Galvannealed steel sheet and its manufacturing method

Abstract

To obtain a high-strength galvannealed steel sheet having a good surface appearance with suppressed formation of irregularities on the plated surface when a steel containing B is used as a steel sheet (base steel sheet). The steel sheet contains a specific amount of B in addition to C, Si, Mn, P, S, sol. Al, and N, and the Cmin / Cbase obtained by GDS analysis in the thickness direction from the surface of the galvannealed layer is 0.50, XPS analysis was performed on the steel sheet surface after the galvannealed layer was removed. The semi-quantitative values ​​of Fe, B, and N are I Fe , I B , I N When this is done, the element concentration ratios of Fe, B, and N are as follows: (I B +I N ) / I Fe ≦0.10 and the surface of the galvannealed layer has a surface roughness Ra of 1.20 μm or less and a lightness L value of 50 or more.

Description

[Technical Field]

[0001] The present invention relates to a high-strength galvannealed steel sheet having a good surface appearance (coating appearance) and a method for producing the same. [Background technology]

[0002] With the aim of reducing CO2 emissions by reducing the weight of vehicles and improving crashworthiness by increasing the strength of the body, efforts are underway to make thin steel sheets for automobiles while also increasing their strength. For example, in order to increase the strength of the body, there are increasing cases of high-strength steel sheets with a tensile strength (TS) of 590 MPa or higher being used in the main structural parts that form the framework of the automobile cabin. A commonly used method for increasing the strength of steel is to add hardening elements such as C, Mn, B, Cr, and Mo. Among these, B has the advantage of being able to significantly improve hardenability even in small amounts, allowing steel to be strengthened at low cost, and it is characterized by the fact that it hardly produces inclusions that degrade bendability and delayed fracture resistance. For these reasons, B is widely used as an additive element in high-strength steel plates.

[0003] With regard to high-strength hot-dip galvanized steel sheets in which B is added to the base steel sheet, for example, Patent Document 1 discloses a technology in which, when continuous annealing and hot-dip galvanizing are performed on a base steel sheet of a predetermined chemical composition, the temperature in the annealing furnace during continuous annealing is set to a temperature range of 750°C or higher and the dew point of the atmosphere is set to -40°C or lower, thereby lowering the oxygen potential at the interface between the steel sheet and the atmosphere, preventing the formation of internal oxidation, suppressing surface segregation of Si, Mn, etc., and achieving an excellent coating appearance. Furthermore, Patent Document 2 discloses a technology in which the ratio of the amount of concentrated Si to the amount of concentrated Mn in the surface layer of a base steel sheet is set to 0.7 or more and 1.3 or less, and when annealing the cold-rolled steel sheet, the cold-rolled steel sheet is heated to the maximum temperature reached, and the dew point of the atmosphere in that region is maintained at -40°C or less, thereby achieving excellent galvanizability. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Japanese Patent Application Laid-Open No. 2010-255100 [Patent Document 2] International Publication No. 2020 / 170542 Summary of the Invention [Problem to be solved by the invention]

[0005] From the investigations of the present inventors, it has been found that the hot-dip galvanized steel sheet described in Patent Document 1 has minute surface defects that are different from the "surface defects caused by reduced plating wettability due to the generation of Si, Mn-based oxides" that is the subject of Patent Document 1, i.e., defects in which Si, Mn-based oxides repel the plating, resulting in areas where the plating does not adhere, and that there is a new challenge of suppressing the generation of these surface defects. It was also found that the hot-dip galvanized steel sheet described in Patent Document 2, particularly steel containing B, frequently suffers from similar surface defects caused by peeling of the coating. Furthermore, conventional hot-dip galvanized steel sheets sometimes have black mottled patterns due to unevenness on the plated surface, and it has not been possible to sufficiently suppress the occurrence of black mottled patterns on the plated surface. In other words, there has been a demand for improving the outer surface, including suppressing the occurrence of black mottled patterns on the plated surface.

[0006] The present invention has been made to solve the above-mentioned newly discovered problems, and its object is to provide a high-strength galvannealed steel sheet in which the steel sheet (base steel sheet) contains B and has a good surface appearance (coating appearance) in which the formation of irregularities on the coating surface is suppressed, and a method for producing the same.

[0007] In the present invention, high strength refers to a tensile strength TS of 590 MPa or more obtained by carrying out a tensile test in accordance with JIS Z2241 (2022). In addition, a good surface appearance (plating appearance) means that the surface roughness Ra measured in accordance with JIS B0601 (2013) is 1.20 μm or less, and the brightness L value measured in accordance with JIS Z8781-4 (2013) is 50 or more. [Means for solving the problem]

[0008] Through extensive research, the present inventors have discovered that the above-mentioned surface defects are caused by the formation of nitrides of B, and have found that by suppressing the formation of nitrides, minute surface defects can be improved. Furthermore, it was found that in order to suppress the formation of nitrides, it is effective to lower the dew point in the temperature range of 300 to 500°C in the annealing process before plating to create a reducing atmosphere of Fe, thereby suppressing the formation of iron oxide and thereby suppressing the generation of ammonia; and to quickly heat the steel sheet to a temperature range of 500°C or above, at which point ammonia generation becomes significant and radical nitrogen begins to penetrate the steel sheet, thereby suppressing the generation of ammonia and nitriding the steel sheet.Furthermore, it was found that shortening the annealing time in the temperature range of 750°C or above and setting the dew point to -55°C or above will fix some of the B as an oxide and thereby suppress the formation of nitrides due to the diffusion of B to the steel sheet surface during annealing. However, although the above-mentioned method was able to improve minute surface defects where the coating did not adhere by suppressing the formation of nitrides of B, the problem of black mottled patterns occurring on the coating surface remained, and it was not yet sufficient in terms of improving the surface appearance. This black mottled pattern deteriorates the appearance of the hot-dip galvanized steel sheet, even though the coating adheres, so it is preferable to suppress it. It was also found that this black mottled pattern is caused by unevenness on the coating surface, and that the color tone of the convex and concave parts changes when only the convex parts come into contact with the rolls during temper rolling, resulting in the concave parts turning black.

[0009] Furthermore, the present inventors have conducted extensive research to solve the above problems and have come to the following findings. (1) As mentioned above, it was found that the method of improving the annealing conditions described above did not completely suppress the formation of nitrides of B. Furthermore, it was found that the unevenness of the coating was caused by the remaining nitrides of B inhibiting the alloying reaction between the base steel sheet and molten zinc, resulting in uneven alloying. (2) B nitrides are formed when radical nitrogen (N) generated from ammonia in the atmosphere adsorbs or penetrates into the steel sheet, mainly during the heating process, and then reacts with B in the steel during heating or soaking in an atmosphere with a low oxygen potential. Previous studies have attempted to suppress the formation of B nitrides by suppressing the generation of ammonia, but this was not effective enough to suppress unevenness in the coating. Therefore, in order to further suppress the formation of B nitrides, the inventors attempted to suppress the surface diffusion of B in the steel in addition to controlling ammonia.

[0010] (3) As a method for suppressing the surface diffusion of B, we investigated slowing the diffusion of B. It has been reported that the diffusion rate of B in austenite is 1 / 5 times faster than in ferrite. Therefore, it is thought that the diffusion of B can be suppressed by reducing the ferrite layer in the steel sheet surface during recrystallization annealing (soaking treatment). Furthermore, in high-strength steel sheets, ferrite forms in the surface layer due to decarburization during recrystallization annealing. Therefore, we attempted to suppress the formation of ferrite and delay the diffusion of B to the steel sheet surface by suppressing decarburization in the steel sheet surface.

[0011] (4) Decarburization during recrystallization annealing is promoted by the gases in the annealing atmosphere. Here, O2 and H2O are known as oxidizing gases, and CO and CH4 are known as carburizing gases. Based on this, we conducted extensive research and found that by controlling the ratio of oxidizing gases (O2, H2O) to carburizing gases (CO, CH4) and maintaining a certain proportion of carburizing gases, it is possible to suppress the formation of a decarburized layer on the steel sheet surface during recrystallization annealing. We also found that suppressing the decarburized layer can suppress the formation of B nitrides and the unevenness of the coating surface, which were the targets. In this way, we found that suppressing the surface diffusion of B and the formation of nitrides and suppressing uneven alloying between the steel sheet and molten zinc can achieve a beautiful coating appearance with minimal unevenness and no black spots.

[0012] The present invention has been made based on the above findings and has the following gist. [1] A galvannealed steel sheet having a steel sheet and a galvannealed layer on the steel sheet, The composition of the steel plate 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 Regarding [%Mn], which is the Mn content (mass%) of the steel plate, and [%Si], which is the Si content (mass%), [%Mn] / [%Si] is 2.50 or more, Additionally, as an optional ingredient, 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 Contains one or more selected from the balance being Fe and unavoidable impurities; GDS analysis is performed from the surface of the galvannealed layer in the thickness direction, Fe detection intensity ≥ Maximum Fe intensity × 0.95 The average value of the C intensity in the range that satisfies the above is defined as Cbase. Fe detection intensity ≥ Maximum Fe intensity × 0.80 When the minimum value of C intensity in the range that satisfies the above is Cmin, Cmin / Cbase≧0.50, XPS analysis is performed on the surface of the galvannealed steel sheet after the galvannealed layer has been peeled off, The semi-quantitative values ​​of Fe, B, and N are I Fe , I B , I N When this is done, the element concentration ratios of Fe, B, and N are as follows: (I B +I N ) / I Fe Satisfies ≦0.10, The galvannealed steel sheet has a surface roughness Ra of 1.20 μm or less and a lightness L value of 50 or more on the surface of the galvannealed layer. [2] The galvannealed steel sheet according to [1], wherein the composition of the steel sheet is such that [%Mn] / [%Si] is 12.00 or more. [3] A method for producing a galvannealed steel sheet, comprising the steps of: continuously annealing a cold-rolled steel sheet having the component composition according to [1] or [2]; galvanizing the cold-rolled steel sheet; and then alloying the cold-rolled steel sheet. In the continuous annealing, The steel sheet is heated in a temperature range of 300°C or higher and 500°C or lower in an atmosphere containing 3% or more by volume of H2, a dew point of -20°C or lower, and an O2 content of 500 ppm by volume or lower, After the heating, the steel sheet is heated in a temperature range of 500°C or higher and 750°C or lower in an atmosphere containing 5% by volume or more of H2 and having a dew point of -40°C or lower at an average heating rate of 1°C / s or higher, After the heating, the steel sheet is soaked at a temperature of 750°C to 950°C in an atmosphere containing 5% or more by volume of H2, with a dew point of -55°C to -40°C inclusive, and where the partial pressures (Pa) of O2, H2O, CO and CH4, respectively, of pO2, pH2O, pCO and pCH4, satisfy the relationship Log(pO2 + pH2O) - Log(pCO + pCH4) ≦ 3.0, for a holding time of 20 seconds to 300 seconds. [4] The method for producing a galvannealed steel sheet according to the above [3], wherein the ammonia concentration in the atmosphere for the soaking treatment is 0.010% by volume or less. [Effects of the Invention]

[0013] According to the present invention, it is possible to produce a high-strength galvannealed steel sheet having a good surface appearance (coating appearance) with reduced generation of unevenness on the coating surface, in which the steel sheet (base steel sheet) contains B. The high-strength galvannealed steel sheet produced by the present invention is suitable for structural members such as automobile parts, and by applying it to this application, it is possible to reduce the weight of the vehicle body and thereby improve fuel efficiency. [Brief explanation of the drawings]

[0014] [Figure 1] Figure 1(A) is a graph illustrating the Fe detection intensity obtained by GDS analysis, where the Fe detection intensity is greater than or equal to the maximum Fe intensity × 0.95 and the Fe detection intensity is greater than or equal to the maximum Fe intensity × 0.80. Figure 1(B) is a graph illustrating Cmin and Cbase. DETAILED DESCRIPTION OF THE INVENTION

[0015] [Galvannealed steel sheet] The galvannealed steel sheet of the present invention is a galvannealed steel sheet having a steel sheet and a galvannealed layer on the steel sheet, the steel sheet having a chemical composition containing, 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, and B: 0.0001% or more and 0.0050% or less, and wherein [%Mn] is the Mn content (mass%) of the steel sheet and [%Si] is the Si content (mass%) of the steel sheet, the ratio [%Mn] / [%Si] is 2.50 or more, and further contains, as optional components, Cr: 1.00% or less, Ti: 0.2 00% 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 Bottom, 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 , containing one or more selected from 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; GDS analysis is performed from the surface of the galvannealed layer in the thickness direction, and the average value of the C intensity in the range where the detected Fe intensity is greater than or equal to the maximum Fe intensity x 0.95 is defined as Cbase. When Cmin is the minimum C intensity in the range that satisfies the Fe detection intensity ≥ maximum Fe intensity × 0.80, Cmin / Cbase≧0.50, XPS analysis was performed on the surface of the galvannealed steel sheet after the galvannealed layer was removed, and the semi-quantitative values ​​of Fe, B, and N were calculated as I Fe , I B , I N When this is done, the element concentration ratios of Fe, B, and N are as follows: (I B +I N ) / I Fe Satisfies ≦0.10, The surface of the galvannealed layer has a surface roughness Ra of 1.20 μm or less and a lightness L value of 50 or more.

[0016] The object of the present invention to be produced is a high-strength galvannealed steel sheet having a hot-dip galvanized layer on one or both sides of a steel sheet (base steel sheet) and being subjected to an alloying treatment after hot-dip galvanizing. Here, the composition of the hot-dip galvannealed layer to be alloyed (hereinafter referred to as the galvannealed layer) is not particularly limited, and may be a common one. For example, the galvannealed layer may have a composition containing 20 mass% or less of Fe, 0.001 mass% to 1.0 mass% of Al, and 0 mass% to 3.5 mass% in total of one or more elements selected from Pb, Sb, Si, Sn, Mg, Mn, Ni, Cr, Co, Ca, Cu, Li, Ti, Be, Bi, and REM, with the remainder being Zn and unavoidable impurities. The Fe content in the coating layer of the galvannealed steel sheet (GA) is preferably 7% by mass or more, more preferably 8% by mass or more. The Fe content in the coating layer of the galvannealed steel sheet (GA) is preferably 15% by mass or less, more preferably 13% by mass or less.

[0017] <Component composition> The chemical composition of the steel sheet (base steel sheet) and the reasons for its limitations will be explained below. In the following explanation, "%" representing the content of the component elements of the steel sheet means "mass%" unless otherwise specified. Furthermore, tensile strength is referred to as TS.

[0018] ·C: 0.050% or more and 0.300% or less C is an effective element for generating the desired amount of quenched martensite and tempered martensite to achieve a TS of 590 MPa or more and obtain excellent dimensional accuracy during forming. If the C content is less than 0.050%, the area fraction of quenched martensite decreases and the area fractions of ferrite and bainite increase, making it difficult to achieve a TS of 590 MPa or more. On the other hand, if the C content exceeds 0.300%, the carbon concentration in quenched martensite and tempered martensite increases, increasing their hardness. As a result, the difference in hardness between the soft phases ferrite and bainite and the hard phases quenched martensite and tempered martensite increases, resulting in poor punchability, stretch flangeability, and bendability. Therefore, the C content is set to 0.050% or more and 0.300% or less. Furthermore, the C content is preferably set to 0.060% or more to ensure a TS of 780 MPa or more, and more preferably set to 0.090% or more to ensure a TS of 980 MPa or more. Since a high C content degrades weldability, the C content is preferably set to 0.250% or less, more preferably 0.220% or less.

[0019] ·Si: 1.20% or less Si is an element that is effective in strengthening steel to obtain good material properties, and is also an element that is effective in improving ductility. On the other hand, if the Si content exceeds 1.20%, the amount of Si concentrated on the steel sheet surface during annealing increases, and Si oxides that cause unplated defects are formed on the steel sheet surface, making it difficult to obtain a good surface appearance. By setting the Si content to 1.20% or less, the formation of Si-based composite oxides is suppressed, making it possible to effectively utilize the Mn in the steel sheet as Mn-B composite oxides, thereby suppressing gray spot defects and black spot defects. Therefore, the Si content is set to 1.20% or less. From the above-mentioned perspectives, the Si content is preferably 0.80% or less, and more preferably 0.55% or less. There is no particular lower limit for the Si content. That is, the Si content may be 0%, but reducing it to less than 0.01% increases refining costs, so the Si content is preferably 0.01% or more. 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. From the viewpoint of obtaining particularly high ductility, the Si content is even more preferably 0.15% or more.

[0020] ·Mn: 2.00% or more and 3.50% or less Mn is an element necessary for suppressing gray and black spot defects and achieving a good surface appearance. By including 2.00% or more Mn, Mn-based composite oxides that have little adverse effect on the coating appearance quality are formed, making it possible to suppress the formation of B nitrides, thereby suppressing gray and black spot defects. Mn is also an effective element for generating the desired amount of quenched martensite and tempered martensite, achieving a TS of 590 MPa or more. If the Mn content is less than 2.00%, the amount of B that forms composite oxides with Mn during annealing decreases, increasing the amount of BN formed, making it impossible to sufficiently suppress black and gray spot defects. On the other hand, if the Mn content exceeds 3.50%, the area ratio of tempered martensite increases and the area ratios of ferrite and bainite decrease, resulting in reduced dimensional accuracy during forming. Furthermore, the amount of Mn enriched on the steel sheet surface during annealing increases, resulting in the formation of a large amount of Mn oxide, which causes unplated defects, on the steel sheet surface, making it difficult to obtain a good surface appearance. Therefore, the Mn content is set to 2.00% or more and 3.50% or less. Furthermore, from the viewpoint of suppressing the occurrence of gray spot defects and black spot defects, the Mn content is preferably set to 2.30% or more, more preferably 2.50% or more, and even more preferably 2.60% or more. Furthermore, from the above viewpoints, the Mn content is preferably set to 3.30% or less, more preferably 3.00% or less.

[0021] ·P:0.100% or less P has a solid solution strengthening effect and is an element that increases the strength of steel sheet. However, if the P content exceeds 0.100%, P segregates at prior austenite grain boundaries, embrittling the grain boundaries and reducing punchability and stretch flangeability. Therefore, the P content is set to 0.100% or less. From the above-mentioned viewpoints, the P content is preferably set to 0.050% or less, and more preferably set to 0.030% or less. There is no lower limit for the P content. That is, the P content may be 0%, but since controlling the P content to less than 0.001% increases refining costs, the P content is preferably 0.001% or more. The P content is more preferably 0.003% or more, and even more preferably 0.005% or more.

[0022] ·S:0.0100% or less S exists as sulfide in steel, and if the S content exceeds 0.0100%, it reduces the ultimate deformability of the steel sheet, resulting in reduced punchability, stretch flangeability, and bendability. Therefore, the S content is set to 0.0100% or less. The lower limit of the S content is not particularly specified. That is, the S content may be 0%, but since controlling the S content to less than 0.0001% increases refining costs, the S content is preferably 0.0001% or more. From the above-mentioned viewpoint, the S content is preferably 0.0050% or less. The S content is more preferably 0.0040% or less, and further preferably 0.0030% or less.

[0023] ·sol.Al: 1.00% or less Al can be used as a deoxidizer. In this case, the content of sol. Al in the steel is preferably 0.01% or more. Furthermore, in steel sheets to which B is added, the addition of Al fixes the N in the steel as AlN, making it possible to utilize the added B as solid-solute B, which is effective in increasing strength. Furthermore, Al has the effect of suppressing the formation of carbides 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%, bare spots occur, so the sol.Al content is set to 1.00% or less. From the above-mentioned viewpoint, the sol.Al content is preferably set to 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, resulting in reduced punchability, stretch flangeability, and bendability. Therefore, the N content is set to 0.0200% or less. From the above-mentioned viewpoint, the N content is preferably set to 0.0080% or less. There is no lower limit for the N content. That is, the N content may be 0%, but controlling it to less than 0.0005% increases refining costs, and therefore, in view of constraints on production technology, the N content is preferably 0.0005% or more. The N content is more preferably 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. Adding B to steel can suppress the formation and grain growth of ferrite during annealing and cooling. To achieve this effect, the B content must be 0.0001% or more. On the other hand, if the B content exceeds 0.0050%, a large amount of nitrides is formed on the steel sheet surface, which deteriorates the plating adhesion and causes poor appearance due to plating peeling. Therefore, the B content is set to 0.0001% or more and 0.0050% or less. From the above-mentioned viewpoint, the B content is preferably set to 0.0002% or more. Similarly, from the above-mentioned viewpoint, the B content is preferably set to 0.0030% or less.

[0026] ·[%Mn] / [%Si]:2.50 or more [%Mn] is the Mn content (mass%) of the steel sheet, and [%Si] is the Si content (mass%) of the steel sheet. By setting the [%Mn] / [%Si] ratio to 2.50 or more, the formation of oxides of simple Si is suppressed, suppressing the occurrence of bare spots. Furthermore, the annealing method of the present invention allows the formation of Mn and B composite oxides, which further suppresses the occurrence of bare spots. Therefore, the [%Mn] / [%Si] ratio is set to 2.50 or more. Furthermore, from the viewpoint of suppressing bare spots, the [%Mn] / [%Si] ratio is preferably 4.00 or more, more preferably 6.00 or more, and even more preferably 12.00 or more. In particular, by making the [%Mn] / [%Si] ratio 12.00 or more, the formation of Mn,B composite oxides is promoted, and the formation of B nitrides, which cause bare spots in the annealing method of the present invention, can be more effectively suppressed. Furthermore, by making the [%Mn] / [%Si] ratio 14.00 or more, the formation of Mn,B composite oxides is particularly promoted, and minute unevenness, which is a sign of bare spots, can also be suppressed. Therefore, it is even more preferable to make the [%Mn] / [%Si] ratio 14.00 or more. Although there is no particular upper limit, [%Mn] / [%Si] may be set to 200.00 or less, or may be set to 150.00 or less.

[0027] ·Optional addition elements In addition to the above-mentioned chemical composition, the steel sheet used in the present invention may further contain, as optional components, in mass%, 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, It may contain one or more elements selected from 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 element that is added as needed, and the effects of the present invention can be obtained even if the content of each of the above elements 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 improves hardenability, and is an effective element for generating a desired amount of quenched martensite or tempered martensite, making the TS 590 MPa or more, and obtaining excellent dimensional accuracy during forming. However, if the Cr content exceeds 1.00%, the coating appearance quality deteriorates, the area ratio of quenched martensite and tempered martensite increases, and the area ratio of ferrite and bainite decreases, resulting in a decrease in dimensional accuracy during forming. Therefore, if Cr is contained, the Cr content is set to 1.00% or less. From the viewpoint of improving the coating appearance quality, the Cr content is preferably set to 0.75% or less. Furthermore, in order to obtain the effect of Cr in improving hardenability, the Cr content is preferably set to 0.02% or more. The Cr content is more preferably set to 0.03% or more, and even more preferably set to 0.04% or more.

[0029] ·Ti: 0.200% or less Ti increases the 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 formed. If the steel sheet contains diffusible hydrogen, these precipitates and inclusions become the starting points for cracks during bending tests, resulting in reduced bendability. Therefore, if Ti is contained, the Ti content is set to 0.200% or less. From the above-mentioned viewpoints, the Ti content is preferably set to 0.100% or less. To obtain the above-mentioned effects, the Ti content is preferably set to 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 formed, and if the steel sheet contains diffusible hydrogen, these precipitates and inclusions become the starting points for cracks during bending tests, resulting in reduced bendability. Therefore, when Nb is contained, the Nb content is set to 0.200% or less. From the above-mentioned viewpoints, the Nb content is preferably set to 0.100% or less. Furthermore, to obtain the above-mentioned effects, the Nb content is preferably set to 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 formed, and if the steel sheet contains diffusible hydrogen, these precipitates and inclusions become the starting points for cracks during bending tests, resulting in reduced bendability. Therefore, when V is contained, the V content is set to 0.200% or less. From the above-mentioned viewpoints, the V content is preferably set to 0.100% or less. Furthermore, to obtain the above-mentioned effects, the V content is preferably set to 0.005% or more, and more preferably 0.010% or more.

[0032] Mo: 2.000% or less Mo is an element that increases hardenability and is effective in keeping the area ratio of quenched martensite and tempered martensite within a more suitable range, thereby increasing TS and improving dimensional accuracy during forming. 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 more, and the dimensional accuracy during forming decreases. Furthermore, the number of coarse precipitates and inclusions increases, and if the steel sheet contains diffusible hydrogen, these precipitates and inclusions become the starting point for cracks during bending tests, resulting in decreased bendability. Therefore, if Mo is contained, the Mo content is set to 2.000% or less. Furthermore, from the above-mentioned viewpoint, the Mo content is preferably set to 0.500% or less. The Mo content is more preferably set to 0.400% or less, and even more preferably set to 0.250% or less. In order to obtain the above effects, the Mo content is preferably 0.005% or more, more preferably 0.020% or more, 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 increases hardenability and is effective in keeping the area ratio of quenched martensite and tempered martensite within a more suitable range, thereby increasing TS and improving dimensional accuracy during forming. 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 excellent dimensional accuracy during forming. Furthermore, the number of coarse precipitates and inclusions increases. If diffusible hydrogen is contained in the steel sheet, these precipitates and inclusions become the starting point for cracks during bending tests, resulting in reduced bendability. Therefore, when Cu is contained, the Cu content is set to 1.000% or less. From the above perspectives, the Cu content is preferably set to 0.200% or less. To achieve the above effects, the Cu content is preferably set to 0.005% or more, more preferably 0.020% or more. The Cu content is more preferably set to 0.040% or more, and even more preferably 0.060% or more.

[0034] ·Ni: 0.500% or less Ni is an element that increases hardenability and is effective in keeping the area ratio of quenched martensite and tempered martensite within a more suitable range, thereby increasing TS and improving dimensional accuracy during forming. However, if the Ni content exceeds 0.500%, the area ratio of quenched martensite and tempered martensite increases, resulting in a decrease in TS and dimensional accuracy during forming. Furthermore, the number of coarse precipitates and inclusions increases. If diffusible hydrogen is contained in the steel sheet, these precipitates and inclusions become the starting point for cracks during bending tests, resulting in a decrease in bendability. Therefore, when Ni is contained, the Ni content is set to 0.500% or less. From the above viewpoints, the Ni content is preferably set to 0.200% or less. To achieve the above effects, the Ni content is preferably set to 0.005% or more, and more preferably 0.020% or more.

[0035] ·Sn: 0.200% or less Sn is an element that is effective in suppressing oxidation of the surface of the base steel sheet during annealing and in obtaining better plating properties. However, if the Sn content exceeds 0.200%, the amount of coarse precipitates and inclusions increases, and if diffusible hydrogen is contained in the base steel sheet, the precipitates and inclusions become the starting points for cracks during bending tests, resulting in reduced bendability. Therefore, when Sn is contained, the Sn content is set to 0.200% or less. From the above-mentioned viewpoints, the Sn content is preferably set to 0.050% or less. The Sn content is more preferably set to 0.040% or less, and even more preferably set to 0.025% or less. In order to obtain the above-mentioned effects, the Sn content is preferably set to 0.001% or more, and more preferably set to 0.005% or more.

[0036] ·Sb:0.200% or less Sb is an element known as a nitriding inhibitor. Adding Sb can suppress the adsorption or penetration of radical nitrogen into steel sheets in the temperature range of 500°C to 750°C, and can also suppress the formation of B nitrides, which are a cause of gray and black spot defects. However, if the Sb content exceeds 0.200%, slab cracking occurs during hot rolling. Therefore, when Sb is contained, the Sb content is set to 0.200% or less. From the above-mentioned viewpoints, the Sb content is preferably set to 0.100% or less. The Sb content is more preferably set to 0.080% or less, and even more preferably set to 0.050% or less. In order to obtain the above-mentioned effects, the Sb content is preferably set to 0.005% or more, and more preferably set to 0.007% or more.

[0037] ·Mg:0.0100% or less Mg is an element that is effective in spheroidizing the shape of inclusions such as sulfides and oxides, improving the ultimate deformability of the steel sheet, and improving stretch flangeability. However, if the Mg content exceeds 0.0100%, a large amount of coarse precipitates and inclusions are formed. If the steel sheet contains diffusible hydrogen, these precipitates and inclusions can become crack initiation sites during bending tests, resulting in reduced bendability. Therefore, when Mg is contained, the Mg content is set to 0.0100% or less. From the above-mentioned viewpoints, the Mg content is preferably set to 0.0050% or less. The Mg content is more preferably set to 0.0040% or less, and even more preferably set to 0.0025% or less. To achieve the above-mentioned effects, the Mg content is preferably set to 0.0001% or more, and more preferably set to 0.0005% or more.

[0038] ·Ca:0.0100% or less Ca exists as inclusions in the base steel sheet. If the Ca content exceeds 0.0100%, and if the base steel sheet contains diffusible hydrogen, the inclusions become the starting points for cracks during a bending test, resulting in a decrease in bendability. Therefore, if Ca is contained, the Ca content is set to 0.0100% or less. From the above-mentioned 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 constraints on production technology, the Ca content is preferably 0.0001% or more, more preferably 0.0002% or more, and even more preferably 0.0003% or more.

[0039] ·Zn: 0.100% or less Zn is an element that is effective in improving stretch flangeability by making the shape of inclusions spheroidal 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 formed, and if the steel sheet contains diffusible hydrogen, these precipitates and inclusions become the starting points for cracks during bending tests, resulting in reduced bendability. Therefore, if Zn is contained, the Zn content is set to 0.100% or less. Furthermore, from the above-mentioned viewpoint, the Zn content is preferably set to 0.020% or less, and more preferably 0.010% or less. In order to obtain the above effects, the Zn content is preferably 0.001% or more, more preferably 0.002% or more, more preferably 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 stretch flangeability by spheroidizing the shape of inclusions and improving the ultimate deformability of steel sheets. However, if the Co content exceeds 0.200%, a large amount of coarse precipitates and inclusions are formed. If the steel sheet contains diffusible hydrogen, these precipitates and inclusions can become crack initiation points during bending tests, resulting in reduced bendability. Therefore, if Co is contained, the Co content is set to 0.200% or less. From the above-mentioned perspectives, the Co content is preferably set to 0.010% or less. To achieve the above-mentioned effects, the Co content is preferably set to 0.001% or more, and more preferably 0.005% or more.

[0041] ·Zr: 0.200% or less Zr is also an effective element for improving stretch flangeability by spheroidizing the shape of inclusions and improving the ultimate deformability of steel sheets. However, if the Zr content exceeds 0.200%, a large amount of coarse precipitates and inclusions are formed. If the steel sheet contains diffusible hydrogen, these precipitates and inclusions can become crack initiation sites during bending tests, resulting in reduced bendability. Therefore, if Zr is contained, the Zr content is set to 0.200% or less. From the above perspective, the Zr content is preferably set to 0.050% or less, and more preferably set to 0.010% or less. To achieve the above effects, the Zr content is preferably set to 0.001% or more, and more preferably set to 0.005% or more.

[0042] ·Ta: 0.10% or less Ta is an element effective in increasing the strength of the base steel sheet, and can be contained as needed. Ta has the effect of improving strength when contained in an amount of 0.005% or more, so the Ta content is preferably 0.005% or more. The Ta content is more preferably 0.02% or more, and even more preferably 0.05% or more. On the other hand, in order to prevent an increase in costs, if Ta is contained, the Ta content is set to 0.10% or less.

[0043] ·Te: 0.10% or less Since the Te content of 0.001% or more can control the morphology of sulfides and improve ductility and toughness, the Te content is preferably 0.001% or more, more preferably 0.01% or more, and even more preferably 0.03% or more. On the other hand, in order to prevent an increase in costs, when Te is contained, the content of Te is set to 0.10% or less.

[0044] ·As: 0.10% or less Since the morphology of sulfides can be controlled and ductility and toughness can be improved by including 0.001% or more of As, the As content is preferably 0.001% or more, more preferably 0.02% or more, and even more preferably 0.04% or more. On the other hand, in order to prevent an increase in costs, when As is contained, the As content is set to 0.10% or less.

[0045] ·Hf:0.10% or less Since the Hf content of 0.01% or more can control the morphology of sulfides and improve ductility and toughness, the Hf content is preferably 0.01% or more, more preferably 0.02% or more, and even more preferably 0.04% or more. On the other hand, in order to prevent an increase in costs, if Hf is contained, the Hf content is set to 0.10% or less.

[0046] ·Bi:0.20% or less When Bi is contained in an amount of 0.001% or more, grain boundary segregation can be suppressed and ductility and toughness can be improved. Bi also has the effect of improving machinability and smoothness of cut edges, and has the effect of improving delayed fracture resistance of cut edges. For these reasons, the Bi content is preferably 0.001% or more. The Bi content is more preferably 0.02% or more, and even more preferably 0.04% or more. On the other hand, when Bi is contained, the Bi content is set to 0.20% or less, preferably 0.10% or less, from the viewpoint of preventing an increase in costs.

[0047] ·Pb: 0.20% or less When Pb is contained in an amount of 0.001% or more, grain boundary segregation can be suppressed, and ductility and toughness can be improved. Pb also has the effect of improving machinability and smoothness of cut edges, and has the effect of improving delayed fracture resistance of cut edges. For these reasons, the Pb content is preferably 0.001% or more. The Pb content is more preferably 0.02% or more, and even more preferably 0.04% or more. On the other hand, in order to prevent an increase in costs, if Pb is contained, the Pb content is set to 0.20% or less, and preferably 0.10% or less.

[0048] ·Ge: 0.10% or less Since Ge does not significantly affect mechanical properties or surface quality even if it is contained in an amount of 0.001% or more, the Ge content may be set to 0.001% or more, more preferably 0.01% or more, and even more preferably 0.02% or more. On the other hand, in order to prevent an increase in costs, when Ge is contained, the Ge content is set to 0.10% or less.

[0049] ·Sr: 0.10% or less Even if the Sr content is 0.001% or more, it does not have a significant effect on the mechanical properties or surface quality, so the Sr content may be 0.001% or more, more preferably 0.01% or more, and even more preferably 0.02% or more. On the other hand, in order to prevent an increase in costs, if Sr is contained, the Sr content is set to 0.10% or less.

[0050] ·Cs: 0.10% or less Since Cs does not significantly affect mechanical properties or surface quality even if it is contained at 0.001% or more, the Cs content may be set to 0.001% or more, more preferably 0.01% or more, and even more preferably 0.03% or more. On the other hand, in order to prevent an increase in costs, if Cs is contained, the Cs content is set to 0.10% or less.

[0051] ·REM: 0.0100% or less REM is an element that effectively improves stretch flangeability by spheroidizing the shape of inclusions and improving the ultimate deformability of steel sheets. However, if the total REM content exceeds 0.0100%, a large amount of coarse precipitates and inclusions are formed. If the steel sheet contains diffusible hydrogen, these precipitates and inclusions become the starting points for cracks during bending tests, resulting in reduced bendability. Therefore, if REM is contained, its total content should be 0.0100% or less. Furthermore, from the above-mentioned perspective, it is preferable that the total REM content be 0.0080% or less. To obtain the above effects, the total REM content is preferably 0.0001% or more, more preferably 0.0005% or more, more preferably 0.0010% or more, and even more preferably 0.0015% or more. In the present invention, REM refers to scandium (Sc) with atomic number 21, yttrium (Y) with atomic number 39, and the lanthanoid elements from lanthanum (La) with atomic number 57 to lutetium (Lu) with atomic number 71. The REM content in the present invention refers to the total content of one or more elements selected from the above-mentioned REM. The REM is not particularly limited, but is preferably La and / or Ce.

[0052] The balance other than the above components is Fe and unavoidable impurities.

[0053] [Manufacturing method of galvannealed steel sheet] Next, the manufacturing conditions of the method for manufacturing the galvannealed steel sheet of the present invention will be described. In the production method of the present invention, a steel sheet (cold-rolled steel sheet) having the above-described component composition is introduced, for example, into continuous hot-dip galvanizing equipment, and then subjected to continuous annealing in the equipment, followed by hot-dip galvanizing and further alloying treatment to obtain a galvannealed steel sheet. In general, continuous hot-dip galvanizing equipment is composed of an annealing furnace and a hot-dip galvanizing device located downstream of the annealing furnace. The hot-dip galvanizing device includes a hot-dip galvanizing bath and a snout connected to the steel strip outlet side of the annealing furnace, with its tip immersed in the hot-dip galvanizing bath. A typical continuous hot-dip galvanizing line (CGL) configured to continuously perform a series of processes including heating, cooling, hot-dip galvanizing, and hot-dip galvanizing alloying can be used as such continuous hot-dip galvanizing equipment. Specific annealing conditions are as follows. The number of annealing steps is not particularly limited, but from the viewpoint of production costs, a single annealing step (single-step annealing method) is preferred.

[0054] The occurrence of a black mottled surface appearance due to plating irregularities, which is a 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, and has not been recognized as a problem in the past, but is a problem newly discovered by the present inventors. As mentioned above, the present inventors conducted extensive research to produce a galvannealed steel sheet in which the occurrence of such surface defects is suppressed and which has a good surface appearance (plating appearance), and as a result, they have obtained the following findings. (1) It was found that the related technology methods, which improved the conditions of the annealing process in various ways, were unable to completely suppress the formation of nitrides of B. Furthermore, it was found that the unevenness of the coating was caused by the remaining nitrides of B inhibiting the alloying reaction between the base steel sheet and molten zinc, resulting in uneven alloying. (2) B nitrides are formed when radical nitrogen (N) generated from ammonia in the atmosphere adsorbs onto or penetrates the steel sheet, primarily during the heating process, and then reacts with B in the steel during heating or soaking in an atmosphere with a low oxygen potential. Previous studies have attempted to suppress the generation of ammonia and the formation of B nitrides by inhibiting the formation of iron oxide, which acts as a catalyst for ammonia generation, and pure iron, which is the result of the reduction of iron oxide. However, this was not effective enough to suppress unevenness in the coating. Therefore, in order to further suppress the formation of B nitrides, the inventors attempted to suppress the surface diffusion of B in the steel in addition to controlling ammonia.

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

[0056] (4) Decarburization during recrystallization annealing is promoted by the gases in the annealing atmosphere. Here, O2 and H2O are known as oxidizing gases, and CO and CH4 are known as carburizing gases. Based on this, after extensive research, we found that by controlling the ratio of oxidizing gases (O2, H2O) to carburizing gases (CO, CH4) and keeping the proportion of carburizing gases above a certain level, it is possible to suppress the formation of a decarburized layer on the surface of steel sheets during recrystallization annealing. We also discovered that suppressing the decarburized layer makes it possible to suppress the formation of BN and the unevenness of the plating surface, as we aimed for.

[0057] (5) Based on the above, we have determined that suppressing the formation of a decarburized layer during recrystallization annealing is an important requirement for suppressing the formation of B nitrides and obtaining a coating with a good appearance. Furthermore, we have found that the amount of remaining B nitrides can be determined by alkaline stripping (stripping with an alkaline solution) of galvannealed steel sheets after hot-dip galvanizing and alloying treatments, followed by XPS analysis. Therefore, we have found that by controlling the ratio of decarburizing gas to carburizing gas in the atmospheric gas during recrystallization annealing, the formation of a decarburized layer on the steel sheet surface can be suppressed. This, along with controlling the ammonia content, suppresses the surface diffusion of B and nitride formation, thereby suppressing uneven alloying between the steel sheet and molten zinc, resulting in a beautiful coating appearance with minimal unevenness and no mottled patterns.

[0058] For this reason, in the present invention, continuous annealing is performed under a series of conditions optimized to obtain the effect of (5) above, and the optimized series of annealing conditions is an important requirement in the present invention.

[0059] Based on the above findings, the method for producing a galvannealed steel sheet of the present invention comprises continuous annealing a cold-rolled steel sheet having the above-mentioned chemical composition, hot-dip galvanizing the cold-rolled steel sheet, and then alloying the cold-rolled steel sheet. In the continuous annealing, the steel sheet is heated in a temperature range of 300°C or higher and 500°C or lower in an atmosphere containing 3% by volume or more of H2, a dew point of -20°C or lower, and an O2 content of 500 ppm by volume or lower. After the heating, the steel sheet is heated in a temperature range of 500°C or higher and 750°C or lower in an atmosphere containing 5% by volume of H2, a dew point of -20°C or lower, and an O2 content of 500 ppm by volume or lower. The steel sheet is heated at an average heating rate of 1°C / s or more in an atmosphere containing 5% or more by volume of H2, with a dew point of -55°C or more and -40°C or less, after which the steel sheet is soaked at a temperature of 750°C or more and 950°C or less in an atmosphere containing 5% or more by volume of H2, with a dew point of -55°C or more and -40°C or less, and where pO2, pH2O, pCO and pCH4, which are the partial pressures (Pa) of O2, H2O, CO and CH4, respectively, satisfy the relationship Log(pO2 + pH2O) - Log(pCO + pCH4) ≦ 3.0, for a holding time of 20 seconds or more and 300 seconds or less.

[0060] (Continuous annealing) In the continuous annealing of the present invention, first, the steel sheet is heated in a temperature range of 300°C or higher and 500°C or lower in an atmosphere containing 3% or more by volume of H2, with a dew point of -20°C or lower and 500 ppm or less by volume of O2. In this temperature range, the atmosphere is reduced by Fe, which suppresses the generation of iron oxide, which acts as a catalyst for ammonia generation, and suppresses the generation of ammonia, which is a cause of the formation of B nitrides (the generation of ammonia becomes particularly noticeable in a temperature range of 500°C or higher).

[0061] Atmospheric hydrogen concentration (H2 concentration): 3% by volume or more Hydrogen is a reducing gas, and therefore can suppress oxidation of the steel sheet surface during annealing. To fully obtain the oxidation suppression effect, the hydrogen concentration in the atmosphere is set to 3% by volume or more, and preferably 5% by volume or more. There is no particular upper limit to the hydrogen concentration, but from the viewpoint of suppressing cost increases, the hydrogen concentration is preferably set to 30% by volume or less.

[0062] Ambient dew point: -20°C or below In the low temperature range of 300 to 500°C, if the dew point exceeds -20°C, oxidation of iron occurs on the steel sheet surface, and this iron oxide acts as a catalyst, promoting the generation of ammonia in the temperature range of 500°C or higher. For this reason, it is important to suppress the oxidation of Fe on the steel sheet surface in order to suppress the generation of ammonia, and in order to fully obtain the effect of suppressing the generation of iron oxide, the dew point is set to -20°C or lower. Furthermore, from the viewpoint of further suppressing the generation of ammonia, the dew point is preferably set to -30°C or lower. The dew point of the atmosphere here is preferably −70° C. or higher, more preferably −60° C. or higher, from the viewpoint of reducing production costs.

[0063] Atmospheric oxygen concentration: 500 ppm by volume or less In order to suppress oxidation of Fe during annealing, the oxygen concentration is set to 500 ppm by volume or less, preferably 400 ppm by volume or less, and more preferably 200 ppm by volume or less. The balance of the atmospheric gas other than hydrogen (H), HO, and oxygen (O) is preferably N gas and unavoidable impurities, and part of the N gas may be replaced with one or more of CO gas, CO gas, and Ar gas. In this case, the proportion of the replacement gas in the atmospheric gas is preferably 30% by volume or less. From the viewpoint of reducing the manufacturing cost, the oxygen concentration in the atmosphere is preferably 3 ppm by volume or more, and more preferably 10 ppm by volume or more.

[0064] Average heating rate in the temperature range of 300°C to 500°C (preferable condition) As mentioned above, the formation of ammonia, which is a nitriding source for B, is promoted by the action of iron oxide and pure iron reduced from the iron oxide as a catalyst. Therefore, from the viewpoint of suppressing the occurrence of gray and black spot defects, it is preferable to set the average heating rate to 10°C / s or less and ensure sufficient annealing time in an Fe reducing atmosphere to reduce iron oxide on the steel sheet surface. This allows Mn and B oxides to form on the steel sheet surface as early as possible in the subsequent heating temperature range of 500°C or higher, covering the steel sheet surface and suppressing the exposure of the pure iron layer on the steel sheet surface during heating and soaking. On the other hand, in consideration of productivity, the average heating rate is preferably 1°C / s or more. From the above-mentioned viewpoint, the average heating rate is more preferably 2°C / s or more. The average heating rate is more preferably 7°C / s or less. Here, the average heating rate is calculated by "(500-300) (°C) / heating time (s) from 300°C to 500°C".

[0065] In continuous annealing, the steel sheet is heated in the subsequent temperature range of 500°C to 750°C in an atmosphere containing 5% or more by volume of H2 and with a dew point of -40°C or less at an average heating rate of 1°C / s or more. This temperature range is where ammonia generation becomes significant and radical nitrogen begins to adsorb to and penetrate into the steel sheet, so this suppresses the generation of ammonia and the penetration of radical nitrogen, and suppresses the generation of B nitrides in the subsequent soaking treatment process.

[0066] Atmospheric hydrogen concentration (H2 concentration): 5% by volume or more Hydrogen is a reducing gas, so it can suppress the formation of oxides of Si and Mn during annealing and prevent oxide-induced non-plating defects. To fully achieve the effect of suppressing oxide formation, the hydrogen concentration in the atmosphere is set to 5% by volume or more, preferably 6% by volume or more. There is no particular upper limit to the hydrogen concentration, but from the perspective of suppressing cost increases, it is preferable that the hydrogen concentration be 30% by volume or less.

[0067] Ambient dew point: -40°C or below If the dew point is higher than -40°C, large amounts of Si and Mn oxides are formed on the steel sheet surface, resulting in oxide-induced bare spots. In addition, the generation of new Fe oxidation results in insufficient suppression of ammonia generation during the heating process at 500°C or higher and the soaking process at 750°C or higher, resulting in insufficient suppression of gray spot defects and black spot defects. For this reason, the dew point is set to -40°C or lower. From the perspective of suppressing gray spot defects and black spot defects, it is more preferable that the dew point of the atmosphere be -42°C or lower. The dew point of the atmosphere here is preferably −70° C. or higher, more preferably −60° C. or higher, from the viewpoint of reducing production costs.

[0068] The balance of the atmospheric gas other than hydrogen (H), HO, and oxygen (O) is preferably N gas and unavoidable impurities, and part of the N gas may be replaced with one or more of CO gas, CO gas, and Ar gas. In this case, the proportion of the replacement gas in the atmospheric gas is preferably 30% by volume or less.

[0069] ·Average heating rate: 1℃ / s or more The nitriding reaction due to the formation of ammonia is likely to occur in the temperature range of 500°C to 750°C. Therefore, in order to suppress gray and black spot defects, it is necessary to heat the steel sheet quickly within this temperature range. At an average heating rate of less than 1°C / s, the time required to raise the temperature to the specified temperature increases, increasing the amount of radical nitrogen penetrating the steel sheet. This leads to the formation of nitrides of B in the subsequent soaking treatment process, resulting in poor appearance due to gray and black spot defects. Therefore, 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 less, more preferably 30° C. / s or less. Here, the average heating rate is calculated by "(750-500) (°C) / heating time (s) from 500°C to 750°C".

[0070] In continuous annealing, after the above heating, the steel sheet is soaked at a temperature of 750°C to 950°C in an atmosphere containing 5% or more by volume of H2 and having a dew point of -55°C to -40°C for a holding time of 20 seconds to 300 seconds. Furthermore, the present invention is characterized in that in the above atmosphere, the partial pressures (Pa) of O2, H2O, CO and CH4, respectively, pO2, pH2O, pCO and pCH4, satisfy Log(pO2+pH2O)-Log(pCO+pCH4)≦3.0. This soaking process suppresses the nitriding reaction of B caused by ammonia and radical nitrogen that has penetrated into the steel, and also fixes some of the B as oxides on the steel sheet surface or inside the steel sheet, thereby suppressing the formation of nitrides due to diffusion of B to the steel sheet surface during annealing. These measures suppress the amount of B nitrides that are formed.

[0071] Atmospheric hydrogen concentration (H2 concentration): 5% by volume or more Hydrogen is a reducing gas, so it can suppress the formation of oxides of Si and Mn during annealing and prevent oxide-induced non-plating defects. To fully achieve the effect of suppressing oxide formation, the hydrogen concentration in the atmosphere is set to 5% by volume or more, preferably 6% by volume or more. There is no particular upper limit to the hydrogen concentration, but from the perspective of suppressing cost increases, the hydrogen concentration is preferably 30% by volume or less. The hydrogen concentration is more preferably 20% by volume or less, and even more preferably 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, large amounts of oxides of Si and Mn are formed on the steel sheet surface, causing unplated defects due to the oxides. On the other hand, if the dew point is lower than -55°C, a portion of B is fixed as an oxide, and the effect of suppressing the formation of nitrides due to diffusion of B to the steel sheet surface during annealing cannot be sufficiently obtained. In addition, the low oxygen potential and high nitrogen potential of the annealing atmosphere stabilize the formation of nitrides, accelerating the formation of B nitrides on the steel sheet surface. As a result, the occurrence of gray spot defects and black spot defects cannot be adequately suppressed. For this reason, the dew point is set to -55°C or higher, preferably -50°C or higher. In addition, the dew point is set to -40°C or lower, preferably -45°C or lower.

[0073] ·Log(pO2+pH2O)-Log(pCO+pCH4)≦3.0 By controlling the partial pressures (pO2 (Pa), pH2O (Pa), pCO (Pa), and pCH4 (Pa)) of the oxidizing gas (O2, HO) and the carburizing gas (CO, CH4) so ​​as to satisfy the above formula, it is possible to suppress the formation of a decarburized layer in the steel sheet surface during recrystallization annealing. This suppresses the formation of a ferrite layer, through which B diffuses rapidly, in the steel sheet surface, thereby suppressing the diffusion of B to the surface. This also makes it possible to suppress the formation of B nitrides on the steel sheet surface. On the other hand, if the ratio of oxidizing gas increases and the above formula is not satisfied, the diffusion of B to the steel sheet surface cannot be suppressed completely, and trace amounts of B nitrides form on the surface, resulting in the formation of plating irregularities due to uneven alloying. Therefore, from the viewpoint of the effect of suppressing the formation of B nitrides, Log(pO2 + pH2O) - Log(pCO + pCH4) is set to 3.0 or less, and preferably 2.8 or less. Although there is no particular lower limit, Log(pO2+pH2O)-Log(pCO+pCH4) is preferably 0.3 or more, and more preferably 0.6 or more. The partial pressure can be calculated by measuring the pressure in the annealing furnace and the concentration of the gas in the furnace. The ratio of this oxidizing gas to the carburizing gas can be adjusted by introducing carburizing gas (CO, CH4) 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 becomes insufficient, resulting in an increase in the area ratio of ferrite and bainite, making it difficult to achieve a TS of 590 MPa or more. On the other hand, if the holding time exceeds 300 seconds, although part of B is fixed as an oxide, part of the remaining B forms nitrides, and therefore the amount of nitrides of B formed on the steel sheet surface increases, making it impossible to appropriately suppress the occurrence of gray spot defects and black spot defects. Therefore, the holding time is set to 20 seconds or more, preferably 30 seconds or more. The holding time is set to 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 the steel sheet remains in (passes through) the above-mentioned atmosphere at a temperature of 750°C or more and 950°C or less.

[0075] Atmospheric ammonia concentration (preferable requirements) In the present invention, it is preferable to reduce the ammonia concentration in the atmosphere during the soaking treatment step to 0.010% by volume or less. Ammonia may be present in the soaking zone atmosphere if ammonia gas generated in the heating zone is carried into the soaking zone or if ammonia-contaminated exhaust gas is reused in the soaking zone, resulting in ammonia contamination. To mitigate the adverse effects of ammonia contamination or generation, it is important to improve the sealing performance of the partition between the heating zone and the soaking zone, reduce the exhaust gas reuse rate, and increase the flow rate of new high-purity gas from the rear to the front in the steel sheet traveling direction (from the exit side to the entry side of the soaking zone). Reducing the ammonia concentration in the soaking zone further suppresses the formation of B nitrides, thereby further suppressing poor appearance due to gray and black spot defects. Therefore, it is preferable to set the ammonia concentration in the atmosphere to 0.010% by volume or less. There is no particular lower limit, but the ammonia concentration in the atmosphere is preferably 0.001% by volume or more, and more preferably 0.002% by volume or more.

[0076] (hot-dip galvanizing) In the present invention, the steel sheet (cold-rolled steel sheet) continuously annealed under the above conditions is cooled and then subjected to hot-dip galvanizing treatment, for example, by immersing it in a hot-dip galvanizing bath. The temperature reached during the cooling is preferably 200 to 520°C, and it is preferable to heat the steel sheet as needed before immersing it in the hot-dip galvanizing bath. The bath temperature of the hot-dip galvanizing bath is preferably 440°C or higher. In addition, 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 may contain, for example, an Al content of 0.10% by mass to 0.23% by mass, and one or more elements selected from Pb, Sb, Si, Sn, Mg, Mn, Ni, Cr, Co, Ca, Cu, Li, Ti, Be, Bi, and REM in a total amount of 0% by mass to 3.5% by mass, with the remainder consisting of Zn and unavoidable impurities. To prevent changes in the bath temperature, the temperature of the steel sheet before galvanizing (immersion temperature) is preferably set to the bath temperature or higher. The temperature of the steel sheet before galvanizing (immersion temperature) is preferably set to the bath temperature + 50°C or lower.

[0077] (Alloying treatment) After the above-mentioned hot-dip galvanizing treatment, a galvannealing treatment is further performed to form a hot-dip galvannealed 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. If the alloying temperature is lower than 480°C, the Zn-Fe alloying rate becomes excessively slow, making alloying extremely difficult. On the other hand, if the alloying temperature exceeds 570°C, untransformed austenite may transform into pearlite, resulting in a decrease in TS and El. The alloying treatment is more preferably carried out in a temperature range of 490°C or higher. The alloying treatment is more preferably carried out in a temperature range of 560°C or less, and even more preferably in a temperature range of 530°C or less. The alloying treatment time is preferably 1 second or more, more preferably 5 seconds or more, and is preferably 150 seconds or less, more preferably 80 seconds or less.

[0078] The coating weight of galvannealed steel sheet (GA) is 20g / m per side. 2 More than 120g / m 2 The coating weight of the galvannealed steel sheet (GA) is preferably 20 g / m or less per side. 2 More than 80g / m 2 More preferably, it is: The coating weight can be adjusted by performing gas wiping or the like after hot dip galvanizing.

[0079] After the alloying treatment of hot dip galvanizing is carried out as described above, it is preferable to cool the steel sheet to a cooling stop temperature of not less than room temperature (0 to 50°C) and not more than 350°C. From the viewpoint of improving ductility, it is more preferable to cool to a range of 150° C. or more and 350° C. or less. On the other hand, from the viewpoint of improving strength (TS), it is preferable to cool to room temperature. The average cooling rate when cooling to a range of 150°C or higher and 350°C or lower is not particularly specified, but from the viewpoint of ensuring a high TS and improving ductility, it is preferable that the average cooling rate to 350°C after alloying treatment is 3°C / s or higher and 20°C / s or lower. Furthermore, the average cooling rate when cooling to room temperature is not particularly specified, but in order to further increase TS, it is preferable that the average cooling rate to 50°C after alloying treatment is 5°C / s or higher. On the other hand, due to constraints on production technology, it is preferable that the average cooling rate to 50°C is 40°C / s or lower. Furthermore, this average cooling rate to 50°C is more preferably 7°C / s or higher and 30°C / s or lower. Furthermore, the cooling rate below 50°C is not particularly limited, and cooling to the predetermined temperature can be performed by any method.

[0080] As a cooling method for cooling after the alloying treatment, gas jet cooling, mist cooling, water cooling, air cooling, etc. can be appropriately applied. Note that, usually, high-strength galvannealed steel sheets are cooled to room temperature before being traded.

[0081] The galvannealed steel sheet cooled to 350°C or less after alloying treatment may be rolled at a predetermined elongation rate. The elongation rate in this rolling is preferably 0.05% or more and 1.00% or less. By setting the elongation rate in this rolling to 0.05% or more, cracks can be introduced into the galvanized layer. By introducing cracks into the galvanized layer, the amount of diffusible hydrogen in the steel sheet can be reduced, and as a result, bendability and hole expandability can be improved. On the other hand, if the elongation rate of rolling exceeds 1.00%, the YS increases and the dimensional accuracy during forming may decrease. The elongation rate of rolling is more preferably 0.70% or less. The elongation rate of rolling is more preferably 0.10% or more. The rolling may be performed online in a device connected to the continuous hot-dip galvanizing facility, or may be performed offline from the continuous hot-dip galvanizing facility. The target elongation (e.g., 0.05% or more and 1.00% or less) may be achieved in a single rolling run, or the target elongation may be achieved by multiple rolling runs. As the rolling, temper rolling is generally carried out, but rolling by a method such as processing with a leveler may also be used as long as it can impart an elongation rate equivalent to that of temper rolling.

[0082] After cooling to 350°C or less and performing the above rolling as necessary, the steel sheet may be kept at room temperature or in a temperature range above room temperature but not exceeding 450°C. By keeping the steel sheet at room temperature or in a temperature range above room temperature but not exceeding 450°C, the amount of diffusible hydrogen in the steel sheet can be reduced, and bendability and hole expandability can be improved. The holding time at room temperature may be 3 days or more. The holding time at room temperature may be 10 months or less. The holding time above room temperature may be 5 minutes or more. The holding time above room temperature may be 14 days or less.

[0083] The manufacturing conditions other than those mentioned above can be the same as those in the ordinary method.

[0084] <Tensile strength TS> The high-strength galvannealed steel sheet produced by the present invention can have a TS of 590 MPa or more. Furthermore, when further increasing the strength, the TS can be increased to 780 MPa or more, or even 980 MPa or more. The TS is measured in accordance with JIS Z2241 (2022) as follows. A JIS No. 5 test piece is taken from the galvannealed steel sheet so that the longitudinal direction is perpendicular to the rolling direction of the steel sheet. Using this test piece, a crosshead displacement rate Vc of 1.67 × 10 -1 A tensile test is carried out under the condition of mm / s, and TS is measured.

[0085] <Thickness> The thickness of the galvannealed steel sheet produced in the present invention is not particularly limited, but is preferably 0.3 mm or more, and is preferably 2.8 mm or less.

[0086] <Cmin / Cbase≧0.50> As described above, the present invention is characterized in that the formation of a decarburized layer in the surface layer of the steel sheet (surface layer of the base steel sheet) is suppressed in order to suppress black mottled patterns caused by plating unevenness. Here, whether or not the formation of a decarburized layer is sufficiently suppressed can be evaluated by the amount of suppression of the decarburized layer in the surface layer of the steel sheet (Cmin / Cbase). The amount of suppression of the decarburized layer in the surface layer of the steel sheet can be determined by the following method using GDS (glow discharge optical emission) analysis. Specifically, a 25mm x 25mm GDS analysis sample was first taken from the center of the width of the galvannealed steel sheet, and GDS analysis (JIS K0144(2018)) was performed from the surface of the galvannealed layer to a position 50μm in the sheet thickness direction at 0.1 second intervals to measure the Fe intensity and C intensity. The excitation source for the GDS analysis was high frequency, with a high frequency output of 35W, and Ar as the discharge gas, at a gas pressure of 600Pa.

[0087] Figure 1(A) is a graph illustrating the Fe detection intensity obtained by GDS analysis, where the Fe detection intensity is greater than or equal to the maximum Fe intensity × 0.95 and the Fe detection intensity is greater than or equal to the maximum Fe intensity × 0.80. Figure 1(B) is a graph illustrating Cmin and Cbase. In the present invention, the detected intensity of Fe is equal to or greater than the maximum intensity of Fe × 0.95. The average value of the C strength in the range that satisfies the above is taken as the C strength of the base material (Cbase). Here, Cbase (average C intensity) can be determined by calculating the average value of the C analysis results (total detected intensity of all data / total number of data) as shown in Figure 1(B) based on the Fe analysis results as shown in Figure 1(A) in a range that satisfies the relationship: Fe detected intensity ≥ maximum Fe intensity × 0.95. The Fe detection intensity is the Fe intensity (au) obtained by GDS analysis corresponding to each measurement time. Fe maximum strength (Fe max ) is the maximum intensity (au) of Fe obtained by GDS analysis over the entire measurement time. Furthermore, in the present invention, the detected intensity of Fe≧maximum intensity of Fe×0.80 The minimum value of the C strength in the range including the surface layer of the steel plate that satisfies the above is defined as Cmin. Here, Cmin (minimum C intensity) can be determined by extracting the minimum C intensity at the relevant measurement time, as shown in Figure 1(B), in the range where the Fe detection intensity is greater than or equal to the maximum Fe intensity × 0.80, based on the Fe analysis results as shown in Figure 1(A).

[0088] If Cmin / Cbase is 0.50 or more, the decarburized layer in the steel sheet surface is sufficiently small, and the above-mentioned effect of retarding the surface diffusion of B by suppressing ferrite formation can be sufficiently obtained. Furthermore, from the viewpoint of improving the effect of suppressing BN formation, Cmin / Cbase is more preferably 0.60 or more. On the other hand, although there is no particular upper limit, from the viewpoint of preventing hardening of the steel sheet surface layer due to excessive concentration of C on the surface, Cmin / Cbase is preferably 1.50 or less, and more preferably 1.20 or less.

[0089] <(I B +I N ) / I Fe <0.10> The amount of nitride formation of B can be determined by taking a 10mm x 10mm sample from the center of the width of the galvannealed steel sheet, removing only the coating layer 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 radiation, the X-ray tube voltage was 15kV, the pass energy was 112eV, the exit angle was 45°, the measurement energy interval was 0.1eV, and the spot diameter was 100μm. For alkaline stripping, the sample was immersed in an 8 vol% NaOH-2 vol% triethanolamine-1 vol% H2O2 aqueous solution. Peeling of the plating layer was confirmed visually. The semi-quantitative values ​​of Fe, B, and N obtained by XPS analysis were calculated as I Fe , I B , I N Then, the relationship between the element concentrations of Fe, B, and N is (I B +I N ) / I Fe When is used as an index of the amount of nitride formed by B, (I B +I N ) / I Fe ≦0.10 In this case, the amount of nitride formed by B is sufficiently small to obtain a good plating appearance. B +I N ) / I Fe is preferably 0.08 or less. On the other hand, the lower limit is not particularly limited, and (I B +I N ) / I Fe ) may be 0.00 or greater Here, the semi-quantitative value is an element concentration (atomic %) calculated based on the XPS intensity ratio of each element on the steel sheet surface.

[0090] In addition, the element concentration ratio of N and B, I N / I B but, I N / I B ≦0.30 In this case, what is mainly formed is not nitride of B but oxide of B, and a good surface appearance (plating appearance) can be obtained. From the viewpoint of obtaining a better plating appearance, I N / I B is preferably 0.25 or less. N / I B is more preferably 0.20 or less, and even more preferably 0.15 or less. On the other hand, the lower limit is not particularly limited, and I N / I B may be 0.00 or greater.

[0091] <Surface roughness Ra: 1.20 μm or less and brightness L value: 50 or more> The unevenness and appearance of the coating layer surface were evaluated using surface roughness Ra (coating surface roughness Ra) and lightness L value. Measurement samples were cut out to a size of 80 mm x 80 mm from the center of the width direction of the galvannealed steel sheet. Surface roughness Ra can be measured based on JIS B0601 (2013), and a large value indicates a large difference in unevenness per unit length. Here, if there is no unevenness due to alloying unevenness, Ra will be 1.20 μm or less. Furthermore, from the viewpoint of obtaining a 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, although there is no particular lower limit, 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 lightness L value can be measured based on JIS Z 8781-4 (2013), and the closer this 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, the larger the L value, the fewer depressions there are. From the above, when the L value is 50 or more, there are fewer black depressions, resulting in a good plating appearance. Furthermore, from the viewpoint of obtaining an even better plating appearance, the L value is preferably 55 or more, and more preferably 60 or more. The upper limit is not particularly limited and may be 100, but from the viewpoint of suppressing an increase in production costs, the L value is preferably 90 or less, and more preferably 80 or less.

[0092] In the present invention described above, when a steel sheet (base steel sheet) containing B is recrystallized (soaked), the ratio of carburizing gas to oxidizing gas in the atmosphere is set to a certain level or higher, thereby suppressing the formation of a decarburized layer through which B diffuses rapidly. Furthermore, in the annealing process before galvanizing, the dew point is lowered in the temperature range of 300 to 500°C to create a reducing atmosphere of Fe, thereby suppressing the formation of iron oxide and thereby suppressing the formation of ammonia. Furthermore, by quickly heating the steel sheet to a temperature range of 500°C or higher, at which ammonia formation becomes significant and radical nitrogen penetration into the steel sheet begins, the formation of ammonia and the nitridation of the steel sheet are suppressed. 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, thereby fixing a portion of the B as an oxide. This suppresses the formation of nitrides due to the diffusion of B into the steel sheet surface during annealing more than ever before, thereby suppressing the occurrence of black mottled patterns caused by uneven alloying, and resulting in a high-strength galvannealed steel sheet with a good surface appearance. [Example]

[0093] [Example 1] A steel material having the chemical composition shown in Table 1 (balance Fe and unavoidable impurities) was melted in a converter and continuously cast into a steel slab. This steel slab was heated to 1250°C and roughly rolled, then finish rolled at a finish rolling temperature of 900°C, and coiled at a coiling temperature of 400 to 600°C to obtain a hot-rolled steel sheet. This hot-rolled steel sheet was pickled and then 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 to 4. Then, hot-dip galvanizing was performed under the conditions shown in Tables 2 to 4, and further alloying treatment was performed, followed by cooling to 50°C or less. Thereafter, temper rolling was performed at an elongation rate of 0.1%, to obtain a galvannealed steel sheet (GA).

[0094] The hot-dip galvanizing bath used contained 0.14 mass% Al, with the remainder consisting of Zn and unavoidable impurities. The coating weight was 45 to 55 g / m per side. 2 (Double-sided plating) The composition of the GA plating layer contained 7 to 15 mass % of Fe, 0.1 to 1.0 mass % of Al, and the remainder was Zn and unavoidable impurities.

[0095] The galvannealed steel sheets obtained as described above were evaluated as follows, and the results are shown in Tables 2 to 4 together with the production conditions.

[0096] According to the above-mentioned procedure, the obtained galvannealed steel sheets were subjected to GDS analysis, and the suppression amount of the decarburized layer in the surface layer of the steel sheet (Cmin / Cbase) was calculated.

[0097] Following the procedure described above, the surface of the obtained galvannealed steel sheet was subjected to XPS analysis after the coating was removed, and the element concentration ratio of N and B (I N / I B ) and the relationship between the element concentrations of B, N, and Fe (I B +I N ) / I Fe was evaluated.

[0098] According to the above-mentioned procedures, the roughness Ra and lightness L value of the coating surface of the obtained galvannealed steel sheets were measured. Ra was measured using a stylus surface roughness meter. Lightness L value was measured using a color difference meter.

[0099] Tensile strength TS The tensile test was carried out in accordance with JIS Z2241 (2022). JIS No. 5 test pieces were taken from the obtained steel sheets so that the longitudinal direction was perpendicular to the rolling direction of the steel sheets. Using these test pieces, a crosshead displacement rate Vc of 1.67 × 10 -1 A tensile test was carried out under the condition of mm / s, and TS was measured.

[0100] As shown in Tables 2 to 4, the galvannealed steel sheets of the invention examples all had little decarburized layer, which suppressed the formation of BN and resulted in small coating irregularities and good appearance. Furthermore, a high strength of TS: 590 MPa or more was achieved, resulting in galvannealed steel sheets that could achieve both high strength and excellent coating quality. On the other hand, the galvannealed steel sheets of the comparative examples had poor appearance due to coating irregularities.

[0101] [Table 1]

[0102] [Table 2]

[0103] [Table 3]

[0104] [Table 4]

[0105] [Example 2] From the viewpoint of further reducing the ammonia concentration in the atmosphere during the soaking treatment step (soaking zone), galvannealed steel sheets were produced under conditions in which the flow rate of high-purity gas within the soaking zone was increased. Using steel materials A, D, I, H, N, O, and P in Table 1, galvannealed steel sheets were produced under the same production conditions as in Example 1 (hot rolling, cold rolling, continuous annealing, hot-dip galvanizing, galvannealing treatment, and temper rolling). The tensile properties, the amount of suppression of the decarburized layer, and the coating irregularities of the obtained galvannealed steel sheets were evaluated using the same methods as in Example 1. The results, along with the production conditions, are shown in Table 5. The ammonia gas concentration was measured at the top of the furnace in the pass including the center of the line length of the CGL equalization zone using ion chromatography (JIS K0099(2020)). As shown in Table 5, by reducing the ammonia concentration in the soaking treatment step (soaking zone) to 0.010% by volume or less, it is clear that the coating irregularities of the galvannealed steel sheet are further suppressed and the coating appearance is improved.

[0106] [Table 5]

Claims

1. A galvannealed steel sheet having a steel sheet and a galvannealed layer on the steel sheet, The composition of the steel plate 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 plate, and [%Si], which is the Si content (mass%) of the steel plate, [%Mn] / [%Si] is 2.50 or more; Additionally, as an optional ingredient, 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 Contains one or more selected from the balance being Fe and unavoidable impurities; GDS analysis is performed from the surface of the galvannealed layer in the thickness direction, Detected intensity of Fe ≥ Maximum intensity of Fe × 0.95 The average value of the C intensity in the range satisfying the above is defined as Cbase, Detected intensity of Fe ≥ Maximum intensity of Fe × 0.80 When the minimum value of the C intensity in the range satisfying the above is Cmin, Cmin / Cbase≧0.50; XPS analysis is performed on the surface of the galvannealed steel sheet after the galvannealed layer has been peeled off, The semi-quantitative values ​​of Fe, B, and N are expressed as I Ｆｅ , I Ｂ , I Ｎ When the element concentration ratios of Fe, B, and N are as follows: (I Ｂ +I Ｎ ) / I Ｆｅ ≦0.10 is satisfied, The galvannealed steel sheet has a surface roughness Ra of 1.20 μm or less and a lightness L value of 50 or more on the surface of the galvannealed layer.

2. The galvannealed steel sheet according to claim 1, wherein the steel sheet has a chemical composition in which [% Mn] / [% Si] is 12.00 or more.

3. A method for producing a galvannealed steel sheet, comprising the steps of: continuously annealing a cold-rolled steel sheet having the component composition according to claim 1 or 2; galvanizing the cold-rolled steel sheet; and then alloying the cold-rolled steel sheet. In the continuous annealing, The temperature range of 300°C to 500°C is H ２ : Contains 3% or more by volume, dew point: -20°C or less, O ２ : The steel sheet is heated in an atmosphere of 500 volume ppm or less, After the heating, the temperature range of 500°C to 750°C is ２ : 5 vol% or more of carbon black, the steel sheet is heated in an atmosphere having a dew point of -40°C or less at an average heating rate of 1°C / s or more, After the heating, H ２ : Contains 5% by volume or more, dew point: -55°C or more and -40°C or less, and O ２ , H ２ O, CO and CH ４ pO is the partial pressure (Pa) of each of ２ , pH ２ O, pCO and pCH ４ Log(pO ２ + pH ２ O)-Log(pCO+pCH ４ )≦3.0 for a holding time of 20 seconds or more and 300 seconds or less.

4. The method for producing a galvannealed steel sheet according to claim 3, wherein an ammonia concentration in the atmosphere for the soaking treatment is set to 0.010% by volume or less.

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