Plated steel sheet

Abstract

The present invention relates to a plated steel sheet having high LME resistance. This plated steel sheet is characterized by having a tensile strength of 780 MPa or more and having a prescribed chemical composition, the plating layer containing Zn, a depth of 8 [mu] m or more at which a C concentration measured by GDS in the thickness direction of the steel sheet is 0.05% or less from the interface between the steel sheet and the plating layer, and a depth of 8 [mu] m or more at which a C concentration measured by GDS in the thickness direction of the steel sheet is 0.05% or less from the interface between the steel sheet and the plating layer. The thickness of a layer having a ferrite phase area ratio of 90% or more is 8 [mu] m or more, and the total solid solution amount of metal Si and metal Al in the ferrite phase is 0.3% or more at a position of 4 [mu] m in the thickness direction of the steel sheet from the interface between the steel sheet and the plating layer.

Description

TECHNICAL FIELD

[0001] The present application relates to plated steel sheets. More specifically, the present application relates to plated steel sheets having high LME resistance. BACKGROUND

[0002] In recent years, high-strength steel sheets are being used in various fields such as automobiles, home electric appliances, and building materials. For example, in the automobile field, in order to improve fuel efficiency, the use of high-strength steel sheets is increasing in order to reduce the weight of the vehicle body.

[0003] In welding of steel sheets on which zinc-based plating has been performed, particularly high-strength steel sheets, a decrease in weldability caused by liquid metal embrittlement (LME) cracking sometimes becomes a problem, as described in Patent Literature 1. LME cracking is thought to occur because the surface layer of the steel sheet is transformed into austenite at the time of welding, molten zinc that has intruded into the grain boundaries of the steel sheet embrittles the steel sheet, and further, a tensile stress is applied to the steel sheet at the time of welding.

[0004] In Patent Literature 2, as a steel sheet in which the weldability is improved by suppressing LME cracking, a steel sheet in which Si oxide particles having a particle diameter of 20 nm or more are present at a number density of 3000 to 6000 / mm 2 at the surface layer of the steel sheet at an appropriate particle diameter.

[0005] PRIOR ART DOCUMENTS PATENT LITERATURE Patent Literature 1: International Publication No. 2019 / 116531 Patent Literature 2: International Publication No. 2020 / 218575 SUMMARY

[0006] PROBLEMS TO BE SOLVED BY THE INVENTION In order to prevent LME cracking, it is effective to suppress the intrusion of Zn or the like contained in the plating layer into the steel sheet after the austenite transformation. In this regard, there is room for improvement.

[0007] The present application was made in view of such actual circumstances, and aims to provide plated steel sheets having high LME resistance.

[0008] MEANS FOR SOLVING THE PROBLEMS The present inventors have conducted intensive studies on means for solving the above problems. As a result, it has been found that by forming a decarburized ferrite phase containing Si and Al in a layer shape at the surface layer of the steel sheet, LME can be suppressed.

[0009] The present application was made in view of such actual circumstances, and aims to provide plated steel sheets having high LME resistance.

[0010] (1) A plated steel sheet characterized by comprising: a steel sheet; and a plated layer provided on one side or both sides of the steel sheet, wherein the tensile strength of the plated steel sheet is 780 MPa or more, the steel sheet contains, in mass%, C: 0.05 to 0.40%, Si: 0.7 to 3.0%, Mn: 0.1 to 5.0%, sol. Al: 0.5 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 the balance being Fe and impurities, and the total content of Si and sol. Al is 1.3% or more, the plated layer contains Zn, the depth at which the C concentration measured by GDS is 0.05% or less in the thickness direction of the steel sheet from the interface between the steel sheet and the plated layer is 8 μm or more, the thickness of the layer in which the area ratio of ferrite phase is 90% or more in the thickness direction of the steel sheet from the interface between the steel sheet and the plated layer is 8 μm or more, and the total solid solution amount of metallic Si and metallic Al in the ferrite phase at a position 4 μm from the interface between the steel sheet and the plated layer in the thickness direction of the steel sheet is 0.3% or more.

[0011] (2) The plated steel sheet according to the above (1), characterized in that the depth at which the C concentration measured by GDS is 0.05% or less in the thickness direction of the steel sheet from the interface between the steel sheet and the plated layer is 15 μm or more.

[0012] (3) The plated steel sheet according to the above (1) or (2), characterized in that the thickness of the layer in which the area ratio of ferrite is 90% or more in the thickness direction of the steel sheet from the interface between the steel sheet and the plated layer is 20 μm or more, and the total solid solution amount of metallic Si and metallic Al in the ferrite phase is 0.5% or more.

[0013] (4) The plated steel sheet according to any one of the above (1) to (3), characterized in that the plated layer contains, in mass%, Fe: 0 to 3.0%, Al: 0 to 30.0%, and Mg: 0 to 10.0%, and the balance being Zn and impurities.

[0014] (5) The plated steel sheet according to the above (4), characterized in that the plated layer contains, in mass%, Al: 10.0 to 30.0%, and Mg: 4.5 to 10.0%.

[0015] Inventive Effects According to the present application, a plated steel sheet having high LME resistance can be obtained. BRIEF DESCRIPTION OF DRAWINGS

[0016] Figure 1 is a schematic view illustrating a layered ferrite phase in the plated steel sheet of the present application.

[0017] Figure 2 is a view illustrating the position of a crack as an object in the LME resistance evaluation of the examples. DETAILED DESCRIPTION

[0018] Hereinafter, one embodiment of the present application will be described. The present application is not limited to the following embodiment. First, an outline of improving LME resistance in the present embodiment will be described.

[0019] It is considered that LME cracking is due to the fact that the surface layer portion of a steel sheet is transformed into austenite when heated at the time of spot welding, molten zinc in the grain boundary of the austenite, which is caused by the fact that the plating layer is molten and intrudes into the grain boundary, makes the steel sheet brittle, and further, a tensile stress is applied to the steel sheet at the time of welding, and the like. As a method of improving LME resistance, the present inventors have conceived that the structure of the surface layer portion of the steel sheet near the interface with the plating layer is effectively utilized. Specifically, it has been conceived that the structure of the surface layer of the steel sheet is made into a structure of the center of ferrite having low C concentration and low LME sensitivity, and further, the ferrite is stabilized by solid-solutionizing metal Si and metal Al in the ferrite phase, thereby suppressing the occurrence of LME.

[0020] In the present embodiment, when the steel sheet is annealed, in the first half of the annealing, decarburization is performed at the surface layer of the steel sheet, and internal oxidation toward the inside of the steel sheet is performed at the surface layer of the steel sheet, and in the latter half, only decarburization is performed, and Si and Al are diffused from the inside of the steel sheet to the surface side, and thereby are solid-solutionized in the ferrite phase at the surface layer of the steel sheet. The present embodiment is completed on the basis of the insight that by imparting strain to the surface layer of the steel sheet and performing annealing at an appropriate dew point, decarburization and internal oxidation as described above can be performed. In addition, in the present specification, the "surface layer" refers to a range from the surface of the steel sheet to a depth of about 100 μm.

[0021] Hereinafter, the present embodiment will be described in detail.

[0022] Plated Steel Sheet The plated steel sheet of the present embodiment has a steel sheet and a plating layer provided on one face or both faces of the steel sheet.

[0023] [Tensile Strength] The plated steel sheet of the present embodiment has a tensile strength of 780 MPa or more. The present embodiment suppresses LME that occurs in a high-strength steel sheet, and thus the plated steel sheet of the present embodiment is high-strength. Specifically, it has a tensile strength of 780 MPa or more. The upper limit of the tensile strength is not particularly limited, and from the viewpoint of securing toughness, it can be, for example, 2000 MPa or less. The tensile strength is measured by taking a JIS No. 5 tensile test piece with a direction at right angles to the rolling direction as the length direction, in accordance with JIS Z 2241:2011. The tensile strength can be 980 MPa or more, 1180 MPa or more.

[0024] [Chemical composition of steel sheet] Hereinafter, the chemical composition of the plated steel sheet of the present embodiment will be described. Hereinafter, "%" with respect to the chemical composition of the steel sheet means "mass %". Also, in the numerical range of the chemical composition, the numerical range indicated using "~" means a range including the numerical values recited before and after the "~" as the lower limit value and the upper limit value.

[0025] (C: 0.05 to 0.40%) C (carbon) is an element that secures the strength of steel. In order to obtain the tensile strength of 780 MPa or more, which is the object of the present embodiment, the content of C is set to 0.05% or more. In order not to make the C concentration of the surface layer described later too high, and also in consideration of weldability, the content of C is set to 0.40% or less. The content of C can be 0.08% or more, 0.10% or more, 0.15% or more. The content of C can be 0.37% or less, 0.35% or less, 0.30% or less.

[0026] (Si: 0.7 to 3.0%) Si (silicon) is an element that promotes ferrite stabilization and decarburization by being added in combination with Al (aluminum). In the past, it has been known that the addition of Si to steel reduces the LME resistance, but as a result of the present inventors' studies, it was found that, contrary to the past understanding, the LME resistance is improved by containing a large amount of Si and Al at the same time. It is believed that this is because, by the heat treatment described later, decarburization is performed in the surface layer portion, and the ferrite is stabilized in the surface layer portion, and thus the effect of the decarburization and the stabilization of the ferrite on the improvement of the LME resistance is greater than the reduction of the LME resistance caused by Si. In order to obtain this effect, the content of Si is set to 0.7% or more. When the content of Si is too much, even if high dew point annealing is performed, external oxidation will proceed and oxides (scale) will be formed on the surface of the steel sheet, and on the contrary, the decarburization of the outermost surface is suppressed, and the effect of the improvement of the LME resistance becomes small. In view of this, the content of Si is set to 3.0% or less. The content of Si can be 0.8% or more, 0.9% or more, 1.0% or more. The content of Si can be 2.5% or less, 2.0% or less, 1.5% or less.

[0027] (Mn: 0.1 to 5.0%) Mn (manganese) is an element effective to improve the strength of steel by obtaining a hard structure. The content of Mn is set to 0.1% or more in consideration of the strength of steel. In addition, the content of Mn is set to 5.0% or less in consideration of the decrease in workability due to Mn segregation. The content of Mn can be 0.5% or more, 1.0% or more, 1.5% or more. The content of Mn can be 4.5% or less, 4.0% or less, 3.5% or less.

[0028] (sol.Al: 0.5 to 2.0%) Al (aluminum) is an element that is solid-solved in steel by being added in combination with Si (silicon), and promotes ferrite stabilization and decarburization as well as Si. sol.Al refers to acid-soluble Al that does not become an oxide such as Al2O3 and is soluble in acid, and is found as Al measured as a residue that is not dissolved on a filter paper in an analysis process of excluding Al. In order to obtain the above effects, the content of sol.Al is set to 0.5% or more. When the content of sol.Al is too much, even if high dew point annealing is performed, external oxidation will proceed, and an oxide (scale) will be formed on the surface layer of the steel sheet, which will rather suppress decarburization on the outermost surface, and the effect of improving LME resistance will be small. In consideration of this, the content of sol.Al is set to 3.0% or less. The content of sol.Al can be 0.6% or more, 0.7% or more, 0.8% or more, 1.0% or more. The content of sol.Al can be 1.8% or less, 1.6% or less, 1.5% or less.

[0029] (Si + sol.Al: 1.3% or more) In order to obtain the effect of improving LME resistance as described above, the total content of Si and sol.Al is set to 1.3% or more. In order to increase the content of metal Si and metal Al that are solid-solved in the ferrite phase as described later, the total content of Si and sol.Al is set to 1.3% or more. The total of the contents of Si and sol.Al can be 1.4% or more, 1.5% or more, 1.6% or more, 1.8% or more, 2.0% or more. The upper limit of the contents of Si and sol.Al can be set to the total of the upper limits of the contents of Si and sol.Al, respectively.

[0030] (P: 0.0300% or less) P (phosphorus) is generally an impurity contained in steel. When the content of P exceeds 0.0300%, the weldability can decrease. Therefore, the content of P is set to 0.0300% or less. The content of P can be 0.0200% or less, 0.0100% or less, or 0.0050% or less. It is preferable that P not be contained, and the lower limit of the content of P is 0. From the viewpoint of dephosphorization cost, the content of P can be more than 0%, 0.0001% or more, or 0.0005% or more.

[0031] (S: 0.0300% or less) S (sulfur) is generally an impurity contained in steel. When the content of S exceeds 0.0300%, the weldability decreases, and further, the amount of MnS precipitates increases, and the workability such as bendability can decrease. Therefore, the content of S is set to 0.0300% or less. The content of S can be 0.0100% or less, 0.0050% or less, or 0.0020% or less. It is preferable that S not be contained, and the lower limit of the content of S is 0. From the viewpoint of desulfurization cost, the content of S can be more than 0%, 0.0001% or more, or 0.0005% or more.

[0032] (N: 0.0100% or less) N (nitrogen) is generally an impurity contained in steel. When the content of N exceeds 0.0100%, the weldability can decrease. Therefore, the content of N is set to 0.0100% or less. The content of N can be 0.0080% or less, 0.0050% or less, or 0.0030% or less. It is preferable that N not be contained, and the lower limit of the content of N is 0. From the viewpoint of manufacturing cost, the content of N can be more than 0%, 0.0005% or more, or 0.0010% or more.

[0033] (B: 0 to 0.0100%) B (boron) is an element that contributes to an increase in strength by improving hardenability, and further, improves toughness by segregating at grain boundaries and strengthening the grain boundaries, and therefore, can be contained as necessary. Since it is not an essential element, the lower limit of the content of B is 0. The effect can be obtained even with a small amount, but when contained, the content of B is preferably 0.0001% or more. Further, from the viewpoint of ensuring toughness, the content of B is set to 0.0100% or less. The content of B can be 0.0002% or more, 0.0003% or more, or 0.0005% or more. The content of B can be 0.0090% or less, 0.0080% or less, 0.0060% or less, 0.0040% or less, 0.0035% or less, 0.0030% or less, or 0.0020% or less.

[0034] (Ti: 0 to 0.1500%) Ti (titanium) is an element that contributes to an increase in strength by precipitating as TiC during cooling of the steel, and thus can be contained as needed. Since it is not an essential element, the lower limit of the content of Ti is 0. The effect can be obtained even with a trace amount, but the content of Ti when contained is preferably 0.0001% or more. On the other hand, if it is contained in excess, coarse TiN is generated and toughness can be impaired, and thus the content of Ti can be 0.0002% or more, 0.0003% or more, 0.0004% or more, or 0.0005% or more. The content of Ti is set to 0.1500% or less. The content of Ti can be 0.1200% or less, 0.1000% or less, 0.0500% or less, 0.0300% or less, 0.0150% or less, 0.0100% or less, 0.0050% or less, 0.0030% or less, or 0.0020% or less.

[0035] (Nb: 0 to 0.150%) Nb (niobium) is an element that contributes to an increase in strength through an increase in hardenability, and thus can be contained as needed. Since it is not an essential element, the lower limit of the content of Nb is 0. The effect can be obtained even with a trace amount, but the content of Nb when contained is preferably 0.001% or more. On the other hand, from the viewpoint of securing toughness, the content of Nb is set to 0.150% or less. The content of Nb can be 0.002% or more, 0.003% or more, 0.005% or more, 0.007% or more, or 0.010% or more. The content of Nb can be 0.120% or less, 0.100% or less, 0.060% or less, 0.050% or less, 0.030% or less, or 0.020% or less.

[0036] (V: 0 to 0.150%) V (vanadium) is an element that contributes to an increase in strength through an increase in hardenability, and thus can be contained as needed. Since it is not an essential element, the lower limit of the content of V is 0. The effect can be obtained even with a trace amount, but the content of V when contained is preferably 0.001% or more. On the other hand, from the viewpoint of securing toughness, the content of V is set to 0.150% or less. The content of V can be 0.002% or more, 0.003% or more, 0.005% or more, or 0.008% or more. The content of V can be 0.120% or less, 0.100% or less, 0.060% or less, 0.050% or less, 0.040% or less, 0.030% or less, or 0.020% or less.

[0037] (Cr: 0 to 2.00%) Cr (chromium) is effective for improving the hardenability of the steel and increasing the strength of the steel, and thus can be contained as needed. Since it is not an essential element, the lower limit of the content of Cr is 0. The effect can be obtained even with a trace amount, but the content of Cr when contained is preferably 0.01% or more. On the other hand, if it is contained in excess, Cr carbide is formed in a large amount, and conversely the hardenability can be impaired, and thus the content of Cr is set to 2.00% or less. The content of Cr can be 0.02% or more, 0.04% or more, 0.05% or more, 0.07% or more, or 0.10% or more. The content of Cr can be 1.80% or less, 1.50% or less, 1.20% or less, 0.80% or less, 0.60% or less, 0.50% or less, 0.30% or less, or 0.20% or less.

[0038] (Ni: 0 ~ 2.00%) Ni (nickel) is effective for improving the hardenability of the steel and increasing the strength of the steel, and thus can be contained as needed. Since it is not an essential element, the lower limit of the content of Ni is 0. The effect can be obtained even with a trace amount, but the content of Ni when contained is preferably 0.001% or more. On the other hand, excessive addition of Ni increases the cost, and thus the content of Ni is set to 2.00% or less. The content of Ni can be 0.02% or more, 0.03% or more, 0.04% or more, or 0.05% or more. The content of Ni can be 1.80% or less, 1.50% or less, 1.20% or less, 0.80% or less, 0.60% or less, 0.50% or less, 0.40% or less, 0.20% or less, or 0.15% or less.

[0039] (Cu: 0 ~ 2.0000%) Cu (copper) is effective for improving the hardenability of the steel and increasing the strength of the steel, and thus can be contained as needed. Since it is not an essential element, the lower limit of the content of Cu is 0. The effect can be obtained even with a trace amount, but the content of Cu when contained is preferably 0.0001% or more. On the other hand, from the viewpoint of suppressing a decrease in toughness, cracking of a slab after casting, and a decrease in weldability, the content of Cu is set to 2.0000% or less. The content of Cu can be 0.0002% or more, 0.0004% or more, or 0.0005% or more. The content of Cu can be 1.8000% or less, 1.5000% 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.

[0040] (Mo: 0 ~ 1.00%) Mo (molybdenum) is effective for improving the hardenability of the steel and increasing the strength of the steel, and thus can be contained as needed. Since it is not an essential element, the lower limit of the content of Mo is 0. The effect can be obtained even with a trace amount, but the content of Mo when contained is preferably 0.01% or more. On the other hand, from the viewpoint of suppressing a decrease in toughness, the content of Mo is set to 1.00% or less. The content of Mo can be 0.02% or more, 0.03% or more, 0.05% or more, or 0.06% or more. The content of Mo can be 0.90% or less, 0.80% or less, 0.60% or less, 0.40% or less, 0.30% or less, or 0.20% or less.

[0041] (W: 0 ~ 1.000%) W (tungsten) is effective for improving the hardenability of the steel and increasing the strength of the steel, and thus can be contained as needed. Since it is not an essential element, the lower limit of the content of W is 0. The effect can be obtained even with a trace amount, but the content of W when contained is preferably 0.001% or more. On the other hand, from the viewpoint of suppressing a decrease in toughness, the content of W is set to 1.000% or less. The content of W can be 0.002% or more or 0.003% or more. The content of W can 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, 0.030% or less, 0.020% or less, 0.015% or less, or 0.010% or less.

[0042] (Ca: 0 ~ 0.1000%) Ca (calcium) is an element that contributes to inclusion control, particularly contributes to the fine dispersion of inclusions, and has an effect of increasing toughness, and thus can be contained as needed. Since it is not an essential element, the lower limit of the content of Ca is 0. The effect can be obtained even with a trace amount, but the content of Ca when contained is preferably 0.0001% or more. On the other hand, if it is contained in excess, deterioration of surface properties sometimes becomes apparent, and thus the content of Ca is set to 0.1000% or less. The content of Ca can be 0.0002% or more or 0.0003% or more. The content of Ca can be 0.0900% or less, 0.0800% or less, 0.0600% or less, 0.0500% 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.

[0043] (Mg: 0 ~ 0.100%) Mg (magnesium) is an element that contributes to inclusion control, particularly to the fine dispersion of inclusions, and has the effect of improving toughness, and thus can be contained as needed. Since it is not an essential element, the lower limit of the content of Mg is 0. The effect can be obtained even with a trace amount, but the content of Mg when contained is preferably 0.0001% or more. On the other hand, if it is contained in excess, deterioration of the surface properties sometimes becomes apparent, and thus the content of Mg is set to 0.100% or less. The content of Mg can be 0.0003% or more, 0.0005% or more, or 0.0008% or more. The content of Mg can 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.

[0044] (Zr: 0 to 0.100%) Zr (zirconium) is an element that contributes to inclusion control, particularly to the fine dispersion of inclusions, and has the effect of improving toughness, and thus can be contained as needed. Since it is not an essential element, the lower limit of the content of Zr is 0. The effect can be obtained even with a trace amount, but the content of Zr when contained is preferably 0.001% or more. On the other hand, if it is contained in excess, deterioration of the surface properties sometimes becomes apparent, and thus the content of Zr is set to 0.100% or less. The content of Zr can be 0.003% or more, 0.005% or more, 0.008% or more, or 0.010% or more. The content of Zr can be 0.090% or less, 0.080% or less, 0.060% or less, 0.050% or less, 0.040% or less, or 0.030% or less.

[0045] (Hf: 0 to 0.100%) Hf (hafnium) is an element that contributes to inclusion control, particularly to the fine dispersion of inclusions, and has the effect of improving toughness, and thus can be contained as needed. Since it is not an essential element, the lower limit of the content of Hf is 0. The effect can be obtained even with a trace amount, but the content of Hf when contained is preferably 0.0001% or more. On the other hand, when contained in excess, deterioration of the surface properties sometimes becomes apparent, and thus the content of Hf is set to 0.100% or less. The content of Hf can be 0.0002% or more, 0.0003% or more, 0.0005% or more, or 0.0008% or more. The content of Hf can be 0.090% or less, 0.080% or less, 0.060% or less, 0.050% 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.

[0046] (REM: 0 to 0.1000%) REM (rare earth element) is an element that contributes to the control of inclusions, particularly to the fine dispersion of inclusions, and has an effect of improving toughness, and thus can be contained as needed. Since it is not an essential element, the lower limit of the content of REM is 0. The effect can be obtained even with a small amount, but when contained, the content of REM is preferably 0.0001% or more. On the other hand, if it is contained in excess, deterioration of surface properties sometimes becomes apparent, and thus the content of REM is set to 0.1000% or less. The content of REM can be 0.0003% or more, 0.0004% or more, or 0.0005% or more. The content of REM can be 0.0900% or less, 0.0800% or less, 0.0600% or less, 0.0500% or less, 0.0400% or less, 0.0300% or less, 0.0200% or less, 0.0100% or less, 0.0050% or less, or 0.0020% or less. Also, the so-called REM is an abbreviation of Rare Earth Metal, and refers to an element belonging to the lanthanide series. REM is usually added as a mixed rare earth metal.

[0047] In the steel sheet of the present embodiment, the remaining portion other than the above-described chemical components is composed of Fe and impurities. Here, the impurities refer to components that are mixed due to various factors of the manufacturing process, with raw materials such as ores, scrap iron, and the like as representatives, in the industrial production of steel sheets, and are contained within a range that does not adversely affect the LME resistance of the steel sheet of the present embodiment, i.e., within a range in which the LME resistance required of the steel sheet of the present embodiment can be obtained. As specific elements, for example, O (oxygen) can be cited. The content of O contained as an impurity can be, for example, 0.0500% or less, 0.0300% or less, 0.0200% or less, or 0.0100% or less. However, from the viewpoint of manufacturing cost, the lower limit of the content of O can be 0.00001%, 0.00005%, or 0.0001%.

[0048] For the analysis of the chemical components of the steel sheet, an elemental analysis method known to those skilled in the art can be used, for example, by inductively coupled plasma mass spectrometry (ICP-MS method). Among these, for C and S, combustion-infrared absorption method can be used for measurement, and for N, non-active gas melting-thermal conductivity method can be used for measurement. For O, non-active gas melting-infrared absorption method is used. These analyses can be performed using a method according to JIS G0417: 1999 with a sample collected from the steel sheet.

[0049] [Plating layer] The plated steel sheet of the present embodiment has a plated layer on the above-described steel sheet. The plated layer can be formed on one side of the steel sheet, or on both sides. The plated layer of the plated steel sheet of the present embodiment is not particularly limited as long as it is a Zn-based plated layer containing Zn, and is mainly used in the automobile field. As elements other than Zn, for example, elements such as Fe, Al, Mg, Si, Ni, Sn, and the like that are normally contained in Zn plated layers can be contained. In addition, elements contained in the steel sheet can also be diffused and contained in the plated layer. The content of Zn can be 50% or more, or 55% or more, or 60% or more.

[0050] [Chemical composition of plated layer] Hereinafter, one example of the chemical composition of the plated layer of the plated steel sheet suitable for the present embodiment will be described. The "%" relating to the content of elements means "mass %" unless otherwise specified. In the numerical range regarding the chemical composition of the plated layer, the numerical range indicated using "~" means a range including the numerical values recited before and after the "~" as lower limit values and upper limit values, unless otherwise specified.

[0051] (Fe: 0 to 3.0%) In the case where the plated steel sheet is subjected to heat treatment after forming the plated layer containing Zn on the steel sheet, Fe can be contained in the plated layer by diffusing from the steel sheet. The content of Fe can be 0%. In addition, the content of Fe is set to 3.0% or less. The content of Fe can be 2.0% or less, or 1.0% or less.

[0052] (Al: 0 to 30.0%) Al is an element that improves the corrosion resistance of the plated layer by being contained together with Zn, and thus can be contained as needed. The content of Al can be 0%. In order to form a plated layer containing Zn and Al, the content of Al is preferably 0.01% or more. When the content of Al is too much, the effect of improving the corrosion resistance is saturated, and thus the content of Al is set to 30.0% or less. The content of Al can be 1.0% or more, or 3.0% or more, or 5.0% or more, or 10.0% or more, or 15.0% or more. The content of Al can be 25.0% or less, or 20.0% or less.

[0053] (Mg: 0 to 10.0%) Mg is an element that improves the corrosion resistance of the plated layer by being contained together with Zn and Al, and thus can be contained as needed. The content of Mg can be 0%. In order to form a plated layer containing Zn, Al, and Mg, the content of Mg is preferably 0.01% or more. When the content of Mg is too much, appearance defects and non-plating can sometimes occur, and thus the content of Mg is set to 10.0% or less. The content of Mg can be 1.0% or more, or 2.0% or more, or 3.0% or more, or 4.5% or more, or 5.0% or more. The content of Mg can be 8.0% or less, or 6.0% or less.

[0054] In one example of the chemical composition of the plated layer of the plated steel sheet according to the present embodiment, the remainder other than the above-described components consists of Zn and impurities. The impurities in the plated layer refer to components mixed in due to various reasons of the manufacturing process as represented by raw materials at the time of manufacturing the plated layer, and are not components intentionally added to the plated layer. For example, when elements other than Zn, Fe, Al, and Mg contained in the steel sheet diffuse and are contained in the plated layer, they correspond to impurities. In one example of the chemical composition of the plated layer of the plated steel sheet according to the present embodiment, as impurities, elements other than the essential components and the optional added components described above can also be contained in a trace amount within a range not to hinder the effects of the present embodiment.

[0055] The chemical composition of the plated layer can be determined by dissolving the plated layer in an acid solution to which an inhibitor for suppressing corrosion of the steel sheet is added, and measuring the obtained solution by ICP (inductively coupled plasma) emission spectrometry. As the acid solution to which the inhibitor is added, for example, a 10 mass% hydrochloric acid solution to which 0.06 mass% of the inhibitor (IBIT710K manufactured by Asahi Chemical Industry Co., Ltd.) is added can be used.

[0056] The thickness of the plated layer can be, for example, 3 to 50 μm. In addition, the attached amount of the plated layer is not particularly limited, and can be, for example, 10 to 170 g / m 2 In the present embodiment, the attached amount of the plated layer is determined from the weight change before and after pickling and stripping of the plated layer by dissolving the plated layer in an acid solution to which an inhibitor for suppressing corrosion of the steel sheet is added. The steel sheet after the removal of the plated layer is subjected to water washing and drying. The thickness of the plated layer can be 5 μm or more, 7 μm or more, or 10 μm or more. The thickness of the plated layer can be 45 μm or less, 40 μm or less, 35 μm or less, or 30 μm or less. The attached amount of the plated layer can be 15 g / m 2 20 g / m 2 25 g / m 2 30 g / m 2 or more. The attached amount of the plated layer can be 160 g / m 2 140 g / m 2 120 g / m 2 100 g / m 2 or less.

[0057] [Surface Layer] Next, the surface layer of the plated steel sheet will be described.

[0058] (C concentration of surface layer) In the plated steel sheet of the present embodiment, the depth at which the C concentration measured by GDS (glow discharge spectroscopy) is 0.05% or less in the thickness direction of the steel sheet from the interface between the steel sheet and the plated layer is 8 μm or more.

[0059] If the C concentration is lowered, the susceptibility to LME is reduced, and thus the LME resistance is improved by lowering the C concentration in the surface layer. In addition, C is an austenite stabilizing element, and thus by lowering the amount thereof, the layer having a low LME susceptibility described later is stabilized.

[0060] As to such a surface layer structure, the steel sheet can be obtained by having the chemical composition as described above and by the pretreatment and heat treatment described later.

[0061] If the depth at which the C concentration is 0.05% or less is 8 μm or more, the effect of improving the LME resistance is obtained, and thus the upper limit of the depth at which the C concentration is 0.05% or less is not particularly limited. The depth at which the C concentration is 0.05% or less can be, for example, 50 μm or less, 40 μm or less, or 30 μm or less. The depth at which the C concentration is 0.02% or less is preferably 10 μm or more, more preferably 12 μm or more, and further preferably 15 μm or more or 20 μm or more.

[0062] GDS measurement was performed five times in the thickness direction of the sheet, and the average thereof was taken as the C concentration. The measurement conditions are described below.

[0063] Apparatus: High-frequency glow discharge emission spectroscopy apparatus (LECO Japan Co., Ltd., Model "GDS850A" Ar gas pressure: 0.3 MPa Anode diameter: 4 mmφ RF output: 30 W Measurement time: 200 to 1500 seconds Note that the interface between the steel sheet and the plated layer in the present embodiment is defined as the position at which the content of Fe measured by GDS is 93% of the content of Fe at a depth of 150 μm.

[0064] (high ferrite layer) In the plated steel sheet of the present embodiment, the thickness of a layer in which the area ratio of ferrite phase is 90% or more in the thickness direction of the steel sheet from the interface between the steel sheet and the plated layer (hereinafter referred to as "high ferrite layer") is 8 μm or more. Figure 1 is a schematic view of the vicinity of the surface layer of the steel sheet of the present embodiment. Figure 1 is a cross section in the thickness direction of the plated steel sheet 11, and the upper side is the surface of the plated steel sheet 11. In the drawing, the high ferrite layer 12 is shown as a layer in which the area ratio of ferrite phase is 90% or more in the vicinity of the surface of the plated steel sheet 11. Figure 1In the present embodiment, the plated steel sheet 11 has a steel sheet 12 and a plated layer 13 having a thickness of, for example, 3 to 50 μm. A high ferrite layer 14 is present in a surface layer portion of the steel sheet 12 (interface side with the plated layer 13) and has a thickness of 8 μm or more. An inner side 15 of the steel sheet is a structure mainly composed of martensite and containing ferrite.

[0065] If the thickness of the high ferrite layer is 8 μm or more, an effect of improving LME resistance can be obtained, and thus the upper limit of the thickness is not particularly limited. For example, it can be 100 μm or less, 80 μm or less, 60 μm or less, or 40 μm or less. The thickness of the high ferrite layer is preferably 10 μm or more, more preferably 12 μm or more, and further preferably 15 μm or more, 20 μm or more, or 25 μm or more.

[0066] The structure other than ferrite in the high ferrite layer is not limited. For example, it can be any one or more of martensite, bainite, and cementite.

[0067] For the thickness of the high ferrite layer, the thickness of the steel sheet is etched with nitric acid ethanol, and SEM observation is performed at a magnification of 1000 times. Ferrite is distinguished from hard structures such as martensite and bainite containing much cementite based on the structure pattern, and the thickness of the high ferrite layer is obtained. For the thickness of the high ferrite layer, a range of 500 μm in a direction perpendicular to the thickness direction of the steel sheet is taken as a measurement range, and measurement is performed at intervals of 1000 μm in the direction perpendicular to the thickness direction of the steel sheet in five measurement ranges, and the average value is taken. Here, the area ratio of ferrite refers to the area ratio obtained by observing the above-mentioned thickness section. In the case of observing a part in the middle of the thickness direction, for example, even if there is a part where the area ratio of ferrite is less than 90%, it is not a problem as long as the area ratio of ferrite is 90% or more in the thickness section from the surface to a depth of 8 μm.

[0068] For the area ratio of ferrite, a section of the steel sheet in the thickness direction orthogonal to the rolling direction is cut out, mirror polished, and then the steel structure is visualized using a nitric acid ethanol solution, and a secondary electron image is captured using a field emission type scanning electron microscope, and thus the area ratio of ferrite is obtained. The observation position is set to a range of 500 μm from the surface of the thickness or the interface between the plated layer and the steel sheet, and five fields of view are observed at equal intervals. For the obtained structure photograph, the fraction of each structure is calculated by a point counting method. More specifically, first, an equidistant grid is drawn on the structure photograph. Next, it is determined which of tempered martensite, pearlite, ferrite, primary martensite or residual austenite, or bainite each grid point belongs to. By dividing the number of grid points corresponding to each structure by the total number of grid points, the fraction of each structure can be measured. The more the total number of grid points, the more accurately the volume fraction can be obtained. In the present embodiment, the grid interval is 2 μm x 2 μm, and the total number of grid points is set to 1500 points.

[0069] A criterion for judging tempered martensite, pearlite, ferrite, primary martensite or residual austenite, or bainite is shown. A region in which a lower structure (lath boundary, lath block boundary) is present in the grain and a carbide is precipitated with a plurality of variants is judged to be tempered martensite. In addition, a region in which cementite is precipitated in a layer is judged to be pearlite. A region in which the brightness is small and no lower structure is confirmed is judged to be ferrite. A region in which the brightness is large and no lower structure is visualized by etching is judged to be primary martensite or residual austenite. A region which does not correspond to any of the above is judged to be bainite. In short, if it is distinguished from ferrite and other structures, the area ratio of the ferrite phase can be obtained.

[0070] (Metallic Si, metallic Al in the ferrite phase) In the ferrite phase in the high ferrite layer, metallic Si and metallic Al are in total solid solution of 0.3% or more at a position of 4 μm in the thickness direction of the steel sheet from the interface of the steel sheet and the plated layer. Thereby, the ferrite phase is stabilized, and even if heat generated by spot welding, it is difficult to transform to the austenite phase which is high in LME sensitivity. The ferrite is low in LME sensitivity, and thus it is preferable to suppress the phase transformation to austenite at the time of welding from the viewpoint of improving the LME resistance. Such a surface layer structure can be obtained by setting the chemical composition of the steel sheet as described above and by the pretreatment and heat treatment described later. The content of the metallic Si and the metallic Al solid-solved in the ferrite phase can be 0.4% or more, 0.5% or more, 0.6% or more.

[0071] The amount of the metallic Si solid-solved in the ferrite phase is calculated as follows: the ratio of the peak area intensity of Si (Si-Si, Fe-Si) to Si (SiO x ) + Si (Si-Si, Fe-Si) is found using an X-ray photoelectron spectrometer, and multiplied by the content of Si in the steel at a position of 4 μm in the thickness direction of the steel sheet from the interface of the steel sheet and the plated layer. Note that this Si content can be measured by the GDS described earlier. The amount of Al solid-solved in the ferrite phase is found as Al measured by subtracting the insoluble residue on the filter paper generated in the process of analyzing Al.

[0072] In addition, it is preferable that the amount of the metallic Si and the metallic Al solid-solved in the ferrite phase is measured by TEM-EDS as follows.

[0073] First, as a sample for TEM observation, a thin film sample having a cross section of the thickness of the steel sheet perpendicular to the L direction (rolling direction) as an observation surface was produced by a FIB (Focused Ion Beam) method. The thin film sample was produced using NB5000 manufactured by Hitachi High-Tech Corporation, and the acceleration voltage at the time of processing was set to 5 to 40 kV. Further, 10 kV or less was used for finishing. C vapor deposition was performed on the outermost surface of the sample for protection. A Mo mesh was used.

[0074] Next, the produced sample was observed with a transmission electron microscope (TEM), and line analysis was performed at a position 4 μm from the surface of the steel sheet using TEM-EDS (Energy Dispersive X-ray Spectroscopy). Quantitative analysis was performed at 10 points at random. The elements for quantitative analysis were set to Fe, Si, Al, Mn, and O. That is, the proportions of the metallic Si and the metallic Al in the ferrite phase in the present application each refer to the proportions of Si and Al in the ferrite phase in the total concentration of Fe, Si, Al, Mn, and O. The concentration was set to the average of the measurement results of the 10 points. Here, in the case where the O concentration was 5% or more, it was determined that the structure at the analysis position was an internal oxide, and was not included in the measurement points, and the analysis was performed at 10 points having an O concentration of less than 5%.

[0075] The TEM can use JEM-2100F manufactured by JEOL Ltd., and the acceleration voltage at the time of observation was set to 200 kV. The EDS can use JED-2300T manufactured by JEOL Ltd., and the acceleration voltage at the time of analysis was set to 200 kV.

[0076] [Sheet thickness] The sheet thickness of the plated steel sheet of the present embodiment is not particularly limited. For example, it can be 0.6 to 3.2 mm. The sheet thickness can be 0.8 mm or more or 1.0 mm or more. The sheet thickness can be 3.0 mm or less, 2.6 mm or less, 2.5 mm or less, 2.4 mm or less, 2.2 mm or less, 2.0 mm or less, or 1.8 mm or less.

[0077] [Manufacturing method] Next, the manufacturing method of the steel sheet of the present embodiment will be described.

[0078] The steel sheet of the present embodiment can be obtained, for example, by a production method including a casting step of casting molten steel having a chemical composition adjusted to form a steel billet, a hot rolling step of hot rolling the steel billet to obtain a hot-rolled steel sheet, a coiling step of coiling the hot-rolled steel sheet, a cold rolling step of cold rolling the hot-rolled steel sheet after coiling to obtain a cold-rolled steel sheet, a pretreatment step of performing a brush grinding treatment on the cold-rolled steel sheet, and an annealing step of annealing the cold-rolled steel sheet after the pretreatment. Alternatively, the hot-rolled steel sheet can be pickled and directly subjected to cold rolling without being coiled after the hot rolling step.

[0079] <casting step> The conditions of the casting step are not particularly limited. For example, after melting by a blast furnace, an electric furnace, or the like, various secondary smelting is performed, and then casting is performed by a method such as a usual continuous casting, a casting by an ingot casting method, or the like.

[0080] <hot rolling step> The steel billet obtained by casting can be hot-rolled to obtain a hot-rolled steel sheet. The hot rolling step is performed by directly or temporarily cooling the steel billet after casting, reheating, and hot-rolling. In the case where reheating is performed, the heating temperature of the steel billet can be, for example, 1100 to 1250°C. In the hot rolling step, usually, rough rolling and finish rolling are performed. The temperature of each rolling and the reduction rate can be appropriately changed depending on the desired metal structure and the sheet thickness. For example, the finish rolling can be performed at a finish rolling end temperature of 900 to 1050°C, and the reduction rate of the finish rolling can be 10 to 50%.

[0081] <coiling step> The hot-rolled steel sheet can be coiled at a predetermined temperature. The coiling temperature can be appropriately changed depending on the desired metal structure or the like, and can be, for example, 500 to 800°C. The hot-rolled steel sheet can be subjected to a predetermined heat treatment before or after coiling. Alternatively, the hot-rolled steel sheet can be pickled after the hot rolling step without being coiled, and then subjected to cold rolling described later.

[0082] <cold rolling step> After the hot-rolled steel sheet is subjected to pickling or the like, the hot-rolled steel sheet can be cold-rolled to obtain a cold-rolled steel sheet. The reduction rate of cold rolling can be appropriately changed depending on the desired metal structure and the sheet thickness, and can be, for example, 20 to 80%. After the cold rolling step, for example, air cooling can be performed to cool to room temperature.

[0083] <pretreatment step> In order to obtain the structure of the surface layer of the steel sheet as described above, a predetermined pretreatment is required, and then annealing is performed.

[0084] The pre-treatment includes abrading the surface of the cold-rolled steel sheet with an abrasive brush (brush grinding treatment). As an abrasive brush that can be used, for example, M-33 manufactured by Hotani Co., Ltd. can be listed. By this, a strain can be introduced into the surface layer of the steel. At the time of grinding, a 1.0 to 5.0% NaOH aqueous solution can be applied to the surface of the steel sheet. The brush pressure can be 0.5 to 10.0 mm, and the rotation speed can be 100 to 1000 rpm. By performing the brush grinding treatment under such application conditions of the solution, brush pressure, and rotation speed, decarburization can be promoted in the annealing process described later, and a ferrite-stable structure can be formed in the surface layer of the steel sheet.

[0085] Note that the smaller the roughness of the steel sheet after the pre-treatment process, the smaller the stress concentration, and the more the LME resistance is improved, and thus the roughness of the steel sheet after the pre-treatment process is more preferably smaller. As for the unevenness of the surface of the steel sheet, according to JIS B0601:2013, 10 points are randomly selected on the surface on the side of the surface layer portion, the surface profile is measured at each point using a contact-type surface roughness meter, the surface roughness of these points is arithmetically averaged, and the obtained value is taken as the arithmetic average roughness Ra. The smaller the Ra, the more preferable, and it is preferably less than 3.0 μm, and more preferably 2.5 μm or less, 2.0 μm or less.

[0086] <Annealing Process> The cold-rolled steel sheet on which the pre-treatment has been performed is subjected to annealing. In the present embodiment, the steel sheet to which a strain has been imparted by the pre-treatment is held at a high dew point. Specifically, in order to promote decarburization and internal oxidation, the holding temperature of the annealing process is set to 760 to 900°C. The holding temperature can be 770 to 870°C. If the holding temperature is low, decarburization and internal oxidation do not proceed, and thus the depth at which the C concentration is 0.05% or less becomes shallow, and the thickness of the high ferrite layer becomes thin. The holding time at the holding temperature of the annealing process is set to 20 to 300 seconds. The holding time can be 50 to 200 seconds. The atmosphere is preferably a non-oxidizing atmosphere, and for example, N2-1 to 10 vol% H2, N2-2 to 4 vol% H2 can be set. The temperature increase rate up to the holding temperature is not particularly limited, and can be performed at 1 to 10°C / second.

[0087] In the annealing step in the manufacturing method of the plated steel sheet of the present embodiment, the dew point of the atmosphere is changed in the first half and the second half of the holding time. The first half of the holding time refers to the period from the start of the holding to 40 ± 1% of the entire holding time, and the dew point of the annealing atmosphere is set to -20 to 20°C in the first half of the holding time. Thereby, both decarburization and internal oxidation of the surface layer of the steel sheet are performed. As a result, Si and Al of the surface layer become oxides, and the solid solution amount of Si and Al decreases. If the dew point of the first half of the holding time is low, decarburization and internal oxidation do not proceed, and thus the depth at which the C concentration is 0.05% or less becomes shallower, and the thickness of the ferrite layer becomes thinner. If the dew point of the first half of the holding time is high, external oxidation proceeds, and decarburization is suppressed, the depth at which the C concentration is 0.05% or less becomes shallower, and the thickness of the ferrite layer becomes thinner. In addition, if the proportion of the first half of the holding time with respect to the entire holding time is large, Si and Al cannot sufficiently diffuse from the inside of the steel sheet to the surface side, and the content of solid solution Si and solid solution Al in the ferrite phase becomes low.

[0088] Thereafter, the dew point of the annealing atmosphere is set to -40 to -20°C in the second half of the holding time after 40 ± 1% of the holding time has elapsed. Further, the dew point of the second half is lower than the dew point of the first half by 10°C or more. Thereby, only decarburization proceeds, and Si and Al diffuse from the inside of the steel sheet to the surface side of the steel sheet. Thereby, Si and Al in the ferrite phase that are solid-solved in the ferrite layer increase. If the dew point of the second half of the holding time is low, decarburization does not proceed, and thus the depth at which the C concentration is 0.05% or less becomes shallower, and the thickness of the ferrite layer becomes thinner. If the dew point of the second half of the holding time is high, external oxidation proceeds, and decarburization is suppressed, the depth at which the C concentration is 0.05% or less becomes shallower, and the thickness of the ferrite layer becomes thinner. In addition, if the proportion of the second half of the holding time with respect to the entire holding time is small, decarburization does not proceed in the surface layer portion, the depth at which the C concentration is 0.05% or less becomes shallower, and the thickness of the ferrite layer becomes thinner. If the proportion of the first half of the holding time with respect to the entire holding time is small, the proportion of the second half of the holding time with respect to the entire holding time becomes relatively large, and in the second half of the holding time, C diffuses from the inside of the steel sheet to the surface, and in the first half of the holding time, the layer in which the surface layer is decarburized disappears, the depth at which the C concentration is 0.05% or less becomes shallower, and the thickness of the ferrite layer becomes thinner.

[0089] The dew point during the first half of the holding time can be -15°C or higher, -10°C or higher. In addition, the dew point during the first half of the holding time can be 15°C or lower, 10°C or lower. The dew point of the second half of the holding time can be -37°C or higher, -35°C or higher. In addition, the dew point of the second half of the holding time can be -22°C or lower, -25°C or lower.

[0090] By making the annealing conditions be in the above-described ranges, decarburization can be promoted to reduce the C concentration of the surface layer, and in addition, the fraction of the ferrite phase in which Si and Al are solid-solved can be increased.

[0091] The annealing is performed, for example, in a state where a tension of 1 to 20 MPa is applied. If a tension is applied at the time of annealing, strain can be more effectively introduced to the steel sheet, and decarburization of the surface layer is promoted.

[0092] The plated steel sheet of the present embodiment can be obtained by performing a plating treatment of forming a plated layer on the steel sheet manufactured as described above.

[0093] <Plating treatment step> The plating treatment is performed as long as it is performed according to a method known to those skilled in the art. The plating treatment can be performed, for example, by a hot-dip plating method, or can be performed by an electroplating method, a vapor deposition method, a sputtering method, a cold spray method. The plating treatment is preferably performed by a hot-dip plating method. The conditions of the plating treatment can be appropriately set in consideration of the chemical composition, the thickness, and the adhesion amount of the plated layer, and the like, which are desired.

[0094] The plated steel sheet of the present embodiment is high in strength, and has high LME resistance, and thus can be suitably used in a wide range of fields such as automobiles, home electric appliances, building materials, and the like. It is particularly preferable to use in the field of automobiles. The plated steel sheet for automobiles is mostly spot-welded, and in this case, LME cracking becomes a significant problem. Therefore, in the case where the plated steel sheet of the present embodiment is used as a steel sheet for automobiles, the effect of the present embodiment of having high LME resistance can be suitably exerted.

[0095] Example Hereinafter, the present application will be described in more detail by examples. The present application is not limited to these examples.

[0096] <Example No. 1> A molten steel was smelted by a blast furnace, and casting was performed by continuous casting, to obtain a steel slab having the chemical composition described in No. 1 of Table 1. The obtained steel slab was heated to 1200°C, the finish temperature of finish rolling was set to 950°C, and the reduction rate of finish rolling was set to 30% to perform hot rolling, to obtain a hot-rolled steel sheet. The obtained hot-rolled steel sheet was coiled at a coiling temperature of 650°C, and after pickling, cold rolling was performed at a reduction rate of 50% to obtain a cold-rolled steel sheet. The sheet thickness of the cold-rolled steel sheet was set to 1.6 mm.

[0097] Next, a NaOH 2.0% aqueous solution was applied to the cold-rolled steel sheet, and a pre-treatment of brush grinding was performed. As a grinding brush, M-33 manufactured by Hotani Corporation was used, and the brush grinding was performed at a brush pressure of 2.0 mm and a rotation speed of 600 rpm (condition "A" of Table 2).

[0098] Then, a steel sheet sample was produced by performing an annealing treatment in a furnace having an oxygen concentration of 20 ppm or less under an N2-4 vol% H2 gas atmosphere at a holding temperature of 800°C and a holding time of 100 seconds. The temperature increase rate during annealing was set to 6.0°C / sec up to 500°C and 2.0°C / sec from 500°C to the holding temperature. The dew point of the annealing atmosphere was set to 0°C for the first half of 40 seconds and -20°C for the second half of 60 seconds up to the holding time. In addition, the annealing treatment was performed under a tension of 15 MPa.

[0099] Further, the annealed steel sheet was immersed in a hot dip galvanizing bath (Zn-0.2% Al) at 450°C for 3 seconds, and then drawn at 100 mm / sec, and the plating adhesion amount was controlled to 50 g / m 2 using N2 wiping gas, to obtain a plated steel sheet having a plating layer on both sides.

[0100] <Example Nos. 2 to 38> A plated steel sheet was produced under the same conditions as in Example 1, except that the chemical components were as described in Table 1, the brush grinding conditions, the annealing treatment conditions, and the plating conditions were as described in Table 2. In No. 37, brush grinding was performed using D-100 (condition "B" in Table 2) manufactured by Hotani Co. as a grinding brush. D-100 is a brush having a higher grinding force than M-33. Also, in Table 2, the plating type, "a" means Zn-0.2% Al, and "b" means Zn-22% Al-8% Mg.

[0101] (Surface roughness after brush grinding) When the plated steel sheet was produced, the surface roughness of the hot-rolled steel sheet after brush grinding was measured. Regarding the surface roughness, 10 points were randomly selected on the surface on the side of the surface layer portion, the surface profile was measured at each point using a contact-type surface roughness meter, and the surface roughness at these points was arithmetically averaged to obtain the arithmetic average roughness Ra in accordance with JIS B 0601:2013. The surface roughness after brush grinding was evaluated in accordance with the following evaluation criteria and is shown in Table 2.

[0102] Evaluation AA: less than 2.0 μm Evaluation A: 2.0 μm or more and less than 3.0 μm Evaluation B: 3.0 μm or more (Tensile strength evaluation) For each steel sheet, a JIS No. 5 tensile test piece having a direction at right angles to the rolling direction as the length direction was collected, a tensile test was performed in accordance with JIS Z 2241:2011, the tensile strength was obtained, and the evaluation was performed as follows.

[0103] 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 (surface layer structure) A sample cut to 30 mm x 30 mm was collected, and GDS measurement was performed in the thickness direction 5 times using the above method, and the depth at which the C concentration was 0.05% or less was calculated and recorded in "C≤0.05% depth" in Table 3.

[0104] In addition, a sample cut to 25 mm x 15 mm was collected, and nitric acid ethanol etching was performed, and the thickness of the layer in which the ferrite area ratio was 90% or more was measured using SEM observation for the T section of each sample. The thickness was measured at 5 points at equal intervals in the range of 500 μm in the T direction, and the average value was taken. Here, the starting point of the "depth" was the interface between the plated layer and the steel sheet.

[0105] In addition, the amount of solid-solved Si in the ferrite phase of the layer in which the ferrite area ratio was 90% or more (high ferrite layer) was calculated using an X-ray photoelectron spectrometer, the ratio of the peak area intensity of Si (Si-Si, Fe-Si) to Si (SiO x ) + Si (Si-Si, Fe-Si) was calculated, and multiplied by the Si content in the steel at a position 4 μm in the thickness direction of the steel sheet from the interface between the steel sheet and the plated layer, thereby calculating the amount of solid-solved Si. The Si content was calculated by the previously described GDS measurement. Further, the amount of solid-solved Al was calculated as Al measured by subtracting the insoluble residue on the filter paper generated during the analysis of Al.

[0106] (resistance to LME) Two samples cut to a size of 50 mm x 100 mm were collected from each plated steel sheet, and for the two samples, spot welding was performed using a dome radius type welding electrode with a tip diameter of 8 mm at an angle of 2°, a pressing force of 5.0 kN, a current time of 1.2 seconds, and a current of 12 kA, and a welded joint was produced.

[0107] Reference Figure 2 The evaluation of the resistance to LME will be described. The resistance to LME was evaluated by the length of the LME cracking (crack 23 of the shoulder) generated at the shoulder 22 of the welded portion formed by overlapping two steel sheets 21 and performing spot welding. The shoulder refers to the inclined portion of the periphery of the concave portion generated by spot welding. According to the length of the crack 23 of the shoulder, the evaluation was as follows.

[0108] Evaluation AAA: 0 μm Evaluation AA: more than 0 μm and less than 50 μm Evaluation A: 50 μm or more and less than 160 μm Evaluation B: 160 μm or more The results of each evaluation are shown in Table 3. "C < 0.05% depth" in Table 3 means the depth where the C concentration is 0.05% or less, "high α layer thickness" means the thickness of the layer where the ferrite area ratio is 90% or more, and "Si + Al in α phase" means the total of the solid solution Si amount and the solid solution Al amount in the ferrite phase.

[0109] Nos. 1 to 23 are examples of the present application, and have high LME resistance.

[0110] The steel sheet of No. 24 has a large amount of C. Therefore, the C amount near the surface layer is also large, and even if decarburization is performed, the C amount of the surface layer does not become low, the depth where the C concentration measured by GDS is 0.05% or less is shallow, and the thickness of the high ferrite layer becomes thin, and as a result, the LME resistance is poor.

[0111] The steel sheet of No. 25 has a small amount of Si. Therefore, decarburization of the surface layer portion is not performed, the depth where the C concentration measured by GDS is 0.05% or less is shallow, and the contents of the solid solution Si and the solid solution Al in the ferrite phase become low. As a result, the LME resistance is poor.

[0112] The steel sheet of No. 26 has a large amount of Si. Therefore, external oxidation is performed, decarburization of the surface is inhibited, and the depth where the C concentration measured by GDS is 0.05% or less becomes shallow. As a result, the LME resistance is poor.

[0113] The steel sheet of No. 27 has a small amount of sol. Al. Therefore, decarburization of the surface layer portion is not performed, the depth where the C concentration measured by GDS is 0.05% or less becomes shallow, and the thickness of the high ferrite layer becomes thin. In addition, the contents of the solid solution Si and the solid solution Al in the ferrite phase become low. As a result, the LME resistance is poor.

[0114] The steel sheet of No. 28 has a large amount of Al. Therefore, external oxidation is performed, decarburization of the surface is inhibited, and the depth where the C concentration measured by GDS is 0.05% or less becomes shallow. As a result, the LME resistance is poor.

[0115] The steel sheet of No. 29 has a small total amount of Si and Al. Therefore, decarburization of the surface layer portion is not performed, the depth where the C concentration measured by GDS is 0.05% or less is shallow, the thickness of the high ferrite layer is thin, and the contents of the solid solution Si and the solid solution Al in the ferrite phase become low. As a result, the LME resistance is poor.

[0116] The dew point during the first half of the holding at the annealing of No. 30 is low. Therefore, decarburization of the surface layer and internal oxidation do not proceed, the depth of the C concentration of 0.05% or less measured by GDS is 0, and the thickness of the high ferrite layer is 0. Therefore, the contents of solid-solved Si and solid-solved Al in the ferrite phase cannot be measured. As a result, the LME resistance is poor.

[0117] The dew point during the first half of the holding at the annealing of No. 31 is high. Therefore, external oxidation proceeds, decarburization of the surface is suppressed, the depth of the C concentration of 0.05% or less measured by GDS becomes shallow, and the thickness of the high ferrite layer becomes thin. As a result, the LME resistance is poor.

[0118] The dew point during the second half of the holding at the annealing of No. 32 is low. Therefore, decarburization of the surface layer does not proceed, the depth of the C concentration of 0.05% or less measured by GDS becomes shallow, and the thickness of the high ferrite layer becomes thin. As a result, the LME resistance is poor.

[0119] The dew point during the second half of the holding at the annealing of No. 33 is high. External oxidation proceeds, decarburization of the surface is suppressed, the depth of the C concentration of 0.05% or less measured by GDS becomes shallow, and the thickness of the high ferrite layer becomes thin. As a result, the LME resistance is poor.

[0120] The holding temperature at the annealing of No. 34 is low. Therefore, decarburization of the surface layer and internal oxidation do not proceed, the depth of the C concentration of 0.05% or less measured by GDS is shallow, and the thickness of the high ferrite layer becomes thin. As a result, the LME resistance is poor.

[0121] The holding time at the annealing of No. 35 is 30 seconds, which is within the above range, but the dew point is switched at the time of 20 seconds, so the proportion of the first half of the holding time is large, and the proportion of the second half of the holding time is relatively small. Therefore, Si and Al do not sufficiently diffuse from the inside of the steel sheet to the surface side, and the contents of solid-solved Si and solid-solved Al in the ferrite phase become low. As a result, the LME resistance is poor.

[0122] The holding time at the annealing of No. 36 is 34 seconds, which is within the above range, but the dew point is switched at the time of 4 seconds, so the proportion of the first half of the holding time is small. Therefore, decarburization of the surface layer does not proceed, the depth of the C concentration of 0.05% or less measured by GDS becomes shallow, and the thickness of the high ferrite layer becomes thin. As a result, the LME resistance is poor.

[0123] The grinding force of the brush used in the brush grinding of No. 37 is large. Therefore, the grinding amount of the surface layer of the steel sheet becomes large, and the strain into the surface layer becomes small. Therefore, decarburization of the surface layer does not proceed, the depth of the C concentration of 0.05% or less measured by GDS becomes shallow, and the thickness of the high ferrite layer becomes thin. In addition, the contents of solid-solved Si and solid-solved Al in the ferrite phase become low. As a result, the LME resistance is poor.

[0124] No. 38 did not change the dew point at the front half and the back half at the time of annealing, and was a constant dew point. Therefore, Si and Al did not sufficiently diffuse from the inside of the steel sheet to the surface side, and the content of solid-solved Si and solid-solved Al in the ferrite phase became low. As a result, the LME resistance was poor.

[0125] Industrial Applicability According to the present application, a plated steel sheet having high LME resistance can be provided, which can be suitably used for automobile, household electric appliance, building material, and the like, particularly for automobile use. Therefore, the present application is an application having extremely high industrial applicability.

[0126] Explanation of Reference Signs 11 plated steel sheet 12 steel sheet 13 plated layer 14 high ferrite layer 15 inner side of steel sheet 21 steel sheet 22 welded portion 23 crack of shoulder portion

Claims

1. A plated steel sheet characterized by, It has: a steel sheet, and a plated layer provided on one side or both sides of the steel sheet, wherein the plated steel sheet has a tensile strength of 780 MPa or more, the steel sheet contains, in mass%: C：0.05~0.40%、 Si: 0.7 to 3.0%, Mn: 0.1 to 5.0%, sol. Al: 0.5 to 2.0%, P: 0.0300% or less, S: 0.0300% or less, N: 0.0100% or less, B：0~0.0100%、 Ti: 0 to 0.1500%, Nb: 0 to 0.150%, V：0~0.150%、 Cr：0~2.00%、 Ni: 0 to 2.00%, Cu: 0 to 2.0000%, Mo: 0 to 1.00%, W：0~1.000%、 Ca: 0 to 0.1000%, Mg: 0 to 0.100%, Zr：0~0.100%、 Hf: 0 to 0.100%, REM: 0 to 0.1000%, the remainder being Fe and impurities, and the total content of Si and sol. Al is 1.3% or more, the plated layer contains Zn, the depth at which the C concentration measured by GDS is 0.05% or less in the thickness direction of the steel sheet from the interface between the steel sheet and the plated layer is 8 μm or more, the thickness of the layer in which the area fraction of ferrite phase is 90% or more in the thickness direction of the steel sheet from the interface between the steel sheet and the plated layer is 8 μm or more, the total solid solution amount of metallic Si and metallic Al in the ferrite phase is 0.3% or more at a position 4 μm in the thickness direction of the steel sheet from the interface between the steel sheet and the plated layer.

2. The plated steel sheet according to claim 1, characterized by, the depth at which the C concentration measured by GDS is 0.05% or less in the thickness direction of the steel sheet from the interface between the steel sheet and the plated layer is 15 μm or more.

3. The plated steel sheet according to claim 1, characterized by, the thickness of the layer in which the area fraction of ferrite is 90% or more in the thickness direction of the steel sheet from the interface between the steel sheet and the plated layer is 20 μm or more, and the total solid solution amount of metallic Si and metallic Al in the ferrite phase is 0.5% or more.

4. The plated steel sheet according to any one of claims 1 to 3, characterized by, the plated layer contains, in mass%:

5. The galvanized steel sheet according to claim 4, characterized in that, Fe: 0 to 3.0%, Al: 0 to 30.0%, and Mg: 0 to 10.0%, the remainder being Zn and impurities. the plated layer contains, in mass%: Al: 10.0 to 30.0%, and Mg: 4.5 to 10.0%.

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