Fe-based electroplated steel sheet, electrodeposition-coated steel sheet, automobile parts, method for producing electrodeposition-coated steel sheet, and method for producing Fe-based electroplated steel sheet

Abstract

The present invention provides an Fe-based electroplated steel sheet having excellent resistance to resistance welding cracking of welded portions when the plate set is a galvanized steel sheet. The Fe-based electroplated steel sheet comprises: a Si-containing cold-rolled steel sheet containing 0.1% to 3.0% Si by mass; and a coating having an adhesion weight of 5.0 g / m2 formed on at least one surface of the Si-containing cold-rolled steel sheet. 2 In the intensity distribution of the above-mentioned Fe-based electroplating layer measured by glow discharge emission spectroscopy, a peak of luminescence intensity representing the wavelength of Si is detected in a range greater than 0.2 μm in the thickness direction from the surface plate of the above-mentioned Fe-based electroplating layer and less than the thickness of the Fe-based electroplating layer, and the average value of the C concentration in the range of 10 μm to 20 μm in the thickness direction from the surface plate of the above-mentioned Fe-based electroplating layer is less than 0.10 mass%.

Description

Technical Field

[0001] The present invention relates to an Fe-based electroplated steel sheet, an electrodeposition-coated steel sheet, an automobile part, a method for producing the electrodeposition-coated steel sheet, and a method for producing the Fe-based electroplated steel sheet having excellent resistance to resistance welding cracking. Background Art

[0002] In recent years, there has been a strong demand for improved fuel efficiency in automobiles to protect the global environment. Furthermore, there is a strong demand for enhanced vehicle safety to ensure passenger safety in the event of a collision. To meet these demands, both lightweight and high-strength automobile bodies are essential. Consequently, cold-rolled steel sheets, the raw materials for automotive parts, are being actively developed to achieve thinner thicknesses to achieve higher strength. However, since most automotive parts are manufactured using steel sheet forming, these sheets must possess both high strength and excellent formability.

[0003] There are various methods for increasing the strength of steel sheets. Among them, solid solution strengthening by adding Si is a method that can achieve high strength without significantly compromising the formability of the steel sheet. On the other hand, in the manufacture of automotive parts, press-formed parts are often assembled by resistance welding (spot welding). When the resistance-welded parts include high-strength galvanized steel sheets, residual stress is generated near the weld during resistance welding, causing the zinc in the coating to melt and diffuse into the grain boundaries, leading to liquid metal embrittlement (LME) and grain boundary cracking (LME cracking) in the steel sheet. In particular, when resistance welding is performed with the welding electrode at an angle to the steel sheet, residual stress increases, causing cracking. Residual stress is believed to increase with increasing steel sheet strength, leading to concerns about the occurrence of LME cracking as steel sheet strength increases. Even if the high-strength steel sheet is ungalvanized, if the steel sheet to be welded is galvanized, the galvanized layer will melt, causing LME cracking in ungalvanized steel sheets. Such a problem of LME cracking is particularly significant in steel sheets containing Si.

[0004] Based on the above, a high-strength steel sheet having excellent resistance welding cracking resistance of the welded portion (hereinafter also simply referred to as "resistance welding cracking resistance of the welded portion") is sought when the sheet set object is a galvanized steel sheet.

[0005] Methods for improving the above-mentioned problem have been reported in the past. Patent Document 1 discloses a steel plate having an internal oxide layer in which at least a portion of the grain boundaries are covered with oxide from the surface of the base material to a depth of 5.0 μm or more, and wherein the grain boundary coverage of the oxide in the region from the surface of the base material to a depth of 5.0 μm is 60% or more.

[0006] Prior art literature

[0007] Patent Literature

[0008] Patent Document 1: Japanese Patent No. 6388099 Summary of the Invention

[0009] In the steel sheet described in Patent Document 1, since the depth of the internal oxide layer, that is, grain boundary oxidation, is too great, it is difficult to completely suppress cracking during resistance welding.

[0010] As described above, the actual situation is that a Si-containing cold-rolled steel sheet that satisfies the resistance welding cracking characteristics of the welded portion at a high level has not yet been developed.

[0011] The present invention has been made in view of the above-mentioned problems existing in Si-containing cold-rolled steel sheets, and an object of the present invention is to provide a steel sheet having excellent resistance to resistance welding cracking at welded portions.

[0012] Previously, the present inventors attempted to improve the resistance to welding cracking by controlling only the surface layer of the steel sheet, but found that it was difficult to achieve a high level of resistance to welding cracking by controlling only the surface layer of the steel sheet. Therefore, they considered forming an Fe-based electroplating layer on the steel sheet and controlling its characteristics through heat treatment. Furthermore, the present inventors conducted repeated and in-depth research to solve the above-mentioned problem and found that in order to achieve a high level of resistance to welding cracking of the weld, it is important to have a surface of the Si-containing cold-rolled steel sheet before continuous annealing after cold rolling with an adhesion amount of 5.0 g / m2 on one side. 2 The above method forms an Fe-based electroplated layer to prepare an Fe-based electroplated steel sheet before annealing. The Fe-based electroplated steel sheet before annealing is subjected to a heating process of 400°C to 650°C at an average heating rate of 10°C / second or more and an annealing process in which the dew point of the atmosphere is greater than -30°C. Internal oxides are formed on the grain boundaries of the Fe-based electroplated layer, and at the interface between the Fe-based electroplated layer and the Si-containing cold-rolled steel sheet, the number of grain boundaries of the Fe-based electroplated layer in contact with the Si-containing cold-rolled steel sheet is 10 or more per 10μm width of the Si-containing cold-rolled steel sheet. By using an adhesion amount of 5.0g / m2 on a single side of the Si-containing cold-rolled steel sheet, the Fe-based electroplated layer is formed on the grain boundaries of the Fe-based electroplated layer. 2The above method forms a soft Fe-based electroplating layer, and the average heating rate in the temperature range of 400°C to 650°C during the heating process is set to 10°C / second or more, thereby minimizing the growth of grains in the Fe-based electroplating layer during the heating process. Furthermore, it was discovered that by setting the dew point of the atmosphere in the subsequent annealing process to above -30°C, the Si that diffuses from the Si-containing cold-rolled steel sheet into the Fe-based electroplating layer during annealing forms an oxide within the Fe-based electroplating layer, acting as a solid-solution Si depletion layer and suppressing the reduction in toughness caused by Si solid solution. Furthermore, the number of grain boundaries in the Fe-based electroplating layer that contact the interface between the Fe-based electroplating layer and the Si-containing cold-rolled steel sheet is set to 10 or more per 10μm width of the Si-containing cold-rolled steel sheet. This refines the grains of the Fe-based electroplating layer that contact the interface between the Si-containing cold-rolled steel sheet and the Fe-based electroplating layer, resulting in a dispersed path for molten zinc to penetrate the Fe-based electroplating layer. As a result, the time it takes for molten zinc to reach the grain boundaries of the Si-containing cold-rolled steel sheet during welding can be delayed, improving the resistance to resistance weld cracking of the weld. Furthermore, during the annealing step, the average carbon concentration of the Fe-based electroplated layer within a range of 10 μm to 20 μm from the surface toward the thickness of the plate is set to 0.10 mass% or less. Thus, by setting the average carbon concentration of the Fe-based electroplated layer within a range of 10 μm to 20 μm from the surface toward the thickness of the plate to 0.10 mass% or less, resistance welding cracking resistance can be further improved. The present inventors have discovered that when the Fe-based electroplated layer is formed before annealing, the carbon concentration within a range of 10 μm to 20 μm from the surface toward the thickness of the plate can be further reduced, thereby more effectively achieving the effect of improving resistance welding cracking resistance. Consequently, they have completed the present invention.

[0013] The present invention has been completed based on the above findings. Specifically, the gist of the present invention is as follows.

[0014] [1] An Fe-based electroplated steel sheet having:

[0015] Si-containing cold-rolled steel sheet containing 0.1 mass% to 3.0 mass% of Si, and

[0016] The Fe-based electroplating layer is formed on at least one side of the Si-containing cold-rolled steel sheet, and the coating weight on the single side is 5.0 g / m 2 above;

[0017] In the intensity distribution measured by glow discharge emission spectroscopy, a peak of luminescence intensity representing the wavelength of Si is detected within a range of more than 0.2 μm in the thickness direction of the Fe-based electroplating layer from the surface plate and less than the thickness of the Fe-based electroplating layer.

[0018] The average C concentration of the Fe-based electroplated layer in the range of 10 μm to 20 μm from the surface plate in the thickness direction is 0.10 mass % or less.

[0019] At the interface between the Fe-based electroplated layer and the Si-containing cold-rolled steel sheet, the number of grain boundaries where the Fe-based electroplated layer contacts the Si-containing cold-rolled steel sheet is 10 or more per 10 μm in the width direction of the Si-containing cold-rolled steel sheet in the observation field.

[0020] [2] The Fe-based electroplated steel sheet according to [1] above, wherein the Si-containing cold-rolled steel sheet contains 0.50% by mass to 3.0% by mass of Si.

[0021] [3] The Fe-based electroplated steel sheet according to [1] or [2] above, wherein the surface layer of the Fe-based electroplated steel sheet is a decarburized layer.

[0022] [4] The Fe-based electroplated steel sheet according to [3] above, wherein the thickness of the decarburized layer is 30 μm or more.

[0023] [5] The Fe-based electroplated steel sheet according to [3] or [4], wherein the coating weight CW on one side of the Fe-based electroplated layer is Fe1 (g / m 2 ) and the thickness C of the decarburized layer d (μm) satisfies the following formula (1).

[0024] 1.6×(CW Fe1 )+(C d )≥77···(1)

[0025] [6] The Fe-based electroplated steel sheet according to any one of [1] to [5] above, wherein the Si-containing cold-rolled steel sheet has the following composition in addition to the Si: 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 remainder being Fe and unavoidable impurities.

[0026] [7] The Fe-based electroplated steel sheet according to [6] above, wherein the above-mentioned component composition further contains, in terms of mass%, one or more 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: 0.020% 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.

[0027] [8] An Fe-based electroplated steel sheet according to any one of [1] to [7] above, wherein the Fe-based electroplated layer has the following composition: containing a total of less than 10 mass% of one or more elements selected from B, C, P, N, O, Ni, Mn, Mo, Zn, W, Pb, Sn, Cr, V and Co, and the remainder is composed of Fe and unavoidable impurities.

[0028] [9] An electrodeposition-coated steel sheet, comprising the Fe-based electroplated steel sheet according to any one of [1] to [8], further comprising a chemical conversion coating formed in contact with the Fe-based electroplating layer and an electrodeposition coating formed on the chemical conversion coating.

[0029]

[10] An automobile part, at least a portion of which is made of the electrodeposition-coated steel sheet described in [9].

[0030]

[11] A method for manufacturing an electrodeposited coated steel plate, comprising the following steps:

[0031] a chemical conversion treatment step of subjecting the Fe-based electroplated steel sheet described in any one of [1] to [8] above to a chemical conversion treatment without performing an additional plating treatment, thereby obtaining a chemical conversion treated steel sheet having a chemical conversion treatment film formed in contact with the Fe-based electroplated layer;

[0032] The electrodeposition coating step is to subject the chemical conversion treated steel sheet to an electrodeposition coating treatment to obtain an electrodeposition coated steel sheet having an electrodeposition coating film formed on the chemical conversion treatment film.

[0033]

[12] A method for manufacturing an Fe-based electroplated steel sheet, comprising: performing Fe-based electroplating on a cold-rolled steel sheet containing 0.1% to 3.0% Si by mass, so as to form a Fe-based electroplated steel sheet with a coating weight of 5.0 g / m2 on at least one side. 2 The above Fe-based electroplated steel sheet before annealing of the Fe-based electroplated layer before annealing,

[0034] Next, the Fe-based electroplated steel sheet before annealing is heated at an average heating rate of 10°C / s or more in a temperature range of 400°C to 650°C, maintained in an atmosphere with a dew point greater than -30°C in the heated temperature range, and then cooled to obtain a Fe-based electroplated steel sheet.

[0035]

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

[12] above, wherein the cold-rolled steel sheet contains 0.50% by mass to 3.0% by mass of Si.

[0036]

[14] The method for manufacturing an Fe-based electroplated steel sheet according to

[12] or

[13] , wherein the adhesion weight CW on one side of the Fe-based electroplated layer before annealing isFe0 (g / m 2 ) and the above-mentioned dew point (DP) satisfy the following formula (2).

[0037] (CW Fe0 )+(DP)≥0···(2)

[0038]

[15] A method for manufacturing an Fe-based electroplated steel sheet according to any one of

[12] to

[14] above, wherein the Fe-based electroplating is carried out using an Fe-based electroplating bath containing one or more elements selected from B, C, P, N, O, Ni, Mn, Mo, Zn, W, Pb, Sn, Cr, V and Co, so that the total content of these elements in the Fe-based electroplating layer before the above-mentioned annealing is less than 10 mass%.

[0039] According to the present invention, it is possible to provide a Si-containing cold-rolled steel sheet having excellent resistance to resistance welding cracking of welded portions even when the sheet set object is a galvanized steel sheet. BRIEF DESCRIPTION OF THE DRAWINGS

[0040] Figure 1 This is a diagram schematically showing a cross section of an Fe-based electroplated steel sheet.

[0041] Figure 2A This is a diagram showing an example of raw data obtained by analyzing the intensity distribution of luminescence intensity of Si at different wavelengths using glow discharge emission spectroscopy.

[0042] Figure 2B This is a diagram showing an example of smoothed data obtained by glow discharge emission spectroscopy analysis of the intensity distribution of light emission intensity of Si at different wavelengths.

[0043] Figure 3 (a) A perspective view and (b) an AA cross-sectional view of an overview of an observation sample for measuring the number of grain boundaries of the Fe-based electroplated layer in contact with the Si-containing cold-rolled steel sheet at the interface between the Fe-based electroplated layer and the Si-containing cold-rolled steel sheet.

[0044] Figure 4 This is a diagram explaining a method for measuring the number of grain boundaries of the Fe-based electroplated layer in contact with the Si-containing cold-rolled steel sheet at the interface between the Fe-based electroplated layer and the Si-containing cold-rolled steel sheet.

[0045] Figure 5 is Figure 4 An enlarged view of the portion enclosed by the quadrilateral.

[0046] Figure 6 This is a diagram showing an observed image of the interface between the Fe-based electroplated layer and the Si-containing cold-rolled steel sheet of Example No. 33.

[0047] Figure 7This is a diagram showing an image in which the boundary line and the positions of the grain boundaries at the interface between the Fe-based electroplated layer and the Si-containing cold-rolled steel sheet in Invention Example No. 33 are plotted.

[0048] Figure 8 This is a diagram showing an observed image of the interface between the Fe-based electroplated layer and the Si-containing cold-rolled steel sheet of Invention Example No. 36.

[0049] Figure 9 This is a diagram showing an image in which the boundary line and the position of the grain boundary at the interface between the Fe-based electroplated layer and the Si-containing cold-rolled steel sheet in Invention Example No. 36 are plotted.

[0050] Figure 10 This is a diagram showing an example of raw data of the distribution of C concentration in the depth direction of the plate thickness analyzed by an electron beam microanalyzer.

[0051] Figure 11 This is a diagram showing an example of smoothed data of the depth distribution of the C concentration in the plate thickness direction analyzed by an electron beam microanalyzer.

[0052] Figure 12 (a) is a diagram illustrating a method for evaluating resistance welding cracking characteristics of a welded portion, (b) the upper diagram is a plan view of a plate assembly after welding in the evaluation, and the lower diagram is a BB cross-sectional view of the upper diagram. DETAILED DESCRIPTION

[0053] The aforementioned LME cracking can be broadly divided into "cracks occurring on the surface in contact with the electrode (hereinafter referred to as surface cracking)" and "cracks occurring near the plastic metal ring region between the steel sheets (hereinafter referred to as internal cracking)." Surface cracking is known to occur more readily during resistance welding in the high current range that generates spatter. By limiting the current range to an appropriate level that prevents spatter, surface cracking can be suppressed. On the other hand, internal cracking can occur even when the resistance welding current is within an appropriate range that prevents spatter. Furthermore, while surface cracking is easily detected during visual inspection during the manufacturing process, internal cracking is difficult to detect. For these reasons, internal cracking is a particularly significant issue among LME cracking. When resistance welding is performed with the welding electrode at a certain angle to the steel sheet, residual stress increases, potentially leading to internal cracking. Residual stress increases with increasing steel sheet strength, potentially leading to internal cracking. The present disclosure improves resistance to resistance welding cracking, particularly the ability to prevent this internal cracking.

[0054] Hereinafter, embodiments of the present invention will be described.

[0055] In the following description, the unit of the content of each element in the composition of the Si-containing cold-rolled steel sheet and the content of each element in the coating composition is "mass %" and is simply expressed as "%" unless otherwise specified. In addition, in this specification, the numerical range expressed using "to" is a range that includes the numerical values before and after the "to" as the lower limit and upper limit. In this specification, "high strength" of a 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.

[0056] Figure 1 The schematic cross section of the Fe-based electroplated steel sheet 1 of this embodiment is shown in FIG. Figure 1 As shown, the Fe-based plated steel sheet 1 has an Fe-based plated layer 3 on at least one surface of a Si-containing cold-rolled steel sheet 2. First, the chemical composition of the Si-containing cold-rolled steel sheet will be described.

[0057] Si: 0.1% to 3.0%

[0058] Si has a significant effect in increasing the strength of steel through solid solution (solution strengthening) without significantly impairing workability, making it an effective element for achieving high strength in steel sheets. On the other hand, Si is also an element that negatively impacts the resistance to resistance welding cracking of welds. When Si is added to achieve high strength in steel sheets, it is necessary to add at least 0.1%. Furthermore, as long as Si is present, internal oxides of Si can form at the grain boundaries of the Fe-based electroplated layer, as described later. However, when Si is less than 0.50%, the resistance to resistance welding cracking of welds in conventional welding with a hold time of approximately 0.24 seconds presents no particular problem. However, from a production cost perspective, the production cycle time during spot welding in the assembly process of automotive parts becomes a challenge. Even when measures are taken to reduce hold times, the resistance to resistance welding cracking of welds may become insufficient, even if Si levels are less than 0.5%. On the other hand, if the Si content exceeds 3.0%, hot and cold rolling properties are significantly reduced, potentially adversely affecting productivity or leading to a decrease in the ductility of the steel sheet itself. Therefore, Si is added within a 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, as it has a greater impact on the resistance welding cracking properties of the weld. Furthermore, the Si content is preferably 2.5% or less, more preferably 2.0% or less.

[0059] In the Si-containing cold-rolled steel sheet of this embodiment, it is essential that Si is contained within the above-mentioned range. Other components are permitted as long as they fall within the composition ranges of conventional Si-containing cold-rolled steel sheets, and there are no particular restrictions. However, when the Si-containing cold-rolled steel sheet of this embodiment is to have a high strength with a tensile strength (TS) of 590 MPa or more, the following composition is preferably used.

[0060] C: 0.8% or less (excluding 0%)

[0061] C improves workability by forming martensite and other elements as a steel structure. When C is contained, the amount is preferably 0.8% or less, more preferably 0.3% or less, to achieve good weldability. While there is no particular lower limit for C, the amount is preferably greater than 0%, more preferably 0.03% or more, and even more preferably 0.08% or more, to achieve good workability.

[0062] Mn: 1.0% to 12.0%

[0063] Mn is an element that strengthens steel by solid solution strengthening, thereby increasing its strength, improving hardenability, and promoting the formation of retained austenite, bainite, and martensite. This effect is 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 preferably 12.0% or less. The Mn content is more preferably 1.3% or more, further preferably 1.5% or more, and most preferably 1.8% or more. In addition, the Mn content is more preferably 3.5% or less, and further preferably 3.3% or less.

[0064] P: 0.1% or less (excluding 0%)

[0065] Controlling the P content prevents a decrease in weldability. Furthermore, preventing P from segregating at grain boundaries can prevent deterioration in ductility, bendability, and toughness. Furthermore, adding large amounts of P promotes ferrite transformation and increases grain size. Therefore, the P content is preferably 0.1% or less. While the lower limit of P is not particularly limited, it can be greater than 0%, and can be 0.001% or more, depending on production technology constraints.

[0066] S: 0.03% or less (excluding 0%)

[0067] The S content is preferably 0.03% or less, more preferably 0.02% or less. Reducing the S content prevents a decrease in weldability, prevents a decrease in hot ductility, suppresses hot cracking, and significantly improves the surface shape. Furthermore, by reducing the S content, coarse sulfides are formed as impurities, preventing a decrease in the ductility, bendability, and stretch flangeability of the steel sheet. These problems become significant when the S content exceeds 0.03%, so the S content is preferably minimized. While the lower limit of S is not particularly limited, it can be greater than 0%, and can be 0.0001% or more, depending on production technology constraints.

[0068] N: 0.010% or less (excluding 0%)

[0069] The N content is preferably 0.010% or less. When the N content is 0.010% or less, N forms coarse nitrides with Ti, Nb, and V at high temperatures, thereby preventing the high strength effect of the steel plate caused by the addition of Ti, Nb, and V from being impaired. In addition, when the N content is 0.010% or less, a decrease in toughness can also be prevented. Furthermore, when the N content is 0.010% or less, cracking of the slab during hot rolling and the generation of surface defects 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. Considering the constraints of production technology, it can be greater than 0%, and can be 0.0005% or more.

[0070] Al: 1.0% or less (excluding 0%)

[0071] Al is thermodynamically most susceptible to oxidation, and therefore oxidizes before Si and Mn. This has the effect of inhibiting oxidation of Si and Mn in the outermost layer of the steel sheet and promoting oxidation of Si and Mn within the steel sheet. This effect is achieved when the Al content is 0.01% or greater. On the other hand, an Al content exceeding 1.0% leads to increased 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 can be greater than 0%, and can be 0.001% or more.

[0072] The component composition may further arbitrarily contain one or more selected from B: less than 0.005%, Ti: less than 0.2%, Cr: less than 1.0%, Cu: less than 1.0%, Ni: less than 1.0%, Mo: less than 1.0%, Nb: less than 0.20%, V: less than 0.5%, Sb: less than 0.200%, Ta: less than 0.1%, W: less than 0.5%, Zr: less than 0.1%, Sn: less than 0.20%, Ca: less than 0.005%, Mg: less than 0.005% and REM: less than 0.005%.

[0073] B: 0.005% or less

[0074] B is an element effective in improving the hardenability of steel. To improve hardenability, the B content is preferably 0.0003% or more, more preferably 0.0005% or more. However, if excessive B is added, formability decreases, so the B content is preferably 0.005% or less.

[0075] Ti: 0.2% or less

[0076] Ti is effective for precipitation strengthening of steel. While the lower limit of Ti is not particularly limited, it is preferably 0.005% or more to achieve the effect of strength adjustment. However, if Ti is added in excess, the hard phase becomes too large, deteriorating formability. Therefore, when Ti is added, the amount is preferably 0.2% or less, and more preferably 0.05% or less.

[0077] Cr: 1.0% or less

[0078] The Cr content is preferably 0.005% or more. By increasing the Cr content to 0.005% or more, hardenability can be improved, and the balance between strength and ductility can be improved. When added, the Cr content is preferably 1.0% or less from the perspective of preventing cost increases.

[0079] Cu: 1.0% or less

[0080] The Cu content is preferably 0.005% or more. A Cu content of 0.005% or more promotes the formation of a retained γ phase. Furthermore, when adding Cu, the Cu content is preferably 1.0% or less from the perspective of preventing cost increases.

[0081] Ni: 1.0% or less

[0082] The Ni content is preferably 0.005% or more. A Ni content of 0.005% or more promotes the formation of a retained γ phase. Furthermore, when Ni is added, the Ni content is preferably 1.0% or less from the perspective of preventing cost increases.

[0083] Mo: 1.0% or less

[0084] The amount of Mo is preferably 0.005% or more. By setting the amount of Mo to 0.005% or more, the strength adjustment effect can be achieved. The amount of Mo is more preferably 0.05% or more. Furthermore, when adding Mo, the amount of Mo is preferably 1.0% or less from the perspective of preventing cost increases.

[0085] Nb: 0.20% or less

[0086] The effect of improving strength is achieved by containing 0.005% or more of Nb. When Nb is contained, the amount of Nb is preferably 0.20% or less from the viewpoint of preventing cost increases.

[0087] V: 0.5% or less

[0088] The effect of improving strength is achieved by containing 0.005% or more of V. When V is contained, the amount of V is preferably 0.5% or less from the viewpoint of preventing cost increases.

[0089] Sb: 0.020% or less

[0090] Sb may be contained to suppress oxidation on the steel sheet surface. By suppressing oxidation, Sb improves chemical conversion treatability. To achieve this effect, the Sb content is preferably 0.001% or greater. Furthermore, Sb suppresses the formation of a decarburized layer. To achieve excellent resistance to resistance welding cracking, the Sb content is preferably 0.02% or less. The Sb content is more preferably 0.015% or less, and even more preferably 0.012% or less.

[0091] Ta: 0.1% or less

[0092] The effect of improving strength is achieved by containing Ta in an amount of 0.001% or more. When Ta is contained, the amount of Ta is preferably 0.1% or less from the viewpoint of preventing cost increases.

[0093] W: 0.5% or less

[0094] The effect of improving strength is achieved by containing 0.005% or more of W. When W is contained, the amount of W is preferably 0.5% or less from the viewpoint of preventing cost increase.

[0095] Zr: 0.1% or less

[0096] The effect of improving strength is achieved by containing Zr in an amount of 0.0005% or more. When Zr is contained, the amount of Zr is preferably 0.1% or less from the viewpoint of preventing cost increases.

[0097] Sn: 0.20% or less

[0098] Sn is an element that effectively suppresses denitrification, deboronization, etc., thereby suppressing the decrease in steel strength. To achieve these effects, the Sn content is preferably 0.002% or more. On the other hand, to obtain good impact resistance, the Sn content is preferably 0.20% or less.

[0099] Ca: 0.005% or less

[0100] By containing 0.0005% or more of Ca, the morphology of sulfides can be controlled, thereby improving ductility and toughness. In order to obtain good ductility, the amount of Ca is preferably 0.005% or less.

[0101] Mg: 0.005% or less

[0102] By containing 0.0005% or more of Mg, the morphology of sulfides can be controlled, thereby improving ductility and toughness. Furthermore, when Mg is contained, the amount of Mg is preferably 0.005% or less from the viewpoint of preventing cost increases.

[0103] REM: 0.005% or less

[0104] By containing 0.0005% or more of REM, the morphology of sulfides can be controlled, thereby improving ductility and toughness. Furthermore, when REM is contained, the amount of REM is preferably 0.005% or less from the viewpoint of obtaining good toughness.

[0105] The balance other than the above-mentioned components of the Si-containing cold-rolled steel sheet according to the present embodiment is Fe and inevitable impurities.

[0106] Next, the Fe-based electroplating layer formed on at least one surface of the above-mentioned Si-containing cold-rolled steel sheet will be described.

[0107] Fe-based electroplating layer: 5.0g / m 2 above

[0108] By having an adhesion of 5.0g / m on one side 2 The above Fe-based electroplating layer functions as a soft layer, which can relieve the stress applied to the steel plate surface during welding and reduce the residual stress of the resistance welded portion, thereby improving the resistance welding cracking characteristics of the welded portion, especially improving the characteristics of preventing internal cracking (stress relief effect). In addition, by making the dew point greater than -30°C, Si diffused from the steel plate to the Fe-based electroplating layer during annealing is formed as an oxide inside the Fe-based electroplating layer, and the amount of solid-solubilized Si is reduced, so that a steel plate with excellent resistance to resistance welding cracking characteristics of the welded portion can be obtained. The adhesion amount on one side is 5.0g / m 2The mechanism by which the above-mentioned Fe-based electroplating layer improves the resistance to resistance welding cracking of the weld is not yet clear, but it is believed that when the amount of dissolved Si on the surface of the steel plate is large, the toughness of the weld is reduced and the resistance to resistance welding cracking of the weld is deteriorated. In contrast, when there is a certain amount of Fe-based electroplating layer on the surface of the steel plate and the dew point is controlled above a certain level, oxides are formed inside the Fe-based electroplating layer and act as a solid-solution Si depletion layer, reducing the amount of Si dissolved in the weld, thereby suppressing the reduction in the toughness of the weld, improving the resistance to resistance welding cracking of the weld, and especially improving the property of preventing internal cracking (toughness reduction suppression effect). On the other hand, after the Fe-based electroplating layer is formed, when the Fe-based electroplated steel sheet before annealing is annealed in an atmosphere with a low dew point below -30°C, the grains of the Fe-based electroplating layer may coarsen. Therefore, molten zinc easily invades the grain boundaries of the Si-containing cold-rolled steel sheet through the grain boundaries of the Fe-based electroplating layer. In this embodiment, by controlling the dew point of the atmosphere during annealing to be greater than -30°C, the Si diffused from the Si-containing cold-rolled steel sheet to the Fe-based electroplating layer during annealing is formed as an internal oxide at the grain boundary of the Fe-based electroplating layer. The internal oxide of Si (hereinafter also referred to as Si internal oxide) hinders the crystal growth of the Fe-based electroplating layer in the annealing process, and refines the crystals of the Fe-based electroplating layer. It is believed that by refining the crystals and forming a large number of grain boundaries in the Fe-based electroplating layer, the path of molten zinc intrusion during resistance welding is dispersed, which can delay the time for molten zinc to reach the grain boundary of the Si-containing cold-rolled steel sheet during resistance welding, improve the resistance welding cracking characteristics of the weld, and especially improve the characteristics of preventing internal cracking (zinc grain boundary intrusion inhibition effect). The contribution of these Fe-based electroplating layers to the resistance welding cracking characteristics of stress relaxation effect, toughness reduction inhibition effect and zinc grain boundary intrusion inhibition effect is complex, and therefore has not yet been quantitatively elucidated. It is believed that the composite effect improves the resistance welding cracking characteristics. In order to improve the resistance to resistance cracking of the welded portion, the coating weight of the Fe-based plating layer on one side must be 5.0 g / m 2 There is no particular upper limit on the amount of Fe-based electroplating layer deposited on one side, but from a cost perspective, the amount of Fe-based electroplating layer deposited on one side is preferably 60 g / m 2 The Fe-based electroplating layer preferably has a deposition weight of 50 g / m 2 Below, more preferably 40g / m 2 Below, more preferably 30g / m 2 The Fe-based electroplated steel sheet preferably has an Fe-based electroplated layer on both sides of the Si-containing cold-rolled steel sheet. By making the Fe-based electroplated layer have an adhesion amount of 5.0 g / m 2 More than 5.0 g / m 2 The resistance to resistance welding cracking of the welded part is particularly good. The adhesion amount of the Fe-based electroplating layer on one side can be 8g / m2 Above, 10g / m 2 above.

[0109] In addition, the thickness of the Fe-based electroplating layer is measured as follows. A sample of 10×15 mm in size is taken from the annealed Si-containing cold-rolled steel sheet and embedded in a resin to make a cross-section embedded sample. A scanning electron microscope (SEM) is used to observe any three positions of the cross section at an accelerating voltage of 15 kV and a magnification of 2000 to 10000 times according to the thickness of the Fe-based electroplating layer. The average value of the thickness of the three viewing fields is multiplied by the density of iron to convert it into the adhesion amount of a single side of the Fe-based electroplating layer.

[0110] As the Fe-based electroplating layer, in addition to pure Fe, alloy plating layers such as Fe-B alloy, Fe-C alloy, Fe-P alloy, Fe-N alloy, Fe-O alloy, Fe-Ni alloy, Fe-Mn alloy, Fe-Mo alloy, and Fe-W alloy can be used. The composition of the Fe-based electroplating layer is not particularly limited, but it is preferably composed of a composition containing a total of 10% by mass or less of one or more elements selected from B, C, P, N, O, Ni, Mn, Mo, Zn, W, Pb, Sn, Cr, V, and Co, and the remainder is composed of Fe and unavoidable impurities. By making the amount of elements other than Fe less than 10% by mass in total, it is possible to prevent the reduction of electrolysis efficiency and form the Fe-based electroplating layer at low cost. In the case of Fe-C alloy, the C content is preferably 0.08% by mass or less.

[0111] Furthermore, the Si-containing cold-rolled steel sheet of this embodiment preferably has no surface coating other than Fe-based electroplating. Since the Si-containing cold-rolled steel sheet has no surface coating other than Fe-based electroplating, it is possible to provide, at low cost, components that do not require excessive galvanized steel sheeting for rust prevention, or components used in environments with mild corrosion environments that do not require excessive rust prevention.

[0112] The Fe-based electroplated layer contains Si internal oxides at at least a portion of the grain boundaries. It is believed that the Si internal oxides hinder the crystal growth of the Fe-based electroplated layer during the annealing process, resulting in a finer grained Fe-based electroplated layer. As a result, numerous grain boundaries are formed in the Fe-based electroplated layer, which disperses the intrusion paths for molten zinc. This delays the time it takes for molten zinc to reach the grain boundaries of the Si-containing cold-rolled steel sheet during resistance welding, improving the resistance to resistance weld cracking at the weld, particularly its ability to prevent internal cracking.

[0113] The presence or absence of Si internal oxides in the Fe-based electroplating layer is determined as follows: when analyzing the luminescence intensity representing the wavelength of Si in the depth direction (thickness direction) from the surface of the Fe-based electroplating layer using glow discharge optical emission spectroscopy (GD-OES), is one or more peaks of luminescence intensity representing the wavelength of Si appear within a range greater than 0.2 μm from the surface of the Fe-based electroplating layer and below the thickness of the Fe-based electroplating layer. The measurement conditions are an Ar gas pressure of 600 Pa, a high-frequency output of 35 W, a measurement diameter of 4 mmΦ, and a sampling interval of 0.1 seconds. In addition, after analyzing a Si-containing cold-rolled steel sheet without Fe-based electroplating using glow discharge optical emission spectroscopy under these conditions, the depth of the spatter mark is measured to calculate the spatter velocity, and the horizontal axis of the intensity distribution representing the wavelength of Si is converted into depth corresponding to each time. The depth of the spatter mark is measured using a non-contact surface profile measuring device (NewView 7300: manufactured by Zygo Corporation). The presence or absence of a peak of luminescence intensity representing the wavelength of Si is determined as follows. First, the raw data of the intensity distribution obtained is smoothed using the Savitzky-Golay method. In this case, the value of m is preferably 15 or greater. Next, the intensity of the Fe-based electroplated layer at a depth of 0.2 μm from the surface and the intensity of the Fe-based electroplated layer / Si-containing cold-rolled steel sheet interface are smoothed using the Savitzky-Golay method. Figure 2B The peaks above the straight line L in the smoothed intensity distribution are determined to be one peak. The number of peaks representing the emission intensity at the wavelength of Si may be one or more, and the upper limit is not particularly limited, but is preferably three or less.

[0114] use Figure 2A and Figure 2B , a representative example of analyzing the Si peak indicating the emission intensity of Si at the wavelength observed in this embodiment will be described. Figure 2A The results of analyzing the raw data of the intensity distribution of the luminescence intensity at the wavelength representing Si for the Fe-based electroplated steel sheets of Examples No. 32 (dew point of the annealing atmosphere -37°C), 33 (dew point -13°C), and 36 (dew point +11°C) described later are shown. Figure 2B The results after smoothing treatment (m=20) are shown. In Examples No. 32, 33 and 36, an Fe-based electroplated layer (referred to as "Fe plating" in the figure for convenience) with a thickness of about 2 μm is formed on the surface of the Si-containing cold-rolled steel sheet (referred to as "base iron" in the figure for convenience). The thickness of the Fe-based electroplated layer here is determined by the cross-sectional SEM observation mentioned above. Figure 2B As shown, in Example No. 32 where the annealing step was performed in a low dew point atmosphere, the peak P derived from the Si external oxide was observed only within 0.2 μm from the surface of the Fe-based electroplated layer.ex On the other hand, in Example No. 33 in which the annealing process was performed in an atmosphere with a dew point greater than -30°C, a peak P derived from Si internal oxide was observed within a range greater than 0.2 μm from the surface of the Fe-based electroplated layer and less than the thickness of the Fe-based electroplated layer. in In addition, in Example No. 36, in which the annealing process was performed at a high dew point of +11°C, there were two peaks P derived from Si internal oxides in the range of more than 0.2 μm from the surface of the Fe-based electroplated layer and less than the thickness of the Fe-based electroplated layer. in . Thus, the presence of one or more peaks of luminescence intensity representing the wavelength of Si in the range of more than 0.2 μm from the surface of the Fe-based electroplating layer and less than the thickness of the Fe-based electroplating layer indicates that internal oxidation occurs in the Fe-based electroplating layer, and Si internal oxides exist in the range of more than 0.2 μm from the surface of the Fe-based electroplating layer and less than the thickness of the Fe-based electroplating layer. Here, the thickness of the Fe-based electroplating layer is a value measured by the above-mentioned cross-sectional observation. In the steel sheet having Si internal oxides within the above-mentioned depth range, the growth of grains in the Fe-based electroplating layer is suppressed by the internal oxides. Therefore, even if an annealing process is performed after the Fe-based electroplating treatment, the grain coarsening of the Fe-based electroplating layer can be prevented, and a large number of grain boundaries are formed in the Fe-based electroplating layer. As a result, the intrusion path of molten zinc is dispersed, delaying the time for molten zinc to reach the grain boundaries of the Si-containing cold-rolled steel sheet during resistance welding, and having excellent resistance to resistance welding cracking characteristics.

[0115] Furthermore, when analyzing the depth direction from the surface of the Fe-based electroplating layer by glow discharge emission spectroscopy, it is possible to have a peak of luminescence intensity representing the wavelength of Si in two ranges: more than 0.2 μm from the surface of the Fe-based electroplating layer and less than the thickness of the Fe-based electroplating layer, and 0.0 μm to 0.2 μm from the surface of the Fe-based electroplating layer. Figure 2B In Examples No. 33 and 36, peaks of luminescence intensity at a wavelength indicative of Si were observed in two ranges: a range of more than 0.2 μm from the surface of the Fe-based electroplating layer and less than the thickness of the Fe-based electroplating layer, and a range of 0.0 μm to 0.2 μm from the surface of the Fe-based electroplating layer. The presence of peaks of luminescence intensity at a wavelength indicative of Si in two ranges of more than 0.2 μm from the surface of the Fe-based electroplating layer and less than the thickness of the Fe-based electroplating layer, and a range of 0.0 μm to 0.2 μm from the surface of the Fe-based electroplating layer indicates that Si internal oxides are present in the Fe-based electroplating layer and Si external oxides are present in the surface layer of the Fe-based electroplating layer.

[0116] At the interface between the Fe-based electroplating layer and the Si-containing cold-rolled steel sheet, the number of grain boundaries of the Fe-based electroplating layer in contact with the Si-containing cold-rolled steel sheet is 10 or more per 10 μm in the width direction of the Si-containing cold-rolled steel sheet in the observation field. If, at the interface between the Fe-based electroplating layer and the Si-containing cold-rolled steel sheet, the number of grain boundaries of the Fe-based electroplating layer in contact with the Si-containing cold-rolled steel sheet is 10 or more per 10 μm in the width direction of the Si-containing cold-rolled steel sheet, the crystals of the Fe-based electroplating layer are sufficiently refined. It is believed that by refining, a large number of grain boundaries are formed in the Fe-based electroplating layer, and as a result, the intrusion of molten zinc is dispersed, which can delay the time of reaching the grain boundaries of the Si-containing cold-rolled steel sheet during welding, improve the resistance welding cracking characteristics of the weld, and especially improve the characteristics of preventing internal cracking. At the interface between the Fe-based electroplated layer and the Si-containing cold-rolled steel sheet, the number of grain boundaries of the Fe-based electroplated layer in contact with the Si-containing cold-rolled steel sheet may preferably be 16 or more per 10 μm in the width direction of the Si-containing cold-rolled steel sheet in the field of view. More preferably, the number of grain boundaries of the Fe-based electroplated layer in contact with the Si-containing cold-rolled steel sheet may be 20 or more per 10 μm in the width direction of the Si-containing cold-rolled steel sheet in the field of view.

[0117] Here, at the interface between the Fe-based electroplated layer and the Si-containing cold-rolled steel sheet, the number of grain boundaries of the Fe-based electroplated layer in contact with the Si-containing cold-rolled steel sheet is measured as follows. First, a sample of 10×10 mm in size is taken from the Fe-based electroplated steel sheet. A focused ion beam (FIB) device is used to process an arbitrary position of the sample, and a 45° cross-section with a width of 30 μm in the rolling right-angle direction and a length of 50 μm in the 45° direction relative to the T-section direction (a cross-section parallel to the rolling right-angle direction of the steel sheet and perpendicular to the steel sheet surface) is formed at this position as an observation sample. Figure 3 An overview of the observation sample is shown in FIG. Figure 3 (a) is a perspective view of a sample for observation. Figure 3 (b) Yes Figure 3 (a) is the AA cross-sectional view of the observation sample shown. Next, a scanning ion microscope (SIM) was used to observe the central portion of the width and length directions of the 45° cross-section of the observation sample at a magnification of 5000 times, and a SIM image was taken. An example of such a SIM image is shown in FIG. Figure 4 . Figure 4 This is a SIM image of Example No. 32 described below. A 10 μm area in the width direction of the Si-containing cold-rolled steel sheet ( Figure 4 The part surrounded by the quadrilateral in the figure). For illustration, Figure 5 Shown in Figure 4 An enlarged view of the part surrounded by the quadrilateral. Figure 5As shown in the SIM image, a boundary line is drawn at the interface between the Fe-based electroplating layer and the Si-containing cold-rolled steel sheet in a 10 μm region in the width direction of the Si-containing cold-rolled steel sheet ( Figure 5 The number of grain boundaries of the Fe-based electroplated layer on the boundary line was measured and used as the "number of grain boundaries of the Fe-based electroplated layer in contact with the Si-containing cold-rolled steel sheet at the interface between the Fe-based electroplated layer and the Si-containing cold-rolled steel sheet."

[0118] Figure 6 SIM images of the interface between the Fe-based electroplated layer and the Si-containing cold-rolled steel sheet of Inventive Example No. 33 described later in the Examples are shown in FIG. 1 and FIG. 2 show images of the interface between the Fe-based electroplated layer and the Si-containing cold-rolled steel sheet of Inventive Example No. 33 described later in the Examples. Figure 7 In Invention Example No. 33, there are grain boundaries on the measurement boundary line at 15 positions indicated by arrows every 10 μm in the width direction of the Si-containing cold-rolled steel sheet. Therefore, in Invention Example No. 33, at the interface between the Fe-based electroplated layer and the Si-containing cold-rolled steel sheet, the number of grain boundaries of the Fe-based electroplated layer in contact with the Si-containing cold-rolled steel sheet is 15 per 10 μm in the width direction of the Si-containing cold-rolled steel sheet. In addition, Figure 8 SIM images of the interface between the Fe-based electroplated layer and the Si-containing cold-rolled steel sheet of Inventive Example No. 36 of the Examples described later are shown in FIG. The image in which the boundary line and the measurement boundary line are drawn in the central portion of the SIM image is shown in FIG. Figure 9 In Invention Example No. 36, grain boundaries on the measurement boundary line exist at 20 positions indicated by arrows every 10 μm across the width of the Si-containing cold-rolled steel sheet. Therefore, in Invention Example No. 36, at the interface between the Fe-based electroplated layer and the Si-containing cold-rolled steel sheet, the number of grain boundaries in the Fe-based electroplated layer in contact with the Si-containing cold-rolled steel sheet is 20 per 10 μm across the width of the Si-containing cold-rolled steel sheet.

[0119] The thickness of the Fe-based electroplated steel sheet of the present embodiment is not particularly limited, but is usually 0.5 mm or more, and may be 3.2 mm or less.

[0120] Next, the C concentration of the surface layer of the Fe-based electroplated steel sheet will be described. In this embodiment, it is important that the average C concentration of the Fe-based electroplated layer in the range of 10μm to 20μm from the surface in the plate thickness direction is 0.10 mass% or less, and more preferably 0.07 mass% or less, through the above-mentioned annealing. It is further preferred that a decarburized layer is formed on the surface of the Fe-based electroplated steel sheet during the above-mentioned annealing. In addition, the decarburized layer refers to an area near the surface of the Fe-based electroplated steel sheet where the C concentration is lower than the concentration in the steel, and may be formed by the detachment of C from the surface of the steel sheet during annealing. If the average C concentration of the Fe-based electroplated layer in the range of 10μm to 20μm from the surface in the plate thickness direction is 0.10 mass% or less, the area of the Fe-based electroplated layer in the range of 10μm to 20μm from the surface in the plate thickness direction is soft. Therefore, the stress applied from the welding electrode during resistance welding is relaxed, thereby improving the resistance to resistance welding cracking characteristics.

[0121] In this embodiment, annealing is performed after forming the Fe-based electroplated layer. This allows the average carbon concentration of the Fe-based electroplated layer, measured over a range of 10 μm to 20 μm from the surface toward the thickness of the plate, to be further reduced compared to when the Fe-based electroplated layer is absent. Furthermore, when a decarburized layer is formed, the carbon concentration in the decarburized layer can be further reduced, provided the thickness of the decarburized layer remains the same. Furthermore, when Ni, Co, Sn, and other metals are electroplated alone, the solid solubility of carbon in these metals is extremely low, and since carbon does not dissolve in the solid solution, the decarburization promoting effect is not achieved.

[0122] When an Fe-based electroplated layer is formed, the reason why the C concentration decreases in the range of 10 μm to 20 μm from the surface of the Fe-based electroplated layer toward the thickness of the sheet is still uncertain. The present inventors speculate as follows. Specifically, it is believed that this is because the Fe-based electroplated layer contains almost no C, which induces C to diffuse from the Si-containing cold-rolled steel sheet. In addition, as described above, the Fe-based electroplated layer is fine-grained, which increases the diffusion paths for C to escape from the Fe-based electroplated layer to the outside.

[0123] Furthermore, the softening effect achieved by reducing the carbon concentration in the Fe-based electroplated layer within a range of 10 to 20 μm from the surface toward the thickness of the plate saturates below a certain carbon concentration level within the range of 10 to 20 μm in the thickness direction, thus limiting the improvement in resistance welding cracking resistance achieved through softening. In this embodiment, by further reducing the carbon concentration in the Fe-based electroplated layer within a range of 10 to 20 μm from the surface toward the thickness of the plate, resistance welding cracking resistance is effectively improved even when the decarburized layer is thin. This demonstrates that, in addition to softening, other effects, such as an increase in melting point due to reduced carbon concentration, can be achieved.

[0124] When a decarburized layer is formed, the thickness of the decarburized layer is preferably 30 μm or greater. While the upper limit of the decarburized layer thickness is not particularly limited, it is preferably 130 μm or less to maintain a good tensile strength. The thickness of the decarburized layer is defined as the thickness of the region in the surface layer of the Fe-plated steel sheet where the carbon concentration is 80% or less of that in the steel, as determined by analyzing the carbon concentration of the Fe-plated steel sheet along the thickness direction from the surface of the Fe-plated layer.

[0125] Here, the average carbon concentration of the Fe-based electroplated layer in the range of 10μm to 20μm from the surface to the thickness of the plate and the thickness of the decarburized layer are measured as follows. The elemental distribution near the surface layer is analyzed by surface or line analysis using an electron probe microanalyzer (EPMA) on the cross-sectioned sample. First, the steel plate embedded in resin is polished, and after observing the cross section perpendicular to the rolling direction, the sample is removed from the resin to prepare the measurement sample. With an accelerating voltage of 7kV and an irradiation current of 50nA, surface or line analysis of the sample cross section is performed in a 300×300μm area including the outermost layer of the Fe-based electroplated steel plate with a step size of 1μm to measure the carbon intensity. To prevent contamination, a plasma cleaner is used in both the measurement room and the sample preparation room to remove hydrocarbons from the surface and surrounding areas of the sample before the measurement begins. Furthermore, to prevent the accumulation of hydrocarbons during the measurement, the sample is heated on the workbench and maintained at a maximum temperature of 100°C. The sample temperature is preferably above 90°C. The C intensity was converted to C concentration (mass %) using a calibration curve prepared by separately measuring standard samples. Due to the contamination suppression effect, the C detection limit was confirmed to be sufficiently lower than 0.10 mass %. The apparatus used and the contamination suppression method are described in detail in Reference 1 below.

[0126] Reference 1: Yamashita et al., “Carbon distribution in the early stage of proeutectoid ferrite transformation of low carbon steel using high-precision FE-EPMA,” Iron and Steel, Vol. 103 (2017) No. 11, p. 14-20

[0127] The necessity of anti-contamination measures during measurement depends on the type of equipment and conditions used, so the above configuration is not essential. In other words, the measurement conditions only need to be able to obtain sufficient accuracy, and the measurement conditions have no essential relationship with the effect of the present invention.

[0128] In the obtained concentration distribution, the line distribution in the thickness direction is extracted from the surface of the Fe-based electroplated steel sheet, and the distribution of the C concentration in the thickness depth direction is obtained by taking 300 points parallel to the surface of the steel sheet and averaging them. The obtained distribution of the C concentration in the thickness depth direction is smoothed using the simple moving average method. At this time, the number of smoothing points is preferably about 21 points. When the number of smoothing points near the surface of the sample is less than 10 points on one side, it is preferable to smooth the measurement points that can be taken on one side. Then, in the intensity distribution after smoothing, the thickness of the surface of the Fe-based electroplated steel sheet where the C concentration is less than 80% of that in steel is evaluated as the thickness of the decarburized layer. In addition, in the range of 10μm to 20μm from the surface of the Fe-based electroplated steel sheet in the thickness direction, the C concentration values of 11 points with a spacing of 1μm are averaged to obtain the C concentration in the range of 10μm to 20μm in the thickness direction. The above evaluation was applied to the measurement results of two viewing fields of each sample, and the average values were taken as the average C concentration of the Fe-based electroplated layer in the range of 10 μm to 20 μm from the surface to the plate thickness direction and the evaluation value of the decarburized layer thickness.

[0129] use Figure 10 、 11 A representative example of the distribution of the C concentration in the plate thickness direction analyzed by an electron beam microanalyzer will be described. Figure 10 The raw data of the distribution of the depth in the plate thickness direction of the C concentration obtained by analyzing the surface of the Fe-based electroplated steel sheets Nos. 32, 33, and 36 of Examples described later are shown. Figure 11 Indicates the use of a simple moving average method with a smoothing point of 21 points. Figure 10 The data after smoothing (m=21) the original data. Figure 11 As shown, in Example No. 36, a decarburized layer having a C concentration of 80% or less in the steel existed, and the thickness of the decarburized layer was 81 μm.

[0130] As described above, the internal crack prevention property of the Fe-based electroplating layer is a composite effect of the zinc grain boundary intrusion suppression effect, stress relaxation effect, toughness reduction suppression effect, and the surface layer C concentration reduction effect obtained by the Fe-based electroplating layer promoting decarburization. These composite effects cannot be clearly quantified, but the Fe-based electroplating layer is preferably deposited at a weight CW of Fe1 (g / m 2 ) and the thickness of the decarburized layer C d (μm) satisfies the following formula (1).

[0131] 1.6×(CW Fe1 )+(C d )≥77···(1)

[0132] This is because as long as the Fe-based electroplating layer has a deposition weight CWFe1 (g / m 2 ) and the thickness of the decarburized layer C d (μm) satisfies the above formula (1), and the resistance to resistance welding cracking is particularly good.

[0133] According to the present disclosure, a high-strength Fe-based electroplated steel sheet having a tensile strength TS of 590 MPa or more as measured according to JIS Z 2241 (2011) can be provided. The strength of the Fe-based electroplated steel sheet is more preferably 800 MPa or more.

[0134] <Method for producing Fe-based electroplated steel sheet>

[0135] Next, a method for producing an Fe-based electroplated steel sheet will be described.

[0136] The method for producing an Fe-based electroplated steel sheet is as follows: a cold-rolled steel sheet containing 0.1% to 3.0% Si by mass is subjected to Fe-based electroplating treatment to obtain a coating weight of 5.0 g / m2 on one side. 2 The above-mentioned pre-annealing Fe-based electroplated steel sheet has the Fe-based electroplated layer formed on at least one side.

[0137] Next, the Fe-based electroplated steel sheet before annealing is heated at an average heating rate of 10°C / s or more in a temperature range of 400°C to 650°C, maintained in an atmosphere with a dew point greater than -30°C in the heated temperature range, and then cooled to obtain a Fe-based electroplated steel sheet.

[0138] First, a cold-rolled steel sheet containing 0.1% to 3.0% Si by mass is manufactured. Alternatively, the cold-rolled steel sheet may contain 0.50% to 3.0% Si by mass. The cold-rolled steel sheet may be manufactured according to a conventional method for manufacturing cold-rolled steel sheets. In one example, the cold-rolled steel sheet is manufactured as follows: a steel slab having the above-described composition is hot-rolled to form a hot-rolled sheet, the hot-rolled sheet is pickled, and then the hot-rolled sheet is cold-rolled to form a cold-rolled steel sheet.

[0139] Next, the surface of the cold-rolled steel sheet is subjected to an Fe-based electroplating treatment to obtain a Fe-based electroplated steel sheet before annealing. The Fe-based electroplating treatment method is not particularly limited. For example, a sulfuric acid bath, a hydrochloric acid bath, or a mixture of the two can be used as the Fe-based electroplating bath. In addition, the Fe-based electroplating treatment can also be performed without performing an oxidation treatment in a preheating furnace or the like on the cold-rolled steel sheet after cold rolling. In addition, the Fe-based electroplated steel sheet before annealing is a method in which the Fe-based electroplated layer does not undergo an annealing process, and the method of pre-annealing the cold-rolled steel sheet before the Fe-based electroplating treatment is not excluded.

[0140] The Fe ion content in the Fe-based electroplating bath before power on is expressed as Fe 2+If the Fe ion content in the Fe-based electroplating bath is expressed as Fe 2+ A sufficient Fe deposition amount can be obtained when the Fe deposition amount is 0.5 mol / L or more. In order to obtain a sufficient Fe deposition amount, the Fe ion content in the Fe-based electroplating bath before the start of energization is preferably 2.0 mol / L or less.

[0141] The Fe-based electroplating bath may contain Fe ions and at least one element selected from 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 such that the total content of these elements in the Fe-based electroplated layer before annealing is 10% by mass or less. Furthermore, metal elements may be contained as metal ions, and non-metallic elements may be contained as part of boric acid, phosphoric acid, nitric acid, an organic acid, or the like. Furthermore, the iron sulfate plating solution may contain a conductive additive such as sodium sulfate or potassium sulfate, a chelating agent, or a pH buffer.

[0142] Other conditions of the Fe-based electroplating bath are not particularly limited. Considering the ability to maintain constant temperature, the temperature of the Fe-based electroplating solution is preferably above 30°C and preferably below 85°C. The pH of the Fe-based electroplating bath is also not particularly specified, but from the perspective of preventing a decrease in current efficiency due to hydrogen generation, it is preferably above 1.0. In addition, considering the conductivity of the Fe-based electroplating bath, it is preferably below 3.0. From the perspective of productivity, the current density is preferably 10A / dm 2 From the perspective of ease, the coating weight of the Fe-based electroplating layer is preferably controlled to be 150 A / dm 2 The sheet feeding speed is preferably 5 mpm or higher from the viewpoint of productivity, and is preferably 150 mpm or lower from the viewpoint of stably controlling the adhesion amount.

[0143] In addition, as treatments before the Fe-based electroplating treatment is implemented, degreasing treatment and water washing for cleaning the surface of the cold-rolled steel sheet, and pickling treatment and water washing for activating the surface of the cold-rolled steel sheet can be implemented. After these pre-treatments, the Fe-based electroplating treatment is implemented. The methods of degreasing treatment and water washing are not particularly limited, and conventional methods can be used. In the pickling treatment, various acids such as sulfuric acid, hydrochloric acid, nitric acid and mixtures thereof can be used. Among them, sulfuric acid, hydrochloric acid or mixtures thereof are preferred. The concentration of the acid is not particularly specified. If the ability to remove the oxide film and the prevention of rough skin (surface defects) caused by excessive pickling are taken into consideration, it is preferably about 1 to 20 mass%. In addition, the pickling treatment solution may contain a defoaming agent, a pickling accelerator, a pickling inhibitor, etc.

[0144] Average heating rate: above 10℃ / second

[0145] Next, the pre-annealed Fe-based electroplated steel sheet is heated to a temperature range of 650°C to 900°C at an average heating rate of at least 10°C / second in the temperature range of 400°C to 650°C (heating step). By setting the average heating rate in the heating step to at least 10°C / second, the growth of grains in the Fe-based electroplated layer during the heating step is minimized. This is because, as will be described later, Si internal oxidation hardly forms at the grain boundaries of the Fe-based electroplated layer during the heating step. Therefore, if the heating rate is less than an average of 10°C / second, grain growth cannot be suppressed. While minimizing the growth of grains in the Fe-based electroplated layer during the heating step, annealing is performed in an atmosphere with a dew point greater than -30°C, as will be described later, to refine the crystals of the Fe-based electroplated layer. The heating zone in the heating step can be, for example, a direct fired furnace (DFF) or a non-oxidizing furnace (NOF). In the case of a radiant tube type heating furnace, a preheating zone such as an IH (Induction Heater) may be provided in the front stage. The average heating rate is based on the temperature measured on the surface of the Fe-based electroplated steel sheet.

[0146] Next, an annealing process is performed in which the Fe-based electroplated steel sheet before annealing is kept in a reducing atmosphere with a dew point greater than -30°C and a hydrogen concentration of 1.0% to 30.0% by volume in a temperature range of 650°C to 900°C for 30 seconds to 600 seconds and then cooled to obtain the Fe-based electroplated steel sheet. The annealing process is performed to eliminate the strain of the cold-rolled steel sheet generated by the rolling process, recrystallize the structure and improve the strength of the steel sheet. At this time, internal oxides are formed at the grain boundaries of the Fe-based electroplated layer, thereby suppressing the growth of the grains of the Fe-based electroplated layer, making it possible to refine the grains, and forming a decarburized layer on the surface of the Fe-based electroplated steel sheet, thereby reducing the C concentration of the surface layer.

[0147] Hydrogen concentration: 1.0% to 30% by volume

[0148] The annealing process is carried out in a reducing atmosphere with a hydrogen concentration of 1.0 volume % to 30.0 volume %. Hydrogen inhibits the oxidation of Fe on the surface of the Fe-based electroplated steel sheet before annealing in the annealing process, and plays a role in activating the surface of the steel sheet. If the hydrogen concentration is 1.0 volume % or more, it is possible to avoid the deterioration of the chemical conversion treatment properties when a chemical conversion treatment film is set as described later due to the oxidation of Fe on the surface of the steel sheet. Therefore, it is preferred that the annealing process be carried out in a reducing atmosphere with a hydrogen concentration of 1.0 volume % or more, and more preferably in a reducing atmosphere with a hydrogen concentration of 2.0 volume % or more. There is no particular upper limit on the hydrogen concentration in the annealing process, but from a cost point of view, the hydrogen concentration is preferably 30.0 volume % or less, and more preferably 20.0 volume % or less. The remainder of the annealing atmosphere other than hydrogen is preferably nitrogen.

[0149] Dew point greater than -30℃

[0150] By setting the dew point of the annealing atmosphere in the annealing process to be greater than -30°C, Si internal oxides are formed at the grain boundaries of the Fe-based electroplating layer. The control of the dew point greater than -30°C is preferably carried out in the temperature range of 650°C to 900°C. As a result, the average heating rate in the heating process is set to an average of more than 10°C / second, and the growth of the grains in the Fe-based electroplating layer is suppressed as much as possible, so that Si internal oxides can be formed at the grain boundaries of the Fe-based electroplating layer. In addition, by setting the dew point of the annealing atmosphere in the annealing process to be greater than -30°C, the decarburization reaction is promoted and the C concentration of the surface layer can be reduced. The Si internal oxides present at the grain boundaries of the Fe-based electroplating layer suppress the growth of the grains of the Fe-based electroplating layer in the annealing process due to the pinning effect. The Si internal oxides are obtained by diffusion of Si from the cold-rolled steel sheet, and the pinning effect of the Si internal oxides is particularly strong on the cold-rolled steel sheet side of the Fe-based electroplating layer. As a result, the crystal grain size at the interface between the Fe-based electroplated layer and the Si-containing cold-rolled steel sheet is particularly small, and the number of grain boundaries of the Fe-based electroplated layer in contact with the interface of the Si-containing cold-rolled steel sheet increases. In addition, the pinning effect here refers to the Zener drag mechanism. Second-phase particles are dispersed in the structure, and when grain boundaries intersect with second-phase particles, energy is required for the grain boundaries to detach from the second-phase particles. In other words, the pinning force acts between the particles and the grain boundaries to prevent the grain boundaries from moving, thereby inhibiting grain growth. Carbides and sulfides are well-known as second-phase particles. It is not clear whether Si internal oxides exhibit this pinning effect, but based on experimental facts, it is inferred that a pinning effect is exhibited. The dew point of the annealing atmosphere is preferably above -20°C, and more preferably above -5°C. By setting the dew point of the annealing atmosphere to above -5°C, the resistance to resistance weld cracking of the weld is particularly good. In particular, the property of preventing internal cracking is good. The upper limit of the dew point of the annealing atmosphere is not particularly limited, but the dew point of the annealing atmosphere is preferably 30° C. or lower in order to appropriately prevent oxidation of the Fe-based electroplated layer surface and improve chemical conversion treatability when a chemical conversion treatment film is formed as described later.

[0151] Holding time in the temperature range of 650°C to 900°C: 30 seconds to 600 seconds

[0152] During the annealing process, the holding time in the temperature range of 650°C to 900°C is preferably 30 seconds to 600 seconds. By setting the holding time in this temperature range to 30 seconds or longer, the natural Fe oxide film formed on the surface of the Fe-based electroplating layer before annealing can be appropriately removed, thereby improving the chemical conversion treatability when a chemical conversion treatment film is provided on the surface of the steel sheet as described later. Therefore, the holding time in this temperature range is preferably 30 seconds or longer. There is no particular upper limit to the holding time in this temperature range, but from the perspective of productivity, the holding time in this temperature range is preferably 600 seconds or shorter.

[0153] The maximum temperature of Fe-based electroplated steel sheet before annealing: 650℃~900℃

[0154] The maximum temperature of the Fe-based electroplated steel sheet before annealing is not particularly limited, but is preferably between 650°C and 900°C. 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 smoothly, allowing the desired strength to be achieved. Furthermore, by appropriately reducing the natural Fe oxide film formed on the surface of the Fe-based electroplated layer before annealing, chemical conversion treatability can be improved when a chemical conversion treatment coating is applied to the steel sheet surface, as described later. Furthermore, by setting the maximum temperature of the Fe-based electroplated steel sheet before annealing to 900°C or lower, the diffusion rate of Si and Mn in the steel can be prevented from increasing excessively, preventing Si and Mn from diffusing to the steel sheet surface, thereby improving chemical conversion treatability when a chemical conversion treatment coating is applied to the steel sheet surface, as described later. Furthermore, if the maximum temperature of the Fe-based electroplated steel sheet before annealing is 900°C or lower, damage to the heat treatment furnace body can be prevented, reducing costs. Therefore, the maximum temperature of the Fe-based electroplated steel sheet is preferably 900°C or lower. The maximum attainable temperature is based on the temperature measured on the surface of the Fe-based electroplated steel sheet.

[0155] The preferred adhesion amount CW of the Fe-based electroplated layer on one side before annealing is Fe0 (g / m 2 ) and the dew point DP (°C) of the annealing atmosphere satisfy the following formula (2). If the following formula (2) is satisfied, the effect of suppressing the grain boundary intrusion of molten zinc, the stress relaxation effect, the effect of suppressing the reduction in toughness, and the effect of reducing the C concentration in the surface layer due to the accelerated decarburization of the Fe-based electroplating layer act in combination, and a more significant improvement in the resistance to resistance welding cracking can be achieved.

[0156] (CW Fe0 )+(DP)≥0···(2)

[0157] As described above, if the result satisfies the above formula (2), the resistance welding cracking characteristics of the weld can be further improved. As another embodiment, when the above formula (2) is not satisfied, a process of changing the dew point DP (°C) of the annealing atmosphere in order to satisfy the above formula (2) can be further provided. In this way, the resistance welding cracking characteristics of the weld can be more reliably improved. As an example of performing this process in operation, consider a process having an adhesion amount CW on a single side of the Fe-based electroplating layer before annealing. Fe0 (g / m 2 ) value in the annealing process to satisfy the above formula (2), and in the annealing process, the atmosphere dew point is controlled in such a way that it becomes a certain dew point. Specifically, the adhesion amount CW on one side of the Fe-based electroplating layer before annealing is set toFe0 (g / m 2 ) is substituted into the above formula (2) to determine the dew point DP in the above annealing process in a manner that satisfies the formula (2). Here, the adhesion amount CW on one side of the Fe-based electroplating layer before annealing is Fe0 (g / m 2 ) into the above formula (2) is not limited to the method of strictly substituting the same formula as the above formula (2). It also includes the method of substituting an inequality that always satisfies the narrower range of formula (2). By performing such control, even if the product style of the steel plate is changed, for example, the adhesion amount CW of the Fe-based electroplating layer on one side before annealing is Fe0 (g / m 2 ) changes significantly and does not satisfy the above formula (3) (when the formula is not actually satisfied, or a situation occurs where the formula is no longer satisfied), automatic control can also be performed to satisfy the formula.

[0158] In addition, the control responsiveness of the dew point DP is better than the adhesion amount CW of the Fe-based electroplating layer on one side before annealing. Fe0 Therefore, from the perspective of control responsiveness, it is preferable to change the adhesion amount CW of the Fe-based electroplating layer on one side before annealing in accordance with the value of the dew point DP so as to satisfy the above formula (2). Fe0 In the case of a continuous annealing furnace, the amount of Fe plating on one side before annealing is changed according to the dew point DP value of the annealing process. Fe0 However, for changing the adhesion amount CW of the Fe-based electroplating layer on one side of the steel plate before annealing Fe0 The position can be manufactured under the condition of satisfying the above formula (2).

[0159] The adhesion amount CW of the Fe-based electroplated layer on one side before annealing is changed so as to satisfy the above formula (2). Fe0 When welding Si-containing cold-rolled steel sheets of different product specifications for continuous passing, it is preferable to change the adhesion amount CW of the Fe-based electroplating layer on one side before annealing according to the welding position. Fe0 Or the dew point DP. As described above, the dew point DP has poor responsiveness, so when changing the dew point DP, it is more preferable to perform feedforward control on the amount of humidification in the furnace so as to satisfy this equation.

[0160] Here, the "adhesion weight CW of Fe-based electroplating layer on one side before annealing" is Fe0 (g / m 2The "value of )" is the adhesion amount (target value) obtained under the conditions used in the Fe-based electroplating, and can also be the adhesion amount (measured value) of the Fe-based electroplating layer actually obtained. Similarly, the "value of the dew point DP" can be either the target value or the measured value.

[0161] The above describes an example of the operation of the method for manufacturing an Fe-based electroplated steel sheet. However, it can also be implemented as a method for determining the manufacturing conditions of an Fe-based electroplated steel sheet as follows: before starting the operation, the adhesion weight CW of the Fe-based electroplated layer on one side before pre-annealing is confirmed. Fe0 (g / m 2 ) and the target value of the dew point DP satisfy the above formula (2). If not, change the adhesion amount CW of the single side of the Fe-based electroplating layer before annealing in advance. Fe0 (g / m 2 ) and the target value of the dew point DP. Such a manufacturing condition determination method can be implemented as part of the process of the manufacturing method of the Fe-based electroplated steel sheet, or as a separate process.

[0162] <Electrodeposition-coated steel sheets>

[0163] In addition, according to this embodiment, it is also possible to provide an electrodeposited steel sheet further having a chemical conversion treatment film formed in contact with the above-mentioned Fe-based electroplating layer and an electrodeposition coating film formed on the chemical conversion film on the above-mentioned Fe-based electroplated steel sheet. The Fe-based electroplated steel sheet of this embodiment has excellent chemical conversion treatment properties, corrosion resistance after coating, and resistance to resistance welding cracking of the welded portion. Therefore, the electrodeposited steel sheet formed using this Fe-based electroplated steel sheet is particularly suitable for use in automotive parts. The electrodeposited steel sheet of this embodiment preferably forms a chemical conversion treatment film directly on the Fe-based electroplating layer. In other words, the electrodeposited steel sheet of this embodiment preferably has no additional coating layer other than the Fe-based electroplating layer. The types of the chemical conversion treatment film and the electrodeposition coating film are not particularly limited, and can be well-known chemical conversion treatment films and electrodeposition coating films. As the chemical conversion treatment film, a zinc phosphate film, a zirconium film, etc. can be used. As the electrodeposition coating film, there is no particular limitation as long as it is an electrodeposited film for automobiles. The thickness of the electrodeposited film varies depending on the application, but the dry film is preferably about 10 μm to 30 μm. In addition, according to this embodiment, an Fe-based electroplated steel sheet for electrodeposition coating that can be used for electrodeposition coating can also be provided.

[0164] <Method for producing electrodeposition-coated steel sheets>

[0165] Next, the manufacturing method of the above-mentioned electrodeposition-coated steel sheet is described. The above-mentioned electrodeposition-coated steel sheet can be manufactured by a manufacturing method of an electrodeposition-coated steel sheet comprising the following steps: a chemical conversion treatment step, in which a chemical conversion treatment is performed on the Fe-based electroplated steel sheet without performing an additional plating treatment, thereby obtaining a chemical conversion-treated steel sheet having a chemical conversion treatment film formed in contact with the above-mentioned Fe-based electroplating layer; an electroplating coating step, in which an electrodeposition coating is performed on the above-mentioned chemical conversion-treated steel sheet, thereby obtaining an electrodeposition-coated steel sheet having an electrodeposition coating film formed on the above-mentioned chemical conversion treatment film. The chemical conversion treatment and the electroplating coating treatment can utilize known methods. In addition, as treatments before the chemical conversion treatment is performed, degreasing treatment, water washing, and surface adjustment treatment as needed for cleaning the surface of the Fe-based electroplated steel sheet can be performed. After these pre-treatments, the chemical conversion treatment is performed. The methods of the degreasing treatment and water washing are not particularly limited, and conventional methods can be used. In the surface adjustment treatment, a surface adjustment agent having Ti colloid or zinc phosphate colloid can be used. When implementing these surface conditioning agents, no special process is required, and they can be implemented according to conventional methods. For example, the desired surface conditioning agent is dissolved in a specified deionized water, and after sufficient stirring, a treatment solution of a predetermined temperature (usually room temperature, 25 to 30°C) is prepared, and the steel plate is immersed in the treatment solution for a specified time (20 to 30 seconds). Then, the chemical conversion treatment of the next process is carried out without drying. The chemical conversion treatment can also be implemented according to conventional methods. For example, the desired chemical conversion treatment agent is dissolved in a specified deionized water, and after sufficient stirring, a treatment solution of a predetermined temperature (usually 35 to 45°C) is prepared, and the steel plate is immersed in the treatment solution for a specified time (60 to 120 seconds). As chemical conversion treatment agents, for example, zinc phosphate treatment agents for steel, zinc phosphate treatment agents for steel and aluminum, and zirconium treatment agents can be used. Then, the next process is electrolytic deposition coating. Electrolytic deposition coating can also be implemented according to conventional methods. After pretreatment such as water washing as needed, the steel sheet is immersed in a well-stirred electrodeposition coating to obtain an electrodeposition coating of the desired thickness. Both cationic and anionic electrodeposition coatings can be used as electrodeposition coatings. Depending on the intended application, a topcoat or other coating may be applied after electrodeposition coating.

[0166] <Automotive Parts>

[0167] In addition, according to the present embodiment, it is possible to provide automobile parts that are at least partially made of the above-mentioned electroplated steel sheet. The Fe-based electroplated steel sheet of the present embodiment is excellent in chemical conversion treatability, corrosion resistance after coating, and resistance to resistance welding cracking of the welded portion, so the electroplated steel sheet using the Fe-based electroplated steel sheet is particularly suitable for automobile parts. Automobile parts made of electroplated steel sheets may include steel plates other than the electroplated steel sheet of the present embodiment as blanks. The resistance to resistance welding cracking of the welded portion of the electroplated steel sheet of the present embodiment is excellent, so the automobile parts made of the Fe-based electroplated steel sheet can appropriately prevent secondary cracking of the welded portion even when the welded portion contains a high-strength hot-dip galvanized steel sheet as a welding object. The type of automobile parts that are at least partially made of electroplated steel sheets is not particularly limited, but can be, for example, side beam parts, pillar parts, automobile bodies, etc.

[0168] Hereinafter, the present invention will be described in detail based on examples.

[0169] Example 1

[0170] Steels having the chemical compositions shown in Tables 1 and 3 were melted to obtain cast slabs, which were then hot-rolled, pickled, and cold-rolled to produce cold-rolled steel sheets having a thickness of 1.6 mm.

[0171]

[0172]

[0173] Next, the cold-rolled steel sheet was degreased with an alkali solution. Following this, an electrolytic treatment was performed using the steel sheet as the cathode under the conditions shown below, producing a pre-annealed Fe-plated steel sheet with a pre-annealed Fe-plated layer on one side. The amount of the pre-annealed Fe-plated layer adhered was controlled by the duration of the current flow. Next, the pre-annealed Fe-plated steel sheet was heated to 800°C at the average heating rate shown in Tables 2-1 and 2-2. Reduction annealing was then performed using 15% H₂-N₂ in a soaking zone at 800°C, with the dew point adjusted as shown in Tables 2-1, 2-2, and 4. The reduction annealing was performed for 100 seconds.

[0174] 〔Electrolysis Conditions〕

[0175] Bath temperature: 50°C

[0176] pH: 2.0

[0177] Current density: 45A / dm 2

[0178] Fe-based electroplating bath: contains 1.5 mol / L of Fe 2+ ion

[0179] Electrode (anode): iridium oxide electrode

[0180] From the Fe-based electroplated steel sheet prepared as described above, the adhesion amount of the Fe-based electroplated layer on a single side, the number of Si intensity peaks, and the number of grain boundaries of the Fe-based electroplated layer in contact with the Si-containing cold-rolled steel sheet at the interface between the Fe-based electroplated layer and the Si-containing cold-rolled steel sheet are calculated according to the above method.

[0181] The Fe-based electroplated steel sheets prepared as described above were used to measure the C intensity by surface analysis of the sample cross section according to the above method. The average C concentration of the Fe-based electroplated layer in the range of 10 μm to 20 μm from the surface to the thickness direction and the thickness of the decarburized layer were evaluated.

[0182] Furthermore, the resistance cracking characteristics of the welded portions of the Fe-based electroplated steel sheets were investigated. The measurement and evaluation methods are shown below.

[0183] <Resistance Welding Cracking Characteristics of Welded Joints When Galvanized Steel Sheets Are Used>

[0184] For Fe-based electroplated steel sheets, the evaluation targets are: a holding time of 0.18 seconds, resistance welding cracking resistance (not a problem), Si content less than 0.50%, tensile strength of 980 MPa, and a single-sided adhesion weight of 50 g / m 2 Resistance welding cracking characteristics of the welded portion of alloyed hot-dip galvanized steel sheet (thickness 1.6mm). Figure 12 A method for evaluating the resistance welding cracking characteristics of a welded portion will be described.

[0185] A test piece 6 of 50×150 mm was cut out with the rolling direction (TD) as the long side and the rolling direction as the short side. The adhesion amount of the hot-dip galvanized layer on one side of the same size was 50 g / m 2 The plate group is assembled by overlapping the test alloyed hot-dip galvanized steel plates 5. The plate group is assembled so that the evaluation target surface (Fe-based electroplating layer) of the test piece 6 and the galvanized layer of the test alloyed hot-dip galvanized steel plate 5 face each other. The plate group is fixed to the fixing table 8 via a 2.0mm thick gasket 7. The gasket 7 is a pair of steel plates with a length of 50mm × a width of 45mm × a thickness of 2.0mm. Figure 12 As shown in (a), the long-side end faces of a pair of steel plates are aligned with the short-side end faces of the plate group. Therefore, the distance between the pair of steel plates is 60 mm. The fixing table 8 is a single plate with a hole in the center.

[0186] Next, a servo motor pressurized single-phase AC (50 Hz) resistance welding machine was used to pressurize the plate group with a pair of electrodes 9 (front end diameter: 6 mm) to bend the plate group. Under the conditions of a pressing force of 3.5 kN, a holding time of 0.18 seconds or 0.24 seconds, and a welding time of 0.36 seconds, resistance welding was performed with a welding current of a weld nugget diameter r of 5.9 mm to form a plate group with a welded portion. At this time, a pair of electrodes 9 pressurize the plate group from the top and bottom in the vertical direction, and the lower electrode pressurizes the test piece 6 through the hole of the fixed table 8. The lower electrode and the fixed table 8 are fixed so that the upper electrode is movable. In addition, the upper electrode contacts the central part of the alloyed hot-dip galvanized steel plate 5 for the test. In addition, the plate group is welded while being tilted 5° along the long side direction of the plate group relative to the horizontal direction. In addition, the holding time refers to the time from the end of the flow of the welding current to the beginning of the release of the electrode. Here, refer to Figure 12 (b) In the lower figure, the nugget diameter r refers to the distance between the ends of the nugget 10 in the longitudinal direction of the plate group.

[0187] Next, the plate assembly with the welded portion is moved along the center of the welded portion including the nugget 10. Figure 12 (b) Cut along line BB in the figure above, and observe the cross section of the weld using an optical microscope (200x magnification). The weld's resistance to resistance welding cracking was evaluated according to the following criteria. A ◎ or ○ rating indicates excellent resistance to resistance welding cracking. A × rating indicates poor resistance to resistance welding cracking.

[0188] ◎: No cracks larger than 0.1 mm were observed during the holding time of 0.18 seconds.

[0189] ○: Cracks of 0.1 mm or longer were observed at a holding time of 0.18 seconds, but no cracks of 0.1 mm or longer were observed at a holding time of 0.24 seconds.

[0190] ×: Cracks of 0.1 mm or longer were observed during the holding time of 0.24 seconds

[0191] in addition, Figure 12 (b) The following figure schematically shows cracks in test piece 6, indicated by reference numeral 11. Furthermore, when cracks occur in the test steel sheet (the alloyed hot-dip galvannealed steel sheet used for testing), stress dispersion in the evaluation steel sheets (the steel sheets of each inventive example and comparative example) is lost, preventing appropriate evaluation. Therefore, data showing no cracks in the test steel sheet are used as examples.

[0192] The results of the above tests are summarized in Tables 2-1, 2-2, and 4. These results show that the welded portions of the Fe-based electroplated steel sheets of the invention examples, in which the Fe-based electroplated layer was formed under conditions suitable for the present invention before continuous annealing, all exhibited excellent resistance to resistance welding cracking. Furthermore, for Reference Examples 1 and 2, since the Si content was less than 0.5%, there were no particular problems with chemical conversion treatability or resistance welding cracking resistance of the welded portions. In each of the invention examples satisfying equations (1) and (2), no cracks exceeding 0.1 mm in length were observed even under the holding time of 0.18 seconds, indicating that the resistance to resistance welding cracking of the welded portions was particularly excellent. In addition, for the examples in the table where no Fe-based electroplating layer is formed, the adhesion amount is expressed as "-", and the peak of the luminous intensity at the wavelength of Si (referred to as "Si intensity peak" for convenience in the table) and the number of grain boundaries of the Fe-based electroplating layer in contact with the Si-containing cold-rolled steel sheet at the interface between the Fe-based electroplating layer and the above-mentioned Si-containing cold-rolled steel sheet (referred to as "the number of grain boundaries in contact with the base iron" for convenience in the table) cannot be measured and are expressed as "-". For steel sheets with a decarburized layer thickness of less than 10 μm on the surface of the Fe-based electroplated steel sheet, the decarburized layer thickness is expressed as "-". In addition, when each variable on the left side of formula (1) and (2) is "-", the variable is calculated as 0 to calculate the left side of formula (1) and (2).

[0193]

[0194]

[0195]

[0196] Example 2

[0197] Steel having the chemical composition shown in Table 5 was melted to obtain a cast slab, which was then hot-rolled, pickled, and cold-rolled to produce a cold-rolled steel sheet having a thickness of 1.6 mm.

[0198]

[0199] Next, the cold-rolled steel sheet was degreased with an alkaline solution. Following this, an electrolytic treatment was performed using the steel sheet as the cathode under the following conditions to produce a pre-annealed Fe-plated steel sheet having a pre-annealed Fe-plated layer on one side. The amount of the pre-annealed Fe-plated layer adhered was controlled by the duration of the current flow. Next, the pre-annealed Fe-plated steel sheet was heated to 800°C at the average heating rate shown in Table 6. Reduction annealing was then performed using a 15% H₂-N₂ atmosphere with a soaking zone temperature of 800°C, with the dew point adjusted as shown in Table 6. Reduction annealing was performed for 100 seconds.

[0200] 〔Electrolysis Conditions〕

[0201] Bath temperature: 50°C

[0202] pH: 2.0

[0203] Current density: 45A / dm 2

[0204] Fe-based electroplating bath: contains 1.5 mol / L of Fe 2+ ion

[0205] Electrode (anode): iridium oxide electrode

[0206] From the Fe-based electroplated steel sheet prepared as described above, the adhesion amount of the Fe-based electroplated layer on a single side, the number of Si intensity peaks, and the number of grain boundaries of the Fe-based electroplated layer in contact with the Si-containing cold-rolled steel sheet at the interface between the Fe-based electroplated layer and the Si-containing cold-rolled steel sheet are calculated according to the above method.

[0207] The Fe-based electroplated steel sheets prepared as described above were used to measure the C strength by surface analysis of the sample cross section according to the above method. The average C concentration of the Fe-based electroplated layer in the range of 10 μm to 20 μm from the surface to the thickness direction and the thickness of the decarburized layer were evaluated.

[0208] Furthermore, the resistance cracking characteristics of the welded portions of the Fe-based electroplated steel sheets were investigated. The measurement and evaluation methods are shown below.

[0209] <Resistance Welding Cracking Characteristics of Welded Joints When Galvanized Steel Sheets Are Used>

[0210] For Fe-based electroplated steel sheets, the evaluation targets are: a holding time of 0.14 seconds, resistance welding cracking resistance, Si content less than 0.1%, tensile strength of 590 MPa, and a single-sided adhesion weight of 50 g / m 2 Resistance welding cracking characteristics of the welded portion of alloyed hot-dip galvanized steel sheet (thickness 1.6mm). Figure 12 A method for evaluating the resistance welding cracking characteristics of a welded portion will be described.

[0211] A test piece 6 of 50×150 mm was cut with the rolling perpendicular direction (TD) as the long side and the rolling direction as the short side, and a hot-dip galvanized layer of the same size was cut to a coating weight of 50 g / m 2 The plate group is assembled by overlapping the test alloyed hot-dip galvanized steel plates 5. The plate group is assembled in such a way that the evaluation target surface (Fe-based electroplating layer) of the test piece 6 faces the galvanized layer of the test alloyed hot-dip galvanized steel plate 5. The plate group is fixed to the fixing table 8 via a 2.0mm thick gasket 7. The gasket 7 is a pair of steel plates with a length of 50mm × a width of 45mm × a thickness of 2.0mm. Figure 12As shown in (a), the long-side end faces of the pair of steel plates are aligned with the short-side end faces of the plate group. Therefore, the distance between the pair of steel plates is 60 mm. The fixing table 8 is a single steel plate with a hole in the center.

[0212] Next, a servo motor pressurized single-phase AC (50 Hz) resistance welding machine was used to pressurize the plate group with a pair of electrodes 9 (front end diameter: 6 mm) to bend the plate group. Under the conditions of a pressing force of 3.5 kN, a holding time of 0.14 seconds or 0.16 seconds and a welding time of 0.36 seconds, resistance welding was performed with a welding current of a weld nugget diameter r of 5.9 mm to form a plate group with a welded portion. At this time, a pair of electrodes 9 pressurize the plate group from the top and bottom in the vertical direction, and the lower electrode pressurizes the test piece 6 through the hole of the fixed table 8. The lower electrode and the fixed table 8 are fixed so that the upper electrode is movable. In addition, the upper electrode contacts the central part of the alloyed hot-dip galvanized steel plate 5 for the test. In addition, the plate group is welded while being tilted 5° along the long side direction of the plate group relative to the horizontal direction. In addition, the holding time refers to the time from the end of the flow of the welding current to the beginning of the release of the electrode. Here, Figure 12 (b) Referring to the figure below, the nugget diameter r refers to the distance between the ends of the nugget 10 in the longitudinal direction of the plate group.

[0213] Next, the plate assembly with the welded portion is moved along the center of the welded portion including the nugget 10. Figure 12 (b) Cut along line BB in the figure above, and observe the cross section of the weld using an optical microscope (200x magnification). The weld's resistance to resistance welding cracking was evaluated according to the following criteria. A ◎ or ○ rating indicates excellent resistance to resistance welding cracking. A × rating indicates poor resistance to resistance welding cracking.

[0214] ◎: No cracks larger than 0.1 mm were observed during the holding time of 0.14 seconds.

[0215] ○: Cracks of 0.1 mm or longer were observed at a holding time of 0.14 seconds, but no cracks of 0.1 mm or longer were observed at a holding time of 0.16 seconds.

[0216] ×: Cracks of 0.1 mm or longer were observed during the holding time of 0.16 seconds

[0217] in addition, Figure 12 (b) The cracks that occurred in the test piece 6 are schematically indicated by reference numeral 11 in the figure below. Furthermore, when cracks occurred in the test steel sheet (the alloyed hot-dip galvannealed steel sheet used for testing), stress dispersion in the evaluation steel sheets (the steel sheets of each of the inventive examples and comparative examples) precluded proper evaluation. Therefore, data showing no cracks in the test steel sheet were used as examples.

[0218] The results of the above test are recorded in Table 6. From the results, it can be seen that the resistance to resistance welding cracking of the welded parts of the Fe-based electroplated steel sheets of the invention examples in which the Fe-based electroplated layer was formed under the conditions suitable for the present invention before continuous annealing was excellent. In each invention example that satisfies formulas (1) and (2), no cracks with a length of 0.1 mm or more were observed even under the condition of a holding time of 0.14 seconds, and the resistance to resistance welding cracking of the welded parts was particularly good. In addition, for the examples in the table where no Fe-based electroplated layer was formed, the adhesion amount is indicated as "-", and the peak of the luminous intensity representing the wavelength of Si (for convenience, referred to as "Si intensity peak" in the table) and the number of grain boundaries of the Fe-based electroplated layer in contact with the Si-containing cold-rolled steel sheet at the interface between the Fe-based electroplated layer and the above-mentioned Si-containing cold-rolled steel sheet (for convenience, referred to as "the number of grain boundaries in contact with the base iron" in the table) cannot be measured, so they are indicated as "-". For steel sheets with a decarburized layer thickness of less than 10 μm in the surface layer of the Fe-based electroplated steel sheet, the decarburized layer thickness is indicated as "-". When each variable on the left side of the equations (1) and (2) is "-", the left side of the equations (1) and (2) is calculated with the variable being 0.

[0219]

[0220] Industrial applicability

[0221] The Fe-based electroplated steel sheet produced by the present invention not only has excellent resistance to resistance welding cracking of the welded portion when the plate group object is a galvanized steel sheet, but also has high strength and excellent workability. It can be used not only as a blank material for automobile parts, but also as a blank material for applications requiring similar properties in the fields of home appliances, building parts, etc.

[0222] Explanation of symbols

[0223] 1 Fe-based electroplated steel sheet

[0224] 2 Si-containing cold-rolled steel sheet

[0225] 3 Fe-based electroplating layer

[0226] 5 Test alloyed hot-dip galvanized steel sheet

[0227] 6 test pieces

[0228] 7 Gasket

[0229] 8 fixed platform

[0230] 9 electrodes

[0231] 10 nugget

[0232] 11 Crack

Claims

1. An Fe-based electroplated steel sheet having: Si-containing cold-rolled steel sheet containing 0.1 mass % to 3.0 mass % of Si, and The Fe-based electroplating layer is formed on at least one side of the Si-containing cold-rolled steel sheet, and the coating weight on the single side is 5.0 g / m 2 above; The surface layer of the Fe-based electroplated steel sheet is a decarburized layer, and the thickness of the decarburized layer is greater than 30 μm. In the intensity distribution measured by glow discharge emission spectroscopy, a peak of luminescence intensity representing the wavelength of Si is detected in a range of more than 0.2 μm from the surface of the Fe-based electroplating layer in the plate thickness direction and not more than the thickness of the Fe-based electroplating layer. The average carbon concentration of the Fe-based electroplated layer in the range of 10 μm to 20 μm from the surface in the thickness direction is 0.10 mass % or less. At the interface between the Fe-based electroplated layer and the Si-containing cold-rolled steel sheet, the number of grain boundaries of the Fe-based electroplated layer in contact with the Si-containing cold-rolled steel sheet is 10 or more per 10 μm in the width direction of the Si-containing cold-rolled steel sheet in the observation field, The adhesion amount CW of the Fe-based electroplating layer on one side Fe1 The thickness of the decarburized layer C d Satisfies the following formula (1), where CW Fe1 The unit is g / m 2 , C d The unit is μm, 1.6×(C.W. Fe1 )+(C d )≥77・・・(1)。 2. The Fe-based electroplated steel sheet according to claim 1, wherein The Si-containing cold-rolled steel sheet contains 0.50 mass % to 3.0 mass % of Si.

3. The Fe-based electroplated steel sheet according to claim 1 or 2, wherein: The Si-containing cold-rolled steel sheet has the following composition in addition to the Si: in terms of mass %, it contains C: less than 0.8%, Mn: 1.0% to 12.0%, P: less than 0.1%, S: less than 0.03%, N: less than 0.010% and Al: less than 1.0%, with the remainder being Fe and unavoidable impurities.

4. The Fe-based electroplated steel sheet according to claim 3, wherein: The above-mentioned component composition further contains, in terms of mass%, 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: 0.020% 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.

5. The Fe-based electroplated steel sheet according to claim 1 or 2, wherein: The Fe-based electroplating layer has the following composition Composition: Contains a total of less than 10% by mass of one or more elements selected from B, C, P, N, O, Ni, Mn, Mo, Zn, W, Pb, Sn, Cr, V and Co, and the remainder is composed of Fe and unavoidable impurities. 6 . An electrodeposition-coated steel sheet comprising the Fe-based electroplated steel sheet according to claim 1 , further comprising a chemical conversion coating formed in contact with the Fe-based electroplated layer and an electrodeposition coating formed on the chemical conversion coating.

7. An automobile part, at least a portion of which is formed using the electrodeposition-coated steel sheet according to claim 6.

8. A method for manufacturing an electrodeposited coated steel sheet, comprising the following steps: a chemical conversion treatment step of subjecting the Fe-based electroplated steel sheet according to any one of claims 1 to 5 to a chemical conversion treatment without performing an additional plating treatment, thereby obtaining a chemical conversion treated steel sheet having a chemical conversion treatment film formed in contact with the Fe-based electroplated layer; The electrodeposition coating step is to perform an electrodeposition coating on the chemical conversion treated steel sheet to obtain an electrodeposition coated steel sheet having an electrodeposition coating film formed on the chemical conversion treatment film.

9. A method for producing an Fe-based electroplated steel sheet, the method for producing an Fe-based electroplated steel sheet according to any one of claims 1 to 5, A cold rolled steel sheet containing 0.1 mass % to 3.0 mass % of Si is subjected to Fe-based electroplating to form a coating having a deposition weight of 5.0 g / m on at least one surface. 2 The above Fe-based electroplated steel sheet before annealing of the Fe-based electroplated layer before annealing, Next, the Fe-based electroplated steel sheet before annealing is heated at an average heating rate of 10°C / second or more in a temperature range of 400°C to 650°C, maintained in an annealing atmosphere with a dew point greater than -30°C in the heated temperature range, and then cooled to obtain a Fe-based electroplated steel sheet. in, The adhesion amount CW of the Fe-based electroplating layer on one side before annealing Fe0 The dew point DP satisfies the following formula (2), where CW Fe0 The unit is g / m 2 , (C.W. Fe0 )+(D.P.)≥0・・・(2)。

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