Fe-based electroplated steel sheet and its manufacturing method

Abstract

Provided is an Fe-based electroplated steel sheet that not only has excellent chemical conversion treatability or plating appearance when hot-dip galvanized, but also has excellent resistance weld crack resistance. The Fe-based plated steel sheet of the present invention comprises a cold-rolled steel sheet having a chemical composition containing 0.1 mass % to 3.0 mass % of Si, and a coating weight of 1.0 g / m2 formed on one or both sides of the cold-rolled steel sheet. 2 The thickness of the inner oxide layer is 2.00 μm or less, and the average C concentration in the range of 10 μm to 20 μm from the surface of the Fe-based electroplated layer in the sheet thickness direction is 0.10 mass% or less.

Description

Technical Field

[0001] The present invention relates to an Fe-based electroplated steel sheet, an alloyed hot-dip galvanized steel sheet, and a method for producing these. In the present invention, the "Fe-based electroplated steel sheet" includes both (i) a simple Fe-based electroplated steel sheet obtained by subjecting a cold-rolled steel sheet to Fe-based electroplating (hereinafter sometimes simply referred to as "CR"), and (ii) a hot-dip galvanized steel sheet obtained by subjecting the simple Fe-based electroplated steel sheet of (i) to hot-dip galvanizing, in which the hot-dip galvanized layer is not alloyed (hereinafter sometimes referred to as "GI"). In the present invention, the "alloyed hot-dip galvanized steel sheet" means an alloyed hot-dip galvanized steel sheet (hereinafter sometimes referred to as "GA") obtained by heating and alloying the hot-dip galvanized layer of the hot-dip galvanized steel sheet of (ii).

Background Art

[0002] In recent years, from the perspective of global environmental conservation, improving the fuel efficiency of automobiles has become an important issue. For this reason, there has been an active movement to reduce the weight of automobile bodies by increasing the strength and reducing the thickness of the steel sheets used as materials for automobile members. However, since increasing the strength of steel sheets leads to a decrease in formability, the development of steel sheets having both high strength and high formability is desired.

[0003] As a method of increasing the strength of a steel sheet without significantly impairing its formability, solid solution strengthening by adding Si to the steel sheet can be mentioned. However, Si added for increasing the strength of the steel sheet forms oxides on the steel sheet surface during annealing. When the steel sheet is used without subjecting it to hot-dip galvanizing, this oxide degrades the chemical conversion treatment property. Further, when producing a hot-dip galvanized steel sheet by subjecting the steel sheet to hot-dip galvanizing, this oxide degrades the wettability between the hot-dip zinc and the steel sheet, resulting in non-plating. For the purpose of ensuring such chemical conversion treatment property or plating appearance when hot-dip galvanizing is performed, a technique of subjecting the steel sheet surface to Fe-based electroplating (Fe-based pre-plating) before annealing the steel sheet is known.

[0004] Patent Document 1 describes the steps of "preparing a base steel sheet and the amount of plating deposited on the prepared base steel sheet being 0.2 to 2 g / m²." 2 A method for manufacturing a hot-dip galvanized steel sheet is described, comprising the steps of: forming an Fe plating layer; oxidizing and heating the steel sheet on which the Fe plating layer is formed at 600 to 800°C; maintaining the heated steel sheet at 750 to 900°C for 5 seconds or more in a reducing atmosphere containing 20 ppm or less of oxygen, 1 to 20 vol% of H2, the remainder being N2 and other unavoidable gases, and having a dew point of -30 to 5°C; cooling the maintained steel sheet; and immersing the cooled steel sheet in a hot-dip galvanizing bath at 445 to 480°C for plating (Claim 1).

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

[0006] [Patent Document 1] Patent No. 6025867 [Patent Document 2] International Publication No. 2019 / 116531 [Overview of the Initiative] [Problems that the invention aims to solve]

[0007] In the manufacturing of automotive parts, press-formed parts are often assembled by resistance welding (spot welding). When at least one of the multiple parts being resistance-welded contains hot-dip galvanized steel, there is a concern that during resistance welding, residual stress is generated near the weld, causing the zinc in the plating layer to melt and diffuse into the grain boundaries of the steel sheets constituting each part, leading to liquid metal embrittlement (LME) and resulting in grain boundary cracking (LME cracking) in the steel sheet. In particular, if welding is performed with the welding electrode at an angle to the steel sheet, residual stress increases, potentially leading to crack formation. Since residual stress is thought to increase with increasing steel sheet strength, there is a concern that LME cracking may occur as steel sheets become stronger due to Si addition.

[0008] However, our investigations have revealed that while the method for manufacturing hot-dip galvanized steel sheets described in Patent Document 1 can prevent unplated areas and ensure excellent plated surface appearance and plating adhesion, the resistance to welding cracks is insufficient. Furthermore, the steel sheets described in Patent Document 2 also exhibit insufficient resistance to welding cracks because the depth of the internal oxide layer, i.e., grain boundary oxidation, is too great.

[0009] Therefore, in view of the above problems, the present invention aims to provide an Fe-based electroplated steel sheet that is excellent not only in chemical conversion treatment properties or plating appearance when hot-dip galvanizing is applied, but also in resistance to welding cracking properties, along with a suitable manufacturing method. Furthermore, the present invention aims to provide an alloyed hot-dip galvanized steel sheet that is excellent not only in plating appearance but also in resistance to welding cracking properties, along with a suitable manufacturing method. [Means for solving the problem]

[0010] The inventors of this invention have conducted extensive research to solve the above problems and have discovered the following findings: When forming an Fe-based electroplating layer with a predetermined adhesion amount A for the purpose of ensuring chemical conversion treatment or plating appearance when hot-dip galvanizing is performed, the sum of the adhesion amount A and the dew point B during subsequent annealing correlates with the resistance to welding cracking characteristics. Specifically, when A + B is greater than or equal to a predetermined value, excellent resistance to welding cracking characteristics can be achieved.

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

[0012] First, we will explain in detail the cases of CR and GI. In order to satisfy the resistance cracking characteristics of the weld at a high level, it is important to form an Fe-based electroplating layer with an adhesion amount A on the cold-rolled steel sheet, and then perform an annealing process under conditions that satisfy A + B ≥ 3.0, thereby forming internal oxides within the Fe-based electroplating layer, and keeping the depth of the internal oxide layer from a depth of 0.10 μm from the surface of the Fe-based electroplating layer toward the cold-rolled steel sheet to within 2.00 μm. When A + B ≥ 3.0 is satisfied, Si that diffuses from the cold-rolled steel sheet to the Fe-based electroplating layer during annealing can be formed as oxides within the Fe-based electroplating layer. In this way, by partially forming oxides within the Fe-based electroplating layer, the depth of the internal oxide layer from a depth of 0.10 μm from the surface of the Fe-based electroplating layer toward the cold-rolled steel sheet can be kept within 2.00 μm. Therefore, when zinc reaches the grain boundaries of the cold-rolled steel sheet, penetration in the depth direction of the cold-rolled steel sheet can be suppressed as much as possible, and the resistance cracking characteristics of the weld can be improved.

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

[0014] Furthermore, in the heating process that follows the annealing (soaking) process, by setting the average heating rate of the Fe-based electroplated steel sheet in the temperature range of 400°C to 650°C to 10°C / second or more, the growth of crystal grains in the Fe-based electroplated layer during the heating process is suppressed as much as possible, and the number of crystal grain boundaries of the Fe-based electroplated layer in contact with the cold-rolled steel sheet at the interface between the Fe-based electroplated layer and the cold-rolled steel sheet is set to 10 or more per 10 μm of width of the cold-rolled steel sheet. This further improves the resistance to welding cracks. When the average heating rate in the temperature range of 400°C to 650°C during the heating process is set to 10°C / second or more, the crystal grains of the Fe-based electroplated layer in contact with the interface between the Fe-based electroplated layer and the cold-rolled steel sheet are refined, and the pathways for molten zinc to penetrate the Fe-based electroplated layer are dispersed. In other words, the time it takes for molten zinc to reach the crystal grain boundaries of the cold-rolled steel sheet during welding can be delayed, further improving the resistance to welding cracks.

[0015] Next, we will explain the case of GA in detail. In the case of GA, the Fe-based electroplating layer is incorporated into the hot-dip galvanized layer and disappears during the alloying process, so the stress relaxation effect of the Fe-based electroplating layer cannot be expected. In order to satisfy the resistance to welding cracking characteristics at the weld at a high level even when the Fe-based electroplating layer disappears, it is important to form an Fe-based electroplating layer with an adhesion amount A on the cold-rolled steel sheet, and then perform an annealing process under conditions that satisfy A+B≧5.0 to form internal oxides within the Fe-based electroplating layer, and then reduce the amount of internal oxides in contact with the alloyed hot-dip galvanized layer by incorporating some of the Si internal oxides into the alloyed hot-dip galvanized layer during the alloying process. When A+B≧5.0 is satisfied, Si that diffuses from the cold-rolled steel sheet to the Fe-based electroplating layer during annealing can be converted into oxides within the Fe-based electroplating layer. In this way, by partially forming oxides within the Fe-based electroplating layer, the amount of internal oxides in contact with the alloyed hot-dip galvanized layer can be reduced. Therefore, it is possible to suppress the intrusion of zinc from the grain boundaries of the internal oxide layer in contact with the alloyed hot-dip galvanized layer. As a result, the time it takes for molten zinc to reach the grain boundaries of the cold-rolled steel sheet during welding can be delayed, thereby improving the resistance to welding cracks in the welded area.

[0016] Furthermore, during the annealing process, the average C concentration in the alloyed hot-dip galvanized steel sheet in the range of 10 μm to 20 μm from the surface of the cold-rolled steel sheet in the thickness direction is set to 0.10 mass% or less. This further improves the resistance to welding cracks. The inventors have found that when an Fe-based electroplating layer is formed before annealing, the C concentration in the range of 10 μm to 20 μm from the surface of the cold-rolled steel sheet in the thickness direction can be further reduced, and the effect of improving resistance to welding cracks can be obtained more effectively.

[0017] Based on the above findings, the gist of the present invention is as follows. [1] Cold-rolled steel sheet having a composition containing 0.1% by mass or more and 3.0% by mass or less of Si, The amount of adhesive formed on one or both sides of the cold-rolled steel sheet, with an adhesion amount of 1.0 g / m² per side.2 The above Fe-based electroplating layers, Fe-based electroplated steel sheet having, In the emission intensity profile of the wavelength indicating Si, measured in the depth direction from the surface of the Fe-based electroplating layer by glow discharge emission spectrometry, (i) the average Si intensity (I) in the range of 10.0 ± 0.1 μm from the interface between the Fe-based electroplating layer and the cold-rolled steel sheet Si (ii) there exists a peak with a greater luminescence intensity than (ii) the average Si intensity (I Si The depth at which this becomes equal is located within a range of 0.10 μm to 2.00 μm from the surface of the Fe-based electroplating layer. The average value of the C concentration in the range of 10 μm to 20 μm in the thickness direction from the surface of the Fe-based electroplating layer is 0.10 mass% or less. Fe-based electroplated steel sheet.

[0018] [2] The Fe-based electroplated steel sheet according to [1], having a non-alloyed hot-dip galvanized layer formed in contact with the Fe-based electroplated layer.

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

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

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

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

[0023] [7] The Fe-based electroplated steel sheet according to any one of the above [1] to [6], wherein at the interface between the Fe-based electroplated layer and the cold-rolled steel sheet, the number of crystal 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 width direction of the steel sheet in the observation field of view of the cold-rolled steel sheet.

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

[0025] [9] The above component composition is further, in mass%, B: 0.005% or less, Ti: 0.2% or less, Cr: 1.0% or less, Cu: 1.0% or less, Ni: 1.0% or less, Mo: 1.0% or less, Nb: 0.20% or less, V: 0.5% or less, Sb: 0.020% or less, Ta: 0.1% or less, W: 0.5% or less, Zr: 0.1% or less, Sn: 0.20% or less, Ca: 0.005% or less, Mg: 0.005% or less, REM: 0.005% or less The Fe-based electroplated steel sheet according to [8] above, comprising at least one element selected from the group consisting of the following.

[0026]

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

[0027]

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

[10] , wherein the Fe-based electroplated layer contains a total of 10% by mass or less 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 being Fe and unavoidable impurities.

[0028]

[12] Cold-rolled steel sheet having a composition containing 0.1% by mass or more and 3.0% by mass or less of Si, A hot-dip galvanized alloy layer formed on one or both sides of the cold-rolled steel sheet, A hot-dip galvanized steel sheet having the above characteristics, and having no Fe-based electroplating layer between the alloyed hot-dip galvanized layer and the cold-rolled steel sheet, In the emission intensity profile of the Si wavelength measured in the depth direction from the surface of the alloyed hot-dip galvanized layer by glow discharge emission spectrometry, the average Si intensity (I) in the range of +0.5 μm from the interface between the alloyed hot-dip galvanized layer and the cold-rolled steel sheet toward the cold-rolled steel sheet is Si,Fe ) is the average Si strength (I) in the cold-rolled steel sheet. Si,bulk The value obtained by dividing by (I Si,Fe ) / (I Si,bulk ) is 0.90 or less, The average value of the C concentration in the range of 10 μm to 20 μm in the thickness direction from the interface between the alloyed hot-dip galvanized layer and the cold-rolled steel sheet is 0.10 mass% or less. Alloyed hot-dip galvanized steel sheet.

[0029]

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

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

[0030]

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

[12] or

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

[0031]

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

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

[0032]

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

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

[0033]

[17] The component composition of the cold-rolled steel sheet is, in mass%, C: 0.8% or less, Si: 0.1% or more and 3.0% or less, Mn: 1.0% or more and 12.0% or less, P: 0.1% or less, S: 0.03% or less, N: 0.010% or less, and Al: 1.0% or less Alloyed hot-dip galvanized steel sheet according to any one of the above

[12] to

[16] , comprising the above, with the remainder being Fe and unavoidable impurities.

[0034]

[18] The above component composition is further, in mass%, B: 0.005% or less, Ti: 0.2% or less, Cr: 1.0% or less, Cu: 1.0% or less, Ni: 1.0% or less, Mo: 1.0% or less, Nb: 0.20% or less, V: 0.5% or less, Sb: 0.020% or less, Ta: 0.1% or less, W: 0.5% or less, Zr: 0.1% or less, Sn: 0.20% or less, Ca: 0.005% or less, Mg: 0.005% or less, REM: 0.005% or less The alloyed hot-dip galvanized steel sheet according to

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

[0035]

[19] In the above component composition, the alloyed hot-dip galvanized steel sheet according to any one of

[12] to

[18] above, wherein the Si content is 0.9% by mass or more and 1.7% by mass or less.

[0036]

[20] The alloyed hot-dip galvanized steel sheet according to any one of

[12] to

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

[0037]

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

[11] above, or the alloyed hot-dip galvanized layer of the alloyed hot-dip galvanized steel sheet according to any one of

[12] to

[19] above.

[0038]

[22] An electrocoated steel sheet having an electrocoated film formed in contact with the chemical conversion coating of the chemical conversion coating steel sheet according to

[21] above.

[0039]

[23] An automotive part made by using at least a part of the electrocoated steel sheet according to

[22] above.

[0040]

[24] Fe-based electroplating is applied to a cold-rolled steel sheet having a component composition containing 0.1% by mass or more and 3.0% by mass or less of Si, and on one or both sides of the cold-rolled steel sheet, the adhesion amount A (g / m 2 ) per side is 1.0 g / m 2 or more to form an Fe-based electroplated layer, to obtain an Fe-based electroplated steel sheet; Then, the Fe-based electroplated steel sheet is held at 650 °C or higher and 900 °C or lower in an atmosphere where the dew point B (°C) satisfies the following formula (1); A manufacturing method of an Fe-based electroplated steel sheet having A + B ≧ 3.0 ··· (1)

[0041]

[25] The method for manufacturing an Fe-based electroplated steel sheet according to

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

[0042]

[26] The method for manufacturing Fe-based electroplated steel sheets according to

[24] or

[25] , wherein the annealing step is carried out in an atmosphere where the dew point B (°C) satisfies the following formula (1)'. A + B ≥ 8.0 ···(1)'

[0043]

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

[24] to

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

[0044]

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

[24] to

[27] , wherein the value is less than

[24] .

[0045]

[29] The component composition of the cold-rolled steel sheet is, in mass%, C: 0.8% or less, Si: 0.1% or more and 3.0% or less, Mn: 1.0% or more and 12.0% or less, P: 0.1% or less, S: 0.03% or less, N: 0.010% or less, and Al: 1.0% or less A method for manufacturing an Fe-based electroplated steel sheet according to any one of the above

[24] to

[28] , comprising the above, with the remainder being Fe and unavoidable impurities.

[0046]

[30] The above component composition is further, in mass%, B: 0.005% or less, Ti: 0.2% or less, Cr: 1.0% or less, Cu: 1.0% or less, Ni: 1.0% or less, Mo: 1.0% or less, Nb: 0.20% or less, V: 0.5% or less, Sb: 0.020% or less, Ta: 0.1% or less, W: 0.5% or less, Zr: 0.1% or less, Sn: 0.20% or less, Ca: 0.005% or less, Mg: 0.005% or less, REM: 0.005% or less A method for producing an Fe-based electroplated steel sheet according to

[29] above, comprising at least one element selected from the group consisting of the following.

[0047]

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

[24] to

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

[0048]

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

[24] to

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

[0049]

[33] When formula (1) or formula (1)' is not satisfied, the amount of adhesion A (g / m) is adjusted to satisfy formula (1) or formula (1)'. 2 A method for manufacturing an Fe-based electroplated steel sheet according to any one of the above

[24] to

[32] , further comprising the step of changing at least one of the following: ) or dew point B (°C).

[0050]

[34] A cold-rolled steel sheet having a composition containing 0.1% by mass or more and 3.0% by mass or less of Si is subjected to Fe-based electroplating, and one or both sides of the cold-rolled steel sheet are coated with an amount A (g / m²) per side. 2) is 1.0 g / m 2 A process to obtain an Fe-based electroplated steel sheet having the above Fe-based electroplating layer formed on it, Subsequently, the Fe-based electroplated steel sheet is subjected to an annealing process in which it is held at a temperature of 650°C to 900°C in an atmosphere where the dew point B (°C) satisfies the following formula (2), Subsequently, the Fe-based electroplated steel sheet is subjected to a hot-dip galvanizing process to form a non-alloyed hot-dip galvanized layer on the surface of the Fe-based electroplating layer. Subsequently, the hot-dip galvanized layer is heated and alloyed to obtain an alloyed hot-dip galvanized steel sheet in which an alloyed hot-dip galvanized layer is formed on one or both sides of the cold-rolled steel sheet. A method for manufacturing alloyed hot-dip galvanized steel sheets having the following characteristics. A + B ≥ 5.0 ···(2)

[0051]

[35] The method for manufacturing alloyed hot-dip galvanized steel sheet according to

[34] above, wherein the annealing step is carried out in an atmosphere where the dew point B (°C) satisfies the following formula (2)'. A + B ≥ 10.0 ···(2)'

[0052]

[36] A method for manufacturing an alloyed hot-dip galvanized steel sheet according to

[34] or

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

[0053]

[37] The amount of adhesion A is 5.0 g / m 2 A method for manufacturing alloyed hot-dip galvanized steel sheet according to any one of the above

[34] to

[36] , wherein the amount is less than

[34] .

[0054]

[38] The component composition of the cold-rolled steel sheet is, in mass%, C: 0.8% or less, Si: 0.1% or more and 3.0% or less, Mn: 1.0% or more and 12.0% or less, P: 0.1% or less, S: 0.03% or less, N: 0.010% or less, and Al: 1.0% or less A method for producing an alloyed hot-dip galvanized steel sheet according to any one of the above

[34] to

[37] , comprising the above, with the remainder being Fe and unavoidable impurities.

[0055]

[39] The above component composition is further, in mass%, B: 0.005% or less, Ti: 0.2% or less, Cr: 1.0% or less, Cu: 1.0% or less, Ni: 1.0% or less, Mo: 1.0% or less, Nb: 0.20% or less, V: 0.5% or less, Sb: 0.020% or less, Ta: 0.1% or less, W: 0.5% or less, Zr: 0.1% or less, Sn: 0.20% or less, Ca: 0.005% or less, Mg: 0.005% or less, REM: 0.005% or less A method for producing an alloyed hot-dip galvanized steel sheet according to

[38] above, comprising at least one element selected from the group consisting of the following:

[0056]

[40] A method for producing an alloyed hot-dip galvanized steel sheet according to any one of the above

[34] to

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

[0057]

[41] A method for producing an alloyed hot-dip galvanized steel sheet according to any one of the above

[34] to

[40] , wherein in the Fe-based electroplating, 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 included in the plating bath such that the total content of these elements in the alloyed hot-dip galvanized layer is 1% by mass or less.

[0058]

[42] When formula (2) or formula (2)' is not satisfied, the amount of adhesion A (g / m) is adjusted to satisfy formula (2) or formula (2)'. 2 A method for manufacturing an alloyed hot-dip galvanized steel sheet according to any one of the above

[34] to

[41] , further comprising the step of changing at least one of the following: ) or dew point B (°C).

[0059]

[43] A method for manufacturing Fe-based electroplated steel sheets as described in any one of the above items

[24] to

[33] , or a method for manufacturing alloyed hot-dip galvanized steel sheets as described in any one of the above items

[34] to

[42] , Subsequently, the Fe-based electroplated steel sheet or the alloyed hot-dip galvanized steel sheet is subjected to a chemical conversion treatment to obtain a chemically converted steel sheet in which a chemical conversion treatment film is formed in contact with the Fe-based electroplated steel sheet or the alloyed hot-dip galvanized steel sheet. A method for manufacturing chemically treated steel sheets having the following characteristics.

[0060]

[44] The method for manufacturing chemically treated steel sheets as described in

[43] above, A step of applying electrodeposition coating to the chemically treated steel sheet to obtain an electrodeposited steel sheet in which an electrodeposited coating film is formed in contact with the chemically treated film, A method for manufacturing electrodeposited coated steel sheets.

[0061]

[45] The method for manufacturing electrodeposited steel sheet described in

[44] above, A process for manufacturing automotive parts using the aforementioned electrodeposited steel sheet as part of the process, A method for manufacturing automotive parts. [Effects of the Invention]

[0062] The Fe-based electroplated steel sheet of the present invention not only exhibits excellent chemical conversion treatment properties or plating appearance when hot-dip galvanizing is applied, but also has excellent resistance to welding cracking. Furthermore, the alloyed hot-dip galvanized steel sheet of the present invention not only exhibits excellent plating appearance, but also has excellent resistance to welding cracking. [Brief explanation of the drawing]

[0063] [Figure 1]This graph shows the relationship between the amount of Fe-based electroplating layer A and the dew point B during annealing, and the evaluation results of resistance to welding cracking, in the example of alloyed hot-dip galvanized steel sheet (GA) of Example 1. [Figure 2] This graph shows the relationship between the amount of Fe-based electroplating layer A and the dew point B during annealing, and the evaluation results of resistance to welding cracking, in the example of hot-dip galvanized steel sheet (GI) of Example 1. [Figure 3] This graph shows the relationship between the amount of Fe-based electroplating layer A and the dew point B during annealing, and the evaluation results of resistance to welding cracking, in the example of Fe-based electroplated steel sheet (CR) without hot-dip galvanizing in Example 1. [Figure 4] (a) is a diagram illustrating the evaluation method for resistance welding cracking characteristics in a welded joint, the upper part of (b) is a top view of the plate assembly after welding in the same evaluation, and the lower part is an enlarged view of the BB cross section in the upper part of (b). [Figure 5] This is raw data of the emission intensity profile at the wavelength indicating Si, obtained by glow discharge emission analysis, for some examples of Fe-based electroplated steel sheets (CR) without hot-dip galvanizing, as measured by Example 1. [Figure 6] (a) is a perspective view and (b) is a cross-sectional view of AA showing an outline of an observation sample for measuring the number of grain boundaries in the Fe-based electroplating layer in contact with the cold-rolled steel sheet at the interface between the Fe-based electroplating layer and the cold-rolled steel sheet. [Figure 7] This figure illustrates a method for measuring the number of grain boundaries in the Fe-based electroplating layer that is in contact with the cold-rolled steel sheet at the interface between the Fe-based electroplating layer and the cold-rolled steel sheet. [Figure 8] This is a magnified view of the area enclosed by the rectangle in Figure 7. [Figure 9] These are raw data of the emission intensity profiles at wavelengths indicating Si and Zn, obtained by glow discharge emission analysis, for some examples of alloyed hot-dip galvanized steel sheets (GA) from Example 1. [Figure 10A] This is raw data of the C concentration profile in the thickness direction of a sheet, analyzed by an electron beam microanalyzer, for some examples of Fe-based electroplated steel sheets (CR) without hot-dip galvanizing as described in Example 1. [Figure 10B] This is the data after smoothing the profile in Figure 10A. [Figure 11A] This is raw data of the C concentration profile in the thickness direction of a sample of alloyed hot-dip galvanized steel sheet (GA) from Example 1, analyzed with an electron beam microanalyzer. [Figure 11B] This is the data after smoothing the profile in Figure 11A. [Modes for carrying out the invention]

[0064] (Fe-based electroplated steel sheet and method for manufacturing the same) A method for manufacturing an Fe-based electroplated steel sheet according to one embodiment of the present invention comprises the steps of: applying Fe-based electroplating to a cold-rolled steel sheet having a component composition containing 0.1% to 3.0% by mass of Si to obtain an Fe-based electroplated steel sheet in which an Fe-based electroplating layer is formed on one or both sides of the cold-rolled steel sheet; and then annealing the Fe-based electroplated steel sheet. If a hot-dip galvanizing step is not performed thereafter, a simple Fe-based plated steel sheet (CR) can be obtained. It is preferable to apply the Fe-based electroplating to the cold-rolled steel sheet without annealing. That is, the Fe-based electroplated steel sheet (CR) according to one embodiment of the present invention comprises a cold-rolled steel sheet having a component composition containing 0.1% to 3.0% by mass of Si, and an Fe-based electroplating layer formed on one or both sides of the cold-rolled steel sheet, and does not have an unalloyed hot-dip galvanized layer or an alloyed hot-dip galvanized layer.

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

[0066] (Alloyed hot-dip galvanized steel sheet and method for manufacturing the same) A method for manufacturing an alloyed hot-dip galvanized steel sheet according to one embodiment of the present invention comprises the steps of: applying Fe-based electroplating to a cold-rolled steel sheet having a component composition containing 0.1% by mass or more and 3.0% by mass or less of Si to obtain an Fe-based electroplated steel sheet in which an Fe-based electroplating layer is formed on one or both sides of the cold-rolled steel sheet; then annealing the Fe-based electroplated steel sheet; then applying hot-dip galvanizing to the Fe-based electroplated steel sheet to form a non-alloyed hot-dip galvanized layer on the surface of the Fe-based electroplating layer; and then heating and alloying the hot-dip galvanized layer to obtain an alloyed hot-dip galvanized steel sheet (GA) in which an alloyed hot-dip galvanized layer is formed on one or both sides of the cold-rolled steel sheet. In this case, the Fe-based electroplating layer is incorporated into the hot-dip galvanized layer and disappears during the alloying step. In other words, an alloyed hot-dip galvanized steel sheet (GA) according to one embodiment of the present invention comprises a cold-rolled steel sheet having a component composition containing 0.1% by mass or more and 3.0% by mass or less of Si, and an alloyed hot-dip galvanized layer formed on one or both sides of the cold-rolled steel sheet, and there is no Fe-based electroplating layer between the alloyed hot-dip galvanized layer and the cold-rolled steel sheet.

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

[0068] [Component composition of cold-rolled steel sheet] This section explains the component composition of cold-rolled steel sheets. Hereafter, "mass%" will be simply abbreviated as "%".

[0069] Si: 0.1% or more and 3.0% or less Si is an effective element for achieving high strength in steel plates because it significantly enhances the strength of steel through solid solution without greatly impairing workability (solid solution strengthening ability). However, Si also negatively affects the resistance to welding cracks in welded areas. When the Si content is less than 0.1%, high strength of the steel plate cannot be achieved, and no particular problems arise with the resistance to welding cracks in welded areas, making the application of the present invention unnecessary. This problem of resistance to welding cracks in welded areas becomes particularly apparent when the Si content is 0.5% or more. However, when the cycle time during spot welding in the assembly process of automobile parts becomes a challenge from the standpoint of production costs, and measures are taken to reduce the hold time, the problem of resistance to welding cracks in welded areas may occur even when the Si content is between 0.1% and 0.5%. Therefore, in the present invention, the Si content is set to 0.1% or more, preferably 0.5% or more, more preferably 0.7% or more, and even more preferably 0.9% or more. On the other hand, if the Si content is excessive, the hot and cold rolling properties will decrease significantly, which may negatively affect productivity and lead to a decrease in the ductility of the steel sheet itself. Therefore, the Si content should be 3.0% or less, preferably 2.5% or less, more preferably 2.0% or less, and most preferably 1.7% or less.

[0070] In this embodiment, the Si content of the cold-rolled steel sheet is a mandatory requirement, but other components can be any composition that is typical of cold-rolled steel sheets and are not particularly limited. However, in the case of a high-strength cold-rolled steel sheet with a tensile strength (TS) of 590 MPa or higher measured in accordance with JIS Z 2241 (2011), the following component composition is preferable.

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

[0072] Mn: 1.0% or more and 12.0% or less Mn is an effective element for strengthening steel through solid solution, increasing its strength, improving its hardenability, and promoting the formation of retained austenite, bainite, and martensite. From this viewpoint, the amount of Mn is preferably 1.0% or more, more preferably 1.3% or more, even more preferably 1.5% or more, and most preferably 1.8% or more. On the other hand, from the viewpoint of obtaining the above effects without increasing costs, the amount of Mn is preferably 12.0% or less, more preferably 3.5% or less, and even more preferably 3.3% or less.

[0073] P: 0.1% or less (excluding 0%) By suppressing the amount of phosphorus (P), a decrease in weldability can be prevented. Furthermore, by preventing P from segregating at grain boundaries, deterioration of ductility, bendability, and toughness can be prevented. Also, adding a large amount of P promotes ferrite transformation, which increases the grain size. For this reason, it is preferable to keep the amount of P to 0.1% or less. The lower limit of P is not particularly limited, and due to production technology constraints, the amount of P may be greater than 0% and may be 0.001% or more.

[0074] S: 0.03% or less (excluding 0%) The sulfur content is preferably 0.03% or less, and more preferably 0.02% or less. By suppressing the sulfur content, a decrease in weldability is prevented, as is a decrease in ductility during hot working, thereby suppressing hot cracking and significantly improving surface properties. Furthermore, by suppressing the sulfur content, the formation of coarse sulfides as impurity elements is avoided, preventing a decrease in the ductility, bendability, and elongation flangeability of the steel sheet. The lower limit of sulfur is not particularly limited, and due to production technology constraints, the sulfur content may be greater than 0% and may be 0.0001% or more.

[0075] N: 0.010% or less (excluding 0%) N forms coarse nitrides with Ti, Nb, and V at high temperatures and does not contribute much to strength. This not only reduces the strength-enhancing effect of adding Ti, Nb, and V, but also leads to a decrease in toughness. Furthermore, if the amount of N is excessive, slab cracking may occur during hot rolling, potentially resulting in surface defects. Therefore, it is preferable to keep the amount of N to 0.010% or less. Preferably, the amount of N is 0.005% or less, more preferably 0.003% or less, and even more preferably 0.002% or less. The lower limit of the amount of N is not particularly limited, and due to production technology constraints, the amount of N may be greater than 0% and may be 0.0005% or more.

[0076] Al: 1.0% or less (excluding 0%) Since Al is thermodynamically the most easily oxidized element, it oxidizes before Si and Mn, suppressing the oxidation of Si and Mn on the outermost surface of the steel sheet and promoting the oxidation of Si and Mn within the steel sheet. On the other hand, if the amount of Al exceeds 1.0%, the cost increases. Therefore, when adding Al, it is preferable to keep the amount of Al to 1.0% or less. The lower limit of Al is not particularly limited, and the amount of Al can be greater than 0% and may be 0.001% or more. However, from the viewpoint of obtaining the above effect, it is preferable that the amount of Al be 0.01% or more.

[0077] The remainder of the mixture consists of Fe and unavoidable impurities. However, it may optionally contain at least one element selected from the following:

[0078] B: 0.005% or less B is an effective element for improving the hardenability of steel. To improve hardenability, the amount of B is preferably 0.0003% or more, and more preferably 0.0005% or more. Furthermore, to avoid impairing formability, the amount of B is preferably 0.005% or less.

[0079] Ti: 0.2% or less Ti is effective in strengthening steel by precipitation. For this reason, the Ti content is preferably 0.005% or more. Furthermore, to avoid impairing formability, the Ti content is preferably 0.2% or less, and more preferably 0.05% or less.

[0080] Cr:1.0% or less It is preferable that the Cr content be 0.005% or more. By having a Cr content of 0.005% or more, hardenability can be improved and the balance between strength and ductility can be improved. When adding Cr, it is preferable to keep the Cr content at 1.0% or less from the viewpoint of preventing cost increases.

[0081] Cu: 1.0% or less It is preferable that the amount of Cu be 0.005% or more. By having an amount of Cu of 0.005% or more, the formation of the residual γ phase can be promoted. When adding Cu, it is preferable that the amount of Cu be 1.0% or less from the viewpoint of preventing cost increases.

[0082] Ni: 1.0% or less The amount of Ni is preferably 0.005% or more. A Ni amount of 0.005% or more can promote the formation of the residual γ phase. When Ni is added, from the viewpoint of preventing cost increases, the Ni amount is preferably 1.0% or less.

[0083] Mo: 1.0% or less The amount of Mo is preferably 0.005% or more. By setting the amount of Mo to 0.005% or more, the effect of adjusting the strength can be obtained, and the amount of Mo is more preferably 0.05% or more. When Mo is added, from the viewpoint of preventing cost increases, the amount of Mo is preferably 1.0% or less.

[0084] Nb: 0.20% or less From the viewpoint of obtaining the effect of improving strength, it is preferable that the Nb content be 0.005% or more. If Nb is included, from the viewpoint of preventing cost increases, it is preferable that the Nb content be 0.20% or less.

[0085] V: 0.5% or less From the viewpoint of obtaining the effect of improving strength, it is preferable that the amount of V be 0.005% or more. If V is included, from the viewpoint of preventing cost increases, it is preferable that the amount of V be 0.5% or less.

[0086] Sb: 0.020% or less Sb can be included from the viewpoint of suppressing oxidation of the steel sheet surface. By suppressing oxidation of the steel sheet, Sb improves the wettability of the plating and the chemical conversion treatment properties of cold-rolled steel sheets. To obtain such effects, it is preferable that the amount of Sb be 0.001% or more. On the other hand, Sb suppresses the formation of a decarburized layer. To obtain good resistance to welding cracking properties, it is preferable that the amount of Sb be 0.020% or less, more preferably 0.015% or less, and even more preferably 0.012% or less.

[0087] Ta: 0.1% or less From the viewpoint of obtaining the effect of improving strength, it is preferable that the amount of Ta be 0.001% or more. If Ta is included, from the viewpoint of preventing cost increases, it is preferable that the amount of Ta be 0.1% or less.

[0088] W: 0.5% or less From the viewpoint of obtaining the effect of improving strength, it is preferable that the amount of W be 0.005% or more. If W is included, from the viewpoint of preventing cost increases, it is preferable that the amount of W be 0.5% or less.

[0089] Zr: 0.1% or less From the viewpoint of obtaining the effect of improving strength, it is preferable that the amount of Zr be 0.0005% or more. If Zr is included, from the viewpoint of preventing cost increases, it is preferable that the amount of Zr be 0.1% or less.

[0090] Sn: 0.20% or less Sn is an effective element in suppressing the reduction in steel strength by inhibiting denitrification, deboration, etc. To obtain these effects, it is preferable that the Sn content be 0.002% or more. To ensure impact resistance, if Sn is included, it is preferable that the Sn content be 0.20% or less.

[0091] Ca: 0.005% or less By setting the Ca content to 0.0005% or more, the morphology of the sulfide can be controlled, improving ductility and toughness. From the viewpoint of obtaining good ductility, it is preferable that the Ca content be 0.005% or less when Ca is present.

[0092] Mg: 0.005% or less By setting the Mg content to 0.0005% or more, the morphology of the sulfides can be controlled, improving ductility and toughness. When Mg is included, it is preferable to keep the Mg content to 0.005% or less to prevent cost increases.

[0093] REM: 0.005% or less By setting the REM content to 0.0005% or more, the morphology of the sulfide can be controlled, improving ductility and toughness. From the viewpoint of obtaining good toughness, it is preferable that the REM content be 0.005% or less when REM is included.

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

[0095] [Degreasing / pickling] In this embodiment, it is preferable to degrease and then pickle the cold-rolled steel sheet as a pretreatment for Fe-based electroplating. Specifically, it is preferable to degrease and rinse with water to clean the surface of the steel sheet, and then pickle and rinse with water to activate the surface of the steel sheet. Degreasing and rinsing are not particularly limited, and known or arbitrary methods and conditions can be used. Various acids such as sulfuric acid, hydrochloric acid, nitric acid, and mixtures thereof can be used in the pickling treatment. Among these, sulfuric acid, hydrochloric acid, and mixtures thereof are preferable. The concentration of the acid is not particularly specified, but considering the ability to remove oxide films and prevent surface roughness due to over-pickling, a concentration of about 1 to 20% by mass is desirable. The pickling solution may also contain an antifoaming agent, a pickling accelerator, a pickling inhibitor, etc.

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

[0097] The Fe ion content in the plating bath before the start of current application is, from the viewpoint of obtaining a sufficient amount of Fe-based electroplating layer, 2+ The concentration is preferably 0.5 mol / L or more and 2.0 mol / L or less. Other conditions for Fe-based electroplating are not particularly limited. The temperature of the plating solution is preferably 30°C or more and 85°C or less, considering the ability to maintain a constant temperature. The pH of the plating solution is not particularly limited, but it is preferably 1.0 or higher from the viewpoint of preventing a decrease in current efficiency due to hydrogen generation, and preferably 3.0 or lower from the viewpoint of ensuring conductivity. The current density is 10 A / dm² from the viewpoint of productivity. 2 It is preferable to set it to the above, and from the viewpoint of facilitating control of the amount of Fe-based electroplating layer deposited, 150 A / dm 2 The following is preferable: The plate passing speed is preferably 5 mpm or higher from the viewpoint of productivity, and preferably 150 mpm or lower from the viewpoint of stably controlling the amount of adhesion.

[0098] As the Fe-based electroplating layer, in addition to pure Fe, alloy plating layers such as Fe-B alloy, Fe-C alloy, Fe-P alloy, Fe-N alloy, Fe-O alloy, Fe-Ni alloy, Fe-Mn alloy, Fe-Mo alloy, and Fe-W alloy can be used. The plating bath may contain at least one element selected from the group consisting of B, C, P, N, O, Ni, Mn, Mo, Zn, W, Pb, Sn, Cr, V, and Co. In the case of CR and GI, it is preferable that the total content of these elements in the Fe-based plating layer be 10% by mass or less, and in the case of GA, since it is considered that the Fe-based plating layer will not remain, it is preferable that the total content of these elements in the alloyed hot-dip galvanized layer be 1% by mass or less. In the case of CR or GI, and when the Fe-based electroplating layer is an Fe-C alloy, it is preferable that the C content be 0.08% by mass or less. Metal elements may be included as metal ions, while nonmetal elements may be included as part of boric acid, phosphoric acid, nitric acid, organic acids, etc. Furthermore, the plating bath may contain conductivity enhancers such as sodium sulfate and potassium sulfate, as well as chelating agents and pH buffers.

[0099] Amount of Fe-based electroplated layer deposited on one side: A: 1.0 g / m² 2 That's all. In all manufacturing processes for CR, GI, and GA, the amount of Fe electroplated layer A per side is 1.0 g / m². 2 If the amount is less than 1.0 g / m², the amount of Fe-based electroplating layer will be insufficient, resulting in poor chemical conversion treatment or an unsatisfactory appearance when hot-dip galvanizing is applied. Therefore, the amount of Fe-based electroplating layer A per side should be 1.0 g / m². 2 The above is preferable, preferably 2.0 g / m 2 The amount exceeding this limit is considered to be the upper limit. However, from the viewpoint of suppressing the lengthening of the manufacturing line and the increase in power costs, the amount of adhesion A per side of the Fe-based electroplating layer is set to 5.0 g / m². 2 It is preferable to keep it below 4.5 g / m². 2The following is more preferable. In CR and GI of this embodiment, the amount of Fe-based electroplating layer deposited on one side is maintained at the amount A deposited immediately after Fe-based electroplating. On the other hand, in GA, the Fe-based electroplating layer disappears.

[0100] The Fe-based electroplating layer also contributes to the resistance to welding cracks in the welded area. Although the mechanism is not clear, it is thought that during annealing, as described later, the Fe-based electroplating layer promotes decarburization from the surface layer of the steel sheet, and the softened surface layer resulting from decarburization relieves residual stress during welding, thereby suppressing cracking in the welded area.

[0101] Compared to cold-rolled steel sheets without an Fe-based electroplating layer, sheets with an Fe-based electroplating layer of 1.0 g / m² 2 In Fe-based electroplated steel sheets possessing the above characteristics, annealing further promotes the formation of a decarburized layer, improving resistance to welding cracks in the welded area. Although the mechanism is not clear, in the absence of the Fe-based electroplating layer, decarburization proceeds during annealing by a reaction in which dissolved carbon reacts with H2O in the furnace to produce CO and CO2 on the surface of the steel sheet. In contrast, with the presence of the Fe-based electroplating layer, the dissolved carbon in the steel sheet can diffuse into the Fe-based electroplating layer while remaining dissolved carbon. Therefore, the rate-determining process is different, and it is presumed that the rate of decarburization is faster with the Fe-based electroplating layer. Note that in the case of electroplating with Ni, Co, Sn, etc. alone, the solid solubility of carbon in these metal elements is extremely low, and since carbon does not dissolve, the effect of promoting decarburization is not obtained.

[0102] The amount of Fe-based electroplating layer deposited is measured as follows: A 10 × 15 mm sample is taken from an Fe-based electroplated steel sheet and embedded in resin to create a cross-sectional embedded sample. Three arbitrary locations on this cross-section are observed using a scanning electron microscope (SEM) at an acceleration voltage of 15 kV, with magnifications ranging from 2000 to 10000 times depending on the thickness of the Fe-based electroplating layer. The amount of Fe-based electroplating layer deposited per side is calculated by multiplying the average thickness of the three fields of view by the density of iron.

[0103] [Heating process] In the manufacturing processes for CR, GI, and GA, it is preferable to heat the Fe-based electroplated steel sheet in the heating step before annealing (soaking) at an average heating rate of 10°C / second or more in the temperature range of 400°C to 650°C. By setting the average heating rate to 10°C / second or more, the growth of crystal grains in the Fe-based electroplated layer during the heating step is suppressed as much as possible. This is because, as will be described later, internal oxidation of Si does not proceed much at the crystal grain boundaries of the Fe-based electroplated layer during the heating step, so if the average heating rate is less than 10°C / second, the growth of crystal grains cannot be suppressed. With the growth of crystal grains in the Fe-based electroplated layer suppressed as much as possible during the heating step, as will be described later, the dew point B (°C) of the atmosphere during annealing is such that the amount of adhesion A (g / m²) per side of the Fe-based electroplated layer 2 In relation to the above, the crystals of the Fe-based electroplating layer can be made finer by performing annealing to satisfy predetermined conditions. For the heating zone in the heating process, for example, a direct-fired furnace (DFF) or a non-oxidizing furnace (NOF) can be used. In the case of a radiant tube type heating furnace, a preheating zone such as an induction heater (IH) may be provided in the preceding stage.

[0104] [Annealing] Next, the Fe-based electroplated steel sheet is subjected to an annealing process. The annealing process is performed to adjust the strength of the steel sheet by removing the strain caused by the rolling process and recrystallizing the microstructure.

[0105] Hydrogen concentration: 1.0% by volume or more and 30.0% by volume or less The annealing process can be carried out, for example, in a reducing atmosphere with a hydrogen concentration of 1.0% by volume or more and 30.0% by volume or less. Hydrogen plays a role in suppressing the oxidation of Fe on the surface of the Fe-based electroplated steel sheet during the annealing process and activating the steel sheet surface. If the hydrogen concentration is 1.0% by volume or more, the oxidation of Fe on the steel sheet surface is suppressed, and plating adhesion can be ensured when applying hot-dip galvanizing. Therefore, it is preferable to carry out the annealing process in a reducing atmosphere with a hydrogen concentration of 1.0% by volume or more, and more preferably in a reducing atmosphere with a hydrogen concentration of 2.0% by volume or more. There is no particular upper limit to the hydrogen concentration in the annealing process, but from the viewpoint of cost, it is preferable to set the hydrogen concentration to 30.0% by volume or less, and more preferably to 20.0% by volume or less. It is preferable that the remainder of the annealing atmosphere other than hydrogen be nitrogen.

[0106] In this embodiment, the dew point B (°C) of the atmosphere during annealing is equal to the amount of Fe-based electroplating layer A (g / m²) deposited on one side. 2 In relation to the above, it is crucial to satisfy the specified conditions, which improves the resistance to welding cracks in the welded joint. This is thought to be because decarburization is promoted when Fe-based electroplated steel sheets are annealed, and the softened surface layer due to decarburization relieves residual stress during welding, thereby suppressing cracking in the welded joint.

[0107] Specifically, when manufacturing simple Fe-based plated steel sheets (CR) or hot-dip galvanized steel sheets (GI) in which the hot-dip galvanized layer is not alloyed, it is important that the following formula (1) is satisfied, and it is preferable that the following formula (1)' is satisfied. A + B ≥ 3.0 ···(1) A + B ≥ 8.0 ···(1)'

[0108] Furthermore, when manufacturing alloyed hot-dip galvanized steel sheets (GA) in which the hot-dip galvanized layer is alloyed, it is important that the following formula (2) is satisfied, and it is preferable that the following formula (2)' is satisfied. A + B ≥ 5.0 ···(2) A + B ≥ 10.0 ···(2)'

[0109] The LME cracks described above can be broadly classified into "cracks occurring on the surface in contact with the electrode (hereinafter referred to as surface cracks)" and "cracks occurring near the corona bond between steel plates (hereinafter referred to as internal cracks)." Surface cracks are known to be more likely to occur in resistance welding in high-current ranges where spatter is generated, and surface cracks can be suppressed by maintaining an appropriate current range that does not generate spatter. On the other hand, internal cracks can occur even when the current during resistance welding is within an appropriate range that does not generate spatter. Furthermore, while surface cracks are easily detected by visual inspection during the manufacturing process, internal cracks are difficult to detect by visual inspection. For these reasons, internal cracks are a particularly significant problem among LME cracks. When resistance welding is performed with the welding electrode at an angle to the steel plate, residual stress increases, which may lead to the formation of internal cracks. Since residual stress is thought to increase with increasing steel plate strength, there are concerns about the occurrence of internal cracks as steel plates become stronger. In this disclosure, it is possible to improve the resistance welding crack resistance characteristics, particularly the characteristics that prevent internal cracks.

[0110] As described above, if formula (1) or formula (2) is ultimately satisfied, the resistance to welding cracking of the welded joint will be improved. In other embodiments, in the manufacturing process of CR and GI, when formula (1) or formula (1)' is not satisfied, the amount of adhesion A (g / m) is adjusted to satisfy formula (1) or formula (1)'. Similarly, in the manufacturing process of GA, when formula (2) or formula (2)' is not satisfied, the amount of adhesion A (g / m) is adjusted to satisfy formula (2) or formula (2)'. 2The process may further include a step of changing at least one of the following: ) or dew point B (°C). This makes it possible to more reliably improve the resistance to welding cracks of the welded part. As an example of performing the process during operation, the process may include a step of changing the dew point B in the annealing process to satisfy formula (1) or formula (1)' or formula (2) or formula (2)' according to the value of the amount A of the Fe-based electroplating layer obtained in Fe-based electroplating, and controlling the atmospheric dew point in the annealing process to achieve the changed dew point B. Specifically, the value of the amount A of the Fe-based electroplating layer obtained in Fe-based electroplating is substituted into formula (1) or formula (1)' or formula (2) or formula (2)', and the dew point B in the annealing process is determined to satisfy the substituted formula. Here, "substituting the value of the amount A of the Fe-based plating layer into equation (1) or equation (1)', or equation (2) or equation (2)'" is not limited to substituting into the exact same equation as (1) or equation (1)', or equation (2) or equation (2)', but also includes substituting into a narrower range of inequalities that always satisfy these equations. By performing such control, for example, when the product specifications of steel sheets being passed through the machine change and the amount A changes significantly, causing equation (1) or equation (1)', or equation (2) or equation (2)' to no longer be satisfied (when the equations are actually no longer satisfied, or when circumstances arise that cause the equations to no longer be satisfied), automatic control can be performed to satisfy the equations.

[0111] Furthermore, since the control response of the dew point B is worse than that of the adhesion amount A, it is preferable from the viewpoint of control response to change the adhesion amount A in accordance with the value of the dew point B so as to satisfy equation (1) or equation (1)', or equation (2) or equation (2)'. In the case of a continuous annealing furnace, the adhesion amount A in the Fe-based electroplating process upstream of the annealing process will be changed in accordance with the value of the dew point B in the annealing process, and the parts of the steel sheet that are passed through continuously in which the adhesion amount A has been changed will be manufactured under conditions that satisfy equation (1) or equation (1)', or equation (2) or equation (2)'.

[0112] As for the timing of changing at least one of the adhesion amount A or dew point B to satisfy formula (1) or formula (1)', or formula (2) or formula (2)', it is more preferable to change the adhesion amount A or dew point B in accordance with the passage of the welded area when welding steel plates of different product specifications and passing them through continuously. As mentioned above, the responsiveness of dew point B is poor, so when changing dew point B, it is more preferable to feedforward control of the amount of humidification in the furnace to satisfy the formula.

[0113] The "value of adhesion amount A" here may be either the adhesion amount (target value) that can be obtained under the conditions adopted for Fe-based electroplating, or the actual adhesion amount (measured value) of the Fe-based electroplating layer obtained. Similarly, the "value of dew point B" may be either the target value or the measured value.

[0114] The above describes examples of the operational processes in the manufacturing methods of CR, GI, and GA. However, it is also possible to implement a method for determining the manufacturing conditions of CR, GI, and GA by first checking whether the target value of adhesion amount A and the target value of dew point B satisfy formula (1) or formula (1)', or formula (2) or formula (2)' before starting operations, and if they do not satisfy the conditions, changing either the target value of adhesion amount A or the target value of dew point B in advance. Such a method for determining manufacturing conditions may be implemented as part of the manufacturing process of CR, GI, and GA, or as a standalone process.

[0115] While there is no particular upper limit to the dew point of the annealing atmosphere, it is preferable that the dew point of the annealing atmosphere be 20°C or lower in order to effectively prevent oxidation of the surface of the Fe-based electroplating layer, suppress variations in the dew point, and improve the chemical conversion treatment properties or the plating adhesion when hot-dip galvanizing is applied. Furthermore, there is no particular lower limit to the dew point B of the annealing atmosphere as long as it satisfies formula (1) or formula (2) above, but it is preferable that the dew point B is above 0°C, and more preferably above 5°C.

[0116] Holding time in the temperature range of 650°C to 900°C: 30 seconds to 600 seconds In the annealing process, it is preferable to hold the material in the temperature range of 650°C to 900°C for 30 seconds or more. This effectively removes the native oxide film of Fe formed on the surface of the Fe-based electroplating layer, improving the chemical conversion treatment properties or the plating adhesion when hot-dip galvanizing is applied. There is no particular upper limit for the holding time in this temperature range, but from the viewpoint of productivity, it is preferable to hold the material in this temperature range for 600 seconds or less.

[0117] Maximum temperature achievable for Fe-based electroplated steel sheets: 650°C to 900°C The maximum temperature achievable by the Fe-based electroplated steel sheet is not particularly limited, but it is preferably between 650°C and 900°C. By setting the maximum temperature of the Fe-based electroplated steel sheet to 650°C or higher, the recrystallization of the steel sheet structure proceeds favorably, allowing for the acquisition of desired strength. Furthermore, the native oxide film of Fe formed on the surface of the Fe-based electroplating layer is suitably reduced, improving the suitability for chemical conversion treatment or the adhesion of the plating when hot-dip galvanizing is applied. Additionally, if the maximum temperature of the Fe-based electroplated steel sheet is 900°C or lower, it prevents excessive increases in the diffusion rate of Si and Mn in the steel, thus preventing the diffusion of Si and Mn to the steel sheet surface, and improving the suitability for chemical conversion treatment or the adhesion of the plating when hot-dip galvanizing is applied. Moreover, if the maximum temperature is 900°C or lower, damage to the heat treatment furnace can be prevented, leading to cost reduction. The above maximum temperature is based on the temperature measured on the surface of the Fe-based electroplated steel sheet.

[0118] [Hot-dip galvanizing] In the manufacturing process of GI and GA, after the annealing process, a further step is performed in which the Fe-based electroplated steel sheet is hot-dip galvanized to form a hot-dip galvanized layer on the surface of the Fe-based electroplated steel sheet. After the annealing process, the Fe-based electroplated steel sheet is cooled and immersed in a hot-dip galvanizing bath to apply hot-dip galvanizing to the surface of the steel sheet. The hot-dip galvanizing bath consists of Al, Zn, and unavoidable impurities. The composition of the hot-dip galvanizing bath is not specifically defined, but generally the Al concentration in the bath is between 0.05% by mass and 0.250% by mass. If the Al concentration in the bath is 0.05% by mass or higher, the occurrence of bottom dross can be prevented, preventing dross from adhering to the steel sheet and becoming a defect. Also, by keeping the Al concentration in the bath below 0.250% by mass, the increase of top dross can be prevented, preventing dross from adhering to the steel sheet and becoming a defect, and also leads to cost reduction. Other conditions for the hot-dip galvanizing process are not limited, but for example, the bath temperature of the hot-dip galvanizing bath is typically in the range of 440 to 500°C, and the steel plate is immersed in the hot-dip galvanizing bath at a plate temperature of 440 to 550°C.

[0119] The amount of hot-dip galvanized plating deposited on one side is 25-80 g / m². 2 The amount of adhesion is controlled to 25 g / m². 2 By doing so, corrosion resistance can be further improved, and the amount of plating deposited can be easily controlled. Furthermore, the amount of plating deposited is 80 g / m². 2 The following conditions indicate good plating adhesion. The amount of adhesion can be adjusted by general gas wiping.

[0120] [Heating alloying] In the manufacturing process of GA, after hot-dip galvanizing, the hot-dip galvanized layer is heated and alloyed to produce alloyed hot-dip galvanized steel sheet (GA). The method of alloying is not particularly limited, but it can be performed using an IH, gas furnace, etc., and the maximum plate temperature reached during alloying is preferably 460 to 600°C. If the temperature is 460°C or higher, sufficient alloying will be performed, and if it is 600°C or lower, the alloying will not be excessive and the adhesion of the plating will not be impaired.

[0121] The Fe% in the alloyed hot-dip galvanized layer of alloyed hot-dip galvanized steel sheets is preferably 7 to 15% by mass. If it is 7% by mass or more, the deterioration of press formability due to the remaining η phase can be suppressed, and if it is 15% by mass or less, the plating adhesion is not impaired. When controlled within this range, the Fe-based electroplated layer is completely alloyed with zinc and disappears. Even in this case, alloyed hot-dip galvanized steel sheets have excellent resistance to welding cracking.

[0122] This document explains how to calculate the amount of zinc plating layer and the Fe% for GI and GA. The amount of zinc plating deposited is measured in accordance with JIS H 0401 or ISO 17925. First, two 20 x 25 mm samples are taken from GI or GA. After measuring the weight of each sample, the plating is removed from one side using the test solution specified in JIS H 0401 or ISO 17925, and the weight is measured again. The amount of plating deposited can be calculated by subtracting the weight after plating removal from the weight before plating removal and dividing by the surface area of ​​the removed portion. Here, the average value of the two measurements is used as the amount of plating deposited.

[0123] Fe% measurement is performed in accordance with ISO 17925. The test solution after plating removal described above is analyzed using an inductively coupled plasma (ICP) emission spectrometer, and the Fe% can be calculated by dividing the amount of Fe by the total amount of Fe, Zn, and Al contained in the test solution. Here, the average value from two locations is used as the Fe%.

[0124] [Internal oxidized layer in CR and GI] In the CR and GI of this embodiment, it is crucial to keep the depth of the internal oxide layer from a depth of 0.10 μm from the surface of the Fe-based electroplating layer toward the cold-rolled steel sheet to within 2.00 μm. This can be achieved by performing annealing that satisfies equation (1) A + B ≥ 3.0. If the depth of the internal oxide layer from a depth of 0.10 μm from the surface of the Fe-based electroplating layer toward the cold-rolled steel sheet exceeds 2.00 μm, the resistance to welding cracks deteriorates. Although the mechanism is not clear, it is thought that by keeping the depth of the internal oxide layer to within 2.00 μm, the penetration of zinc into the depth direction of the cold-rolled steel sheet when it reaches the grain boundaries of the cold-rolled steel sheet can be suppressed as much as possible, thereby improving the resistance to welding cracks at the weld. On the other hand, forming an internal oxide suppresses the formation of oxides on the surface during annealing, which plays a role in improving the chemical conversion treatment properties in CR and the plating appearance in GI. In order to exhibit such effects, it is preferable that the depth of the internal oxide layer be 0.10 μm or more.

[0125] The condition that the depth of the internal oxide layer in the direction of the cold-rolled steel sheet from a depth of 0.10 μm from the surface of the Fe-based electroplating layer is within 2.00 μm is defined as satisfying the following conditions: That is, in the emission intensity profile of the wavelength indicating Si measured in the depth direction from the surface of the Fe-based electroplating layer by glow discharge optical emission spectrometry (GD-OES), (i) the average Si intensity (I) in the range of 10.0 ± 0.1 μm from the interface between the Fe-based electroplating layer and the cold-rolled steel sheet Si (ii) there exists a peak with a greater emission intensity than (ii) the peak of the peak is located at a depth greater than 0.10 μm from the surface of the Fe-based electroplating layer, and (iii) the emission intensity gradually decreases in the depth direction from the peak of the peak only when the average Si intensity (I SiThe depth at which the sputtering marks are equal to the specified value is located within a range of 0.10 μm to 2.00 μm from the surface of the Fe-based electroplating layer. The measurement conditions were Ar gas pressure of 600 Pa, high-frequency output of 35 W, measurement diameter of 4 mmΦ, and sampling interval of 0.1 seconds. For the analysis of cold-rolled steel sheets without Fe-based electroplating under the same conditions using glow discharge emission spectrometry, the sputtering rate was calculated by measuring the depth of the sputtering marks, and the horizontal axis of the wavelength intensity profile showing Si was converted to the depth corresponding to each time. A non-contact surface shape measuring device (NewView 7300: Zygo Corporation) was used to measure the depth of the sputtering marks.

[0126] Using Figure 5, we will explain a typical example of the Si peak obtained by analyzing the emission intensity at wavelengths indicating Si as observed in this embodiment. Figure 5 shows Comparative Example No. 21 (no Fe-based electroplating, dew point B in annealing atmosphere: +4.7℃) and Invention Example No. 23 (amount of Fe-based electroplating A: 4.7g / m²) in Example 1 (Table 4) described later. 2 (Dew point B: -1.3℃), and Invention Example No. 27 (Adhesion amount A of Fe-based electroplating: 3.9g / m²) 2 This is raw data of the emission intensity profile at a wavelength indicating Si with a dew point B: +9.8℃. In Invention Examples No. 23 and 27, an Fe-based electroplating layer (referred to as "Fe plating" in Figure 5 for convenience) with a thickness of approximately 0.50 to 0.60 μm is formed on the surface of the cold-rolled steel sheet. In Comparative Example No. 21, a peak Pex originating from external Si oxide is observed within 0.10 μm from the surface of the cold-rolled steel sheet. In addition, a peak Pin originating from internal Si oxide is observed at a depth of more than 0.10 μm from the surface. In Invention Examples No. 23 and 27 as well, a peak Pex originating from external Si oxide is observed within 0.10 μm from the surface of the Fe-based electroplating layer, and a peak Pin originating from internal Si oxide is also observed at a depth of more than 0.10 μm from the surface. The Pin is observed spanning both the Fe-based electroplating layer and the cold-rolled steel sheet, which means that internal Si oxide is also formed in the Fe-based electroplating layer. The luminescence intensity gradually decreases in the depth direction from the peak of this mountain Pin, and only then does the average Si intensity (I) begin to decrease. SiThe internal oxide layer is defined as the depth to which the luminescence intensity becomes equal to (I). Since Pex exists in the range from the surface (the surface of the cold-rolled steel sheet in Comparative Example No. 21, and the surface of the Fe-based electroplating layer in Invention Examples No. 23 and 27) to a depth of 0.10 μm, this range is excluded, and the "thickness of the internal oxide layer" is defined as the depth from a depth of 0.10 μm from the surface, where the luminescence intensity gradually decreases in the depth direction from the peak of the mountain Pin, and only then is the average Si intensity (I) equal to (I) Si The thickness is set to a depth equal to ). In Comparative Example No. 21, the thickness of the internal oxide layer was 2.39 μm. In Invention Examples No. 23 and 27, the thickness of the internal oxide layer was 0.56 μm and 0.52 μm, respectively.

[0127] Here, the thickness of the Fe-based electroplating layer is the value measured by the cross-sectional observation described above. In steel sheets in which oxide is partially formed inside the Fe-based electroplating layer, the growth of crystal grains in the Fe-based electroplating layer is suppressed by the internal oxide. Therefore, even if annealing is performed after Fe-based electroplating, it is possible to prevent the crystal grains of the Fe-based electroplating layer from becoming coarser, and as a result of the formation of many crystal grain boundaries in the Fe-based electroplating layer, the penetration paths of molten zinc are dispersed, delaying the time it takes for molten zinc to reach the crystal grain boundaries of the cold-rolled steel sheet during resistance welding, resulting in excellent resistance welding crack resistance characteristics. Furthermore, by partially forming oxide inside the Fe-based electroplating layer, the depth of the internal oxide layer from a depth of 0.10 μm from the surface of the Fe-based electroplating layer toward the cold-rolled steel sheet can be kept within 2.00 μm, which minimizes the penetration of zinc into the depth direction of the cold-rolled steel sheet when it reaches the crystal grain boundaries of the cold-rolled steel sheet, resulting in even better resistance welding crack resistance characteristics.

[0128] Furthermore, when analyzed by glow discharge emission spectrometry in the depth direction from the surface, peaks of emission intensity at wavelengths indicating Si may be present both above 0.10 μm from the surface of the Fe-based electroplating layer and between 0.00 μm and 0.10 μm from the surface. In all of the above-mentioned figures 5, No. 21, 23, and 27, peaks of emission intensity at wavelengths indicating Si are observed both above 0.10 μm from the surface and between 0.00 μm and 0.10 μm from the surface. This indicates that the material has both internal Si oxide and external Si oxide on the surface.

[0129] [Amount of internal oxides in GA] In this embodiment of GA, it is important to reduce the amount of internal oxides in contact with the alloyed hot-dip galvanized layer. Specifically, in the emission intensity profile of the wavelength indicating Si, measured from the surface of the alloyed hot-dip galvanized layer in the depth direction (plate thickness direction) using glow discharge optical emission spectrometry (GD-OES), the average Si intensity (I) in the range of +0.5 μm toward the cold-rolled steel sheet from the interface between the alloyed hot-dip galvanized layer and the cold-rolled steel sheet is measured. Si,Fe ) is the average Si strength (I) in cold-rolled steel sheet. Si,bulk The value obtained by dividing by (I Si,Fe ) / (I Si,bulk It is crucial that ) is 0.90 or less. This can be achieved by performing annealing that satisfies equation (2) A + B ≥ 5.0.

[0130] (I Si,Fe ) / (I Si,bulkA value of 0.90 or less means that the Si that diffuses from the cold-rolled steel sheet into the Fe-based electroplating layer during annealing can be converted into oxide within the Fe-based electroplating layer. In this way, by partially forming oxide within the Fe-based electroplating layer, the amount of internal oxide in contact with the alloyed hot-dip galvanized layer can be reduced. Therefore, the intrusion of zinc from the grain boundaries of the internal oxide layer in contact with the alloyed hot-dip galvanized layer can be suppressed. As a result, the time it takes for molten zinc to reach the crystal grain boundaries of the cold-rolled steel sheet during welding can be delayed, and the resistance to welding cracks in the weld can be improved. Si,Fe ) / (I Si,bulk ) is preferably 0.85 or less, and more preferably 0.80 or less. Also, (I Si,Fe ) / (I Si,bulk ) is preferably 0.50 or higher, and more preferably 0.60 or higher.

[0131] The measurement conditions were: Ar gas pressure 600 Pa, high-frequency output 35 W, measurement diameter 4 mmΦ, and sampling interval 0.1 seconds. The average Si intensity for each measurement is the average value of all Si intensities sampled within each range. Cold-rolled steel sheets without Fe-based electroplating and hot-dip galvanizing were analyzed under the same conditions using glow discharge emission spectrometry. The sputtering rate was calculated by measuring the depth of the sputter marks, and the horizontal axis of the intensity profile of the wavelength showing Si was converted to the depth corresponding to each time point. A non-contact surface shape measuring device (NewView 7300: Zygo Corporation) was used to measure the depth of the sputter marks. The sputtering rate of the alloyed hot-dip galvanized layer differs from that of the Fe-based electroplated layer and the cold-rolled steel sheet. That is, the criteria for depth conversion differ due to the difference in elements. Therefore, the interface between the alloyed hot-dip galvanized layer and the cold-rolled steel sheet is determined as follows. Figure 9 illustrates the method for identifying the interface between the alloyed hot-dip galvanized layer and the cold-rolled steel sheet. First, calculate the average Zn intensity (IZn) in the range of 0.5 ± 0.1 μm in the thickness direction from the surface of the alloyed hot-dip galvanized layer, and divide the result by 2. Next, define the depth in the thickness direction at which the Zn intensity is the above-mentioned value (IZn / 2) as the depth of the interface between the alloyed hot-dip galvanized layer and the cold-rolled steel sheet. Then, move +0.5 μm in the thickness direction toward the cold-rolled steel sheet from the interface between the alloyed hot-dip galvanized layer and the cold-rolled steel sheet. Si,Fe The sputtering rate in the alloyed hot-dip galvanized layer differs from that in the Fe-based electroplated layer and the cold-rolled steel sheet. Therefore, the horizontal axis of the intensity profile does not accurately correspond to the position of the interface between the alloyed hot-dip galvanized layer and the cold-rolled steel sheet that can be visually observed in cross-section. Furthermore, it is generally known that in measurements by glow discharge emission spectrometry, a broad profile is obtained at interfaces consisting of two or more materials due to reasons such as unevenness and sputtering non-uniformity. For this reason, the depth in the thickness direction where the average Zn intensity (IZn) in the range of 0.5 ± 0.1 μm from the surface of the alloyed hot-dip galvanized layer in the thickness direction is divided by 2 to obtain IZn / 2 is defined as the depth of the interface between the alloyed hot-dip galvanized layer and the cold-rolled steel sheet.

[0132] Using Figure 9, we will explain a representative example of the analysis of the emission intensity at wavelengths showing Si and Zn observed in this embodiment. Figure 9 shows Comparative Example No. 36 (no Fe-based electroplating, dew point B in annealing atmosphere: +7.4℃) and Invention Example No. 41 (amount of Fe-based electroplating A: 3.5g / m²) in Example 1 (Table 2) described later. 2 (Dew point B: +7.1℃), and Invention Example No. 45 (Adhesion amount of Fe-based electroplating: 4.2g / m²) 2 These are raw data of the emission intensity profiles at wavelengths showing Si and Zn with a dew point B: +7.6°C. In Comparative Example No. 36, (I Si,Fe ) / (I Si,bulk ) was 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.

[0133] [C concentration in CR, GI, and GA] From the viewpoint of further improving resistance to welding cracking characteristics, in the CR and GI of this embodiment, it is important to reduce the average value of C concentration in the range of 10 μm to 20 μm in the thickness direction from the surface of the Fe-based electroplating layer to 0.10 mass% or less by annealing, preferably to 0.06 mass% or less, and more preferably to 0.04 mass% or less. Similarly, in the GA of this embodiment, it is important to reduce the average value of C concentration in the range of 10 μm to 20 μm in the thickness direction from the interface between the alloyed hot-dip galvanized layer and the cold-rolled steel sheet to 0.10 mass% or less by annealing, preferably to 0.06 mass% or less, and more preferably to 0.04 mass% or less. On the other hand, if the C concentration is too low, the fatigue strength may decrease. Therefore, in the CR and GI of this embodiment, it is preferable to reduce the average value of C concentration in the range of 10 μm to 20 μm in the thickness direction from the surface of the Fe-based electroplating layer to 0.01 mass% or more. Similarly, in the GA of this embodiment, it is preferable that the average value of the C concentration in the range of 10 μm to 20 μm in the thickness direction from the interface between the alloyed hot-dip galvanized layer and the cold-rolled steel sheet is 0.01% by mass or more.

[0134] During annealing, a decarburized layer is formed on the surface of the Fe-based electroplated steel sheet. The decarburized layer is a region near the surface of the steel sheet where the carbon concentration is lower than that in the steel, and can be formed because carbon is removed from the surface of the steel sheet during annealing. In the CR and GI of this embodiment, as described above, if the average carbon concentration in the range of 10 μm to 20 μm in the thickness direction from the surface of the Fe-based electroplated layer is 0.10 mass% or less, that region becomes soft. Similarly, in the GA of this embodiment, if the average carbon concentration in the range of 10 μm to 20 μm in the thickness direction from the interface between the alloyed hot-dip galvanized layer and the cold-rolled steel sheet is 0.10 mass% or less, that region becomes soft. As a result, the stress applied from the welding electrode during resistance welding is relaxed, and the resistance welding crack resistance characteristics are improved.

[0135] In the CR and GI of this embodiment, by forming an Fe-based electroplating layer and then performing annealing, the average C concentration in the range of 10 μm to 20 μm in the thickness direction from the surface of the Fe-based electroplating layer can be further reduced compared to when there is no Fe-based electroplating layer. Similarly, in the GA of this embodiment, by forming an Fe-based electroplating layer and then performing annealing, the average C concentration in the range of 10 μm to 20 μm in the thickness direction from the interface between the alloyed hot-dip galvanized layer and the cold-rolled steel sheet can be further reduced compared to when there is no Fe-based electroplating layer. Note that in the case of electroplating with Ni, Co, Sn, etc. alone, the solid solubility of C in these metal elements is extremely low, and since C does not dissolve, the effect of promoting decarburization cannot be obtained.

[0136] The reason why the carbon (C) concentration decreases in the range of 10 μm to 20 μm in the thickness direction from the surface of the Fe-based electroplating layer when an Fe-based electroplating layer is formed, and also the reason why the carbon concentration decreases in the range of 10 μm to 20 μm in the thickness direction from the interface between the alloyed hot-dip galvanized layer and the cold-rolled steel sheet, is not clear, but the inventors speculate as follows: That is, the Fe-based electroplating layer contains almost no carbon, and the diffusion of carbon from the cold-rolled steel sheet is induced.

[0137] Furthermore, softening by reducing the C concentration in the thickness direction from the surface of the Fe-based electroplating layer or the interface between the alloyed hot-dip galvanized layer and the cold-rolled steel sheet, in a range of 10 μm to 20 μm, causes the C concentration in that range to saturate below a certain level, thus limiting the improvement of resistance welding cracking characteristics through softening. In this embodiment, by further lowering the C concentration in the thickness direction from the surface of the Fe-based electroplating layer or the interface between the alloyed hot-dip galvanized layer and the cold-rolled steel sheet, resistance welding cracking characteristics are effectively improved even when the decarburized layer is shallow. This suggests that, in addition to softening, other effects such as an increase in the melting point due to the reduction in C concentration may be occurring.

[0138] In the CR, GI, and GA of this embodiment, the depth of the decarburized layer from the surface of the Fe-based electroplating layer in CR and GI, and the thickness of the decarburized layer from the interface between the alloyed hot-dip galvanized layer and the cold-rolled steel sheet in GA, is preferably 30 μm or more, and more preferably 80 μm or more. There is no particular upper limit to the thickness of the decarburized layer, but in order to keep the tensile strength within a good range, it is preferable that the thickness of the decarburized layer be 130 μm or less. The thickness of the decarburized layer is defined as the thickness of the region in the surface part of the Fe-based electroplated steel sheet where the C concentration is 80% or less of the steel, when the C concentration of CR and GI is analyzed from the surface of the Fe-based electroplating layer in the thickness direction of the sheet. In GA, it is defined as the thickness of the region in the surface part of the cold-rolled steel sheet where the C concentration is 80% or less of the steel, when the C concentration is analyzed from the interface between the alloyed hot-dip galvanized layer and the cold-rolled steel sheet (i.e., the surface of the cold-rolled steel sheet).

[0139] Here, the average C concentration in the range of 10 μm to 20 μm in the thickness direction from the interface between the Fe-based electroplating layer on the surface in CR and GI, or between the alloyed hot-dip galvanized layer and the cold-rolled steel sheet in GA, and the thickness of the decarburized layer near the surface of the Fe-based electroplating layer in CR and GI, or near the interface between the alloyed hot-dip galvanized layer and the cold-rolled steel sheet in GA, are measured by performing surface or line analysis of the elemental distribution near the surface of the cross-sectioned sample using an electron probe microanalyzer (EPMA). First, the resin-embedded steel sheet is polished to prepare a cross-section perpendicular to the rolling direction for observation, and then removed from the resin to be used as a sample for measurement. The acceleration voltage is 7 kV and the irradiation current is 50 nA. Surface or line analysis of the sample cross-section is performed in 1 μm steps over a 300 × 300 μm area including the outermost layer of the Fe-based electroplating layer in CR and GI, and including the outermost layer of the cold-rolled steel sheet in GA, to measure the C intensity. In this process, to suppress contamination, a plasma cleaner is used to remove hydrocarbons from the surface and surrounding area of ​​the sample in both the measurement room and the sample preparation room before the start of measurement. Furthermore, to suppress the accumulation of hydrocarbons during measurement, the sample temperature is heated and maintained at 100°C on the stage during the measurement. A calibration curve prepared by separately measuring a standard sample is used to convert the C intensity to C concentration (mass%). It is confirmed that the C detection limit is lower than 0.04 mass% due to the effect of contamination suppression. Details of the equipment used and the contamination suppression method are described in Reference 1 below.

[0140] Reference 1: Yamashita et al., "Carbon distribution in the initial stages of proto-ferrite transformation of low-carbon steel by high-precision FE-EPMA," Iron and Steel, Vol. 103 (2017) No. 11, pp. 14-20. However, the need for contamination prevention measures during measurement depends on the equipment and conditions used, so the above configuration is not necessarily required. In other words, it is sufficient that the measurement conditions are such that sufficient accuracy can be obtained, and the measurement conditions are not essential to the effectiveness of this invention.

[0141] In the obtained C concentration map, line profiles in the thickness direction are extracted from the surface of the Fe-based electroplating layer in CR and GI, and from the interface between the alloyed hot-dip galvanized layer and the cold-rolled steel sheet in GA. These are then averaged over 300 points in a direction parallel to the surface of the cold-rolled steel sheet to obtain a C concentration profile in the thickness direction. The position on the surface of the cold-rolled steel sheet can be determined from the secondary electron image or backscattered electron image obtained simultaneously. The obtained C concentration profile in the thickness direction is then smoothed using a simple moving average method. In this case, it is preferable to use approximately 21 smoothing points. If the number of smoothing points near the surface of the sample is less than 10 on one side, it is preferable to perform smoothing on all available measurement points on the other side. Subsequently, in the strength profile after smoothing, the thickness direction range in which the C concentration is 80% or less of the steel is identified in the surface layer of the Fe-based electroplated steel sheet, including the Fe-based electroplating layer and the cold-rolled steel sheet, in CR and GI, and in the surface layer of the cold-rolled steel sheet, in GA, and is defined as the thickness of the decarburized layer. Furthermore, for CR and GI, the carbon concentration values ​​of 11 points at 1 μm intervals are averaged from the surface of the Fe-based electroplating layer, and for GA, from the interface between the alloyed hot-dip galvanized layer and the cold-rolled steel sheet, within a range of 10 μm to 20 μm in the thickness direction. This average is then used as the carbon concentration for the range of 10 μm to 20 μm in the thickness direction. The above evaluation is applied to the measurement results of two fields for each sample, and the average is used as the average carbon concentration in the range of 10 μm to 20 μm in the thickness direction, and as the evaluation value for the thickness of the decarburized layer.

[0142] Representative examples of carbon concentration profiles in the thickness direction, analyzed by an electron beam microanalyzer, are described using Figures 10A, B and 11A, B. Figure 10A shows the raw data of carbon concentration profiles in the thickness direction obtained by analyzing Fe-based electroplated steel sheets No. 21, 23, and 27 of Example 1 (Table 4), which will be described later. Figure 11A shows the raw data of carbon concentration profiles in the thickness direction obtained by analyzing alloyed hot-dip galvanized steel sheets No. 36, 41, and 45 of Example 1 (Table 2). In the case of alloyed hot-dip galvanized steel sheets, the alloyed hot-dip galvanized layer was removed before measurement. Figures 10B and 11B show the data after smoothing the raw data of Figures 10A and 11A, respectively, using a simple moving average method with 21 smoothing points (m=21). As shown in Figures 10B and 11B, in No. 27 of Table 4 and No. 45 of Table 2, a decarburized layer existed in which the C concentration was 80% or less of the steel, and the thickness of this decarburized layer was 77 μm and 81 μm, respectively.

[0143] [Crystal grains in CR and GI] In the CR and GI of this embodiment, it is preferable that the number of grain boundaries in the Fe-based electroplating layer in contact with the cold-rolled steel sheet at the interface between the Fe-based electroplating layer and the cold-rolled steel sheet is 10 or more per 10 μm in the width direction of the steel sheet in the observation field of view of the cold-rolled steel sheet. In this case, the crystals of the Fe-based electroplating layer are sufficiently fine-grained. As a result of the formation of many grain boundaries in the Fe-based electroplating layer due to the fine-graining, the penetration of molten zinc is dispersed, delaying the time it takes to reach the grain boundaries of the cold-rolled steel sheet during welding, improving the resistance to welding cracks in the welded area, and in particular preventing internal cracking. The number of grain boundaries per 10 μm is more preferably 20 or more, and even more preferably 25 or more. On the other hand, if the number of grain boundaries is too large, the fatigue strength may decrease. Therefore, it is preferable that the number of grain boundaries per 10 μm be 40 or less.

[0144] Here, at the interface between the Fe-based electroplating layer and the cold-rolled steel sheet, the number of grain boundaries in the Fe-based electroplating layer in contact with the cold-rolled steel sheet is measured as follows. First, a 10 × 10 mm sample is taken from the Fe-based electroplated steel sheet. An arbitrary location on the sample is processed using a focused ion beam (FIB) apparatus to form a 45° cross section at that location, with a width of 30 μm in the direction perpendicular to the rolling direction and a length of 50 μm in the direction 45° to the T-section (a cross section parallel to the rolling direction of the steel sheet and perpendicular to the surface of the steel sheet), and this is used as the observation sample. Figure 6 shows an overview of the observation sample. Figure 6(a) is a perspective view of the observation sample. Figure 6(b) is a cross-sectional view AA of the observation sample shown in Figure 6(a). Next, a scanning ion microscope (SIM) is used to observe the central part of the 45° cross-section of the observation sample in both the width and length directions at a magnification of 4000x, and a SIM image is taken. An example of such a SIM image is shown in Figure 7. Figure 7 is a SIM image taken as described above for No. 11 of Example 3 (Table 11), which will be described later. From the SIM image, a 10 μm region in the width direction of the cold-rolled steel sheet (the area enclosed by a rectangle in Figure 7) is extracted. For explanation, Figure 8 shows a magnified view of the area enclosed by the rectangle in Figure 7. As shown in Figure 8, for the SIM image, a boundary line (dashed line in Figure 8) is drawn at the interface between the Fe-based electroplating layer and the cold-rolled steel sheet in a 10 μm region in the width direction of the cold-rolled steel sheet. The number of grain boundaries of the Fe-based electroplating layer on the boundary line is measured and defined as "the number of grain boundaries of the Fe-based electroplating layer in contact with the cold-rolled steel sheet at the interface between the Fe-based electroplating layer and the cold-rolled steel sheet." In Invention Example No. 11, at the interface between the Fe-based electroplating layer and the cold-rolled steel sheet, the number of grain boundaries in the Fe-based electroplating layer in contact with the cold-rolled steel sheet was 20 per 10 μm in the width direction of the cold-rolled steel sheet.

[0145] (Chemical conversion treated steel sheet and its manufacturing method) By subjecting an Fe-based electroplated steel sheet (CR or GI) or an alloyed hot-dip galvanized steel sheet (GA) to a chemical conversion treatment, a chemical conversion treatment film can be formed on the surface of the Fe-based electroplated steel sheet or alloyed hot-dip galvanized steel sheet to obtain a chemical conversion treated steel sheet. In this case, as a pretreatment for the chemical conversion treatment, degreasing, washing with water, and, if necessary, surface conditioning treatment can be performed to clean the surface of the Fe-based electroplated steel sheet or alloyed hot-dip galvanized steel sheet. Following these pretreatments, the chemical conversion treatment is carried out. The methods of degreasing and washing with water are not particularly limited, and conventional methods can be used. For the surface conditioning treatment, surface conditioning agents containing Ti colloid or zinc phosphate colloid can be used. When applying these surface conditioning agents, it is not necessary to set up a special process, and it can be carried out according to conventional methods. For example, the desired surface conditioning agent is dissolved in a predetermined amount of deionized water, stirred thoroughly, and then prepared as a treatment solution at a predetermined temperature (usually room temperature, 25-30°C), and the steel sheet is immersed in the treatment solution for a predetermined time (20-30 seconds). The next step, chemical conversion treatment, is carried out without drying. The chemical conversion treatment can also be carried out according to conventional methods. For example, the desired chemical conversion agent is dissolved in a predetermined amount of deionized water, stirred thoroughly, and then heated to a predetermined temperature (usually 35-45°C) to create a treatment solution, in which the steel plate is immersed for a predetermined time (60-120 seconds). As chemical conversion agents, for example, zinc phosphate treatment agents for steel, zinc phosphate treatment agents for both steel and aluminum, and zirconium treatment agents can be used.

[0146] (Electrodeposited coated steel sheet and its manufacturing method) Subsequently, electrodeposition coating can be applied to the chemically treated steel sheet to obtain an electrodeposited steel sheet in which an electrodeposited coating film is formed in contact with the chemically treated film. Electrodeposition coating can also be carried out according to conventional methods. After pretreatment such as washing with water as necessary, the steel sheet is immersed in a well-stirred electrodeposited coating and an electrodeposited coating film of the desired thickness is obtained by electrodeposition. In addition to cationic electrodeposition coating, anionic electrodeposition coating can also be used. Furthermore, depending on the application, a topcoat coating may be applied after electrodeposition coating. The thickness of the electrodeposited coating film varies depending on the application, but it is preferable to have a thickness of 10 μm to 30 μm in a dry state.

[0147] (Automotive parts and their manufacturing methods) Automotive parts can be manufactured using electrodeposited steel sheets in at least a portion of the material. Because the Fe-based electroplated steel sheets and alloyed hot-dip galvanized steel sheets according to this embodiment exhibit excellent resistance to welding cracking at welded joints, electrodeposited steel sheets using these Fe-based electroplated steel sheets and alloyed hot-dip galvanized steel sheets are particularly suitable for application to automotive parts. The type of automotive part is not particularly limited, but examples include side sill parts, pillar parts, and automobile bodies. [Examples]

[0148] (Example 1) A slab of steel having the component composition shown in Table 1 (the remainder being Fe and unavoidable impurities) was melted and hot-rolled to obtain a hot-rolled steel sheet. This hot-rolled steel sheet was then pickled and cold-rolled to obtain a cold-rolled steel sheet with a thickness of 1.6 mm.

[0149] [Table 1]

[0150] Using these cold-rolled steel sheets, various types of GA shown in Table 2, various types of GI shown in Table 3, and various types of CR shown in Table 4 were produced.

[0151] First, cold-rolled steel sheets were degreased with alkali, and then electrolytic treatment was performed using the cold-rolled steel sheets as the cathode under the conditions shown below to produce Fe-based electroplated steel sheets. The amount A of the Fe-based electroplating layer shown in Tables 2-4 was calculated using the method described above and controlled by the energizing time. [Electrolysis conditions] Bath temperature: 50℃ pH: 2.0 Current density: 45A / dm 2 Plating bath: Fe 2+ Sulfuric acid bath containing 1.5 mol / L of ions Anode: Iridium oxide electrode

[0152] Subsequently, the Fe-based electroplated steel sheet was heated at the average heating rate shown in Tables 2-4 in the temperature range of 400°C to 650°C. Then, the steel sheet was annealed at a uniform temperature of 800°C in a reducing atmosphere having the atmospheric dew point B shown in Tables 2-4, containing 15% by volume of hydrogen, with the remainder being N2 and unavoidable impurities. The holding time at the maximum temperature reached by the steel sheet (800°C) was 100 seconds. In the examples shown in Table 4, Fe-based electroplated steel sheets (CR) were obtained in this manner.

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

[0154] In the examples shown in Table 2, the hot-dip galvanized layer was further heated and alloyed by an alloying treatment at 510°C to produce alloyed hot-dip galvanized steel sheet (GA). The Fe% in the alloyed hot-dip galvanized layer was controlled by varying the alloying treatment time.

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

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

[0157] In the GA and GI examples, the appearance of the zinc plating layer (Evaluation 1) and the resistance to welding cracks in the welded area (Evaluation 3) were evaluated, and the results are shown in Tables 2 and 3. In the CR example, the chemical conversion treatment properties and corrosion resistance after painting (Evaluation 2) and the resistance to welding cracks in the welded area (Evaluation 3) were evaluated, and the results are shown in Table 4.

[0158] [Evaluation 1: Evaluation of the appearance of the zinc plating layer (hot-dip galvanized layer or alloyed hot-dip galvanized layer)] The presence or absence of external defects (unplated areas, uneven appearance) was visually assessed and evaluated according to the following criteria. ○: No cosmetic defects. △: Although there are cosmetic defects, when observed at 3000x magnification with an SEM, no exposure of the Fe-based electroplating layer or cold-rolled steel sheet is observed. ×: There is a defect in appearance, and when observed at 3000x magnification with an SEM, the Fe-based electroplating layer or cold-rolled steel sheet is exposed, indicating that the plated surface is unplated. Furthermore, the presence or absence of an exposed Fe-based electroplating layer or cold-rolled steel sheet can be determined by the difference in contrast between Zn and Fe in SEM. More specifically, it can be determined by analyzing with EDX (Energy Dispersive X-ray Spectroscopy) to see whether or not Fe is detected.

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

[0160] Next, a surface conditioning agent was sprayed onto the degreased test specimens, and surface conditioning treatment was performed under the following standard conditions. [Surface conditioning treatment] • Surface conditioning agent: Preparen XG (PL-XG; manufactured by Nippon Parkerizing Co., Ltd.) pH: 9.5 • Processing temperature: Room temperature Processing time: 20 seconds

[0161] Next, the test specimens after surface preparation were immersed in a chemical conversion agent and subjected to chemical conversion treatment under the following standard conditions. [Chemical treatment] • Chemical treatment agent: Palbond PB-SX35 (manufactured by Nippon Parkerizing Co., Ltd.) • Temperature of chemical treatment solution: 35℃ Processing time: 90 seconds

[0162] Using the chemical conversion treatment test specimens prepared as described above, the chemical conversion treatment properties, as described later, were measured.

[0163] (2) Electrodeposition coating treatment The surface of the chemical conversion treated test specimen was electrodeposited with Kansai Paint's GT-100 electrodeposition coating to a film thickness of 15 μm to create an electrodeposited test specimen. This electrodeposited test specimen was subjected to the salt water immersion test described later.

[0164] <Chemical treatment properties> The surface of the above chemical conversion treated test specimen (n=1) was observed by SEM at a magnification of 1000x and evaluated according to the following criteria. ◎ or ○ indicated excellent chemical conversion treatment performance. ◎: The particle size of the chemically converted crystals is 5 μm or less, and no undeposited areas are observed. ○: The particle size of the chemically converted crystals exceeds 5 μm, but no undeposited areas are observed. ×: The particle size of the chemically converted crystals exceeds 5 μm, and undeposited areas are observed.

[0165] <Saltwater immersion test> A 45mm long cross-cut was made on the surface of the electrodeposited coating test specimen (n=1) using a cutter. The specimen was then immersed in a 5 mass% NaCl solution (60°C) for 360 hours, washed with water, and dried. Next, cellophane tape was applied to the cross-cut area of ​​the specimen, and a tape peel test was performed by peeling it off. The maximum peel width of the electrodeposited coating film, combining both sides of the cross-cut area, was measured. The maximum peel width of the electrodeposited coating film was evaluated according to the following criteria. A score of ◎ or ○ indicated excellent corrosion resistance after coating. ◎: Maximum peeling width is 3.0 mm or less ○: Maximum peel width is 5.0 mm or less ×: Maximum peel width exceeds 5.0 mm

[0166] [Evaluation 3: Evaluation of resistance to welding cracks in the welded joint] Referring to Figure 4(a), test pieces 6 were cut from the steel sheets (CR, GI, GA) of each inventive example and comparative example, with the longitudinal direction (TD) being the longitudinal direction and the rolling direction being the short direction, measuring 150 mm in the longitudinal direction and 50 mm in the short direction. The amount of zinc plating layer on one side of a piece cut to the same size was 50 g / m². 2 A test alloyed hot-dip galvanized steel sheet 5 (thickness: 1.6 mm, TS: 980 MPa class) was stacked with the test specimen 6 to form a plate assembly. The plate assembly was assembled so that the evaluation surface of the test specimen 6 (Fe plating layer in the case of CR, 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 assembly was fixed to a fixing base 8 via a spacer 7 with a thickness of 2.0 mm. The spacer 7 is a pair of steel plates measuring 50 mm in the longitudinal direction × 45 mm in the transverse direction × 2.0 mm in thickness, and as shown in Figure 4(a), the longitudinal end faces of each pair of steel plates were aligned with the transverse end faces of the plate assembly. Therefore, the distance between the pair of steel plates is 60 mm. The fixing base 8 is a single plate with a hole in the center. Next, using a servo-motor-operated single-phase AC (50Hz) resistance welding machine, resistance welding was performed on the plate assembly with a pair of electrodes 9 (tip diameter: 6mm) while the assembly was flexed, under the conditions of pressurized force: 3.5kN, hold time: 0.18 seconds or 0.24 seconds, and welding time: 0.36 seconds, using a welding current that resulted in a nugget diameter r of 5.9mm, to create a plate assembly with a welded joint. At this time, the pair of electrodes 9 pressed the plate assembly from above and below in the vertical direction, and the lower electrode pressed the test piece 6 through a hole in the fixed base 8. When pressing, the lower electrode and the fixed base 8 were fixed so that the lower electrode of the pair of electrodes 9 was in contact with the plane extending from the surface where the spacer 7 and the fixed base 8 are in contact, while the upper electrode was made movable. The upper electrode was also positioned to be in contact with the center of the test alloyed hot-dip galvanized steel sheet 5. Furthermore, welding was performed with the plate assembly tilted 5° to the longitudinal side of the plate assembly with respect to the horizontal direction. Hold time refers to the time from when the welding current has finished flowing until the electrodes are released. Referring to the lower part of Figure 4(b), the nugget diameter r means the distance between the ends of the nuggets 10 in the longitudinal direction of the plate assembly. Next, the plate assembly with the weld was cut along the BB line in the upper diagram of Figure 4(b), including the center of the weld, which includes the nugget 10. The cross-section of the weld was then observed with an optical microscope (200x magnification), and the resistance cracking characteristics of the weld were evaluated according to the following criteria. A ◎ or ○ indicates that the resistance cracking characteristics of the weld are excellent. A × indicates that the resistance cracking characteristics of the weld are poor. ◎: No cracks longer than 0.1 mm were observed during a hold time of 0.18 seconds. ○: A crack longer than 0.1 mm is observed at a hold time of 0.18 seconds, but no crack longer than 0.1 mm is observed at a hold time of 0.24 seconds. ×: A crack longer than 0.1 mm was observed with a hold time of 0.24 seconds. In Figure 4(b), the lower diagram schematically shows the crack that occurred in test piece 6 as reference numeral 11. Note that if a crack occurs in the mating steel plate (test alloyed hot-dip galvanized steel plate), the stress in the steel plate under evaluation (steel plates of each invention example and comparative example) will be dispersed, resulting in an inappropriate evaluation. Therefore, data in which no cracks occurred in the mating steel plate were used as examples.

[0167] [Table 2] TIFF0007845208000003.tif245153

[0168] [Table 3]

[0169] [Table 4]

[0170] Figure 1 shows the relationship between the amount of Fe-based electroplating layer A and the dew point B during annealing, and the evaluation results of resistance weld cracking characteristics, in the GA example shown in Table 2. Figure 2 shows the relationship between the amount of Fe-based electroplating layer A and the dew point B during annealing, and the evaluation results of resistance weld cracking characteristics, in the GI example shown in Table 3. Figure 3 shows the relationship between the amount of Fe-based electroplating layer A and the dew point B during annealing, and the evaluation results of resistance weld cracking characteristics, in the CR example shown in Table 4.

[0171] As is clear from Tables 2 and 3 and Figures 1 and 2, the inventive examples in the GA and GI embodiments achieved good appearance of the zinc plating layer and excellent resistance to welding cracking. Furthermore, as is clear from Table 4 and Figure 3, the inventive examples in the CR embodiment achieved good chemical conversion treatment properties and corrosion resistance after painting, as well as excellent resistance to welding cracking.

[0172] From the results of Fig. 1(GA), it can be seen that when compared at the same dew point, the resistance welding crack resistance property is improved by increasing the adhesion amount A of the Fe-based electroplated layer before annealing. Also, the boundary line (solid line) that divides whether the resistance welding crack resistance property is improved shows linearity only in the region where the adhesion amount A of the Fe-based electroplated layer before annealing is 1.0 g / m 2 or more, and it can be seen that in particular, the resistance welding crack resistance property is improved at a lower dew point in the region where the adhesion amount A of the Fe-based electroplated layer before annealing is 1.0 g / m 2 or more. In GA, since all the Fe-based electroplated layers are alloyed with zinc plating by alloying and do not remain, the resistance welding crack resistance property is presumed to be improved by the modification of the surface layer of the underlying cold-rolled steel sheet, particularly by the promotion of decarburization due to the combination of the Fe-based electroplated layer with a predetermined adhesion amount or more and a high dew point.

[0173] On the other hand, in the case of GI and CR, since the Fe-based electroplated layer remains, the Fe-based electroplated layer further acts as a surface soft phase, and when compared at the same dew point and the adhesion amount of the Fe-based electroplated layer before annealing, it is presumed that the resistance welding crack resistance property is improved more than that of GA.

[0174] (Example 2) An ingot obtained by melting a steel having the component composition shown in Table 5 (the balance is Fe and inevitable impurities) was hot-rolled to obtain a hot-rolled steel sheet, and this hot-rolled steel sheet was pickled and cold-rolled to obtain a cold-rolled steel sheet with a thickness of 1.6 mm.

[0175]

Table 5

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

[0177] First, cold-rolled steel sheets were degreased with alkali, and then electrolytic treatment was performed using the cold-rolled steel sheets as the cathode under the conditions shown below to produce Fe-based electroplated steel sheets. The amount A of the Fe-based electroplating layer shown in Tables 6-8 was calculated using the method described above and controlled by the energizing time. [Electrolysis conditions] Bath temperature: 50℃ pH: 2.0 Current density: 45A / dm 2 Plating bath: Fe 2+ Sulfuric acid bath containing 1.5 mol / L of ions Anode: Iridium oxide electrode

[0178] Subsequently, the Fe-based electroplated steel sheet was heated in the temperature range of 400°C to 650°C, with the average heating rate set to the values ​​shown in Tables 6-8. Then, the steel sheet was annealed in a reducing atmosphere with a dew point B as shown in Tables 6-8, containing 15% by volume of hydrogen, with the remainder being N2 and unavoidable impurities, at a uniform temperature of 800°C. The holding time at the maximum temperature reached by the steel sheet (800°C) was 100 seconds. In the examples shown in Table 8, Fe-based electroplated steel sheets (CR) were obtained in this manner.

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

[0180] In the examples shown in Table 6, the hot-dip galvanized layer was further heated and alloyed by an alloying treatment at 510°C to produce alloyed hot-dip galvanized steel sheets (GA). The Fe% in the alloyed hot-dip galvanized layer was controlled by varying the alloying treatment time.

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

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

[0183] In the GA and GI examples, the appearance of the zinc plating layer (Evaluation 1) and the resistance to welding cracks in the welded area (Evaluation 3) were evaluated, and the results are shown in Tables 6 and 7. In the CR example, the chemical conversion treatment properties and corrosion resistance after painting (Evaluation 2) and the resistance to welding cracks in the welded area (Evaluation 3) were evaluated, and the results are shown in Table 8. The evaluation methods and criteria for Evaluations 1 to 3 are the same as in Example 1.

[0184] [Table 6]

[0185] [Table 7]

[0186] [Table 8]

[0187] As is clear from Tables 6 and 7, the inventive examples in the GA and GI embodiments achieved good appearance of the zinc plating layer and excellent resistance to welding cracking. Furthermore, as is clear from Table 8, the inventive examples in the CR embodiment achieved good chemical treatment properties and corrosion resistance after painting, as well as excellent resistance to welding cracking.

[0188] (Example 3) A steel slab having the component composition shown in Table 1 (the balance being Fe and inevitable impurities) was melted and hot-rolled to obtain a hot-rolled steel sheet. This hot-rolled steel sheet was subjected to pickling and cold rolling to obtain a cold-rolled steel sheet with a thickness of 1.6 mm.

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

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

[0191] Subsequently, the Fe-based electroplated steel sheet was heated with the average heating rate in the temperature range of 400 °C or higher and 650 °C or lower being the values shown in Tables 9 to 11. Subsequently, annealing was carried out by heating the steel sheet at a soaking zone temperature of 800 °C in a reducing atmosphere having the atmospheric dew point B shown in Tables 9 to 11, containing 15% by volume of hydrogen, and the balance being N2 and inevitable impurities. The holding time at the maximum temperature (800 °C) of the steel sheet was 100 seconds. In the examples shown in Table 11, an Fe-based electroplated steel sheet (CR) was obtained in this way.

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

[0193] In the examples shown in Table 9, the hot-dip galvanized layer was further heated and alloyed by an alloying treatment at 510°C to produce alloyed hot-dip galvanized steel sheet (GA). The Fe% in the alloyed hot-dip galvanized layer was controlled by varying the alloying treatment time.

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

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

[0196] In the GA and GI examples, the appearance of the zinc plating layer (Evaluation 1) and the resistance to welding cracks in the welded area (Evaluation 3) were evaluated, and the results are shown in Tables 9 and 10. In the CR example, the chemical conversion treatment properties and corrosion resistance after painting (Evaluation 2) and the resistance to welding cracks in the welded area (Evaluation 3) were evaluated, and the results are shown in Table 11. The evaluation methods and criteria for Evaluations 1 to 3 are the same as in Example 1.

[0197] [Table 9]

[0198] [Table 10]

[0199] [Table 11]

[0200] As is clear from Tables 9 and 10, the inventive examples in the GA and GI models exhibited good appearance of the zinc plating layer and excellent resistance to welding cracking. Furthermore, as is clear from Table 11, the inventive examples in the CR model exhibited good chemical treatment properties and corrosion resistance after painting, as well as excellent resistance to welding cracking.

[0201] (Example 4) A slab of steel having the component composition shown in Table 5 (the remainder being Fe and unavoidable impurities) was melted and hot-rolled to obtain a hot-rolled steel sheet. This hot-rolled steel sheet was then pickled and cold-rolled to obtain a cold-rolled steel sheet with a thickness of 1.6 mm.

[0202] Using these cold-rolled steel sheets, various types of GA shown in Table 12, various types of GI shown in Table 13, and various types of CR shown in Table 14 were produced.

[0203] First, cold-rolled steel sheets were degreased with alkali, and then electrolytic treatment was performed using the cold-rolled steel sheets as the cathode under the conditions shown below to produce Fe-based electroplated steel sheets. The amount A of the Fe-based electroplating layer shown in Tables 12-14 was calculated using the method described above and controlled by the energizing time. [Electrolysis conditions] Bath temperature: 50℃ pH: 2.0 Current density: 45A / dm 2 Plating bath: Fe 2+ Sulfuric acid bath containing 1.5 mol / L of ions Anode: Iridium oxide electrode

[0204] Subsequently, the Fe-based electroplated steel sheet was heated in the temperature range of 400°C to 650°C, with the average heating rate set to the values ​​shown in Tables 12-14. Then, the steel sheet was annealed in a reducing atmosphere with a dew point B as shown in Tables 12-14, containing 15% by volume of hydrogen, with the remainder being N2 and unavoidable impurities, at a uniform temperature of 800°C. The holding time at the maximum temperature reached by the steel sheet (800°C) was 100 seconds. In the examples shown in Table 14, Fe-based electroplated steel sheets (CR) were obtained in this manner.

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

[0206] In the examples shown in Table 12, the hot-dip galvanized layer was further heated and alloyed by an alloying treatment at 510°C to produce alloyed hot-dip galvanized steel sheets (GA). The Fe% in the alloyed hot-dip galvanized layer was controlled by varying the alloying treatment time.

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

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

[0209] In the GA and GI examples, the appearance of the zinc plating layer (Evaluation 1) and the resistance to welding cracks in the welded area (Evaluation 3) were evaluated, and the results are shown in Tables 12 and 13. In the CR example, the chemical conversion treatment properties and corrosion resistance after painting (Evaluation 2) and the resistance to welding cracks in the welded area (Evaluation 3) were evaluated, and the results are shown in Table 14. The evaluation methods and criteria for Evaluations 1 to 3 are the same as in Example 1.

[0210] [Table 12]

[0211] [Table 13]

[0212] [Table 14]

[0213] As is clear from Tables 12 and 13, the inventive examples in the GA and GI models exhibited good appearance of the zinc plating layer and excellent resistance to welding cracking. Furthermore, as is clear from Table 14, the inventive examples in the CR model exhibited good chemical conversion treatment properties and corrosion resistance after painting, as well as excellent resistance to welding cracking.

[0214] (Example 5) A slab of steel having the component composition shown in Table 15 (the remainder being Fe and unavoidable impurities) was melted and hot-rolled to obtain a hot-rolled steel sheet. This hot-rolled steel sheet was then pickled and cold-rolled to obtain a cold-rolled steel sheet with a thickness of 1.6 mm.

[0215] [Table 15]

[0216] Using these cold-rolled steel sheets, various types of GA shown in Table 16, various types of GI shown in Table 17, and various types of CR shown in Table 18 were produced.

[0217] First, cold-rolled steel sheets were degreased with alkali, and then electrolytic treatment was performed using the cold-rolled steel sheets as the cathode under the conditions shown below to produce Fe-based electroplated steel sheets. The amount A of the Fe-based electroplating layer shown in Tables 16-18 was calculated using the method described above and controlled by the energizing time. [Electrolysis conditions] Bath temperature: 50℃ pH: 2.0 Current density: 45A / dm 2 Plating bath: Fe 2+ Sulfuric acid bath containing 1.5 mol / L of ions Anode: Iridium oxide electrode

[0218] Subsequently, the Fe-based electroplated steel sheet was heated in the temperature range of 400°C to 650°C, with the average heating rate set to the values ​​shown in Tables 16-18. Then, the steel sheet was annealed in a reducing atmosphere having the atmospheric dew point B shown in Tables 16-18, containing 15% by volume of hydrogen, with the remainder being N2 and unavoidable impurities, at a uniform temperature of 800°C. The holding time at the maximum temperature reached by the steel sheet (800°C) was 100 seconds. In the examples shown in Table 18, Fe-based electroplated steel sheets (CR) were obtained in this manner.

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

[0220] In the examples shown in Table 16, the hot-dip galvanized layer was further heated and alloyed by an alloying treatment at 510°C to produce alloyed hot-dip galvanized steel sheet (GA). The Fe% in the alloyed hot-dip galvanized layer was controlled by varying the alloying treatment time.

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

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

[0223] In the GA and GI examples, the appearance of the zinc plating layer (Evaluation 1) and the resistance to welding cracks in the welded area (Evaluation 3) were evaluated, and the results are shown in Tables 16 and 17. In the CR example, the chemical conversion treatment properties and corrosion resistance after painting (Evaluation 2) and the resistance to welding cracks in the welded area (Evaluation 3) were evaluated, and the results are shown in Table 18. The evaluation methods and criteria for Evaluations 1 and 2 are the same as in Example 1. The evaluation methods and criteria for Evaluation 3 are as follows.

[0224] [Evaluation 3: Evaluation of resistance to welding cracks in the welded joint] Referring to Figure 4(a), test pieces 6 were cut from the steel sheets (CR, GI, GA) of each inventive example and comparative example, with the longitudinal direction (TD) being the longitudinal direction and the rolling direction being the short direction, measuring 150 mm in the longitudinal direction and 50 mm in the short direction. The amount of zinc plating layer on one side of a piece cut to the same size was 50 g / m². 2A test alloyed hot-dip galvanized steel sheet 5 (thickness: 1.6 mm, TS: 980 MPa class) was stacked with the test specimen 6 to form a plate assembly. The plate assembly was assembled so that the evaluation surface of the test specimen 6 (Fe plating layer in the case of CR, 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 assembly was fixed to a fixing base 8 via a spacer 7 with a thickness of 2.0 mm. The spacer 7 is a pair of steel plates measuring 50 mm in the longitudinal direction × 45 mm in the transverse direction × 2.0 mm in thickness, and as shown in Figure 4(a), the longitudinal end faces of each pair of steel plates were aligned with the transverse end faces of the plate assembly. Therefore, the distance between the pair of steel plates is 60 mm. The fixing base 8 is a single plate with a hole in the center. Next, using a servo-motor-operated single-phase AC (50Hz) resistance welding machine, resistance welding was performed on the plate assembly with a pair of electrodes 9 (tip diameter: 6mm) while the assembly was flexed, under the conditions of pressurized force: 3.5kN, hold time: 0.14 seconds or 0.16 seconds, and welding time: 0.36 seconds, using a welding current that resulted in a nugget diameter r of 5.9mm, to create a plate assembly with a welded joint. At this time, the pair of electrodes 9 pressed the plate assembly from above and below in the vertical direction, and the lower electrode pressed the test piece 6 through a hole in the fixed base 8. When pressing, the lower electrode and the fixed base 8 were fixed so that the lower electrode of the pair of electrodes 9 was in contact with a plane extending from the surface where the spacer 7 and the fixed base 8 are in contact, while the upper electrode was made movable. The upper electrode was also positioned to be in contact with the center of the test alloyed hot-dip galvanized steel sheet 5. Furthermore, the plate assembly was tilted 5° to the longitudinal side of the plate assembly with respect to the horizontal direction when welding was performed. Hold time refers to the time from when the welding current has finished flowing until the electrodes are released. Referring to the lower part of Figure 4(b), the nugget diameter r means the distance between the ends of the nuggets 10 in the longitudinal direction of the plate assembly. Next, the plate assembly with the weld was cut along the BB line in the upper diagram of Figure 4(b), including the center of the weld, which includes the nugget 10. The cross-section of the weld was then observed with an optical microscope (200x magnification), and the resistance cracking characteristics of the weld were evaluated according to the following criteria. A ◎ or ○ indicates that the resistance cracking characteristics of the weld are excellent. A × indicates that the resistance cracking characteristics of the weld are poor. ◎: No cracks longer than 0.1 mm were observed during a hold time of 0.14 seconds. ○: A crack longer than 0.1 mm is observed at a hold time of 0.14 seconds, but no crack longer than 0.1 mm is observed at a hold time of 0.16 seconds. ×: A crack with a length of 0.1 mm or more was observed with a hold time of 0.16 seconds. In Figure 4(b), the lower diagram schematically shows the crack that occurred in test piece 6 as reference numeral 11. Note that if a crack occurs in the mating steel plate (test alloyed hot-dip galvanized steel plate), the stress in the steel plate under evaluation (steel plates of each invention example and comparative example) will be dispersed, resulting in an inappropriate evaluation. Therefore, data in which no cracks occurred in the mating steel plate were used as examples.

[0225] [Table 16]

[0226] [Table 17]

[0227] [Table 18]

[0228] As is clear from Tables 16 and 17, the inventive examples in the GA and GI models exhibited good appearance of the zinc plating layer and excellent resistance to welding cracking. Furthermore, as is clear from Table 18, the inventive examples in the CR model exhibited good chemical treatment properties and corrosion resistance after painting, as well as excellent resistance to welding cracking.

[0229] (Example 6) A slab of steel having the component composition shown in Table 15 (the remainder being Fe and unavoidable impurities) was melted and hot-rolled to obtain a hot-rolled steel sheet. This hot-rolled steel sheet was then pickled and cold-rolled to obtain a cold-rolled steel sheet with a thickness of 1.6 mm.

[0230] Using these cold-rolled steel sheets, various types of GA shown in Table 19, various types of GI shown in Table 20, and various types of CR shown in Table 21 were produced.

[0231] First, cold-rolled steel sheets were degreased with alkali, and then electrolytic treatment was performed using the cold-rolled steel sheets as the cathode under the conditions shown below to produce Fe-based electroplated steel sheets. The amount A of the Fe-based electroplating layer shown in Tables 19-21 was calculated using the method described above and controlled by the energizing time. [Electrolysis conditions] Bath temperature: 50℃ pH: 2.0 Current density: 45A / dm 2 Plating bath: Fe 2+ Sulfuric acid bath containing 1.5 mol / L of ions Anode: Iridium oxide electrode

[0232] Subsequently, the Fe-based electroplated steel sheet was heated in the temperature range of 400°C to 650°C at the average heating rate shown in Tables 19-21. Then, the steel sheet was annealed at a uniform temperature of 800°C in a reducing atmosphere having the atmospheric dew point B shown in Tables 19-21, containing 15% by volume of hydrogen, with the remainder being N2 and unavoidable impurities. The holding time at the maximum temperature reached by the steel sheet (800°C) was 100 seconds. In the examples shown in Table 21, Fe-based electroplated steel sheets (CR) were obtained in this manner.

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

[0234] In the examples shown in Table 19, the hot-dip galvanized layer was further heated and alloyed by an alloying treatment at 510°C to produce alloyed hot-dip galvanized steel sheet (GA). The Fe% in the alloyed hot-dip galvanized layer was controlled by varying the alloying treatment time.

[0235] Tables 19 and 20 show the amount of hot-dip galvanized layer or alloyed hot-dip galvanized layer and Fe% on each steel sheet, as determined by the method described above.

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

[0237] In the GA and GI examples, the appearance of the zinc plating layer (Evaluation 1) and the resistance to welding cracks in the welded area (Evaluation 3) were evaluated, and the results are shown in Tables 19 and 20. In the CR example, the chemical conversion treatment properties and corrosion resistance after painting (Evaluation 2) and the resistance to welding cracks in the welded area (Evaluation 3) were evaluated, and the results are shown in Table 21. The evaluation methods and criteria for Evaluations 1 to 3 are the same as in Example 5.

[0238] [Table 19]

[0239] [Table 20]

[0240] [Table 21]

[0241] As is clear from Tables 19 and 20, the inventive examples in the GA and GI models exhibited good appearance of the zinc plating layer and excellent resistance to welding cracks. Furthermore, as is clear from Table 21, the inventive examples in the CR model exhibited good chemical treatment properties and corrosion resistance after painting, as well as excellent resistance to welding cracks. [Industrial applicability]

[0242] The Fe-based electroplated steel sheet of the present invention not only exhibits excellent chemical conversion treatment properties or plating appearance when hot-dip galvanizing is applied, but also has excellent resistance to welding cracking. Furthermore, the alloyed hot-dip galvanized steel sheet of the present invention not only exhibits excellent plating appearance, but also has excellent resistance to welding cracking. Therefore, by applying the Fe-based electroplated steel sheet or alloyed hot-dip galvanized steel sheet of the present invention to, for example, automotive structural components, it is possible to improve fuel efficiency by reducing the weight of the vehicle body, and it can also be applied to applications such as home appliances and building materials. [Explanation of Symbols]

[0243] 1 Fe-type electroplated steel sheet 2 Cold rolled steel plate 3 Fe-based electroplating layer 5. Test alloyed hot-dip galvanized steel sheet 6 Test specimens 7 Spacers 8 Fixed base 9 electrodes 10 nuggets 11 cracks

Claims

1. A cold-rolled steel sheet having a composition containing 0.1% to 3.0% by mass of Si and 0.05% to 0.8% by mass of C, The amount of adhesive formed on one or both sides of the cold-rolled steel sheet, with an amount of 1.0 g / m² per side. ２ The above Fe-based electroplating layers, Fe-based electroplated steel sheet having, In the emission intensity profile of the wavelength indicating Si, measured in the depth direction from the surface of the Fe-based electroplating layer by glow discharge emission spectrometry, (i) the average Si intensity (I) in the range of 10.0 ± 0.1 μm from the interface between the Fe-based electroplating layer and the cold-rolled steel sheet Ｓｉ (ii) There exists a peak with a greater luminescence intensity than (ii), (iii) The peak of the peak is located at a depth greater than 0.10 μm from the surface of the Fe-based electroplating layer, and (iii) The luminescence intensity gradually decreases in the depth direction from the peak of the peak only after the average Si intensity (I Ｓｉ The depth at which this becomes equal is located within a range of 0.10 μm to 2.00 μm from the surface of the Fe-based electroplating layer. The average value of the C concentration in the range of 10 μm to 20 μm in the thickness direction from the surface of the Fe-based electroplating layer is 0.04% by mass or less. The surface layer of the Fe-based electroplated steel sheet is a decarburized layer, and the thickness of the decarburized layer is 80 μm or more. Fe-based electroplated steel sheet.

2. The Fe-based electroplated steel sheet according to claim 1, having a non-alloyed hot-dip galvanized layer formed in contact with the Fe-based electroplated layer.

3. The Fe-based electroplated steel sheet according to claim 1 or 2, wherein at the interface between the Fe-based electroplated layer and the cold-rolled steel sheet, the number of crystal 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 width direction of the steel sheet in the observation field of view of the cold-rolled steel sheet.

4. The Fe-based electroplated steel sheet according to claim 1 or 2, wherein at the interface between the Fe-based electroplated layer and the cold-rolled steel sheet, the number of crystal grain boundaries of the Fe-based electroplated layer in contact with the cold-rolled steel sheet is 20 or more per 10 μm in the width direction of the steel sheet in the observation field of view of the cold-rolled steel sheet.

5. The component composition of the aforementioned cold-rolled steel sheet is, in mass%, C: 0.05% or more and 0.8% or less, Si: 0.1% or more and 3.0% or less, Mn: 1.0% or more and 12.0% or less, P: 0.1% or less, S: 0.03% or less, N: 0.010% or less, and Al: 1.0% or less Fe-based electroplated steel sheet according to any one of claims 1 to 4, comprising the above, with the remainder being Fe and unavoidable impurities.

6. The aforementioned component composition is further expressed in mass%, B: 0.005% or less, Ti: 0.2% or less, Cr: 1.0% or less, Cu: 1.0% or less, Ni: 1.0% or less, Mo: 1.0% or less Nb: 0.20% or less, V: 0.5% or less, Sb: 0.020% or less, Ta: 0.1% or less, W: 0.5% or less, Zr: 0.1% or less, Sn: 0.20% or less, Ca: 0.005% or less, Mg: 0.005% or less, REM: 0.005% or less The Fe-based electroplated steel sheet according to claim 5, comprising at least one element selected from the group consisting of the following.

7. The Fe-based electroplated steel sheet according to any one of claims 1 to 6, wherein the Si content in the aforementioned component composition is 0.9% by mass or more and 1.7% by mass or less.

8. The Fe-based electroplated steel sheet according to any one of claims 1 to 7, wherein the Fe-based electroplated layer contains a total of 10% by mass or less 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, and the remainder consists of Fe and unavoidable impurities.

9. A chemically treated steel sheet having a chemical conversion coating formed on the surface of an Fe-based electroplated steel sheet according to any one of claims 1 to 8.

10. An electrodeposited steel sheet having an electrodeposited coating film formed in contact with the chemical conversion coating of the chemical conversion treated steel sheet according to claim 9.

11. An automobile part comprising at least a portion of the electrodeposited steel sheet described in claim 10.

12. A method for manufacturing an Fe-based electroplated steel sheet according to Claim 1, A cold-rolled steel sheet having a composition containing 0.1% to 3.0% by mass of Si and 0.05% to 0.8% by mass of C is subjected to Fe-based electroplating without annealing, thereby coating one or both sides of the cold-rolled steel sheet with an amount A (g / m²) per side. ２ ) is 1.0 g / m ２ The process of obtaining an Fe-based electroplated steel sheet having the above Fe-based electroplating layer formed on it, Subsequently, the Fe-based electroplated steel sheet is subjected to an annealing process in which it is held at 650°C to 900°C in an atmosphere where the dew point B (°C) satisfies the following formula (1)', A method for manufacturing Fe-based electroplated steel sheets having [a specific characteristic]. A + B ≧8.0...(1)'

13. The method for manufacturing an Fe-based electroplated steel sheet according to claim 12, further comprising the step of applying hot-dip galvanizing to the Fe-based electroplated steel sheet after the annealing step to form a non-alloyed hot-dip galvanized layer on the surface of the Fe-based electroplated layer.

14. A method for manufacturing an Fe-based electroplated steel sheet according to claim 12 or 13, comprising a step of heating the Fe-based electroplated steel sheet at an average heating rate of 10°C / second or more in a temperature range of 400°C to 650°C before the annealing step.

15. A method for manufacturing an Fe-based electroplated steel sheet according to any one of claims 12 or 13, comprising a step of heating the Fe-based electroplated steel sheet at an average heating rate of 15°C / second or more in a temperature range of 400°C to 650°C before the annealing step.

16. The aforementioned adhesion amount A is 5.0 g / m ２ A method for manufacturing an Fe-based electroplated steel sheet according to any one of claims 12 to 15, wherein the amount is less than [amount missing].

17. The component composition of the aforementioned cold-rolled steel sheet is, in mass%, C: 0.05% or more and 0.8% or less, Si: 0.1% or more and 3.0% or less, Mn: 1.0% or more and 12.0% or less, P: 0.1% or less, S: 0.03% or less, N: 0.010% or less, and Al: 1.0% or less A method for manufacturing an Fe-based electroplated steel sheet according to any one of claims 12 to 16, comprising the above, with the remainder being Fe and unavoidable impurities.

18. The aforementioned component composition is further expressed in mass%, B: 0.005% or less, Ti: 0.2% or less, Cr: 1.0% or less, Cu: 1.0% or less, Ni: 1.0% or less, Mo: 1.0% or less Nb: 0.20% or less, V: 0.5% or less, Sb: 0.020% or less, Ta: 0.1% or less, W: 0.5% or less, Zr: 0.1% or less, Sn: 0.20% or less, Ca: 0.005% or less, Mg: 0.005% or less, REM: 0.005% or less A method for manufacturing an Fe-based electroplated steel sheet according to claim 17, comprising at least one element selected from the group consisting of the following.

19. A method for manufacturing an Fe-based electroplated steel sheet according to any one of claims 12 to 18, wherein the Si content in the aforementioned component composition is 0.9% by mass or more and 1.7% by mass or less.

20. A method for producing an Fe-based electroplated steel sheet according to any one of claims 12 to 19, wherein in the Fe-based electroplating, 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 included in the plating bath such that the total content of these elements in the Fe-based electroplating layer is 10% by mass or less.

21. A method for manufacturing an Fe-based electroplated steel sheet according to any one of claims 12 to 20, Subsequently, the Fe-based electroplated steel sheet is subjected to a chemical conversion treatment to obtain a chemically converted steel sheet in which a chemical conversion treatment film is formed in contact with the Fe-based electroplated steel sheet. A method for manufacturing chemically treated steel sheets having the following characteristics.

22. A method for manufacturing a chemically treated steel sheet according to claim 21, A step of applying electrodeposition coating to the chemically treated steel sheet to obtain an electrodeposited steel sheet in which an electrodeposited coating film is formed in contact with the chemically treated film, A method for manufacturing electrodeposited coated steel sheets.

23. A method for manufacturing an electrodeposited steel sheet according to claim 22, A process for manufacturing automotive parts using the aforementioned electrodeposited steel sheet as part of the process, A method for manufacturing automotive parts.

Images