterioration of the surface properties sometimes surfaces, therefore the content of Zr is 0.100% or less. The content of Zr may also be 0.090% or less, 0.085% or less, 0.065% or less, 0.050% or less, 0.040% or less, or 0.030% or less.

(Hf: 0 to 0.100%)

Hf (hafnium) is an element having the action of contributing to control of inclusions, in particular fine dispersion of the inclusions and raising the toughness, therefore may be included in accordance with need. It is not an essential element, therefore the lower limit of the content of Hf is 0%. The effect is obtained even with inclusion in a trace amount. The content of Hf if included is preferably 0.0001% or more. The content of Hf may also be 0.0002% or more, 0.0003% or more, or 0.0005% or more. On the other hand, if excessively contained, deterioration of the surface properties sometimes surfaces, therefore the content of Hf is 0.100% or less. The content of Hf may also be0.090% or less, 0.080% or less, 0.060% or less, 0.040% or less, 0.030% or less, 0.020% or less, 0.010% or less, 0.005% or less, 0.003% or less, or 0.002% or less.

(REM: 0 to 0.1000%)

An REM (rare earth metal) is an element having the action of contributing to control of inclusions, in particular fine dispersion of the inclusions and raising the toughness, therefore may be included in accordance with need. It is not an essential element, therefore the lower limit of the content of an REM is 0%. The effect is obtained even with inclusion in a trace amount. The content of REM if included is preferably 0.0001% or more. The content of REM may also be 0.0003% or more, 0.0004% or more, or 0.0005% or more. On the other hand, if excessively contained, deterioration of the surface properties sometimes surfaces, therefore the content of an REM is 0.1000% or less. The content of an REM may also be 0.0900% or less, 0.0800% or less, 0.0600% or less, 0.0400% or less, 0.0300% or less, 0.0200% or less, 0.0100% or less, 0.0040% or less, 0.0025% or less, or 0.0015% or less. Note that, REM an abbreviation of "rare earth metal" and means elements belonging to the lanthanides. An REM is usually added as a misch metal.
In the steel sheet of the present invention, the balance besides the above chemical composition is comprised of Fe and impurities. Here, the "impurities" are constituents entering due to the ore, scrap, or other raw materials and other various factors in the production process when industrially producing steel sheet and not having a detrimental effect on the LME resistance of the steel sheet according to the present invention, that is, is contained in an extent where the LME resistance sought in the steel sheet of the present invention is obtained.
The chemical composition of the steel sheet may be analyzed using an elemental analysis method known to persons skilled in the art. For example, it is analyzed by inductively coupled plasma-mass spectrometry (ICP-MS method). However, C and S may be measured using the combustion-infrared absorption method, while N can be measured using the inert gas melting-thermal conductivity method. These analyses may be performed on a sample taken from the steel sheet by a method based on JIS G0417: 1999.
Next, the surface layer part of the steel sheet will be explained.

[C Concentration]

In the steel sheet of the present invention, in the depth direction from the surface of the steel sheet, the depth where the C concentration measured by GDS (glow discharge spectrometry) is 0.01% or less is 3 µm or more.
The sensitivity to LME becomes lower if the C concentration becomes lower, therefore if the C concentration of the surface layer is low, the LME resistance is improved. Further, C is an element stabilizing austenite, therefore by this being low, the layer with a low LME sensitivity, explained later, stabilizes.
Such a surface layer structure can be obtained by making the chemical composition of the steel sheet the one explained above and performing the later explained pretreatment and heat treatment.
If the depth at which the C concentration is 0.01% or less is 3 µm or more, the effect of improvement of the LME resistance is obtained, therefore the upper limit of the depth is not limited. The depth at which the C concentration is 0.01% or less may also be for example 50 µm or less, 40 µm or less, or 30 µm or less. The depth at which the C concentration is 0.01% or less may also be for example 5 µm or more, 7 µm or more, 10 µm or more, 15 µm or more, or 20 µm or more.
GDS measurement is performed five times in the sheet thickness direction and the average value of these is made the C concentration. The measurement conditions are made the following. The starting point of "depth" in the case of steel sheet which is not plated is the surface of the steel sheet or is the interface of the steel sheet and plating layer in the case of steel sheet which is plated. The interface of the steel sheet and the plating layer is made the position where the concentration of Fe measured by GDS measurement becomes 93% of the concentration of Fe at the depth of 150 µm.

Apparatus: high frequency glow discharge optical emission spectrometry (made by LECO Japan Corporation, Model No. "GDS850A"
Ar gas pressure: 0.3 MPa
Anode size: 4 mmφ
RF output: 30W
Measurement time: 200 to 1500 seconds

[Low Cementite Layer]

In the steel sheet of the present invention, in a depth direction from the steel sheet surface, the thickness of the layer with an area ratio of cementite of 10% or less (below, referred to as the "low cementite layer") is 5 µm or more. FIG. 1 is a structural photograph by an SEM capturing the vicinity of the surface layer of the steel sheet of the present invention by a power of 1000X. FIG. 1 is a cross-section parallel to the thickness direction of the steel sheet. The top side of the figure is the front surface of the steel sheet. The steel sheet surface layer has a low cementite layer 11 comprised of a layer which is low in C concentration, is mainly comprised of ferrite, and has an area ratio of cementite of 10% or less. At the inside of the steel sheet from the low cementite layer 11 (bottom side of drawing), there are hard structures 12 including a relatively large amount of cementite. The ferrite and the cementite can be judged by the difference in brightness at the SEM image. When classifying the steel sheet part of FIG. 1 into parts with a relatively high brightness (bright parts) and parts with a relatively low brightness (dark parts), the parts with relatively high brightness can be judged to be cementite. The parts with relatively low brightness can be judged to be ferrite.
Cementite easily segregates at the grain boundaries, therefore if cementite increases, the molten plating penetrates the grain boundaries and LME easily occurs. Therefore, by making the low cementite layer thickly present, even if the plating melts, LME hardly occurs and the LME resistance can be improved. Such a surface layer structure can be obtained by making the chemical composition of the steel sheet one as explained above and performing the later explained pretreatment and heat treatment.
If the thickness of the low cementite layer is 5 µm or more, the effect of improvement of the LME resistance is obtained, therefore the upper limit of the thickness is not particularly prescribed. The thickness of the low cementite layer may also be, for example, 50 µm or less, 40 µm or less, or 30 µm or less. The thickness of the low cementite layer may be 10 µm or more or 20 µm or more.
The structure other than the cementite of the low cementite layer is not limited. For example, it may be made one or more of any of martensite, bainite, and ferrite. Ferrite is low in LME sensitivity, so being a structure mainly comprised of ferrite is preferable from the viewpoint of the improvement of the LME resistance.
The thickness of the low cementite layer is found by etching by Nital the C-cross-section of the steel sheet (sheet thickness cross-section parallel to rolling direction (L-direction)) and examining a 50 µm×50 µm field containing the surface layer of the steel sheet at a power of 1000X. From the state of the structure on the SEM image obtained by examination under an SEM, it is possible to differentiate between the martensite, bainite, and other hard structures containing relatively large amounts of cementite from ferrite. The thickness of the low cementite layer is measured in a 500 µm measurement range in the L-direction. Five measurement ranges are set at 1000 µm intervals in the L-direction. In the five measurement ranges, the average value of the thicknesses of the low cementite layers in the sheet thickness direction is found. Here, the "area ratio of cementite" means the area ratio found by examining the C-cross-section. If examining locally the L-cross-section (sheet thickness cross-section perpendicular to rolling direction) in the middle of the thickness direction, even if there is a location with a cross-section of the cementite of more than 10%, there is no problem if the area ratio of the cementite is 10% or less in the C-cross-section at depth down to 5 µm.
The steel sheet of the present invention can have a plating layer as explained later. If having a plating layer, the starting points of the depth with a C concentration of 0.01% or less by GDS measurement and the thickness of the layer with a cementite area ratio of 10% or less are the interface of the steel sheet and plating layer.

[Surface Roughness]

In the steel sheet of the present invention, the surface roughness is an arithmetic average roughness Ra defined by JIS B0601: 2013 of 3.0 µm or less. If the roughness becomes larger, stress concentration causes cracking to more easily occur, therefore the LME resistance falls. In the case of having the later explained plating layer, this roughness is the roughness of the interface of the steel sheet and plating layer.

[Tensile Strength]

The present invention suppresses the LME which occurs in high strength steel sheet. High strength steel sheet is specifically steel sheet having a 780 MPa or more tensile strength. The upper limit of the tensile strength is not particularly prescribed, but from the viewpoint of securing toughness, it may for example be 2000 MPa or less. The tensile strength may be measured by taking a JIS No. 5 tensile test piece having a direction perpendicular to the rolling direction and sheet thickness direction as a long direction and performing a test based on JIS Z2241:2011. 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 also be 1900 MPa or less or 1800 MPa or less.
If the rolling direction of the steel sheet is not clear, as the method of identifying the rolling direction of the steel sheet, for example, the following method is employed. The sheet thickness cross-section of the steel sheet is polished to a mirror surface to finish it, then an electron probe micro analyzer (EPMA) is used to measure the S concentration. The measurement conditions are made an acceleration voltage of 15 kV and a measurement pitch of 1 µm. An image of distribution of a range of 500 µm square at the center part of sheet thickness is measured. At this time, a stretched region with a high S concentration is judged as an MnS or other inclusion. At the time of examination, a plurality of fields may also be examined. Next, based on the sheet thickness cross-section first examined by the above method, surfaces parallel to surfaces rotated at 5° increments in the 0° to 180° range about the sheet thickness direction are examined in cross-section by the above method. The average value of the lengths of the long axes of the plurality of inclusions at each obtained cross-section is calculated for each cross-section and the cross-section giving the largest average value of the long axes of the inclusions is identified. The direction parallel to the long axis directions of the inclusions at that cross-section is judged to be the rolling direction.
Note that, when a test piece for measurement of the tensile strength cannot be taken from the steel sheet forming a welded joint, alternatively, the hardness (Vickers hardness) of the steel sheet at a non-heat affected zone at a distance of 5 mm or more from an outside end of the nugget of the spot welded part can be measured and the value of the tensile strength may be estimated from the following correlation formula (Correlation Between Static Strength Parameters, Fumihiko Hasegawa, Junichi Arai, Tsuneshichi Tanaka,"Materials", Vol. 39, No. 442, P. 859 to 863). Here, a "heat affected zone" is a not melted part of the steel sheet at which welding heat causes a change in the structure, metallurgical properties, mechanical properties, etc. while a "non-heat affected zone" is a part other than a heat affected zone. A portion 5 mm or more separated from the outside end of the nugget of the spot welded part may be judged to be a non-heat affected zone. $Hv = 0.301 \times TS + 5.701$ (where, Hv is the Vickers hardness and TS is the tensile strength (unit: MPa))
That is, if the hardness if 240 Hv or so or more, the tensile strength may be deemed 780 MPa or more.

[Plating Layer]

The steel sheet according to the present invention has a plating layer containing Zn on the steel sheet according to the present invention explained above. The plating layer is formed on at least part of the surface of the steel sheet. It may be formed on one surface of the steel sheet or may be formed on both surfaces. The plating layer may be alloyed.

[Chemical Composition of Plating Layer]

The chemical composition of the plating layer is not limited so long as containing Zn. The plating layer containing Zn may be 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.
The chemical composition of the plating layer can be determined by dissolving the plating layer in an acid solution to which an inhibitor is added for suppressing corrosion of the steel sheet and measuring the obtained solution by ICP (inductively coupled plasma) emission spectrometry. The acid solution to which the inhibitor is added may for example be a 10 mass% hydrochloric acid solution to which 0.06 mass% of an inhibitor (ibit made by Asahi Chemical Corporation) is added.
The thickness of the plating layer may for example be 3 to 50 µm. Further, the amount of deposition of the plating layer is not particularly limited, but for example may be 10 to 170 g/m² per side. In the present invention, the amount of deposition of the plating layer is determined from the change in weight before and after dissolving the plating layer in an acid solution to which an inhibitor for suppressing corrosion of the steel sheet is added and pickling and removal of the plating layer. 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 also be 40 µm or less or 30 µm or less. The amount of deposition of the plating layer may be 20 g/m² or more, 30 g/m² or more, 40 g/m² or more, or 50 g/m² or more per side. The amount of deposition of the plating layer may also be 150 g/m² or less, 130 g/m² or less, 120 g/m² or less, or 100 g/m² or less per side.
The roughness of the interface of the steel sheet and the plating layer becomes the roughness of the surface of the above-mentioned steel sheet, so is an arithmetic average roughness Ra of 3.0 µm or less. If considering adhesion of the plating, Ra may be 2.5 µm or less or 2.0 µm or less. The roughness of the interface of the steel sheet and plating layer may be made the surface roughness of the steel sheet measured after dissolving away the plating.
Note that the steel sheet of the present invention can realize the effect of improvement of the LME resistance even if not provided with zinc plating. In general, if spot welding together steel sheets which are not zinc plated, LME cracking will never occur unless there is contact with melted zinc in the vicinity of the spot welded part. However, if spot welding one steel sheet which is zinc plated and another steel sheet which is not zinc plated, at the time of welding, melted zinc will be formed at the overlaid surfaces of the steel sheets. For this reason, the melted zinc may possibly contact the surface of the steel sheet which is not plated and cause LME cracking. Further, if using a welding electrode on which melted zinc had deposited when spot welding a steel sheet provided with zinc plating so as to spot weld another steel sheet which is not plated, the zinc plating deposited on the welding electrode will sometimes contact the surface of that other steel sheet and cause LME cracking. If using the steel sheet of the present invention as the steel sheet which has not been plated, even if melted zinc may contact it at the time of spot welding such as explained above, the C concentration of the surface layer is low and the surface layer is a low cementite layer, so LME cracking can be suppressed.
If superposing a plurality of steel sheets and joining them by spot welding to prepare a welded joint, by using the steel sheet of the present invention as the steel sheet of the outermost layer, it is possible to suppress the occurrence of LME cracking at the outermost layer of the welded joint. LME cracking at the outermost layer of a welded joint can occur in the case where the steel sheet of the outermost layer is high strength steel sheet with a high C concentration and has zinc plating at the surface layer side, the case where melted zinc deposits at a welding electrode, etc. As LME cracking at the outermost layer of a welded joint, for example, there are cracking at an indentation part due to a welding electrode (cracking right under welding electrode), cracking at a slanted part (shoulder part) formed at a peripheral edge part of an indentation part (cracking of weld shoulder part), etc. These can be suppressed by using the steel sheet of the present invention as the steel sheet of the outermost layer.
Further, in the above-mentioned such welded joint, even in the case where at the surfaces where the steel sheets are overlaid (overlaid surfaces), one steel sheet has zinc plating and the other steel sheet is high strength steel sheet not having zinc plating, by using the steel sheet of the present invention as that high strength steel sheet, LME cracking can be suppressed. As the LME cracking at such overlaid surfaces, for example, there are cracking at a part near the outside of the part where the steel sheets are pressure contacted by the spot welding (cracking right outside pressure contact zone) etc. These can be suppressed by using the steel sheet of the present invention as the above-mentioned high strength steel sheet.
The thickness of the steel sheet and plated steel sheet of the present invention is not particularly limited. For example, it can be 0.6 to 3.2 mm. The sheet thickness may also be 0.8 mm or more or 1.0 mm or more. The sheet thickness may also 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, the method of production of the steel sheet according to the present invention will be explained.
The steel sheet according to the present invention can, for example, be obtained by a method of production comprising a casting step of casting molten steel adjusted in chemical composition to form a steel slab, a hot rolling step of hot rolling the steel slab to obtain hot rolled steel sheet, a coiling step of coiling the hot rolled steel sheet, a cold rolling step of cold rolling the coiled hot rolled steel sheet to obtain cold rolled steel sheet, a pretreatment step of shot blasting the cold rolled steel sheet, and an annealing step of annealing the pretreated cold rolled steel sheet. Alternatively, after the hot rolling step, it is also possible to not coil up the hot rolled steel sheet but to pickle it and cold roll it as is.

[Casting Step]

The conditions of the casting step are not particularly limited. For example, after smelting by a blast furnace, electric furnace, etc., various secondary refining operations may be performed, then casting may be performed by a method such as usual continuous casting, casting by the ingot method, etc.

[Hot Rolling Step]

The steel slab obtained by casting can be hot rolled to obtain hot rolled steel sheet. The hot rolling step is performed by hot rolling the cast steel slab directly or after cooling once, then reheating. If reheating, the heating temperature of the steel slab may be, for example, 1100 to 1250°C. In the hot rolling step, usually rough rolling and finish rolling are performed. The temperatures and reduction rates of the rolling operations may be suitably changed in accordance with the desired microstructure and sheet thickness. For example, the end temperature of the finish rolling may be 900 to 1050°C, and the rolling reduction of the finish rolling may be 10 to 50%.

[Coiling Step]

The hot rolled steel sheet can be coiled at a predetermined temperature. The coiling temperature may be suitably changed in accordance with the desired microstructure etc. and for example may be 500 to 800°C. It is also possible apply predetermined heat treatment to hot rolled steel sheet before coiling or after coiling, then uncoiling. Alternatively, it is also possible to not perform coiling, but pickle the hot rolled steel sheet after the hot rolling step and then perform the later explained cold rolling.

[Cold Rolling Step]

After pickling etc. the hot rolled steel sheet, the hot rolled steel sheet can be cold rolled to obtain cold rolled steel sheet. The reduction rate of the cold rolling may be suitably changed in accordance with the desired microstructure and sheet thickness. For example, it may be 20 to 80%. After the cold rolling step, for example, the steel sheet may be air cooled to cool it down to room temperature.

[Pretreatment Step]

To obtain the structure of the surface layer of the steel sheet explained above, predetermined pretreatment is performed, then annealing has to be performed.
The pretreatment includes using a spherical abrasive to shot blast the surface of the cold rolled steel sheet. The abrasive which can be used is not particularly limited, but for example it is possible to use steel balls with a center particle size of 40 to 450 µm, preferably 120 to 420 µm, more preferably 180 to 350 µm. For example, TSH30 made by WINOA IKK JAPAN may be mentioned. The blasting amount of the shot blasting may be 5 to 400 kg/m². Due to this, it is possible to introduce strain without increasing the roughness of the surface. By performing such shot blasting, in the later explained annealing step, decarburization is promoted and a structure reduced in cementite can be efficiently formed at the surface layer of the steel sheet. Note that, the amount blasted per unit time/unit area on the level of 400 kg/m² is 4.0×10^{- 4} kg/(mm² ·min). The roughness of the surface of the steel sheet after pretreatment is preferably an arithmetic average roughness Ra of 3.0 µm or less. The roughness may also be an Ra of 2.5 µm or less or 2.0 µm or less. Note that the surface roughness of the steel sheet after pretreatment is maintained in the steel sheet and plated steel sheet according to the present invention after the later explained annealing step and plating step (including alloying step).

[Annealing Step]

After the pretreatment, the cold rolled steel sheet is annealed. The annealing is performed in a state applying a 1 to 20 MPa tension. If applying tension at the time of annealing, the steel sheet can be more effectively be given strain and decarburization of the surface layer is promoted.
The holding temperature of the annealing step is 750 to 900°C. The holding temperature may be 770 to 870°C. By setting such a range, decarburization is promoted, the C concentration of the surface layer is lowered, and the cementite can be reduced. The speed of temperature rise to the holding temperature is not particularly limited, but may be 1 to 10°C/s.
The holding time at the holding temperature of the annealing step is 40 to 300 seconds. The holding time may also be 50 to 250 seconds. By setting such a range, decarburization is promoted, the C concentration of the surface layer is lowered, and the cementite can be reduced.
The atmosphere of the annealing step is made the dew point-30 to 20°C. The dew point may also be -10 to 5°C. The atmosphere may, for example, be N₂-1 to 10 vol%H₂ or N₂-2 to 4 vol%H₂ . By making it such a range, decarburization is promoted, the C concentration of the surface layer is lowered, and cementite can be reduced. Further, if the dew point is too high or too low, at the outside part of the steel sheet, a layer including oxides of Si, Mn, Al, etc. is formed, decarburization is no longer promoted, and mutual diffusion of the plating constituents and the steel constituents is obstructed and the plateability sometimes becomes insufficient.
Due to the method of production provided with the above-mentioned steps, at the surface layer of the steel sheet, decarburization is promoted and steel sheet reduced in cementite can be obtained.

The plated steel sheet according to the present invention can be obtained by a method of production provided with a plating step for forming a plating layer on the steel sheet produced as explained above.
The plating step may be performed by the hot dip coating method known to persons skilled in the art. The conditions of the plating step may be suitably set considering the chemical composition, thickness, amount of deposition, etc. of the desired plating layer. For example, the sheet may be dipped in a 420 to 480°C hot dip galvanization bath adjusted in chemical composition for 1 to 10 seconds, pulled out after dipping by 20 to 200 mm/s, and wiped by N₂ gas to control the amount of plating deposition. After the plating step, a known alloying step may be performed to alloy the plating. The alloying may be performed at 500 to 550°C for 10 to 60 seconds.
The steel sheet and plated steel sheet according to the present invention are high strength and have high LME resistance, therefore can be suitably used in automobiles, household electric appliances, building materials, and other broad fields. In particular, they are preferably used in the automobile field. Steel sheet and plated steel sheet used for automobiles are often spot welded. In that case, LME cracking can become a remarkable problem. For this reason, if using the steel sheet and plated steel sheet according to the present invention as steel sheet for automotive use, the effect of the present invention of giving high LME resistance is optimally realized.

EXAMPLES

Below, examples will be used to explain the present invention in more detail. The present invention is not limited to these examples.

(Preparation of Steel Sheet Sample)

Molten steel adjusted to the chemical composition described in No. 1 of Table 1 was smelted in a blast furnace and cast by continuous casting to obtain a steel slab. The obtained steel slab was heated to 1200°C and hot rolled with an end temperature of the finish rolling of 950°C and a reduction rate of the finish rolling of 30% to obtain hot rolled steel sheet. The obtained hot rolled steel sheet was coiled at a cooling temperature of 650°C, pickled, then cold rolled by a rolling reduction of 50% to obtain cold rolled steel sheet. The sheet thickness of the cold rolled steel sheet was made 1.6 mm.
Next, the cold rolled steel sheet was shot blasted using an abrasive of TSH30 made by WINOA IKK JAPAN and a blasting amount of 5 kg/m².
After the pretreatment step and before the annealing step, the surface roughness of the steel sheet was measured based on JIS B 0601: 2013. That is, at the surface of the surface layer part side, 10 locations were randomly selected. At each location, the surface profile was measured by a contact type surface roughness meter. The surface roughnesses at these locations were arithmetically averaged. The arithmetic average roughness Ra was evaluated in the following way.

Evaluation AA: 2.0 µm or less
Evaluation A: more than 2.0 µm and 3.0 µm or less
Evaluation B: more than 3.0 µm

After that, the steel sheet was annealed in an oxygen concentration 20 ppm or less furnace in a N₂ -4%H₂ gas atmosphere by a dew point of 0°C, holding temperature of 800°C, and holding time of 40 seconds and a steek sheet sample was prepared. The speed of temperature rise at the time of annealing was made 6.0°C/s up to 500°C and 2.0°C/s from 500°C to the holding temperature. The annealing was performed in a state applying a 5.0 MPa tension.
After the annealing, plating was performed to obtain a plated steel sheet. The plating was dipping in a 450°C hot dip galvanization bath (Zn-0.14%Al) for 3 seconds. After dipping, the steel sheet was pulled out at 100 mm/s and wiped by N₂ gas to control the amount of plating deposition to 50 g/m² . After the plating, alloying was performed at 520°C for 30 seconds.

Except for making each chemical composition the one described in Table 1, making the conditions of the pretreatment step and annealing step the ones described in Table 2, and making the plating type the one described in Table 3, the same procedure was followed as in Example 1 to produce a welded joint and evaluate the LME resistance at the time of production. Note that, in No. 32, the shot blasting was omitted. In No. 33, steel sheet increased in surface roughness by skin pass rolling was used. In No. 37, instead of shot blasting, grinding by a brush rolls was used for surface treatment.
In the plating types of Table 3, "a" indicates Zn-0.14%Al hot dip galvannealing, "b" indicates Zn-0.14%Al hot dip galvanizing without alloying, and "c" indicates Zn-1.5%Al-1.5%Mg. Further, "nonplated" indicates no plating applied.

[Table 1]

Table 1

No.	Class	Chemical composition (mass%), balance: Fe and impurities
No.	Class	C	Si	Mn	sol. Al	P	S	N	B	Ti	Others	Si+sol.Al
1	Ex.	0.08	1.2	2.2	0.0	0.0002	0.0011	0.0006				1.2
2	Ex.	0.08	1.3	2.1	0.0	0.0003	0.0010	0.0006				1.3
3	Ex.	0.10	1.4	2.0	0.2	0.0080	0.0003	0.0002	0.0002	0.0001		1.6
4	Ex.	0.10	1.0	2.0	0.5	0.0080	0.0005	0.0007	0.0010	0.0006	Hf:0.001	1.5
5	Ex.	0.10	1.0	2.0	0.5	0.0070	0.0003	0.0008	0.0008	0.0006		1.5
6	Ex.	0.10	0.9	2.2	0.3	0.0020	0.0004	0.0006	0.0006	0.0009		1.2
7	Ex.	0.20	1.0	2.0	0.5	0.0100	0.0002	0.0001	0.0004	0.0003	Mg:0.001	1.5
8	Ex.	0.20	1.0	2.0	0.4	0.0020	0.0009	0.0002	0.0002	0.0006	Zr:0.015	1.4
9	Ex.	0.20	0.8	2.2	0.7	0.0008	0.0008	0.0007	0.0008	0.0009		1.5
10	Ex.	0.20	1.0	2.5	0.5	0.0017	0.0004	0.0009	0.0004	0.0007	Cr:0.10	1.5
11	Ex.	0.20	0.9	2.3	0.4	0.0011	0.0002	0.0008	0.0005	0.0003	Cu:0.0007	1.3
12	Ex.	0.20	1.0	2.3	0.5	0.0031	0.0002	0.0007	0.0001	0.0003		1.5
13	Ex.	0.20	1.0	2.2	0.0	0.0065	0.0004	0.0006	0.0006	0.0005	Ni:0.08	1.0
14	Ex.	0.20	0.8	5.0	0.7	0.0090	0.0007	0.0008	0.0006	0.0006		1.5
15	Ex.	0.25	1.0	2.2	0.5	0.0012	0.0008	0.0009	0.0009	0.0006	Nb:0.010	1.5
16	Ex.	0.23	2.0	2.2	0.6	0.0009	0.0001	0.0006	-	-	V:0.010	2.6
17	Ex.	0.30	0.8	5.0	0.7	0.0099	0.0006	0.0008	0.0005	0.0004		1.5
18	Ex.	0.30	0.8	0.3	0.7	0.0110	0.0005	0.0001	0.0005	0.0007		1.5
19	Ex.	0.35	0.8	3.0	0.7	0.0092	0.0004	0.0006	0.0008	0.0005	Mo:0.09	1.5
20	Ex.	0.35	0.8	2.2	0.7	0.0091	0.0010	0.0004	0.0004	0.0005	REM:0.0008	1.5
21	Ex.	0.40	0.8	2.2	0.7	0.0045	0.0003	0.0005	0.0004	0.0005		1.5
22	Ex.	0.40	0.8	2.2	0.7	0.0035	0.0008	0.0001	0.0002	0.0002	W:0.005	1.5
23	Ex.	0.40	0.8	2.2	0.7	0.0082	0.0009	0.0004	0.0004	0.0010		1.5
24	Ex.	0.40	0.8	2.2	0.7	0.0080	0.0008	0.0003	0.0007	0.0004	Ca:0.0005	1.5
25	Comp. ex.	0.44	1.0	2.0	0.3	0.0100	0.0003	0.0003	0.0001	0.0001		1.3
26	Comp. ex.	0.20	0.3	2.0	0.3	0.0100	0.0010	0.0002	0.0001	0.0001		0.6
27	Comp. ex.	0.20	2.2	2.0	0.4	0.0100	0.0008	0.0006	0.0005	0.0008		2.6
28	Comp. ex.	0.20	1.0	2.0	2.1	0.0100	0.0003	0.0003	0.0001	0.0007		3.1
29	Comp. ex.	0.20	1.0	2.0	0.4	0.0100	0.0009	0.0006	0.0007	0.0002		1.4
30	Comp. ex.	0.08	1.0	2.0	0.4	0.0100	0.0005	0.0003	0.0009	0.0009		1.4
31	Comp. ex.	0.20	1.0	2.0	0.5	0.0100	0.0002	0.0009	0.0001	0.0005		1.5
32	Comp. ex.	0.20	1.0	2.0	0.5	0.0100	0.0004	0.0006	0.0003	0.0001		1.5
33	Comp. ex.	0.20	1.0	2.0	0.5	0.0100	0.0009	0.0004	0.0004	0.0009		1.5
34	Comp. ex.	0.10	1.0	2.0	0.5	0.0100	0.0005	0.0005	0.0004	0.0002		1.5
35	Comp. ex.	0.20	1.0	2.0	0.5	0.0100	0.0005	0.0005	0.0004	0.0002		1.5
36	Comp. ex.	0.20	1.0	2.0	0.5	0.0100	0.0005	0.0005	0.0004	0.0002		1.5
37	Comp. ex.	0.20	1.0	2.0	0.5	0.0100	0.0005	0.0005	0.0004	0.0002	-	1.5

Underlines indicate outside scope of present invention.

[Table 2]

Table 2

No.	Class	Pretreatment step		Annealing step
No.	Class	Blasted amount (kg/m²)	Surface roughness after pretreatment	Holding temp. (°C)	Holding time (s)	Dew point (°C)
1	Ex.	5	A	800	40	0
2	Ex.	5	A	800	60	0
3	Ex.	15	A	800	60	0
4	Ex.	20	A	800	80	0
5	Ex.	50	AA	820	100	0
6	Ex.	100	AA	840	100	0
7	Ex.	100	AA	860	100	0
8	Ex.	100	AA	860	100	0
9	Ex.	100	AA	860	100	0
10	Ex.	100	AA	860	100	0
11	Ex.	100	AA	860	100	0
12	Ex.	35	AA	860	100	0
13	Ex.	100	AA	860	100	0
14	Ex.	400	AA	830	100	0
15	Ex.	100	AA	860	100	0
16	Ex.	100	AA	840	80	0
17	Ex.	300	AA	860	100	0
18	Ex.	100	AA	860	100	0
19	Ex.	100	AA	860	100	0
20	Ex.	100	A	860	100	0
21	Ex.	100	AA	860	150	0
22	Ex.	100	AA	860	100	-20
23	Ex.	100	AA	860	100	0
24	Ex.	100	AA	860	100	0
25	Comp. ex.	15	A	800	60	0
26	Comp. ex.	15	A	800	60	0
27	Comp. ex.	15	A	800	60	0
28	Comp. ex.	15	A	800	60	0
29	Comp. ex.	15	A	720	60	0
30	Comp. ex.	15	A	920	60	0
31	Comp. ex.	15	A	800	10	0
32	Comp. ex.	0	A	800	60	0
33	Comp. ex.	15	B	800	60	0
34	Comp. ex.	500000	A	800	60	0
35	Comp. ex.	15	A	800	60	-35
36	Comp. ex.	15	A	800	60	25
37	Comp. ex.	Grinding	A	800	60	0

Underlines indicate outside scope of preferable method of production.

Each obtained steel sheet was evaluated for surface layer structure, roughness of the steel sheet surface or steel sheet/plating interface, tensile strength, and LME resistance.

(Evaluation of Surface Layer Structure)

A sample cut to 30 mm×30 mm was taken and measured by GDS five times in the sheet thickness direction. The depth with a C concentration of 0.01% or less was found and shown as "C≤0.01% depth" in Table 3.
Further, a sample cut to a 25 mm×15 mm sheet shape was taken and etched by Nital. The C-cross-section of each sample (sheet thickness cross-section parallel to rolling direction (L-direction)) was examined by an SEM. The thickness of the low cementite layer was measured and shown by "cementite 10% or less thickness" in Table 3. Here, the starting point of the "depth" was the surface of the steel sheet in the case of steel sheet which has not been plated and the interface of the steel sheet and plating layer in the case of steel sheet which has been plated. The interface of the steel sheet and the plating layer was made the position where the concentration of Fe measured by GDS measurement became 93% of the concentration of Fe at the depth of 150 µm.
Further, the roughness of the surface of the steel sheet in the case of steel sheet which has not been plated and the surface of the steel sheet exposed by removal of the plating by a 10 mass% hydrochloric acid solution to which 0.06 mass% of an inhibitor (ibit made by Asahi Chemical Corporation) is added in the case of steel sheet which has been plated was measured by a method similar to before annealing and shown as "steel sheet surface or steel sheet/plating interface roughness" in Table 3.

(Evaluation of Tensile Strength)

In each steel sheet, a JIS No. 5 tensile test piece having a direction perpendicular to the rolling direction as the long direction was taken. A tensile test was performed based on JIS Z 2241: 2011 to find the tensile strength. This was evaluated as follows:

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

(Evaluation of LME Resistance)

From each steel sheet, two samples cut to 50 mm×100 mm size were taken. These two samples were spot welded using dome radius type tip diameter 8 mm welding electrodes by an electrode angle of 2°, squeeze force of 4.0kN, energizing time of 0.5 second, and energizing current of 12 kA to prepare a welded joint. In Table 3, when using a "nonplated" steel sheet to produce a welded joint, spot welding was performed by spot welding a steel sheet given a plating containing zinc in advance 10 times or more so as to use a welding electrode on which zinc was deposited and make that welding electrode abut against the "nonplated" steel sheet.
Referring to FIG. 2, the evaluation of the LME resistance will be explained. The LME resistance was evaluated by superposing two steel sheets 21, spot welding them, the measuring the lengths of the LME cracking formed at the shoulder parts and shoulder outsides of the spot welded part 22 formed (cracking 23 of shoulder parts and shoulder outsides). The "shoulder parts" mean the slanted parts of the valleys of the indentations formed due to the spot welding, while the "shoulder outsides" mean the outsides of the shoulder parts. The lengths of the cracking 23 of the shoulder parts were used for evaluation as follows: In the present embodiment, if the evaluation A or more, it was judged that the LME resistance was excellent and the problem of the present invention was solved.

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

The results of the evaluation are shown in Table 3.

[Table 3]

Table 3

No.	Class	Steel surface layer structure		Steel sheet surface or steel sheet/plating interface roughness (µm)	Plating type	Performance
No.	Class	C≤0.01% depth (um)	Cementite 10% or less thickness (µm)		Plating type	Tensile strength (MPa)	LME resistance
1	Ex.	3	5	2.9	a	A	A
2	Ex.	3	6	3.0	Nonplated	A	A
3	Ex.	4	7	2.3	b	AA	A
4	Ex.	5	10	2.4	b	A	A
5	Ex.	7	15	1.9	b	AA	AAA
6	Ex.	12	21	1.7	b	AA	AAA
7	Ex.	19	22	1.5	b	AA	AAA
8	Ex.	22	24	1.6	b	AA	AAA
9	Ex.	23	22	1.4	c	AA	AAA
10	Ex.	21	24	1.1	b	AA	AAA
11	Ex.	22	25	1.7	b	AA	AAA
12	Ex.	6	14	1.3	b	AA	AA
13	Ex.	22	24	1.1	b	AA	AAA
14	Ex.	25	27	1.5	b	AAA	AAA
15	Ex.	22	24	1.6	b	AAA	AAA
16	Ex.	22	25	1.3	b	AAA	AAA
17	Ex.	40	21	1.7	b	AAA	AAA
18	Ex.	22	24	1.8	b	AA	AAA
19	Ex.	23	23	1.3	b	AAA	AAA
20	Ex.	22	24	2.2	b	AA	AA
21	Ex.	29	27	1.0	b	AAA	AAA
22	Ex.	22	22	1.9	b	AAA	AAA
23	Ex.	22	24	1.2	c	AAA	AAA
24	Ex.	22	24	1.6	b	AAA	AAA
25	Comp. ex.	2	4	2.5	b	AAA	B
26	Comp. ex.	2	3	2.7	b	AAA	B
27	Comp. ex.	2	3	2.2	b	AAA	B
28	Comp. ex.	3	3	2.5	b	AAA	B
29	Comp. ex.	3	2	2.6	b	AAA	B
30	Comp. ex.	2	6	2.4	b	AAA	B
31	Comp. ex.	2	4	2.2	b	AAA	B
32	Comp. ex.	2	4	2.7	b	AAA	B
33	Comp. ex.	12	14	3.3	b	AAA	B
34	Comp. ex.	44	82	2.5	b	B	A
35	Comp. ex.	2	3	2.6	b	AAA	B
36	Comp. ex.	2	2	2.6	b	AAA	B
37	Comp. ex.	2	4	2.7	b	AAA	B

Underlines indicate outside scope of present invention or desired properties not obtained.

No. 25 is a comparative example with a large content of C of the steel sheet. Since the content of C of the steel sheet is large, even if performing high dew point annealing, it is believed that the C concentration at the surface layer of the steel sheet did not fall. Therefore, the depth where the C concentration was 0.01% or less and the thickness of the layer with a cementite area ratio of 10% or less were small. As a result, the LME resistance became inferior.
No. 26 is a comparative example with a small content of Si of the steel sheet. Since the content of Si of the steel sheet was small, even if performing high dew point annealing, it is believed decarburization at the surface layer did not progress. Therefore, the depth where the C concentration was 0.01% or less and the thickness of the layer with a cementite area ratio of 10% or less were small. As a result, the LME resistance became inferior.
No. 27 is a comparative example with a large content of Si of the steel sheet. Since the content of Si of the steel sheet was large, even if performing high dew point annealing, it is believed external oxidation proceeded and oxides (scale) were formed at the surface layer of the steel sheet and decarburization at the surface was suppressed. Therefore, the depth where the C concentration was 0.01% or less and the thickness of the layer with a cementite area ratio of 10% or less were small. As a result, the LME resistance became inferior.
No. 28 is a comparative example with a large content of sol. Al of the steel sheet. Since the content of sol. Al of the steel sheet was large, even if performing high dew point annealing, it is believed external oxidation proceeded and oxides (scale) were formed at the surface layer of the steel sheet and decarburization at the surface was suppressed. Therefore, the thickness of the layer with a cementite area ratio of 10% or less was small. As a result, the LME resistance became inferior.
No. 29 was low in holding temperature at the time of annealing, therefore it is believed decarburization was not sufficiently promoted at the time of annealing. For this reason, the thickness of the layer with a cementite area ratio of 10% or less was small. As a result, the LME resistance became inferior.
No. 30 was high in holding temperature at the time of annealing, therefore it is believed decarburization was not sufficiently promoted at the time of annealing. Therefore, the depth where the C concentration was 0.01% or less became small. As a result, LME resistance became inferior.
No. 31 was short in holding time at the time of annealing, therefore decarburization was not sufficiently promoted at the time of annealing. Therefore, the depth where the C concentration was 0.01% or less and the thickness of the layer with a cementite area ratio of 10% or less were small. As a result, the LME resistance became inferior.
No. 32 was not subjected to shot blasting in the pretreatment step, so it is believed strain was not introduced to the surface of the steel sheet and decarburization did not proceed at the time of annealing. Therefore, the depth where the C concentration was 0.01% or less and the thickness of the layer with a cementite area ratio of 10% or less were small. As a result, the LME resistance became inferior.
No. 33 used steel sheet with a large surface roughness, therefore the roughness of the steel sheet/plating interface after annealing became greater and stress concentration easily occurred. As a result, the LME resistance became inferior.
No. 34 had a large amount of abrasive in the shot blasting, therefore, it is believed decarburization proceeded too much and the tensile strength of the steel sheet fell.
No. 35 was low in dew point at the time of annealing, therefore it is believed that the outside part of the steel sheet was formed with a layer including oxides of Si, Mn, Al, etc. and decarburization did not proceed. Therefore, the depth where the C concentration was 0.01% or less and the thickness of the layer with a cementite area ratio of 10% or less were small. As a result, the LME resistance became inferior.
No. 36 was high in dew point at the time of annealing, therefore it is believed that the outside part of the steel sheet was formed with a layer including oxides of Si, Mn, Al, etc. and decarburization was not promoted. Therefore, the depth where the C concentration was 0.01% or less and the thickness of the layer with a cementite area ratio of 10% or less were small. As a result, the LME resistance became inferior.
In No. 37, instead of shot blasting, surface treatment was performed by grinding using brush rolls, therefore it is believed sufficient strain was not introduced into the surface of the steel sheet and decarburization at the time of annealing did not proceed. Therefore, the depth where the C concentration was 0.01% or less and the thickness of the layer with a cementite area ratio of 10% or less were small. As a result, the LME resistance became inferior.
On the other hand, Nos. 1 to 24 are examples of the present invention and had high LME resistance. It was confirmed that in the examples where the depth where the C concentration was 0.01% or less and the thickness of the layer with a cementite area ratio of 10% or less were large, there was particularly excellent LME resistance.

[INDUSTRIAL APPLICABILITY]

According to the present invention, high strength steel sheet and a plated steel sheet having high LME resistance can be provided. The steel sheet and plated steel sheet can be suitably used for automobiles, household electric appliances, building materials, and other applications, in particular for automobiles. Therefore, the present invention is an invention with extremely high industrial applicability.

REFERENCE SIGNS LIST

11 low cementite layer
12 hard structures
21 steel sheet
22 welded part
23 cracking of shoulder part and shoulder outside

Claims

A steel sheet with a tensile strength of 780 MPa or more, wherein
a chemical composition comprises, by mass%,

C: 0.08 to 0.40%,

Si: 0.4 to 2.0%,

Mn: 0.1 to 5.0%,

sol. Al: 0 to 2.0%,

P: 0.0300% or less,

S: 0.0300% or less,

N: 0.0100% or less,

B: 0 to 0.0100%,

Ti: 0 to 0.1500%,

Nb: 0 to 0.150%,

V: 0 to 0.150%,

Cr: 0 to 2.00%,

Ni: 0 to 2.00%,

Cu: 0 to 2.0000%,

Mo: 0 to 1.00%,

W: 0 to 1.000%,

Ca: 0 to 0.1000%,

Mg: 0 to 0.100%,

Zr: 0 to 0.100%,

Hf: 0 to 0.100%,

REM: 0 to 0.1000% and

a balance of Fe and impurities,

in a depth direction from the steel sheet surface, a depth with a C concentration measured by GDS of 0.01% or less is 3 µm or more,

in a depth direction from the steel sheet surface, a thickness of a layer with an area ratio of cementite of 10% or less is 5 µm or more, and

a surface roughness of the steel sheet is an arithmetic average roughness Ra of 3.0 µm or less.
The steel sheet according to claim 1, wherein a total value of the contents of Si and sol. Al is less than 1.8%.
The steel sheet according to claim 1, wherein in a depth direction from the steel sheet surface, a depth with a C concentration measured by GDS of 0.01% or less is 5 µm or more.
The steel sheet according to claim 1, wherein in a depth direction from the steel sheet surface, a depth with a C concentration measured by GDS of 0.01% or less is 7 µm or more.
The steel sheet according to claim 1, wherein a surface roughness of the steel sheet is an arithmetic average roughness Ra of 2.0 µm or less.
A plated steel sheet comprising the steel sheet according to any one of claims 1 to 5 and a plating layer containing Zn at least at part of the surface of the steel sheet.