Alloyed hot-dipped galvanized steel sheet and method for manufacturing same

Abstract

Provided is a high-strength alloyed hot-dipped galvanized steel sheet which has good surface appearance in which the formation of rugged parts on a plating surface is suppressed when steel containing B is used as a steel sheet (base steel sheet).　The alloyed hot-dipped galvanized steel sheet contains a specific amount of B in addition to C, Si, Mn, P, S, sol. Al, and N, wherein Bmin / Bbase obtained by performing GDS analysis in the thickness direction from the surface of the alloyed hot-dipped galvanization layer is 0.80 or less, and the maximum plating thickness Tmax (µm) and the minimum plating thickness Tmin (µm) satisfy Tmin / Tmax ≥ 0.50.

Description

Alloyed hot-dip galvanized steel sheet and method for manufacturing the same

[0001] The present invention relates to a high-strength alloyed hot-dip galvanized steel sheet having a good surface appearance (plated appearance) and a method for producing the same.

[0002] CO2 reduction due to vehicle weight reduction ２ To achieve both emission reduction and improved collision resistance through increased vehicle body strength, attempts are being made to increase the strength of automotive thin steel sheets and then thin them. For example, there is a growing trend to apply high-strength steel sheets with a tensile strength (TS) of 590 MPa or higher to major structural components forming the framework of automobile cabins, with the aim of increasing vehicle body strength. As a method for increasing the strength of steel, the addition of quenching elements such as C, Mn, B, Cr, and Mo is commonly used. Among these, B has the advantage of providing a high hardenability improvement effect even in small amounts, allowing for low-cost high-strength steel, and it also has the characteristic of hardly causing the drawbacks of generating inclusions and degrading bendability or delayed fracture resistance. For this reason, B is widely used as an additive element in high-strength steel sheets.

[0003] Regarding high-strength hot-dip galvanized steel sheets with B added to the base steel sheet, for example, Patent Document 1 describes a technique in which, when performing continuous annealing and hot-dip galvanizing on a base steel sheet with a predetermined component composition, the dew point of the atmosphere is set to -40°C or lower in the temperature range of the annealing furnace temperature of 750°C or higher during continuous annealing. This reduces the oxygen potential at the interface between the steel sheet and the atmosphere, suppresses surface enrichment of Si, Mn, etc., without forming internal oxidation, and allows for the acquisition of an excellent plated appearance. Furthermore, Patent Document 2 describes a technique in which the ratio of Si enrichment to Mn enrichment on the surface layer of the base steel sheet is set to 0.7 or more and 1.3 or less, and when annealing a cold-rolled steel sheet, the dew point of the atmosphere in that region is set to -40°C or lower after heating the cold-rolled steel sheet to the highest attainable temperature, thereby achieving excellent plating properties.

[0004] Japanese Patent Publication No. 2010-255100, International Publication No. 2020 / 170542

[0005] From our investigations, we found that the hot-dip galvanized steel sheet described in Patent Document 1 has minute surface defects that are different from the "surface defects caused by the formation of Si, Mn-based oxides and the resulting decrease in plating wettability" targeted in Patent Document 1, that is, defects where Si, Mn-based oxides repel the plating and create areas where the plating does not adhere. This indicates a new challenge: suppressing the occurrence of these surface defects. Furthermore, we found that the hot-dip galvanized steel sheet described in Patent Document 2 also experiences similar surface defects caused by plating peeling at a high frequency, particularly in steel containing B. In addition, conventional hot-dip galvanized steel sheets sometimes exhibit black mottled patterns due to irregularities on the plated surface, and the suppression of these black mottled patterns on the plated surface has not been sufficient. In other words, there has been a desire to improve the surface appearance, including the suppression of black mottled patterns on the plated surface.

[0006] The present invention was made to solve the newly identified problems described above. Specifically, its objective is to provide a high-strength alloyed hot-dip galvanized steel sheet in which the steel sheet (base steel sheet) contains B, and which has a good surface appearance (plated appearance) with suppressed formation of irregularities on the plated surface, and a method for manufacturing the same.

[0007] In this invention, high strength refers to a tensile strength TS of 590 MPa or higher obtained by performing a tensile test in accordance with JIS Z2241 (2022). Furthermore, good surface appearance (plating appearance) refers to the maximum plating thickness Tmax (μm) and minimum plating thickness Tmin (μm) in the width direction of the alloyed hot-dip galvanized layer satisfying Tmin / Tmax ≥ 0.50.

[0008] Through diligent research by the inventors, it was discovered that the above-mentioned surface defects are due to the formation of nitrides of B, and that minute surface defects can be improved by suppressing nitride formation. Furthermore, it was found that suppressing nitride formation is effective by lowering the dew point in the temperature range of 300 to 500°C during the annealing process before plating to create a Fe reducing atmosphere, thereby suppressing the formation of ammonia by suppressing the formation of iron oxide, and by rapidly heating to the temperature range of 500°C or higher, where ammonia formation becomes significant and radical nitrogen penetration into the steel sheet begins, thereby suppressing ammonia formation and nitriding of the steel sheet. In addition, it was found that shortening the annealing time in the temperature range of 750°C or higher and setting the dew point to -55°C or higher to fix a portion of B as an oxide suppresses the formation of nitrides due to the diffusion of B to the surface of the steel sheet during annealing. However, although the above method improved minute surface defects that prevented plating from adhering by suppressing the formation of nitrides of B, the problem of black mottled patterns appearing on the plated surface remained, and it was not yet sufficient in terms of improving the surface appearance. This black mottled pattern is preferable to suppress because, although the plating is present, it degrades the appearance of the hot-dip galvanized steel sheet. Furthermore, it was found that this black mottled pattern is caused by irregularities on the plated surface, and that only the raised areas come into contact with the rolls during temper rolling, resulting in a change in color tone between the raised and recessed areas, with the recessed areas appearing black.

[0009] Furthermore, the inventors conducted extensive research to solve the above problems and obtained the following findings: (1) As described above, it was found that the improved annealing process conditions did not completely suppress the formation of nitrides of B. Furthermore, it was found that the unevenness of the plating was caused by alloying irregularities resulting from residual nitrides of B inhibiting the alloying reaction between the base steel sheet and molten zinc. (2) Nitrides of B are formed when radical nitrogen (N) generated from ammonia in the atmosphere is adsorbed or penetrates the steel sheet during the heating process, and then reacts with B in the steel during the heating or soaking process in an atmosphere with a low oxygen potential. In previous studies, the formation of nitrides of B was suppressed by suppressing the generation of ammonia, but this was insufficient to suppress the unevenness of the plating. Therefore, in order to further suppress the formation of nitrides of B, the inventors attempted to suppress the surface diffusion of B in the steel in addition to controlling ammonia.

[0010] (3) As a method to suppress surface diffusion of B, we investigated forming a B-deficient layer on the surface of the steel sheet (base steel sheet surface). Specifically, we investigated forming a B-deficient reduced iron layer by first oxidizing the surface of the base steel sheet, which had not been done in previous studies, and then performing reduction annealing. As a result, we found that the reduced iron layer suppresses the diffusion of B and can further suppress the formation of B nitrides. In this way, by suppressing the formation of B nitrides and suppressing alloying unevenness between the steel sheet and molten zinc, it was found that a beautiful plated appearance with small irregularities and no black mottled patterns can be obtained.

[0011] The present invention is based on the above findings and is summarized as follows: [1] An alloyed hot-dip galvanized steel sheet having a steel sheet and an alloyed hot-dip galvanized layer on the steel sheet, wherein the component composition of the steel sheet is, in mass%, C: 0.050% or more and 0.300%, Si: 1.20% or less, Mn: 2.00% or more and 3.50%, P: 0.100% or less, S: 0.0100% or less, sol. It contains Al: 1.00% or less, N: 0.0200% or less, B: 0.0001% to 0.0050%, and the ratio of Mn content (mass%) [%Mn] and Si content (mass%) [%Si] of the steel sheet is 2.50 or more, and further, as optional components, it contains Cr: 1.00% or less, Ti: 0.200% or less, Nb: 0.200% or less, V: 0.200% or less, Mo: 2.000% or less, Cu: 1.000% or less, Ni: 0.500% or less, Sn: 0.200% or less, Sb: 0.200% or less, Mg: 0.0100% or less, Ca: 0.0100% or less, Zn: 0.100% or less. It contains one or more elements selected from Co: 0.200% or less, Zr: 0.200% or less, Ta: 0.10% or less, Te: 0.10% or less, As: 0.10% or less, Hf: 0.10% or less, Bi: 0.20% or less, Pb: 0.20% or less, Ge: 0.10% or less, Sr: 0.10% or less, Cs: 0.10% or less, and REM: 0.0100% or less, with the remainder being Fe and unavoidable impurities. When GDS analysis is performed in the thickness direction from the surface of the alloyed hot-dip galvanized layer, Bbase is defined as the average value of the B intensity in the range where the detection intensity of Fe ≥ the maximum intensity of Fe × 0.95, and Bmin is defined as the minimum value of the B intensity in the range where the detection intensity of Fe ≥ the maximum intensity of Fe × 0.80, [1] An alloyed hot-dip galvanized steel sheet having a Bmin / Bbase of 0.80 or less, and the maximum plating thickness Tmax (μm) and minimum plating thickness Tmin (μm) in the width direction of the alloyed hot-dip galvanized layer satisfying Tmin / Tmax ≥ 0.50. [2] The alloyed hot-dip galvanized steel sheet according to [1], wherein the component composition of the steel sheet has a [%Mn] / [%Si] ratio of 12.00 or more.[3] A method for manufacturing an alloyed hot-dip galvanized steel sheet, comprising: continuously annealing a cold-rolled steel sheet having the component composition described in [1] or [2] above; then hot-dip galvanizing the cold-rolled steel sheet; and then alloying the cold-rolled steel sheet, wherein the continuous annealing includes an oxidation treatment step and a reduction annealing step, and in the oxidation treatment step, N. ２ , O 1000 ppm or more by volume ２ In an atmosphere consisting of H and unavoidable impurities, the steel plate is heated to 500°C or more and 700°C or less, and in the subsequent reduction annealing step after the oxidation treatment step, the temperature range of 300°C or more and 500°C or less is H ２ Contains 3% or more by volume, dew point: -20°C or lower, O ２ The steel plate is heated in an atmosphere of 500 ppm by volume or less, and after heating, the temperature range is 500°C to 750°C. ２ The steel plate is heated in an atmosphere containing 5% or more by volume and with a dew point of -40°C or lower, at an average heating rate of 1°C / s or higher, and after heating, at a temperature of 750°C to 950°C, H ２ A method for manufacturing alloyed hot-dip galvanized steel sheets, comprising: soaking the steel sheet in an atmosphere containing 5% or more by volume and with a dew point of -55°C to -40°C for a holding time of 20 seconds to 300 seconds. [4] The method for manufacturing alloyed hot-dip galvanized steel sheets according to [3], wherein the ammonia concentration in the soaking atmosphere is 0.010% by volume or less.

[0012] According to the present invention, it is possible to manufacture a high-strength alloyed hot-dip galvanized steel sheet in which the steel sheet (base steel sheet) contains B, and which has a good surface appearance (plated appearance) with suppressed formation of irregularities on the plated surface. The high-strength alloyed hot-dip galvanized steel sheet manufactured according to the present invention is suitable for structural members such as automobile parts, and by applying it to such applications, it is possible to improve fuel efficiency by reducing the weight of the vehicle body.

[0013] Figure 1(A) is a graph illustrating the detection intensity of Fe obtained by GDS analysis, specifically for the cases where Fe detection intensity ≥ maximum Fe intensity × 0.95 and Fe detection intensity ≥ maximum Fe intensity × 0.80. Figure 1(B) is a graph illustrating Bmin and Bbase.

[0014] [Alloyed Hot-Dip Galvanized Steel Sheet] The alloyed hot-dip galvanized steel sheet of the present invention is an alloyed hot-dip galvanized steel sheet having a steel sheet and an alloyed hot-dip galvanized layer on the steel sheet, wherein the component composition of the steel sheet is, in mass%, C: 0.050% or more and 0.300%, Si: 1.20% or less, Mn: 2.00% or more and 3.50%, P: 0.100% or less, S: 0.0100% or less, sol. Al: 1.00% or less, N: 0.0200% or less, B: 0.0001% or more and 0.0050% or less, and the ratio of [%Mn] / [%Si] for the Mn content (mass%) of the steel sheet ([%Mn]) and Si content (mass%) ([%Si]) is 2.50 or more, and further, as an optional component, Cr: 1.00% or less, Ti: 0.200% or less, Nb: 0.20% 0% or less, V: 0.200% or less, Mo: 2.000% or less, Cu: 1.000% or less, Ni: 0.500% or less, Sn: 0.200% or less, Sb: 0.200% or less, Mg: 0.0 100% or less, Ca: 0.0100% or less, Zn: 0.100% or less, Co: 0.200% or less, Zr: 0.200% or less, Ta: 0.10% or less, Te: 0.10% or less, As: 0 The alloy contains one or more elements selected from 10% or less, Hf: 0.10% or less, Bi: 0.20% or less, Pb: 0.20% or less, Ge: 0.10% or less, Sr: 0.10% or less, Cs: 0.10% or less, and REM: 0.0100% or less, with the remainder being Fe and unavoidable impurities. When GDS analysis is performed in the thickness direction from the surface of the alloyed hot-dip galvanized layer, Bbase is defined as the average value of the B intensity in the range where the detection intensity of Fe ≥ the maximum intensity of Fe × 0.95, and Bmin is defined as the minimum value of the B intensity in the range where the detection intensity of Fe ≥ the maximum intensity of Fe × 0.80, then Bmin / Bbase is 0.80 or less, and the maximum plating thickness Tmax (μm) and minimum plating thickness Tmin (μm) in the width direction of the alloyed hot-dip galvanized layer satisfy Tmin / Tmax ≥ 0.50.

[0015] The present invention aims to manufacture a high-strength alloyed hot-dip galvanized steel sheet having a hot-dip galvanized layer on one or both sides of a steel sheet (base steel sheet), which is alloyed after hot-dip galvanizing. Here, the composition of the hot-dip galvanized layer to be alloyed (hereinafter referred to as the alloyed hot-dip galvanized layer) is not particularly limited and can be any general composition. For example, the alloyed hot-dip galvanized layer may contain Fe: 20% by mass or less, Al: 0.001% by mass or more and 1.0% by mass or less, and further contain one or more selected from Pb, Sb, Si, Sn, Mg, Mn, Ni, Cr, Co, Ca, Cu, Li, Ti, Be, Bi, and REM in a total of 0% by mass or more and 3.5% by mass or less, with the remainder being Zn and unavoidable impurities. The Fe content in the plating layer of the alloyed hot-dip galvanized steel sheet (GA) is preferably 7% by mass or more, more preferably 8% by mass or more. The Fe content in the plating layer of the alloyed hot-dip galvanized steel sheet (GA) is preferably 15% by mass or less, more preferably 13% by mass or less.

[0016] <Component Composition> The component composition of the steel sheet (base steel sheet) and the reasons for its limitation are explained below. In the following explanation, "%" representing the content of the component elements of the steel sheet means "mass %" unless otherwise specified. Also, tensile strength is referred to as TS.

[0017] • C: 0.050% to 0.300% Carbon (C) is an effective element for generating the desired amount of quenched or tempered martensite, achieving a total strength (TS) of 590 MPa or higher, and obtaining excellent dimensional accuracy during forming. If the C content is less than 0.050%, the area ratio of quenched martensite decreases, and the area ratio of ferrite and bainite increases, making it difficult to achieve a TS of 590 MPa or higher. On the other hand, if the C content exceeds 0.300%, the carbon concentration in the quenched or tempered martensite increases, and the hardness of the quenched or tempered martensite increases. As a result, the hardness difference between the soft phases of ferrite and bainite and the hard phases of quenched or tempered martensite becomes large, reducing punchability, flexibility, and bendability. Therefore, the C content should be between 0.050% and 0.300%. Furthermore, the carbon content is preferably 0.060% or more in order to achieve a TS of 780 MPa or more, and more preferably 0.090% or more in order to achieve a TS of 980 MPa or more. Also, since a high carbon content degrades weldability, the carbon content is preferably 0.250% or less, and more preferably 0.220% or less.

[0018] • Si: 1.20% or less Si is an effective element for strengthening steel and obtaining good material properties, and is also an effective element for improving ductility. On the other hand, if the Si content exceeds 1.20%, the amount of Si concentration on the steel sheet surface increases during annealing, and Si oxides that cause non-plating defects are formed on the steel sheet surface, making it difficult to obtain a good surface appearance. By keeping the Si content at 1.20% or less, the formation of Si,Mn-based composite oxides is suppressed, and Mn in the steel sheet can be effectively utilized as Mn,B composite oxides, making it possible to suppress gray spot defects and black spot defects. Therefore, the Si content should be 1.20% or less. Also, from the above viewpoint, the Si content is preferably 0.80% or less, and more preferably 0.55% or less. There is no particular lower limit for the Si content. That is, the Si content may be 0%, but since the refining cost increases when it is reduced to less than 0.01%, it is preferable that the Si content be 0.01% or more. Furthermore, from the viewpoint of achieving both high strength and improved ductility, the Si content is preferably 0.05% or more, and more preferably 0.10% or more. Moreover, from the viewpoint of obtaining particularly high ductility, the Si content is even more preferably 0.15% or more.

[0019] - Mn: 2.00% or more and 3.50% or less. Mn is an element necessary to suppress gray dot defects and black dot defects and obtain a good surface appearance. By containing 2.00% or more of Mn, it becomes possible to form an Mn, B-based composite oxide with little adverse effect on the plating appearance quality and suppress the formation of B nitride, so gray dot defects and black dot defects are suppressed. Also, Mn is an element effective in generating a desired amount of quenched martensite or tempered martensite to make TS 590 MPa or more. If the content of Mn is less than 2.00%, B that forms a composite oxide with Mn during annealing decreases, and the formation amount of BN increases, so black dot defects and gray dot defects cannot be sufficiently suppressed. On the other hand, when the content of Mn exceeds 3.50%, the area ratio of tempered martensite increases, and the area ratios of ferrite and bainite decrease, resulting in a decrease in dimensional accuracy during forming. Furthermore, the amount of Mn enrichment on the steel plate surface during annealing increases, and a large amount of Mn oxide that causes non-plating defects is formed on the steel plate surface, making it difficult to obtain a good surface appearance. Therefore, the content of Mn is 2.00% or more and 3.50% or less. Also, from the viewpoint of suppressing the occurrence of gray dot defects and black dot defects, the content of Mn is preferably 2.30% or more, more preferably 2.50% or more, and even more preferably 2.60% or more. Also, from the above viewpoints, the content of Mn is preferably 3.30% or less, more preferably 3.00% or less.

[0020] - P: 0.100% or less. P has the effect of solid solution strengthening and is an element for increasing the strength of the steel plate. However, when the content of P exceeds 0.100%, P segregates at the prior austenite grain boundary and embrittles the grain boundary, so the punching property and elongation flange property decrease. Therefore, the content of P is 0.100% or less. Also, from the above viewpoints, the content of P is preferably 0.050% or less, more preferably 0.030% or less. The lower limit of the content of P is not set. That is, the content of P may be 0%, but the refining cost increases to control it to less than 0.001%, so the content of P is preferably 0.001% or more. The content of P is more preferably 0.003% or more, and even more preferably 0.005% or more.

[0021] - S: 0.0100% or less. S exists as sulfide in steel. When the content of S exceeds 0.0100%, the ultimate deformation capacity of the steel sheet decreases, resulting in reduced punching performance, stretch flangeability and bendability. Therefore, the content of S should be 0.0100% or less. The lower limit of the content of S is not particularly specified. That is, the content of S may be 0%, but the refining cost increases to control it below 0.0001%, so the content of S is preferably 0.0001% or more. Also, from the above-mentioned perspective, the content of S is preferably 0.0050% or less. The content of S is more preferably 0.0040% or less, and even more preferably 0.0030% or less.

[0022] - sol.Al: 1.00% or less. Al can be used as a deoxidizer. In this case, the content of sol.Al in the steel is preferably 0.01% or more. Also, in the steel sheet added with B, by adding Al, N in the steel can be fixed as AlN, and the added B can be utilized as solid-solution B effective for strength increase. Also, Al has the effect of suppressing carbide formation during annealing and increasing the volume fraction of retained austenite. The generated retained austenite has the effect of improving ductility. To obtain the effect of fixing N as AlN, sol.Al is preferably contained at 0.02% or more. Also, from the perspective of obtaining the effect of improving ductility, sol.Al is more preferably contained at 0.05% or more. On the other hand, when the content of sol.Al exceeds 1.00%, non-plating occurs, so the content of sol.Al should be 1.00% or less. Also, from the above-mentioned perspective, the content of sol.Al is preferably 0.10% or less, and more preferably 0.08% or less.

[0023] ・N: 0.0200% or less. N exists as nitride in steel, and if the content exceeds 0.0200%, it reduces the ultimate deformability of the steel sheet, thus reducing punchability, elongation flangeability, and bendability. For this reason, the N content should be 0.0200% or less. Also, from the above viewpoint, the N content should preferably be 0.0080% or less. There is no lower limit for the N content. That is, the N content may be 0%, but since refining costs increase when controlling it to less than 0.0005%, it is preferable to have an N content of 0.0005% or more from the standpoint of production technology constraints. The N content should more preferably be 0.0007% or more, and even more preferably 0.0010% or more.

[0024] ・B: 0.0001% or more and 0.0050% or less. B is an element that can improve hardenability by segregating at austenite grain boundaries. By adding B to steel, it is possible to suppress the formation of ferrite and grain growth during annealing cooling. In order to obtain these effects, it is necessary for the B content to be 0.0001% or more. On the other hand, if the B content exceeds 0.0050%, a large amount of nitride will be formed on the surface of the steel sheet, the adhesion of the plating will deteriorate, and appearance defects caused by plating peeling will occur. Therefore, the B content should be 0.0001% or more and 0.0050% or less. Also, from the above viewpoint, the B content is preferably 0.0002% or more. Similarly, from the above viewpoint, the B content is preferably 0.0030% or less.

[0025] • [%Mn] / [%Si]: 2.50 or higher [%Mn] is the Mn content (mass%) of the steel sheet, and [%Si] is the Si content (mass%) of the steel sheet. By setting [%Mn] / [%Si] to 2.50 or higher, the formation of oxides of Si alone is suppressed, thereby suppressing non-plating. Furthermore, in the annealing method of the present invention, the formation of Mn,B composite oxides further suppresses non-plating. Therefore, [%Mn] / [%Si] should be 2.50 or higher. Also, from the viewpoint of suppressing non-plating, [%Mn] / [%Si] is preferably 4.0 or higher, more preferably 6.00 or higher, and even more preferably 12.00 or higher. In particular, by setting [%Mn] / [%Si] to 12.00 or higher, the formation of Mn,B composite oxides is promoted, and the formation of B nitrides, which cause non-plating in the annealing method of the present invention, can be suppressed more effectively. Furthermore, by setting the [%Mn] / [%Si] ratio to 14.00 or higher, the formation of Mn,B composite oxides is particularly significantly promoted, and minute unevenness that could be a precursor to non-plating can be suppressed. For this reason, it is even more preferable to set the [%Mn] / [%Si] ratio to 14.00 or higher. There is no particular upper limit, but the [%Mn] / [%Si] ratio may be 200.00 or lower, or 150.00 or lower.

[0026] Optional Additives The steel sheet used in this invention, in addition to the above component composition, may further contain optional components in mass%, such as: Cr: 1.00% or less, Ti: 0.200% or less, Nb: 0.200% or less, V: 0.200% or less, Mo: 2.000% or less, Cu: 1.000% or less, Ni: 0.500% or less, Sn: 0.200% or less, Sb: 0.200% or less, Mg: 0.0100% or less, Ca: 0.0100% The following elements may be included: Zn: 0.100% or less, Co: 0.200% or less, Zr: 0.200% or less, Ta: 0.10% or less, Te: 0.10% or less, As: 0.10% or less, Hf: 0.10% or less, Bi: 0.20% or less, Pb: 0.20% or less, Ge: 0.10% or less, Sr: 0.10% or less, Cs: 0.10% or less, and REM: 0.0100% or less. In other words, each of the above elements is an optional additive that can be added as needed, and the effects of the present invention can be obtained even if the content is 0%, so the content of each of the above elements may be 0%.

[0027] ・Cr: 1.00% or less Cr is an element that greatly increases hardenability and is an effective element for generating a desired amount of quenched martensite or tempered martensite, thereby achieving a TS of 590 MPa or higher and obtaining excellent dimensional accuracy during forming. However, if the Cr content exceeds 1.00%, the plating appearance quality deteriorates, the area ratio of quenched martensite and tempered martensite increases, the area ratio of ferrite and bainite decreases, and the dimensional accuracy during forming deteriorates. Therefore, when Cr is included, the Cr content should be 1.00% or less. Furthermore, from the viewpoint of improving the plating appearance quality, it is preferable that the Cr content be 0.75% or less. In addition, in order to obtain the effect of improving hardenability by Cr, it is preferable that the Cr content be 0.02% or more. It is more preferable that the Cr content be 0.03% or more, and even more preferable that be 0.04% or more.

[0028] - Ti: 0.200% or less Ti increases TS by forming fine carbides, nitrides, or carbonitrides during hot rolling or annealing. However, if the Ti content exceeds 0.200%, a large amount of coarse precipitates and inclusions are generated, and if the steel sheet contains diffusible hydrogen, these precipitates and inclusions become crack initiation points during bending tests, thus reducing bendability. Therefore, when Ti is included, the Ti content should be 0.200% or less. Furthermore, from the above viewpoint, it is preferable that the Ti content be 0.100% or less. In addition, to obtain the above effects, it is preferable that the Ti content be 0.005% or more, and more preferably 0.010% or more.

[0029] • Nb: 0.200% or less Nb also increases TS by forming fine carbides, nitrides, or carbonitrides during hot rolling or annealing. However, if the Nb content exceeds 0.200%, a large amount of coarse precipitates and inclusions are generated, and if the steel sheet contains diffusible hydrogen, these precipitates and inclusions become crack initiation points during bending tests, thus reducing bendability. Therefore, if Nb is included, the Nb content should be 0.200% or less. Furthermore, from the above viewpoint, it is preferable that the Nb content be 0.100% or less. In addition, to obtain the above effects, it is preferable that the Nb content be 0.005% or more, and more preferably 0.010% or more.

[0030] • V: 0.200% or less V also increases TS by forming fine carbides, nitrides, or carbonitrides during hot rolling or annealing. However, if the V content exceeds 0.200%, a large amount of coarse precipitates and inclusions are generated, and if the steel sheet contains diffusible hydrogen, these precipitates and inclusions become crack initiation points during bending tests, thus reducing bendability. Therefore, when V is included, the V content should be 0.200% or less. Furthermore, from the above viewpoint, it is preferable that the V content be 0.100% or less. In addition, to obtain the above effects, it is preferable that the V content be 0.005% or more, and more preferably 0.010% or more.

[0031] ・Mo: 2.000% or less Mo is an element that greatly increases hardenability and is an effective element for further increasing TS and improving dimensional accuracy during forming by keeping the area ratio of quenched martensite and tempered martensite within a more suitable range. However, if the Mo content exceeds 2.000%, the area ratio of quenched martensite and tempered martensite increases, making it difficult to achieve a TS of 590 MPa or higher, and reducing dimensional accuracy during forming. Furthermore, coarse precipitates and inclusions increase, and if the steel sheet contains diffusible hydrogen, these precipitates and inclusions become crack initiation points during bending tests, thus reducing bendability. Therefore, when Mo is included, the Mo content should be 2.000% or less. Also, from the above viewpoint, it is preferable that the Mo content be 0.500% or less. It is more preferable that the Mo content be 0.400% or less, and even more preferable that it be 0.250% or less. Furthermore, in order to obtain the above effects, the Mo content is preferably 0.005% or more, more preferably 0.020% or more. The Mo content is even more preferably 0.050% or more, and even more preferably 0.080% or more.

[0032] ・Cu: 1.000% or less Cu is an element that greatly increases hardenability and is an effective element for further increasing TS and improving dimensional accuracy during forming by keeping the area ratio of quenched martensite and tempered martensite within a more suitable range. However, if the Cu content exceeds 1.000%, the area ratio of quenched martensite and tempered martensite increases, making it difficult to achieve a TS of 590 MPa or more and to obtain excellent dimensional accuracy during forming. In addition, coarse precipitates and inclusions increase, and if the steel sheet contains diffusible hydrogen, these precipitates and inclusions become crack initiation points during bending tests, thus reducing bendability. Therefore, when Cu is included, the Cu content should be 1.000% or less. Furthermore, from the above viewpoint, it is preferable that the Cu content be 0.200% or less. In addition, to obtain the above effects, it is preferable that the Cu content be 0.005% or more, and more preferably 0.020% or more. The Cu content is more preferably 0.040% or more, and even more preferably 0.060% or more.

[0033] Ni: 0.500% or less Ni is an element that greatly increases hardenability and is effective in raising the total strength (TS) and improving dimensional accuracy during forming by keeping the area ratio of quenched martensite and tempered martensite within a more suitable range. However, if the Ni content exceeds 0.500%, the area ratio of quenched martensite and tempered martensite increases, and the TS and dimensional accuracy during forming decrease. In addition, coarse precipitates and inclusions increase, and if the steel sheet contains diffusible hydrogen, these precipitates and inclusions become crack initiation points during bending tests, thus reducing bendability. Therefore, when Ni is included, the Ni content should be 0.500% or less. Furthermore, from the above viewpoint, it is preferable that the Ni content be 0.200% or less. In addition, to obtain the above effects, it is preferable that the Ni content be 0.005% or more, and more preferably 0.020% or more.

[0034] ・Sn: 0.200% or less Sn is an effective element for suppressing oxidation of the surface of the base steel sheet during annealing and obtaining better plating properties. However, if the Sn content exceeds 0.200%, the amount of coarse precipitates and inclusions increases, and if the base steel sheet contains diffusible hydrogen, these precipitates and inclusions become crack initiation points during bending tests, thus reducing the bendability. Therefore, when Sn is included, the Sn content should be 0.200% or less. Furthermore, from the above viewpoint, it is preferable that the Sn content be 0.050% or less. It is more preferable that the Sn content be 0.040% or less, and even more preferable that it be 0.025% or less. Furthermore, in order to obtain the above effects, it is preferable that the Sn content be 0.001% or more, and more preferable that it be 0.005% or more.

[0035] ・Sb: 0.200% or less Sb is an element known as a nitriding-inhibiting element. By adding Sb, the adsorption or penetration of radical nitrogen into the steel sheet in the temperature range of 500°C to 750°C can be suppressed, and the formation of nitrides of B, which are a cause of gray spot defects and black spot defects, can be suppressed. However, if the Sb content exceeds 0.200%, slab cracking will occur during hot rolling. Therefore, if Sb is included, the Sb content should be 0.200% or less. Also, from the above viewpoint, it is preferable that the Sb content be 0.100% or less. It is more preferable that the Sb content be 0.080% or less, and even more preferable that it be 0.050% or less. Furthermore, in order to obtain the above effects, it is preferable that the Sb content be 0.005% or more, and more preferable that it be 0.007% or more.

[0036] ・Mg: 0.0100% or less Mg is an effective element for improving the ultimate deformability of steel sheets and enhancing flexural flangeability by spheroidizing the shape of inclusions such as sulfides and oxides. However, if the Mg content exceeds 0.0100%, a large amount of coarse precipitates and inclusions are generated, and if the steel sheet contains diffusible hydrogen, these precipitates and inclusions become crack initiation points during bending tests, thus reducing bendability. Therefore, if Mg is included, the Mg content should be 0.0100% or less. Furthermore, from the above viewpoint, it is preferable that the Mg content be 0.0050% or less. It is more preferable that the Mg content be 0.0040% or less, and even more preferable that it be 0.0025% or less. Furthermore, in order to obtain the above effects, it is preferable that the Mg content be 0.0001% or more, and more preferable that it be 0.0005% or more.

[0037] Ca: 0.0100% or less. Ca exists as an inclusion in the base steel sheet. If the Ca content exceeds 0.0100%, and the base steel sheet contains diffusible hydrogen, the inclusion will become the starting point for cracks during bending tests, thus reducing the bendability. Therefore, if Ca is included, the Ca content should be 0.0100% or less. Also, from the above viewpoint, it is preferable that the Ca content be 0.0020% or less. The lower limit of the Ca content may be 0.0000%, but due to production technology constraints, it is preferable that the Ca content be 0.0001% or more. It is more preferable that the Ca content be 0.0002% or more, and even more preferable that it be 0.0003% or more.

[0038] Zn: 0.100% or less. Zn is an effective element for improving tensile flangeability by spheroidizing the shape of inclusions and improving the ultimate deformability of the steel sheet. However, if the Zn content exceeds 0.100%, a large amount of coarse precipitates and inclusions are generated, and if the steel sheet contains diffusible hydrogen, these precipitates and inclusions become crack initiation points during bending tests, thus reducing bendability. Therefore, if Zn is included, the Zn content should be 0.100% or less. Furthermore, from the above viewpoint, it is preferable that the Zn content be 0.020% or less, and more preferably 0.010% or less. In addition, to obtain the above effects, it is preferable that the Zn content be 0.001% or more, and more preferably 0.002% or more. It is even more preferable that the Zn content be 0.003% or more, and even more preferably 0.005% or more.

[0039] ・Co: 0.200% or less Co is also an effective element for improving tensile flangeability by spheroidizing the shape of inclusions and improving the ultimate deformability of the steel sheet. However, if the Co content exceeds 0.200%, a large amount of coarse precipitates and inclusions are generated, and if the steel sheet contains diffusible hydrogen, these precipitates and inclusions become the starting points for cracks during bending tests, thus reducing the bendability. Therefore, when Co is included, the Co content should be 0.200% or less. Furthermore, from the above viewpoint, it is preferable that the Co content be 0.010% or less. In addition, to obtain the above effects, it is preferable that the Co content be 0.001% or more, and more preferable that be 0.005% or more.

[0040] • Zr: 0.200% or less Zr is also an effective element for improving flexural flangeability by spheroidizing the shape of inclusions and improving the ultimate deformability of the steel sheet. However, if the Zr content exceeds 0.200%, a large amount of coarse precipitates and inclusions are generated, and if the steel sheet contains diffusible hydrogen, these precipitates and inclusions become crack initiation points during bending tests, thus reducing the bendability. Therefore, if Zr is included, the Zr content should be 0.200% or less. Furthermore, from the above viewpoint, it is preferable that the Zr content be 0.050% or less, and more preferably 0.010% or less. In addition, to obtain the above effects, it is preferable that the Zr content be 0.001% or more, and more preferably 0.005% or more.

[0041] • Ta: 0.10% or less. Ta is an effective element for increasing the strength of the base steel sheet and can be included as needed. Since a strength improvement effect can be obtained by including 0.005% or more of Ta, it is preferable that the Ta content be 0.005% or more. It is more preferable that the Ta content be 0.02% or more, and even more preferable that be 0.05% or more. On the other hand, from the viewpoint of preventing cost increases, if Ta is included, the Ta content should be 0.10% or less.

[0042] • Te: 0.10% or less. A Te content of 0.001% or more is preferable because it controls the sulfide morphology and improves ductility and toughness. A Te content of 0.01% or more is more preferable, and 0.03% or more is even preferable. On the other hand, from the viewpoint of preventing cost increases, if Te is included, the Te content should be 0.10% or less.

[0043] • As: 0.10% or less. It is preferable that the As content be 0.001% or more, as this allows for control of the sulfide morphology and improves ductility and toughness. A more preferable As content is 0.02% or more, and even more preferable is 0.04% or more. On the other hand, from the viewpoint of preventing cost increases, if As is included, the As content should be 0.10% or less.

[0044] • Hf: 0.10% or less. It is preferable that the Hf content be 0.01% or more, as this allows for control of the sulfide morphology and improves ductility and toughness. A more preferable Hf content is 0.02% or more, and even more preferable is 0.04% or more. On the other hand, from the viewpoint of preventing cost increases, if Hf is included, the Hf content should be 0.10% or less.

[0045] • Bi: 0.20% or less. Including 0.001% or more of Bi suppresses grain boundary segregation and improves ductility and toughness. In addition, Bi has the effect of improving machinability and the smoothness of the cut surface, and improving the delayed fracture resistance of the cut surface. For these reasons, it is preferable that the Bi content be 0.001% or more. It is more preferable that the Bi content be 0.02% or more, and even more preferable that be 0.04% or more. On the other hand, from the viewpoint of preventing cost increases, if Bi is included, the Bi content should be 0.20% or less. It is preferable that the Bi content be 0.10% or less.

[0046] • Pb: 0.20% or less. Including 0.001% or more of Pb suppresses grain boundary segregation and improves ductility and toughness. In addition, Pb has the effect of improving machinability and the smoothness of the cut surface, and improving the delayed fracture resistance of the cut surface. For these reasons, it is preferable that the Pb content be 0.001% or more. It is more preferable that the Pb content be 0.02% or more, and even more preferable that be 0.04% or more. On the other hand, from the viewpoint of preventing cost increases, if Pb is included, the Pb content should be 0.20% or less. It is preferable that the Pb content be 0.10% or less.

[0047] • Ge: 0.10% or less. Even if Ge is present in a concentration of 0.001% or more, it does not significantly affect the mechanical properties or surface quality, so the Ge content may be 0.001% or more. It is more preferable for the Ge content to be 0.01% or more, and even more preferable for it to be 0.02% or more. On the other hand, from the viewpoint of preventing cost increases, if Ge is present, the Ge content should be 0.10% or less.

[0048] • Sr: 0.10% or less. Even if the Sr content is 0.001% or more, it does not significantly affect the mechanical properties or surface quality, so the Sr content may be 0.001% or more. It is more preferable for the Sr content to be 0.01% or more, and even more preferable for it to be 0.02% or more. On the other hand, from the viewpoint of preventing cost increases, if Sr is included, the Sr content should be 0.10% or less.

[0049] • Cs: 0.10% or less. Even if Cs is present in a concentration of 0.001% or more, it does not significantly affect the mechanical properties or surface quality, so the Cs content may be 0.001% or more. It is more preferable for the Cs content to be 0.01% or more, and even more preferable for it to be 0.03% or more. On the other hand, from the viewpoint of preventing cost increases, if Cs is included, the Cs content should be 0.10% or less.

[0050] - REM: 0.0100% or less. REM is an effective element for improving tensile flangeability by spheroidizing the shape of inclusions and improving the ultimate deformability of the steel sheet. However, if the total REM content exceeds 0.0100%, a large amount of coarse precipitates and inclusions are generated, and if the steel sheet contains diffusible hydrogen, these precipitates and inclusions become crack initiation points during bending tests, thus reducing bendability. Therefore, if REM is included, its total content should be 0.0100% or less. Also, from the above viewpoint, it is preferable that the total REM content be 0.0080% or less. Furthermore, in order to obtain the above effects, it is preferable that the total REM content be 0.0001% or more, and more preferably 0.0005% or more. It is even more preferable that the REM content be 0.0010% or more, and even more preferably 0.0015% or more. In this invention, REM refers to scandium (Sc) with atomic number 21, yttrium (Y) with atomic number 39, and lanthanide elements from lanthanum (La) with atomic number 57 to lutetium (Lu) with atomic number 71. In this invention, the REM content refers to the total content of one or more elements selected from the above-mentioned REM. The REM is not particularly limited, but La and / or Ce is preferred.

[0051] The remainder of the mixture, aside from the components mentioned above, consists of Fe and unavoidable impurities.

[0052] [Method for Manufacturing Alloyed Hot-Dip Galvanized Steel Sheets] Next, the manufacturing conditions for the method for manufacturing alloyed hot-dip galvanized steel sheets of the present invention will be described. In the manufacturing method of the present invention, a steel sheet (cold-rolled steel sheet) having the above-mentioned component composition is introduced into, for example, a continuous hot-dip galvanizing facility, continuously annealed in the same facility, then hot-dip galvanized, and further alloyed to obtain an alloyed hot-dip galvanized steel sheet. Generally, a continuous hot-dip galvanizing facility consists of an annealing furnace and a hot-dip galvanizing apparatus located downstream of the annealing furnace. This hot-dip galvanizing apparatus includes a hot-dip galvanizing bath and a snout connected to the steel strip exit side of the annealing furnace, with its tip immersed in the hot-dip galvanizing bath. As such a continuous hot-dip galvanizing facility, a general continuous galvanizing line (CGL) can be applied, which is configured to continuously perform a series of processes including heating, cooling, hot-dip galvanizing, and alloying treatment of the hot-dip galvanized steel. The specific annealing conditions are as follows. The number of annealing steps is not particularly limited, but from the viewpoint of manufacturing cost, it is preferably one step (single-step annealing method).

[0053] The occurrence of a black, mottled surface appearance due to plating irregularities, which is the problem to be solved by the present invention, is a phenomenon specific to cases where B is added to the base steel sheet and the dew point during annealing before plating is low. This problem had not been recognized conventionally and was newly discovered by the present inventors. As mentioned above, in order to produce alloyed hot-dip galvanized steel sheets with a good surface appearance (plating appearance) that can suppress the occurrence of such surface defects, the present inventors have conducted diligent studies and obtained the following findings: (1) It was found that methods of related technologies with various improvements to the annealing process conditions could not completely suppress the formation of B nitrides. Furthermore, it was found that the plating irregularities were caused by alloying unevenness due to residual B nitrides inhibiting the alloying reaction between the base steel sheet and molten zinc. (2) B nitrides are formed when radical nitrogen (N) generated from ammonia in the atmosphere is mainly adsorbed or penetrated into the steel sheet during the heating process, and then reacts with B in the steel during the heating or soaking process in an atmosphere with a low oxygen potential. In previous studies, suppressing the formation of iron oxide and pure iron, which are reduced from iron oxide and act as catalysts for ammonia production, thereby suppressing ammonia generation and thus inhibiting the formation of nitrides of B, was insufficient to suppress the unevenness of the plating. Therefore, in order to further suppress the formation of nitrides of B, the inventors attempted to suppress the surface diffusion of B in the steel in addition to controlling ammonia.

[0054] (3) As a method to suppress surface diffusion of B, we investigated increasing the distance over which B diffuses to the steel sheet surface. Specifically, we investigated forming an iron oxide layer on the surface by oxidation treatment of the base steel sheet surface, which had not been performed in previous studies, and then forming a reduced iron layer deficient in B by performing reduction annealing. As a result, we found that the reduced iron layer suppresses the diffusion of B and can further suppress the formation of nitrides of B.

[0055] (4) Based on the above, the inventors have found that the presence of a B-deficient layer on the surface layer of the steel sheet during reduction annealing suppresses the formation of B nitride, which is an important requirement for obtaining plating with good appearance. Furthermore, it has been found that the presence or absence of this B-deficient layer can be determined by performing GDS analysis in the thickness direction from the surface of the plating layer to the inside of the base steel sheet (plating penetration GDS analysis) on the alloyed hot-dip galvanized steel sheet after hot-dip galvanizing and alloying treatment. Therefore, by forming a B-deficient layer through oxidation treatment and reduction treatment, suppressing the surface diffusion of B and the formation of nitride by controlling this and ammonia, and suppressing the uneven alloying of the steel sheet and molten zinc, it has been found that a beautiful plating appearance with small unevenness and no mottled pattern can be obtained.

[0056] For this reason, in the present invention, continuous annealing is performed under a series of conditions optimized so as to obtain the action of (4) above, and the series of optimized annealing conditions are important requirements in the present invention.

[0057] The method for manufacturing an alloyed hot-dip galvanized steel sheet according to the present invention based on the above findings is a method for manufacturing an alloyed hot-dip galvanized steel sheet in which a cold-rolled steel sheet having the above component composition is continuously annealed, then hot-dip galvanized, and then alloyed. The continuous annealing includes an oxidation treatment step and a reduction annealing step. In the oxidation treatment step, the steel sheet is heated to 500°C or higher and 700°C or lower in an atmosphere consisting of N ２ , O of 1000 volume ppm or more ２ and unavoidable impurities. After the oxidation treatment step, in the subsequent reduction annealing step, a temperature range of 300°C or higher and 500°C or lower is used, and the steel sheet is heated in an atmosphere containing H ２ : 3 volume % or more, dew point: -20°C or lower, O ２ : 500 volume ppm or less. After this heating, a temperature range of 500°C or higher and 750°C or lower is used, and the steel sheet is heated in an atmosphere containing H ２ : 5 volume % or more, dew point: -40°C or lower at an average heating rate of 1°C / s or more. After this heating, at a temperature of 750°C or higher and 950°C or lower, the steel sheet is homogenized in an atmosphere containing H ２ : 5 volume % or more, dew point: -55°C or higher and -40°C or lower for a holding time of 20 seconds or more and 300 seconds or less.

[0058] (Continuous Annealing) <Oxidation Treatment Process> In the oxidation treatment process of the continuous annealing of the present invention, first, for example, in the stage before the heating zone, N ２ , O 1000 ppm or more by volume ２ The steel plate is heated to a temperature between 500°C and 700°C in an atmosphere containing unavoidable impurities and oxidized.

[0059] ・O ２ : 1000 ppm or more by volume; heating temperature: 500°C to 700°C ２ If the concentration is less than 1000 ppm by volume, the surface of the steel plate will not be sufficiently oxidized, and the formation of a B-deficient layer, which is important for suppressing BN formation, will be insufficient. Therefore, O ２ The concentration is 1000 ppm by volume or more. From the viewpoint of suppressing BN formation, O ２ The concentration is preferably 1200 ppm by volume or more. ２ There is no upper limit to the concentration, but it becomes difficult to reduce the surface of the steel plate in the subsequent reducing annealing, O ２ The concentration of is preferably 5000 ppm by volume or less. In the oxidation treatment process, the steel sheet is heated to 500°C or higher and 700°C or lower. If the temperature during oxidation (heating temperature in the oxidation treatment) is lower than 500°C, the surface of the steel sheet will not be sufficiently oxidized, and the formation of the B-deficient layer, which is important for suppressing BN formation, will be insufficient. Therefore, the temperature during oxidation (heating temperature in the oxidation treatment) is 500°C or higher. From the viewpoint of suppressing BN formation, the temperature during oxidation is preferably 550°C or higher. On the other hand, if the temperature during oxidation is higher than 700°C, the oxidation of the steel sheet will be excessive, and the reduction will not be completed in the subsequent reduction annealing, causing oxides to peel off and adhere to the rolls, a phenomenon called pickup. When pickup occurs on the rolls, indentations will be made in the steel sheet, greatly impairing the appearance of the galvanized steel sheet. Therefore, the temperature during oxidation (heating temperature in the oxidation treatment) is 700°C or lower.

[0060] O ２ The remaining portion is N ２ It consists of and unavoidable impurities.

[0061] <Reduction Annealing Process> Next, in the reduction annealing process, first, the temperature range of 300°C to 500°C is H ２ Contains 3% or more by volume, dew point: -20°C or lower, O２ The steel plate is heated in an atmosphere of 500 ppm by volume or less. In this temperature range, the Fe reducing atmosphere suppresses the formation of iron oxide, which acts as a catalyst for ammonia production, and also suppresses the production of ammonia, which is a factor in the formation of B nitrides (particularly the production of ammonia that becomes significant in the temperature range above 500°C).

[0062] • Hydrogen concentration in the atmosphere (H ２ Concentration: 3% by volume or more. Since hydrogen is a reducing gas, it can suppress oxidation of the steel sheet surface during annealing. To obtain a sufficient oxidation suppression effect, the hydrogen concentration in the atmosphere should be 3% by volume or more, preferably 5% by volume or more. There is no particular upper limit to the hydrogen concentration, but from the viewpoint of suppressing cost increases, it is preferable that the hydrogen concentration be 30% by volume or less.

[0063] - Dew point of the atmosphere: Below -20°C In the low temperature range of 300 to 500°C, it is possible to reduce iron by setting the dew point to below -10°C. If the iron is not sufficiently reduced, the remaining iron oxide acts as a catalyst in the temperature range of 500°C or higher, promoting the production of ammonia. In this invention, the base steel sheet is oxidized first, and in order to obtain a sufficient reduction effect on iron, the dew point of the atmosphere is set to below -20°C. Furthermore, in order to obtain an even greater effect in suppressing the production of iron oxide, it is preferably set to below -25°C, and more preferably to below -40°C. From the viewpoint of reducing manufacturing costs, the dew point of the atmosphere here is preferably above -70°C, and more preferably above -60°C.

[0064] • Oxygen concentration of the atmosphere (O ２ Concentration): 500 ppm by volume or less. In order to suppress the oxidation of Fe during annealing, the oxygen concentration is set to 500 ppm by volume or less, preferably 400 ppm by volume or less, and more preferably 200 ppm by volume or less. Hydrogen (H) in the atmospheric gas. ２ ), H ２ O, oxygen (O ２ The remaining portion is N ２ It is preferable that the gas be an unavoidable impurity, and furthermore, the N ２ A portion of the gas is CO gas, CO ２The gas may be replaced with one or more of the following: gas or Ar gas. In this case, it is preferable that the proportion of the replacement gas in the atmosphere gas be 30 volume% or less. From the viewpoint of reducing manufacturing costs, the oxygen concentration of the atmosphere here is preferably 3 volume ppm or more, and more preferably 10 volume ppm or more.

[0065] ・Average heating rate in the temperature range of 300°C to 500°C (preferred requirement) As mentioned above, the ammonia that serves as the nitriding source for B is promoted by using iron oxide and pure iron, which is obtained by reducing iron oxide, as catalysts. Therefore, from the viewpoint of suppressing the occurrence of gray spot defects and black spot defects, it is preferable to set the average heating rate to 10°C / s or less, to ensure sufficient annealing time in a Fe reducing atmosphere, and to reduce the iron oxide on the surface of the steel sheet. This allows Mn and B oxides to be formed on the surface of the steel sheet as early as possible in the subsequent heating temperature range of 500°C or higher, thereby covering the surface of the steel sheet and suppressing the exposure of the pure iron layer on the surface of the steel sheet during heating and maintaining uniform heat. On the other hand, considering productivity, it is preferable that the average heating rate be 1°C / s or more. Furthermore, from the viewpoint described above, the average heating rate is more preferably 2°C / s or more. Furthermore, the average heating rate is more preferably 7°C / s or less. Here, the average heating rate is obtained by "(500 - 300) (°C) / heating time from 300°C to 500°C (s)".

[0066] In the reduction annealing process of continuous annealing, the subsequent temperature range of 500°C to 750°C is H ２ The steel plate is heated in an atmosphere containing 5% or more by volume and with a dew point of -40°C or lower, at an average heating rate of 1°C / s or higher. This temperature range is where ammonia generation becomes significant and the adsorption and penetration of radical nitrogen into the steel plate begins. Therefore, the generation of ammonia and the penetration of radical nitrogen are suppressed, thereby suppressing the formation of nitrides of B in the subsequent soaking treatment process.

[0067] • Hydrogen concentration in the atmosphere (H ２Concentration: 5% by volume or more. Since hydrogen is a reducing gas, it can suppress the formation of oxides of Si and Mn during annealing and prevent non-plating defects caused by oxides. To obtain a sufficient effect in suppressing oxide formation, the hydrogen concentration in the atmosphere should be 5% by volume or more, preferably 6% by volume or more. There is no particular upper limit to the hydrogen concentration, but from the viewpoint of suppressing cost increases, it is preferable that the hydrogen concentration be 30% by volume or less.

[0068] - Dew point of the atmosphere: Below -40°C. If the dew point is higher than -40°C, a large amount of Si and Mn oxides will form on the surface of the steel sheet, causing non-plating defects due to oxides. In addition, new oxidation of Fe will occur, resulting in insufficient suppression of ammonia generation during the heating process above 500°C and the soaking process above 750°C, and thus insufficient suppression of gray spot defects and black spot defects. For this reason, the dew point should be below -40°C. From the viewpoint of suppressing gray spot defects and black spot defects, it is more preferable that the dew point of the atmosphere be below -42°C. Here, from the viewpoint of reducing manufacturing costs, it is preferable that the dew point of the atmosphere be above -70°C, and more preferable that be above -60°C.

[0069] Hydrogen (H) in the atmospheric gas ２ ), H ２ O, oxygen (O ２ The remaining portion is N ２ It is preferable that the gas be an unavoidable impurity, and furthermore, the N ２ A portion of the gas is CO gas, CO ２ The gas may be replaced with one or more of the following: gas or Ar gas. In this case, it is preferable that the proportion of the replacement gas in the atmospheric gas is 30% by volume or less.

[0070] Average heating rate: 1°C / s or higher. The nitriding reaction with ammonia is likely to occur in the temperature range of 500°C to 750°C. Therefore, from the viewpoint of suppressing gray spot defects and black spot defects, it is necessary to heat this temperature range quickly. At an average heating rate of less than 1°C / s, the time required to raise the temperature to the predetermined temperature increases, increasing the amount of radical nitrogen penetration into the steel sheet, and nitrides of B are formed in the subsequent soaking treatment process, resulting in appearance defects due to gray spot defects and black spot defects. For this reason, the average heating rate should be 1°C / s or higher, preferably 1.5°C / s or higher, and more preferably 5°C / s or higher. The average heating rate is preferably 50°C / s or lower, and more preferably 30°C / s or lower. Here, the average heating rate is obtained by "(750 - 500) (°C) / heating time from 500°C to 750°C (s)".

[0071] In the reduction annealing step of continuous annealing, after the above heating, H is used at a temperature of 750°C to 950°C. ２ The steel sheet is subjected to a soaking heat treatment in an atmosphere containing 5% or more by volume of : and with a dew point of -55°C to -40°C, for a holding time of 20 seconds to 300 seconds. In this soaking heat treatment process, the nitriding reaction of B by ammonia and radical nitrogen that has penetrated into the steel is suppressed, and a portion of B is fixed as an oxide on the surface or inside the steel sheet, thereby suppressing the formation of nitrides due to the diffusion of B to the surface of the steel sheet during annealing. In this way, the amount of nitrides formed by B is suppressed.

[0072] • Hydrogen concentration in the atmosphere (H ２ Concentration: 5% by volume or more. Since hydrogen is a reducing gas, it can suppress the formation of oxides of Si and Mn during annealing and prevent non-plating defects caused by oxides. To obtain a sufficient effect in suppressing oxide formation, the hydrogen concentration in the atmosphere should be 5% by volume or more, preferably 6% by volume or more. There is no particular upper limit to the hydrogen concentration, but from the viewpoint of suppressing cost increases, it is preferable that the hydrogen concentration be 30% by volume or less. It is more preferable that the hydrogen concentration be 20% by volume or less, and even more preferable that it be 15% by volume or less.

[0073] - Dew point of the atmosphere: -55°C or higher and -40°C or lower. If the dew point is higher than -40°C, a large amount of Si and Mn oxides will form on the steel sheet surface, causing non-plating defects due to oxides. On the other hand, if the dew point is below -55°C, a portion of B will be fixed as an oxide, and the effect of suppressing the formation of nitrides by the diffusion of B to the steel sheet surface during annealing will not be sufficiently obtained. In addition, the low oxygen potential and high nitrogen potential of the annealing atmosphere will stabilize the formation of nitrides, and the formation of B nitrides on the steel sheet surface will be promoted. As a result, the occurrence of gray spot defects and black spot defects cannot be adequately suppressed. For this reason, the dew point should be -55°C or higher, preferably -50°C or higher. Also, the dew point should be -40°C or lower, preferably -45°C or lower.

[0074] - Holding time: 20 seconds or more and 300 seconds or less. If the holding time is less than 20 seconds, the proportion of austenite generated during heating in the two-phase region of ferrite and austenite will be insufficient, so the area ratio of ferrite and bainite will increase, making it difficult to achieve a TS of 590 MPa or more. On the other hand, if the holding time exceeds 300 seconds, although some of B is fixed as an oxide, some of the remaining B will form nitrides, so the amount of B nitrides formed on the surface of the steel sheet will increase, and the occurrence of gray spot defects and black spot defects will not be adequately suppressed. Therefore, the holding time should be 20 seconds or more, preferably 30 seconds or more. Also, the holding time should be 300 seconds or less, preferably 200 seconds or less, and more preferably 100 seconds or less. Note that this holding time refers to the time during which the steel sheet remains (passes through) the above-mentioned atmosphere at a temperature of 750°C or more and 950°C or less.

[0075] ・Ammonia concentration in the atmosphere (preferred requirement) In the present invention, it is preferable to reduce the ammonia concentration in the atmosphere during the soaking process to 0.010 volume% or less. Reasons for ammonia being present in the atmosphere of the soaking zone include the introduction of ammonia gas generated in the heating zone into the soaking zone, and the introduction of ammonia when exhaust gas containing ammonia is reused in the soaking zone. To mitigate the harmful effects of such ammonia contamination and generation, it is important to improve the sealing performance of the partition wall between the heating zone and the soaking zone, reduce the reuse rate of exhaust gas, and increase the flow rate of new high-purity gas from the rear to the front in the direction of steel plate travel (from the outlet to the inlet of the soaking zone). By further suppressing the formation of nitrides of B by reducing the ammonia concentration in the soaking zone, it is possible to further suppress appearance defects due to gray spot defects and black spot defects. For this reason, it is preferable to set the ammonia concentration in the atmosphere to 0.010 volume% or less. The lower limit is not particularly limited, but it is preferable that the ammonia concentration in the atmosphere be 0.001 volume% or more, and more preferably 0.002 volume% or more.

[0076] (Hot-dip galvanizing) In the present invention, after cooling a steel sheet (cold-rolled steel sheet) that has been continuously annealed under the above conditions, it is subjected to hot-dip galvanizing treatment by, for example, immersing it in a hot-dip galvanizing bath. The temperature reached during the above cooling is preferably 200 to 520°C, and it is preferable to heat it before immersing it in the hot-dip galvanizing bath if necessary. The bath temperature of the hot-dip galvanizing bath is preferably 440°C or higher. Furthermore, the bath temperature of the hot-dip galvanizing bath is preferably 500°C or lower. The hot-dip galvanizing bath is not particularly limited, but for example, a bath with an Al content of 0.10% by mass or more and 0.23% by mass or less, and further containing one or more selected from Pb, Sb, Si, Sn, Mg, Mn, Ni, Cr, Co, Ca, Cu, Li, Ti, Be, Bi, and REM in a total of 0% by mass or more and 3.5% by mass or less, with the remainder being Zn and unavoidable impurities, can be used. Furthermore, in order to prevent changes in the temperature of the plating bath, it is preferable that the temperature of the steel plate before plating (immersion plate temperature) be equal to or higher than the plating bath temperature. Also, it is preferable that the temperature of the steel plate before plating (immersion plate temperature) be 50°C or less above the plating bath temperature.

[0077] (Alloying Treatment) After the hot-dip galvanizing treatment described above, a further alloying treatment of zinc plating is performed to form an alloyed hot-dip galvanized layer. The alloying treatment is preferably performed in a temperature range of 480°C or higher. Furthermore, the alloying treatment is preferably performed in a temperature range of 570°C or lower. If the alloying temperature is below 480°C, the Zn-Fe alloying rate becomes excessively slow, making alloying extremely difficult. On the other hand, if the alloying temperature exceeds 570°C, the untransformed austenite may transform into pearlite, and the TS and total elongation El may decrease. The alloying treatment is more preferably performed in a temperature range of 490°C or higher. The alloying treatment is more preferably performed in a temperature range of 560°C or lower, and even more preferably in a temperature range of 530°C or lower. The alloying treatment time is preferably 1 second or more, and more preferably 5 seconds or more. Furthermore, the alloying treatment time is preferably 150 seconds or less, and more preferably 80 seconds or less.

[0078] The amount of plating deposited on alloyed hot-dip galvanized steel sheet (GA) is 20 g / m² per side. ２ 120g / m or more ２ The following is preferable: The amount of plating on the alloyed hot-dip galvanized steel sheet (GA) is 20 g / m² per side. ２ 80g / m or more ２ The following is more preferable: The amount of plating can be adjusted by performing gas wiping or the like after hot-dip galvanizing.

[0079] As described above, after applying the hot-dip galvanizing alloy treatment, it is preferable to cool the material to a cooling stop temperature between room temperature (0-50°C) and 350°C. From the viewpoint of improving ductility, it is more preferable to cool the material to a range of 150°C to 350°C. On the other hand, from the viewpoint of improving strength (TS), it is preferable to cool the material to room temperature. There is no particular requirement for the average cooling rate when cooling to a range of 150°C to 350°C, but from the viewpoint of ensuring a high TS and improving ductility, it is preferable to set the average cooling rate from the alloy treatment up to 350°C to 3°C to 20°C. Also, there is no particular requirement for the average cooling rate when cooling to room temperature, but in order to further increase TS, it is preferable to set the average cooling rate from the alloy treatment up to 50°C to 5°C to 5°C to 5°C to 5°C or higher. On the other hand, from the constraints of production technology, it is preferable to set the average cooling rate up to 50°C to 40°C or lower. Furthermore, this average cooling rate up to 50°C is more preferably 7°C or higher and 30°C or lower. Furthermore, the cooling rate below 50°C is not particularly limited, and the temperature can be cooled to a predetermined temperature by any method.

[0080] For cooling after alloying, gas jet cooling, mist cooling, water cooling, and air cooling can be applied as appropriate. Typically, high-strength alloyed hot-dip galvanized steel sheets are traded after they have cooled to room temperature.

[0081] The alloyed hot-dip galvanized steel sheet, cooled to 350°C or below after alloying treatment, may be rolled at a predetermined elongation rate. Preferably, this elongation rate is 0.05% or more and 1.00% or less. A roll elongation rate of 0.05% or more allows for the introduction of cracks in the zinc plating layer. Introducing cracks in the zinc plating layer reduces the amount of diffusible hydrogen in the steel sheet, thereby improving bendability and hole-expanding properties. On the other hand, if the roll elongation rate exceeds 1.00%, the yield speed (YS) increases, which may reduce dimensional accuracy during forming. More preferably, this roll elongation rate is 0.70% or less. More preferably, this roll elongation rate is 0.10% or more. The above rolling may be performed online in a device continuous with the continuous hot-dip galvanizing equipment, or offline in the continuous hot-dip galvanizing equipment. Furthermore, the target elongation rate (for example, 0.05% to 1.00%) may be achieved in a single rolling process, or it may be achieved by rolling multiple times. Generally, temper rolling is performed as the above rolling process, but rolling by methods such as processing with a leveler may also be used as long as an elongation rate equivalent to that of temper rolling can be achieved.

[0082] After cooling to 350°C or below and performing the rolling as necessary, the sheet may be kept warm at room temperature or above room temperature but below 450°C. Keeping the sheet warm at room temperature or above room temperature but below 450°C can reduce the amount of diffusible hydrogen in the steel sheet, thereby improving its bendability and hole-expanding properties. The holding time at room temperature may be 3 days or more. Alternatively, the holding time at room temperature may be 10 months or less. The holding time above room temperature may be 5 minutes or more. Alternatively, the holding time above room temperature may be 14 days or less.

[0083] Furthermore, manufacturing conditions other than those mentioned above can be met by conventional methods.

[0084] <Tensile Strength TS> The high-strength alloyed hot-dip galvanized steel sheet manufactured according to the present invention can have a TS of 590 MPa or higher. Furthermore, for even higher strength, the TS can be increased to 780 MPa or higher, and even further to 980 MPa or higher. The TS is measured in accordance with JIS Z2241 (2022) as follows: A JIS No. 5 test piece is taken from the alloyed hot-dip galvanized steel sheet so that its longitudinal direction is perpendicular to the rolling direction of the steel sheet. Using this test piece, the crosshead displacement velocity Vc is 1.67 × 10⁻⁶. －１ A tensile test is performed under the condition of mm / s, and the TS is measured.

[0085] <Sheet Thickness> The sheet thickness of the alloyed hot-dip galvanized steel sheet manufactured according to the present invention is not particularly limited, but it is preferably 0.3 mm or more. Furthermore, the sheet thickness of the alloyed hot-dip galvanized steel sheet is preferably 2.8 mm or less.

[0086] <Bmin / Bbase: 0.80 or less> As described above, the present invention is characterized by sufficiently forming a B-deficient layer in order to suppress the black mottled pattern caused by the unevenness of the plating. Here, whether or not the formation of the B-deficient layer is sufficient can be evaluated as the degree of B deficiency by the amount of B-deficient layer (Bmin / Bbase) on the surface layer of the steel sheet (surface layer of the base steel sheet). This amount of B-deficient layer (degree of B deficiency) on the surface layer of the steel sheet can be determined using GDS (glow discharge emission) analysis in the following way. Specifically, first, a 25 mm x 25 mm GDS analysis sample is taken from the center in the width direction of the alloyed hot-dip galvanized steel sheet, and GDS analysis (JIS K0144 (2018)) is performed from the surface of the alloyed hot-dip galvanized layer in the thickness direction at intervals of 0.1 seconds up to a position of 50 μm in the thickness direction, and the Fe intensity and B intensity are measured. For the GDS analysis, a high-frequency excitation source was used, with a high-frequency output of 35 W, Ar as the discharge gas, and a gas pressure of 600 Pa.

[0087] Figure 1(A) is a graph illustrating the detection intensity of Fe obtained by GDS analysis, specifically the values ​​Fe detection intensity ≥ maximum Fe intensity × 0.95 and Fe detection intensity ≥ maximum Fe intensity × 0.80. Figure 1(B) is a graph illustrating Bmin and Bbase. In this invention, the average value of the B intensity within the range satisfying Fe detection intensity ≥ maximum Fe intensity × 0.95 is defined as the B intensity of the base material (Bbase). Here, Bbase (average value of B intensity) can be determined by calculating the average value of the B analysis results (total detection intensity of all data / total number of data) as shown in Figure 1(B), within the range satisfying Fe detection intensity ≥ maximum Fe intensity × 0.95, based on the Fe analysis results shown in Figure 1(A). Note that Fe detection intensity refers to the Fe intensity (a.u.) obtained by GDS analysis corresponding to each measurement time. Fe maximum intensity (Fe ｍａｘ ) is the maximum Fe intensity (a.u.) obtained from GDS analysis over the entire measurement time.

[0088] Furthermore, in this invention, Bmin is defined as the minimum value of B intensity in the range including the steel plate surface layer where the detected Fe intensity ≥ the maximum Fe intensity × 0.80. Here, Bmin (the minimum value of B intensity) can be determined by extracting the minimum value of B intensity at the corresponding measurement time, as shown in Figure 1(B), within the range where the detected Fe intensity ≥ the maximum Fe intensity × 0.80, based on the Fe analysis results shown in Figure 1(A).

[0089] If Bmin / Bbase is 0.80 or less, a sufficient B-deficient layer is formed on the surface of the steel sheet, and the BN formation suppression effect by the aforementioned B-deficient layer can be fully obtained. Furthermore, from the viewpoint of improving the BN formation suppression effect, it is more preferable that Bmin / Bbase be 0.70 or less. On the other hand, there is no particular lower limit, but from the viewpoint of reducing manufacturing costs, it is preferable that Bmin / Bbase be 0.10 or more, and more preferable that it be 0.20 or more.

[0090] <Tmin / Tmax≧0.50> The surface irregularities of the plating can be determined using a scanning electron microscope (SEM) in the following way: Take 10 mm × 10 mm SEM observation samples from each of the five equal positions along the width of the alloyed hot-dip galvanized steel sheet, embed them in resin so that the cross-section in the width direction can be observed, and then mechanically polish them. The preparation of the cross-sectional observation samples is not limited to the above method; other conventional methods may be used. Take three consecutive SEM images of the width direction from the obtained cross-sectional observation samples at a magnification of 1500x, with a field of view of 65 μm vertically × 85 μm horizontally. Measure the maximum and minimum plating thickness in each of these fields to the first decimal place. Furthermore, similarly measure the maximum and minimum plating thickness for the remaining four samples taken from different positions. The average of the maximum plating thickness obtained in this way is defined as Tmax (μm), and the average of the minimum plating thickness is defined as Tmin (μm). If Tmin / Tmax is 0.50 or higher, an alloyed hot-dip galvanized steel sheet can be obtained with a good plating appearance, having few irregularities on the plated surface and no mottled patterns. Furthermore, from the viewpoint of improving the appearance, it is more preferable that Tmin / Tmax be 0.60 or higher, and even more preferable that it be 0.70 or higher. On the other hand, there is no particular upper limit, but from the viewpoint of reducing manufacturing costs, it is preferable that Tmin / Tmax be 0.98 or lower, and more preferable that it be 0.95 or lower.

[0091] In the present invention described above, a reduced iron layer is formed on the surface of a steel sheet containing B (base steel sheet) by oxidation and reduction treatment, thereby suppressing the surface diffusion of B. Furthermore, in the annealing process before plating, the dew point in the temperature range of 300 to 500°C is lowered to create a Fe reducing atmosphere, thereby suppressing the formation of ammonia by suppressing the formation of iron oxide. Furthermore, heating in the temperature range of 500°C or higher, where ammonia formation becomes significant and radical nitrogen penetration into the steel sheet begins, is performed quickly to suppress ammonia formation and nitriding of the steel sheet. Furthermore, the annealing time in the temperature range of 750°C or higher is shortened, and the dew point is set to -55°C or higher to fix a portion of B as oxide. As a result, the formation of nitrides due to the diffusion of B to the surface of the steel sheet during annealing is suppressed more than in conventional methods, thereby suppressing the occurrence of black mottled patterns caused by alloying unevenness, and a high-strength alloyed hot-dip galvanized steel sheet with a good surface appearance is obtained.

[0092] [Example 1] A steel material with the component composition shown in Table 1 (the remainder being Fe and unavoidable impurities) was melted in a converter and continuously cast to form a steel slab. This steel slab was heated to 1250°C and roughly rolled, then finished rolling at a finish rolling temperature of 900°C, and wound at a winding temperature of 400-600°C to obtain a hot-rolled steel sheet. After pickling this hot-rolled steel sheet, it was cold-rolled to obtain a cold-rolled steel sheet with a thickness of 1.4 mm. This cold-rolled steel sheet was annealed in a Continuous Galvanizing Line (CGL) under the conditions shown in Tables 2-4. Next, it was hot-dip galvanized under the conditions shown in Tables 2-4, then alloyed, and cooled to below 50°C. After that, temper rolling was performed at an elongation rate of 0.1% to obtain an alloyed hot-dip galvanized steel sheet (GA).

[0093] The molten zinc plating bath used contained 0.14% by mass of Al, with the remainder being Zn and unavoidable impurities. The plating adhesion amount was 45 to 55 g / m² per side. ２ The plating was applied to a degree equivalent to double-sided plating. The composition of the GA plating layer contained Fe: 7-15% by mass, Al: 0.1-1.0% by mass, with the remainder being Zn and unavoidable impurities.

[0094] The alloyed hot-dip galvanized steel sheets obtained as described above were evaluated as follows. The results, along with the manufacturing conditions, are shown in Tables 2 to 4.

[0095] Following the procedure described above, GDS analysis was performed on the obtained alloyed hot-dip galvanized steel sheets, and the amount of B-deficient layer on the surface of the steel sheets (Bmin / Bbase) was calculated.

[0096] Following the procedure described above, the resulting alloyed hot-dip galvanized steel sheet was subjected to cross-sectional SEM observation in the width direction, and the surface irregularities (Tmin / Tmax) of the steel sheet were evaluated.

[0097] • Tensile Strength (TS) The tensile test was conducted in accordance with JIS Z2241 (2022). From the obtained steel plate, a JIS No. 5 test specimen was taken so that the longitudinal direction was perpendicular to the rolling direction of the steel plate. Using this test specimen, the crosshead displacement velocity Vc: 1.67 × 10⁻⁶ －１ A tensile test was performed under conditions of mm / s, and the TS was measured.

[0098] As shown in Tables 2-4, all of the alloyed hot-dip galvanized steel sheets of the inventive examples had a sufficiently formed B-deficient layer, resulting in minimal plating irregularities and a good appearance. Furthermore, a high strength of TS: 590 MPa or higher was achieved, demonstrating that alloyed hot-dip galvanized steel sheets with both high strength and excellent plating appearance were obtained. On the other hand, plating irregularities were observed in the alloyed hot-dip galvanized steel sheets of the comparative examples.

[0099]

[0100]

[0101]

[0102]

[0103] [Example 2] From the viewpoint of further reducing the ammonia concentration in the atmosphere of the soaking process (soaking zone), alloyed hot-dip galvanized steel sheets were manufactured under conditions in which the flow rate of high-purity gas in the soaking zone was increased. Using steel materials A, D, I, H, N, O, and P shown in Table 1, alloyed hot-dip galvanized steel sheets were manufactured under the same manufacturing conditions as in Example 1 (hot rolling, cold rolling, continuous annealing, hot-dip galvanizing, alloying treatment, temper rolling). The tensile properties, B-deficient layer amount, and plating surface irregularities of the obtained alloyed hot-dip galvanized steel sheets were evaluated using the same method as in Example 1. The results, along with the manufacturing conditions, are shown in Table 5. The ammonia gas concentration was measured in the upper part of the furnace in a pass including the central part of the longitudinal section of the CGL soaking zone line. Ion chromatography (JIS K0099 (2020)) was used to measure the ammonia gas concentration. As shown in Table 5, it can be seen that reducing the ammonia concentration in the soaking process (soaking tubing) to 0.010 volume% or less further suppresses the plating irregularities of the alloyed hot-dip galvanized steel sheet, improving the appearance of the plating.

[0104]

Claims

1. A hot-dip galvanized steel sheet comprising a steel sheet and a hot-dip galvanized alloy layer on the steel sheet, wherein the composition of the steel sheet is, in mass%, C: 0.050% or more and 0.300%, Si: 1.20% or less, Mn: 2.00% or more and 3.50%, P: 0.100% or less, S: 0.0100% or less, sol. It contains Al: 1.00% or less, N: 0.0200% or less, B: 0.0001% to 0.0050%, and the ratio of Mn content (mass%) [%Mn] and Si content (mass%) [%Si] of the steel sheet is 2.50 or more, and further, as optional components, it contains Cr: 1.00% or less, Ti: 0.200% or less, Nb: 0.200% or less, V: 0.200% or less, Mo: 2.000% or less, Cu: 1.000% or less, Ni: 0.500% or less, Sn: 0.200% or less, Sb: 0.200% or less, Mg: 0.0100% or less, Ca: 0.0100% or less, Zn: 0.100% or less. It contains one or more elements selected from Co: 0.200% or less, Zr: 0.200% or less, Ta: 0.10% or less, Te: 0.10% or less, As: 0.10% or less, Hf: 0.10% or less, Bi: 0.20% or less, Pb: 0.20% or less, Ge: 0.10% or less, Sr: 0.10% or less, Cs: 0.10% or less, and REM: 0.0100% or less, with the remainder being Fe and unavoidable impurities. When GDS analysis is performed in the thickness direction from the surface of the alloyed hot-dip galvanized layer, Bbase is defined as the average value of the B intensity in the range where the detection intensity of Fe ≥ the maximum intensity of Fe × 0.95, and Bmin is defined as the minimum value of the B intensity in the range where the detection intensity of Fe ≥ the maximum intensity of Fe × 0.80, A hot-dip galvanized steel sheet having Bmin / Bbase of 0.80 or less, and where the maximum plating thickness Tmax (μm) and minimum plating thickness Tmin (μm) in the width direction of the alloyed hot-dip galvanized layer satisfy Tmin / Tmax ≥ 0.

50.

2. The alloyed hot-dip galvanized steel sheet according to claim 1, wherein the component composition of the steel sheet is such that [%Mn] / [%Si] is 12.00 or more.

3. A method for manufacturing an alloyed hot-dip galvanized steel sheet, comprising: continuously annealing a cold-rolled steel sheet having the component composition described in claim 1 or 2; then hot-dip galvanizing the cold-rolled steel sheet; and subsequently performing an alloying treatment, wherein the continuous annealing includes an oxidation treatment step and a reduction annealing step, and in the oxidation treatment step, N ２ , O 1000 ppm or more by volume ２ In an atmosphere consisting of H and unavoidable impurities, the steel plate is heated to 500°C or more and 700°C or less, and in the subsequent reduction annealing step after the oxidation treatment step, the temperature range of 300°C or more and 500°C or less is H ２ Contains 3% or more by volume, dew point: -20°C or lower, O ２ The steel plate is heated in an atmosphere of 500 ppm by volume or less, and after heating, the temperature range is 500°C to 750°C. ２ The steel plate is heated in an atmosphere containing 5% or more by volume and with a dew point of -40°C or lower, at an average heating rate of 1°C / s or higher, and after heating, at a temperature of 750°C to 950°C, H ２ A method for manufacturing alloyed hot-dip galvanized steel sheets, comprising: soaking a steel sheet in an atmosphere containing 5% or more by volume and with a dew point of -55°C to -40°C for a holding time of 20 seconds to 300 seconds.

4. The method for producing an alloyed hot-dip galvanized steel sheet according to claim 3, wherein the ammonia concentration in the atmosphere for the soaking treatment is 0.010 volume% or less.

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