Fe-based electrogalvanized steel sheet and galvannealed steel sheet and method for manufacturing the same

Abstract

The present invention provides an Fe-based plated steel sheet which is excellent not only in chemical conversion treatment properties or plating appearance when hot-dip galvanizing is performed, but also in resistance welding crack resistance. The Fe-based plated steel sheet of the present invention has: a cold-rolled steel sheet having a composition containing 0.1 to 3.0 mass% of Si, and an Fe-based plating layer formed on one side or both sides of the cold-rolled steel sheet, the plating layer having an adhesion amount of 1.0 g / m 2 The above Fe-based plated layer; the thickness of the internal oxide layer is 2.00 μm or less, and the average value of the C concentration in the range of 10 to 20 μm from the surface of the Fe-based plated layer toward the sheet thickness direction is 0.10 mass% or less.

Description

TECHNICAL FIELD

[0001] The present application relates to Fe-based electroplated steel sheets and galvannealed steel sheets, and a method for producing the same. Note that in the present application, "Fe-based electroplated steel sheets" include both (i) simple Fe-based electroplated steel sheets (hereinafter also sometimes referred to as "CR") obtained by performing Fe-based electroplating on a cold-rolled steel sheet and (ii) hot-dip galvanized steel sheets (hereinafter also sometimes referred to as "GI") in which the hot-dip galvanized layer is not alloyed, obtained by performing hot-dip galvanizing on the simple Fe-based electroplated steel sheets of (i) above. In addition, in the present application, "galvannealed steel sheets" refer to galvannealed steel sheets (hereinafter also sometimes referred to as "GA") in which the hot-dip galvanized layer of the hot-dip galvanized steel sheets of (ii) above is heated and alloyed. BACKGROUND

[0002] In recent years, from the viewpoint of protecting the global environment, improving the fuel efficiency of automobiles has become an important issue. Therefore, measures to make the steel sheets used as materials for automobile members high-strength and thin, thereby making automobile bodies lightweight, are becoming more active. However, high-strength of steel sheets leads to a decrease in formability, and thus development of a steel sheet that has both high-strength and high-formability is desired.

[0003] As a method for achieving high-strength without significantly impairing the formability of steel sheets, solid solution strengthening by adding Si to the steel sheet can be cited. However, Si added for high-strength of the steel sheet forms an oxide on the surface of the steel sheet in annealing. In the case where the steel sheet is used without performing hot-dip galvanizing on the steel sheet, the oxide deteriorates the chemical conversion treatment properties. In addition, when hot-dip galvanizing is performed on the steel sheet to produce a hot-dip galvanized steel sheet, the oxide deteriorates the wettability of molten zinc with the steel sheet, and uncoating occurs. For the purpose of securing such chemical conversion treatment properties or plating appearance when hot-dip galvanizing is performed, a technique of performing Fe-based electroplating (Fe-based pre-plating) on the surface of the steel sheet before annealing of the steel sheet is known.

[0004] Patent Document 1 describes a method for producing a hot-dip galvanized steel sheet, including: "a stage of preparing a base steel sheet; a stage of forming an Fe-plated layer having a plating adhesion amount of 0.2 to 2 g / m 2 2 of Patent Document 1; a stage of oxidizing and heating the steel sheet on which the Fe-plated layer is formed at 600 to 800°C; a stage of maintaining the heated steel sheet in a reducing atmosphere containing oxygen at 20 ppm or less, H2: 1 to 20 vol%, the remaining portion being N2 and other unavoidable gases, and having a dew point of -30 to 5°C, at 750 to 900°C for 5 seconds or more; a stage of cooling the maintained steel sheet; and a stage of immersing the cooled steel sheet in a hot-dip galvanizing bath at 445 to 480°C to perform plating (claim 1)".

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

[0006] Prior Art Documents

[0007] Patent Document

[0008] Patent Document 1: Japanese Patent No. 6025867

[0009] Patent Document 2: International Publication No. 2019 / 116531 SUMMARY

[0010] In the production of automobile parts, the press-formed parts are often combined by resistance welding (spot welding). In the case where at least one of the parts subjected to resistance welding contains a hot-dip galvanized steel sheet, during resistance welding, zinc of the plated layer is molten and diffuses into the grain boundaries of the steel sheet constituting each part in a state where residual stress is generated in the vicinity of the welded portion, thereby causing liquid metal embrittlement (LME) and possibly generating a grain boundary crack (LME crack) in the steel sheet. In particular, if welding is performed in a state where an electrode for welding exists at an angle with respect to the steel sheet, the residual stress can increase and a crack can be generated. It is considered that the residual stress increases as the steel sheet is strengthened, and thus an LME crack can be generated as the steel sheet is strengthened by the addition of Si.

[0011] However, according to the research by the present inventors and others, it was found that in the production method of the hot-dip galvanized steel sheet described in Patent Document 1, although it is possible to prevent non-plating and ensure an excellent plated surface appearance and plating adhesion, the resistance welding crack resistance characteristics are insufficient. In addition, it was found that in the steel sheet described in Patent Document 2, because the depth of the internal oxidation layer, i.e., grain boundary oxidation, is too large, the resistance welding crack resistance characteristics are still insufficient.

[0012] Therefore, the present application aims to provide an Fe-based plated steel sheet in which not only the chemical conversion treatment properties or the plated appearance when hot-dip galvanizing are excellent, but also the resistance welding crack resistance characteristics are excellent, and a preferred production method thereof. In addition, the present application aims to provide an alloyed hot-dip galvanized steel sheet in which not only the plated appearance is excellent, but also the resistance welding crack resistance characteristics are excellent, and a preferred production method thereof.

[0013] The present inventors have made intensive studies to solve the above problem and found the following many insights. When the Fe-based plating layer having a prescribed adhesion amount A is formed for the purpose of ensuring the chemical conversion treatment property or the plating appearance when hot-dip galvanizing is performed, the sum of the adhesion amount A and the dew point B at the time of subsequent annealing is related to the resistance welding crack resistance. Specifically, in the case where A+B is a prescribed value or more, excellent resistance welding crack resistance can be exhibited.

[0014] The prescribed value of A+B is equal in the case of CR and GI. In the case of CR and GI, by forming a soft Fe-based plating layer, the stress applied to the surface of the cold-rolled steel sheet at the time of welding is mitigated. On the other hand, in the case of GA, since the Fe-based plating layer is incorporated into the hot-dip galvanized layer in the alloying step, the prescribed value of A+B rises.

[0015] First, the case of CR and GI will be described in detail. In order to satisfy the resistance welding crack resistance of the welded portion at a high level, it is important to form the Fe-based plating layer having the adhesion amount A on the cold-rolled steel sheet, and then perform the annealing step under the condition that A+B≥3.0, thereby forming an internal oxide within the Fe-based plating layer, and making the depth of the internal oxide layer in the direction of the cold-rolled steel sheet from the depth of 0.10 μm from the surface of the Fe-based plating layer be within 2.00 μm. In the case where A+B≥3.0, Si diffused from the cold-rolled steel sheet to the Fe-based plating layer at the time of annealing forms an oxide within the Fe-based plating layer. In this way, by partially forming an oxide within the Fe-based plating layer, the depth of the internal oxide layer in the direction of the cold-rolled steel sheet from the depth of 0.10 μm from the surface of the Fe-based plating layer can be made to be within 2.00 μm. Therefore, the penetration of zinc in the depth direction of the cold-rolled steel sheet when zinc reaches the grain boundary of the cold-rolled steel sheet can be suppressed as much as possible, and the resistance welding crack resistance of the welded portion can be improved.

[0016] Further, in the annealing step, the average value of the C concentration in the range of 10 μm to 20 μm in the thickness direction of the Fe-based plated steel sheet from the surface of the Fe-based plating layer is made to be 0.10 mass% or less. Thereby, the resistance welding crack resistance can be further improved. The present inventors have found that, in the case where the Fe-based plating layer is formed before annealing, the C concentration in the range of 10 μm to 20 μm in the thickness direction from the surface of the Fe-based plating layer can be further reduced, and the effect of improving the resistance welding crack resistance can be more effectively obtained.

[0017] In addition, in the heating step following the annealing (soaking) step, by making the average temperature increase rate of the Fe-based plated steel sheet in the temperature range of 400°C to 650°C be 10°C / sec or more, the growth of the crystal grains of the Fe-based plated layer in the temperature increase step is suppressed as much as possible, and the number of grain boundaries of the Fe-based plated layer that is in contact with the cold-rolled steel sheet at the interface between the Fe-based plated layer and the cold-rolled steel sheet is made to be 10 or more per 10 μm width of the cold-rolled steel sheet. Thus, the resistance welding crack resistance can be further improved. In the case where the average temperature increase rate in the temperature range of 400°C to 650°C in the temperature increase step is made to be 10°C / sec or more, the crystal grains of the Fe-based plated layer that is in contact with the cold-rolled steel sheet at the interface between the Fe-based plated layer and the cold-rolled steel sheet are made to be fine, and the invasion path of the molten zinc into the Fe-based plated layer is dispersed. That is, the time until the molten zinc reaches the grain boundaries of the cold-rolled steel sheet at the time of welding can be delayed, and the resistance welding crack resistance can be further improved.

[0018] Next, the case of the GA will be described in detail. In the case of the GA, since the Fe-based plated layer is incorporated into the hot-dip zinc layer and disappears in the alloying step, the stress relaxation effect of the Fe-based plated layer cannot be expected. In order to satisfy the resistance welding crack resistance of the welded portion at a high level even in the case where the Fe-based plated layer disappears, it is important to form the Fe-based plated layer having the attached amount A on the cold-rolled steel sheet, and then perform the annealing step under the condition that A+B≥5.0, thereby forming internal oxides in the Fe-based plated layer, and incorporating a part of the Si internal oxides into the alloyed hot-dip zinc layer in the alloying step, thereby reducing the amount of the internal oxides that are in contact with the alloyed hot-dip zinc layer. In the case where A+B≥5.0 is satisfied, Si that diffuses from the cold-rolled steel sheet to the Fe-based plated layer at the time of annealing forms oxides inside the Fe-based plated layer. In this way, by partially forming oxides inside the Fe-based plated layer, the amount of the internal oxides that are in contact with the alloyed hot-dip zinc layer can be reduced. Thus, the invasion of zinc from the grain boundaries of the internal oxide layer that is in contact with the alloyed hot-dip zinc layer can be suppressed. As a result, the time until the molten zinc reaches the grain boundaries of the cold-rolled steel sheet at the time of welding can be delayed, and the resistance welding crack resistance of the welded portion can be improved.

[0019] Further, in the annealing step, the average value of the C concentration in the range of 10 μm to 20 μm from the surface of the cold-rolled steel sheet in the thickness direction of the alloyed hot-dip zinc steel sheet is made to be 0.10% by mass or less. Thus, the resistance welding crack resistance can be further improved. The present inventors have found that, in the case where the Fe-based plated 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 thickness direction can be further reduced, and the effect of improving the resistance welding crack resistance can be more effectively obtained.

[0020] The main configuration of the present application completed based on the above insight is as follows.

[0021] [1] An Fe-based electroplated steel sheet having:

[0022] a cold-rolled steel sheet having a composition containing 0.1 to 3.0 mass% of Si, and

[0023] an Fe-based electroplated layer formed on one side or both sides of the above cold-rolled steel sheet, the adhesion amount per side being 1.0 g / m 2 or more;

[0024] in a profile of the luminescence intensity of the wavelength indicating Si, which is measured from the surface of the above Fe-based electroplated layer in the depth direction, (i) there is a mountain of the luminescence intensity larger than the average Si intensity (I Si ) in the range of the depth of 10.0 ± 0.1 μm from the interface of the above Fe-based electroplated layer and the above cold-rolled steel sheet, (ii) the peak of the above mountain is located deeper than 0.10 μm from the surface of the above Fe-based electroplated layer, (iii) the above luminescence intensity gradually decreases in the depth direction from the peak of the above mountain and the depth at which the above luminescence intensity first becomes equal to the above average Si intensity (I Si ) is located in the range of the depth of 0.10 μm to 2.00 μm or less from the surface of the above Fe-based electroplated layer,

[0025] the average value of the C concentration in the range of 10 to 20 μm in the sheet thickness direction from the surface of the above Fe-based electroplated layer is 0.10 mass% or less.

[0026] [2] The Fe-based electroplated steel sheet according to the above [1], wherein a hot-dip galvanized layer not alloyed is formed in contact with the above Fe-based electroplated layer.

[0027] [3] The Fe-based electroplated steel sheet according to the above [1] or [2], wherein the average value of the C concentration in the range of 10 to 20 μm in the sheet thickness direction from the surface of the above Fe-based electroplated layer is 0.04 mass% or less.

[0028] [4] The Fe-based electroplated steel sheet according to any one of the above [1] to [3], wherein the surface layer portion of the above Fe-based electroplated steel sheet is a decarburized layer.

[0029] [5] The Fe-based electroplated steel sheet according to the above [4], wherein the thickness of the above decarburized layer is 30 μm or more.

[0030] [6] The Fe-based electroplated steel sheet according to the above [4], wherein the thickness of the above decarburized layer is 80 μm or more.

[0031] [7] The Fe-based electroplated steel sheet according to any one of the above [1] to [6], wherein, at the interface between the Fe-based electroplated layer and the cold-rolled steel sheet, the number of grain boundaries of the Fe-based electroplated layer in contact with the cold-rolled steel sheet is 10 or more per 10 μm in the steel sheet width direction in the field of view of the cold-rolled steel sheet.

[0032] [8] The Fe-based electroplated steel sheet according to any one of the above [1] to [7], wherein the composition of the cold-rolled steel sheet contains, in mass %, C: 0.8% or less, Si: 0.1% to 3.0%, Mn: 1.0% to 12.0%, P: 0.1% or less, S: 0.03% or less, N: 0.010% or less, and Al: 1.0% or less, with the remainder consisting of Fe and unavoidable impurities.

[0033] [9] The Fe-based electroplated steel sheet according to the above [8], wherein the composition further contains, in mass %, at least one element selected from the group consisting of B: 0.005% or less, Ti: 0.2% or less, Cr: 1.0% or less, Cu: 1.0% or less, Ni: 1.0% or less, Mo: 1.0% or less, Nb: 0.20% or less, V: 0.5% or less, Sb: 0.020% or less, Ta: 0.1% or less, W: 0.5% or less, Zr: 0.1% or less, Sn: 0.20% or less, Ca: 0.005% or less, Mg: 0.005% or less, and REM: 0.005% or less.

[0034]

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

[0035]

[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.

[0036]

[12] An alloyed hot-dip galvanized steel sheet having:

[0037] a cold-rolled steel sheet having a composition containing 0.1 mass % to 3.0 mass % of Si, and

[0038] an alloyed hot-dip galvanized layer formed on one side or both sides of the cold-rolled steel sheet;

[0039] no Fe-based electroplated layer between the alloyed hot-dip galvanized layer and the cold-rolled steel sheet,

[0040] In the luminescence intensity distribution of the wavelength indicating Si measured from the surface of the above-mentioned alloyed hot-dip galvanized layer in the depth direction by the glow discharge emission spectroscopy, the average Si intensity (I Si，Fe ) from the interface of the above-mentioned alloyed hot-dip galvanized layer and the above-mentioned cold-rolled steel sheet to the range of +0.5 μm of the above-mentioned cold-rolled steel sheet side is 0.90 or less, Si，bulk Si，Fe Si，bulk

[0041] The average value of the C concentration in the range of 10 μm to 20 μm in the sheet thickness direction from the interface of the above-mentioned alloyed hot-dip galvanized layer and the above-mentioned cold-rolled steel sheet is 0.10 mass% or less.

[0042]

[13] The alloyed hot-dip galvanized steel sheet according to the above-mentioned

[12] , wherein the average value of the C concentration in the range of 10 μm to 20 μm in the sheet thickness direction from the interface of the above-mentioned alloyed hot-dip galvanized layer and the above-mentioned cold-rolled steel sheet is 0.04 mass% or less.

[0043]

[14] The alloyed hot-dip galvanized steel sheet according to the above-mentioned

[12] or

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

[0044]

[15] The alloyed hot-dip galvanized steel sheet according to the above-mentioned

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

[0045]

[16] The alloyed hot-dip galvanized steel sheet according to the above-mentioned

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

[0046]

[17] The alloyed hot-dip galvanized steel sheet according to any one of the above-mentioned

[12] to

[16] , wherein the component composition of the above-mentioned cold-rolled steel sheet contains, in mass%, C: 0.8% or less, Si: 0.1% to 3.0%, Mn: 1.0% to 12.0%, P: 0.1% or less, S: 0.03% or less, N: 0.010% or less, and Al: 1.0% or less, with the remainder consisting of Fe and inevitable impurities.

[0047] ​​​

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

[17] , wherein the component composition further contains at least one element selected from the group consisting of B: 0.005% or less, Ti: 0.2% or less, Cr: 1.0% or less, Cu: 1.0% or less, Ni: 1.0% or less, Mo: 1.0% or less, Nb: 0.20% or less, V: 0.5% or less, Sb: 0.020% or less, Ta: 0.1% or less, W: 0.5% or less, Zr: 0.1% or less, Sn: 0.20% or less, Ca: 0.005% or less, Mg: 0.005% or less, and REM: 0.005% or less, in mass%.

[0048]

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

[12] to

[18] , wherein the Si amount in the component composition is 0.9 mass% to 1.7 mass%.

[0049]

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

[12] to

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

[0050]

[21] A chemical conversion treated steel sheet having a chemical conversion treated film formed on the surface of the galvannealed layer of the Fe-based electrogalvanized steel sheet according to any one of the above items [1] to

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

[12] to

[19] .

[0051]

[22] An electrodeposited coated steel sheet having an electrodeposited coated film formed in contact with the chemical conversion treated film of the chemical conversion treated steel sheet according to the above item

[21] .

[0052]

[23] An automobile member made of at least a part of the electrodeposited coated steel sheet according to the above item

[22] .

[0053]

[24] A method of manufacturing an Fe-based electrogalvanized steel sheet, comprising:

[0054] Fe-based electrogalvanizing is performed on a cold-rolled steel sheet having a component composition containing 0.1 mass% to 3.0 mass% of Si to obtain an Fe-based electrogalvanized layer having an adhesion amount A (g / m 2 ) of 1.0 g / m 2 2 on one side or both sides of the cold-rolled steel sheet, and

[0055] Then, the Fe-based electrogalvanized steel sheet is subjected to an annealing process at 650°C to 900°C in an atmosphere in which the dew point B (°C) satisfies the following formula (1).

[0056] A + B ≥ 3.0... (1)

[0057]

[25] The method of manufacturing the Fe-based electrogalvanized steel sheet according to the above-mentioned

[24] , further comprising a step of forming a hot-dip galvanized layer that is not alloyed on the surface of the Fe-based electrogalvanized layer after the annealing step.

[0058]

[26] The method of manufacturing the Fe-based electrogalvanized steel sheet according to the above-mentioned

[24] or

[25] , wherein the annealing step is performed in an atmosphere in which the dew point B (°C) satisfies the following formula (1)'.

[0059] A + B ≥ 8.0... (1)'

[0060]

[27] The method of manufacturing the Fe-based electrogalvanized steel sheet according to any one of the above-mentioned

[24] to

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

[0061]

[28] The method of manufacturing the Fe-based electrogalvanized steel sheet according to any one of the above-mentioned

[24] to

[27] , wherein the attached amount A is less than 5.0 g / m 2 .

[0062]

[29] The method of manufacturing the Fe-based electrogalvanized steel sheet according to any one of the above-mentioned

[24] to

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

[0063]

[30] The method of manufacturing the Fe-based electrogalvanized steel sheet according to the above-mentioned

[29] , wherein the composition further contains, in mass%, at least one element selected from the group consisting of B: 0.005% or less, Ti: 0.2% or less, Cr: 1.0% or less, Cu: 1.0% or less, Ni: 1.0% or less, Mo: 1.0% or less, Nb: 0.20% or less, V: 0.5% or less, Sb: 0.020% or less, Ta: 0.1% or less, W: 0.5% or less, Zr: 0.1% or less, Sn: 0.20% or less, Ca: 0.005% or less, Mg: 0.005% or less, and REM: 0.005% or less.

[0064]

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

[24] to

[30] , wherein the Si content in the above composition is 0.9 to 1.7 mass%.

[0065]

[32] The method of producing a Fe-based electroplated steel sheet according to any one of the above

[24] to

[31] , wherein 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 is contained in the plating bath in the above Fe-based plating and the total content of these elements is 10 mass% or less in the above Fe-based plated layer.

[0066]

[33] The method of producing a 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 adhesion amount A (g / m 2 ) or the dew point B (°C) so as to satisfy the above formula (1) or (1)' when the above formula (1) or (1)' is not satisfied.

[0067]

[34] A method of producing an alloyed hot-dip galvanized steel sheet, comprising:

[0068] Fe-based plating is performed on a cold-rolled steel sheet having a composition containing 0.1 to 3.0 mass% of Si to obtain a Fe-based electroplated steel sheet having an Fe-based plated layer formed on one side or both sides of the above cold-rolled steel sheet, the adhesion amount A (g / m 2 ) per one side being 1.0 g / m 2 2.0 g / m

[0069] Then, the above Fe-based electroplated steel sheet is held at 650 to 900°C in an atmosphere in which the dew point B (°C) satisfies the following formula (2) in an annealing step,

[0070] Then, hot-dip galvanizing is performed on the above Fe-based electroplated steel sheet to form a hot-dip galvanized layer which is not alloyed on the surface of the above Fe-based plated layer, and

[0071] Then, the above hot-dip galvanized layer is heated and alloyed to obtain an alloyed hot-dip galvanized steel sheet having an alloyed hot-dip galvanized layer formed on one side or both sides of the above cold-rolled steel sheet.

[0072] A + B ≥ 5.0... (2)

[0073]

[35] The method of producing an alloyed hot-dip galvanized steel sheet according to the above

[34] , wherein the above annealing step is performed in an atmosphere in which the dew point B (°C) satisfies the following formula (2)'.

[0074] A + B ≥ 10.0... (2)'

[0075]

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

[34] or

[35] , wherein having a step of heating the Fe-based plated steel sheet at an average temperature increasing rate of 10°C / sec or more in a temperature range of 400°C to 650°C before the annealing step.

[0076]

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

[34] to

[36] , wherein the attached amount A is less than 5.0 g / m 2 .

[0077]

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

[34] to

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

[0078]

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

[38] , wherein the component composition further contains, in mass%, at least one element selected from the group consisting of B: 0.005% or less, Ti: 0.2% or less, Cr: 1.0% or less, Cu: 1.0% or less, Ni: 1.0% or less, Mo: 1.0% or less, Nb: 0.20% or less, V: 0.5% or less, Sb: 0.020% or less, Ta: 0.1% or less, W: 0.5% or less, Zr: 0.1% or less, Sn: 0.20% or less, Ca: 0.005% or less, Mg: 0.005% or less, and REM: 0.005% or less.

[0079]

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

[24] to

[39] , wherein, in the component composition, the amount of Si is 0.9 mass% to 1.7 mass%.

[0080]

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

[34] to

[40] , wherein, in the Fe-based plating, 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 is contained in the plating bath, and the total content of these elements is 1 mass% or less in the galvannealed layer.

[0081]

[42] The method of producing the galvannealed steel sheet according to any one of the above

[34] to

[41] , further comprising a step of changing at least one of the attached amount A (g / m 2 ) or the dew point B (°C) so as to satisfy the above formula (2) or (2)' when the above formula (2) or (2)' is not satisfied.

[0082]

[43] A method of producing a chemical conversion treated steel sheet, comprising:

[0083] the method of producing the Fe-based electrogalvanized steel sheet according to any one of the above

[24] to

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

[34] to

[42] , and

[0084] then performing chemical conversion treatment on the above Fe-based electrogalvanized steel sheet or the above galvannealed steel sheet to obtain a chemical conversion treated steel sheet having a chemical conversion treated film formed in contact with the above Fe-based electrogalvanized steel sheet or the above galvannealed steel sheet.

[0085]

[44] A method of producing an electrodeposited coated steel sheet, comprising:

[0086] the method of producing the chemical conversion treated steel sheet according to the above

[43] , and

[0087] then performing electrodeposition coating on the above chemical conversion treated steel sheet to obtain an electrodeposited coated steel sheet having an electrodeposited coated film formed in contact with the above chemical conversion treated film.

[0088]

[45] A method of producing an automobile component, comprising:

[0089] the method of producing the electrodeposited coated steel sheet according to the above

[44] , and

[0090] a step of producing an automobile component using the above electrodeposited coated steel sheet as a part thereof.

[0091] The Fe-based electrogalvanized steel sheet of the present application is not only excellent in chemical conversion treatability or plated appearance when hot-dip galvanizing is performed, but also excellent in resistance to resistance welding cracks. In addition, the galvannealed steel sheet of the present application is not only excellent in plated appearance, but also excellent in resistance to resistance welding cracks. BRIEF DESCRIPTION OF DRAWINGS

[0092] Figure 1 is a graph showing the relationship between the attached amount A of the Fe-based plated layer and the dew point B at the time of annealing and the evaluation results of the resistance to resistance welding cracks in the case of the example of the galvannealed steel sheet (GA) of Example 1.

[0093] Figure 2is a graph showing the relationship between the adhesion amount A of the Fe-based plated layer and the dew point B at the time of annealing and the evaluation results of the resistance welding crack resistance in the case of the hot-dip galvanized steel sheet (GI) of Example 1.

[0094] Figure 3 is a graph showing the relationship between the adhesion amount A of the Fe-based plated layer and the dew point B at the time of annealing and the evaluation results of the resistance welding crack resistance in the case of the Fe-based plated steel sheet (CR) of Example 1 which is not subjected to hot-dip galvanizing.

[0095] Figure 4 (a) of is a graph for explaining the evaluation method of the resistance welding crack resistance of the welded portion, and the upper drawing of (b) is a plan view of the welded sheet set after the evaluation, and the lower drawing is an enlarged view of the B-B section of the upper drawing.

[0096] Figure 5 is raw data based on the glow discharge emission analysis method showing the emission intensity distribution of the wavelength of Si in the case of a part of the Fe-based plated steel sheet (CR) of Example 1 which is not subjected to hot-dip galvanizing.

[0097] Figure 6 is a (a) perspective view and (b) A-A sectional view showing the outline of the observation sample for measuring the number of grain boundaries of the Fe-based plated layer which is in contact with the cold-rolled steel sheet at the interface of the Fe-based plated layer and the cold-rolled steel sheet.

[0098] Figure 7 is a graph for explaining the measurement method of the number of grain boundaries of the Fe-based plated layer which is in contact with the cold-rolled steel sheet at the interface of the Fe-based plated layer and the cold-rolled steel sheet.

[0099] Figure 8 is Figure 7 is an enlarged view of the portion enclosed by the frame of

[0100] Figure 9 is raw data based on the glow discharge emission analysis method showing the emission intensity distribution of the wavelength of Si and Zn in the case of a part of the galvannealed steel sheet (GA) of Example 1.

[0101] Figure 10A is raw data of the distribution of the depth of the C concentration in the sheet thickness direction obtained by analysis using an electron probe microanalyzer in the case of a part of the Fe-based plated steel sheet (CR) of Example 1 which is not subjected to hot-dip galvanizing.

[0102] Figure 10B is Figure 10A is data after smoothing processing of the distribution of

[0103] Figure 11ARaw data of the distribution of the C concentration in the sheet thickness direction obtained by analysis using an electron probe microanalyzer of a part of the alloyed hot-dip galvanized steel sheet (GA) of Example 1.

[0104] Figure 11B Smoothed data of the distribution of Figure 11A after smoothing processing. DETAILED DESCRIPTION

[0105] (Fe-based plated steel sheet and method for manufacturing the same)

[0106] The method for manufacturing the Fe-based plated steel sheet of one embodiment of the present application has a step of performing Fe-based plating on a cold-rolled steel sheet having a composition containing 0.1 to 3.0 mass% of Si to obtain an Fe-based plated steel sheet in which an Fe-based plated layer is formed on one side or both sides of the above cold-rolled steel sheet, and a step of then annealing the above Fe-based plated steel sheet. Then, a simple Fe-based plated steel sheet (CR) can be obtained without a hot-dip galvanizing step. Note that it is preferable that the above Fe-based plating be performed without annealing the above cold-rolled steel sheet. That is, the Fe-based plated steel sheet (CR) of one embodiment of the present application has a cold-rolled steel sheet having a composition containing 0.1 to 3.0 mass% of Si, and an Fe-based plated layer formed on one side or both sides of the above cold-rolled steel sheet; and does not have a non-alloyed hot-dip galvanized layer and an alloyed hot-dip galvanized layer.

[0107] The method for manufacturing the Fe-based plated steel sheet of another embodiment of the present application can further have a step of, after the above annealing step, performing hot-dip galvanizing on the above Fe-based plated steel sheet to form a non-alloyed hot-dip galvanized layer on the surface of the above Fe-based plated layer. Then, a hot-dip galvanized steel sheet (GI) in which the hot-dip galvanized layer is not alloyed can be obtained without a step of heating and alloying the hot-dip galvanized layer. That is, the Fe-based plated steel sheet (GI) of another embodiment of the present application has a cold-rolled steel sheet having a composition containing 0.1 to 3.0 mass% of Si, an Fe-based plated layer formed on one side or both sides of the above cold-rolled steel sheet, and a non-alloyed hot-dip galvanized layer formed in contact with the above Fe-based plated layer.

[0108] (Alloyed hot-dip galvanized steel sheet and method for manufacturing the same)

[0109] The manufacturing method of the galvannealed steel sheet according to one embodiment of the present application has: a step of performing Fe plating on a cold-rolled steel sheet having a composition containing 0.1 to 3.0 mass% of Si to obtain an Fe-plated steel sheet having an Fe-plated layer formed on one or both surfaces of the cold-rolled steel sheet; a step of annealing the Fe-plated steel sheet; a step of performing hot-dip galvanizing on the Fe-plated steel sheet to form a hot-dip galvanized layer not alloyed on the surface of the Fe-plated layer; and a step of heating and alloying the hot-dip galvanized layer to obtain a galvannealed steel sheet (GA) having a galvannealed layer formed on one or both surfaces of the cold-rolled steel sheet. In this case, the Fe-plated layer is incorporated into the hot-dip galvanized layer and disappears in the alloying step. That is, the galvannealed steel sheet (GA) according to one embodiment of the present application has: a cold-rolled steel sheet having a composition containing 0.1 to 3.0 mass% of Si, and a galvannealed layer formed on one or both surfaces of the cold-rolled steel sheet; and does not have an Fe-plated layer between the galvannealed layer and the cold-rolled steel sheet.

[0110] [The cold-rolled steel sheet]

[0111] The step of obtaining the cold-rolled steel sheet is not particularly limited, and a publicly known or arbitrary procedure and conditions can be employed. For example, a slab having a desired composition can be hot-rolled to obtain a hot-rolled steel sheet, the hot-rolled steel sheet can be subjected to degreasing and pickling following the degreasing, and then the hot-rolled steel sheet can be cold-rolled to obtain a cold-rolled steel sheet.

[0112] [Composition of the cold-rolled steel sheet]

[0113] The composition of the cold-rolled steel sheet is described below. Hereinafter, "mass%" is simply referred to as "%".

[0114] Si: 0.1 to 3.0%

[0115] Si does not significantly impair workability, but has a large effect (solid solution strengthening ability) of increasing the strength of the steel by solid solution, and thus is an effective element for achieving high-strength steel sheets, but is also an element that adversely affects the resistance to resistance welding cracking of the welded portion. In the case where the amount of Si is less than 0.1%, high-strength steel sheets cannot be achieved, and in addition, the resistance to resistance welding cracking of the welded portion is not particularly problematic, and the necessity of applying the present application is insufficient. The problem of the resistance to resistance welding cracking of the welded portion is particularly significant when the amount of Si is 0.5% or more. However, from the viewpoint of production cost, the tact time at the time of spot welding in the assembly process of automobile parts becomes a problem, and when measures to reduce the holding time are taken, even when the amount of Si is 0.1% or more and less than 0.5%, the problem of the resistance to resistance welding cracking of the welded portion sometimes occurs. Therefore, in the present application, the amount of Si is 0.1% or more, preferably 0.5% or more, more preferably 0.7% or more, and further preferably 0.9% or more. On the other hand, when the amount of Si is too much, the hot workability and the cold workability are greatly reduced, adversely affecting the productivity, or the ductility of the steel sheet itself is reduced. Therefore, the amount of Si is 3.0% or less, preferably 2.5% or less, more preferably 2.0% or less, and most preferably 1.7% or less.

[0116] The cold-rolled steel sheet of the present embodiment requires that the amount of Si be in the above range, and as for other components, any component composition that is possessed by a general cold-rolled steel sheet can be permitted without particular limitation. However, in the case of a high-strength cold-rolled steel sheet having a tensile strength (TS) of 590 MPa or more as measured in accordance with JIS Z2241 (2011), the following component composition is preferable.

[0117] C: 0.8% or less (0% not included)

[0118] C is an effective element for securing mechanical properties and strength by forming martensite or the like as the steel structure. From this viewpoint, the amount of C is preferably more than 0%, more preferably 0.03% or more, more preferably 0.05% or more, and further preferably 0.08% or more. On the other hand, from the viewpoint of obtaining good weldability, the amount of C is preferably 0.8% or less, and more preferably 0.3% or less.

[0119] Mn: 1.0% to 12.0%

[0120] Mn is an element effective for solid solution strengthening of the steel to increase the strength, and for increasing the hardenability, and for promoting the generation of residual austenite, bainite and martensite. From this viewpoint, the amount of Mn is preferably 1.0% or more, more preferably 1.3% or more, further preferably 1.5% or more, most preferably 1.8% or more. On the other hand, from the viewpoint of obtaining the above effects without causing an increase in cost, the amount of Mn is preferably 12.0% or less, more preferably 3.5% or less, further preferably 3.3% or less.

[0121] P: 0.1% or less (0% not included)

[0122] By suppressing the amount of P, it is possible to prevent a decrease in weldability. Furthermore, it is possible to prevent P from segregating at grain boundaries, and to prevent deterioration in ductility, bendability and toughness. In addition, if P is added in a large amount, ferrite transformation is promoted, and thus the crystal grain size also becomes large. Therefore, the amount of P is preferably 0.1% or less. The lower limit of P is not particularly limited, and due to constraints in production technology, the amount of P can exceed 0%, and can be 0.001% or more.

[0123] S: 0.03% or less (0% not included)

[0124] The amount of S is preferably 0.03% or less, more preferably 0.02% or less. By suppressing the amount of S, it is possible to prevent a decrease in weldability, and to prevent a decrease in ductility at high temperatures, to suppress hot cracking, and to significantly improve surface properties. Furthermore, by suppressing the amount of S, it is possible to avoid the formation of coarse sulfides as impurity elements, and to prevent a decrease in ductility, bendability and stretch flangeability of the steel sheet. The lower limit of S is not particularly limited, and due to constraints in production technology, the amount of S can exceed 0%, and can be 0.0001% or more.

[0125] N: 0.010% or less (0% not included)

[0126] N forms coarse nitrides with Ti, Nb and V at high temperatures, and does not contribute much to strength, and thus not only reduces the effects of high strength brought about by the addition of Ti, Nb and V, but also causes a decrease in toughness. Furthermore, when the amount of N is excessive, slab cracking occurs during hot rolling, and surface defects can occur. Therefore, the amount of N is preferably 0.010% or less. The amount of N is preferably 0.005% or less, more preferably 0.003% or less, further preferably 0.002% or less. The lower limit of the amount of N is not particularly limited, and due to constraints in production technology, the amount of N can exceed 0%, and can be 0.0005% or more.

[0127] Al: 1.0% or less (0% not included)

[0128] Al is oxidized most easily from the viewpoint of thermodynamics, and thus oxidizes before Si and Mn, and has an effect of inhibiting oxidation of the most surface of the steel sheet and promoting oxidation of the inside of the steel sheet of Si and Mn. On the other hand, if the Al amount exceeds 1.0%, the cost increases. Therefore, in the case where Al is added, the Al amount is preferably 1.0% or less. The lower limit of Al is not particularly limited, and the Al amount can exceed 0%, and can be 0.001% or more. However, from the viewpoint of obtaining the above effect, the Al amount is preferably 0.01% or more.

[0129] The remainder other than the above components is Fe and inevitable impurities. However, it is also possible to arbitrarily include an element selected from at least one of the following.

[0130] B: 0.005% or less

[0131] B is an element effective for improving the hardenability of the steel. In order to improve the hardenability, the B amount is preferably 0.0003% or more, and more preferably 0.0005% or more. In addition, in order not to impair formability, the B amount is preferably 0.005% or less.

[0132] Ti: 0.2% or less

[0133] Ti is effective for precipitation strengthening of the steel. Therefore, the Ti amount is preferably 0.005% or more. In addition, in order not to impair formability, the Ti amount is preferably 0.2% or less, and more preferably 0.05% or less.

[0134] Cr: 1.0% or less

[0135] The Cr amount is preferably 0.005% or more. By making the Cr amount 0.005% or more, it is possible to improve the hardenability and improve the balance between strength and ductility. In the case where Cr is added, from the viewpoint of preventing an increase in cost, the Cr amount is preferably 1.0% or less.

[0136] Cu: 1.0% or less

[0137] The Cu amount is preferably 0.005% or more. By making the Cu amount 0.005% or more, it is possible to promote the formation of residual γ phase. In the case where Cu is added, from the viewpoint of preventing an increase in cost, the Cu amount is preferably 1.0% or less.

[0138] Ni: 1.0% or less

[0139] The Ni amount is preferably 0.005% or more. By making the Ni amount 0.005% or more, it is possible to promote the formation of residual γ phase. In the case where Ni is added, from the viewpoint of preventing an increase in cost, the Ni amount is preferably 1.0% or less.

[0140] Mo: 1.0% or less

[0141] The Mo amount is preferably 0.005% or more. By making the Mo amount 0.005% or more, the effect of adjusting the strength can be obtained, and the Mo amount is more preferably 0.05% or more. In the case of adding Mo, the Mo amount is preferably 1.0% or less from the viewpoint of preventing an increase in cost.

[0142] Nb: 0.20% or less

[0143] The Nb amount is preferably 0.005% or more from the viewpoint of obtaining the effect of improving the strength. In the case of containing Nb, the Nb amount is preferably 0.20% or less from the viewpoint of preventing an increase in cost.

[0144] V: 0.5% or less

[0145] The V amount is preferably 0.005% or more from the viewpoint of obtaining the effect of improving the strength. In the case of containing V, the V amount is preferably 0.5% or less from the viewpoint of preventing an increase in cost.

[0146] Sb: 0.020% or less

[0147] Sb can be contained from the viewpoint of suppressing oxidation of the surface of the steel sheet. Sb improves the wettability of plating and the chemical conversion treatment property of the cold-rolled steel sheet by suppressing oxidation of the steel sheet. In order to obtain such effects, the Sb amount is preferably 0.001% or more. On the other hand, Sb suppresses the formation of a decarburized layer. In order to obtain good resistance to resistance welding cracks, the Sb amount is preferably 0.020% or less, more preferably 0.015% or less, and further preferably 0.012% or less.

[0148] Ta: 0.1% or less

[0149] The Ta amount is preferably 0.001% or more from the viewpoint of obtaining the effect of improving the strength. In the case of containing Ta, the Ta amount is preferably 0.1% or less from the viewpoint of preventing an increase in cost.

[0150] W: 0.5% or less

[0151] The W amount is preferably 0.005% or more from the viewpoint of obtaining the effect of improving the strength. In the case of containing W, the W amount is preferably 0.5% or less from the viewpoint of preventing an increase in cost.

[0152] Zr: 0.1% or less

[0153] The Zr amount is preferably 0.0005% or more from the viewpoint of obtaining the effect of improving the strength. In the case of containing Zr, the Zr amount is preferably 0.1% or less from the viewpoint of preventing an increase in cost.

[0154] Sn: 0.20% or less

[0155] Sn is an element effective to suppress the decrease in strength of steel by suppressing denitrogenation, deboronization, and the like. To obtain such an effect, the Sn amount is preferably 0.002% or more. To ensure impact resistance, in the case where Sn is contained, the Sn amount is preferably 0.20% or less.

[0156] Ca: 0.005% or less

[0157] By making the Ca amount 0.0005% or more, it is possible to control the morphology of sulfides, improve ductility, and improve toughness. From the viewpoint of obtaining good ductility, in the case where Ca is contained, the Ca amount is preferably 0.005% or less.

[0158] Mg: 0.005% or less

[0159] By making the Mg amount 0.0005% or more, it is possible to control the morphology of sulfides, improve ductility, and improve toughness. In the case where Mg is contained, from the viewpoint of preventing an increase in cost, the Mg amount is preferably 0.005% or less.

[0160] REM: 0.005% or less

[0161] By making the REM amount 0.0005% or more, it is possible to control the morphology of sulfides, improve ductility, and improve toughness. From the viewpoint of obtaining good toughness, in the case where REM is contained, the REM amount is preferably 0.005% or less.

[0162] [Thickness of cold-rolled steel sheet]

[0163] The sheet thickness of the cold-rolled steel sheet of the present embodiment is not particularly limited, and is typically 0.5 mm or more, and can be 3.2 mm or less.

[0164] [Degreasing and pickling]

[0165] In the present embodiment, as a pretreatment for Fe-based plating, degreasing is preferably performed on the cold-rolled steel sheet, and then pickling is performed after the degreasing. Specifically, degreasing and water washing are preferably performed in order to clean the surface of the steel sheet, and then pickling and water washing are performed in order to activate the surface of the steel sheet. The degreasing and water washing are not particularly limited, and a publicly known or arbitrary method and conditions can be employed. In the pickling treatment, various acids such as sulfuric acid, hydrochloric acid, nitric acid, and mixtures thereof can be used. Among them, sulfuric acid, hydrochloric acid, and mixtures thereof are preferred. The concentration of the acid is not particularly specified, and is preferably around 1 to 20 mass% if the ability to remove the oxidation film, prevention of surface roughening due to excessive pickling, and the like are taken into consideration. In addition, a defoaming agent, a pickling accelerator, a pickling inhibitor, and the like can be contained in the pickling solution.

[0166] [Fe-based plating]

[0167] Next, Fe-based plating is performed on the cold-rolled steel sheet to obtain an Fe-based plated steel sheet in which an Fe-based plated layer having a prescribed adhesion amount is formed on one side or both sides of the cold-rolled steel sheet. Due to the presence of the Fe-based plated layer, excellent chemical conversion treatment properties can be obtained in a simple Fe-based plated steel sheet (CR), and good plating appearance can be obtained in a hot-dip galvanized steel sheet (GI). In an alloyed hot-dip galvanized steel sheet (GA), the Fe-based plated layer disappears, but performing Fe-based plating in the manufacturing process of the GA is a necessary condition for obtaining good plating appearance. The specific method and conditions of the Fe-based plating treatment are not particularly limited. For example, as the plating bath, a sulfuric acid bath, a hydrochloric acid bath, or a mixture of both, or the like can be used.

[0168] From the viewpoint of obtaining a sufficient adhesion amount of the Fe-based plated layer, the Fe ion content in the plating bath before the start of the power supply is preferably 0.5 mol / L to 2.0 mol / L. Other conditions related to the Fe-based plating are not particularly limited. If the temperature of the plating solution is considered, the temperature is preferably 30°C to 85°C. The pH of the plating solution is not particularly limited, but from the viewpoint of preventing a decrease in current efficiency due to hydrogen generation, it is preferably 1.0 or more, and from the viewpoint of ensuring electrical conductivity, it is preferably 3.0 or less. The current density is preferably 10 A / dm2 or more from the viewpoint of productivity, and preferably 150 A / dm2 or less from the viewpoint of easily controlling the adhesion amount of the Fe-based plated layer. 2+ The temperature of the plating solution is not particularly limited, but from the viewpoint of productivity, it is preferably 30°C to 85°C. The pH of the plating solution is not particularly limited, but from the viewpoint of preventing a decrease in current efficiency due to hydrogen generation, it is preferably 1.0 or more, and from the viewpoint of ensuring electrical conductivity, it is preferably 3.0 or less. The current density is preferably 10 A / dm2 or more from the viewpoint of productivity, and preferably 150 A / dm2 or less from the viewpoint of easily controlling the adhesion amount of the Fe-based plated layer. 2 The temperature of the plating solution is not particularly limited, but from the viewpoint of productivity, it is preferably 30°C to 85°C. The pH of the plating solution is not particularly limited, but from the viewpoint of preventing a decrease in current efficiency due to hydrogen generation, it is preferably 1.0 or more, and from the viewpoint of ensuring electrical conductivity, it is preferably 3.0 or less. The current density is preferably 10 A / dm2 or more from the viewpoint of productivity, and preferably 150 A / dm2 or less from the viewpoint of easily controlling the adhesion amount of the Fe-based plated layer. 2 The temperature of the plating solution is not particularly limited, but from the viewpoint of productivity, it is preferably 30°C to 85°C. The pH of the plating solution is not particularly limited, but from the viewpoint of preventing a decrease in current efficiency due to hydrogen generation, it is preferably 1.0 or more, and from the viewpoint of ensuring electrical conductivity, it is preferably 3.0 or less. The current density is preferably 10 A / dm2 or more from the viewpoint of productivity, and preferably 150 A / dm2 or less from the viewpoint of easily controlling the adhesion amount of the Fe-based plated layer.

[0169] As the Fe-based plated layer, in addition to pure Fe, an alloy plated layer of Fe-B alloy, Fe-C alloy, Fe-P alloy, Fe-N alloy, Fe-O alloy, Fe-Ni alloy, Fe-Mn alloy, Fe-Mo alloy, Fe-W alloy, or the like can be used. At least one element selected from B, C, P, N, O, Ni, Mn, Mo, Zn, W, Pb, Sn, Cr, V, and Co can be contained in the plating bath. With respect to the total content of these elements in the plating bath, in the case of CR and GI, the total content of these elements in the Fe-based plated layer is preferably 10 mass% or less, and in the case of GA, it is considered that the Fe-based plated layer does not remain, and therefore, the total content of these elements in the alloyed hot-dip galvanized layer is preferably 1 mass% or less. Note that in the case of CR or GI and in the case where the Fe-based plated layer is an Fe-C alloy, the content of C is preferably 0.08 mass% or less. Note that the metal elements can be contained as metal ions, and the non-metal elements can be contained as part of boric acid, phosphoric acid, nitric acid, organic acid, or the like. In addition, a conductivity aid such as sodium sulfate, potassium sulfate, a chelating agent, a pH buffer can be contained in the plating bath.

[0170] The attached amount A per one side of the Fe-based plated layer: 1.0 g / m 2 The above

[0171] In any of the manufacturing processes of the CR, the GI, and the GA, in the case where the attached amount A per one side of the Fe-based plated layer is less than 1.0 g / m 2 , the attached amount of the Fe-based plated layer is insufficient, and the chemical conversion treatment property or the plated appearance when hot-dip galvanizing is insufficient. Therefore, the attached amount A per one side of the Fe-based plated layer is 1.0 g / m 2 or more, preferably 2.0 g / m 2 or more. The upper limit of the attached amount A is not particularly limited, but from the viewpoint of suppressing the elongation of the production line and the increase in the electric power cost, the attached amount A per one side of the Fe-based plated layer is preferably less than 5.0 g / m 2 , more preferably 4.5 g / m 2 or less. Note that in the CR and the GI of the present embodiment, the attached amount per one side of the Fe-based plated layer is maintained as the attached amount A after the Fe-based plating. On the other hand, in the GA, the Fe-based plated layer disappears.

[0172] The Fe-based plated layer also contributes to the resistance welding crack resistance property of the welded portion. The mechanism is not yet clear, but it is considered that decarburization from the surface layer of the steel sheet is promoted by the Fe-based plated layer at the time of annealing described later, and the surface layer softening layer resulting from the decarburization alleviates the residual stress at the time of welding, thereby suppressing the cracking of the welded portion.

[0173] The Fe-based plated steel sheet having the Fe-based plated layer of 1.0 g / m 2 or more has the resistance welding crack resistance property of the welded portion improved by further promoting the formation of the decarburized layer by annealing. The mechanism is not yet clear, but it is presumed that in the case where the Fe-based plated layer is not present, decarburization is performed by the reaction of solid-solution C in the steel sheet reacting with H2O in the furnace to generate CO and CO2 in the annealing, and in contrast, in the case where the Fe-based plated layer is present, the solid-solution C in the steel sheet can diffuse into the Fe-based plated layer while maintaining the state of the solid-solution C, and thus the rate-controlling process is different, and the decarburization is faster in the case where the Fe-based plated layer is present. Note that in the case of the individual plating of Ni, Co, Sn, or the like, the solid-solution degree of C in these metal elements is extremely low, and C is not solid-solved, and thus the effect of promoting decarburization is not obtained.

[0174] The attached amount of the Fe-based plated layer was determined as follows. A sample of 10 x 15 mm in size was taken from the Fe-based plated steel sheet, embedded in a resin, and a cross-section embedded sample was prepared. A scanning electron microscope (SEM) was used to observe three random positions of the cross-section at an acceleration voltage of 15 kV and a magnification of 2000 to 10000 times according to the thickness of the Fe-based plated layer, and the attached amount per one side of the Fe-based plated layer was converted by multiplying the average of the thicknesses of the three fields by the density of iron.

[0175] [temperature increasing process]

[0176] In any of the manufacturing processes of the CR, GI, and GA, in the temperature increasing process before annealing (soaking), it is also preferable to heat the Fe-based plated steel sheet at an average temperature increasing rate of 10°C / sec or more in the temperature range of 400°C to 650°C. By making the average temperature increasing rate 10°C / sec or more, the growth of the crystal grains in the Fe-based plated layer in the temperature increasing process is strongly suppressed. This is because in the temperature increasing process, as described later, the Si internal oxidation hardly proceeds at the grain boundaries of the Fe-based plated layer, and therefore if the average temperature increasing rate is less than 10°C / sec, the growth of the crystal grains cannot be suppressed. By annealing in the temperature increasing process in a state where the growth of the crystal grains in the Fe-based plated layer is strongly suppressed, as described later, in a manner such that the relationship between the dew point B (°C) of the atmosphere at the time of annealing and the attached amount A (g / m2) per one side of the Fe-based plated layer satisfies a prescribed condition, the crystal of the Fe-based plated layer can be made fine. The heating zone of the temperature increasing process can use, for example, a direct fired furnace (DFF) or a non oxidizing furnace (NOF). In the case of a radiant tube type heating furnace, an IH (induction heater) or the like pre-heating zone can be provided in the front stage. 2 ) of the Fe-based plated layer satisfies a prescribed condition, the crystal of the Fe-based plated layer can be made fine. The heating zone of the temperature increasing process can use, for example, a direct fired furnace (DFF) or a non oxidizing furnace (NOF). In the case of a radiant tube type heating furnace, an IH (induction heater) or the like pre-heating zone can be provided in the front stage.

[0177] [annealing]

[0178] Then, a process of annealing the Fe-based plated steel sheet is performed. The annealing process is performed in order to recrystallize the structure by removing the strain of the cold-rolled steel sheet generated by the rolling process, thereby adjusting the strength of the steel sheet.

[0179] Hydrogen concentration: 1.0 vol% to 30.0 vol%

[0180] The annealing process can be performed, for example, in a reducing atmosphere having a hydrogen concentration of 1.0 vol% to 30.0 vol%. Hydrogen functions to inhibit oxidation of Fe on the surface of the Fe-based plated steel sheet during the annealing process, and activates the surface of the steel sheet. If the hydrogen concentration is 1.0 vol% or more, oxidation of Fe on the surface of the steel sheet can be inhibited, and plating adhesion can be ensured when hot-dip galvanizing is performed. Therefore, the annealing process is preferably performed in a reducing atmosphere having a hydrogen concentration of 1.0 vol% or more, and more preferably in a reducing atmosphere having a hydrogen concentration of 2.0 vol% or more. The upper limit of the hydrogen concentration in the annealing process is not particularly limited, but from the viewpoint of cost, the hydrogen concentration is preferably 30.0 vol% or less, and more preferably 20.0 vol% or less. The remainder of the annealing atmosphere other than hydrogen is preferably nitrogen.

[0181] In the present embodiment, it is important that the relationship between the dew point B (°C) of the atmosphere at the time of annealing and the adhesion amount A (g / m 2 ) per one surface of the Fe-based plated layer satisfies a prescribed condition, whereby the resistance welded crack resistance of the welded portion can be improved. It is considered that this is because decarburization is promoted when the Fe-based plated steel sheet is annealed, and the surface layer softening layer resulting from the decarburization mitigates residual stress at the time of welding, whereby cracking of the welded portion is inhibited.

[0182] Specifically, in the case of manufacturing 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 that the following formula (1) is satisfied, and preferably the following formula (1)' is satisfied.

[0183] A + B ≥ 3.0... (1)

[0184] A + B ≥ 8.0... (1)'

[0185] In addition, in the case of manufacturing an alloyed hot-dip galvanized steel sheet (GA) in which the hot-dip galvanized layer is alloyed, it is important that the following formula (2) is satisfied, and preferably the following formula (2)' is satisfied.

[0186] A + B ≥ 5.0... (2)

[0187] A + B ≥ 10.0... (2)'

[0188] The LME cracks described above can be roughly classified into "cracks generated on a surface in contact with an electrode (hereinafter, surface cracks)" and "cracks generated between steel sheets in the vicinity of a plastic metal corona bond (hereinafter, internal cracks)". It is known that surface cracks are easily generated in resistance welding in a large current range in which sputtering occurs, and by setting to an appropriate current range in which sputtering does not occur, surface cracks can be suppressed. On the other hand, even if the current at the time of resistance welding is made to be in an appropriate range in which sputtering does not occur, internal cracks are generated. In addition, surface cracks are easily found by appearance inspection of the manufacturing process, and in contrast, internal cracks are not easily found by appearance inspection. For these reasons, among LME cracks, internal cracks become a particularly large problem. If resistance welding is performed in a state in which an electrode for welding exists at an angle with respect to a steel sheet, there is a possibility that residual stress increases and internal cracks are generated. It is considered that residual stress increases accompanying the high strength of a steel sheet, and thus there is a possibility that internal cracks are generated accompanying the high strength of a steel sheet. In the present disclosure, a property of resistance welding crack resistance, particularly a property of preventing this internal crack, can be improved.

[0189] As described above, if the result satisfies the above formula (1) or formula (2), the resistance welding crack resistance property of the welded portion is improved. As other embodiments, a process can be further provided in which, in the manufacturing process of the CR and the GI, at least one of the attached amount A (g / m 2 ) or the dew point B (°C) is changed in such a way that the above formula (1) or formula (1)' is satisfied when the above formula (1) or formula (1)' is not satisfied, and in the manufacturing process of the GA, at least one of the attached amount A (g / m 2) or dew point B (°C). Thus, the resistance welded crack resistance of the welded portion can be more reliably improved. As an example of performing this process in operation, a process of changing the dew point B in the annealing process in such a manner that the value of the attached amount A of the Fe-based plated layer obtained in the Fe-based plating satisfies the above-described equation (1) or equation (1') or equation (2) or equation (2') is cited. Specifically, the value of the attached amount A of the Fe-based plated layer obtained in the Fe-based plating is substituted into the above-described equation (1) or equation (1') or equation (2) or equation (2'), and the dew point B in the above-described annealing process is determined in such a manner that the substituted equation is satisfied. Here, "the value of the attached amount A of the Fe-based plated layer is substituted into the above-described equation (1) or equation (1') or equation (2) or equation (2')" is not limited to a manner of strictly substituting into the same equation as the above-described equation (1) or equation (1') or equation (2) or equation (2'), but also includes a manner of substituting into a narrower range of inequalities that always satisfy these equations. By performing such control, even when the above-described equation (1) or equation (1') or equation (2) or equation (2') is not satisfied due to, for example, a change in the product specification of the steel sheet being continuously passed through the annealing furnace, a large change in the attached amount A, and the like (when the equation is not actually satisfied, when a situation in which the equation is not satisfied occurs), automatic control can be performed in such a manner that the equation is satisfied.

[0190] Note that since the control responsiveness of the dew point B is poorer than that of the attached amount A, from the viewpoint of control responsiveness, it is preferable to change the attached amount A in such a manner that the value of the dew point B satisfies the above-described equation (1) or equation (1') or equation (2) or equation (2'). In the case of a continuous annealing furnace, the attached amount A in the Fe-based plating process that is more upstream than the annealing process is changed in accordance with the value of the dew point B of the annealing process, but the portion of the continuously passed steel sheet in which the attached amount A is changed is manufactured under the condition that the above-described equation (1) or equation (1') or equation (2) or equation (2') is satisfied.

[0191] As the timing of changing at least one of the attached amount A or the dew point B in such a manner that the above-described equation (1) or equation (1') or equation (2) or equation (2') is satisfied, in the case of welding steel sheets of different product specifications and continuously passing through the steel sheet, it is more preferable to change the attached amount A or the dew point B in accordance with the passage of the welded portion. As described above, since the responsiveness of the dew point B is poor, in the case of changing the dew point B, it is more preferable to perform feedforward control of the amount of humidification in the furnace in such a manner that the equation is satisfied.

[0192] Here, the "value of the attached amount A" can be the attached amount obtained under the conditions employed in the Fe-based plating (target value), or can be the attached amount of the Fe-based plated layer actually obtained (measured value). Similarly, the "value of the dew point B" can be either of the target value or the measured value.

[0193] The above describes an example of the operation of the manufacturing method of CR, GI, and GA, but the manufacturing condition determination method of CR, GI, and GA can also be implemented as a process that, before the operation starts, preliminarily confirms whether the target values of the attachment amount A and the dew point B satisfy the above formula (1) or formula (1)' or formula (2) or formula (2)', and preliminarily changes either of the target values of the attachment amount A and the dew point B in the case of non-satisfaction. Such a manufacturing condition determination method can be implemented as a process that is part of the manufacturing method of CR, GI, and GA, or can be implemented as a separate process.

[0194] The upper limit of the dew point of the annealing atmosphere is not particularly limited, but in order to properly prevent oxidation of the surface of the Fe-based plated layer, suppress deviation of the dew point, and make the chemical conversion treatment property or plating adhesion when hot dip galvanizing good, the dew point of the annealing atmosphere is preferably 20°C or lower. In addition, the lower limit of the dew point B of the annealing atmosphere is not particularly limited as long as the above formula (1) or formula (2) is satisfied, but the dew point B is preferably over 0°C, and more preferably over 5°C.

[0195] Retention time in the temperature range of 650°C to 900°C: 30 seconds to 600 seconds

[0196] In the annealing process, it is preferable that the retention time in the temperature range of 650°C to 900°C be 30 seconds or more. Thereby, the natural oxide film of Fe formed on the surface of the Fe-based plated layer can be properly removed, and the chemical conversion treatment property or plating adhesion when hot dip galvanizing can be improved. The upper limit of the retention time in this temperature range is not particularly limited, but from the viewpoint of productivity, the retention time in this temperature range is preferably 600 seconds or less.

[0197] Maximum temperature reached by the Fe-based plated steel sheet: 650°C to 900°C

[0198] The maximum temperature reached by the Fe-based plated steel sheet is not particularly limited, but is preferably 650°C to 900°C. By making the maximum temperature reached by the Fe-based plated steel sheet 650°C or higher, recrystallization of the steel sheet structure properly proceeds, and a desired strength can be obtained. In addition, the natural oxide film of Fe formed on the surface of the Fe-based plated layer can be properly reduced, and the chemical conversion treatment property or plating adhesion when hot dip galvanizing can be improved. In addition, if the maximum temperature reached by the Fe-based plated steel sheet is 900°C or lower, the diffusion speed of Si and Mn in the steel can be prevented from excessively increasing, and diffusion of Si and Mn to the surface of the steel sheet can be prevented, and thus the chemical conversion treatment property or plating adhesion when hot dip galvanizing can be improved. In addition, if the maximum temperature reached is 900°C or lower, the furnace body of the heat treatment furnace can be prevented from being damaged, and costs can be reduced. Note that the above maximum temperature reached is based on the temperature measured at the surface of the Fe-based plated steel sheet.

[0199] [hot dip galvanizing]

[0200] In the manufacturing process of the GI and the GA, the following process is further performed: after the annealing process, hot dip galvanizing is performed on the Fe-based plated steel sheet to form a hot dip galvanizing layer on the surface of the Fe-based plated steel sheet. As long as the Fe-based plated steel sheet is cooled after the annealing process, immersed in a hot dip galvanizing bath, and the surface of the steel sheet is subjected to hot dip galvanizing. The hot dip galvanizing bath is composed of Al, Zn, and unavoidable impurities. The composition of the hot dip galvanizing bath is not particularly specified, but generally the Al concentration in the bath is 0.05 mass% to 0.250 mass%. If the Al concentration in the bath is 0.05 mass% or more, the generation of bottom dross is prevented, and the dross is prevented from adhering to the steel sheet to become a defect. In addition, by making the Al concentration in the bath 0.250 mass% or less, the increase in top dross is prevented, the dross is prevented from adhering to the steel sheet to become a defect, and the cost is reduced. The other conditions of the hot dip galvanizing treatment are not limited, for example, the bath temperature of the hot dip galvanizing bath is in the range of generally 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.

[0201] The plating adhesion amount per single side of the hot dip galvanizing layer is controlled to be 25 to 80 g / m 2 By making the adhesion amount 25 g / m 2 or more, the corrosion resistance can be further improved, and the plating adhesion amount is easily controlled. In addition, if the adhesion amount is 80 g / m 2 or less, the plating adhesion is good. The adjustment of the adhesion amount can be performed by general gas wiping.

[0202] [heat alloying]

[0203] In the manufacturing process of the GA, after the hot dip galvanizing treatment, the hot dip galvanizing layer is heat alloyed to produce an alloyed hot dip galvanizing steel sheet (GA). The method of performing the alloying treatment is not particularly limited, and can be performed using IH, a gas furnace, or the like, and the maximum sheet temperature reached at the time of alloying is preferably 460 to 600°C. If it is 460°C or more, the alloying is sufficiently performed, and if it is 600°C or less, the alloying is not excessive, and the plating adhesion is not impaired.

[0204] The %Fe in the alloyed hot dip galvanizing layer of the alloyed hot dip galvanizing steel sheet is preferably 7 to 15 mass%. If it is 7 mass% or more, the deterioration of the press formability due to the residual of the η phase is suppressed, and if it is 15 mass% or less, the plating adhesion is not impaired. Note that, in the case where it is controlled to be in this range, the Fe-based plated layer is entirely alloyed with zinc and disappears. In this case, the resistance welded crack resistance of the alloyed hot dip galvanizing steel sheet is also excellent.

[0205] The calculation method of the adhesion amount and the %Fe of the zinc plated layer in the GI and the GA will be described.

[0206] The measurement of the attached amount of the zinc plating layer is performed in accordance with JIS H 0401 or ISO 17925. First, a sample of 20 x 25 mm size is taken from two sites of GI or GA. After measuring the respective weights, one-side plating peeling is performed using a test solution prescribed in JIS H 0401 or ISO 17925, and the weight measurement is performed again. The attached amount can be calculated by subtracting the weight after the plating peeling from the weight before the plating peeling and dividing by the surface area of the plating-peeled portion. Here, the average of the two sites is taken as the plating attached amount.

[0207] The measurement of the Fe% is performed in accordance with ISO 17925. The amount of Fe is calculated by dividing the amount of Fe in the test solution after the plating peeling by the total of the amounts of Fe, Zn, and Al contained in the test solution, using an inductively coupled plasma (ICP) emission analysis device. Here, the average of the two sites is taken as the Fe%.

[0208] [Internal Oxidation Layer in CR and GI]

[0209] In the CR and GI of the present embodiment, it is important that the depth of the internal oxidation layer in the direction of the cold-rolled steel sheet from the depth of 0.10 μm from the surface of the Fe-based plating layer be 2.00 μm or less. This can be achieved by performing annealing that satisfies formula (1) A+B≥3.0. In the case where the depth of the internal oxidation layer in the direction of the cold-rolled steel sheet from the depth of 0.10 μm from the surface of the Fe-based plating layer exceeds 2.00 μm, the resistance welding crack resistance characteristics deteriorate. The mechanism thereof is not clear, but it is considered that by making the depth of the internal oxidation layer 2.00 μm or less, the penetration of zinc in the depth direction of the cold-rolled steel sheet when zinc reaches the grain boundary of the cold-rolled steel sheet is strongly suppressed, and the resistance welding crack resistance characteristics of the welded portion can be improved. On the other hand, by forming the internal oxide, the formation of an oxide on the surface at the time of annealing can be suppressed, and the effect of improving the chemical conversion treatment property in the CR and the plating appearance in the GI can be exerted. In order to exhibit such effects, the depth of the internal oxidation layer is preferably 0.10 μm or more.

[0210] The definition that the depth of the internal oxidation layer in the direction of the cold-rolled steel sheet from the depth of 0.10 μm from the surface of the Fe-based plating layer is 2.00 μm or less is defined as satisfying the following conditions. That is, in the luminescence intensity distribution of the wavelength indicating Si measured in the depth direction from the surface of the Fe-based plating layer by a glow discharge optical emission spectrometry (GD-OES), (i) the luminescence intensity of Si at the depth of 10.0±0.1 μm from the interface of the Fe-based plating layer and the cold-rolled steel sheet is 0.5 or more times the average Si intensity (I Si) large mountains, (ii) the peaks of the mountains are located at a depth of 0.10 μm or more from the surface of the Fe-based plated layer, (iii) the luminescence intensity gradually decreases from the peaks of the mountains in the depth direction and first becomes equal to the average Si intensity (I Si ) at a depth of 0.10 μm or more from the surface of the Fe-based plated layer. The measurement conditions were an Ar gas pressure of 600 Pa, a high-frequency output of 35 W, a measurement diameter of 4 mm Φ, a sampling interval of 0.1 seconds. Note that, after a cold-rolled steel sheet that had not been subjected to Fe-based plating was analyzed by the glow discharge luminescence analysis method under the same conditions, the depth of a sputter mark was measured, and the sputtering speed was calculated, and the horizontal axis of the intensity distribution of the wavelength representing Si was converted into the depth corresponding to each time. The depth of the sputter mark was measured using a non-contact surface shape measuring device (NewView 7300: manufactured by Zygo Corporation).

[0211] Using Figure 5 , representative examples of the peaks of Si obtained by analyzing the luminescence intensity at the wavelength representing Si observed in the present embodiment will be described. Figure 5 are the raw data of the luminescence intensity distribution at the wavelength representing Si in Comparative Example No. 21 (no Fe-based plating, dew point B of the annealing atmosphere: +4.7°C), Inventive Example No. 23 (Fe-based plated layer with an attached amount A of 4.7 g / m 2 , dew point B: -1.3°C), and Inventive Example No. 27 (Fe-based plated layer with an attached amount A of 3.9 g / m 2 , dew point B: +9.8°C) in Example 1 (Table 4) described later. In Inventive Examples No. 23 and 27, an Fe-based plated layer (referred to as "plated Fe" in Example 1 for convenience) with a thickness of about 0.50 to 0.60 μm was formed on the surface of the cold-rolled steel sheet. Figure 5 In Comparative Example No. 21, mountains Pex from Si external oxides were observed at a depth of 0.10 μm or less from the surface of the cold-rolled steel sheet. In addition, mountains Pin from Si internal oxides were observed at a depth of more than 0.10 μm from the surface. In Inventive Examples No. 23 and 27, peaks Pex from Si external oxides were observed at a depth of 0.10 μm or less from the surface of the Fe-based plated layer, and mountains Pin from Si internal oxides were observed at a depth of more than 0.10 μm from the surface. Pin was observed in a form that spanned the Fe-based plated layer and the cold-rolled steel sheet, which means that Si internal oxides were also formed in the Fe-based plated layer. The luminescence intensity gradually decreased from the peaks of the mountains Pin in the depth direction and first became equal to the average Si intensity (I Si) becomes equal is defined as the internal oxidation layer. Since Pex exists in the range of the depth of 0.10 μm from the surface (the surface of the cold-rolled steel sheet in Comparative Example No. 21, the surface of the Fe-based plated layer in Inventive Examples Nos. 23 and 27), it is excluded, and the "thickness of the internal oxidation layer" is the thickness of the depth of 0.10 μm from the surface to the depth at which the luminescence intensity gradually decreases from the peak of the luminescence intensity of Si to the depth direction and first becomes equal to the average Si intensity (I Si ) becomes equal. In Comparative Example No. 21, the thickness of the internal oxidation layer was 2.39 μm. In Inventive Examples Nos. 23 and 27, the thickness of the internal oxidation layer was 0.56 μm and 0.52 μm, respectively.

[0212] Here, the thickness of the Fe-based plated layer is a value measured by the cross-sectional observation described above. For the steel sheet in which the oxide is partially formed in the internal portion of the Fe-based plated layer, the growth of the crystal grains in the Fe-based plated layer is suppressed by the internal oxide. Therefore, even if annealing is performed after the Fe-based plating treatment, the crystal grains of the Fe-based plated layer can be prevented from coarsening, a plurality of crystal boundaries are formed in the Fe-based plated layer, as a result, the invasion path of the molten zinc is dispersed, the time until the molten zinc reaches the crystal boundary of the cold-rolled steel sheet at the time of resistance welding is delayed, and excellent resistance welding crack resistance characteristics are obtained. Furthermore, by partially forming the oxide in the internal portion of the Fe-based plated layer, the depth of the internal oxidation layer from the depth of 0.10 μm from the surface of the Fe-based plated layer to the internal portion of the cold-rolled steel sheet can be made to be within 2.00 μm, the invasion in the depth direction of the cold-rolled steel sheet at the time when the zinc reaches the crystal boundary of the cold-rolled steel sheet is suppressed as much as possible, and more excellent resistance welding crack resistance characteristics are obtained.

[0213] Note that, when the analysis is performed in the depth direction from the surface by the glow discharge luminescence analysis method, a peak of the luminescence intensity of the wavelength indicating Si can be observed in both the range of more than 0.10 μm from the surface and the range of 0.00 μm to 0.10 μm from the surface. In the above Figure 5 In any one of Comparative Examples Nos. 21, 23, and 27, a peak of the luminescence intensity of the wavelength indicating Si was observed in both the range of more than 0.10 μm from the surface and the range of 0.00 μm to 0.10 μm from the surface. This indicates that the Si internal oxide is present and the Si external oxide is present in the surface layer.

[0214] [Amount of Internal Oxide in GA]

[0215] In the GA of the present embodiment, it is important to reduce the amount of internal oxides that are in contact with the galvannealed layer. Specifically, in the intensity distribution of the luminescence of the wavelength indicating Si that is measured from the surface of the galvannealed layer in the depth direction (the sheet thickness direction) by the Glow Discharge Optical Emission Spectrometry (GD-OES), it is important that the average Si intensity (I Si，Fe ) in the range of +0.5 μm from the interface of the galvannealed layer and the cold-rolled steel sheet toward the cold-rolled steel sheet side be divided by the average Si intensity (I Si，bulk ) in the cold-rolled steel sheet to give a value (I Si，Fe ) / (I Si，bulk ) of 0.90 or less. This can be achieved by performing annealing that satisfies Equation (2) A+B≥5.0.

[0216] (I Si，Fe ) / (I Si，bulk ) of 0.90 or less means that Si that diffuses from the cold-rolled steel sheet toward the Fe-based plated layer at the time of annealing forms oxides inside the Fe-based plated layer, and thus means that by partially forming oxides inside the Fe-based plated layer, it is possible to reduce the amount of internal oxides that are in contact with the galvannealed layer. Therefore, it is possible to suppress the invasion of zinc from the grain boundaries of the internal oxide layer that is in contact with the galvannealed layer. As a result, it is possible to delay the time at which molten zinc reaches the grain boundaries of the cold-rolled steel sheet at the time of welding, and to improve the resistance to welding crack characteristics of the welded portion.(I Si，Fe ) / (I Si，bulk ) is preferably 0.85 or less, and more preferably 0.80 or less. In addition, (I Si，Fe ) / (I Si，bulk ) is preferably 0.50 or more, and more preferably 0.60 or more.

[0217] The measurement conditions were an Ar gas pressure of 600 Pa, a high-frequency output of 35 W, a measurement diameter of 4 mm Φ, and a sampling interval of 0.1 seconds. Each average Si intensity was the average of all Si intensities sampled in each range. Note that after a cold-rolled steel sheet that had not been subjected to Fe-based plating and hot-dip plating was analyzed by the Glow Discharge Optical Emission Spectrometry under the same conditions, the depth of the sputter mark was measured, and thus the sputter speed was calculated, and the horizontal axis of the intensity distribution of the wavelength indicating Si was converted into the depth corresponding to each time. The depth of the sputter mark was measured using a non-contact surface shape measuring device (NewView 7300: manufactured by Zygo Corporation). The sputter speed of the galvannealed layer was different from the sputter speeds of the Fe-based plated layer and the cold-rolled steel sheet. That is, the reference differed due to the difference in elements used for depth conversion. Therefore, the interface of the galvannealed layer and the cold-rolled steel sheet was determined as follows. The intensity distribution of the wavelength indicating Si was measured using a GD-OES (Horizon II: manufactured by Horiba, Ltd.), and the depth of the sputter mark was measured using a non-contact surface shape measuring device (NewView 7300: manufactured by Zygo Corporation). Figure 9This section describes a method for determining the interface between the alloyed hot-dip galvanized layer and the cold-rolled steel sheet. First, the average Zn strength (IZn) within a range of 0.5 ± 0.1 μm from the surface of the alloyed hot-dip galvanized layer towards the sheet thickness is calculated and divided by 2. Next, the thickness-direction depth where the Zn strength reaches the aforementioned value (IZn / 2) is defined as the depth of the interface between the alloyed hot-dip galvanized layer and the cold-rolled steel sheet. Then, the thickness-direction depth is shifted by +0.5 μm from the interface between the alloyed hot-dip galvanized layer and the cold-rolled steel sheet towards the cold-rolled steel sheet side. The average Si strength (ISi) from the interface L1 to the position L2 at the +0.5 μm shift is calculated. Si，Fe Because the sputtering velocity of the alloyed hot-dip galvanized layer differs from that of the Fe-based electroplated layer and the cold-rolled steel sheet, the horizontal axis of the intensity distribution does not accurately correspond to the position of the interface between the alloyed hot-dip galvanized layer and the cold-rolled steel sheet, which can be visually observed through cross-section. Furthermore, in measurements based on glow discharge luminescence analysis, it is generally known that the intensity distribution is wide at interfaces composed of two or more materials due to unevenness, sputtering inhomogeneity, etc. Therefore, here, the thickness-direction depth of the interface between the alloyed hot-dip galvanized layer and the cold-rolled steel sheet is defined as the value IZn / 2, obtained by dividing the average Zn intensity (IZn) in the range of 0.5 ± 0.1 μm from the surface of the alloyed hot-dip galvanized layer towards the thickness direction.

[0218] use Figure 9 A representative example of the luminescence intensity representing the wavelengths of Si and Zn observed in this embodiment will be explained. Figure 9 Comparative Example No. 36 (Fe-free electroplating, dew point B of annealing atmosphere: +7.4°C) and Invention Example No. 41 (Fe-based electroplating adhesion amount A: 3.5 g / m) in Example 1 (Table 2) described later. 2 (Dew point B: +7.1℃) and Invention Example No. 45 (Fe-based electroplating adhesion amount 4.2 g / m 2 The raw data representing the luminescence intensity distribution of Si and Zn at wavelengths with dew point B: +7.6℃ are shown. In Comparative Example No. 36, (I Si，Fe ) / (I Si，bulk The value is 0.99. On the other hand, in Invention Examples No. 41 and 45, (I Si，Fe ) / (I Si，bulk The values ​​were 0.80 and 0.77, respectively.

[0219] [C concentration in CR, GI, and GA]

[0220] From the viewpoint of further improving the resistance to resistance welding cracks, in the CR and the GI of the present embodiment, it is important that the average value of the C concentration in the range of 10 μm to 20 μm from the surface of the Fe-based plated layer toward the thickness direction of the sheet be 0.10% by mass or less, preferably 0.06% by mass or less, and more preferably 0.04% by mass or less by annealing. Likewise, in the GA of the present embodiment, it is important that the average value of the C concentration in the range of 10 μm to 20 μm from the interface of the galvannealed layer with the cold-rolled steel sheet toward the thickness direction of the sheet be 0.10% by mass or less, preferably 0.06% by mass or less, and more preferably 0.04% by mass or less by annealing. On the other hand, in the case where the C concentration is too low, the fatigue strength can possibly decrease. Therefore, in the CR and the GI of the present embodiment, it is preferable that the average value of the C concentration in the range of 10 μm to 20 μm from the surface of the Fe-based plated layer toward the thickness direction of the sheet be 0.01% by mass or more. Likewise, in the GA of the present embodiment, it is preferable that the average value of the C concentration in the range of 10 μm to 20 μm from the interface of the galvannealed layer with the cold-rolled steel sheet toward the thickness direction of the sheet be 0.01% by mass or more.

[0221] At the time of annealing, a decarburized layer is formed in the surface layer portion of the steel sheet on which the Fe-based plating has been performed. The decarburized layer is a region in which the C concentration is lower than that in the steel near the surface of the steel sheet, and can be formed at the time of annealing in order for C to be removed from the surface of the steel sheet. In the CR and the GI of the present embodiment, as described above, if the average value of the C concentration in the range of 10 μm to 20 μm from the surface of the Fe-based plated layer toward the thickness direction of the sheet is 0.10% by mass or less, the region is soft. Likewise, in the GA of the present embodiment, if the average value of the C concentration in the range of 10 μm to 20 μm from the interface of the galvannealed layer with the cold-rolled steel sheet toward the thickness direction of the sheet is 0.10% by mass or less, the region is soft. Therefore, the stress applied from the electrode for welding at the time of resistance welding is mitigated, and the resistance to resistance welding cracks is improved.

[0222] In the CR and the GI of the present embodiment, by performing annealing after the formation of the Fe-based plated layer, the average value of the C concentration in the range of 10 μm to 20 μm from the surface of the Fe-based plated layer toward the thickness direction of the sheet can be further reduced as compared with the case where there is no Fe-based plated layer. Likewise, in the GA of the present embodiment, by performing annealing after the formation of the Fe-based plated layer, the average value of the C concentration in the range of 10 μm to 20 μm from the interface of the galvannealed layer with the cold-rolled steel sheet toward the thickness direction of the sheet can be further reduced as compared with the case where there is no Fe-based plated layer. Note that, in the case of individual plating of Ni, Co, Sn, or the like, the solid solubility of C in these metal elements is extremely low, and C is not solid-solved, so that the effect of promoting decarburization is not obtained.

[0223] The reason for the decrease in the C concentration in the range of 10 to 20 μm in the plate thickness direction from the surface of the Fe-based plated layer and the reason for the decrease in the C concentration in the range of 10 to 20 μm in the plate thickness direction from the interface of the galvannealed layer and the cold-rolled steel sheet are not clear, but the present inventors and others have conjectured as follows. That is, it is considered that C is induced to diffuse from the cold-rolled steel sheet because the Fe-based plated layer contains almost no C.

[0224] In addition, by softening due to the decrease in the C concentration in the range of 10 to 20 μm in the plate thickness direction from the surface of the Fe-based plated layer or the interface of the galvannealed layer and the cold-rolled steel sheet, the C concentration in the range of 10 to 20 μm in the plate thickness direction from the surface of the Fe-based plated layer or the interface of the galvannealed layer and the cold-rolled steel sheet is saturated at a certain level or less, and thus the improvement in the resistance spot weld crack resistance due to softening is limited. In the present embodiment, by further decreasing the C concentration in the range of 10 to 20 μm in the plate thickness direction from the surface of the Fe-based plated layer or the interface of the galvannealed layer and the cold-rolled steel sheet, the resistance spot weld crack resistance is effectively improved even in the case of a shallow decarburized layer, and thus it is shown that not only softening but also other effects such as an increase in the melting point due to a decrease in the C concentration are exhibited.

[0225] In the CR, GI, and GA of the present embodiment, the depth of the decarburized layer from the surface of the Fe-based plated layer for the CR and GI, and the depth of the decarburized layer from the interface of the galvannealed layer and the cold-rolled steel sheet for the GA, that is, the thickness of the decarburized layer is preferably 30 μm or more, and more preferably 80 μm or more. The upper limit of the thickness of the decarburized layer is not particularly limited, but in order to have a tensile strength in a good 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 the region in which the C concentration is 80% or less of that in the steel in the surface layer portion of the Fe-based plated steel sheet, by analyzing the C concentration in the plate thickness direction from the surface of the Fe-based plated layer for the CR and GI. In the GA, it is defined as the thickness of the region in which the C concentration is 80% or less of that in the steel in the surface layer portion of the cold-rolled steel sheet, by analyzing from the interface of the galvannealed layer and the cold-rolled steel sheet (that is, the surface of the cold-rolled steel sheet).

[0226] Here, the average value of the C concentration in the range of 10 to 20 pm from the surface of the Fe-based plated layer in the CR and GI or from the interface of the galvannealed layer with the cold-rolled steel sheet in the GA, and the thickness of the decarburized layer in the vicinity of the surface of the Fe-based plated layer in the CR and GI or in the vicinity of the interface of the galvannealed layer with the cold-rolled steel sheet in the GA are determined by performing surface analysis or line analysis of the element distribution in the vicinity of the surface layer using an electron probe micro analyzer (EPMA) on a sample after cross-section processing. First, the resin-embedded steel sheet is ground, and the cross-section perpendicular to the rolling direction is finished for observation, and then the resin is removed to obtain a sample for measurement. The acceleration voltage is 7 kV, the irradiation current is 50 nA, and the surface analysis or line analysis of the sample cross-section is performed in the range of 300 x 300 pm of the outermost layer including the Fe-based plated layer in the CR and GI, and the outermost layer including the cold-rolled steel sheet in the GA, at 1 pm steps, and the measurement of the C intensity is performed. At this time, in order to suppress contamination, a plasma cleaner is used to remove hydrocarbons from the surface and periphery of the sample in the measurement chamber and the sample preparation chamber before the measurement starts. In addition, in order to suppress the accumulation of hydrocarbons during measurement, the sample temperature is heated and maintained at 100°C on the workbench during measurement. A calibration line prepared by measuring a standard sample is used to convert the C intensity to the C concentration (mass %). As a result of the effect of contamination suppression, it was confirmed that the lower limit of C detection was lower than 0.04 mass %.

[0227] Reference 1: Yamanaka et al., "Distribution of Carbon at the Initial Stage of Proeutectoid Ferrite Transformation in Low Carbon Steel Based on High-precision FE-EPMA", Iron and Steel, Vol. 103 (2017) No. 11. p14-20

[0228] However, the necessity of the contamination countermeasures at the time of measurement depends on the model and conditions used, and therefore the above-described configuration is not necessarily essential. That is, as long as it can be confirmed that the measurement conditions are sufficient in terms of accuracy, the measurement conditions are essentially unrelated to the effects of the present application.

[0229] In the obtained C concentration maps, a line profile along the thickness direction is extracted from the surface of the Fe-based electroplated layer in CR and GI, and from the interface between the alloyed hot-dip galvanized layer and the cold-rolled steel sheet in GA. The average value of 300 points along the parallel direction of the cold-rolled steel sheet surface is calculated, thus obtaining the thickness-direction distribution of C concentration. The location on the surface of the cold-rolled steel sheet can be determined based on simultaneously obtained secondary electron images or reflected electron images. The obtained thickness-direction distribution of C concentration is smoothed using a simple moving average method. The number of smoothing points is preferably around 21. If the number of smoothing points near the surface of the sample is less than 10 points on one side, it is preferable to smooth the measurement points that can be taken on one side. Next, in the intensity distribution after smoothing, the thickness of the decarburized layer is determined as the range in the thickness direction where the C concentration is less than 80% of the steel in the surface portion of the Fe-based electroplated steel sheet containing the Fe-based electroplated layer and the cold-rolled steel sheet in CR and GI, and in the surface portion of the cold-rolled steel sheet in GA. Furthermore, for the range of 10 μm to 20 μm in the thickness direction from the surface of the Fe-based electroplated layer in CR and GI, and from the interface between the alloyed hot-dip galvanized layer and the cold-rolled steel sheet in GA, the average C concentration values ​​at 1 μm intervals were taken as the C concentration for the 10 μm to 20 μm thickness direction. The above evaluation was applied to the measurement results of two fields of view for each sample, and the average was taken as the average C concentration and the evaluation value for the thickness of the decarburized layer in the 10 μm to 20 μm thickness direction.

[0230] use Figure 10A B and Figure 11A Sections B and B illustrate representative examples of the thickness-direction distribution of C concentration analyzed using an electron probe microanalyzer. Figure 10A The data are the original data on the thickness-direction distribution of C concentration obtained by analyzing Fe-based electroplated steel sheets No. 21, 23 and 27 of Examples 1 (Table 4) described later. Figure 11A These are raw data on the thickness-direction distribution of C concentration obtained from analyzing alloyed hot-dip galvanized steel sheets No. 36, 41, and 45 of Example 1 (Table 2). It should be noted that the measurements were performed on the alloyed hot-dip galvanized steel sheets after peeling off the alloyed hot-dip galvanized layer. Figure 10B and Figure 11B These represent the effects of a simple moving average method with 21 points of smoothing on... Figure 10A and Figure 11A The original data after smoothing (m=21). For example... Figure 10B and Figure 11B As shown, in No. 27 of Table 4 and No. 45 of Table 2, there are decarburized layers with a C concentration of less than 80% in the steel, and the thicknesses of the decarburized layers are 77 μm and 81 μm, respectively.

[0231] [Crystal grains in CR and GI]

[0232] In the CR and GI of the present embodiment, at the interface between the Fe-based plating layer and the cold-rolled steel sheet, the number of grain boundaries of the Fe-based plating layer in contact with the cold-rolled steel sheet is preferably 10 or more per 10 μm in the steel sheet width direction in the field of view for observation of the cold-rolled steel sheet. In this case, the crystals of the Fe-based plating layer are sufficiently fine-grained. It is considered that by fine-graining, a plurality of grain boundaries are formed in the Fe-based plating layer, as a result of which the intrusion of molten zinc is dispersed, the time until the grain boundaries of the cold-rolled steel sheet are reached at the time of welding is delayed, the resistance spot weld crack resistance of the welded portion is improved, and in particular, internal cracks can be prevented. The number of grain boundaries per 10 μm is more preferably 20 or more, and further preferably 25 or more. On the other hand, in the case where the number of grain boundaries is excessively large, the fatigue strength can possibly decrease. Therefore, the number of grain boundaries per 10 μm is preferably 40 or less.

[0233] Here, the number of grain boundaries of the Fe-based plating layer in contact with the cold-rolled steel sheet at the interface between the Fe-based plating layer and the cold-rolled steel sheet is determined as follows. First, a sample of 10 x 10 mm size is taken from the Fe-based plated steel sheet. An arbitrary portion of the sample is processed using a focused ion beam (FIB) device, and a 45° cross section having an angle of 45° with respect to the T cross section (a cross section parallel to the rolling right angle direction of the steel sheet and perpendicular to the steel sheet surface) direction, a width of 30 μm in the rolling right angle direction, and a length of 50 μm in the 45° direction with respect to the T cross section direction is formed at the portion as an observation sample. In Figure 6 a summary of the observation sample is shown. Figure 6 (a) of FIG. 10 is a perspective view of the observation sample. Figure 6 (b) of FIG. 10 is a plan view of the observation sample shown in (a) of FIG. 10. Figure 6 (a) of FIG. 10 is a perspective view of the observation sample. Figure 7 Figure 7 is a SIM image obtained by photographing No. 11 of Example 3 (Table 11) described later as described above. A region of 10 μm in the width direction of the cold-rolled steel sheet (a portion surrounded by a frame in Figure 7 is extracted from the SIM image. For the purpose of explanation, an enlarged view of the portion surrounded by the frame in Figure 8 is shown in Figure 7 (a) of FIG. 13. As shown in Figure 8 , for the SIM image, a boundary line is drawn at the interface between the Fe-based plating layer and the cold-rolled steel sheet in a region of 10 μm in the sheet width direction of the cold-rolled steel sheet. Figure 8 ​The number of grain boundaries of the Fe-based plated layer on the boundary line was measured as the "number of grain boundaries of the Fe-based plated layer in contact with the cold-rolled steel sheet at the interface between the Fe-based plated layer and the cold-rolled steel sheet". In Inventive Example No. 11, the number of grain boundaries of the Fe-based plated layer in contact with the cold-rolled steel sheet at the interface between the Fe-based plated layer and the cold-rolled steel sheet was 20 per 10 μm in the width direction of the cold-rolled steel sheet.

[0234] (Chemically converted steel sheet and method for producing the same)

[0235] A chemical conversion treatment can be applied to the Fe-based plated steel sheet (CR or GI) or the galvannealed steel sheet (GA) to obtain a chemically converted steel sheet in which a chemical conversion coating is formed on the surface of the Fe-based plated steel sheet or the galvannealed steel sheet. At this time, as pretreatments for the chemical conversion treatment, a degreasing treatment for cleaning the surface of the Fe-based plated steel sheet or the galvannealed steel sheet, water washing, and a surface conditioning treatment as necessary can be applied. The chemical conversion treatment is then applied after these pretreatments. The method of the degreasing treatment and the water washing is not particularly limited, and a general method can be used. In the surface conditioning treatment, a surface conditioner having a Ti colloid or a zinc phosphate colloid, or the like can be used. When these surface conditioners are applied, a special process need not be provided, and a general method can be applied. For example, a desired surface conditioner is dissolved in prescribed deionized water, and after being sufficiently stirred, a treatment liquid at a prescribed temperature (generally, room temperature, 25 to 30°C) is prepared, and the steel sheet is immersed in the treatment liquid for a prescribed time (20 to 30 seconds). The next process of the chemical conversion treatment is then performed without drying. In the chemical conversion treatment, a general method can also be applied. For example, a desired chemical conversion treatment agent is dissolved in prescribed deionized water, and after being sufficiently stirred, a treatment liquid at a prescribed temperature (generally, 35 to 45°C) is prepared, and the steel sheet is immersed in the treatment liquid for a prescribed time (60 to 120 seconds). As the chemical conversion treatment agent, for example, a zinc phosphate treatment agent for steel, a zinc phosphate treatment agent for steel and aluminum, a zirconium treatment agent, and the like can be used.

[0236] (Electrodeposited coated steel sheet and method for producing the same)

[0237] Next, the chemical conversion-treated steel sheet can be subjected to electrodeposition coating to obtain an electrodeposition-coated steel sheet in which an electrodeposition coating film is formed in contact with the chemical conversion film. The electrodeposition coating can also be performed according to a conventional method. After performing a pretreatment such as water washing treatment as needed, the steel sheet is immersed in an electrodeposition paint subjected to sufficient agitation, and an electrodeposition coating film having a desired thickness is obtained by electrodeposition treatment. As the electrodeposition coating, in addition to a cationic electrodeposition coating, an anionic electrodeposition coating can also be used. Furthermore, according to the use, an overcoat coating or the like can be performed after the electrodeposition coating. The thickness of the electrodeposition coating film varies according to the use, but is preferably about 10 to 30 μm in the dry state.

[0238] (Automobile component and method for manufacturing the same)

[0239] An automobile component can be manufactured using at least a part of the electrodeposition-coated steel sheet. The Fe-based plated steel sheet and the welded portion of the galvannealed steel sheet of the present embodiment are excellent in resistance to resistance welding cracking, and thus the electrodeposition-coated steel sheet using the Fe-based plated steel sheet and the galvannealed steel sheet is particularly suitable for use in an automobile component. The type of the automobile component is not particularly limited, and can be, for example, a side beam component, a pillar component, an automobile body, or the like.

[0240] Example

[0241] (Example 1)

[0242] A cast slab obtained by melting a steel having a composition shown in Table 1 (the remainder being Fe and inevitable impurities) was hot-rolled to obtain a hot-rolled steel sheet, and the hot-rolled steel sheet was subjected to pickling and cold-rolling to obtain a cold-rolled steel sheet having a sheet thickness of 1.6 mm.

[0243] [Table 1]

[0244]

[0245] Various GAs shown in Table 2, various GIs shown in Table 3, and various CRs shown in Table 4 were produced using these cold-rolled steel sheets.

[0246] First, the cold-rolled steel sheet was subjected to degreasing treatment using an alkali, and then the cold-rolled steel sheet was subjected to electrolysis treatment under the conditions shown below with the cold-rolled steel sheet serving as a cathode to produce an Fe-based plated steel sheet. The attachment amount A of the Fe-based plated layer shown in Tables 2 to 4 was calculated by the above method, and was controlled by the current application time.

[0247] [Electrolysis conditions]

[0248] Bath temperature: 50°C

[0249] pH: 2.0

[0250] Current density: 45 A / dm 2

[0251] Plating bath: containing 1.5 mol / L of Fe 2+ Sulfuric acid bath containing ions

[0252] Anode: Iridium oxide electrode

[0253] Next, the Fe-based plated steel sheet was heated at an average temperature- increasing rate in the range of 400°C to 650°C at the values shown in Tables 2 to 4, and then annealing was performed in which the steel sheet was heated at 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 vol% of hydrogen, and the remainder consisting of N2and inevitable impurities. The holding time of the steel sheet at the maximum reaching temperature (800°C) was 100 seconds. In the examples shown in Table 4, the Fe-based plated steel sheet (CR) was obtained in this manner.

[0254] In the examples shown in Tables 2 and 3, after the obtained Fe-based plated steel sheet was cooled to 440 to 550°C, hot dip galvanizing treatment was performed on the Fe-based plated steel sheet using a hot dip galvanizing bath of 460°C having an effective Al concentration in the bath of 0.132 mass% and the remainder consisting of Zn and inevitable impurities, and then the attachment amount of the hot dip galvanizing layer per one side surface was adjusted by gas wiping. In the examples shown in Table 3, the hot dip galvanizing steel sheet (GI) was obtained in this manner.

[0255] In the examples shown in Table 2, further, a process of heating and alloying the hot dip galvanizing layer by alloying treatment at 510°C was further performed, and an alloyed hot dip galvanizing steel sheet (GA) was manufactured. The time of the alloying treatment was changed, and the % of Fe in the alloyed hot dip galvanizing layer was controlled.

[0256] The attachment amount and the % of Fe of the hot dip galvanizing layer or the alloyed hot dip galvanizing layer of each steel sheet obtained by the above-described method are shown in Tables 2 and 3.

[0257] The values of (I Si，Fe ) / (I Si，bulk ), the average value of the C concentration, and the thickness of the decarburized layer obtained by the above-described method are shown in Table 2. The thickness of the internal oxidation layer, the average value of the 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 obtained by the above-described method are shown in Tables 3 and 4.

[0258] In the examples of GA and GI, the appearance of the plated layer (Evaluation 1) and the resistance welding crack resistance of the welded portion (Evaluation 3) were evaluated, and the results are shown in Tables 2 and 3. In the examples of CR, the chemical conversion treatment property and the post-painting corrosion resistance (Evaluation 2) and the resistance welding crack resistance of the welded portion (Evaluation 3) were evaluated, and the results are shown in Table 4.

[0259] [Assessment 1: Evaluation of the appearance of the zinc plating layer (hot-dip galvanized layer or alloyed hot-dip galvanized layer)]

[0260] The presence or absence of appearance defects (non-plating, appearance unevenness) was visually determined and evaluated according to the following criteria.

[0261] O: No appearance defects.

[0262] Δ: Although there were appearance defects, no Fe-based plating layer or exposed cold-rolled steel sheet unevenness was confirmed when observed at 3000 times using SEM.

[0263] X: There were appearance defects, and non-plating of the Fe-based plating layer or exposed cold-rolled steel sheet was confirmed when observed at 3000 times using SEM.

[0264] Note that the presence or absence of exposure of the Fe-based plating layer or cold-rolled steel sheet can be determined based on the difference in contrast between Zn and Fe in SEM. Further specifically, analysis is performed using EDX (Energy Dispersive X-ray Spectroscopy) to determine whether Fe is detected or not.

[0265] [Assessment 2: Evaluation of chemical conversion treatment and post-coating corrosion resistance]

[0266] (1) Chemical conversion treatment

[0267] A chemical conversion treatment test piece having a chemical conversion treatment coating film on both the front and back surfaces of the test piece was produced by performing degreasing treatment, surface conditioning treatment, and chemical conversion treatment on a test piece taken from the above Fe-based plated steel sheet. First, the test piece taken from the above Fe-based plated steel sheet was immersed in a degreasing agent, and degreasing treatment was performed under the following standard conditions.

[0268] [Degreasing treatment]

[0269] • Degreasing agent: FC-E2011 (manufactured by Japan Parkerizing Co., Ltd.)

[0270] • Treatment temperature: 43°C

[0271] • Treatment time: 120 seconds

[0272] Next, the test piece after degreasing treatment was sprayed with a surface conditioner, and surface conditioning treatment was performed under the following standard conditions.

[0273] [Surface conditioning treatment]

[0274] • Surface conditioner: Prepalene XG (PL-XG; manufactured by Japan Parkerizing Co., Ltd.)

[0275] • pH: 9.5

[0276] • Treatment temperature: room temperature

[0277] • Treatment time: 20 seconds

[0278] Next, the test piece after the surface adjustment treatment was immersed in a chemical conversion treatment agent, and a chemical conversion treatment was performed under the following standard conditions.

[0279] [Chemical Conversion Treatment]

[0280] • Chemical conversion treatment agent: Palbond PB-SX35 (manufactured by Japan Parkerizing Co.)

[0281] • Temperature of chemical conversion treatment liquid: 35°C

[0282] • Treatment time: 90 seconds

[0283] The chemical conversion treatment properties described below were measured using the chemical conversion treatment test piece manufactured as described above.

[0284] (2) Electrodeposition Coating Treatment

[0285] An electrodeposition coating test piece was manufactured by performing electrodeposition coating on the surface of the above-described chemical conversion treatment test piece using an electrodeposition coating material (GT-100) manufactured by Kansai Paint Co., Ltd., so that the film thickness became 15 μm. This electrodeposition coating test piece was subjected to the salt hot water immersion test described below.

[0286] <Chemical Conversion Treatment Properties>

[0287] The surface of the above-described chemical conversion treatment test piece (n = 1) was observed by SEM at a magnification of 1000 times, and evaluated according to the following criteria. Note that if it was, or O, it was judged that the chemical conversion treatment properties were excellent.

[0288] : The particle diameter of the chemical conversion crystals was 5 μm or less, and no unprecipitated portions were confirmed.

[0289] : The particle diameter of the chemical conversion crystals exceeded 5 μm, but no unprecipitated portions were confirmed.

[0290] : The particle diameter of the chemical conversion crystals exceeded 5 μm, and unprecipitated portions were confirmed.

[0291] <Salt Hot Water Immersion Test>

[0292] After the cross-cut scratch of 45 mm in length was imparted to the surface of the electrodeposited coated test piece (n = 1) described above with a knife, the test piece was immersed in a 5 mass% NaCl solution (60°C) for 360 hours, followed by water washing and drying. Subsequently, the tape peeling test was performed in which a cellophane tape was pasted to the cross-cut scratch portion of the test piece and then peeled, and the maximum total peeling width of the electrodeposited coating film combined with the cross-cut scratch portion on the left and right was measured. The maximum total peeling width of the electrodeposited coating film was evaluated in accordance with the following criteria. Note that if it is, the corrosion resistance after coating is judged to be excellent.

[0293] : The maximum total peeling width is 3.0 mm or less

[0294] : The maximum total peeling width is 5.0 mm or less

[0295] : The maximum total peeling width exceeds 5.0 mm

[0296] [Evaluation 3: Evaluation of resistance welded crack resistance of the welded portion]

[0297] Referring to Figure 4 (a), a test piece 6 of 150 mm in lengthwise direction x 50 mm in shortwise direction was cut from each of the steel sheets (CR, GI, GA) of the inventive examples and the comparative examples with the rolling right angle direction (TD) as the long side and the rolling direction as the short side, and a test alloyed hot-dip galvanized steel sheet 5 (sheet thickness: 1.6 mm, TS: 980 MPa grade) of which the attached amount of the zinc plating layer on each single side was 50 g / m 2 was cut to the same size, and the plate set was made by superimposing them. The plate set was assembled so that the evaluation target surface of the test piece 6 (Fe plating layer in the case of CR, and zinc plating layer in the case of GI and GA) and the zinc plating layer of the test alloyed hot-dip galvanized steel sheet 5 faced each other. The plate set was fixed to a fixing table 8 via a spacer 7 of 2.0 mm in thickness. The spacer 7 was a pair of steel sheets of 50 mm in lengthwise direction x 45 mm in shortwise direction x 2.0 mm in thickness, and as shown in (a) of Figure 4 , the pair of steel sheets were arranged so that each of the lengthwise direction end surfaces aligned with both of the shortwise direction end surfaces of the plate set. Thus, the distance between the pair of steel sheets was 60 mm. The fixing table 8 was a plate having a hole in the center portion.

[0298] Next, using a servo motor-driven single-phase AC (50Hz) resistance welding machine, the plate assembly was stacked while being pressurized using a pair of electrodes 9 (front diameter: 6mm). Resistance welding was performed at a welding current with a weld nugget diameter r of 5.9mm under the following conditions: pressure: 3.5kN, holding time: 0.18 seconds or 0.24 seconds, and welding time: 0.36 seconds. At this time, the pair of electrodes 9 pressurize the plate assembly vertically from top to bottom, with the lower electrode pressing the test piece 6 through a hole in the fixing platform 8. During pressurization, the lower electrode of the pair of electrodes 9 is fixed to the fixing platform 8 by contacting a plane extending from the surface where the separator 7 contacts the fixing platform 8, allowing the upper electrode to move. The upper electrode is also in contact with the central portion of the test alloyed hot-dip galvanized steel plate 5. Furthermore, the plate assembly was welded while tilted 5° towards the long side of the plate assembly relative to the horizontal direction. It should be noted that the holding time refers to the time from the end of the welding current flow to the start of electrode release. Here, refer to... Figure 4 In Figure (b) below, the diameter r of the melt core refers to the distance between the ends of the melt cores 10 along the long side of the plate assembly.

[0299] Next, the aforementioned plate assembly with the welded portion is arranged along the center of the welded portion containing the weld nugget 10. Figure 4 (b) The cross-section of the welded section was observed using an optical microscope (200x) on the B-B wire cut in the above figure. The resistance to weld cracking characteristics of the welded section were evaluated according to the following criteria. It should be noted that if it is ◎ or ○, the resistance to weld cracking characteristics of the welded section are judged to be excellent. If it is ×, the resistance to weld cracking characteristics of the welded section are judged to be poor.

[0300] ◎: No cracks longer than 0.1 mm were detected when the holding time was 0.18 seconds.

[0301] ○: A crack longer than 0.1 mm was confirmed when the holding time was 0.18 seconds, but no crack longer than 0.1 mm was confirmed when the holding time was 0.24 seconds.

[0302] ×: A crack longer than 0.1 mm was detected when the holding time was 0.24 seconds.

[0303] It should be noted that, in Figure 4 (b) In the figure below, the crack generated in test piece 6 is schematically shown by symbol 11. It should be noted that when cracks occur in the target-side steel plate (the alloyed hot-dip galvanized steel plate used in the test), it is impossible to properly evaluate the stress dispersion of the target steel plate (the steel plates of each inventive example and comparative example). Therefore, data in which no cracks occurred in the target-side steel plate are used as examples.

[0304] [Table 2]

[0305]

[0306]

[0307] [Table 3]

[0308]

[0309] [Table 4]

[0310]

[0311] The relationship between the adhesion amount A of the Fe-based plated layer and the dew point B at the time of annealing in the examples of GA shown in Table 2 and the evaluation results of the resistance welding crack resistance characteristics is shown in Figure 1 . In addition, the relationship between the adhesion amount A of the Fe-based plated layer and the dew point B at the time of annealing in the examples of GI shown in Table 3 and the evaluation results of the resistance welding crack resistance characteristics is shown in Figure 2 . In addition, the relationship between the adhesion amount A of the Fe-based plated layer and the dew point B at the time of annealing in the examples of CR shown in Table 4 and the evaluation results of the resistance welding crack resistance characteristics is shown in Figure 3 .

[0312] From Tables 2, 3 and Figure 1 , 2 It is clear that in the inventive examples of the examples of GA and GI, the appearance of the zinc-plated layer is good, and excellent resistance welding crack resistance characteristics can be achieved. In addition, from Tables 4 and Figure 3 it is clear that in the inventive examples of the examples of CR, the chemical conversion treatment properties and the post-painting corrosion resistance are good, and excellent resistance welding crack resistance characteristics can be achieved.

[0313] From the results of Figure 1 (GA), when compared at the same dew point, by increasing the adhesion amount A of the Fe-based plated layer before annealing, the resistance welding crack resistance characteristics are improved. In addition, it is known that the boundary line (solid line) that distinguishes whether the resistance welding crack resistance characteristics are improved or not is only at the adhesion amount A of the Fe-based plated layer before annealing of 1.0 g / m 2 The above region shows linearity, and in the region where the adhesion amount A of the Fe-based plated layer before annealing is 1.0 g / m 2 or more, the resistance welding crack resistance characteristics are improved, particularly at a lower dew point. In GA, by alloying, the Fe-based plated layer is alloyed with the zinc-plated alloy without remaining, and thus it is presumed that by modification of the surface layer of the cold-rolled steel sheet that becomes the base, particularly by combining the Fe-based plated layer of the prescribed adhesion amount or more and a high dew point, decarburization is promoted, and thus the resistance welding crack resistance characteristics are improved.

[0314] On the other hand, in the case of GI and CR, since the Fe-based plated layer remains, the Fe-based plated layer further functions as a surface layer soft phase, and when the Fe-based plated layer adhesion amount before annealing is compared at the same dew point, it is presumed that the resistance welded crack resistance is improved more than in the case of GA.

[0315] (Example 2)

[0316] A cast slab obtained by melting a steel having the composition shown in Table 5 (the remainder being Fe and unavoidable impurities) was hot-rolled to obtain a hot-rolled steel sheet, and the hot-rolled steel sheet was subjected to pickling and cold-rolling to obtain a cold-rolled steel sheet having a sheet thickness of 1.6 mm.

[0317] [Table 5]

[0318]

[0319] 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.

[0320] First, the cold-rolled steel sheet was subjected to degreasing treatment using alkali, and then the cold-rolled steel sheet was subjected to electrolytic treatment under the conditions shown below as a cathode to produce an Fe-based plated steel sheet. The Fe-based plated layer adhesion amount A shown in Tables 6 to 8 was calculated by the above-described method, and was controlled by the current application time.

[0321] [Electrolysis conditions]

[0322] Bath temperature: 50°C

[0323] pH: 2.0

[0324] Current density: 45 A / dm 2

[0325] Plating bath: sulfuric acid bath containing 1.5 mol / L of Fe ion 2+

[0326] Anode: iridium oxide electrode

[0327] Next, the Fe-based plated steel sheet was heated at an average temperature increase rate in the temperature range of 400°C to 650°C shown in Tables 6 to 8, and then annealing was performed in which the steel sheet was heated at 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 vol% of hydrogen, and the remainder consisting of N2 and unavoidable impurities. The holding time of the steel sheet at the maximum temperature reached (800°C) was 100 seconds. In the example shown in Table 8, the Fe-based plated steel sheet (CR) was obtained in this manner.

[0328] ​In the examples shown in Tables 6, 7, the obtained Fe-based plated steel sheet was cooled to 440 to 5500C, and then hot dip galvanizing treatment was performed on the Fe-based plated steel sheet using a 4600C hot dip galvanizing bath having an effective Al concentration of 0.132 mass% and the remaining portion consisting of Zn and unavoidable impurities, and then the attachment amount of the hot dip galvanizing layer per one side face was adjusted by gas wiping. In the examples shown in Table 7, the hot dip galvanizing steel sheet (GI) was obtained in this manner.

[0329] In the examples shown in Table 6, furthermore, the process of heating and alloying the hot dip galvanizing layer by alloying treatment at 5100C was further performed, and an alloyed hot dip galvanizing steel sheet (GA) was manufactured. The time of the alloying treatment was changed, and the Fe% in the alloyed hot dip galvanizing layer was controlled.

[0330] The attachment amount and the Fe% of the hot dip galvanizing layer or the alloyed hot dip galvanizing layer of each steel sheet obtained by the above-described method are shown in Tables 6 and 7.

[0331] The values of (I Si，Fe ) / (I Si，bulk ), the average value of the C concentration, and the thickness of the decarburized layer obtained by the above-described method are shown in Table 6. The thickness of the internal oxidation layer, the average value of the 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 obtained by the above-described method are shown in Tables 7 and 8.

[0332] In the examples of GA and GI, the appearance of the plated layer (Evaluation 1) and the resistance welding crack resistance of the welded portion (Evaluation 3) were evaluated, and the results are shown in Tables 6 and 7. In the examples of CR, the chemical conversion treatment property and the post-painting corrosion resistance (Evaluation 2) and the resistance welding crack resistance of the welded portion (Evaluation 3) were evaluated, and the results are shown in Table 8. Note that the evaluation methods and the evaluation criteria of Evaluations 1 to 3 were the same as in Example 1.

[0333] [Table 6]

[0334]

[0335] [Table 7]

[0336]

[0337] [Table 8]

[0338]

[0339] As is clear from Tables 6 and 7, in the inventive examples of the examples of GA and GI, the appearance of the zinc plating layer was good, and excellent resistance to resistance welding cracks could be achieved. In addition, as is clear from Table 8, in the inventive examples of the examples of CR, the chemical conversion treatment property and the post-painting corrosion resistance were good, and excellent resistance to resistance welding cracks could be achieved.

[0340] (Example 3)

[0341] A cast slab obtained by melting a steel having the composition shown in Table 1 (the remainder being Fe and unavoidable impurities) was hot-rolled to obtain a hot-rolled steel sheet, and the hot-rolled steel sheet was subjected to pickling and cold-rolling to obtain a cold-rolled steel sheet having a sheet thickness of 1.6 mm.

[0342] 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.

[0343] First, the cold-rolled steel sheet was subjected to degreasing treatment using alkali, and then, the cold-rolled steel sheet was subjected to electrolytic treatment under the conditions shown below with the cold-rolled steel sheet as the cathode to produce an Fe-based plated steel sheet. The attached amount A of the Fe-based plated layer shown in Tables 9 to 11 was calculated by the above method, and was controlled by the current application time.

[0344] [Electrolysis conditions]

[0345] Bath temperature: 50°C

[0346] pH: 2.0

[0347] Current density: 45 A / dm 2

[0348] Plating bath: sulfuric acid bath containing 1.5 mol / L of Fe ions 2+

[0349] Anode: iridium oxide electrode

[0350] Next, the Fe-based plated steel sheet was heated at an average temperature increase rate in the temperature range of 400°C to 650°C at the value shown in Tables 9 to 11, and then, annealing was performed in which the steel sheet was heated at 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 vol% of hydrogen, and the remainder consisting of N2 and unavoidable impurities. The holding time of the steel sheet at the maximum temperature reached (800°C) was 100 seconds. In the examples shown in Table 11, the Fe-based plated steel sheet (CR) was obtained in this manner.

[0351] ​In the examples shown in Tables 9 and 10, after the obtained Fe-based plated steel sheet was cooled to 440 to 5500C, then hot dip galvanizing treatment was performed on the Fe-based plated steel sheet using a 4600C hot dip galvanizing bath having an effective Al concentration of 0.132 mass% and the remaining portion consisting of Zn and unavoidable impurities, and then the attachment amount of the hot dip galvanizing layer per one side surface was adjusted by gas wiping. In the examples shown in Table 10, the hot dip galvanizing steel sheet (GI) was obtained in this manner.

[0352] In the examples shown in Table 9, furthermore, a process of heating and alloying the hot dip galvanizing layer by alloying treatment at 5100C was further performed, and an alloyed hot dip galvanizing steel sheet (GA) was manufactured. The time of the alloying treatment was changed, and the Fe% in the alloyed hot dip galvanizing layer was controlled.

[0353] The attachment amount and the Fe% of the hot dip galvanizing layer or the alloyed hot dip galvanizing layer of each steel sheet obtained by the above-described method are shown in Tables 9 and 10.

[0354] The values of (I Si，Fe ) / (I Si，bulk ), the average value of the C concentration, and the thickness of the decarburized layer are shown in Table 9. The thickness of the internal oxidation layer, the average value of the 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 obtained by the above-described method are shown in Tables 10 and 11.

[0355] In the examples of GA and GI, the appearance of the plated layer (Evaluation 1) and the resistance welding crack resistance of the welded portion (Evaluation 3) were evaluated, and the results are shown in Tables 9 and 10. In the examples of CR, the chemical conversion treatment property and the post-painting corrosion resistance (Evaluation 2) and the resistance welding crack resistance of the welded portion (Evaluation 3) were evaluated, and the results are shown in Table 11. Note that the evaluation methods and the evaluation criteria of Evaluations 1 to 3 were the same as in Example 1.

[0356] [Table 9]

[0357]

[0358] [Table 10]

[0359]

[0360] [Table 11]

[0361]

[0362] As is clear from Tables 9 and 10, in the inventive examples of the examples of GA and GI, the appearance of the zinc plating layer was good, and excellent resistance to resistance welding cracks could be achieved. In addition, as is clear from Table 11, in the inventive examples of the examples of CR, the chemical conversion treatment property and the post-painting corrosion resistance were good, and excellent resistance to resistance welding cracks could be achieved.

[0363] (Example 4)

[0364] A cast slab obtained by melting a steel having the composition shown in Table 5 (the remainder being Fe and unavoidable impurities) was hot-rolled to obtain a hot-rolled steel sheet, and the hot-rolled steel sheet was subjected to pickling and cold-rolling to obtain a cold-rolled steel sheet having a sheet thickness of 1.6 mm.

[0365] 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.

[0366] First, the cold-rolled steel sheet was subjected to degreasing treatment using alkali, and then, the cold-rolled steel sheet was subjected to electrolytic treatment under the conditions shown below with the cold-rolled steel sheet as the cathode to produce an Fe-based plated steel sheet. The attached amount A of the Fe-based plated layer shown in Tables 12 to 14 was calculated by the above method, and was controlled by the current application time.

[0367] [Electrolysis conditions]

[0368] Bath temperature: 50°C

[0369] pH: 2.0

[0370] Current density: 45 A / dm 2

[0371] Plating bath: sulfuric acid bath containing 1.5 mol / L of Fe ions 2+

[0372] Anode: iridium oxide electrode

[0373] Next, the Fe-based plated steel sheet was heated at an average temperature increase rate in the temperature range of 400°C to 650°C shown in Tables 12 to 14, and then, annealing was performed in which the steel sheet was heated at 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 vol% of hydrogen, and the remainder consisting of N2 and unavoidable impurities. The holding time of the steel sheet at the maximum temperature reached (800°C) was 100 seconds. In the examples shown in Table 14, the Fe-based plated steel sheet (CR) was obtained in this manner.

[0374] ​In the examples shown in Tables 12 and 13, after the obtained Fe-based plated steel sheet was cooled to 440 to 5500C, then hot dip galvanizing treatment was performed on the Fe-based plated steel sheet using a 4600C hot dip galvanizing bath having an effective Al concentration of 0.132 mass% and the remaining portion consisting of Zn and unavoidable impurities, and then the attachment amount of the hot dip galvanizing layer per one side face was adjusted by gas wiping. In the examples shown in Table 13, the hot dip galvanizing steel sheet (GI) was obtained in this manner.

[0375] In the examples shown in Table 12, furthermore, then further a step of heating and alloying the hot dip galvanizing layer by alloying treatment at 5100C was performed, and an alloyed hot dip galvanizing steel sheet (GA) was manufactured. The time of the alloying treatment was changed, and the Fe% in the alloyed hot dip galvanizing layer was controlled.

[0376] The attachment amount and the Fe% of the hot dip galvanizing layer or the alloyed hot dip galvanizing layer of each steel sheet obtained by the above-described method are shown in Tables 12 and 13.

[0377] The values of (I Si，Fe ) / (I Si，bulk ), the average value of the C concentration, and the thickness of the decarburized layer are shown in Table 12. The thickness of the internal oxidized layer, the average value of the 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 obtained by the above-described method are shown in Tables 13 and 14.

[0378] In the examples of GA and GI, the appearance of the plated layer (Evaluation 1) and the resistance welding crack resistance of the welded portion (Evaluation 3) were evaluated, and the results are shown in Tables 12 and 13. In the examples of CR, the chemical conversion treatment property and the post-painting corrosion resistance (Evaluation 2) and the resistance welding crack resistance of the welded portion (Evaluation 3) were evaluated, and the results are shown in Table 14. Note that the evaluation methods and the evaluation criteria of Evaluations 1 to 3 were the same as in Example 1.

[0379] [Table 12]

[0380]

[0381] [Table 13]

[0382]

[0383] [Table 14]

[0384]

[0385] As is clear from Tables 12 and 13, in the inventive examples of the examples of GA and GI, the appearance of the zinc plating layer was good, and excellent resistance to resistance welding cracks could be achieved. In addition, as is clear from Table 14, in the inventive examples of the examples of CR, the chemical conversion treatment property and the post-painting corrosion resistance were good, and excellent resistance to resistance welding cracks could be achieved.

[0386] (Example 5)

[0387] A cast slab obtained by melting a steel having the composition shown in Table 15 (the remainder being Fe and unavoidable impurities) was hot-rolled to obtain a hot-rolled steel sheet, and the hot-rolled steel sheet was subjected to pickling and cold-rolling to obtain a cold-rolled steel sheet having a sheet thickness of 1.6 mm.

[0388] [Table 15]

[0389]

[0390] 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.

[0391] First, the cold-rolled steel sheet was subjected to degreasing treatment using alkali, and then, the cold-rolled steel sheet was subjected to electrolytic treatment under the conditions shown below with the cold-rolled steel sheet as the cathode to produce an Fe-based plated steel sheet. The attached amount A of the Fe-based plated layer shown in Tables 16 to 18 was calculated by the above method, and was controlled by the current application time.

[0392] [Electrolysis conditions]

[0393] Bath temperature: 50°C

[0394] pH: 2.0

[0395] Current density: 45 A / dm 2

[0396] Plating bath: sulfuric acid bath containing 1.5 mol / L of Fe ions 2+

[0397] Anode: iridium oxide electrode

[0398] Next, the Fe-based plated steel sheet was heated at an average temperature increase rate in the temperature range of 400°C to 650°C at the values shown in Tables 16 to 18, and then, annealing was performed in which the steel sheet was heated at 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 vol% of hydrogen, and the remainder consisting of N2 and unavoidable impurities. The holding time of the steel sheet at the maximum reaching temperature (800°C) was 100 seconds. In the examples shown in Table 18, the Fe-based plated steel sheet (CR) was obtained in this manner.

[0399] ​In the examples shown in Tables 16, 17, the obtained Fe-based plated steel sheet was cooled to 440 to 550°C, and then hot dip galvanizing treatment was performed on the Fe-based plated steel sheet using a 460°C hot dip galvanizing bath having an effective Al concentration of 0.132 mass% and the remaining portion consisting of Zn and unavoidable impurities, and then the attachment amount of the hot dip galvanizing layer per one side was adjusted by gas wiping. In the examples shown in Table 17, a hot dip galvanizing steel sheet (GI) was obtained in this manner.

[0400] In the examples shown in Table 16, furthermore, a step of heating and alloying the hot dip galvanizing layer by alloying treatment at 510°C was further performed, and an alloyed hot dip galvanizing steel sheet (GA) was manufactured. The time of the alloying treatment was changed, and the %Fe in the alloyed hot dip galvanizing layer was controlled.

[0401] The attachment amount and the %Fe of the hot dip galvanizing layer or the alloyed hot dip galvanizing layer of each steel sheet obtained by the above-described method are shown in Tables 16 and 17.

[0402] The values of (I Si，Fe ) / (I Si，bulk ), the average value of the C concentration, and the thickness of the decarburized layer obtained by the above-described method are indicated in Table 16. The thickness of the internal oxidation layer, the average value of the 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 obtained by the above-described method are shown in Tables 17 and 18.

[0403] In the examples of the GA and the GI, the appearance of the plated layer (Evaluation 1) and the resistance welding crack resistance of the welded portion (Evaluation 3) were evaluated, and the results are shown in Tables 16 and 17. In the examples of the CR, the chemical conversion treatment property and the post-painting corrosion resistance (Evaluation 2) and the resistance welding crack resistance of the welded portion (Evaluation 3) were evaluated, and the results are shown in Table 18. Note that the evaluation methods and the evaluation criteria of Evaluations 1 and 2 were the same as in Example 1. The evaluation methods and the evaluation criteria of Evaluation 3 were as follows.

[0404] [Evaluation 3: Evaluation of Resistance Welding Crack Resistance of Welded Portion]

[0405] With reference to (a) of Figure 4 , test pieces 6 of 150 mm in the long direction x 50 mm in the short direction were cut from the steel sheets (CR, GI, GA) of each of the inventive examples and the comparative examples with the rolling right angle direction (TD) as the long side and the rolling direction as the short side, and test pieces of the same size in which the attachment amount of the plated layer per one side was 50 g / m 2The test alloyed hot-dip galvanized steel sheet 5 (thickness: 1.6 mm, TS: 980 MPa grade) was stacked to form a plate assembly. The evaluation surface of the plate assembly (Fe coating in the case of CR, zinc coating in the cases of GI and GA) was aligned with the zinc coating of the test alloyed hot-dip galvanized steel sheet 5. The plate assembly was fixed to the mounting platform 8 by a 2.0 mm thick spacer 7. The spacer 7 consisted of a pair of steel plates, each 50 mm long × 45 mm short × 2.0 mm thick. Figure 4 As shown in (a), the long side end faces of a pair of steel plates are aligned with the short side end faces of the plate assembly. Therefore, the distance between the pair of steel plates is 60 mm. The fixing platform 8 is a plate with a hole in the center.

[0406] Next, using a servo motor-driven single-phase AC (50Hz) resistance welding machine, the plate assembly was stacked while being pressurized using a pair of electrodes 9 (front diameter: 6mm). Resistance welding was performed at a welding current with a weld nugget diameter r of 5.9mm under the following conditions: pressure: 3.5kN; holding time: 0.14 seconds or 0.16 seconds; welding time: 0.36 seconds. At this time, the pair of electrodes 9 pressurize the plate assembly vertically from top to bottom, with the lower electrode pressing the test piece 6 through a hole in the fixing platform 8. During pressurization, the lower electrode of the pair of electrodes 9 is fixed to the fixing platform 8 by contacting a plane extending from the surface where the separator 7 contacts the fixing platform 8, allowing the upper electrode to move. The upper electrode is also in contact with the central portion of the test alloyed hot-dip galvanized steel plate 5. Furthermore, the plate assembly was welded while tilted 5° towards the long side of the plate assembly relative to the horizontal direction. It should be noted that the holding time refers to the time from the end of the welding current flow to the start of electrode release. Here, refer to... Figure 4 In Figure (b) below, the diameter r of the melt core refers to the distance between the ends of the melt cores 10 along the long side of the plate assembly.

[0407] Next, the aforementioned plate assembly with the welded portion is arranged along the center of the welded portion containing the weld nugget 10. Figure 4 (b) The cross-section of the welded section was observed using an optical microscope (200x) on the B-B wire cut in the above figure. The resistance to weld cracking characteristics of the welded section were evaluated according to the following criteria. It should be noted that if it is ◎ or ○, the resistance to weld cracking characteristics of the welded section are judged to be excellent. If it is ×, the resistance to weld cracking characteristics of the welded section are judged to be poor.

[0408] ◎: No cracks longer than 0.1 mm were detected when the holding time was 0.14 seconds.

[0409] O: A crack of 0.1 mm or more in length was confirmed at a holding time of 0.14 seconds, but a crack of 0.1 mm or more in length was not confirmed at a holding time of 0.16 seconds.

[0410] X: A crack of 0.1 mm or more in length was confirmed at a holding time of 0.16 seconds.

[0411] Note that, in the (b) of the figure, a crack generated in the test piece 6 is schematically shown by a symbol 11. Note that, in a case where a crack is generated in the object-side steel sheet (test alloyed hot-dip galvanized steel sheet), stress dispersion of the evaluation object steel sheet (steel sheet of each of the inventive examples and comparative examples) cannot be evaluated properly. Therefore, data in which a crack is not generated in the object-side steel sheet was adopted as the examples. Figure 4

[0412] [Table 16]

[0413]

[0414] [Table 17]

[0415]

[0416] [Table 18]

[0417]

[0418] As is clear from Tables 16 and 17, in the inventive examples of the examples of GA and GI, the appearance of the galvanized layer was good, and excellent resistance to resistance welding cracking characteristics could be achieved. In addition, as is clear from Table 18, in the inventive examples of the examples of CR, the chemical conversion treatment property and the post-painting corrosion resistance were good, and excellent resistance to resistance welding cracking characteristics could be achieved.

[0419] (Example 6)

[0420] A cast slab obtained by melting a steel having a composition shown in Table 15 (the remainder being Fe and inevitable impurities) was hot-rolled to obtain a hot-rolled steel sheet, and the hot-rolled steel sheet was subjected to pickling and cold-rolling to obtain a cold-rolled steel sheet having a sheet thickness of 1.6 mm.

[0421] 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.

[0422] First, the cold-rolled steel sheet was subjected to degreasing treatment with an alkali, and then, the cold-rolled steel sheet was subjected to electrolytic treatment under the conditions shown below as a cathode to produce an Fe-based plated steel sheet. The attachment amount A of the Fe-based plated layer shown in Tables 19 to 21 was calculated by the above method, and was controlled by the power application time.

[0423] [Electrolysis Conditions] ​

[0424] Bath temperature: 50℃

[0425] pH: 2.0

[0426] Current density: 45A / dm 2

[0427] Plating bath: Contains 1.5 mol / L Fe 2+ Ionic sulfuric acid bath

[0428] Anode: Iridium oxide electrode

[0429] Next, the Fe-based electroplated steel sheet was heated at an average heating rate within a temperature range of 400°C to 650°C, as shown in Tables 19-21. Then, annealing was performed on the steel sheet at a soaking temperature of 800°C in a reducing atmosphere having an atmosphere dew point B as shown in Tables 19-21, containing 15% by volume hydrogen, with the remainder consisting of N2 and unavoidable impurities. The holding time of the steel sheet at the highest reached temperature (800°C) was 100 seconds. In the embodiment shown in Table 21, the Fe-based electroplated steel sheet (CR) was obtained in this manner.

[0430] In the embodiments shown in Tables 19 and 20, the obtained Fe-based electroplated steel sheet was cooled to 440–550°C, and then hot-dip galvanized in a 460°C hot-dip galvanizing bath with an effective Al concentration of 0.132% by mass and the remainder consisting of Zn and unavoidable impurities. The adhesion of the hot-dip galvanized layer on each side was then adjusted by gas wiping. In the embodiments shown in Table 20, hot-dip galvanized steel sheet (GI) was obtained in this manner.

[0431] In the embodiments shown in Table 19, the hot-dip galvanized layer is further alloyed by a heating process at 510°C to produce alloyed hot-dip galvanized steel sheet (GA). The Fe content in the alloyed hot-dip galvanized layer is controlled by varying the alloying time.

[0432] Tables 19 and 20 show the adhesion amount and Fe% of the hot-dip galvanized or alloyed hot-dip galvanized coating for each steel plate, determined by the above method.

[0433] Table 19 shows the (I) obtained by the above method. Si，Fe ) / (I Si，bulk The values ​​of C, the average C concentration, and the thickness of the decarburized layer are shown in Tables 20 and 21. 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 plate are shown in Tables 20 and 21.

[0434] In the examples of the GA and the GI, the appearance of the zinc plating layer (Evaluation 1) and the resistance welding crack resistance of the welded portion (Evaluation 3) were evaluated, and the results are shown in Table 19 and Table 20. In the example of the CR, the chemical conversion treatment property and the post-painting corrosion resistance (Evaluation 2) and the resistance welding crack resistance of the welded portion (Evaluation 3) were evaluated, and the results are shown in Table 21. Note that the evaluation methods and evaluation criteria of Evaluations 1 to 3 were the same as in Example 5.

[0435] [Table 19]

[0436]

[0437] [Table 20]

[0438]

[0439] [Table 21]

[0440]

[0441] As is clear from Tables 19 and 20, in the inventive examples of the examples of the GA and the GI, the appearance of the zinc plating layer was good, and excellent resistance welding crack resistance could be achieved. In addition, as is clear from Table 21, in the inventive example of the example of the CR, the chemical conversion treatment property and the post-painting corrosion resistance were good, and excellent resistance welding crack resistance could be achieved.

[0442] Industrial Applicability

[0443] The Fe-based plated steel sheet of the present application is not only excellent in the chemical conversion treatment property or the plating appearance when hot-dip galvanizing is performed, but also excellent in the resistance welding crack resistance. In addition, the galvannealed steel sheet of the present application is not only excellent in the plating appearance, but also excellent in the resistance welding crack resistance. Therefore, by applying the Fe-based plated steel sheet or the galvannealed steel sheet of the present application to, for example, an automobile structural member, it is possible to achieve improvement in fuel efficiency due to lightening of the vehicle body, and it is also possible to apply to uses such as household electrical appliances, building members, and the like.

[0444] Explanation of Symbols

[0445] 1 Fe-based plated steel sheet

[0446] 2 Cold-rolled steel sheet

[0447] 3 Fe-based plated layer

[0448] 5 Test galvannealed steel sheet

[0449] 6 Test piece

[0450] 7 Spacer

[0451] 8 Fixing stand

[0452] 9 electrode

[0453] 10 nugget

[0454] 11 crack

Claims

1. A method for manufacturing Fe-based electroplated steel sheet, comprising: Fe-based electroplating is performed on cold-rolled steel sheets having a composition containing 0.1% to 3.0% by mass of Si to obtain an adhesion amount A (g / m²) on one or both sides of the cold-rolled steel sheet. 2 (Exceeding 2.0g / m) 2 The process of Fe-based electroplating of steel sheets with Fe-based electroplating layers, and Then, the Fe-based electroplated steel sheet is annealed at 650°C to 900°C in an atmosphere where the dew point B (°C) exceeds 5°C and is below 20°C, and the dew point B (°C) satisfies the following formula (1). Furthermore, the following operational condition change process is implemented: when the condition no longer satisfies equation (1), the adhesion amount A (g / m³) is changed in a manner that satisfies equation (1). 2 ) or at least one of the dew point B (°C), A+B≥8.0···(1) 2. The method for manufacturing Fe-based electroplated steel sheet according to claim 1, wherein, The process further includes the following steps: after the annealing process, the Fe-based electroplated steel sheet is subjected to hot-dip galvanizing to form an unalloyed hot-dip galvanized layer on the surface of the Fe-based electroplated layer.

3. The method for manufacturing Fe-based electroplated steel sheet according to claim 1, wherein, The process includes the following steps: before the annealing process, the Fe-based electroplated steel sheet is heated at an average heating rate of 10°C / second or more within a temperature range of 400°C to 650°C.

4. The method for manufacturing Fe-based electroplated steel sheet according to claim 1, wherein, The adhesion amount A is less than 5.0 g / m 2 .

5. The method for manufacturing Fe-based electroplated steel sheet according to claim 1, wherein, The composition of the cold-rolled steel sheet, by mass%, includes C: less than 0.8%, Si: 0.1% to 3.0%, Mn: 1.0% to 12.0%, P: less than 0.1%, S: less than 0.03%, N: less than 0.010%, and Al: less than 1.0%, with the remainder consisting of Fe and unavoidable impurities.

6. The method for manufacturing Fe-based electroplated steel sheet according to claim 5, wherein, The composition further contains, by mass%, at least one element selected from the following: B: less than 0.005%, Ti: less than 0.2%, Cr: less than 1.0%, Cu: less than 1.0%, Ni: less than 1.0%, Mo: less than 1.0%, Nb: less than 0.20%, V: less than 0.5%, Sb: less than 0.020%, Ta: less than 0.1%, W: less than 0.5%, Zr: less than 0.1%, Sn: less than 0.20%, Ca: less than 0.005%, Mg: less than 0.005%, and REM: less than 0.005%.

7. The method for manufacturing Fe-based electroplated steel sheet according to claim 1, wherein, In the composition, the amount of Si is 0.9% to 1.7% by mass.

8. The method for manufacturing Fe-based electroplated steel sheet according to claim 1, wherein, In the Fe-based electroplating, the plating bath contains at least one element selected from B, C, P, N, O, Ni, Mn, Mo, Zn, W, Pb, Sn, Cr, V, and Co, and the total content of these elements in the Fe-based plating layer is less than 10% by mass.

9. A method for manufacturing alloyed hot-dip galvanized steel sheet, comprising: Fe-based electroplating is performed on cold-rolled steel sheets having a composition containing 0.1% to 3.0% by mass of Si to obtain an adhesion amount A (g / m²) on one or both sides of the cold-rolled steel sheet. 2 (The value exceeds 2.0 g / m) 2 The process of Fe-based electroplating of steel sheets with Fe-based electroplating layers. Then, the Fe-based electroplated steel sheet is subjected to an annealing process at 650°C to 900°C in an atmosphere where the dew point B (°C) exceeds 5°C but is below 20°C and satisfies the following formula (2). Then, the Fe-based electroplated steel sheet is subjected to hot-dip galvanizing, forming an unalloyed hot-dip galvanized layer on the surface of the Fe-based electroplated layer, and... Then, the hot-dip galvanized layer is heated and alloyed to obtain an alloyed hot-dip galvanized steel sheet with an alloyed hot-dip galvanized layer formed on one or both sides of the cold-rolled steel sheet. Furthermore, the following operational condition change process is implemented: when the condition no longer satisfies equation (2), the adhesion amount A (g / m³) is changed in a manner that satisfies equation (2). 2 ) or at least one of the dew point B (°C), A+B≥10.0···(2) 10. The method for manufacturing alloyed hot-dip galvanized steel sheet according to claim 9, wherein, The process includes the following steps: before the annealing process, the Fe-based electroplated steel sheet is heated at an average heating rate of 10°C / second or more within a temperature range of 400°C to 650°C.

11. The method for manufacturing alloyed hot-dip galvanized steel sheet according to claim 9, wherein, The adhesion amount A is less than 5.0 g / m 2 .

12. The method for manufacturing alloyed hot-dip galvanized steel sheet according to claim 9, wherein, The composition of the cold-rolled steel sheet, by mass%, includes C: less than 0.8%, Si: 0.1% to 3.0%, Mn: 1.0% to 12.0%, P: less than 0.1%, S: less than 0.03%, N: less than 0.010%, and Al: less than 1.0%, with the remainder consisting of Fe and unavoidable impurities.

13. The method for manufacturing alloyed hot-dip galvanized steel sheet according to claim 12, wherein, The composition further contains, by mass%, at least one element selected from the following: B: less than 0.005%, Ti: less than 0.2%, Cr: less than 1.0%, Cu: less than 1.0%, Ni: less than 1.0%, Mo: less than 1.0%, Nb: less than 0.20%, V: less than 0.5%, Sb: less than 0.020%, Ta: less than 0.1%, W: less than 0.5%, Zr: less than 0.1%, Sn: less than 0.20%, Ca: less than 0.005%, Mg: less than 0.005%, and REM: less than 0.005%.

14. The method for manufacturing alloyed hot-dip galvanized steel sheet according to claim 9, wherein, In the composition, the amount of Si is 0.9% to 1.7% by mass.

15. The method for manufacturing alloyed hot-dip galvanized steel sheet according to claim 9, wherein, In the Fe-based electroplating, the plating bath contains at least one element selected from B, C, P, N, O, Ni, Mn, Mo, Zn, W, Pb, Sn, Cr, V, and Co, and the total content of these elements in the alloyed hot-dip galvanized layer is less than 1% by mass.

16. A method for manufacturing a chemically converted steel plate, comprising: The manufacturing method of Fe-based electroplated steel sheet according to any one of claims 1 to 8, or the manufacturing method of alloyed hot-dip galvanized steel sheet according to any one of claims 9 to 15, and Then, the Fe-based electroplated steel sheet or the alloyed hot-dip galvanized steel sheet is subjected to chemical conversion treatment to obtain a chemically converted steel sheet that is in contact with the Fe-based electroplated steel sheet or the alloyed hot-dip galvanized steel sheet to form a chemical conversion coating.

17. A method for manufacturing an electrodeposited coated steel sheet, comprising: The method for manufacturing the chemically converted steel sheet according to claim 16, and The process of performing electrodeposition coating on the chemically converted steel sheet to obtain an electrodeposited coated steel sheet with an electrodeposited coating film in contact with the chemically converted film.

18. A method for manufacturing an automotive component, comprising: The method for manufacturing the electrodeposited coated steel sheet according to claim 17, and The process of manufacturing automotive parts by using the electrodeposited coated steel sheet as part of the process.

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