Manufacturing methods for galvanized steel sheets, electrodeposited coated steel sheets, automotive parts, and galvanized steel sheets.

Abstract

This invention provides a steel sheet with excellent resistance to resistance welding cracking in welded parts. The galvanized steel sheet comprises: a Si-containing cold-rolled steel sheet containing 0.1% to 3.0% by mass of Si; and an Fe-based electroplating layer formed on at least one side of the Si-containing cold-rolled steel sheet, wherein the adhesion amount on one side is 5.0 g / m². 2 The above; a zinc plating layer, which is formed on the above-mentioned Fe-based electroplating layer; and, in the intensity distribution measured by glow discharge emission spectroscopy, (I Si，Fe ) / (I Si，bulk The average C concentration in the range of 10 μm to 20 μm from the interface between the zinc plating layer and the Fe-based electroplating layer toward the Fe-based electroplating layer is 0.10% by mass or less.

Description

Technical Field

[0001] This invention relates to galvanized steel sheets with excellent resistance to resistance welding cracking, electrodeposited coated steel sheets, automotive parts, methods for manufacturing electrodeposited coated steel sheets, and methods for manufacturing galvanized steel sheets. Background Technology

[0002] In recent years, from the perspective of protecting the Earth's environment, there has been a strong demand to improve automobile fuel efficiency. Furthermore, from the perspective of ensuring the safety of occupants in the event of a collision, there has also been a strong demand to improve automobile safety. To meet these demands, it is necessary to achieve both lightweight and high strength in automobile bodies. In the cold-rolled steel sheets used as raw materials for automobile components, the trend towards thinner walls based on high strength is actively developing. However, since most automobile components are manufactured by forming steel sheets, these steel sheets, in addition to high strength, also require excellent formability.

[0003] Various methods exist for improving the strength of steel sheets. Among them, solid solution strengthening by adding silicon (Si) is an example of a method that achieves high strength without severely compromising the formability of the steel sheet. On the other hand, in the manufacture of automotive parts, pressure-formed components are mostly assembled by resistance welding (spot welding). When resistance-welded components contain high-strength galvanized steel sheets, residual stress is generated near the weld area during resistance welding. This causes the zinc coating to melt and diffuse into the grain boundaries, resulting in liquid metal embrittlement (LME), and grain boundary cracking (LME cracking) in the steel sheet. Cracking occurs particularly when resistance welding is performed with the welding electrode at an angle to the steel sheet, as residual stress increases. Considering that residual stress increases with the increase of steel sheet strength, there is concern about LME cracking occurring with the increase of steel sheet strength. This problem of LME cracking is particularly significant in steel sheets containing Si.

[0004] Based on the above, we are seeking high-strength steel plates with excellent resistance to resistance welding cracking characteristics in the welded parts.

[0005] Previously, strategies for improving the above-mentioned problems have been reported. For example, Patent Document 1 discloses a hot-dip galvanized steel sheet having an internal oxide layer from the surface of the base material to a depth of 5.0 μm or more, wherein at least a portion of the grain boundaries are covered by oxides, and the grain boundary coverage of the oxides in the region from the surface of the base material to a depth of 5.0 μm is 60% or more.

[0006] Existing technical documents

[0007] Patent documents

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

[0009] In the steel plate described in Patent Document 1, because the depth of the internal oxide layer, i.e. the grain boundary oxide, is too large, it is difficult to completely suppress cracking during resistance welding.

[0010] Thus, in reality, no galvanized steel sheet has yet been developed that meets the high standards for resistance welding crack resistance characteristics of welded parts.

[0011] The present invention was made in view of the above-mentioned problems of galvanized steel sheets, and its object is to provide a steel sheet with excellent resistance to resistance welding cracking characteristics of the welded parts.

[0012] Previously, the inventors attempted to form an Fe-based electroplating layer as a protective layer to suppress grain boundary oxidation depth, improving resistance to weld cracking characteristics solely by controlling the surface layer on the steel plate side. However, in reality, it is difficult to achieve high-level resistance to weld cracking characteristics simply by controlling the surface layer on the steel plate side. Therefore, a scheme to control the characteristics of the Fe-based electroplating layer through heat treatment was considered. Furthermore, in order to solve the above-mentioned problem, the inventors conducted repeated and in-depth research and found that in order to achieve high-level resistance to weld cracking characteristics in the welded part, it is important to pre-plat the Si-containing cold-rolled steel plate before forming a zinc coating on the surface after continuous annealing following cold rolling, with an adhesion amount of 5.0 g / m² on one side. 2 The above-mentioned Fe-based electroplating layer is used to produce Fe-based electroplated steel sheet before annealing. This Fe-based electroplated steel sheet is then subjected to a heating process at an average heating rate of 10°C / second or higher between 400°C and 650°C, followed by an annealing process at an atmosphere with a dew point greater than -30°C. This process causes internal oxides to form at the grain boundaries of the Fe-based electroplating layer, and at the interface between the Fe-based electroplating layer and the Si-containing cold-rolled steel sheet, the number of grain boundaries in contact between the Fe-based electroplating layer and the Si-containing cold-rolled steel sheet is 10 or more per 10 μm width of the Si-containing cold-rolled steel sheet. The adhesion amount on one side of the cold-rolled steel sheet is 5.0 g / m. 2The above process forms a soft Fe-based electroplated layer, which alleviates the stress applied to the surface of the Si-containing cold-rolled steel sheet during welding. Furthermore, by making the average heating rate in the temperature range of 400°C to 650°C during the heating process 10°C / second or more, the growth of grains in the Fe-based electroplated layer during the heating process is strongly suppressed. Moreover, by making the dew point of the atmosphere in the subsequent annealing process greater than -30°C, the Si diffused from the cold-rolled steel sheet to the Fe-based electroplated layer during annealing is oxide inside the Fe-based electroplated layer, which acts as a Si depletion layer, suppressing the reduction in toughness caused by Si solid solution. Furthermore, the number of grain boundaries of the Fe-based electroplated layer in contact with the interface between the Fe-based electroplated layer and the cold-rolled steel sheet is more than 10 per 10μm width of the Si-containing cold-rolled steel sheet, that is, the grains of the Fe-based electroplated layer in contact with the interface between the cold-rolled steel sheet and the Fe-based electroplated layer are refined. As a result, the intrusion path of molten zinc into the Fe-based electroplated layer is dispersed. As a result, it was found that the time it takes for molten zinc to reach the grain boundaries of the cold-rolled steel sheet during welding can be delayed, thereby improving the resistance to resistance welding cracking characteristics of the welded part. Furthermore, in the aforementioned annealing process, the average C concentration in the range of 10 μm to 20 μm in the thickness direction from the interface between the zinc plating layer and the Fe-based electroplating layer toward the Fe-based electroplating layer is 0.10% by mass or less. Thus, by keeping the average C concentration in the range of 10 μm to 20 μm in the thickness direction from the interface between the zinc plating layer and the Fe-based electroplating layer toward the Fe-based electroplating layer to 0.10% by mass or less, the resistance to resistance welding cracking characteristics can be further improved. The inventors discovered that by further reducing the C concentration in the range of 10 μm to 20 μm in the thickness direction from the interface between the zinc plating layer and the Fe-based electroplating layer toward the Fe-based electroplating layer when forming the Fe-based electroplating layer before annealing, the effect of improving resistance to resistance welding cracking characteristics can be obtained more effectively, thus completing the present invention.

[0013] This invention is based on the above insights. Specifically, the essence of this invention is as follows.

[0014] [1] A galvanized steel sheet, comprising:

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

[0016] An Fe-based electroplated layer is formed on at least one side of the aforementioned Si-containing cold-rolled steel sheet, with an adhesion amount of 5.0 g / m² on each side. 2 above;

[0017] A zinc plating layer is formed on the aforementioned Fe-based electroplating layer;

[0018] In the intensity distribution measured using glow discharge emission spectroscopy, the average Si intensity (I) from the interface between the zinc plating layer and the Fe-based electroplating layer to the interface between the Fe-based electroplating layer and the Si-containing cold-rolled steel sheet is... Si，FeDivide by the average Si strength (I) in Si-containing cold-rolled steel sheets Si，bulk The value obtained (I) Si，Fe ) / (I Si，bulk The value is above 0.50.

[0019] The average C concentration in the range of 10 μm to 20 μm along the thickness direction from the interface between the zinc plating layer and the Fe-based electroplating layer toward the Fe-based electroplating layer is 0.10% by mass or less.

[0020] At the interface between the Fe-based electroplated layer and the Si-containing cold-rolled steel sheet, the number of grain boundaries in contact between the Fe-based electroplated layer and the Si-containing cold-rolled steel sheet is more than 10 per 10 μm in the width direction of the steel sheet in the field of view of the Si-containing cold-rolled steel sheet.

[0021] [2] According to the galvanized steel sheet described in [1] above, wherein the above-mentioned Si-containing cold-rolled steel sheet contains 0.50% to 3.0% by mass of Si.

[0022] [3] According to the galvanized steel sheet described in [1] or [2] above, a decarburized layer is provided on the Fe-based electroplating layer side starting from the interface between the galvanized layer and the Fe-based electroplating layer.

[0023] [4] According to the galvanized steel sheet described in [3] above, the thickness of the decarburized layer is 30 μm or more.

[0024] [5] According to the galvanized steel sheet described in [3] or [4] above, wherein the adhesion amount CW of the Fe-based electroplating layer on one side is... Fe1 (g / m 2 ) and the thickness C of the aforementioned decarburized layer d (μm) satisfies the following equation (1).

[0025] 1.6×(CW Fe1 )+(C d )≥77···(1)

[0026] [6] The galvanized steel sheet according to any one of [1] to [5] above, wherein the Si-containing cold-rolled steel sheet, in addition to the Si mentioned above, has the following composition: containing, by mass %, 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 consisting of Fe and unavoidable impurities.

[0027] [7] According to the galvanized steel sheet described in [6] above, wherein the above composition further contains, by mass %, one or more of the following: 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 0.020%, 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%.

[0028] [8] According to any one of [1] to [7] above, the Fe-based electroplated layer has the following composition, containing 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.

[0029] [9] An electrodeposited coated steel sheet, wherein the galvanized steel sheet described in any one of [1] to [8] above further has a chemical conversion treatment film and an electrodeposited coating film formed on the chemical conversion treatment film.

[0030]

[10] An automotive component, at least a portion of which is made of the electrodeposited coated steel sheet described above [9].

[0031]

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

[0032] The chemical conversion treatment process involves subjecting the galvanized steel sheet described in any one of [1] to [8] above to a chemical conversion treatment process to obtain a chemically converted steel sheet with a chemically converted coating.

[0033] In the electrodeposition coating process, the above-mentioned chemically converted steel plate is subjected to electrodeposition coating treatment to obtain an electrodeposition coated steel plate with an electrodeposition coating film formed on the above-mentioned chemically converted film.

[0034]

[12] A method for manufacturing galvanized steel sheet, wherein a cold-rolled steel sheet containing 0.1% to 3.0% by mass of Si is subjected to Fe-based electroplating treatment to produce a coating with an adhesion amount of 5.0 g / m on at least one side. 2 The above-mentioned Fe-based electroplated steel sheet before annealing, with Fe-based electroplating layer before annealing.

[0035] Next, the Fe-based electroplated steel sheet before annealing is heated at an average heating rate of 10°C / second or higher in a temperature range of 400°C to 650°C. After heating, it is held in an atmosphere with a dew point greater than -30°C and then cooled to produce an Fe-based electroplated steel sheet.

[0036] Next, the Fe-based electroplated steel sheet is galvanized to obtain a galvanized steel sheet.

[0037]

[13] According to the manufacturing method of the galvanized steel sheet described in

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

[0038]

[14] According to the manufacturing method of galvanized steel sheet described in

[12] or

[13] above, wherein the adhesion amount CW of the Fe-based electroplated layer on one side before annealing is... Fe0 (g / m 2 The above dew point (DP) and the above dew point satisfy the following equation (2).

[0039] (CW Fe0 )+(DP)≥0···(2)

[0040]

[15] The method for manufacturing galvanized steel sheet according to any one of

[12] to

[14] above, wherein the above-mentioned Fe-based electroplating is performed 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, such that the total content of these elements in the Fe-based electroplating layer before annealing is 10% by mass or less.

[0041] According to the present invention, a galvanized steel sheet with excellent resistance to resistance welding cracking characteristics of the welded part can be provided. Attached Figure Description

[0042] Figure 1 It is a diagram showing a summary of the cross-section of a galvanized steel sheet.

[0043] Figure 2A This is a figure representing an example of the raw data showing the intensity distribution of luminescence intensity at wavelengths representing Si and Zn, analyzed using glow discharge emission spectroscopy.

[0044] Figure 2B This is a figure representing an example of raw data showing the intensity distribution of the luminescence intensity of Si and Zn analyzed using glow discharge emission spectroscopy.

[0045] Figure 2C Peak Intensity I Zn The diagram illustrates the definition method.

[0046] Figure 3These are (a) a perspective view and (b) an A-A cross-sectional view of a sample used for observation to determine 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 coating and the Si-containing cold-rolled steel sheet.

[0047] Figure 4 This diagram illustrates the method for determining the number of grain boundaries at the interface between an Fe-based electroplated layer and a Si-containing cold-rolled steel sheet.

[0048] Figure 5 yes Figure 4 An enlarged view of the quadrilateral-enclosed portion.

[0049] Figure 6 This is a diagram showing an observation image of the interface between the Fe-based electroplated layer and the cold-rolled steel sheet in Example No. 37.

[0050] Figure 7 It is a diagram showing the boundary line and the location of grain boundaries at the interface between the Fe-based electroplated layer and the Si-containing cold-rolled steel sheet in Invention Example No. 37.

[0051] Figure 8 This is a diagram showing an observation image of the interface between the Fe-based electroplated layer and the Si-containing cold-rolled steel sheet of Invention Example No. 39.

[0052] Figure 9 It is a diagram showing the boundary lines and the positions of grain boundaries at the interface between the Fe-based electroplated layer and the cold-rolled steel sheet in Invention Example No. 39.

[0053] Figure 10 (a) is a graph showing an example of the raw data of the distribution of C concentration along the thickness direction of the plate as analyzed by an electron beam microanalyzer, and (b) is a graph showing an example of the data after smoothing.

[0054] Figure 11 (a) is a diagram illustrating the evaluation method for the resistance to welding cracking characteristics of the welded part, (b) is a top view of the welded plate assembly in this evaluation, and (c) is a B-B cross-sectional view of the top view. Detailed Implementation

[0055] The aforementioned LME cracking can be broadly categorized into "cracking occurring on the surface in contact with the electrode (hereinafter, surface cracking)" and "cracking occurring near the ductile metal ring region between steel plates (hereinafter, internal cracking)." It is known that surface cracking is easily induced in resistance welding in high-current domains that generate spatter, and surface cracking can be suppressed by achieving a suitable current range that prevents spattering. On the other hand, internal cracking can occur even when the current during resistance welding is within a suitable range that prevents spattering. Furthermore, surface cracking is easily detected during visual inspection in the manufacturing process, while internal cracking is difficult to detect during visual inspection. For these reasons, internal cracking is a particularly significant problem in LME cracking. When resistance welding is performed with the welding electrode at a certain angle to the steel plate, residual stress increases, potentially leading to internal cracking. It is believed that residual stress increases with the strengthening of the steel plate, and therefore, internal cracking may occur along with the strengthening of the steel plate. This disclosure improves the resistance welding cracking resistance characteristics, and in particular, improves the characteristics that prevent internal cracking.

[0056] The embodiments of the present invention will be described below.

[0057] Furthermore, in the following description, the content of each element in the composition of Si-containing cold-rolled steel sheet and the content of each element in the coating composition are all expressed in "mass%" unless otherwise specified. Additionally, in this specification, the numerical range indicated by "~" includes the values ​​listed before and after "~" as the lower and upper limits. Furthermore, in this specification, "high strength" steel sheet refers to a tensile strength TS of 590 MPa or higher as measured according to JIS Z 2241 (2011).

[0058] Figure 1 A schematic cross-section of the galvanized steel sheet 1 of this embodiment is shown. Figure 1 As shown, the galvanized steel sheet 1 has at least one side of the Si-containing cold-rolled steel sheet 2 having an Fe-based electroplating layer 3 and a zinc-plated layer 4 formed on the Fe-based electroplating layer. First, the composition of the Si-containing cold-rolled steel sheet will be explained.

[0059] Si: 0.1%–3.0%

[0060] Si (Si) significantly enhances the strength of steel through solid solution without significantly impairing processability (solid solution strengthening ability), making it an effective element for achieving high strength in steel sheets. However, Si also negatively impacts the resistance to welding cracking characteristics of welded sections. When adding Si to achieve high strength in steel sheets, an addition of 0.1% or more is required. Furthermore, as described later, Si can form internal oxides at the grain boundaries of Fe-based electroplating layers. However, when Si content is less than 0.50%, the resistance to welding cracking characteristics of welded sections in welding with a holding time of approximately 0.24 seconds are not particularly problematic. However, from a production cost perspective, the production interval during spot welding in the assembly process of automotive parts becomes a challenge. Even with Si content less than 0.50%, the resistance to welding cracking characteristics in welded sections may be insufficient when measures are taken to reduce holding time. On the other hand, if the Si content is greater than 3.0%, hot-rollability and cold-rollability are significantly reduced, potentially negatively impacting productivity or leading to a decrease in the ductility of the steel sheet itself. Therefore, Si is added in the range of 0.1% to 3.0%. The amount of Si is preferably 0.50% or more, more preferably 0.7% or more, and even more preferably 0.9% or more. In addition, the amount of Si is preferably 2.5% or less, more preferably 2.0% or less, and even more preferably 1.7% or less.

[0061] In the Si-containing cold-rolled steel sheet of this embodiment, the presence of Si within the above-mentioned range is a necessary requirement. Other components are permissible as long as they fall within the composition range typically found in cold-rolled steel sheets, and there are no particular limitations. However, when the Si-containing cold-rolled steel sheet of this embodiment has a high strength of 590 MPa or more in tensile strength (TS), the following composition is preferred.

[0062] C: Below 0.8% (excluding 0%)

[0063] C improves workability by forming martensite and other structures within the steel. When C is present, the amount of C is preferably 0.8% or less, more preferably 0.3% or less, to obtain good weldability. There is no particular limitation on the lower limit of C, but to obtain good workability, the amount of C is preferably greater than 0%, more preferably 0.03% or more, and even more preferably 0.08% or more.

[0064] Mn: 1.0%~12.0%

[0065] Mn is an element that strengthens steel through solid solution treatment, increases its strength, improves hardenability, and promotes the formation of retained austenite, bainite, and martensite. This effect is achieved by adding 1.0% or more of Mn. Conversely, if the Mn content is 12.0% or less, the aforementioned effects can be obtained without increasing costs. Therefore, the Mn content is preferably 1.0% or more, more 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. Furthermore, the Mn content is more preferably 3.5% or less, more preferably 3.3% or less.

[0066] P: Below 0.1% (excluding 0%)

[0067] By controlling the phosphorus (P) content, a decrease in weldability can be prevented. Furthermore, preventing P segregation at grain boundaries helps prevent deterioration in ductility, flexibility, and toughness. Additionally, adding a large amount of P promotes ferrite transformation and increases crystal size. Therefore, the P content is preferably 0.1% or less. There is no particular lower limit for the P content; considering production technology constraints, it can be greater than 0% or more than 0.001%.

[0068] S: Less than 0.03% (excluding 0%)

[0069] The sulfur content is preferably 0.03% or less, more preferably 0.02% or less. By suppressing the sulfur content, it is possible to prevent a decrease in weldability and a decrease in hot ductility, suppress hot cracking, and significantly improve the surface shape. Furthermore, by suppressing the sulfur content, coarse sulfides are formed as impurity elements, which can prevent a decrease in the ductility, bendability, and tensile flangeability of the steel sheet. These problems become significant when the sulfur content is greater than 0.03%, and the sulfur content is preferably minimized. There is no particular limitation on the lower limit of sulfur; considering production technology constraints, it can be greater than 0% or more than 0.0001%.

[0070] N: Less than 0.010% (excluding 0%)

[0071] The nitrogen (N) content is preferably 0.010% or less. With an N content of 0.010% or less, N forms coarse nitrides with Ti, Nb, and V at high temperatures, thereby preventing damage to the high-strength effect of the steel sheet caused by the addition of Ti, Nb, and V. Furthermore, an N content of 0.010% or less also prevents a decrease in toughness. Moreover, an N content of 0.010% or less prevents slab cracking and surface defects during hot rolling. The N content is preferably 0.005% or less, more preferably 0.003% or less, and even more preferably 0.002% or less. There is no particular limitation on the lower limit of the N content; considering production technology constraints, it can be greater than 0% or more than 0.0005%.

[0072] A1: Below 1.0% (excluding 0%)

[0073] Al is thermodynamically the most readily oxidized, thus oxidizing before Si and Mn. This inhibits the oxidation of Si and Mn on the outermost layer of the steel sheet and promotes the oxidation of Si and Mn within the steel sheet. This effect is achieved when the Al content is 0.01% or higher. On the other hand, if the Al content exceeds 1.0%, it leads to increased costs. Therefore, when adding Al, the Al content is preferably 1.0% or less. More preferably, the Al content is 0.1% or less. There is no particular limitation on the lower limit of Al; it can be greater than 0% or more, and can be 0.001% or higher.

[0074] The composition may further contain one or more of the following components: 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%.

[0075] B: Below 0.005%

[0076] Boron (B) is an effective element for improving the hardenability of steel. To improve hardenability, the amount of B is preferably 0.0003% or more, more preferably 0.0005% or more. However, if B is added in excess, the formability will decrease, and the amount of B is preferably 0.005% or less.

[0077] Ti: below 0.2%

[0078] Ti is effective in precipitation strengthening of steel. There is no particular limit to the lower limit of Ti, but in order to achieve the effect of strength adjustment, it is preferable to be 0.005% or more. However, if too much Ti is added, the hard phase becomes too large and the formability deteriorates. Therefore, when adding Ti, the amount of Ti is preferably 0.2% or less, and more preferably 0.05% or less.

[0079] Cr: less than 1.0%

[0080] The Cr content is preferably 0.005% or more. By ensuring the Cr content is 0.005% or more, hardenability can be improved, and the balance between strength and ductility can be enhanced. When adding Cr, from the viewpoint of preventing increased costs, the Cr content is preferably 1.0% or less.

[0081] Cu: below 1.0%

[0082] The Cu content is preferably 0.005% or more. By making the Cu content 0.005% or more, the formation of the residual γ phase can be promoted. In addition, from the viewpoint of preventing cost increases, the Cu content is preferably 1.0% or less.

[0083] Ni: below 1.0%

[0084] The Ni content is preferably 0.005% or more. By making the Ni content 0.005% or more, the formation of the residual γ phase can be promoted. In addition, from the viewpoint of preventing cost increases, the Ni content is preferably 1.0% or less when adding Ni.

[0085] Mo: 1.0% or less

[0086] The Mo content is preferably 0.005% or more. By making the Mo content 0.005% or more, the strength adjustment effect can be obtained. The Mo content is more preferably 0.05% or more. In addition, from the viewpoint of preventing cost increase, the Mo content is preferably 1.0% or less when adding Mo.

[0087] Nb: below 0.20%

[0088] The strength is improved by containing 0.005% or more of Nb. Furthermore, from the viewpoint of preventing increased costs, the amount of Nb is preferably 0.20% or less.

[0089] V: Below 0.5%

[0090] The strength is improved by containing 0.005% or more of V. Furthermore, from the viewpoint of preventing increased costs, the amount of V is preferably 0.5% or less when it is present.

[0091] Sb: below 0.020%

[0092] Sb can be included from the viewpoint of suppressing oxidation of the steel plate surface. By suppressing the oxidation of the steel plate, Sb improves the wettability of the plating. To achieve this effect, the Sb content is preferably 0.001% or more. On the other hand, Sb suppresses the formation of a decarburized layer. To obtain good resistance to welding cracking characteristics, the Sb content is preferably 0.020% or less. The Sb content is more preferably 0.015% or less, and even more preferably 0.012% or less.

[0093] Ta: below 0.1%

[0094] Ta increases strength by containing 0.001% or more. Furthermore, from the viewpoint of preventing cost increases, the amount of Ta is preferably 0.1% or less when it is present.

[0095] W: below 0.5%

[0096] W increases strength by containing 0.005% or more. Furthermore, from the viewpoint of preventing cost increases, the amount of W is preferably 0.5% or less when it is present.

[0097] Zr: below 0.1%

[0098] Zr content of 0.0005% or more results in increased strength. Furthermore, from the viewpoint of preventing increased costs, the Zr content is preferably 0.1% or less when Zr is present.

[0099] Sn: below 0.20%

[0100] Sn is an effective element for inhibiting the reduction of steel strength by suppressing denitrification and deboration. To achieve this effect, the 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.

[0101] Ca: below 0.005%

[0102] By controlling the morphology of sulfides with a content of 0.0005% or more, Ca can improve ductility and toughness. Furthermore, from the viewpoint of obtaining good ductility, the Ca content is preferably 0.005% or less.

[0103] Mg: less than 0.005%

[0104] By controlling the morphology of sulfides with Mg at a concentration of 0.0005% or more, ductility and toughness can be improved. Furthermore, from the viewpoint of preventing cost increases, the Mg content is preferably 0.005% or less when Mg is present.

[0105] REM: below 0.005%

[0106] REM, by controlling the morphology of sulfides with 0.0005% or more, can improve ductility and toughness. Furthermore, from the viewpoint of obtaining good toughness, the amount of REM is preferably 0.005% or less when it contains REM.

[0107] In this embodiment, the remaining components of the Si-containing cold-rolled steel sheet, other than those described above, are Fe and unavoidable impurities.

[0108] Next, the Fe-based electroplating layer formed on at least one side of the Si-containing cold-rolled steel sheet described above will be explained.

[0109] Fe-based electroplating layer: 5.0 g / m 2 above

[0110] It is estimated that the amount of material adhering to one side is 5.0 g / m². 2The above-mentioned Fe-based electroplating layer functions as a soft layer, mitigating the stress applied to the steel plate surface during welding and reducing residual stress in the resistance weld area. This improves the resistance welding crack resistance of the weld area, particularly enhancing its ability to prevent internal cracking (stress mitigation effect). Furthermore, by setting the dew point above -30°C, Si diffused from the steel plate into the Fe-based electroplating layer during annealing forms as oxides within the Fe-based electroplating layer, reducing the amount of dissolved Si. This results in a steel plate with excellent resistance welding crack resistance of the weld area. The adhesion amount on one side is 5.0 g / m². 2 The mechanism by which the Fe-based electroplating layer improves the resistance to resistance welding cracking in the welded section is not yet clear. However, it is believed that when the amount of dissolved Si on the steel plate surface is high, the toughness of the welded section decreases, and its resistance to resistance welding cracking deteriorates. Conversely, it is believed that when the Fe-based electroplating layer on the steel plate surface has a certain amount and is controlled at a certain dew point, oxides form inside the Fe-based electroplating layer, acting as a Si depletion layer. This reduces the amount of dissolved Si in the welded section, thus suppressing the decrease in weld toughness and improving the resistance to resistance welding cracking, especially improving the ability to prevent internal cracking (toughness reduction suppression effect). On the other hand, after the formation of the Fe-based electroplating layer, when the Fe-based electroplated steel plate is annealed in a low dew point atmosphere below -30°C, the grains of the Fe-based electroplating layer may coarsen. Therefore, molten zinc can easily invade the grain boundaries of the Si-containing cold-rolled steel plate 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, Si diffusing from the Si-containing cold-rolled steel sheet to the Fe-based electroplating layer during annealing is formed as internal oxides at the grain boundaries of the Fe-based electroplating layer. These internal oxides of Si (hereinafter also referred to as Si internal oxides) hinder the crystal growth of the Fe-based electroplating layer during the annealing process, resulting in a finer grain structure in the Fe-based electroplating layer. It is believed that by forming a large number of grain boundaries in the Fe-based electroplating layer through grain refinement, the path of molten zinc intrusion during resistance welding is dispersed, delaying the time it takes for molten zinc to reach the grain boundaries of the Si-containing cold-rolled steel sheet during resistance welding, thus improving the resistance welding crack resistance of the welded portion, particularly improving the characteristics preventing internal cracking (zinc grain boundary intrusion inhibition effect). The contributions of these Fe-based electroplating layers to the resistance welding crack resistance characteristics—including stress mitigation, toughness reduction inhibition, and zinc grain boundary intrusion inhibition effects—are complex and have not yet been quantitatively elucidated; it is believed that they improve resistance welding crack resistance characteristics through a composite effect. To improve the resistance to resistance welding cracking of the welded joint, the adhesion amount of the Fe-based electroplated layer on one side must be 5.0 g / m. 2 The above describes the upper limit of the Fe-based electroplating layer's adhesion on one side. While there is no specific limit, from a cost perspective, a preferred adhesion amount of 60 g / m² on one side of the Fe-based electroplating layer is... 2The following is a preferred adhesion amount for Fe-based electroplated layers: 50 g / m². 2 The following is more preferably 40g / m 2 The following is a further preferred value: 30g / m 2 The following is preferred: Fe-based electroplated steel sheet has an Fe-based electroplating layer on both sides of a Si-containing cold-rolled steel sheet. The Fe-based electroplating layer adhesion amount is set to 5.0 g / m². 2 Above, and further greater than 5.0 g / m 2 The welded parts exhibit exceptional resistance to resistance welding cracking. The adhesion amount of the Fe-based electroplated layer on one side can reach 7.0 g / m². 2 Above, 10.0g / m 2 above.

[0111] Furthermore, the thickness of the Fe-based electroplated layer was measured as follows. A 10×15mm sample was taken from a galvanized steel sheet and embedded in resin to create a cross-section of the embedded sample. Using a scanning electron microscope (SEM) at an accelerating voltage of 15kV and magnifications of 2000–10000x based on the thickness of the Fe-based electroplated layer, three arbitrary locations on this cross-section were observed. The thickness of the Fe-based electroplated layer on one side was calculated by multiplying the average thickness of the three fields of view by the density of iron.

[0112] As Fe-based electroplating layers, in addition to pure Fe, alloy plating layers such as Fe-B alloys, Fe-C alloys, Fe-P alloys, Fe-N alloys, Fe-O alloys, Fe-Ni alloys, Fe-Mn alloys, Fe-Mo alloys, and Fe-W alloys can also be used. The composition of Fe-based electroplating layers is not particularly limited, but preferably contains one or more elements selected from B, C, P, N, O, Ni, Mn, Mo, Zn, W, Pb, Sn, Cr, V, and Co, totaling 10% or less, with the remainder consisting of Fe and unavoidable impurities. By ensuring that the amount of elements other than Fe is 10% or less in total, a decrease in electrolysis efficiency can be prevented, allowing for the formation of Fe-based electroplating layers at low cost. In the case of Fe-C alloys, the C content is preferably 0.08% or less by mass.

[0113] Fe-based electroplated layers contain Si internal oxides at at least a portion of the grain boundaries. It is believed that these Si internal oxides hinder crystal growth in the Fe-based electroplated layer during the annealing process, resulting in a finer crystal grain structure. Consequently, numerous grain boundaries are formed within the Fe-based electroplated layer, dispersing the intrusion path of 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 welding crack resistance of the weld, particularly enhancing the prevention of internal cracking.

[0114] The presence or absence of Si internal oxides in Fe-based electroplated layers is determined as follows: When analyzing the luminescence intensity (representing the wavelength of Si) along the depth direction (thickness direction) from the surface of the Fe-based electroplated layer using glow discharge optical emission spectroscopy (GD-OES), the average Si intensity (IL) from the interface between the zinc plating layer and the Fe-based electroplated layer to the interface between the Fe-based electroplated layer and the Si-containing cold-rolled steel sheet is used. Si，Fe The average Si intensity (I) in the thickness direction within a range of 20 μm ± 0.5 μm from the interface between the Fe-based electroplated layer and the Si-containing cold-rolled steel sheet towards the Si-containing cold-rolled steel sheet side. Si，bulk The ratio of (I) Si，Fe / I Si，bulk Whether it is above 0.50. The measurement conditions are: Ar gas pressure 600Pa, high frequency output 35W, measurement diameter 4mmΦ, and sampling interval 0.1 seconds. For any average Si intensity, the average value of all Si intensities sampled within the range is calculated. In addition, after analyzing Si-containing cold-rolled steel sheets without Fe-based electroplating and zinc plating under the same conditions using glow discharge emission spectroscopy, the spatter depth is measured, the spatter velocity is calculated, and the horizontal axis representing the intensity distribution of Si wavelength is converted into the depth corresponding to each time. The spatter depth is measured using a non-contact surface shape measuring device (NewView 7300: manufactured by Zygo). The spatter velocity of the zinc plating layer is different from that of the Fe-based electroplating layer and the Si-containing cold-rolled steel sheet. That is, the reference used for depth conversion is different due to different elements, so the horizontal axis of the intensity distribution does not completely correspond to the position that can be visually observed in cross-section observation. Furthermore, in glow discharge emission spectroscopy, it is well known that due to unevenness, splashing, and other inhomogeneities, the peak intensity at interfaces composed of two or more substances becomes broad. Therefore, here we use the Zn intensity and set the peak intensity I... Zn Half the strength is defined as the interface between the zinc plating and the Fe-based electroplating layer. (Using...) Figure 2C For peak intensity I Zn The definition method is explained as follows: (1) First, find and draw the tangent line with point 2 in the Zn intensity distribution. (2) Moreover, at each inference depth, subtract the Zn intensity of the tangent line from the Zn intensity of the Zn intensity distribution. (3) Define the Zn intensity of the Zn intensity distribution corresponding to the inference depth with the largest difference as I. Zn Calculate I Zn The value obtained by dividing by 2. Next, when observed from the side of the Si-containing cold-rolled steel sheet, the Zn strength first reaches the above value (I). Zn / 2) The thickness-direction depth is defined as the depth of the interface between the galvanized layer and the Fe-based electroplating layer. Furthermore, the average Si strength (I) is calculated from the interface between the galvanized layer and the Fe-based electroplating layer to the interface between the Fe-based electroplating layer and the Si-containing cold-rolled steel sheet.Si，Fe In addition, the thickness-direction depth of the interface between the Fe-based electroplated layer and the Si-containing cold-rolled steel sheet was measured using the aforementioned cross-sectional observation.

[0115] use Figure 2A Section B will explain representative examples of the luminescence intensity representing the wavelengths of Si and Zn, as analyzed and observed in this embodiment. Figure 2A B represents the raw data results of the intensity distribution analysis of the wavelength luminescence intensity of Si and Zn in the galvanized steel sheets of Examples No. 32 (dew point of annealing atmosphere -7°C), 34 (dew point of -4°C), 36 (dew point of annealing atmosphere -37°C), 37 (dew point of -13°C), and 39 (dew point of +11°C) described later. In the intensity distribution, the solid line represents the wavelength luminescence intensity of Si, and the dashed line represents the wavelength luminescence intensity of Zn. The depths of the interface between the galvanized layer and the Fe-based electroplated layer, and the interface between the Fe-based electroplated layer and the Si-containing cold-rolled steel sheet in these examples are represented as L1 and L2, respectively. In Example No. 36, where the annealing process is performed in a low dew point atmosphere, I... Si，Fe / I Si，bulk The value is 0.42. On the other hand, in Examples 32, 34, 37, and 39, where the annealing process is carried out in an atmosphere with a dew point greater than -30°C, I... Si，Fe / I Si，bulk The values ​​were 0.97, 0.86, 0.61, and 0.65, respectively. Si，Fe / I Si，bulk A value of 0.50 or higher indicates that internal oxidation has occurred in the Fe-based electroplating layer, resulting in the presence of Si internal oxides. Here, the thickness of the Fe-based electroplating layer is the value measured using the aforementioned cross-sectional observation. In steel sheets with Si internal oxides within the aforementioned depth range, grain growth in the Fe-based electroplating layer is inhibited by the internal oxides. Therefore, even if an annealing process is performed after Fe-based electroplating, grain coarsening in the Fe-based electroplating layer can be prevented, and many grain boundaries are formed in the Fe-based electroplating layer. As a result, the penetration path of molten zinc is dispersed, delaying the time it takes for the grain boundaries of the Si-containing cold-rolled steel sheet to reach the molten zinc during resistance welding, thus exhibiting excellent resistance welding crack resistance characteristics.

[0116] At the interface between the Fe-based electroplated layer and the Si-containing cold-rolled steel sheet, the number of grain boundaries in contact between the Fe-based electroplated layer and the Si-containing cold-rolled steel sheet is more than 10 per 10 μm in the width direction of the steel sheet in the observation field of the Si-containing cold-rolled steel sheet. If the number of grain boundaries in contact between the Fe-based electroplated layer and the Si-containing cold-rolled steel sheet is more than 10 per 10 μm in the width direction of the Si-containing cold-rolled steel sheet, the grain size of the Fe-based electroplated layer is sufficiently refined. It is believed that by refining the grain size of the Fe-based electroplated layer to form a large number of grain boundaries, the intrusion of molten zinc is dispersed, which can delay the time for the zinc to reach the grain boundaries of the Si-containing cold-rolled steel sheet during welding, improve the resistance to welding cracking characteristics of the weld, and in particular, 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 in contact between the Fe-based electroplated layer and the Si-containing cold-rolled steel sheet is preferably 16 or more per 10 μm in the width direction of the steel sheet in the observation field of the Si-containing cold-rolled steel sheet. More preferably, the number is 20 or more per 10 μm in the width direction of the steel sheet in the observation field of the Si-containing cold-rolled steel sheet.

[0117] Here, at the interface between the Fe-based electroplated layer and the Si-containing cold-rolled steel sheet, the number of grain boundaries in contact between the Fe-based electroplated layer and the Si-containing cold-rolled steel sheet was determined as follows. First, a 10×10mm sample was taken from the galvanized steel sheet. A focused ion beam (FIB) device was used to process an arbitrary location on the sample, forming a 30μm wide section at a 45° angle to the T-section (a section parallel to the rolling right-angle direction of the steel sheet and perpendicular to the steel sheet surface), with a length of 50μm at the 45° angle to the T-section direction. This section served as the observation sample. Figure 3 The image shows a summary of the sample used for this observation. Figure 3 (a) is a three-dimensional view of the sample for observation. Figure 3 (b) is Figure 3 (a) shows the A-A cross-sectional view of the sample used for observation. Next, a SIM image was taken of the central portion of the sample's width and length directions at a 45° cross-section using a scanning ion microscope (SIM) at 5000x magnification. An example of such a SIM image is shown below. Figure 4 . Figure 4 This refers to the IM image captured as described above for Embodiment No. 36 described later. A 10 μm region in the width direction of the Si-containing cold-rolled steel sheet was extracted from the SIM image. Figure 4 (The part enclosed by the quadrilateral). To illustrate, Figure 5 The middle shows Figure 4 An enlarged view of the quadrilateral-enclosed portion. (See image below.) Figure 5As shown, for SIM images, a boundary line is drawn at the interface between the Fe-based electroplated layer and the Si-containing cold-rolled steel sheet in a 10 μm region along the width direction of the Si-containing cold-rolled steel sheet. Figure 5 (Middle dashed line). The number of grain boundaries of the Fe-based electroplated layer on the boundary line is determined as "the number of grain boundaries in contact between the Fe-based electroplated layer and the Si-containing cold-rolled steel sheet at the interface between the Fe-based electroplated layer and the Si-containing cold-rolled steel sheet".

[0118] Figure 6 The image shown is a SIM image of the interface between an Fe-based electroplated layer and a Si-containing cold-rolled steel sheet, as described later in Invention Example No. 37. An image showing the boundary line drawn in the center of this SIM image as described above, along with an image of the boundary line used for measurement, is also shown. Figure 7 In Invention Example No. 37, the grain boundaries measured on the boundary line are 15 at each 10 μm along the width direction of the Si-containing cold-rolled steel sheet, as indicated by the arrow. Therefore, in Invention Example No. 37, at the interface between the Fe-based electroplated layer and the Si-containing cold-rolled steel sheet, the number of grain boundaries contacting the Fe-based electroplated layer and the Si-containing cold-rolled steel sheet is 15 per 10 μm along the width direction of the Si-containing cold-rolled steel sheet. Furthermore, Figure 8 The image shown is a SIM image of the interface between an Fe-based electroplated layer and a Si-containing cold-rolled steel sheet, according to Invention Example No. 39 of the following embodiment. An image showing the boundary line drawn in the center of this SIM image as described above, and an image of the boundary line for measurement, is also shown. Figure 9 In Invention Example No. 39, the grain boundaries measured on the boundary line are 18 in every 10 μm of the Si-containing cold-rolled steel sheet, as indicated by the arrow. Therefore, in Invention Example No. 39, at the interface between the Fe-based electroplated layer and the Si-containing cold-rolled steel sheet, the number of grain boundaries in contact between the Fe-based electroplated layer and the Si-containing cold-rolled steel sheet is 18 per 10 μm in the width direction of the Si-containing cold-rolled steel sheet.

[0119] The thickness of the galvanized steel sheet in this embodiment is not particularly limited, but it is usually 0.5 mm or more, and can also be 3.2 mm or less.

[0120] Next, the carbon concentration on the surface of the Fe-based electroplated layer will be explained. In this embodiment, it is important that the average carbon concentration in the range of 10 μm to 20 μm in the thickness direction of the Fe-based electroplated layer from the interface between the zinc plating layer and the Fe-based electroplated layer is 0.10% by mass or less, more preferably 0.07% by mass or less, during the annealing process described above. It is even more preferable that a decarburized layer is formed on the Fe-based electroplated layer side, starting from the interface between the zinc plating layer and the Fe-based electroplated layer. That is, a decarburized layer is formed on the surface of the galvanized steel sheet excluding the zinc plating layer during annealing. In addition, the decarburized layer is a region near the interface between the zinc plating layer and the Fe-based electroplated layer (the surface of the galvanized steel sheet excluding the zinc plating layer) where the carbon concentration is lower than that in the steel, and it may be formed by carbon detaching from the surface of the Fe-based electroplated steel sheet during annealing. If the average C concentration in the range of 10 μm to 20 μm from the interface between the zinc plating layer and the Fe-based electroplating layer towards the thickness of the Fe-based electroplating layer is 0.10% by mass or less, then the region of 10 μm to 20 μm from the interface between the zinc plating layer and the Fe-based electroplating layer towards the thickness of the Fe-based electroplating layer is soft. Therefore, it has the effect of mitigating the stress applied from the welding electrode during resistance welding and improving the resistance welding crack resistance characteristics.

[0121] In this embodiment, by annealing after forming an Fe-based electroplated layer, the average carbon concentration in the range of 10 μm to 20 μm from the interface between the zinc plating layer and the Fe-based electroplated layer towards the thickness of the Fe-based electroplated layer can be reduced compared to when no Fe-based electroplated layer is present. Furthermore, when forming a decarburized layer from the interface between the zinc plating layer and the Fe-based electroplated layer towards the Fe-based electroplated layer, the carbon concentration of the decarburized layer can be further reduced even with the same thickness. Therefore, the resistance to resistance welding cracking can be improved without deteriorating the mechanical properties of the steel sheet. Additionally, when Ni, Co, Sn, etc., are electroplated individually, the carbon solubility in these metal elements is extremely low, and the decarburization-promoting effect is not achieved due to the lack of carbon solubility.

[0122] When an Fe-based electroplating layer is formed, the reason for the decrease in C concentration from the interface between the zinc plating layer and the Fe-based electroplating layer to a range of 10 μm to 20 μm in the thickness direction of the Fe-based electroplating layer is not yet clear. The inventors speculate as follows: It is considered that because the Fe-based electroplating layer contains almost no C, it induces C to diffuse from the Si-containing cold-rolled steel sheet. Therefore, as mentioned above, the Fe-based electroplating layer is finer, resulting in more diffusion paths for C to detach to the outside through the Fe-based electroplating layer.

[0123] Furthermore, the softening effect achieved by reducing the C concentration in the range of 10 μm to 20 μm from the interface between the zinc plating layer and the Fe-based electroplating layer towards the thickness of the Fe-based electroplating layer side plate is limited because the C concentration in this range saturates below a certain level. Therefore, the improvement in resistance welding crack resistance characteristics through softening is limited. In this embodiment, by further reducing the C concentration in the range of 10 μm to 20 μm from the interface between the zinc plating layer and the Fe-based electroplating layer towards the thickness of the Fe-based electroplating layer side plate, resistance welding crack resistance characteristics are effectively improved even with a thin decarburized layer. This demonstrates that not only softening may occur, but other effects such as an increase in melting point due to the reduction in C concentration may also be observed.

[0124] When forming the decarburized layer, the thickness of the decarburized layer from the interface between the zinc plating layer and the Fe-based electroplating layer toward the Fe-based electroplating layer is preferably 15 μm or more, more preferably 30 μm or more. There is no particular upper limit to the thickness of the decarburized layer, but to ensure good tensile strength, the thickness of the decarburized layer from the interface between the zinc plating layer and the Fe-based electroplating layer toward the Fe-based electroplating layer is preferably 130 μm or less. The thickness of the decarburized layer is determined by analyzing the C concentration along the thickness direction from the interface between the zinc plating layer and the Fe-based electroplating layer of the galvanized steel sheet, and is defined as the thickness from the interface between the zinc plating layer and the Fe-based electroplating layer in the region where the C concentration in the surface layer of the steel sheet (excluding the zinc plating layer) is 80% or less of that in the steel, toward the Fe-based electroplating layer.

[0125] Here, the average C concentration and the thickness of the decarburized layer in a range of 10 μm to 20 μm from the interface between the zinc plating layer and the Fe-based electroplating layer towards the thickness of the Fe-based electroplating layer side plate were determined as follows. For the cross-sectioned specimens, the elemental distribution near the interface between the zinc plating layer and the Fe-based electroplating layer (described later) was analyzed using an electron beam microanalyzer (EPMA). First, the steel plate embedded in resin was ground, and after observing the cross-section perpendicular to the rolling direction, it was removed from the resin to prepare the specimen for testing. The accelerating voltage was 7 kV, the irradiation current was 50 nA, and the cross-section of the specimen was analyzed using a 300 × 300 μm range including the interface between the zinc plating layer and the Fe-based electroplating layer in 1 μm increments to determine the C intensity. At this time, in order to suppress contamination, a plasma cleaner was used in two places, the testing chamber and the specimen preparation chamber, to remove hydrocarbons from the surface and surrounding area of ​​the specimen before the start of the test. In addition, to suppress the accumulation of hydrocarbons during the measurement, the sample temperature is maintained at a maximum of 100°C on the worktable. The sample temperature is preferably 90°C or higher. A calibration curve prepared using another standard sample is used to convert C intensity to C concentration (mass %). Due to the contamination suppression effect, the C detection limit is confirmed to be sufficiently lower than 0.10% by mass. Details of the apparatus used and the above-described contamination suppression method are described in Reference 1 below.

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

[0127] The necessity of contamination countermeasures during measurement depends on the type of equipment and conditions used; therefore, the above-mentioned components are not essential. In other words, measurement conditions only need to achieve sufficient accuracy, and the measurement conditions are not fundamentally related to the effectiveness of the present invention.

[0128] In the obtained concentration distribution, the thickness-direction line distribution is extracted from the interface between the zinc plating layer and the Fe-based electroplating layer. This line distribution is averaged at 300 points parallel to the steel plate surface to obtain the thickness-direction distribution of C concentration. The obtained thickness-direction distribution of C concentration is then smoothed using a simple moving average method. The number of smoothing points is preferably around 21. If the number of smoothing points near the surface of the sample is less than 10 on one side, it is preferable to smooth only the measurement points that can be taken on one side. Next, in the smoothed strength distribution, the thickness-direction depth of the interface between the zinc plating layer and the Fe-based electroplating layer in the region where the C concentration is less than 80% of that in the steel is evaluated as the thickness of the decarburized layer. Furthermore, for a range of 10 μm to 20 μm from the interface between the zinc plating layer and the Fe-based electroplating layer towards the thickness of the Fe-based electroplating layer, the C concentration values ​​at 11 points with 1 μm spacing were averaged to obtain the C concentration for this range. The above evaluation was applied to the measurement results from two fields of view for each sample, and the average was taken as the average C concentration and the evaluation value for the thickness of the decarburized layer for the range of 10 μm to 20 μm from the interface between the zinc plating layer and the Fe-based electroplating layer towards the thickness of the Fe-based electroplating layer.

[0129] use Figure 10 A representative example of the distribution of C concentration along the thickness direction of the plate, as analyzed by EPMA, is presented. Figure 10 (a) shows the results of raw data on the distribution of C concentration in the thickness direction of the galvanized steel sheet obtained by analyzing the galvanized steel sheet of Example No. 39 described later. It should be noted that the measurement was performed by peeling off the galvanized layer. Figure 10 (b) indicates the application of a simple moving average method with a smoothing point of 21. Figure 10 (a) is the original data after smoothing. Figure 10 As shown in (b), in this embodiment, there is a decarburized layer with a C concentration of less than 80% in the steel, and the thickness direction depth of the decarburized layer from the interface between the zinc plating layer and the Fe-based electroplating layer is 82 μm.

[0130] As described above, the ability of Fe-based electroplating to prevent internal cracking is due to the combined effects of zinc plating's grain boundary intrusion suppression, stress mitigation, toughness reduction suppression, and the reduction of carbon concentration in the surface layer, which promotes decarburization. These combined effects cannot be clearly quantified, but the preferred adhesion amount (CW) of the galvanized Fe-based electroplating layer is... Fe1 (g / m 2 ) and the thickness C of the decarburized layer d (μm) satisfies the following equation (1).

[0131] 1.6×(CW Fe1 )+(Cd )≥77···(1)

[0132] This is because as long as the adhesion amount of the Fe-based electroplated layer after zinc plating is CW Fe1 (g / m 2 ) and the thickness C of the decarburized layer d (μm) satisfies the above formula (1), and has particularly good resistance to resistance welding cracking characteristics.

[0133] Next, the zinc plating layer formed on the aforementioned Fe-based electroplating layer will be explained. Here, "zinc plating layer" refers to a zinc film formed on the surface of the steel plate. It is not limited to zinc film formation methods such as plating, thermal spraying, and cold spraying; any zinc film formed on the surface of the steel plate is included in the "zinc plating layer".

[0134] Excellent corrosion resistance can be achieved by forming a galvanized layer on the surface of the steel sheet. Conversely, during resistance welding, zinc may melt and penetrate into the grain boundaries of the Si-containing cold-rolled steel sheet, making it difficult to improve the resistance welding crack resistance of the welded area. As mentioned above, by applying a single-sided coating of 5.0 g / m² to the surface of the cold-rolled steel sheet before the annealing process after cold rolling... 2 The formation of an Fe-based electroplating layer as a pre-coating before forming a zinc plating layer can improve the resistance to resistance weld cracking characteristics of welded parts in galvanized steel sheets. If an Fe-based electroplating layer is formed as a pre-coating for forming the aforementioned zinc plating layer, the resistance to resistance weld cracking characteristics of welded parts in galvanized steel sheets can be improved regardless of the type of zinc plating layer. The zinc plating layer can be, for example, a hot-dip galvanized layer, an electroplated layer, a zinc spray coating, or a cold-sprayed coating. The composition of the zinc plating layer is not particularly limited, but in the case of a hot-dip galvanized layer, it consists of Al, Zn, and unavoidable impurities. The Al content in the zinc plating layer is not particularly specified; in one example, the Al content in a hot-dip galvanized layer is 0.05% to 0.250% by mass. Furthermore, the zinc plating layer is not an alloyed zinc plating layer.

[0135] The adhesion amount of the galvanized layer on one side can be 25g / m². 2 Alternatively, it can be 80g / m 2 The following is an example of achieving an adhesion amount of 25g / m² on one side of the galvanized layer. 2 The above measures further improve corrosion resistance and make it easier to control the coating adhesion. Additionally, if the adhesion of the zinc coating on one side is 80 g / m²... 2 The coating exhibits good adhesion. The preferred adhesion amount on one side of the zinc coating is 35 g / m². 2 That's all. Furthermore, the preferred adhesion amount of the galvanized layer on one side is 60 g / m². 2 the following.

[0136] According to this disclosure, it is possible to provide high-strength galvanized steel sheets with a tensile strength (TS) of 590 MPa or higher as measured according to JIS Z 2241 (2011). More preferably, the strength of the galvanized steel sheet is 800 MPa or higher.

[0137] <Manufacturing Method of Galvanized Steel Sheet>

[0138] Next, the manufacturing method of galvanized steel sheet will be explained.

[0139] One embodiment of the method for manufacturing galvanized steel sheet may be as follows: Fe-based electroplating is performed on a cold-rolled steel sheet containing 0.1% to 3.0% by mass of Si to produce a coating with an adhesion amount of 5.0 g / m² on at least one side. 2 The above-mentioned Fe-based electroplated steel sheet before annealing, with Fe-based electroplating layer before annealing.

[0140] Next, the Fe-based electroplated steel sheet before annealing is heated at an average heating rate of 10°C / second or higher in a temperature range of 400°C to 650°C. After heating, it is held in an atmosphere with a dew point greater than -30°C and then cooled to produce an Fe-based electroplated steel sheet.

[0141] Next, the Fe-based electroplated steel sheet is galvanized to obtain a galvanized steel sheet.

[0142] First, a cold-rolled steel sheet containing 0.1% to 3.0% by mass of Si is manufactured. Alternatively, the cold-rolled steel sheet may contain 0.50% to 3.0% by mass of Si. The manufacturing method of the cold-rolled steel sheet can follow the conventional method for manufacturing cold-rolled steel sheets. In one example, the cold-rolled steel sheet is manufactured as follows: a steel billet having the above-mentioned composition is hot-rolled to produce a hot-rolled sheet; the hot-rolled sheet is then pickled; and finally, the hot-rolled sheet is cold-rolled to produce a cold-rolled steel sheet.

[0143] Next, the surface of the cold-rolled steel sheet is subjected to Fe-based electroplating treatment to produce Fe-based electroplated steel sheet before annealing. There are no particular limitations on the Fe-based electroplating method. For example, a sulfuric acid bath, a hydrochloric acid bath, or a mixture of both can be used as the Fe-based electroplating bath. Alternatively, Fe-based electroplating treatment can be performed without preheating the cold-rolled steel sheet in an oxidation furnace or similar process. Furthermore, Fe-based electroplated steel sheet before annealing refers to a Fe-based electroplated layer that does not undergo an annealing process; however, it is not excluded that the cold-rolled steel sheet before Fe-based electroplating treatment may be annealed.

[0144] The Fe ion content in the Fe-based electroplating bath before energization is determined by Fe. 2+ The preferred concentration is 0.5 mol / L or higher. If the Fe ion content in the Fe-based electroplating bath is... 2+A concentration of 0.5 mol / L or higher is required to achieve a sufficient amount of Fe adhesion. Furthermore, to ensure a sufficient amount of Fe adhesion, the Fe ion content in the Fe-based electroplating bath before energizing is preferably below 2.0 mol / L.

[0145] Furthermore, 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 10% by mass or less in the Fe-based electroplated layer before annealing. Additionally, metallic elements may be contained as metallic ions, and non-metallic elements may be contained as part of boric acid, phosphoric acid, nitric acid, organic acids, etc. Furthermore, the ferric sulfate plating solution may contain conductive additives such as sodium sulfate and potassium sulfate, chelating agents, and pH buffers.

[0146] Other conditions for the Fe-based electroplating bath are not particularly limited. Considering temperature stability, the temperature of the Fe-based electroplating bath is preferably 30°C or higher, and more preferably 85°C or lower. The pH of the Fe-based electroplating bath is not particularly specified, but from the viewpoint of preventing a decrease in current efficiency due to hydrogen generation, it is preferably 1.0 or higher. Furthermore, considering the conductivity of the Fe-based electroplating bath, it is preferably 3.0 or lower. From a productivity viewpoint, the current density is preferably 10 A / dm³. 2 From the perspective of easily controlling the adhesion amount of Fe-based electroplated layers, 150 A / dm is preferred. 2 The following applies to the plate speed. From a productivity point of view, a speed of 5 mpm or higher is preferred, and from the point of view of stable control of the amount of adhesion, a speed of 150 mpm or lower is preferred.

[0147] In addition, as a pretreatment before Fe-based electroplating, degreasing and rinsing can be performed to clean the surface of the cold-rolled steel sheet, and pickling and rinsing can be performed to activate the surface of the cold-rolled steel sheet. Fe-based electroplating is then performed after these pretreatments. The methods for degreasing and rinsing are not particularly limited, and conventional methods can be used. For pickling, various acids such as sulfuric acid, hydrochloric acid, nitric acid, and mixtures thereof can be used. Sulfuric acid, hydrochloric acid, or mixtures thereof are preferred. The concentration of the acid is not particularly specified, but considering the ability to remove the oxide film and to prevent rough skin (surface defects) caused by over-pickling, a concentration of about 1 to 20 mass% is preferred. Furthermore, the pickling solution may contain defoamers, pickling accelerators, pickling inhibitors, etc.

[0148] Next, after Fe-based electroplating, an annealing process is performed: the Fe-based electroplated steel sheet before annealing is held in a reducing atmosphere with a dew point greater than -30°C and a hydrogen concentration of 1.0 vol% to 30.0 vol% at a temperature range of 650°C to 900°C for 30 to 600 seconds, and then cooled to obtain the Fe-based electroplated steel sheet. The annealing process is to remove the strain of the Fe-based electroplated steel sheet before annealing caused by the rolling process, to allow the microstructure to recrystallize, and to improve the strength of the steel sheet.

[0149] Average heating rate: 10℃ / second or higher

[0150] Next, the Fe-based electroplated steel sheet before annealing is heated to a temperature range of 650°C to 900°C at an average heating rate of 10°C / second or higher within the temperature range of 400°C to 650°C (heating process). By ensuring an average heating rate of 10°C / second or higher in the heating process, grain growth in the Fe-based electroplated layer is strongly suppressed. This is because, during the heating process, almost no internal oxidation of Si occurs at the grain boundaries of the Fe-based electroplated layer, as described later; therefore, if the average heating rate is less than 10°C / second, grain growth cannot be suppressed. Under conditions that strongly suppress grain growth in the Fe-based electroplated layer during the heating process, annealing is performed in an atmosphere with a dew point greater than -30°C, as described later, to refine the grain size of the Fe-based electroplated layer. The heating zone in the heating process can be, for example, a direct-fired furnace (DFF) or a non-oxidizing furnace (NOF). In the case of a radiant tube furnace, a preheating zone such as an IH (Induction Heater) can be set up at the front. Furthermore, the aforementioned average heating rate is based on the temperature measured on the surface of an Fe-based electroplated steel sheet.

[0151] Next, an annealing process is performed: the Fe-based electroplated steel sheet before annealing is held in a reducing atmosphere with a dew point greater than -30°C and a hydrogen concentration of 1.0 vol% to 30.0 vol% at a temperature range of 650°C to 900°C for 30 to 600 seconds, and then cooled to obtain the Fe-based electroplated steel sheet. The annealing process is to remove the strain from the cold-rolled steel sheet produced by the rolling process, allowing the microstructure to recrystallize and thus increasing the strength of the steel sheet. During this process, because internal oxides are formed at the grain boundaries of the Fe-based electroplated layer, the grain growth of the Fe-based electroplated layer is inhibited, enabling crystal refinement, and a decarburized layer is formed on the surface of the Fe-based electroplated steel sheet, reducing the C concentration on the surface.

[0152] Hydrogen concentration: 1.0 vol% to 30.0 vol%

[0153] The annealing process is carried out in a reducing atmosphere with a hydrogen concentration of 1.0 vol% to 30.0 vol%. Hydrogen inhibits the oxidation of Fe on the surface of the Fe-based electroplated steel sheet before annealing, thus activating the steel sheet surface. If the hydrogen concentration is 1.0 vol% or higher, it can prevent the deterioration of the plating adhesion when setting the zinc plating layer, as described later, due to Fe oxidation on the steel sheet surface. Therefore, the annealing process is preferably carried out in a reducing atmosphere with a hydrogen concentration of 1.0 vol% or higher, more preferably in a reducing atmosphere with a hydrogen concentration of 2.0 vol% or higher. There is no particular upper limit to the hydrogen concentration in the annealing process, but from a cost perspective, the hydrogen concentration is preferably 30.0 vol% or less, more preferably 20.0 vol% or less. The remainder of the annealing atmosphere other than hydrogen is preferably nitrogen.

[0154] Dew point greater than -30℃

[0155] 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 electroplated layer. The dew point of -30°C or greater is preferably controlled within a temperature range of 650°C to 900°C. This allows Si internal oxides to form at the grain boundaries of the Fe-based electroplated layer while suppressing grain growth in the Fe-based electroplated layer by maintaining an average heating rate of 10°C / second or higher in the heating process. Furthermore, setting the dew point of the annealing atmosphere in the annealing process to be greater than -30°C promotes the decarburization reaction and reduces the C concentration on the surface layer. The Si internal oxides present at the grain boundaries of the Fe-based electroplated layer inhibit grain growth in the Fe-based electroplated layer during the annealing process due to a pinning effect. These Si internal oxides are formed due to the diffusion of Si from the cold-rolled steel sheet, exhibiting a particularly strong pinning effect on the cold-rolled steel sheet side of the Fe-based electroplated layer. As a result, it was found that the grain size at the interface between the Fe-based electroplated layer and the Si-containing cold-rolled steel sheet was particularly small, and the number of grain boundaries at the interface contact between the Fe-based electroplated layer and the Si-containing cold-rolled steel sheet increased. Furthermore, the pinning effect here refers to the Zener drag mechanism. Second-phase particles are dispersed in the microstructure, and when grain boundaries intersect with second-phase particles, energy is required for the grain boundaries to detach from the second-phase particles. That is, pinning force acts between the particles and the grain boundaries to prevent grain boundary movement, thereby inhibiting grain growth. Carbides and sulfides are well-known as second-phase particles. Whether the internal oxides of Si exhibit this pinning effect is unclear; based on experimental evidence, it is speculated that they do. The dew point of the annealing atmosphere is preferably -20°C or higher, more preferably -5°C or higher. By setting the dew point of the annealing atmosphere to -5°C or higher, the resistance to welding cracking characteristics of the welded part, especially the characteristics preventing internal cracking, are excellent. There is no particular upper limit to the dew point of the annealing atmosphere, but in order to properly prevent oxidation of the Fe-based electroplated layer surface and improve the coating adhesion when setting the zinc plating layer as described later, the dew point of the annealing atmosphere is preferably below 30°C.

[0156] Holding time in the temperature range of 650℃ to 900℃: 30 seconds to 600 seconds

[0157] In the annealing process, the holding time in the temperature range of 650°C to 900°C is preferably 30 to 600 seconds. By holding this temperature range for more than 30 seconds, the natural oxide film of Fe formed on the surface of the Fe-based electroplated layer before annealing is appropriately removed, which improves the adhesion of the plating when setting the zinc plating layer as described later. Therefore, the holding time in this temperature range is preferably more than 30 seconds. There is no particular upper limit to the holding time in this temperature range, but from a productivity point of view, the holding time in this temperature range is preferably less than 600 seconds.

[0158] Maximum temperature reached for Fe-based electroplated steel sheets before annealing: 650℃~900℃

[0159] The maximum temperature reached before annealing of Fe-based electroplated steel sheets is not particularly limited, but is preferably 650°C to 900°C. By setting the maximum temperature of the Fe-based electroplated steel sheets before annealing to 650°C or higher, the recrystallization of the steel sheet structure proceeds smoothly, and the desired strength can be obtained. Furthermore, the natural oxide film of Fe formed on the surface of the Fe-based electroplated layer before annealing can be appropriately reduced, improving the coating adhesion when hot-dip galvanizing is applied to the surface of the Fe-based electroplated steel sheets as described later. Additionally, if the maximum temperature reached before annealing of the Fe-based electroplated steel sheets is 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, thus improving the coating adhesion when hot-dip galvanizing is applied to the surface of the Fe-based electroplated steel sheets as described later. Furthermore, if the maximum temperature reached before annealing is 900°C or lower, damage to the heat treatment furnace can be prevented, and costs can be reduced. Therefore, the maximum temperature reached before annealing of Fe-based electroplated steel sheets is preferably 900°C or lower. In addition, the above-mentioned maximum temperature is based on the temperature measured on the surface of the Fe-based electroplated steel sheet before annealing.

[0160] CW of Fe-based electroplated layer on one side before annealing Fe0 (g / m 2 The dew point DP (°C) of the annealing atmosphere preferably satisfies the following formula (2). If the following formula (2) is satisfied, the combined effects of the grain boundary intrusion suppression effect of molten zinc, the stress relief effect, the toughness reduction suppression effect, and the C concentration reduction effect of the surface layer promoted by the Fe-based electroplating layer can more significantly improve the resistance to welding cracking characteristics.

[0161] (CW Fe0 )+(DP)≥0···(2)

[0162] As described above, if the result satisfies equation (2), the resistance to resistance welding cracking characteristics of the welded part can be further improved. As another embodiment, if equation (2) is not satisfied, a process can be further included to change the dew point DP (°C) of the annealing atmosphere in order to satisfy equation (2). This allows for a more reliable improvement in the resistance to resistance welding cracking characteristics of the welded part. As an example of performing this process, consider a case where the adhesion amount CW of the Fe-based electroplated layer on one side before annealing is... Fe0 (g / m 2 The value of ) is changed in a way that satisfies the above formula (2) to change the dew point DP in the annealing process. In the annealing process, the atmosphere dew point is controlled in a way that makes the dew point a certain value. Specifically, the amount of Fe-based electroplated layer CW on one side before annealing is reduced. Fe0 (g / m 2 Substituting the value of ) into the above equation (2), the dew point DP in the above annealing process is determined in a manner that satisfies equation (2). Here, the adhesion amount CW of the Fe-based electroplated layer on one side before annealing is used. Fe0 (g / m 2 Substituting the value of ) into equation (2) above is not limited to substituting into an equation that is strictly the same as equation (2) above. It includes substituting into an inequality that always satisfies a narrower range of equation (2). By performing such control, even if, for example, the product specifications of continuously circulating steel plates are switched, the adhesion amount CW of the Fe-based electroplated layer on one side before annealing can be controlled. Fe0 (g / m 2 When the changes are significant and the above formula (3) is no longer satisfied (when the formula is not actually satisfied, or when the formula is no longer satisfied), automatic control can be performed to satisfy the formula.

[0163] In addition, the control responsiveness of dew point DP is better than that of the adhesion amount CW on one side of the Fe-based electroplated layer before annealing. Fe0 Even worse, from the point of view of controlling responsiveness, it is preferable to change the amount of Fe-based electroplated layer on one side before annealing according to the dew point DP value in a manner that satisfies the above equation (2). Fe0 In the case of a continuous annealing furnace, the adhesion amount CW of the Fe-based electroplated layer on one side before annealing in the Fe-based electroplating treatment upstream of the annealing process is changed according to the dew point DP value of the annealing process. Fe0 However, for continuously plated steel sheets, the adhesion amount CW of the Fe-based electroplated layer on one side before annealing is changed. Fe0 The position can be manufactured under the condition that the above formula (2) is satisfied.

[0164] As a way to satisfy the above formula (2), the amount of Fe-based electroplated layer CW on one side before annealing is changed. Fe0Alternatively, when at least one of the dew point DPs is used, and Si-containing cold-rolled steel sheets of different product specifications are welded and continuously passed through, it is preferable to adjust the adhesion amount CW of the Fe-based electroplated layer on one side before annealing in conjunction with the passage through the welding position. Fe0 Alternatively, dew point DP. As mentioned above, dew point DP has poor responsiveness, therefore, when changing dew point DP, it is more preferable to feedforward control of the humidification amount in the furnace in a manner that satisfies this formula.

[0165] The "attachment amount CW of the Fe-based electroplated layer on one side before annealing" mentioned here Fe0 (g / m 2 The "value" can be the amount of adhesion obtained under the conditions used in Fe-based electroplating (target value) or the actual amount of adhesion of the Fe-based electroplated layer (measured value). Similarly, the "value of dew point DP" can be either a target value or a measured value.

[0166] The above examples illustrate operational procedures in the manufacturing process of galvanized steel sheets and can also be used as a method for determining manufacturing conditions for galvanized steel sheets: Before starting the operation, confirm the adhesion amount (CW) of the Fe-based electroplated layer on one side before pre-annealing. Fe0 (g / m 2 If the target values ​​of the Fe-based electroplating layer and the dew point DP satisfy the above formula (2), and if not, the adhesion amount CW on one side of the Fe-based electroplating layer before annealing is changed in advance. Fe0 (g / m 2 The target value is either the target value of the galvanized steel sheet or the target value of the dew point (DP). This method of determining manufacturing conditions can be implemented as part of the manufacturing process of galvanized steel sheets, or as a separate process.

[0167] Next, the galvanizing process will be explained.

[0168] Following the aforementioned annealing process, galvanizing is performed on the Fe-based electroplated steel sheet. The galvanizing method is not particularly limited, but can include, for example, hot-dip galvanizing, electroplating, cold spraying, and plasma spraying. In one example of hot-dip galvanizing, the Fe-based electroplated steel sheet can be cooled after the annealing process and immersed in a hot-dip galvanizing bath to perform hot-dip galvanizing on the steel sheet surface. The hot-dip galvanizing bath consists of Al, Zn, and unavoidable impurities. The composition of the hot-dip galvanizing bath is not particularly specified; generally, the Al concentration in the bath is 0.05% to 0.250% by mass. If the Al concentration in the bath is 0.05% by mass or higher, bottom slag formation can be prevented, and floating slag can be prevented from adhering to the steel sheet and becoming a defect. Furthermore, by keeping the Al concentration in the bath below 0.250% by mass, top slag accumulation is prevented, floating slag adhering to the steel sheet and becoming a defect is prevented, and costs are also reduced. There are no restrictions on other conditions for hot-dip galvanizing. For example, the bath temperature of the hot-dip galvanizing bath is usually in the range of 440 to 500°C, so that the steel plate is immersed in the hot-dip galvanizing bath at a plate temperature of 440 to 550°C.

[0169] The preferred coating thickness on one side of the zinc plating layer is 25 g / m². 2 The above, and preferably 80g / m 2 The following is an example of achieving a zinc coating adhesion of 25 g / m² on one side. 2 The above measures further improve corrosion resistance and facilitate control of the coating adhesion. Additionally, if the coating adhesion on one side of the zinc plating layer is 80 g / m²... 2 The following coatings exhibit good adhesion.

[0170] After galvanizing, the coating thickness can be adjusted appropriately. There are no particular limitations on the method for adjusting the coating thickness, but in hot-dip galvanizing, for example, it is generally adjusted by gas purging. In one example, the coating thickness is adjusted using the gas pressure during purging and the distance between the purging nozzle and the steel plate. Additionally, after galvanizing, the galvanized layer is alloyed.

[0171] <Electrodeposition Coated Steel Sheet>

[0172] Furthermore, according to this embodiment, an electrodeposited coated steel sheet can also be provided, which further comprises a chemical conversion coating formed on the zinc plating layer and an electrodeposited coating formed on the chemical conversion coating. The Fe-based electroplated steel sheet of this embodiment exhibits excellent resistance to resistance welding cracking at the welded portion, therefore, the electrodeposited coated steel sheet formed using this galvanized steel sheet is particularly suitable for automotive parts. The types of chemical conversion coating and electrodeposited coating are not particularly limited, and can be any known chemical conversion coating or electrodeposited coating. As a chemical conversion coating, zinc phosphate coatings, zirconium coatings, etc., can be used. As for the electrodeposited coating, there are no particular limitations as long as it is an electrodeposited coating for automotive applications. The thickness of the electrodeposited coating varies depending on the application, but the coating in the dry state is preferably about 10 μm to 30 μm. Furthermore, according to this embodiment, an electrodeposited coated galvanized steel sheet for performing electrodeposited coating can also be provided.

[0173] <Manufacturing Method of Electrodeposited Coated Steel Sheets>

[0174] Next, the manufacturing method of the above-mentioned electrodeposited coated steel sheet will be described. The above-mentioned electrodeposited coated steel sheet can be manufactured by a manufacturing method having the following steps: a chemical conversion treatment step, in which a galvanized steel sheet undergoes chemical conversion treatment to obtain a chemically converted steel sheet with a chemically converted coating film formed on the galvanized layer; and an electrodeposition coating step, in which the chemically converted steel sheet undergoes electrodeposition coating treatment to obtain an electrodeposited coated steel sheet with an electrodeposited coating film formed on the chemically converted coating film. The chemical conversion treatment and electrodeposition coating treatment can be performed using known methods. Furthermore, as a pretreatment before the chemical conversion treatment, degreasing, washing, and surface conditioning treatment as needed can be performed to clean the surface of the galvanized steel sheet. The chemical conversion treatment is performed after these pretreatments. The methods for degreasing and washing are not particularly limited, and conventional methods can be used. In the surface conditioning treatment, surface conditioning agents containing Ti colloid or zinc phosphate colloid can be used. When using these surface conditioning agents, no special steps are required, and conventional methods can be followed. For example, the desired surface conditioner is dissolved in a specified amount of deionized water, stirred thoroughly, and then the solution is prepared at a predetermined temperature (usually room temperature, 25–30°C). The steel sheet is then immersed in this solution for a specified time (20–30 seconds). Following this, the next step, chemical conversion treatment, is performed without drying. Chemical conversion treatment can also be carried out using conventional methods. For example, the desired chemical conversion agent is dissolved in a specified amount of deionized water, stirred thoroughly, and then the solution is prepared at a specified temperature (usually 35–45°C). The steel sheet is then immersed in this solution for a specified time (60–120 seconds). Examples of chemical conversion agents include zinc phosphate for steel, zinc phosphate for steel and aluminum, and zirconium. Next, the next step, electrodeposition coating, is performed. Electrodeposition coating can also be carried out using conventional methods. After pretreatment such as water washing as needed, the steel sheet is immersed in a thoroughly stirred electrodeposition coating and electrodeposition is performed to obtain an electrodeposition coating of the desired thickness. As an electrodeposition coating, cationic and anionic electrodeposition coatings can be used. Furthermore, depending on the application, a topcoat can be applied after the electrodeposition coating.

[0175] <Automotive Parts>

[0176] Furthermore, according to this embodiment, it is possible to provide automotive parts made at least partially using the electrodeposited coated steel sheet described above. The galvanized steel sheet of this embodiment exhibits excellent resistance to resistance weld cracking at its welded portions, therefore the electrodeposited coated steel sheet using this galvanized steel sheet is particularly suitable for automotive parts. Automotive parts made using the electrodeposited coated steel sheet may include steel sheets other than those of this embodiment as blanks. The electrodeposited coated steel sheet of this embodiment exhibits excellent resistance to resistance weld cracking at its welded portions, thus appropriately preventing LME cracking in the welded portions of automotive parts made using this galvanized steel sheet. The types of automotive parts made at least partially using the electrodeposited coated steel sheet are not particularly limited, but for example, they may be side beam components, pillar components, automotive bodies, etc.

[0177] The present invention will now be described in detail based on embodiments.

[0178] Example 1

[0179] The steel with the chemical compositions shown in Tables 1 and 3 was melted to obtain castings. The castings were then hot-rolled, pickled, and cold-rolled to produce cold-rolled steel sheets with a thickness of 1.6 mm.

[0180]

[0181]

[0182] Next, the cold-rolled steel sheet was degreased with an alkaline solution. Then, it was electrolyzed under the conditions shown below, using the steel sheet as the cathode, to produce a pre-annealed Fe-based electroplated steel sheet with a pre-annealed Fe-based electroplating layer on one side. The adhesion amount of the pre-annealed Fe-based electroplating layer was controlled by the energizing time. Next, the pre-annealed Fe-based electroplated steel sheet was subjected to reduction annealing with 15% H2-N2, a soaking zone temperature of 800°C, and dew points adjusted according to Tables 2-1, 2-2, and 4 to obtain the Fe-based electroplated steel sheet. After cooling the obtained Fe-based electroplated steel sheet to 440–550°C, it was further hot-dip galvanized in a 460°C hot-dip galvanizing bath with an effective Al concentration of 0.197% by mass and the balance consisting of Zn and unavoidable impurities. The weight per unit area was then adjusted to approximately 50 g / m² per side by gas purging. 2 Samples of galvanized steel sheets were made.

[0183] [Electrolysis conditions]

[0184] Bath temperature: 50℃

[0185] pH: 2.0

[0186] Current density: 45A / dm 2

[0187] Fe-based electroplating bath: containing 1.5 mol / L Fe 2+ ion

[0188] Electrode (anode): Iridium oxide electrode

[0189] The adhesion amount and I of the Fe-based electroplated layer on one side are determined using the method described above from the galvanized steel sheet manufactured as described above. Si，Fe / I Si，bulk And the number of grain boundaries at the interface between the Fe-based electroplated layer and the Si-containing cold-rolled steel sheet.

[0190] Using galvanized steel sheets prepared as described above, the C strength was determined by surface analysis of the sample cross-section according to the above method. The average C concentration and the depth of the decarburized layer were evaluated in the range of 10 μm to 20 μm from the interface between the galvanized layer and the Fe-based electroplated layer to the thickness of the Fe-based electroplated layer.

[0191] The resistance to weld cracking of galvanized steel sheets and welded joints obtained above was investigated. The methods for determining and evaluating the resistance to weld cracking characteristics of welded joints are shown below.

[0192] <Characteristics of resistance welding cracking resistance of welded parts>

[0193] For galvanized steel sheets, the evaluation criteria included a holding time of 0.18 seconds, resistance welding cracking characteristics (not a key factor), a Si content of less than 0.50%, a tensile strength of 980 MPa, and a single-sided adhesion of 50 g / m². 2 The resistance to weld cracking characteristics of welded joints when using alloyed hot-dip galvanized steel sheets (1.6 mm thick). Figure 11 The evaluation method for the resistance to cracking of welded parts is explained.

[0194] Test piece 6, cut with the rolling right-angle direction (TD) as the long side and the rolling direction as the short side, and a single side with the same size hot-dip galvanized coating cut out, showed an adhesion amount of 50 g / m². 2 The test alloyed hot-dip galvanized steel sheets 5 are overlapped to form a plate assembly. The plate assembly is assembled with the evaluation surface of the test piece 6 (the side with the Fe-based electroplating layer) facing the galvanized layer of the test hot-dip galvanized steel sheet 5. This plate assembly is fixed to the mounting platform 8 with 2.0 mm thick spacers 7 in between. Spacers 7 are a pair of steel plates, each 50 mm long x 45 mm short x 2.0 mm thick. Figure 11 As shown in (a), the long side end faces of each pair of steel plates are aligned with the short side end faces of the plate assembly. Therefore, the distance between the pair of steel plates is 60 mm. The fixing platform 8 is a steel plate with a hole in the center.

[0195] Next, using a servo motor-driven single-phase AC (50Hz) resistance welding machine, while bending the plate assembly under pressure with a pair of electrodes 9 (front diameter: 6mm), resistance welding was performed with a welding current of 5.9mm weld nugget diameter r under the conditions of a pressure of 3.5kN, a holding time of 0.18 seconds or 0.24 seconds, and a welding time of 0.36 seconds, to produce a plate assembly with a welded section. At this time, the pair of electrodes 9 applied pressure to the plate assembly from above and below in the vertical direction, with the lower electrode applying pressure to the test piece 6 through a hole in the fixing stage 8. During pressure application, the lower electrode of the pair of electrodes 9 was fixed to the fixing stage 8 by contacting the plane extending from the surface where the gasket 7 contacts the fixing stage 8, while the upper electrode was movable. Furthermore, the upper electrode contacted the center of the test alloyed hot-dip galvanized steel plate 5. Additionally, the plate assembly was welded while tilted 5° to the long side of the plate assembly relative to the horizontal direction. Additionally, the holding time refers to the time from the end of the welding current flow to the start of electrode release. Here, refer to... Figure 11 (b) In the figure below, the diameter r of the melt core refers to the distance between the ends of the melt cores 10 in the long side direction of the plate assembly.

[0196] Next, the aforementioned plate assembly with welded portions is arranged along the center of the welded portion, including the weld nugget 10. Figure 11 (b) Cut along line B-B in the above figure and observe the cross-section of the weld using an optical microscope (200x). Evaluate the resistance to welding cracking characteristics of the weld according to the following criteria. Furthermore, if the result is ◎ or ○, the weld is judged to have excellent resistance to welding cracking characteristics. If the result is ×, the weld is judged to have poor resistance to welding cracking characteristics.

[0197] ◎: No cracks longer than 0.1 mm were observed during a holding time of 0.18 seconds.

[0198] ○: Cracks longer than 0.1 mm were observed after a holding time of 0.18 seconds, but no cracks longer than 0.1 mm were observed after a holding time of 0.24 seconds.

[0199] ×: Cracks longer than 0.1 mm were observed after a holding time of 0.24 seconds.

[0200] in addition, Figure 11 (b) In the figure below, the crack generated in test piece 6 is schematically represented by symbol 11. Furthermore, when cracking occurs in the target-side steel plate (the alloyed hot-dip galvanized steel plate used in the test), the stress dispersion of the target steel plate (the steel plates of each invention example and comparative example) cannot be properly evaluated. Therefore, data showing that no cracking occurred in the target-side steel plate are used as examples.

[0201] The results of the above tests are listed in Tables 2-1, 2-2, and 4. These results show that the welded portions of the galvanized steel sheets of the inventive examples, which formed the Fe-based electroplating layer under conditions suitable for this invention before continuous annealing, exhibit excellent resistance to weld cracking. Furthermore, in Reference Examples 1 and 2, Si content was less than 0.50%, so there were no particular problems with the resistance to weld cracking characteristics of the welded portions. In the inventive examples satisfying equations (1) and (2), no cracks longer than 0.1 mm were observed even under a holding time of 0.18 seconds, indicating particularly good resistance to weld cracking characteristics of the welded portions. Additionally, for examples where no Fe-based electroplating layer was formed, the amount of adhesion is represented as "-", indicating that the peak of the luminescence intensity of Si (referred to as "Si intensity peak" in the table for convenience) and the number of grain boundaries (referred to as "number of grain boundaries in contact with the base iron" in the table for convenience) at the interface between the Fe-based electroplating layer and the Si-containing cold-rolled steel sheet could not be determined, and are therefore represented as "-". For steel plates with a decarburized layer thickness of less than 10 μm, the decarburized layer thickness is represented as "-". In addition, when the variables on the left side of equations (1) and (2) are "-", the variable is treated as 0 to calculate the left side of equations (1) and (2).

[0202]

[0203]

[0204]

[0205] Example 2

[0206] The steel with the chemical composition shown in Table 5 was melted to obtain a casting. The casting was then hot-rolled, pickled, and cold-rolled to produce a cold-rolled steel sheet with a thickness of 1.6 mm.

[0207]

[0208] Next, the cold-rolled steel sheet was degreased with an alkaline solution. Then, it was electrolyzed under the conditions shown below, using the steel sheet as the cathode, to produce a pre-annealed Fe-based electroplated steel sheet with an Fe-based electroplating layer on one side. The adhesion amount of the Fe-based electroplating layer was controlled by the energizing time. Next, the pre-annealed Fe-based electroplated steel sheet was reduced annealed with 15% H2-N2, a soaking zone temperature of 800°C, and an atmosphere adjusted to the dew point shown in Table 2 to obtain the Fe-based electroplated steel sheet. After cooling the obtained Fe-based electroplated steel sheet to 440–550°C, it was then hot-dip galvanized in a 460°C hot-dip galvanizing bath with an effective Al concentration of 0.197% by mass and the balance consisting of Zn and unavoidable impurities. The weight per unit area was then adjusted to approximately 50 g / m² per side by gas purging. 2 Samples of galvanized steel sheets were made.

[0209] [Electrolysis conditions]

[0210] Bath temperature: 50℃

[0211] pH: 2.0

[0212] Current density: 45A / dm 2

[0213] Fe-based electroplating bath: containing 1.5 mol / L Fe 2+ ion

[0214] Electrode (anode): Iridium oxide electrode

[0215] The amount of Fe-based electroplated coating on one side of the galvanized steel sheet manufactured as described above is determined using the method described above. Si，Fe / I Si，bulk And the number of grain boundaries at the interface between the Fe-based electroplated layer and the Si-containing cold-rolled steel sheet.

[0216] Using galvanized steel sheets prepared as described above, the C strength was determined by surface analysis of the sample cross-section according to the above method. The average C concentration and the depth of the decarburized layer were evaluated in the range of 10 μm to 20 μm from the interface between the galvanized layer and the Fe-based electroplated layer to the thickness of the Fe-based electroplated layer.

[0217] The resistance to resistance welding cracking characteristics of the welded parts were investigated for the galvanized steel sheets obtained above. The following describes the methods for determining and evaluating the resistance to resistance welding cracking characteristics of the welded parts.

[0218] <Characteristics of resistance welding cracking resistance of welded parts>

[0219] For galvanized steel sheets, the evaluation criteria included resistance to resistance welding cracking characteristics during a holding time of 0.14 seconds, Si content less than 0.1%, tensile strength of 590 MPa, and adhesion of 50 g / m² on one side. 2 The resistance to weld cracking characteristics of welded joints when using alloyed hot-dip galvanized steel sheets (1.6 mm thick). Figure 11 The evaluation method for the resistance to cracking of welded parts is explained.

[0220] Test piece 6, cut at a length of 50×150mm along the rolling right angle (TD) and with the rolling direction as the short side, was compared with a single-sided hot-dip galvanized layer of the same size. The adhesion amount was 50g / m². 2The test alloyed hot-dip galvanized steel sheets 5 were overlapped to form a plate assembly. The plate assembly was assembled with the evaluation surface of the test piece 6 (the side with the Fe-based electroplating layer) facing the galvanized layer of the test hot-dip galvanized steel sheet 5. This plate assembly was fixed to the mounting table 8 with 2.0 mm thick shims 7 in between. The shims 7 were a pair of steel plates, each 50 mm long x 45 mm short x 2.0 mm thick. Figure 11 As shown in (a), the long side end faces of each pair of steel plates are aligned with the short side end faces of the plate assembly. Therefore, the distance between the pair of steel plates is 60 mm. The fixing platform 8 is a steel plate with a hole in the center.

[0221] Next, using a servo motor-driven single-phase AC (50Hz) resistance welding machine, while bending the plate assembly under pressure with a pair of electrodes 9 (front diameter: 6mm), resistance welding was performed with a welding current of 5.9mm weld nugget diameter r under the conditions of a pressure of 3.5kN, a holding time of 0.14 seconds or 0.16 seconds, and a welding time of 0.36 seconds, to produce a plate assembly with a welded section. At this time, the pair of electrodes 9 applied pressure to the plate assembly from above and below in the vertical direction, with the lower electrode applying pressure to the test piece 6 through a hole in the fixing stage 8. During pressure application, the lower electrode of the pair of electrodes 9 was fixed to the fixing stage 8 by contacting the plane formed by extending the surface of the pad 7 that contacts the fixing stage 8, while the upper electrode was movable. Furthermore, the upper electrode contacted the center of the test alloyed hot-dip galvanized steel plate 5. Additionally, the plate assembly was welded while tilted 5° to the long side of the plate assembly relative to the horizontal direction. Additionally, the holding time refers to the time from the end of the welding current flow to the start of electrode release. Here, refer to... Figure 11 (b) In the figure below, the diameter r of the melt core refers to the distance between the ends of the melt cores 10 in the long side direction of the plate assembly.

[0222] Next, the aforementioned plate assembly with welded portions is arranged along the center of the welded portion, including the weld nugget 10. Figure 11 (b) Cut along line B-B in the above figure and observe the cross-section of the weld using an optical microscope (200x). Evaluate the resistance to weld cracking characteristics of the weld according to the following benchmarks. Furthermore, if the mark is ◎ or ○, the weld is judged to have excellent resistance to weld cracking characteristics. If the mark is ×, the weld is judged to have poor resistance to weld cracking characteristics.

[0223] ◎: No cracks longer than 0.1 mm were observed during a holding time of 0.14 seconds.

[0224] ○: Cracks longer than 0.1 mm were observed after a holding time of 0.14 seconds, but no cracks longer than 0.1 mm were observed after a holding time of 0.16 seconds.

[0225] ×: Cracks longer than 0.1 mm were observed after a holding time of 0.16 seconds.

[0226] in addition, Figure 11 (b) In the figure below, the crack generated in test piece 6 is schematically represented by symbol 11. Furthermore, when cracking occurs in the target-side steel plate (the alloyed hot-dip galvanized steel plate used in the test), the stress dispersion of the target steel plate (the steel plates of each inventive example and comparative example) cannot be properly evaluated. Therefore, data showing that no cracking occurred in the target-side steel plate are used as examples.

[0227] The results of the above tests are listed in Table 6. These results show that the welded portions of the galvanized steel sheets of the inventive examples, which formed the Fe-based electroplating layer under conditions suitable for this invention before continuous annealing, exhibit excellent resistance to weld cracking. In each inventive example satisfying equations (1) and (2), no cracks longer than 0.1 mm were observed even with a holding time of 0.14 seconds, indicating particularly good resistance to weld cracking in the welded portions. Furthermore, for examples where no Fe-based electroplating layer was formed, the amount of adhesion is represented as "-", indicating that the peak of the luminescence intensity of Si (referred to as "Si intensity peak" in the table for convenience) and the number of grain boundaries (referred to as "number of grain boundaries in contact with the base iron" in the table for convenience) at the interface between the Fe-based electroplating layer and the Si-containing cold-rolled steel sheet could not be determined, and are therefore represented as "-". For steel sheets with a decarburized layer thickness of less than 10 μm, the decarburized layer thickness is represented as "-". In addition, if the variables on the left side of equations (1) and (2) are “-”, the variable is treated as 0 to calculate the left side of equations (1) and (2).

[0228]

[0229] Industrial availability

[0230] The galvanized steel sheet manufactured by this invention not only has excellent resistance to resistance welding cracking characteristics in the welded parts, especially in preventing internal cracking, but also has high strength and excellent workability. It can be used not only as a blank material for automotive parts, but also as a blank material for applications requiring the same characteristics in the fields of home appliances, building components and the like.

[0231] Symbol Explanation

[0232] 1. Galvanized steel sheet

[0233] 2 Si-containing cold-rolled steel plates

[0234] 3Fe-based electroplating layer

[0235] 4 zinc coatings

[0236] 5. Alloyed hot-dip galvanized steel sheets used in the experiment

[0237] 6 test pieces

[0238] 7 gaskets

[0239] 8 fixed platforms

[0240] 9 electrodes

[0241] 10 molten cores

[0242] 11 cracks

Claims

1. A galvanized steel sheet, comprising: Si-containing cold-rolled steel sheet, containing 0.1% to 3.0% by mass of Si, An Fe-based electroplated layer is formed on at least one side of the Si-containing cold-rolled steel sheet, with an adhesion amount of 5.0 g / m² on each side. 2 above; A zinc plating layer is formed on the Fe-based electroplating layer; In the intensity distribution determined using glow discharge emission spectroscopy, the average Si intensity I from the interface between the zinc plating layer and the Fe-based electroplating layer to the interface between the Fe-based electroplating layer and the Si-containing cold-rolled steel sheet is... Si，Fe Divided by the average Si strength I in Si-containing cold-rolled steel sheets Si，bulk The obtained value (I) Si，Fe ) / (I Si，bulk The value is above 0.

50. The average C concentration in the range of 10 μm to 20 μm in the thickness direction from the interface between the zinc plating layer and the Fe-based electroplating layer toward the Fe-based electroplating layer is less than 0.10% by mass. At the interface between the Fe-based electroplated layer and the Si-containing cold-rolled steel sheet, the number of grain boundaries in contact between the Fe-based electroplated layer and the Si-containing cold-rolled steel sheet is more than 10 per 10 μm in the width direction of the steel sheet in the field of view of the Si-containing cold-rolled steel sheet. Furthermore, a decarburization layer extends from the interface between the zinc plating layer and the Fe-based electroplating layer towards the Fe-based electroplating layer. The adhesion amount CW on one side of the Fe-based electroplated layer Fe1 With respect to the thickness C of the decarburized layer d The following equation (1) is satisfied, 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 galvanized steel sheet according to claim 1, wherein, The Si-containing cold-rolled steel sheet contains 0.50% to 3.0% by mass of Si.

3. The galvanized steel sheet according to claim 1, wherein, The thickness of the decarburized layer is 30 μm or more.

4. The galvanized steel sheet according to any one of claims 1 to 3, wherein, The Si-containing cold-rolled steel sheet, in addition to Si, has the following composition: by mass % 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 consisting of Fe and unavoidable impurities.

5. The galvanized steel sheet according to claim 4, wherein, The composition further comprises, by mass%, one or more of the following: 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 0.020%, 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%.

6. The galvanized steel sheet according to any one of claims 1 to 3, wherein, The Fe-based electroplated layer has the following composition Composition: Contains one or more elements selected from B, C, P, N, O, Ni, Mn, Mo, Zn, W, Pb, Sn, Cr, V and Co, totaling less than 10% by mass, with the remainder consisting of Fe and unavoidable impurities.

7. An electrodeposited coated steel sheet, further comprising a chemical conversion treatment film and an electrodeposited coating film formed on the galvanized steel sheet according to any one of claims 1 to 6.

8. An automotive component, at least a portion of which is made using the electrodeposited coated steel sheet of claim 7.

9. A method for manufacturing an electrodeposited coated steel sheet, comprising the following steps: The chemical conversion treatment process involves subjecting the galvanized steel sheet according to any one of claims 1 to 6 to a chemical conversion treatment process to obtain a chemically converted steel sheet with a chemically converted coating. The electrodeposition coating process involves applying an electrodeposition coating treatment to the chemically converted steel plate to form an electrodeposited coated steel plate with an electrodeposited coating film on the chemically converted film.

10. A method for manufacturing galvanized steel sheet, comprising subjecting a cold-rolled steel sheet containing 0.1% to 3.0% by mass of Si to Fe-based electroplating treatment to produce an Fe-based electroplated steel sheet before annealing, wherein the Fe-based electroplated steel sheet before annealing has at least one side having an adhesion amount of 5.0 g / m² on one side. 2 The above 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 higher in a temperature range of 400°C to 650°C. After heating, it is held in an atmosphere with a dew point greater than -30°C and then cooled to produce an Fe-based electroplated steel sheet. Next, the Fe-based electroplated steel sheet is galvanized to obtain a galvanized steel sheet; The adhesion amount CW on one side of the Fe-based electroplated layer before annealing Fe0 The dew point DP satisfies the following equation (2), CW Fe0 The unit is g / m 2 , (C.W. Fe0 )+(D.P.)≥0···(2)。 11. The method for manufacturing galvanized steel sheet according to claim 10, wherein, The cold-rolled steel sheet contains 0.50% to 3.0% by mass of Si.

12. The method for manufacturing galvanized steel sheet according to claim 10 or 11, wherein, The Fe-based electroplating is performed 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, such that the total content of these elements in the Fe-based electroplating layer before annealing is less than 10% by mass.

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