Method for manufacturing high-strength, hot-dip galvanized steel sheet

Abstract

This high-strength, hot-dip galvanized steel sheet having a favorable surface appearance and in which the occurrence of gray spot defects and black spot defects in a plating surface thereof are suppressed is manufactured by using, as a base material steel sheet, steel to which B has been added.　In an annealing step prior to plating, an Fe reducing atmosphere is created by lowering the dew point to a temperature range of 300-500°C to achieve suppression of generation of iron oxide and thereby suppress generation of ammonia. Furthermore, heating to a temperature range of 500°C or higher, which is the range in which generation of ammonia becomes noticeable and penetration of nitrogen into the steel sheet starts, is carried out rapidly to thereby suppress generation of ammonia and nitridization of the steel sheet. Also, the annealing time at a temperature range of 750°C or higher is shortened and the dew point is set to -55°C or higher to fix a portion of the B as an oxide to thereby suppress the generation of a nitride resulting from the diffusion of the B to the steel sheet surface during annealing. As a result, generation of a nitride of B, which is a factor in the occurrence of gray spot defects and black spot defects, is suppressed.

Description

Manufacturing method for high-strength hot-dip galvanized steel sheet

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

[0002] CO2 emissions from lighter vehicles ２ Aiming to simultaneously reduce carbon emissions and improve crashworthiness by increasing the strength of the vehicle body, efforts are underway to increase the strength and thickness of automotive steel sheets. For example, in order to increase the strength of the vehicle body, there has been an increase in the use of high-strength steel sheets with a tensile strength (TS) of 590 MPa or higher for the main structural components that form the framework of the automobile cabin. Adding hardening elements such as C, Mn, B, Cr, and Mo is a commonly used method for increasing the strength of steel. Among these, B is widely used as an additive element for high-strength steel sheets because it has the advantages of being able to highly improve hardenability even with a small amount, enabling steel to be strengthened at low cost, and having almost no adverse effects such as the formation of inclusions that deteriorate bendability and delayed fracture resistance.

[0003] Regarding high-strength hot-dip galvanized steel sheets containing B added to the base steel sheet, for example, Patent Document 1 discloses a technology in which, when a base steel sheet having a predetermined composition is subjected to continuous annealing and hot-dip galvanization, the temperature range in an annealing furnace of 750°C or higher during continuous annealing is set to a dew point of −40°C or lower in the atmosphere, thereby lowering the oxygen potential at the interface between the steel sheet and the atmosphere, suppressing surface segregation of Si, Mn, etc. without forming internal oxidation, and achieving an excellent coating appearance. Patent Document 2 also discloses a technology in which the ratio of the amount of Si segregation to the amount of Mn segregation in the surface layer of the base steel sheet is set to 0.7 to 1.3, and the cold-rolled steel sheet heated to the maximum temperature during annealing is maintained at a dew point of −40°C or lower in that region, thereby achieving excellent coating properties.

[0004] JP 2010-255100 A International Publication No. 2020 / 170542

[0005] However, after detailed investigations, the present inventors found that the hot-dip galvanized steel sheet obtained in Patent Document 1 exhibited minute surface defects other than the "surface defects caused by reduced plating wettability due to the formation of Si- and Mn-based oxides" addressed in Patent Document 1, i.e., defects in which Si- and Mn-based oxides repel the plating, resulting in areas where the plating does not adhere. This finding indicated a new challenge: suppressing the occurrence of these surface defects. In the production of hot-dip galvanized steel sheets, even if the steel sheet does not repel the plating, a phenomenon occurs in which the plating once adhered to the steel sheet peels off from the steel sheet due to weak adhesion between the steel sheet and the plating. The above-mentioned surface defects are considered to be minute defects caused by such a phenomenon. It was found that such surface defects caused by plating peeling occur frequently, particularly in steels containing B. It was also found that similar surface defects caused by plating peeling occur frequently in the hot-dip galvanized steel sheets obtained in Patent Document 2, particularly in steels containing B. The surface defects described above are point-like defects with a diameter of about 0.1 to 1.0 mm, and are classified as gray defects and black defects, which will be referred to hereinafter as gray point defects and black point defects, respectively.

[0006] The present invention has been made to solve the above-mentioned newly discovered problems, that is, an object of the present invention is to provide a method for manufacturing a high-strength hot-dip galvanized steel sheet in which B is added to a base steel sheet, and which has a good surface appearance (coating appearance) in which the occurrence of gray spot defects and black spot defects on the coating surface is suppressed.

[0007] The present inventors conducted extensive research to solve the above-mentioned problems, and as a result, have made the following findings. (1) Gray spot defects are concave defects caused by the coating initially adhering to the steel sheet surface after immersion treatment without being repelled by the steel sheet surface, but peeling off and adhering to the roll when the coating comes into contact with a transport roll due to weak adhesion between the coating and the steel sheet. Black spot defects are convex defects caused by the coating adhering to the roll peeling off from the roll and re-adhering to the steel sheet. These surface defects are unique phenomena that occur when B is added to the base steel sheet and the dew point during annealing before plating is low, and therefore have not been recognized as a problem in the past. (2) As a result of investigating and studying the reason why the above-mentioned surface defects are unique phenomena when B is added to the base steel sheet and the dew point during annealing before plating is low, it was found that this is because B nitrides are formed on the steel sheet surface only when the dew point during annealing is low. In other words, when the dew point during annealing is high, Si- and Mn-based oxides are preferentially formed on the steel sheet surface, and this phenomenon rarely occurs. Furthermore, if B is not added, the formation of Si- and Mn-based oxides is suppressed when the dew point is low, improving the appearance quality of the plating surface. However, if B is added and the dew point is low, although the formation of Si- and Mn-based oxides is suppressed, nitrides can remain stable, resulting in defects due to the formation of B nitrides.

[0008] (3) B nitrides are thought to be formed primarily due to the following two factors, and countermeasures corresponding to each factor can be considered. (i) Nitrogen (N) generated from ammonia in the atmosphere is adsorbed onto or penetrates the steel sheet mainly during the heating process, and then reacts with B during the heating or soaking process in an atmosphere with a low oxygen potential. The generation of ammonia is significantly promoted by the catalytic action of iron oxide or pure iron obtained by reducing iron oxide. Therefore, the generation of B nitrides can be suppressed by sufficiently suppressing the generation of iron oxide from the early stage of temperature rise during annealing and reducing the ammonia concentration in the atmosphere. (ii) When B does not form an oxide under low oxygen potential conditions, B diffuses to the steel sheet surface in a solid solution state and forms nitrides there. Even under low oxygen potential conditions, the generation of B nitrides can be suppressed by fixing B on the steel sheet surface or inside the steel sheet as an oxide that does not adversely affect the coating appearance quality.

[0009] (4) Based on the above, the inventors have found that by controlling the annealing conditions in the annealing step before galvanizing as described below, it is possible to effectively suppress the formation of B nitrides, thereby suppressing the occurrence of gray and black spot defects and obtaining a hot-dip galvanized steel sheet with a good surface appearance (galvanized appearance): (i) In the temperature range of 300 to 500°C, which was not controlled in conventional technology, the dew point is lowered to create a reducing atmosphere of Fe, thereby sufficiently suppressing the formation of iron oxide and thereby suppressing the amount of ammonia produced, which is a cause of the formation of B nitrides; (ii) Furthermore, by rapidly heating in the temperature range of 500 to 750°C, where ammonia production becomes significant and nitrogen adsorption or infiltration into the steel sheet begins, the adsorption or infiltration of nitrogen into or into the steel sheet is suppressed; (iii) Furthermore, by shortening the annealing time in the temperature range of 750°C or higher and optimizing the dew point to fix a portion of B as an oxide, the formation of B nitrides due to the diffusion of B to the steel sheet surface during annealing is suppressed. It was also found that the addition of Sb is effective in suppressing the adsorption or penetration of nitrogen into the steel sheet in the temperature range of 500°C or higher and 750°C or lower, and that controlling the Mn / Si content ratio, which is the ratio of Mn to Si, within a predetermined range makes it easier to generate Mn-B composite oxides, thereby achieving the effect of fixing the above-mentioned B as an oxide, and that these contribute to suppressing the occurrence of gray spot defects and black spot defects.

[0010] The present invention has been made based on the above findings and provides the following: [1] 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. and further containing: Al: 1.00% or less, N: 0.0200% or less, B: 0.0001% or more and 0.0050% or less, Sb: 0.001% or more and 0.200% or less, and having [%Mn] / [%Si] of 2.5 or more, and optionally further containing: 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, 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, 1. A method for producing a high-strength hot-dip galvanized steel sheet, comprising continuously annealing a steel sheet having a chemical composition containing one or more elements selected from the group consisting of REM: 0.0100% 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, and Cs: 0.10% or less, with the balance being Fe and unavoidable impurities, and then immersing the steel sheet in a hot-dip galvanizing bath to be hot-dip galvanized, and then subjecting the steel sheet to an alloying treatment as required, wherein in the continuous annealing step, the steel sheet is heated in a temperature range of 300°C or more and 500°C or less in an atmosphere containing 3% or more by volume of hydrogen, having a dew point of -10°C or less, and having 500 ppm or less by volume of oxygen, A method for manufacturing a high-strength hot-dip galvanized steel sheet, comprising: heating a steel sheet in a temperature range of 500°C or higher and 750°C or lower in an atmosphere containing 5% or more by volume of hydrogen and having a dew point of -40°C or lower at an average heating rate of 1°C / s or higher; and then soaking the steel sheet at a temperature of 750°C or higher and 950°C or lower in an atmosphere containing 5% or more by volume of hydrogen and having a dew point of -55°C or higher and -40°C or lower for a holding time of 20 to 300 seconds.

[0011] [2] The method for producing a high-strength hot-dip galvanized steel sheet according to the above [1], wherein the steel sheet has a chemical composition in which [%Mn] / [%Si] is 12.0 or more. [3] The method for producing a high-strength hot-dip galvanized steel sheet according to the above [1] or [2], wherein the step of soaking the steel sheet at a temperature of 750°C or more and 950°C or less in continuous annealing is performed in an atmosphere containing 0.010% by volume of ammonia.

[0012] According to the present invention, it is possible to produce a high-strength hot-dip galvanized steel sheet in which B is added to a base steel sheet, and which has a good surface appearance (coating appearance) in which the generation of gray spot defects and black spot defects on the coating surface is suppressed. The high-strength hot-dip galvanized steel sheet produced by the present invention is suitable for structural members such as automobile parts, and by applying it to such applications, it is possible to achieve improvement in fuel efficiency by reducing the vehicle body weight.

[0013] The present invention targets high-strength galvanized steel sheets having a hot-dip galvanized layer on one or both sides of a steel sheet (base steel sheet), including alloyed hot-dip galvanized steel sheets that are subjected to an alloying treatment after hot-dip galvanization. The composition of the hot-dip galvanized layer is not particularly limited, and may be a common one. For example, the hot-dip galvanized layer may contain 20% by mass or less of Fe, 0.001% by mass to 1.0% by mass of Al, and 0% by mass to 3.5% by mass 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 balance being Zn and unavoidable impurities. Generally, in the case of hot-dip galvanized steel sheet (GI), the Fe content in the coating layer is less than 7 mass%, and in the case of alloyed hot-dip galvanized steel sheet (GA), the Fe content in the coating layer is 7 mass% or more and 15 mass% or less, preferably 8 mass% or more and 13 mass% or less. The chemical composition of the steel sheet (base steel sheet) and the reasons for limiting this content are 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.

[0014] C: 0.050% or more and 0.300% or less. C is an effective element for generating the desired amount of quenched martensite or tempered martensite, achieving a TS of 590 MPa or more, and achieving excellent dimensional accuracy during forming. If the C content is less than 0.050%, the area ratio of quenched martensite decreases, and the area ratios 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 or tempered martensite increases, increasing the hardness of the quenched martensite or tempered martensite. As a result, the hardness difference between the soft phases ferrite and bainite and the hard phases quenched martensite or tempered martensite becomes large, resulting in poor punchability, stretch flangeability, and bendability. Therefore, the C content is set to 0.050% or more and 0.300% or less. The C content is preferably 0.060% or more to achieve a TS of 780 MPa or more, and more preferably 0.090% or more to achieve a TS of 980 MPa or more. Since a high C content deteriorates weldability, the C content is preferably 0.250% or less, and more preferably 0.220% or less.

[0015] Si: 1.20% or less. Si is an effective element for strengthening steel to obtain good material properties and for 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, resulting in the formation of Si oxides that cause unplated defects on the steel sheet surface, making it difficult to achieve good galvanizability. By setting the Si content to 1.20% or less, the formation of Si-based composite oxides can be suppressed, enabling the Mn in the steel sheet to be effectively utilized 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 viewpoints, the Si content is preferably 0.80% or less, 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 more preferably 0.15% or more.

[0016] Mn: 2.00% or more and 3.50% or less. Mn is an element necessary for suppressing gray and black spot defects and achieving good surface quality. By including 2.00% or more Mn, Mn-based composite oxides that have little adverse effect on the coating appearance quality are formed, and the formation of B nitrides is suppressed, thereby suppressing gray and black spot defects. Mn is also an effective element for generating a desired amount of quenched martensite and tempered martensite to achieve 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, the area ratios of ferrite and bainite decrease, and dimensional accuracy during forming decreases. Furthermore, the amount of Mn enriched on the steel sheet surface increases during annealing, resulting in the formation of a large amount of Mn oxides on the steel sheet surface that cause unplated defects, making it difficult to achieve good galvanizability. Therefore, the Mn content is set to 2.00% or more and 3.50% or less. 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. From the above viewpoints, the Mn content is preferably set to 3.30% or less, more preferably 3.00% or less.

[0017] P: 0.100% or less. P has the effect of solid solution strengthening and is an element that increases the strength of steel sheets. However, if the P content exceeds 0.100%, P segregates at prior austenite grain boundaries, embrittling the grain boundaries and thereby 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, more preferably 0.030% or less. No lower limit is set for the P content. That is, the P content may be 0%, but controlling it to less than 0.001% increases refining costs, so the P content is preferably set to 0.001% or more. S: 0.0100% or less. S exists in steel as sulfides, and if the S content exceeds 0.0100%, it reduces the ultimate deformability of the steel sheet, thereby reducing 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 the refining cost, the S content is preferably set to 0.0001% or more. Furthermore, from the above-mentioned viewpoint, the S content is preferably set to 0.0050% or less.

[0018] Sol. Al: 1.00% or less. Al can be used as a deoxidizer. In this case, the amount of sol. Al in the steel is preferably 0.01% or more. Furthermore, in steel sheets containing B, the addition of Al fixes N in the steel as AlN, allowing the added B to be used as solute B, which is effective in increasing strength. Furthermore, Al suppresses carbide formation during annealing and increases the volume fraction of retained austenite. The resulting retained austenite has the effect of improving ductility. To achieve the effect of fixing N as AlN, it is preferable to include sol. Al in an amount of 0.02% or more. Furthermore, from the viewpoint of improving ductility, it is even more preferable to include sol. Al in an amount of 0.05% or more. However, since a sol. Al content exceeding 1.00% results in bare spots, the sol. Al content is set to 1.00% or less. From the above viewpoints, the sol. Al content is preferably set to 0.10% or less, more preferably 0.08% or less.

[0019] N: 0.0200% or less. N exists as a 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. While there is no specific lower limit for the N content, due to production technology constraints, the N content is preferably set to 0.0005% or more. From the above-mentioned perspective, 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, so the N content is preferably set to 0.0005% or more. B: 0.0001% or more and 0.0050% or less. B is an element that segregates at austenite grain boundaries and thereby improves hardenability. 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 coating adhesion and causes poor appearance due to coating peeling. Therefore, the B content is set to 0.0001% or more and 0.0050% or less. From the above-mentioned viewpoints, the B content is preferably set to 0.0002% or more. Similarly, from the above-mentioned viewpoints, the B content is preferably set to 0.0030% or less. Sb: 0.001% or more and 0.200% or less Sb is known as an element that suppresses nitriding. Adding Sb can suppress the adsorption or penetration of nitrogen into or into the steel sheet in the temperature range of 500°C or more and 750°C or less, thereby suppressing the formation of B nitrides, which are a cause of gray spot defects and black spot defects. To achieve these effects, the Sb content must be 0.001% or more. On the other hand, since Sb has the effect of grain boundary embrittlement, if the Sb content exceeds 0.200%, slab cracking occurs during hot rolling. Therefore, the Sb content is set to 0.001% or more and 0.200% or less. Furthermore, from the above-mentioned viewpoint, the Sb content is preferably set to 0.005% or more. Similarly, from the above-mentioned viewpoint, the Sb content is preferably set to 0.100% or less.

[0020] [%Mn] / [%Si]: 2.5 or more. [%Mn] is the Mn content, and [%Si] is the Si content. By setting the [%Mn] / [%Si] ratio to 2.5 or more, the formation of oxides of simple Si is suppressed, suppressing non-plating. Furthermore, in the annealing method of the present invention, Mn-B composite oxides are formed, suppressing gray spot defects and black spot defects. Therefore, [%Mn] / [%Si] is set to 2.5 or more. Furthermore, from the viewpoint of suppressing gray spot defects and black spot defects, [%Mn] / [%Si] is preferably 4.0 or more, more preferably 6.0 or more, and even more preferably 12.0 or more. In particular, by setting the [%Mn] / [%Si] ratio to 12.0 or more, the formation of Mn-B composite oxides is promoted, and the formation of B nitrides, which cause gray spot defects and black spot defects, can be more effectively suppressed. Furthermore, by making the [%Mn] / [%Si] 14.0 or more, the formation of Mn, B complex oxides is particularly promoted and minute unevenness that is a sign of gray spot defects and black spot defects can also be suppressed, so it is even more preferable to make the [%Mn] / [%Si] 14.0 or more. There is no particular upper limit, but in order to suppress the formation of excessive Mn, B complex oxides, it is preferable that the [%Mn] / [%Si] be 300.0 or less.

[0021] Optionally Added Elements In addition to the above-described chemical composition, the high-strength steel plate used in the present invention further contains, 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, Mg: 0.0100% or less, Ca: 0.0100% or less, and Zn: 0.100%. It may contain one or more elements selected from the group consisting of Co: 0.200% or less, Zr: 0.200% or less, REM: 0.0100% 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, and Cs: 0.10% or less. That is, 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 is 0%, so each of the above elements may be 0%.

[0022] Cr is an element that enhances hardenability and is effective for generating a desired amount of quenched martensite or tempered martensite, achieving a TS of 590 MPa or more, and achieving excellent dimensional accuracy during forming. However, if the Cr content exceeds 1.00%, the coating appearance quality deteriorates, the area ratio of quenched martensite or tempered martensite increases, and the area ratio of ferrite and bainite decreases, resulting in reduced dimensional accuracy during forming. Therefore, when Cr is added, its content is preferably 1.00% or less. Furthermore, from the viewpoint of improving the coating appearance quality, the Cr content is more preferably 0.75% or less. Furthermore, to obtain the effect of Cr in improving hardenability, the Cr content is preferably 0.02% or more.

[0023] Ti increases TS by forming fine carbides, nitrides, or carbonitrides during hot rolling or annealing. However, if the Ti content exceeds 0.200%, a large amount of coarse precipitates and inclusions are formed. If diffusible hydrogen is contained in the steel sheet, these precipitates and inclusions become the starting points for cracks during bending tests, resulting in reduced bendability. Therefore, when Ti is added, its content is preferably 0.200% or less. From the above-mentioned viewpoints, the Ti content is more preferably 0.100% or less. To achieve the above-mentioned effects, the Ti content is preferably 0.005% or more, more preferably 0.010% or more.

[0024] 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. If diffusible hydrogen is contained in the steel sheet, these precipitates and inclusions become the starting points for cracks during bending tests, resulting in reduced bendability. Therefore, when Nb is added, its content is preferably 0.200% or less. From the above-mentioned viewpoints, the Nb content is more preferably 0.100% or less. To obtain the above-mentioned effects, the Nb content is preferably 0.005% or more, more preferably 0.010% or more.

[0025] 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. If diffusible hydrogen is contained in the steel sheet, these precipitates and inclusions become the starting points for cracks during bending tests, resulting in reduced bendability. Therefore, when V is added, its content is preferably 0.200% or less. From the above-mentioned viewpoints, the V content is more preferably 0.100% or less. To obtain the above-mentioned effects, the V content is preferably 0.005% or more, more preferably 0.010% or more.

[0026] Mo is an element that improves hardenability and is effective in maintaining the area ratio of quenched martensite and tempered martensite within a preferred 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 TS of 590 MPa or more and reducing dimensional accuracy during forming. Furthermore, the number of coarse precipitates and inclusions increases, and if diffusible hydrogen is contained in the steel sheet, these precipitates and inclusions become the initiation points for cracks during bending tests, thereby reducing bendability. Therefore, when Mo is added, its content is preferably 2.000% or less. From the above-mentioned perspective, the Mo content is more preferably 0.500% or less. To achieve the above-mentioned effects, the Mo content is preferably 0.005% or more, and more preferably 0.020% or more.

[0027] Cu is an element that improves hardenability and is effective in maintaining the area ratio of quenched martensite and tempered martensite within a preferred 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 amount 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 reduced bendability. Therefore, when Cu is added, its content is preferably 1.000% or less. From the above-mentioned perspective, the Cu content is more preferably 0.200% or less. To achieve the above-mentioned effects, the Cu content is preferably 0.005% or more, and more preferably 0.020% or more.

[0028] Ni is an element that improves hardenability and is effective in maintaining the area ratio of quenched martensite and tempered martensite within a preferred 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 added, its content is preferably 0.500% or less. From the above-mentioned perspective, the Ni content is more preferably 0.200% or less. To achieve the above-mentioned effects, the Ni content is preferably 0.005% or more, and more preferably 0.020% or more.

[0029] Sn is an effective element for suppressing oxidation of the base steel sheet surface during annealing and achieving better galvanizability. However, if the Sn content exceeds 0.200%, coarse precipitates and inclusions increase, and if the base steel sheet contains diffusible hydrogen, these precipitates and inclusions become the starting points for cracks during bending tests, resulting in reduced bendability. Therefore, when Sn is added, the Sn content is preferably 0.200% or less. From the above-mentioned perspective, the Sn content is more preferably 0.050% or less. To achieve the above-mentioned effects, the Sn content is preferably 0.001% or more, more preferably 0.005% or more.

[0030] Mg is an effective element for spheroidizing the shape of inclusions such as sulfides and oxides, thereby improving the ultimate deformability of steel sheets 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 become the starting point for cracks during bending tests, resulting in reduced bendability. Therefore, when Mg is added, its content is preferably 0.0100% or less. From the above-mentioned perspective, the Mg content is more preferably 0.0050% or less. To achieve the above-mentioned effects, the Mg content is preferably 0.0001% or more, more preferably 0.0005% or more. Ca exists as inclusions in the base steel sheet. If the Ca content exceeds 0.0100%, when diffusible hydrogen is contained in the base steel sheet, the inclusions become the starting points for cracks during bending tests, resulting in reduced bendability. Therefore, the Ca content is preferably 0.0100% or less. Although the lower limit of the Ca content may be 0.0000%, due to constraints on production technology, the Ca content is preferably 0.0001% or more. Furthermore, from the above-mentioned viewpoint, the Ca content is more preferably 0.0020% or less.

[0031] Zn is an effective element for improving stretch flangeability by spheroidizing the shape of inclusions and improving the ultimate deformability of steel sheets. However, if the Zn content exceeds 0.100%, 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, when Zn is added, its content is preferably 0.100% or less. From the above-mentioned viewpoints, the Zn content is more preferably 0.020% or less, and even more preferably 0.010% or less. To achieve the above-mentioned effects, the Zn content is preferably 0.001% or more, and more preferably 0.002% or more.

[0032] 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 become the starting points for cracks during bending tests, resulting in reduced bendability. Therefore, when Co is added, its content is preferably 0.200% or less. From the above-mentioned viewpoints, the Co content is more preferably 0.010% or less. To achieve the above-mentioned effects, the Co content is preferably 0.001% or more, and more preferably 0.005% or more.

[0033] 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 become the starting point for cracks during bending tests, resulting in reduced bendability. Therefore, when Zr is added, its content is preferably 0.200% or less. From the above-mentioned viewpoints, the Zr content is more preferably 0.010% or less. To achieve the above-mentioned effects, the Zr content is preferably 0.001% or more, and more preferably 0.005% or more.

[0034] REM is an element effective in improving 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 is 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, when REM is added, its total content is preferably 0.0100% or less. From the above-mentioned perspective, the total REM content is more preferably 0.0080% or less. To achieve the above-mentioned effects, the total REM content is preferably 0.0001% or more, and more preferably 0.0005% or more.

[0035] Ta is an element effective in increasing the strength of the base steel sheet and can be contained as necessary. Ta can be effectively improved by containing 0.005% or more of Ta, but from the viewpoint of preventing an increase in costs, if Ta is contained, the Ta content is preferably 0.10% or less.

[0036] When Te is contained in an amount of 0.001% or more, the form of sulfides can be controlled and ductility and toughness can be improved. However, from the viewpoint of preventing an increase in costs, when Te is contained, the Te content is preferably 0.10% or less.

[0037] When As is contained in an amount of 0.001% or more, the morphology of sulfides can be controlled and ductility and toughness can be improved. However, from the viewpoint of preventing an increase in costs, when As is contained, the As content is preferably 0.10% or less.

[0038] When Hf is contained in an amount of 0.01% or more, the form of sulfides can be controlled and ductility and toughness can be improved. However, from the viewpoint of preventing an increase in costs, when Hf is contained, the Hf content is preferably 0.10% or less.

[0039] 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 improving the smoothness of the cut end surface, and has the effect of improving the delayed fracture resistance of the cut surface. When Bi is contained, the Bi content is preferably 0.10% or less from the viewpoint of preventing an increase in cost.

[0040] When Pb is contained in an amount of 0.001% or more, it is possible to suppress grain boundary segregation and improve ductility and toughness. Pb also has the effect of improving machinability and improving the smoothness of cut edges, and has the effect of improving the delayed fracture resistance of cut edges. When Pb is contained, the Pb content is preferably 0.10% or less from the viewpoint of preventing an increase in costs.

[0041] Even if Ge is contained in an amount of 0.001% or more, it does not have a significant effect on mechanical properties or surface quality. From the viewpoint of preventing an increase in cost, if Ge is contained, the Ge content is set to 0.10% or less.

[0042] Even if Sr is contained in an amount of 0.001% or more, it does not have a significant effect on mechanical properties or surface quality. From the viewpoint of preventing an increase in costs, if Sr is contained, the Sr content is set to 0.10% or less.

[0043] Even if Cs is contained in an amount of 0.001% or more, it does not have a significant effect on mechanical properties or surface quality. From the viewpoint of preventing an increase in costs, if Cs is contained, the Cs content is set to 0.10% or less. The balance other than the above-mentioned components is Fe and unavoidable impurities.

[0044] Next, the manufacturing conditions of the method of the present invention will be described. In the manufacturing method of the present invention, a steel sheet (cold-rolled steel sheet or hot-rolled steel sheet) having the above-mentioned composition is introduced into a continuous hot-dip galvanizing facility, where it is continuously annealed, then hot-dip galvanized, and further, if necessary, subjected to an alloying treatment to obtain a hot-dip galvanized steel sheet. A continuous hot-dip galvanizing facility generally comprises an annealing furnace and a hot-dip galvanizing apparatus located downstream of the annealing furnace. The hot-dip galvanizing apparatus includes a hot-dip galvanizing bath and a snout connected to the steel strip outlet side of the annealing furnace, the tip of which is 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 treatment can be used as such a continuous hot-dip galvanizing facility. Among the above processes, the hot-dip galvanizing alloying treatment is performed as needed and is not necessarily required. The steel sheet introduced into the continuous hot-dip galvanizing facility is annealed while passing through an annealing furnace having a heating zone, a soaking zone, and a cooling zone provided in this order. Specific annealing conditions are as follows. The number of annealing passes is not particularly limited, but in the present invention, one annealing pass is preferable because gray spot defects and black spot defects can be suppressed with one annealing pass.

[0045] The occurrence of surface defects (gray spot defects and black spot defects), which is a problem to be solved by the present invention, is a phenomenon specific to when B is added to the base steel sheet and the dew point during annealing before galvanizing 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 studies to produce a hot-dip galvanized steel sheet that suppresses the occurrence of such surface defects and has a good surface appearance (galvanized appearance), and as a result, they have made the following findings. (1) Gray spot defects and black spot defects occur because nitrides of B are formed on the steel sheet surface when B is added to the base steel sheet and the dew point during annealing is low. Therefore, by suppressing the formation of these nitrides of B, the occurrence of gray spot defects and black spot defects can be prevented.

[0046] (2) It is believed that nitrides of B are formed mainly due to two factors, namely, (i) nitrogen (N) generated from ammonia in the atmosphere penetrates or is adsorbed into the steel sheet and reacts with B under conditions of low oxygen potential; (ii) when B does not form oxides under conditions of low oxygen potential, B diffuses in a solid solution state to the surface layer of the steel sheet and forms nitrides in the surface layer. Regarding the factor (i), ammonia reacts with N in the atmosphere. ２ and H ２ The above-mentioned raw material is iron oxide, and in the temperature range of 500°C or higher and 750°C or lower, the catalytic action of iron oxide and pure iron to which the iron oxide is reduced significantly promotes its generation, which in turn increases the penetration and adsorption of nitrogen into the steel sheet. Therefore, if the generation of iron oxide is suppressed in the temperature range of 500°C or lower and the amount of ammonia generated is reduced, the generation of B nitrides can be suppressed. Furthermore, with regard to the above factor (ii), if B is fixed on the surface or inside the steel sheet as an oxide that is less likely to adversely affect the coating appearance quality even under conditions of low oxygen potential, the generation of B nitrides can be suppressed.

[0047] (3) In the continuous annealing process before plating, by controlling the annealing conditions as follows, it is possible to effectively suppress the formation of B nitrides, thereby effectively suppressing the occurrence of gray spot defects and black spot defects. (i) In the temperature range of 300 to 500°C, which was not controlled in the conventional technology, the dew point is lowered to create a reducing atmosphere of Fe, thereby sufficiently suppressing the formation of iron oxide, and thereby suppressing the formation of ammonia, which is a cause of the formation of B nitrides; (ii) Furthermore, by rapidly heating in the temperature range of 500 to 750°C, where ammonia formation becomes significant and nitrogen penetration and adsorption into the steel sheet begins, the nitriding of the steel sheet is suppressed, and the formation of B nitrides in the temperature range of 750°C or higher is suppressed; (iii) Furthermore, by shortening the annealing time in the temperature range of 750°C or higher (soaking zone) and optimizing the dew point to fix a portion of B as an oxide, the formation of nitrides due to the diffusion of B to the steel sheet surface during annealing is suppressed. Therefore, in the present invention, continuous annealing is carried out under a series of conditions optimized to obtain the effect of (3) above, and the series of optimized annealing conditions is an extremely important requirement in the present invention.

[0048] In the continuous annealing process of the present invention, first, a 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 hydrogen, with a dew point of −10°C or lower and an oxygen concentration of 500 ppm by volume or lower. In this temperature range, the reducing atmosphere of Fe suppresses the generation of iron oxide and the generation of ammonia, which is a cause of the formation of B nitrides (the generation of ammonia becomes particularly pronounced in a temperature range of 500°C or higher). Hydrogen concentration in the atmosphere: 3% or more by volume. Hydrogen is a reducing gas, so it is possible to 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% or more by volume, preferably 5% or more by volume. There is no particular upper limit to the hydrogen concentration, but it is preferably set to 30% or less by volume in order to suppress cost increases.

[0049] Atmosphere dew point: -10°C or less In the low temperature range of 300 to 500°C, if the dew point exceeds -10°C, oxidation of iron occurs on the steel sheet surface, accelerating the generation of ammonia in the temperature range of 500°C or more. By setting the dew point to -10°C or less, it is possible to suppress the oxidation of Fe on the steel sheet surface. For this reason, the dew point is set to -10°C or less, and in order to fully obtain the effect of suppressing the generation of iron oxide, it is preferably set to -40°C or less. There is no particular lower limit, but from the viewpoint of preventing an increase in costs due to a lower dew point, it is preferably set to -60°C or more. Atmosphere oxygen concentration: 500 volume ppm or less In order to suppress oxidation of Fe during annealing, the oxygen concentration is set to 500 volume ppm or less, preferably 200 volume ppm or less. The atmospheric gas contains hydrogen, H ２ O, the balance other than oxygen (95% by volume or less) is N ２ It is preferable that the N ２ Part of the gas is CO gas, CO ２The atmospheric gas may be replaced with one or more of nitrogen gas, iron oxide, and Ar gas. In this case, the proportion of the replacement gas in the atmospheric gas is preferably 30% by volume or less. While the lower limit is not particularly specified, 0.01% by volume or more is preferred from the viewpoint of preventing cost increases due to the removal of unavoidable impurities. Average Heating Rate in the Temperature Range of 300°C to 500°C: As mentioned above, the generation of ammonia, which serves as a nitriding source for B, is promoted by iron oxide and pure iron obtained by reducing the iron oxide as a catalyst. Therefore, from the viewpoint of suppressing the occurrence of gray spot defects and black spot defects, it is preferable to set the average heating rate to 10°C / s or less 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, from the viewpoint of productivity, the average heating rate is preferably 1°C / s or more. From the above viewpoint, the average heating rate is more preferably in the range of 2° C. / s or more and 7° C. / s or less.

[0050] In the continuous annealing process, the steel sheet is heated at an average heating rate of 1°C / s or more in a subsequent temperature range of 500°C to 750°C in an atmosphere containing 5% or more by volume of hydrogen and having a dew point of -40°C or less. This temperature range is where ammonia generation becomes significant and nitrogen begins to adsorb to and penetrate the steel sheet. Therefore, the generation of ammonia and the penetration of nitrogen are suppressed, thereby suppressing the formation of B nitrides in the subsequent soaking treatment process. - Hydrogen concentration in the atmosphere: 5% or more by volume. Hydrogen is a reducing gas, so it can suppress the formation of oxides of Si and Mn during annealing and prevent oxide-related non-plating defects. To fully suppress the oxide formation, the hydrogen concentration in the atmosphere is set to 5% or more by volume, preferably 6% or more by volume. There is no particular upper limit to the hydrogen concentration, but it is preferably set to 30% or less by volume to prevent cost increases.

[0051] Atmospheric dew point: -40°C or less If the dew point is higher than -40°C, a large amount of oxides of Si and Mn are formed on the steel sheet surface, resulting in oxide-induced non-plating defects. In addition, new oxidation of Fe occurs, which results in insufficient suppression of ammonia generation in the heating step at 500°C or higher and the soaking step 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 less. Furthermore, from the viewpoint of suppressing gray spot defects and black spot defects, it is more preferable that the atmospheric dew point is -42°C or less. There is no particular lower limit, but from the viewpoint of preventing an increase in costs due to a lower dew point, it is preferable that it is -60°C or higher. The atmospheric gases hydrogen, H ２ O, the balance other than oxygen (95% by volume or less) is N ２ It is preferable that the N ２ Part of the gas is CO gas, CO ２ The atmospheric gas may be replaced with one or more of nitrogen gas and Ar gas. In this case, the proportion of the replacement gas in the atmospheric gas is preferably 30% by volume or less. While the lower limit is not particularly specified, a value of 0.01% by volume or more is preferred to prevent increased costs due to the removal of unavoidable impurities. Average heating rate: 1°C / s or more. Nitriding reactions through the formation of ammonia are likely to occur in the temperature range of 500 to 750°C. Therefore, to suppress gray spot defects and black spot defects, rapid heating within this temperature range is necessary. At a heating rate of less than 1°C / s, the time required to heat the steel sheet to the specified temperature increases, increasing the amount of nitrogen penetration into 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 spot defects and black spot defects. For this reason, the average heating rate is set to 1°C / s or more, preferably 1.5°C / s or more, and more preferably 5°C / s or more. While the upper limit is not particularly specified, a rate of 20°C / s or less, which is feasible in a typical annealing furnace, is preferred.

[0052] In the continuous annealing process, the steel sheet is soaked at a temperature of 750°C to 950°C in an atmosphere containing 5% or more by volume of hydrogen and having a dew point of -55°C to -40°C for a holding time of 20 to 300 seconds. This soaking process suppresses the nitriding reaction of B caused by ammonia and nitrogen infiltrated into the steel, and also fixes a portion of the B as an oxide on the steel sheet surface or within the steel sheet, thereby suppressing the formation of nitrides due to the diffusion of B to the steel sheet surface during annealing. These measures suppress the amount of B nitride formation. - Hydrogen concentration in the atmosphere: 5% or more by volume. Hydrogen is a reducing gas, so it can suppress the formation of oxides of Si and Mn during annealing and prevent oxide-related non-plating defects. To fully suppress the oxide formation, the hydrogen concentration in the atmosphere is set to 5% or more by volume, preferably 6% or more by volume. There is no particular upper limit to the hydrogen concentration, but it is preferably set to 30% or less by volume to prevent cost increases.

[0053] Atmosphere dew point: -55°C or higher and -40°C or lower. 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. 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 the 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. 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, while some of the B is fixed as an oxide, a portion of the remaining B forms nitrides. This increases the amount of B nitrides formed on the steel sheet surface, making it impossible to adequately 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. Furthermore, the holding time is set to 300 seconds or less, 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.

[0054] Ammonia Concentration in the Atmosphere In the present invention, it is preferable to reduce the ammonia concentration in the atmosphere during the soaking process to 0.010 vol% 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 spot and black spot defects. For this reason, it is preferable to set the ammonia concentration in the atmosphere to 0.010 vol% or less. Although there is no particular lower limit, a concentration of 0.0001 vol% or more is preferable from the viewpoint of preventing increased costs due to ammonia removal.

[0055] In the present invention, the steel sheet continuously annealed under the above conditions is cooled and then immersed in a hot-dip galvanizing bath to undergo hot-dip galvanizing. The cooling temperature is preferably 200 to 520°C, and the steel sheet is heated as needed before immersion in the hot-dip galvanizing bath. The bath temperature of the hot-dip galvanizing bath is generally about 440 to 500°C. The hot-dip galvanizing bath is not particularly limited, but may be, for example, one having an Al content of 0.10% by mass to 0.23% by mass, and further containing 0% by mass to 3.5% by mass 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 balance being Zn and unavoidable impurities. In order to prevent a change in the temperature of the coating bath, the temperature of the steel sheet before coating (immersion sheet temperature) is preferably set to the coating bath temperature or higher and the coating bath temperature + 50°C or lower.

[0056] After the above-mentioned hot-dip galvanizing treatment, a further alloying treatment of galvanization may be performed to form a galvannealed layer. The alloying treatment is preferably performed in a temperature range of 480°C or higher and 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 to pearlite, resulting in a decrease in TS and El. The alloying treatment is more preferably performed in a temperature range of 490°C or higher and 560°C or lower, and even more preferably in a temperature range of 490°C or higher and 530°C or lower. The coating weight of the hot-dip galvanized steel sheet (GI) and the alloyed hot-dip galvanized steel sheet (GA) is 20 to 80 g / m per side. ２ The coating weight can be adjusted by performing gas wiping or the like after hot dip galvanizing.

[0057] After hot-dip galvanizing or further alloying treatment as described above, the steel sheet is cooled to a temperature range of 150°C to 350°C. From the viewpoint of improving ductility, it is preferable to cool the steel sheet to a temperature range of 150°C to 350°C, and then heat it as necessary and hold it at a predetermined temperature. On the other hand, from the viewpoint of improving strength, it is preferable to cool the steel sheet to room temperature. The cooling rate when cooling to a temperature range of 150°C to 350°C is not particularly specified. However, from the viewpoint of ensuring a high TS and improving ductility, it is preferable to set the cooling rate to 350°C after hot-dip galvanizing or further alloying treatment to 3°C / s or more and 20°C / s or less. Furthermore, the cooling rate when cooling to room temperature is not particularly specified. However, to further increase TS, it is preferable to set the average cooling rate to 50°C to 5°C or more after hot-dip galvanizing or further alloying treatment to 50°C to 5°C or more. On the other hand, due to production technology constraints, it is preferable to set the average cooling rate to 50°C to 40°C / s or less. The average cooling rate to 50°C is more preferably 7°C / s or more and 30°C / s or less. The cooling rate below 50°C is not particularly limited, and cooling to a predetermined temperature can be performed by any method. As a cooling method for cooling after hot-dip galvanizing or after alloying treatment, gas jet cooling, mist cooling, water cooling, air cooling, etc. can be appropriately applied. Note that high-strength hot-dip galvanized steel sheets are usually traded after being cooled to room temperature.

[0058] A hot-dip galvanized steel sheet cooled to 350°C or less after hot-dip galvanizing or 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, resulting in improved bendability and hole expandability. On the other hand, if the elongation rate in the rolling exceeds 1.00%, the YS increases and the dimensional accuracy during forming decreases. The elongation rate in this rolling is more preferably 0.70% or less and 0.10% or more. The rolling may be performed online in a device connected to a continuous hot-dip galvanizing facility, or offline from the continuous hot-dip galvanizing facility. Furthermore, the target elongation rate (e.g., 0.05% or more and 1.00% or less) may be achieved in a single rolling pass, or the target elongation rate may be achieved by performing multiple rolling passes. 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.

[0059] After hot-dip galvanizing or alloying, the steel sheet may be cooled to 350°C or below, and optionally rolled. Then, the steel sheet may be held at room temperature or maintained at a temperature higher than room temperature but lower than 450°C. By maintaining the steel sheet at room temperature or maintaining the temperature higher than room temperature but lower than 450°C, the amount of diffusible hydrogen in the steel sheet can be reduced, improving its bendability and hole expandability. Furthermore, from the viewpoint of improving ductility, when the steel sheet is cooled to a temperature range of 150°C or higher but lower than 350°C as described above, it is preferable to heat the steel sheet after cooling as necessary and maintain (maintain) it at a temperature range of 300°C or higher but lower than 450°C. The holding time at room temperature is typically about 3 days to 10 months, and the holding time above room temperature is typically about 1 minute to 14 days. Conventional manufacturing conditions other than those described above can be used. The high-strength hot-dip galvanized steel sheet manufactured by the present invention can have a TS of 590 MPa or higher. Furthermore, for even higher strength, the TS can be set to 780 MPa or higher, or even 980 MPa or higher. The TS is measured in accordance with JIS Z2241 as follows: A JIS No. 5 test piece is taken from a hot-dip galvanized 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 The tensile strength (TS) of the hot-dip galvanized steel sheet produced in the present invention is measured by performing a tensile test under the condition of 1 / 3000 tensile strength (TS) ...). The thickness of the hot-dip galvanized steel sheet produced in the present invention is not particularly limited, but is usually about 0.3 mm to 2.8 mm.

[0060] Example 1 A steel material having the chemical composition shown in Table 1 (the balance being 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 CGL under the conditions shown in Tables 2 to 5, then hot-dip galvanized under the conditions shown in Tables 2 to 5, with some steel sheets further subjected to an alloying treatment after hot-dip galvanizing and cooled to 50°C or below. Subsequently, the steel sheet was subjected to temper rolling at an elongation rate of 0.1%, to obtain a high-strength hot-dip galvannealed steel sheet (GI) and a high-strength alloyed hot-dip galvannealed steel sheet (GA). In addition, the alloying treatment column in Tables 2 to 4 is marked with "-" for those that were not subjected to alloying treatment.

[0061] The hot-dip galvanizing bath used for producing GI was one containing 0.20 mass% Al, with the balance being Zn and unavoidable impurities. The hot-dip galvanizing bath used for producing GA was one containing 0.14 mass% Al, with the balance being Zn and unavoidable impurities. The coating weight for GI was 45 to 72 g / m per side. ２ (Both sides plating) and when manufacturing GA, 45 to 55 g / m per side ２ The plating layer of the GI contained 0.1 to 1.0 mass% Fe, 0.2 to 1.0 mass% Al, and the balance was Fe and unavoidable impurities. The plating layer of the GA contained 7 to 15 mass% Fe, 0.1 to 1.0 mass% Al, and the balance was Fe and unavoidable impurities.

[0062] The high-strength hot-dip galvanized steel sheets and high-strength galvannealed steel sheets obtained as described above were evaluated for tensile properties and coating appearance as follows. The results are shown in Tables 2 to 5 along with the manufacturing conditions. Tensile strength: Tensile tests were conducted in accordance with JIS Z2241. 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 was used. －１ A tensile test was carried out under the condition of 1 / 1000 mm / s to measure TS.

[0063] - Plating Appearance The appearance of the plated steel sheets was visually inspected for the presence or absence of gray and black spot defects. Observations were performed on the front and back surfaces of steel sheets measuring 1000 mm in the coil width direction and 1000 mm in the coil length direction, with N = 5 sheets. A score of 3 was given for a steel sheet with no defects observed in the observed area; a score of 2 was given for a steel sheet with only minute gray or black spot defects of 0.2 mm or less; a score of 1 was given for a steel sheet with gray or black spot defects larger than 0.2 mm; and a score of 0 was given for a steel sheet with conventional bare spots. The higher the score, the better the plating appearance, and a score of 2 or higher was deemed acceptable. Furthermore, a steel sheet with no gray or black spot defects and a particularly beautiful surface with no visible minute plating irregularities that could be a sign of these defects was given a score of 3+.

[0064] As shown in Tables 2 to 5, the hot-dip galvanized steel sheets of the invention examples all had good coating appearance. They also achieved high strength of TS: 590 MPa or more, indicating that hot-dip galvanized steel sheets were capable of combining high strength with excellent coating quality. On the other hand, the hot-dip galvanized steel sheets of the comparative examples had poor appearance due to gray and black spot defects.

[0065]

[0066]

[0067]

[0068]

[0069]

[0070] Example 2 In order to further reduce the ammonia concentration in the atmosphere during the soaking treatment step (soaking zone), hot-dip galvanized steel sheets were produced under conditions in which the flow rate of high-purity gas in the soaking zone was increased. Using steel materials A, C, G, I, S, and T in Table 1, hot-dip galvanized steel sheets were produced under the same production conditions as in Example 1 (hot rolling, cold rolling, continuous annealing, hot-dip galvanizing, alloying treatment, and temper rolling). The tensile properties and coating appearance of the resulting hot-dip galvanized steel sheets were evaluated using the same methods as in Example 1. The results, along with the production conditions, are shown in Table 6. The ammonia gas concentration was measured at the upper part of the furnace in the pass that included the longitudinal center of the CGL soaking zone. Ion chromatography was used for the measurement. As shown in Table 6, it can be seen that the surface quality (coating appearance) of hot-dip galvanized steel sheets was further improved by reducing the ammonia concentration in the soaking treatment step (soaking zone) to 0.010 vol% or less.

[0071]

Claims

1. In mass percent, 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. and further containing: Al: 1.00% or less, N: 0.0200% or less, B: 0.0001% or more and 0.0050% or less, Sb: 0.001% or more and 0.200% or less, and [%Mn] / [%Si] being 2.5 or more, and optionally further containing: 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, 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, 1. A method for producing a high-strength hot-dip galvanized steel sheet, comprising: continuously annealing a steel sheet having a component composition containing one or more elements selected from among REM: 0.0100% 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, and Cs: 0.10% or less, with the balance being Fe and unavoidable impurities, and then immersing the steel sheet in a hot-dip galvanizing bath for hot-dip galvanizing, and then subjecting the steel sheet to an alloying treatment as required, wherein in the continuous annealing step, the steel sheet is heated in a temperature range of 300° C. or more and 500° C. or less in an atmosphere containing 3 vol % or more of hydrogen, having a dew point of −10° C. or less and having oxygen of 500 vol ppm or less, A method for manufacturing a high-strength hot-dip galvanized steel sheet, comprising heating a steel sheet at a temperature range of 500°C or higher and 750°C or lower in an atmosphere containing 5 vol% or more of hydrogen and having a dew point of -40°C or lower at an average heating rate of 1°C / s or higher, and then soaking the steel sheet at a temperature of 750°C or higher and 950°C or lower in an atmosphere containing 5 vol% or more of hydrogen and having a dew point of -55°C or higher and -40°C or lower for a holding time of 20 to 300 seconds.

2. A method for producing a high-strength hot-dip galvanized steel sheet according to claim 1, wherein in the chemical composition of the steel sheet, [%Mn] / [%Si] is 12.0 or more.

3. A method for producing high-strength hot-dip galvanized steel sheet according to claim 1 or 2, wherein in the continuous annealing step of soaking the steel sheet at a temperature of 750°C or higher and 950°C or lower, an atmosphere containing 0.010% by volume or less of ammonia is used.

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