Alloyed galvanized steel sheet, electrodeposition-coated steel sheet, automobile part, method for manufacturing electrodeposition-coated steel sheet, and method for manufacturing alloyed galvanized steel sheet

Abstract

The present invention provides a steel sheet having excellent resistance to resistance welding cracking at welded portions. An alloyed galvanized steel sheet comprising: a Si-containing cold-rolled steel sheet containing 0.1% to 3.0% Si by mass; an Fe-based electroplated layer formed on at least one surface of the Si-containing cold-rolled steel sheet; and an alloyed galvanized layer formed on the Fe-based electroplated layer; in an intensity distribution measured by glow discharge emission spectroscopy, (I Si，Fe ) / (I Si，bulk ) is 0.30 or more, and the average C concentration in the range of 10 μm to 20 μm in the thickness direction from the interface between the alloyed zinc plating layer and the Fe-based electroplating layer to the Fe-based electroplating layer side is 0.10 mass % or less.

Description

Technical Field

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

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

[0003] There are various methods to improve the strength of steel sheets, but as a method that can achieve high strength without seriously damaging the formability of the steel sheets, solid solution strengthening by adding Si can be cited. On the other hand, in the manufacture of automotive parts, press-formed parts are mostly assembled by resistance welding (spot welding). When the resistance-welded parts include high-strength galvanized steel sheets, during resistance welding, the zinc in the coating melts and diffuses into the grain boundaries under the condition that residual stress is generated near the weld, thereby causing liquid metal embrittlement (Liquid Metal Embrittlement: LME) and grain boundary cracking (LME cracking) of the steel sheet. In particular, when resistance welding is performed in a state where the welding electrode and the steel sheet are at an angle, the residual stress increases and cracking occurs. The residual stress increases as the steel sheet becomes stronger, so LME cracking may occur as the steel sheet becomes stronger. Such LME cracking problems are particularly significant in steel sheets containing Si.

[0004] In view of the above, a high-strength steel sheet having excellent resistance to resistance welding cracking at the welded portion is being sought.

[0005] In the past, there have been reports on strategies to improve the above-mentioned problems. For example, Patent Document 1 discloses a steel plate in which the grain boundary oxidation depth of the hot-rolled steel plate is reduced to 5.0 μm or less, and a protective layer is applied with a thickness of 3 g / m2 as a protective layer to suppress the grain boundary oxidation depth. 2The above Fe-based electroplating treatment is performed, so that the grain boundary oxidation depth after alloyed hot-dip galvanizing treatment is less than 5.0μm. Patent Document 1 states that resistance welding requires a severe processing environment, so it is difficult to suppress cracking when the grain boundary oxidation depth is large. Moreover, by making the grain boundary oxidation depth on the steel plate side less than 5.0μm, cracking during resistance welding can be suppressed. In addition, Patent Document 2 discloses a melt-alloyed hot-dip galvanized steel plate, which has an internal oxide layer in which at least a portion of the grain boundaries are covered with oxides from the surface of the base material to a depth of more than 5.0μm, and the grain boundary coverage of the oxide in the region from the surface of the base material to a depth of 5.0μm is more than 60%.

[0006] Prior art literature

[0007] Patent Literature

[0008] Patent Document 1: Japanese Patent No. 4791992

[0009] Patent Document 2: Japanese Patent No. 6388099 Summary of the Invention

[0010] The steel sheet described in Patent Document 1 forms an Fe-based electroplated layer as a protective layer to suppress the depth of grain boundary oxidation. This attempt improves resistance to resistance welding cracking by controlling only the depth of grain boundary oxidation on the steel sheet side, without focusing on controlling the properties of the Fe-based electroplated layer. Furthermore, as suggested in Patent Document 1, the steel sheet described in Patent Document 2 has an excessively deep internal oxide layer, i.e., grain boundary oxidation, making it difficult to completely suppress cracking during resistance welding.

[0011] As described above, the actual situation is that no galvannealed steel sheet that satisfies the resistance welding cracking characteristics of the welded portion at a high level has been developed.

[0012] The present invention has been made in view of the above-mentioned problems existing in galvannealed steel sheets, and an object of the present invention is to provide a steel sheet having excellent resistance to resistance welding cracking at welded portions.

[0013] The present inventors have previously attempted to improve resistance welding cracking resistance by forming an Fe-based electroplated layer as a protective layer to suppress the depth of grain boundary oxidation. However, controlling the depth of grain boundary oxidation only on the steel sheet side made it difficult to achieve a high level of resistance welding cracking resistance. Consequently, the inventors developed the idea of ​​controlling the properties of the Fe-based electroplated layer through a specific heat treatment. Furthermore, the present inventors discovered that by performing a specific heat treatment after forming the Fe-based electroplated layer, the carbon concentration in the steel sheet surface layer could be further reduced, thereby achieving an improvement in resistance welding cracking resistance.

[0014] Furthermore, the inventors have repeatedly conducted in-depth research to solve the above-mentioned problems. As a result, it was found that in order to meet the resistance welding cracking characteristics of the weld at a high level, it is important to form an Fe-based electroplating layer as a pre-plating on the surface of the Si-containing cold-rolled steel sheet before continuous annealing after cold rolling before forming the galvanized layer, to produce a Fe-based electroplated steel sheet before annealing, and to subject the Fe-based electroplated steel sheet before annealing to a temperature rise process of 400°C to 650°C with an average heating rate of more than 10°C / second and annealing in an atmosphere with a dew point greater than -30°C, to form internal oxides on the grain boundaries of the Fe-based electroplating layer, and further to make the number of grain boundaries where the Fe-based electroplating layer contacts the Si-containing cold-rolled steel sheet at the interface between the Fe-based electroplating layer and the Si-containing cold-rolled steel sheet be more than 10 per 10μm width of the Si-containing cold-rolled steel sheet. It was also found that it is important to perform the subsequent galvanizing process and alloying treatment to produce an alloyed galvanized steel sheet, so that the Fe-based electroplating layer after the alloying treatment has an adhesion amount on a single side of more than 0g / m 2 By making the soft Fe-based electroplating layer adhere to one side of the cold-rolled steel sheet with a coverage greater than 0 g / m 2 The method further reduces the residual amount of zinc in the Fe-based electroplating layer by making the average heating rate in the temperature range of 400°C to 650°C in the heating process at 10°C / second or more, thereby minimizing the growth of grains in the Fe-based electroplating layer during the heating process. Furthermore, by making the dew point of the atmosphere in the subsequent annealing process greater than -30°C, the Si diffused from the cold-rolled steel sheet into the Fe-based electroplating layer during annealing is converted into oxides within the Fe-based electroplating layer. This reduces the number of grain boundaries in the Fe-based electroplating layer contacting the interface between the Fe-based electroplating layer and the Si-containing cold-rolled steel sheet to 10 or more per 10μm width of the Si-containing cold-rolled steel sheet. This refines the grains of the Fe-based electroplating layer contacting the interface between the Si-containing cold-rolled steel sheet and the Fe-based electroplating layer, resulting in a dispersed intrusion path for molten zinc into the Fe-based electroplating layer. As a result, it was found that the time it takes for molten zinc to reach the grain boundaries of the cold-rolled steel sheet during welding can be delayed, thereby improving the resistance to resistance weld cracking of the weld. Furthermore, during the annealing step, the average carbon concentration in the range of 10 μm to 20 μm in the thickness direction from the interface between the zinc-alloyed plating layer and the Fe-based electroplating layer to the Fe-based electroplating layer side is set to 0.10 mass% or less. Thus, by setting the average carbon concentration in the range of 10 μm to 20 μm in the thickness direction from the interface between the zinc-alloyed plating layer and the Fe-based electroplating layer to 0.10 mass% or less, the resistance to resistance welding cracking can be further improved.

[0015] The present invention has been completed based on the above findings. Specifically, the gist of the present invention is as follows.

[0016] [1] An alloyed galvanized steel sheet having:

[0017] Si-containing cold-rolled steel sheet containing 0.1% to 3.0% by mass of Si,

[0018] The Fe-based electroplating layer is formed on at least one surface of the Si-containing cold-rolled steel sheet.

[0019] An alloyed zinc plating layer formed on the Fe-based electroplating layer;

[0020] Among them, in the intensity distribution measured by glow discharge emission spectrometry, the Fe content in the alloyed zinc plating layer is converted into the thickness of the plating layer, and the average Si intensity (I) of the range from the position of the interface between the alloyed zinc plating layer and the Fe-based electroplating layer moving toward the alloyed zinc plating layer by the thickness as the starting point to the interface between the Fe-based electroplating layer and the Si-containing cold-rolled steel sheet as the end point is Si，Fe ) divided by the average Si intensity in the Si-containing cold-rolled steel sheet (I Si，bulk ) and the value (I Si，Fe ) / (I Si，bulk ) is 0.30 or more,

[0021] The average carbon concentration in the range of 10 μm to 20 μm in the thickness direction from the interface between the alloyed zinc plating layer and the Fe-based electroplating layer to the Fe-based electroplating layer side is 0.10 mass % or less.

[0022] At the interface between the Fe-based electroplated layer and the Si-containing cold-rolled steel sheet, the number of grain boundaries where the Fe-based electroplated layer and the Si-containing cold-rolled steel sheet are in contact is 10 or more per 10 μm in the width direction of the Si-containing cold-rolled steel sheet in the observation field.

[0023] [2] The galvannealed steel sheet according to [1] above, wherein the Si-containing cold-rolled steel sheet contains 0.50% to 3.0% by mass of Si.

[0024] [3] The galvannealed steel sheet according to [1] or [2], wherein a decarburized layer is provided starting from the interface between the galvannealed layer and the Fe-based electroplated layer and extending toward the Fe-based electroplated layer.

[0025] [4] The galvannealed steel sheet according to [3] above, wherein the thickness of the decarburized layer is 30 μm or more.

[0026] [5] The alloyed galvannealed steel sheet according to [3] or [4], wherein the coating weight CW of the Fe-based electroplating layer on one side is Fe1 (g / m 2 ) and the thickness C of the above decarburized layer d (μm) satisfies the following formula (1).

[0027] 0.8×(CW Fe1 )+(C d )≥70···(1)

[0028] [6] The alloyed galvannealed steel sheet according to any one of [1] to [5], wherein the coating weight CW on one side of the Fe-based electroplating layer is Fe1 (g / m 2 ) is 2g / m 2 above.

[0029] [7] The alloyed galvannealed steel sheet according to any one of [1] to [6] above, wherein the Si-containing cold-rolled steel sheet has the following composition, which, in addition to the Si, contains, in mass%, C: less than 0.8%, Mn: 1.0% to 12.0%, P: less than 0.1%, S: less than 0.03%, N: less than 0.010% and Al: less than 1.0%, with the remainder being Fe and unavoidable impurities.

[0030] [8] The alloyed galvannealed steel sheet according to [7] above, wherein the above-mentioned component composition further contains, in terms of mass%, one or more selected from the group consisting of B: less than 0.005%, Ti: less than 0.2%, Cr: less than 1.0%, Cu: less than 1.0%, Ni: less than 1.0%, Mo: less than 1.0%, Nb: less than 0.20%, V: less than 0.5%, Sb: less than 0.020%, Ta: less than 0.1%, W: less than 0.5%, Zr: less than 0.1%, Sn: less than 0.20%, Ca: less than 0.005%, Mg: less than 0.005% and REM: less than 0.005%.

[0031] [9] The alloyed galvannealed steel sheet according to any one of [1] to [8] above, wherein the Fe-based electroplating layer has the following composition: containing a total of less than 10 mass% of one or more elements selected from B, C, P, N, O, Ni, Mn, Mo, Zn, W, Pb, Sn, Cr, V and Co, and the remainder is composed of Fe and unavoidable impurities.

[0032]

[10] An electrodeposition-coated steel sheet comprising the galvannealed steel sheet according to any one of [1] to [9] above, further comprising a chemical conversion coating and an electrodeposition coating formed on the chemical conversion coating.

[0033]

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

[10] .

[0034]

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

[0035] a chemical conversion treatment step of subjecting the alloyed galvannealed steel sheet described in any one of [1] to [9] above to chemical conversion treatment to obtain a chemical conversion treated steel sheet having a chemical conversion treatment film formed thereon;

[0036] The electrodeposition coating step is to subject the chemical conversion treated steel sheet to an electrodeposition coating treatment to obtain an electrodeposition coated steel sheet having an electrodeposition coating film formed on the chemical conversion treatment film.

[0037]

[13] A method for producing an alloyed galvanized steel sheet, comprising: subjecting a cold-rolled steel sheet containing 0.1% to 3.0% by mass of Si to Fe-based electroplating to produce a pre-annealed Fe-based electroplated steel sheet having a pre-annealed Fe-based electroplated layer formed on at least one surface.

[0038] Next, the Fe-based electroplated steel sheet before annealing is heated at an average heating rate of 10°C / second or more in a temperature range of 400°C to 650°C, and the heated temperature range is maintained in an atmosphere with a dew point greater than -30°C to produce the Fe-based electroplated steel sheet.

[0039] Next, the Fe-based electroplated steel sheet is galvanized to produce a galvanized steel sheet.

[0040] Then, the galvanized steel sheet is further alloyed to obtain a sheet with an adhesion amount greater than 0 g / m 2 An alloyed galvanized steel sheet comprising an Fe-based electroplating layer and an alloyed zinc plating layer formed on the Fe-based electroplating layer.

[0041]

[14] The method for manufacturing a galvannealed steel sheet according to

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

[0042]

[15] The method for manufacturing an alloyed galvannealed steel sheet according to

[13] or

[14] , wherein the adhesion amount CW on one side of the Fe-based electroplating layer before annealing is Fe0 (g / m 2 ) satisfies the following formula (2).

[0043] (CW Fe0 )-(CW Zn )×[mass%Fe] / 100 >0··· (2)

[0044] Here, CW Zn : The target value of the adhesion amount of the above-mentioned alloyed zinc plating layer on one side (g / m 2 )

[0045] Among them, 25.0g / m 2 ≤CW Zn ≤80.0g / m2

[0046] [mass%Fe]: target value of Fe content in the above-mentioned alloyed zinc plating layer (mass %)

[0047]

[16] The method for manufacturing an alloyed galvannealed steel sheet according to any one of

[13] to

[15] , wherein the adhesion amount CW on one side of the Fe-based electroplating layer before annealing is Fe0 (g / m 2 ) and the above dew point (DP) satisfy the following formula (3).

[0048] 0.83×{(CW Fe0 )-(CW Zn )×[mass%Fe] / 100}+(DP)≥-5···(3)

[0049] Here, CW Zn : The target value of the adhesion amount of the above-mentioned alloyed zinc plating layer on one side (g / m 2 )

[0050] Among them, 25.0g / m 2 ≤CW Zn ≤80.0g / m 2

[0051] [mass%Fe]: target value of Fe content in the above-mentioned alloyed zinc plating layer (mass %)

[0052]

[17] The method for manufacturing an alloyed galvannealed steel sheet according to any one of

[13] to

[16] , wherein the adhesion amount CW on one side of the Fe-based electroplating layer before annealing is Fe0 (g / m 2 ) satisfies the following formula (4).

[0053] (CW Fe0 )-(CW Zn )×[mass%Fe] / 100 ≥2··· (4)

[0054] Here, CW Zn : The target value of the adhesion amount of the above-mentioned alloyed zinc plating layer on one side (g / m 2 )

[0055] Among them, 25.0g / m 2 ≤CW Zn ≤80.0g / m 2

[0056] [mass%Fe]: target value of Fe content in the above-mentioned alloyed zinc plating layer (mass %)

[0057]

[18] A method for manufacturing an alloyed galvanized steel sheet according to any one of

[13] to

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

[0058] According to the present invention, it is possible to provide a galvannealed steel sheet having excellent resistance to resistance welding cracking at a welded portion. BRIEF DESCRIPTION OF THE DRAWINGS

[0059] Figure 1 This is a diagram schematically showing a cross section of a galvannealed steel sheet.

[0060] Figure 2A This is a diagram showing an example of raw data of intensity distribution obtained by analyzing the emission intensity at wavelengths representing Si and Zn using glow discharge emission spectroscopy.

[0061] Figure 2B This is a diagram showing an example of raw data of intensity distribution obtained by analyzing the emission intensity at wavelengths representing Si and Zn using glow discharge emission spectroscopy.

[0062] Figure 3 (a) A perspective view and (b) an AA cross-sectional view schematically show an observation sample for measuring the number of grain boundaries in contact between the Fe-based electroplated layer and the Si-containing cold-rolled steel sheet at the interface between the Fe-based electroplated layer and the Si-containing cold-rolled steel sheet.

[0063] Figure 4 This is a diagram explaining a method for measuring the number of grain boundaries at the interface between the Fe-based electroplated layer and the Si-containing cold-rolled steel sheet, where the Fe-based electroplated layer and the Si-containing cold-rolled steel sheet are in contact.

[0064] Figure 5 yes Figure 4 An enlarged view of the quadrilateral-enclosed portion.

[0065] Figure 6 This is a diagram showing an observed image of the interface between the Fe-based electroplated layer and the cold-rolled steel sheet in Example No. 36.

[0066] Figure 7 This is a diagram showing an image of the boundary line and the position of the grain boundary at the interface between the Fe-based electroplated layer and the Si-containing cold-rolled steel sheet in Invention Example No. 36.

[0067] Figure 8 This is a diagram showing an observed image of the interface between the Fe-based electroplated layer and the cold-rolled steel sheet of Invention Example No. 38.

[0068] Figure 9 This is a diagram showing an image of the boundary line and the position of the grain boundary at the interface between the Fe-based electroplated layer and the Si-containing cold-rolled steel sheet in Invention Example No. 38.

[0069] Figure 10 (a) is a diagram showing an example of raw data of the depth distribution of the C concentration in the plate thickness direction analyzed by an electron beam microanalyzer, and (b) is a diagram showing an example of smoothed data.

[0070] Figure 11 (a) is a diagram illustrating a method for evaluating resistance welding cracking characteristics of a welded portion, the upper diagram of (b) is a plan view of a plate assembly after welding in the evaluation, and the lower diagram is a BB cross-sectional view of the upper diagram. DETAILED DESCRIPTION

[0071] The aforementioned LME cracking can be broadly divided into "cracks occurring on the surface in contact with the electrode (hereinafter referred to as surface cracking)" and "cracks occurring near the plastic metal ring zone between the steel sheets (hereinafter referred to as internal cracking)." Surface cracking is known to occur more readily during resistance welding in the high current range that produces spatter. By keeping the current within an appropriate range that prevents spatter, surface cracking can be suppressed. On the other hand, internal cracking can occur even when the current during resistance welding is within an appropriate range that prevents spatter. Furthermore, while surface cracking is easily detected during visual inspection during the manufacturing process, internal cracking is difficult to detect. For these reasons, internal cracking is a particularly significant issue among LME cracking. When resistance welding is performed with the welding electrode at a certain angle to the steel sheet, residual stress increases, potentially leading to internal cracking. It is believed that residual stress increases with increasing steel sheet strength, potentially leading to internal cracking as steel sheet strength increases. The present disclosure improves resistance to resistance welding cracking, particularly the ability to prevent internal cracking.

[0072] Hereinafter, embodiments of the present invention will be described.

[0073] In the following description, the unit of the content of each element in the composition of the Si-containing cold-rolled steel sheet and the content of each element in the coating composition is "mass %" and is simply expressed as "%" unless otherwise specified. In addition, in this specification, the numerical range expressed using "to" is a range that includes the numerical values ​​before and after the "to" as the lower limit and upper limit. In this specification, "high strength" of a steel sheet means that the tensile strength TS of the steel sheet measured in accordance with JIS Z 2241 (2011) is 590 MPa or more.

[0074] Figure 1The schematic cross section of the galvannealed steel sheet 1 according to this embodiment is shown. Figure 1 As shown, the galvannealed steel sheet 1 includes an Fe-based electroplated layer 3 and a galvannealed layer 4 formed on the Fe-based electroplated layer on at least one surface of a Si-containing cold-rolled steel sheet 2. First, the composition of the Si-containing cold-rolled steel sheet will be described.

[0075] Si: 0.1% to 3.0%

[0076] Si has a significant effect in increasing the strength of steel through solid solution (solution strengthening) without significantly impairing workability, making it an effective element for achieving high strength in steel sheets. On the other hand, Si is also an element that negatively impacts the resistance to resistance welding cracking in welds. When Si is added to achieve high strength in steel sheets, it is necessary to add at least 0.1%. Furthermore, as long as Si is present, internal Si oxides can form at the grain boundaries of the Fe-based electroplated layer, as described later. When Si content is less than 0.50%, there are no particular issues with the resistance to resistance welding cracking in welds with conventional hold times of approximately 0.24 seconds. However, from a production cost perspective, the production cycle time during spot welding in the assembly process of automotive parts becomes a challenge. Even when measures are taken to reduce hold times, resistance to resistance welding cracking in welds may be insufficient, even if Si content is less than 0.50%. On the other hand, if Si content exceeds 3.0%, hot and cold rolling properties are significantly reduced, potentially adversely affecting productivity or leading to a decrease in the ductility of the steel sheet itself. Therefore, Si is added within a range of 0.1% to 3.0%. The Si content is preferably 0.50% or more, more preferably 0.7% or more, and even more preferably 0.9% or more, as it has a greater impact on the resistance welding cracking properties of the weld. Furthermore, the Si content is preferably 2.5% or less, more preferably 2.0% or less, and even more preferably 1.7% or less.

[0077] In the Si-containing cold-rolled steel sheet of this embodiment, it is essential to contain Si within the above-mentioned range. Other components are permitted as long as they are within the composition range of ordinary cold-rolled steel sheets, and there are no particular restrictions. However, when the Si-containing cold-rolled steel sheet of this embodiment has a high strength with a tensile strength (TS) of 590 MPa or more, the following composition is preferably used.

[0078] C: 0.8% or less (excluding 0%)

[0079] C improves workability by forming martensite and other elements as a steel structure. When C is contained, the amount is preferably 0.8% or less, more preferably 0.3% or less, to achieve good weldability. While the lower limit of C is not particularly limited, to achieve good workability, the amount is preferably 0% or greater, more preferably 0.03% or greater, and even more preferably 0.08% or greater.

[0080] Mn: 1.0% to 12.0%

[0081] Mn is an element that strengthens steel by solid solution strengthening, thereby increasing its strength, improving hardenability, and promoting the formation of retained austenite, bainite, and martensite. This effect is achieved by adding 1.0% or more of Mn. On the other hand, if the Mn content is 12.0% or less, the above effects can be achieved without increasing costs. Therefore, the Mn content is preferably 1.0% or more, preferably 12.0% or less. The Mn content is more preferably 1.3% or more, preferably 1.5% or more, and most preferably 1.8% or more. In addition, the Mn content is more preferably 3.5% or less, and even more preferably 3.3% or less.

[0082] P: 0.1% or less (excluding 0%)

[0083] Controlling the P content prevents a decrease in weldability. Furthermore, preventing P from segregating at grain boundaries can prevent deterioration in ductility, bendability, and toughness. Furthermore, adding large amounts of P promotes ferrite transformation and increases grain size. Therefore, a P content of 0.1% or less is preferred. While the lower limit of P is not particularly limited, it can be greater than 0%, and can be 0.001% or more, depending on production technology constraints.

[0084] S: 0.03% or less (excluding 0%)

[0085] The S content is preferably 0.03% or less, more preferably 0.02% or less. Reducing the S content prevents a decrease in weldability, prevents a decrease in hot ductility, suppresses hot cracking, and significantly improves the surface shape. Furthermore, by reducing the S content, coarse sulfides are formed as impurities, preventing a decrease in the ductility, bendability, and stretch flangeability of the steel sheet. These problems become significant when the S content exceeds 0.03%, so the S content is preferably minimized. While the lower limit of S is not particularly limited, it can be greater than 0%, and can be 0.0001% or more, depending on production technology constraints.

[0086] N: 0.010% or less (excluding 0%)

[0087] The N content is preferably 0.010% or less. When the N content is 0.010% or less, N forms coarse nitrides with Ti, Nb, and V at high temperatures, thereby preventing the high strength effect of the steel plate caused by the addition of Ti, Nb, and V from being impaired. In addition, when the N content is 0.010% or less, a decrease in toughness can also be prevented. Furthermore, when the N content is 0.010% or less, cracking of the slab during hot rolling and the generation of surface defects can be prevented. The N content is preferably 0.005% or less, more preferably 0.003% or less, and even more preferably 0.002% or less. The lower limit of the N content is not particularly limited. Considering the constraints of production technology, it can be greater than 0%, and can be 0.0005% or more.

[0088] Al: 1.0% or less (excluding 0%)

[0089] Because Al is thermodynamically most susceptible to oxidation, it oxidizes before Si and Mn, inhibiting oxidation of Si and Mn in the outermost layer of the steel sheet and promoting oxidation of Si and Mn within the steel sheet. This effect is achieved when the Al content is 0.01% or greater. On the other hand, an Al content exceeding 1.0% leads to increased costs. Therefore, when added, the Al content is preferably 1.0% or less. More preferably, the Al content is 0.1% or less. The lower limit of Al is not particularly limited and can be greater than 0%, or even 0.001% or greater.

[0090] The component composition may further arbitrarily contain one or more selected from B: less than 0.005%, Ti: less than 0.2%, Cr: less than 1.0%, Cu: less than 1.0%, Ni: less than 1.0%, Mo: less than 1.0%, Nb: less than 0.20%, V: less than 0.5%, Sb: less than 0.200%, Ta: less than 0.1%, W: less than 0.5%, Zr: less than 0.1%, Sn: less than 0.20%, Ca: less than 0.005%, Mg: less than 0.005% and REM: less than 0.005%.

[0091] B: 0.005% or less

[0092] B is an element effective in improving the hardenability of steel. To improve hardenability, the B content is preferably 0.0003% or more, more preferably 0.0005% or more. However, if excessive B is added, formability decreases, so the B content is preferably 0.005% or less.

[0093] Ti: 0.2% or less

[0094] Ti is effective for precipitation strengthening of steel. While the lower limit of Ti is not particularly limited, it is preferably 0.005% or more to achieve the effect of strength adjustment. However, if Ti is added in excess, the hard phase becomes too large, deteriorating formability. Therefore, when Ti is added, the amount is preferably 0.2% or less, and more preferably 0.05% or less.

[0095] Cr: 1.0% or less

[0096] The amount of Cr is preferably 0.005% or more. By making the amount of Cr 0.005% or more, the hardenability can be improved and the balance between strength and ductility can be improved. When added, from the perspective of preventing cost increase,

[0097] The Cr content is preferably 1.0% or less.

[0098] Cu: 1.0% or less

[0099] The Cu content is preferably 0.005% or more. A Cu content of 0.005% or more promotes the formation of a retained γ phase. Furthermore, when adding Cu, the Cu content is preferably 1.0% or less from the perspective of preventing cost increases.

[0100] Ni: 1.0% or less

[0101] The Ni content is preferably 0.005% or more. A Ni content of 0.005% or more promotes the formation of a retained γ phase. Furthermore, when Ni is added, the Ni content is preferably 1.0% or less from the perspective of preventing cost increases.

[0102] Mo: 1.0% or less

[0103] The amount of Mo is preferably 0.005% or more. By setting the amount of Mo to 0.005% or more, the strength adjustment effect can be achieved. The amount of Mo is more preferably 0.05% or more. Furthermore, when adding Mo, the amount of Mo is preferably 1.0% or less from the perspective of preventing cost increases.

[0104] Nb: 0.20% or less

[0105] The effect of improving strength is achieved by containing Nb in an amount of 0.005% or more. When Nb is contained, the amount of Nb is preferably 0.20% or less from the viewpoint of preventing cost increases.

[0106] V: 0.5% or less

[0107] The effect of improving strength is achieved by containing 0.005% or more of V. When V is contained, the amount of V is preferably 0.5% or less from the viewpoint of preventing cost increases.

[0108] Sb: 0.020% or less

[0109] Sb may be contained to suppress oxidation on the steel sheet surface. By suppressing oxidation of the steel sheet, Sb improves the wettability of the plating. To achieve this effect, the Sb content is preferably 0.001% or greater. Furthermore, Sb inhibits the formation of a decarburized layer. To achieve excellent resistance to resistance welding cracking, the Sb content is preferably 0.020% or less. The Sb content is more preferably 0.015% or less, and even more preferably 0.012% or less.

[0110] Ta: 0.1% or less

[0111] The effect of improving strength is achieved by containing Ta in an amount of 0.001% or more. When Ta is contained, the amount of Ta is preferably 0.1% or less from the viewpoint of preventing cost increases.

[0112] W: 0.5% or less

[0113] The effect of improving strength is achieved by containing 0.005% or more of W. When W is contained, the amount of W is preferably 0.5% or less from the viewpoint of preventing cost increase.

[0114] Zr: 0.1% or less

[0115] The effect of improving strength is achieved by containing Zr in an amount of 0.0005% or more. When Zr is contained, the amount of Zr is preferably 0.1% or less from the viewpoint of preventing cost increases.

[0116] Sn: 0.20% or less

[0117] Sn is an element that effectively suppresses denitrification, deboronization, etc., thereby suppressing the decrease in steel strength. To achieve these effects, the Sn content is preferably 0.002% or more. On the other hand, to obtain good impact resistance, the Sn content is preferably 0.20% or less.

[0118] Ca: 0.005% or less

[0119] By containing 0.0005% or more of Ca, the morphology of sulfides can be controlled, thereby improving ductility and toughness. In order to obtain good ductility, the amount of Ca is preferably 0.005% or less.

[0120] Mg: 0.005% or less

[0121] By containing 0.0005% or more of Mg, the morphology of sulfides can be controlled, thereby improving ductility and toughness. Furthermore, when Mg is contained, the amount of Mg is preferably 0.005% or less from the viewpoint of preventing cost increases.

[0122] REM: 0.005% or less

[0123] By containing 0.0005% or more of REM, the morphology of sulfides can be controlled, thereby improving ductility and toughness. Furthermore, when REM is contained, the amount of REM is preferably 0.005% or less from the viewpoint of obtaining good toughness.

[0124] The balance other than the above-mentioned components of the Si-containing cold-rolled steel sheet according to the present embodiment is Fe and inevitable impurities.

[0125] Next, the Fe-based electroplating layer formed on at least one surface of the alloyed galvannealed steel sheet after galvanizing by pre-plating the Si-containing cold-rolled steel sheet before forming the galvannealed layer will be described.

[0126] Fe-based electroplating layer: greater than 0g / m 2

[0127] In the alloyed galvanized steel sheet, the Fe-based electroplating layer must have a deposition weight of more than 0 g / m on one side. 2 . On the other hand, after the Fe-based electroplated layer is formed, the Fe-based electroplated steel sheet before annealing is annealed in an atmosphere with a low dew point below -30°C, and the crystal grain size of the Fe-based electroplated layer may coarsen. Therefore, molten zinc easily invades the grain boundary of the Si-containing cold-rolled steel sheet through the grain boundary of the Fe-based electroplated layer. In this embodiment, by controlling the dew point of the atmosphere during annealing to be greater than -30°C, the Si diffused from the Si-containing cold-rolled steel sheet to the Fe-based electroplated layer during annealing forms an internal oxide at the grain boundary of the Fe-based electroplated layer. The internal oxide of Si (hereinafter also referred to as Si internal oxide) hinders the crystal growth of the Fe-based electroplated layer in the annealing process, and refines the crystals of the Fe-based electroplated layer. It is believed that by making the crystals finer, a large number of grain boundaries are formed in the Fe-based electroplating layer and remain even after alloying. As a result, the path of molten zinc intrusion during resistance welding is dispersed, which can delay the time for molten zinc to reach the grain boundaries of the Si-containing cold-rolled steel sheet during resistance welding, thereby improving the resistance welding cracking characteristics of the weld, especially improving the characteristics of preventing internal cracking (zinc grain boundary intrusion inhibition effect). The Fe-based electroplating layer functions as a soft layer, which can relax the stress applied to the steel plate surface during welding and reduce the residual stress of the resistance welding portion, thereby improving the resistance welding cracking characteristics of the weld, especially improving the characteristics of preventing internal cracking (stress relaxation effect). In addition, by making the dew point greater than -30°C, the Si diffused from the steel plate to the Fe-based electroplating layer during annealing is formed as an oxide inside the Fe-based electroplating layer, and the amount of solid-solution Si becomes less, thereby obtaining a steel plate with excellent resistance welding cracking characteristics of the weld. By making the adhesion amount on one side greater than 0g / m 2The mechanism by which the Fe-based electroplating layer improves the resistance welding cracking characteristics of the weld is not yet clear, but it is believed that when the amount of dissolved Si on the surface of the steel plate is large, the toughness of the weld is reduced and the resistance welding cracking characteristics of the weld are deteriorated. In contrast, it is believed that when there is a certain amount of Fe-based electroplating layer on the surface of the steel plate and the dew point is controlled to be above a certain level, an oxide is formed inside the Fe-based electroplating layer, which acts as a solid-solution Si depletion layer, and the amount of Si dissolved in the weld is reduced, so the reduction in the toughness of the weld is suppressed, and the resistance welding cracking characteristics of the weld can be improved, especially the characteristics of preventing internal cracking (toughness reduction inhibition effect). It is believed that the contribution of these Fe-based electroplating layers to the resistance welding cracking characteristics of stress relaxation effect, toughness reduction inhibition effect and zinc grain boundary intrusion inhibition effect is very complex, so it cannot be quantitatively explained, and the composite effect improves the resistance welding cracking characteristics. In order to produce the effect of improving the resistance welding cracking characteristics of the weld, in the galvanized steel sheet after alloying, the adhesion amount of the single side of the Fe-based electroplating layer must be greater than 0g / m 2 The upper limit of the amount of Fe-based electroplating layer deposited on one side is not particularly limited, but from the perspective of cost, the amount of Fe-based electroplating layer deposited on one side is preferably 60 g / m 2 The Fe-based electroplating layer preferably has a deposition weight of 50 g / m 2 Below, more preferably 40g / m 2 Below, more preferably 30g / m 2 The Fe-based electroplated steel sheet preferably has an Fe-based electroplated layer on both sides of the Si-containing cold-rolled steel sheet. 2 The resistance to resistance welding cracking of the welded part is particularly good. The adhesion of the Fe-based electroplating layer is greater than 0g / m 2 The above-mentioned grain boundary intrusion suppression effect, stress relaxation effect, and toughness reduction suppression effect of galvanizing are shown to prevent internal cracking. However, in order to further realize the grain boundary intrusion suppression effect, the deposition amount of the Fe-based electroplating layer on one side is more preferably 2.0 g / m 2 Above. The adhesion of Fe-based electroplating layer is greater than 0g / m 2 , and make the crystals finer, as mentioned above, the path for the molten zinc to penetrate during resistance welding is dispersed. This is because it is believed that by further increasing the deposition amount of the Fe-based electroplating layer on one side to 2.0 g / m 2 As described above, the diffusion path can be further prolonged, and the time for the molten zinc to reach the grain boundary of the Si-containing cold-rolled steel sheet during resistance welding can be further delayed.

[0128] In addition, the thickness of the Fe-based electroplated layer is measured as follows. After the alloying galvanized steel sheet is taken from the alloying galvanized steel sheet, a sample of 10×15 mm in size is embedded in resin to prepare a cross-section embedded sample. Using a scanning electron microscope (SEM), an accelerating voltage of 15 kV and a magnification of 2000 to 10000 times according to the thickness of the Fe-based electroplated layer are observed at any three positions of the cross section. The average value of the thickness of the three viewing fields is multiplied by the density of iron to convert it into the adhesion amount of the Fe-based electroplated layer on a single side.

[0129] As an Fe-based electroplating layer, in addition to pure Fe, alloy plating layers such as Fe-B alloy, Fe-C alloy, Fe-P alloy, Fe-N alloy, Fe-O alloy, Fe-Ni alloy, Fe-Mn alloy, Fe-Mo alloy, and Fe-W alloy can also be used. The composition of the Fe-based electroplating layer is not particularly limited, but it is preferably composed of a composition containing a total of 10% by mass or less of one or more elements selected from B, C, P, N, O, Ni, Mn, Mo, Zn, W, Pb, Sn, Cr, V, and Co, with the remainder being Fe and unavoidable impurities. By reducing the amount of elements other than Fe to a total of 10% by mass or less, the electrolysis efficiency is prevented from decreasing, and the Fe-based electroplating layer can be formed at low cost. In the case of an Fe-C alloy, the C content is preferably 0.08% by mass or less.

[0130] The Fe-based electroplated layer contains Si internal oxides at at least a portion of the grain boundaries. It is believed that the Si internal oxides hinder the crystal growth of the Fe-based electroplated layer during the annealing process, resulting in a finer grain size of the Fe-based electroplated layer. This results in the formation of numerous grain boundaries in the Fe-based electroplated layer, which disperses the intrusion paths for molten zinc. This delays the time it takes for the grain boundaries of the Si-containing cold-rolled steel sheet to reach the molten zinc during resistance welding, improving the resistance to resistance weld cracking of the weld, particularly its ability to prevent internal cracking.

[0131] The presence or absence of Si internal oxides in the Fe-based electroplating layer is determined as follows: when analyzing the luminescence intensity of the wavelength representing Si from the surface of the alloyed zinc plating layer in the depth direction (plate thickness direction) by glow discharge optical emission spectroscopy (GD-OES), the Fe content in the alloyed zinc plating layer is converted into the thickness of the plating layer from the interface between the Fe-based electroplating layer and the Si-containing cold-rolled steel sheet, and the average Si intensity (I) in the range starting from the position shifted by the thickness from the interface between the alloyed zinc plating layer and the Fe-based electroplating layer toward the alloyed zinc plating layer and ending at the interface between the Fe-based electroplating layer and the Si-containing cold-rolled steel sheet is determined. Si，Fe) and the average Si intensity (I Si，bulk ) ratio (I Si，Fe / I Si，bulk ) is above 0.30. The measuring conditions are Ar gas pressure 600Pa, high frequency output 35W, measuring diameter 4mmΦ, and sampling interval 0.1 second. For any average Si intensity, the average value of all Si intensities sampled within the range is obtained. In addition, after the Si-containing cold-rolled steel sheet that has not been subjected to Fe-based electroplating and alloyed galvanizing is analyzed by glow discharge emission spectrometry under the same conditions, the splash velocity is calculated by measuring the depth of the splash mark, and the horizontal axis of the intensity distribution representing the wavelength of Si is converted into the depth corresponding to each time. The depth of the splash mark is measured using a non-contact surface shape measuring device (NewView 7300: manufactured by Zygo Company). For the splash velocity of the alloyed galvanized layer, the Fe-based electroplated layer and the Si-containing cold-rolled steel sheet are different, that is, the benchmark for depth conversion is different due to the difference in elements, so the interface of the alloyed galvanized layer and the Fe-based electroplated layer is judged as follows. The evaluation method of the interface of the alloyed galvanized layer and the Fe-based electroplated layer is described using Figure 2. The solid line in the intensity distribution is the luminescence intensity at the wavelength of Si, and the dotted line is the luminescence intensity at the wavelength of Zn. First, the average Zn intensity (I Zn ) divided by 2. Next, the Zn intensity is the above value (I Zn The thickness direction depth of galvanized layer ( / 2) is defined as the depth of the interface of galvanized layer and Fe system electroplating layer.And, the Fe diffusion amount in galvanized layer is used to convert alloying and sneak into the thickness of the Fe system electroplating layer in galvanized layer, and the interface from galvanized layer and Fe system electroplating layer moves this thickness to galvanized layer side along the thickness direction.First, the adhesion amount of galvanized layer obtained according to the assay method described later is multiplied by the Fe concentration (quality %) in galvanized layer, obtain the Fe content (quality) in galvanized layer divided by 100, then from the Fe content in galvanized layer, deduct the proportion of iron, thus the Fe content in galvanized layer is converted to the thickness of Fe system electroplating layer.Thereafter, calculate the average Si intensity (I) from the position L3 of above-mentioned thickness that the interface L1 of galvanized layer and Fe system electroplating layer moves to the interface L2 of Fe system electroplating layer and Si-containing cold rolled steel sheet. Si，Fe). It should be noted that the interface L2 between the Fe-based electroplating layer and the Si-containing cold-rolled steel sheet is the position of the thickness measured by moving from L3 to the cold-rolled steel sheet side through the above-mentioned cross-sectional observation. Because the splashing speed of the alloyed galvanized layer is different from the splashing speed in the Fe-based electroplated layer and the Si-containing cold-rolled steel sheet, the horizontal axis of the intensity distribution does not completely correspond to the position of the interface between the alloyed galvanized layer and the Fe-based electroplated layer that can be visually observed in the cross-sectional observation. In addition, it is well known that in the measurement of glow discharge emission spectrometry, the interface composed of two or more substances is widely distributed due to unevenness of concavities and convexities, splashing, etc. Therefore, here, the average Zn intensity (I Zn ) divided by 2 to get the value I Zn The depth in the plate thickness direction of / 2 is defined as the depth of the interface between the alloyed zinc plating layer and the Fe-based electroplating layer.

[0132] use Figure 2A , B will describe a representative example of analyzing the emission intensities of the wavelengths representing Si and Zn observed in this embodiment. Figure 2A , B represents the raw data of intensity distribution analysis of the luminescence intensity of the wavelengths representing Si and Zn of the alloyed galvanized steel sheets of Examples No. 31 (dew point of annealing atmosphere -10°C), 33 (dew point -3°C), 35 (dew point -39°C), 36 (dew point -14°C), and 38 (dew point +12°C) described later. In Example No. 35, in which the annealing process was performed in a low dew point atmosphere, I Si，Fe / I Si，bulk The value of is 0.18. On the other hand, in Examples No. 31, 33, 36 and 38 in which the annealing process was performed in an atmosphere with a dew point greater than -30°C, I Si，Fe / I Si，bulk The values ​​of are 0.57, 0.50, 0.38 and 0.45 respectively. Si，Fe / I Si，bulk A value of 0.30 or greater indicates that internal oxidation has occurred in the Fe-based electroplated layer or in the region where Fe in the Fe-based electroplated layer diffuses into the alloyed zinc-plated layer, resulting in the presence of Si internal oxides. In steel sheets containing Si internal oxides within the aforementioned depth range, grain growth in the Fe-based electroplated layer is suppressed by the internal oxides. Therefore, even if an annealing step is performed after the Fe-based electroplating treatment, grain coarsening of the Fe-based electroplated layer can be prevented, and numerous grain boundaries are formed in the Fe-based electroplated layer. Consequently, the intrusion paths for molten zinc are dispersed, delaying the time it takes for the grain boundaries of the Si-containing cold-rolled steel sheet to reach molten zinc during resistance welding, resulting in excellent resistance to resistance welding cracking.

[0133] At the interface between the Fe-based electroplated layer and the Si-containing cold-rolled steel sheet, the number of grain boundaries of the Fe-based electroplated layer in contact with the Si-containing cold-rolled steel sheet is 10 or more per 10 μm in the width direction of the Si-containing cold-rolled steel sheet in the observation field. If, at the interface between the Fe-based electroplated layer and the Si-containing cold-rolled steel sheet, the number of grain boundaries of the Fe-based electroplated layer in contact with the Si-containing cold-rolled steel sheet is 10 or more per 10 μm in the width direction of the Si-containing cold-rolled steel sheet, the crystals of the Fe-based electroplated layer are sufficiently refined. By refining, a large number of grain boundaries are formed in the Fe-based electroplated layer, and as a result, the intrusion of molten zinc is dispersed, which can delay the time it takes to reach the grain boundaries of the Si-containing cold-rolled steel sheet during welding, thereby improving the resistance to resistance welding cracking characteristics of the weld, and in particular, improving the characteristics of preventing internal cracking. At the interface between the Fe-based electroplated layer and the Si-containing cold-rolled steel sheet, the number of grain boundaries where the Fe-based electroplated layer contacts the Si-containing cold-rolled steel sheet may preferably be 16 or more per 10 μm in the width direction of the Si-containing cold-rolled steel sheet in the observation field. More preferably, the number of grain boundaries may be 20 or more per 10 μm in the width direction of the Si-containing cold-rolled steel sheet in the observation field.

[0134] Here, at the interface between the Fe-based electroplated layer and the Si-containing cold-rolled steel sheet, the number of grain boundaries at which the Fe-based electroplated layer contacts the Si-containing cold-rolled steel sheet is measured as follows. First, a sample of 10×10 mm in size is taken from the alloyed galvanized steel sheet. A focused ion beam (FIB) device is used to process an arbitrary position of the sample, and a 45° cross-section with a width of 30 μm in the rolling right-angle direction and a length of 50 μm in the 45° direction relative to the T-section direction (a cross-section parallel to the rolling right-angle direction of the steel sheet and perpendicular to the steel sheet surface) is formed at this position to form an observation sample. Figure 3 An overview of the observation sample is shown in FIG. Figure 3 (a) is a perspective view of the observation sample. Figure 3 (b) Yes Figure 3 (a) is the AA cross-sectional view of the observation sample shown. Next, a scanning ion microscope (SIM) was used to observe the central portion of the width and length directions of the 45° cross-section of the observation sample at a magnification of 5000 times, and a SIM image was taken. An example of such a SIM image is shown in FIG. Figure 4 . Figure 4 This is a SIM image taken as described above for Example No. 35 described later. From the SIM image, a 10 μm region in the width direction of the Si-containing cold-rolled steel sheet ( Figure 4 For illustration, Figure 5 Shown in Figure 4 An enlarged view of the quadrilateral-enclosed portion. Figure 5As shown, for the SIM image, a boundary line is drawn at the interface between the Fe-based electroplating layer and the Si-containing cold-rolled steel sheet in a 10 μm region in the width direction of the Si-containing cold-rolled steel sheet ( Figure 5 The number of grain boundaries of the Fe-based electroplated layer on the boundary line was measured as "the number of grain boundaries where the Fe-based electroplated layer and the Si-containing cold-rolled steel sheet are in contact at the interface between the Fe-based electroplated layer and the Si-containing cold-rolled steel sheet".

[0135] Figure 6 36 of the embodiment described later, the SIM image of the interface between the Fe-based electroplated layer and the Si-containing cold-rolled steel sheet is shown in FIG. Figure 7 In Invention Example No. 36, there are 13 grain boundaries on the measurement boundary line as indicated by arrows per 10 μm in the width direction of the Si-containing cold-rolled steel sheet. Therefore, in Invention Example No. 36, at the interface between the Fe-based electroplated layer and the Si-containing cold-rolled steel sheet, the number of grain boundaries where the Fe-based electroplated layer contacts the Si-containing cold-rolled steel sheet is 13 per 10 μm in the width direction of the Si-containing cold-rolled steel sheet. In addition, Figure 8 The SIM image of the interface between the Fe-based electroplated layer and the Si-containing cold-rolled steel sheet of Inventive Example No. 38 described later in the Examples is shown. The image in which the boundary line and the measurement boundary line are drawn as described above in the center of the SIM image is shown in FIG. Figure 9 In Invention Example No. 38, there are 17 grain boundaries on the measurement boundary line, as indicated by arrows, per 10 μm across the width of the Si-containing cold-rolled steel sheet. Therefore, in Invention Example No. 38, at the interface between the Fe-based electroplated layer and the Si-containing cold-rolled steel sheet, the number of grain boundaries where the Fe-based electroplated layer and the Si-containing cold-rolled steel sheet are in contact is 17 per 10 μm across the width of the Si-containing cold-rolled steel sheet.

[0136] The thickness of the galvannealed steel sheet of the present embodiment is not particularly limited, but is usually 0.5 mm or more, and may be 3.2 mm or less.

[0137] Next, the C concentration of the surface layer of the Fe-based electroplating layer is described. In the present embodiment, it is important that the above-mentioned annealing is performed so that the average value of the C concentration in the range of 10 μm to 20 μm from the interface of the galvannealed layer and the Fe-based electroplating layer to the side plate thickness direction of the Fe-based electroplating layer is below 0.10 mass %, more preferably below 0.06 mass %. Further preferably, during the above-mentioned annealing, a decarburized layer is formed on the Fe-based electroplating layer side with the interface of the galvannealed layer and the Fe-based electroplating layer as a starting point. That is, during annealing, a decarburized layer is formed on the surface portion of the steel sheet other than the galvannealed layer of the galvannealed steel sheet. In addition, the decarburized layer is a region where the C concentration is lower than the concentration in the steel near the interface of the galvannealed layer and the Fe-based electroplating layer (the surface of the steel sheet other than the galvannealed layer of the galvannealed steel sheet), and may be formed by detaching from the surface of the Fe-based electroplated steel sheet during annealing. If the average carbon concentration in the region of 10 μm to 20 μm in the thickness direction from the interface between the zinc-alloyed plating layer and the Fe-based electroplating layer to the Fe-based electroplating layer side is 0.10 mass% or less, the region of 10 μm to 20 μm in the thickness direction from the interface between the zinc-alloyed plating layer and the Fe-based electroplating layer to the Fe-based electroplating layer side is soft. This reduces the stress applied by the welding electrodes during resistance welding, improving resistance to resistance weld cracking.

[0138] In the present embodiment, by annealing after forming the Fe-based electroplating layer, the average value of the C concentration in the range of 10 μm to 20 μm from the interface of the alloyed zinc-plating layer and the Fe-based electroplating layer to the plate thickness direction of the above-mentioned Fe-based electroplating layer side can be reduced compared to when the Fe-based electroplating layer is not present. In addition, when a decarburized layer is formed toward the Fe-based electroplating layer side with the interface of the alloyed zinc-plating layer and the Fe-based electroplating layer as a starting point, even if the thickness of the decarburized layer formed is the same, the C concentration in the decarburized layer can be further reduced. Therefore, it is possible to improve the resistance welding cracking characteristics without deteriorating the mechanical properties of the steel sheet. In addition, when Ni, Co, Sn, etc. are electroplated alone, the C solid solubility in these metal elements is extremely low, and C does not dissolve in solid, so the effect of promoting decarburization cannot be obtained.

[0139] When an Fe-based electroplated layer is formed, the reason why the C concentration decreases from the interface between the alloyed zinc plating layer and the Fe-based electroplated layer toward the Fe-based electroplated layer in the thickness direction within a range of 10 μm to 20 μm is unclear, but the present inventors speculate as follows. Specifically, it is believed that this is because the Fe-based electroplated layer contains almost no C, which induces the diffusion of C from the Si-containing cold-rolled steel sheet. Furthermore, the refinement of the Fe-based electroplated layer as described above increases the number of diffusion paths for C to escape from the Fe-based electroplated layer to the outside.

[0140] Furthermore, softening is achieved by reducing the carbon concentration in the range of 10 μm to 20 μm from the interface between the zinc-alloyed plating layer and the Fe-based electroplating layer toward the Fe-based electroplating layer in the thickness direction. However, the carbon concentration in the range of 10 μm to 20 μm from the interface between the zinc-alloyed plating layer and the Fe-based electroplating layer toward the Fe-based electroplating layer in the thickness direction saturates below a certain level, so there is a limit to how much improvement in resistance welding cracking resistance can be achieved through softening. In this embodiment, by further reducing the carbon concentration in the range of 10 μm to 20 μm from the interface between the zinc-alloyed plating layer and the Fe-based electroplating layer toward the Fe-based electroplating layer in the thickness direction, resistance welding cracking resistance can be effectively improved even with a thin decarburized layer. This not only achieves softening but also exhibits other effects, such as an increase in melting point due to the reduced carbon concentration.

[0141] When a decarburized layer is formed, the thickness of the decarburized layer of the Fe-based electroplated steel sheet is preferably 30 μm or greater, more preferably 40 μm or greater. While the upper limit of the decarburized layer thickness is not particularly limited, it is preferably 130 μm or less to maintain a good tensile strength. The thickness of the decarburized layer is defined as the thickness of the region in the surface layer of the galvanized steel sheet, excluding the galvanized layer, where the carbon concentration is less than 80% of that in the steel, as determined by analyzing the carbon concentration along the thickness direction from the interface between the galvanized layer and the Fe-based electroplated layer.

[0142] Here, the average value of the C concentration in the range of 10μm to 20μm from the interface of the alloyed zinc plating layer and the Fe-based electroplating layer to the thickness direction of the Fe-based electroplating layer side and the thickness of the decarburized layer are measured as follows. For the cross-sectioned sample, the element distribution near the interface of the alloyed zinc plating layer and the Fe-based electroplating layer described later is analyzed by surface analysis or line analysis using an electron beam microanalyzer (Electron Probe Micro Analyzer: EPMA). First, the steel plate embedded in the resin is polished, and after observing the cross section perpendicular to the rolling direction, it is removed from the resin to prepare a sample for measurement. The accelerating voltage is 7kV, the irradiation current is 50nA, and the surface analysis or line analysis of the sample cross section is performed in a range of 300×300μm including the outermost layer of the steel plate with a step size of 1μm to implement the measurement of C intensity. At this time, in order to suppress contamination, a plasma cleaner is used in the measurement room and the sample preparation room to remove hydrocarbons on the surface and surrounding areas of the sample before the measurement begins. To prevent hydrocarbon accumulation during measurement, the sample was heated and maintained at a maximum temperature of 100°C on a benchtop. The sample temperature was preferably 90°C or higher. A calibration curve prepared by separately measuring a standard sample was used to convert the C intensity into C concentration (mass %). Due to the contamination suppression effect, the C detection limit was confirmed to be well below 0.10% by mass. Details of the apparatus used and the contamination suppression method are described in Reference 1 below.

[0143] Reference 1: Yamashita et al., “Carbon distribution in the early stage of proeutectoid ferrite transformation of low carbon steel using high-precision FE-EPMA,” Iron and Steel, Vol. 103 (2017) No. 11, p. 14-20

[0144] The necessity of anti-contamination measures during measurement depends on the type of equipment and conditions used, so the above configuration is not essential. In other words, the measurement conditions only need to be able to obtain sufficient accuracy, and the measurement conditions have no essential relationship with the effect of the present invention.

[0145] In the obtained concentration distribution, the line distribution in the plate thickness direction is extracted from the interface between the alloyed zinc plating layer and the Fe-based electroplating layer, and 300 points are taken in the direction parallel to the steel plate surface for averaging to obtain the distribution of the C concentration in the plate thickness direction. The obtained distribution of the C concentration in the plate thickness direction is smoothed using the simple moving average method. At this time, the number of smoothing points is preferably about 21 points. When the number of smoothing points near the surface of the sample is less than 10 points on one side, it is preferred to smooth the measurement points that can be taken on one side. Then, in the intensity distribution after smoothing, the thickness at which the C concentration is less than 80% of that in the steel is evaluated as the thickness of the decarburized layer. In addition, for the range of 10μm to 20μm in the plate thickness direction from the interface between the alloyed zinc plating layer and the Fe-based electroplating layer to the above-mentioned Fe-based electroplating layer side, the C concentration values ​​of 11 points with a spacing of 1μm are averaged as the C concentration in the range of 10μm to 20μm in the plate thickness direction. The above evaluation was applied to the measurement results of two viewing fields of each sample, and the averages were taken as the average C concentration in the range of 10 μm to 20 μm in the plate thickness direction and the evaluation values ​​of the decarburized layer thickness.

[0146] use Figure 10 A representative example of the distribution of the C concentration and thickness analyzed by EPMA will be described. Figure 10 (a) shows the results of raw data of the distribution of the thickness of the C concentration obtained by analyzing the galvannealed steel sheet No. 38 of Example described later. It should be noted that the measurement was performed by peeling off the galvannealed layer. Figure 10 (b) shows the use of a simple moving average method with 21 smoothing points. Figure 10 (a) The original data is smoothed. Figure 10 As shown in (b), in this example, there is a decarburized layer having a C concentration of 80% or less in the steel, and the thickness of the interface between the alloyed zinc plating layer and the Fe-based electroplating layer from the decarburized layer is 93 μm.

[0147] As mentioned above, the characteristics of the Fe-based electroplating layer that prevent internal cracking are the combined effects of the galvanizing's grain boundary intrusion suppression effect, stress relaxation effect, and toughness reduction suppression effect, as well as the Fe-based electroplating layer's accelerated decarburization effect to reduce the C concentration in the surface layer. It is not possible to clearly quantify these combined effects, but the adhesion weight CW on one side of the alloyed Fe-based electroplating layer is Fe1 (g / m 2 ) and the thickness of the decarburized layer C d (μm) preferably satisfies the following formula (1).

[0148] 0.8×(CW Fe1 )+(C d )≥70···(1)

[0149] This is because if the adhesion amount CW on one side of the Fe-based electroplating layer after alloying Fe1 (g / m 2 ) and the thickness of the decarburized layer C d (μm) satisfies the formula (1), and the resistance to resistance welding cracking is particularly good.

[0150] Next, the alloyed zinc plating layer formed on the above-mentioned Fe-based electroplating layer is described. Here, the "alloyed zinc plating layer" refers to a zinc film formed on the surface of the steel sheet by zinc plating. The alloyed zinc plating layer may contain one or more selected from the group consisting of ζ phase, δ1 phase, Γ phase and Γ1 phase. By forming a zinc plating layer on the surface of the steel sheet, excellent corrosion resistance can be obtained. As described above, by pre-plating the Fe-based electroplating layer before annealing on the surface of the cold-rolled steel sheet before the annealing process after cold rolling before forming the zinc plating layer, the adhesion amount on one side is greater than 0 g / m 2 The Fe-based electroplated layer in the alloyed galvanized steel sheet that is alloyed after galvanizing can improve the resistance to resistance welding cracking characteristics of the weld in the alloyed galvanized steel sheet. The composition of the alloyed galvanized layer is not particularly limited, but for example, in the case of an alloyed hot-dip galvanized layer, it is composed of Al, Zn, Fe and inevitable impurities. The Al content in the alloyed galvanized layer is not particularly specified. In one example, the Al content in the alloyed hot-dip galvanized layer is 0.05% to 0.250% by mass. The Fe content in the alloyed galvanized layer is also not particularly specified, but in one example, the Fe content in the alloyed galvanized layer is 7.0% by mass or more, and in another example, it is 15.0% by mass or less. The Fe content in the alloyed galvanized layer is more preferably 13% by mass or less.

[0151] The coating weight on one side of the alloyed zinc plating layer is not particularly limited, but can be 25 g / m 2 Above, can also be 80g / m 2 By making the coating weight of the alloyed zinc plating layer on one side 25g / m2 The above can further improve the corrosion resistance and make it easy to control the coating adhesion. In addition, if the adhesion of the single side of the alloyed zinc plating layer is 80g / m 2 The coating adhesion is good. The coating weight on one side of the alloyed zinc plating layer is more preferably 35 g / m 2 In addition, the adhesion amount of the alloyed zinc plating layer on one side can be more preferably 60g / m 2 the following.

[0152] According to the present disclosure, a high-strength galvannealed steel sheet having a tensile strength TS of 590 MPa or more as measured according to JIS Z 2241 (2011) can be provided. The strength of the galvannealed steel sheet is more preferably 800 MPa or more.

[0153] <Method for manufacturing alloyed galvanized steel sheet>

[0154] Next, a method for producing a galvannealed steel sheet will be described.

[0155] A method for producing a galvanized steel sheet according to one embodiment may be a method for producing an alloyed galvanized steel sheet as follows: a cold-rolled steel sheet containing 0.1% to 3.0% by mass of Si is subjected to Fe-based electroplating to produce a pre-annealed Fe-based electroplated steel sheet having a pre-annealed Fe-based electroplated layer formed on at least one surface;

[0156] Next, the Fe-based electroplated steel sheet before annealing is heated at an average heating rate of 10°C / second or more in a temperature range of 400°C to 650°C, and maintained in an atmosphere with a dew point greater than -30°C in the temperature range after heating to produce the Fe-based electroplated steel sheet.

[0157] Next, the Fe-based electroplated steel sheet is galvanized to produce a galvanized steel sheet.

[0158] Then, the galvanized steel sheet is further alloyed to obtain a sheet with an adhesion amount greater than 0 g / m 2 An alloyed galvanized steel sheet comprising an Fe-based electroplating layer and an alloyed zinc plating layer formed on the Fe-based electroplating layer.

[0159] First, a cold-rolled steel sheet containing 0.1% to 3.0% Si by mass is manufactured. Alternatively, the cold-rolled steel sheet may contain 0.50% to 3.0% Si by mass. The cold-rolled steel sheet may be manufactured according to a conventional method for manufacturing cold-rolled steel sheets. In one example, the cold-rolled steel sheet is manufactured by hot-rolling a steel slab having the above-described composition to form a hot-rolled sheet, then pickling the hot-rolled sheet, and then cold-rolling the hot-rolled sheet to form a cold-rolled steel sheet.

[0160] Next, the surface of the cold-rolled steel sheet is subjected to an Fe-based electroplating treatment to produce a Fe-based electroplated steel sheet before annealing. The Fe-based electroplating treatment method is not particularly limited. For example, as the Fe-based electroplating bath, a sulfuric acid bath, a hydrochloric acid bath, or a mixture of the two can be used. In addition, the cold-rolled steel sheet after cold rolling can also be subjected to an Fe-based electroplating treatment without undergoing an oxidation treatment such as a preheating furnace. In addition, the Fe-based electroplated steel sheet before annealing refers to a method in which the Fe-based electroplated layer does not undergo an annealing process, and does not exclude the method of pre-annealing the cold-rolled steel sheet before the Fe-based electroplating treatment.

[0161] The adhesion amount of the Fe-based electroplated layer before annealing is greater than 0 g / m on one side of the Fe-based electroplated layer in the alloyed galvanized steel sheet obtained after alloying treatment. 2 The amount of Fe-based electroplating layer deposited before annealing is adjusted by the power-on time, etc.

[0162] Adhesion weight CW of Fe-based electroplating layer on one side before annealing Fe0 (g / m 2 ) preferably satisfies the following formula (2). Thus, taking into account the amount of Fe-based plating consumed by alloying, the Fe-based plating layer can be applied in a manner such that the Fe-based plating layer remains after alloying.

[0163] (CW Fe0 )-(CW Zn )×[mass%Fe] / 100>0···(2)

[0164] Here, CW Zn : The target value of the adhesion amount of the above-mentioned alloyed zinc plating layer on one side (g / m 2 )

[0165] However, 25.0g / m 2 ≤CW Zn ≤80.0g / m 2

[0166] [mass%Fe]: target value of Fe content in the above-mentioned alloyed zinc plating layer (mass %)

[0167] In other words, if the galvannealed layer forming the galvannealed steel sheet contains more Fe than the target value (g / m 2 ) of the Fe-based electroplated layer before annealing, it is possible to obtain a Fe-based electroplated layer greater than 0 g / m after alloying. 2 Fe-based electroplating layer.

[0168] Here, the target value CW of the coating weight on one side of the galvannealed steel sheet is ZnThe target value of the Fe content in the alloyed zinc plating layer is determined according to the predetermined specifications for each application. In addition, the adhesion amount of the Fe-based electroplating layer on one side can be calculated by the cross-sectional observation method described above.

[0169] The Fe ion content in the Fe-based electroplating bath before power on is expressed as Fe 2+ If the Fe ion content in the Fe-based electroplating bath is expressed as Fe 2+ A sufficient Fe deposition amount can be obtained when the Fe deposition amount is 0.5 mol / L or more. In order to obtain a sufficient Fe deposition amount, the Fe ion content in the Fe-based electroplating bath before the start of energization is preferably 2.0 mol / L or less.

[0170] The Fe-based electroplating bath may contain Fe ions and at least one element selected from B, C, P, N, O, Ni, Mn, Mo, Zn, W, Pb, Sn, Cr, V, and Co. The total content of these elements in the Fe-based electroplating bath is preferably such that the total content of these elements in the Fe-based electroplated layer before annealing is 10% by mass or less. Furthermore, metal elements may be contained as metal ions, and non-metallic elements may be contained as part of boric acid, phosphoric acid, nitric acid, an organic acid, or the like. Furthermore, the iron sulfate plating solution may contain a conductive additive such as sodium sulfate or potassium sulfate, a chelating agent, or a pH buffer.

[0171] Other conditions of the Fe-based electroplating bath are not particularly limited. Considering the ability to maintain constant temperature, the temperature of the Fe-based electroplating solution is preferably above 30°C and preferably below 85°C. The pH of the Fe-based electroplating bath is also not particularly specified, but from the perspective of preventing the reduction of current efficiency by utilizing hydrogen generation, it is preferably above 1.0. In addition, considering the conductivity of the Fe-based electroplating bath, it is preferably below 3.0. From the perspective of productivity, the current density is preferably 10A / dm 2 From the viewpoint of easy control of the adhesion amount of the Fe-based electroplating layer, 150 A / dm is preferred. 2 The sheet feeding speed is preferably 5 mpm or higher from the viewpoint of productivity, and is preferably 150 mpm or lower from the viewpoint of stably controlling the amount of adhesion.

[0172] In addition, as treatments before the Fe-based electroplating treatment, degreasing treatment and water washing for cleaning the surface of the cold-rolled steel sheet, and pickling treatment and water washing for activating the surface of the cold-rolled steel sheet can be carried out. After these pre-treatments, the Fe-based electroplating treatment is carried out. The methods of degreasing and water washing are not particularly limited, and conventional methods can be used. In the pickling treatment, various acids such as sulfuric acid, hydrochloric acid, nitric acid and mixtures thereof can be used. Among them, sulfuric acid, hydrochloric acid or mixtures thereof are preferred. The concentration of the acid is not particularly specified, but considering the ability to remove the oxide film and prevent rough skin (surface defects) caused by excessive pickling, etc., it is preferably about 1 to 20 mass%. In addition, the pickling treatment solution may contain a defoaming agent, a pickling accelerator, a pickling inhibitor, etc.

[0173] Next, after the Fe-based electroplating treatment, an annealing step is performed: the pre-annealed Fe-based electroplated steel sheet is held in a reducing atmosphere with a dew point greater than -30°C and a hydrogen concentration of 1.0% to 30.0% by volume in a temperature range of 650°C to 900°C for 30 to 600 seconds, followed by cooling to obtain the Fe-based electroplated steel sheet. The annealing step is performed to remove the strain in the pre-annealed Fe-based electroplated steel sheet caused by the rolling process, thereby recrystallizing the structure and improving the steel sheet strength.

[0174] Average heating rate: above 10℃ / second

[0175] Next, the pre-annealed Fe-based electroplated steel sheet is heated to a temperature range of 650°C to 900°C at an average heating rate of 10°C / second or more in the temperature range of 400°C to 650°C (heating step). By setting the average heating rate of the heating step to an average of 10°C / second or more, the growth of grains in the Fe-based electroplated layer during the heating step is minimized. This is because, as will be described later, Si internal oxidation is almost non-existent at the grain boundaries of the Fe-based electroplated layer during the heating step. Therefore, if the heating rate is less than 10°C / second on average, grain growth cannot be suppressed. While the growth of grains in the Fe-based electroplated layer is minimized during the heating step, annealing is performed in an atmosphere with a dew point greater than -30°C, as will be described later, to refine the crystals of the Fe-based electroplated layer. The heating zone in the heating step can be, for example, a direct fired furnace (DFF) or a non-oxidizing furnace (NOF). In the case of a radiant tube type heating furnace, a preheating zone such as an IH (Induction Heater) may be provided in the front stage. The average heating rate is based on the temperature measured on the surface of the Fe-based electroplated steel sheet.

[0176] Next, an annealing process is performed: the Fe-based electroplated steel sheet before annealing is kept at a temperature range of 650°C to 900°C for 30 seconds to 600 seconds in a reducing atmosphere with a dew point greater than -30°C and a hydrogen concentration of 1.0% to 30.0% by volume, and then cooled to obtain the Fe-based electroplated steel sheet. The annealing process is performed to remove the strain of the cold-rolled steel sheet generated by the rolling process, recrystallize the structure, and improve the strength of the steel sheet. At this time, almost no internal oxides are formed at the grain boundaries of the Fe-based electroplated layer, the growth of the grains of the Fe-based electroplated layer is suppressed, the crystals are refined, and a decarburized layer is formed on the surface of the Fe-based electroplated steel sheet, and the C concentration of the surface layer is reduced.

[0177] Hydrogen concentration: 1.0% to 30% by volume

[0178] The annealing process is carried out in a reducing atmosphere with a hydrogen concentration of 1.0 volume % to 30.0 volume %. Hydrogen inhibits the oxidation of Fe on the surface of the Fe-based electroplated steel sheet before annealing in the annealing process, and plays a role in activating the surface of the steel sheet. If the hydrogen concentration is 1.0 volume % or more, it is possible to avoid the deterioration of the coating adhesion when the galvanized layer is set as described later due to the oxidation of Fe on the surface of the steel sheet. Therefore, the annealing process is preferably carried out in a reducing atmosphere with a hydrogen concentration of 1.0 volume % or more, more preferably a reducing atmosphere with a hydrogen concentration of 2.0 volume % or more. The upper limit of the hydrogen concentration in the annealing process is not particularly limited, but from the perspective of cost, the hydrogen concentration is preferably 30.0 volume % or less, more preferably 20.0 volume % or less. The remainder of the annealing atmosphere other than hydrogen is preferably nitrogen.

[0179] Dew point greater than -30℃

[0180] By setting the dew point of the annealing atmosphere in the annealing process to be greater than -30°C, Si internal oxides are formed at the grain boundaries of the Fe-based electroplating layer. The control of the dew point to be greater than -30°C is preferably carried out in the temperature range of 650°C to 900°C. As a result, Si internal oxides can be formed at the grain boundaries of the Fe-based electroplating layer while the average heating rate in the heating process is set to an average of more than 10°C / second to suppress the growth of the grains in the Fe-based electroplating layer as much as possible. In addition, by setting the dew point of the annealing atmosphere in the annealing process to be greater than -30°C, the decarburization reaction can be promoted and the C concentration of the surface layer can be reduced. The Si internal oxides present at the grain boundaries of the Fe-based electroplating layer inhibit the growth of the grains of the Fe-based electroplating layer in the annealing process due to the pinning effect. The Si internal oxides are generated by the diffusion of Si from the cold-rolled steel sheet, and the pinning effect of the Si internal oxides is particularly strong on the cold-rolled steel sheet side of the Fe-based electroplating layer. As a result, it is believed that the crystal grain size on the interface side of the Fe-based electroplating layer and the Si-containing cold-rolled steel sheet is particularly small, and the number of grain boundaries in contact with the interface of the Fe-based electroplating layer and the Si-containing cold-rolled steel sheet increases. In addition, the pinning effect here refers to the Zenerdrag mechanism. The second phase particles are dispersed in the structure, and when the grain boundaries intersect with the second phase particles, energy is required for the grain boundaries to detach from the second phase particles. That is, the pinning force acts between the particles and the grain boundaries to prevent the grain boundaries from moving and inhibit the growth of the grains. Carbides and sulfides are well known as second phase particles. Although it is not clear whether the Si internal oxides reflect this pinning effect, it is speculated that the pinning effect is reflected based on experimental facts. The dew point of the annealing atmosphere is preferably above -20°C, and more preferably above -5°C. By setting the dew point of the annealing atmosphere to above -5°C, the resistance welding cracking characteristics of the weld, especially the characteristics of preventing internal cracking, are good. The upper limit of the dew point of the annealing atmosphere is not particularly limited, but is preferably 30° C. or lower in order to appropriately prevent oxidation of the Fe-based electroplated layer surface and improve plating adhesion when a zinc plating layer is provided as described below.

[0181] Holding time in the temperature range of 650°C to 900°C: 30 seconds to 600 seconds

[0182] In the annealing process, the holding time in the temperature range of 650°C to 900°C is preferably 30 seconds to 600 seconds. By making the holding time in this temperature range 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, and the coating adhesion can be improved when the zinc coating layer is provided 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 limited, but from the perspective of productivity, the holding time in this temperature range is preferably 600 seconds or less.

[0183] The maximum temperature of Fe-based electroplated steel sheet before annealing: 650℃~900℃

[0184] The maximum temperature of the Fe-based electroplated steel sheet before annealing is not particularly limited, but is preferably 650°C to 900°C. By setting the maximum temperature of the Fe-based electroplated steel sheet before annealing to 650°C or above, the recrystallization of the steel sheet structure proceeds smoothly, and the desired strength can be obtained. In addition, the natural Fe oxide film formed on the surface of the Fe-based electroplated layer before annealing can be appropriately reduced, improving the plating adhesion when a zinc layer is provided on the surface of the Fe-based electroplated steel sheet as described later. In addition, if the maximum temperature of the Fe-based electroplated steel sheet is below 900°C, the diffusion rate of Si and Mn in the steel can be prevented from increasing excessively, and Si and Mn can be prevented from diffusing to the steel sheet surface, which can improve the plating adhesion when a zinc layer is provided on the surface of the Fe-based electroplated steel sheet as described later. In addition, if the maximum temperature is below 900°C, damage to the furnace body of the heat treatment furnace can be prevented, and costs can also be reduced. Therefore, the maximum temperature of the Fe-based electroplated steel sheet before annealing is preferably below 900°C. The maximum attainable temperature is based on the temperature measured on the surface of the Fe-based electroplated steel sheet before annealing.

[0185] Adhesion weight CW of Fe-based electroplating layer on one side before annealing Fe0 (g / m 2 ) and the dew point DP (°C) of the annealing atmosphere preferably satisfy the following formula (3). If the following formula (3) is satisfied, the effects of suppressing the grain boundary intrusion of molten zinc, the stress relaxation effect, the toughness reduction suppression effect, and the effect of promoting decarburization of the Fe-based electroplating layer to reduce the C concentration in the surface layer act in a complex manner, and the effect of improving the resistance to resistance welding cracking characteristics can be more significantly achieved.

[0186] 0.83×{(CW Fe0 )-(CW Zn )×[mass%Fe] / 100}+(DP)≥-5···(3)

[0187] Here, CW Zn : The target value of the adhesion amount of the above-mentioned alloyed zinc plating layer on one side (g / m 2 )

[0188] Among them, 25.0g / m 2 ≤CW Zn ≤80.0g / m 2

[0189] [mass%Fe]: target value of Fe content in the above-mentioned alloyed zinc plating layer (mass %)

[0190] In addition, the adhesion amount CW of the Fe-based electroplating layer on one side before annealing Fe0 (g / m 2 ) more preferably satisfies the following formula (4).

[0191] (CW Fe0 )-(CW Zn )×[mass%Fe] / 100≥2···(4)

[0192] Among them, 25.0g / m 2 ≤CW Zn ≤80.0g / m 2

[0193] [mass%Fe]: target value of Fe content in the above-mentioned alloyed zinc plating layer (mass %)

[0194] In other words, if the target value (g / m2) of Fe content in the galvannealed layer of the galvannealed steel sheet is achieved, 2 )+2g / m 2 The Fe-based electroplated layer before annealing can obtain 2g / m 2 The above Fe-based electroplating layer.

[0195] As described above, if the result satisfies the above formula (3), the resistance welding cracking characteristics of the weld can be further improved. As another embodiment, when the above formula (3) is not satisfied, a process of changing the dew point DP (°C) of the annealing atmosphere can be further provided to satisfy the above formula (3). In this way, the resistance welding cracking characteristics of the weld can be more reliably improved. As an example of performing this process in operation, it can be considered to have a method according to the adhesion amount CW of the single side of the Fe-based electroplating layer before annealing. Fe0 (g / m 2 ) value in the annealing process to change the dew point DP in the annealing process to satisfy the above formula (3). In the annealing process, the atmosphere dew point is controlled to become the determined dew point. Specifically, the adhesion amount CW on one side of the Fe-based electroplating layer before annealing is Fe0 (g / m 2 ) is substituted into the above formula (3) to determine the dew point DP in the above annealing process in a manner that satisfies the formula (3). Here, the adhesion amount CW on one side of the Fe-based electroplating layer before annealing is Fe0 (g / m 2 ) into the above formula (3) is not limited to the method of substituting the same formula as the above formula (3). It also includes the method of substituting an inequality that always satisfies the narrower range of formula (3). By performing such control, even if the product specifications of the steel plate that is continuously passed through are changed, the adhesion amount CW of the Fe-based electroplating layer on one side before annealing is Fe0 (g / m 2 ) changes significantly and does not satisfy the above formula (3) (when the formula is not actually satisfied or a situation occurs where the formula is no longer satisfied), automatic control can also be performed to satisfy the formula.

[0196] In addition, the control responsiveness of the dew point DP is better than the adhesion amount CW of the Fe-based electroplating layer on one side before annealing. Fe0 Therefore, from the perspective of control responsiveness, it is preferable to change the adhesion amount CW of the Fe-based electroplating layer on one side before annealing in accordance with the value of the dew point DP so as to satisfy the above formula (3). Fe0 In the case of a continuous annealing furnace, the adhesion weight CW of the Fe-based electroplating layer on one side before annealing in the Fe-based electroplating treatment upstream of the annealing process is changed according to the dew point DP value of the annealing process. Fe0 However, for the steel plate that is continuously passed through, the adhesion amount CW of the Fe-based electroplating layer on one side before annealing is changed. Fe0 The position can be manufactured under the condition of satisfying the above formula (3).

[0197] The adhesion amount CW of the Fe-based electroplated layer on one side before annealing is changed so as to satisfy the above formula (3). Fe0 When at least one of the dew point DP is changed, when welding Si-containing cold-rolled steel sheets of different product specifications and continuously passing the sheets, it is preferable to change the adhesion amount CW of the Fe-based electroplating layer on one side before annealing according to the welding position. Fe0 Or dew point DP. As described above, since the responsiveness of the dew point DP is poor, when changing the dew point DP, it is more preferable to perform feedforward control on the amount of humidification in the furnace so as to satisfy this equation.

[0198] Here, the "adhesion weight CW of Fe-based electroplating layer on one side before annealing" is Fe0 (g / m 2 The "value of the dew point DP" may be the adhesion amount obtained under the conditions adopted in the Fe-based electroplating (target value), or the adhesion amount of the Fe-based electroplating layer actually obtained (measured value). Similarly, the "value of the dew point DP" may be either the target value or the measured value.

[0199] The above is an example of the operation in the method for manufacturing the alloyed galvanized steel sheet, which can also be implemented as the following method for determining the manufacturing conditions of the alloyed galvanized steel sheet: before the start of the operation, the adhesion weight CW of the Fe-based electroplating layer on one side before annealing is confirmed in advance. Fe0 (g / m 2 ) and the target value of the dew point DP satisfy the above formula (3). If not, change the adhesion amount CW of the Fe-based electroplating layer on one side in advance. Fe0 (g / m 2) and the target value of the dew point DP. Such a manufacturing condition determination method can be implemented as a part of the manufacturing method of the alloyed galvanized steel sheet or as a separate process.

[0200] Next, the galvanizing treatment will be described.

[0201] After the above-mentioned annealing process, the Fe-based electroplated steel sheet is galvanized. After the annealing process, the Fe-based electroplated steel sheet is cooled and immersed in a hot-dip galvanizing bath to galvanize the surface of the steel sheet. Galvanizing can be, for example, hot-dip galvanizing. The hot-dip galvanizing bath is composed of Al, Zn, and inevitable impurities. The composition of the hot-dip galvanizing bath is not particularly specified, and the Al concentration in the bath is generally 0.05% to 0.250% by mass. If the Al concentration in the bath is 0.05% by mass or more, the formation of bottom dross can be prevented, and dross can be prevented from adhering to the steel sheet and causing defects. In addition, by setting the Al concentration in the bath to 0.250% by mass or less, the increase of top dross can be prevented, and dross can be prevented from adhering to the steel sheet and causing defects, and costs can also be reduced. Other conditions for the hot-dip galvanizing treatment are not limited. For example, the bath temperature of the hot-dip galvanizing bath is generally in the range of 440 to 500°C, and the steel sheet is immersed in the hot-dip galvanizing bath at a sheet temperature of 440 to 550°C.

[0202] The coating weight of each side of the galvanized layer is controlled at 25-80g / m 2 By making the coating weight of each single side of the zinc coating layer 25g / m 2 The above can further improve the corrosion resistance and make it easy to control the coating adhesion. In addition, if the coating adhesion of each single side of the zinc coating is 80g / m 2 Below, the plating adhesion is good.

[0203] After galvanizing, the coating weight can be adjusted appropriately. While there are no specific limitations on the method, hot-dip galvanizing typically utilizes gas purging to adjust the coating weight. In one example, the coating weight is adjusted by factors such as the gas pressure of the gas purging process and the distance between the purging nozzle and the steel sheet.

[0204] After galvanizing, alloying treatment is further performed to produce an alloyed galvanized steel sheet. The galvanized layer is heated and alloyed during the alloying treatment. The alloying treatment is preferably performed at a temperature of 450°C to 600°C. By performing the alloying treatment at a temperature of 450°C or higher, a steel sheet with excellent press formability can be provided without residual η phase. Furthermore, by performing the alloying treatment at a temperature of 600°C or lower, excellent plating adhesion is achieved. Furthermore, the alloying time is preferably 5s to 60s.

[0205] Next, a method for calculating the coating weight per single surface of the zinc alloy plating layer will be described.

[0206] First, take samples of 20×25 mm size at two locations from the alloyed galvanized steel sheet. After measuring the weight respectively, use the test solution specified in JIS H 0401 (2013) or ISO17925 (2004) to peel off the alloyed galvanized layer on one side with the Fe-based electroplating layer, and measure the weight again. Subtract the weight after the plating is peeled off from the weight before the plating is peeled off, and divide it by the surface area of ​​the plating peeled off portion to calculate the plating adhesion. Here, the average value of the two locations is used as the plating adhesion per single side of the alloyed galvanized layer.

[0207] The Fe content (mass %) in the alloyed zinc plating layer was determined in accordance with ISO 17925 (2004). The test solution after stripping the coating was analyzed using an inductively coupled plasma (ICP) emission spectrometer. The Fe content in the test solution was divided by the total of the Fe, Zn, and Al components in the test solution that make up the alloyed zinc plating layer, and the resultant value was multiplied by 100 to calculate the Fe content in the alloyed zinc plating layer. The average value of the two values ​​was used as the Fe content in the alloyed zinc plating layer.

[0208] <Electrodeposition-coated steel sheets>

[0209] In addition, according to this embodiment, it is also possible to provide an electrodeposited steel sheet having a chemical conversion coating formed on the alloyed zinc coating layer and an electrodeposited coating formed on the chemical conversion coating on the alloyed zinc coating. Since the alloyed zinc coating of this embodiment has excellent resistance to resistance welding cracking of the welded portion, the electrodeposited steel sheet formed using the alloyed zinc coating is particularly suitable for automotive parts. The types of chemical conversion coating and electrodeposited coating are not particularly limited, and can be known chemical conversion coatings and electrodeposited coatings. As the chemical conversion coating, zinc phosphate coating, zirconium coating, etc. can be used. As the electrodeposited coating, there is no particular limitation as long as it is an electrodeposited coating for automobiles. The thickness of the electrodeposited coating varies depending on the application, and the coating in the dry state is preferably about 10μm to 30μm. In addition, according to this embodiment, it is also possible to provide an alloyed zinc coating steel sheet for electrodeposited coating for electroplating.

[0210] <Method for Manufacturing Electrodeposition-Coated Steel Sheets>

[0211] Next, the manufacturing method of the above-mentioned electrodeposition-coated steel sheet will be described. The above-mentioned electrodeposition-coated steel sheet can be manufactured using a manufacturing method of electrodeposition-coated steel sheet having the following steps: a chemical conversion treatment step, in which a galvanized steel sheet is subjected to a chemical conversion treatment to obtain a chemical conversion-treated steel sheet having a chemical conversion treatment film formed on the galvanized steel sheet; and an electrodeposition coating step, in which a galvanized steel sheet is subjected to an electrodeposition coating to obtain an electrodeposition-coated steel sheet having an electrodeposition coating film formed on the chemical conversion treatment film. The chemical conversion treatment and the electrodeposition coating treatment can be carried out using known methods. In addition, as treatments before the chemical conversion treatment, degreasing treatment, water washing, and surface conditioning treatment as needed can be carried out to clean the surface of the galvanized steel sheet. After these pre-treatments, the chemical conversion treatment is carried out. The degreasing and water washing methods are not particularly limited, and conventional methods can be used. Surface conditioning agents such as Ti colloid or zinc phosphate colloid can be used in the surface conditioning treatment. When implementing these surface conditioning agents, no special steps are required, and the treatment can be carried out according to conventional methods. For example, the desired surface conditioner is dissolved in a predetermined deionized water, and after sufficient stirring, a treatment solution of a predetermined temperature (usually room temperature, 25 to 30°C) is prepared, and the steel plate is immersed in the treatment solution for a predetermined time (20 to 30 seconds). Then, the chemical conversion treatment of the next process is carried out without drying. The chemical conversion treatment can also be carried out according to conventional methods. For example, the desired chemical conversion treatment agent is dissolved in a predetermined deionized water, and after sufficient stirring, a treatment solution of a predetermined temperature (usually 35 to 45°C) is prepared, and the steel plate is immersed in the treatment solution for a predetermined time (60 to 120 seconds). As the chemical conversion treatment agent, for example, zinc phosphate treatment agent for steel, zinc phosphate treatment agent for steel and aluminum combination, and zirconium treatment agent can be used. Continuing, the next process of electroplating is carried out. Electroplating can also be carried out according to conventional methods. After pre-treatment such as water washing treatment is carried out as needed, the steel plate is immersed in the fully stirred electroplating paint, and the electroplating coating of the desired thickness is obtained by electroplating treatment. As the electrodeposition coating, cationic electrodeposition coating and anionic electrodeposition coating can be used. In addition, a top coating or the like may be applied after the electrodeposition coating, depending on the application.

[0212] <Automotive Parts>

[0213] In addition, according to the present embodiment, an automobile part can be provided, at least part of which is made of the above-mentioned electroplated coated steel sheet. Since the alloyed galvanized steel sheet of the present embodiment has excellent resistance to resistance welding cracking at the weld portion, the electroplated coated steel sheet using the alloyed galvanized steel sheet is particularly suitable for automobile parts. Automobile parts made of electroplated coated steel sheets may include steel plates other than the electroplated coated steel sheet of the present embodiment as blanks. Since the electrodeposited coated steel sheet of the present embodiment has excellent resistance to resistance welding cracking at the weld portion, LME cracking in the weld portion of automobile parts made of the alloyed galvanized steel sheet can be appropriately prevented. The type of automobile parts at least part of which is made of the electroplated coated steel sheet is not particularly limited, but may be, for example, side beam parts, pillar parts, automobile bodies, etc.

[0214] Hereinafter, the present invention will be described in detail based on examples.

[0215] Example 1

[0216] Steels having the chemical compositions shown in Tables 1 and 3 were melted to obtain cast slabs, which were then hot-rolled, pickled, and cold-rolled to produce cold-rolled steel sheets having a thickness of 1.6 mm.

[0217]

[0218]

[0219] Next, the cold-rolled steel sheet is degreased with an alkali solution, and then electrolytically treated with the steel sheet as the cathode under the conditions shown below to produce a pre-annealed Fe-based electroplated steel sheet having an Fe-based electroplated layer on one side. The adhesion amount of the Fe-based electroplated layer is calculated as described above and controlled by the power-on time. Next, the pre-annealed Fe-based electroplated steel sheet is reduction annealed with 15% H2-N2, a soaking zone temperature of 800°C, and the dew point of the atmosphere adjusted as shown in Tables 2-1, 2-2, and 4 to obtain a Fe-based electroplated steel sheet. After the obtained Fe-based electroplated steel sheet is cooled to 440-550°C, the Fe-based electroplated steel sheet is hot-dip galvanized using a 460°C hot-dip galvanizing bath with an effective Al concentration of 0.132% by mass in the bath and the balance consisting of Zn and unavoidable impurities. The weight per unit area is then adjusted to approximately 50 g / m2 per side by gas purging. 2 Next, the galvanized steel sheets were alloyed at 490°C for varying periods of time to produce samples of alloyed hot-dip galvanized steel sheets with different Fe contents (Fe%) diffused into the alloyed galvanneal layer.

[0220] 〔Electrolysis Conditions〕

[0221] Bath temperature: 50°C

[0222] pH: 2.0

[0223] Current density: 45A / dm 2

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

[0225] Electrode (anode): iridium oxide electrode

[0226] From the alloyed hot-dip galvanized steel sheet prepared as described above, the adhesion amount of the Fe-based electroplating layer on one side, the adhesion amount of the alloyed galvanized layer on one side, the Fe concentration in the alloyed galvanized layer, I Si，Fe / I Si，bulk And at the interface between the Fe-based electroplated layer and the Si-containing cold-rolled steel sheet, the number of grain boundaries where the Fe-based electroplated layer and the Si-containing cold-rolled steel sheet are in contact.

[0227] Using the alloyed hot-dip galvanized steel sheet produced as described above, the C strength was measured by surface analysis of the sample cross section according to the above method, and the average value of the C concentration in the range of 10μm to 20μm in the thickness direction from the interface between the alloyed galvanized layer and the Fe-based electroplated layer to the Fe-based electroplated layer side and the depth of the decarburized layer were evaluated.

[0228] The resistance cracking characteristics of the welded portions of the alloyed hot-dip galvannealed steel sheets obtained above were investigated. The following describes a method for measuring and evaluating the resistance cracking characteristics of the welded portions.

[0229] Resistance cracking characteristics of welded joints

[0230] For alloyed hot-dip galvanized steel sheets, the evaluation targets are those with resistance to resistance welding cracking characteristics at a holding time of 0.18 seconds, a Si content of 0.50% or less, a tensile strength of 980 MPa, and a single-sided adhesion weight of 50 g / m 2 The resistance cracking characteristics of the welded portion of the test were measured using alloyed hot-dip galvanized steel sheets (thickness 1.6 mm). Figure 11 A method for evaluating the resistance welding cracking characteristics of a welded portion will be described.

[0231] A test piece 6 was cut out of 50×150 mm with the rolling perpendicular direction (TD) as the long side and the rolling direction as the short side, and a hot-dip galvanized layer of the same size was cut out with an adhesion amount of 50 g / m 2The plate group is assembled in such a way that the evaluation target surface of the test piece 6 (the side with the Fe-based electroplating layer) faces the galvanized layer of the test alloyed hot-dip galvanized steel plate 5. The plate group is fixed to the fixing table 8 via a 2.0mm thick gasket 7. The gasket 7 is a pair of steel plates with a length of 50mm, a width of 45mm, and a thickness of 2.0mm. Figure 11 As shown in (a), the pair of steel plates are arranged so that their respective longitudinal end faces are aligned with both end faces of the plate group in the transverse direction. Therefore, the distance between the pair of steel plates is 60 mm. The fixing table 8 is a single steel plate with a hole in the center.

[0232] Next, using a servo motor-operated, single-phase AC (50 Hz) resistance welding machine, the plate assembly was bent by applying pressure to the plate assembly with a pair of electrodes 9 (tip diameter: 6 mm). Resistance welding was performed under the conditions of a pressure of 3.5 kN, a hold time of 0.18 seconds or 0.24 seconds, and a weld time of 0.36 seconds, using a welding current with a nugget diameter r of 5.9 mm to produce a plate assembly with a welded portion. The pair of electrodes 9 applied pressure to the plate assembly from above and below in a vertical direction, with the lower electrode pressing against the test piece 6 through a hole in the fixture 8. During pressure application, the lower electrode of the pair of electrodes 9 was fixed to the fixture 8, with the lower electrode in contact with a flat surface extending from the contact surface between the gasket 7 and the fixture 8. The upper electrode was movable. The upper electrode was in contact with the center of the test alloyed hot-dip galvannealed steel sheet 5. The plate assembly was welded while being tilted 5° relative to the horizontal along the longitudinal direction of the plate assembly. In addition, the holding time refers to the time from the end of the welding current to the start of electrode release. Figure 11 (b) Referring to the figure below, the nugget diameter r refers to the distance between the ends of the nugget 10 in the longitudinal direction of the plate group.

[0233] Next, the plate group with the welded portion is moved along the center of the welded portion including the nugget 10. Figure 11 (b) Cut along line BB in the figure above, and observe the cross section of the weld using an optical microscope (200x magnification). Evaluate the weld's resistance to resistance welding cracking according to the following criteria. A ◎ or ○ rating indicates excellent resistance to resistance welding cracking. A × rating indicates poor resistance to resistance welding cracking.

[0234] ◎: No cracks larger than 0.1 mm were observed during the holding time of 0.18 seconds.

[0235] ○: Cracks of 0.1 mm or longer were observed at a holding time of 0.18 seconds, but no cracks of 0.1 mm or longer were observed at a holding time of 0.24 seconds.

[0236] ×: Cracks of 0.1 mm or longer were observed during the holding time of 0.24 seconds

[0237] in addition, Figure 11 (b) The cracks that occurred in the test piece 6 are schematically indicated by reference numeral 11 in the figure below. Furthermore, when cracks occurred in the test steel sheet (the alloyed hot-dip galvannealed steel sheet used for testing), stress dispersion in the evaluation steel sheets (the steel sheets of each inventive example and comparative example) was insufficient, preventing appropriate evaluation. Therefore, data showing no cracks in the test steel sheet were used as examples.

[0238] The results of the above tests are shown in Tables 2-1, 2-2, and 4. According to the results, the resistance cracking characteristics of the welded portions of the alloyed galvanized steel sheets of the invention examples in which the Fe-based electroplating layer was formed under conditions suitable for the present invention before continuous annealing were all excellent. In addition, there were no particular problems with the resistance cracking characteristics of the welded portions of Reference Examples 1 and 2. In each invention example that satisfied equations (1) and (3), no cracks longer than 0.1 mm were observed even under the condition of a holding time of 0.18 seconds, and the resistance cracking characteristics of the welded portions were particularly good. In addition, in the table, for the examples where the Fe-based electroplating layer was not formed, the adhesion amount is indicated as "-", and the peak of the luminescence intensity at the wavelength of Si (referred to as "Si intensity peak" for convenience in the table), the interface between the Fe-based electroplating layer and the above-mentioned Si-containing cold-rolled steel sheet, and the number of grain boundaries where the Fe-based electroplating layer contacts the Si-containing cold-rolled steel sheet (referred to as "the number of grain boundaries in contact with the base steel" for convenience in the table) cannot be measured and are therefore indicated as "-". For steel sheets with a decarburized layer thickness of less than 10 μm, the decarburized layer thickness is expressed as “-”. In addition, when each variable on the left side of equations (1) and (3) is “-”, the left side of equations (1) and (3) is calculated as 0.

[0239]

[0240]

[0241]

[0242] Example 2

[0243] Steel having the chemical components shown in Table 5 was melted to obtain a cast slab, which was then hot-rolled, pickled, and cold-rolled to produce a cold-rolled steel sheet having a thickness of 1.6 mm.

[0244]

[0245] Next, the cold-rolled steel sheet is degreased with an alkali solution, and then, electrolytic treatment is performed with the steel sheet as the cathode under the conditions shown below to produce a pre-annealed Fe-based electroplated steel sheet having an Fe-based electroplated layer on one side. The adhesion amount of the Fe-based electroplated layer is calculated according to the above method and controlled by the power-on time. Next, the pre-annealed Fe-based electroplated steel sheet is reduction annealed with 15% H2-N2, a soaking zone temperature of 800°C, and the dew point of the atmosphere adjusted as shown in Table 6 to obtain a Fe-based electroplated steel sheet. After the obtained Fe-based electroplated steel sheet is cooled to 440-550°C, the Fe-based electroplated steel sheet is then hot-dip galvanized using a 460°C hot-dip galvanizing bath with an effective Al concentration of 0.132% by mass in the bath and the balance consisting of Zn and unavoidable impurities. The weight per unit area is then adjusted to about 50 g / m2 per single side by gas purging. 2 Next, the galvanized steel sheets were alloyed at 490°C for varying periods of time to produce samples of alloyed hot-dip galvanized steel sheets with different Fe contents (Fe%) diffused into the alloyed galvanneal layer.

[0246] 〔Electrolysis Conditions〕

[0247] Bath temperature: 50°C

[0248] pH: 2.0

[0249] Current density: 45A / dm 2

[0250] Fe-based electroplating bath: contains 1.5 mol / L of Fe 2+ ion

[0251] Electrode (anode): iridium oxide electrode

[0252] The above-mentioned method was used to determine the adhesion amount of the Fe-based electroplating layer on one side, the adhesion amount of the alloyed zinc plating layer on one side, the Fe concentration in the alloyed zinc plating layer, and the I Si，Fe / I Si，bulk Also, at the interface between the Fe-based electroplated layer and the Si-containing cold-rolled steel sheet, the number of grain boundaries of the Fe-based electroplated layer in contact with the Si-containing cold-rolled steel sheet.

[0253] Using the alloyed hot-dip galvanized steel sheets prepared as described above, the C strength was measured by surface analysis of the sample cross section according to the above method, and the average value of the C concentration in the range of 10μm to 20μm in the thickness direction from the interface between the alloyed galvanized layer and the Fe-based electroplated layer to the Fe-based electroplated layer side and the depth of the decarburized layer were evaluated.

[0254] The resistance cracking characteristics of the welded portions of the alloyed hot-dip galvannealed steel sheets obtained above were investigated. The following describes the measurement and evaluation methods for the resistance cracking characteristics of the welded portions.

[0255] Resistance cracking characteristics of welded joints

[0256] Alloyed hot-dip galvanized steel sheets, the evaluation targets are: resistance to resistance welding cracking characteristics within a holding time of 0.14 seconds, Si content less than 0.1%, tensile strength of 980 MPa, and adhesion weight of 50 g / m on one side. 2 The resistance cracking characteristics of the welded portion of the test were measured using alloyed hot-dip galvanized steel sheets (thickness 1.6 mm). Figure 11 A method for evaluating the resistance welding cracking characteristics of a welded portion will be described.

[0257] A test piece 6 of 50×150 mm was cut with the rolling direction (TD) as the long side and the rolling direction as the short side, and a hot-dip galvanized layer of the same size was cut to a coating weight of 50 g / m 2 The plate group is assembled by overlapping the test alloyed hot-dip galvanized steel plates 5. The plate group is assembled in such a way that the evaluation target surface of the test piece 6 (the side with the Fe-based electroplating layer) faces the galvanized layer of the test alloyed hot-dip galvanized steel plate 5. The plate group is fixed to the fixing table 8 via a 2.0mm thick gasket 7. The gasket 7 is a pair of steel plates with a length of 50mm × a width of 45mm × a thickness of 2.0mm. Figure 11 As shown in (a), the long-side end faces of the pair of steel plates are aligned with the short-side end faces of the plate group. Therefore, the distance between the pair of steel plates is 60 mm. The fixing table 8 is a single steel plate with a hole in the center.

[0258] Next, using a servo motor-operated, single-phase AC (50 Hz) resistance welding machine, the plate assembly was bent by applying pressure to the plate assembly with a pair of electrodes 9 (tip diameter: 6 mm). Resistance welding was performed under the conditions of a pressure of 3.5 kN, a hold time of 0.14 seconds or 0.16 seconds, and a weld time of 0.36 seconds, using a welding current with a nugget diameter r of 5.9 mm to produce a plate assembly with a welded portion. The pair of electrodes 9 applied pressure to the plate assembly from above and below in a vertical direction, with the lower electrode pressing against the test piece 6 through a hole in the fixture 8. During pressure application, the lower electrode of the pair of electrodes 9 was fixed to the fixture 8, with the lower electrode in contact with the flat surface extending from the contact surface of the gasket 7 and the fixture 8, while the upper electrode was movable. The upper electrode was in contact with the center of the test alloyed hot-dip galvannealed steel sheet 5. The plate assembly was welded while being tilted 5° relative to the horizontal along the longitudinal direction of the plate assembly. In addition, the holding time refers to the time from the end of the welding current to the start of releasing the electrode. Figure 11 (b) In the lower figure, the nugget diameter r refers to the distance between the ends of the nugget 10 in the longitudinal direction of the plate group.

[0259] Next, the plate group with the welded portion is moved along the center of the welded portion including the nugget 10. Figure 11 (b) Cut along line BB in the figure above, and observe the cross section of the weld using an optical microscope (200x magnification). Evaluate the weld's resistance to resistance welding cracking according to the following criteria. A ◎ or ○ rating indicates excellent resistance to resistance welding cracking. A × rating indicates poor resistance to resistance welding cracking.

[0260] ◎: No cracks larger than 0.1 mm were observed during the holding time of 0.14 seconds.

[0261] ○: Cracks of 0.1 mm or longer were observed at a holding time of 0.14 seconds, but no cracks of 0.1 mm or longer were observed at a holding time of 0.16 seconds.

[0262] ×: Cracks of 0.1 mm or longer were observed during the holding time of 0.16 seconds

[0263] in addition, Figure 11 (b) The cracks that occurred in the test piece 6 are schematically indicated by reference numeral 11 in the figure below. Furthermore, when cracks occurred in the test steel sheet (the alloyed hot-dip galvannealed steel sheet used for testing), stress dispersion in the evaluation steel sheets (the steel sheets of each of the inventive examples and comparative examples) was insufficient, preventing appropriate evaluation. Therefore, data showing no cracks in the test steel sheet were used as examples.

[0264] The results of the above test are recorded in Table 6. According to the results, the resistance to resistance welding cracking characteristics of the welded parts of the alloyed galvanized steel sheets of the invention examples in which the Fe-based electroplating layer was formed under the conditions suitable for the present invention before continuous annealing were all excellent. In each invention example that satisfied equations (1) and (3), no cracks with a length of 0.1 mm or more were observed even under the condition of a holding time of 0.14 seconds, and the resistance to resistance welding cracking characteristics of the welded parts were particularly good. In addition, in the table, for the examples without the Fe-based electroplating layer, the adhesion amount is indicated as "-", and the peak of the luminescence intensity at the wavelength of Si (for convenience, referred to as "Si intensity peak" in the table) and the number of grain boundaries where the Fe-based electroplating layer contacts the Si-containing cold-rolled steel sheet at the interface between the Fe-based electroplating layer and the above-mentioned Si-containing cold-rolled steel sheet (for convenience, referred to as "the number of grain boundaries in contact with the base steel") cannot be measured and are therefore indicated as "-". For steel sheets with a decarburized layer thickness of less than 10 μm, the decarburized layer thickness is indicated as "-". In addition, when each variable on the left side of the equations (1) and (3) is "-", the left side of the equations (1) and (3) is calculated with the variable being 0.

[0265]

[0266] Industrial applicability

[0267] The alloyed galvanized steel sheet produced by the present invention not only has excellent resistance to resistance welding cracking of the welded portion, especially excellent resistance to internal cracking, but also has high strength and excellent workability. It can be used not only as a blank for automobile parts, but also as a blank for applications requiring similar properties in the fields of home appliances, building parts, etc.

[0268] Explanation of symbols

[0269] 1 Alloyed galvanized steel sheet

[0270] 2 Si-containing cold-rolled steel sheet

[0271] 3 Fe-based electroplating layer

[0272] 4 Alloyed zinc coating

[0273] 5 Test alloyed hot-dip galvanized steel sheet

[0274] 6 test pieces

[0275] 7 Gasket

[0276] 8 fixed platform

[0277] 9 electrodes

[0278] 10 nugget

[0279] 11 Crack

Claims

1. An alloyed galvanized steel sheet having: Si-containing cold-rolled steel sheet, containing 0.1 mass % to 3.0 mass % of Si, An Fe-based electroplating layer is formed on at least one surface of the Si-containing cold-rolled steel sheet. An alloyed zinc plating layer is formed on the Fe-based electroplating layer; In the intensity distribution measured by glow discharge emission spectrometry, the Fe content in the alloyed zinc plating layer is converted into the thickness of the plating layer, and the average Si intensity I in the range from the position of the thickness shifted from the interface between the alloyed zinc plating layer and the Fe-based electroplating layer toward the alloyed zinc plating layer side is taken as the starting point and the interface between the Fe-based electroplating layer and the Si-containing cold-rolled steel sheet is taken as the end point. Si，Fe Divided by the average Si intensity I in Si-containing cold-rolled steel sheet Si，bulk The value obtained (I Si，Fe ) / (I Si，bulk ) is 0.30 or more, Furthermore, the average value of the C concentration in the range of 10 μm to 20 μm in the thickness direction from the interface between the alloyed zinc plating layer and the Fe-based electroplating layer to the Fe-based electroplating layer side is 0.10 mass % or less. At the interface between the Fe-based electroplated layer and the Si-containing cold-rolled steel sheet, the number of grain boundaries at which the Fe-based electroplated layer contacts the Si-containing cold-rolled steel sheet is 10 or more per 10 μm in the width direction of the Si-containing cold-rolled steel sheet in the observation field, A decarburized layer is provided starting from the interface between the alloyed zinc plating layer and the Fe-based electroplating layer and extending toward the Fe-based electroplating layer. The adhesion amount CW of the Fe-based electroplating layer on one side Fe1 and the thickness C of the decarburized layer d Satisfying the following formula (1), CW Fe1 The unit is g / m 2 , C d The unit is μm, 0.8×(C.W. Fe1 )+(C d )≥70···(1)。 2. The alloyed galvannealed steel sheet according to claim 1, wherein: The Si-containing cold-rolled steel sheet contains 0.50 mass % to 3.0 mass % of Si.

3. The alloyed galvannealed steel sheet according to claim 1 or 2, wherein: The thickness of the decarburized layer is greater than 30 μm.

4. The alloyed galvannealed steel sheet according to claim 1 or 2, wherein: The adhesion amount CW of the Fe-based electroplating layer on one side Fe1 2g / m 2 above.

5. The alloyed galvannealed steel sheet according to claim 3, wherein: The adhesion amount CW of the Fe-based electroplating layer on one side Fe1 2g / m 2 above.

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

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

8. The alloyed galvannealed steel sheet according to claim 4, wherein: The Si-containing cold-rolled steel sheet has the following composition in addition to the Si: in terms of mass %, it contains C: less than 0.8%, Mn: 1.0% to 12.0%, P: less than 0.1%, S: less than 0.03%, N: less than 0.010% and Al: less than 1.0%, with the remainder being Fe and unavoidable impurities.

9. The alloyed galvannealed steel sheet according to claim 5, wherein: The Si-containing cold-rolled steel sheet has the following composition in addition to the Si: in terms of mass %, it contains C: less than 0.8%, Mn: 1.0% to 12.0%, P: less than 0.1%, S: less than 0.03%, N: less than 0.010% and Al: less than 1.0%, with the remainder being Fe and unavoidable impurities.

10. The alloyed galvannealed steel sheet according to claim 6, wherein: The above-mentioned component composition further contains, in terms of mass%, one or more elements selected from the group consisting of B: 0.005% or less, Ti: 0.2% or less, Cr: 1.0% or less, Cu: 1.0% or less, Ni: 1.0% or less, Mo: 1.0% or less, Nb: 0.20% or less, V: 0.5% or less, Sb: 0.020% or less, Ta: 0.1% or less, W: 0.5% or less, Zr: 0.1% or less, Sn: 0.20% or less, Ca: 0.005% or less, Mg: 0.005% or less, and REM: 0.005% or less.

11. The alloyed galvannealed steel sheet according to claim 7, wherein: The above-mentioned component composition further contains, in terms of mass%, one or more elements selected from the group consisting of B: 0.005% or less, Ti: 0.2% or less, Cr: 1.0% or less, Cu: 1.0% or less, Ni: 1.0% or less, Mo: 1.0% or less, Nb: 0.20% or less, V: 0.5% or less, Sb: 0.020% or less, Ta: 0.1% or less, W: 0.5% or less, Zr: 0.1% or less, Sn: 0.20% or less, Ca: 0.005% or less, Mg: 0.005% or less, and REM: 0.005% or less.

12. The alloyed galvannealed steel sheet according to claim 8, wherein: The above-mentioned component composition further contains, in terms of mass%, one or more elements selected from the group consisting of B: 0.005% or less, Ti: 0.2% or less, Cr: 1.0% or less, Cu: 1.0% or less, Ni: 1.0% or less, Mo: 1.0% or less, Nb: 0.20% or less, V: 0.5% or less, Sb: 0.020% or less, Ta: 0.1% or less, W: 0.5% or less, Zr: 0.1% or less, Sn: 0.20% or less, Ca: 0.005% or less, Mg: 0.005% or less, and REM: 0.005% or less.

13. The alloyed galvannealed steel sheet according to claim 9, wherein: The above-mentioned component composition further contains, in terms of mass%, one or more elements selected from the group consisting of B: 0.005% or less, Ti: 0.2% or less, Cr: 1.0% or less, Cu: 1.0% or less, Ni: 1.0% or less, Mo: 1.0% or less, Nb: 0.20% or less, V: 0.5% or less, Sb: 0.020% or less, Ta: 0.1% or less, W: 0.5% or less, Zr: 0.1% or less, Sn: 0.20% or less, Ca: 0.005% or less, Mg: 0.005% or less, and REM: 0.005% or less.

14. The alloyed galvannealed steel sheet according to claim 1 or 2, wherein: The Fe-based electroplating layer has the following composition Composition: Contains a total of less than 10% by mass of one or more elements selected from B, C, P, N, O, Ni, Mn, Mo, Zn, W, Pb, Sn, Cr, V and Co, and the remainder is composed of Fe and unavoidable impurities.

15. The alloyed galvannealed steel sheet according to claim 3, wherein: The Fe-based electroplating layer has the following composition Composition: Contains a total of less than 10% by mass of one or more elements selected from B, C, P, N, O, Ni, Mn, Mo, Zn, W, Pb, Sn, Cr, V and Co, and the remainder is composed of Fe and unavoidable impurities.

16. The alloyed galvannealed steel sheet according to claim 4, wherein: The Fe-based electroplating layer has the following composition Composition: Contains a total of less than 10% by mass of one or more elements selected from B, C, P, N, O, Ni, Mn, Mo, Zn, W, Pb, Sn, Cr, V and Co, and the remainder is composed of Fe and unavoidable impurities.

17. The alloyed galvannealed steel sheet according to claim 5, wherein: The Fe-based electroplating layer has the following composition Composition: Contains a total of less than 10% by mass of one or more elements selected from B, C, P, N, O, Ni, Mn, Mo, Zn, W, Pb, Sn, Cr, V and Co, and the remainder is composed of Fe and unavoidable impurities.

18. The alloyed galvannealed steel sheet according to claim 6, wherein: The Fe-based electroplating layer has the following composition Composition: Contains a total of less than 10% by mass of one or more elements selected from B, C, P, N, O, Ni, Mn, Mo, Zn, W, Pb, Sn, Cr, V and Co, and the remainder is composed of Fe and unavoidable impurities.

19. The alloyed galvannealed steel sheet according to claim 7, wherein: The Fe-based electroplating layer has the following composition Composition: Contains a total of less than 10% by mass of one or more elements selected from B, C, P, N, O, Ni, Mn, Mo, Zn, W, Pb, Sn, Cr, V and Co, and the remainder is composed of Fe and unavoidable impurities.

20. The alloyed galvannealed steel sheet according to claim 8, wherein: The Fe-based electroplating layer has the following composition Composition: Contains a total of less than 10% by mass of one or more elements selected from B, C, P, N, O, Ni, Mn, Mo, Zn, W, Pb, Sn, Cr, V and Co, and the remainder is composed of Fe and unavoidable impurities.

21. The alloyed galvannealed steel sheet according to claim 9, wherein: The Fe-based electroplating layer has the following composition Composition: Contains a total of less than 10% by mass of one or more elements selected from B, C, P, N, O, Ni, Mn, Mo, Zn, W, Pb, Sn, Cr, V and Co, and the remainder is composed of Fe and unavoidable impurities.

22. The alloyed galvannealed steel sheet according to claim 10, wherein: The Fe-based electroplating layer has the following composition Composition: Contains a total of less than 10% by mass of one or more elements selected from B, C, P, N, O, Ni, Mn, Mo, Zn, W, Pb, Sn, Cr, V and Co, and the remainder is composed of Fe and unavoidable impurities.

23. The alloyed galvannealed steel sheet according to claim 11, wherein: The Fe-based electroplating layer has the following composition Composition: Contains a total of less than 10% by mass of one or more elements selected from B, C, P, N, O, Ni, Mn, Mo, Zn, W, Pb, Sn, Cr, V and Co, and the remainder is composed of Fe and unavoidable impurities.

24. The alloyed galvannealed steel sheet according to claim 12, wherein: The Fe-based electroplating layer has the following composition Composition: Contains a total of less than 10% by mass of one or more elements selected from B, C, P, N, O, Ni, Mn, Mo, Zn, W, Pb, Sn, Cr, V and Co, and the remainder is composed of Fe and unavoidable impurities.

25. The alloyed galvannealed steel sheet according to claim 13, wherein: The Fe-based electroplating layer has the following composition Composition: Contains a total of less than 10% by mass of one or more elements selected from B, C, P, N, O, Ni, Mn, Mo, Zn, W, Pb, Sn, Cr, V and Co, and the remainder is composed of Fe and unavoidable impurities. 26 . An electrodeposition-coated steel sheet comprising the galvannealed steel sheet according to claim 1 , further comprising a chemical conversion coating and an electrodeposition coating formed on the chemical conversion coating.

27. An automobile part, at least a portion of which is formed using the electrodeposition-coated steel sheet according to claim 26.

28. A method for manufacturing an electrodeposited coated steel plate, comprising the following steps: A chemical conversion treatment step of subjecting the alloyed galvannealed steel sheet according to any one of claims 1 to 25 to a chemical conversion treatment to obtain a chemical conversion treated steel sheet having a chemical conversion treatment film formed thereon. The electrodeposition coating step is to perform an electrodeposition coating on the chemical conversion treated steel sheet to obtain an electrodeposition coated steel sheet having an electrodeposition coating film formed on the chemical conversion treatment film.

29. A method for producing an alloyed galvanized steel sheet, comprising: subjecting a cold-rolled steel sheet containing 0.1% to 3.0% by mass of Si to Fe-based electroplating to produce a pre-annealed Fe-based electroplated steel sheet having a pre-annealed Fe-based electroplated layer formed on at least one surface thereof; Next, the Fe-based electroplated steel sheet before annealing is heated at an average heating rate of 10°C / second or more in a temperature range of 400°C to 650°C, and maintained in an atmosphere with a dew point greater than -30°C in the temperature range after heating to produce the Fe-based electroplated steel sheet. Next, the Fe-based electroplated steel sheet is galvanized to produce a galvanized steel sheet. Then, the galvanized steel sheet is further alloyed to obtain a sheet with an adhesion amount greater than 0 g / m 2 An alloyed galvanized steel sheet comprising an Fe-based electroplating layer and an alloyed zinc plating layer formed on the Fe-based electroplating layer, in, The adhesion amount CW of the Fe-based electroplating layer on one side before annealing Fe0 and the dew point DP satisfies the following formula (3), CW Fe0 The unit is g / m 2 , 0.83×{(C.W. Fe0 )-(C.W. Zn )×[mass%Fe] / 100}+(D.P.)≥-5···(3) Among them, CW Zn : The target value of the adhesion amount of the alloyed zinc coating on one side, in g / m 2 , Among them, 25.0g / m 2 ≤CW Zn ≤80.0g / m 2 , [mass% Fe]: target value of the Fe content in the alloyed zinc plating layer, expressed in mass%.

30. The method for manufacturing the alloyed galvannealed steel sheet according to claim 29, wherein: The cold-rolled steel sheet contains 0.50 mass % to 3.0 mass % of Si.

31. The method for manufacturing a galvannealed steel sheet according to claim 29 or 30, wherein: The adhesion amount CW of the Fe-based electroplating layer on one side before annealing Fe0 Satisfying the following formula (2), (C.W. Fe0 )-(C.W. Zn )×[mass%Fe] / 100>0···(2) Among them, CW Zn : The target value of the adhesion amount of the alloyed zinc coating on one side, in g / m 2 , Among them, 25.0g / m 2 ≤CW Zn ≤80.0g / m 2 , [mass% Fe]: target value of the Fe content in the alloyed zinc plating layer, expressed in mass %.

32. The method for manufacturing the alloyed galvannealed steel sheet according to claim 29 or 30, wherein: The adhesion amount CW of the Fe-based electroplating layer on one side before annealing Fe0 Satisfying the following formula (4), (C.W. Fe0 )-(C.W. Zn )×[mass%Fe] / 100≥2···(4), Among them, CW Zn : The target value of the adhesion amount of the alloyed zinc coating on one side, in g / m 2 , Among them, 25.0g / m 2 ≤CW ZN ≤80.0g / m 2 , [mass% Fe]: target value of the Fe content in the alloyed zinc plating layer, expressed in mass%.

33. The method for manufacturing the alloyed galvannealed steel sheet according to claim 31, wherein: The adhesion amount CW of the Fe-based electroplating layer on one side before annealing Fe0 Satisfying the following formula (4), (C.W. Fe0 )-(C.W. Zn )×[mass%Fe] / 100≥2···(4), Among them, CW Zn : The target value of the adhesion amount of the alloyed zinc coating on one side, in g / m 2 , Among them, 25.0g / m 2 ≤CW ZN ≤80.0g / m 2 , [mass% Fe]: target value of the Fe content in the alloyed zinc plating layer, expressed in mass%.

34. The method for manufacturing the alloyed galvannealed steel sheet according to claim 29 or 30, wherein: The Fe-based electroplating is carried out using an Fe-based electroplating bath containing one or more elements selected from B, C, P, N, O, Ni, Mn, Mo, Zn, W, Pb, Sn, Cr, V and Co, so that the total content of these elements in the Fe-based electroplating layer before annealing is 10% by mass or less.

35. The method for manufacturing the alloyed galvannealed steel sheet according to claim 31, wherein: The Fe-based electroplating is carried out using an Fe-based electroplating bath containing one or more elements selected from B, C, P, N, O, Ni, Mn, Mo, Zn, W, Pb, Sn, Cr, V and Co, so that the total content of these elements in the Fe-based electroplating layer before annealing is 10% by mass or less.

36. The method for manufacturing the alloyed galvannealed steel sheet according to claim 32, wherein: The Fe-based electroplating is carried out using an Fe-based electroplating bath containing one or more elements selected from B, C, P, N, O, Ni, Mn, Mo, Zn, W, Pb, Sn, Cr, V and Co, so that the total content of these elements in the Fe-based electroplating layer before annealing is 10% by mass or less.

37. The method for manufacturing the alloyed galvannealed steel sheet according to claim 33, wherein: The Fe-based electroplating is carried out using an Fe-based electroplating bath containing one or more elements selected from B, C, P, N, O, Ni, Mn, Mo, Zn, W, Pb, Sn, Cr, V and Co, so that the total content of these elements in the Fe-based electroplating layer before annealing is 10% by mass or less.

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