Galvanized steel sheet, electrodeposited coated steel sheet, automobile part, method for manufacturing electrodeposited coated steel sheet, and method for manufacturing galvanized steel sheet

Abstract

A galvanized steel sheet in which the resistance to cracking of a welded portion is excellent even when the crystal orientation of a Fe-based plated layer and a cold-rolled steel sheet is integrated at a high ratio at the interface of the Fe-based plated layer and the cold-rolled steel sheet can be provided. The above-described galvanized steel sheet has a Si-containing cold-rolled steel sheet containing 0.1 to 3.0 mass% of Si, a Fe-based plated layer formed on at least one side of the above-described Si-containing cold-rolled steel sheet, the adhesion amount of each side of which exceeds 20.0 g / m 2 2, a galvanized layer formed on the above-described Fe-based plated layer; at the interface of the above-described Fe-based plated layer and the above-described Si-containing cold-rolled steel sheet, the ratio of the integration of the crystal orientation of the above-described Fe-based plated layer and the above-described Si-containing cold-rolled steel sheet exceeds 50%.

Description

Technical Field

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

[0002] In recent years, from the perspective of protecting the Earth's environment, there has been a strong demand to improve the fuel efficiency of automobiles. Furthermore, from the perspective of ensuring occupant safety 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. Therefore, in cold-rolled steel sheets used as materials for automobile components, efforts have been actively made to reduce the wall thickness based on high strength. However, since most automobile components are manufactured by forming steel sheets, these steel sheets require not only high strength but also excellent formability.

[0003] Various methods exist to improve the strength of steel sheets, but one method that can achieve high strength without significantly compromising the formability of the steel sheet is solid solution strengthening by adding silicon (Si). 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 joint during resistance welding. This causes the zinc coating to melt and diffuse into the grain boundaries, leading to liquid metal embrittlement (LME) and resulting in intergranular cracking (LME cracking) in the steel sheet. Especially when welding is performed with the welding electrode at an angle relative to the steel sheet, residual stress may increase, leading to cracking. It is believed that residual stress increases with increasing steel sheet strength, and therefore LME cracking may occur as the steel sheet strengthens. This LME cracking problem is particularly pronounced in steel sheets containing Si.

[0004] The above requires high-strength steel plates with excellent resistance to resistance welding cracking characteristics in the welded parts.

[0005] Previously, improvements to address the aforementioned problems have been reported. For example, Patent Document 1 discloses an internal oxide layer with at least a portion of grain boundaries covered by oxides extending from the surface of the base material to a depth of 5.0 μm or more, and hot-dip galvanized steel sheet in which the oxides have a grain boundary coverage of 60% or more in the region extending from the surface of the base material to a depth of 5.0 μm.

[0006] Existing technical documents

[0007] Patent documents

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

[0009] The inventors have recently discovered that forming an Fe-based electroplating layer on the surface of cold-rolled steel sheet can improve resistance welding cracking resistance. However, they have also found that when the steel sheet is annealed after forming the Fe-based electroplating layer, the crystal orientation of the Fe-based electroplating layer and the cold-rolled steel sheet is highly integrated at the interface, depending on the annealing conditions. The inventors have found that in cold-rolled steel sheets with such a high degree of crystal orientation integration, molten zinc during resistance welding can easily penetrate through the grain boundaries of the Fe-based electroplating layer into the grain boundaries of the Si-containing cold-rolled steel sheet. These phenomena are not investigated at all in Patent Document 1.

[0010] Therefore, the object of the present invention is to provide a galvanized steel sheet that exhibits excellent resistance to resistance welding cracking characteristics at the welded portion, even when the crystal orientation of the Fe-based electroplated layer and the cold-rolled steel sheet is integrated in a high proportion at the interface between the Fe-based electroplated layer and the cold-rolled steel sheet.

[0011] In order to solve the above-mentioned problems, the inventors conducted repeated and in-depth research, and found that in order to meet the resistance welding cracking characteristics of the welded part at a high level, it is important that the surface of the cold-rolled steel sheet before the annealing process after cold rolling has an adhesion amount of more than 20.0 g / m² per single side. 2 An Fe-based electroplating layer was formed as a pre-plating layer before the formation of the zinc plating layer. It was found that the adhesion amount per single side of the cold-rolled steel sheet exceeded 20.0 g / m². 2 The invention is completed by forming a soft Fe-based electroplating layer, which alleviates the stress applied to the surface of the steel plate during welding. In the case of cold-rolled steel plate containing Si, the Fe-based electroplating layer acts as a Si-deficient solid solution layer to suppress the reduction in toughness caused by Si solid solution, thereby improving the resistance to resistance welding cracking characteristics of the welded part.

[0012] This invention was made based on the above circumstances. That is, the main components of this invention are as follows.

[0013] [1] A galvanized steel sheet comprising: a Si-containing cold-rolled steel sheet containing 0.1% to 3.0% by mass of Si;

[0014] Fe-based electroplating layer, formed on at least one side of the aforementioned Si-containing cold-rolled steel sheet, with an adhesion amount exceeding 20.0 g / m² on each side. 2 ;as well as

[0015] A zinc plating layer is formed on the aforementioned Fe-based electroplating layer;

[0016] At the interface between the Fe-based electroplated layer and the Si-containing cold-rolled steel sheet, the proportion of crystal orientation integration between the Fe-based electroplated layer and the Si-containing cold-rolled steel sheet exceeds 50%.

[0017] [2] According to the Fe-based electroplated 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.

[0018] [3] According to the galvanized steel sheet described in [1] or [2] above, the adhesion amount of the Fe-based electroplating layer on each single side is 25.0 g / m². 2 above.

[0019] [4] The galvanized steel sheet according to any one of [1] to [3] above, wherein the Si-containing cold-rolled steel sheet has the following composition: in addition to the Si above, it contains, 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%, and the remainder consists of Fe and unavoidable impurities.

[0020] [5] According to the galvanized steel sheet described in [4] above, wherein the composition further contains 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 0.200%, 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%.

[0021] [6] The galvanized steel sheet according to any one of [1] to [5] above, wherein the Fe-based electroplating layer has the following composition: containing 1 or more elements selected from B, C, P, N, O, Ni, Mn, Mo, Zn, W, Pb, Sn, Cr, V and Co in total of less than 10% by mass, and the remainder is composed of Fe and unavoidable impurities.

[0022] [7] A galvanized steel sheet, comprising: cold-rolled steel sheet;

[0023] Fe-based electroplated coatings are formed on at least one side of the aforementioned cold-rolled steel sheet, with an adhesion amount exceeding 20.0 g / m² on each side. 2 ;as well as

[0024] A zinc plating layer is formed on the aforementioned Fe-based electroplating layer;

[0025] At the interface between the Fe-based electroplated layer and the cold-rolled steel sheet, the proportion of crystal orientation integration between the Fe-based electroplated layer and the cold-rolled steel sheet exceeds 50%.

[0026] Here, the aforementioned cold-rolled steel sheet is the following: a test piece cut to 50×150mm with the rolling right angle as the long side, and a hot-dip galvanized layer of the same size with an adhesion amount of 50g / m² on each single side. 2 The experiment used alloyed hot-dip galvanized steel sheets stacked together to form a plate assembly.

[0027] Next, using a servo motor-driven single-phase AC (50Hz) resistance welding machine, the plate assembly was tilted at 5° relative to the perpendicular face of the line connecting the central axes of the electrode pairs (6mm front diameter) of the resistance welding machine to the long side of the plate assembly. A gap of 60mm in the long side direction and 2.0mm in the thickness direction of the plate assembly was provided between the lower electrode of the electrode pair and the test piece. With the lower electrode and the plate assembly fixed and the upper electrode of the electrode pair movable, resistance welding was performed on the plate assembly under the following conditions: applied pressure: 3.5kN; holding time: 0.16 seconds; welding current and welding time: 5.9mm weld nugget diameter. This produced a plate assembly with a welded section.

[0028] Next, the plate assembly with the welded part was cut in half along the long side of the cold-rolled steel plate, including the welded part. When the cross-section of the welded part was observed using an optical microscope (200x magnification), cracks with a length of more than 0.1 mm were observed.

[0029] [8] According to the galvanized steel sheet described in [7] above, the cold-rolled steel sheet is subjected to resistance welding to obtain the plate assembly with the welded part. When the cross section of the welded part is observed using an optical microscope (magnification 200x), a crack with a length of 0.1 mm or more is observed.

[0030] [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.

[0031]

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

[0032]

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

[0033] 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, thereby obtaining a chemically converted steel sheet having a chemically converted coating formed on the galvanized layer; and

[0034] In the electrodeposition coating process, the above-mentioned chemically converted steel sheet is subjected to electrodeposition coating treatment to obtain an electrodeposition coated steel sheet with an electrodeposition coating film formed on the above-mentioned chemically converted film.

[0035]

[12] A method for manufacturing galvanized steel sheet, wherein an Fe-based electroplating is performed on a cold-rolled steel sheet containing 0.1% to 3.0% by mass of Si, to form at least one side with an adhesion amount exceeding 20.0 g / m² on each side. 2 Fe-based electroplated steel sheet before annealing, Fe-based electroplating layer before annealing

[0036] Next, the Fe-based electroplated steel sheet before annealing is annealed in an atmosphere with a dew point below -30°C to produce an Fe-based electroplated steel sheet.

[0037] Next, the Fe-based electroplated steel sheet is galvanized to obtain a galvanized steel sheet.

[0038]

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

[12] above, the cold-rolled steel sheet contains 0.5% to 3.0% by mass of Si.

[0039]

[14] A method for manufacturing galvanized steel sheet, wherein a cold-rolled steel sheet is subjected to Fe-based electroplating to form an adhesion layer on at least one side with an adhesion amount exceeding 20.0 g / m² on each side. 2 Fe-based electroplated steel sheet before annealing, Fe-based electroplating layer before annealing

[0040] Next, the Fe-based electroplated steel sheet before annealing is annealed to produce an Fe-based electroplated steel sheet.

[0041] Next, the above-mentioned Fe-based electroplated steel sheet is galvanized to obtain galvanized steel sheet.

[0042] Here, the aforementioned cold-rolled steel sheet is the following: a test piece cut to 50×150mm with the rolling right angle as the long side, and a hot-dip galvanized layer of the same size with an adhesion amount of 50g / m² on each single side. 2 The experiment used alloyed hot-dip galvanized steel sheets stacked together to form a plate assembly.

[0043] Next, using a servo motor-driven single-phase AC (50Hz) resistance welding machine, the plate assembly was tilted at 5° relative to the perpendicular face of the line connecting the central axes of the electrode pairs (6mm front diameter) of the resistance welding machine to the long side of the plate assembly. With a gap of 60mm in the long side direction and 2.0mm in the thickness direction of the plate assembly between the lower electrode of the electrode pair and the test piece, the lower electrode and the plate assembly were fixed while the upper electrode of the electrode pair was movable. Resistance welding was then performed on the plate assembly under the following conditions: applied pressure: 3.5kN; holding time: 0.16 seconds; welding current and welding time: 5.9mm weld nugget diameter. This produced a plate assembly with a welded section.

[0044] Next, the plate assembly with the welded part was cut in half along the long side of the cold-rolled steel plate, including the welded part. When the cross-section of the welded part was observed using an optical microscope (200x magnification), cracks with a length of more than 0.1 mm were observed.

[0045]

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

[14] above, when the cold-rolled steel sheet is observed with a cross section of the welded part under the condition that the holding time is set to 0.24 seconds, a crack with a length of 0.1 mm or more is observed.

[0046]

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

[12] to

[15] above, wherein the above-mentioned Fe-based electroplating is carried out using an Fe-based electroplating bath, the 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 the above-mentioned annealing is 10% by mass or less.

[0047] According to the present invention, a galvanized steel sheet with excellent resistance to resistance welding cracking characteristics can be provided even when the crystal orientation of the Fe-based electroplated layer and the cold-rolled steel sheet is integrated in a high proportion at the interface between the Fe-based electroplated layer and the cold-rolled steel sheet. Attached Figure Description

[0048] Figure 1 This is a diagram showing a general outline of the cross-section of an Fe-based electroplated steel sheet.

[0049] Figure 2 These are (a) a perspective view and (b) an A-A cross-sectional view of an observation sample used to determine the proportion of crystal orientation integration.

[0050] Figure 3The figures are used to illustrate the evaluation method for the proportion of crystal orientation integration. (a) is a figure depicting the boundary line at the interface between the Fe-based electroplated layer and the Si-containing cold-rolled steel sheet in the SIM image. (b) is a figure depicting the boundary line and the decision region in the binarized image. (c) is an enlarged view of the position enclosed by the box in (b).

[0051] Figure 4 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 Comparative Example No. 30.

[0052] Figure 5 In Comparative Example No. 30, a diagram showing the boundary line and decision region depicted after binarization of the interface between the Fe-based electroplated layer and the Si-containing cold-rolled steel sheet is provided.

[0053] Figure 6 This is an image showing an observation of the interface between the Fe-based electroplated layer and the Si-containing cold-rolled steel sheet of Invention Example No. 31.

[0054] Figure 7 This is a diagram showing the boundary line and determination region depicted after binarization processing of the interface between the Fe-based electroplated layer and the Si-containing cold-rolled steel sheet in Invention Example No. 31.

[0055] Figure 8 (a) is a diagram illustrating the evaluation method for the resistance to cracking characteristics of welded parts, and (b) is a top view of the welded plate assembly in this evaluation, and a B-B sectional view of the top view. Detailed Implementation

[0056] The aforementioned LME cracking can be broadly categorized into "cracking at the surface in contact with the electrode (hereinafter, surface cracking)" and "cracking near the ductile metal ring region between steel plates (hereinafter, internal cracking)." Surface cracking is known to easily occur in resistance welding with high current ranges where spatter occurs. By setting the current to an appropriate range that prevents spatter, surface cracking can be suppressed. On the other hand, internal cracking occurs even when the current during resistance welding is within an appropriate range that prevents spatter. Furthermore, surface cracking is easily detected by visual inspection during the manufacturing process, while internal cracking is not easily detected by 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 an angle relative to the steel plate, residual stress may increase, leading to internal cracking. Considering that residual stress increases with the strengthening of the steel plate, internal cracking may occur as the steel plate strengthens. In this disclosure, resistance welding cracking resistance characteristics can be improved, and in particular, the characteristics preventing internal cracking can be improved.

[0057] The embodiments of the present invention will be described below.

[0058] It should be noted that 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. Furthermore, in this specification, the numerical range indicated by "~" refers to the range of values ​​listed before and after "~" as the lower and upper limits. Additionally, in this specification, "high strength" steel sheet means that the tensile strength TS of the steel sheet measured according to JIS Z 2241 (2011) is 590 MPa or higher.

[0059] [Implementation Method 1]

[0060] Figure 1 This is a summary of the cross-section of the galvanized steel sheet 1 in this embodiment. (As 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.

[0061] Si: 0.1%~3.0%

[0062] Si (Si) significantly improves 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. On the other hand, Si negatively impacts the resistance to resistance weld cracking characteristics in welded sections. When adding Si to achieve high strength in steel sheets, an addition of 0.1% or more is required. With Si content less than 0.50%, resistance to resistance weld cracking characteristics in welded sections are not particularly problematic in welding with a holding time of approximately 0.24 seconds. However, from a production cost perspective, the production interval during spot welding in the assembly process of automotive parts becomes a concern. Even with Si content less than 0.50%, insufficient resistance to resistance weld cracking characteristics in welded sections can occur when measures are taken to reduce holding time. Conversely, if the Si content exceeds 3.0%, hot-rollability and cold-rollability decrease significantly, negatively impacting productivity or reducing the ductility of the steel sheet itself. Therefore, Si is added in the range of 0.1% to 3.0%. The Si content is preferably 0.50% or more, more preferably 0.7% or more, and even more preferably 0.9% or more, which has a greater impact on the resistance welding cracking characteristics of the welded part. Furthermore, the Si content is preferably 2.5% or less, more preferably 2.0% or less, and even more preferably 1.7% or less.

[0063] The Si-containing cold-rolled steel sheet of this embodiment requires that it contain Si within the above-mentioned range, but other components are permissible as long as they are within the composition range of ordinary cold-rolled steel sheets and are not particularly limited. Among them, when the Si-containing cold-rolled steel sheet of this embodiment is set to a high strength of 590 MPa or more tensile strength (TS), the following composition is preferred.

[0064] C: Below 0.8% (excluding 0%)

[0065] C improves workability by forming martensite and other structures within the steel. When C is present, the C content 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 C content is preferably more than 0%, more preferably more than 0.03%, and even more preferably more than 0.08%.

[0066] Mn: 1.0%~12.0%

[0067] Mn is an element that increases the strength of steel through solid solution strengthening and also 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 are 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.

[0068] P: Below 0.1% (excluding 0%)

[0069] By controlling the phosphorus (P) content, a decrease in weldability can be prevented. Furthermore, P segregation at grain boundaries can be prevented, thus preventing deterioration in ductility, flexibility, and toughness. Additionally, if a large amount of P is added, the crystal grain size increases by promoting the ferrite phase transformation. 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 exceed 0% and can be 0.001% or more.

[0070] S: Less than 0.03% (excluding 0%)

[0071] The sulfur content is preferably 0.03% or less, more preferably 0.02% or less. By suppressing the sulfur content, the reduction in weldability and the reduction in ductility during hot rolling are prevented, hot cracking is suppressed, and surface properties are significantly improved. Furthermore, by suppressing the sulfur content, coarse sulfides, which are impurity elements, are formed, thereby preventing the reduction in the ductility, bendability, and tensile flangeability of the steel sheet. These problems become significant when the sulfur content exceeds 0.03%, and it is preferable to minimize the sulfur content as much as possible. There is no particular limitation on the lower limit of sulfur; due to limitations in production technology, it can exceed 0% and can be 0.0001% or more.

[0072] N: less than 0.010% (excluding 0%)

[0073] The nitrogen (N) content is preferably 0.010% or less. By setting the N content to 0.010% or less, N forms coarse nitrides with Ti, Nb, and V at high temperatures, preventing damage to the high strength effect of the steel sheet caused by the addition of Ti, Nb, and V. Furthermore, setting the N content to 0.010% or less also prevents a decrease in toughness. Moreover, setting the N content to 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 exceed 0% and can be 0.0005% or more.

[0074] A1: Below 1.0% (excluding 0%)

[0075] 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 their oxidation 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%, the cost decreases. Therefore, when added, the Al content is preferably 1.0% or less. More preferably, it is 0.1% or less. The lower limit for Al is not particularly limited; it can exceed 0% and can be 0.001% or more.

[0076] The composition may further optionally contain 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 0.200%, 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%.

[0077] B: Below 0.005%

[0078] 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 decreases, so the amount of B is preferably 0.005% or less.

[0079] Ti: below 0.2%

[0080] Ti is effective in precipitation strengthening of steel. There is no particular lower limit for the amount of Ti, but to achieve the strength adjustment effect, it is preferably 0.005% or more. However, if excessive Ti is added, the hard phase becomes too large, reducing formability. Therefore, when adding Ti, the amount is preferably 0.2% or less, more preferably 0.05% or less.

[0081] Cr: less than 1.0%

[0082] The Cr content is preferably 0.005% or more. By setting the Cr content to 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.

[0083] Cu: below 1.0%

[0084] The Cu content is preferably 0.005% or more. Setting the Cu content to 0.005% or more promotes the formation of the residual γ phase. Furthermore, when adding Cu, from the viewpoint of preventing cost increases, the Cu content is preferably 1.0% or less.

[0085] Ni: below 1.0%

[0086] The Ni content is preferably 0.005% or more. Setting the Ni content to 0.005% or more promotes the formation of the residual γ phase. Furthermore, when adding Ni, from the viewpoint of preventing cost increases, the Ni content is preferably 1.0% or less.

[0087] Mo: 1.0% or less

[0088] The Mo content is preferably 0.005% or more. By setting the Mo content to 0.005% or more, the strength adjustment effect can be obtained. In addition, when adding Mo, from the viewpoint of preventing cost increase, the Mo content is preferably 1.0% or less.

[0089] Nb: below 0.20%

[0090] The strength is improved by containing 0.005% or more of Nb. Furthermore, when Nb is present, from the viewpoint of preventing increased costs, the amount of Nb is preferably 0.20% or less.

[0091] V: Below 0.5%

[0092] 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 V is present.

[0093] Sb: below 0.200%

[0094] Sb can be included from the viewpoint of suppressing decarburization in the tens of micrometers of the steel plate surface caused by nitriding, oxidation, or oxidation. By suppressing nitriding and oxidation on the steel plate surface, Sb prevents a reduction in the amount of martensite formed on the steel plate surface, thereby improving the fatigue properties and surface quality of the steel plate. To achieve this effect, the Sb content is preferably 0.001% or more. On the other hand, to obtain good toughness, the Sb content is preferably 0.200% or less.

[0095] Ta: Below 0.1%

[0096] Ta increases strength by containing 0.001% or more. Furthermore, when Ta is present, from the viewpoint of preventing increased costs, the amount of Ta is preferably 0.1% or less.

[0097] W: Below 0.5%

[0098] W increases strength by containing 0.005% or more. Furthermore, when W is present, from the viewpoint of preventing increased costs, the amount of W is preferably 0.5% or less.

[0099] Zr: below 0.1%

[0100] Zr content of 0.0005% or more results in increased strength. Furthermore, when Zr is present, from the viewpoint of preventing increased costs, the Zr content is preferably 0.1% or less.

[0101] Sn: below 0.20%

[0102] Sn is an element that effectively inhibits the reduction in strength of steel 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.

[0103] Ca: below 0.005%

[0104] Ca, by containing 0.0005% or more, suppresses the morphology of sulfides, thereby improving ductility and toughness. Furthermore, from the viewpoint of achieving good ductility, the Ca content is preferably 0.005% or less.

[0105] Mg: less than 0.005%

[0106] By controlling the morphology of sulfides with Mg at a content of 0.0005% or more, ductility and toughness can be improved. Furthermore, when Mg is present, from the viewpoint of preventing increased costs, the Mg content is preferably 0.005% or less.

[0107] REM: below 0.005%

[0108] REM, by containing 0.0005% or more, can control the morphology of sulfides and 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.

[0109] In the Si-containing cold-rolled steel sheet of this embodiment, the remaining components other than those mentioned above are Fe and unavoidable impurities.

[0110] Next, the Fe-based electroplating layer formed on at least one side of the aforementioned Si-containing cold-rolled steel sheet will be described.

[0111] Fe-based electroplating: exceeding 20.0 g / m 2

[0112] With an adhesion amount exceeding 20.0 g / m² per single side 2The 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 the Fe-based plating layer functions as a soft layer, mitigating the stress applied to the steel plate surface during welding. By reducing the residual stress in the resistance welded section, it can improve the resistance to resistance welding cracking characteristics, especially the characteristics preventing internal cracking (stress mitigation effect). Furthermore, it is believed that when there is a high amount of dissolved Si on the steel plate surface, the toughness of the welded section decreases, and the resistance to resistance welding cracking characteristics of the welded section deteriorate. Conversely, when there is a certain amount or more of Fe-based electroplating layer on the steel plate surface, this Fe-based electroplating layer functions as a layer lacking dissolved Si. The amount of dissolved Si in the welded section is reduced, thus suppressing the reduction in toughness of the welded section utilizing dissolved Si, and improving the resistance to resistance welding cracking characteristics, especially the characteristics preventing internal cracking (toughness reduction suppression effect). On the other hand, in this embodiment, as described later, annealing is performed after the Fe-based electroplating layer is formed. By performing annealing after forming the Fe-based electroplating layer, indentations called pick-up chips due to surface oxides such as Si and Mn formed during annealing can be suppressed on the surface of the Fe-based electroplated steel sheet. Furthermore, at the interface between the Fe-based electroplating layer and the Si-containing cold-rolled steel sheet, the crystal orientations of the Fe-based electroplating layer and the Si-containing cold-rolled steel sheet are integrated at a ratio exceeding 50%. Therefore, molten zinc easily penetrates into the grain boundaries of the Si-containing cold-rolled steel sheet via the grain boundaries of the Fe-based electroplating layer. Therefore, in this embodiment, a coating with a density exceeding 20.0 g / m³ is formed. 2 The Fe-based electroplated layer with an adhesion amount exceeding 20.0 g / m² is considered to be formed. 2 Fe-based electroplating layers, with sufficient adhesion, can delay the time it takes for molten zinc to reach the grain boundaries of Si-containing cold-rolled steel sheets during resistance welding, thereby improving the resistance weld cracking resistance of the weld, particularly its ability to prevent internal cracking (due to the grain boundary intrusion inhibition effect of zinc). The effects of these Fe-based electroplating layers on stress mitigation, toughness reduction inhibition, and the grain boundary intrusion inhibition effect of zinc plating on resistance weld cracking resistance are complex and have not yet been quantitatively determined, but it is believed that they improve resistance weld cracking resistance through a combination of factors. To achieve the effect of improving the resistance weld cracking resistance of the weld, the adhesion amount of the Fe-based electroplating layer per single side needs to exceed 20.0 g / m². 2 There is no particular upper limit to the amount of Fe-based electroplated coating applied per side. However, from a cost perspective, it is preferable to set the amount of Fe-based electroplated coating applied per side to 60.0 g / m². 2 The preferred adhesion amount of the Fe-based electroplated layer is 25.0 g / m². 2 The above, more preferably 30.0 g / m 2 The above is further preferably 35.0 g / m 2The above describes the preferred method for galvanized steel sheets, which involves applying an Fe-based electroplating layer to both the front and back surfaces of the Si-containing cold-rolled steel sheet. The Fe-based electroplating layer is set to an adhesion amount of 25.0 g / m². 2 The resistance to resistance welding cracking in the welded parts is particularly good.

[0113] It should be noted that the thickness of the Fe-based electroplating layer was measured as follows. A 10×15mm sample was taken from a hot-dip galvanized steel sheet, embedded in resin, and a cross-section was created. Using a scanning electron microscope (SEM), observations were made at any three locations on this cross-section at an accelerating voltage of 15kV and magnifications ranging from 2000 to 10000 times, depending on the thickness of the Fe-based electroplating layer. The thickness per side of the Fe-based electroplating layer was calculated by multiplying the average thickness from the three fields of view by the density of iron.

[0114] As an Fe-based electroplating layer, 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 be used. The composition of the Fe-based electroplating layer is not particularly limited, but it preferably has the following composition: containing a total of 10% by mass or less of one or more elements selected from B, C, P, N, O, Ni, Mn, Mo, Zn, W, Pb, Sn, Cr, V, and Co, with the remainder consisting of Fe and unavoidable impurities. By setting the total amount of elements other than Fe to 10% by mass or less, it is possible to prevent a decrease in electrolysis efficiency and form an Fe-based electroplating layer at low cost. In the case of an Fe-C alloy, the C content is preferably 0.08% by mass or less.

[0115] At the interface between the Fe-based electroplated layer and the Si-containing cold-rolled steel sheet, the proportion of crystal orientation integration between the Fe-based electroplated layer and the Si-containing cold-rolled steel sheet exceeds 50%. This is because when the proportion of crystal orientation integration between the Fe-based electroplated layer and the Si-containing cold-rolled steel sheet exceeds 50%, molten zinc can easily penetrate into the grain boundaries of the Si-containing cold-rolled steel sheet through the grain boundaries of the Fe-based electroplated layer. The Fe-based electroplated layer of this embodiment significantly improves this effect. In the high-strength hot-dip galvanized steel sheet of this embodiment, the proportion of crystal orientation integration between the Fe-based electroplated layer and the Si-containing cold-rolled steel sheet at the interface can be 70% or more, or 75% or more. It should be noted that there is no particular upper limit to the proportion of crystal orientation integration between the Fe-based electroplated layer and the Si-containing cold-rolled steel sheet at the interface; it can be 100%.

[0116] As described above, at the interface between the Fe-based electroplated layer and the Si-containing cold-rolled steel sheet, the higher the proportion of crystal orientation integration between the Fe-based electroplated layer and the Si-containing cold-rolled steel sheet, the easier it is for molten zinc to penetrate into the grain boundaries of the Si-containing cold-rolled steel sheet via the grain boundaries of the Fe-based electroplated layer. This trend becomes particularly pronounced when the proportion of crystal orientation integration between the Fe-based electroplated layer and the Si-containing cold-rolled steel sheet exceeds 50%. In this embodiment, after Fe-based electroplating of the Si-containing cold-rolled steel sheet, annealing is performed, and as described later, annealing is carried out using a low dew point atmosphere, thus achieving a high proportion of crystal orientation integration between the Fe-based electroplated layer and the Si-containing cold-rolled steel sheet. Therefore, by forming a layer with a crystal orientation integration exceeding 20.0 g / m², [the desired effect is achieved]. 2 The amount of Fe-based electroplating layer attached can be expected to allow molten zinc to penetrate into the grain boundaries of Si-containing cold-rolled steel sheets through the grain boundaries of the Fe-based electroplating layer, and can even be expected to further improve the resistance to resistance welding cracking characteristics of the welded parts, especially the characteristics of preventing internal cracking.

[0117] Here, at the interface between the Fe-based electroplated layer and the Si-containing cold-rolled steel sheet, the proportion of crystal orientation integration between the Fe-based electroplated layer and the Si-containing cold-rolled steel sheet was determined as follows. A 10×10mm sample was taken from the galvanized steel sheet. The sample was processed at an arbitrary location using a focused ion beam (FIB) device to form a 45° cross-section with a width of 30μm in the rolling right-angle direction and a length of 50μm in the 45° direction relative to the T-section (a cross-section parallel to the rolling right angle direction of the steel sheet and perpendicular to the steel sheet surface). This cross-section was used for observation. Figure 2 The image shows a summary of the sample used for this observation. Figure 2 (a) is a three-dimensional view of the sample used for observation. Figure 2 (b) is Figure 2 (a) shows the A-A cross-sectional view of the sample for observation. Next, using a scanning ion microscope (SIM), the central part of the 45° section of the sample for observation was observed at 5000x magnification, and an 8-bit SIM image with a width of 1024 × height of 943 pixels was captured. Based on the following formula (1), the proportion of crystal orientation integration between the Fe-based electroplated layer and the Si-containing cold-rolled steel sheet at the interface of the SIM image taken at the 45° section was determined. It should be noted that decimals are rounded.

[0118] (The proportion of the crystal orientation integration between the Fe-based electroplated layer and the Si-containing cold-rolled steel sheet at the interface) = (the length of the position where the crystal orientation of the Fe-based electroplated layer and the Si-containing cold-rolled steel sheet is integrated at the interface) ÷ (the length of the interface in the field of view) × 100……(1)

[0119] At the interface between the Fe-based electroplated layer and the Si-containing cold-rolled steel sheet, whether the crystal orientation of the Fe-based electroplated layer and the Si-containing cold-rolled steel sheet is integrated is determined through image processing. Figure 3 The evaluation method for the proportion of crystal orientation integration is explained. First, as... Figure 3 As shown in (a), at the interface between the Fe-based electroplated layer 3 and the Si-containing cold-rolled steel sheet 2 in the aforementioned SIM image, the boundary line B was depicted using a scanning electron microscope. Next, the SIM image was processed separately from the image with the depicted boundary line to create an image. Specifically, firstly, the grain boundaries of the captured 8-bit SIM image (1024 x 943 pixels) were emphasized using a Sobel filter. Then, the image with emphasized grain boundaries was smoothed using a Gaussian filter (radius (R): 10 pixels). Next, the smoothed image was binarized (threshold: 17). Then, the boundary line B of the image depicting the cross-section was transferred to the binarized image. Then, as... Figure 3 As shown in (b), in the binarized image, along the 40-pixel-wide decision region centered on boundary line B ( Figure 3 The boundary line B on the binarized image (the region enclosed by L1 and L2 in (b)) is depicted. The length of the boundary line B is considered as the length of the portion where the interface between the Fe-based electroplated layer and the Si-containing cold-rolled steel sheet (the white and black boundary in the binarized image) does not exist within the determination region. Here, the total length of the boundary line is calculated as follows: First, the entire determination region is explored to divide it into approximately rectangular areas such that the two normals of the boundary line B contain only either white or black. Then, the maximum distance between the intersections of the boundary line and the two normals of this portion is summed over the entire determination region to obtain the total length of the boundary line in the length of the boundary line that contains the interface between the Fe-based electroplated layer and the Si-containing cold-rolled steel sheet. It should be noted that the length of the portion where the crystal orientation is integrated can be obtained by subtracting the length of the non-integrated portion from the length of the cross section in the field of view. For explanation, Figure 3 (c) shows Figure 3(b) is an enlarged view of the area enclosed by the box. First, as... Figure 3 As shown in (c), exploring the entire decision region can roughly divide the decision region into a rectangle so that the two normals of the boundary line B ( Figure 3 In (c), l1 and l2, l3 and l4, l5 and l6, l7 and l8, and l9 and l 10 The region includes both white and black areas. Next, the maximum distance between the intersections of the boundary line of this region and the two normals is summed across the entire judgment region. This sum is taken as the length of the boundary line, which includes the length of the interface between the Fe-based electroplated layer and the Si-containing cold-rolled steel sheet within the judgment region. The length of the region with integrated crystal orientation is obtained by subtracting this length (i.e., the length of the region where the crystal orientation is not integrated) from the length of the cross-section in the observation field.

[0120] Figure 4 The image shown is a SIM image of the interface between the Fe-based electroplated layer and the Si-containing cold-rolled steel sheet of Comparative Example No. 30, which is described later. Figure 5 The image shown is a binarized version of the SIM image as described above. In Comparative Example No. 30, at the interface between the Fe-based electroplated layer and the Si-containing cold-rolled steel sheet, the proportion of crystal orientation integration between the Fe-based electroplated layer and the Si-containing cold-rolled steel sheet is 94%. Furthermore, 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, according to Invention Example No. 31 of the embodiments described later. Figure 7 The image shown is a binarized version of the SIM image as described above. In Invention Example No. 31, at the interface between the Fe-based electroplated layer and the Si-containing cold-rolled steel sheet, the proportion of crystal orientation integration between the Fe-based electroplated layer and the Si-containing cold-rolled steel sheet is 92%.

[0121] Next, the zinc plating layer formed on the Fe-based electroplating layer will be explained. Here, "zinc plating layer" refers to the zinc coating formed on the surface of the steel plate. It is not limited to zinc coating formation methods such as plating, thermal spraying, and cold spraying; any zinc coating formed on the surface of the steel plate is included in "zinc plating layer".

[0122] Excellent corrosion resistance can be obtained by forming a galvanized layer on the surface of the steel sheet. Conversely, during resistance welding, molten zinc may penetrate into the grain boundaries of the Si-containing cold-rolled steel sheet through the grain boundaries of the Fe-based electroplating layer, making it generally difficult to improve the resistance weld crack resistance of the welded part. As mentioned above, as a pre-coating before forming a galvanized layer on the surface of the cold-rolled steel sheet after the annealing process following cold rolling, the adhesion amount per single side exceeds 20.0 g / m². 2By forming an Fe-based electroplating layer, the resistance to resistance weld cracking of the welded parts in galvanized steel sheets can be improved. As described above, if an Fe-based electroplating layer is formed as a pre-coating before forming the galvanized layer, the resistance to resistance weld cracking of the welded parts in the galvanized steel sheet can be improved regardless of the type of galvanized layer. The galvanized layer can be, for example, a hot-dip galvanized layer, an electroplated layer, a zinc thermal spray coating, or a cold spray coating. The composition of the galvanized layer is not particularly limited; for example, in the case of a hot-dip galvanized layer, it consists of Al, Zn, and unavoidable impurities. The Al content in the galvanized layer is not particularly specified; in one example, the Al content of a hot-dip galvanized layer is 0.05% by mass to 0.250% by mass. It should be noted that the galvanized layer can be an alloyed galvanized layer.

[0123] The coating thickness per single side of the zinc plating can be 25 g / m². 2 The above, and can be 80g / m 2 The following is an example of setting the coating adhesion amount per single side of the galvanized layer to 25 g / m. 2 The above measures further improve corrosion resistance and facilitate control of the coating adhesion. Additionally, if the coating adhesion per single side of the zinc plating layer is 80 g / m²... 2 The following coatings exhibit good adhesion.

[0124] According to this disclosure, high-strength galvanized steel sheets with a tensile strength (TS) of 590 MPa or higher, as measured according to JIS Z 2241 (2011), can be provided. More preferably, the strength of the galvanized steel sheet is 800 MPa or higher.

[0125] The thickness of the galvanized steel sheet in this embodiment is not particularly limited, and can generally be 0.5 mm or more, or 3.2 mm or less.

[0126] <Manufacturing Method of Galvanized Steel Sheet>

[0127] Next, the manufacturing method of galvanized steel sheet will be explained.

[0128] 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 exceeding 20.0 g / m² on at least one side. 2 Fe-based electroplated steel sheet before annealing, Fe-based electroplating layer before annealing

[0129] Next, the Fe-based electroplated steel sheet before annealing is annealed in an atmosphere with a dew point below -30°C to produce an Fe-based electroplated steel sheet.

[0130] Next, the Fe-based electroplated steel sheet is galvanized to obtain a galvanized steel sheet.

[0131] First, a cold-rolled steel sheet containing 0.1% to 3.0% by mass of Si is manufactured. It should be noted that 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 be carried out according to the usual 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, then the hot-rolled sheet is pickled, and then the hot-rolled sheet is cold-rolled to produce a cold-rolled steel sheet.

[0132] 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, sulfuric acid baths, hydrochloric acid baths, or a mixture of both can be used as the Fe-based electroplating bath. It should be noted that Fe-based electroplated steel sheet before annealing refers to steel sheet where the Fe-based electroplating layer has not undergone an annealing process; however, this does not preclude the possibility of pre-annealing the cold-rolled steel sheet before Fe-based electroplating treatment.

[0133] 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 sufficient to achieve adequate Fe adhesion. Furthermore, to obtain sufficient Fe adhesion, the Fe ion content in the Fe-based electroplating bath before energization is preferably below 2.0 mol / L.

[0134] Furthermore, the Fe-based electroplating bath may contain elements selected from Fe ions and at least one of B, C, P, N, O, Ni, Mn, Mo, Zn, W, Pb, Sn, Cr, V, and Co. The total content of these elements in the Fe-based electroplating bath is preferably less than 10% by mass in the Fe-based coating before annealing. It should be noted that 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. Additionally, the ferric sulfate plating solution may contain conductive agents such as sodium sulfate and potassium sulfate, chelating agents, and pH buffers.

[0135] There are no particular limitations on other conditions for the Fe-based electroplating bath. 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, and considering the electrical 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 controlling the adhesion amount of easily deposited Fe-based electroplated layers, an A / dm² is preferred. 2The following applies. From a productivity point of view, the plate speed is preferably 5 mpm or higher, and from the point of view of stable control of the amount of adhesion, it is preferably 150 mpm or lower.

[0136] It should be noted that, as a pretreatment before Fe-based electroplating, degreasing and rinsing for cleaning the surface of the cold-rolled steel sheet, and pickling and rinsing for activating the surface of the cold-rolled steel sheet can be performed. 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 surface roughness (surface defects) due to over-pickling, it is preferably around 1 to 20 mass%. Furthermore, the pickling solution may contain defoamers, pickling accelerators, pickling inhibitors, etc.

[0137] Next, after Fe-based electroplating treatment, the Fe-based electroplated steel sheet before annealing is subjected to an annealing process in a reducing atmosphere with a dew point below -30°C and a hydrogen concentration of 1.0% to 30.0% by volume, held 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 performed to remove the strain of the Fe-based electroplated steel sheet before annealing caused by the rolling process, allowing the microstructure to recrystallize and thereby improving the strength of the steel sheet.

[0138] Dew point: below -30℃

[0139] In this embodiment, the dew point of the annealing atmosphere in the annealing process is a low dew point below -30°C, under conditions where no additional equipment such as humidification equipment is required. Controlling the dew point below -30°C is preferably performed within a temperature range of 650°C to 900°C. Through independent research, the inventors have discovered a correlation between the proportion of crystal orientation integration 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, and the dew point of the annealing atmosphere in the annealing process after the formation of the Fe-based electroplated layer. Specifically, when annealing the Fe-based electroplated steel sheet before annealing after the formation of the Fe-based electroplated layer, the lower the dew point of the annealing atmosphere, the higher the proportion of crystal orientation integration between the Fe-based electroplated layer and the Si-containing cold-rolled steel sheet obtained after annealing; conversely, the higher the dew point of the annealing atmosphere, the lower the proportion of crystal orientation integration between the Fe-based electroplated layer and the Si-containing cold-rolled steel sheet. The reason for the correlation between the proportion of crystal orientation integration between the Fe-based electroplated layer and the Si-containing cold-rolled steel sheet and the dew point is not yet clear, but it can be speculated as follows: When the dew point is controlled at a certain high level, elements that diffuse from the steel sheet into the Fe-based electroplated layer during annealing form oxides inside the Fe-based electroplated layer. These oxides hinder the crystal growth of the Fe particles, resulting in grain refinement. On the other hand, when annealing is performed in a low dew point atmosphere after the formation of the Fe-based electroplated layer, it is difficult for the aforementioned oxides to form, and the crystal grain size of the Fe-based electroplated layer becomes coarser. Therefore, it can be considered that when annealing is performed at a low dew point, the crystal orientation of the Fe-based electroplated layer and the crystal orientation of the Si-containing cold-rolled steel sheet are integrated to a high proportion. Due to reasons such as the cost of the humidification equipment in the annealing furnace, when the dew point of the annealing atmosphere in the annealing process is set to below -30°C, the proportion of crystal orientation integration between the Fe-based electroplated layer and the Si-containing cold-rolled steel sheet at the interface increases. As a result, when galvanized steel sheets are manufactured, molten zinc during resistance welding can easily penetrate the grain boundaries of the Si-containing cold-rolled steel sheet via the grain boundaries of the Fe-based electroplating layer. In this embodiment, by forming an Fe-based electroplating layer with a specific adhesion amount, when the sheet assembly is galvanized steel sheet, the time it takes for molten zinc to reach the grain boundaries of the Si-containing cold-rolled steel sheet via the grain boundaries of the Fe-based electroplating layer during resistance welding is delayed, thereby improving the resistance welding crack resistance characteristics of the welded portion. The lower limit of the dew point of the annealing atmosphere is not particularly determined; however, achieving a dew point below -80°C is difficult industrially, so -80°C or higher is preferred. The dew point of the annealing atmosphere is more preferably -55°C or higher.

[0140] Hydrogen concentration: 1.0 vol% to 30.0 vol%

[0141] The annealing process is carried out in a reducing atmosphere with a hydrogen concentration of 1.0 vol% to 30.0 vol%. Hydrogen serves to suppress the oxidation of Fe on the surface of the Fe-based electroplated steel sheet before annealing and to activate the steel sheet surface. If the hydrogen concentration is 1.0 vol% or higher, the Fe on the steel sheet surface will oxidize, which, as described later, can prevent deterioration of the plating adhesion when the zinc plating layer is applied. 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; 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.

[0142] Holding time in the temperature range of 650℃~900℃: 30 seconds~600 seconds

[0143] In the annealing process, it is preferable to set the holding time in the temperature range of 650°C to 900°C to be 30 to 600 seconds. By setting the holding time in this temperature range to 30 seconds or more, the natural oxide film of Fe formed on the surface of the Fe-based electroplated layer before annealing can be appropriately removed, thereby improving the coating adhesion when a zinc plating layer is applied to the surface of the Fe-based electroplated steel sheet, as described later. Therefore, the holding time in this temperature range is preferably 30 seconds or more. The upper limit of the holding time in this temperature range is not particularly determined, but from a productivity point of view, the holding time in this temperature range is preferably 600 seconds or less.

[0144] Maximum temperature reached for Fe-based electroplated steel sheets before annealing: 650℃~900℃

[0145] 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 reached before annealing of Fe-based electroplated steel sheets to 650°C or higher, the recrystallization of the steel sheet structure can be appropriately carried out, resulting in the desired strength. Furthermore, the natural oxide film of Fe formed on the surface of the Fe-based electroplated layer before annealing can be appropriately reduced, which, as will be explained later, improves the adhesion of the coating when hot-dip galvanizing is applied to the surface of the Fe-based electroplated steel sheet. Additionally, if the maximum temperature reached before annealing of 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, and the diffusion of Si and Mn to the steel sheet surface can be prevented. Therefore, as will be explained later, the adhesion of the coating when hot-dip galvanizing is applied to the surface of the Fe-based electroplated steel sheet can be improved. 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 cold-rolled steel sheets is preferably 900°C or lower. It should be noted that the above-mentioned maximum temperature is based on the temperature measured on the surface of the cold-rolled steel sheet.

[0146] Next, the galvanizing process will be explained.

[0147] Following the aforementioned annealing process, the Fe-based electroplated steel sheet is galvanized. The galvanizing method is not particularly limited; for example, it can be hot-dip galvanizing, electroplating, cold spraying, or plasma spraying. In the case of hot-dip galvanizing, in one example, the Fe-based electroplated steel sheet can be cooled after the annealing process and immersed in a hot-dip galvanizing bath to achieve 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 specifically defined; generally, the Al concentration in the bath is 0.05% to 0.250% by mass. Setting the Al concentration in the bath to 0.05% by mass or higher prevents the formation of bottom dross, preventing dross from adhering to the steel sheet and becoming a defect. Furthermore, setting the Al concentration in the bath to 0.250% by mass or less prevents the increase of top dross, preventing dross from adhering to the steel sheet and forming defects, and resulting in cost reduction. Other conditions for hot-dip galvanizing are not restricted, but for example, the bath temperature of the hot-dip galvanizing bath is usually in the range of 440 to 500°C, and the steel plate is immersed in the hot-dip galvanizing bath at a plate temperature of 440 to 550°C.

[0148] The amount of zinc coating applied per single side can be 25g / m². 2 The above, and can be 80g / m 2 The following is an example of setting the adhesion amount of the galvanized layer on each single side to 25 g / m². 2 The above measures further improve corrosion resistance and make the coating adhesion easy to control. Additionally, if the adhesion of the zinc coating on each single side is 80 g / m²... 2 The following results in good adhesion of the galvanized layer. The preferred adhesion amount per single side of the galvanized layer is 35 g / m². 2 That's all. Furthermore, the preferred adhesion amount of the galvanized layer per single side is 60 g / m². 2 the following.

[0149] After galvanizing, the coating adhesion can be adjusted appropriately. There are no particular limitations on the method for adjusting the coating adhesion; for example, in hot-dip galvanizing, the coating adhesion is generally adjusted by gas wiping. In one example, the coating adhesion is adjusted by factors such as the gas pressure during wiping and the distance between the wiping nozzle and the steel plate. It should be noted that the galvanized layer is not alloyed after galvanizing.

[0150] <Electrodeposited Coated Steel Sheets>

[0151] Furthermore, according to this embodiment, an electrodeposited coated steel sheet can also be provided, which further comprises a chemical conversion coating formed on the galvanized layer and an electrodeposited coating formed on the chemical conversion coating. Because the galvanized steel sheet of this embodiment exhibits excellent resistance to resistance weld cracking at the welded portion, 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 known chemical conversion coatings and electrodeposited coatings can be formed. Zinc phosphate coatings, zirconium coatings, etc., can be used as chemical conversion coatings. As 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; preferably, the coating in the dry state is 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.

[0152] <Manufacturing Method of Electrodeposited Coated Steel Sheets>

[0153] 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 including the following engineering: a chemical conversion treatment process in which a chemical conversion treatment film is formed on the galvanized layer by performing a chemical conversion treatment on a galvanized steel sheet; and an electrodeposition coating process in which an electrodeposition coating film is formed on the chemical conversion treatment film by performing an electrodeposition coating treatment on the chemical conversion treatment steel sheet. The chemical conversion treatment and the electrodeposition coating treatment can be performed using known methods. It should be noted that, as a pretreatment before performing the chemical conversion treatment, degreasing treatment for cleaning the surface of the galvanized steel sheet, water washing, and surface conditioning treatment as needed can be performed. The chemical conversion treatment is performed after these pretreatments. The methods for degreasing treatment and water 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 applying these surface modifiers, no special procedures are required; common methods can be used. For example, after dissolving the desired surface modifier in a specified amount of deionized water and stirring thoroughly, a treatment solution is prepared at a specified temperature (usually room temperature, 25–30°C). A steel plate is then immersed in this treatment solution for a specified time (20–30 seconds). Next, without drying, the subsequent chemical conversion treatment is performed. The chemical conversion treatment can also be performed using common methods. For example, after dissolving the desired chemical conversion agent in a specified amount of deionized water and stirring thoroughly, a treatment solution is prepared at a specified temperature (usually 35–45°C). A steel plate is then immersed in this treatment 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 subsequent electrodeposition coating process is performed. Electrodeposition coating can also be performed using common methods. As needed, after pretreatment such as water washing, the steel sheet is impregnated with a thoroughly stirred electrodeposited coating, and the desired thickness of the electrodeposited coating is obtained through electrodeposition. In addition to cationic electrodeposited coatings, anionic electrodeposited coatings can also be used. Furthermore, depending on the application, a top coat can be applied after the electrodeposited coating.

[0154] <Automotive Parts>

[0155] Furthermore, according to this embodiment, an automotive component using at least a portion of the electrodeposited coated steel sheet described above can be provided. The galvanized steel sheet of this embodiment exhibits excellent resistance to resistance weld cracking in its welded portions, therefore the electrodeposited coated steel sheet using this galvanized steel sheet is particularly suitable for automotive component applications. Automotive components made using the electrodeposited coated steel sheet may include steel sheets other than those of this embodiment as materials. Because the electrodeposited coated steel sheet of this embodiment has excellent resistance to resistance weld cracking in its welded portions, LME cracking in the welded portions of automotive components made using this galvanized steel sheet can be appropriately prevented. The type of automotive component using at least a portion of the electrodeposited coated steel sheet is not particularly limited; for example, it may be a side beam component, a pillar component, or an automotive body.

[0156] [Implementation Method 2]

[0157] Next, the galvanized steel sheet of Embodiment 2 of the present invention will be described.

[0158] The galvanized steel sheet of this embodiment has:

[0159] Cold-rolled steel sheet,

[0160] The amount of adhesion formed on at least one side of the aforementioned cold-rolled steel sheet exceeds 20.0 g / m² per side. 2 Fe-based electroplated layers, and

[0161] A zinc plating layer formed on the above-mentioned Fe-based electroplating layer;

[0162] At the interface between the Fe-based electroplated layer and the cold-rolled steel sheet, the proportion of crystal orientation integration between the Fe-based electroplated layer and the cold-rolled steel sheet exceeds 50%.

[0163] Here, the aforementioned cold-rolled steel sheet is a steel sheet in which a test piece of 50×150mm is cut with the rolling right angle as the long side, and a hot-dip galvanized layer of the same size is applied to each single side with an adhesion amount of 50g / m². 2 The experiment used alloyed hot-dip galvanized steel sheets stacked together to form a plate assembly.

[0164] Next, using a servo motor-driven single-phase AC (50Hz) resistance welding machine, the plate assembly was tilted at 5° relative to the perpendicular face of the line connecting the central axes of the electrode pairs (6mm front diameter) of the resistance welding machine to the long side of the plate assembly. A gap of 60mm in the long side direction and 2.0mm in the thickness direction of the plate assembly was provided between the lower electrode of the electrode pair and the test piece. With the lower electrode fixed to the plate assembly and the upper electrode of the electrode pair movable, resistance welding was performed on the plate assembly under the conditions of a pressure of 3.5kN, a holding time of 0.16 seconds, a welding current with a weld nugget diameter of 5.9mm, and a welding time to produce a plate assembly with a welded section.

[0165] Next, the plate assembly with the aforementioned welded portion was cut in half along the long side of the test piece, including the welded portion. When the cross-section of the welded portion was observed using an optical microscope (200x magnification), cracks with a length of 0.1 mm or more were observed.

[0166] The cold-rolled steel sheet in this embodiment is not particularly limited as long as it is a steel sheet with poor resistance to resistance weld cracking characteristics in the welded part when the sheet group is galvanized steel sheet and the following test evaluation is conducted. The composition of the cold-rolled steel sheet is also not particularly limited. The inventors found that if the cold-rolled steel sheet has a Si content of 0.1% by mass or more, the resistance to resistance weld cracking characteristics in the welded part are poor as evaluated in the following test.

[0167] Cold-rolled steel sheet can be a sheet assembly with a welded section obtained by resistance welding with a holding time of 0.24 seconds, where cracks of 0.1 mm or longer are observed in the cross-section of the welded section under an optical microscope (200x magnification). It should be noted that, generally, the resistance weld cracking resistance of the welded section deteriorates as the holding time decreases, even for the same cold-rolled steel sheet. Therefore, if a cold-rolled steel sheet exhibits cracks of 0.1 mm or longer when subjected to the following test with a holding time of 0.24 seconds, then even when resistance welding is performed with a holding time of 0.16 seconds, cracks of 0.1 mm or longer are also observed in the cross-section of the welded section under an optical microscope (200x magnification). If the cold-rolled steel sheet has a Si content of 0.50% by mass or more, the resistance weld cracking resistance of the welded part is poor as evaluated by the following test. However, even for cold-rolled steel sheets with a Si content of less than 0.50% by mass, examples of poor resistance weld cracking resistance in the welded part have been confirmed as evaluated by the following test.

[0168] <Resistance to resistance welding cracking characteristics of welded parts>

[0169] use Figure 8The evaluation method for the resistance to cracking characteristics of welded parts is explained. Test pieces 6, cut with the rolling right-angle direction (TD) as the long side and the rolling direction as the short side, and hot-dip galvanized coatings of the same size, with an adhesion amount of 50 g / m² per single side, are used. 2 The test alloyed hot-dip galvanized steel sheets 5 were stacked to form a plate assembly. The plate assembly was assembled with the evaluation surface (the Fe-based electroplated layer side) of the test piece 6 facing the galvanized layer of the test alloyed hot-dip galvanized steel sheet 5. This plate assembly was fixed to the mounting platform 8 via a 2.0 mm thick spacer 7. The spacer 7 consisted of a pair of steel plates, each 50 mm long x 45 mm short x 2.0 mm thick. Figure 8 As shown in (a), the long side end faces of a pair of steel plates are aligned with the short side end faces of the plate assembly. Therefore, the distance between the pair of steel plates is 60 mm. The fixing platform 8 is a plate with a hole in the center.

[0170] Next, using a servo motor-driven single-phase AC (50Hz) resistance welding machine, while bending the plate assembly by applying pressure with a pair of electrodes 9 (front diameter: 6mm), resistance welding was performed at a welding current and welding time of 5.9mm weld nugget diameter r under the conditions of a pressure of 3.5kN and a holding time of 0.18 seconds or 0.24 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 vertically upwards and downwards, with the lower electrode applying pressure to the test piece 6 through the hole in the fixing table 8. During pressure application, the lower electrode of the pair of electrodes 9 was fixed in such a way that it contacted the plane extending the surface that contacts the spacer 7 and the fixing table 8, while the upper electrode was movable. In addition, the upper electrode contacted the center of the test alloyed hot-dip galvanized steel plate 5. Furthermore, the plate assembly is a vertical plane relative to the line connecting the central axis of the electrode pair of the resistance welding machine ( Figure 8 (a) Welding is performed with the plate group tilted at 5° towards its long side (horizontal direction). A gap of 60 mm along the long side and 2.0 mm along the thickness direction of the plate group is formed between the lower electrode and the test piece 6 using the aforementioned separator. It should be noted that the holding time refers to the time from the end of the welding current flow until the electrode is opened. Here, refer to... Figure 8 In (b) 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.

[0171] Next, the aforementioned plate assembly with welded portions is arranged along the center of the welded portion, including the weld nugget 10. Figure 8(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 resistance welding cracking characteristics of the weld according to the following criteria. It should be noted that if it is ◎ or ○, the weld is judged to have excellent resistance to resistance welding cracking characteristics. If it is ×, the weld is judged to have poor resistance to resistance welding cracking characteristics.

[0172] ◎: No cracks longer than 0.1 mm were observed during a holding time of 0.14 seconds.

[0173] ○: Cracks longer than 0.1 mm were observed when the holding time was 0.14 seconds, but no cracks longer than 0.1 mm were observed when the holding time was 0.16 seconds.

[0174] ×: Cracks longer than 0.1 mm were observed when the holding time was 0.16 seconds.

[0175] In addition, as a more stable welding condition, the resistance to resistance welding cracking characteristics of the welded part can be evaluated using the following benchmarks.

[0176] ◎: No cracks longer than 0.1 mm were observed during a holding time of 0.18 seconds.

[0177] ○: Cracks longer than 0.1 mm were observed when the holding time was 0.18 seconds, but no cracks longer than 0.1 mm were observed when the holding time was 0.24 seconds.

[0178] ×: Cracks longer than 0.1 mm were observed during a holding time of 0.24 seconds.

[0179] It should be noted that, in Figure 8 (b) In the figure below, symbol 11 schematically represents an example of a crack produced in test piece 6.

[0180] The Fe-based electroplated layer and the zinc layer of the galvanized steel sheet in this embodiment are the same as in Embodiment 1, so the description is omitted here. Furthermore, at the interface between the Fe-based electroplated layer and the cold-rolled steel sheet, the proportion of crystal orientation integration between the Fe-based electroplated layer and the cold-rolled steel sheet is also greater than 50%, similar to Embodiment 1. Details regarding the proportion of crystal orientation integration between the Fe-based electroplated layer and the cold-rolled steel sheet at the interface are the same as in Embodiment 1, so the description is omitted here.

[0181] Next, the manufacturing method of the galvanized steel sheet of Embodiment 2 will be described.

[0182] One embodiment of the method for manufacturing galvanized steel sheet may be as follows: Fe-based electroplating is performed on cold-rolled steel sheet to form a coating on at least one side with an adhesion amount exceeding 20.0 g / m² per side. 2Fe-based electroplated steel sheet before annealing, Fe-based electroplating layer before annealing

[0183] Next, the Fe-based electroplated steel sheet before annealing is annealed to produce an Fe-based electroplated steel sheet.

[0184] Next, the above-mentioned Fe-based electroplated steel sheet is galvanized to obtain galvanized steel sheet.

[0185] Here, the aforementioned cold-rolled steel sheet is a cold-rolled steel sheet as follows: a test piece cut into 50×150mm sections with the rolling right angle as the long side, and hot-dip galvanized sheets of the same size, with an adhesion amount of 50g / m² on each single side. 2 The experiment used alloyed hot-dip galvanized steel sheets stacked together to form a plate assembly.

[0186] Next, using a servo motor-driven single-phase AC (50Hz) resistance welding machine, the aforementioned plate assembly was tilted at 5° relative to the side perpendicular to the central axis of the electrode pair (6mm front diameter) of the resistance welding machine, which connects to each other. A gap of 60mm in the long side direction and 2.0mm in the thickness direction of the plate assembly was provided between the lower electrode of the electrode pair and the test piece. With the lower electrode fixed to the plate assembly and the upper electrode of the electrode pair movable, resistance welding was performed on the plate assembly under the following conditions: applied pressure: 3.5kN; holding time: 0.16 seconds; welding current and welding time: 5.9mm weld nugget diameter. This produced a plate assembly with a welded section.

[0187] Next, the test piece was cut in half along the long side of the plate assembly containing the welded part. When the cross-section of the welded part was observed using an optical microscope (200x magnification), a crack with a length of more than 0.1 mm was observed.

[0188] First, cold-rolled steel sheets are manufactured. The manufacturing method for cold-rolled steel sheets can be based on conventional methods for manufacturing cold-rolled steel sheets. In one example, a steel billet is hot-rolled to produce a hot-rolled sheet, which is then pickled and subsequently cold-rolled to produce a cold-rolled steel sheet.

[0189] In this embodiment, the cold-rolled steel sheet is not particularly limited as long as it passes the above-described test evaluation and, when the sheet assembly is galvanized steel sheet, exhibits poor resistance to resistance weld cracking in the welded portion. The composition of the cold-rolled steel sheet is also not particularly limited; however, if the cold-rolled steel sheet has a Si content of 0.1% by mass or more, then the resistance to resistance weld cracking in the welded portion will be poor as evaluated in the above-described test.

[0190] The cold-rolled steel sheet can be an assembly of plates with welded sections obtained by resistance welding with a holding time of 0.24 seconds, where cracks of 0.1 mm or more in length are observed in the cross-section of the welded section under an optical microscope (200x magnification). It should be noted that, generally, the resistance weld cracking resistance of the welded section deteriorates as the holding time decreases, even for the same cold-rolled steel sheet. Therefore, if a cold-rolled steel sheet with cracks of 0.1 mm or more in length are observed in the cross-section of the welded section when resistance welding is performed with a holding time of 0.24 seconds, then when resistance welding is performed with a holding time of 0.16 seconds, cracks of 0.1 mm or more in length are also observed in the cross-section of the welded section under an optical microscope (200x magnification). If the cold-rolled steel sheet has a Si content of 0.50% by mass or more, the resistance weld cracking resistance of the welded part evaluated by the above test is poor. However, even for cold-rolled steel sheets with a Si content of less than 0.50% by mass, examples of poor resistance weld cracking resistance of the welded part evaluated by the above test have been confirmed.

[0191] Next, the surface of the cold-rolled steel sheet is subjected to Fe-based electroplating treatment to obtain Fe-based electroplated steel sheet before annealing. Details of the Fe-based electroplating treatment are as described above and will not be elaborated further.

[0192] Next, the Fe-based electroplated steel sheet before annealing is subjected to an annealing process in a reducing atmosphere with a dew point below -30°C and a hydrogen concentration of 1.0% to 30.0% by volume, held 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. Details of the annealing process are as described above, so they are omitted here.

[0193] Similar to Embodiment 1 described above, galvanizing is performed on the Fe-based electroplated steel sheet in this embodiment to obtain a galvanized steel sheet. The details of the galvanizing process are the same as in Embodiment 1, and therefore are omitted here.

[0194] In this embodiment, similar to Embodiment 1 described above, an electrodeposited coated steel sheet can also be provided, which further comprises a chemical conversion coating formed on the galvanized layer and an electrodeposited coating film formed on the chemical conversion coating. Additionally, an electrodeposited coated galvanized steel sheet for performing electrodeposited coating can also be provided. Details regarding the electrodeposited coated steel sheet and its manufacturing method are the same as in Embodiment 1 described above, and therefore are omitted here.

[0195] Furthermore, in this embodiment, a car component can be provided in the same manner as in Embodiment 1 described above. Details regarding the car component are as described above, and therefore will not be repeated here.

[0196] The present invention will now be described in detail based on specific embodiments.

[0197] Example 1

[0198] The steel with the chemical compositions shown in Tables 1 and 3 was melted to obtain castings, which were then hot-rolled, pickled, and cold-rolled to produce cold-rolled steel sheets with a thickness of 1.6 mm.

[0199]

[0200]

[0201] Next, the cold-rolled steel sheet was degreased using alkali, and then electrolyzed using the steel sheet as the cathode under the conditions shown below 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 subjected to reduction annealing with 15% H2-N2, a soaking 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 then hot-dip galvanized in a 460°C hot-dip galvanizing bath with an effective Al concentration of 0.197% by mass and the remainder consisting of Zn and unavoidable impurities. The weight per unit area was then adjusted to approximately 50 g / m² per side using gas wiping. 2 Samples of galvanized steel sheets were made.

[0202] [Electrolysis conditions]

[0203] Bath temperature: 50℃

[0204] pH: 2.0

[0205] Current density: 45 A / dm 2

[0206] Fe-based electroplating bath: containing 1.5 mol / L Fe 2＋ ion

[0207] Electrode (anode): Iridium oxide electrode

[0208] The amount of Fe-based electroplating coating on each side and the proportion of crystal orientation integration between the Fe-based electroplating coating and the Si-containing cold-rolled steel sheet at the interface of the Fe-based electroplating coating and the Si-containing cold-rolled steel sheet are determined by the above method.

[0209] The resistance to resistance weld cracking characteristics of the welded parts of the galvanized steel sheets obtained above were investigated. The following describes the methods for determining and evaluating the resistance to resistance weld cracking characteristics of the welded parts.

[0210] <Characteristics of resistance welding crack resistance in welded parts>

[0211] For galvanized steel sheets, according to the above method, the evaluation criteria for the sheet group are: resistance to resistance welding cracking characteristics not being an issue during a holding time of 0.18 seconds; tensile strength of 980 MPa or less with Si content less than 0.5%; and adhesion weight per single side of 50 g / m². 2 The resistance welding cracking characteristics of the welded portion were investigated using alloyed hot-dip galvanized steel sheet (1.6 mm thick) as the test material. The welding time was 0.36 seconds, and the holding time was 0.18 to 0.24 seconds. Corresponding to Example No., the welding current was varied, and the weld nugget diameter was measured. Evaluation was conducted using a welding current with a weld nugget diameter of 5.9 mm. Furthermore, as example data, an example was used where the alloyed hot-dip galvanized steel sheet used as the test material did not crack. This is because if cracking occurs in the test material, the stress distribution to the galvanized steel sheet in the evaluation object cannot be properly assessed.

[0212] The results of the above tests are listed in Tables 2-1, 2-2, and 4. The results show that, prior to the annealing process, the galvanized steel sheet of the inventive example, with an Fe-based electroplating layer formed under conditions suitable for this invention, exhibits excellent resistance to resistance weld cracking in the welded portion. It should be noted that for Reference Examples 1 and 2, since Si is less than 0.5%, no particular problem arises in the resistance to resistance weld cracking characteristics of the welded portion. The Fe-based electroplating layer adhesion amount was set to 25.0 g / m². 2 In all the above-mentioned examples, even under a holding time of 0.18 seconds, no cracks longer than 0.1 mm were observed, demonstrating particularly good resistance to resistance weld cracking in the welded sections. It should be noted that in Tables 2-1 and 2-2, for examples where no Fe-based electroplating layer was formed, the amount of Fe-based electroplating layer attached is indicated as "-". Furthermore, in Reference Examples No. 17, 29, and 45, where a high dew point annealing process was performed followed by hot-dip galvanizing, due to the high dew point annealing, the proportion of crystal orientation integration between the Fe-based electroplating layer and the Si-containing cold-rolled steel sheet at the interface was low, resulting in good resistance to resistance weld cracking in the welded sections. It should be noted that in these reference examples, the Fe-based electroplated steel sheet before annealing was heated to a soaking temperature of 800°C at an average heating rate of 10°C / second or higher within a temperature range of 400°C to 650°C, and then subjected to reduction annealing.

[0213]

[0214]

[0215]

[0216] Example 2

[0217] The steel with the chemical composition shown in Table 5 was melted to obtain castings, which were then hot-rolled, pickled, and cold-rolled to produce cold-rolled steel sheets with a thickness of 1.6 mm.

[0218]

[0219] Next, the cold-rolled steel sheet was degreased using alkali. Then, under the conditions shown below, the steel sheet was electrolyzed as the cathode to produce a pre-annealed Fe-based electroplated steel sheet with a 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 subjected to reduction annealing at 15% H2-N2, a soaking temperature of 800°C, and dew points adjusted according to the atmosphere shown in Table 6 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, the remainder consisting of Zn and unavoidable impurities. Finally, the weight per unit area was adjusted to approximately 50 g / m² per side using gas wiping. 2 Samples of galvanized steel sheets were made.

[0220] [Electrolysis conditions]

[0221] Bath temperature: 50℃

[0222] pH: 2.0

[0223] Current density: 45 A / dm 2

[0224] Fe-based electroplating bath: containing 1.5 mol / L Fe 2＋ ion

[0225] Electrode (anode): Iridium oxide electrode

[0226] The amount of Fe-based electroplating coating on each side and the proportion of crystal orientation integration between the Fe-based electroplating coating and the Si-containing cold-rolled steel sheet at the interface of the Fe-based electroplating coating and the Si-containing cold-rolled steel sheet are determined by the above method.

[0227] The resistance to resistance weld 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 weld cracking characteristics of the welded parts.

[0228] <Characteristics of resistance welding crack resistance in welded parts>

[0229] For galvanized steel sheets, the evaluation criteria based on the above method are: a holding time of 0.14 seconds, resistance welding cracking resistance characteristics (not a problem), tensile strength of 590 MPa (Si < 0.1%), and adhesion weight per single side of 50 g / m². 2 The resistance welding cracking characteristics of the welded section were investigated using alloyed hot-dip galvanized steel sheet (1.6 mm thick) as the test material. The welding time was 0.36 seconds, and the holding time was 0.14 seconds and 0.16 seconds. The weld nugget diameter was measured by changing the welding current, corresponding to Example No. 1, and the evaluation was performed using a welding current with a weld nugget diameter of 5.9 mm. Furthermore, as example data, an example was presented where no cracking occurred using the alloyed hot-dip galvanized steel sheet as the test material. This is because when cracking occurs in the test material, the stress distribution to the galvanized steel sheet is reduced, resulting in a poor evaluation.

[0230] Table 6 records the results of the above tests. According to the results, the galvanized steel sheet of the invention example, formed with an Fe-based electroplating layer under conditions suitable for this invention before the annealing process, exhibits excellent resistance to resistance weld cracking in the welded section. The adhesion amount of the Fe-based electroplating layer was set to 25.0 g / m². 2 In all the above-described examples, no cracks longer than 0.1 mm were observed even under a holding time of 0.14 seconds, demonstrating particularly good resistance to resistance welding cracking in the welded parts. It should be noted that in Table 6, for examples where no Fe-based electroplating layer was formed, the amount of Fe-based electroplating layer attached is represented as "-".

[0231]

[0232] Industrial availability

[0233] 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. Therefore, it can be used not only as a material for automotive parts, but also appropriately used in applications requiring the same characteristics in fields such as home appliances and building components.

[0234] Symbol Explanation

[0235] 1: Galvanized steel sheet

[0236] 2: Si-containing cold-rolled steel sheet

[0237] 3: Fe-based electroplating layer

[0238] 4: Zinc plating layer

[0239] 5: Experimental alloyed hot-dip galvanized steel sheet

[0240] 6: Test piece

[0241] 7: Isolation material

[0242] 8: Fixed platform

[0243] 9: Electrode

[0244] 10: Molten Core

[0245] 11: Cracks

Claims

1. A galvanized steel sheet having: a Si-containing cold-rolled steel sheet containing 0.1 to 3.0 mass% of Si, a Fe-based plated layer formed on the Si-containing cold-rolled steel sheet, and a zinc plating layer formed on the Fe-based plated layer, wherein at an interface between the Fe-based plated layer and the Si-containing cold-rolled steel sheet, a proportion of crystal orientation integration of the Fe-based plated layer and the Si-containing cold-rolled steel sheet exceeds 50%, wherein the proportion of crystal orientation integration is determined by: taking a sample of 10 x 10 mm in size from the galvanized steel sheet, machining an arbitrary position of the sample using a focused ion beam device to form a 45° cross section having an angle of 45° with respect to a T cross section direction, a width of 30 μm in a rolling right angle direction, and a length of 50 μm in a 45° direction with respect to the T cross section direction, to prepare an observation sample, the T cross section being a cross section parallel to a rolling right angle direction of the steel sheet and perpendicular to a steel sheet surface, subsequently observing a central portion of the 45° cross section of the observation sample using a scanning ion microscope at a magnification of 5000 times, and taking a SIM image of 1024 x 943 pixels in width x height, 8 bits, from the SIM image taken in units of the 45° cross section, based on the following formula (1), at the interface between the Fe-based plated layer and the Si-containing cold-rolled steel sheet, the proportion of crystal orientation integration of the Fe-based plated layer and the Si-containing cold-rolled steel sheet is calculated, (Proportion of crystal orientation integration of Fe-based plated layer and Si-containing cold-rolled steel sheet at interface) = (Length of position of crystal orientation integration of Fe-based plated layer and Si-containing cold-rolled steel sheet in interface between Fe-based plated layer and Si-containing cold-rolled steel sheet) ÷ (Length of interface in observation field of view) x 100... (1). The Si-containing cold-rolled steel sheet contains 0.50 to 3.0 mass% of Si. Fe-based plating layer formed on at least one side of the Si-containing cold-rolled steel sheet, the adhesion amount of each side being 25.0 g / m 2 The above, and The Si-containing cold-rolled steel sheet has a composition consisting of, in mass%, C: 0.8% or less, Mn: 1.0 to 12.0%, P: 0.1% or less, S: 0.03% or less, N: 0.010% or less, and Al: 1.0% or less, in addition to the Si, with the remainder consisting of Fe and inevitable impurities. The composition further contains one or two or more kinds selected from B: 0.005% or less, Ti: 0.2% or less, Cr: 1.0% or less, Cu: 1.0% or less, Ni: 1.0% or less, Mo: 1.0% or less, Nb: 0.20% or less, V: 0.5% or less, Sb: 0.200% or less, Ta: 0.1% or less, W: 0.5% or less, Zr: 0.1% or less, Sn: 0.20% or less, Ca: 0.005% or less, Mg: 0.005% or less, and REM: 0.005% or less. The Fe-based plated layer has a composition consisting of one or two or more kinds of elements selected from B, C, P, N, O, Ni, Mn, Mo, Zn, W, Pb, Sn, Cr, V, and Co, in a total of 10 mass% or less, with the remainder consisting of Fe and inevitable impurities.

7. A galvanized steel sheet having: ​ ​ ​ ​ 2. Zinc-coated steel sheet according to claim 1, wherein, ​ 3. Galvanised steel sheet according to claim 1 or 2 wherein, The attachment amount of the Fe-based plating layer per single side is 30.0 g / m 2 The above.

4. Galvanised steel sheet according to claim 1 or 2 wherein, ​ 5. Galvanised steel sheet according to claim 4 wherein, ​ 6. Galvanised steel sheet according to claim 1 or 2 wherein, ​ ​ A cold-rolled steel sheet, Fe-based plating layer formed on at least one side of the cold-rolled steel sheet, the adhesion amount of each side being 25.0 g / m 2 The above, and A zinc plating layer formed on the Fe-based electroplating layer; At the interface between the Fe-based electroplating layer and the cold-rolled steel sheet, the proportion of crystal orientation integration of the Fe-based electroplating layer and the cold-rolled steel sheet exceeds 50%, Here, the cold-rolled steel sheet is a cold-rolled steel sheet in which a test piece of 50 x 150 mm cut in a rolling diagonal direction is overlapped with a test alloyed hot-dip galvanized steel sheet of 50 g / m2 of the attached amount of a hot-dip galvanized layer per single side cut in the same size 2 to make a sheet set. Next, using a servo motor pressurization type 50 Hz single-phase alternating current resistance welding machine, the sheet set is inclined 5° to the long side direction of the sheet set with respect to the perpendicular plane of the line connecting the center axes of the electrode pair of the resistance welding machine having a front end diameter of 6 mm, a 60 mm long side direction of the sheet set x 2.0 mm thickness direction of the sheet set gap is provided between the lower side electrode of the electrode pair and the sheet set, the lower side electrode and the sheet set are fixed, and the upper side electrode of the electrode pair is made movable, and resistance welding is performed on the sheet set under the conditions of a pressurization force of 3.5 kN, a holding time of 0.16 seconds, and a welding current and a welding time in which the nugget diameter is 5.9 mm, to produce a sheet set with a welded portion, Next, the sheet set with the welded portion is cut in half along the long side direction of the cold-rolled steel sheet to include the welded portion, and when the cross section of the welded portion is observed using an optical microscope at a magnification of 200 times, a crack having a length of 0.1 mm or more is observed. The proportion of crystal orientation integration is determined as follows, A sample having a size of 10 x 10 mm is taken from the zinc-plated steel sheet, A focused ion beam device is used to process an arbitrary position of the sample, and a 45° cross section having an angle of 45° with respect to the T cross section direction, a width of 30 μm in the rolling right angle direction, and a length of 50 μm in the 45° direction with respect to the T cross section direction are formed at the position, to produce an observation sample, Next, a central portion of the 45° cross section of the observation sample is observed using a scanning ion microscope at a magnification of 5000 times, and a SIM image having a width of 1024 x height of 943 pixels and 8 bits is captured, From the SIM image captured in units of 45° cross sections, the proportion of crystal orientation integration of the Fe-based electroplating layer and the Si-containing cold-rolled steel sheet at the interface between the Fe-based electroplating layer and the Si-containing cold-rolled steel sheet is calculated based on the following formula (1), (The proportion of crystal orientation integration of the Fe-based electroplating layer and the Si-containing cold-rolled steel sheet at the interface between the Fe-based electroplating layer and the Si-containing cold-rolled steel sheet) = (the length of the position of crystal orientation integration of the Fe-based electroplating layer and the Si-containing cold-rolled steel sheet at the interface between the Fe-based electroplating layer and the Si-containing cold-rolled steel sheet) ÷ (the length of the interface in the observation field of view) x 100 … (1).

8. Galvanised steel sheet according to claim 7 wherein, The cold-rolled steel sheet is a cold-rolled steel sheet in which the sheet set with the welded portion is obtained by performing the resistance welding under the condition that the holding time is set to 0.24 seconds, and when the cross section of the welded portion is observed using an optical microscope at a magnification of 200 times, a crack having a length of 0.1 mm or more is observed.

9. An electrodeposited coated steel sheet further having a chemical conversion treatment film and an electrodeposited coating film formed on the chemical conversion treatment film on the zinc-plated steel sheet of any one of claims 1 to 8.

10. An automobile part made of at least a part of the electrogalvanized steel sheet according to claim 9.

11. A method for producing an electrogalvanized steel sheet, comprising the steps of: a chemical conversion treatment step of subjecting the galvanized steel sheet according to any one of claims 1 to 8 to a chemical conversion treatment to obtain a chemical conversion-treated steel sheet having a chemical conversion coating formed on the galvanized layer; and an electrogalvanizing step of subjecting the chemical conversion-treated steel sheet to an electrogalvanizing treatment to obtain an electrogalvanized steel sheet having an electrogalvanizing coating formed on the chemical conversion coating. Next, the annealed Fe-based plated steel sheet is subjected to galvanizing to obtain a galvanized steel sheet. The cold-rolled steel sheet contains 0.5 to 3.0 mass% of Si.

12. A method of producing a galvanized steel sheet, which produces a Fe-based plated steel sheet having a cold-rolled steel sheet containing 0.1 to 3.0 mass% of Si subjected to Fe-based plating, so as to form a Fe-based plated layer having a thickness of 5.0 to 25.0 g / m2 per one side on at least one side of the cold-rolled steel sheet. 2 the above Fe-based plated steel sheet before annealing of the Fe-based plated layer before annealing, Next, the annealed Fe-based plated steel sheet is subjected to galvanizing to obtain a galvanized steel sheet. Next, the annealed Fe-based plated steel sheet is subjected to galvanizing to obtain a galvanized steel sheet.

13. The method of producing a galvanized steel sheet according to claim 12, wherein, Next, the plate set is cut in half along the long side direction of the cold-rolled steel sheet in a manner that the weld portion is included, and when the cross section of the weld portion is observed with an optical microscope at a magnification of 200 times, a crack having a length of 0.1 mm or more is observed.

14. A method of producing a galvanized steel sheet, which comprises forming an Fe-based electroplating layer on a cold-rolled steel sheet to produce a steel sheet having an Fe-based electroplating layer with an adhesion amount of 25.0 g / m2 or more per one side on at least one side of the steel sheet. 2 the Fe-based electroplated steel sheet before annealing of the Fe-based electroplating layer before annealing described above, The cold-rolled steel sheet is a cold-rolled steel sheet in which, when the cross section of the weld portion is observed with an optical microscope at a magnification of 200 times under the condition that the holding time is set to 0.24 seconds, a crack having a length of 0.1 mm or more is observed. The Fe-based plating is performed using an Fe-based plating bath containing one or two or more elements selected from the group consisting of B, C, P, N, O, Ni, Mn, Mo, Zn, W, Pb, Sn, Cr, V, and Co, such that the total content of these elements in the pre-annealing Fe-based plated layer is 10 mass% or less. Here, the cold-rolled steel sheet is a cold-rolled steel sheet in which a test piece of 50 x 150 mm cut in a rolling diagonal direction is overlapped with a test alloyed hot-dip galvanized steel sheet of 50 g / m2 of the attached amount of a hot-dip galvanized layer per single side cut in the same size 2 to make a sheet set. ​ ​ 15. The method of producing a galvanized steel sheet according to claim 14, wherein, ​ 16. The method of producing a galvannealed steel sheet according to any one of claims 12 to 15, wherein, ​

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