Hot-dip galvannealed steel sheet and method for producing same

Abstract

To provide a galvannealed steel sheet excellent not only in plating appearance but also in resistance to resistance welding cracking.SOLUTION: An alloyed hot-dip galvanized steel sheet according to the present invention includes a cold-rolled steel sheet having a composition containing 0.1 mass% or more and 3.0 mass% or less of Si and 0.05 mass% or more and 0.8 mass% or less of C, and an alloyed hot-dip galvanized layer formed on one side or both sides of the cold-rolled steel sheet, wherein an emission intensity profile of a wavelength indicating Si measured in a depth direction from a surface of the alloyed hot-dip galvanized layer by a glow discharge optical emission spectrometry does not satisfy a predetermined relationship, and a surface layer portion of the cold-rolled steel sheet has a predetermined decarburized layer.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to an Fe-based electroplated steel sheet and a galvannealed steel sheet, as well as methods for producing them. 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). [Background technology]

[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, which involves adding Si to steel sheets. However, Si added to steel sheets to increase their strength forms oxides on the steel sheet surface during annealing. When steel sheets are used without hot-dip galvanizing, these oxides deteriorate their 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. To ensure the 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.

[0004] Patent Document 1 describes a process for preparing a base steel sheet, and applying a coating weight of 0.2 to 2 g / m2 to the prepared base steel sheet. 2 the steps of forming an Fe-plated layer on the steel sheet at 600-800°C by oxidation heating; maintaining the heated steel sheet at 750-900°C for 5 seconds or more in a reducing atmosphere containing 20 ppm or less of oxygen, 1-20 vol% of H2, the balance being N2 and other unavoidable gases, and having a dew point of -30-5°C; cooling the maintained steel sheet; and immersing the cooled steel sheet in a hot-dip galvanizing bath at 445-480°C to coat it (Claim 1).

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

[0006] [Patent Document 1] Patent No. 6025867 [Patent Document 2] International Publication No. 2019 / 116531 Summary of the Invention [Problem to be solved by the invention]

[0007] In the manufacture of automotive parts, press-formed components are often assembled by resistance welding (spot welding). When at least one of the components to be resistance-welded contains hot-dip galvanized steel sheet, residual stresses are generated near the weld during resistance welding. The zinc in the galvanized layer melts and diffuses into the grain boundaries of the steel sheets that make up each component, potentially causing liquid metal embrittlement (LME) and intergranular cracking (LME cracking) in the steel sheet. In particular, welding performed with a welding electrode at an angle to the steel sheet increases residual stress, potentially leading to cracking. Because residual stress is thought to increase with increasing strength of steel sheet, LME cracking is a concern due to the addition of Si.

[0008] However, according to the investigations of the present inventors, it was found that the manufacturing method of hot-dip galvanized steel sheet described in Patent Document 1 can prevent ungalvanized areas 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., intergranular 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. [Means for solving the problem]

[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 to ensure chemical conversion treatability or coating 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, which increases the predetermined value of A+B.

[0012] First, we will explain the cases of CR and GI in detail. To achieve high resistance weld cracking resistance at the weld, it is important to form an Fe-based electroplated layer with a coating weight of A on a cold-rolled steel sheet and then perform an annealing process under conditions that satisfy A + B ≥ 3.0. This allows for the formation of an internal oxide within the Fe-based electroplated layer, and limits 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 within 2.00 μm. When A + B ≥ 3.0 is satisfied, Si that diffuses from the cold-rolled steel sheet into 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, 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 within 2.00 μm. This minimizes the penetration of zinc into the grain boundaries of the cold-rolled steel sheet, thereby improving the resistance weld cracking resistance at the weld.

[0013] Furthermore, in the annealing step, the average C concentration in the area 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 inventors have found that when an Fe-based electroplated layer is formed before annealing, the C concentration in the area of ​​10 μm to 20 μm from the surface of the Fe-based electroplated layer in the sheet thickness direction can be further reduced, making it possible to more effectively obtain the effect of improving resistance weld cracking resistance.

[0014] Furthermore, in the heating process following the annealing (soaking) process, 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 / s or more. This minimizes grain growth in the Fe-based electroplated layer during the heating process, and 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 is set to 10 or more per 10 μm of cold-rolled steel sheet width. This further improves resistance weld cracking resistance. Setting the average heating rate to 10°C / s or more in the temperature range of 400°C to 650°C during the heating process refines the grains in the Fe-based electroplated layer that contact the interface between the Fe-based electroplated layer and the cold-rolled steel sheet, 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, further improving resistance weld cracking resistance.

[0015] Next, we will explain the case of GA in detail. In the case of GA, the Fe-based electroplated layer is absorbed into the hot-dip galvanized layer during the alloying process and disappears, so the stress relief effect of the Fe-based electroplated layer cannot be expected. To maintain high resistance weld cracking 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 of 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. During the alloying process, a portion of the Si internal oxide is incorporated 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 into the Fe-based electroplated layer during annealing can be converted into an oxide within the Fe-based electroplated layer. In this way, forming a portion of the oxide within the Fe-based electroplated layer reduces the amount of internal oxide in contact with the galvannealed layer. This suppresses zinc penetration through the grain boundaries of the internal oxide layer in contact with the galvannealed layer. As a result, the time it takes for molten zinc to reach the grain boundaries of the cold-rolled steel sheet during welding can be delayed, and the resistance weld crack resistance characteristics of the welded portion can be improved.

[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 electroplated 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 and configuration of the present invention, which has been completed based on the above findings, is as follows. [1] A cold-rolled steel sheet having a composition containing 0.1 mass% or more and 3.0 mass% or less of Si; The coating weight per side formed on one or both sides of the cold-rolled steel sheet is 1.0 g / m2 The above Fe-based electroplated layer, An Fe-based electroplated steel sheet having In the emission intensity profile of the wavelength indicating Si measured in the depth direction from the surface of the Fe-based electroplating layer by glow discharge optical emission spectrometry, (i) the average Si intensity (I 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 a depth of 0.10 μm to 2.00 μm from the surface of the Fe-based electroplated layer, the average C concentration in the area of ​​10 μm or more and 20 μm or less from the surface of the Fe-based electroplated layer in the sheet thickness direction is 0.10 mass% or less; Fe-based electroplated steel sheet.

[0018] [2] An 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 area of ​​10 μm or more and 20 μm or less 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 the surface layer of the Fe-based electroplated steel sheet is a decarburized layer.

[0021] [5] The Fe-based electroplated steel sheet according to [4] above, 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 the above [1] to [6], 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 composition of the cold-rolled steel sheet is, in mass%, 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 The Fe-based electroplated steel sheet according to any one of the above [1] to [7], comprising the above and the remainder consisting of Fe and unavoidable impurities.

[0025] [9] The component composition further comprises, in mass%, 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 The Fe-based electroplated steel sheet according to [8] above, containing at least one element selected from the group consisting of:

[0026]

[10] The Fe-based electroplated steel sheet according to any one of the above [1] to [9], 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 the above [1] to

[10] , wherein the Fe-based electroplated layer has a 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 remainder consisting of Fe and unavoidable impurities.

[0028]

[12] A cold-rolled steel sheet having a composition containing 0.1 mass% or more and 3.0 mass% or less of Si; a galvannealed layer formed on one or both sides of the cold-rolled steel sheet; and wherein no Fe-based electroplating layer is present between the galvannealed layer and the cold-rolled steel sheet, In the emission intensity profile of a wavelength showing Si measured in the depth direction from the surface of the galvannealed layer by glow discharge optical emission spectrometry, the 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, the average C concentration in the 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 0.10 mass% or less; Galvannealed steel sheet.

[0029]

[13] The galvannealed steel sheet according to

[12] , wherein the average C concentration in an area 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 the surface layer 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 composition of the cold-rolled steel sheet is, in mass%, 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 The galvannealed steel sheet according to any one of the above

[12] to

[16] , comprising the above and the remainder consisting of Fe and unavoidable impurities.

[0034]

[18] The component composition further comprises, in mass%, 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 The galvannealed steel sheet according to

[17] above, containing at least one element selected from the group consisting of:

[0035]

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

[12] to

[18] , wherein the Si content in the chemical 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 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 in a total amount of 1 mass% or less.

[0037]

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

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

[12] to

[19] .

[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 in part, 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 1.0g / m 2 a step of obtaining an Fe-based electroplated steel sheet having the above-mentioned Fe-based electroplated layer formed thereon; Thereafter, an 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 method for producing an Fe-based electroplated steel sheet. A + B ≧ 3.0 (1)

[0041]

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

[24] , further comprising the 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 the above

[24] to

[26] , 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 adhesion amount A is 5.0 g / m 2 The method for producing an Fe-based electroplated steel sheet according to any one of the above

[24] to

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

[0045]

[29] The composition of the cold-rolled steel sheet is, in mass%, 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 and the balance consisting of Fe and unavoidable impurities.

[0046]

[30] The component composition further comprises, in mass%, 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 The method for producing an Fe-based electroplated steel sheet according to

[29] above, wherein the Fe-based electroplated steel sheet contains at least one element selected from the group consisting of:

[0047]

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

[24] to

[30] , wherein the Si content in the 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 the above

[24] to

[31] , 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 (1)' is not satisfied, the coating amount A (g / m) is adjusted so as to satisfy the formula (1) or (1'). 2 The method for producing an Fe-based 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) and 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 weight A (g / m) per side is applied to one or both sides of the cold-rolled steel sheet. 2) is 1.0g / m 2 a step of obtaining an Fe-based electroplated steel sheet having the above-mentioned Fe-based electroplated layer formed thereon; Thereafter, an 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); Thereafter, the Fe-based electroplated steel sheet is subjected to hot-dip galvanization to form an unalloyed hot-dip galvanized layer on the surface of the Fe-based electroplated layer; Thereafter, the hot-dip galvanized layer is heated and alloyed to obtain a galvannealed steel sheet in which a galvannealed layer is formed on one side or both sides of the cold-rolled steel sheet; A method for producing a galvannealed steel sheet having the above structure. 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] , further 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 adhesion 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 composition of the cold-rolled steel sheet is, in mass%, 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 and the balance consisting of Fe and unavoidable impurities.

[0055]

[39] The component composition further comprises, in mass%, 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 The method for producing a galvannealed steel sheet according to

[38] above, wherein the steel sheet contains at least one element selected from the group consisting of:

[0056]

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

[24] to

[39] , wherein the Si content in the chemical 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, 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, so that the total content of this element in the galvannealed layer is 1 mass% or less.

[0058]

[42] When the formula (2) or (2)' is not satisfied, the coating amount A (g / m) is adjusted so as to satisfy the formula (2) or (2)'. 2 The 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 the dew point B (°C).

[0059]

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

[24] to

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

[34] to

[42] , thereafter, 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; A method for producing a chemically treated steel sheet having the above structure.

[0060]

[44] A method for producing a chemically treated steel sheet according to

[43] above; a step of applying electrodeposition coating to the chemical conversion treated steel sheet to obtain an electrodeposition coated steel sheet having an electrodeposition coated film formed in contact with the chemical conversion treatment film; A method for producing an electrodeposition coated steel sheet comprising the steps of:

[0061]

[45] A method for producing an electrodeposition coated steel sheet according to

[44] above; a step of manufacturing an automobile part using the electrodeposition coated steel sheet as a part; A method for manufacturing an automobile part having the above structure. [Effects of the Invention]

[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. [Brief explanation of the drawings]

[0063] [Figure 1]1 is a graph showing the relationship between the coating weight A and annealing dew point B of the Fe-based electroplated layer and the evaluation results of resistance weld cracking resistance in the example of galvannealed steel sheet (GA) of Example 1. [Figure 2] 1 is a graph showing the relationship between the coating weight A and annealing dew point B of the Fe-based electroplated layer and the evaluation results of resistance weld crack resistance in the example of hot-dip galvanized steel sheet (GI) of Example 1. [Figure 3] 1 is a graph showing the relationship between the coating weight A and annealing dew point B of the Fe-based electroplated layer and the evaluation results of resistance weld cracking resistance in an example of an Fe-based electroplated steel sheet (CR) that is not hot-dip galvanized in Example 1. [Figure 4] (a) is a diagram for explaining a method for evaluating the resistance weld cracking resistance characteristics of a welded portion, the upper diagram of (b) is a top view of the plate assembly after welding in the same evaluation, and the lower diagram is an enlarged view of the BB cross section of the upper diagram. [Figure 5] 1 shows raw data of emission intensity profiles of wavelengths showing Si, measured by glow discharge optical emission spectroscopy, for some examples of Fe-based electroplated steel sheets (CR) that are not hot-dip galvanized in Example 1. [Figure 6] FIG. 1 shows an outline of an observation sample for measuring 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. (a) is a perspective view, and (b) is an AA cross-sectional view. [Figure 7] FIG. 2 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. [Figure 8] FIG. 8 is an enlarged view of the area enclosed by the square in FIG. 7. [Figure 9] 1 shows raw data of emission intensity profiles of wavelengths showing Si and Zn, measured by glow discharge optical emission spectroscopy, for some examples of galvannealed steel sheets (GA) of Example 1. [Figure 10A] 1 shows raw data of the profile of C concentration in the sheet thickness direction against depth in some examples of the Fe-based electroplated steel sheet (CR) in Example 1 that is not hot-dip galvanized, analyzed by an electron probe microanalyzer. [Figure 10B] This is data after smoothing processing of the profile in FIG. 10A. [Figure 11A] 1 shows raw data of the profile of C concentration versus depth in the sheet thickness direction in some examples of the galvannealed steel sheet (GA) of Example 1, analyzed by an electron probe microanalyzer. [Figure 11B] This is data after smoothing processing of the profile in FIG. 11A. DETAILED DESCRIPTION OF THE INVENTION

[0064] (Fe-based electroplated steel sheet and its manufacturing method) A method for producing 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 chemical composition containing 0.1 to 3.0% by mass of 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 a subsequent hot-dip galvanizing step is not performed, a simple Fe-based electroplated steel sheet (CR) can be obtained. 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 chemical composition containing 0.1 to 3.0% by mass of 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] A method for producing an Fe-based plated steel sheet according to another embodiment of the present invention 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, the Fe-based electroplated steel sheet (GI) according to another embodiment of the present invention includes a cold-rolled steel sheet having a chemical composition containing 0.1% to 3.0% by mass 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 its manufacturing method) A method for producing 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% by mass of 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; 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 absorbed 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 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 plate] The process for obtaining a cold-rolled steel sheet is not particularly limited, and any known or arbitrary process 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] [Cold-rolled steel sheet composition] The chemical composition of the cold-rolled steel sheet will be described below. Hereinafter, "mass %" will be simply referred to 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 significantly increases the strength of steel through solid solution (solid solution strengthening ability) without significantly impairing workability. However, Si also has a negative effect on resistance weld crack resistance in welded parts. If the Si content is less than 0.1%, high strength cannot be achieved in the steel sheet, and no particular problems with resistance weld crack resistance in welded parts occur, making the application of the present invention unnecessary. 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, resistance weld crack resistance in welded parts may occur even with a Si content of 0.1% or more but 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 may be significantly reduced, which may adversely affect productivity and may cause a decrease 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 common to cold-rolled steel sheets. 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 that is 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, even 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, and 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 achieving 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% and 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. There is no particular lower limit for S, 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 of adding Ti, Nb, and V, but also reduces toughness. Furthermore, excessive N content can cause slab cracking during hot rolling, resulting in 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. There is no particular lower limit for the N content, 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%) Since 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 greater than 0% or may be 0.001% or more. However, from the viewpoint of obtaining the above 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 making the Cr content 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 to prevent an increase in costs.

[0081] Cu: 1.0% or less The Cu content is preferably 0.005% or more. By making the Cu content 0.005% or more, the formation of the residual γ phase can be promoted. 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 making the Ni content 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 cost, 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 cost, 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 thus suppressing a decrease in the strength of steel. To achieve this effect, the Sn content is preferably 0.002% or more. To ensure impact resistance, if 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, it is possible to control the morphology of sulfides and improve ductility and toughness. From the viewpoint of obtaining good ductility, when Ca is contained, the Ca content is preferably 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 / 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 mass % in consideration of 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] Next, the cold-rolled steel sheet is subjected to an Fe-based electroplating process to obtain an Fe-based electroplated steel sheet having a predetermined coating weight of an 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 simple Fe-based plated steel sheets (CR) and good plating appearance in hot-dip galvanized steel sheets (GI). Although the Fe-based electroplating layer disappears in galvannealed steel sheets (GA), performing Fe-based electroplating in the GA manufacturing process is a necessary condition for obtaining good plating appearance. The specific method and conditions for the Fe-based electroplating process 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 Fe ion content in the plating bath before the start of current application is set to Fe 0.5% in order to obtain a sufficient amount of Fe-based electroplating layer. 2+ The concentration is preferably 0.5 mol / L or more and 2.0 mol / L or less. Other conditions for Fe-based electroplating are not particularly limited. The temperature of the plating solution is preferably 30°C or more and 85°C or less, taking into consideration the ability to maintain 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 coating weight 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 adhesion amount.

[0098] The Fe-based electroplated layer can 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 can 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. For CR and GI, the total content of these elements in the Fe-based electroplated layer is preferably 10% by mass or less. For GA, the total content of these elements in the galvannealed layer is preferably 1% by mass or less, since it is believed that no Fe-based electroplated layer will remain. For CR or GI, if the Fe-based electroplated layer is an Fe-C alloy, the C content is preferably 0.08% by mass or less. Metal elements may be contained as metal ions, and non-metal 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] Fe-based electroplated layer coating weight per side A: 1.0 g / m 2 End In the manufacturing processes of 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 coating 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 thickness is less than 4.5 g / m 2It is more preferable that the following is satisfied. In the CR and GI of this embodiment, the coating weight of the Fe-based electroplated layer per side is maintained at the coating 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 steel sheet surface during annealing (described below), and the softened surface layer caused by decarburization relieves residual stress during welding, thereby suppressing cracking in the weld.

[0101] Compared with cold-rolled steel sheets without an Fe-based electroplated layer, 2 In Fe-based electroplated steel sheets with the above properties, annealing further promotes the formation of a decarburized layer, improving resistance weld cracking resistance at the weld. While the mechanism is unclear, it is believed that without an Fe-based electroplated layer, decarburization occurs through a reaction in which solute C reacts with HO in the furnace during annealing to produce CO and CO. In contrast, with an Fe-based electroplated layer, the solute C in the steel sheet can diffuse into the Fe-based electroplated layer without solidifying. This leads to a different rate-determining process, resulting in a faster decarburization rate. Furthermore, when electroplating is performed using elements such as Ni, Co, or Sn alone, the solubility of C in these metal elements is extremely low, and C does not dissolve, resulting in no decarburization-promoting effect.

[0102] The deposition weight of the Fe-based electroplated layer is measured as follows: A 10 x 15 mm sample is taken from the Fe-based electroplated steel sheet and embedded in resin to create 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 2000 to 10,000 times, depending on the thickness of the Fe-based electroplated 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 electroplated layer per side.

[0103] [Temperature increase process] In the manufacturing processes for CR, GI, and GA, 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 in the heating step before annealing (soaking). By setting the average heating rate to 10°C / sec or more, the growth of crystal grains in the Fe-based electroplated layer during the heating step is minimized. This is because, as will be described later, internal oxidation of Si hardly progresses at the grain boundaries of the Fe-based electroplated layer during the heating step, 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 during the heating step, the dew point B (°C) of the atmosphere during annealing is set to a value that satisfies the coating weight A (g / m) of the Fe-based electroplated layer per side, as will be described later. 2 In relation to the above, by performing annealing so as to satisfy certain conditions, the crystal grains of the Fe-based electroplated layer can be refined. For example, a direct-fired furnace (DFF) or a non-oxidizing furnace (NOF) can be used as the heating zone in the temperature-raising step. In the case of a radiant tube-type heating furnace, a pre-heating zone such as an induction heater (IH) can be provided in the upstream stage.

[0104] [Annealing] The Fe-based electroplated steel sheet is then annealed to remove the strain that has occurred in the cold-rolled steel sheet during the rolling process and to recrystallize the structure, thereby adjusting the strength of the steel sheet.

[0105] Hydrogen concentration: 1.0% by volume or more and 30.0% by volume or less The annealing step can be carried out, 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 step 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 step is preferably carried out in a reducing atmosphere with a hydrogen concentration of 1.0 vol% or more, more preferably 2.0 vol% or more. There are no particular restrictions on the upper limit of the hydrogen concentration in the annealing step, but from the viewpoint of cost, the hydrogen concentration is preferably 30.0 vol% or less, more preferably 20.0 vol% or less. The balance 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 adjusted to the coating 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 Fe-based electroplated steel sheet is annealed, and the softened surface layer caused by decarburization relieves residual stress during welding, thereby suppressing cracking in 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 cracks 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 a 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. Resistance welding performed with a welding electrode angled relative to the steel sheet can increase residual stress, 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 weld is improved. In another embodiment, in the manufacturing process of CR and GI, the coating weight 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 weight 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 is to change 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 to control 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 no longer actually 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 response 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 the coating weight (target value) that would be obtained under the conditions adopted in Fe-based electroplating, or the coating weight (measured value) of the Fe-based electroplated layer that has actually been obtained. Similarly, the "value of dew point B" may be either a target value or a measured value.

[0114] Although the above describes an example of an operation of the CR, GI, and GA manufacturing method, it is also possible to implement a method for determining manufacturing conditions for CR, GI, and GA by checking in advance before starting operations 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, changing in advance either the target value of adhesion amount A or the target value of dew point B. 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 set, in order to effectively prevent oxidation of the Fe-based electroplated layer surface, suppress dew point variations, and improve 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 effectively 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, the holding time in this temperature range is preferably 600 seconds or less.

[0117] Maximum temperature of Fe-based electroplated steel sheet: 650°C to 900°C 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 maintaining the maximum temperature of the Fe-based electroplated steel sheet at 650°C or higher, the recrystallization of the steel sheet structure can be favorably promoted, resulting in the desired strength. Furthermore, the natural Fe oxide film formed on the surface of the Fe-based electroplated layer can be favorably reduced, thereby improving the chemical conversion treatability or coating adhesion when hot-dip galvanizing is performed. Furthermore, maintaining the maximum temperature of the Fe-based electroplated steel sheet at 900°C or lower can prevent an excessive increase in the diffusion rate of Si and Mn in the steel and prevent the diffusion of Si and Mn to the steel sheet surface, thereby improving the chemical conversion treatability or coating adhesion when hot-dip galvanizing is performed. Furthermore, maintaining the maximum temperature at 900°C or lower can prevent damage to the heat treatment furnace body and reduce costs. The above 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 further step is performed in which the Fe-based electroplated steel sheet is subjected to hot-dip galvanization to form a hot-dip galvanized layer on the surface of the Fe-based electroplated steel sheet. After the annealing step, the Fe-based electroplated steel sheet is cooled and immersed in a hot-dip galvanizing bath to apply hot-dip galvanization to the steel sheet surface. The hot-dip galvanizing bath contains Al, Zn, and unavoidable impurities. The components of the hot-dip galvanizing bath are not particularly specified, but the Al concentration in the bath is generally 0.05% by mass or more and 0.250% by mass or less. Setting the Al concentration in the bath to 0.05% by 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% by 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] [Heating alloying] In the manufacturing process of GA, after hot-dip galvanizing, the hot-dip galvanized layer is heated and 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 the coating adhesion is not impaired.

[0121] The Fe content in the galvannealed layer of a galvannealed steel sheet is preferably 7 to 15 mass%. If it is 7 mass% or more, deterioration of press formability due to remaining η phase can be suppressed, and if it 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] This section explains how to calculate the coating weight and Fe% of the zinc plating layer for GI and GA. 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, the plating is stripped from one side using the 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 area. 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 points 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 so that the formula (1) A + B ≥ 3.0 is satisfied. 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 grain boundaries of the cold-rolled steel sheet in the depth direction, thereby improving the resistance weld crack resistance of the weld. On the other hand, forming an internal oxide layer can suppress the formation of oxide on the surface during annealing, thereby improving the chemical conversion treatability in CR and the plating appearance in GI. To achieve such effects, 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 is defined as being within 2.00 μm if the following condition is satisfied: In the emission intensity profile of the wavelength indicating Si measured in the depth direction from the surface of the Fe-based electroplating layer using glow discharge optical emission spectrometry (GD-OES), (i) the average Si intensity (I 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 SiThe depth at which the sputtering mark is equal to the value of the surface roughness of the Fe-based electroplated layer is located within a range of 0.10 μm to 2.00 μm from the surface. Measurement conditions were Ar gas pressure 600 Pa, high-frequency output 35 W, measurement diameter 4 mm, and sampling interval 0.1 seconds. A cold-rolled steel sheet without Fe-based electroplating was analyzed under the same conditions using glow discharge optical emission spectroscopy, and the sputtering rate was calculated by measuring the depth of the sputtering mark. 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 mark.

[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. The results are shown in Table 4 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 weight A: 4.7 g / m). 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 representing Si at a dew point B of +9.8°C. In 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 the Si outer oxide was observed within 0.10 μm from the surface of the cold-rolled steel sheet. Furthermore, a peak Pin derived from the Si inner oxide was also observed at a depth of more than 0.10 μm from the surface. In Examples 23 and 27, a peak Pex derived from the Si outer oxide was observed within 0.10 μm from the surface of the Fe-based electroplated layer, and a peak Pin derived from the 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, indicating that an Si inner oxide was also formed in the Fe-based electroplated layer. The emission intensity gradually decreases from the peak in the depth direction, and then 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 in the range of 0.10 μm deep 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 0.10 μm deep where the emission intensity gradually decreases from the peak of P in the depth direction. Si 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 cross-sectional observation described above. Steel sheets with partial oxide formation within the Fe-based electroplated layer have internal oxides that inhibit grain growth in the Fe-based electroplated layer. Therefore, coarsening of the Fe-based electroplated layer can be prevented even when annealed after Fe-based electroplating. 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, resulting in excellent resistance weld cracking resistance. Furthermore, by partially forming oxides within 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 within 2.00 μm. This minimizes the penetration of zinc into the cold-rolled steel sheet when it reaches the grain boundaries, resulting in even better resistance weld cracking 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 within a range of 0.00 μm to 0.10 μm from the surface. In all of Nos. 21, 23, and 27 in Figure 5, peaks of emission intensity at wavelengths indicative of Si are observed both more than 0.10 μm from the surface and within a range of 0.00 μm to 0.10 μm from the surface. This indicates that the layer has both an inner Si oxide and an outer Si oxide in the surface layer.

[0129] [Amount of internal oxide in GA] In the GA of this embodiment, it is important to reduce the amount of internal oxide in contact with the galvannealed layer. Specifically, in the emission intensity profile of the 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 that diffuses from the cold-rolled steel sheet into the Fe-based electroplated layer during annealing can be oxidized inside the Fe-based electroplated layer, and by partially forming an oxide inside the Fe-based electroplated layer in this way, the amount of internal oxide in contact with the galvannealed hot-dip coated layer can be reduced. This makes it possible to suppress the penetration of zinc through the grain boundaries of the internal oxide layer in contact with the galvannealed hot-dip coated layer. As a result, the time it takes for molten zinc to reach the grain boundaries of the cold-rolled steel sheet during welding can be delayed, improving the resistance weld cracking resistance of the weld. (I 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: Ar gas pressure 600 Pa, high-frequency output 35 W, measurement diameter 4 mm, sampling interval 0.1 s. The average Si intensity was the average of all Si intensities sampled within each range. After analyzing the Fe-based electroplated and non-galvanized cold-rolled steel sheets using glow discharge optical emission spectroscopy under the same conditions, the sputtering rate was calculated by measuring the depth of the sputtering marks. The horizontal axis of the wavelength intensity profile representing 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 sputtering rate of the galvannealed layer. The sputtering rate of the galvannealed layer differs from that of the Fe-based electroplated layer and the cold-rolled steel sheet. In other words, the standard for depth conversion differs 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 is explained using Figure 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 the 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, moving +0.5 μm in the sheet thickness direction from the interface between the galvannealed layer and the cold-rolled steel sheet toward the cold-rolled steel sheet, the average Si intensity (I) is calculated from the interface L1 between the galvannealed layer and the cold-rolled steel sheet to a position L2 that is +0.5 μm away. Si,Fe ) is calculated. Because the sputtering rate in the galvannealed layer differs from that 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 seen visually in cross-sectional observation. Furthermore, it is generally known that measurements using glow discharge optical emission spectroscopy produce broad profiles at interfaces consisting of two or more materials due to factors such as unevenness and uneven spatter. Therefore, here, the depth in the sheet thickness direction at which the average Zn intensity (IZn) within a range of 0.5±0.1 μm from the surface of the galvannealed layer in the sheet thickness direction is divided by 2, yielding 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 results of the analysis of the emission intensity of wavelengths representing Si and Zn observed in Example 1 (Table 2) described later, for Comparative Example No. 36 (no Fe-based electroplating, dew point B of annealing atmosphere: +7.4°C), Invention Example No. 41 (Fe-based electroplating coating mass A: 3.5 g / m 2 , dew point B: +7.1°C), and invention example No. 45 (Fe-based electroplating coating weight 4.2 g / m 2 , dew point B: +7.6°C). Si,Fe ) / (I Si,bulk On the other hand, in Examples 41 and 45, (I Si,Fe ) / (I Si,bulk ) were 0.80 and 0.77, respectively.

[0133] [C concentration in CR, GI and GA] To further improve 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 electroplated 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 electroplated 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 surface of the steel sheet 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, this 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, this 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 steel sheets of the present embodiment, by forming an Fe-based electroplated layer and then annealing, the average C concentration in the 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 steel sheets of the present embodiment, by forming an Fe-based electroplated layer and then annealing, the average C concentration in the 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 other metal elements alone, the solid solubility of C in these metal elements is extremely low, and C does not dissolve in these metal elements, so the effect of promoting decarburization cannot be obtained.

[0136] The reasons why the C concentration decreases in the area 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 the reasons why the C concentration decreases in the area 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 inventors speculate as follows: Namely, it is thought that this is because the Fe-based electroplated layer contains almost no C, and the diffusion of C from the cold-rolled steel sheet is induced.

[0137] Furthermore, softening by reducing the C concentration in the area of ​​10 μm to 20 μm in the sheet thickness direction from the surface of the Fe-based electroplated layer or the interface between the galvannealed layer and the cold-rolled steel sheet saturates the C concentration in the area 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 cracking resistance due to softening. In this embodiment, by lowering the C concentration in the area of ​​10 μm to 20 μm in the sheet thickness direction from the surface of the Fe-based electroplated layer or the interface between the galvannealed layer and the cold-rolled steel sheet, resistance weld cracking resistance is effectively improved even when the decarburized layer is shallow, suggesting that in addition to softening, other effects, such as an increase in melting point due to the reduced C concentration, may also be occurring.

[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 in the case of CR and GI, and from the interface between the galvannealed layer and the cold-rolled steel sheet in the case of GA, i.e., the thickness of the decarburized layer, is preferably 30 μm or more, and more preferably 80 μm or more. There is no particular upper limit to the thickness of the decarburized layer, but to maintain tensile strength within a favorable range, the thickness of the decarburized layer is preferably 130 μm or less. 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 steel sheets is analyzed in the sheet thickness direction from the surface of the Fe-based electroplated layer. In the case of 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 the C concentration is 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 carbon concentration in the thickness direction of the Fe-based electroplated layer in CR and GI, or in the area 10 to 20 μm 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, were measured by area or line analysis of the element distribution near the surface of the cross-sectioned specimen using an electron probe microanalyzer (EPMA). First, the resin-embedded steel sheet was polished to prepare a cross section perpendicular to the rolling direction for observation, and then removed from the resin to prepare the measurement specimen. The accelerating voltage was 7 kV and the probe current was 50 nA. Area or line analysis of the specimen cross section was performed in 1 μm increments over a 300 × 300 μm area, including the outermost layer of the Fe-based electroplated layer in CR and GI, and the outermost layer of the cold-rolled steel sheet in GA, to measure the carbon intensity. To prevent contamination, a plasma cleaner is used to remove hydrocarbons from the surface and surrounding area of ​​the sample before starting the measurement in both the measurement chamber and the sample preparation chamber. Furthermore, to prevent hydrocarbon accumulation during the measurement, the sample is heated and maintained at 100°C on the stage during the measurement. A calibration curve, prepared by measuring a separate standard sample, is used to convert the C intensity into a C concentration (mass%). Due to the effect of contamination prevention, it is confirmed that the lower limit of C detection is lower than 0.04 mass%. Details of the equipment used and the contamination prevention method are explained in Reference 1 below.

[0140] Reference 1: Yamashita et al., "Carbon partitioning during 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 of contamination prevention measures during measurement depends on the model and conditions used, so the above configuration is not necessarily required. In other words, the measurement conditions only need to be sufficient to ensure sufficient accuracy, and the measurement conditions are not essentially related to the effects of the present invention.

[0141] From the obtained carbon concentration map, a line profile in the sheet thickness direction was 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 was then averaged over 300 points in the direction parallel to the cold-rolled steel sheet surface to obtain a carbon concentration profile in the sheet thickness direction. The position of the cold-rolled steel sheet surface can be determined from the secondary electron image or backscattered electron image obtained at the same time. The obtained carbon concentration profile in the sheet thickness direction was smoothed using the simple moving average method. 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 smooth only the available measurement points on one side. Next, in the strength profile after smoothing, the thickness direction range where the carbon concentration is 80% or less of the total carbon concentration in the steel is identified 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 in the surface layer of the cold-rolled steel sheet for GA, and this range is determined as the thickness of the decarburized layer. For CR and GI, the C concentration values ​​at 11 points at 1 μm intervals were taken from the surface of the Fe-based electroplated layer, and for GA, the C concentration was taken from the interface between the galvannealed layer and the cold-rolled steel sheet in the thickness direction of 10 μm to 20 μm, and this was taken as the C concentration for the thickness direction of 10 μm to 20 μm. 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 for the thickness direction of 10 μm to 20 μm, and the evaluation value for the thickness of the decarburized layer.

[0142] Representative examples of C concentration profiles through the sheet thickness direction analyzed with an electron probe microanalyzer will be described using Figures 10A and 10B and Figures 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 Fig. 10B and Fig. 11B, in No. 27 in Table 4 and No. 45 in Table 2, there were decarburized layers in which the C concentration was 80% or less of the total carbon concentration in the steel, and the thicknesses of the decarburized layers were 77 µm and 81 µm, respectively.

[0143] [Grains in CR and GI] In the CR and GI of this embodiment, 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 is preferably 10 or more per 10 μm in the width direction of the steel sheet in an 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] 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 is measured as follows. First, a 10 × 10 mm sample is taken from the Fe-based electroplated steel sheet. A 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 direction 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 overview of the observation sample. FIG. 6(a) is a perspective view of the observation sample. FIG. 6(b) is an AA cross section 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 and length directions at 4000x magnification, and SIM images were taken. An example of such a SIM image is shown in FIG. 7. FIG. 7 is a SIM image of No. 11 of Example 3 (Table 11), which will be described later, taken as described above. 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 explanatory purposes, FIG. 8 shows an enlarged view of the area surrounded by a square in FIG. 7. As shown in FIG. 8, in the SIM image, a boundary line (a dashed line in FIG. 8) was drawn at the interface between the Fe-based electroplated layer and the cold-rolled steel sheet in the 10 μm region in the sheet width direction of the cold-rolled steel sheet. The number of grain boundaries of the Fe-based electroplated layer on the boundary line was measured, and this was defined as "the number of grain boundaries of 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 Example No. 11, 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 was 20 per 10 μm in the width direction of the cold-rolled steel sheet.

[0145] (Chemical conversion treated steel sheet and its manufacturing method) Chemical conversion treatment can be performed on Fe-based electroplated steel sheet (CR or GI) or galvannealed steel sheet (GA) to obtain a chemical conversion treated steel sheet in which a chemical conversion coating is formed on the surface of the Fe-based electroplated steel sheet or galvannealed steel sheet. In this case, as pretreatment for the chemical conversion treatment, degreasing treatment to clean the surface of the Fe-based electroplated steel sheet or galvannealed steel sheet, water rinsing, and, if necessary, surface conditioning treatment can be performed. These pretreatments are followed by the chemical conversion treatment. The degreasing and water rinsing methods are not particularly limited, and conventional methods can be used. Surface conditioning treatment can be performed using a surface conditioner containing Ti colloid or zinc phosphate colloid, for example. No special process is required for applying these surface conditioners; they can be performed according to conventional methods. For example, a desired surface conditioner is dissolved in a predetermined amount of deionized water and thoroughly stirred. After the treatment, 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). The next step, chemical conversion treatment, is then carried out without drying. 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 amount of deionized water, thoroughly stirred, and then heated to a predetermined temperature (usually 35 to 45°C) to prepare a treatment solution. The steel sheet is then 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, the chemical conversion treated steel sheet is subjected to electrodeposition coating to obtain 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 conventional methods. After performing pretreatment such as water washing if 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 top coat 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] (Automobile parts and their manufacturing methods) 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 in welded joints, and therefore, the electrodeposition-coated steel sheet using the Fe-based electroplated steel sheet and galvannealed steel sheet is particularly suitable for application to automotive parts. The type of automotive part is not particularly limited, but may be, for example, a side sill part, a pillar part, an automotive body, or the like. [Example]

[0148] Example 1 Steel having the chemical composition shown in Table 1 (the balance being Fe and unavoidable impurities) was melted and cast, and the resulting slab 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.

[0149] [Table 1]

[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 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 2 to 4 was calculated by the method described above and controlled by the current application time. [Electrolysis conditions] Bath temperature: 50℃ pH: 2.0 Current density: 45A / dm 2 Plating bath: Fe 2+ Sulfuric acid bath containing 1.5 mol / L of ions Anode: Iridium oxide electrode

[0152] The Fe-based electroplated steel sheet was then heated at an average heating rate in the temperature range of 400°C to 650°C, both inclusive, at the values ​​shown in Tables 2 to 4. Subsequently, the steel sheet was annealed by heating it to a soaking zone temperature of 800°C in a reducing atmosphere having an atmospheric dew point B shown in Tables 2 to 4, containing 15% by volume of hydrogen, with the remainder consisting of N2 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, the Fe-based electroplated steel sheet (CR) was 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 hot-dip galvanization using a 460°C hot-dip galvanizing bath with an effective Al concentration of 0.132 mass% 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 sheet (GI) was obtained in this manner.

[0154] In the examples shown in Table 2, a subsequent step of heating and alloying the hot-dip galvanized layer in an 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 for 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 average C concentration, and the thickness of the decarburized layer. Tables 3 and 4 show the thickness 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 steel sheet width direction, all determined by the methods described above.

[0157] For the GA and GI examples, the appearance of the zinc-plated layer (evaluation 1) and resistance weld crack resistance at the welded joint (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 joint (evaluation 3) were evaluated, and the results are shown in Table 4.

[0158] [Evaluation 1: Evaluation of the appearance of the zinc-plated layer (hot-dip galvanized layer or galvannealed hot-dip galvanized layer)] The presence or absence of appearance defects (bare spots, uneven appearance) was visually determined and evaluated according to the following criteria. ○: No defects in appearance. △: Although there is a poor appearance, when observed with an SEM at 3000x, the appearance is uneven and no exposed Fe-based electroplated layer or cold-rolled steel sheet is observed. ×: Poor appearance, unplated with exposed Fe-based electroplated layer or cold-rolled steel sheet when observed at 3000x with an SEM. The presence or absence of exposed Fe-based electroplated layer or cold-rolled steel sheet can be determined by the difference in contrast between Zn and Fe under SEM, and more specifically, by whether Fe is detected in EDX (Energy Dispersive X-ray Spectroscopy) analysis.

[0159] [Evaluation 2: Evaluation of chemical conversion treatment properties and corrosion resistance after painting] (1) Chemical conversion treatment Test pieces taken from the Fe-based electroplated steel sheets were subjected to degreasing, surface conditioning, and chemical conversion treatment to produce chemically 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 Fe-based electroplated steel sheets were immersed in a degreasing agent and subjected to degreasing treatment under the following standard conditions. [Degreasing process] Degreasing agent: FC-E2011 (Nihon Parkerizing Co., Ltd.) Processing temperature: 43°C Processing time: 120 seconds

[0160] Next, a surface conditioner was sprayed onto the degreased test piece, and the surface was conditioned under the following standard conditions. [Surface conditioning treatment] Surface conditioner: Preparen XG (PL-XG; manufactured by Nihon Parkerizing Co., Ltd.) pH: 9.5 Processing temperature: Room temperature Processing 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.) Chemical conversion treatment solution temperature: 35℃ Processing 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 The surfaces of the chemically treated test pieces were subjected to electrodeposition coating to a film thickness of 15 μm using GT-100 electrodeposition paint manufactured by Kansai Paint Co., Ltd. The electrodeposition coated test pieces were subjected to the hot salt water immersion test described below.

[0164] <Chemical conversion treatment> The surface of the chemical conversion treated test piece (n=1) was observed under an SEM at a magnification of 1000 times and evaluated according to the following criteria: ⊚ or ◯ was judged to be excellent in chemical conversion treatment. ◎: The grain size of the formed crystals is 5 μm or less and no unprecipitated areas are observed. ○: The grain size of the formed crystals exceeds 5 μm, but no unprecipitated areas are observed. ×: The grain size of the formed crystals exceeds 5 μm and unprecipitated portions are observed.

[0165] <Hot salt water immersion test> A 45 mm long cross-cut scratch was made on the surface of each 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. A tape peel test was then conducted in which cellophane tape was applied to the cross-cut scratch on the test piece and then peeled off, and the maximum overall peeled width of the electrodeposition coated film, measured by combining the left and right sides of the cross-cut scratch, was measured. The maximum overall peeled width of the electrodeposition coated film was evaluated according to the following criteria. A rating of ⊚ or ○ indicated excellent post-painting corrosion resistance. ◎: Maximum peel width is 3.0mm or less ○: Maximum peel width is 5.0 mm or less ×: Maximum peel width exceeds 5.0 mm

[0166] [Evaluation 3: Evaluation of resistance weld crack resistance in welded joints] 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 transverse direction being the rolling direction (TD) as the longitudinal direction. The test pieces 6 were cut out to the same size, and the coating weight of the zinc coating layer per side was 50 g / m 2 The test specimen 6 was placed on a galvannealed test steel sheet 5 (thickness: 1.6 mm, TS: 980 MPa) 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-coated layer in the case of CR, and the galvanized layer in the case of GI and GA) faced the galvanized layer of the test galvannealed test steel sheet 5. The sheet assembly was fixed to a fixing base 8 via a 2.0 mm-thick spacer 7. The spacer 7 was a pair of steel sheets 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 spacer 7 was positioned so that the longitudinal end faces of each of the pair of steel sheets were aligned with the transverse end faces of the sheet assembly. Therefore, the distance between the pair of steel sheets was 60 mm. The fixing base 8 was a single plate with a hole in the center. Next, using a servomotor-operated, single-phase AC (50 Hz) resistance welding machine, the sheet assembly was pressed with a pair of electrodes 9 (tip diameter: 6 mm) while bending the sheet assembly. 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 fixture 8 were fixed so that the lower electrode of the pair of electrodes 9 was in contact with the plane extending from the contact surface between the spacer 7 and 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 is finished flowing to when the electrodes start to be released. Here, referring to the lower diagram of Figure 4(b), the nugget diameter r means the distance between the ends of the nugget 10 in the longitudinal direction of the sheet assembly. Next, the plate assembly with the weld was cut along line BB in the upper diagram of Figure 4(b) so as to include the center of the weld including the nugget 10, and the cross section of the weld was observed with an optical microscope (200x magnification) and the resistance weld crack resistance of the weld was evaluated according to the following criteria. A rating of ◎ or ○ indicated that the resistance weld crack resistance of the weld was excellent. A rating of × indicated that the resistance weld crack resistance of the weld was poor. ◎: No cracks longer than 0.1 mm were observed with a hold time of 0.18 seconds. ◯: Cracks of 0.1 mm or more in length are observed at a hold time of 0.18 seconds, but no cracks of 0.1 mm or more in length are observed at a hold time of 0.24 seconds. ×: A crack of 0.1 mm or more in length was observed at a hold time of 0.24 seconds. In the lower diagram of Fig. 4(b), a crack that occurred in the test piece 6 is shown schematically as reference numeral 11. If a crack occurs in the counterpart 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) will be dispersed, making it impossible to perform an appropriate evaluation. For this reason, data in which no cracks occurred in the counterpart steel sheet was used as an example.

[0167] [Table 2] TIFF2026034821000004.tif245161

[0168] [Table 3]

[0169] [Table 4]

[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 achieve 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 achieve excellent resistance weld cracking resistance.

[0172] The results in Figure 1 (GA) show that, when compared at the same dew point, increasing the coating weight A of the Fe-based electroplated layer before annealing improves resistance weld cracking resistance. Also, the boundary line (solid line) that separates whether or not resistance weld cracking resistance is improved is when the coating weight A of the Fe-based electroplated layer before annealing is 1.0 g / m 2 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 range. With GA, the Fe-based electroplated layer is completely alloyed with the zinc coating and no longer remains, so it is presumed that 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 amount of Fe-based electroplated layer or more and a high dew point.

[0173] On the other hand, in the case of GI and CR, the Fe-based electroplated layer remains, and this layer acts as a soft surface phase. When compared at the same dew point and with the same amount of Fe-based electroplated layer before annealing, it is estimated that the resistance weld cracking resistance is 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] [Table 5]

[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℃ pH: 2.0 Current density: 45A / dm 2 Plating bath: Fe 2+ Sulfuric acid bath containing 1.5 mol / L of ions Anode: Iridium oxide electrode

[0178] The Fe-based electroplated steel sheet was then heated at an average heating rate in the temperature range of 400°C to 650°C, both inclusive, at the values ​​shown in Tables 6 to 8. Subsequently, the steel sheet was annealed by heating it to a soaking zone temperature of 800°C in a reducing atmosphere having an atmospheric dew point B shown in Tables 6 to 8, containing 15% by volume of hydrogen, with the remainder consisting of N2 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, the Fe-based electroplated steel sheet (CR) was obtained in this manner.

[0179] In the examples shown in Tables 6 and 7, 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 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 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 7, hot-dip galvanized steel sheet (GI) was obtained in this manner.

[0180] In the examples shown in Table 6, a subsequent step of heating and alloying the hot-dip galvanized layer was further carried out in an alloying treatment at 510°C 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 for 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 average C concentration, and the thickness of the decarburized layer. Tables 7 and 8 show the thickness 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, all of which were determined by the methods described above.

[0183] For the GA and GI examples, the appearance of the zinc-coated layer (evaluation 1) and resistance weld crack resistance at the welded joint (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 joint (evaluation 3) were evaluated, and the results are shown in Table 8. The evaluation methods and evaluation criteria for evaluations 1 to 3 were the same as those for Example 1.

[0184] [Table 6]

[0185] [Table 7]

[0186] [Table 8]

[0187] As is clear from Tables 6 and 7, the invention examples in the examples of GA and GI had good appearance of the zinc plating layer and were able to achieve excellent resistance weld cracking resistance. Also, as is clear from Table 8, the invention 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.

[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 slab 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 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 9 to 11 was calculated by the method described above and controlled by the current application time. [Electrolysis conditions] Bath temperature: 50℃ pH: 2.0 Current density: 45A / dm 2 Plating bath: Fe 2+ Sulfuric acid bath containing 1.5 mol / L of ions Anode: Iridium oxide electrode

[0191] The Fe-based electroplated steel sheet was then heated at an average heating rate in the temperature range of 400°C to 650°C, both inclusive, at the values ​​shown in Tables 9 to 11. Subsequently, the steel sheet was annealed by heating it to a soaking zone temperature of 800°C in a reducing atmosphere having an atmospheric dew point B shown in Tables 9 to 11, containing 15% by volume of hydrogen, with the remainder consisting of N2 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 sheet (CR) was obtained in this manner.

[0192] In the examples shown in Tables 9 and 10, 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 hot-dip galvanization using a 460°C hot-dip galvanizing bath with an effective Al concentration of 0.132 mass% 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 10, hot-dip galvanized steel sheet (GI) was obtained in this manner.

[0193] In the examples shown in Table 9, a subsequent step of heating and alloying the hot-dip galvanized layer was further carried out in an alloying treatment at 510°C 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 for 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 average C concentration, and the thickness of the decarburized layer. Tables 10 and 11 show the thickness 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 steel sheet width direction, all of which were determined by the methods described above.

[0196] For the GA and GI examples, the appearance of the zinc-coated layer (evaluation 1) and resistance weld crack resistance at the welded joints (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 joints (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] [Table 9]

[0198] [Table 10]

[0199] [Table 11]

[0200] As is clear from Tables 9 and 10, the GA and GI examples of the invention had good zinc plating layer appearance and excellent resistance weld cracking resistance. Also, as is clear from Table 11, the CR examples of the invention had good chemical conversion treatability and corrosion resistance after painting and 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℃ pH: 2.0 Current density: 45A / dm 2 Plating bath: Fe 2+ Sulfuric acid bath containing 1.5 mol / L of ions Anode: Iridium oxide electrode

[0204] The Fe-based electroplated steel sheet was then heated at an average heating rate in the temperature range of 400°C to 650°C, both inclusive, at the values ​​shown in Tables 12 to 14. Subsequently, the steel sheet was annealed by heating it to a soaking zone temperature of 800°C in a reducing atmosphere having an atmospheric dew point B shown in Tables 12 to 14, containing 15% by volume of hydrogen, with the remainder consisting of N2 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, the Fe-based electroplated steel sheet (CR) was obtained in this manner.

[0205] In the examples shown in Tables 12 and 13, 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 hot-dip galvanization using a 460°C hot-dip galvanizing bath with an effective Al concentration of 0.132 mass% 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 13, hot-dip galvanized steel sheets (GI) were obtained in this manner.

[0206] In the examples shown in Table 12, a subsequent step of heating and alloying the hot-dip galvanized layer in an 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.

[0207] Tables 12 and 13 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.

[0208] Table 12 shows the results obtained by the method described above (I Si,Fe ) / (I Si,bulk ), the average C concentration, and the thickness of the decarburized layer. Tables 13 and 14 show the thickness 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, all of which were determined by the methods described above.

[0209] For the GA and GI examples, the appearance of the zinc-coated layer (evaluation 1) and resistance weld crack resistance at the welded joints (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 joints (evaluation 3) were evaluated, and the results are shown in Table 14. The evaluation methods and evaluation criteria for Evaluations 1 to 3 were the same as those for Example 1.

[0210] [Table 12]

[0211] [Table 13]

[0212] [Table 14]

[0213] As is clear from Tables 12 and 13, the GA and GI examples of the invention had good zinc plating layer appearance and excellent resistance weld cracking resistance. Also, as is clear from Table 14, the CR examples of the invention had good chemical conversion treatability and corrosion resistance after painting and also 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] [Table 15]

[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℃ pH: 2.0 Current density: 45A / dm 2 Plating bath: Fe 2+ Sulfuric acid bath containing 1.5 mol / L of ions Anode: Iridium oxide electrode

[0218] The Fe-based electroplated steel sheet was then heated at an average heating rate in the temperature range of 400°C to 650°C, both inclusive, at the values ​​shown in Tables 16 to 18. Subsequently, the steel sheet was annealed by heating it to a soaking zone temperature of 800°C in a reducing atmosphere having an atmospheric dew point B shown in Tables 16 to 18, containing 15% by volume of hydrogen, with the remainder consisting of N2 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 sheet (CR) was 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 hot-dip galvanizing using a 460°C hot-dip galvanizing bath with an effective Al concentration of 0.132 mass% 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 17, hot-dip galvanized steel sheets (GI) were obtained in this manner.

[0220] In the examples shown in Table 16, a subsequent step of heating and alloying the hot-dip galvanized layer was further carried out in an alloying treatment at 510°C 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 for 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 average C concentration, and the thickness of the decarburized layer. Tables 17 and 18 show the thickness 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 steel sheet width direction, all of which were determined by the methods described above.

[0223] For the GA and GI examples, the appearance of the zinc-coated layer (evaluation 1) and resistance weld crack resistance at the welded joints (evaluation 3) were evaluated, and the results are shown in Tables 16 and 17. For the CR example, the chemical conversion treatability and corrosion resistance after painting (evaluation 2) and resistance weld crack resistance at the welded joints (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 in welded joints] 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 transverse direction being the rolling direction (TD) as the longitudinal direction. The test pieces 6 were cut out to the same size, and the coating weight of the zinc coating layer per side was 50 g / m 2The test specimen 6 was placed on a galvannealed test steel sheet 5 (thickness: 1.6 mm, TS: 980 MPa) 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-coated layer in the case of CR, and the galvanized layer in the case of GI and GA) faced the galvanized layer of the test galvannealed test steel sheet 5. The sheet assembly was fixed to a fixing base 8 via a 2.0 mm-thick spacer 7. The spacer 7 was a pair of steel sheets 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 spacer 7 was positioned so that the longitudinal end faces of each of the pair of steel sheets were aligned with the transverse end faces of the sheet assembly. Therefore, the distance between the pair of steel sheets was 60 mm. The fixing base 8 was a single plate with a hole in the center. Next, using a servomotor-operated, single-phase AC (50 Hz) resistance welding machine, the sheet assembly was pressed with a pair of electrodes 9 (tip diameter: 6 mm) while bending the sheet assembly. 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 fixture 8 were fixed so that the lower electrode of the pair of electrodes 9 was in contact with the plane extending from the contact surface between the spacer 7 and 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° longitudinally relative to the horizontal. The hold time refers to the time from when the welding current is finished flowing to when the electrodes start to be released. Here, referring to the lower diagram of Figure 4(b), the nugget diameter r means the distance between the ends of the nugget 10 in the longitudinal direction of the sheet assembly. Next, the plate assembly with the weld was cut along line BB in the upper diagram of Figure 4(b) so as to include the center of the weld including the nugget 10, and the cross section of the weld was observed with an optical microscope (200x magnification) and the resistance weld crack resistance of the weld was evaluated according to the following criteria. A rating of ◎ or ○ indicated that the resistance weld crack resistance of the weld was excellent. A rating of × indicated that the resistance weld crack resistance of the weld was poor. ◎: No cracks longer than 0.1 mm were observed with a hold time of 0.14 seconds. ◯: Cracks of 0.1 mm or more in length are observed at a hold time of 0.14 seconds, but no cracks of 0.1 mm or more in length are observed at a hold time of 0.16 seconds. ×: A crack of 0.1 mm or more in length was observed at a hold time of 0.16 seconds. In the lower diagram of Fig. 4(b), a crack that occurred in the test piece 6 is shown schematically as reference numeral 11. If a crack occurs in the counterpart 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) will be dispersed, making it impossible to perform an appropriate evaluation. For this reason, data in which no cracks occurred in the counterpart steel sheet was used as an example.

[0225] [Table 16]

[0226] [Table 17]

[0227] [Table 18]

[0228] As is clear from Tables 16 and 17, the invention examples in the examples of GA and GI had good appearance of the zinc plating layer and were able to achieve excellent resistance weld cracking resistance. Also, as is clear from Table 18, the invention 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.

[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℃ pH: 2.0 Current density: 45A / dm 2 Plating bath: Fe 2+ Sulfuric acid bath containing 1.5 mol / L of ions Anode: Iridium oxide electrode

[0232] The Fe-based electroplated steel sheet was then heated at an average heating rate in the temperature range of 400°C to 650°C, both inclusive, at the values ​​shown in Tables 19 to 21. Subsequently, the steel sheet was annealed by heating it to a soaking zone temperature of 800°C in a reducing atmosphere having an atmospheric dew point B shown in Tables 19 to 21, containing 15% by volume of hydrogen, with the balance being N 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 sheet (CR) was obtained in this manner.

[0233] In the examples shown in Tables 19 and 20, 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 hot-dip galvanization using a 460°C hot-dip galvanizing bath with an effective Al concentration of 0.132 mass% 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 20, hot-dip galvanized steel sheets (GI) were obtained in this manner.

[0234] In the examples shown in Table 19, a subsequent step of heating and alloying the hot-dip galvanized layer was further carried out in an alloying treatment at 510°C 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 average C concentration, and the thickness of the decarburized layer. Tables 20 and 21 show the thickness 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 steel sheet width direction, all of which were determined by the methods described above.

[0237] For the GA and GI examples, the appearance of the zinc-coated layer (evaluation 1) and resistance weld crack resistance at the welded joints (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 joints (evaluation 3) were evaluated, and the results are shown in Table 21. The evaluation methods and criteria for evaluations 1 to 3 were the same as those for Example 5.

[0238] [Table 19]

[0239] [Table 20]

[0240] [Table 21]

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

[0242] The Fe-based electroplated steel sheet of the present invention not only exhibits excellent chemical conversion treatability or coating appearance after hot-dip galvanizing, but also has excellent resistance weld crack resistance. Furthermore, the galvannealed steel sheet of the present invention not only exhibits excellent coating 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. [Explanation of symbols]

[0243] 1. Fe-based electroplated steel sheet 2 Cold rolled steel plate 3. Fe-based electroplating layer 5. Galvannealed steel sheets for testing 6 Test pieces 7 spacers 8 Fixed stand 9 electrodes 10 Nuggets 11 Crack

Claims

1. A cold-rolled steel sheet having a component composition containing 0.1 mass% or more and 3.0 mass% or less of Si and 0.05 mass% or more and 0.8 mass% or less of C; a galvannealed layer formed on one or both sides of the cold-rolled steel sheet; and wherein no Fe-based electroplating layer is present between the galvannealed layer and the cold-rolled steel sheet, In 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 spectrometry, the 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, an average 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 a sheet thickness direction is 0.04 mass% or less, The surface layer of the cold-rolled steel sheet is a decarburized layer, and the thickness of the decarburized layer is 80 μm or more. Galvannealed steel sheet.

2. The cold-rolled steel sheet has a composition, in mass%, of C: 0.05% or more and 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 The galvannealed steel sheet according to claim 1, comprising:

3. The component composition further comprises, in mass%, 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 The galvannealed steel sheet according to claim 2, further comprising at least one element selected from the group consisting of:

4. The galvannealed steel sheet according to any one of claims 1 to 3, wherein, in the chemical composition, the amount of Si is 0.9 mass% or more and 1.7 mass% or less.

5. 5. The galvannealed steel sheet according to claim 1, 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.

6. A chemical conversion treated steel sheet having a chemical conversion coating formed on a surface of the galvannealed layer of the galvannealed steel sheet according to any one of claims 1 to 5.

7. 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 claim 6.

8. An automobile part, at least in part of which is made of the electrodeposition coated steel sheet according to claim 7.

9. A method for producing the galvannealed steel sheet according to claim 1, A cold-rolled steel sheet having a composition containing 0.1 mass% or more and 3.0 mass% or less of Si and 0.05 mass% or more and 0.8 mass% or less of C is subjected to Fe-based electroplating without annealing, and a coating weight 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 step of obtaining an Fe-based electroplated steel sheet having the above Fe-based electroplated layer formed thereon; Thereafter, an 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)'; Thereafter, the Fe-based electroplated steel sheet is subjected to hot-dip galvanization to form an unalloyed hot-dip galvanized layer on the surface of the Fe-based electroplated layer; Thereafter, the hot-dip galvanized layer is heated and alloyed to obtain a galvannealed steel sheet in which a galvannealed layer is formed on one side or both sides of the cold-rolled steel sheet; A method for producing a galvannealed steel sheet having the above structure. A + B ≧10.0...(2)'

10. 10. The method for producing a galvannealed steel sheet according to claim 9, further comprising, before the annealing step, a step of heating the Fe-based electroplated steel sheet at an average heating rate of 10°C / second or more in a temperature range of 400°C or higher and 650°C or lower.

11. 10. The method for producing a galvannealed steel sheet according to claim 9, further comprising, before the annealing step, a step of heating the Fe-based electroplated steel sheet at an average heating rate of 15°C / second or more in a temperature range of 400°C or higher and 650°C or lower.

12. The deposition amount A is 5.0 g / m ２ The method for producing a galvannealed steel sheet according to any one of claims 9 to 11, wherein the total mass of the galvannealed steel sheet is less than 10 ...

13. The cold-rolled steel sheet has a composition, in mass%, of C: 0.05% or more and 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 The method for producing a galvannealed steel sheet according to any one of claims 9 to 12, comprising the remainder consisting of Fe and unavoidable impurities.

14. The component composition further comprises, in mass%, 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 The method for producing a galvannealed steel sheet according to claim 13, wherein the galvannealed steel sheet contains at least one element selected from the group consisting of:

15. The method for producing a galvannealed steel sheet according to any one of claims 9 to 14, wherein, in the component composition, the Si content is 0.9 mass% or more and 1.7 mass% or less.

16. The method for producing a galvannealed steel sheet according to any one of claims 9 to 15, 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 a total content of this element in the galvannealed layer is 1 mass% or less.

17. A method for producing a galvannealed steel sheet according to any one of claims 9 to 16; Thereafter, a step of subjecting 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 galvannealed steel sheet; A method for producing a chemically treated steel sheet having the above structure.

18. The method for producing a chemically treated steel sheet according to claim 17; a step of applying electrodeposition coating to the chemical conversion treated steel sheet to obtain an electrodeposition coated steel sheet having an electrodeposition coated film formed in contact with the chemical conversion treatment film; A method for producing an electrodeposition coated steel sheet comprising the steps of:

19. The method for producing an electrodeposition coated steel sheet according to claim 18; a step of manufacturing an automobile part using the electrodeposition coated steel sheet as a part; A method for manufacturing an automobile part having the above structure.

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