Electroplated steel plates with an Fe base, and galvanized steel plates, and the methods of manufacturing these steel plates.

Abstract

DEPCT6626 / 07 / 2566 To provide electroplated steel sheets with a Fe base that are not only capable... Excellent chemical transformation or excellent coating appearance when subjected to plating. Hot-dip galvanizing is not only effective but also offers excellent resistance to cracking when used in welding. The revealed resistance is to a steel plate coated with an Fe-based material, which includes... Cold-rolled steel sheets with a chemical composition containing 0.1% or more Si by mass. 3.0% by mass or less, and an electroplated coating layer with a Fe base that has been formed on it. One or both surfaces of cold-rolled steel sheets with a coating weight per surface of 1.0 g / m². Or more, in which case the thickness of the internal oxidation layer is 2.00 micrometers or less and The average concentration of C is in the range of 10 micrometers to 20 micrometers in the thickness direction. From the surface of an electroplated coating with an Fe base of 0.10% by mass or less;

Description

Fe-based electroplated steel sheet and galvannealed steel sheet, and manufacturing method thereof

[0001] The present invention relates to an Fe-based electroplated steel sheet and a galvannealed steel sheet, as well as methods for producing the same. In the present invention, the term "Fe-based electroplated steel sheet" includes both (i) a simple Fe-based electroplated steel sheet (hereinafter sometimes simply referred to as "CR") obtained by applying an Fe-based electroplating to a cold-rolled steel sheet, and (ii) a hot-dip galvanized steel sheet (hereinafter sometimes referred to as "GI") obtained by applying hot-dip galvanizing to the simple Fe-based electroplated steel sheet (i) and having an unalloyed hot-dip galvanized layer. In the present invention, the term "galvannealed steel sheet" refers to a galvannealed steel sheet (hereinafter sometimes referred to as "GA") obtained by heating and alloying the hot-dip galvanized layer of the hot-dip galvanized steel sheet (ii).

[0002] In recent years, improving the fuel efficiency of automobiles has become an important issue from the perspective of preserving the global environment. For this reason, there has been an active movement to reduce the weight of automobile bodies by increasing the strength and reducing the thickness of steel sheets, which are the raw materials for automobile components. However, increasing the strength of steel sheets leads to a decrease in formability, so there is a need to develop steel sheets that combine high strength and high formability.

[0003] One method for increasing the strength of steel sheets without significantly impairing their formability is solid-solution strengthening by adding Si to the steel sheet. However, Si added to increase the strength of steel sheets forms oxides on the steel sheet surface during annealing. When steel sheets are used without hot-dip galvanizing, these oxides deteriorate chemical conversion treatability. Furthermore, when hot-dip galvanizing steel sheets to produce hot-dip galvanized steel sheets, these oxides deteriorate the wettability of the steel sheet with molten zinc, resulting in bare areas. In order to ensure chemical conversion treatability or the plating appearance after hot-dip galvanizing, a technique is known in which an Fe-based electroplating (Fe-based pre-plating) is applied to the steel sheet surface before annealing the steel sheet.

[0004] Patent Document 1 describes a method for manufacturing a steel plate by applying a plating coating amount of 0.2 to 2 g / m to the steel plate. ２and subjecting the steel sheet on which the Fe plating layer is formed to oxidation heating at 600 to 800°C. The heated steel sheet is then heated in an atmosphere containing 20 ppm or less of oxygen and H ２ :1-20vol%, remainder N ２ and other unavoidable gases, maintaining the steel sheet at 750 to 900°C for 5 seconds or more in a reducing atmosphere having a dew point of -30 to 5°C, cooling the maintained steel sheet, and immersing the cooled steel sheet in a hot-dip galvanizing bath at 445 to 480°C to coat it (Claim 1)

[0005] Patent Document 2 describes "a steel sheet (claim 1) having an internal oxide layer in which at least a portion of the grain boundaries is covered with an oxide from the surface of the base material to a depth of 5.0 μm or more, and in a region from the surface of the base material to a depth of 5.0 μm, the grain boundary coverage rate of the oxide is 60% or more."

[0006] Patent No. 6025867 International Publication No. 2019 / 116531

[0007] In the manufacture of automotive parts, press-formed parts are often assembled by resistance welding (spot welding). When at least one of the parts to be resistance-welded includes a hot-dip galvanized steel sheet, residual stress is generated near the weld during resistance welding. This melts the zinc from the galvanized layer and diffuses into the grain boundaries of the steel sheets that make up each part. This can lead to liquid metal embrittlement (LME) and intergranular cracking (LME cracking) of the steel sheet. In particular, welding performed with a welding electrode angled relative to the steel sheet can increase residual stress, potentially leading to cracking. Because residual stress is thought to increase with increasing strength of the steel sheet, there is concern about the occurrence of LME cracking due to the increased strength of the steel sheet caused by the addition of Si.

[0008] However, according to the investigations of the present inventors, it was found that the manufacturing method of the hot-dip galvanized steel sheet described in Patent Document 1 can prevent ungalvanized portions and ensure excellent coating surface appearance and coating adhesion, but the resistance weld cracking resistance is insufficient. Also, it was found that the steel sheet described in Patent Document 2 also has insufficient resistance weld cracking resistance because the depth of the internal oxidation layer, i.e., the grain boundary oxidation, is too large.

[0009] In view of the above problems, the present invention aims to provide an Fe-based electroplated steel sheet that not only has excellent chemical conversion treatability or coating appearance after hot-dip galvanizing but also has excellent resistance weld crack resistance, together with a suitable manufacturing method thereof. Another object of the present invention is to provide an alloyed hot-dip galvanized steel sheet that not only has excellent coating appearance but also has excellent resistance weld crack resistance, together with a suitable manufacturing method thereof.

[0010] The present inventors have conducted extensive research to solve the above-mentioned problems and have discovered the following: When an Fe-based electroplated layer with a predetermined coating weight A is formed for the purpose of ensuring chemical conversion treatability or plating appearance after hot-dip galvanizing, the sum of the coating weight A and the dew point B during subsequent annealing correlates with resistance weld cracking resistance. Specifically, excellent resistance weld cracking resistance can be exhibited when A + B is equal to or greater than a predetermined value.

[0011] The predetermined value of A+B is the same for CR and GI. In the case of CR and GI, the formation of a soft Fe-based electroplated layer relieves the stress applied to the surface of the cold-rolled steel sheet during welding. On the other hand, in the case of GA, the Fe-based electroplated layer is incorporated into the hot-dip galvanized layer during the alloying process, so the predetermined value of A+B increases.

[0012] First, the cases of CR and GI will be described in detail. To achieve high resistance weld crack resistance at the weld, it is important to form an Fe-based electroplated layer with a coating weight A on a cold-rolled steel sheet and then perform an annealing process under conditions that satisfy A + B ≥ 3.0, thereby forming an internal oxide within the Fe-based electroplated layer and limiting the depth of the internal oxide layer from a depth of 0.10 μm from the surface of the Fe-based electroplated layer toward the cold-rolled steel sheet to 2.00 μm or less. When A + B ≥ 3.0 is satisfied, Si that diffuses from the cold-rolled steel sheet to the Fe-based electroplated layer during annealing can be converted into an oxide within the Fe-based electroplated layer. By partially forming an oxide within the Fe-based electroplated layer in this way, the depth of the internal oxide layer from a depth of 0.10 μm from the surface of the Fe-based electroplated layer toward the cold-rolled steel sheet can be limited to 2.00 μm or less. Therefore, when zinc reaches the grain boundaries of the cold-rolled steel sheet, it is possible to minimize penetration of the cold-rolled steel sheet in the depth direction, thereby improving the resistance weld cracking resistance characteristics of the welded portion.

[0013] Furthermore, in the annealing step, the average C concentration in the range of 10 μm to 20 μm from the surface of the Fe-based electroplated layer in the Fe-based electroplated steel sheet in the sheet thickness direction is set to 0.10 mass% or less. This makes it possible to further improve resistance weld cracking resistance. The present inventors have found that when the Fe-based electroplated layer is formed before annealing, the C concentration in the range of 10 μm to 20 μm from the surface of the Fe-based electroplated layer in the sheet thickness direction can be further reduced, and the effect of improving resistance weld cracking resistance can be more effectively obtained.

[0014] Furthermore, in the heating step following the annealing (soaking) step, the average heating rate of the Fe-based electroplated steel sheet in the temperature range of 400°C to 650°C is set to 10°C / sec or more. This minimizes the growth of crystal grains in the Fe-based electroplated layer during the heating step, and the number of crystal grain boundaries in the Fe-based electroplated layer that contact the cold-rolled steel sheet at the interface between the Fe-based electroplated layer and the cold-rolled steel sheet is set to 10 or more per 10 μm width of the cold-rolled steel sheet. This further improves resistance weld cracking resistance. When the average heating rate is set to 10°C / sec or more during the heating step in the temperature range of 400°C to 650°C, the crystal grains in the Fe-based electroplated layer that contact the interface between the Fe-based electroplated layer and the cold-rolled steel sheet are refined, thereby dispersing the penetration paths of molten zinc into the Fe-based electroplated layer. In other words, the time it takes for molten zinc to reach the grain boundaries of the cold-rolled steel sheet during welding can be delayed, thereby further improving resistance weld cracking resistance.

[0015] Next, the case of GA will be described in detail. In the case of GA, the Fe-based electroplated layer is absorbed into the hot-dip galvanized layer and disappears during the alloying process, so the stress relaxation effect of the Fe-based electroplated layer cannot be expected. To maintain high resistance weld crack resistance at the weld even when the Fe-based electroplated layer disappears, it is important to form an Fe-based electroplated layer with a coating weight A on a cold-rolled steel sheet and then perform an annealing process under conditions that satisfy A + B ≥ 5.0 to form an internal oxide within the Fe-based electroplated layer. In the alloying process, a portion of the Si internal oxide is absorbed into the galvannealed layer, thereby reducing the amount of internal oxide in contact with the galvannealed layer. When A + B ≥ 5.0 is satisfied, Si that diffuses from the cold-rolled steel sheet to the Fe-based electroplated layer during annealing can be converted into an oxide within the Fe-based electroplated layer. In this way, by partially forming an oxide within the Fe-based electroplated layer, the amount of internal oxide in contact with the galvannealed layer can be reduced. This makes it possible to suppress the penetration of zinc from the grain boundaries of the internal oxide layer in contact with the galvannealed layer, thereby delaying the time it takes for molten zinc to reach the grain boundaries of the cold-rolled steel sheet during welding and improving the resistance weld cracking resistance of the weld.

[0016] Furthermore, in the annealing step, the average C concentration in the range of 10 μm to 20 μm from the surface of the cold-rolled steel sheet in the sheet thickness direction of the galvannealed steel sheet is set to 0.10 mass% or less. This makes it possible to further improve resistance weld cracking resistance. The present inventors have found that when an Fe-based electroplating layer is formed before annealing, the C concentration in the range of 10 μm to 20 μm from the surface of the cold-rolled steel sheet in the sheet thickness direction can be further reduced, and the effect of improving resistance weld cracking resistance can be more effectively obtained.

[0017] The gist of the present invention, which has been completed based on the above findings, is as follows: [1] A cold-rolled steel sheet having a chemical composition containing 0.1 mass % to 3.0 mass % of Si, and a coating amount of 1.0 g / m2 formed on one or both sides of the cold-rolled steel sheet. ２ and an Fe-based electroplated steel sheet having the above Fe-based electroplated layer, wherein an emission intensity profile of a wavelength indicative of Si measured in a depth direction from the surface of the Fe-based electroplated layer by glow discharge optical emission spectroscopy shows: (i) an average Si intensity (I) in a depth range of 10.0±0.1 μm from the interface between the Fe-based electroplated layer and the cold-rolled steel sheet; Ｓｉ (ii) the peak of the peak is located at a depth of more than 0.10 μm from the surface of the Fe-based electroplated layer; and (iii) the emission intensity gradually decreases in the depth direction from the peak of the peak to reach the average Si intensity (I Ｓｉ ) is located within a range of 0.10 μm to 2.00 μm from the surface of the Fe-based electroplated layer, and the average C concentration in a range of 10 μm to 20 μm from the surface of the Fe-based electroplated layer in the sheet thickness direction is 0.10 mass % or less.

[0018] [2] The Fe-based electroplated steel sheet according to the above [1], which has an unalloyed hot-dip galvanized layer formed in contact with the Fe-based electroplated layer.

[0019] [3] The Fe-based electroplated steel sheet according to [1] or [2] above, wherein the average C concentration in the range of 10 μm to 20 μm from the surface of the Fe-based electroplated layer in the sheet thickness direction is 0.04 mass% or less.

[0020] [4] The Fe-based electroplated steel sheet according to any one of the above [1] to [3], wherein a surface layer portion of the Fe-based electroplated steel sheet is a decarburized layer.

[0021] [5] The Fe-based electroplated steel sheet according to the above [4], wherein the thickness of the decarburized layer is 30 μm or more.

[0022] [6] The Fe-based electroplated steel sheet according to [4] above, wherein the thickness of the decarburized layer is 80 μm or more.

[0023] [7] The Fe-based electroplated steel sheet according to any one of [1] to [6] above, wherein the number of grain boundaries of the Fe-based electroplated layer in contact with the cold-rolled steel sheet at the interface between the Fe-based electroplated layer and the cold-rolled steel sheet is 10 or more per 10 μm in the steel sheet width direction in an observation field of the cold-rolled steel sheet.

[0024] [8] The Fe-based electroplated steel sheet according to any one of the above [1] to [7], wherein the cold-rolled steel sheet has a chemical composition, in mass%, of C: 0.8% or less, Si: 0.1% to 3.0%, Mn: 1.0% to 12.0%, P: 0.1% or less, S: 0.03% or less, N: 0.010% or less, and Al: 1.0% or less, with the balance being Fe and unavoidable impurities.

[0025] [9] The Fe-based electroplated steel sheet according to [8] above, wherein the composition further contains, in mass %, at least one element 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.

[0026]

[10] The Fe-based electroplated steel sheet according to any one of [1] to [9] above, wherein the Si content in the chemical composition is 0.9 mass% or more and 1.7 mass% or less.

[0027]

[11] The Fe-based electroplated steel sheet according to any one of [1] to

[10] above, wherein the Fe-based electroplated layer has a component composition containing 10 mass% or less in total of at least one element selected from the group consisting of B, C, P, N, O, Ni, Mn, Mo, Zn, W, Pb, Sn, Cr, V, and Co, with the balance consisting of Fe and unavoidable impurities.

[0028]

[12] A galvannealed steel sheet comprising: a cold-rolled steel sheet having a chemical composition containing 0.1 mass % or more and 3.0 mass % or less of Si; and a galvannealed layer formed on one or both surfaces of the cold-rolled steel sheet, wherein no Fe-based electroplating layer is present between the galvannealed layer and the cold-rolled steel sheet, wherein an emission intensity profile of a wavelength indicating Si, measured in a depth direction from the surface of the galvannealed layer by glow discharge optical emission spectroscopy, shows an average Si intensity (I Ｓｉ，Ｆｅ ) is the average Si intensity (I Ｓｉ，ｂｕｌｋ ) divided by (I Ｓｉ，Ｆｅ ) / (I Ｓｉ，ｂｕｌｋ ) is 0.90 or less, and an average C concentration in a range of 10 μm or more and 20 μm or less from an interface between the galvannealed layer and the cold-rolled steel sheet in a sheet thickness direction is 0.10 mass% or less.

[0029]

[13] The galvannealed steel sheet according to the above

[12] , wherein an average value of the C concentration in a range of 10 μm or more and 20 μm or less from the interface between the galvannealed layer and the cold-rolled steel sheet in the sheet thickness direction is 0.04 mass% or less.

[0030]

[14] The galvannealed steel sheet according to the above

[12] or

[13] , wherein a surface layer portion of the cold-rolled steel sheet is a decarburized layer.

[0031]

[15] The galvannealed steel sheet according to the above

[14] , wherein the thickness of the decarburized layer is 30 μm or more.

[0032]

[16] The galvannealed steel sheet according to the above

[14] , wherein the thickness of the decarburized layer is 80 μm or more.

[0033]

[17] The galvannealed steel sheet according to any one of the above

[12] to

[16] , wherein the cold-rolled steel sheet has a chemical composition, in mass%, of C: 0.8% or less, Si: 0.1% to 3.0%, Mn: 1.0% to 12.0%, P: 0.1% or less, S: 0.03% or less, N: 0.010% or less, and Al: 1.0% or less, with the balance consisting of Fe and unavoidable impurities.

[0034]

[18] The galvannealed steel sheet according to the above item

[17] , wherein the chemical composition further contains, in mass %, at least one element 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.

[0035]

[19] The galvannealed steel sheet according to any one of

[12] to

[18] , wherein the Si content in the component composition is 0.9 mass% or more and 1.7 mass% or less.

[0036]

[20] The galvannealed steel sheet according to any one of the above

[12] to

[19] , wherein the galvannealed layer contains 1 mass% or less in total of at least one element selected from the group consisting of B, C, P, N, O, Ni, Mn, Mo, Zn, W, Pb, Sn, Cr, V, and Co.

[0037]

[21] A chemical conversion treated steel sheet having a chemical conversion coating formed on a surface of the Fe-based electroplated steel sheet according to any one of [1] to

[11] above, or the galvannealed layer of the galvannealed steel sheet according to any one of

[12] to

[19] above.

[0038]

[22] An electrodeposition coated steel sheet having an electrodeposition coating formed in contact with the chemical conversion coating of the chemical conversion treated steel sheet according to

[21] above.

[0039]

[23] An automobile part at least partially using the electrodeposition coated steel sheet according to

[22] above.

[0040]

[24] A cold-rolled steel sheet having a composition containing 0.1 mass% or more and 3.0 mass% or less of Si is subjected to Fe-based electroplating, and a coating amount A (g / m) per side is applied to one or both sides of the cold-rolled steel sheet. ２ ) is 1.0 g / m ２ A method for producing an Fe-based electroplated steel sheet, comprising: a step of obtaining an Fe-based electroplated steel sheet having the above-described Fe-based electroplated layer formed thereon; and a subsequent annealing step of holding the Fe-based electroplated steel sheet at 650°C or higher and 900°C or lower in an atmosphere having a dew point B (°C) that satisfies the following formula (1): A + B ≧ 3.0 (1)

[0041]

[25] The method for producing a Fe-based electroplated steel sheet according to the above

[24] , further comprising a step of hot-dip galvanizing the Fe-based electroplated steel sheet after the annealing step to form an unalloyed hot-dip galvanized layer on a surface of the Fe-based electroplated layer.

[0042]

[26] The method for producing an Fe-based electroplated steel sheet according to the above

[24] or

[25] , wherein the annealing step is carried out in an atmosphere having a dew point B (°C) that satisfies the following formula (1)': A + B ≧ 8.0 (1)'

[0043]

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

[24] to

[26] above, further comprising, prior to the annealing step, a step of heating the Fe-based electroplated steel sheet at an average heating rate of 10°C / sec or more in a temperature range of 400°C or more and 650°C or less.

[0044]

[28] The deposition amount A is 5.0 g / m ２ The method for producing an Fe-based electroplated steel sheet according to any one of

[24] to

[27] above, wherein the tensile strength is less than 1000 MPa.

[0045]

[29] The method for producing an Fe-based electroplated steel sheet according to any one of the above

[24] to

[28] , wherein the cold-rolled steel sheet has a chemical composition, in mass%, of C: 0.8% or less, Si: 0.1% to 3.0%, Mn: 1.0% to 12.0%, P: 0.1% or less, S: 0.03% or less, N: 0.010% or less, and Al: 1.0% or less, with the balance consisting of Fe and unavoidable impurities.

[0046]

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

[29] above, wherein the chemical composition further contains, in mass %, at least one element 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.

[0047]

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

[24] to

[30] above, wherein the Si content in the component composition is 0.9 mass% or more and 1.7 mass% or less.

[0048]

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

[24] to

[31] above, wherein in the Fe-based electroplating, the plating bath contains at least one element selected from the group consisting of B, C, P, N, O, Ni, Mn, Mo, Zn, W, Pb, Sn, Cr, V, and Co, such that the total content of this element in the Fe-based plating layer is 10 mass% or less.

[0049]

[33] When the formula (1) or the formula (1)' is not satisfied, the deposition amount A (g / m) is adjusted so as to satisfy the formula (1) or the formula (1)'. ２ The method for producing an ferrous electroplated steel sheet according to any one of the above

[24] to

[32] , further comprising a step of changing at least one of the temperature (°C) of the ferrous metal layer (W) or the dew point B (°C).

[0050]

[34] A cold-rolled steel sheet having a composition containing 0.1 mass% or more and 3.0 mass% or less of Si is subjected to Fe-based electroplating, and a coating amount A (g / m) per side is applied to one or both sides of the cold-rolled steel sheet. ２ ) is 1.0 g / m ２ A method for producing a galvannealed steel sheet, comprising the steps of: obtaining a Fe-based electroplated steel sheet having the above-described Fe-based electroplated layer formed thereon; a subsequent annealing step of holding the Fe-based electroplated steel sheet at 650°C or higher and 900°C or lower in an atmosphere having a dew point B (°C) that satisfies the following formula (2); a subsequent hot-dip galvanizing step of hot-dip galvanizing the Fe-based electroplated steel sheet to form an unalloyed hot-dip galvanized layer on the surface of the Fe-based electroplated layer; and a subsequent hot-dip alloying step of heating the hot-dip galvanized layer to obtain a galvannealed steel sheet having a galvannealed layer formed on one or both sides of the cold-rolled steel sheet. A + B ≧ 5.0 (2)

[0051]

[35] The method for producing a galvannealed steel sheet according to the above

[34] , wherein the annealing step is carried out in an atmosphere having a dew point B (°C) that satisfies the following formula (2)': A + B ≧ 10.0 (2)'

[0052]

[36] The method for producing a galvannealed steel sheet according to the above

[34] or

[35] , comprising, before the annealing step, a step of heating the Fe-based electroplated steel sheet at an average heating rate of 10°C / sec or more in a temperature range of 400°C or more and 650°C or less.

[0053]

[37] The deposition amount A is 5.0 g / m ２ The method for producing a galvannealed steel sheet according to any one of the above

[34] to

[36] , wherein the galvannealed steel sheet is less than 100%.

[0054]

[38] The method for producing a galvannealed steel sheet according to any one of the above

[34] to

[37] , wherein the cold-rolled steel sheet has a chemical composition, in mass%, of C: 0.8% or less, Si: 0.1% to 3.0%, Mn: 1.0% to 12.0%, P: 0.1% or less, S: 0.03% or less, N: 0.010% or less, and Al: 1.0% or less, with the balance consisting of Fe and unavoidable impurities.

[0055]

[39] The method for producing a galvannealed steel sheet according to the above item

[38] , wherein the chemical composition further contains, in mass %, at least one element 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.

[0056]

[40] The method for producing a galvannealed steel sheet according to any one of

[24] to

[39] , wherein the Si content in the component composition is 0.9 mass% or more and 1.7 mass% or less.

[0057]

[41] The method for producing a galvannealed steel sheet according to any one of the above

[34] to

[40] , wherein in the Fe-based electroplating, a plating bath contains at least one element selected from the group consisting of B, C, P, N, O, Ni, Mn, Mo, Zn, W, Pb, Sn, Cr, V, and Co, such that the total content of this element in the galvannealed layer is 1 mass% or less.

[0058]

[42] When the formula (2) or the formula (2)' is not satisfied, the deposition amount A (g / m) is adjusted so as to satisfy the formula (2) or the formula (2)'. ２The method for producing a galvannealed steel sheet according to any one of the above

[34] to

[41] , further comprising a step of changing at least one of the temperature (°C) and dew point B (°C).

[0059]

[43] A method for producing a chemically treated steel sheet, comprising: the method for producing a Fe-based electroplated steel sheet according to any one of the above

[24] to

[33] , or the method for producing a galvannealed steel sheet according to any one of the above

[34] to

[42] ; and a subsequent step of subjecting the Fe-based electroplated steel sheet or the galvannealed steel sheet to a chemical conversion treatment to obtain a chemically treated steel sheet having a chemical conversion coating formed in contact with the Fe-based electroplated steel sheet or the galvannealed steel sheet.

[0060]

[44] A method for producing an electrodeposition-coated steel sheet, comprising: the method for producing a chemical conversion-treated steel sheet according to the above

[43] ; and a step of applying electrodeposition coating to the chemical conversion-treated steel sheet to obtain an electrodeposition-coated steel sheet having an electrodeposition coating film formed in contact with the chemical conversion coating film.

[0061]

[45] A method for producing an automobile part, comprising: the method for producing an electrodeposition coated steel sheet according to the above

[44] ; and a step of producing an automobile part using the electrodeposition coated steel sheet as a part.

[0062] The Fe-based electroplated steel sheet of the present invention not only has excellent chemical conversion treatability or coating appearance when hot-dip galvanized, but also has excellent resistance weld crack resistance. Also, the galvannealed steel sheet of the present invention not only has excellent coating appearance, but also has excellent resistance weld crack resistance.

[0063] 1 is a graph showing the relationship between the coating weight A of the Fe-based electroplating layer and the annealing dew point B, and the evaluation results of resistance weld crack resistance, for an example of a galvannealed steel sheet (GA) of Example 1.

[0023] FIG. 1 is a graph showing the relationship between the coating weight A of the Fe-based electroplating layer and the annealing dew point B, and the evaluation results of resistance weld crack resistance, for an example of a hot-dip galvanized steel sheet (GI) of Example 1.

[0024] FIG. 1 is a graph showing the relationship between the coating weight A of the Fe-based electroplating layer and the annealing dew point B, and the evaluation results of resistance weld crack resistance, for an example of a non-hot-dip galvanized Fe-based electroplated steel sheet (CR) of Example 1. (a) is a diagram for explaining a method for evaluating resistance weld crack resistance in a welded portion; (b) is a top view of a sheet assembly after welding in the evaluation; and (b) is an enlarged view of the B-B cross section in the upper diagram.

[0025] FIG. 1 is raw data of an emission intensity profile at a wavelength indicating Si, measured by glow discharge optical emission spectroscopy, for a part of an example of a non-hot-dip galvanized Fe-based electroplated steel sheet (CR) of Example 1. 10A is a perspective view and FIG. 10B is an A-A cross-sectional view showing an outline of an observation sample for measuring the number of grain boundaries in the Fe-based electroplated layer that contacts the cold-rolled steel sheet at the interface between the Fe-based electroplated layer and the cold-rolled steel sheet.

[0034] FIG. 10B is a diagram for explaining a method for measuring the number of grain boundaries in the Fe-based electroplated layer that contacts the cold-rolled steel sheet at the interface between the Fe-based electroplated layer and the cold-rolled steel sheet.

[0035] FIG. 10C is an enlarged view of the area surrounded by a square in FIG. 7.

[0036] FIG. 10D is raw data of emission intensity profiles of wavelengths indicating Si and Zn obtained by glow discharge optical emission spectroscopy in a portion of the galvannealed steel sheet (GA) of Example 1.

[0037] FIG. 10E is raw data of the carbon concentration profile in the thickness direction of a portion of the non-galvannealed Fe-based electroplated steel sheet (CR) of Example 1, analyzed with an electron probe microanalyzer.

[0038] FIG. 10F is data after smoothing treatment of the profile in FIG. 10A.

[0039] FIG. 10G is raw data of the carbon concentration profile in the thickness direction of a portion of the galvannealed steel sheet (GA) of Example 1, analyzed with an electron probe microanalyzer. This is data after smoothing processing of the profile in FIG. 11A.

[0064] (Fe-based electroplated steel sheet and manufacturing method thereof) A manufacturing method of an Fe-based electroplated steel sheet according to one embodiment of the present invention includes the steps of: applying Fe-based electroplating to a cold-rolled steel sheet having a composition containing 0.1 to 3.0 mass% Si to obtain an Fe-based electroplated steel sheet having an Fe-based electroplated layer formed on one or both sides of the cold-rolled steel sheet; and then annealing the Fe-based electroplated steel sheet. If the subsequent hot-dip galvanizing step is not performed, a simple Fe-based electroplated steel sheet (CR) can be obtained. Note that it is preferable to apply the Fe-based electroplating to the cold-rolled steel sheet without annealing it. That is, the Fe-based electroplated steel sheet (CR) according to one embodiment of the present invention includes a cold-rolled steel sheet having a composition containing 0.1 to 3.0 mass% Si and an Fe-based electroplated layer formed on one or both sides of the cold-rolled steel sheet, but does not include an unalloyed hot-dip galvanized layer or an alloyed hot-dip galvanized layer.

[0065] According to another embodiment of the present invention, a method for producing a Fe-based plated steel sheet may further include, after the annealing step, a step of hot-dip galvanizing the Fe-based electroplated steel sheet to form an unalloyed hot-dip galvanized layer on the surface of the Fe-based electroplated layer. If the subsequent step of heating and alloying the hot-dip galvanized layer is not performed, a hot-dip galvanized steel sheet (GI) having an unalloyed hot-dip galvanized layer can be obtained. That is, according to another embodiment of the present invention, the Fe-based electroplated steel sheet (GI) includes a cold-rolled steel sheet having a chemical composition containing 0.1% by mass or more and 3.0% by mass or less of Si, an Fe-based electroplated layer formed on one or both sides of the cold-rolled steel sheet, and an unalloyed hot-dip galvanized layer formed in contact with the Fe-based electroplated layer.

[0066] (Galvannealed Steel Sheet and Manufacturing Method Thereof) A manufacturing method of a galvannealed steel sheet according to one embodiment of the present invention includes the steps of: electroplating a cold-rolled steel sheet having a chemical composition containing 0.1 to 3.0 mass% of Si with an Fe-based electroplating layer to obtain a Fe-based electroplated steel sheet having an Fe-based electroplated layer formed on one or both sides of the cold-rolled steel sheet; annealing the Fe-based electroplated steel sheet; hot-dip galvanizing the Fe-based electroplated steel sheet to form an unalloyed hot-dip galvanized layer on the surface of the Fe-based electroplated layer; and alloying the hot-dip galvanized layer by heating to obtain a galvannealed steel sheet (GA) having an alloyed hot-dip galvanized layer formed on one or both sides of the cold-rolled steel sheet. In this case, the Fe-based electroplated layer is incorporated into the hot-dip galvanized layer and disappears during the alloying step. That is, a galvannealed steel sheet (GA) according to one embodiment of the present invention comprises a cold-rolled steel sheet having a chemical composition containing 0.1 mass % or more and 3.0 mass % or less of Si, and a galvannealed layer formed on one side or both sides of the cold-rolled steel sheet, and does not have an Fe-based electroplated layer between the galvannealed layer and the cold-rolled steel sheet.

[0067] [Cold-rolled steel sheet] The process for obtaining a cold-rolled steel sheet is not particularly limited, and known or arbitrary processes and conditions can be adopted. For example, a slab having a desired chemical composition is hot-rolled to obtain a hot-rolled steel sheet, which is then degreased and pickled, and then cold-rolled to obtain a cold-rolled steel sheet.

[0068] [Composition of Cold-Rolled Steel Sheet] The composition of the cold-rolled steel sheet will be described below. Hereinafter, "mass %" will be simply abbreviated as "%".

[0069] Si: 0.1% or more and 3.0% or less. Si is an effective element for achieving high strength in steel sheets because it has a significant effect of increasing the strength of steel through solid solution (solid solution strengthening ability) without significantly impairing workability. However, Si also has a negative effect on the resistance weld crack resistance characteristics of welded parts. If the Si content is less than 0.1%, high strength of the steel sheet cannot be achieved, and no particular problems with the resistance weld crack resistance characteristics of welded parts occur, making the application of the present invention less necessary. The problem of resistance weld crack resistance in welded parts becomes particularly pronounced when the Si content is 0.5% or more. However, if the takt time during spot welding in the assembly process of automobile parts becomes an issue from the perspective of production costs and measures such as reducing the hold time are taken, the problem of resistance weld crack resistance in welded parts may occur even with a Si content of 0.1% or more and less than 0.5%. Therefore, in the present invention, the Si content is set to 0.1% or more, preferably 0.5% or more, more preferably 0.7% or more, and even more preferably 0.9% or more. On the other hand, if the Si content is excessive, the hot rolling property and cold rolling property are significantly reduced, which may adversely affect productivity and may cause a reduction in the ductility of the steel sheet itself. Therefore, the Si content is set to 3.0% or less, preferably 2.5% or less, more preferably 2.0% or less, and most preferably 1.7% or less.

[0070] The cold-rolled steel sheet in this embodiment is required to have the Si content in the above range, but other components are not particularly limited and can have any component composition that is found in a typical cold-rolled steel sheet. However, when a high-strength cold-rolled steel sheet having a tensile strength (TS) of 590 MPa or more measured in accordance with JIS Z 2241 (2011) is to be obtained, the following component composition is preferred.

[0071] C: 0.8% or less (excluding 0%) C is an element effective in ensuring mechanical properties and strength by forming martensite or the like as a steel structure. From this viewpoint, the C content is preferably more than 0%, more preferably 0.03% or more, more preferably 0.05% or more, and even more preferably 0.08% or more. On the other hand, from the viewpoint of obtaining good weldability, the C content is preferably 0.8% or less, more preferably 0.3% or less.

[0072] Mn: 1.0% or more and 12.0% or less Mn is an element that effectively strengthens steel through solid solution strengthening, improves hardenability, and promotes the formation of retained austenite, bainite, and martensite. From this perspective, the Mn content is preferably 1.0% or more, more preferably 1.3% or more, even more preferably 1.5% or more, and most preferably 1.8% or more. On the other hand, from the perspective of obtaining the above effects without increasing costs, the Mn content is preferably 12.0% or less, more preferably 3.5% or less, and even more preferably 3.3% or less.

[0073] P: 0.1% or less (excluding 0%) By suppressing the P content, it is possible to prevent a decrease in weldability. Furthermore, it is possible to prevent P from segregating at grain boundaries, thereby preventing deterioration of ductility, bendability, and toughness. Furthermore, adding a large amount of P promotes ferrite transformation, which increases the grain size. Therefore, it is preferable that the P content be 0.1% or less. There is no particular lower limit for P, and due to constraints on production technology, the P content may be more than 0% or may be 0.001% or more.

[0074] S: 0.03% or less (excluding 0%) The S content is preferably 0.03% or less, and more preferably 0.02% or less. By suppressing the S content, it is possible to prevent a decrease in weldability and a decrease in ductility during hot rolling, thereby suppressing hot cracking and significantly improving surface properties. Furthermore, by suppressing the S content, it is possible to avoid the formation of coarse sulfides as an impurity element and prevent a decrease in the ductility, bendability, and stretch flangeability of the steel sheet. The lower limit of S is not particularly limited, and due to constraints on production technology, the S content may be more than 0% and may be 0.0001% or more.

[0075] N: 0.010% or less (excluding 0%) N forms coarse nitrides with Ti, Nb, and V at high temperatures and does not contribute much to strength. This not only reduces the strength-enhancing effect achieved by adding Ti, Nb, and V, but also reduces toughness. Furthermore, excessive N content can cause slab cracking during hot rolling and surface defects. Therefore, the N content is preferably 0.010% or less. The N content is preferably 0.005% or less, more preferably 0.003% or less, and even more preferably 0.002% or less. The lower limit of the N content is not particularly limited, and due to production technology constraints, the N content can be more than 0% and can be 0.0005% or more.

[0076] Al: 1.0% or less (excluding 0%) Because Al is thermodynamically most easily oxidized, it oxidizes before Si and Mn, suppressing the oxidation of Si and Mn at the outermost surface of the steel sheet and promoting the oxidation of Si and Mn inside the steel sheet. On the other hand, an Al content exceeding 1.0% increases costs. Therefore, when Al is added, the Al content is preferably 1.0% or less. There is no particular lower limit for Al, and the Al content may be more than 0% or may be 0.001% or more. However, from the viewpoint of obtaining the above-mentioned effects, the Al content is preferably 0.01% or more.

[0077] The balance other than the above components is Fe and unavoidable impurities, but may optionally contain at least one element selected from the following:

[0078] B: 0.005% or less B is an element effective in improving the hardenability of steel. To improve hardenability, the B content is preferably 0.0003% or more, and more preferably 0.0005% or more. In order not to impair formability, the B content is preferably 0.005% or less.

[0079] Ti: 0.2% or less Ti is effective for precipitation strengthening of steel. Therefore, the Ti content is preferably 0.005% or more. In order not to impair formability, the Ti content is preferably 0.2% or less, and more preferably 0.05% or less.

[0080] Cr: 1.0% or less The Cr content is preferably 0.005% or more. By setting the Cr content to 0.005% or more, hardenability can be improved and the balance between strength and ductility can be improved. When Cr is added, the Cr content is preferably 1.0% or less from the viewpoint of preventing an increase in costs.

[0081] Cu: 1.0% or less The Cu content is preferably 0.005% or more. By setting the Cu content to 0.005% or more, the formation of the residual γ phase can be promoted. When Cu is added, the Cu content is preferably 1.0% or less from the viewpoint of preventing an increase in costs.

[0082] Ni: 1.0% or less The Ni content is preferably 0.005% or more. By setting the Ni content to 0.005% or more, the formation of the residual γ phase can be promoted. When Ni is added, the Ni content is preferably 1.0% or less from the viewpoint of preventing an increase in costs.

[0083] Mo: 1.0% or less The Mo content is preferably 0.005% or more. By making the Mo content 0.005% or more, the effect of adjusting strength can be obtained, and the Mo content is more preferably 0.05% or more. When Mo is added, the Mo content is preferably 1.0% or less from the viewpoint of preventing an increase in costs.

[0084] Nb: 0.20% or less From the viewpoint of obtaining the effect of improving strength, the Nb content is preferably 0.005% or more. When Nb is contained, from the viewpoint of preventing an increase in cost, the Nb content is preferably 0.20% or less.

[0085] V: 0.5% or less From the viewpoint of obtaining the effect of improving strength, the V content is preferably 0.005% or more. When V is contained, from the viewpoint of preventing an increase in costs, the V content is preferably 0.5% or less.

[0086] Sb: 0.020% or less Sb can be added from the viewpoint of suppressing oxidation of the steel sheet surface. Sb suppresses oxidation of the steel sheet, thereby improving the wettability of plating and the chemical conversion treatability of the cold-rolled steel sheet. To achieve this effect, the Sb content is preferably 0.001% or more. On the other hand, Sb suppresses the formation of a decarburized layer. To obtain good resistance weld cracking resistance, the Sb content is preferably 0.020% or less, more preferably 0.015% or less, and even more preferably 0.012% or less.

[0087] Ta: 0.1% or less From the viewpoint of obtaining the effect of improving strength, the Ta content is preferably 0.001% or more. When Ta is contained, from the viewpoint of preventing an increase in cost, the Ta content is preferably 0.1% or less.

[0088] W: 0.5% or less From the viewpoint of obtaining the effect of improving strength, the W content is preferably 0.005% or more. When W is contained, from the viewpoint of preventing an increase in cost, the W content is preferably 0.5% or less.

[0089] Zr: 0.1% or less From the viewpoint of obtaining the effect of improving strength, the Zr content is preferably 0.0005% or more. When Zr is contained, from the viewpoint of preventing an increase in costs, the Zr content is preferably 0.1% or less.

[0090] Sn: 0.20% or less Sn is an element that is effective in suppressing denitrification, deboronation, etc., and suppressing a decrease in the strength of steel. To achieve this effect, the Sn content is preferably 0.002% or more. In order to ensure impact resistance, when Sn is contained, the Sn content is preferably 0.20% or less.

[0091] Ca: 0.005% or less By setting the Ca content to 0.0005% or more, the morphology of sulfides can be controlled and ductility and toughness can be improved. From the viewpoint of obtaining good ductility, when Ca is contained, the Ca content is preferably set to 0.005% or less.

[0092] Mg: 0.005% or less By setting the Mg content to 0.0005% or more, the morphology of sulfides can be controlled and ductility and toughness can be improved. When Mg is contained, the Mg content is preferably set to 0.005% or less from the viewpoint of preventing an increase in costs.

[0093] REM: 0.005% or less By setting the REM content to 0.0005% or more, the morphology of sulfides can be controlled and ductility and toughness can be improved. From the viewpoint of obtaining good toughness, when REM is contained, the REM content is preferably set to 0.005% or less.

[0094] [Thickness of Cold-Rolled Steel Sheet] The thickness of the cold-rolled steel sheet in this embodiment is not particularly limited, but may usually be 0.5 mm or more and 3.2 mm or less.

[0095] [Degreasing and Pickling] In this embodiment, as a pretreatment for Fe-based electroplating, the cold-rolled steel sheet is preferably degreased and then pickled. Specifically, it is preferable to perform degreasing and water rinsing to clean the steel sheet surface, followed by pickling and water rinsing to activate the steel sheet surface. The degreasing and water rinsing are not particularly limited, and known or arbitrary methods and conditions can be used. Various acids can be used in the pickling treatment, such as sulfuric acid, hydrochloric acid, nitric acid, and mixtures thereof. Among these, sulfuric acid, hydrochloric acid, and mixtures thereof are preferred. The acid concentration is not particularly specified, but is preferably about 1 to 20% by mass, taking into consideration the ability to remove oxide films and the prevention of surface roughness due to excessive pickling. The pickling solution may also contain an antifoaming agent, a pickling accelerator, a pickling inhibitor, etc.

[0096] [Fe-based Electroplating] Subsequently, the cold-rolled steel sheet is subjected to Fe-based electroplating to obtain an Fe-based electroplated steel sheet having a predetermined coating weight of Fe-based plating layer formed on one or both sides of the cold-rolled steel sheet. The presence of the Fe-based electroplating layer enables excellent chemical conversion treatability in a simple Fe-based plated steel sheet (CR) and a good plating appearance in a hot-dip galvanized steel sheet (GI). Although the Fe-based electroplating layer disappears in a galvannealed steel sheet (GA), performing Fe-based electroplating in the GA manufacturing process is a necessary condition for obtaining a good plating appearance. The specific method and conditions for the Fe-based electroplating treatment are not particularly limited. For example, a sulfuric acid bath, a hydrochloric acid bath, or a mixture of both can be used as the plating bath.

[0097] The content of Fe ions in the plating bath before the start of current application is set to Fe in order to obtain a sufficient amount of Fe-based electroplating layer. ２＋ The concentration of the electrolytic solution is preferably 0.5 mol / L or more and 2.0 mol / L or less. Other conditions related to Fe-based electroplating are not particularly limited. The temperature of the plating solution is preferably 30°C or more and 85°C or less, in consideration of maintaining a constant temperature. The pH of the plating solution is not particularly limited, but is preferably 1.0 or more in order to prevent a decrease in current efficiency due to hydrogen generation, and is preferably 3.0 or less in order to ensure electrical conductivity. The current density is preferably 10 A / dm ２ It is preferable that the current is 150 A / dm or more, and from the viewpoint of facilitating control of the deposition amount of the Fe-based electroplating layer, ２ The sheet threading speed is preferably 5 mpm or more from the viewpoint of productivity, and is preferably 150 mpm or less from the viewpoint of stable control of the amount of adhesion.

[0098] The Fe-based electroplated layer may be pure Fe or an alloy plating layer such as an Fe—B alloy, an Fe—C alloy, an Fe—P alloy, an Fe—N alloy, an Fe—O alloy, an Fe—Ni alloy, an Fe—Mn alloy, an Fe—Mo alloy, or an Fe—W alloy. The plating bath may contain at least one element selected from the group consisting of B, C, P, N, O, Ni, Mn, Mo, Zn, W, Pb, Sn, Cr, V, and Co. The total content of these elements in the plating bath is preferably 10% by mass or less in the Fe-based plating layer for CR and GI. In the case of GA, since it is believed that no Fe-based plating layer will remain, the total content of these elements in the galvannealed layer is preferably 1% by mass or less. In the case of CR or GI, and when the Fe-based electroplated layer is an Fe—C alloy, the C content is preferably 0.08 mass% or less. Metal elements may be contained as metal ions, and nonmetal elements may be contained as part of boric acid, phosphoric acid, nitric acid, organic acids, etc. The plating bath may also contain conductivity aids such as sodium sulfate and potassium sulfate, chelating agents, and pH buffers.

[0099] Coating weight A of Fe-based electroplated layer per side: 1.0 g / m ２ In any of the manufacturing processes for CR, GI, and GA, the coating weight A of the Fe electroplated layer per side is 1.0 g / m ２ If the coating weight A of the Fe-based electroplating layer is less than 1.0 g / m, the coating weight A of the Fe-based electroplating layer per side will be insufficient, resulting in insufficient chemical conversion treatment properties or plating appearance when hot-dip galvanizing is performed. ２ More preferably, 2.0 g / m ２ Although there is no particular upper limit to the coating weight A, from the viewpoint of suppressing the lengthening of the production line and the increase in power costs, the coating weight A of the Fe-based electroplated layer per side is set to 5.0 g / m ２ It is preferable that the density is less than 4.5 g / m ２In the CR and GI of this embodiment, the deposition weight of the Fe-based electroplated layer per side is maintained at the deposition weight A immediately after the Fe-based electroplating. On the other hand, in the GA, the Fe-based electroplated layer disappears.

[0100] The Fe-based electroplated layer also contributes to the resistance weld cracking resistance of the weld. Although the mechanism is not clear, it is thought that the Fe-based electroplated layer promotes decarburization from the surface layer of the steel sheet during annealing, which will be described later, and the softened surface layer caused by decarburization relieves residual stress during welding, thereby suppressing cracking of the weld.

[0101] Compared with a cold-rolled steel sheet without an Fe-based electroplated layer, the Fe-based electroplated layer was ２ In Fe-based electroplated steel sheets having the above properties, annealing further promotes the formation of a decarburized layer, improving the resistance weld cracking resistance of the weld. Although the mechanism behind this is unclear, in the absence of an Fe-based electroplated layer, the dissolved C in the steel sheet surface layer is converted into H in the furnace during annealing. ２ Reacts with O to form CO and CO ２ In contrast, when an Fe-based electroplated layer is present, the solute C in the steel sheet can diffuse into the Fe-based electroplated layer as solute C, and so the rate-determining process is different, and it is presumed that the rate of decarburization is faster when an Fe-based electroplated layer is present. Note that when electroplating is performed using Ni, Co, Sn, etc. alone, the solid solubility of C in these metal elements is extremely low, and C does not dissolve, so the effect of promoting decarburization cannot be obtained.

[0102] The deposition weight of the Fe-based electroplating layer is measured as follows: A 10 × 15 mm sample is taken from the Fe-based electroplated steel sheet and embedded in resin to form a cross-section embedded sample. Three randomly selected locations on the cross-section are observed using a scanning electron microscope (SEM) at an accelerating voltage of 15 kV and a magnification of 2,000 to 10,000 times depending on the thickness of the Fe-based electroplating layer. The average thickness of the three fields is multiplied by the iron density to convert it into the deposition weight of the Fe-based electroplating layer per side.

[0103] [Heating Step] In any of the manufacturing processes for CR, GI, and GA, in the heating step before annealing (soaking), it is preferable to heat the Fe-based electroplated steel sheet at an average heating rate of 10°C / sec or more in the temperature range of 400°C or more and 650°C or less. By setting the average heating rate to 10°C / sec or more, the growth of crystal grains in the Fe-based electroplated layer in the heating step is minimized. This is because, in the heating step, internal oxidation of Si hardly progresses at the grain boundaries of the Fe-based electroplated layer, as will be described later, and therefore, if the average heating rate is less than 10°C / sec, the growth of crystal grains cannot be suppressed. With the growth of crystal grains in the Fe-based electroplated layer minimized in the heating step, the dew point B (°C) of the atmosphere during annealing is set to be equal to or greater than the deposition weight A (g / m) of the Fe-based electroplated layer per side, as will be described later. ２ ), by performing annealing so as to satisfy predetermined conditions in relation to the above, it is possible to refine the crystal grains of the Fe-based electroplated layer. For the heating zone in the temperature-raising step, for example, a direct-fired furnace (DFF) or a non-oxidizing furnace (NOF) can be used. In the case of a radiant tube-type heating furnace, a preheating zone such as an induction heater (IH) may be provided in the upstream stage.

[0104] [Annealing] The Fe-based electroplated steel sheet is then subjected to an annealing step, which is performed to remove strain in the cold-rolled steel sheet caused by the rolling step and to recrystallize the structure, thereby adjusting the strength of the steel sheet.

[0105] Hydrogen concentration: 1.0 vol% or more and 30.0 vol% or less The annealing process can be performed, for example, in a reducing atmosphere with a hydrogen concentration of 1.0 vol% or more and 30.0 vol% or less. Hydrogen suppresses oxidation of Fe on the surface of the Fe-based electroplated steel sheet during the annealing process and plays a role in activating the steel sheet surface. A hydrogen concentration of 1.0 vol% or more suppresses oxidation of Fe on the steel sheet surface, ensuring coating adhesion when hot-dip galvanizing is performed. Therefore, the annealing process is preferably performed in a reducing atmosphere with a hydrogen concentration of 1.0 vol% or more, more preferably 2.0 vol% or more. There is no particular upper limit for the hydrogen concentration in the annealing process, but from the perspective of cost, the hydrogen concentration is preferably 30.0 vol% or less, more preferably 20.0 vol% or less. The remainder of the annealing atmosphere other than hydrogen is preferably nitrogen.

[0106] In this embodiment, the dew point B (°C) of the atmosphere during annealing is determined by the deposition amount A (g / m) of the Fe-based electroplated layer per side. ２ ), it is essential to satisfy certain conditions, which can improve the resistance weld cracking resistance of the weld. This is thought to be because decarburization is promoted when the Fe-based electroplated steel sheet is annealed, and the softened surface layer caused by decarburization relieves the residual stress during welding, thereby suppressing cracking of the weld.

[0107] Specifically, when producing a simple Fe-based plated steel sheet (CR) or a hot-dip galvanized steel sheet (GI) in which the hot-dip galvanized layer is not alloyed, it is important to satisfy the following formula (1), and it is preferable to satisfy the following formula (1)': A + B ≧ 3.0 (1) A + B ≧ 8.0 (1)'

[0108] Furthermore, when producing a galvannealed steel sheet (GA) in which the hot-dip galvanized layer is alloyed, it is important to satisfy the following formula (2), and it is preferable to satisfy the following formula (2)': A + B ≧ 5.0 (2) A + B ≧ 10.0 (2)'

[0109] The LME cracking described above can be broadly classified into "cracks occurring on the surface in contact with the electrode (hereinafter referred to as surface cracks)" and "cracks occurring near the corona bond between steel sheets (hereinafter referred to as internal cracks)." Surface cracks are known to be prone to occur during resistance welding at high currents that generate spatter. Surface cracks can be suppressed by maintaining the current within an appropriate range that does not generate spatter. Internal cracks, on the other hand, can occur even when the resistance welding current is within an appropriate range that does not generate spatter. Furthermore, while surface cracks are easily detected by visual inspection during the manufacturing process, internal cracks are difficult to detect by visual inspection. For these reasons, internal cracks are a particularly significant issue among LME cracks. When resistance welding is performed with a welding electrode angled relative to the steel sheet, residual stress increases, potentially leading to internal cracks. Since residual stress is thought to increase with increasing strength of the steel sheet, internal cracks are a concern as the steel sheet becomes stronger. The present disclosure can improve resistance weld crack resistance, particularly the ability to prevent internal cracks.

[0110] As described above, if the formula (1) or (2) is ultimately satisfied, the resistance weld crack resistance of the welded portion is improved. In another embodiment, in the manufacturing process of CR and GI, the coating amount A (g / m) is adjusted so that the formula (1) or (1)' is satisfied when the formula (1) or (1)' is not satisfied, and in the manufacturing process of GA, the coating amount A (g / m) is adjusted so that the formula (2) or (2)' is satisfied when the formula (2) or (2)' is not satisfied. ２) or dew point B (°C). This more reliably improves the resistance weld cracking resistance of the weld. An example of performing this step during operation includes a step of changing the dew point B in the annealing step so as to satisfy the formula (1) or (1)', or the formula (2) or (2)', depending on the value of the coating weight A of the Fe-based electroplating layer obtained in the Fe-based electroplating, and controlling the atmospheric dew point so as to achieve the changed dew point B in the annealing step. Specifically, the value of the coating weight A of the Fe-based electroplating layer obtained in the Fe-based electroplating is substituted into the formula (1) or (1)', or the formula (2) or (2)', and the dew point B in the annealing step is determined so as to satisfy the substituted formula. Here, "substituting the value of the coating weight A of the Fe-based coating layer into the formula (1) or (1)', or the formula (2) or (2)'" does not necessarily mean substituting the value into the exact same formula as the formula (1) or (1)', or the formula (2) or (2)', but also includes substituting the value into an inequality with a narrower range that always satisfies these formulas. By performing such control, automatic control can be performed to satisfy the formulas even when, for example, the product specifications of continuously threaded steel sheets change and the coating weight A changes significantly, causing the formula (1) or (1)', or the formula (2) or (2)' to not be satisfied (when the formulas are actually no longer satisfied, or when circumstances arise that cause the formulas to not be satisfied).

[0111] Note that, since the control response of the dew point B is poorer than that of the coating weight A, it is preferable from the viewpoint of control response to change the coating weight A so as to satisfy the above formula (1) or (1)', or formula (2) or (2)', depending on the value of the dew point B. In the case of a continuous annealing furnace, the coating weight A in the Fe-based electroplating process upstream of the annealing process is changed depending on the value of the dew point B in the annealing process, and the portion of the continuously threaded steel sheet where the coating weight A is changed is produced under conditions that satisfy the above formula (1) or (1)', or formula (2) or (2)'.

[0112] As for the timing for changing at least one of the adhesion amount A or the dew point B so as to satisfy the formula (1) or (1)', or the formula (2) or (2)', when steel sheets of different product specifications are welded and passed through continuously, it is more preferable to change the adhesion amount A or the dew point B in accordance with the passage of the welded portion. As described above, the responsiveness of the dew point B is poor, so when the dew point B is changed, it is more preferable to perform feedforward control of the humidification amount inside the furnace so as to satisfy the formula.

[0113] The "value of coating weight A" referred to here may be a coating weight (target value) that would be obtained under the conditions employed in Fe-based electroplating, or may be a coating weight (measured value) of the Fe-based electroplating layer that is actually obtained. Similarly, the "value of dew point B" may be either a target value or a measured value.

[0114] Although an example during operation of the CR, GI, and GA manufacturing method has been described above, the method may also be implemented as a manufacturing condition determination method for CR, GI, and GA, in which, before starting operation, it is confirmed whether the target value of adhesion amount A and the target value of dew point B satisfy the above formula (1) or formula (1)', or formula (2) or formula (2)', and if they do not, one of the target value of adhesion amount A and the target value of dew point B is changed in advance. Such a manufacturing condition determination method may be implemented as part of a process in the CR, GI, and GA manufacturing method, or may be implemented as a separate process.

[0115] Although the upper limit of the dew point of the annealing atmosphere is not particularly determined, in order to suitably prevent oxidation of the surface of the Fe-based electroplated layer, suppress variations in the dew point, and improve the plating adhesion when chemical conversion treatment or hot-dip galvanization is performed, the dew point of the annealing atmosphere is preferably 20° C. or less. Furthermore, the lower limit of the dew point B of the annealing atmosphere is not particularly limited as long as the formula (1) or (2) is satisfied, but the dew point B is preferably above 0° C., and more preferably above 5° C.

[0116] Holding time in the temperature range of 650°C to 900°C: 30 seconds to 600 seconds In the annealing step, the holding time in the temperature range of 650°C to 900°C is preferably 30 seconds or more. This allows the natural oxide film of Fe formed on the surface of the Fe-based electroplated layer to be suitably removed, improving chemical conversion treatability or plating adhesion when hot-dip galvanizing is performed. There is no particular upper limit to the holding time in this temperature range, but from the viewpoint of productivity, it is preferable that the holding time in this temperature range be 600 seconds or less.

[0117] Maximum temperature of Fe-based electroplated steel sheet: 650°C or higher and 900°C or lower. The maximum temperature of the Fe-based electroplated steel sheet is not particularly limited, but is preferably 650°C or higher and 900°C or lower. By setting the maximum temperature of the Fe-based electroplated steel sheet to 650°C or higher, the recrystallization of the steel sheet structure can be favorably promoted, resulting in the desired strength. Furthermore, the natural oxide film of Fe formed on the surface of the Fe-based electroplated layer can be favorably reduced, thereby improving chemical conversion treatability or coating adhesion when hot-dip galvanizing is performed. Furthermore, if the maximum temperature of the Fe-based electroplated steel sheet is 900°C or lower, the diffusion rate of Si and Mn in the steel can be prevented from excessively increasing, preventing the diffusion of Si and Mn to the steel sheet surface, thereby improving chemical conversion treatability or coating adhesion when hot-dip galvanizing is performed. Furthermore, if the maximum temperature is 900°C or lower, damage to the heat treatment furnace can be prevented, leading to cost reduction. The maximum temperature is based on the temperature measured on the surface of the Fe-based electroplated steel sheet.

[0118] [Hot-dip galvanizing] In the manufacturing process of GI and GA, after the annealing step, a step of hot-dip galvanizing the Fe-based electroplated steel sheet to form a hot-dip galvanized layer on the surface of the Fe-based electroplated steel sheet is further carried out. After the annealing step, the Fe-based electroplated steel sheet is cooled and immersed in a hot-dip galvanizing bath to hot-dip galvanize the steel sheet surface. The hot-dip galvanizing bath contains Al, Zn, and inevitable impurities. The components of the hot-dip galvanizing bath are not particularly specified, but the Al concentration in the bath is generally 0.05 mass% or more and 0.250 mass% or less. Setting the Al concentration in the bath to 0.05 mass% or more can prevent the generation of bottom dross and prevent the dross from adhering to the steel sheet and causing defects. Setting the Al concentration in the bath to 0.250 mass% or less can prevent the increase in top dross and prevent the dross from adhering to the steel sheet and causing defects, while also reducing costs. Other conditions for the hot-dip galvanizing treatment are not limited, but for example, the bath temperature of the hot-dip galvanizing bath is usually in the range of 440 to 500°C, and the steel sheet is immersed in the hot-dip galvanizing bath at a sheet temperature of 440 to 550°C.

[0119] The coating weight of the hot-dip galvanized layer on one side is 25 to 80 g / m ２ The deposition amount is controlled to 25 g / m ２ By setting the coating weight at 80 g / m or more, it is possible to further improve corrosion resistance and to easily control the coating weight. ２ The coating weight can be adjusted by general gas wiping.

[0120] [Heat Alloying] In the manufacturing process of GA, after hot-dip galvanizing, the hot-dip galvanized layer is heat-alloyed to produce a galvannealed steel sheet (GA). The method for alloying is not particularly limited, but it can be performed using an induction heater, a gas furnace, or the like, and the maximum sheet temperature reached during alloying is preferably 460 to 600°C. If the temperature is 460°C or higher, alloying is sufficient, and if the temperature is 600°C or lower, excessive alloying is not achieved and coating adhesion is not impaired.

[0121] The Fe content in the galvannealed layer of the galvannealed steel sheet is preferably 7 to 15 mass %. If the Fe content is 7 mass % or more, deterioration of press formability due to residual η phase can be suppressed, and if the Fe content is 15 mass % or less, coating adhesion is not impaired. When controlled within this range, the Fe-based electroplated layer is completely alloyed with zinc and disappears. Even in this case, the galvannealed steel sheet has excellent resistance weld cracking resistance.

[0122] The method for calculating the coating weight and Fe% of the zinc plating layer for GI and GA is explained below. The coating weight of the zinc plating layer is measured in accordance with JIS H 0401 or ISO 17925. First, two 20 x 25 mm samples are taken from the GI or GA. After measuring the weight of each sample, one side of the plating is stripped using a test solution specified in JIS H 0401 or ISO 17925, and the weight is measured again. The coating weight can be calculated by subtracting the weight after plating stripping from the weight before plating stripping and dividing the result by the surface area of ​​the stripped portion. Here, the average value of the two locations is taken as the coating weight.

[0123] The Fe% is measured in accordance with ISO 17925. The test solution after the plating removal is analyzed using an inductively coupled plasma (ICP) emission spectrometer, and the Fe% is calculated by dividing the amount of Fe by the total amount of Fe, Zn, and Al contained in the test solution. Here, the average value of the two measurements is taken as the Fe%.

[0124] [Internal Oxide Layer in CR and GI] In the CR and GI of this embodiment, it is essential that the depth of the internal oxide layer from a depth of 0.10 μm from the surface of the Fe-based electroplated layer toward the cold-rolled steel sheet be 2.00 μm or less. This can be achieved by performing annealing that satisfies the formula (1) A + B ≥ 3.0. If the depth of the internal oxide layer from a depth of 0.10 μm from the surface of the Fe-based electroplated layer toward the cold-rolled steel sheet exceeds 2.00 μm, resistance weld crack resistance deteriorates. Although the mechanism is unclear, it is believed that by keeping the depth of the internal oxide layer within 2.00 μm, it is possible to minimize the penetration of zinc into the cold-rolled steel sheet when it reaches the grain boundaries of the cold-rolled steel sheet, thereby improving the resistance weld crack resistance of the weld. On the other hand, forming an internal oxide can suppress the formation of oxide on the surface during annealing, thereby improving the chemical conversion treatability of the CR and the plating appearance of the GI. In order to achieve such an effect, the depth of the internal oxide layer is preferably 0.10 μm or more.

[0125] The depth of the internal oxide layer from a depth of 0.10 μm from the surface of the Fe-based electroplating layer toward the cold-rolled steel sheet being within 2.00 μm is defined as satisfying the following condition: In the emission intensity profile of a wavelength indicating Si measured in the depth direction from the surface of the Fe-based electroplating layer by glow discharge optical emission spectrometry (GD-OES), (i) the average Si intensity (I ) in the range of a depth of 10.0±0.1 μm from the interface between the Fe-based electroplating layer and the cold-rolled steel sheet is 0.0±0.1 μm. Ｓｉ (ii) the peak of the mountain is located at a depth of more than 0.10 μm from the surface of the Fe-based electroplated layer; and (iii) the emission intensity gradually decreases in the depth direction from the peak of the mountain until the average Si intensity (I Ｓｉ) is located within a range of 0.10 μm to 2.00 μm from the surface of the Fe-based electroplating layer. The measurement conditions were an Ar gas pressure of 600 Pa, a high-frequency output of 35 W, a measurement diameter of 4 mm, and a sampling interval of 0.1 seconds. A cold-rolled steel sheet not subjected to Fe-based electroplating was analyzed by glow discharge optical emission spectroscopy under the same conditions, and the sputtering rate was calculated by measuring the depth of the sputtering marks. The horizontal axis of the intensity profile of the wavelength indicating Si was converted to the depth corresponding to each time. A non-contact surface profiler (NewView 7300, manufactured by Zygo) was used to measure the depth of the sputtering marks.

[0126] A representative example of a Si peak obtained by analyzing the emission intensity of the wavelength indicating Si observed in this embodiment will be described with reference to Fig. 5. Fig. 5 shows the results of analyzing the emission intensity of the wavelength indicating Si observed in this embodiment for Comparative Example No. 21 (no Fe-based electroplating, dew point B of annealing atmosphere: +4.7°C) and Invention Example No. 23 (Fe-based electroplating coating amount A: 4.7 g / m) in Example 1 (Table 4) described later. ２ , dew point B: -1.3°C), and Invention Example No. 27 (Fe-based electroplating coating weight A: 3.9 g / m ２ 5 shows raw data of emission intensity profiles of wavelengths showing Si at a temperature of 1000 K (dew point B: +9.8°C). In Inventive Examples 23 and 27, an Fe-based electroplated layer (referred to as "Fe plating" in FIG. 5 for convenience) having a thickness of approximately 0.50 to 0.60 μm was formed on the surface of the cold-rolled steel sheet. In Comparative Example 21, a peak Pex derived from an Si outer oxide was observed within 0.10 μm from the surface of the cold-rolled steel sheet. Furthermore, a peak Pin derived from an Si inner oxide was also observed at a depth of more than 0.10 μm from the surface. In Inventive Examples 23 and 27, a peak Pex derived from an Si outer oxide was observed within 0.10 μm from the surface of the Fe-based electroplated layer, and a peak Pin derived from an Si inner oxide was also observed at a depth of more than 0.10 μm from the surface. Pin was observed across the Fe-based electroplated layer and the cold-rolled steel sheet, which means that Si internal oxides were also formed in the Fe-based electroplated layer. The emission intensity gradually decreased from the peak of Pin in the depth direction, and the average Si intensity (I ＳｉThe internal oxide layer is defined as the depth up to which the average Si intensity (I) is reached. Since Pex exists within a range of 0.10 μm from the surface (the surface of the cold-rolled steel sheet in Comparative Example No. 21, and the surface of the Fe-based electroplated layer in Inventive Examples Nos. 23 and 27), this range is excluded. The "thickness of the internal oxide layer" is defined as the depth from the surface of 0.10 μm to the point where the emission intensity gradually decreases from the peak of P in the depth direction. Ｓｉ The thickness of the internal oxide layer is defined as the depth at which the internal oxide layer reaches a thickness equal to the thickness of the internal oxide layer. In Comparative Example No. 21, the thickness of the internal oxide layer was 2.39 μm. In Inventive Examples Nos. 23 and 27, the thicknesses of the internal oxide layer were 0.56 μm and 0.52 μm, respectively.

[0127] Here, the thickness of the Fe-based electroplated layer is the value measured by the above-mentioned cross-sectional observation. In a steel sheet having an oxide partially formed inside the Fe-based electroplated layer, the growth of crystal grains in the Fe-based electroplated layer is suppressed by the internal oxide. Therefore, even when annealing is performed after the Fe-based electroplating process, coarsening of the crystal grains in the Fe-based electroplated layer can be prevented. The formation of numerous grain boundaries in the Fe-based electroplated layer disperses the penetration paths of molten zinc, delaying the time it takes for molten zinc to reach the grain boundaries of the cold-rolled steel sheet during resistance welding, thereby providing excellent resistance weld crack resistance. Furthermore, by partially forming an oxide inside the Fe-based electroplated layer, the depth of the internal oxide layer from a depth of 0.10 μm from the surface of the Fe-based electroplated layer toward the cold-rolled steel sheet can be limited to 2.00 μm or less. This minimizes the penetration of zinc into the cold-rolled steel sheet when it reaches the grain boundaries of the cold-rolled steel sheet, resulting in even better resistance weld crack resistance.

[0128] When analyzed by glow discharge optical emission spectroscopy in the depth direction from the surface, the Fe-based electroplated layer may have peaks of emission intensity at wavelengths indicative of Si both more than 0.10 μm from the surface and in the range of 0.00 μm to 0.10 μm from the surface. In all of Nos. 21, 23, and 27 in Figure 5 described above, peaks of emission intensity at wavelengths indicative of Si are observed both more than 0.10 μm from the surface and in the range of 0.00 μm to 0.10 μm from the surface. This indicates that the Fe-based electroplated layer has an inner Si oxide and an outer Si oxide in the surface layer.

[0129] [Amount of Internal Oxides in GA] In the GA of this embodiment, it is important to reduce the amount of internal oxides in contact with the galvannealed layer. Specifically, in an emission intensity profile of a wavelength indicating Si measured in the depth direction (sheet thickness direction) from the surface of the galvannealed layer by glow discharge optical emission spectrometry (GD-OES), the average Si intensity (I Ｓｉ，Ｆｅ ) is the average Si intensity (I Ｓｉ，ｂｕｌｋ ) divided by (I Ｓｉ，Ｆｅ ) / (I Ｓｉ，ｂｕｌｋ It is essential that the value of A+B is 0.90 or less. This can be achieved by performing annealing that satisfies the formula (2) A+B≧5.0.

[0130] (I Ｓｉ，Ｆｅ ) / (I Ｓｉ，ｂｕｌｋ ) being 0.90 or less means that Si diffusing from the cold-rolled steel sheet to the Fe-based electroplated layer during annealing can be oxidized inside the Fe-based electroplated layer, and thus, by partially forming oxides inside the Fe-based electroplated layer, the amount of internal oxide in contact with the galvannealed hot-dip coated layer can be reduced. Therefore, it is possible to suppress the penetration of zinc from the grain boundaries of the internal oxide layer in contact with the galvannealed hot-dip coated layer. As a result, it is possible to delay the time it takes for molten zinc to reach the grain boundaries of the cold-rolled steel sheet during welding, and it is possible to improve the resistance weld cracking resistance of the weld. Ｓｉ，Ｆｅ ) / (I Ｓｉ，ｂｕｌｋ ) is preferably 0.85 or less, more preferably 0.80 or less. Ｓｉ，Ｆｅ ) / (I Ｓｉ，ｂｕｌｋ ) is preferably 0.50 or more, more preferably 0.60 or more.

[0131] The measurement conditions were an Ar gas pressure of 600 Pa, a high-frequency output of 35 W, a measurement diameter of 4 mm, and a sampling interval of 0.1 seconds. Each average Si intensity was the average value of all Si intensities sampled within each range. After analyzing a cold-rolled steel sheet without Fe-based electroplating or hot-dip galvanizing under the same conditions using glow discharge optical emission spectroscopy, the sputtering rate was calculated by measuring the depth of the sputtering marks, and the horizontal axis of the wavelength intensity profile indicating Si was converted to the depth corresponding to each time. A non-contact surface profiler (NewView 7300, manufactured by Zygo) was used to measure the depth of the sputtering marks. The sputtering rate of the galvannealed layer differs from that of the Fe-based electroplating layer and the cold-rolled steel sheet. That is, the criteria for depth conversion differ due to the difference in elements. Therefore, the interface between the galvannealed layer and the cold-rolled steel sheet was determined as follows. A method for identifying the interface between the galvannealed layer and the cold-rolled steel sheet will be described using FIG. 9 . First, the average Zn intensity (IZn) in the range of 0.5±0.1 μm from the surface of the galvannealed layer in the sheet thickness direction is divided by 2 to calculate a value. Next, the depth in the sheet thickness direction at which the Zn intensity reaches the above-mentioned value (IZn / 2) is defined as the depth of the interface between the galvannealed layer and the cold-rolled steel sheet. Then, the average Si intensity (I) is calculated by moving +0.5 μm from the interface between the galvannealed layer and the cold-rolled steel sheet toward the cold-rolled steel sheet in the sheet thickness direction. Ｓｉ，Ｆｅ ) is calculated. Since the sputtering rate in the galvannealed layer is different from the sputtering rate in the Fe-based electroplated layer and the cold-rolled steel sheet, the horizontal axis of the intensity profile does not accurately correspond to the position of the interface between the galvannealed layer and the cold-rolled steel sheet, which can be visually observed in cross-sectional observation. It is also generally known that measurements by glow discharge optical emission spectroscopy result in a broad profile at an interface consisting of two or more substances due to unevenness, non-uniform sputtering, etc. Therefore, here, the depth in the sheet thickness direction at which the average Zn intensity (IZn) in a range of 0.5±0.1 μm from the surface of the galvannealed layer in the sheet thickness direction is divided by 2 to obtain IZn / 2 is defined as the depth of the interface between the galvannealed layer and the cold-rolled steel sheet.

[0132] A representative example of the analysis of the emission intensity of wavelengths representing Si and Zn observed in this embodiment will be described with reference to Fig. 9. Fig. 9 shows the emission intensity of Comparative Example No. 36 (no Fe-based electroplating, dew point B of annealing atmosphere: +7.4°C) and Invention Example No. 41 (Fe-based electroplating coating mass A: 3.5 g / m) in Example 1 (Table 2) described later. ２ , dew point B: +7.1°C), and Invention Example No. 45 (Fe-based electroplating coating amount 4.2 g / m ２ , dew point B: +7.6°C). Ｓｉ，Ｆｅ ) / (I Ｓｉ，ｂｕｌｋ On the other hand, in invention examples Nos. 41 and 45, (I Ｓｉ，Ｆｅ ) / (I Ｓｉ，ｂｕｌｋ ) were 0.80 and 0.77, respectively.

[0133] [C Concentration in CR, GI, and GA] From the viewpoint of further improving resistance weld crack resistance, in the CR and GI of this embodiment, it is essential that the average C concentration in the range of 10 μm to 20 μm from the surface of the Fe-based electroplating layer in the sheet thickness direction be 0.10 mass% or less by annealing, preferably 0.06 mass% or less, and more preferably 0.04 mass% or less. Similarly, in the GA of this embodiment, it is essential that the average C concentration in the range of 10 μm to 20 μm from the interface between the galvannealed layer and the cold-rolled steel sheet in the sheet thickness direction be 0.10 mass% or less by annealing, preferably 0.06 mass% or less, and more preferably 0.04 mass% or less. On the other hand, if the C concentration is too low, fatigue strength may be reduced. Therefore, in the CR and GI of this embodiment, it is preferable that the average C concentration in the range of 10 μm to 20 μm from the surface of the Fe-based electroplating layer in the sheet thickness direction be 0.01 mass% or more. Similarly, in the GA of this embodiment, the average C concentration in the range of 10 μm to 20 μm from the interface between the galvannealed layer and the cold-rolled steel sheet in the sheet thickness direction is preferably 0.01 mass % or more.

[0134] During annealing, a decarburized layer is formed in the surface layer of a steel sheet subjected to Fe-based electroplating. The decarburized layer is a region near the steel sheet surface where the C concentration is lower than the concentration in the steel. This region is formed due to desorption of C from the steel sheet surface during annealing. In the CR and GI of this embodiment, as described above, if the average C concentration in the range of 10 μm to 20 μm in the sheet thickness direction from the surface of the Fe-based electroplating layer is 0.10 mass% or less, the region is considered soft. Similarly, in the GA of this embodiment, if the average C concentration in the range of 10 μm to 20 μm in the sheet thickness direction from the interface between the galvannealed layer and the cold-rolled steel sheet is 0.10 mass% or less, the region is considered soft. This reduces the stress applied from the welding electrode during resistance welding, improving resistance weld cracking resistance.

[0135] In the CR and GI of this embodiment, by forming an Fe-based electroplated layer and then performing annealing, the average C concentration in the range of 10 μm to 20 μm in the sheet thickness direction from the surface of the Fe-based electroplated layer can be further reduced compared to when the Fe-based electroplated layer is not present. Similarly, in the GA of this embodiment, by forming an Fe-based electroplated layer and then performing annealing, the average C concentration in the range of 10 μm to 20 μm in the sheet thickness direction from the interface between the galvannealed layer and the cold-rolled steel sheet can be further reduced compared to when the Fe-based electroplated layer is not present. Note that when electroplating is performed using Ni, Co, Sn, or the like alone, the solid solubility of C in these metal elements is extremely low, and C does not form a solid solution, so the effect of promoting decarburization cannot be obtained.

[0136] The reasons why the C concentration decreases in the range of 10 μm to 20 μm in the sheet thickness direction from the surface of the Fe-based electroplated layer when the Fe-based electroplated layer is formed, and why the C concentration decreases in the range of 10 μm to 20 μm in the sheet thickness direction from the interface between the galvannealed layer and the cold-rolled steel sheet, are unclear, but the present inventors speculate as follows: That is, it is thought that this is because the Fe-based electroplated layer contains almost no C, and diffusion of C from the cold-rolled steel sheet is induced.

[0137] Furthermore, softening by reducing the C concentration in the surface of the Fe-based electroplated layer or in the range of 10 μm to 20 μm in the sheet thickness direction from the interface between the galvannealed layer and the cold-rolled steel sheet saturates the C concentration in the surface of the Fe-based electroplated layer or in the range of 10 μm to 20 μm in the sheet thickness direction from the interface between the galvannealed layer and the cold-rolled steel sheet below a certain level, so there is a limit to the improvement in resistance weld crack resistance due to softening. In this embodiment, by lowering the C concentration in the surface of the Fe-based electroplated layer or in the range of 10 μm to 20 μm in the sheet thickness direction from the interface between the galvannealed layer and the cold-rolled steel sheet, resistance weld crack resistance is effectively improved even when the decarburized layer is shallow, suggesting the possibility that not only softening but also other effects, such as an increase in melting point due to the reduced C concentration, are exhibited.

[0138] In the CR, GI, and GA of this embodiment, the depth of the decarburized layer from the surface of the Fe-based electroplated layer for CR and GI, and from the interface between the galvannealed layer and the cold-rolled steel sheet for GA, i.e., the thickness of the decarburized layer, is preferably 30 μm or more, more preferably 80 μm or more. While there is no particular upper limit to the thickness of the decarburized layer, the thickness of the decarburized layer is preferably 130 μm or less to maintain tensile strength within a favorable range. The thickness of the decarburized layer is defined as the thickness of a region in the surface layer of the Fe-based electroplated steel sheet where the C concentration is 80% or less of the steel, when the C concentration of the CR and GI is analyzed in the sheet thickness direction from the surface of the Fe-based electroplated layer. In the GA, the thickness is defined as the thickness of a region in the surface layer of the cold-rolled steel sheet where the C concentration is 80% or less of the steel, when analyzed from the interface between the galvannealed layer and the cold-rolled steel sheet (i.e., the surface of the cold-rolled steel sheet).

[0139] Here, the average C concentration on the surface of the Fe-based electroplated layer in CR and GI, or in the range of 10 μm to 20 μm in the sheet thickness direction from the interface between the galvannealed layer and the cold-rolled steel sheet in GA, and the thickness of the decarburized layer near the surface of the Fe-based electroplated layer in CR and GI, or near the interface between the galvannealed layer and the cold-rolled steel sheet in GA, are measured by area or line analysis of the element distribution near the surface layer using an electron probe microanalyzer (EPMA) for a cross-sectionally processed sample. First, the resin-embedded steel sheet is polished to prepare a cross section perpendicular to the rolling direction for observation, and then removed from the resin to prepare a measurement sample. The acceleration voltage was 7 kV and the probe current was 50 nA. Area or line analysis of the sample cross section was performed in 1 μm increments over a 300 × 300 μm area, including the outermost layer of the Fe-based electroplating layer for CR and GI, and the outermost layer of the cold-rolled steel sheet for GA, to measure the carbon intensity. To prevent contamination, a plasma cleaner was used to remove hydrocarbons from the sample surface and periphery before starting the measurement in two locations: the measurement chamber and the sample preparation chamber. Furthermore, to prevent hydrocarbon accumulation during the measurement, the sample was heated and maintained at 100°C on the stage during the measurement. A calibration curve prepared by measuring a separate standard sample was used to convert the carbon intensity to a carbon concentration (mass%). Due to the effect of contamination suppression, it was confirmed that the carbon detection limit was lower than 0.04 mass%. Details of the equipment used and the contamination suppression method are described in Reference 1 below.

[0140] Reference 1: Yamashita et al., "Carbon Distribution in the Early Stage of Proeutectoid Ferrite Transformation in Low Carbon Steel Using High-Precision FE-EPMA," Iron and Steel, Vol. 103 (2017) No. 11, pp. 14-20. However, the necessity for contamination prevention measures during measurement depends on the model and conditions used, so the above configuration is not necessarily required. In other words, it is sufficient as long as it is confirmed that sufficient measurement accuracy is obtained, and the measurement conditions are not essentially related to the effects of the present invention.

[0141] From the obtained C concentration map, a line profile in the sheet thickness direction is extracted from the surface of the Fe-based electroplated layer for CR and GI, and from the interface between the galvannealed layer and the cold-rolled steel sheet for GA. The line profile is then averaged over 300 points in a direction parallel to the surface of the cold-rolled steel sheet to obtain a C concentration profile in the sheet thickness direction. The position of the cold-rolled steel sheet surface can be determined from a secondary electron image or a backscattered electron image obtained at the same time. The obtained C concentration profile in the sheet thickness direction is smoothed using a simple moving average method. In this case, the number of smoothing points is preferably approximately 21. If the number of smoothing points on one side is less than 10 near the surface of the sample, it is preferable to perform the smoothing process on as many measurement points as possible on one side. Next, in the strength profile after smoothing, the thickness direction range in the surface layer of the Fe-based electroplated steel sheet including the Fe-based electroplated layer and the cold-rolled steel sheet for CR and GI, and the surface layer of the cold-rolled steel sheet for GA, where the C concentration is 80% or less of the total C in the steel, is identified and used as the thickness of the decarburized layer. For CR and GI, the C concentration values ​​at 11 points at 1-μm intervals were taken in a range of 10 μm to 20 μm in the sheet thickness direction from the surface of the Fe-based electroplated layer, and for GA, the C concentration was taken as the C concentration in the range of 10 μm to 20 μm in the sheet thickness direction. The above evaluation was applied to the measurement results of two fields for each sample, and the average was taken as the average C concentration in the range of 10 μm to 20 μm in the sheet thickness direction and the evaluation value of the thickness of the decarburized layer.

[0142] Representative examples of C concentration profiles through the sheet thickness direction analyzed with an electron probe microanalyzer are described below using Figures 10A and 11A and 11B. Figure 10A shows raw data of C concentration profiles through the sheet thickness direction obtained by analyzing Fe-based electroplated steel sheets Nos. 21, 23, and 27 in Example 1 (Table 4) described below. Figure 11A shows raw data of C concentration profiles through the sheet thickness direction obtained by analyzing galvannealed steel sheets Nos. 36, 41, and 45 in Example 1 (Table 2). Note that for the galvannealed steel sheets, the galvannealed layer was removed before measurement. Figures 10B and 11B show data after smoothing the raw data of Figures 10A and 11A, respectively, using a simple moving average method with 21 smoothing points (m = 21). As shown in Figures 10B and 11B, the raw data of No. 27 in Table 4 and No. 45 in Table 2 are obtained. In No. 45, there was a decarburized layer in which the C concentration was 80% or less of the total carbon concentration in the steel, and the thickness of the decarburized layer was 77 μm and 81 μm, respectively.

[0143] [Crystal Grains in CR and GI] In the CR and GI of this embodiment, at the interface between the Fe-based electroplated layer and the cold-rolled steel sheet, the number of grain boundaries in the Fe-based electroplated layer that contact the cold-rolled steel sheet is preferably 10 or more per 10 μm in the steel sheet width direction in the observation field of the cold-rolled steel sheet. In this case, the crystals in the Fe-based electroplated layer are sufficiently refined. The refined grains result in the formation of many grain boundaries in the Fe-based electroplated layer, which disperses the penetration of molten zinc and delays the time it takes for the molten zinc to reach the grain boundaries of the cold-rolled steel sheet during welding. This is thought to improve the resistance weld cracking resistance of the weld, and in particular, prevent internal cracking. The number of grain boundaries per 10 μm is more preferably 20 or more, and even more preferably 25 or more. On the other hand, if the number of grain boundaries is too large, fatigue strength may be reduced. Therefore, the number of grain boundaries per 10 μm is preferably 40 or less.

[0144] Here, the number of grain boundaries in the Fe-based electroplated layer in contact with the cold-rolled steel sheet at the interface between the Fe-based electroplated layer and the cold-rolled steel sheet is measured as follows. First, a 10 × 10 mm sample is taken from the Fe-based electroplated steel sheet. An arbitrary location of the sample is processed using a focused ion beam (FIB) device to form a 45° cross section at that location, which is angled at 45° with respect to the T-section (a cross section parallel to the rolling direction of the steel sheet and perpendicular to the surface of the steel sheet). The cross section is 30 μm wide in the direction perpendicular to the rolling and 50 μm long in the 45° direction with respect to the T-section direction, and serves as an observation sample. FIG. 6 shows an outline of the observation sample. FIG. 6( a) is a perspective view of the observation sample. FIG. 6( b) is a cross-sectional view taken along the line A-A of the observation sample shown in FIG. 6( a). Next, a scanning ion microscope (SIM) was used to observe the central portions of the 45° cross section of the observation sample in the width direction and length direction of the sample at a magnification of 4000x, and SIM images were taken. An example of such a SIM image is shown in FIG. 7. FIG. 7 is a SIM image taken as described above for No. 11 of Example 3 (Table 11) described below. A 10 μm region in the width direction of the cold-rolled steel sheet (the area surrounded by a square in FIG. 7) was extracted from the SIM image. For illustrative purposes, FIG. 8 shows an enlarged view of the area surrounded by a square in FIG. 7. As shown in FIG. 8, a boundary line (a dashed line in FIG. 8) was drawn at the interface between the Fe-based electroplating layer and the cold-rolled steel sheet in the 10 μm region in the width direction of the cold-rolled steel sheet in the SIM image. The number of grain boundaries in the Fe-based electroplated layer on the boundary lines was measured and defined as "the number of grain boundaries in the Fe-based electroplated layer that contact the cold-rolled steel sheet at the interface between the Fe-based electroplated layer and the cold-rolled steel sheet." In Inventive Example No. 11, the number of grain boundaries in the Fe-based electroplated layer that contact the cold-rolled steel sheet at the interface between the Fe-based electroplated layer and the cold-rolled steel sheet was 20 per 10 μm in the width direction of the cold-rolled steel sheet.

[0145] (Chemical Conversion Treated Steel Sheet and Manufacturing Method Thereof) A chemical conversion treatment can be performed on an Fe-based electroplated steel sheet (CR or GI) or a galvannealed steel sheet (GA) to obtain a chemical conversion treated steel sheet having a chemical conversion coating formed on the surface of the Fe-based electroplated steel sheet or galvannealed steel sheet. In this case, as pretreatments for the chemical conversion treatment, degreasing treatment, water rinsing, and, if necessary, surface conditioning treatment can be performed to clean the surface of the Fe-based electroplated steel sheet or galvannealed steel sheet. These pretreatments are followed by the chemical conversion treatment. The methods for the degreasing treatment and water rinsing are not particularly limited, and conventional methods can be used. In the surface conditioning treatment, a surface conditioner containing Ti colloid or zinc phosphate colloid can be used. The application of these surface conditioners does not require any special steps, and can be performed according to conventional methods. For example, a desired surface conditioner is dissolved in a predetermined deionized water, the mixture is thoroughly stirred, and then a treatment solution is prepared at a predetermined temperature (usually room temperature, 25 to 30°C), and the steel sheet is immersed in the treatment solution for a predetermined time (20 to 30 seconds). Subsequently, without drying, the next step, chemical conversion treatment, is carried out. The chemical conversion treatment may also be carried out according to a conventional method. For example, a desired chemical conversion treatment agent is dissolved in a predetermined deionized water, the mixture is thoroughly stirred, and then a treatment solution is prepared at a predetermined temperature (usually 35 to 45°C), and the steel sheet is immersed in the treatment solution for a predetermined time (60 to 120 seconds). Examples of chemical conversion treatment agents that can be used include zinc phosphate treatment agents for steel, zinc phosphate treatment agents for steel and aluminum, and zirconium treatment agents.

[0146] (Electrodeposition-coated steel sheet and its manufacturing method) Subsequently, electrodeposition coating is applied to the chemical conversion treated steel sheet, thereby obtaining an electrodeposition-coated steel sheet having an electrodeposition coating film formed in contact with the chemical conversion treatment film. Electrodeposition coating may also be performed according to a conventional method. After performing pretreatment such as water washing as necessary, the steel sheet is immersed in a thoroughly stirred electrodeposition paint, and an electrodeposition coating film of the desired thickness is obtained by electrodeposition treatment. As the electrodeposition coating, in addition to cationic electrodeposition coating, anionic electrodeposition coating can be used. Furthermore, a topcoat or the like may be applied after electrodeposition coating depending on the application. The thickness of the electrodeposition coating film varies depending on the application, but it is preferably about 10 μm to 30 μm in the dry state.

[0147] (Automotive Parts and Manufacturing Method Thereof) Automotive parts can be manufactured at least in part using the electrodeposition-coated steel sheet. The Fe-based electroplated steel sheet and galvannealed steel sheet according to this embodiment have excellent resistance weld cracking resistance properties in welded parts, and therefore, electrodeposition-coated steel sheets using the Fe-based electroplated steel sheet and galvannealed steel sheet are particularly suitable for application to automotive parts. The type of automotive part is not particularly limited, and may be, for example, a side sill part, a pillar part, an automobile body, etc.

[0148] Example 1 A steel having the chemical composition shown in Table 1 (the balance being Fe and unavoidable impurities) was melted and cast, and the resulting cast was hot-rolled to obtain a hot-rolled steel sheet. The hot-rolled steel sheet was then pickled and cold-rolled to obtain a cold-rolled steel sheet having a thickness of 1.6 mm.

[0149]

[0150] Using these cold-rolled steel sheets, various GAs shown in Table 2, various GIs shown in Table 3, and various CRs shown in Table 4 were produced.

[0151] First, the cold-rolled steel sheet was subjected to an alkali degreasing treatment, and then electrolysis was carried out using the cold-rolled steel sheet as the cathode under the conditions shown below to produce an Fe-based electroplated steel sheet. The coating weight A of the Fe-based electroplated layer shown in Tables 2 to 4 was calculated by the method described above and controlled by the current application time. [Electrolysis conditions] Bath temperature: 50°C pH: 2.0 Current density: 45 A / dm ２ Plating bath: Fe ２＋ Sulfuric acid bath containing 1.5 mol / L of ions Anode: iridium oxide electrode

[0152] Subsequently, the Fe-based electroplated steel sheet was heated at an average heating rate in the temperature range of 400°C or higher and 650°C or lower, as shown in Tables 2 to 4, and then heated in an atmosphere having a dew point B shown in Tables 2 to 4, containing 15% by volume of hydrogen and the balance being N. ２ Annealing was performed by heating the steel sheet at a soaking temperature of 800°C in a reducing atmosphere containing iron and unavoidable impurities. The holding time at the maximum temperature (800°C) of the steel sheet was 100 seconds. In the examples shown in Table 4, Fe-based electroplated steel sheets (CR) were obtained in this manner.

[0153] In the examples shown in Tables 2 and 3, the obtained Fe-based electroplated steel sheet was cooled to 440 to 550°C, and then the Fe-based electroplated steel sheet was subjected to a hot-dip galvanizing treatment using a 460°C hot-dip galvanizing bath having an effective Al concentration of 0.132 mass% in the bath and the remainder being Zn and unavoidable impurities, and then the coating weight of the hot-dip galvanized layer per side was adjusted by gas wiping. In the examples shown in Table 3, hot-dip galvanized steel sheets (GI) were obtained in this manner.

[0154] In the examples shown in Table 2, a subsequent step of heating and alloying the hot-dip galvanized layer by alloying treatment at 510°C was further carried out to produce a galvannealed steel sheet (GA). The alloying treatment time was changed to control the Fe% in the galvannealed layer.

[0155] Tables 2 and 3 show the coating weight and Fe % of the hot-dip galvanized layer or galvannealed hot-dip galvanized layer of each steel sheet, determined by the method described above.

[0156] Table 2 shows the results obtained by the method described above (I Ｓｉ，Ｆｅ ) / (I Ｓｉ，ｂｕｌｋ The values ​​of the internal oxide layer, the average C concentration, the thickness of the decarburized layer, and the number of grain boundaries per 10 μm in the width direction of the steel sheet, which were obtained by the above-mentioned methods, are shown in Tables 3 and 4.

[0157] For the GA and GI examples, the appearance of the zinc-coated layer (evaluation 1) and resistance weld crack resistance at the welded portion (evaluation 3) were evaluated, and the results are shown in Tables 2 and 3. For the CR examples, the chemical conversion treatability and corrosion resistance after painting (evaluation 2) and resistance weld crack resistance at the welded portion (evaluation 3) were evaluated, and the results are shown in Table 4.

[0158] [Evaluation 1: Evaluation of Appearance of Galvanized Layer (Hot-Dip Galvanized Layer or Galvannealed Hot-Dip Galvanized Layer)] The presence or absence of poor appearance (bare surface, uneven appearance) was visually determined and evaluated according to the following criteria. ∘: No poor appearance. Δ: Poor appearance, but uneven appearance where no exposed Fe-based electroplated layer or cold-rolled steel sheet is observed when observed with an SEM at 3000x magnification. ×: Poor appearance, bare surface where exposed Fe-based electroplated layer or cold-rolled steel sheet is observed when observed with an SEM at 3000x magnification. The presence or absence of exposed Fe-based electroplated layer or cold-rolled steel sheet can be determined from the difference in contrast between Zn and Fe in an SEM. More specifically, it can be determined by whether Fe is detected in an analysis using EDX (Energy Dispersive X-ray Spectroscopy).

[0159] [Evaluation 2: Evaluation of chemical conversion treatability and corrosion resistance after painting] (1) Chemical conversion treatment Test pieces taken from the above Fe-based electroplated steel sheets were subjected to degreasing, surface conditioning, and chemical conversion treatment to produce chemical conversion treated test pieces having chemical conversion coatings on both the front and back sides of the test pieces. First, the test pieces taken from the above Fe-based electroplated steel sheets were immersed in a degreasing agent and subjected to degreasing under the following standard conditions. [Degreasing treatment] Degreasing agent: FC-E2011 (manufactured by Nihon Parkerizing Co., Ltd.) Treatment temperature: 43°C Treatment time: 120 seconds

[0160] Next, a surface conditioner was sprayed onto the degreased test piece, and the test piece was subjected to a surface conditioning treatment under the following standard conditions: [Surface conditioning treatment] Surface conditioner: Preparen XG (PL-XG; manufactured by Nihon Parkerizing Co., Ltd.) pH: 9.5 Treatment temperature: room temperature Treatment time: 20 seconds

[0161] Next, the test piece after the surface conditioning treatment was immersed in a chemical conversion treatment agent and subjected to a chemical conversion treatment under the following standard conditions: [Chemical Conversion Treatment] Chemical conversion treatment agent: Palbond PB-SX35 (manufactured by Nihon Parkerizing Co., Ltd.) Temperature of chemical conversion treatment solution: 35°C Treatment time: 90 seconds

[0162] The chemical conversion treated test pieces produced as described above were used to measure the chemical conversion treatability described below.

[0163] (2) Electrodeposition Coating Treatment Electrodeposition coating was performed on the surface of the above chemical conversion treated test piece using GT-100 electrodeposition paint manufactured by Kansai Paint Co., Ltd. to a film thickness of 15 μm to obtain an electrodeposition coated test piece. The electrodeposition coated test piece was subjected to the hot salt water immersion test described later.

[0164] <Chemical conversion treatability> The surface of the above chemical conversion treated test piece (n=1) was observed with an SEM at 1000x magnification and evaluated according to the following criteria. A rating of ⊚ or ◯ was considered to be excellent chemical conversion treatability. ⊚: The particle size of the chemical conversion crystals is 5 μm or less and no unprecipitated areas are observed. ◯: The particle size of the chemical conversion crystals is more than 5 μm, but no unprecipitated areas are observed. ×: The particle size of the chemical conversion crystals is more than 5 μm and unprecipitated areas are observed.

[0165] <Warm Salt Water Immersion Test> A 45 mm long crosscut flaw was created on the surface of the electrodeposition coated test piece (n=1) using a cutter, and the test piece was then immersed in a 5 mass % NaCl solution (60°C) for 360 hours, then rinsed with water, and dried. Next, a tape peel test was performed in which cellophane tape was attached to the crosscut flaw of the test piece and then peeled off, and the maximum total peel width of the electrodeposition coated film, combining the left and right sides of the crosscut flaw, was measured. The maximum total peel width of the electrodeposition coated film was evaluated according to the following criteria. A rating of ⊚ or ◯ indicated excellent corrosion resistance after painting. ⊚: Maximum total peel width was 3.0 mm or less. ○: Maximum total peel width was 5.0 mm or less. ×: Maximum total peel width was more than 5.0 mm.

[0166] [Evaluation 3: Evaluation of resistance weld crack resistance at welded parts] Referring to Fig. 4(a), test pieces 6 were cut out from the steel sheets (CR, GI, GA) of each of the invention examples and comparative examples, measuring 150 mm in the longitudinal direction and 50 mm in the transverse direction, with the direction perpendicular to the rolling (TD) as the longitudinal direction and the rolling direction as the transverse direction. Test pieces 6 were cut out to the same size, and the coating weight of the galvanized layer per side was 50 g / m ２The test specimen 6 was stacked with a test galvannealed steel sheet 5 (sheet thickness: 1.6 mm, TS: 980 MPa class) of the same material to form a sheet assembly. The sheet assembly was assembled so that the evaluation surface of the test specimen 6 (the Fe-plated layer in the case of CR, and the zinc-plated layer in the case of GI and GA) faced the zinc-plated layer of the test galvannealed steel sheet 5. The sheet assembly was fixed to a fixing base 8 via a spacer 7 having a thickness of 2.0 mm. The spacer 7 was a pair of steel plates measuring 50 mm in the longitudinal direction, 45 mm in the transverse direction, and 2.0 mm in thickness. As shown in Figure 4(a), the longitudinal end faces of each of the pair of steel plates were arranged so as to be aligned with both transverse end faces of the sheet assembly. Therefore, the distance between the pair of steel plates was 60 mm. The fixing base 8 was a single plate with a hole in the center. Next, using a servomotor-pressurized, single-phase AC (50 Hz) resistance welding machine, the sheet assembly was pressed with a pair of electrodes 9 (tip diameter: 6 mm) while being deflected. Resistance welding was performed with a welding current that resulted in a nugget diameter r of 5.9 mm under the following conditions: a pressure of 3.5 kN, a hold time of 0.18 or 0.24 seconds, and a welding time of 0.36 seconds, to obtain a sheet assembly with a weld. The pair of electrodes 9 pressed the sheet assembly from above and below in the vertical direction, and the lower electrode pressed the test piece 6 through a hole in the fixture 8. During pressing, the lower electrode and the fixture 8 were fixed so that the lower electrode of the pair of electrodes 9 was in contact with a plane extending from the contact surface between the spacer 7 and the fixture 8, and the upper electrode was movable. The upper electrode was in contact with the center of the test galvannealed steel sheet 5. The sheet assembly was also welded while tilted 5° toward the longitudinal direction of the sheet assembly relative to the horizontal. The hold time refers to the time from when the welding current has finished flowing to when the electrodes begin to be released. Referring to the lower diagram of Figure 4(b), the nugget diameter r refers to the distance between the ends of the nugget 10 in the longitudinal direction of the sheet assembly. The sheet assembly with the weld was then cut along line B-B in the upper diagram of Figure 4(b) so as to include the center of the weld including the nugget 10. The cross section of the weld was observed with an optical microscope (200x magnification), and the resistance weld cracking resistance of the weld was evaluated according to the following criteria. A rating of ◎ or ○ indicates that the resistance weld cracking resistance of the weld is excellent. A rating of × indicates that the resistance weld cracking resistance of the weld is poor.◎: No cracks of 0.1 mm or more in length were observed at a hold time of 0.18 seconds. ○: Cracks of 0.1 mm or more in length were observed at a hold time of 0.18 seconds, but no cracks of 0.1 mm or more in length were observed at a hold time of 0.24 seconds. ×: Cracks of 0.1 mm or more in length were observed at a hold time of 0.24 seconds. In the lower diagram of Figure 4(b), a crack that occurred in test piece 6 is schematically shown as reference numeral 11. If cracks occurred in the mating steel sheet (galvannealed steel sheet for testing), the stress in the steel sheets to be evaluated (steel sheets of each invention example and comparative example) would be dispersed, preventing an appropriate evaluation. For this reason, data in which no cracks occurred in the mating steel sheet were used as examples.

[0167]

[0168]

[0169]

[0170] The relationship between the coating weight A of the Fe-based electroplated layer and the annealing dew point B and the evaluation results of resistance weld cracking resistance for the GA examples shown in Table 2 is shown in Figure 1. The relationship between the coating weight A of the Fe-based electroplated layer and the annealing dew point B and the evaluation results of resistance weld cracking resistance for the GI examples shown in Table 3 is shown in Figure 2. The relationship between the coating weight A of the Fe-based electroplated layer and the annealing dew point B and the evaluation results of resistance weld cracking resistance for the CR examples shown in Table 4 is shown in Figure 3.

[0171] As is clear from Tables 2 and 3 and Figures 1 and 2, the invention examples in the examples of GA and GI had good appearance of the zinc plating layer and were able to realize excellent resistance weld cracking resistance. Also, as is clear from Table 4 and Figure 3, the invention examples in the examples of CR had good chemical conversion treatability and corrosion resistance after painting and were able to realize excellent resistance weld cracking resistance.

[0172] The results in Figure 1 (GA) show that, when compared at the same dew point, resistance weld crack resistance improves as the coating weight A of the Fe-based electroplated layer before annealing increases. The boundary line (solid line) that distinguishes whether resistance weld crack resistance improves or not is the boundary line when the coating weight A of the Fe-based electroplated layer before annealing is 1.0 g / m２ The linearity is only observed in the above range, and the coating weight A of the Fe-based electroplated layer before annealing is 1.0 g / m ２ It can be seen that resistance weld cracking resistance is improved particularly at lower dew points in the above ranges. In GA, the Fe-based electroplated layer is all alloyed with the galvanized coating and does not remain, so it is presumed that the resistance weld cracking resistance is improved by modifying the surface layer of the underlying cold-rolled steel sheet, particularly by promoting decarburization through a combination of a specified coating weight or more of the Fe-based electroplated layer and a high dew point.

[0173] On the other hand, in the case of GI and CR, the Fe-based electroplated layer remains, and the Fe-based electroplated layer further acts as a soft surface phase. Therefore, when compared at the same dew point and with the same amount of Fe-based electroplated layer before annealing, it is presumed that the resistance weld cracking resistance characteristics are improved compared to GA.

[0174] Example 2 Steel having the chemical composition shown in Table 5 (the balance being Fe and unavoidable impurities) was melted and cast, and the resulting cast was hot-rolled to obtain a hot-rolled steel sheet. This hot-rolled steel sheet was then pickled and cold-rolled to obtain a cold-rolled steel sheet having a thickness of 1.6 mm.

[0175]

[0176] Using these cold-rolled steel sheets, various GAs shown in Table 6, various GIs shown in Table 7, and various CRs shown in Table 8 were produced.

[0177] First, the cold-rolled steel sheet was degreased with alkali, and then electrolytic treatment was carried out using the cold-rolled steel sheet as the cathode under the conditions shown below to produce an Fe-based electroplated steel sheet. The coating weight A of the Fe-based electroplated layer shown in Tables 6 to 8 was calculated by the method described above and controlled by the current application time. [Electrolysis conditions] Bath temperature: 50°C pH: 2.0 Current density: 45 A / dm ２ Plating bath: Fe ２＋ Sulfuric acid bath containing 1.5 mol / L of ions Anode: iridium oxide electrode

[0178] Subsequently, the Fe-based electroplated steel sheet was heated at an average heating rate in the temperature range of 400°C or higher and 650°C or lower, as shown in Tables 6 to 8, and then heated in an atmosphere having an atmospheric dew point B shown in Tables 6 to 8, containing 15% by volume of hydrogen and the balance being N. ２ Annealing was performed by heating the steel sheet at a soaking temperature of 800°C in a reducing atmosphere containing iron and unavoidable impurities. The holding time at the maximum temperature (800°C) of the steel sheet was 100 seconds. In the examples shown in Table 8, Fe-based electroplated steel sheets (CR) were obtained in this manner.

[0179] In the examples shown in Tables 6 and 7, the obtained Fe-based electroplated steel sheets were cooled to 440 to 550°C, and then the Fe-based electroplated steel sheets were subjected to a hot-dip galvanizing treatment using a 460°C hot-dip galvanizing bath with an effective Al concentration of 0.132 mass% in the bath and the remainder consisting of Zn and unavoidable impurities, and then the coating weight of the hot-dip galvanized layer per side was adjusted by gas wiping. In the examples shown in Table 7, hot-dip galvanized steel sheets (GI) were obtained in this manner.

[0180] In the examples shown in Table 6, a step of heating and alloying the hot-dip galvanized layer by alloying treatment at 510°C was subsequently carried out to produce a galvannealed steel sheet (GA). The alloying treatment time was changed to control the Fe% in the galvannealed layer.

[0181] Tables 6 and 7 show the coating weight and Fe % of the hot-dip galvanized layer or galvannealed hot-dip galvanized layer of each steel sheet, determined by the method described above.

[0182] Table 6 shows the results obtained by the method described above (I Ｓｉ，Ｆｅ ) / (I Ｓｉ，ｂｕｌｋ The values ​​of the internal oxide layer, the average C concentration, the thickness of the decarburized layer, and the number of grain boundaries per 10 μm in the width direction of the steel sheet, which were obtained by the above-mentioned methods, are shown in Tables 7 and 8.

[0183] For the GA and GI examples, the appearance of the zinc-coated layer (evaluation 1) and resistance weld crack resistance at the welded portion (evaluation 3) were evaluated, and the results are shown in Tables 6 and 7. For the CR example, the chemical conversion treatability and corrosion resistance after painting (evaluation 2) and resistance weld crack resistance at the welded portion (evaluation 3) were evaluated, and the results are shown in Table 8. The evaluation methods and criteria for evaluations 1 to 3 were the same as those for Example 1.

[0184]

[0185]

[0186]

[0187] As is clear from Tables 6 and 7, the inventive examples in the examples of GA and GI had good appearance of the zinc plating layer and were able to realize excellent resistance weld cracking resistance. Also, as is clear from Table 8, the inventive examples in the examples of CR had good chemical conversion treatability and corrosion resistance after painting and were able to realize excellent resistance weld cracking resistance.

[0188] Example 3 Steel having the chemical composition shown in Table 1 (the balance being Fe and unavoidable impurities) was melted and cast, and the resulting cast was hot-rolled to obtain a hot-rolled steel sheet. This hot-rolled steel sheet was then pickled and cold-rolled to obtain a cold-rolled steel sheet having a thickness of 1.6 mm.

[0189] Using these cold-rolled steel sheets, various GAs shown in Table 9, various GIs shown in Table 10, and various CRs shown in Table 11 were produced.

[0190] First, the cold-rolled steel sheet was subjected to an alkali degreasing treatment, and then electrolysis was carried out using the cold-rolled steel sheet as the cathode under the conditions shown below to produce an Fe-based electroplated steel sheet. The coating weight A of the Fe-based electroplated layer shown in Tables 9 to 11 was calculated by the method described above and controlled by the current application time. [Electrolysis conditions] Bath temperature: 50°C pH: 2.0 Current density: 45 A / dm ２ Plating bath: Fe ２＋ Sulfuric acid bath containing 1.5 mol / L of ions Anode: iridium oxide electrode

[0191] Subsequently, the Fe-based electroplated steel sheet was heated at an average heating rate in the temperature range of 400°C or higher and 650°C or lower, as shown in Tables 9 to 11, and then heated in an atmosphere having an atmospheric dew point B shown in Tables 9 to 11, containing 15% by volume of hydrogen and the balance being N. ２ Annealing was performed by heating the steel sheet at a soaking temperature of 800°C in a reducing atmosphere containing iron, zinc, and unavoidable impurities. The holding time at the maximum temperature (800°C) of the steel sheet was 100 seconds. In the examples shown in Table 11, Fe-based electroplated steel sheets (CR) were obtained in this manner.

[0192] In the examples shown in Tables 9 and 10, the obtained Fe-based electroplated steel sheets were cooled to 440 to 550°C, and then the Fe-based electroplated steel sheets were subjected to a hot-dip galvanizing treatment using a 460°C hot-dip galvanizing bath with an effective Al concentration of 0.132 mass% in the bath and the remainder consisting of Zn and unavoidable impurities, and then the coating weight per side of the hot-dip galvanized layer was adjusted by gas wiping. In the examples shown in Table 10, hot-dip galvanized steel sheets (GI) were obtained in this manner.

[0193] In the examples shown in Table 9, a step of heating and alloying the hot-dip galvanized layer by alloying treatment at 510°C was subsequently carried out to produce a galvannealed steel sheet (GA). The alloying treatment time was changed to control the Fe% in the galvannealed layer.

[0194] Tables 9 and 10 show the coating weight and Fe % of the hot-dip galvanized layer or galvannealed hot-dip galvanized layer of each steel sheet, determined by the method described above.

[0195] Table 9 shows the results obtained by the method described above (I Ｓｉ，Ｆｅ ) / (I Ｓｉ，ｂｕｌｋ The values ​​of the internal oxide layer, the average C concentration, the thickness of the decarburized layer, and the number of grain boundaries per 10 μm in the width direction of the steel sheet, which were obtained by the above-mentioned methods, are shown in Tables 10 and 11.

[0196] For the GA and GI examples, the appearance of the zinc-coated layer (evaluation 1) and resistance weld crack resistance at the welded portion (evaluation 3) were evaluated, and the results are shown in Tables 9 and 10. For the CR examples, the chemical conversion treatability and corrosion resistance after painting (evaluation 2) and resistance weld crack resistance at the welded portion (evaluation 3) were evaluated, and the results are shown in Table 11. The evaluation methods and evaluation criteria for evaluations 1 to 3 were the same as those for Example 1.

[0197]

[0198]

[0199]

[0200] As is clear from Tables 9 and 10, the inventive examples in the examples of GA and GI had good appearance of the zinc plating layer and were able to realize excellent resistance weld cracking resistance. Also, as is clear from Table 11, the inventive examples in the examples of CR had good chemical conversion treatability and corrosion resistance after painting and were able to realize excellent resistance weld cracking resistance.

[0201] Example 4 Steel having the chemical composition shown in Table 5 (the balance being Fe and unavoidable impurities) was melted and cast, and the resulting cast was hot-rolled to obtain a hot-rolled steel sheet. This hot-rolled steel sheet was then pickled and cold-rolled to obtain a cold-rolled steel sheet having a thickness of 1.6 mm.

[0202] Using these cold-rolled steel sheets, various GAs shown in Table 12, various GIs shown in Table 13, and various CRs shown in Table 14 were produced.

[0203] First, the cold-rolled steel sheet was degreased with alkali, and then electrolytic treatment was carried out using the cold-rolled steel sheet as the cathode under the conditions shown below to produce an Fe-based electroplated steel sheet. The coating weight A of the Fe-based electroplated layer shown in Tables 12 to 14 was calculated by the method described above and controlled by the current application time. [Electrolysis conditions] Bath temperature: 50°C pH: 2.0 Current density: 45 A / dm ２ Plating bath: Fe ２＋ Sulfuric acid bath containing 1.5 mol / L of ions Anode: iridium oxide electrode

[0204] Subsequently, the Fe-based electroplated steel sheet was heated at an average heating rate in the temperature range of 400°C or higher and 650°C or lower, as shown in Tables 12 to 14, and then heated in an atmosphere having an atmospheric dew point B shown in Tables 12 to 14, containing 15% by volume of hydrogen and the balance being N. ２ Annealing was performed by heating the steel sheet at a soaking temperature of 800°C in a reducing atmosphere containing iron, zinc, and unavoidable impurities. The holding time at the maximum temperature (800°C) of the steel sheet was 100 seconds. In the examples shown in Table 14, Fe-based electroplated steel sheets (CR) were obtained in this manner.

[0205] In the examples shown in Tables 12 and 13, the obtained Fe-based electroplated steel sheets were cooled to 440 to 550°C, and then the Fe-based electroplated steel sheets were subjected to hot-dip galvanizing using a 460°C hot-dip galvanizing bath with an effective Al concentration of 0.132 mass% in the bath and the remainder consisting of Zn and unavoidable impurities, and then the coating weight per side of the hot-dip galvanized layer was adjusted by gas wiping. In the examples shown in Table 13, hot-dip galvanized steel sheets (GI) were obtained in this manner.

[0206] In the examples shown in Table 12, a step of heating and alloying the hot-dip galvanized layer by alloying treatment at 510°C was subsequently carried out to produce a galvannealed steel sheet (GA). The alloying treatment time was changed to control the Fe% in the galvannealed layer.

[0207] Tables 12 and 13 show the coating weight and Fe % of the hot-dip galvanized layer or galvannealed hot-dip galvanized layer of each steel sheet, determined by the method described above.

[0208] Table 12 shows the results obtained by the method described above (I Ｓｉ，Ｆｅ ) / (I Ｓｉ，ｂｕｌｋ The values ​​of the internal oxide layer, the average C concentration, the thickness of the decarburized layer, and the number of grain boundaries per 10 μm in the width direction of the steel sheet, which were obtained by the above-mentioned methods, are shown in Tables 13 and 14.

[0209] For the GA and GI examples, the appearance of the zinc-coated layer (evaluation 1) and resistance weld crack resistance at the welded portion (evaluation 3) were evaluated, and the results are shown in Tables 12 and 13. For the CR examples, the chemical conversion treatability and corrosion resistance after painting (evaluation 2) and resistance weld crack resistance at the welded portion (evaluation 3) were evaluated, and the results are shown in Table 14. The evaluation methods and criteria for evaluations 1 to 3 were the same as those for Example 1.

[0210]

[0211]

[0212]

[0213] As is clear from Tables 12 and 13, the inventive examples in the examples of GA and GI had good appearance of the zinc-plated layer and were able to achieve excellent resistance weld cracking resistance. Also, as is clear from Table 14, the inventive examples in the examples of CR had good chemical conversion treatability and corrosion resistance after painting and were able to achieve excellent resistance weld cracking resistance.

[0214] Example 5 Steel having the chemical composition shown in Table 15 (the balance being Fe and unavoidable impurities) was melted and cast, and the resulting cast was hot-rolled to obtain a hot-rolled steel sheet. This hot-rolled steel sheet was then pickled and cold-rolled to obtain a cold-rolled steel sheet having a thickness of 1.6 mm.

[0215]

[0216] Using these cold-rolled steel sheets, various GAs shown in Table 16, various GIs shown in Table 17, and various CRs shown in Table 18 were produced.

[0217] First, the cold-rolled steel sheet was degreased with alkali, and then electrolytic treatment was carried out using the cold-rolled steel sheet as the cathode under the conditions shown below to produce an Fe-based electroplated steel sheet. The coating weight A of the Fe-based electroplated layer shown in Tables 16 to 18 was calculated by the method described above and controlled by the current application time. [Electrolysis conditions] Bath temperature: 50°C pH: 2.0 Current density: 45 A / dm ２ Plating bath: Fe ２＋ Sulfuric acid bath containing 1.5 mol / L of ions Anode: iridium oxide electrode

[0218] Subsequently, the Fe-based electroplated steel sheet was heated at an average heating rate in the temperature range of 400°C or higher and 650°C or lower, as shown in Tables 16 to 18, and then heated in an atmosphere having an atmospheric dew point B shown in Tables 16 to 18, containing 15% by volume of hydrogen and the balance being N. ２ Annealing was performed by heating the steel sheet at a soaking temperature of 800°C in a reducing atmosphere containing iron, zinc, and unavoidable impurities. The holding time at the maximum temperature (800°C) of the steel sheet was 100 seconds. In the examples shown in Table 18, Fe-based electroplated steel sheets (CR) were obtained in this manner.

[0219] In the examples shown in Tables 16 and 17, the obtained Fe-based electroplated steel sheets were cooled to 440 to 550°C, and then the Fe-based electroplated steel sheets were subjected to a hot-dip galvanizing treatment using a 460°C hot-dip galvanizing bath with an effective Al concentration of 0.132 mass% in the bath and the remainder consisting of Zn and unavoidable impurities, and then the coating weight per side of the hot-dip galvanized layer was adjusted by gas wiping. In the examples shown in Table 17, hot-dip galvanized steel sheets (GI) were obtained in this manner.

[0220] In the examples shown in Table 16, a step of heating and alloying the hot-dip galvanized layer by alloying treatment at 510°C was subsequently carried out to produce a galvannealed steel sheet (GA). The alloying treatment time was changed to control the Fe% in the galvannealed layer.

[0221] Tables 16 and 17 show the coating weight and Fe % of the hot-dip galvanized layer or galvannealed hot-dip galvanized layer of each steel sheet, determined by the method described above.

[0222] Table 16 shows the results obtained by the method described above (I Ｓｉ，Ｆｅ ) / (I Ｓｉ，ｂｕｌｋ The values ​​of the internal oxidation layer, the average C concentration, the thickness of the decarburized layer, and the number of grain boundaries per 10 μm in the width direction of the steel sheet, which were obtained by the above-mentioned methods, are shown in Tables 17 and 18.

[0223] For the GA and GI examples, the appearance of the zinc-coated layer (evaluation 1) and the resistance weld crack resistance of the welded parts (evaluation 3) were evaluated, and the results are shown in Tables 16 and 17. For the CR examples, the chemical conversion treatability and corrosion resistance after painting (evaluation 2) and the resistance weld crack resistance of the welded parts (evaluation 3) were evaluated, and the results are shown in Table 18. The evaluation methods and evaluation criteria for Evaluations 1 and 2 were the same as those for Example 1. The evaluation method and evaluation criteria for Evaluation 3 were as follows:

[0224] [Evaluation 3: Evaluation of resistance weld crack resistance at welded parts] Referring to Fig. 4(a), test pieces 6 were cut out from the steel sheets (CR, GI, GA) of each of the invention examples and comparative examples, measuring 150 mm in the longitudinal direction and 50 mm in the transverse direction, with the direction perpendicular to the rolling (TD) as the longitudinal direction and the rolling direction as the transverse direction. Test pieces 6 were cut out to the same size, and the coating weight of the galvanized layer per side was 50 g / m ２The test specimen 6 was stacked with a test galvannealed steel sheet 5 (sheet thickness: 1.6 mm, TS: 980 MPa class) of the same material to form a sheet assembly. The sheet assembly was assembled so that the evaluation surface of the test specimen 6 (the Fe-plated layer in the case of CR, and the zinc-plated layer in the case of GI and GA) faced the zinc-plated layer of the test galvannealed steel sheet 5. The sheet assembly was fixed to a fixing base 8 via a spacer 7 having a thickness of 2.0 mm. The spacer 7 was a pair of steel plates measuring 50 mm in the longitudinal direction, 45 mm in the transverse direction, and 2.0 mm in thickness. As shown in Figure 4(a), the longitudinal end faces of each of the pair of steel plates were arranged so as to be aligned with both transverse end faces of the sheet assembly. Therefore, the distance between the pair of steel plates was 60 mm. The fixing base 8 was a single plate with a hole in the center. Next, using a servomotor-pressurized, single-phase AC (50 Hz) resistance welding machine, the sheet assembly was pressed with a pair of electrodes 9 (tip diameter: 6 mm) while being deflected. Resistance welding was performed with a welding current that resulted in a nugget diameter r of 5.9 mm under the following conditions: a pressure of 3.5 kN, a hold time of 0.14 or 0.16 seconds, and a welding time of 0.36 seconds, to obtain a sheet assembly with a weld. The pair of electrodes 9 pressed the sheet assembly from above and below in the vertical direction, and the lower electrode pressed the test piece 6 through a hole in the fixture 8. During pressing, the lower electrode and the fixture 8 were fixed so that the lower electrode of the pair of electrodes 9 was in contact with a plane extending from the contact surface between the spacer 7 and the fixture 8, and the upper electrode was movable. The upper electrode was also in contact with the center of the test galvannealed steel sheet 5. The sheet assembly was also welded while tilted 5° toward the longitudinal direction of the sheet assembly relative to the horizontal. The hold time refers to the time from when the welding current has finished flowing to when the electrodes begin to be released. Referring to the lower diagram of Figure 4(b), the nugget diameter r refers to the distance between the ends of the nugget 10 in the longitudinal direction of the sheet assembly. The sheet assembly with the weld was then cut along line B-B in the upper diagram of Figure 4(b) so as to include the center of the weld including the nugget 10. The cross section of the weld was observed with an optical microscope (200x magnification), and the resistance weld cracking resistance of the weld was evaluated according to the following criteria. A rating of ◎ or ○ indicates that the resistance weld cracking resistance of the weld is excellent. A rating of × indicates that the resistance weld cracking resistance of the weld is poor.◎: No cracks of 0.1 mm or more in length were observed at a hold time of 0.14 seconds. ○: Cracks of 0.1 mm or more in length were observed at a hold time of 0.14 seconds, but no cracks of 0.1 mm or more in length were observed at a hold time of 0.16 seconds. ×: Cracks of 0.1 mm or more in length were observed at a hold time of 0.16 seconds. In the lower diagram of Figure 4(b), a crack that occurred in test piece 6 is schematically shown as reference numeral 11. If cracks occurred in the mating steel sheet (galvannealed steel sheet for testing), the stress in the steel sheets to be evaluated (steel sheets of each invention example and comparative example) would be dispersed, preventing an appropriate evaluation. For this reason, data in which no cracks occurred in the mating steel sheet were used as examples.

[0225]

[0226]

[0227]

[0228] As is clear from Tables 16 and 17, the inventive examples in the examples of GA and GI had good appearance of the zinc plating layer and were able to realize excellent resistance weld cracking resistance. Also, as is clear from Table 18, the inventive examples in the examples of CR had good chemical conversion treatability and corrosion resistance after painting and were able to realize excellent resistance weld cracking resistance.

[0229] Example 6 Steel having the chemical composition shown in Table 15 (the balance being Fe and unavoidable impurities) was melted and cast, and the resulting cast was hot-rolled to obtain a hot-rolled steel sheet. This hot-rolled steel sheet was then pickled and cold-rolled to obtain a cold-rolled steel sheet having a thickness of 1.6 mm.

[0230] Using these cold-rolled steel sheets, various GAs shown in Table 19, various GIs shown in Table 20, and various CRs shown in Table 21 were produced.

[0231] First, the cold-rolled steel sheet was degreased with alkali, and then electrolytic treatment was carried out using the cold-rolled steel sheet as the cathode under the conditions shown below to produce an Fe-based electroplated steel sheet. The coating weight A of the Fe-based electroplated layer shown in Tables 19 to 21 was calculated by the method described above and controlled by the current application time. [Electrolysis conditions] Bath temperature: 50°C pH: 2.0 Current density: 45 A / dm ２Plating bath: Fe ２＋ Sulfuric acid bath containing 1.5 mol / L of ions Anode: iridium oxide electrode

[0232] Subsequently, the Fe-based electroplated steel sheet was heated at an average heating rate in the temperature range of 400°C or higher and 650°C or lower, as shown in Tables 19 to 21, and then heated in an atmosphere having an atmospheric dew point B shown in Tables 19 to 21, containing 15% by volume of hydrogen and the balance being N. ２ Annealing was performed by heating the steel sheet at a soaking temperature of 800°C in a reducing atmosphere containing iron and unavoidable impurities. The holding time at the maximum temperature (800°C) of the steel sheet was 100 seconds. In the examples shown in Table 21, Fe-based electroplated steel sheets (CR) were obtained in this manner.

[0233] In the examples shown in Tables 19 and 20, the obtained Fe-based electroplated steel sheets were cooled to 440 to 550°C, and then the Fe-based electroplated steel sheets were subjected to a hot-dip galvanizing treatment using a 460°C hot-dip galvanizing bath with an effective Al concentration of 0.132 mass% in the bath and the remainder consisting of Zn and unavoidable impurities, and then the coating weight per side of the hot-dip galvanized layer was adjusted by gas wiping. In the examples shown in Table 20, hot-dip galvanized steel sheets (GI) were obtained in this manner.

[0234] In the examples shown in Table 19, a step of heating and alloying the hot-dip galvanized layer by alloying treatment at 510°C was further subsequently carried out to produce a galvannealed steel sheet (GA). The alloying treatment time was changed to control the Fe% in the galvannealed layer.

[0235] Tables 19 and 20 show the coating weight and Fe % of the hot-dip galvanized layer or galvannealed hot-dip galvanized layer for each steel sheet, determined by the method described above.

[0236] Table 19 shows the results obtained by the method described above (I Ｓｉ，Ｆｅ ) / (I Ｓｉ，ｂｕｌｋ The values ​​of the internal oxidation layer, the average C concentration, the thickness of the decarburized layer, and the number of grain boundaries per 10 μm in the width direction of the steel sheet, which were obtained by the above-mentioned methods, are shown in Tables 20 and 21.

[0237] For the GA and GI examples, the appearance of the zinc-coated layer (evaluation 1) and resistance weld crack resistance at the welded portion (evaluation 3) were evaluated, and the results are shown in Tables 19 and 20. For the CR examples, the chemical conversion treatability and corrosion resistance after painting (evaluation 2) and resistance weld crack resistance at the welded portion (evaluation 3) were evaluated, and the results are shown in Table 21. The evaluation methods and evaluation criteria for evaluations 1 to 3 were the same as those for Example 5.

[0238]

[0239]

[0240]

[0241] As is clear from Tables 19 and 20, the inventive examples in the examples of GA and GI had good appearance of the zinc-plated layer and were able to realize excellent resistance weld cracking resistance. Also, as is clear from Table 21, the inventive examples in the examples of CR had good chemical conversion treatability and corrosion resistance after painting and were able to realize excellent resistance weld cracking resistance.

[0242] The Fe-based electroplated steel sheet of the present invention not only exhibits excellent chemical conversion treatability or plating appearance when hot-dip galvanized, but also has excellent resistance weld crack resistance. Furthermore, the galvannealed steel sheet of the present invention not only exhibits excellent plating appearance, but also has excellent resistance weld crack resistance. Therefore, by applying the Fe-based electroplated steel sheet or galvannealed steel sheet of the present invention to, for example, automotive structural members, it is possible to reduce the vehicle body weight and thereby improve fuel economy, and the steel sheet can also be used in applications such as home appliances and building members.

[0243] REFERENCE SIGNS LIST 1 Fe-based electroplated steel sheet 2 Cold-rolled steel sheet 3 Fe-based electroplated layer 5 Galvannealed steel sheet for test 6 Test piece 7 Spacer 8 Fixing base 9 Electrode 10 Nugget 11 Crack

Claims

DEPCT6626 / 07 / 25661. Electric Fe-based coated steel sheets composed of cold-rolled steel sheets containing a chemical alloy of 0.1% by mass or more and 3.0% by mass or less Si; and a Fe-based electroplating layer formed on one or both surfaces of the cold-rolled steel sheets with a coating weight per surface of 1.0 g / m² or more, where in the indicative wavelength emission intensity profile of Si is measured in the depth direction from the surface of the Fe-based electroplating layer by glow discharge. (i) Peaks with emission intensity greater than the mean Si (ISi) intensity were found at a depth of 10.0 ± 0.1 µm from the interface between the Fe-based electroplating layer and the cold-rolled steel sheet. (ii) The highest peaks of the peaks were found to be more than 0.10 µm below the surface of the Fe-based electroplating layer, and (iii) the depth at which the emission intensity equals the mean Si (ISi) intensity for the first time after gradual decrease in the depth direction from the highest peak of the peaks was within 2.

1. A Fe-based electroplated steel plate under Relief Clause 1 where the average concentration of C in the range of 10 micrometers to 20 micrometers in the thickness direction from the surface of the Fe-based electroplated plate is 0.10% by mass or less.

2. A Fe-based electroplated steel plate under Relief Clause 1 where an integrated, non-alloyed, hot-dip galvanized coating is formed in contact with the Fe-based electroplated plate.

3. A Fe-based electroplated steel plate under Relief Clause 1 or 2 where the average concentration of C in the range of 10 micrometers to 20 micrometers in the thickness direction from the surface of the Fe-based electroplated plate is 0.04% by mass or less.

4. A Fe-based electroplated steel plate under Relief Clauses 1 through 3 where the Fe-based electroplated steel plate has a substrate layer in which a carbon reduction layer is formed. 5.

6. Electricly coated steel plates with a Fe base under claim 4 where the carbon reduction layer is 30 micrometers or thicker.

7. Electricly coated steel plates with a Fe base under claim 4 where the carbon reduction layer is 80 micrometers or thicker.

8. Electricly coated steel plates with a Fe base under any one of claims 1 through 6 where the number of crystalline grain boundaries of the Fe-based electroplating layer in contact with the cold-rolled steel plate at the interlayer interface is 80 micrometers or thicker.

8. Electroplated steel sheets with a Fe base, according to any one of claims 1 to 7, in which the chemical composition of the cold-rolled steel sheet, as a percentage by mass, is C: 0.8% or less, Si: 0.1% or more and 3.0% or less, Mn: 1.0% or more and 12.0% or less, P: 0.1% or less, S: 0.03% or less, N: 0.010% or less, and Al: 1.0% or less.0% or less with a balancing factor of Fe and unavoidable impurities.

9. Electroplated steel plates with a Fe base according to claim 8, where the chemical composition, in % by mass, is further regulated by at least one element selected from the group of elements including 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 10. Electroplated steel plates with a Fe base according to any one of claims 1 to 9 in which the amount of Si in the chemical mixture is 0.9% by mass or more and 1.7% by mass or less 11.Electroplated steel plates with a Fe base under any one of the claims 1 to 10, where the Fe-based electroplating layer contains at least one chemical element chosen from the group of B, C, P, N, O, Ni, Mn, Mo, Zn, W, Pb, Sn, Cr, V, and Co in a total amount of 10% by mass or less, with a Fe balancing agent and unavoidable impurities.

12. Galvanized steel plates incorporated with cold-rolled steel plates containing a chemical composition of Si in an amount of 0.1% by mass or more and 3.0% by mass or more. Less than and with a galvanene layer formed on one or both surfaces of cold-rolled steel sheet where a Fe-based electroplating layer is absent between the galvanene layer and the cold-rolled steel sheet, in the indicative Si emission intensity profile at the indicated wavelength, measured in the depth direction from the surface of the galvanene layer by glow discharge optical emission spectrometry, the value of (ISi,Fe) / (ISi,Bulk) is 0.90 or less, where (ISi,Fe) indicates the average Si intensity in the range from the interface between the galvanene layer and the cold-rolled steel sheet to +0.5 micrometers to cold-rolled steel sheet and (ISi, bulk) indicates the average Si concentration in the cold-rolled steel sheet and the average C concentration in the range of 10 micrometers to 20 micrometers in the thickness direction from the bond between the galvanyl layer and the cold-rolled steel sheet is 0.10% by mass or less.

13. Galvanyl steel sheet according to claim 12 where the average C concentration in the range of 10 micrometers to 20 micrometers in the thickness direction from the bond between the galvanyl layer and the cold-rolled steel sheet is 0.04% by mass or less.

14. Galvanyl steel sheet according to claim 12 14. Galvanyl steel sheet under claim 12 or 13 where the cold-rolled steel sheet has a surface layer in which a carbon reduction layer is formed.

15. Galvanyl steel sheet under claim 14 where the carbon reduction layer is 30 micrometers or thicker.

16. Galvanyl steel sheet under claim 14 where the carbon reduction layer is 80 micrometers or thicker.

17. Galvanyl steel sheet under any one of claims 12 through 16 where the chemical composition of the cold-rolled steel sheet, by percentage by mass, is C: 0.8% or less, Si: 0.1% or more, and Mn: 1% or less.0% or more and 12.0% or less, P: 0.1% or less, S: 0.03% or less, N: 0.010% or less, and Al: 1.0% or less, with a balancing factor of Fe and unavoidable impurities.

18. Galvanized steel plates pursuant to claim 17, where the chemical composition, in % by mass, is further saturated with at least one element chosen from the group of elements consisting of B: 0.005% or less, Ti: 0.2% or less, Cr: 1.0% or less, Cu: 1.0% or less.

19. Galvanized steel plates according to any one of claims 12 to 18 in which the chemical composition contains 0.9% by mass or more of Si and 1.7% by mass or less of Si.

21. Galvanized steel plates under any of the claims 12 to 19 where the galvanized layer contains at least one element selected from the group of B, C, P, N, O, Ni, Mn, Mo, Zn, W, Pb, Sn, Cr, V, and Co in a total amount of 1% by mass or less.

22. Steel plates undergoing chemical transformation treatments incorporating a chemical transformation layer formed on the surface of a Fe-based electroplated steel plate as described in any of the claims 1 to 11 or on the surface of a galvanized layer.

21. Galvanized steel sheets as described in any one of Claims 12 through 19.

22. Electrodeposition-coated steel sheets incorporating an electrodeposition coating layer formed into contact with a chemical transformation layer of the steel sheet through a chemical transformation operation as described in Claim 21.

23. Automotive parts made at least partially from electrodeposition-coated steel sheets as described in Claim 22. 24.The method of producing Fe-based electroplated steel sheets involves taking cold-rolled steel sheets with a chemical composition containing 0.1% by mass or more of Si and 3.0% by mass or less, and subjecting them to a Fe-based electroplating process to obtain Fe-based electroplated steel sheets with a Fe-based electroplating layer formed on one or both surfaces of the cold-rolled steel sheet with a coating weight A (g / m²) per surface of 1.0 g / m² or more, and then subjecting the Fe-based electroplated steel sheets to an annealing process in which the Fe-based electroplated steel sheets are held in a temperature range of 650 °C to 900 °C in an atmosphere with a dew point B (°C) corresponding to the following formula (1), where A+B ≥ 3.0(1)25.

26. The method of producing Fe-based electroplated steel sheets under claim 24, which is incorporated after annealing, involves subjecting the Fe-based electroplated steel sheets to hot-dip galvanizing to form a non-alloyed hot-dip galvanizing layer on the surface of the Fe-based electroplated steel.

27. The method of producing Fe-based electroplated steel sheets under claim 24 or 25, where the annealing is performed in an atmosphere with a dew point B(°C) that corresponds to the following formula (1)', A+B greater than or equal to 8.0(1).

28. The method of producing Fe-based electroplated steel sheets under one of the incorporated claims 24 to 26, which is incorporated before annealing, involves heating the Fe-based electroplated steel sheets at an average heating rate of 10°C / s or higher in the temperature range of 400°C to 650°C. 29.

29. Methods of producing electroplated steel sheets with Fe-based coatings under any of the claims 24 to 27 where the coating weight A is less than 5.0 g / m².

29. Methods of producing electroplated steel sheets with Fe-based coatings under any of the claims 24 to 28 where the chemical composition of the cold-rolled steel sheet, in percentage by mass, is C: 0.8% or less, Si: 0.1% or more and 3.0% or less, Mn: 1.0% or more and 12.0% or less, P: 0.1% or less, S: 0.03% or less, N: 0.010% or less, and Al: 1.0% or less.

30. A method for the production of Fe-based electroplated steel plates according to claim 29 where the chemical composition, in % by mass, is further maintained with at least one element selected from the group of elements 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 31. Methods of producing Fe-based electroplated steel plates according to any one of claims 24 to 30 in which the chemical composition contains 0.9% by mass or more and 1.7% by mass or less.

32. Methods of producing Fe-based electroplated steel plates according to any one of claims 24 to 31 in which the Fe-based electroplating is obtained. The process of electroplating steel plates with a coating containing at least one element selected from the group comprising B, C, P, N, O, Ni, Mn, Mo, Zn, W, Pb, Sn, Cr, V, and Co, so that at least one element in the electroplating layer, which is Fe-based, is present in a total amount of 10% by mass or less.

33. The method of producing Fe-based electroplated steel plates according to any one of the claims 24 to 32, combined with at least one change in the weight of coating A(a). / square meter) or dew point B (degrees Celsius) to be consistent with formula (1) or (1)' in the case that it is not consistent with formula (1) or (1)'34. The method of producing galvanized steel sheets which involves taking cold-rolled steel sheets with a chemical alloy containing Si in an amount of 0.1% by mass or more and 3.0% by mass or less and subjecting them to an electroplating with a Fe base to obtain an electroplated steel sheet with a Fe base electroplating layer formed on one or both surfaces of the cold-rolled steel sheet with a coating weight A (g / square meter) per surface of 1.0g. / square meters or more and then the Fe-based electroplated steel sheet is subjected to an annealing process in which the Fe-based electroplated steel sheet is maintained in a temperature range of 650°C to 900°C in an atmosphere with a dew point B(°C) corresponding to the following formula(2), and then the Fe-based electroplated steel sheet is subjected to hot-dip galvanizing to form a non-alloyed hot-dip galvanizing layer on the surface of Electroplating of Fe-based and subsequent heating and alloyation of hot-dip zinc coatings to obtain galvanized steel sheets with a galvanized layer formed on one or both surfaces of cold-rolled steel sheets A+B greater than or equal to 5.0(2)35. The method of producing galvanized steel sheets according to claim 34 in which the annealing is performed in an atmosphere with a dew point B(°C) corresponding to this formula(2)' A+B greater than or equal to 10.0(2)'36.

37. Methods of manufacturing galvanized steel plates under any of the claims 34 or 35, which include, prior to annealing, heating of the Fe-based electroplated steel plate at an average heating rate of 10°C / s or higher in the temperature range of 400°C to 650°C.

37. Methods of manufacturing galvanized steel plates under any of the claims 34 to 36, where the weight of coating A is less than 5.0 g / m².

38. Methods of manufacturing galvanized steel plates under any of the claims 34 to 37, where the composition The chemical composition of cold-rolled steel sheets, in percentage by mass, contains C: 0.8% or less, Si: 0.1% or more and 3.0% or less, Mn: 1.0% or more and 12.0% or less, P: 0.1% or less, S: 0.03% or less, N: 0.010% or less, and Al: 1.0% or less, with a balancing factor of Fe and unavoidable impurities.

39. The method of manufacturing galvanized steel sheets according to claim 38, where the chemical composition, in percentage by mass, also contains at least one element chosen 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.

40. Method of manufacturing galvanized steel plates according to any one of claims 24 to 39 in which the chemical composition of Si is 0.9% by mass or more and 1.7% by mass or less.

41. Method of manufacturing steel plates 42. Methods of producing galvanyl steel plates pursuant to any of the claims 34 to 40 in which an electroplating of a Fe-based coating is performed in a coating bath containing at least one element selected from the group comprising B, C, P, N, O, Ni, Mn, Mo, Zn, W, Pb, Sn, Cr, V, and Co, so that at least one of that element is present in the galvanyl layer in a total amount of 1% by mass or less. / square meter) or dew point B (degrees Celsius) to be true according to formula (2) or (2)' in the case where it is not true according to formula (2) or (2)'43. The method of producing steel plates through chemical transformation operations which includes the method of producing Fe-based electroplated steel plates as described in any of Claims 24 to 33 or the method of producing galvanyl steel plates as described in any of Claims 34 to 42 and then subjecting the Fe-based electroplated steel plate or galvanyl steel plate to a chemical transformation operation to obtain a chemical transformation plate in which a chemical transformation layer is formed in contact with the Fe-based electroplated steel plate or galvanyl steel plate.44.The method of manufacturing electroplated steel sheets which includes the method of manufacturing steel sheets through chemical transformation treatment as described in claim 43 and the application of the chemical transformation treatment to the electroplated steel sheets to obtain electroplated steel sheets with an electroplated coating layer formed in contact with the chemical transformation layer.

45. The method of manufacturing automobile parts which includes the method of manufacturing electroplated steel sheets as described in claim 44 and the manufacture of automobile parts using at least one part of electroplated steel sheets;