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

Technical Field

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

Background Art

[0002] For the purpose of achieving both reduction of CO2 emissions by vehicle weight reduction and improvement of collision resistance performance by increasing the strength of the vehicle body, attempts have been made to increase the strength of thin steel sheets for automobiles and then reduce the thickness. For example, for the purpose of increasing the vehicle body strength, the number of cases where high-strength steel sheets with a tensile strength (TS) of 590 MPa or more are applied to the main structural parts forming the skeleton of the automobile cabin is increasing. As a method for increasing the strength of steel, a method of adding hardening elements such as C, Mn, B, Cr, and Mo is generally used. Among them, B has the advantage of obtaining a high hardening effect with a small amount, and can increase the strength of steel at low cost. Since there are almost no adverse effects of generating inclusions and deteriorating bendability and stress corrosion cracking characteristics, it is widely used as an additive element for high-strength steel sheets.

[0003] Regarding a high-strength hot-dip galvanized steel sheet in which B is added to a base steel sheet, for example, in Patent Document 1, when performing continuous annealing and hot-dip galvanizing on a base steel sheet having a predetermined component composition, the temperature in the annealing furnace during continuous annealing: By setting the dew point in the atmosphere in the temperature range of 750 °C or higher to -40 °C or lower, the oxygen potential at the interface between the steel sheet and the atmosphere is reduced, surface enrichment of Si, Mn, etc. is suppressed without forming internal oxidation, and an excellent plating appearance is obtained. technology has been shown. Further, in Patent Document 2, the ratio of the Si enrichment amount to the Mn enrichment amount on the surface layer of the 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 heated to the maximum temperature reached is set to the dew point of the atmosphere in that region. By holding at -40 °C or lower, a technique for obtaining excellent plating properties has been shown.

Prior Art Documents

Patent Documents

[0004] [Patent Document 1] Japanese Unexamined Patent Application Publication No. 2010-255100 [Patent Document 2] International Publication No. 2020 / 170542 [Summary of the Invention] [Problems to be Solved by the Invention]

[0005] However, as a result of the inventors' detailed examination, it was found that the hot-dip galvanized steel sheet obtained in Patent Document 1 has minute surface defects different from the "surface defects caused by the formation of Si, Mn-based oxides and the resulting decrease in plating wettability" targeted in Patent Document 1, that is, defects in which Si, Mn-based oxides repel the plating and create areas where the plating does not adhere. It was also found that there is a new problem of suppressing the occurrence of these surface defects. In the production of hot-dip plated steel sheets, even if the steel sheet does not repel the plating, there is a phenomenon in which the plating that has once adhered to the steel sheet peels off due to the 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. In particular, in steels containing B, it was found that such surface defects caused by plating peeling occur frequently. Also, regarding the hot-dip galvanized steel sheet obtained in Patent Document 2, especially in steels containing B, it was found that similar surface defects caused by plating peeling occur frequently. The above-mentioned surface defects are dot-like defects with a diameter of about 0.1 to 1.0 mm, and there are defects showing gray and defects showing black, which are hereinafter referred to as gray dot defects and black dot defects, respectively.

[0006] The present invention has been made to solve the newly discovered problems as described above. That is, the object is to provide a method for producing a high-strength hot-dip galvanized steel sheet in which B is added to the base steel sheet, and which has a good surface appearance (plating appearance) with suppressed occurrence of gray dot defects and black dot defects on the plating surface. [Means for Solving the Problems]

[0007] As a result of intensive studies to solve the above problems, the present inventors have obtained the following findings. (1) Gray dot defects are defects in the form of concave shapes that occur when, although the plating once adheres to the steel sheet without being repelled on the surface of the steel sheet after the immersion treatment, the adhesion between the plating and the steel sheet is weak, and thus the plating peels off and adheres to the roll when it comes into contact with the conveying roll. Further, black dot defects are convex defects that occur when the plating adhering to the roll peels off from the roll and re-adheres to the steel sheet. Since these surface defects are specific phenomena that occur when B is added to the base steel sheet and the dew point during annealing before plating is low, they have not been recognized as problems in the past. (2) As a result of investigating and examining the reason why the above surface defects are specific phenomena when B is added to the base steel sheet and the dew point during annealing before plating is low, it was found that nitride of B is formed on the surface of the steel sheet only when the dew point during annealing is low. That is, when the dew point during annealing is high, oxides of Si and Mn are preferentially formed on the surface of the steel sheet, so this phenomenon hardly occurs. Further, if B is not added, the formation of oxides of Si and Mn is suppressed when the dew point is low, so the appearance quality of the plating surface is improved. However, when B is added and the dew point is low, although the oxides of Si and Mn are suppressed, nitrides can exist stably, so defects occur due to the formation of nitride of B.

[0008] (3) It is considered that nitride of B is mainly caused by the following two factors, and countermeasures corresponding to each factor can be considered. (i) Nitrogen (N) generated from ammonia in the atmosphere is mainly adsorbed or infiltrated into the steel sheet in the heating process, and then reacts with B in the process of being heated or soaked in an atmosphere with a low oxygen potential. Since the generation of ammonia is remarkably promoted by the catalytic action of iron oxide or pure iron obtained by reducing iron oxide, if the generation of iron oxide is sufficiently suppressed from the initial stage of temperature rise during annealing and the ammonia concentration in the atmosphere is reduced, it is possible to suppress the generation of nitride of B. (ii) When B does not form an oxide under conditions of low oxygen potential, B diffuses to the steel sheet surface in a solid solution state and forms a nitride on the surface. Even under conditions of low oxygen potential, if B is fixed on the steel sheet surface or inside the steel sheet as an oxide that hardly affects the plating appearance quality, it is possible to suppress the formation of B nitride.

[0009] (4) Based on the above, in the annealing process before plating, by controlling a series of annealing conditions as follows, the formation of B nitride can be effectively suppressed. As a result, it was found that the occurrence of gray dot defects and black dot defects can be suppressed, and a hot-dip galvanized steel sheet with good surface appearance (plating appearance) can be obtained. (i) For the temperature range of 300 to 500 °C that was not controlled in the prior art, by reducing the dew point to create a reducing atmosphere of Fe, the formation of iron oxide is sufficiently suppressed, thereby suppressing the amount of ammonia generated, which is a factor in the formation of B nitride; (ii) Further, by quickly heating in the temperature range of 500 °C or higher and 750 °C or lower where the generation of ammonia becomes significant and the adsorption or intrusion of nitrogen into the steel sheet begins, the adsorption or intrusion of nitrogen into the steel sheet is suppressed; (iii) Further, by shortening the annealing time in the temperature range of 750 °C or higher and optimizing the dew point to fix a part of B as an oxide, the formation of B nitride due to the diffusion of B to the steel sheet surface during annealing is suppressed. Also, it was found that the addition of Sb is effective in suppressing the adsorption or intrusion of nitrogen into the steel sheet in the temperature range of 500 °C or higher and 750 °C or lower, and by controlling the ratio of the contents of Mn and Si, that is, Mn / Si, within a predetermined range, it becomes easier to form a Mn,B composite oxide, and the effect of fixing B as an oxide as described above can be obtained, and these contribute to the suppression of the occurrence of gray dot defects and black dot defects.

[0010] The present invention has been made based on the above findings and has the following gist. [1] By mass%, C: 0.050% or more and 0.300% or less, Si: 1.20% or less, Mn: not less than 2.00% and not more than 3.50%, P: not more than 0.100%, S: not more than 0.0100%, sol.Al: not more than 1.00%, N: not more than 0.0200%, B: not less than 0.0001% and not more than 0.0050% Sb: not less than 0.001% and not more than 0.200% containing, and [%Mn] / [%Si] is 2.5 or more, optionally, further, Cr: not more than 1.00%, Ti: not more than 0.200%, Nb: not more than 0.200%, V: not more than 0.200%, Mo: not more than 2.000%, Cu: not more than 1.000%, Ni: not more than 0.500%, Sn: not more than 0.200%, Mg: not more than 0.0100%, Ca: not more than 0.0100%, Zn: not more than 0.100%, Co: not more than 0.200%, Zr: not more than 0.200%, REM: not more than 0.0100%, Ta: not more than 0.10%, Te: not more than 0.10%, As: not more than 0.10%, Hf: not more than 0.10%, Bi: not more than 0.20%, Pb: not more than 0.20%, Ge: not more than 0.10%, Sr: not more than 0.10%, Cs: not more than 0.10%, containing one or more selected from among, a method for manufacturing a high-strength hot-dip galvanized steel sheet, in which a steel sheet having a component composition consisting of the balance being Fe and inevitable impurities is continuously annealed, then immersed in a hot-dip zinc bath for hot-dip galvanizing, and then subjected to an alloying treatment as necessary, in the continuous annealing step, Heat the steel sheet in an atmosphere with a temperature range of above 300°C and below 500°C, containing 3% by volume or more of hydrogen, a dew point of -10°C or lower, and oxygen of 500 ppm by volume or lower. Heat the steel sheet in an atmosphere with a temperature range of above 500°C and below 750°C, containing 5% by volume or more of hydrogen and a dew point of -40°C or lower at an average heating rate of 1°C / s or more. A method for manufacturing a high-strength hot-dip galvanized steel sheet, in which the steel sheet is soaked at a temperature of above 750°C and below 950°C in an atmosphere containing 5% by volume 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.

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

Advantages of the Invention

[0012] According to the present invention, it is possible to manufacture a high-strength hot-dip galvanized steel sheet in which B is added to the base steel sheet and which has a good surface appearance (coating appearance) with the generation of gray dot defects and black dot defects on the coating surface suppressed. The high-strength hot-dip galvanized steel sheet manufactured by the present invention is suitable for, for example, structural members such as automotive parts, and by applying it to the same use, it is possible to improve fuel efficiency by reducing the vehicle body weight.

Embodiments for Carrying Out the Invention

[0013] What the present invention aims to manufacture is a high-strength hot-dip galvanized steel sheet having a hot-dip galvanized layer on one side or both sides of a steel sheet (base steel sheet), including an alloyed hot-dip galvanized steel sheet that is alloyed after hot-dip galvanizing. Here, the composition of the hot-dip galvanized layer is not particularly limited and may be a general one. For example, the hot-dip galvanized layer contains Fe: 20 mass% or less, Al: 0.001 mass% or more and 1.0 mass% or less, and further contains in total 0 mass% or more and 3.5 mass% or less of one or more selected from Pb, Sb, Si, Sn, Mg, Mn, Ni, Cr, Co, Ca, Cu, Li, Ti, Be, Bi, and REM, with the balance being composed of Zn and inevitable impurities. Generally, in the case of a hot-dip galvanized steel sheet (GI), the Fe content in the plating layer is less than 7 mass%, and in the case of an alloyed hot-dip galvanized steel sheet (GA), the Fe content in the plating layer is 7 mass% or more and 15 mass% or less, preferably 8 mass% or more and 13 mass% or less. Hereinafter, the component composition of the steel sheet (base steel sheet) and the reasons for its limitation will be described. In the following description, "%" representing the content of the component elements of the steel sheet means "mass%" unless otherwise specified. Also, the tensile strength is referred to as TS.

[0014] · C: 0.050% or more and 0.300% or less C is an element effective for producing a desired amount of quenched martensite and tempered martensite, making TS 590 MPa or more, and obtaining excellent dimensional accuracy during forming. When 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 make TS 590 MPa or more. On the other hand, when the C content exceeds 0.300%, the carbon concentration in quenched martensite and tempered martensite increases, and the hardness of quenched martensite and tempered martensite increases. As a result, the hardness difference between the soft phases of ferrite and bainite and the hard phases of quenched martensite and tempered martensite increases, resulting in a decrease in punching property, elongation flangeability, and bendability. Therefore, the C content is set to 0.050% or more and 0.300% or less. Also, the C content is preferably 0.060% or more to make TS 780 MPa or more, and more preferably 0.090% or more to make TS 980 MPa or more. Also, since the weldability deteriorates as the C content increases, 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 element effective for strengthening steel to obtain good material properties and is also an element effective for improving ductility. On the other hand, when the Si content exceeds 1.20%, the amount of Si enrichment on the steel sheet surface during annealing increases, and silicon oxides that cause non-plating defects are formed on the steel sheet surface, making it difficult to achieve good plating properties. By setting the Si content to 1.20% or less, it becomes possible to suppress the formation of Si,Mn-based composite oxides and effectively use Mn in the steel sheet as Mn,B composite oxides, and it becomes possible to suppress gray point defects and black point defects. Therefore, the Si content is set to 1.20% or less. Also, from the above viewpoints, the Si content is preferably 0.80% or less, and more preferably 0.55% or less. The lower limit of the Si content is not specifically set. That is, the Si content may be 0%, but since the refining cost increases when it is reduced to less than 0.01%, the Si content is preferably 0.01% or more. Also, from the perspective of achieving both high strength and improved ductility, the Si content is preferably 0.05% or more, more preferably 0.10% or more. Further, from the perspective 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 to suppress gray dot defects and black dot defects and obtain good surface quality. By containing 2.00% or more of Mn, it is possible to form an Mn, B-based composite oxide with little adverse effect on the plating appearance quality and suppress the formation of B nitride, so that gray dot defects and black dot defects are suppressed. Also, Mn is an element effective in generating a desired amount of quenched martensite and tempered martensite to make the TS 590 MPa or more. If the Mn content is less than 2.00%, B that forms a composite oxide with Mn during annealing decreases, and the formation amount of BN increases, so that black dot defects and gray dot defects cannot be sufficiently suppressed. On the other hand, when the Mn content exceeds 3.50%, the area ratio of tempered martensite increases, and the area ratios of ferrite and bainite decrease, resulting in a decrease in dimensional accuracy during forming. Further, the amount of Mn concentration on the steel sheet surface during annealing increases, and a large amount of Mn oxide that causes non-plating defects is formed on the steel sheet surface, making it difficult to achieve good plating properties. Therefore, the Mn content is 2.00% or more and 3.50% or less. Also, from the perspective of suppressing the occurrence of gray dot defects and black dot defects, the Mn content is preferably 2.30% or more, more preferably 2.50% or more, and even more preferably 2.60% or more. Also, from the above-mentioned perspective, the Mn content is preferably 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 for increasing the strength of the steel sheet. However, when the content of P exceeds 0.100%, P segregates at the prior austenite grain boundaries and embrittles the grain boundaries, resulting in a decrease in punchability and elongation flange formability. Therefore, the content of P should be 0.100% or less. Also, from the above viewpoints, the content of P is preferably 0.050% or less, more preferably 0.030% or less. No lower limit for the content of P is provided. That is, the content of P may be 0%, but since the refining cost increases to control it below 0.001%, the content of P is preferably 0.001% or more. ·S: 0.0100% or less S exists as sulfide in the steel. When the content exceeds 0.0100%, the ultimate deformability of the steel sheet decreases, resulting in a decrease in punchability, elongation flange formability, and bendability. Therefore, the content of S should be 0.0100% or less. Note that no specific lower limit for the content of S is specified. That is, the content of S may be 0%, but since the refining cost increases to control it below 0.0001%, the content of S is preferably 0.0001% or more. Also, from the above viewpoints, the content of S is preferably 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. Also, in the steel sheet added with B, by adding Al, N in the steel can be fixed as AlN, and the added B can be utilized as solid-solution B effective for strength increase. Further, Al also has the effect of suppressing carbide formation during annealing and increasing the volume ratio of retained austenite. The generated retained austenite has the effect of improving ductility. In order to obtain the effect of fixing N as AlN, it is preferable to contain sol.Al at 0.02% or more. Also, from the viewpoint of obtaining the effect of improving ductility, it is more preferable to contain sol.Al at 0.05% or more. However, when the amount of sol.Al exceeds 1.00%, non-plating occurs, so the amount of sol.Al is set to 1.00% or less. Also, from the above-mentioned viewpoints, the content of sol.Al is preferably 0.10% or less, more preferably 0.08% or less.

[0019] ·N: 0.0200% or less N exists as a nitride in the steel. When the content exceeds 0.0200%, the ultimate deformability of the steel sheet decreases, resulting in a decrease in punchability, elongation flangeability, and bendability. Therefore, the content of N is set to 0.0200% or less. Although the lower limit of the content of N is not particularly specified, due to production technology constraints, the content of N is preferably 0.0005% or more. Also, from the above-mentioned viewpoints, the content of N is preferably 0.0080% or less. The lower limit of the content of N is not provided. That is, the content of N may be 0%, but refining costs increase to control it below 0.0005%, so the content of N is preferably 0.0005% or more. ·B: 0.0001% or more and 0.0050% or less B is an element that can improve hardenability by segregating at austenite grain boundaries. By adding B to steel, it is possible to suppress the formation and grain growth of ferrite during annealing and cooling. To obtain such an effect, it is necessary to set the content of B to 0.0001% or more. On the other hand, when the content of B exceeds 0.0050%, a large amount of nitrides are formed on the surface of the steel sheet, the plating adhesion deteriorates, and appearance defects caused by plating peeling occur. Therefore, the content of B is set to 0.0001% or more and 0.0050% or less. Also, from the above-described viewpoints, the content of B is preferably 0.0002% or more. Similarly, from the above-described viewpoints, the content of B is preferably 0.0030% or less. · Sb: 0.001% or more and 0.200% or less Sb is an element known as a nitrogen inhibition element. By adding Sb, the adsorption or penetration of nitrogen into the steel sheet in the temperature range of 500°C or more and 750°C or less can be suppressed, and the formation of B nitrides, which are the causes of gray point defects and black point defects, can be suppressed. To obtain such an effect, it is necessary to set the content of Sb to 0.001% or more. On the other hand, since Sb has the effect of grain boundary embrittlement, when the content of Sb exceeds 0.200%, slab cracking during hot rolling occurs. Therefore, the content of Sb is set to 0.001% or more and 0.200% or less. Also, from the above-described viewpoints, the content of Sb is preferably 0.005% or more. Similarly, from the above-described viewpoints, the content of Sb is preferably 0.100% or less.

[0020] · [%Mn] / [%Si]: 2.5 or more [%Mn] represents the content of Mn, and [%Si] represents the content of Si. By setting [%Mn] / [%Si] to 2.5 or more, the formation of the oxide of elemental Si is suppressed, non-plating is suppressed, and in the annealing method of the present invention, an Mn,B composite oxide is formed, and gray dot defects and black dot defects are suppressed. Therefore, [%Mn] / [%Si] should be 2.5 or more. Also, from the viewpoint of suppressing gray dot defects and black dot 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 [%Mn] / [%Si] to 12.0 or more, the formation of the Mn,B composite oxide is promoted, and the formation of B nitride, which causes gray dot defects and black dot defects, can be more effectively suppressed. Furthermore, by setting [%Mn] / [%Si] to 14.0 or more, the formation of the Mn,B composite oxide is particularly significantly promoted, and minute unevenness that is a sign of gray dot defects and black dot defects can also be suppressed. Therefore, it is even more preferable to set [%Mn] / [%Si] to 14.0 or more. Although the upper limit is not particularly defined, in order to suppress the formation of excessive Mn,B composite oxide, it is preferably 300.0 or less.

[0021] ·Optional additive element In addition to the above component composition, the high-strength steel sheet used in the present invention may further contain, in mass%, one or more selected from the following: 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, 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, Cs: 0.10% or less. That is, each of the above elements is an optional additive element added as needed, and the effects of the present invention can be obtained even when the content is 0%. Therefore, each of the above elements may be 0%.

[0022] Cr is an element that significantly enhances hardenability. It is effective in producing a desired amount of hardened martensite and tempered martensite, achieving a TS of 590 MPa or higher, and obtaining excellent dimensional accuracy during forming. However, when the Cr content exceeds 1.00%, the plating appearance quality deteriorates, the area ratio of hardened martensite and tempered martensite increases, the area ratio of ferrite and bainite decreases, and the dimensional accuracy during forming decreases. Therefore, when adding Cr, its content is preferably 1.00% or less. Also, from the perspective of improving the plating appearance quality, the Cr content is more preferably 0.75% or less. Further, to obtain the effect of improving hardenability by Cr, the Cr content is preferably 0.02% or more.

[0023] Ti increases the TS by forming fine carbides, nitrides, or carbonitrides during hot rolling or annealing. However, when the Ti content exceeds 0.200%, a large amount of coarse precipitates and inclusions are generated. When the steel sheet contains diffusible hydrogen, these precipitates and inclusions become the starting points of cracks during the bending test, resulting in a decrease in bendability. Therefore, when adding Ti, its content is preferably 0.200% or less. Also, from the above-mentioned perspective, the Ti content is more preferably 0.100% or less. Further, to obtain the above effects, the Ti content is preferably 0.005% or more, and more preferably 0.010% or more.

[0024] Nb also increases the TS by forming fine carbides, nitrides, or carbonitrides during hot rolling or annealing. However, when the Nb content exceeds 0.200%, a large amount of coarse precipitates and inclusions are generated. When the steel sheet contains diffusible hydrogen, these precipitates and inclusions become the starting points of cracks during the bending test, resulting in a decrease in bendability. Therefore, when adding Nb, its content is preferably 0.200% or less. Also, from the above-mentioned perspective, the Nb content is more preferably 0.100% or less. Further, to obtain the above effects, the Nb content is preferably 0.005% or more, and more preferably 0.010% or more.

[0025] V also increases TS by forming fine carbides, nitrides or carbonitrides during hot rolling or annealing. However, when the V content exceeds 0.200%, a large amount of coarse precipitates and inclusions are generated. When the steel sheet contains diffusible hydrogen, these precipitates and inclusions become the starting points of cracks during the bending test, resulting in a decrease in bendability. Therefore, when adding V, its content is preferably 0.200% or less. Also, from the above viewpoints, the V content is more preferably 0.100% or less. Also, in order to obtain the above effects, the V content is preferably 0.005% or more, and more preferably 0.010% or more.

[0026] Mo is an element that increases hardenability. It is an effective element for making the area ratio of hardened martensite and tempered martensite within a more suitable range, increasing TS more, and further improving the dimensional accuracy during forming. However, when the Mo content exceeds 2.000%, the area ratio of hardened martensite and tempered martensite increases, making it difficult to make TS 590 MPa or more, and the dimensional accuracy during forming decreases. Furthermore, when a large amount of coarse precipitates and inclusions increase and the steel sheet contains diffusible hydrogen, these precipitates and inclusions become the starting points of cracks during the bending test, resulting in a decrease in bendability. Therefore, when adding Mo, its content is preferably 2.000% or less. Also, from the above viewpoints, the Mo content is more preferably 0.500% or less. Also, in order to obtain the above effects, the Mo content is preferably 0.005% or more, and more preferably 0.020% or more.

[0027] Cu is an element that increases hardenability. It is an effective element for making the area ratio of quenched martensite and tempered martensite fall within a more suitable range, increasing the TS, and further improving the dimensional accuracy during forming. However, when 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 higher and excellent dimensional accuracy during forming. Also, when coarse precipitates and inclusions increase and the steel sheet contains diffusible hydrogen, these precipitates and inclusions become the starting points of cracks during the bending test, resulting in a decrease in bendability. Therefore, when adding Cu, its content is preferably 1.000% or less. From the above viewpoints, the Cu content is more preferably 0.200% or less. Also, to obtain the above effects, the Cu content is preferably 0.005% or more, and more preferably 0.020% or more.

[0028] Ni is an element that increases hardenability. It is an effective element for making the area ratio of quenched martensite and tempered martensite fall within a more suitable range, increasing the TS, and further improving the dimensional accuracy during forming. However, when the Ni content exceeds 0.500%, the area ratio of quenched martensite and tempered martensite increases, and the TS and dimensional accuracy during forming decrease. Also, when coarse precipitates and inclusions increase and the steel sheet contains diffusible hydrogen, these precipitates and inclusions become the starting points of cracks during the bending test, resulting in a decrease in bendability. Therefore, when adding Ni, its content is preferably 0.500% or less. From the above viewpoints, the Ni content is more preferably 0.200% or less. Also, to obtain the above effects, the Ni content is preferably 0.005% or more, and more preferably 0.020% or more.

[0029] Sn is an element effective for suppressing oxidation of the surface of the base steel plate during annealing and obtaining better plating properties. However, when the Sn content exceeds 0.200%, the amount of coarse precipitates and inclusions increases. When the base steel plate contains diffusible hydrogen, these precipitates and inclusions become the starting points of cracks during the bending test, resulting in a decrease in bendability. Therefore, when adding Sn, the Sn content is preferably 0.200% or less. Also, from the above viewpoints, the Sn content is more preferably 0.050% or less. Further, in order to obtain the above effects, the Sn content is preferably 0.001% or more, and more preferably 0.005% or more.

[0030] Mg is an element effective for spheroidizing the shape of inclusions such as sulfides and oxides, improving the ultimate deformability of the steel plate, and improving the stretch flange formability. However, when the Mg content exceeds 0.0100%, a large amount of coarse precipitates and inclusions are generated. When the steel plate contains diffusible hydrogen, these precipitates and inclusions become the starting points of cracks during the bending test, resulting in a decrease in bendability. Therefore, when adding Mg, the content is preferably 0.0100% or less. Also, from the above viewpoints, the Mg content is more preferably 0.0050% or less. Further, in order to obtain the above effects, the Mg content is preferably 0.0001% or more, and more preferably 0.0005% or more. Ca exists as an inclusion in the base steel plate. When the Ca content exceeds 0.0100%, when the base steel plate contains diffusible hydrogen, the above inclusions become the starting points of cracks during the bending test, resulting in a decrease in bendability. Therefore, the Ca content is preferably 0.0100% or less. Note that the lower limit of the Ca content may be 0.0000%, but due to production technology constraints, the Ca content is preferably 0.0001% or more. Also, from the above viewpoints, the Ca content is more preferably 0.0020% or less.

[0031] Zn is an element effective in improving elongation flangeability by spheroidizing the shape of inclusions and enhancing the ultimate deformability of the steel sheet. However, when the Zn content exceeds 0.100%, a large amount of coarse precipitates and inclusions are generated. When the steel sheet contains diffusible hydrogen, these precipitates and inclusions become the starting points of cracks during the bending test, resulting in a decrease in bendability. Therefore, when adding Zn, 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. Also, in order to obtain the above effects, the Zn content is preferably 0.001% or more, and more preferably 0.002% or more.

[0032] Co is also an element effective in improving elongation flangeability by spheroidizing the shape of inclusions and enhancing the ultimate deformability of the steel sheet. However, when the Co content exceeds 0.200%, a large amount of coarse precipitates and inclusions are generated. When the steel sheet contains diffusible hydrogen, these precipitates and inclusions become the starting points of cracks during the bending test, resulting in a decrease in bendability. Therefore, when adding Co, its content is preferably 0.200% or less. From the above-mentioned viewpoints, the Co content is more preferably 0.010% or less. Also, in order to obtain the above effects, the Co content is preferably 0.001% or more, and more preferably 0.005% or more.

[0033] Zr is also an element effective in improving elongation flangeability by spheroidizing the shape of inclusions and enhancing the ultimate deformability of the steel sheet. However, when the Zr content exceeds 0.200%, a large amount of coarse precipitates and inclusions are generated. When the steel sheet contains diffusible hydrogen, these precipitates and inclusions become the starting points of cracks during the bending test, resulting in a decrease in bendability. Therefore, when adding Zr, its content is preferably 0.200% or less. From the above-mentioned viewpoints, the Zr content is more preferably 0.010% or less. Also, in order to obtain the above effects, the Zr content is preferably 0.001% or more, and more preferably 0.005% or more.

[0034] REM is an element effective for improving the stretch flangeability by spheroidizing the shape of inclusions and improving the ultimate deformability of the steel sheet. However, when the total content of REM exceeds 0.0100%, a large amount of coarse precipitates and inclusions are generated. When the steel sheet contains diffusible hydrogen, these precipitates and inclusions become the starting points of cracks during the bending test, resulting in a decrease in bendability. Therefore, when adding REM, its content is preferably 0.0100% or less in total. Further, from the above-mentioned viewpoints, the content of REM is more preferably 0.0080% or less in total. Also, in order to obtain the above effects, the content of REM is preferably 0.0001% or more in total, and more preferably 0.0005% or more.

[0035] Ta is an element effective for increasing the strength of the base steel sheet and can be contained as required. The effect of improving the strength can be obtained by containing 0.005% or more of Ta. However, from the viewpoint of preventing cost increase, when containing Ta, the Ta content is preferably 0.10% or less.

[0036] Te can control the form of sulfides and improve ductility and toughness by containing 0.001% or more. However, from the viewpoint of preventing cost increase, when containing Te, the Te content is preferably 0.10% or less.

[0037] As can control the form of sulfides and improve ductility and toughness by containing 0.001% or more. However, from the viewpoint of preventing cost increase, when containing As, the As content is preferably 0.10% or less.

[0038] Hf can control the form of sulfides and improve ductility and toughness by containing 0.01% or more. However, from the viewpoint of preventing cost increase, when containing Hf, the Hf content is preferably 0.10% or less.

[0039] Bi can suppress grain boundary segregation and improve ductility and toughness by containing 0.001% or more. In addition, Bi has the effect of improving machinability and the smoothness of the cut end face, and has the function of improving the stress corrosion cracking resistance of the cut surface. When containing Bi, from the viewpoint of preventing cost increase, the Bi content is preferably 0.10% or less.

[0040] Pb can suppress grain boundary segregation and improve ductility and toughness by containing 0.001% or more. In addition, Pb has the effect of improving machinability and the smoothness of the cut end face, and has the function of improving the stress corrosion cracking resistance of the cut surface. When containing Pb, from the viewpoint of preventing cost increase, the Pb content is preferably 0.10% or less.

[0041] Even if Ge contains 0.001% or more, it does not significantly affect the mechanical properties and surface quality. From the viewpoint of preventing cost increase, when containing Ge, the Ge content is set to 0.10% or less.

[0042] Even if Sr contains 0.001% or more, it does not significantly affect the mechanical properties and surface quality. From the viewpoint of preventing cost increase, when containing Sr, the Sr content is set to 0.10% or less.

[0043] Even if Cs contains 0.001% or more, it does not significantly affect the mechanical properties and surface quality. From the viewpoint of preventing cost increase, when containing Cs, the Cs content is set to 0.10% or less. The balance other than the components described above 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 components is introduced into a continuous hot-dip galvanizing facility, continuously annealed in the facility, then hot-dip galvanized, and further subjected to an alloying treatment as necessary to obtain a hot-dip galvanized steel sheet. Generally, a continuous hot-dip galvanizing facility is composed of an annealing furnace and a hot-dip galvanizing device located downstream of this annealing furnace. This hot-dip galvanizing device includes a hot-dip galvanizing bath and a snout that is connected to the strip outlet side of the annealing furnace and has a tip immersed in the hot-dip galvanizing bath. As such a continuous hot-dip galvanizing facility, a general continuous hot-dip galvanizing line (CGL: Continuous Galvanizing Line) configured to continuously perform a series of processes including heating, cooling, hot-dip galvanizing, and alloying treatment of hot-dip galvanizing can be applied. Note that among the above, the alloying treatment of hot-dip galvanizing is implemented as necessary and may not be implemented. The steel sheet introduced into the continuous hot-dip galvanizing facility is annealed while passing through an annealing furnace provided with a heating zone, a soaking zone, and a cooling zone in this order. Specific annealing conditions are as follows. Note that the number of annealing times is not particularly limited, but in the present invention, since gray spot defects and black spot defects can be suppressed by one-time annealing, it is preferably once (single annealing method).

[0045] The occurrence of surface defects (gray spot defects, black spot defects), which is the problem to be solved in the present invention, is a phenomenon peculiar to the case where B is added to the base steel sheet and the dew point during annealing before plating is low. Conventionally, this problem has not been recognized and is a problem newly discovered by the present inventors. As described above, as a result of intensive studies by the present inventors to suppress the occurrence of such surface defects and manufacture a hot-dip galvanized steel sheet having a good surface appearance (plating appearance), the following findings were obtained. (1) Gray spot defects and black spot defects occur because B nitride is 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 this B nitride, the occurrence of gray spot defects and black spot defects can be prevented.

[0046] (2) The nitride of B is mainly caused by two factors, namely, (i) nitrogen (N) generated from ammonia in the atmosphere penetrates or adsorbs onto the steel sheet and reacts with B under conditions of low oxygen potential; (ii) when B does not form an oxide under conditions of low oxygen potential, B diffuses to the surface layer of the steel sheet in a solid solution state and forms a nitride on the surface layer; it is considered to occur by these. And regarding the factor (i) above, ammonia uses N2 and H2 in the atmosphere as raw materials, and its production is significantly promoted by the catalytic action of iron oxide and pure iron obtained by reducing iron oxide in the temperature range of 500 °C or higher and 750 °C or lower. Along with this, the penetration and adsorption of nitrogen into the steel sheet increase. Therefore, if the formation of iron oxide is suppressed in the temperature range of 500 °C or lower and the production amount of ammonia is reduced, the production of the nitride of B can be suppressed. Also, regarding the factor (ii) above, if B is fixed as an oxide that hardly affects the plating appearance quality on the surface of the steel sheet or inside the steel sheet even under conditions of low oxygen potential, the production of the nitride of B can be suppressed.

[0047] (3) In the continuous annealing process before plating, by controlling a series of annealing conditions as follows, the production of the nitride of B can be effectively suppressed, and as a result, the occurrence of gray dot defects and black dot defects can be effectively suppressed. (i) Regarding the temperature range of 300 to 500 °C that was not controlled in the prior art, by lowering the dew point to create a reducing atmosphere of Fe, the formation of iron oxide is sufficiently suppressed, thereby suppressing the production of ammonia, which is a factor in the formation of the nitride of B; (ii) Further, by quickly heating in the temperature range of 500 °C or higher and 750 °C or lower where the production of ammonia becomes significant and the penetration and adsorption of nitrogen into the steel sheet start, nitriding of the steel sheet is suppressed, and the production of the nitride of B in the temperature range of 750 °C or higher is suppressed; (iii) Further, by shortening the annealing time in the temperature range of 750 °C or higher (holding zone) and optimizing the dew point to fix a part of B as an oxide, the production of nitride due to the diffusion of B to the surface of the steel sheet during annealing is suppressed. Therefore, in the present invention, continuous annealing is carried out under a series of conditions optimized so as to obtain the action of the above (3), and the series of optimized annealing conditions are extremely important requirements in the present invention.

[0048] In the continuous annealing process of the present invention, first, in a temperature range of 300°C or higher and 500°C or lower, a steel sheet is heated in an atmosphere containing 3% by volume or more of hydrogen, a dew point of -10°C or lower, and an oxygen concentration of 500 ppm by volume or lower. In this temperature range, by setting the reducing atmosphere of Fe, the formation of iron oxide is suppressed, and the formation of ammonia (especially the formation of ammonia that becomes prominent in the temperature range of 500°C or higher), which is a factor in the formation of B nitride, is suppressed. · Hydrogen concentration in the atmosphere: 3% by volume or more Since hydrogen is a reducing gas, it is possible to suppress the oxidation of the steel sheet surface during annealing. In order to sufficiently obtain the oxidation suppression effect, the hydrogen concentration in the atmosphere is set to 3% by volume or more, preferably 5% by volume or more. The upper limit of the hydrogen concentration is not particularly limited, but from the viewpoint of suppressing cost increase, it is preferably 30% by volume or less.

[0049] · Dew point of the atmosphere: -10°C or lower In the low temperature range of 300 to 500°C, when the dew point exceeds -10°C, oxidation of iron occurs on the steel sheet surface, and the formation of ammonia in the temperature range of 500°C or higher is promoted. By setting the dew point to -10°C or lower, it is possible to suppress the oxidation of Fe on the steel sheet surface. Therefore, the dew point is set to -10°C or lower, and in order to sufficiently obtain the effect of suppressing the formation of iron oxide, it is preferably -40°C or lower. The lower limit is not particularly defined, but from the viewpoint of preventing cost increase due to reducing the dew point, -60°C or higher is preferable. · Oxygen concentration in the atmosphere: 500 ppm by volume or lower In order to suppress the oxidation of Fe during annealing, the oxygen concentration is set to 500 ppm by volume or lower, preferably 200 ppm by volume or lower. The remainder of the atmosphere gas other than hydrogen, H2O, and oxygen (95% by volume or less) is preferably N2 gas and inevitable impurities. Further, a part of the N2 gas may be replaced with one or more of CO gas, CO2 gas, and Ar gas. In this case, the ratio of the replacement gas in the atmosphere gas is preferably 30% by volume or less. The lower limit is not particularly defined, but from the viewpoint of preventing cost increase due to removing inevitable impurities, it is preferably 0.01% by volume or more. ·Average heating rate in the temperature range of 300°C or higher and 500°C or lower As described above, ammonia serving as a nitrogen source for B is promoted in its production using iron oxide and pure iron obtained by reducing iron oxide as catalysts. Therefore, from the viewpoint of suppressing the generation of gray dot defects and black dot defects, it is preferable to set the average heating rate to 10°C / s or less, ensure sufficient annealing time in the reducing atmosphere of Fe, and reduce the iron oxide on the surface of the steel sheet. Thereby, Mn and B oxides can be formed on the surface of the steel sheet as early as possible in the heating temperature range of 500°C or higher to coat the surface of the steel sheet, and the exposure of the pure iron layer in the surface layer of the steel sheet during heating and soaking can be suppressed. On the other hand, considering productivity, the average heating rate is preferably 1°C / s or more. Also, from the above-described viewpoints, a more preferable range of the average heating rate is 2°C / s or more and 7°C / s or less.

[0050] In the continuous annealing process, in the subsequent temperature range of 500°C or higher and 750°C or lower, the steel sheet is heated with an average heating rate of 1°C / s or more in an atmosphere containing 5% by volume or more of hydrogen and having a dew point of -40°C or lower. This temperature range is a temperature range in which the generation of ammonia becomes significant and the adsorption and penetration of nitrogen into the steel sheet start. Therefore, the generation of ammonia and the penetration of nitrogen are suppressed, and the generation of B nitride in the subsequent soaking heat treatment process is suppressed. ·Hydrogen concentration in the atmosphere: 5% by volume or more Since hydrogen is a reducing gas, it is possible to suppress the formation of oxides of Si and Mn during annealing and prevent non-plating defects caused by oxides. To sufficiently obtain the effect of suppressing the formation of oxides, the hydrogen concentration in the atmosphere is set to 5% by volume or more, preferably 6% by volume or more. The upper limit of the hydrogen concentration is not particularly limited, but from the viewpoint of suppressing cost increase, it is preferably 30% by volume or less.

[0051] · Dew point of the atmosphere: -40°C or lower When the dew point is higher than -40°C, a large amount of oxides of Si and Mn are formed on the surface of the steel sheet, resulting in non-plating defects caused by oxides. In addition, due to the occurrence of new oxidation of Fe, the suppression of ammonia generation in the heating process at 500°C or higher and the soaking process at 750°C or higher becomes insufficient, and the suppression of ash point defects and black point defects becomes insufficient. Therefore, the dew point should be -40°C or lower. Also, from the viewpoint of suppressing ash point defects and black point defects, it is more preferable that the dew point of the atmosphere is -42°C or lower. Although the lower limit is not particularly defined, from the viewpoint of preventing cost increase due to reducing the dew point, -60°C or higher is preferable. The remainder (95% by volume or less) of the atmosphere gas other than hydrogen, H2O, and oxygen is preferably N2 gas and inevitable impurities, and a part of the N2 gas may be replaced with one or more of CO gas, CO2 gas, and Ar gas. In this case, the ratio of the replacement gas in the atmosphere gas is preferably 30% by volume or less. Although the lower limit is not particularly defined, from the viewpoint of preventing cost increase due to removing inevitable impurities, 0.01% by volume or more is preferable. · Average heating rate: 1°C / s or more The nitriding reaction through the formation of ammonia easily occurs in the temperature range of 500 to 750°C. Therefore, from the viewpoint of suppressing ash point defects and black point defects, it is necessary to heat this temperature range quickly. When the heating rate is less than 1°C / s, the time required to raise the temperature to a predetermined temperature increases, the amount of nitrogen infiltrated into the steel sheet increases, and nitrides of B are formed in the subsequent soaking heat treatment process, resulting in appearance defects due to ash point defects and black point defects. Therefore, 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. Although the upper limit is not particularly defined, 20°C / s or lower, which can be achieved in a general annealing furnace, is preferable.

[0052] In the continuous annealing process, the steel sheet is soaked at a temperature of 750°C or higher and 950°C or lower, in an atmosphere containing 5% by volume 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. In this soaking heat treatment process, the nitriding reaction of B by ammonia and nitrogen infiltrated into the steel is suppressed, and a part of B is fixed as an oxide on the steel sheet surface or inside the steel sheet, thereby suppressing the formation of nitrides due to the diffusion of B to the steel sheet surface during annealing. By these means, the amount of B nitride formation is suppressed. · Hydrogen concentration in the atmosphere: 5% by volume or more Since hydrogen is a reducing gas, it is possible to suppress the formation of oxides of Si and Mn during annealing and prevent non-plating defects caused by oxides. In order to sufficiently obtain the effect of suppressing the formation of oxides, the hydrogen concentration in the atmosphere is set to 5% by volume or more, preferably 6% by volume or more. The upper limit of the hydrogen concentration is not particularly limited, but from the viewpoint of suppressing cost increase, it is preferably 30% by volume or less.

[0053] · Dew point of the atmosphere: -55°C or higher and -40°C or lower If the dew point is higher than -40°C, a large amount of oxides of Si and Mn are formed on the steel sheet surface, and non-plating defects caused by oxides occur. On the other hand, if the dew point is less than -55°C, the effect of fixing a part of B as an oxide and suppressing the formation of nitrides due to the diffusion of B to the steel sheet surface during annealing cannot be sufficiently obtained. Also, the formation of nitrides becomes stable due to the low oxygen potential and high nitrogen potential in the annealing atmosphere, and the formation of B nitrides on the steel sheet surface is promoted. As a result, the occurrence of gray dot defects and black dot defects cannot be appropriately suppressed. Therefore, the dew point is set to -55°C or higher, preferably -50°C or higher. Also, 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 formation ratio of austenite 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, when the holding time exceeds 300 seconds, although part of B is fixed as an oxide, part of the remaining B forms nitrides, so the amount of B nitrides formed on the steel plate surface increases, and it is impossible to appropriately suppress the occurrence of gray dot defects and black dot defects. Therefore, the holding time should be 20 seconds or more, preferably 30 seconds or more. Also, the holding time should be 300 seconds or less, preferably 100 seconds or less. Note that this holding time refers to the time during which the steel plate stays (passes through) in the above-mentioned atmosphere at a temperature of 750 °C or higher and 950 °C or lower.

[0054] · Ammonia concentration in the atmosphere In the present invention, it is preferable to reduce the ammonia concentration in the atmosphere in the soaking heat treatment step to 0.010% by volume or less. The reasons for ammonia being contained in the atmosphere in the soaking zone include cases where ammonia gas generated in the heating zone is brought into the soaking zone, or cases where ammonia is mixed in when exhaust gas containing ammonia is reused in the soaking zone. In order to reduce the adverse effects of such ammonia mixing and generation, it is important to improve the sealing performance of the partition wall between the heating zone and the soaking zone, reduce the reuse rate of exhaust gas, and increase the flow rate of new high-purity gas from the rear to the front (from the outlet side to the inlet side of the soaking zone) in the steel plate advancing direction. By reducing the ammonia concentration in the soaking zone to further suppress the formation of B nitrides, it is possible to further suppress appearance defects due to gray dot defects and black dot defects. For this reason, it is preferable that the ammonia concentration in the atmosphere is 0.010% by volume or less. The lower limit is not particularly defined, but from the viewpoint of preventing cost increase due to ammonia removal, it is preferably 0.0001% by volume or more.

[0055] In the present invention, after cooling the steel sheet continuously annealed under the above conditions, it is immersed in a hot-dip galvanizing bath to perform a hot-dip galvanizing treatment. The reaching temperature of the cooling is set to 200 to 520°C, and it is preferably immersed in the hot-dip galvanizing bath after heating if necessary. 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. For example, it can have an Al content of 0.10 mass% or more and 0.23 mass% or less, and further contain 0 mass% or more and 3.5 mass% or less in total of one or more 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 inevitable impurities. Also, in order to prevent the change in the bath temperature of the plating bath, the temperature of the steel sheet before the plating treatment (immersion plate temperature) is preferably not less than the plating bath temperature and not more than the plating bath temperature + 50°C.

[0056] After the above-described hot-dip galvanizing treatment, an alloying treatment of the zinc plating may be further performed to form an alloyed hot-dip galvanized layer. The alloying treatment is preferably performed in a temperature range of 480°C or more and 570°C or less. If the alloying temperature is less than 480°C, the Zn-Fe alloying rate becomes excessively slow and alloying becomes extremely difficult. On the other hand, if the alloying temperature exceeds 570°C, untransformed austenite transforms into pearlite, and TS and El may decrease. The alloying treatment is more preferably performed in a temperature range of 490°C or more and 560°C or less, and even more preferably in a temperature range of 490°C or more and 530°C or less. The plating adhesion amount of the hot-dip galvanized steel sheet (GI) and the alloyed hot-dip galvanized steel sheet (GA) is preferably 20 to 80 g / m per side. 2 The adhesion amount of the plating can be adjusted by performing gas wiping or the like after the hot-dip galvanizing.

[0057] After hot-dip galvanizing as described above, or after further alloying treatment, it is cooled to a temperature of 350°C or lower at room temperature or higher. From the perspective of improving ductility, it is preferable to cool to a temperature range of 150°C or higher and 350°C or lower, and then perform heating as necessary and hold at a predetermined temperature. On the other hand, from the perspective of improving strength, it is preferable to cool to room temperature. The cooling rate when cooling to a temperature range of 150°C or higher and 350°C or lower is not particularly specified, but from the perspective of ensuring high TS and improving ductility, after hot-dip galvanizing, or after further alloying treatment, the cooling rate to 350°C is preferably 3°C / s or higher and 20°C / s or lower. Also, the cooling rate when cooling to room temperature is not particularly specified, but in order to further increase TS, after hot-dip galvanizing, or after further alloying treatment, the average cooling rate to 50°C is preferably 5°C / s or higher. On the other hand, due to production technology constraints, the average cooling rate to 50°C is preferably 40°C / s or lower. Also, this average cooling rate to 50°C is more preferably 7°C / s or higher and 30°C / s or lower. Also, the cooling rate below 50°C is not particularly limited, and it can be cooled to a predetermined temperature 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. Usually, high-strength hot-dip galvanized steel sheets are traded after being cooled to room temperature.

[0058] The hot-dip galvanized steel sheet cooled to 350°C or lower after hot-dip galvanizing or after alloying treatment may be rolled at a predetermined elongation rate. This elongation rate of rolling is preferably 0.05% or higher and 1.00% or lower. By setting this elongation rate of rolling to 0.05% or higher, cracks can be introduced into the zinc coating layer. By introducing cracks into the zinc coating layer, the amount of diffusible hydrogen in the steel sheet can be reduced, and as a result, bendability and hole expansion properties can be improved. On the other hand, when the elongation rate of rolling exceeds 1.00%, the YS increases and the dimensional accuracy during forming decreases. This elongation rate of rolling is more preferably 0.70% or lower and 0.10% or higher. The above rolling may be performed online in an apparatus continuous with the continuous hot-dip galvanizing facility, or may be performed offline from the continuous hot-dip galvanizing facility. Also, the target elongation rate (for example, 0.05% or more and 1.00% or less) may be achieved by a single rolling, or the target elongation rate may be achieved by performing multiple rollings. In general, temper rolling is performed as the above rolling, but rolling by a method such as processing with a leveler may be used as long as an elongation rate equivalent to that of temper rolling can be imparted.

[0059] After hot-dip galvanizing or after alloying treatment, it is cooled to 350°C or lower, and if necessary, after performing the above rolling, it may be held at room temperature or heat-treated in a temperature range exceeding room temperature and 450°C or lower. By holding at room temperature or heat-treating in a temperature range exceeding room temperature and 450°C or lower, the amount of diffusible hydrogen in the steel sheet can be reduced, and bendability and hole expansion properties can be improved. Also, from the viewpoint of improving ductility, when cooling in the temperature range of 150°C or higher and 350°C or lower as described above, after cooling, heating is performed if necessary, and it is preferably held (heat-treated) in the temperature range of 300°C or higher and 450°C or lower. Here, the holding time at room temperature is usually about 3 days to 10 months, and the heat treatment time exceeding room temperature is usually about 1 minute to 14 days. In addition, manufacturing conditions other than the above-described conditions can be based on conventional methods. The high-strength hot-dip galvanized steel sheet manufactured by the present invention can have a TS of 590 MPa or more. Also, when further increasing the strength, the TS can be 780 MPa or more, and further 980 MPa or more. The measurement of TS is performed as follows in accordance with JIS Z2241. From the hot-dip galvanized steel sheet, a JIS No. 5 test piece is sampled so that the longitudinal direction is perpendicular to the rolling direction of the steel sheet. Using this test piece, a tensile test is performed under the condition that the crosshead displacement speed Vc is 1.67×10 -1 mm / s, and the TS is measured. Also, the sheet thickness of the hot-dip galvanized steel sheet manufactured by the present invention is not particularly limited, but is usually about 0.3 mm or more and 2.8 mm or less.

Examples

[0060] [Example 1] A steel material with the component 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 - 600°C to obtain a hot-rolled steel sheet. After pickling this hot-rolled steel sheet, it was cold-rolled to obtain a cold-rolled steel sheet with a thickness of 1.4 mm. This cold-rolled steel sheet was annealed under the conditions shown in Tables 2 - 5 in a CGL, then hot-dip galvanized under the conditions shown in Tables 2 - 5. For some steel sheets, an alloying treatment was further performed after hot-dip galvanizing, and then cooled to 50°C or lower. Thereafter, temper rolling was performed at an elongation of 0.1% to obtain high-strength hot-dip galvanized steel sheets (GI) and high-strength alloyed hot-dip galvanized steel sheets (GA). For those without the alloying treatment, the alloying treatment columns in Tables 2 - 4 were marked as "-".

[0061] As the hot-dip galvanizing bath, when manufacturing GI, a hot-dip galvanizing bath containing 0.20 mass% of Al, with the balance being Zn and unavoidable impurities, was used. Also, when manufacturing GA, a hot-dip galvanizing bath containing 0.14 mass% of Al, with the balance being Zn and unavoidable impurities, was used. The plating adhesion amount was about 45 - 72 g / m 2 (double-sided plating) per side for GI, and about 45 - 55 g / m 2 (double-sided plating) per side for GA. Also, the composition of the plating layer of GI contained 0.1 - 1.0 mass% of Fe and 0.2 - 1.0 mass% of Al, with the balance being Fe and unavoidable impurities. The composition of the plating layer of GA contained 7 - 15 mass% of Fe and 0.1 - 1.0 mass% of Al, with the balance being Fe and unavoidable impurities.

[0062] Regarding the high-strength hot-dip galvanized steel sheets and high-strength alloyed hot-dip galvanized steel sheets obtained as described above, the tensile properties and plating appearance were evaluated as follows. The results are shown in Tables 2 - 5 together with the manufacturing conditions. ·Tensile strength The tensile test was conducted in accordance with JIS Z2241. From the obtained steel sheet, a JIS No. 5 test piece was taken such that the longitudinal direction was perpendicular to the rolling direction of the steel sheet. Using this test piece, a tensile test was carried out under the condition of a crosshead displacement speed Vc: 1.67×10 -1 mm / s, and TS was measured.

[0063] · Plating appearance The appearance of the plated steel sheet was visually observed to inspect for the presence or absence of gray dot defects and black dot defects. The observation was carried out on the front and back surfaces of the steel sheet in an area of 1000 mm in the coil width direction × 1000 mm in the coil longitudinal direction, and was performed for N = 5 sheets. In the observed area, if no defects were found, it was rated 3 points; if only minute gray dot defects or black dot defects of 0.2 mm or less were found, it was rated 2 points; if gray dot defects or black dot defects larger than 0.2 mm were found, it was rated 1 point; if conventional non-plating was found, it was rated 0 points. The higher the score of the steel sheet, the better the plating appearance, and a score of 2 or more was judged as passing. Also, in the case where no gray dot defects or black dot defects were found, in the case of a particularly beautiful surface condition where no minute plating unevenness that would be a sign of these defects was found, it was rated 3+ points.

[0064] As shown in Tables 2 to 5, all of the hot-dip galvanized steel sheets of the invention examples had good plating appearances. Also, a high strength of TS: 590 MPa or more was achieved, and a hot-dip galvanized steel sheet capable of achieving both high strength and excellent plating quality was obtained. On the other hand, in the hot-dip galvanized steel sheets of the comparative examples, appearance defects due to gray dot defects and black dot defects were recognized.

[0065]

Table 1

[0066]

Table 2

[0067]

Table 3

[0068]

Table 4

[0069]

Table 5

[0070] [Example 2] From the viewpoint of further reducing the ammonia concentration in the atmosphere of the soaking heat treatment process (soaking zone), hot-dip galvanized steel sheets were produced under the condition of increasing the flow rate of the high-purity gas in the soaking zone. Using the steel materials of Steels A, C, G, I, S, and T in Table 1, hot-dip galvanized steel sheets were produced under the production conditions (hot rolling, cold rolling, continuous annealing, hot-dip galvanizing, alloying treatment, temper rolling) according to Example 1. The tensile properties and plating appearance of the obtained hot-dip galvanized steel sheets were evaluated by the same method as in Example 1. The results are shown in Table 6 together with the production conditions. The concentration of ammonia gas was measured at the upper part of the furnace in the path including the central part of the line longitudinal direction of the CGL soaking zone. The ion chromatography method was used for the measurement method. As shown in Table 6, it can be seen that by reducing the ammonia concentration in the soaking heat treatment process (soaking zone) to 0.010% by volume or less, the surface quality (plating appearance) of the hot-dip galvanized steel sheet is further improved.

[0071]

Table 6

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. 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] is 2.5 or more; If necessary, further 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, 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, Cs: 0.10% or less, Contains one or more selected from A method for producing a high-strength hot-dip galvanized steel sheet, comprising the steps of: continuously annealing a steel sheet having a composition with the balance being Fe and unavoidable impurities; immersing the steel sheet in a hot-dip galvanizing bath to perform hot-dip galvanizing; and then optionally performing an alloying treatment, In the step of performing the continuous annealing, The steel sheet is heated in a temperature range of 300° C. or more and 500° C. or less in an atmosphere containing hydrogen: 3 vol. % or more, a dew point: −10° C. or less, and oxygen: 500 vol. ppm or less, The steel sheet is heated at an average heating rate of 1°C / s or more in an atmosphere containing 5% by volume of hydrogen and having a dew point of -40°C or less in a temperature range of 500°C or more and 750°C or less, A method for producing a high-strength hot-dip galvanized steel sheet, comprising 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.

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

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