Electroplated steel sheets with FE base; Electroplated drop-coating steel sheets; Automotive parts; Methods of manufacturing electroplated steel sheets; and methods of manufacturing FE-based electroplated steel sheets.

Abstract

DEPCT6625 / 07 / 2566 What was provided was a steel plate with excellent resistance to breakage during welding. Resistance at the welded area despite the crystal arrangement of the electroplating layer containing Fe. It is a base and cold-rolled steel sheet with Si integrated at a high ratio at the section. The bonding between an electroplating layer based on Fe and a cold-rolled steel sheet based on Si is... It is supplied as an electroplated steel sheet with a Fe-based coating, using a cold-rolled steel sheet. Si containing 0.1% or more by mass and 3.0% or less by mass, and coating layers. With an electric Fe-based material that is formed on at least one surface of a rolled steel sheet. Cold-pressed Si with a coating weight per surface area greater than 20.0 g / m², where the crystal arrangement of the layers is... Electroplating with Fe as a base and cold-rolled steel sheets with Si were integrated. A ratio greater than 50% at the interface between the Fe-based electroplating layer and... Cold-rolled steel sheet containing Si;

Description

Fe-based electroplated steel sheet, electrodeposition coated steel sheet, automotive parts, manufacturing method of electrodeposition coated steel sheet, and manufacturing method of Fe-based electroplated steel sheet

[0001] The present invention relates to an Fe-based electroplated steel sheet having excellent resistance weld crack resistance, an electrodeposition coated steel sheet, an automobile part, a method for manufacturing an electrodeposition coated steel sheet, and a method for manufacturing an Fe-based electroplated steel sheet.

[0002] In recent years, there has been a strong demand for improved automobile fuel efficiency in order to protect the global environment. Furthermore, there is also a strong demand for improved automobile safety in order to ensure the safety of passengers in the event of a collision. To meet these demands, it is necessary to simultaneously reduce the weight and strength of automobile bodies. Therefore, the cold-rolled steel sheets used in automobile parts are being actively developed to reduce thickness while increasing strength. However, since many automobile parts are manufactured by forming steel sheets, these steel sheets must have excellent formability in addition to high strength.

[0003] There are various methods for increasing the strength of cold-rolled steel sheets. One method that can increase the strength of cold-rolled steel sheets without significantly impairing their formability is solid-solution strengthening through the addition of Si. Meanwhile, in the manufacture of automotive parts, press-formed parts are often assembled by resistance welding (spot welding). When a part to be resistance-welded contains high-strength galvanized steel sheets, residual stresses are generated near the weld during resistance welding. This can cause liquid metal embrittlement (LME) and lead to intergranular cracking (LME cracking) in the steel sheet. In particular, when welding is performed with a welding electrode angled relative to the steel sheet, residual stresses increase, potentially leading to cracking. Since residual stresses are thought to increase with increasing strength of steel sheets, LME cracking is a concern. Even if the high-strength steel sheet does not have a zinc-coated layer, if the steel sheet to be welded is a zinc-coated steel sheet, the zinc-coated layer will melt, and LME cracking can occur even in steel sheets that do not have a zinc-coated layer. This problem of LME cracking is particularly noticeable in steel sheets containing Si.

[0004] For these reasons, there is a demand for high-strength steel sheets that are excellent in resistance weld crack resistance at welded joints when the other sheet is a galvanized steel sheet (hereinafter, also simply referred to as "resistance weld crack resistance at welded joints").

[0005] Conventionally, various solutions to the above problem have been reported. For example, Patent Document 1 discloses a steel sheet having an internal oxide layer in which at least a portion of the grain boundaries is covered with an oxide from the surface of the base material to a depth of 5.0 μm or more, and in which the grain boundary coverage by the oxide is 60% or more in the region from the surface of the base material to a depth of 5.0 μm.

[0006] Patent No. 6388099

[0007] The present inventors have newly discovered that forming an Fe-based electroplated layer on the surface of a cold-rolled steel sheet can improve resistance weld cracking resistance. Furthermore, they have also found that when annealing is performed after the formation of the Fe-based electroplated layer, the crystal orientations of the Fe-based electroplated layer and the cold-rolled steel sheet are highly integrated at the interface between them, depending on the annealing conditions. They have also found that when such an Fe-based electroplated steel sheet is paired with a zinc-plated steel sheet, molten zinc easily penetrates into the grain boundaries of the cold-rolled steel sheet via the grain boundaries of the Fe-based electroplated layer. Patent Document 1 did not consider this phenomenon at all.

[0008] Therefore, an object of the present invention is to provide a steel sheet that has excellent resistance weld cracking resistance at a welded portion even when the crystal orientations of the Fe-based electroplated layer and the Si-containing cold-rolled steel sheet are integrated to a high degree at the interface between the Fe-based electroplated layer and the cold-rolled steel sheet.

[0009] As a result of extensive research conducted by the present inventors to solve the above-mentioned problems, it has been found that in order to achieve a high level of resistance weld crack resistance in a welded portion, it is necessary to form an Fe-based electroplated layer on the surface of a cold-rolled steel sheet after cold rolling and before continuous annealing in a coating amount of 20.0 g / m per side. ２ It was found that it is important to form a soft Fe-based electroplated layer at a coating weight of 20.0 g / m per side of the cold-rolled steel sheet. ２The present inventors have found that forming the cold-rolled steel sheet with an Fe-based electroplating layer of ultra-high purity reduces the stress applied to the steel sheet surface during welding, and when the cold-rolled steel sheet contains Si, the Fe-based electroplating layer acts as a solid-solution Si-depleted layer, suppressing the decrease in toughness due to Si solid solution and improving the resistance weld cracking resistance of the welded joint, thereby completing the present invention.

[0010] The present invention has been made based on the above findings. That is, the gist and configuration of the present invention are as follows.

[0011] [1] A Si-containing cold-rolled steel sheet containing 0.1 mass% or more and 3.0 mass% or less of Si, and a coating amount per side of 20.0 g / m formed on at least one side of the Si-containing cold-rolled steel sheet. ２ and an Fe-based electroplated layer having a crystal orientation greater than 100%.

[0012] [2] The Fe-based electroplated steel sheet according to [1], wherein the Si-containing cold-rolled steel sheet contains 0.50 mass % or more and 3.0 mass % or less of Si.

[0013] [3] The deposition amount of the Fe-based electroplating layer per side is 25.0 g / m ２ The Fe-based electroplated steel sheet according to [1] or [2] above.

[0014] [4] The Fe-based electroplated steel sheet according to any one of [1] to [3], wherein the Si-containing cold-rolled steel sheet contains, in addition to the Si, the following, by mass%, C: 0.8% or less, Mn: 1.0% to 12.0%, P: 0.1% or less, S: 0.03% or less, N: 0.010% or less, and Al: 1.0% or less, with the balance being Fe and unavoidable impurities.

[0015] [5] The Fe-based electroplated steel sheet according to [4], wherein the chemical composition further contains one or more elements selected from the group consisting of B: 0.005% or less, Ti: 0.2% or less, Cr: 1.0% or less, Cu: 1.0% or less, Ni: 1.0% or less, Mo: 1.0% or less, Nb: 0.20% or less, V: 0.5% or less, Sb: 0.200% or less, Ta: 0.1% or less, W: 0.5% or less, Zr: 0.1% or less, Sn: 0.20% or less, Ca: 0.005% or less, Mg: 0.005% or less, and REM: 0.005% or less.

[0016] [6] The Fe-based electroplated steel sheet according to any one of [1] to [5], wherein the Fe-based electroplated layer has a composition containing one or more elements selected from the group consisting of B, C, P, N, O, Ni, Mn, Mo, Zn, W, Pb, Sn, Cr, V, and Co in a total amount of 10 mass% or less, with the remainder consisting of Fe and unavoidable impurities.

[0017] [7] A cold-rolled steel sheet, and a coating amount per side of 20.0 g / m formed on at least one side of the cold-rolled steel sheet. ２ and an Fe-based electroplated layer having a crystal orientation of more than 50% at the interface between the Fe-based electroplated layer and the cold-rolled steel sheet, wherein the rate at which the crystal orientations of the Fe-based electroplated layer and the cold-rolled steel sheet are integrated is more than 50%. Here, the cold-rolled steel sheet is prepared by cutting a test piece of 50 × 150 mm with the longitudinal direction perpendicular to the rolling direction into a size of 50 × 150 mm, and measuring the coating weight of the hot-dip galvanized layer cut to the same size on one side of the test piece. ２Next, using a servo motor pressure type single-phase AC (50 Hz) resistance welding machine, the plate assembly was tilted by 5° in the longitudinal direction of the plate assembly with respect to a plane perpendicular to a line connecting the central axes of an electrode pair (tip diameter 6 mm) of the resistance welding machine, and a gap of 60 mm in the longitudinal direction of the plate assembly and 2.0 mm in the thickness direction of the plate assembly was provided between the lower electrode of the electrode pair and the test piece, and the lower electrode and the plate assembly were fixed, and with the upper electrode of the electrode pair movable, resistance welding was performed on the plate assembly under conditions of a pressure of 3.5 kN, a hold time of 0.16 seconds, and a welding current and welding time such that the nugget diameter was 5.9 mm, to form a plate assembly with a weld. Next, the plate assembly with the welded portion is cut in half along the longitudinal direction of the test piece so as to include the welded portion, and when the cross section of the welded portion is observed under an optical microscope (magnification 200 times), a crack having a length of 0.1 mm or more is observed in the cold-rolled steel sheet.

[0018] [8] The Fe-based electroplated steel sheet according to [7] above, wherein the cold-rolled steel sheet is a cold-rolled steel sheet in which, when the resistance welding is performed under a condition in which the hold time is 0.24 seconds to obtain the sheet assembly with the welded portion and a cross section of the welded portion is observed with the optical microscope (magnification: 200 times), a crack having a length of 0.1 mm or more is observed.

[0019] [9] An electrodeposition-coated steel sheet further comprising, on the Fe-based electroplated steel sheet according to any one of [1] to [8], a chemical conversion coating formed in contact with the Fe-based electroplating layer, and an electrodeposition coating film formed on the chemical conversion coating.

[0020]

[10] An automobile part, at least in part, using the electrodeposition coated steel sheet according to [9] above.

[0021]

[11] A method for producing an electrodeposition-coated steel sheet, comprising: a chemical conversion treatment step of subjecting the Fe-based electroplated steel sheet according to any one of [1] to [8] above to a chemical conversion treatment without additional plating treatment, to obtain a chemical conversion-treated steel sheet having a chemical conversion coating formed in contact with the Fe-based electroplating layer; and an electrodeposition coating step of subjecting the chemical conversion-treated steel sheet to an electrodeposition coating treatment, to obtain an electrodeposition-coated steel sheet having an electrodeposition coating coating formed on the chemical conversion coating.

[0022]

[12] A cold-rolled steel sheet containing 0.1 mass% or more and 3.0 mass% or less of Si is subjected to Fe-based electroplating to obtain a coating weight of 20.0 g / m per side. ２ and subsequently, annealing the pre-annealed Fe-based electroplated steel sheet in an atmosphere having a dew point of −30° C. or less to obtain the Fe-based electroplated steel sheet.

[0023]

[13] The method for producing an Fe-based electroplated steel sheet according to

[12] , wherein the cold-rolled steel sheet contains 0.5 mass % or more and 3.0 mass % or less of Si.

[0024]

[14] A cold-rolled steel sheet is electroplated with an Fe-based coating to a coating weight of 20.0 g / m ２ A method for producing an Fe-based electroplated steel sheet, comprising the steps of: forming a pre-annealed Fe-based electroplated steel sheet having a pre-annealing Fe-based electroplated layer of more than 1000 μm on at least one side thereof; and then annealing the pre-annealed Fe-based electroplated steel sheet to obtain an Fe-based electroplated steel sheet. Here, the cold-rolled steel sheet is a test piece cut into 50 × 150 mm with the longitudinal direction perpendicular to the rolling direction, and then annealing the test piece to obtain an Fe-based electroplated steel sheet. ２ Next, using a servo motor pressure type single-phase AC (50 Hz) resistance welding machine, the plate assembly was tilted by 5° in the longitudinal direction of the plate assembly with respect to a plane perpendicular to a line connecting the central axes of an electrode pair (tip diameter 6 mm) of the resistance welding machine, and a gap of 60 mm in the longitudinal direction of the plate assembly and 2.0 mm in the thickness direction of the plate assembly was provided between the lower electrode of the electrode pair and the test piece, and the lower electrode and the plate assembly were fixed, and with the upper electrode of the electrode pair movable, resistance welding was performed on the plate assembly under conditions of a pressure of 3.5 kN, a hold time of 0.16 seconds, and a welding current and welding time such that the nugget diameter was 5.9 mm, to form a plate assembly with a weld. Next, the plate assembly with the welded portion is cut in half along the longitudinal direction of the test piece so as to include the welded portion, and when the cross section of the welded portion is observed under an optical microscope (magnification 200 times), a crack having a length of 0.1 mm or more is observed in the cold-rolled steel sheet.

[0025]

[15] The method for producing an Fe-based electroplated steel sheet according to

[14] , wherein the cold-rolled steel sheet is a cold-rolled steel sheet in which a crack having a length of 0.1 mm or more is observed when the cross section of the weld is observed with an optical microscope (magnification: 200 times) after the resistance welding is performed under a condition in which the hold time is 0.24 seconds to obtain the sheet assembly with the weld.

[0026]

[16] The method for producing a Fe-based electroplated steel sheet according to any one of

[12] to

[15] above, wherein the Fe-based electroplating is performed using an Fe-based electroplating bath containing one or more elements selected from the group consisting of B, C, P, N, O, Ni, Mn, Mo, Zn, W, Pb, Sn, Cr, V, and Co, such that the total content of these elements in the pre-annealing Fe-based electroplated layer is 10 mass% or less.

[0027] According to the present invention, it is possible to provide a steel sheet that has excellent resistance weld crack resistance at the welded portion, even when the crystal orientations of the Fe-based electroplated layer and the Si-containing cold-rolled steel sheet are integrated to a high degree at the interface between the Fe-based electroplated layer and the Si-containing cold-rolled steel sheet.

[0028] 1 is a diagram showing an outline of a cross section of an Fe-based electroplated steel sheet. (a) A perspective view and (b) an A-A cross-sectional view showing an outline of an observation sample for measuring the degree of integration of crystal orientation.

[0034] Figures for explaining a method for evaluating the degree of integration of crystal orientation, including (a) a diagram in which a boundary line is drawn at the interface between the Fe-based electroplated layer and the Si-containing cold-rolled steel sheet in an SIM image, (b) a diagram in which a boundary line and a judgment area are drawn on a binarized image, and (c) an enlarged view of the area enclosed by a square in (b) above.

[0035] Figures showing an observation image of the interface between the Fe-based electroplated layer and the Si-containing cold-rolled steel sheet of Invention Example No. 31.

[0036] Figures showing an image in which a boundary line and a judgment area are drawn after binarization of the interface between the Fe-based electroplated layer and the Si-containing cold-rolled steel sheet of Invention Example No. 31.

[0037] Figures showing an observation image of the interface between the Fe-based electroplated layer and the Si-containing cold-rolled steel sheet of Invention Example No. 34. 34 shows an image in which a boundary line and a judgment region are drawn after binarization of the interface between the Fe-based electroplated layer and the Si-containing cold-rolled steel sheet. (a) is a diagram for explaining a method for evaluating resistance weld cracking resistance in a welded portion, (b) the upper figure is a top view of the sheet assembly after welding in the same evaluation, and the lower figure is a B-B cross-sectional view of the upper figure.

[0029] The LME cracks described above can be broadly classified into "cracks occurring on the surface in contact with the electrode (hereinafter, "surface cracks")" and "cracks occurring near the corona bond between steel sheets (hereinafter, "internal cracks"). Surface cracks are known to be prone to occur during resistance welding at high currents that generate spatter. Surface cracks can be suppressed by maintaining the current within an appropriate range that does not generate spatter. On the other hand, internal cracks can occur even when the resistance welding current is within an appropriate range that does not generate spatter. Furthermore, while surface cracks are easily detected by visual inspection during the manufacturing process, internal cracks are difficult to detect by visual inspection. For these reasons, internal cracks are a particularly significant issue among LME cracks. Resistance welding performed with a welding electrode angled relative to the steel sheet can increase residual stress, potentially leading to internal cracks. Since residual stress is thought to increase with increasing strength of the steel sheet, internal cracks are a concern as the steel sheet becomes stronger. The present disclosure can improve resistance weld crack resistance, particularly the ability to prevent internal cracks.

[0030] Hereinafter, embodiments of the present invention will be described. In the following description, the unit of the content of each element in the composition of a Si-containing cold-rolled steel sheet and the content of each element in the composition of a coating layer is "mass %," and unless otherwise specified, it is simply referred to as "%." In this specification, a numerical range expressed using "to" means a range that includes the numerical values ​​before and after "to" as the lower and upper limits. In this specification, a "high-strength" steel sheet means that the tensile strength TS of the steel sheet measured in accordance with JIS Z 2241 (2011) is 590 MPa or more.

[0031] [Embodiment 1] Fig. 1 shows an outline of a cross section of an Fe-based electroplated steel sheet 1 according to this embodiment. As shown in Fig. 1, the Fe-based electroplated steel sheet 1 has an Fe-based electroplated layer 3 on at least one surface of an Si-containing cold-rolled steel sheet 2. First, the chemical composition of the Si-containing cold-rolled steel sheet will be described.

[0032] Si: 0.1% or more and 3.0% or less. Si is an effective element for achieving high strength in steel sheets because it has a significant effect of increasing the strength of steel through solid solution (solid solution strengthening ability) without significantly impairing workability. On the other hand, Si also has a negative effect on the resistance weld crack resistance of welded parts. When Si is added to achieve high strength in steel sheets, an addition of 0.1% or more is necessary. When the Si content is less than 0.50%, welding with a conventional hold time of approximately 0.24 seconds does not cause any particular problems with the resistance weld crack resistance of welded parts. However, when the takt time during spot welding in the assembly process of automobile parts becomes an issue from the perspective of production costs and measures such as reducing the hold time are taken, the resistance weld crack resistance of welded parts may be insufficient even with a Si content of less than 0.50%. On the other hand, when the Si content exceeds 3.0%, hot rolling and cold rolling properties are significantly reduced, adversely affecting productivity and causing a decrease in the ductility of the steel sheet itself. Therefore, Si is added in the range of 0.1% to 3.0%. The Si content is preferably 0.50% or more, more preferably 0.7% or more, and even more preferably 0.9% or more. The Si content is preferably 2.5% or less, more preferably 2.0% or less, and even more preferably 1.7% or less.

[0033] The Si-containing cold-rolled steel sheet according to this embodiment must contain Si in the above range, but other elements are permissible as long as they are within the composition ranges of ordinary cold-rolled steel sheets, and are not particularly limited. However, when the Si-containing cold-rolled steel sheet according to this embodiment is to have a high strength of 590 MPa or more in tensile strength (TS), it is preferable that the steel sheet has the following composition.

[0034] C: 0.8% or less (excluding 0%) C improves workability by forming martensite or the like as a steel structure. When C is contained, in order to obtain good weldability, the C content is preferably 0.8% or less, and more preferably 0.3% or less. There is no particular lower limit for C, but in order to obtain good workability, the C content is preferably more than 0%, more preferably 0.03% or more, and even more preferably 0.08% or more.

[0035] Mn: 1.0% or more and 12.0% or less Mn is an element that strengthens steel by solid solution strengthening, improves hardenability, and promotes the formation of retained austenite, bainite, and martensite. These effects are achieved by adding 1.0% or more of Mn. On the other hand, if the Mn content is 12.0% or less, the above effects can be achieved without increasing costs. Therefore, the Mn content is preferably 1.0% or more and 12.0% or less. The Mn content is more preferably 1.3% or more, even more preferably 1.5% or more, and most preferably 1.8% or more. Furthermore, the Mn content is more preferably 3.5% or less, and even more preferably 3.3% or less.

[0036] P: 0.1% or less (excluding 0%). By suppressing the P content, it is possible to prevent a decrease in weldability. Furthermore, it is possible to prevent P from segregating at grain boundaries, thereby preventing deterioration of ductility, bendability, and toughness. Furthermore, adding a large amount of P promotes ferrite transformation, which increases the grain size. Therefore, it is preferable that the P content be 0.1% or less. There is no particular lower limit for P, and it can be more than 0% or 0.001% or more due to constraints on production technology.

[0037] S: 0.03% or less (excluding 0%) The S content is preferably 0.03% or less, and more preferably 0.02% or less. By suppressing the S content, deterioration of weldability is prevented, and deterioration of ductility during hot rolling is prevented, thereby suppressing hot cracking and significantly improving surface properties. Furthermore, suppressing the S content forms coarse sulfides as an impurity element, thereby preventing deterioration of the ductility, bendability, and stretch flangeability of the steel sheet. These problems become significant when the S content exceeds 0.03%, so it is preferable to reduce the S content as much as possible. The lower limit of S is not particularly limited, and may be greater than 0% or greater than 0.0001% due to constraints on production technology.

[0038] N: 0.010% or less (excluding 0%) The N content is preferably 0.010% or less. By setting the N content to 0.010% or less, N can form coarse nitrides with Ti, Nb, and V at high temperatures, preventing the effect of adding Ti, Nb, and V to increase the strength of the steel sheet from being impaired. Furthermore, by setting the N content to 0.010% or less, a decrease in toughness can be prevented. Furthermore, by setting the N content to 0.010% or less, the occurrence of slab cracks and surface defects during hot rolling can be prevented. The N content is preferably 0.005% or less, more preferably 0.003% or less, and even more preferably 0.002% or less. The lower limit of the N content is not particularly limited, and may be greater than 0% or greater than 0.0005% due to production technology constraints.

[0039] Al: 1.0% or less (excluding 0%) Because Al is thermodynamically most easily oxidized, it oxidizes before Si and Mn, suppressing the oxidation of Si and Mn at the outermost layer of the steel sheet and promoting the oxidation of Si and Mn inside the steel sheet. This effect is obtained when the Al content is 0.01% or more. On the other hand, an Al content exceeding 1.0% increases costs. Therefore, when added, the Al content is preferably 1.0% or less. The Al content is more preferably 0.1% or less. The lower limit of Al is not particularly limited and may be more than 0% or 0.001% or more.

[0040] The chemical composition may further optionally contain one or more elements selected from the group consisting of B: 0.005% or less, Ti: 0.2% or less, Cr: 1.0% or less, Cu: 1.0% or less, Ni: 1.0% or less, Mo: 1.0% or less, Nb: 0.20% or less, V: 0.5% or less, Sb: 0.200% or less, Ta: 0.1% or less, W: 0.5% or less, Zr: 0.1% or less, Sn: 0.20% or less, Ca: 0.005% or less, Mg: 0.005% or less, and REM: 0.005% or less.

[0041] B: 0.005% or less B is an element effective in improving the hardenability of steel. To improve hardenability, the B content is preferably 0.0003% or more, and more preferably 0.0005% or more. However, since excessive addition of B reduces formability, the B content is preferably 0.005% or less.

[0042] Ti: 0.2% or less Ti is effective for precipitation strengthening of steel. There is no particular lower limit for Ti, but in order to obtain the effect of adjusting strength, it is preferable to set it to 0.005% or more. However, if Ti is added in excess, the hard phase becomes excessively large and formability decreases, so when Ti is added, the Ti amount is preferably 0.2% or less, and more preferably 0.05% or less.

[0043] Cr: 1.0% or less The Cr content is preferably 0.005% or more. By making the Cr content 0.005% or more, it is possible to improve hardenability and improve the balance between strength and ductility. When Cr is added, it is preferable that the Cr content be 1.0% or less in order to prevent an increase in costs.

[0044] Cu: 1.0% or less The Cu content is preferably 0.005% or more. By setting the Cu content to 0.005% or more, the formation of the residual γ phase can be promoted. Furthermore, when adding Cu, the Cu content is preferably 1.0% or less from the viewpoint of preventing an increase in cost.

[0045] Ni: 1.0% or less The Ni content is preferably 0.005% or more. By setting the Ni content to 0.005% or more, it is possible to promote the formation of the residual γ phase. Furthermore, when Ni is added, the Ni content is preferably 1.0% or less from the viewpoint of preventing an increase in costs.

[0046] Mo: 1.0% or less The Mo content is preferably 0.005% or more. By setting the Mo content to 0.005% or more, the effect of adjusting strength can be obtained. The Mo content is more preferably 0.05% or more. Furthermore, when Mo is added, the Mo content is preferably 1.0% or less from the viewpoint of preventing an increase in costs.

[0047] Nb: 0.20% or less When Nb is contained in an amount of 0.005% or more, the effect of improving strength can be obtained. Furthermore, when Nb is contained, the Nb content is preferably 0.20% or less from the viewpoint of preventing an increase in cost.

[0048] V: 0.5% or less The effect of improving strength can be obtained by adding 0.005% or more of V. Furthermore, when V is added, the V content is preferably 0.5% or less from the viewpoint of preventing an increase in cost.

[0049] Sb: 0.200% or less Sb can be added to suppress nitriding and oxidation of the steel sheet surface, or decarburization of the steel sheet surface in a region of several tens of microns caused by oxidation. By suppressing nitriding and oxidation of the steel sheet surface, Sb prevents a decrease in the amount of martensite formed on the steel sheet surface, improving the fatigue properties and surface quality of the steel sheet. To achieve this effect, the Sb content is preferably 0.001% or more. On the other hand, to obtain good toughness, the Sb content is preferably 0.200% or less.

[0050] Ta: 0.1% or less The effect of improving strength can be obtained by adding 0.001% or more of Ta. Furthermore, when Ta is added, the Ta content is preferably 0.1% or less from the viewpoint of preventing an increase in cost.

[0051] W: 0.5% or less The effect of improving strength can be obtained by adding 0.005% or more of W. Furthermore, when W is added, the W content is preferably 0.5% or less from the viewpoint of preventing an increase in cost.

[0052] Zr: 0.1% or less When Zr is contained in an amount of 0.0005% or more, the effect of improving strength can be obtained. Furthermore, when Zr is contained, the Zr content is preferably 0.1% or less from the viewpoint of preventing an increase in cost.

[0053] Sn: 0.20% or less Sn is an element that is effective in suppressing denitrification, deboronation, etc., and suppressing a decrease in the strength of steel. To obtain these effects, it is preferable that each content be 0.002% or more. On the other hand, to obtain good impact resistance, it is preferable that the Sn content be 0.20% or less.

[0054] Ca: 0.005% or less When Ca is contained in an amount of 0.0005% or more, the morphology of sulfides can be controlled and ductility and toughness can be improved. In addition, from the viewpoint of obtaining good ductility, the Ca content is preferably 0.005% or less.

[0055] Mg: 0.005% or less When Mg is contained in an amount of 0.0005% or more, the morphology of sulfides can be controlled and ductility and toughness can be improved. Furthermore, when Mg is contained, the Mg content is preferably 0.005% or less from the viewpoint of preventing an increase in costs.

[0056] REM: 0.005% or less When REM is contained in an amount of 0.0005% or more, the morphology of sulfides can be controlled and ductility and toughness can be improved. Furthermore, when REM is contained, the REM content is preferably 0.005% or less from the viewpoint of obtaining good toughness.

[0057] The Si-containing cold rolled steel sheet of this embodiment contains the remainder other than the above components, namely Fe and unavoidable impurities.

[0058] Next, the Fe-based electroplated layer formed on at least one surface of the above-mentioned Si-containing cold-rolled steel sheet will be described. Fe-based electroplated layer: 20.0 g / m ２ Ultra-high adhesion amount per side: 20.0 g / m ２Although the mechanism by which the presence of an Fe-based electroplated layer exceeding 100% improves the resistance weld crack resistance of a weld is unclear, it is believed that the Fe-based electroplated layer functions as a soft layer, alleviating stress imparted to the steel sheet surface during welding. This reduces residual stress in the resistance weld, thereby improving the resistance weld crack resistance of the weld, particularly the ability to prevent internal cracking (stress relaxation effect). Furthermore, it is believed that a high amount of solute Si on the steel sheet surface reduces the toughness of the weld, thereby deteriorating the resistance weld crack resistance of the weld. In contrast, when a certain amount of Fe-based electroplated layer is present on the steel sheet surface, the Fe-based electroplated layer acts as a solute Si-deficient layer, reducing the amount of Si dissolved in the weld. This suppresses the decrease in toughness of the weld due to Si solid solution, thereby improving the resistance weld crack resistance of the weld, particularly the ability to prevent internal cracking (toughness reduction suppression effect). Meanwhile, in this embodiment, as described below, annealing is performed after the formation of the Fe-based electroplated layer. Annealing after the formation of the Fe-based electroplated layer can suppress the occurrence of scratches called pickups on the surface of the Fe-based electroplated steel sheet due to surface oxides of Si, Mn, etc. formed during annealing, while integrating the crystal orientations of the Fe-based electroplated layer and the Si-containing cold-rolled steel sheet at a rate of more than 50% at the interface between the Fe-based electroplated layer and the Si-containing cold-rolled steel sheet. Therefore, molten zinc easily penetrates into the grain boundaries of the Si-containing cold-rolled steel sheet via the grain boundaries of the Fe-based electroplated layer. Therefore, in this embodiment, the zinc content of 20.0 g / m ２ An Fe-based electroplated layer having a coating weight of more than 20.0 g / m is formed. ２It is believed that forming an Fe-based electroplated layer with a coating weight of more than 20.0 g / m delays the time it takes for molten zinc to reach the grain boundaries of the Si-containing cold-rolled steel sheet during resistance welding, thereby improving the resistance weld crack resistance characteristics of the weld, particularly the ability to prevent internal cracks (the effect of inhibiting zinc penetration into grain boundaries). The contributions of the stress relaxation effect, the effect of inhibiting toughness reduction, and the effect of inhibiting zinc penetration into grain boundaries to the resistance weld crack resistance characteristics are complex and therefore not quantitatively clear, but it is believed that they act in a combined manner to improve the resistance weld crack resistance characteristics. In order to produce the effect of improving the resistance weld crack resistance characteristics of the weld, the coating weight of the Fe-based electroplated layer per side should be 20.0 g / m. ２ Although there is no particular upper limit to the amount of the Fe-based electroplated layer deposited on one side, from the viewpoint of cost, it is preferable that the amount of the Fe-based electroplated layer deposited on one side be 60.0 g / m or more. ２ The coating weight of the Fe-based electroplating layer is preferably 25.0 g / m or less. ２ More preferably, 30.0 g / m ２ More preferably, 35.0 g / m ２ The Fe-based electroplated steel sheet preferably has an Fe-based electroplated layer on both the front and back sides of a Si-containing cold-rolled steel sheet. The coating weight of the Fe-based electroplated layer is 25.0 g / m or more. ２ By satisfying the above conditions, the resistance weld cracking resistance characteristics of the welded portion become particularly good.

[0059] The thickness of the Fe-based electroplated layer was measured as follows: A 10 × 15 mm sample was taken from the Fe-based electroplated steel sheet and embedded in resin to form a cross-section embedded sample. Three randomly selected locations on the cross-section were observed using a scanning electron microscope (SEM) at an acceleration voltage of 15 kV and a magnification of 2,000 to 10,000 times depending on the thickness of the Fe-based electroplated layer. The average thickness of the three fields of view was multiplied by the iron density to convert it into the deposition weight of the Fe-based electroplated layer per side.

[0060] The Fe-based electroplating layer may be made of pure Fe, or an alloy plating layer such as an Fe-B alloy, an Fe-C alloy, an Fe-P alloy, an Fe-N alloy, an Fe-O alloy, an Fe-Ni alloy, an Fe-Mn alloy, an Fe-Mo alloy, or an Fe-W alloy. The composition of the Fe-based electroplating layer is not particularly limited, but it preferably contains one or more elements selected from the group consisting of B, C, P, N, O, Ni, Mn, Mo, Zn, W, Pb, Sn, Cr, V, and Co in a total amount of 10% by mass or less, with the remainder consisting of Fe and unavoidable impurities. By limiting the total amount of elements other than Fe to 10% by mass or less, a decrease in electrolysis efficiency can be prevented and the Fe-based electroplating layer can be formed at low cost. In the case of an Fe-C alloy, the C content is preferably 0.08% by mass or less.

[0061] The Si-containing cold-rolled steel sheet according to this embodiment preferably has no plating layer other than the Fe-based electroplating on its surface. By not having any plating layer other than the Fe-based electroplating on its surface, the Si-containing cold-rolled steel sheet can provide, at low cost, parts that do not require excessive amounts of galvanized steel sheet for rust prevention or parts that are used in environments where the corrosive environment is mild and no excessive rust prevention is required.

[0062] The proportion of the crystal orientations of the Fe-based electroplated layer and the Si-containing cold-rolled steel sheet that are integrated at the interface between the Fe-based electroplated layer and the Si-containing cold-rolled steel sheet is set to more than 50%. When the proportion of the crystal orientations of the Fe-based electroplated layer and the Si-containing cold-rolled steel sheet that are integrated at the interface between the Fe-based electroplated layer and the Si-containing cold-rolled steel sheet is more than 50%, molten zinc easily penetrates into the grain boundaries of the Si-containing cold-rolled steel sheet via the grain boundaries of the Fe-based electroplated layer, thereby significantly enhancing the effect of providing the Fe-based electroplated layer according to this embodiment. In the Fe-based electroplated steel sheet according to this embodiment, the proportion of the crystal orientations of the Fe-based electroplated layer and the Si-containing cold-rolled steel sheet that are integrated at the interface between the Fe-based electroplated layer and the Si-containing cold-rolled steel sheet may be 70% or more, and may be 75% or more. The upper limit of the percentage of the crystal orientations of the Fe-based electroplated layer and the Si-containing cold-rolled steel sheet that are integrated at the interface between the Fe-based electroplated layer and the Si-containing cold-rolled steel sheet is not particularly limited, and may be 100%.

[0063] As described above, the higher the percentage of integration of the crystal orientations of the Fe-based electroplated layer and the Si-containing cold-rolled steel sheet at the interface between the Fe-based electroplated layer and the Si-containing cold-rolled steel sheet, the more easily molten zinc penetrates into the grain boundaries of the Si-containing cold-rolled steel sheet via the grain boundaries of the Fe-based electroplated layer. This tendency becomes particularly pronounced when the percentage of integration of the crystal orientations of the Fe-based electroplated layer and the Si-containing cold-rolled steel sheet at the interface between the Fe-based electroplated layer and the Si-containing cold-rolled steel sheet exceeds 50%. In this embodiment, the Si-containing cold-rolled steel sheet is subjected to annealing after being subjected to Fe-based electroplating, and the annealing is performed in a low-dew-point atmosphere as described below. Therefore, the percentage of integration of the crystal orientations of the Fe-based electroplated layer and the Si-containing cold-rolled steel sheet is high. ２ By forming an Fe-based electroplated layer having a coating weight of 10 ...

[0064] Here, the percentage of integration of the crystal orientations of the Fe-based electroplated layer and the Si-containing cold-rolled steel sheet at the interface between the Fe-based electroplated layer and the Si-containing cold-rolled steel sheet is measured as follows: A sample of 10 × 10 mm in size is taken from the Fe-based electroplated steel sheet. An arbitrary location of the sample is processed using a focused ion beam (FIB) device to form a 45° cross section at that location, which is angled at 45° with respect to the T-section direction (a cross section parallel to the rolling direction of the steel sheet and perpendicular to the steel sheet surface), and has a width of 30 μm in the direction perpendicular to the rolling direction and a length of 50 μm in the 45° direction with respect to the T-section direction, to serve as an observation sample. Figure 2 shows an outline of the observation sample. Figure 2(a) is a perspective view of the observation sample. Figure 2(b) is a cross-sectional view taken along the line A-A of the observation sample shown in Figure 2(a). Next, a scanning ion microscope (SIM) was used to observe the center of a 45° cross section of the observation sample at a magnification of 5000 times, and an 8-bit SIM image of 1024 pixels wide x 943 pixels high was taken. From the SIM images taken for each of the three 45° cross sections, the percentage of integration of the crystal orientations of the Fe-based electroplated layer and the Si-containing cold-rolled steel sheet at the interface between the Fe-based electroplated layer and the Si-containing cold-rolled steel sheet was calculated based on the following formula (1). Note that decimal points were rounded up. (Proportion of the crystal orientations of the Fe-based electroplated layer and the Si-containing cold-rolled steel sheet integrated at the interface between the Fe-based electroplated layer and the Si-containing cold-rolled steel sheet) = (Length of the portion of the interface between the Fe-based electroplated layer and the Si-containing cold-rolled steel sheet where the crystal orientations of the Fe-based electroplated layer and the Si-containing cold-rolled steel sheet are integrated) ÷ (Length of the interface in the observation field) × 100 ... (1)

[0065] Image processing is used to determine whether the crystal orientations of the Fe-based electroplated layer and the Si-containing cold-rolled steel sheet are integrated at the interface between the Fe-based electroplated layer and the Si-containing cold-rolled steel sheet. A method for evaluating the degree of integration of the crystal orientations will be described with reference to FIG. 3. First, as shown in FIG. 3(a), a boundary line B is drawn using a scanning electron microscope at the interface between the Fe-based electroplated layer and the Si-containing cold-rolled steel sheet in the SIM image described above. Next, a processed image of the SIM image is created, separate from the image in which the boundary line is drawn. Specifically, the captured 8-bit SIM image, measuring 1024 pixels wide and 943 pixels high, is first subjected to a Sobel filter to enhance the grain boundaries. The image with the enhanced grain boundaries is then smoothed using a Gaussian filter (radius (R): 10 pixels). The smoothed image is then subjected to binarization (threshold: 17). Next, the boundary line B of the image depicting the interface is transferred to the binarized image. After that, as shown in FIG. 3(b), in the image after the binarization process, a judgment area with a width of 40 pixels (L in FIG. 3(b)) centered on the boundary line B is created. １ and L ２The area surrounded by the boundary line B is drawn along boundary line B on the binarized image. The total length of the boundary line B where the interface between the Fe-based electroplated layer and the Si-containing cold-rolled steel sheet (the black-and-white boundary on the binarized image) does not exist within the judgment area is considered to be the length of the area where the crystal orientations are integrated. The total length of the boundary line B where the interface between the Fe-based electroplated layer and the Si-containing cold-rolled steel sheet does not exist within the judgment area is calculated as follows: First, a location where the judgment area can be divided into an approximately rectangular shape by two normal lines to boundary line B so that only one color, black or white, is included is found within the entire judgment area. Next, the maximum distances between the intersections of the boundary line and the two normal lines at that location are summed over the entire judgment area to determine the total length of the boundary line where the interface between the Fe-based electroplated layer and the Si-containing cold-rolled steel sheet does not exist within the judgment area. The length of the area where the crystal orientations are integrated may also be calculated by subtracting the length of the area where the crystal orientations are not integrated from the length of the interface in the observation field. For the purpose of explanation, Fig. 3(c) shows an enlarged view of the area enclosed by the square in Fig. 3(b). First, as shown in Fig. 3(c), two normal lines of the boundary line B (in Fig. 3(c), l １ and l ２ , and l ３ and l ４ ) is used to find a location within the entire judgment area where the judgment area can be divided into an approximately rectangular shape so that the two colors, black and white, are included. Next, the maximum distances between the intersections of the boundary lines and the two normal lines at that location are summed over the entire judgment area, and this is taken as the total length of the boundary lines within the judgment area where the interface between the Fe-based electroplated layer and the Si-containing cold-rolled steel sheet exists. The length, i.e., the length of the portion where the crystal orientations are not integrated, is subtracted from the length of the interface in the observation field to determine the length of the portion where the crystal orientations are integrated.

[0066] Figure 4 shows a SIM image of the interface between the Fe-based electroplated layer and the Si-containing cold-rolled steel sheet for Inventive Example No. 31 in the Examples described later. Figure 5 shows an image of the SIM image after image processing and binarization as described above. In Inventive Example No. 31, the percentage of the crystal orientations of the Fe-based electroplated layer and the Si-containing cold-rolled steel sheet that were integrated at the interface between the Fe-based electroplated layer and the Si-containing cold-rolled steel sheet was 97%. Figure 6 shows a SIM image of the interface between the Fe-based electroplated layer and the Si-containing cold-rolled steel sheet for Inventive Example No. 34 in the Examples described later. Figure 7 shows an image of the SIM image after image processing and binarization as described above. In Inventive Example No. 34, the percentage of the crystal orientations of the Fe-based electroplated layer and the Si-containing cold-rolled steel sheet that were integrated at the interface between the Fe-based electroplated layer and the Si-containing cold-rolled steel sheet was 95%.

[0067] According to the present disclosure, it is possible to provide a high-strength Fe-based electroplated steel sheet having a tensile strength TS of 590 MPa or more as measured in accordance with JIS Z 2241 (2011). The strength of the Fe-based electroplated steel sheet is more preferably 800 MPa or more.

[0068] The thickness of the Fe-based electroplated steel sheet according to this embodiment is not particularly limited, but may usually be 0.5 mm or more and 3.2 mm or less.

[0069] <Method for manufacturing an Fe-based electroplated steel sheet> Next, a method for manufacturing an Fe-based electroplated steel sheet will be described. In one embodiment, a cold-rolled steel sheet containing 0.1 mass % to 3.0 mass % of Si is electroplated with an Fe-based plating to form a coating weight of 20.0 g / m ２ and subsequently annealing the pre-annealed Fe-based electroplated steel sheet in an atmosphere having a dew point of −30° C. or less to obtain the Fe-based electroplated steel sheet.

[0070] First, a cold-rolled steel sheet containing 0.1 mass% or more and 3.0 mass% or less of Si is manufactured. Note that the cold-rolled steel sheet may contain 0.50 mass% or more and 3.0 mass% or less of Si. The manufacturing method of the cold-rolled steel sheet can follow a conventional manufacturing method of a cold-rolled steel sheet. In one example, the cold-rolled steel sheet is manufactured by hot-rolling a steel slab having the above-mentioned chemical composition to obtain a hot-rolled sheet, then pickling the hot-rolled sheet, and then cold-rolling the hot-rolled sheet to obtain a cold-rolled steel sheet.

[0071] Next, the surface of the cold-rolled steel sheet is subjected to an Fe-based electroplating treatment to obtain a pre-annealed Fe-based electroplated steel sheet. The Fe-based electroplating method is not particularly limited. For example, a sulfuric acid bath, a hydrochloric acid bath, or a mixture of both can be used as the Fe-based electroplating bath. Note that the pre-annealed Fe-based electroplated steel sheet means that the Fe-based electroplated layer has not been subjected to an annealing step, and does not exclude a state in which the cold-rolled steel sheet has been annealed before the Fe-based electroplating treatment.

[0072] The Fe ion content in the Fe-based electroplating bath before the start of current application was Fe ２＋ The Fe ion content in the Fe-based electroplating bath is preferably 0.5 mol / L or more. ２＋ In order to obtain a sufficient amount of Fe deposited, the Fe ion content in the Fe-based electroplating bath before the start of current application is preferably 2.0 mol / L or less.

[0073] The Fe-based electroplating bath may contain Fe ions and at least one element selected from the group consisting of B, C, P, N, O, Ni, Mn, Mo, Zn, W, Pb, Sn, Cr, V, and Co. The total content of these elements in the Fe-based electroplating bath is preferably 10 mass% or less in the Fe-based electroplated layer before annealing. Metal elements may be contained as metal ions, and nonmetal elements may be contained as part of boric acid, phosphoric acid, nitric acid, organic acids, etc. The iron sulfate plating solution may also contain conductivity aids such as sodium sulfate and potassium sulfate, chelating agents, and pH buffers.

[0074] Other conditions for the Fe-based electroplating bath are not particularly limited. The temperature of the Fe-based electroplating solution is preferably 30°C or higher and 85°C or lower, considering the ability to maintain a constant temperature. The pH of the Fe-based electroplating bath is not particularly specified, but is preferably 1.0 or higher in order to prevent a decrease in current efficiency due to hydrogen generation, and is preferably 3.0 or lower in consideration of the electrical conductivity of the Fe-based electroplating bath. The current density is 10 A / dm from the viewpoint of productivity. ２ It is preferable that the current is 150 A / dm or more from the viewpoint of facilitating control of the deposition amount of the Fe-based electroplating layer. ２ The sheet threading speed is preferably 5 mpm or more from the viewpoint of productivity, and is preferably 150 mpm or less from the viewpoint of stable control of the amount of adhesion.

[0075] Prior to the Fe-based electroplating treatment, the cold-rolled steel sheet may be subjected to a degreasing treatment and water rinsing to clean the surface, and further to a pickling treatment and water rinsing to activate the surface. These pretreatments are followed by the Fe-based electroplating treatment. The degreasing and water rinsing methods are not particularly limited, and conventional methods can be used. Various acids can be used in the pickling treatment, such as sulfuric acid, hydrochloric acid, nitric acid, and mixtures thereof. Among these, sulfuric acid, hydrochloric acid, or mixtures thereof are preferred. The acid concentration is not particularly specified, but a concentration of approximately 1 to 20 mass% is preferred, taking into consideration the ability to remove oxide films and the prevention of surface roughness (surface defects) due to excessive pickling. The pickling solution may also contain an antifoaming agent, a pickling accelerator, a pickling inhibitor, etc.

[0076] Next, after subjecting the steel sheet to Fe-based electroplating treatment, the steel sheet before annealing is subjected to an annealing step in which the steel sheet is held in a reducing atmosphere having a dew point of −30° C. or lower and a hydrogen concentration of 1.0 to 30.0% by volume in a temperature range of 650 to 900° C. for 30 to 600 seconds, and then cooled to obtain a Fe-based electroplated steel sheet. The annealing step is performed to remove strain in the steel sheet before annealing caused by the rolling step and to recrystallize the structure, thereby increasing the strength of the steel sheet.

[0077] Dew point: −30° C. or less In this embodiment, the dew point of the annealing atmosphere in the annealing step is a low dew point of −30° C. or less, which is a condition that does not require additional equipment such as a humidifying equipment. The control of the dew point to −30° C. or less is preferably performed in a temperature range of 650° C. or more and 900° C. or less. Through independent studies, the present inventors have found that there is a correlation between the rate at which the crystal orientations of the Fe-based electroplated layer and the Si-containing cold-rolled steel sheet are integrated at the interface between the Fe-based electroplated layer and the Si-containing cold-rolled steel sheet, and the dew point of the annealing atmosphere in the annealing step after the formation of the Fe-based electroplated layer. That is, when annealing a pre-annealed Fe-based electroplated steel sheet after the formation of the Fe-based electroplating layer, the lower the dew point of the annealing atmosphere, the higher the degree of integration of the crystal orientations of the Fe-based electroplating layer and the Si-containing cold-rolled steel sheet in the Fe-based electroplated steel sheet obtained after annealing. Conversely, the higher the dew point of the annealing atmosphere, the lower the degree of integration of the crystal orientations of the Fe-based electroplating layer and the Si-containing cold-rolled steel sheet. The reason for this correlation between the degree of integration of the crystal orientations of the Fe-based electroplating layer and the Si-containing cold-rolled steel sheet and the dew point is unclear, but can be inferred as follows. When the dew point is controlled to a certain level or higher, elements that diffuse from the steel sheet to the Fe-based electroplating layer during annealing form oxides within the Fe-based electroplating layer. These oxides inhibit crystal growth in the Fe-based electroplating layer, resulting in finer grains. On the other hand, when the Fe-based electroplated layer is annealed in a low-dew-point atmosphere after formation, the oxides described above are less likely to form, and the crystal grain size of the Fe-based electroplated layer becomes coarse. Therefore, it can be considered that when annealing is performed at a low dew point, the crystal orientation of the Fe-based electroplated layer is highly integrated with the crystal orientation of the Si-containing cold-rolled steel sheet. When the dew point of the annealing atmosphere in the annealing process is set to −30°C or lower due to the cost of humidifying equipment in the annealing furnace, the crystal orientations of the Fe-based electroplated layer and the Si-containing cold-rolled steel sheet are highly integrated at the interface between the Fe-based electroplated layer and the Si-containing cold-rolled steel sheet. As a result, when the sheet is assembled with a galvanized steel sheet, molten zinc during resistance welding is more likely to penetrate into the grain boundaries of the Si-containing cold-rolled steel sheet via the grain boundaries of the Fe-based electroplated layer.In this embodiment, by forming an Fe-based electroplated layer having a specific coating weight, when the mating sheet is a galvanized steel sheet, the time it takes for molten zinc to reach the grain boundaries of the Si-containing cold-rolled steel sheet via the grain boundaries of the Fe-based electroplated layer during resistance welding is delayed, thereby improving the resistance weld cracking resistance of the weld. Although there is no particular lower limit for the dew point of the annealing atmosphere, a temperature below -80°C is difficult to achieve industrially, so it is preferably -80°C or higher. The dew point of the annealing atmosphere is more preferably -55°C or higher.

[0078] Hydrogen concentration: 1.0 vol% or more and 30.0 vol% or less The annealing process is performed in a reducing atmosphere with a hydrogen concentration of 1.0 vol% or more and 30.0 vol% or less. Hydrogen suppresses oxidation of Fe on the surface of the Fe-based electroplated steel sheet before annealing during the annealing process and activates the steel sheet surface. A hydrogen concentration of 1.0 vol% or more can prevent oxidation of Fe on the steel sheet surface, which can lead to deterioration of chemical conversion treatability when a chemical conversion coating is applied, as described below. Therefore, the annealing process is preferably performed in a reducing atmosphere with a hydrogen concentration of 1.0 vol% or more, more preferably 2.0 vol% or more. While there are no particular limitations on the upper limit of the hydrogen concentration in the annealing process, from the viewpoint of cost, the hydrogen concentration is preferably 30.0 vol% or less, more preferably 20.0 vol% or less. The remainder of the annealing atmosphere other than hydrogen is preferably nitrogen.

[0079] Holding time in the temperature range of 650°C to 900°C: 30 seconds to 600 seconds In the annealing step, the holding time in the temperature range of 650°C to 900°C is preferably 30 seconds to 600 seconds. By holding for 30 seconds or more in this temperature range, the natural oxide film of Fe formed on the surface of the Fe-based electroplated layer before annealing can be suitably removed, and as described below, the chemical conversion treatability can be improved when a chemical conversion coating is formed on the surface of the Fe-based electroplated steel sheet. Therefore, the holding time in this temperature range is preferably 30 seconds or more. There is no particular upper limit to the holding time in this temperature range, but from the viewpoint of productivity, the holding time in this temperature range is preferably 600 seconds or less.

[0080] Maximum temperature of Fe-based electroplated steel sheet before annealing: 650°C or higher and 900°C or lower. The maximum temperature of the Fe-based electroplated steel sheet before annealing is not particularly limited, but is preferably 650°C or higher and 900°C or lower. By setting the maximum temperature of the Fe-based electroplated steel sheet before annealing to 650°C or higher, recrystallization of the steel sheet structure proceeds favorably, resulting in the desired strength. Furthermore, the natural oxide film of Fe formed on the surface of the Fe-based electroplated layer before annealing is favorably reduced, thereby improving the chemical treatability when a chemical conversion coating is formed on the surface of the Fe-based electroplated steel sheet, as described below. Furthermore, if the maximum temperature of the Fe-based electroplated steel sheet is 900°C or lower, an excessive increase in the diffusion rate of Si and Mn in the steel can be prevented, preventing the diffusion of Si and Mn to the steel sheet surface. This improves the chemical treatability when a chemical conversion coating is formed on the steel sheet surface, as described below. Furthermore, if the maximum temperature is 900°C or lower, damage to the heat treatment furnace can be prevented, resulting in cost reduction. Therefore, the maximum temperature of the Fe-based electroplated steel sheet before annealing is preferably set to 900° C. or less. The maximum temperature is based on the temperature measured on the surface of the Fe-based electroplated steel sheet before annealing.

[0081] <Electrodeposition-Coated Steel Sheet> According to this embodiment, an electrodeposition-coated steel sheet can be provided, further comprising a chemical conversion coating formed on the above-described Fe-based electroplated steel sheet in contact with the Fe-based electroplated layer, and an electrodeposition coating formed on the chemical conversion coating. Because the Fe-based electroplated steel sheet according to this embodiment has excellent resistance weld crack resistance at welds, an electrodeposition-coated steel sheet formed using the Fe-based electroplated steel sheet is particularly suitable for application to automotive parts. Preferably, the electrodeposition-coated steel sheet according to this embodiment has a chemical conversion coating formed directly on the Fe-based electroplated layer. In other words, it is preferable that the electrodeposition-coated steel sheet according to this embodiment does not have an additional plating layer other than the Fe-based electroplated layer. The types of the chemical conversion coating and the electrodeposition coating are not particularly limited, and can be any known chemical conversion coating and electrodeposition coating. Examples of chemical conversion coatings that can be used include zinc phosphate coatings and zirconium coatings. The electrodeposition coating is not particularly limited as long as it is an automotive electrodeposition coating. The thickness of the electrodeposition coating film varies depending on the application, but it is preferably about 10 μm to 30 μm in thickness in a dry state. Furthermore, according to this embodiment, it is also possible to provide an Fe-based electroplated steel sheet for electrodeposition coating.

[0082] <Method for Manufacturing Electrodeposition-Coated Steel Sheet> Next, a method for manufacturing the above-described electrodeposition-coated steel sheet will be described. The above-described electrodeposition-coated steel sheet can be manufactured by a method for manufacturing an electrodeposition-coated steel sheet, including a chemical conversion treatment step in which an Fe-based electroplated steel sheet is subjected to a chemical conversion treatment without an additional plating treatment to obtain a chemical conversion-treated steel sheet having a chemical conversion coating formed in contact with the Fe-based electroplated layer, and an electrodeposition coating step in which the chemical conversion-treated steel sheet is subjected to an electrodeposition coating treatment to obtain an electrodeposition-coated steel sheet having an electrodeposition coating coating formed on the chemical conversion coating. The chemical conversion treatment and the electrodeposition coating treatment can be performed by known methods. Prior to the chemical conversion treatment, the Fe-based electroplated steel sheet can be subjected to a degreasing treatment, water rinsing, and, if necessary, a surface conditioning treatment to clean the surface. Following these pretreatments, the chemical conversion treatment is performed. The methods for the degreasing treatment and water rinsing are not particularly limited, and conventional methods can be used. In the surface conditioning treatment, a surface conditioner containing Ti colloid or zinc phosphate colloid, for example, can be used. The application of these surface conditioners does not require any special process and can be carried out according to conventional methods. For example, the desired surface conditioner is dissolved in a predetermined amount of deionized water, thoroughly stirred, and then a treatment solution is prepared at a predetermined temperature (usually room temperature, 25 to 30°C). The steel sheet is immersed in this treatment solution for a predetermined time (20 to 30 seconds). The subsequent chemical conversion treatment can be carried out without drying. The chemical conversion treatment can also be carried out according to conventional methods. For example, the desired chemical conversion treatment agent is dissolved in a predetermined amount of deionized water, thoroughly stirred, and then a treatment solution is prepared at a predetermined temperature (usually 35 to 45°C). The steel sheet is immersed in this treatment solution for a predetermined time (60 to 120 seconds). Examples of chemical conversion treatment agents that can be used include zinc phosphate treatment agents for steel, zinc phosphate treatment agents for steel and aluminum, and zirconium treatment agents. The subsequent electrodeposition coating process is then carried out according to conventional methods. After performing pretreatment such as water washing as necessary, the steel sheet is immersed in a thoroughly stirred electrodeposition paint, and the electrodeposition coating of the desired thickness is obtained by electrodeposition. As the electrodeposition coating, cationic electrodeposition coating or anionic electrodeposition coating can be used. Furthermore, depending on the application, a top coat or the like may be applied after the electrodeposition coating.

[0083] <Automotive Parts> Furthermore, according to the present embodiment, it is possible to provide automotive parts at least partially using the above-described electrodeposition-coated steel sheet. Because the Fe-based electroplated steel sheet according to the present embodiment has excellent resistance weld crack resistance properties at welded joints, electrodeposition-coated steel sheets using the Fe-based electroplated steel sheet are particularly suitable for application to automotive parts. Automotive parts using the electrodeposition-coated steel sheet may contain, as their base material, steel sheets other than the electrodeposition-coated steel sheet according to the present embodiment. Because the electrodeposition-coated steel sheet according to the present embodiment has excellent resistance weld crack resistance properties at welded joints, even when the automotive part using the Fe-based electroplated steel sheet includes a high-strength hot-dip galvanized steel sheet as a welding partner, back cracking at the welded joint is suitably prevented. The type of automotive part at least partially using the electrodeposition-coated steel sheet is not particularly limited, but may be, for example, a side sill part, a pillar part, an automotive body, or the like.

[0084] [Embodiment 2] Next, an Fe-based electroplated steel sheet according to embodiment 2 of the present invention will be described. The Fe-based electroplated steel sheet according to this embodiment comprises a cold-rolled steel sheet and a coating weight of 20.0 g / m2 formed on at least one surface of the cold-rolled steel sheet. ２ The present invention may be an Fe-based electroplated steel sheet having an Fe-based electroplated layer with a crystal orientation of more than 50% at the interface between the Fe-based electroplated layer and the cold-rolled steel sheet, wherein the Fe-based electroplated layer and the cold-rolled steel sheet have a crystal orientation that is integrated by more than 50%. Here, the cold-rolled steel sheet is a test piece cut into 50 × 150 mm with the longitudinal direction perpendicular to the rolling direction, and the hot-dip galvanized layer cut into the same size has a coating weight of 50 g / m per side. ２Next, using a servo motor pressure type single-phase AC (50 Hz) resistance welding machine, the plate assembly was tilted by 5° in the longitudinal direction of the plate assembly with respect to a plane perpendicular to a line connecting the central axes of an electrode pair (tip diameter 6 mm) of the resistance welding machine, and a gap of 60 mm in the longitudinal direction of the plate assembly and 2.0 mm in the thickness direction of the plate assembly was provided between the lower electrode of the electrode pair and the test piece, and the lower electrode and the plate assembly were fixed, and with the upper electrode of the electrode pair movable, resistance welding was performed on the plate assembly under conditions of a pressure of 3.5 kN, a hold time of 0.16 seconds, and a welding current and welding time such that the nugget diameter was 5.9 mm, to form a plate assembly with a weld. Next, the plate assembly with the welded portion is cut in half along the longitudinal direction of the test piece so as to include the welded portion, and when the cross section of the welded portion is observed under an optical microscope (magnification 200 times), a crack having a length of 0.1 mm or more is observed in the cold-rolled steel sheet.

[0085] The cold-rolled steel sheet according to the present embodiment is not particularly limited as long as it is a steel sheet that is inferior in resistance weld crack resistance at a welded portion when the mating sheet is a galvanized steel sheet, as evaluated by the following test. The chemical composition of the cold-rolled steel sheet is also not particularly limited. The inventors have found that a cold-rolled steel sheet having a Si content of 0.1 mass % or more in the steel is inferior in resistance weld crack resistance at a welded portion, as evaluated by the following test.

[0086] The cold-rolled steel sheet may be a cold-rolled steel sheet that is resistance-welded to obtain a welded sheet assembly under a hold time of 0.24 seconds, and that exhibits cracks of 0.1 mm or more when the cross section of the weld is observed under an optical microscope (magnification 200x). Note that, for the same cold-rolled steel sheet, the resistance weld cracking resistance characteristics of the weld generally deteriorate as the hold time is reduced. Therefore, if a cold-rolled steel sheet exhibits cracks of 0.1 mm or more when the following test is performed under a hold time of 0.24 seconds, even when the cold-rolled steel sheet is resistance-welded under a hold time of 0.16 seconds, cracks of 0.1 mm or more are observed under an optical microscope (magnification 200x). Cold-rolled steel sheets having a Si content of 0.50% by mass or more are inferior in resistance weld crack resistance at the welded portion evaluated by the following test, but there have also been confirmed cases where cold-rolled steel sheets having a Si content of less than 0.50% by mass are also inferior in resistance weld crack resistance at the welded portion evaluated by the following test.

[0087] <Resistance weld crack resistance at welded joints when the mating plate is a galvanized steel sheet> A method for evaluating the resistance weld crack resistance at welded joints will be described with reference to Fig. 8. A test piece 6 was cut out to 50 x 150 mm with the transverse direction (TD) of rolling as the long side and the rolling direction as the short side. A test piece 6 was cut out to the same size with a coating weight of 50 g / m2 of the hot-dip galvanized layer per side. ２ The test piece 6 is placed on a galvannealed steel sheet 5 having a thickness of 2.0 mm, and the test piece 6 is placed on a galvannealed steel sheet 5 having a thickness of 2.0 mm. The test piece 6 is assembled so that the evaluation surface (Fe-based electroplated layer) faces the zinc-plated layer of the galvannealed steel sheet 5. The test piece 6 is fixed to a fixing base 8 via a spacer 7 having a thickness of 2.0 mm. The spacer 7 is a pair of steel plates having a length of 50 mm, a width of 45 mm, and a thickness of 2.0 mm. As shown in Figure 8(a), the spacer 7 is arranged so that the longitudinal end faces of each of the pair of steel plates are aligned with the widthwise end faces of the test piece 6. Therefore, the distance between the pair of steel plates is 60 mm. The fixing base 8 is a single plate with a hole in the center.

[0088] Next, using a servomotor-operated, single-phase AC (50 Hz) resistance welding machine, the sheet assembly was pressed with a pair of electrodes 9 (tip diameter: 6 mm) while being deflected. Resistance welding was performed under conditions of a pressure of 3.5 kN and a hold time of 0.18 or 0.24 seconds, with a welding current and welding time sufficient to produce a nugget diameter r of 5.9 mm, to produce a sheet assembly with a weld. The pair of electrodes 9 pressed the sheet assembly from above and below in the vertical direction, and the lower electrode pressed the test piece 6 through a hole in the fixture 8. During pressing, the lower electrode and fixture 8 were fixed so that the lower electrode of the pair of electrodes 9 was in contact with a plane extending from the contact surface between the spacer 7 and fixture 8, and the upper electrode was movable. The upper electrode was also in contact with the center of the test galvannealed steel sheet 5. The plate assembly was welded with the plate assembly tilted 5° in the longitudinal direction of the plate assembly with respect to a plane perpendicular to the line connecting the center axes of the electrode pair of the resistance welding machine (horizontal in Figure 8(a)). The spacer described above created a gap of 60 mm in the longitudinal direction of the plate assembly and 2.0 mm in the thickness direction of the plate assembly between the lower electrode and the test piece 6. The hold time refers to the time from the end of the welding current flow to the start of the electrode release. Here, referring to the lower diagram in Figure 8(b), the nugget diameter r refers to the distance between the ends of the nugget 10 in the longitudinal direction of the plate assembly.

[0089] Next, the plate assembly with the weld was cut along line B-B in the upper diagram of Figure 8(b) so as to include the center of the weld including the nugget 10, and the cross section of the weld was observed with an optical microscope (200x magnification), and the resistance weld crack resistance characteristics of the weld were evaluated according to the following criteria. A ◎ or ○ indicates that the resistance weld crack resistance characteristics of the weld are excellent. A × indicates that the resistance weld crack resistance characteristics of the weld are poor. ◎: No cracks of 0.1 mm or more in length were observed with a hold time of 0.14 seconds. ○: Cracks of 0.1 mm or more in length were observed with a hold time of 0.14 seconds, but no cracks of 0.1 mm or more in length were observed with a hold time of 0.16 seconds. ×: Cracks of 0.1 mm or more in length were observed with a hold time of 0.16 seconds.

[0090] Furthermore, as a milder welding condition, the resistance weld crack resistance characteristics of the weld may be evaluated in the same manner using the following criteria: ◎: No cracks of 0.1 mm or more in length are observed with a hold time of 0.18 seconds ○: Cracks of 0.1 mm or more in length are observed with a hold time of 0.18 seconds, but no cracks of 0.1 mm or more in length are observed with a hold time of 0.24 seconds ×: Cracks of 0.1 mm or more in length are observed with a hold time of 0.24 seconds

[0091] In the lower diagram of FIG. 12(b), an example of a crack that occurred in the test piece 6 is shown schematically as reference numeral 11.

[0092] The Fe-based electroplated layer of the Fe-based electroplated steel sheet according to this embodiment is the same as that of the above-described embodiment 1, and therefore a description thereof will be omitted here. In addition, the proportion of the crystal orientations of the Fe-based electroplated layer and the cold-rolled steel sheet that are integrated at the interface between the Fe-based electroplated layer and the cold-rolled steel sheet is more than 50%, as in the above-described embodiment 1. The details of the proportion of the crystal orientations of the Fe-based electroplated layer and the cold-rolled steel sheet that are integrated at the interface between the Fe-based electroplated layer and the cold-rolled steel sheet are the same as those of the above-described embodiment 1, and therefore a description thereof will be omitted here.

[0093] Next, a method for manufacturing an Fe-based electroplated steel sheet according to embodiment 2 will be described. In the method for manufacturing an Fe-based electroplated steel sheet according to one embodiment, a cold-rolled steel sheet is electroplated with an Fe-based electroplating agent to obtain a coating weight of 20.0 g / m2 per side. ２ The method for producing an Fe-based electroplated steel sheet comprises the steps of: forming a pre-annealed Fe-based electroplated steel sheet having a pre-annealing Fe-based electroplated layer of more than 1000 μm on at least one side thereof; and then annealing the pre-annealed Fe-based electroplated steel sheet to obtain an Fe-based electroplated steel sheet. Here, the cold-rolled steel sheet is a test piece cut into 50 × 150 mm with the longitudinal direction perpendicular to the rolling direction, and then cutting the test piece into the same size into a hot-dip galvanized layer having a coating weight of 50 g / m per side. ２Next, using a servo motor pressure type single-phase AC (50 Hz) resistance welding machine, the plate assembly was tilted by 5° in the longitudinal direction of the plate assembly with respect to a plane perpendicular to a line connecting the central axes of an electrode pair (tip diameter 6 mm) of the resistance welding machine, and a gap of 60 mm in the longitudinal direction of the plate assembly and 2.0 mm in the thickness direction of the plate assembly was provided between the lower electrode of the electrode pair and the test piece, and the lower electrode and the plate assembly were fixed, and with the upper electrode of the electrode pair movable, resistance welding was performed on the plate assembly under conditions of a pressure of 3.5 kN, a hold time of 0.16 seconds, and a welding current and welding time such that the nugget diameter was 5.9 mm, to form a plate assembly with a weld. Next, the plate assembly with the welded portion is cut in half along the longitudinal direction of the test piece so as to include the welded portion, and when the cross section of the welded portion is observed under an optical microscope (magnification 200 times), a crack having a length of 0.1 mm or more is observed in the cold-rolled steel sheet.

[0094] First, a cold-rolled steel sheet is manufactured. The manufacturing method of the cold-rolled steel sheet can be a method for manufacturing a normal cold-rolled steel sheet. In one example, a steel slab is hot-rolled to form a hot-rolled sheet, which is then pickled, and then cold-rolled to form the cold-rolled steel sheet.

[0095] The cold-rolled steel sheet according to this embodiment is not particularly limited as long as it is a steel sheet that, when evaluated by the above-mentioned test, is poor in resistance weld crack resistance at a welded portion when the mating sheet is a galvanized steel sheet. The chemical composition of the cold-rolled steel sheet is also not particularly limited, and a cold-rolled steel sheet having a Si content of 0.1 mass% or more in the steel will be poor in resistance weld crack resistance at a welded portion evaluated by the above-mentioned test.

[0096] The cold-rolled steel sheet may be a cold-rolled steel sheet in which a sheet assembly with a weld is obtained by the above-mentioned resistance welding under a hold time of 0.24 seconds, and a crack having a length of 0.1 mm or more is observed at the cross section of the weld under an optical microscope (magnification 200x). Note that, for the same cold-rolled steel sheet, the resistance weld cracking resistance characteristics of the weld generally deteriorate as the hold time is shortened. Therefore, if a cold-rolled steel sheet is obtained by resistance welding under a hold time of 0.24 seconds, and a crack having a length of 0.1 mm or more is observed at the cross section of the weld under an optical microscope (magnification 200x), a crack having a length of 0.1 mm or more is observed at the cross section of the weld under an optical microscope (magnification 200x), even when the cold-rolled steel sheet is resistance-welded under a hold time of 0.16 seconds. Cold-rolled steel sheets having a Si content of 0.50% by mass or more are inferior in resistance weld crack resistance at the welded portion evaluated by the above-mentioned test, but there have also been confirmed cases where cold-rolled steel sheets having a Si content of less than 0.50% by mass are also inferior in resistance weld crack resistance at the welded portion evaluated by the above-mentioned test.

[0097] Next, the surface of the cold-rolled steel sheet is subjected to an Fe-based electroplating treatment to obtain a pre-annealed Fe-based electroplated steel sheet. Details of the Fe-based electroplating treatment have been described above, so a detailed description thereof will be omitted here.

[0098] Next, the pre-annealed Fe-based electroplated steel sheet is subjected to an annealing step in which the pre-annealed Fe-based electroplated steel sheet is held in a reducing atmosphere having a dew point of −30° C. or lower and a hydrogen concentration of 1.0 to 30.0% by volume in a temperature range of 650 to 900° C. for 30 to 600 seconds, and then cooled, to obtain a Fe-based electroplated steel sheet. Details of the annealing step have been described above, so a description thereof will be omitted here.

[0099] In this embodiment, as in the above-described Embodiment 1, an electrodeposition-coated steel sheet can be provided, which further comprises, on the Fe-based electroplated steel sheet according to this embodiment, a chemical conversion coating formed in contact with the Fe-based electroplating layer and an electrodeposition coating film formed on the chemical conversion coating. Also, an Fe-based electroplated steel sheet for electrodeposition coating can be provided. Details of the electrodeposition-coated steel sheet and the method for manufacturing the electrodeposition-coated steel sheet are the same as those of the above-described Embodiment 1, and therefore, a description thereof will be omitted here.

[0100] Furthermore, in this embodiment, it is possible to provide automobile parts, similar to the above-described embodiment 1. Details of the automobile parts have been described above, and therefore will not be described here.

[0101] The present invention will be specifically described below based on examples.

[0102] Steels having the chemical compositions shown in Tables 1 and 3 were melted and cast, and the resulting casts were hot-rolled, pickled, and cold-rolled to form cold-rolled steel sheets having a thickness of 1.6 mm.

[0103]

[0104]

[0105] Next, the cold-rolled steel sheet was subjected to an alkali degreasing treatment, and then electrolytic treatment was carried out under the conditions shown below using the steel sheet as the cathode to produce a pre-annealed Fe-based electroplated steel sheet having an Fe-based electroplated layer on one side. The deposition weight of the Fe-based electroplated layer was controlled by the current application time. Subsequently, the pre-annealed Fe-based electroplated steel sheet was subjected to 15% H ２ -N ２ The soaking zone temperature was 800°C, and the atmospheric dew point was adjusted as shown in Tables 2-1, 2-2, and 4, and reduction annealing was performed to obtain Fe-based electroplated steel sheets. The reduction annealing was performed for 100 seconds. [Electrolysis conditions] Bath temperature: 50°C, pH: 2.0, Current density: 45 A / dm ２ Fe-based electroplating bath: Fe ２＋ Contains 1.5 mol / L of ions Electrode (anode): iridium oxide electrode

[0106] The Fe-based electroplated steel sheets prepared as described above were used to determine the coating weight per side of the Fe-based electroplated layer and the proportion of the Fe-based electroplated layer and the Si-containing cold-rolled steel sheet at which the crystal orientations of the Fe-based electroplated layer and the Si-containing cold-rolled steel sheet were integrated at the interface between the Fe-based electroplated layer and the Si-containing cold-rolled steel sheet, according to the method described above.

[0107] The resistance weld crack resistance of the welded portion of the Fe-based electroplated steel sheet obtained as described above was investigated. The methods for measuring and evaluating the resistance weld crack resistance of the welded portion are described below.

[0108] <Resistance weld crack resistance at welded joint when the other sheet is a galvanized steel sheet> For Fe-based electroplated steel sheets, according to the above-mentioned method, a 980 MPa-class tensile strength with less than 0.50% Si and a coating weight per side of 50 g / m2 was used, in which resistance weld crack resistance was not an issue at a hold time of 0.18 seconds. ２ The resistance weld crack resistance characteristics of the welds of test galvannealed steel sheets (sheet thickness 1.6 mm) were evaluated. The welding time was 0.36 seconds, and the hold times were 0.18 seconds and 0.24 seconds. The welding current was changed for each example, and the nugget diameter was measured. Evaluation was performed at the welding current that resulted in a nugget diameter of 5.9 mm. In addition, data from the example was taken from test galvannealed steel sheets that did not have cracks. This is because if cracks had occurred in the test galvannealed steel sheets, the evaluation would not have been appropriate.

[0109] The results of the above tests are shown in Tables 2-1, 2-2 and 4. These results show that the Fe-based electroplated steel sheets of the invention, in which an Fe-based electroplated layer was formed under conditions suitable for the present invention before continuous annealing, exhibited excellent resistance weld crack resistance at the welded portion. In Reference Examples 1 and 2, the Si content was less than 0.5%, so no particular problems arose with respect to resistance weld crack resistance at the welded portion. The coating weight of the Fe-based electroplated layer was 25.0 g / m ２In each of the above-described inventive examples, no cracks longer than 0.1 mm were observed even under the hold time condition of 0.18 seconds, and the resistance weld crack resistance properties of the weld were particularly good. In Tables 2-1 and 2-2, the coating weight of the Fe-based electroplated layer is indicated as "-" for examples in which an Fe-based electroplated layer was not formed. Furthermore, in Reference Examples 17, 30, and 46, in which an annealing process at a high dew point was performed, the annealing at a high dew point reduced the proportion of the crystal orientations of the Fe-based electroplated layer and the Si-containing cold-rolled steel sheet integrated at the interface between them, and the resistance weld crack resistance properties of the weld were good. In these Reference Examples, the Fe-based electroplated steel sheets before annealing were heated to a soaking zone temperature of 800°C at an average heating rate of 10°C / second or more in the temperature range of 400°C to 650°C, and then subjected to reduction annealing.

[0110]

[0111]

[0112]

[0113] Steels having the chemical compositions shown in Table 5 were melted and cast slabs were obtained, which were then hot rolled, pickled, and cold rolled to form cold-rolled steel sheets having a thickness of 1.6 mm.

[0114]

[0115] Next, the cold-rolled steel sheet was subjected to an alkali degreasing treatment, and then electrolytic treatment was carried out under the conditions shown below using the steel sheet as the cathode to produce a pre-annealed Fe-based electroplated steel sheet having an Fe-based electroplated layer on one side. The deposition weight of the Fe-based electroplated layer was controlled by the current application time. Subsequently, the pre-annealed Fe-based electroplated steel sheet was subjected to 15% H ２ -N ２ The soaking zone temperature was 800°C, and the dew point of the atmosphere was adjusted as shown in Table 6, and reduction annealing was performed to obtain an Fe-based electroplated steel sheet. The reduction annealing was performed for 100 seconds. [Electrolysis conditions] Bath temperature: 50°C, pH: 2.0, Current density: 45 A / dm ２ Fe-based electroplating bath: Fe ２＋ Contains 1.5 mol / L of ions Electrode (anode): iridium oxide electrode

[0116] The Fe-based electroplated steel sheets prepared as described above were used to determine the coating weight per side of the Fe-based electroplated layer and the proportion of the Fe-based electroplated layer and the Si-containing cold-rolled steel sheet at which the crystal orientations of the Fe-based electroplated layer and the Si-containing cold-rolled steel sheet were integrated at the interface between the Fe-based electroplated layer and the Si-containing cold-rolled steel sheet, according to the method described above.

[0117] The resistance weld crack resistance of the welded portion of the Fe-based electroplated steel sheet obtained as described above was investigated. The methods for measuring and evaluating the resistance weld crack resistance of the welded portion are described below.

[0118] <Resistance weld crack resistance at welded joint when the other sheet is a galvanized steel sheet> For Fe-based electroplated steel sheets, according to the above-mentioned method, a tensile strength of 590 MPa class with Si of less than 0.1% and a coating weight per side of 50 g / m2, in which resistance weld crack resistance does not become an issue at a hold time of 0.14 seconds, was measured. ２ The resistance weld crack resistance characteristics of the welds of test galvannealed steel sheets (sheet thickness 1.6 mm) were evaluated. The welding time was 0.36 seconds, and the hold times were 0.14 seconds and 0.16 seconds. The welding current was changed for each example, and the nugget diameter was measured. Evaluation was performed at the welding current that resulted in a nugget diameter of 5.9 mm. In addition, data from examples in which no cracks occurred in the test galvannealed steel sheets that were paired with the sheet were used. This is because if cracks occurred in the paired sheet, the stress would be dispersed to the Fe-based electroplated steel sheets being evaluated, preventing an appropriate evaluation.

[0119] The results of the above tests are shown in Table 6. These results show that the Fe-based electroplated steel sheets of the invention, in which the Fe-based electroplated layer was formed under conditions suitable for the present invention before continuous annealing, exhibited excellent resistance weld cracking resistance at the welded joints. ２ In each of the above-described inventive examples, no cracks longer than 0.1 mm were observed even under the condition of a hold time of 0.14 seconds, and the resistance weld crack resistance characteristics of the weld were particularly good. In Table 6, for examples in which an Fe-based electroplated layer was not formed, the deposition amount of the Fe-based electroplated layer is indicated as "-".

[0120]

[0121] The Fe-based electroplated steel sheet produced according to the present invention not only has excellent resistance weld crack resistance, particularly the ability to prevent internal cracks, at welded joints when the mating plate is a galvanized steel sheet, but also has high strength and excellent workability, making it suitable not only for use as a material for automobile parts, but also for applications requiring similar properties in fields such as home appliances and building materials.

[0122] REFERENCE SIGNS LIST 1 Fe-based electroplated steel sheet 2 Si-containing cold-rolled steel sheet 3 Fe-based electroplated layer 5 Galvannealed steel sheet for test 6 Test piece 7 Spacer 8 Fixing base 9 Electrode 10 Nugget 11 Crack