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).　This 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: Cmin / Cbase obtained by performing GDS analysis in the thickness direction from the surface of the alloyed hot-dipped galvanization layer is 0.50; if XPS analysis is performed on the surface of the steel sheet after removing the alloyed hot-dipped galvanization layer, and IFe, IB, and IN are the semi-quantitative values of Fe, B, and N, (IB + IN) / IFe ≤ 0.10 is satisfied as the element concentration ratio of Fe, B, and N; and on the surface of the alloyed hot-dipped galvanization layer, the surface roughness Ra is 1.20 µm or less and the lightness L value is 50 or more.

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 exhibits minute surface defects different from those described in Patent Document 1, namely "surface defects caused by the formation of Si, Mn-based oxides and the resulting decrease in plating wettability," that is, defects where Si, Mn-based oxides repel the plating, creating 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 exhibits 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 unevenness 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 strong desire to improve the outer surface, 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 a surface roughness Ra of 1.20 μm or less, measured according to JIS B0601 (2013), and a lightness L value of 50 or higher, measured according to JIS Z8781-4 (2013).

[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 slowing down the diffusion of B. It has been reported that the diffusion rate of B in austenite is 1 / 5 times faster than in ferrite. For this reason, it is thought that the diffusion of B can be suppressed by reducing the ferrite layer on the surface of the steel sheet during recrystallization annealing (soaking treatment). In addition, in high-strength steel sheets, ferrite is formed on the surface due to decarburization during recrystallization annealing. Therefore, we attempted to suppress the formation of ferrite and delay the diffusion of B to the surface of the steel sheet by suppressing the decarburization of the surface of the steel sheet.

[0011] (4) Decarburization during recrystallization annealing is carried out by the gas in the annealing atmosphere. Here, O ２ and H ２ O is an oxidizing gas, CO and CH ４ It is known as a carburizing gas. Based on this, after thorough investigation, it was determined that the oxidizing gas (O ２ , H ２ O) and carburizing gases (CO, CH) ４It was found that by controlling the ratio of ( ) and keeping the proportion of carburizing gas above a certain level, the formation of a decarburized layer on the surface of the steel sheet during recrystallization annealing can be suppressed. Furthermore, it was discovered that suppressing the decarburized layer makes it possible to suppress the formation of nitrides of B and the unevenness of the plated surface, which were the targets. In this way, it was found that by suppressing the surface diffusion of B and the formation of nitrides, and suppressing alloying unevenness between the steel sheet and molten zinc, a beautiful plated appearance with small irregularities and no black mottled patterns can be obtained.

[0012] The present invention has been made based on the above findings and has the following gist. [1] An alloyed hot-dip galvanized steel sheet having a steel sheet and an alloyed hot-dip zinc plating layer on the steel sheet, wherein the component composition of the steel sheet is in mass%, C: 0.050% or more and 0.300% or less, Si: 1.20% or less, Mn: 2.00% or more and 3.50% or less, 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 with respect to [%Mn] which is the Mn content (mass%) of the steel sheet and [%Si] which is the Si content (mass%) of the steel sheet, [%Mn] / [%Si] is 2.50 or more. Further, as optional components, 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, 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, REM: 0.0100% or less, and the balance consists of Fe and unavoidable impurities. GDS analysis is performed in the thickness direction from the surface of the alloyed hot-dip zinc plating layer. When the average value of the C intensity in the range satisfying Fe detection intensity ≧ maximum Fe intensity × 0.95 is defined as Cbase, and the minimum value of the C intensity in the range satisfying Fe detection intensity ≧ maximum Fe intensity × 0.80 is defined as Cmin, Cmin / Cbase ≧ 0.50. After peeling the alloyed hot-dip zinc plating layer of the alloyed hot-dip galvanized steel sheet, XPS analysis is performed on the steel sheet surface, and when the semi-quantified values of Fe, B, and N are I Ｆｅ , I Ｂ , I Ｎ and, as the elemental concentration ratio of Fe, B, and N, (I Ｂ + I Ｎ ) / I Ｆｅ[1] An alloyed hot-dip galvanized steel sheet that satisfies ≤0.10, and on the surface of the alloyed hot-dip galvanized layer, the surface roughness Ra is 1.20 μm or less and the brightness L value is 50 or more. [2] The alloyed hot-dip galvanized steel sheet according to [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 according to [1] or [2], then applying hot-dip galvanizing to the cold-rolled steel sheet, and then applying an alloying treatment, wherein in the continuous annealing, the temperature range is 300°C to 500°C, 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 : Contains 5% or more by volume, dew point: -55°C or higher and -40°C or lower, and O ２ , H ２ O, CO, and CH ４ The respective partial pressures (Pa) of pO ２ pH ２ O, pCO, and pCH ４ However, Log(pO ２ + pH ２ O)-Log(pCO+pCH ４ A method for manufacturing an alloyed hot-dip galvanized steel sheet, comprising: soaking the steel sheet in an atmosphere satisfying ) ≤ 3.0 for a holding time of 20 seconds or more and 300 seconds or less. [4] The method for manufacturing an alloyed hot-dip galvanized steel sheet according to [3] above, wherein the ammonia concentration in the atmosphere for soaking is 0.010 volume% or less.

[0013] 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.

[0014] 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 Cmin and Cbase.

[0015] [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% or less, Si: 1.20% or less, Mn: 2.00% or more and 3.50% or less, 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] to Si content (mass%) [%Si] of the steel sheet is 2.50 or more. Furthermore, it may contain optional components 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, and Sn: 0.200% or less. The following is a composition containing one or more elements selected from Sb: 0.200% or less, Mg: 0.0100% 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: 0.10% 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, and the average value of the C intensity in the range where the detected Fe intensity ≥ the maximum Fe intensity × 0.95 is taken as Cbase, and the minimum value of the C intensity in the range where the detected Fe intensity ≥ the maximum Fe intensity × 0.80 is taken as Cmin, then Cmin / Cbase ≥ 0.50, and when XPS analysis is performed on the surface of the steel sheet after the alloyed hot-dip galvanized layer has been removed, the semi-quantified values ​​of Fe, B, and N are taken as I. Ｆｅ , I Ｂ , I Ｎ In this case, the elemental concentration ratios of Fe, B, and N are as follows: (I Ｂ +I Ｎ ) / I Ｆｅ The condition ≤ 0.10 is satisfied, and on the surface of the alloyed hot-dip galvanized layer, the surface roughness Ra is 1.20 μm or less, and the brightness L value is 50 or more.

[0016] 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.

[0017] <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.

[0018] • 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.

[0019] • 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 set for the Si content. That is, the Si content may be 0%, but since the refining cost increases when reducing it to less than 0.01%, it is preferable to have a Si content of 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.

[0020] • Mn: 2.00% to 3.50% Mn is an element necessary to suppress gray spot and black spot defects and obtain a good surface appearance. By including 2.00% or more of Mn, it is possible to form Mn,B-based composite oxides that have little adverse effect on the plating appearance quality and to suppress the formation of B nitrides, thereby suppressing gray spot and black spot defects. In addition, Mn is an effective element for generating the desired amount of quenched martensite and tempered martensite, thereby achieving a TS of 590 MPa or more. If the Mn content is less than 2.00%, the amount of B that forms a composite oxide with Mn during annealing decreases, and the amount of BN formed increases, so black spot and gray spot defects cannot be sufficiently suppressed. On the other hand, if the Mn content exceeds 3.50%, the area ratio of tempered martensite increases, and the area ratio of ferrite and bainite decreases, resulting in a decrease in dimensional accuracy during forming. Furthermore, during annealing, the amount of Mn concentrated on the steel sheet surface increases, and a large amount of Mn oxide, which causes non-plating defects, is formed on the steel sheet surface, making it difficult to obtain a good surface appearance. Therefore, the Mn content should be 2.00% or more and 3.50% or less. Also, from the viewpoint of suppressing the occurrence of gray spot defects and black spot defects, the Mn content should preferably be 2.30% or more, more preferably 2.50% or more, and even more preferably 2.60% or more. Also, from the above viewpoint, the Mn content should preferably be 3.30% or less, and more preferably 3.00% or less.

[0021] • P: 0.100% or less. P is an element that has a solid solution strengthening effect and increases the strength of steel sheets. However, if the P content exceeds 0.100%, P segregates at the prior austenite grain boundaries, making the grain boundaries brittle, which reduces punchability and elongation flangeability. Therefore, the P content should be 0.100% or less. Also, from the above viewpoint, the P content should preferably be 0.050% or less, and more preferably 0.030% or less. There is no lower limit for the P content. That is, the P content may be 0%, but since the refining cost increases when controlling it to less than 0.001%, it is preferable that the P content be 0.001% or more. It is more preferable that the P content be 0.003% or more, and even more preferable that be 0.005% or more.

[0022] ・S: 0.0100% or less S exists as a sulfide in steel, and if the S content exceeds 0.0100%, it reduces the ultimate deformability of the steel sheet, thus reducing punchability, elongation flangeability, and bendability. For this reason, the S content should be 0.0100% or less. There is no particular lower limit for the S content. In other words, the S content may be 0%, but since the refining cost increases when controlling it to less than 0.0001%, it is preferable that the S content be 0.0001% or more. Furthermore, from the above viewpoint, the S content is preferably 0.0050% or less. The S content is more preferably 0.0040% or less, and even more preferably 0.0030% or less.

[0023] ・sol. Al: 1.00% or less Al can be used as a deoxidizing agent. In this case, it is preferable that the sol. Al content in the steel be 0.01% or more. In addition, in steel plates to which B has been added, adding Al fixes the N in the steel as AlN, and the added B can be used as solid solution B which is effective in increasing strength. In addition, Al also has the effect of suppressing carbide formation during annealing and increasing the volume fraction of retained austenite. The formed retained austenite has the effect of improving ductility. In order to obtain the effect of fixing N as AlN, it is preferable that the sol. Al content be 0.02% or more. Furthermore, from the viewpoint of obtaining the effect of improving ductility, it is even more preferable that the sol. Al content be 0.05% or more. On the other hand, if the sol. Al content exceeds 1.00%, non-plating will occur, so the sol. Al content should be 1.00% or less. Also, from the viewpoint of the above, sol. The Al content is preferably 0.10% or less, and more preferably 0.08% or less.

[0024] ・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.

[0025] ・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.

[0026] • [%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 4.00 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.

[0027] 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%.

[0028] ・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.

[0029] - 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.

[0030] • 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.

[0031] • 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.

[0032] ・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.

[0033] ・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.

[0034] 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.

[0035] ・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.

[0036] ・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.

[0037] ・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.

[0038] 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. Furthermore, from the above viewpoint, it is more 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.

[0039] 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.

[0040] ・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.

[0041] • Zr: 0.200% or less Zr 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 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 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.

[0042] • 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.

[0043] • 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.

[0044] • 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.

[0045] • 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.

[0046] • 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, when Bi is included, from the viewpoint of preventing cost increases, the Bi content should be 0.20% or less. It is preferable that the Bi content be 0.10% or less.

[0047] • 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.

[0048] • 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.

[0049] • 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.

[0050] • 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.

[0051] - 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.

[0052] The remainder of the mixture, other than the components mentioned above, consists of Fe and unavoidable impurities.

[0053] [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).

[0054] 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.

[0055] (3) As a method to suppress surface diffusion of B, we investigated slowing down the diffusion of B. It has been reported that the diffusion rate of B in austenite is 1 / 5 times faster than in ferrite. For this reason, it is thought that the diffusion of B can be suppressed by reducing the ferrite layer on the surface of the steel sheet during recrystallization annealing (soaking treatment). In addition, in high-strength steel sheets, ferrite is formed on the surface due to decarburization during recrystallization annealing. Therefore, we attempted to suppress the formation of ferrite and delay the diffusion of B to the surface of the steel sheet by suppressing the decarburization of the surface of the steel sheet.

[0056] (4) Decarburization during recrystallization annealing is carried out by the gas in the annealing atmosphere. Here, O ２ and H ２ O is an oxidizing gas, CO and CH ４It is known as a carburizing gas. Based on this, after thorough investigation, it was determined that the oxidizing gas (O ２ H ２ O) and carburizing gases (CO, CH) ４ It was found that by controlling the ratio of ( ) and keeping the proportion of carburizing gas above a certain level, the formation of a decarburized layer on the surface of the steel sheet during recrystallization annealing can be suppressed. Furthermore, it was discovered that suppressing the decarburized layer makes it possible to suppress the formation of BN and the unevenness of the plated surface, which were the targets.

[0057] (5) Based on the above, we found that suppressing the formation of a decarburized layer during recrystallization annealing is an important requirement for suppressing the formation of nitrides of B and obtaining a plating with a good appearance. Furthermore, we found that the amount of residual nitrides of B can be determined by performing XPS analysis after alkali stripping (stripping with an alkaline solution) of the alloyed hot-dip galvanized steel sheet after hot-dip galvanizing and alloying treatment.Therefore, we found that by controlling the ratio of decarburizing gas and carburizing gas in the atmospheric gas during recrystallization annealing, the formation of a decarburized layer on the surface of the steel sheet can be suppressed, and by controlling ammonia, surface diffusion of B and nitride formation can be suppressed, and uneven alloying between the steel sheet and molten zinc can be suppressed, resulting in a beautiful plating appearance with minimal irregularities and no mottled patterns.

[0058] Therefore, in the present invention, continuous annealing is performed under a series of optimized conditions so as to achieve the effect described in (5) above, and these optimized series of annealing conditions are an important requirement in the present invention.

[0059] Based on the above findings, the present invention provides a method for manufacturing alloyed hot-dip galvanized steel sheets, comprising: continuously annealing a cold-rolled steel sheet having the above-mentioned component composition; applying hot-dip galvanizing to the cold-rolled steel sheet; and then performing an alloying treatment, wherein in the continuous annealing, the temperature range is 300°C to 500°C, 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. ２The 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 : Contains 5% or more by volume, dew point: -55°C or higher and -40°C or lower, and O ２ H ２ O, CO, and CH ４ The respective partial pressures (Pa) of pO ２ pH ２ O, pCO, and pCH ４ However, Log(pO ２ + pH ２ O)-Log(pCO+pCH ４ In an atmosphere satisfying ≤ 3.0, the steel plate is subjected to soaking heat treatment for a holding time of 20 seconds to 300 seconds.

[0060] (Continuous Annealing) In the continuous annealing of the present invention, first, a temperature range of 300°C to 500°C is used. 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. 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).

[0061] • 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.

[0062] - Dew point of the atmosphere: Below -20°C In the low temperature range of 300 to 500°C, if the dew point exceeds -20°C, oxidation of iron occurs on the surface of the steel plate, and this iron oxide acts as a catalyst, promoting the production of ammonia in the temperature range above 500°C. For this reason, suppressing the oxidation of Fe on the surface of the steel plate is important for suppressing ammonia production, and in order to obtain a sufficient effect of suppressing the production of iron oxide, the dew point should be below -20°C. Furthermore, from the viewpoint of further suppressing ammonia production, the dew point should preferably be below -30°C. The dew point of the atmosphere here is preferably above -70°C, and more preferably above -60°C, from the viewpoint of reducing manufacturing costs.

[0063] • Oxygen concentration of the atmosphere: 500 ppm by volume or less. In order to suppress the oxidation of Fe during annealing, the oxygen concentration should be 500 ppm by volume or less, preferably 400 ppm by volume or less, and more preferably 200 ppm by volume or less. Hydrogen (H) of 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 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.

[0064] ・Average heating rate in the temperature range of 300°C to 500°C (preferred requirement) As mentioned above, the formation of ammonia, which is the nitriding source of 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 makes it possible to form Mn and B oxides on the surface of the steel sheet as early as possible in the subsequent heating temperature range of 500°C or higher, covering the surface of the steel sheet and suppressing the exposure of the pure iron layer on the surface layer 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 mentioned above, a more preferable range for the average heating rate is 2°C / s or more. Moreover, 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)".

[0065] In 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.

[0066] • 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.

[0067] - 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 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.

[0068] 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.

[0069] Average heating rate: 1°C / s or higher. The nitriding reaction due to ammonia formation 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)".

[0070] In continuous annealing, after the above heating, H is used at a temperature of 750°C to 950°C. ２The steel plate is subjected to uniform heat treatment 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. In addition, in the present invention, in the above atmosphere, O ２ H ２ O, CO, and CH ４ The respective partial pressures (Pa) of pO ２ pH ２ O, pCO, and pCH ４ However, Log(pO ２ + pH ２ O)-Log(pCO+pCH ４ This process is characterized by satisfying the condition ≤ 3.0. In this soaking 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. This suppresses the amount of nitrides formed by B.

[0071] • 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.

[0072] ・Ambient dew point: -55°C or higher and -40°C or lower. If the dew point is higher than -40°C, a large amount of oxides of Si and Mn will be formed on the steel plate surface, resulting in unplated defects caused by oxides. On the other hand, if the dew point is less than -55°C, a part of B is fixed as an oxide, and the effect of suppressing the formation of nitrides due to the diffusion of B on the steel plate surface during annealing cannot be obtained sufficiently. Also, when the oxygen potential of the annealing atmosphere is low and the nitrogen potential is high, the formation of nitrides becomes stable, and the formation of nitrides of B on the steel plate surface is promoted. As a result, the occurrence of gray dot defects and black dot defects cannot be appropriately suppressed. Therefore, 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.

[0073] ・Log(pO ２ + pH ２ O) - Log(pCO + pCH ４ ) ≤ 3.0 Oxidizing gas (O ２ , H ２ O) and carburizing gas (CO, CH ４ ) partial pressures (pO ２ (Pa), pH ２ O (Pa), pCO (Pa) and pCH ４ (Pa)) are controlled to satisfy the above formula, it becomes possible to suppress the formation of decarburized layers in the surface layer of the steel plate during recrystallization annealing. Thereby, in the surface layer of the steel plate, the formation of a ferrite layer where the diffusion of B is fast is suppressed, and the diffusion of B to the surface can be suppressed. Thereby, it becomes possible to suppress the formation of nitrides of B on the steel plate surface. On the other hand, when the ratio of the oxidizing gas becomes large and the above formula is not satisfied, the diffusion of B to the steel plate surface cannot be completely suppressed, a small amount of nitrides of B are formed on the surface, and plating unevenness due to alloying unevenness is formed. Therefore, from the viewpoint of the effect of suppressing the formation of nitrides of B, Log(pO ２ + pH ２ O) - Log(pCO + pCH ４ ) should be 3.0 or less, preferably 2.8 or less. The lower limit is not particularly limited, but Log(pO ２ + pH ２ O) - Log(pCO + pCH ４The partial pressure is preferably 0.3 or higher, and more preferably 0.6 or higher. The partial pressure can be calculated by measuring the pressure inside the annealing furnace and the concentration of the furnace gas. The ratio of oxidizing gas to carburizing gas is such that the carburizing gas (CO, CH) ４ This can be adjusted by introducing a certain component into the annealing furnace.

[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 above-described hot-dip galvanizing treatment, an alloying treatment of the zinc plating is further performed to form an alloyed hot-dip zinc plating layer. The alloying treatment is preferably performed in a temperature range of 480°C or higher. Also, the alloying treatment is preferably performed in a temperature range of 570°C or lower. When the alloying temperature is less than 480°C, the Zn-Fe alloying rate becomes excessively slow, and alloying becomes extremely difficult. On the other hand, when the alloying temperature exceeds 570°C, untransformed austenite transforms into pearlite, and the TS and 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 performed in a temperature range of 530°C or lower. The alloying treatment time is preferably 1 second or longer, and more preferably 5 seconds or longer. Also, the alloying treatment time is preferably 150 seconds or shorter, and more preferably 80 seconds or shorter.

[0078] The plating adhesion amount of the alloyed hot-dip zinc plated steel sheet (GA) is preferably 20 g / m ２ or more and 120 g / m ２ or less per side. The plating adhesion amount of the alloyed hot-dip zinc plated steel sheet (GA) is more preferably 20 g / m ２ or more and 80 g / m ２ or less per side. The plating adhesion amount 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] <Cmin / Cbase≧0.50> As described above, the present invention is characterized by suppressing the formation of a decarburized layer on the surface of the steel sheet (base steel sheet surface) in order to suppress the black mottled pattern caused by the unevenness of the plating. Here, whether or not the formation of the decarburized layer is sufficiently suppressed can be evaluated by the amount of suppression of the decarburized layer on the surface of the steel sheet (Cmin / Cbase). The amount of suppression of the decarburized layer on the surface of the steel sheet can be determined using GDS (glow discharge emission) analysis by the following method. Specifically, first, a 25 mm × 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 C intensity are measured. The excitation source for the GDS analysis was high frequency, the high frequency output was 35 W, Ar was used as the discharge gas, and the gas pressure was 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 ≥ Fe maximum intensity × 0.95 and Fe detection intensity ≥ Fe maximum intensity × 0.80. Figure 1(B) is a graph illustrating Cmin and Cbase. In this invention, the average value of the C intensity within the range satisfying Fe detection intensity ≥ Fe maximum intensity × 0.95 is defined as the C intensity of the base material (Cbase). Here, Cbase (average value of C intensity) can be determined by calculating the average value of the C analysis results shown in Figure 1(B) (total detection intensity of all data / total number of data) within the range satisfying Fe detection intensity ≥ Fe maximum 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 by GDS analysis over the entire measurement time. Furthermore, in this invention, Cmin is defined as the minimum value of C intensity in the range including the steel plate surface layer that satisfies the condition Fe detection intensity ≥ Fe maximum intensity × 0.80. Here, Cmin (minimum value of C intensity) can be determined by extracting the minimum value of C intensity at the corresponding measurement time, as shown in Figure 1(B), within the range that satisfies Fe detection intensity ≥ Fe maximum intensity × 0.80, based on the Fe analysis results shown in Figure 1(A).

[0088] If the Cmin / Cbase is 0.50 or higher, the decarburized layer on the surface of the steel sheet is sufficiently small, and the aforementioned effect of delaying B surface diffusion by suppressing ferrite formation can be fully obtained. Furthermore, from the viewpoint of improving the BN formation suppression effect, it is more preferable that the Cmin / Cbase be 0.60 or higher. On the other hand, although there is no particular upper limit, from the viewpoint of preventing hardening of the surface of the steel sheet due to excessive surface concentration of C, it is preferable that the Cmin / Cbase be 1.50 or lower, and more preferably 1.20 or lower.

[0089] <(I Ｂ +I Ｎ ) / I Ｆｅ≤0.10> The amount of nitride formation in B can be determined by taking a 10 mm × 10 mm sample from the center of the width direction of the alloyed hot-dip galvanized steel sheet, removing only the plating layer with alkali (removal with an alkaline solution), and then performing XPS (X-ray photoelectron spectroscopy) analysis on the surface. The X-ray source used for the analysis was monochromatic AlKα rays, the X-ray tube voltage was 15 kV, the pass energy was 112 eV, the emission angle was 45°, the measurement energy interval was 0.1 eV, and the spot diameter was 100 μm. For alkali removal, 8 vol% NaOH - 2 vol% triethanolamine - 1 vol% H ２ O ２ The sample was immersed in an aqueous solution. The peeling of the plating layer was visually confirmed. The semi-quantified values ​​of Fe, B, and N were obtained by XPS analysis. Ｆｅ , I Ｂ , I Ｎ In this case, the relationship between the elemental concentrations of Fe, B, and N is (I Ｂ +I Ｎ ) / I Ｆｅ When this is used as an indicator of the amount of nitride formation in B, (I Ｂ +I Ｎ ) / I Ｆｅ If ≤ 0.10, the amount of nitride formation in B is sufficiently small and a good plating appearance can be obtained. From the viewpoint of obtaining an even better plating appearance, (I Ｂ +I Ｎ ) / I Ｆｅ It is preferable that it is 0.08 or less. On the other hand, the lower limit is not particularly limited, (I Ｂ +I Ｎ ) / I Ｆｅ ) may be set to 0.00 or greater. Here, the semi-quantified value is the elemental concentration (atomic %) calculated based on the XPS intensity ratio of each element on the steel plate surface.

[0090] Also, the elemental concentration ratio of N to B is I Ｎ / I Ｂ However, I Ｎ / I Ｂ The condition ≤0.30 may also be satisfied. In this case, the main formation is not a nitride of B but an oxide of B, and a good surface appearance (plated appearance) can be obtained. From the viewpoint of obtaining an even better plated appearance, I Ｎ / I ＢIt is preferable that it is 0.25 or less. Ｎ / I Ｂ It is more preferable that it be 0.20 or less, and even more preferable that it be 0.15 or less. On the other hand, the lower limit is not particularly limited, I Ｎ / I Ｂ It may be set to 0.00 or greater.

[0091] <Surface roughness Ra: 1.20 μm or less and brightness L value: 50 or more> The surface roughness and appearance of the plated layer were evaluated by surface roughness Ra (plating surface roughness Ra) and brightness L value. The measurement sample was cut out from the center of the width direction of the alloyed hot-dip galvanized steel sheet to a size of 80 mm x 80 mm. Surface roughness Ra can be measured according to JIS B0601 (2013), and a larger value means that the difference in unevenness per unit length is large. Here, if there are no unevenness due to alloying irregularities, Ra will be 1.20 μm or less. Furthermore, from the viewpoint of obtaining an even more beautiful surface appearance (plating appearance), Ra is preferably 1.00 μm or less, and more preferably 0.80 μm or less. On the other hand, there is no particular lower limit, but if Ra is small, the steel sheet will slip and become difficult to handle, so Ra is preferably 0.20 μm or more, and more preferably 0.40 μm or more. The brightness L value can be measured according to JIS Z 8781-4 (2013), and the closer the value is to 100, the whiter the color, and the closer it is to 0, the blacker the color. Here, since depressions formed due to uneven alloying appear black, a larger L value indicates fewer depressions. From the above, when the L value is 50 or higher, there are fewer black depressions and a good plating appearance. Furthermore, from the viewpoint of obtaining an even better plating appearance, the L value is preferably 55 or higher, and more preferably 60 or higher. The upper limit is not particularly limited and may be 100, but from the viewpoint of suppressing an increase in manufacturing costs, the L value is preferably 90 or lower, and more preferably 80 or lower.

[0092] In the present invention described above, when recrystallizing annealing (soaking treatment) of a steel sheet containing B (base steel sheet), the ratio of carburizing gas to oxidizing gas in the atmosphere is kept above a certain level to suppress the formation of a decarburized layer where B diffuses rapidly. 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.

[0093] [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).

[0094] 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.

[0095] 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.

[0096] Following the procedure described above, GDS analysis was performed on the obtained alloyed hot-dip galvanized steel sheets, and the amount of suppression of the decarburization layer on the surface of the steel sheets (Cmin / Cbase) was calculated.

[0097] - Following the procedure described above, surface XPS analysis was performed on the obtained alloyed hot-dip galvanized steel sheet after plating removal to determine the elemental concentration ratio of N to B (I Ｎ / I Ｂ ) and the relationship between the elemental concentrations of B, N, and Fe (I Ｂ +I Ｎ ) / I Ｆｅ I evaluated it.

[0098] - The surface roughness Ra and lightness L value of the obtained alloyed hot-dip galvanized steel sheet were measured according to the procedure described above. Ra was measured using a stylus-type surface roughness meter. Lightness L value was measured using a colorimeter.

[0099] • 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.

[0100] As shown in Tables 2-4, the alloyed hot-dip galvanized steel sheets of the inventive examples all exhibited minimal decarburization layer formation, thereby suppressing BN formation and resulting in smoother plating with good appearance. Furthermore, 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 quality are obtained. On the other hand, the alloyed hot-dip galvanized steel sheets of the comparative examples had inferior appearance due to plating irregularities.

[0101]

[0102]

[0103]

[0104]

[0105] [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, decarburization layer suppression amount, and plating 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.

[0106]

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, the average value of the C intensity in the range satisfying Fe detection intensity ≥ maximum Fe intensity × 0.95 is defined as Cbase, and the minimum value of the C intensity in the range satisfying Fe detection intensity ≥ maximum Fe intensity × 0.80 is defined as Cmin. Cmin / Cbase ≥ 0.50, and XPS analysis is performed on the surface of the alloyed hot-dip galvanized steel sheet after the alloyed hot-dip galvanizing layer has been removed, and the semi-quantified values ​​of Fe, B, and N are I Ｆｅ , I Ｂ , I Ｎ In this case, the elemental concentration ratios of Fe, B, and N are as follows: (I Ｂ +I Ｎ ) / I Ｆｅ A hot-dip galvanized steel sheet that satisfies ≤0.10, and on the surface of the alloyed hot-dip galvanized layer, the surface roughness Ra is 1.20 μm or less, and the brightness L value is 50 or more.

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, which comprises continuously annealing a cold-rolled steel sheet having the component composition according to claim 1 or 2, then subjecting the cold-rolled steel sheet to hot-dip galvanization, and thereafter subjecting it to an alloying treatment. In the continuous annealing, the steel sheet is heated in an atmosphere having a temperature range of 300°C or higher and 500°C or lower, H ２ : containing 3% by volume or more, dew point: -20°C or lower, O ２ : 500 ppm by volume or less, and after the heating, the steel sheet is heated in an atmosphere having a temperature range of 500°C or higher and 750°C or lower, H ２ : containing 5% by volume or more, dew point: -40°C or lower, at an average heating rate of 1°C / s or more, and after the heating, at a temperature of 750°C or higher and 950°C or lower, H ２ : containing 5% by volume or more, dew point: -55°C or higher and -40°C or lower, and O ２ , H ２ O, CO and CH ４ The partial pressures (Pa) of each of them, pO ２ , pH ２ O, pCO and pCH ４ Satisfy Log(pO ２ + pH ２ O) - Log(pCO + pCH ４ ) ≦ 3.0, and the steel sheet is soaked for a holding time of 20 seconds or more and 300 seconds or less.

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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