Steel sheet, plated steel sheet, and automobile member

Abstract

The present invention has as its technical issue the provision of steel sheet and plated steel sheet having a high LME resistance. The steel sheet of the present invention has a tensile strength of 780 MPa or more, has a predetermined chemical composition, has a surface roughness Ra of the steel sheet of 3.0 µm or less, has a depth in a thickness direction from the surface of the steel sheet wherein a C concentration measured by GDS is 0.02% or less of 3 µm or more, has a thickness in a thickness direction from the surface of the steel sheet of a layer having an area ratio of ferrite of 90% or more of 3 µm or more, and, in grazing incidence X-ray diffraction with an incidence angle of 1° with respect to the steel sheet surface, 0.45 ≤ I(110) / ( I(110) + I(200) + I(211) ) ≤ 0.90 is satisfied, when I(110) denotes a diffraction intensity corresponding to the (110) plane, I(200) denotes a diffraction intensity corresponding to the (200) plane, and I(211) denotes a diffraction intensity corresponding to the (211) plane.

Description

FIELD

[0001] 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 a high LME resistance.BACKGROUND

[0002] In recent years, the steel sheet used in automobiles, household electric appliances, building materials, and other various fields has been made increasingly higher in strength. For example, in the automobile field, use of high strength steel sheet has been increasing with the aim of lightening the weight of car bodies so as to improve fuel efficiency.

[0003] In welding galvanized steel sheet, in particular high strength steel sheet, for example as described in PTL 1, liquid metal embrittlement (LME) cracking sometimes causes the weldability to fall.

[0004] PTL 2 discloses steel sheet suppressed in LME cracking to improve weldability wherein the surface layer part of the steel sheet has size 20 nm or more Si oxide grains present in a 3000 to 6000 / mm 2< number density by a suitable particle size distribution.[CITATION LIST][PATENT LITERATURE]

[0005] [PTL 1] WO2019 / 116531 [PTL 2] WO2020 / 218575 SUMMARY[TECHNICAL PROBLEM]

[0006] To suppress the occurrence of LME cracking, for example, it is effective to keep the Zn etc. contained in a plating layer from penetrating steel sheet with a microstructure transformed to austenite at the time of welding. There is room for improvement on this point.

[0007] The present invention, in consideration of such a situation, has as its technical problem the provision of steel sheet and plated steel sheet having a high LME resistance.[SOLUTION TO PROBLEM]

[0008] The inventors engaged in intensive studies on means for solving the above technical problem. As a result, they discovered that by imparting strain to steel sheet before annealing by a blasting material under suitable conditions to render it a suitable surface condition and annealing it by a high dew point, the surface layer of the steel sheet is decarburized and further a layer with a high ferrite fraction where the ferrite is randomly oriented is formed and, therefore, as a result, it becomes possible to suppress LME.

[0009] The present invention was made based on the above discovery and further by additional study. Its gist is as follows: (1) A steel sheet having a tensile strength of 780 MPa or more, wherein a chemical composition of the steel sheet comprises, by mass%, C: 0.05 to 0.40%, Si: 0.5 to 3.0%, Mn: 0.1 to 5.0%, sol. Al: 0 to 3.0%, 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 0.1500%, 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 comprising Fe and impurities; a surface roughness Ra of the steel sheet is 3.0 µm or less; a depth in a thickness direction from the surface of the steel sheet wherein a C concentration measured by GDS is 0.02% or less is 3 µm or more; a thickness in a thickness direction from the surface of the steel sheet of a layer having an area ratio of ferrite of 90% or more is 3 µm or more; and, in grazing incidence X-ray diffraction with an incidence angle of 1° with respect to the steel sheet surface, 0.45 ≤I(110) / ( I(110) + I(200) + I(211) ) ≤ 0.90 is satisfied, when I(110) denotes a diffraction intensity corresponding to the (110) plane, I(200) denotes a diffraction intensity corresponding to the (200) plane, and I(211) denotes a diffraction intensity corresponding to the (211) plane. (2) The steel sheet of (1), wherein in the thickness direction from the steel sheet surface, the depth with a C concentration of 0.02% or less is 10 µm or more. (3) The steel sheet of (1) or (2), wherein 0.45 ≤ 1(110) / ( I(110) + I(200) + I(211) ) ≤ 0.75 is satisfied. (4) The steel sheet of any of (1) to (3), wherein in the thickness direction from the steel sheet surface, the thickness of the layer with a ferrite area ratio of 90% or more is 8 µm or more. (5) The steel sheet of any of (1) to (4), wherein the surface roughness Ra of the steel sheet is 2.0 µm or less. (6) A plated steel sheet comprising the steel sheet of any of (1) to (5) and a plating layer comprising Zn, provided at least at part of the surface of the steel sheet. (7) An automobile member comprising the steel sheet of any of (1) to (5) or plated steel sheet of (6). [ADVANTAGEOUS EFFECT OF INVENTION]

[0010] 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

[0011] FIG. 1 is a view showing one example of results of measurement of grazing incidence X-ray diffraction in the case where the ferrite phase is randomly oriented and the case where it is not. FIG. 2 is a view explaining a position of cracking covered in evaluation of the LME resistance in an embodiment. EMBODIMENTS OF INVENTION

[0012] Below, the present invention will be explained. The present invention is not limited to the following aspects. First, details on improvement of the LME resistance in the present invention will be explained.

[0013] LME cracking, for example, is due to the microstructure of steel sheet being heated and transforming to austenite at the time of spot welding and penetration of molten zinc formed by the plating melting into the grain boundaries of the austenite at the surface layer part of the steel sheet. It is believed to arise due to the molten zinc which penetrated into the austenite grain boundaries causing the steel sheet to become brittle and further tensile stress being applied to the steel sheet at the time of welding. The inventors came up with the idea of utilizing the microstructure of the surface layer part of the steel sheet as a method for improving the LME resistance. Specifically, by making the structure of the surface layer of the steel sheet a microstructure with a low C concentration and mainly comprised of the low LME sensitivity ferrite phase and, further, by making the ferrite phase randomly oriented, it is possible to suppress the occurrence of LME. Here, in the present invention, "LME resistance" means the property of LME cracking being suppressed in the steel sheet while "LME sensitivity" means the property of LME cracking easily occurring in the steel sheet.

[0014] The "ferrite phase being randomly oriented" means the features of the ferrite grain boundaries being leveled as a whole. In other words, it means the crystal orientations of the ferrite grains of the ferrite phase being randomized. Bhy the crystal orientations of the ferrite grains being randomized, grain boundaries oriented in specific directions are kept from segregating and being continuously or intermittently connected. It is believed that LME cracking occurs due to Zn from the plating concentratedly penetrating the grain boundaries where grain boundary energies locally become lower. That is, if grain boundaries with grain boundary energies locally becoming lower are present in succession, it is believed the Zn from the plating will concentrate there and LME cracking will easily occur. By the features of the grain boundaries being leveled as a whole, it is believed grain boundaries with grain boundary energies becoming locally lower will no longer be connected in succession, Zn from the plating will be kept from locally concentrating, and as a result LME resistance will be improved.

[0015] In the present invention, the "ferrite phase being randomly oriented" is expressed by the following conditional expression. That is, steel sheet satisfying the following conditional expression means the ferrite phase being randomly oriented.(Conditional Expression)

[0016] In grazing incidence X-ray diffraction with an incidence angle of 1° with respect to the steel sheet surface, when a diffraction intensity corresponding to the (110) plane is I(110), a diffraction intensity corresponding to the (200) plane is I(200), and a diffraction intensity corresponding to the (211) plane is I(211), 0.45 ≤ I 110 / I 110 + I 200 + I 211 ≤ 0.90

[0017] By configuring the steel sheet in the above way, in the present invention, when producing the steel sheet, strain is imparted to the steel sheet after cold rolling, then the steel sheet is annealed at a high dew point. Due to this, decarburization is promoted and a ferrite phase becomes easier to form at the steel sheet surface. Furthermore, at this time, the present invention was completed based on the discovery that by controlling the temperature of start of humidification, it is possible to randomize the orientation of the ferrite phase. Below, the present invention will be explained in detail.[Tensile Strength]

[0018] The steel sheet according to the present invention has a tensile strength of 780 MPa or more, that is, is high strength steel sheet. The present invention suppresses the LME occurring at steel sheet with a high strength. The steel sheet according to the present invention specifically has 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, may for example be 2000 MPa or less. The tensile strength may be measured by obtaining a JIS No. 5 tensile test piece having a direction perpendicular to the rolling direction and sheet thickness direction as a longitudinal direction and testing it based on JIS Z 2241: 2011. The tensile strength may be 980 MPa or more or 1180 MPa or more. If the rolling direction at the steel sheet cannot be identified, at the time of measurement of the tensile strength, a JIS No. 5 test piece having any direction on the surface of the steel sheet as the longitudinal direction may be taken.[Chemical Composition]

[0019] Below, the chemical composition of the steel sheet will be explained. The "%" relating to the chemical composition shall mean "mass%". Further, in the numerical ranges in the chemical composition, numerical ranges expressed using "to" mean ranges including the numerical values described before and after the "to" as the lower limit value and upper limit value.(C: 0.05 to 0.40%)

[0020] C (carbon) is an element securing the strength of steel. To obtain the 780 MPa or more tensile strength covered by the present invention, the content of C is 0.05% or more. To keep the C concentration of the surface layer explained later from becoming too high and, further, considering the weldability, the content of C is 0.40% or less. The content of C may also be 0.08% or more, 0.10% or more, or 0.15% or more. The content of C may also be 0.37% or less, 0.35% or less, or 0.30% or less.(Si: 0.5 to 3.0%)

[0021] Si (silicon) is an element promoting ferrite stabilization and decarburization. By Si being included, decarburization proceeds at the surface layer part due to the later explained pretreatment and heat treatment and, further, the ferrite at the surface layer part becomes stabilized, whereby the LME resistance is improved. To obtain this effect, the content of Si is 0.5% or more. If the content of Si is too great, even if performing high dew point annealing, external oxidation will progress and oxides (scale) will form at the surface layer of the steel sheet. Conversely, decarburization at the surfacemost part will be suppressed and the effect of improvement of the LME resistance will become smaller. Considering this point, the content of Si is 3.0% or less. The content of Si may also be 0.6% or more, 0.7% or more, or 0.8% or more. The content of Si may also be 2.5% or less, 2.0% or less, or 1.5% or less.(Mn: 0.1 to 5.0%)

[0022] Mn (manganese) is an element effective for obtaining a hard structure and thereby improving the strength of the steel. Considering the strength of steel, the content of Mn is 0.1% or more. Further, considering the drop in formability due to Mn segregation, the content of Mn is 5.0% or less. 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.(sol. Al: 0 to 3.0%)

[0023] Al (aluminum) is an element which dissolves in steel and, like Si, promotes ferrite stabilization and decarburization. "sol. Al" means acid soluble Al which does not form Al 2 O 3 or other oxides and can dissolve in acid and is produced in the process of analysis of Al. It is found as the Al measured after deducting the insoluble residue on the filter paper. In the steel sheet of the present invention, the role of the sol. Al can also be obtained by inclusion of Si, so 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 will progress and oxides (scale) will form at the surface layer of the steel sheet. Conversely, decarburization at the surfacemost part will be suppressed and the effect of improvement of the LME resistance will become smaller. Considering this point, the content of sol. Al is 3.0% or less. The content of sol. Al may also be 0.1% or more, 0.3% or more, or 0.5% or more. The content of sol. Al may also be 2.0% or less, 1.5% or less, or 1.0% or less.

[0024] Si and sol. Al, as explained above, are also elements which cause the LME resistance to fall if excessively added, therefore the total value of the contents of Si and sol. Al is preferably 1.8% or less. The total of the contents of Si and sol. Al may also be 1.7% or less, 1.6% or less, or 1.5% or less.(P: 0.0300% or less)

[0025] 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 be more than 0%, 0.0001% or more, or 0.0005% or more.(S: 0.0300% or less)

[0026] S (sulfur) is an impurity generally contained in steel. If the content of S is more than 0.0300%, the weldability falls and, further, the amount of precipitation of MnS increases whereby 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 be more than 0%, 0.0001% or more, or 0.0005% or more.(N: 0.0100% or less)

[0027] 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 0.0080% 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 the manufacturing costs, the content of N may be more than 0%, 0.0005% or more, or 0.0010% or more.(O: 0.0030% or less)

[0028] O (oxygen) is an element which forms oxides and causes the formability of the steel sheet to fall. If the content of O is too great, oxides are excessively formed etc. and the formability of the steel sheet easily falls. Therefore, the content of O is 0.0030% or less. The content of O may also be 0.0026% or less, 0.0024% or less, 0.0020% or less, or 0.0018% or less. O is preferably not contained. The lower limit of the content of O is 0%. From the viewpoint of the production costs, the content of O may also be more than 0%, 0.0005% or more, or 0.0010% or more.(B: 0 to 0.0100%)

[0029] B (boron) is an element raising the hardenability and contributing to improvement of the strength and further segregating at the grain boundaries to strengthen the grain boundaries and improve the toughness, therefore may be contained in accordance with need. It is not an essential element, therefore the lower limit of the content of B is 0%. This effect is obtained even when contained in a trace amount, but if included, the content of B is preferably 0.0001% or more. Further, from the viewpoint of securing sufficient toughness, the content of B is 0.0100% or less. The content of B may also be 0.0002% or more, 0.0003% or more, or 0.0005% or more. The content of B may also be 0.0080% or less, 0.0060% or less, 0.0040% or less, or 0.0020% or less.(Ti: 0 to 0.1500%)

[0030] Ti (titanium) is an element which precipitates as TiC during cooling of steel and contributes to improvement of strength, therefore may be contained in accordance with need. It is not an essential element, therefore the lower limit of the content of Ti is 0%. This effect is obtained even when contained in a trace amount, but if contained, the content of Ti is preferably 0.0001% or more. The content of Ti may also be 0.0003% or more or 0.0005% or more. On the other hand, if excessively contained, coarse TiN is produced and the toughness is liable to be harmed, therefore the content of Ti is 0.1500% or less. The content of Ti may also be 0.1000% or less, 0.0500% or less, 0.0050% or less, or 0.0020% or less.(Nb: 0 to 0.1500%)

[0031] Nb (niobium) is an element contributing to improvement of strength through improvement of the hardenability, therefore may be contained in accordance with need. It is not an essential element, therefore the lower limit of the content of Nb is 0%. This effect is obtained even when contained in a trace amount, but if contained, the content of the Nb is preferably 0.0001% or more. The content of Nb may also be 0.0005% or more or 0.0010% or more. On the other hand, from the viewpoint of securing sufficient toughness, the content of Nb is 0.1500% or less. The content of Nb may also be 0.1000% or less, 0.0600% or less, or 0.0200% or less.(V: 0 to 0.150%)

[0032] V (vanadium) is an element contributing to improvement of strength through improvement of the hardenability, therefore may be contained in accordance with need. It is not an essential element, therefore the lower limit of the content of V is 0%. This effect is obtained even when contained in a trace amount, but if contained, the content of the V is preferably 0.001% or more. The content of V may also be 0.003% or more, 0.005% or more, or 0.008% 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 0.100% or less, 0.060% or less, or 0.020% or less.(Cr: 0 to 2.00%)

[0033] Cr (chromium) is an element contributing to improvement of strength through improvement of the hardenability of steel, therefore may be contained in accordance with need. It is not an essential element, therefore the lower limit of the content of Cr is 0%. This effect is obtained even when contained in a trace amount, but if contained, the content of the Cr is preferably 0.001% or more. The content of Cr may also be 0.01% or more, 0.05% or more, or 0.10% 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, 1.50% or less, 0.50% or less, or 0.20% or less.(Ni: 0 to 2.00%)

[0034] Ni (nickel) is effective for raising the hardenability of steel to raise the strength of the steel, therefore may be contained in accordance with need. It is not an essential element, therefore the lower limit of the content of Ni is 0%. This effect is obtained even when contained in a trace amount, but if contained, the content of the Ni is preferably 0.001% or more. The content of Ni may also be 0.01% or more, 0.02% or more, or 0.05% or more. On the other hand, excessive addition of Ni causes the cost to rise, therefore the content of Ni is 2.00% or less. The content of Ni may also be 1.80% or less, 1.50% or less, 0.50% or less, or 0.20% or less.(Cu: 0 to 2.0000%)

[0035] Cu (copper) is effective for raising the hardenability of steel to raise the strength of the steel, therefore may be contained in accordance with need. It is not an essential element, therefore the lower limit of the content of Cu is 0%. This effect is obtained even when contained in a trace amount, but if contained, the content of the Cu is preferably 0.0001% or more. The content of Cu may also be 0.0002% or more or 0.0005% or more. On the other hand, from the viewpoint of suppressing a drop in toughness or cracking of the slab after casting, the content of Cu is 2.0000% or less. The content of Cu may also be 1.8000% or less, 1.5000% or less, 0.0050% or less, or 0.0020% or less.(Mo: 0 to 1.00%)

[0036] Mo (molybdenum) is effective for raising the hardenability of steel to raise the strength of the steel, therefore may be contained in accordance with need. It is not an essential element, therefore the lower limit of the content of Mo is 0%. This effect is obtained even when contained in a trace amount, but if contained, the content of the Mo is preferably 0.001% or more. The content of Mo may also be 0.01% or more, 0.02% or more, or 0.03% 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 0.80% or less, 0.60% or less, or 0.20% or less.(W: 0 to 1.000%)

[0037] W (tungsten) is effective for raising the hardenability of steel to raise the strength of the steel, therefore may be contained in accordance with need. It is not an essential element, therefore the lower limit of the content of W is 0%. This effect is obtained even when contained in a trace amount, but if contained, the content of the W 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 suppressing a drop in toughness, the content of W is 1.000% or less. The content of W may also be 0.800% or less, 0.600% or less, 0.300% or less, 0.100% or less, or 0.020% or less.(Ca: 0 to 0.1000%)

[0038] Ca (calcium) is an element contributing to control of inclusions, in particular to finely dispersing inclusions, and having the action of raising the toughness, therefore may be contained in accordance with need. It is not an essential element, therefore the lower limit of the content of Ca is 0%. This effect is obtained even when contained in a trace amount, but if contained, the content of the Ca 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, excessive inclusion causes deterioration of the surface properties to surface in some cases, therefore the content of Ca is 0.1000% or less. The content of Ca may also be 0.0800% or less, 0.0500% or less, 0.0300% or less, 0.0100% or less, or 0.0010% or less.(Mg: 0 to 0.100%)

[0039] Mg (magnesium) is an element contributing to control of inclusions, in particular to finely dispersing inclusions, and having the action of raising the toughness, therefore may be contained in accordance with need. It is not an essential element, therefore the lower limit of the content of Mg is 0%. This effect is obtained even when contained in a trace amount, but if contained, the content of the Mg is preferably 0.0001% or more. The content of Mg may also be 0.0005% or more or 0.0008% or more. On the other hand, excessive inclusion causes deterioration of the surface properties to surface in some cases, 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, 0.030% or less, 0.010% or less, or 0.002% or less.(Zr: 0 to 0.100%)

[0040] Zr (zirconium) is an element contributing to control of inclusions, in particular to finely dispersing inclusions, and having the action of raising the toughness, therefore may be contained in accordance with need. It is not an essential element, therefore the lower limit of the content of Zr is 0%. This effect is obtained even when contained in a trace amount, but if contained, the content of the Zr is preferably 0.001% or more. The content of Zr may also be 0.005% or more or 0.010% or more. On the other hand, excessive inclusion causes deterioration of the surface properties to surface in some cases, therefore the content of Zr is 0.100% or less. The content of Zr may also be 0.050% or less or 0.030% or less.(Hf: 0 to 0.100%)

[0041] Hf (hafnium) is an element contributing to control of inclusions, in particular to finely dispersing inclusions, and having the action of raising the toughness, therefore may be contained in accordance with need. It is not an essential element, therefore the lower limit of the content of Hf is 0%. This effect is obtained even when contained in a trace amount, but if contained, the content of the Hf is preferably 0.0001% or more. The content of Hf may also be 0.0003% or more or 0.0005% or more. On the other hand, excessive inclusion causes deterioration of the surface properties to surface in some cases, therefore the content of Hf is 0.100% or less. The content of Hf may also be 0.050% or less, 0.030% or less, 0.010% or less, 0.005% or less, or 0.002% or less.(REM: 0 to 0.1000%)

[0042] A REM (rare earth element) is an element contributing to control of inclusions, in particular to finely dispersing inclusions, and having the action of raising the toughness, therefore may be contained in accordance with need. It is not an essential element, therefore the lower limit of the content of a REM is 0%. This effect is obtained even when contained in a trace amount, but if contained, the content of a REM is preferably 0.0001% or more. The content of a REM may also be 0.0003% or more or 0.0005% or more. On the other hand, excessive inclusion causes deterioration of the surface properties to surface in some cases, therefore the content of a REM is 0.1000% or less. The content of a REM may also be 0.0500% or less, 0.0300% or less, 0.0100% or less, 0.0050% or less, or 0.0020% or less. Note that "REM" is an abbreviation for a rare earth metal and means an element belonging to the lanthanoids. A REM is usually added as a misch metal.

[0043] In the steel sheet according to the present invention, the balance beside the above chemical composition comprises Fe and impurities. The balance may consist of Fe and impurities, that is, the balance may comprise only Fe and impurities. Here, "impurities" are constituents entering due to various factors in the production process such as the ore, scraps, and other such starting materials 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, it means something contained to an extent where the LME resistance sought in the steel sheet of the present invention is obtained.

[0044] 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. For O, the inert gas melting-infrared absorption method is used. These analyses may be performed on a sample taken from the steel sheet by a method based on JIS G0417: 1999.

[0045] Next, the surface layer part of the steel sheet will be explained. Here, the "surface layer part of the steel sheet" means the layer-shaped region having a depth from the surface of the steel sheet (sheet surface of steel sheet) down by a predetermined distance in the sheet thickness direction of the steel sheet. In the case of plated steel sheet, the surface of the steel sheet (sheet surface of steel sheet) means the surface of the steel sheet (sheet surface) excluding the plating. Note that the "predetermined distance in the sheet thickness direction" can be made the longer (deeper) length (depth) among the later explained "depth of surface layer with a C concentration of 0.02% or less" or "depth of layer with area ratio of ferrite phase of 90% or more".[Surface Roughness Ra]

[0046] The steel sheet of the present invention has a surface roughness Ra of the steel sheet of 3.0 µm or less. The surface roughness Ra is the arithmetic average roughness Ra defined by JIS B0601: 2013. In the case of plated steel sheet, the surface roughness Ra is the surface roughness of the interface of the steel sheet excluding the plating and the plating layer.

[0047] In the present invention, when measuring the surface roughness Ra, based on JIS B 0601: 2013, 10 measurement locations are selected at the surface of the steel sheet at random so that the interval between measurement locations becomes 1 mm or more. At each of the measurement locations, the surface profile is measured by a laser microscope (for example,"VK-X3000" made by Keyence). Specifically, the laser microscope is used to take a photograph by a power of 20X. At the photographed image, using a standard length of 2000 µm, the arithmetic average roughness (Ra) at each measurement location is found. The arithmetic average value of the arithmetic average roughnesses (Ra) of the 10 points found at the respective measurement locations is made the "surface roughness Ra".[Depth of Surface Layer With C Concentration of 0.02% or Less]

[0048] In the steel sheet of the present invention, the depth from the steel sheet surface in the sheet thickness direction with a C concentration measured by GDS (glow discharge spectrometry) of 0.02% or less is 3 µm or more.

[0049] The LME sensitivity falls if the C concentration becomes lower, therefore by lowering the C concentration of the surface layer, the LME resistance is improved. Further, C is an austenite stabilizing element, therefore by this being less, the layer with a low LME sensitivity is stabilized

[0050] Such a surface layer structure (microstructure at surface layer part of steel sheet) can be obtained as a decarburized layer formed by making the chemical composition of the steel sheet the one explained above and performing the pretreatment step and annealing step explained later.

[0051] If the depth with a C concentration of 0.02% or less is 3 µm or more, this contributes to improvement of the LME resistance, so the upper limit of the depth with a C concentration of 0.02% or less is not particularly prescribed. The depth with a C concentration of 0.02% or less may, for example, also be 50 µm or less, 40 µm or less, or 30 µm or less. The depth with a C concentration of 0.02% or less is preferably 5 µm or more, more preferably 7 µm or more, still more preferably 10 µm or more, 15 µm or more, or 20 µm or more.

[0052] The GDS measurement is performed at five measurement points in the sheet thickness direction. At each of the measurement points, the arithmetic average value of depth of the region with a C concentration of 0.02% or less is made the depth of the surface layer with a C concentration of 0.02% or less. The five measurement points are randomly determined so that the measurement points of the steel sheet surface are separated by 5 mm or more intervals. The measurement conditions are as follows. Only naturally, measurement results can be obtained even without, for example, using the measurement apparatus or other conditions explained below, but if differences arise in the measurement results, the steel sheet according to the present invention is identified by measurement results obtained by the following conditions.

[0053] Apparatus: high frequency glow discharge optical emission spectrometry apparatus (made by LECO Japan Corporation, Model Name "GDS850A" Ar gas pressure: 0.3 MPa Anode diameter: 4 mmφ RF output: 30W Measurement time: 200 to 1500 seconds [Thickness of High Ferrite Layer (Layer With Area Ratio of Ferrite of 90% or More)]

[0054] At the steel sheet of the present invention, the thickness of the layer with an area ratio of the ferrite phase of 90% or more in the thickness direction from the steel sheet surface (below, referred to as the "high ferrite layer") is 3 µm or more.

[0055] If the thickness of the high ferrite layer is 3 µm or more, this contributes to improvement of the LME resistance, therefore the upper limit of the thickness is not particularly prescribed. The thickness of the high ferrite layer may, for example, 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 5 µm or more, more preferably 8 µm or more, still more preferably 10 µm or more or 20 µm or more.

[0056] The structure besides the ferrite in the high ferrite layer is not limited. For example, it may be made one or more of martensite, bainite, and cementite.

[0057] The thickness of the high ferrite layer is measured by finishing a thickness cross-section of the steel sheet by mechanical polishing to a mirror surface, then etching the observed cross-section by a Nital reagent and analyzing the secondary electron image by observation by an SEM. For the observation by an SEM, a field emission scanning electron microscope (for example, "JSM 7000F" made by JEOL, acceleration voltage: 15 kV) is used. In the observed cross-section, the range of depth from the surface of the steel sheet (sheet surface) down to a depth of 500 µm in the sheet thickness direction (vertical direction of observed cross-section) and range of width of 600 µm in the direction perpendicular to the sheet thickness direction (horizontal direction of observed cross-section) is made the observed field. In that observed cross-section, five observed fields in the direction perpendicular to the sheet thickness direction (horizontal direction of observed cross-section) are observed by an SEM so as to have observed fields having 1000 µm or more intervals between them to obtain secondary electron images. The observed resolution is made 1280×960 pixels. Note that, in the case of plated steel sheet, the "surface" of the steel sheet (sheet surface) is the surface of the steel sheet from which the plating has been removed.

[0058] The obtained five secondary electron images are calculated for ratios of ferrite by the point counting method. More specifically, first, equal interval lattices are drawn on the secondary images. Next, by finding the number of lattice points of each lattice with structures at the lattice points comprised of ferrite and dividing this by the total number of lattice points, the fraction of ferrite is measured. The greater the total number of lattice points, the more accurately the area ratio can be found. In the present embodiment, the lattice intervals are made 2 µm×2 µm and the total number of lattice points is made 1500 points.

[0059] In a secondary electron image, regions relatively small in brightness and not having substructures observable can be judged as ferrite. Here, "substructures" mean laths, blocks, or other transformed structures formed inside the prior austenite phase. In a secondary electron image, ferrite is observed as a region relatively low in brightness and having a spread of relatively monotonous brightness or hue. In the present invention, the microstructure other than ferrite does not have to be specially judged, but the judgment criteria for tempered martensite, pearlite, ferrite, fresh martensite, or retained austenite or bainite at the secondary electron image are shown below. A region having substructures (lath boundaries and block boundaries) in the grains and having carbides precipitated with several variants is judged as tempered martensite. Further, a region having cementite precipitated in a lamellar form is judged as pearlite. A region having a large brightness and not having substructures appearing by etching is judged as fresh martensite or retained austenite. A region not corresponding to any of the above is judged as bainite. For simplicity, the area ratio of the ferrite phase can be found if differentiating the ferrite from other structures.

[0060] Ferrite is low in LME sensitivity. The surface layer structure of the steel sheet being a structure mainly comprised of ferrite is preferable from the viewpoint of improvement of the LME resistance. Such a surface layer structure can be obtained by making the chemical composition of the steel sheet the one explained above and performing the pretreatment step and annealing step explained later.[Diffraction Intensity Ratio of Ferrite Phase by Grazing Incidence X-Ray Diffraction (XRD)]

[0061] The steel sheet of the present invention satisfies the conditional expression of 0.45 ≤ I 110 / I 110 + I 200 + I 211 ≤ 0.90 in grazing incidence X-ray diffraction with an incidence angle of 1° with respect to the steel sheet surface when the diffraction intensity corresponding to the (110) plane is I(110), the diffraction intensity corresponding to the (200) plane is I(200), and the diffraction intensity corresponding to the (211) plane is I(211).

[0062] In the conditional expression, the value of the term "I(110) / ( 1(110) + I(200) + I(211) )" is preferably 0.85 or less, more preferably 0.80 or less, and still more preferably 0.75 or less. The conditional expression means that the ferrite phase is randomly oriented. If the ferrite phase is completely randomly oriented, the value of the center term is 0.67.

[0063] Here, the grazing incidence X-ray diffraction (grazing incidence XRD, low angle incidence XRD, glancing angle XRD) is a measurement technique setting the incidence angle of the incident X-rays small and making only the detector scan the surface while maintaining that incidence angle (changing the detection angle). Due to this, it is possible to efficiently detect information on the orientation of crystal grains down to a depth of several µm or so of the surface layer part of a sample. In the present invention, the incidence angle of X-rays is fixed to 1° and the orientation of ferrite at the surface layer part of the steel sheet is detected. Note that the "incidence angle" is the angle formed by the surface of a sample (steel sheet) and the incident direction of the incident X-rays.

[0064] FIG. 1 shows examples of the results of grazing incidence XRD analysis for the case where the ferrite phase changes randomly (b) and the case where it does not (a). (a) shows the results of grazing incidence XRD analysis in an ordinary (conventional) steel sheet. It will be understood that the phase is oriented in the (110) direction. Therefore, the value in the middle term "I(110) / ( I(110) + I(200) + I(211) )" of the conditional expression becomes a relatively large 0.91. (b) shows the results of grazing incidence XRD analysis in the steel sheet of the present invention. Compared with (a), the orientation in the (110) direction becomes smaller. For this reason, the value in the middle term "I(110) / ( I(110) + I(200) + I(211) )" of the conditional expression becomes a relatively small 0.58.[Plating Layer]

[0065] The steel sheet of the present invention, as explained later, can have a plating layer. If having a plating layer, the starting point of the depth with a C concentration of 0.02% or less and the thickness of the layer with an area ratio of the ferrite phase of 90% or more by GDS measurement is the interface of the steel sheet and the plating layer. The interface of the steel sheet and plating layer in the present invention is determined in the following way. First, the Fe content in the thickness direction of the plated steel sheet by GDS measurement is measured. The highest value of this Fe content is made the Fe content of the steel sheet. The point becoming an Fe content of 93% of the Fe content of the steel sheet is defined as the "interface of the steel sheet and the plating layer".<Plated Steel Sheet>

[0066] The plated steel sheet according to the present invention has a plating layer containing Zn on the above-mentioned steel sheet according to the present invention. This plating layer may be formed at one of the surfaces of the steel sheet or may be formed at both surfaces. Further, it may be formed at only part of the surfaces as well. The plating layer may also be treated to alloy it.[Chemical Composition of Plating Layer]

[0067] The chemical composition of the plating layer is not limited so long as containing Zn. As the layer containing Zn, 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, Zn-15%Mg, etc. can be used.

[0068] 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 inhibiting corrosion of the steel sheet and measuring the obtained solution by ICP (high-frequency inductively coupled plasma) optical emission spectroscopy. As the acid solution to which an inhibitor for dissolving the plating layer is added, for example, a 10 mass% hydrochloric acid solution to which 0.06 mass% of an inhibitor for inhibiting corrosion of the base material steel sheet (ibit 710K made by Asahi Chemical Co., Ltd.) has been added may be used.

[0069] 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 / m 2< per surface. In the present invention, the amount of deposition of the plating layer is determined by dissolving the plating layer in an acid solution to which an inhibitor for inhibiting corrosion of the steel sheet has been added and finding the change in weight before and after acid washing and peeling off the plating layer.

[0070] The roughness of the interface of the steel sheet and plating layer (surface roughness Ra) corresponds to the roughness of the surface of the steel sheet explained above. The surface roughness Ra is 3.0 µm or less. If considering the adhesion of the plating, the surface roughness Ra is preferably 2.0 µm or less. The roughness of the interface (surface roughness Ra) may be made the surface roughness of the steel sheet measured after removing the plating layer. The plating layer is acid washed and removed by dissolving the plating layer in an acid solution to which an inhibitor is added.

[0071] Note that, the steel sheet of the present invention can have the effect of improvement of the LME resistance even if not galvanized. In general, if spot welding steel sheets which have not been galvanized, LME cracking will never occur so long as the state of contact with molten zinc does not arise near a spot welded part. However, if one steel sheet which has been galvanized and another steel sheet which has not been galvanized are spot welded, at the time of welding, molten zinc is formed at the overlaid surfaces of the steel sheets. For this reason, the molten zinc contacts the surface of the not plated steel sheet and LME cracking may occur.

[0072] The thickness of the steel sheet and plated steel sheet of the present invention is not particularly limited. For example, it may be 0.1 to 3.2 mm. The sheet thickness may also be 0.2 mm or more, 0.4 mm or more, or 0.6 mm or more. The sheet thickness may also be 3.0 mm or less, 2.5 mm or less, 2.0 mm or less, or 1.8 mm or less.<<Method of Production>>

[0073] 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 be obtained by, for example, 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 up the hot rolled steel sheet, a cold rolling step of cold rolling the coiled up hot rolled steel sheet to obtain cold rolled steel sheet, a pretreatment step of pretreating (shot blasting) the cold rolled steel sheet, and an annealing step of annealing the pretreated cold rolled steel sheet. Typically, after the hot rolling step, the hot rolled steel sheet need not be coiled, but may be pickled then cold rolled as it is.<Casting Step>

[0074] The conditions of the casting step are not particularly limited. For example, after smelting by a blast furnace, electric furnace, etc., various types of secondary refining may be performed, then the usual continuous casting, casting by ingot method, or other method may be used for casting.<Hot Rolling Step>

[0075] 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 it. If reheating, the heating temperature of the steel slab may, for example, be 1100 to 1250°C. In the hot rolling step, usually rough rolling and finish rolling are performed. The temperature and reduction rate of each rolling may be suitably changed in accordance with the desired microstructure or sheet thickness. For example, the end temperature of the finish rolling may be 900 to 1050°C and the reduction rate of the finish rolling may be 10 to 50%.<Coiling Step>

[0076] 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. For example, it may be 500 to 800°C. Before coiling or after coiling, then uncoiling, the hot rolled steel sheet may be subjected to predetermined heat treatment. Typically, the coiling step need not be performed and the hot rolled steel sheet can be pickled after the hot rolling step and the later explained cold rolling step performed.<Cold Rolling Step>

[0077] After pickling 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 or sheet thickness. For example, it may be 20 to 80%. After the cold rolling step, for example, the steel sheet may be air cooled down to room temperature for cooling.<Pretreatment Step>

[0078] To obtain a surface layer structure of steel sheet such as explained above, the predetermined pretreatment must be performed followed by annealing.

[0079] Pretreatment includes shot blasting for blasting the surface of the cold rolled steel sheet using a spherical polishing material. The polishing material used is not particularly limited, but, for example, steel balls (shot) having a center grain size of 40 to 450 µm can be used. As such steel balls (shot), for example, TSH30 made by WINOA IKK JAPAN may be mentioned. The amount of blasting of shot may be 5 to 400 kg / m 2< . Due to this, it is possible to introduce strain in the surface layer part of the steel sheet without increasing the surface roughness Ra of the steel sheet. By performing such shot blasting, in the later explained annealing step, decarburization is promoted and a structure with stable ferrite can be efficiently formed at the surface layer of the steel sheet. The greater the amount of blasting, the greater the effect of improvement of the LME resistance, but if more than a certain amount of blasting, for example, 400 kg / m 2< , the effect becomes saturated. Note that per unit time and unit area of the level of blasting of 400 kg / m 2< is 4.0×10 -4< kg / (mm 2< ·min).<Annealing Step>

[0080] After the pretreatment step, the cold rolled steel sheet is annealed.

[0081] In the present invention, an annealing step is performed of holding the steel sheet given strain by the shot blasting at a predetermined holding temperature and a high dew point. The speed of temperature rise up to the predetermined holding temperature is not particularly limited and may be 1 to 10°C / s. The dew point is controlled by humidification control from 300°C or more, preferably humidification control from 450 to 550°C. That is, the dew point (humidification) control start temperature is 300°C or more and less than 600°C, preferably 450 to 550°C in range. The dew point in the case of no dew point (humidification) control is usually less than -30°C. The dew point at the time of annealing (high dew point) is -30 to 20°C to promote decarburization. The dew point at the time of annealing (high dew point) is preferably - 10°C or more. Further, the dew point at the time of annealing is, preferably 5°C or less. The predetermined holding temperature at the annealing step (highest heating temperature) is 750 to 900°C, preferably 770 to 870°C, to promote decarburization. The holding time at the holding temperature (highest heating temperature) of the annealing step is 20 to 300 seconds, preferably 50 to 200 seconds. Further, the atmosphere is preferably a nonoxidizing atmosphere, for example, can be N 2 -1 to 10 vol%H 2 , N 2 -2 to 4 vol%H 2 .

[0082] By making the dew point, holding temperature, and holding time the above such ranges, it is possible to promote decarburization, lower the C concentration of the surface layer, and suitably control the ferrite phase fraction. Furthermore, by making the dew point (humidification) control start temperature the above such range, decarburization of the surface layer part of the steel sheet is promoted. Together with this, internal oxidation of Si or Mn is promoted, internal oxides are rapidly formed, and the formed internal oxides function as nucleation sites. As a result, the orientation of the ferrite phase is randomized. If the dew point (humidification) control start temperature is too low, external oxidation proceeds and internal oxidation of Si or Mn does not proceed, therefore the orientation of the ferrite phase becomes hard to randomize.

[0083] The annealing is performed in a state applying 1 to 20 MPa of tension. If tension is applied at the time of annealing, it becomes possible to more effectively introduce strain into the steel sheet and decarburization of the surface layer is promoted.

[0084] By performing the above-mentioned steps, decarburization is promoted at the surface layer part of the steel sheet and steel sheet with a surface layer part of the steel sheet made a structure mainly comprised of a randomly oriented ferrite phase can be obtained.<<Method of Production of Plated Steel Sheet>>

[0085] The plated steel sheet according to the present invention can be obtained by a plating step of forming a plating layer on the surface of the steel sheet produced as explained above.<Plating Step>

[0086] The plating may be performed in accordance with a method known to persons skilled in the art. The plating, for example, may be performed by hot dip coating or may be performed by electroplating. Preferably the plating is performed by hot dip coating. The conditions of the plating may be suitably set considering the chemical composition, thickness, amount of deposition, etc. of the desired plating layer. After the plating step, a known alloying step may be performed to obtain an alloyed plating.

[0087] The steel sheet and plated steel sheet according to the present invention are high in strength and have high LME resistances, therefore can be suitably used in automobiles, household electric appliance products, building materials, and other broad fields. In particular, they can be suitably used in the automobile field. The steel sheet and plated steel sheet used for automobiles are often spot welded. In this case, LME cracking easily occurs. For this reason, the effect of the present invention of realizing high LME resistance as an automobile member using the steel sheet and plated steel sheet according to the present invention as the steel sheet for automobile use is suitably exhibited.EXAMPLES

[0088] Below, examples will be used to explain the present invention in more detail. The present invention is not limited to these examples.<Test No. 1>

[0089] Molten steel adjusted to the chemical composition described in Test 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 by an end temperature of finish rolling of 950°C and a reduction rate of finish rolling of 30% to obtain hot rolled steel sheet. The obtained hot rolled steel sheet was coiled at the coiling temperature 650°C, pickled, then cold rolled by a reduction rate of 50% to obtain cold rolled steel sheet. The thickness of the cold rolled steel sheet was 1.6 mm.

[0090] Next, the surface of the obtained cold rolled steel sheet was blasted using as the blasting material TSH30 made by WINOA IKK JAPAN by a blasting rate of 5 kg / m 2< as shot blasting. The surface roughness Ra of the cold rolled steel sheet after the shot blasting was 2.9 µm.

[0091] Next, the shot blasted cold rolled steel sheet was annealed by raising it in temperature in a furnace with an oxygen concentration of 20 ppm or less and N 2 -4%H 2 gas atmosphere by a rate of temperature rise up to 500°C of 6.0°C / s and further raising it in temperature up to 800°C by a rate of temperature rise of 2.0°C / s and held there for 40 seconds. At this time, the dew point started to be controlled from 300°C to give a dew point of 0°C. The annealing was performed in a state applying 5.0 MPa tension to the steel sheet.

[0092] Furthermore, the annealed steel sheet was dipped in a 450°C hot dip galvanization bath (Zn-0.14%Al) for 3 seconds, then was pulled out at 100 mm / s and controlled to an amount of plating deposition of 50 g / m 2< by an N 2 wiping gas. After that, it was alloyed at 520°C for 30 seconds to obtain a hot dip galvannealed steel sheet.<Test Nos. 2 to 55>

[0093] Except for making the chemical compositions of the steel sheets the ones described in Table 1 or Table 2, making the conditions of the pretreatment steps and conditions of the annealing steps the ones described in Table 3, and the plating types the ones described in Table 4, the same procedure was performed under the same conditions of Example 1 to prepare steel sheets or plated steel sheets. Note that, in Test No. 32, the shot blasting was omitted. In Test No. 35, instead of the shot blasting, the surface was treated by grinding using brush rolls. In the types of plating in Table 4, "a" means hot dip galvannealing, "b" means hot dip galvanization with the alloying treatment in Test No. 1 omitted, "c" means making the plating bath Zn-1.5%Al-1.5%Mg and omitting the alloying treatment, and "unplated" means cold rolled steel sheet not plated.[Table 1]

[0094] Table 1Test no.ClassChemical composition (mass%). balance: Fe and impuritiesCSiMnsol.AlPSNOBTiNbOther1Inv. ex.0.051.52.40.00.00010.00100.00050.0013---2Inv. ex.0.051.52.50.00.00010.00030.00070.00080.00020.00080.00013Inv. ex.0.103.03.00.20.00800.00030.00020.00110.00020.00010.00014Inv. ex.0.101.52.00.20.00800.00030.00020.00160.00020.00010.00095Inv. ex.0.101.02.50.50.00800.00050.00010.00120.00100.00060.0009Hf:0.0016Inv. ex.0.101.02.00.50.00800.00050.00070.00110.00010.00060.00017Inv. ex.0.101.02.00.50.00700.00030.00010.00140.00010.00010.00018Inv. ex.0.100.92.20.50.00200.00040.00060.00140.00060.00090.00019Inv. ex.0.201.02.00.50.01000.00020.00010.00110.00040.00010.0004Mg:0.00110Inv. ex.0.201.02.00.40.00200.00090.00020.00110.00020.00060.0008Zr:0.01511Inv. ex.0.200.82.20.70.00080.00080.00070.00080.00080.00090.000112Inv. ex.0.200.92.50.60.00170.00040.00090.00140.00040.00070.0009Cr:0.1013Inv. ex.0.200.92.30.40.00110.00020.00080.00100.00050.00030.0017Cu:0.000714Inv. ex.0.201.02.30.50.00310.00020.00070.00110.00010.00030.003115Inv. ex.0.201.02.20.00.00650.00040.00060.00160.00060.00050.0017Ni:0.0816Inv. ex.0.251.02.20.50.00120.00080.00090.00120.00090.00060.001217Inv. ex.0.251.62.20.50.00400.00010.00050.00180.00070.00030.0099V:0.00918Inv. ex.0.300.85.00.70.00990.00060.00080.00110.00050.00040.009119Inv. ex.0.300.80.30.70.01100.00050.00010.00110.00050.00070.002020Inv. ex.0.351.03.00.70.00920.00040.00060.00300.00080.00050.0080Mo:0.0921Inv. ex.0.350.82.20.70.00910.00100.00040.00110.00040.00050.0003REM:0.000822Inv. ex.0.401.02.20.50.00450.00030.00050.00140.00040.00050.002023Inv. ex.0.400.82.20.70.00350.00080.00010.00210.00020.00020.0700W:0.00524Inv. ex.0.400.82.20.70.00820.00090.00040.00110.00040.00100.008225Inv. ex.0.400.82.20.70.00800.00080.00030.00220.00070.00040.0091Ca:0.000526Comp. ex.0.411.02.30.30.01000.00030.00030.00120.00010.0001-27Comp. ex.0.200.42.30.30.01000.00100.00020.00190.00010.0001-28Comp. ex.0.203.12.30.40.01000.00080.00060.00110.00050.0008-*Underlines indicate outside scope of present invention. [Table 2] Table 2 (continuation of Table 1)Test no.ClassChemical composition (mass%). balance: Fe and impuritiesCSiMnsol. AlPSNOBTiNbOther29Comp. ex.0.201.02.33.20.01000.00030.00030.00120.00010.0007-30Comp. ex.0.201.02.30.30.01000.00090.00060.00170.00070.0002-31Comp. ex.0.201.02.30.30.01000.00050.00030.00170.00090.0009-32Comp. ex.0.201.02.50.30.01000.00040.00060.00160.00030.0001-33Comp. ex.0.201.02.50.30.01000.00050.00050.00110.00040.0002-34Comp. ex.0.201.02.50.30.01000.00050.00050.00120.00040.0002-35Comp. ex.0.201.02.50.30.01000.00050.00050.00110.00040.0002-36Comp. ex.0.201.02.50.30.01000.00050.00050.00130.00040.0002-37Inv. ex.0.200.82.50.60.00180.00060.00100.00180.00050.00070.0008Cr:1.8038Inv. ex.0.200.92.20.70.00170.00040.00090.00170.00040.00080.0007V:0.1539Inv. ex.0.250.82.50.60.00190.00050.00090.00140.00040.00090.0010Ni: 1.9040Inv. ex.0.200.92.20.60.00150.00050.00090.00160.00050.00100.0010Cu:1.900041Inv. ex.0.250.92.50.70.00170.00040.00090.00140.00040.00080.0009Mo:1.0042Inv. ex.0.200.82.50.60.00160.00050.00090.00120.00040.00070.0009W:1.00043Inv. ex.0.200.92.20.70.00170.00040.00090.00140.00040.00070.0008Ca:0.100044Inv. ex.0.250.92.50.60.00180.00050.00100.00170.00050.00080.0009Mg:0.10045Inv. ex.0.201.02.20.60.00170.00050.00090.00140.00040.00110.0010Zr:0.10046Inv. ex.0.200.92.50.60.00170.00040.00090.00170.00040.00080.0009Hf:0.10047Inv. ex.0.250.82.40.60.00180.00080.00110.00160.00060.00070.0008REM:0.100048Inv. ex.0.200.92.52.20.00190.00040.00090.00150.00040.00090.000949Inv. ex.0.200.92.50.60.02890.00080.00080.00130.00040.00080.001050Inv. ex.0.200.82.50.60.00170.02770.00070.00150.00050.00080.001151Inv. ex.0.200.82.40.50.00180.00040.00970.00140.00040.00070.000852Inv. ex.0.200.82.50.70.00170.00040.00110.00240.00030.00070.000953Inv. ex.0.250.92.40.60.00190.00040.00090.00140.00890.00080.001154Inv. ex.0.200.92.50.60.00170.00050.00080.00140.00050.12000.001055Inv. ex.0.200.92.50.60.00170.00050.00080.00140.00050.12000.1100 *Underlines indicate outside scope of present invention. [Table 3]

[0095] Table 3Test no.ClassPretreatment stepAnnealing stepBlasted amount (kg / m 2< )Surface roughness after pretreatmentDew point control start temp. (°C)Dew point (°C)Holding temp. (°C)Holding time (s)1Inv. ex.5A3000800402Inv. ex.5A4000800453Inv. ex.5A4500800604Inv. ex.5A5000800605Inv. ex.20A5000800606Inv. ex.45A5000800507Inv. ex.50AA4500820608Inv. ex.100AA50008401009Inv. ex.100AA550086010010Inv. ex.100AA520086010011Inv. ex.100AA500086010012Inv. ex.100AA500086010013Inv. ex.100AA500086010014Inv. ex.100AA50008602015Inv. ex.100AA500086010016Inv. ex.100AA500086010017Inv. ex.100AA500086010018Inv. ex.300AA500086010019Inv. ex.100AA500086010020Inv. ex.100AA500086010021Inv. ex.100AA500086010022Inv. ex.100AA500086015023Inv. ex.100AA500-2086010024Inv. ex.100AA500086010025Inv. ex.100AA500086010026Comp. ex.15A50008207027Comp. ex.15A50008207028Comp. ex.15A50008207029Comp. ex.15A50008207030Comp. ex.15A50007407031Comp. ex.15A50009157032Comp. ex.0AA50008006033Comp. ex.15A500-358006034Comp. ex.15A500258006035Comp. ex.GrindingA50008006036Comp. ex.15A25008006037Inv. ex.100AA500086010038Inv. ex.100AA500086010039Inv. ex.100AA450086010040Inv. ex.100AA500086010041Inv. ex.100AA500086010042Inv. ex.100AA500086010043Inv. ex.100AA500086010044Inv. ex.100AA550085010045Inv. ex.100AA500086010046Inv. ex.100AA500086010047Inv. ex.100AA500087010048Inv. ex.100AA500086010049Inv. ex.100AA550086010050Inv. ex.100AA500086010051Inv. ex.100AA500086010052Inv. ex.100AA450086010053Inv. ex.100AA500086010054Inv. ex.100AA500086010055Inv. ex.100AA5000860100 Underlines indicate outside scope of preferred conditions of present invention. (Surface Roughness After Pretreatment)

[0096] The "surface roughness after pretreatment" described in Table 3 was the surface roughness Ra of the steel sheet after the pretreatment step and before the annealing step measured based on JIS B 0601: 2013.

[0097] 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

[0098] The annealed steel sheet and plated steel sheet were evaluated as follows:(Surface Roughness Ra)

[0099] After the annealing step or the annealing step and plating step, if the not plated steel sheet, the surface of the steel sheet, while if the plated steel sheet, the surface of the steel sheet exposed by removal of the plating, was measured for surface roughness Ra. The plating layer was removed by dissolving the plating layer in a 10 mass% hydrochloric acid solution to which 0.06 mass% of an inhibitor for inhibiting corrosion of the base material steel sheet (ibit 710K made by Asahi Chemical Co., Ltd.) had been added.(Steel Surface Layer Structure)

[0100] A sample cut to 30 mm×30 mm was taken from the steel sheet. The above-mentioned methods were used for GDS measurement, measurement of the thickness of the high ferrite layer, and grazing incidence XRD analysis. The obtained results are shown in Table 4 in the columns of "C≤0.02% depth", "high ferrite layer thickness", and "grazing incidence XRD (value of center term in conditional expression)".(Tensile Strength)

[0101] For each steel sheet, a JIS No. 5 tensile test piece was taken and subjected to a tensile test based on JIS Z 2241: 2011 to find the tensile strength. This was evaluated as follows in accordance with the obtained value of the tensile strength.

[0102] 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 (LME Resistance)

[0103] From each steel sheet, a sample cut to 50 mm×100 mm size was taken. Further, a counterpart material steel sheet of the same size was prepared. These two samples were spot welded by a weld angle of 5°, a squeezing force of 4.0 kN, a weld time of 1.6 seconds, and a weld current of 13 kA using dome radius type tip diameter 8 mm weld electrodes to produce a welded joint. For the counterpart material steel sheet, the type of steel sheet described in the column "counterpart material" of Table 4 was used. "Same type" of the counterpart material indicates use of the same type of steel sheet as the steel sheet of that test number (test no.) as the counterpart material steel sheet. Further, "unplated, same type" indicates use of the same type of steel sheet as the steel sheet of that test number (test no.) as the counterpart material steel sheet, but not plated, while "GA, same type" indicates use of the same type of steel sheet as the steel sheet of that test number (test no.) as the counterpart material steel sheet which has been galvannealed. Further, "GI270IF" indicates use of commercially available hot dip galvanized steel sheet with a tensile strength of 270 MPa as the counterpart material steel sheet, while "GA590" indicates use of commercially available hot dip galvannealed steel sheet with a tensile strength of 590 MPa as the counterpart material steel sheet.

[0104] Referring to FIG. 2, the method of evaluation of the LME resistance will be explained. The LME resistance was evaluated by overlaying two steel sheets 1 and spot welding them, then determining the length of an LME crack (crack 11 right outside pressure contact zone) formed right outside the pressure contact zone of the welded part 2 formed. The "two steel sheets 1" are the steel sheet of each test number (test no.) and its counterpart material steel sheet. The "right outside the pressure contact zone of the welded part" means a position at the overlaid surfaces of the two steel sheets at the part pressure contacted by spot welding as the part at the outside of the pressure contact zone 3 and near the pressure contact zone 3 (range from end part of pressure contact zone 3 to 1 mm or so to the outside). The crack 11 right outside of the pressure contact zone was evaluated for the length of the crack. Note that the test by spot welding was performed three times and the sample with the longest length of the crack of the crack 11 right outside the pressure contact zone was evaluated. The evaluation criteria were as follows. In the present embodiment, if the evaluation A or more (that is, evaluation A, AA, AAA), it was judged that the LME resistance was excellent.

[0105] 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

[0106] The results of the evaluations are shown in Table 4.[Table 4]

[0107] Table 4Test no.ClassPlating typeSurface roughness Ra (µm)Steel surface layer structurePerformanceCounterpart materialC≤0.02% depth (µm)High ferrite layer thickness (µm)Grazing incidence XRD (value of center term of conditional expression)Tensile strength (MPa)LME resistance1Inv. ex.a2.9350.90AASame type2Inv. ex.Unplated2.9660.82AAGI270IF3Inv. ex.b1.6870.75AAAASame type4Inv. ex.b1.31080.72AAAAASame type5Inv. ex.b1.51390.70AAAAAUnplatedSame type6Inv. ex.b1.515120.72AAAAASame type7Inv. ex.b1.316150.70AAAAASame type8Inv. ex.b1.723200.69AAAAASame type9Inv. ex.b1.525220.68AAAAASame type10Inv. ex.b1.422230.67AAAAASame tvpe11Inv. ex.c1.523240.66AAAAASame type12Inv. ex.b1.621250.65AAAAASame type13Inv. ex.b1.122260.67AAAAASame type14Inv. ex.b1.0970.63AAAASame type15Inv. ex.b1.122220.67AAAAAGA59016Inv. ex.b1.222230.66AAAAAASame type17Inv. ex.Unplated1.222250.64AAAAAAGASame type18Inv. ex.b1.140250.66AAAAAASame type19Inv. ex.b1.022270.66AAAAAASame type20Inv. ex.b1.223280.65AAAAAASame type21Inv. ex.b1.222260.67AAAAASame type22Inv. ex.b1.029280.69AAAAAASame type23Inv. ex.Unplated1.122240.67AAAAAAGASame type24Inv. ex.c1.222290.65AAAAAASame type25Inv. ex.b1.222260.65AAAAAASame type26Comp. ex.b2.4140.83AAABSame type27Comp. ex.b2.4130.83AAABSame type28Comp. ex.b2.4230.82AAABSame type29Comp. ex.b2.3130.81AAABSame type30Comp. ex.b2.2220.82AAABSame type31Comp. ex.b2.3160.81AAABSame type32Comp. ex.b0.5241.02AAABSame type33Comp. ex.b2.4230.79AAABSame type34Comp. ex.b2.5220.82AAABSame type35Comp. ex.b2.4940.95AAABSame type36Comp. ex.b2.3230.97AAABSame type37Inv. ex.b1.620230.64AAAAASame type38Inv. ex.b1.521240.62AAAAASame type39Inv. ex.b1.622250.62AAAAASame type40Inv. ex.b1.421250.61AAAAASame type41Inv. ex.b1.622230.66AAAAASame type42Inv. ex.b1.621240.63AAAAASame type43Inv. ex.b1.523240.65AAAAASame type44Inv. ex.b1.621240.65AAAAASame type45Inv. ex.b1.620250.65AAAAASame type46Inv. ex.b1.621260.64AAAAASame type47Inv. ex.b1.622240.63AAAAASame type48Inv. ex.b1.622230.65AAAAASame type49Inv. ex.b1.621220.65AAAAASame type50Inv. ex.b1.621240.63AAAAASame type51Inv. ex.b1.621250.64AAAAASame type52Inv. ex.b1.622240.65AAAAASame type53Inv. ex.b1.622230.63AAAAASame type54Inv. ex.b1.621240.66AAAAASame type55Inv. ex.b1.621240.66AAAAASame type* Underlines indicate outside scope of present invention or not yielding desired properties. <Test Nos. 5-1 to 5-7>

[0108] Test Nos. 5-1 to 5-7 were performed under the same conditions as Test No. 5 shown in Tables 1, 3, and 4 except for making the holding temperature 860°C as the dew point in the annealing step shown in Table 5 and designating the counterpart material the "same type". The results are shown in Table 5.[Table 5]

[0109] Table 5Test no.ClassDew pointSurface roughness Ra (µm)Steel surface layer structurePerformanceC≤0.02% depth (µm)High ferrite layer thickness (µm)Grazing incidence XRD (value of center term of conditional expression)Tensile strength (MPa)LME resistance5-1Inv. ex.-301.5360.76AAAA5-2Inv. ex.-201.5770.77AAAA5-3Inv. ex.-101.51190.72AAAAA5-4Inv. ex.01.51390.70AAAAA5-5Inv. ex.51.51080.71AAAAA5-6Inv. ex.101.5860.76AAAA5-7Inv. ex.201.5350.78AAAA <Test Nos. 3-1 to 3-7>

[0110] Test Nos. 3-1 to 3-7 were performed under the same conditions as Test No. 3 shown in Tables 1, 3, and 4 except for making the dew point control start temperature in the annealing step the one shown in Table 6 and making the holding temperature 860°C. The results are shown in Table 6.[Table 6]

[0111] Table 6Test no.ClassDew point control start temperatureGrazing incidence XRD (value of center term of conditional expression)LME resistance3-1Comp. ex.2500.97B3-2Inv. ex.3000.90A3-3Inv. ex.4000.82A3-4Inv. ex.4500.75AA3-5Inv. ex.5000.74AA3-6Inv. ex.5500.75AA3-7Comp. ex.6000.95B* Underlines indicate outside scope of present invention or not yielding desired properties.

[0112] The steel sheets of Test Nos. 1 to 25 and 37 to 55 and Test Nos. 3-2 to 3-6 and 5-1 to 5-7 are invention examples and had high LME resistances. On the other hand, the steel sheets of Test Nos. 26 to 36, 3-1, and 3-7 with chemical compositions of the steel sheets or production conditions outside the predetermined conditions in the present invention had LME resistances not reaching the passing criteria.

[0113] In Test No. 32, shot blasting was not performed, therefore strain was not introduced into the surface layer of the steel sheet. For this reason, it is believed that decarburization was not promoted and the depth with a C concentration of 0.02% or less measured by GDS was shallow. Further, it is believed that internal oxidation of Si or Mn did not proceed and the orientation of the ferrite phase did not become randomized. As a result, the LME resistance became inferior.

[0114] In Test No. 35, instead of shot blasting treatment, pretreatment was performed by grinding using brush rolls, therefore strain was not sufficiently introduced into the surface layer. Further, it is believed that in the annealing step, internal oxidation of Si or Mn did not proceed and the orientation of the ferrite phase did not become randomized. As a result, the LME resistance became inferior.

[0115] In Test No. 36, the dew point control start temperature at the annealing step was low, therefore it is believed external oxidation proceeded, decarburization did not proceed, and the depth with a C concentration of 0.02% or less by GDS measurement became shallower. Further, it is believed that internal oxidation of Si or Mn did not proceed and the orientation of the ferrite phase did not become randomized. As a result, the LME resistance became inferior.[INDUSTRIAL APPLICABILITY]

[0116] According to the present invention, it becomes possible to provide high strength steel sheet and plated steel sheet having a high LME resistance. The steel sheet and plated steel sheet can be optimally used for automobiles, household electric appliance products, building materials, and other applications, in particular for automobile use. Therefore, the present invention is an invention with an extremely high industrial applicability.REFERENCE SIGNS LIST

[0117] 1 steel sheet 2 welded part 3 contact zone 11 cracking right outside pressure contact zone

Examples

examples

[0088]Below, examples will be used to explain the present invention in more detail. The present invention is not limited to these examples.

[0089]Molten steel adjusted to the chemical composition described in Test 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 by an end temperature of finish rolling of 950°C and a reduction rate of finish rolling of 30% to obtain hot rolled steel sheet. The obtained hot rolled steel sheet was coiled at the coiling temperature 650°C, pickled, then cold rolled by a reduction rate of 50% to obtain cold rolled steel sheet. The thickness of the cold rolled steel sheet was 1.6 mm.

[0090]Next, the surface of the obtained cold rolled steel sheet was blasted using as the blasting material TSH30 made by WINOA IKK JAPAN by a blasting rate of 5 kg / m 2< as shot blasting. The surface roughness Ra of the cold rolled steel sheet after the shot blasting...

FIELD

[0001] 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 a high LME resistance.BACKGROUND

[0002] In recent years, the steel sheet used in automobiles, household electric appliances, building materials, and other various fields has been made increasingly higher in strength. For example, in the automobile field, use of high strength steel sheet has been increasing with the aim of lightening the weight of car bodies so as to improve fuel efficiency.

[0003] In welding galvanized steel sheet, in particular high strength steel sheet, for example as described in PTL 1, liquid metal embrittlement (LME) cracking sometimes causes the weldability to fall.

[0004] PTL 2 discloses steel sheet suppressed in LME cracking to improve weldability wherein the surface layer part of the steel sheet has size 20 nm or more Si oxide grains present in a 3000 to 6000 / mm 2< number density by a suitable particle size distribution.[CITATION LIST][PATENT LITERATURE]

[0005] [PTL 1] WO2019 / 116531 [PTL 2] WO2020 / 218575 SUMMARY[TECHNICAL PROBLEM]

[0006] To suppress the occurrence of LME cracking, for example, it is effective to keep the Zn etc. contained in a plating layer from penetrating steel sheet with a microstructure transformed to austenite at the time of welding. There is room for improvement on this point.

[0007] The present invention, in consideration of such a situation, has as its technical problem the provision of steel sheet and plated steel sheet having a high LME resistance.[SOLUTION TO PROBLEM]

[0008] The inventors engaged in intensive studies on means for solving the above technical problem. As a result, they discovered that by imparting strain to steel sheet before annealing by a blasting material under suitable conditions to render it a suitable surface condition and annealing it by a high dew point, the surface layer of the steel sheet is decarburized and further a layer with a high ferrite fraction where the ferrite is randomly oriented is formed and, therefore, as a result, it becomes possible to suppress LME.

[0009] The present invention was made based on the above discovery and further by additional study. Its gist is as follows: (1) A steel sheet having a tensile strength of 780 MPa or more, wherein a chemical composition of the steel sheet comprises, by mass%, C: 0.05 to 0.40%, Si: 0.5 to 3.0%, Mn: 0.1 to 5.0%, sol. Al: 0 to 3.0%, 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 0.1500%, 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 comprising Fe and impurities; a surface roughness Ra of the steel sheet is 3.0 µm or less; a depth in a thickness direction from the surface of the steel sheet wherein a C concentration measured by GDS is 0.02% or less is 3 µm or more; a thickness in a thickness direction from the surface of the steel sheet of a layer having an area ratio of ferrite of 90% or more is 3 µm or more; and, in grazing incidence X-ray diffraction with an incidence angle of 1° with respect to the steel sheet surface, 0.45 ≤I(110) / ( I(110) + I(200) + I(211) ) ≤ 0.90 is satisfied, when I(110) denotes a diffraction intensity corresponding to the (110) plane, I(200) denotes a diffraction intensity corresponding to the (200) plane, and I(211) denotes a diffraction intensity corresponding to the (211) plane. (2) The steel sheet of (1), wherein in the thickness direction from the steel sheet surface, the depth with a C concentration of 0.02% or less is 10 µm or more. (3) The steel sheet of (1) or (2), wherein 0.45 ≤ 1(110) / ( I(110) + I(200) + I(211) ) ≤ 0.75 is satisfied. (4) The steel sheet of any of (1) to (3), wherein in the thickness direction from the steel sheet surface, the thickness of the layer with a ferrite area ratio of 90% or more is 8 µm or more. (5) The steel sheet of any of (1) to (4), wherein the surface roughness Ra of the steel sheet is 2.0 µm or less. (6) A plated steel sheet comprising the steel sheet of any of (1) to (5) and a plating layer comprising Zn, provided at least at part of the surface of the steel sheet. (7) An automobile member comprising the steel sheet of any of (1) to (5) or plated steel sheet of (6). [ADVANTAGEOUS EFFECT OF INVENTION]

[0010] 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

[0011] FIG. 1 is a view showing one example of results of measurement of grazing incidence X-ray diffraction in the case where the ferrite phase is randomly oriented and the case where it is not. FIG. 2 is a view explaining a position of cracking covered in evaluation of the LME resistance in an embodiment. EMBODIMENTS OF INVENTION

[0012] Below, the present invention will be explained. The present invention is not limited to the following aspects. First, details on improvement of the LME resistance in the present invention will be explained.

[0013] LME cracking, for example, is due to the microstructure of steel sheet being heated and transforming to austenite at the time of spot welding and penetration of molten zinc formed by the plating melting into the grain boundaries of the austenite at the surface layer part of the steel sheet. It is believed to arise due to the molten zinc which penetrated into the austenite grain boundaries causing the steel sheet to become brittle and further tensile stress being applied to the steel sheet at the time of welding. The inventors came up with the idea of utilizing the microstructure of the surface layer part of the steel sheet as a method for improving the LME resistance. Specifically, by making the structure of the surface layer of the steel sheet a microstructure with a low C concentration and mainly comprised of the low LME sensitivity ferrite phase and, further, by making the ferrite phase randomly oriented, it is possible to suppress the occurrence of LME. Here, in the present invention, "LME resistance" means the property of LME cracking being suppressed in the steel sheet while "LME sensitivity" means the property of LME cracking easily occurring in the steel sheet.

[0014] The "ferrite phase being randomly oriented" means the features of the ferrite grain boundaries being leveled as a whole. In other words, it means the crystal orientations of the ferrite grains of the ferrite phase being randomized. Bhy the crystal orientations of the ferrite grains being randomized, grain boundaries oriented in specific directions are kept from segregating and being continuously or intermittently connected. It is believed that LME cracking occurs due to Zn from the plating concentratedly penetrating the grain boundaries where grain boundary energies locally become lower. That is, if grain boundaries with grain boundary energies locally becoming lower are present in succession, it is believed the Zn from the plating will concentrate there and LME cracking will easily occur. By the features of the grain boundaries being leveled as a whole, it is believed grain boundaries with grain boundary energies becoming locally lower will no longer be connected in succession, Zn from the plating will be kept from locally concentrating, and as a result LME resistance will be improved.

[0015] In the present invention, the "ferrite phase being randomly oriented" is expressed by the following conditional expression. That is, steel sheet satisfying the following conditional expression means the ferrite phase being randomly oriented.(Conditional Expression)

[0016] In grazing incidence X-ray diffraction with an incidence angle of 1° with respect to the steel sheet surface, when a diffraction intensity corresponding to the (110) plane is I(110), a diffraction intensity corresponding to the (200) plane is I(200), and a diffraction intensity corresponding to the (211) plane is I(211), 0.45 ≤ I 110 / I 110 + I 200 + I 211 ≤ 0.90

[0017] By configuring the steel sheet in the above way, in the present invention, when producing the steel sheet, strain is imparted to the steel sheet after cold rolling, then the steel sheet is annealed at a high dew point. Due to this, decarburization is promoted and a ferrite phase becomes easier to form at the steel sheet surface. Furthermore, at this time, the present invention was completed based on the discovery that by controlling the temperature of start of humidification, it is possible to randomize the orientation of the ferrite phase. Below, the present invention will be explained in detail.[Tensile Strength]

[0018] The steel sheet according to the present invention has a tensile strength of 780 MPa or more, that is, is high strength steel sheet. The present invention suppresses the LME occurring at steel sheet with a high strength. The steel sheet according to the present invention specifically has 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, may for example be 2000 MPa or less. The tensile strength may be measured by obtaining a JIS No. 5 tensile test piece having a direction perpendicular to the rolling direction and sheet thickness direction as a longitudinal direction and testing it based on JIS Z 2241: 2011. The tensile strength may be 980 MPa or more or 1180 MPa or more. If the rolling direction at the steel sheet cannot be identified, at the time of measurement of the tensile strength, a JIS No. 5 test piece having any direction on the surface of the steel sheet as the longitudinal direction may be taken.[Chemical Composition]

[0019] Below, the chemical composition of the steel sheet will be explained. The "%" relating to the chemical composition shall mean "mass%". Further, in the numerical ranges in the chemical composition, numerical ranges expressed using "to" mean ranges including the numerical values described before and after the "to" as the lower limit value and upper limit value.(C: 0.05 to 0.40%)

[0020] C (carbon) is an element securing the strength of steel. To obtain the 780 MPa or more tensile strength covered by the present invention, the content of C is 0.05% or more. To keep the C concentration of the surface layer explained later from becoming too high and, further, considering the weldability, the content of C is 0.40% or less. The content of C may also be 0.08% or more, 0.10% or more, or 0.15% or more. The content of C may also be 0.37% or less, 0.35% or less, or 0.30% or less.(Si: 0.5 to 3.0%)

[0021] Si (silicon) is an element promoting ferrite stabilization and decarburization. By Si being included, decarburization proceeds at the surface layer part due to the later explained pretreatment and heat treatment and, further, the ferrite at the surface layer part becomes stabilized, whereby the LME resistance is improved. To obtain this effect, the content of Si is 0.5% or more. If the content of Si is too great, even if performing high dew point annealing, external oxidation will progress and oxides (scale) will form at the surface layer of the steel sheet. Conversely, decarburization at the surfacemost part will be suppressed and the effect of improvement of the LME resistance will become smaller. Considering this point, the content of Si is 3.0% or less. The content of Si may also be 0.6% or more, 0.7% or more, or 0.8% or more. The content of Si may also be 2.5% or less, 2.0% or less, or 1.5% or less.(Mn: 0.1 to 5.0%)

[0022] Mn (manganese) is an element effective for obtaining a hard structure and thereby improving the strength of the steel. Considering the strength of steel, the content of Mn is 0.1% or more. Further, considering the drop in formability due to Mn segregation, the content of Mn is 5.0% or less. 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.(sol. Al: 0 to 3.0%)

[0023] Al (aluminum) is an element which dissolves in steel and, like Si, promotes ferrite stabilization and decarburization. "sol. Al" means acid soluble Al which does not form Al 2 O 3 or other oxides and can dissolve in acid and is produced in the process of analysis of Al. It is found as the Al measured after deducting the insoluble residue on the filter paper. In the steel sheet of the present invention, the role of the sol. Al can also be obtained by inclusion of Si, so 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 will progress and oxides (scale) will form at the surface layer of the steel sheet. Conversely, decarburization at the surfacemost part will be suppressed and the effect of improvement of the LME resistance will become smaller. Considering this point, the content of sol. Al is 3.0% or less. The content of sol. Al may also be 0.1% or more, 0.3% or more, or 0.5% or more. The content of sol. Al may also be 2.0% or less, 1.5% or less, or 1.0% or less.

[0024] Si and sol. Al, as explained above, are also elements which cause the LME resistance to fall if excessively added, therefore the total value of the contents of Si and sol. Al is preferably 1.8% or less. The total of the contents of Si and sol. Al may also be 1.7% or less, 1.6% or less, or 1.5% or less.(P: 0.0300% or less)

[0025] 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 be more than 0%, 0.0001% or more, or 0.0005% or more.(S: 0.0300% or less)

[0026] S (sulfur) is an impurity generally contained in steel. If the content of S is more than 0.0300%, the weldability falls and, further, the amount of precipitation of MnS increases whereby 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 be more than 0%, 0.0001% or more, or 0.0005% or more.(N: 0.0100% or less)

[0027] 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 0.0080% 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 the manufacturing costs, the content of N may be more than 0%, 0.0005% or more, or 0.0010% or more.(O: 0.0030% or less)

[0028] O (oxygen) is an element which forms oxides and causes the formability of the steel sheet to fall. If the content of O is too great, oxides are excessively formed etc. and the formability of the steel sheet easily falls. Therefore, the content of O is 0.0030% or less. The content of O may also be 0.0026% or less, 0.0024% or less, 0.0020% or less, or 0.0018% or less. O is preferably not contained. The lower limit of the content of O is 0%. From the viewpoint of the production costs, the content of O may also be more than 0%, 0.0005% or more, or 0.0010% or more.(B: 0 to 0.0100%)

[0029] B (boron) is an element raising the hardenability and contributing to improvement of the strength and further segregating at the grain boundaries to strengthen the grain boundaries and improve the toughness, therefore may be contained in accordance with need. It is not an essential element, therefore the lower limit of the content of B is 0%. This effect is obtained even when contained in a trace amount, but if included, the content of B is preferably 0.0001% or more. Further, from the viewpoint of securing sufficient toughness, the content of B is 0.0100% or less. The content of B may also be 0.0002% or more, 0.0003% or more, or 0.0005% or more. The content of B may also be 0.0080% or less, 0.0060% or less, 0.0040% or less, or 0.0020% or less.(Ti: 0 to 0.1500%)

[0030] Ti (titanium) is an element which precipitates as TiC during cooling of steel and contributes to improvement of strength, therefore may be contained in accordance with need. It is not an essential element, therefore the lower limit of the content of Ti is 0%. This effect is obtained even when contained in a trace amount, but if contained, the content of Ti is preferably 0.0001% or more. The content of Ti may also be 0.0003% or more or 0.0005% or more. On the other hand, if excessively contained, coarse TiN is produced and the toughness is liable to be harmed, therefore the content of Ti is 0.1500% or less. The content of Ti may also be 0.1000% or less, 0.0500% or less, 0.0050% or less, or 0.0020% or less.(Nb: 0 to 0.1500%)

[0031] Nb (niobium) is an element contributing to improvement of strength through improvement of the hardenability, therefore may be contained in accordance with need. It is not an essential element, therefore the lower limit of the content of Nb is 0%. This effect is obtained even when contained in a trace amount, but if contained, the content of the Nb is preferably 0.0001% or more. The content of Nb may also be 0.0005% or more or 0.0010% or more. On the other hand, from the viewpoint of securing sufficient toughness, the content of Nb is 0.1500% or less. The content of Nb may also be 0.1000% or less, 0.0600% or less, or 0.0200% or less.(V: 0 to 0.150%)

[0032] V (vanadium) is an element contributing to improvement of strength through improvement of the hardenability, therefore may be contained in accordance with need. It is not an essential element, therefore the lower limit of the content of V is 0%. This effect is obtained even when contained in a trace amount, but if contained, the content of the V is preferably 0.001% or more. The content of V may also be 0.003% or more, 0.005% or more, or 0.008% 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 0.100% or less, 0.060% or less, or 0.020% or less.(Cr: 0 to 2.00%)

[0033] Cr (chromium) is an element contributing to improvement of strength through improvement of the hardenability of steel, therefore may be contained in accordance with need. It is not an essential element, therefore the lower limit of the content of Cr is 0%. This effect is obtained even when contained in a trace amount, but if contained, the content of the Cr is preferably 0.001% or more. The content of Cr may also be 0.01% or more, 0.05% or more, or 0.10% 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, 1.50% or less, 0.50% or less, or 0.20% or less.(Ni: 0 to 2.00%)

[0034] Ni (nickel) is effective for raising the hardenability of steel to raise the strength of the steel, therefore may be contained in accordance with need. It is not an essential element, therefore the lower limit of the content of Ni is 0%. This effect is obtained even when contained in a trace amount, but if contained, the content of the Ni is preferably 0.001% or more. The content of Ni may also be 0.01% or more, 0.02% or more, or 0.05% or more. On the other hand, excessive addition of Ni causes the cost to rise, therefore the content of Ni is 2.00% or less. The content of Ni may also be 1.80% or less, 1.50% or less, 0.50% or less, or 0.20% or less.(Cu: 0 to 2.0000%)

[0035] Cu (copper) is effective for raising the hardenability of steel to raise the strength of the steel, therefore may be contained in accordance with need. It is not an essential element, therefore the lower limit of the content of Cu is 0%. This effect is obtained even when contained in a trace amount, but if contained, the content of the Cu is preferably 0.0001% or more. The content of Cu may also be 0.0002% or more or 0.0005% or more. On the other hand, from the viewpoint of suppressing a drop in toughness or cracking of the slab after casting, the content of Cu is 2.0000% or less. The content of Cu may also be 1.8000% or less, 1.5000% or less, 0.0050% or less, or 0.0020% or less.(Mo: 0 to 1.00%)

[0036] Mo (molybdenum) is effective for raising the hardenability of steel to raise the strength of the steel, therefore may be contained in accordance with need. It is not an essential element, therefore the lower limit of the content of Mo is 0%. This effect is obtained even when contained in a trace amount, but if contained, the content of the Mo is preferably 0.001% or more. The content of Mo may also be 0.01% or more, 0.02% or more, or 0.03% 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 0.80% or less, 0.60% or less, or 0.20% or less.(W: 0 to 1.000%)

[0037] W (tungsten) is effective for raising the hardenability of steel to raise the strength of the steel, therefore may be contained in accordance with need. It is not an essential element, therefore the lower limit of the content of W is 0%. This effect is obtained even when contained in a trace amount, but if contained, the content of the W 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 suppressing a drop in toughness, the content of W is 1.000% or less. The content of W may also be 0.800% or less, 0.600% or less, 0.300% or less, 0.100% or less, or 0.020% or less.(Ca: 0 to 0.1000%)

[0038] Ca (calcium) is an element contributing to control of inclusions, in particular to finely dispersing inclusions, and having the action of raising the toughness, therefore may be contained in accordance with need. It is not an essential element, therefore the lower limit of the content of Ca is 0%. This effect is obtained even when contained in a trace amount, but if contained, the content of the Ca 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, excessive inclusion causes deterioration of the surface properties to surface in some cases, therefore the content of Ca is 0.1000% or less. The content of Ca may also be 0.0800% or less, 0.0500% or less, 0.0300% or less, 0.0100% or less, or 0.0010% or less.(Mg: 0 to 0.100%)

[0039] Mg (magnesium) is an element contributing to control of inclusions, in particular to finely dispersing inclusions, and having the action of raising the toughness, therefore may be contained in accordance with need. It is not an essential element, therefore the lower limit of the content of Mg is 0%. This effect is obtained even when contained in a trace amount, but if contained, the content of the Mg is preferably 0.0001% or more. The content of Mg may also be 0.0005% or more or 0.0008% or more. On the other hand, excessive inclusion causes deterioration of the surface properties to surface in some cases, 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, 0.030% or less, 0.010% or less, or 0.002% or less.(Zr: 0 to 0.100%)

[0040] Zr (zirconium) is an element contributing to control of inclusions, in particular to finely dispersing inclusions, and having the action of raising the toughness, therefore may be contained in accordance with need. It is not an essential element, therefore the lower limit of the content of Zr is 0%. This effect is obtained even when contained in a trace amount, but if contained, the content of the Zr is preferably 0.001% or more. The content of Zr may also be 0.005% or more or 0.010% or more. On the other hand, excessive inclusion causes deterioration of the surface properties to surface in some cases, therefore the content of Zr is 0.100% or less. The content of Zr may also be 0.050% or less or 0.030% or less.(Hf: 0 to 0.100%)

[0041] Hf (hafnium) is an element contributing to control of inclusions, in particular to finely dispersing inclusions, and having the action of raising the toughness, therefore may be contained in accordance with need. It is not an essential element, therefore the lower limit of the content of Hf is 0%. This effect is obtained even when contained in a trace amount, but if contained, the content of the Hf is preferably 0.0001% or more. The content of Hf may also be 0.0003% or more or 0.0005% or more. On the other hand, excessive inclusion causes deterioration of the surface properties to surface in some cases, therefore the content of Hf is 0.100% or less. The content of Hf may also be 0.050% or less, 0.030% or less, 0.010% or less, 0.005% or less, or 0.002% or less.(REM: 0 to 0.1000%)

[0042] A REM (rare earth element) is an element contributing to control of inclusions, in particular to finely dispersing inclusions, and having the action of raising the toughness, therefore may be contained in accordance with need. It is not an essential element, therefore the lower limit of the content of a REM is 0%. This effect is obtained even when contained in a trace amount, but if contained, the content of a REM is preferably 0.0001% or more. The content of a REM may also be 0.0003% or more or 0.0005% or more. On the other hand, excessive inclusion causes deterioration of the surface properties to surface in some cases, therefore the content of a REM is 0.1000% or less. The content of a REM may also be 0.0500% or less, 0.0300% or less, 0.0100% or less, 0.0050% or less, or 0.0020% or less. Note that "REM" is an abbreviation for a rare earth metal and means an element belonging to the lanthanoids. A REM is usually added as a misch metal.

[0043] In the steel sheet according to the present invention, the balance beside the above chemical composition comprises Fe and impurities. The balance may consist of Fe and impurities, that is, the balance may comprise only Fe and impurities. Here, "impurities" are constituents entering due to various factors in the production process such as the ore, scraps, and other such starting materials 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, it means something contained to an extent where the LME resistance sought in the steel sheet of the present invention is obtained.

[0044] 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. For O, the inert gas melting-infrared absorption method is used. These analyses may be performed on a sample taken from the steel sheet by a method based on JIS G0417: 1999.

[0045] Next, the surface layer part of the steel sheet will be explained. Here, the "surface layer part of the steel sheet" means the layer-shaped region having a depth from the surface of the steel sheet (sheet surface of steel sheet) down by a predetermined distance in the sheet thickness direction of the steel sheet. In the case of plated steel sheet, the surface of the steel sheet (sheet surface of steel sheet) means the surface of the steel sheet (sheet surface) excluding the plating. Note that the "predetermined distance in the sheet thickness direction" can be made the longer (deeper) length (depth) among the later explained "depth of surface layer with a C concentration of 0.02% or less" or "depth of layer with area ratio of ferrite phase of 90% or more".[Surface Roughness Ra]

[0046] The steel sheet of the present invention has a surface roughness Ra of the steel sheet of 3.0 µm or less. The surface roughness Ra is the arithmetic average roughness Ra defined by JIS B0601: 2013. In the case of plated steel sheet, the surface roughness Ra is the surface roughness of the interface of the steel sheet excluding the plating and the plating layer.

[0047] In the present invention, when measuring the surface roughness Ra, based on JIS B 0601: 2013, 10 measurement locations are selected at the surface of the steel sheet at random so that the interval between measurement locations becomes 1 mm or more. At each of the measurement locations, the surface profile is measured by a laser microscope (for example,"VK-X3000" made by Keyence). Specifically, the laser microscope is used to take a photograph by a power of 20X. At the photographed image, using a standard length of 2000 µm, the arithmetic average roughness (Ra) at each measurement location is found. The arithmetic average value of the arithmetic average roughnesses (Ra) of the 10 points found at the respective measurement locations is made the "surface roughness Ra".[Depth of Surface Layer With C Concentration of 0.02% or Less]

[0048] In the steel sheet of the present invention, the depth from the steel sheet surface in the sheet thickness direction with a C concentration measured by GDS (glow discharge spectrometry) of 0.02% or less is 3 µm or more.

[0049] The LME sensitivity falls if the C concentration becomes lower, therefore by lowering the C concentration of the surface layer, the LME resistance is improved. Further, C is an austenite stabilizing element, therefore by this being less, the layer with a low LME sensitivity is stabilized

[0050] Such a surface layer structure (microstructure at surface layer part of steel sheet) can be obtained as a decarburized layer formed by making the chemical composition of the steel sheet the one explained above and performing the pretreatment step and annealing step explained later.

[0051] If the depth with a C concentration of 0.02% or less is 3 µm or more, this contributes to improvement of the LME resistance, so the upper limit of the depth with a C concentration of 0.02% or less is not particularly prescribed. The depth with a C concentration of 0.02% or less may, for example, also be 50 µm or less, 40 µm or less, or 30 µm or less. The depth with a C concentration of 0.02% or less is preferably 5 µm or more, more preferably 7 µm or more, still more preferably 10 µm or more, 15 µm or more, or 20 µm or more.

[0052] The GDS measurement is performed at five measurement points in the sheet thickness direction. At each of the measurement points, the arithmetic average value of depth of the region with a C concentration of 0.02% or less is made the depth of the surface layer with a C concentration of 0.02% or less. The five measurement points are randomly determined so that the measurement points of the steel sheet surface are separated by 5 mm or more intervals. The measurement conditions are as follows. Only naturally, measurement results can be obtained even without, for example, using the measurement apparatus or other conditions explained below, but if differences arise in the measurement results, the steel sheet according to the present invention is identified by measurement results obtained by the following conditions.

[0053] Apparatus: high frequency glow discharge optical emission spectrometry apparatus (made by LECO Japan Corporation, Model Name "GDS850A" Ar gas pressure: 0.3 MPa Anode diameter: 4 mmφ RF output: 30W Measurement time: 200 to 1500 seconds [Thickness of High Ferrite Layer (Layer With Area Ratio of Ferrite of 90% or More)]

[0054] At the steel sheet of the present invention, the thickness of the layer with an area ratio of the ferrite phase of 90% or more in the thickness direction from the steel sheet surface (below, referred to as the "high ferrite layer") is 3 µm or more.

[0055] If the thickness of the high ferrite layer is 3 µm or more, this contributes to improvement of the LME resistance, therefore the upper limit of the thickness is not particularly prescribed. The thickness of the high ferrite layer may, for example, 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 5 µm or more, more preferably 8 µm or more, still more preferably 10 µm or more or 20 µm or more.

[0056] The structure besides the ferrite in the high ferrite layer is not limited. For example, it may be made one or more of martensite, bainite, and cementite.

[0057] The thickness of the high ferrite layer is measured by finishing a thickness cross-section of the steel sheet by mechanical polishing to a mirror surface, then etching the observed cross-section by a Nital reagent and analyzing the secondary electron image by observation by an SEM. For the observation by an SEM, a field emission scanning electron microscope (for example, "JSM 7000F" made by JEOL, acceleration voltage: 15 kV) is used. In the observed cross-section, the range of depth from the surface of the steel sheet (sheet surface) down to a depth of 500 µm in the sheet thickness direction (vertical direction of observed cross-section) and range of width of 600 µm in the direction perpendicular to the sheet thickness direction (horizontal direction of observed cross-section) is made the observed field. In that observed cross-section, five observed fields in the direction perpendicular to the sheet thickness direction (horizontal direction of observed cross-section) are observed by an SEM so as to have observed fields having 1000 µm or more intervals between them to obtain secondary electron images. The observed resolution is made 1280×960 pixels. Note that, in the case of plated steel sheet, the "surface" of the steel sheet (sheet surface) is the surface of the steel sheet from which the plating has been removed.

[0058] The obtained five secondary electron images are calculated for ratios of ferrite by the point counting method. More specifically, first, equal interval lattices are drawn on the secondary images. Next, by finding the number of lattice points of each lattice with structures at the lattice points comprised of ferrite and dividing this by the total number of lattice points, the fraction of ferrite is measured. The greater the total number of lattice points, the more accurately the area ratio can be found. In the present embodiment, the lattice intervals are made 2 µm×2 µm and the total number of lattice points is made 1500 points.

[0059] In a secondary electron image, regions relatively small in brightness and not having substructures observable can be judged as ferrite. Here, "substructures" mean laths, blocks, or other transformed structures formed inside the prior austenite phase. In a secondary electron image, ferrite is observed as a region relatively low in brightness and having a spread of relatively monotonous brightness or hue. In the present invention, the microstructure other than ferrite does not have to be specially judged, but the judgment criteria for tempered martensite, pearlite, ferrite, fresh martensite, or retained austenite or bainite at the secondary electron image are shown below. A region having substructures (lath boundaries and block boundaries) in the grains and having carbides precipitated with several variants is judged as tempered martensite. Further, a region having cementite precipitated in a lamellar form is judged as pearlite. A region having a large brightness and not having substructures appearing by etching is judged as fresh martensite or retained austenite. A region not corresponding to any of the above is judged as bainite. For simplicity, the area ratio of the ferrite phase can be found if differentiating the ferrite from other structures.

[0060] Ferrite is low in LME sensitivity. The surface layer structure of the steel sheet being a structure mainly comprised of ferrite is preferable from the viewpoint of improvement of the LME resistance. Such a surface layer structure can be obtained by making the chemical composition of the steel sheet the one explained above and performing the pretreatment step and annealing step explained later.[Diffraction Intensity Ratio of Ferrite Phase by Grazing Incidence X-Ray Diffraction (XRD)]

[0061] The steel sheet of the present invention satisfies the conditional expression of 0.45 ≤ I 110 / I 110 + I 200 + I 211 ≤ 0.90 in grazing incidence X-ray diffraction with an incidence angle of 1° with respect to the steel sheet surface when the diffraction intensity corresponding to the (110) plane is I(110), the diffraction intensity corresponding to the (200) plane is I(200), and the diffraction intensity corresponding to the (211) plane is I(211).

[0062] In the conditional expression, the value of the term "I(110) / ( 1(110) + I(200) + I(211) )" is preferably 0.85 or less, more preferably 0.80 or less, and still more preferably 0.75 or less. The conditional expression means that the ferrite phase is randomly oriented. If the ferrite phase is completely randomly oriented, the value of the center term is 0.67.

[0063] Here, the grazing incidence X-ray diffraction (grazing incidence XRD, low angle incidence XRD, glancing angle XRD) is a measurement technique setting the incidence angle of the incident X-rays small and making only the detector scan the surface while maintaining that incidence angle (changing the detection angle). Due to this, it is possible to efficiently detect information on the orientation of crystal grains down to a depth of several µm or so of the surface layer part of a sample. In the present invention, the incidence angle of X-rays is fixed to 1° and the orientation of ferrite at the surface layer part of the steel sheet is detected. Note that the "incidence angle" is the angle formed by the surface of a sample (steel sheet) and the incident direction of the incident X-rays.

[0064] FIG. 1 shows examples of the results of grazing incidence XRD analysis for the case where the ferrite phase changes randomly (b) and the case where it does not (a). (a) shows the results of grazing incidence XRD analysis in an ordinary (conventional) steel sheet. It will be understood that the phase is oriented in the (110) direction. Therefore, the value in the middle term "I(110) / ( I(110) + I(200) + I(211) )" of the conditional expression becomes a relatively large 0.91. (b) shows the results of grazing incidence XRD analysis in the steel sheet of the present invention. Compared with (a), the orientation in the (110) direction becomes smaller. For this reason, the value in the middle term "I(110) / ( I(110) + I(200) + I(211) )" of the conditional expression becomes a relatively small 0.58.[Plating Layer]

[0065] The steel sheet of the present invention, as explained later, can have a plating layer. If having a plating layer, the starting point of the depth with a C concentration of 0.02% or less and the thickness of the layer with an area ratio of the ferrite phase of 90% or more by GDS measurement is the interface of the steel sheet and the plating layer. The interface of the steel sheet and plating layer in the present invention is determined in the following way. First, the Fe content in the thickness direction of the plated steel sheet by GDS measurement is measured. The highest value of this Fe content is made the Fe content of the steel sheet. The point becoming an Fe content of 93% of the Fe content of the steel sheet is defined as the "interface of the steel sheet and the plating layer".<Plated Steel Sheet>

[0066] The plated steel sheet according to the present invention has a plating layer containing Zn on the above-mentioned steel sheet according to the present invention. This plating layer may be formed at one of the surfaces of the steel sheet or may be formed at both surfaces. Further, it may be formed at only part of the surfaces as well. The plating layer may also be treated to alloy it.[Chemical Composition of Plating Layer]

[0067] The chemical composition of the plating layer is not limited so long as containing Zn. As the layer containing Zn, 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, Zn-15%Mg, etc. can be used.

[0068] 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 inhibiting corrosion of the steel sheet and measuring the obtained solution by ICP (high-frequency inductively coupled plasma) optical emission spectroscopy. As the acid solution to which an inhibitor for dissolving the plating layer is added, for example, a 10 mass% hydrochloric acid solution to which 0.06 mass% of an inhibitor for inhibiting corrosion of the base material steel sheet (ibit 710K made by Asahi Chemical Co., Ltd.) has been added may be used.

[0069] 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 / m 2< per surface. In the present invention, the amount of deposition of the plating layer is determined by dissolving the plating layer in an acid solution to which an inhibitor for inhibiting corrosion of the steel sheet has been added and finding the change in weight before and after acid washing and peeling off the plating layer.

[0070] The roughness of the interface of the steel sheet and plating layer (surface roughness Ra) corresponds to the roughness of the surface of the steel sheet explained above. The surface roughness Ra is 3.0 µm or less. If considering the adhesion of the plating, the surface roughness Ra is preferably 2.0 µm or less. The roughness of the interface (surface roughness Ra) may be made the surface roughness of the steel sheet measured after removing the plating layer. The plating layer is acid washed and removed by dissolving the plating layer in an acid solution to which an inhibitor is added.

[0071] Note that, the steel sheet of the present invention can have the effect of improvement of the LME resistance even if not galvanized. In general, if spot welding steel sheets which have not been galvanized, LME cracking will never occur so long as the state of contact with molten zinc does not arise near a spot welded part. However, if one steel sheet which has been galvanized and another steel sheet which has not been galvanized are spot welded, at the time of welding, molten zinc is formed at the overlaid surfaces of the steel sheets. For this reason, the molten zinc contacts the surface of the not plated steel sheet and LME cracking may occur.

[0072] The thickness of the steel sheet and plated steel sheet of the present invention is not particularly limited. For example, it may be 0.1 to 3.2 mm. The sheet thickness may also be 0.2 mm or more, 0.4 mm or more, or 0.6 mm or more. The sheet thickness may also be 3.0 mm or less, 2.5 mm or less, 2.0 mm or less, or 1.8 mm or less.<<Method of Production>>

[0073] 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 be obtained by, for example, 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 up the hot rolled steel sheet, a cold rolling step of cold rolling the coiled up hot rolled steel sheet to obtain cold rolled steel sheet, a pretreatment step of pretreating (shot blasting) the cold rolled steel sheet, and an annealing step of annealing the pretreated cold rolled steel sheet. Typically, after the hot rolling step, the hot rolled steel sheet need not be coiled, but may be pickled then cold rolled as it is.<Casting Step>

[0074] The conditions of the casting step are not particularly limited. For example, after smelting by a blast furnace, electric furnace, etc., various types of secondary refining may be performed, then the usual continuous casting, casting by ingot method, or other method may be used for casting.<Hot Rolling Step>

[0075] 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 it. If reheating, the heating temperature of the steel slab may, for example, be 1100 to 1250°C. In the hot rolling step, usually rough rolling and finish rolling are performed. The temperature and reduction rate of each rolling may be suitably changed in accordance with the desired microstructure or sheet thickness. For example, the end temperature of the finish rolling may be 900 to 1050°C and the reduction rate of the finish rolling may be 10 to 50%.<Coiling Step>

[0076] 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. For example, it may be 500 to 800°C. Before coiling or after coiling, then uncoiling, the hot rolled steel sheet may be subjected to predetermined heat treatment. Typically, the coiling step need not be performed and the hot rolled steel sheet can be pickled after the hot rolling step and the later explained cold rolling step performed.<Cold Rolling Step>

[0077] After pickling 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 or sheet thickness. For example, it may be 20 to 80%. After the cold rolling step, for example, the steel sheet may be air cooled down to room temperature for cooling.<Pretreatment Step>

[0078] To obtain a surface layer structure of steel sheet such as explained above, the predetermined pretreatment must be performed followed by annealing.

[0079] Pretreatment includes shot blasting for blasting the surface of the cold rolled steel sheet using a spherical polishing material. The polishing material used is not particularly limited, but, for example, steel balls (shot) having a center grain size of 40 to 450 µm can be used. As such steel balls (shot), for example, TSH30 made by WINOA IKK JAPAN may be mentioned. The amount of blasting of shot may be 5 to 400 kg / m 2< . Due to this, it is possible to introduce strain in the surface layer part of the steel sheet without increasing the surface roughness Ra of the steel sheet. By performing such shot blasting, in the later explained annealing step, decarburization is promoted and a structure with stable ferrite can be efficiently formed at the surface layer of the steel sheet. The greater the amount of blasting, the greater the effect of improvement of the LME resistance, but if more than a certain amount of blasting, for example, 400 kg / m 2< , the effect becomes saturated. Note that per unit time and unit area of the level of blasting of 400 kg / m 2< is 4.0×10 -4< kg / (mm 2< ·min).<Annealing Step>

[0080] After the pretreatment step, the cold rolled steel sheet is annealed.

[0081] In the present invention, an annealing step is performed of holding the steel sheet given strain by the shot blasting at a predetermined holding temperature and a high dew point. The speed of temperature rise up to the predetermined holding temperature is not particularly limited and may be 1 to 10°C / s. The dew point is controlled by humidification control from 300°C or more, preferably humidification control from 450 to 550°C. That is, the dew point (humidification) control start temperature is 300°C or more and less than 600°C, preferably 450 to 550°C in range. The dew point in the case of no dew point (humidification) control is usually less than -30°C. The dew point at the time of annealing (high dew point) is -30 to 20°C to promote decarburization. The dew point at the time of annealing (high dew point) is preferably - 10°C or more. Further, the dew point at the time of annealing is, preferably 5°C or less. The predetermined holding temperature at the annealing step (highest heating temperature) is 750 to 900°C, preferably 770 to 870°C, to promote decarburization. The holding time at the holding temperature (highest heating temperature) of the annealing step is 20 to 300 seconds, preferably 50 to 200 seconds. Further, the atmosphere is preferably a nonoxidizing atmosphere, for example, can be N 2 -1 to 10 vol%H 2 , N 2 -2 to 4 vol%H 2 .

[0082] By making the dew point, holding temperature, and holding time the above such ranges, it is possible to promote decarburization, lower the C concentration of the surface layer, and suitably control the ferrite phase fraction. Furthermore, by making the dew point (humidification) control start temperature the above such range, decarburization of the surface layer part of the steel sheet is promoted. Together with this, internal oxidation of Si or Mn is promoted, internal oxides are rapidly formed, and the formed internal oxides function as nucleation sites. As a result, the orientation of the ferrite phase is randomized. If the dew point (humidification) control start temperature is too low, external oxidation proceeds and internal oxidation of Si or Mn does not proceed, therefore the orientation of the ferrite phase becomes hard to randomize.

[0083] The annealing is performed in a state applying 1 to 20 MPa of tension. If tension is applied at the time of annealing, it becomes possible to more effectively introduce strain into the steel sheet and decarburization of the surface layer is promoted.

[0084] By performing the above-mentioned steps, decarburization is promoted at the surface layer part of the steel sheet and steel sheet with a surface layer part of the steel sheet made a structure mainly comprised of a randomly oriented ferrite phase can be obtained.<<Method of Production of Plated Steel Sheet>>

[0085] The plated steel sheet according to the present invention can be obtained by a plating step of forming a plating layer on the surface of the steel sheet produced as explained above.<Plating Step>

[0086] The plating may be performed in accordance with a method known to persons skilled in the art. The plating, for example, may be performed by hot dip coating or may be performed by electroplating. Preferably the plating is performed by hot dip coating. The conditions of the plating may be suitably set considering the chemical composition, thickness, amount of deposition, etc. of the desired plating layer. After the plating step, a known alloying step may be performed to obtain an alloyed plating.

[0087] The steel sheet and plated steel sheet according to the present invention are high in strength and have high LME resistances, therefore can be suitably used in automobiles, household electric appliance products, building materials, and other broad fields. In particular, they can be suitably used in the automobile field. The steel sheet and plated steel sheet used for automobiles are often spot welded. In this case, LME cracking easily occurs. For this reason, the effect of the present invention of realizing high LME resistance as an automobile member using the steel sheet and plated steel sheet according to the present invention as the steel sheet for automobile use is suitably exhibited.EXAMPLES

[0088] Below, examples will be used to explain the present invention in more detail. The present invention is not limited to these examples.<Test No. 1>

[0089] Molten steel adjusted to the chemical composition described in Test 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 by an end temperature of finish rolling of 950°C and a reduction rate of finish rolling of 30% to obtain hot rolled steel sheet. The obtained hot rolled steel sheet was coiled at the coiling temperature 650°C, pickled, then cold rolled by a reduction rate of 50% to obtain cold rolled steel sheet. The thickness of the cold rolled steel sheet was 1.6 mm.

[0090] Next, the surface of the obtained cold rolled steel sheet was blasted using as the blasting material TSH30 made by WINOA IKK JAPAN by a blasting rate of 5 kg / m 2< as shot blasting. The surface roughness Ra of the cold rolled steel sheet after the shot blasting was 2.9 µm.

[0091] Next, the shot blasted cold rolled steel sheet was annealed by raising it in temperature in a furnace with an oxygen concentration of 20 ppm or less and N 2 -4%H 2 gas atmosphere by a rate of temperature rise up to 500°C of 6.0°C / s and further raising it in temperature up to 800°C by a rate of temperature rise of 2.0°C / s and held there for 40 seconds. At this time, the dew point started to be controlled from 300°C to give a dew point of 0°C. The annealing was performed in a state applying 5.0 MPa tension to the steel sheet.

[0092] Furthermore, the annealed steel sheet was dipped in a 450°C hot dip galvanization bath (Zn-0.14%Al) for 3 seconds, then was pulled out at 100 mm / s and controlled to an amount of plating deposition of 50 g / m 2< by an N 2 wiping gas. After that, it was alloyed at 520°C for 30 seconds to obtain a hot dip galvannealed steel sheet.<Test Nos. 2 to 55>

[0093] Except for making the chemical compositions of the steel sheets the ones described in Table 1 or Table 2, making the conditions of the pretreatment steps and conditions of the annealing steps the ones described in Table 3, and the plating types the ones described in Table 4, the same procedure was performed under the same conditions of Example 1 to prepare steel sheets or plated steel sheets. Note that, in Test No. 32, the shot blasting was omitted. In Test No. 35, instead of the shot blasting, the surface was treated by grinding using brush rolls. In the types of plating in Table 4, "a" means hot dip galvannealing, "b" means hot dip galvanization with the alloying treatment in Test No. 1 omitted, "c" means making the plating bath Zn-1.5%Al-1.5%Mg and omitting the alloying treatment, and "unplated" means cold rolled steel sheet not plated.[Table 1]

[0094] Table 1Test no.ClassChemical composition (mass%). balance: Fe and impuritiesCSiMnsol.AlPSNOBTiNbOther1Inv. ex.0.051.52.40.00.00010.00100.00050.0013---2Inv. ex.0.051.52.50.00.00010.00030.00070.00080.00020.00080.00013Inv. ex.0.103.03.00.20.00800.00030.00020.00110.00020.00010.00014Inv. ex.0.101.52.00.20.00800.00030.00020.00160.00020.00010.00095Inv. ex.0.101.02.50.50.00800.00050.00010.00120.00100.00060.0009Hf:0.0016Inv. ex.0.101.02.00.50.00800.00050.00070.00110.00010.00060.00017Inv. ex.0.101.02.00.50.00700.00030.00010.00140.00010.00010.00018Inv. ex.0.100.92.20.50.00200.00040.00060.00140.00060.00090.00019Inv. ex.0.201.02.00.50.01000.00020.00010.00110.00040.00010.0004Mg:0.00110Inv. ex.0.201.02.00.40.00200.00090.00020.00110.00020.00060.0008Zr:0.01511Inv. ex.0.200.82.20.70.00080.00080.00070.00080.00080.00090.000112Inv. ex.0.200.92.50.60.00170.00040.00090.00140.00040.00070.0009Cr:0.1013Inv. ex.0.200.92.30.40.00110.00020.00080.00100.00050.00030.0017Cu:0.000714Inv. ex.0.201.02.30.50.00310.00020.00070.00110.00010.00030.003115Inv. ex.0.201.02.20.00.00650.00040.00060.00160.00060.00050.0017Ni:0.0816Inv. ex.0.251.02.20.50.00120.00080.00090.00120.00090.00060.001217Inv. ex.0.251.62.20.50.00400.00010.00050.00180.00070.00030.0099V:0.00918Inv. ex.0.300.85.00.70.00990.00060.00080.00110.00050.00040.009119Inv. ex.0.300.80.30.70.01100.00050.00010.00110.00050.00070.002020Inv. ex.0.351.03.00.70.00920.00040.00060.00300.00080.00050.0080Mo:0.0921Inv. ex.0.350.82.20.70.00910.00100.00040.00110.00040.00050.0003REM:0.000822Inv. ex.0.401.02.20.50.00450.00030.00050.00140.00040.00050.002023Inv. ex.0.400.82.20.70.00350.00080.00010.00210.00020.00020.0700W:0.00524Inv. ex.0.400.82.20.70.00820.00090.00040.00110.00040.00100.008225Inv. ex.0.400.82.20.70.00800.00080.00030.00220.00070.00040.0091Ca:0.000526Comp. ex.0.411.02.30.30.01000.00030.00030.00120.00010.0001-27Comp. ex.0.200.42.30.30.01000.00100.00020.00190.00010.0001-28Comp. ex.0.203.12.30.40.01000.00080.00060.00110.00050.0008-*Underlines indicate outside scope of present invention. [Table 2] Table 2 (continuation of Table 1)Test no.ClassChemical composition (mass%). balance: Fe and impuritiesCSiMnsol. AlPSNOBTiNbOther29Comp. ex.0.201.02.33.20.01000.00030.00030.00120.00010.0007-30Comp. ex.0.201.02.30.30.01000.00090.00060.00170.00070.0002-31Comp. ex.0.201.02.30.30.01000.00050.00030.00170.00090.0009-32Comp. ex.0.201.02.50.30.01000.00040.00060.00160.00030.0001-33Comp. ex.0.201.02.50.30.01000.00050.00050.00110.00040.0002-34Comp. ex.0.201.02.50.30.01000.00050.00050.00120.00040.0002-35Comp. ex.0.201.02.50.30.01000.00050.00050.00110.00040.0002-36Comp. ex.0.201.02.50.30.01000.00050.00050.00130.00040.0002-37Inv. ex.0.200.82.50.60.00180.00060.00100.00180.00050.00070.0008Cr:1.8038Inv. ex.0.200.92.20.70.00170.00040.00090.00170.00040.00080.0007V:0.1539Inv. ex.0.250.82.50.60.00190.00050.00090.00140.00040.00090.0010Ni: 1.9040Inv. ex.0.200.92.20.60.00150.00050.00090.00160.00050.00100.0010Cu:1.900041Inv. ex.0.250.92.50.70.00170.00040.00090.00140.00040.00080.0009Mo:1.0042Inv. ex.0.200.82.50.60.00160.00050.00090.00120.00040.00070.0009W:1.00043Inv. ex.0.200.92.20.70.00170.00040.00090.00140.00040.00070.0008Ca:0.100044Inv. ex.0.250.92.50.60.00180.00050.00100.00170.00050.00080.0009Mg:0.10045Inv. ex.0.201.02.20.60.00170.00050.00090.00140.00040.00110.0010Zr:0.10046Inv. ex.0.200.92.50.60.00170.00040.00090.00170.00040.00080.0009Hf:0.10047Inv. ex.0.250.82.40.60.00180.00080.00110.00160.00060.00070.0008REM:0.100048Inv. ex.0.200.92.52.20.00190.00040.00090.00150.00040.00090.000949Inv. ex.0.200.92.50.60.02890.00080.00080.00130.00040.00080.001050Inv. ex.0.200.82.50.60.00170.02770.00070.00150.00050.00080.001151Inv. ex.0.200.82.40.50.00180.00040.00970.00140.00040.00070.000852Inv. ex.0.200.82.50.70.00170.00040.00110.00240.00030.00070.000953Inv. ex.0.250.92.40.60.00190.00040.00090.00140.00890.00080.001154Inv. ex.0.200.92.50.60.00170.00050.00080.00140.00050.12000.001055Inv. ex.0.200.92.50.60.00170.00050.00080.00140.00050.12000.1100 *Underlines indicate outside scope of present invention. [Table 3]

[0095] Table 3Test no.ClassPretreatment stepAnnealing stepBlasted amount (kg / m 2< )Surface roughness after pretreatmentDew point control start temp. (°C)Dew point (°C)Holding temp. (°C)Holding time (s)1Inv. ex.5A3000800402Inv. ex.5A4000800453Inv. ex.5A4500800604Inv. ex.5A5000800605Inv. ex.20A5000800606Inv. ex.45A5000800507Inv. ex.50AA4500820608Inv. ex.100AA50008401009Inv. ex.100AA550086010010Inv. ex.100AA520086010011Inv. ex.100AA500086010012Inv. ex.100AA500086010013Inv. ex.100AA500086010014Inv. ex.100AA50008602015Inv. ex.100AA500086010016Inv. ex.100AA500086010017Inv. ex.100AA500086010018Inv. ex.300AA500086010019Inv. ex.100AA500086010020Inv. ex.100AA500086010021Inv. ex.100AA500086010022Inv. ex.100AA500086015023Inv. ex.100AA500-2086010024Inv. ex.100AA500086010025Inv. ex.100AA500086010026Comp. ex.15A50008207027Comp. ex.15A50008207028Comp. ex.15A50008207029Comp. ex.15A50008207030Comp. ex.15A50007407031Comp. ex.15A50009157032Comp. ex.0AA50008006033Comp. ex.15A500-358006034Comp. ex.15A500258006035Comp. ex.GrindingA50008006036Comp. ex.15A25008006037Inv. ex.100AA500086010038Inv. ex.100AA500086010039Inv. ex.100AA450086010040Inv. ex.100AA500086010041Inv. ex.100AA500086010042Inv. ex.100AA500086010043Inv. ex.100AA500086010044Inv. ex.100AA550085010045Inv. ex.100AA500086010046Inv. ex.100AA500086010047Inv. ex.100AA500087010048Inv. ex.100AA500086010049Inv. ex.100AA550086010050Inv. ex.100AA500086010051Inv. ex.100AA500086010052Inv. ex.100AA450086010053Inv. ex.100AA500086010054Inv. ex.100AA500086010055Inv. ex.100AA5000860100 Underlines indicate outside scope of preferred conditions of present invention. (Surface Roughness After Pretreatment)

[0096] The "surface roughness after pretreatment" described in Table 3 was the surface roughness Ra of the steel sheet after the pretreatment step and before the annealing step measured based on JIS B 0601: 2013.

[0097] 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

[0098] The annealed steel sheet and plated steel sheet were evaluated as follows:(Surface Roughness Ra)

[0099] After the annealing step or the annealing step and plating step, if the not plated steel sheet, the surface of the steel sheet, while if the plated steel sheet, the surface of the steel sheet exposed by removal of the plating, was measured for surface roughness Ra. The plating layer was removed by dissolving the plating layer in a 10 mass% hydrochloric acid solution to which 0.06 mass% of an inhibitor for inhibiting corrosion of the base material steel sheet (ibit 710K made by Asahi Chemical Co., Ltd.) had been added.(Steel Surface Layer Structure)

[0100] A sample cut to 30 mm×30 mm was taken from the steel sheet. The above-mentioned methods were used for GDS measurement, measurement of the thickness of the high ferrite layer, and grazing incidence XRD analysis. The obtained results are shown in Table 4 in the columns of "C≤0.02% depth", "high ferrite layer thickness", and "grazing incidence XRD (value of center term in conditional expression)".(Tensile Strength)

[0101] For each steel sheet, a JIS No. 5 tensile test piece was taken and subjected to a tensile test based on JIS Z 2241: 2011 to find the tensile strength. This was evaluated as follows in accordance with the obtained value of the tensile strength.

[0102] 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 (LME Resistance)

[0103] From each steel sheet, a sample cut to 50 mm×100 mm size was taken. Further, a counterpart material steel sheet of the same size was prepared. These two samples were spot welded by a weld angle of 5°, a squeezing force of 4.0 kN, a weld time of 1.6 seconds, and a weld current of 13 kA using dome radius type tip diameter 8 mm weld electrodes to produce a welded joint. For the counterpart material steel sheet, the type of steel sheet described in the column "counterpart material" of Table 4 was used. "Same type" of the counterpart material indicates use of the same type of steel sheet as the steel sheet of that test number (test no.) as the counterpart material steel sheet. Further, "unplated, same type" indicates use of the same type of steel sheet as the steel sheet of that test number (test no.) as the counterpart material steel sheet, but not plated, while "GA, same type" indicates use of the same type of steel sheet as the steel sheet of that test number (test no.) as the counterpart material steel sheet which has been galvannealed. Further, "GI270IF" indicates use of commercially available hot dip galvanized steel sheet with a tensile strength of 270 MPa as the counterpart material steel sheet, while "GA590" indicates use of commercially available hot dip galvannealed steel sheet with a tensile strength of 590 MPa as the counterpart material steel sheet.

[0104] Referring to FIG. 2, the method of evaluation of the LME resistance will be explained. The LME resistance was evaluated by overlaying two steel sheets 1 and spot welding them, then determining the length of an LME crack (crack 11 right outside pressure contact zone) formed right outside the pressure contact zone of the welded part 2 formed. The "two steel sheets 1" are the steel sheet of each test number (test no.) and its counterpart material steel sheet. The "right outside the pressure contact zone of the welded part" means a position at the overlaid surfaces of the two steel sheets at the part pressure contacted by spot welding as the part at the outside of the pressure contact zone 3 and near the pressure contact zone 3 (range from end part of pressure contact zone 3 to 1 mm or so to the outside). The crack 11 right outside of the pressure contact zone was evaluated for the length of the crack. Note that the test by spot welding was performed three times and the sample with the longest length of the crack of the crack 11 right outside the pressure contact zone was evaluated. The evaluation criteria were as follows. In the present embodiment, if the evaluation A or more (that is, evaluation A, AA, AAA), it was judged that the LME resistance was excellent.

[0105] 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

[0106] The results of the evaluations are shown in Table 4.[Table 4]

[0107] Table 4Test no.ClassPlating typeSurface roughness Ra (µm)Steel surface layer structurePerformanceCounterpart materialC≤0.02% depth (µm)High ferrite layer thickness (µm)Grazing incidence XRD (value of center term of conditional expression)Tensile strength (MPa)LME resistance1Inv. ex.a2.9350.90AASame type2Inv. ex.Unplated2.9660.82AAGI270IF3Inv. ex.b1.6870.75AAAASame type4Inv. ex.b1.31080.72AAAAASame type5Inv. ex.b1.51390.70AAAAAUnplatedSame type6Inv. ex.b1.515120.72AAAAASame type7Inv. ex.b1.316150.70AAAAASame type8Inv. ex.b1.723200.69AAAAASame type9Inv. ex.b1.525220.68AAAAASame type10Inv. ex.b1.422230.67AAAAASame tvpe11Inv. ex.c1.523240.66AAAAASame type12Inv. ex.b1.621250.65AAAAASame type13Inv. ex.b1.122260.67AAAAASame type14Inv. ex.b1.0970.63AAAASame type15Inv. ex.b1.122220.67AAAAAGA59016Inv. ex.b1.222230.66AAAAAASame type17Inv. ex.Unplated1.222250.64AAAAAAGASame type18Inv. ex.b1.140250.66AAAAAASame type19Inv. ex.b1.022270.66AAAAAASame type20Inv. ex.b1.223280.65AAAAAASame type21Inv. ex.b1.222260.67AAAAASame type22Inv. ex.b1.029280.69AAAAAASame type23Inv. ex.Unplated1.122240.67AAAAAAGASame type24Inv. ex.c1.222290.65AAAAAASame type25Inv. ex.b1.222260.65AAAAAASame type26Comp. ex.b2.4140.83AAABSame type27Comp. ex.b2.4130.83AAABSame type28Comp. ex.b2.4230.82AAABSame type29Comp. ex.b2.3130.81AAABSame type30Comp. ex.b2.2220.82AAABSame type31Comp. ex.b2.3160.81AAABSame type32Comp. ex.b0.5241.02AAABSame type33Comp. ex.b2.4230.79AAABSame type34Comp. ex.b2.5220.82AAABSame type35Comp. ex.b2.4940.95AAABSame type36Comp. ex.b2.3230.97AAABSame type37Inv. ex.b1.620230.64AAAAASame type38Inv. ex.b1.521240.62AAAAASame type39Inv. ex.b1.622250.62AAAAASame type40Inv. ex.b1.421250.61AAAAASame type41Inv. ex.b1.622230.66AAAAASame type42Inv. ex.b1.621240.63AAAAASame type43Inv. ex.b1.523240.65AAAAASame type44Inv. ex.b1.621240.65AAAAASame type45Inv. ex.b1.620250.65AAAAASame type46Inv. ex.b1.621260.64AAAAASame type47Inv. ex.b1.622240.63AAAAASame type48Inv. ex.b1.622230.65AAAAASame type49Inv. ex.b1.621220.65AAAAASame type50Inv. ex.b1.621240.63AAAAASame type51Inv. ex.b1.621250.64AAAAASame type52Inv. ex.b1.622240.65AAAAASame type53Inv. ex.b1.622230.63AAAAASame type54Inv. ex.b1.621240.66AAAAASame type55Inv. ex.b1.621240.66AAAAASame type* Underlines indicate outside scope of present invention or not yielding desired properties. <Test Nos. 5-1 to 5-7>

[0108] Test Nos. 5-1 to 5-7 were performed under the same conditions as Test No. 5 shown in Tables 1, 3, and 4 except for making the holding temperature 860°C as the dew point in the annealing step shown in Table 5 and designating the counterpart material the "same type". The results are shown in Table 5.[Table 5]

[0109] Table 5Test no.ClassDew pointSurface roughness Ra (µm)Steel surface layer structurePerformanceC≤0.02% depth (µm)High ferrite layer thickness (µm)Grazing incidence XRD (value of center term of conditional expression)Tensile strength (MPa)LME resistance5-1Inv. ex.-301.5360.76AAAA5-2Inv. ex.-201.5770.77AAAA5-3Inv. ex.-101.51190.72AAAAA5-4Inv. ex.01.51390.70AAAAA5-5Inv. ex.51.51080.71AAAAA5-6Inv. ex.101.5860.76AAAA5-7Inv. ex.201.5350.78AAAA <Test Nos. 3-1 to 3-7>

[0110] Test Nos. 3-1 to 3-7 were performed under the same conditions as Test No. 3 shown in Tables 1, 3, and 4 except for making the dew point control start temperature in the annealing step the one shown in Table 6 and making the holding temperature 860°C. The results are shown in Table 6.[Table 6]

[0111] Table 6Test no.ClassDew point control start temperatureGrazing incidence XRD (value of center term of conditional expression)LME resistance3-1Comp. ex.2500.97B3-2Inv. ex.3000.90A3-3Inv. ex.4000.82A3-4Inv. ex.4500.75AA3-5Inv. ex.5000.74AA3-6Inv. ex.5500.75AA3-7Comp. ex.6000.95B* Underlines indicate outside scope of present invention or not yielding desired properties.

[0112] The steel sheets of Test Nos. 1 to 25 and 37 to 55 and Test Nos. 3-2 to 3-6 and 5-1 to 5-7 are invention examples and had high LME resistances. On the other hand, the steel sheets of Test Nos. 26 to 36, 3-1, and 3-7 with chemical compositions of the steel sheets or production conditions outside the predetermined conditions in the present invention had LME resistances not reaching the passing criteria.

[0113] In Test No. 32, shot blasting was not performed, therefore strain was not introduced into the surface layer of the steel sheet. For this reason, it is believed that decarburization was not promoted and the depth with a C concentration of 0.02% or less measured by GDS was shallow. Further, it is believed that internal oxidation of Si or Mn did not proceed and the orientation of the ferrite phase did not become randomized. As a result, the LME resistance became inferior.

[0114] In Test No. 35, instead of shot blasting treatment, pretreatment was performed by grinding using brush rolls, therefore strain was not sufficiently introduced into the surface layer. Further, it is believed that in the annealing step, internal oxidation of Si or Mn did not proceed and the orientation of the ferrite phase did not become randomized. As a result, the LME resistance became inferior.

[0115] In Test No. 36, the dew point control start temperature at the annealing step was low, therefore it is believed external oxidation proceeded, decarburization did not proceed, and the depth with a C concentration of 0.02% or less by GDS measurement became shallower. Further, it is believed that internal oxidation of Si or Mn did not proceed and the orientation of the ferrite phase did not become randomized. As a result, the LME resistance became inferior.[INDUSTRIAL APPLICABILITY]

[0116] According to the present invention, it becomes possible to provide high strength steel sheet and plated steel sheet having a high LME resistance. The steel sheet and plated steel sheet can be optimally used for automobiles, household electric appliance products, building materials, and other applications, in particular for automobile use. Therefore, the present invention is an invention with an extremely high industrial applicability.REFERENCE SIGNS LIST

[0117] 1 steel sheet 2 welded part 3 contact zone 11 cracking right outside pressure contact zone

Examples

examples

[0088]Below, examples will be used to explain the present invention in more detail. The present invention is not limited to these examples.

[0089]Molten steel adjusted to the chemical composition described in Test 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 by an end temperature of finish rolling of 950°C and a reduction rate of finish rolling of 30% to obtain hot rolled steel sheet. The obtained hot rolled steel sheet was coiled at the coiling temperature 650°C, pickled, then cold rolled by a reduction rate of 50% to obtain cold rolled steel sheet. The thickness of the cold rolled steel sheet was 1.6 mm.

[0090]Next, the surface of the obtained cold rolled steel sheet was blasted using as the blasting material TSH30 made by WINOA IKK JAPAN by a blasting rate of 5 kg / m 2< as shot blasting. The surface roughness Ra of the cold rolled steel sheet after the shot blasting...

Claims

1. A steel sheet having a tensile strength of 780 MPa or more, wherein a chemical composition of the steel sheet comprises, by mass%, C: 0.05 to 0.40%, Si: 0.5 to 3.0%, Mn: 0.1 to 5.0%, sol. Al: 0 to 3.0%, 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 0.1500%, 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 comprising Fe and impurities; a surface roughness Ra of the steel sheet is 3.0 µm or less; a depth in a thickness direction from the surface of the steel sheet wherein a C concentration is 0.02% or less is 3 µm or more; a thickness in a thickness direction from the surface of the steel sheet of a layer having an area ratio of ferrite of 90% or more is 3 µm or more; and, in grazing incidence X-ray diffraction with an incidence angle of 1° with respect to the steel sheet surface, 0.45 ≤ I(110) / ( 1(110) + I(200) + I(211) ) ≤ 0.90 is satisfied, when I(110) denotes a diffraction intensity corresponding to the (110) plane, I(200) denotes a diffraction intensity corresponding to the (200) plane, and I(211) denotes a diffraction intensity corresponding to the (211) plane.

2. The steel sheet according to claim 1, wherein in the thickness direction from the steel sheet surface, the depth with a C concentration of 0.02% or less is 10 µm or more.

3. The steel sheet according to claim 1, wherein 0.45 ≤ I(110) / ( I(110) + I(200) + I(211) ) ≤ 0.75 is satisfied.

4. The steel sheet according to claim 1, wherein in the thickness direction from the steel sheet surface, the thickness of the layer with a ferrite area ratio of 90% or more is 8 µm or more.

5. The steel sheet according to claim 1, wherein the surface roughness Ra of the steel sheet is 2.0 µm or less.

6. A plated steel sheet comprising the steel sheet according to any one of claims 1 to 5 and a plating layer comprising Zn, provided at least at part of the surface of the steel sheet.

7. An automobile member comprising the steel sheet according to any one of claims 1 to 5.

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