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

Abstract

The purpose of the present invention is to produce a high-strength hot-dip galvanized steel sheet in which steel to which B has been added is used as a base steel sheet, said hot-dip galvanized steel sheet having a good surface appearance in which the occurrence of gray spot defects and black spot defects on the plated surface has been suppressed.　In an annealing step prior to plating, the generation of ammonia and nitriding of the steel sheet are suppressed by lowering the dew point in the 300-500°C temperature range to create an Fe reducing atmosphere, suppressing the generation of iron oxide to suppress the generation of ammonia, and heating quickly to a temperature range at or above 500°C where generation of ammonia becomes prominent and penetration of nitrogen into the steel sheet begins; and the generation of nitrides due to the diffusion of B into the steel sheet surface during annealing is suppressed by also shortening the annealing time in the temperature range at or above 750°C and setting the dew point at -55°C or higher to fix part of the B as an oxide. As a result, the generation of a nitride of B, which is a factor in the formation 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, and has almost no adverse effect of generating inclusions and deteriorating bendability and stress corrosion cracking characteristics. Therefore, it is widely used as an additive element for high-strength steel sheets.

[0003] Regarding a high-strength hot-dip galvanized steel sheet obtained by adding B to a base steel sheet, for example, Patent Document 1 discloses that 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: in a temperature range of 750 °C or higher, the dew point in the atmosphere is set to -40 °C or lower, thereby reducing the oxygen potential at the interface between the steel sheet and the atmosphere, suppressing surface enrichment of Si, Mn, etc. without forming internal oxidation, and obtaining an excellent plating appearance. In addition, Patent Document 2 discloses a technique in which the ratio of the Si enrichment amount to the Mn enrichment amount in the surface layer of the base steel sheet is 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 held with the dew point of the atmosphere in the region being -40 °C or lower, thereby obtaining excellent plating properties.

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 resulting in a 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 from the steel sheet due to weak adhesion between the steel sheet and the plating. The above 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 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 found 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 concavities that occur when, although the plating once adheres to the steel sheet without bouncing off the surface of the steel sheet after immersion treatment, the adhesion between the plating and the steel sheet is weak, so that when it comes into contact with the conveying roll, it peels off and adheres to the 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 that the 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 that 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 that defects occur due to the formation of nitrides of B.

[0008] (3) It is considered that nitrides of B are 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 nitrides of B. (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. Even under conditions of low oxygen potential, if B is fixed on the surface of the steel sheet 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 has been 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 surface of the steel sheet during annealing is suppressed; Also, by controlling the ratio of the contents of Mn and Si, i.e., 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 described above is obtained, contributing 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 percentage, C: 0.050% or more and 0.300% or less, Si: 0.80% or less, Mn: 2.30% 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 containing, and [%Mn] / [%Si] is 3.0 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, 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 unavoidable impurities is continuously annealed, then immersed in a hot-dip zinc bath for hot-dip galvanizing, and then, if necessary, subjected to an alloying treatment, 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 5% by volume or more of hydrogen, a dew point of -20°C or less, and 400 ppm by volume or less of oxygen, The steel sheet is heated at an average heating rate of 1 °C / s or more in an atmosphere with a temperature range of 500 °C or higher and 750 °C or lower, containing 5% by volume or more of hydrogen and a dew point of -40 °C or lower. A method for manufacturing a high-strength hot-dip galvanized steel sheet, wherein 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 200 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 750 °C or higher and 950 °C or lower in continuous annealing, the method for manufacturing a high-strength hot-dip galvanized steel sheet according to [1] or [2] above, wherein the atmosphere contains 0.010% by volume or less of ammonia.

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 (plating appearance) with the generation of gray dot defects and black dot defects on the plating surface suppressed. The high-strength hot-dip galvanized steel sheet manufactured by the present invention is suitable for use as a structural member 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] The object of the present invention to be manufactured is a high-strength hot-dip galvanized steel sheet having a hot-dip galvanized layer on one 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% by mass or less, Al: 0.001% by mass or more and 1.0% by mass or less, and further contains a total of 0% by mass or more and 3.5% by 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, and the balance may have a composition consisting of Zn and unavoidable impurities. Generally, in the case of a hot-dip galvanized steel sheet (GI), the Fe content in the plating layer is less than 7% by mass, and in the case of an alloyed hot-dip galvanized steel sheet (GA), the Fe content in the plating layer is 7% by mass or more and 15% by mass or less, preferably 8% by mass or more and 13% by 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 "% by 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 or 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 the quenched martensite and tempered martensite increases, and the hardness of the 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 punchability, elongation flangeability, and bendability. Therefore, the C content shall be 0.050% or more and 0.300% or less. Also, the C content is preferably 0.060% or more in order to make TS 780 MPa or more, and more preferably 0.090% or more in order to make TS 980 MPa or more. Also, since 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: 0.80% 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 0.80%, the amount of Si enrichment on the steel sheet surface during annealing increases, and silicon oxide, which causes non-plating defects on the steel sheet surface, is formed, making it difficult to achieve good plating properties. By setting the Si content to 0.80% 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, making it possible to suppress gray point defects and black point defects. Therefore, the Si content shall be 0.80% or less. Also, from the above viewpoints, the Si content is preferably 0.65% or less. The lower limit of the Si content is not specifically set. That is, the Si content may be 0%, but refining costs increase when it is reduced to less than 0.01%, so 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: 2.30% 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 2.30% 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 gray dot defects and black dot defects are suppressed. Also, Mn is an element effective for 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 2.30%, B that forms a composite oxide with Mn during annealing decreases, and the formation amount of BN increases, so black dot defects and gray dot defects cannot be sufficiently suppressed. On the other hand, when the 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, which causes non-plating defects on the steel sheet surface, is formed, making it difficult to achieve good plating properties. Therefore, the Mn content is 2.30% 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 2.40% 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 is an element that has the effect of solid-solution strengthening and increases 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-mentioned 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 the refining cost increases to control it below 0.001%, so 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 deformation ability 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 the refining cost increases to control it below 0.0001%, so the content of S is preferably 0.0001% or more. Also, from the above-mentioned 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 sol.B effective for increasing strength. Further, Al also has the effect of suppressing carbide formation during annealing and increasing the volume fraction of retained austenite. The generated retained austenite has the effect of improving ductility. In order to obtain the effect of fixing N as AlN, it is preferable that sol.Al contains 0.02% or more. Also, from the viewpoint of obtaining the effect of improving ductility, it is more preferable that sol.Al contains 0.05% or more. However, if the amount of sol.Al exceeds 1.00%, non-plating occurs, so the amount of sol.Al is 1.00% or less. Also, from the above 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 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 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 the refining cost increases 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 effects, 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 due to 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.

[0020] · [%Mn] / [%Si]: 3.0 or more [%Mn] is the content of Mn, and [%Si] is the content of Si. By setting [%Mn] / [%Si] to 3.0 or more, the formation of simple Si oxides is suppressed, non-plating is suppressed, and in the annealing method of the present invention, Mn,B composite oxides are formed, and gray dot defects and black dot defects are suppressed. Therefore, [%Mn] / [%Si] is set to 3.0 or more. Also, from the viewpoint of suppressing gray dot defects and black dot defects, [%Mn] / [%Si] is preferably 4.2 or more, and more preferably 12.0 or more. In particular, by setting [%Mn] / [%Si] to 12.0 or more, the formation of Mn,B composite oxides is promoted, and the formation of B nitrides, which cause 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 Mn,B composite oxides is particularly remarkably promoted, and minute unevenness that is a sign of gray dot defects and black dot defects can also be suppressed. Therefore, it is more preferable to set [%Mn] / [%Si] to 14.0 or more. The upper limit is not particularly defined, but in order to suppress the formation of excessive Mn,B composite oxides, 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 one or more selected from the following, by 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, 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 necessary, and the effects of the present invention can be obtained even if the content is 0%. Therefore, each of the above elements may be 0%.

[0022] Cr is an element that increases hardenability. It is effective for generating a desired amount of hardened martensite or tempered martensite, making the TS 590 MPa or more, 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 or tempered martensite increases, the area ratio of ferrite or bainite decreases, and the dimensional accuracy during forming decreases. Therefore, when adding Cr, its content is preferably 1.00% or less. Further, from the viewpoint of improving the plating appearance quality, the Cr content is more preferably 0.75% or less. Also, in order to obtain the effect of improving hardenability by Cr, 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, 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. From the above-mentioned viewpoints, the Ti content is more preferably 0.100% or less. Also, in order 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 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. From the above-mentioned viewpoints, the Nb content is more preferably 0.100% or less. Also, in order 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. From the above-mentioned 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 fall within a more suitable range, increasing the 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 set the TS at 590 MPa or more, and the dimensional accuracy during forming decreases. Furthermore, 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 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 hardened martensite and tempered martensite fall within a more suitable range, increasing the TS more, and further improving the dimensional accuracy during forming. However, when the Cu content exceeds 1.000%, the area ratio of hardened martensite and tempered martensite increases, making it difficult to set the TS at 590 MPa or more and obtain 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. Also, from the above viewpoints, the Cu content is more preferably 0.200% or less. Also, in order 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 hardened martensite and tempered martensite fall within a more suitable range, increasing TS, and further improving dimensional accuracy during forming. However, when the Ni content exceeds 0.500%, the area ratio of hardened martensite and tempered martensite increases, and TS and dimensional accuracy during forming decrease. Also, coarse precipitates and inclusions increase. 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 Ni, its content is preferably 0.500% or less. Also, 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 sheet during annealing and obtaining better plating properties. However, when the Sn content exceeds 0.200%, coarse precipitates and inclusions increase. When the base 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 Sn, its content is preferably 0.200% or less. Also, from the above viewpoints, the Sn content is more preferably 0.050% or less. Also, 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 effective element for spheroidizing the shape of inclusions such as sulfides and oxides, improving the ultimate deformability of the steel sheet, and enhancing 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 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 Mg, its 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 sheet. When the Ca content exceeds 0.0100%, when the base steel sheet 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 effective element for improving the stretch flange formability by spheroidizing the shape of inclusions and improving 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. Also, from the above viewpoints, the Zn content is more preferably 0.020% or less, and even more preferably 0.010% or less. Further, 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 the stretch flange formability by spheroidizing the shape of inclusions and improving 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. Also, from the above viewpoints, the Co content is more preferably 0.010% or less. Further, 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 the stretch flange formability by spheroidizing the shape of inclusions and improving 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. Also, from the above viewpoints, the Zr content is more preferably 0.010% or less. Further, 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 also an element effective in improving the stretch flange formability by spheroidizing the shape of inclusions and improving the ultimate deformability of the steel sheet. However, when the total REM content 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 total content is preferably 0.0100% or less. Also, from the above viewpoints, the total REM content is more preferably 0.0080% or less. Further, in order to obtain the above 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 metal steel plate and can be contained as needed. When Ta is contained at 0.005% or more, the effect of strength improvement can be obtained. However, from the viewpoint of preventing cost increase, when Ta is contained, the Ta content is preferably 0.10% or less.

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

[0037] When As is contained at 0.001% or more, the form of sulfide can be controlled and ductility and toughness can be improved. However, from the viewpoint of preventing cost increase, when As is contained, the As content is preferably 0.10% or less.

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

[0039] When Bi is contained at 0.001% or more, grain boundary segregation can be suppressed and ductility and toughness can be improved. In addition, Bi has the effect of improving machinability and the smoothness of the cut end face, and has the effect of improving the anti-delayed fracture characteristics of the cut surface. When Bi is contained, from the viewpoint of preventing cost increase, the Bi content is preferably 0.10% or less.

[0040] When Pb is contained at 0.001% or more, grain boundary segregation can be suppressed and ductility and toughness can be improved. In addition, Pb has the effect of improving machinability and the smoothness of the cut end face, and has the effect of improving the anti-delayed fracture characteristics of the cut surface. When Pb is contained, from the viewpoint of preventing cost increase, the Pb content is preferably 0.10% or less.

[0041] Even if Ge is contained in an amount of 0.001% or more, it does not significantly affect the mechanical properties and surface quality. From the perspective of preventing cost increase, when Ge is contained, the Ge content should be 0.10% or less.

[0042] Even if Sr is contained in an amount of 0.001% or more, it does not significantly affect the mechanical properties and surface quality. From the perspective of preventing cost increase, when Sr is contained, the Sr content should be 0.10% or less.

[0043] Even if Cs is contained in an amount of 0.001% or more, it does not significantly affect the mechanical properties and surface quality. From the perspective of preventing cost increase, when Cs is contained, the Cs content should be 0.10% or less. The balance other than the components described above is Fe and inevitable 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 on the downstream side of this annealing furnace. This hot-dip galvanizing device includes a hot-dip galvanizing bath and a snout connected to the steel strip outlet side of the annealing furnace with its tip immersed in the hot-dip galvanizing bath. 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 in which a heating zone, a soaking zone, and a cooling zone are provided in this order. Specific annealing conditions are as follows. Although the number of annealing times is not particularly limited, in the present invention, since gray spot defects and black spot defects can be suppressed by one-time annealing, it is preferably one time (one-time annealing method).

[0045] The occurrence of surface defects (gray spot defects, black spot defects), which is the problem to be solved by 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, the problem has not been recognized, and it 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) B nitride is mainly caused by two factors, namely, (i) nitrogen (N) generated from ammonia in the atmosphere penetrates or adsorbs into the steel sheet and reacts with B under conditions where the oxygen potential is low; (ii) when B does not form an oxide under conditions where the oxygen potential is low, B diffuses to the steel sheet surface layer in a solid solution state and forms a nitride on the surface layer. It is considered to occur by this. Regarding the factor (i) above, ammonia uses N2 and H2 in the atmosphere as raw materials, and its generation is remarkably 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 amount of ammonia generation is reduced, the formation of B nitride can be suppressed. Regarding the factor (ii) above, even under conditions where the oxygen potential is low, if B is fixed on the steel sheet surface or inside the steel sheet as an oxide that hardly affects the plating appearance quality, the formation of B nitride can be suppressed.

[0047] (3) In the continuous annealing process before plating, by performing a series of controls on the annealing conditions as follows, the formation of nitrides 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) 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 formation of ammonia, which is a factor in the formation of nitrides of B; (ii) Further, by quickly heating in the temperature range of 500 °C or higher and 750 °C or lower where the formation of ammonia becomes significant and the intrusion and adsorption of nitrogen into the steel sheet start, nitriding of the steel sheet is suppressed, and the formation of nitrides 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 ( soaking zone ) and optimizing the dew point to fix a part of B as an oxide, the formation of nitrides due to the diffusion of B on the surface of the steel sheet during annealing is suppressed. Therefore, in the present invention, continuous annealing is performed under a series of conditions optimized so as to obtain the action of (3) above, 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 the temperature range of 300 °C or higher and 500 °C or lower, the steel sheet is heated in an atmosphere containing 5% by volume or more of hydrogen, a dew point of -20 °C or lower, and an oxygen concentration of 400 ppm by volume or lower. In this temperature range, by creating a reducing atmosphere of Fe, the formation of iron oxide is suppressed, and the formation of ammonia ( especially the formation of ammonia that becomes significant in the temperature range of 500 °C or higher ), which is a factor in the formation of nitrides of B, is suppressed. · Hydrogen concentration in the atmosphere: 5% by volume or more Since hydrogen is a reducing gas, it is possible to suppress the oxidation of the surface of the steel sheet during annealing. In order to sufficiently obtain the oxidation suppression effect, 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.

[0049] · Dew point of the atmosphere: -20 °C or lower In the low temperature range of 300 to 500 °C, when the dew point exceeds -20 °C, iron oxidation occurs on the steel plate surface, promoting the generation of ammonia in the temperature range of 500 °C and above. By setting the dew point to -20 °C or lower, it is possible to suppress the oxidation of Fe on the steel plate surface as a reducing atmosphere for Fe. Therefore, the dew point should be -20 °C or lower, and preferably -40 °C or lower to sufficiently obtain the effect of suppressing the formation of iron oxide. The lower limit is not particularly specified, but from the perspective of preventing cost increase due to reducing the dew point, -60 °C or higher is preferable. · Oxygen concentration in the atmosphere: 400 volume ppm or less To suppress the oxidation of Fe during annealing, the oxygen concentration should be 400 volume ppm or less, preferably 200 volume ppm or less. The remainder of the atmosphere gas other than hydrogen, H2O, and oxygen (95 volume % or less) is preferably N2 gas and inevitable impurities. Furthermore, 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 volume % or less. The lower limit is not particularly specified, but from the perspective of preventing cost increase due to removing inevitable impurities, 0.01 volume % or more is preferable. · Average heating rate in the temperature range of 300 °C or higher and 500 °C or lower As described above, ammonia, which is the nitriding source of B, is promoted in its generation using iron oxide and pure iron obtained by reducing iron oxide as a catalyst. Therefore, from the perspective of suppressing the occurrence of gray dot defects and black dot defects, it is preferable to set the average heating rate to 10 °C / s or lower, ensure sufficient annealing time in the reducing atmosphere of Fe, and reduce the iron oxide on the steel plate surface. Thereby, it is possible to form Mn and B oxides on the steel plate surface as early as possible in the heating temperature range of 500 °C and above to coat the steel plate surface, and suppress the exposure of the pure iron layer in the surface layer of the steel plate during heating and soaking holding. On the other hand, considering productivity, the average heating rate is preferably 1 °C / s or higher. Also, from the above-mentioned perspective, a more preferable range of the average heating rate is 2 °C / s or higher and 7 °C / s or lower.

[0050] In the continuous annealing process, the steel sheet is heated at 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 less in a temperature range of 500 °C or more and 750 °C or less. In this temperature range, 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 nitrides 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. 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.

[0051] · Dew point of the atmosphere: -40 °C or less When 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. In addition, due to the occurrence of new oxidation of Fe, the suppression of ammonia generation in this heating process of 500 °C or more and soaking process of 750 °C or more becomes insufficient, and the suppression of ash point defects and black point defects becomes insufficient. Therefore, the dew point is set to -40 °C or less. Further, from the viewpoint of suppressing ash point defects and black point defects, the dew point of the atmosphere is more preferably -42 °C or less. The lower limit is not particularly specified, but from the viewpoint of preventing cost increase due to reducing the dew point, it is preferably -60 °C or more. The remainder (95% by volume or less) of the atmosphere gas other than hydrogen, H2O, and oxygen 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 specified, 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: 1 °C / s or more The nitridation reaction through the formation of ammonia easily occurs in the temperature range of 500 to 750 °C. Therefore, from the perspective of suppressing gray dot defects and black dot defects, it is necessary to rapidly heat this temperature range. 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 infiltrating into the steel sheet increases, and nitrides of B are formed in the subsequent soaking heat treatment process, resulting in appearance defects due to gray dot defects and black dot 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. The upper limit is not particularly defined, but it is preferably 20 °C / s or less which can be achieved in a general annealing furnace.

[0052] In the continuous annealing process, the steel sheet is subjected to soaking heat treatment at a temperature of 750 °C or higher and 950 °C or lower, in an atmosphere containing 5% by volume or more of hydrogen, with a dew point of -55 °C or higher and -40 °C or lower, and a holding time of 20 to 200 seconds. In this soaking heat treatment process, the nitridation reaction of B due to ammonia and nitrogen infiltrated into the steel is suppressed, and by fixing a part of B as an oxide on the steel sheet surface or inside the steel sheet, the generation of nitrides due to the diffusion of B to the steel sheet surface during annealing is suppressed. By these means, the amount of B nitrides generated 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 perspective 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 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 plating defects caused by oxides. On the other hand, when the dew point is less than -55°C, part of B is fixed as an oxide, and the effect of suppressing the formation of nitrides due to the diffusion of B to the surface of the steel sheet during annealing is not sufficiently obtained. In addition, when the oxygen potential of the annealing atmosphere is low and the nitrogen potential is high, the formation of nitrides becomes stable, and the formation of nitrides of B on the surface of the steel sheet is promoted. As a result, the occurrence of gray dot defects and black dot defects cannot be appropriately suppressed. Therefore, the dew point should be -55°C or higher, preferably -50°C or higher. Also, the dew point should be -40°C or lower, preferably -45°C or lower. · Holding time: 20 seconds or more and 200 seconds or less When 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, so the area ratio of ferrite and bainite increases, making it difficult to make TS 590 MPa or more. On the other hand, when the holding time exceeds 200 seconds, although part of B is fixed as an oxide, part of the remaining B generates nitrides, so the amount of nitrides of B formed on the surface of the steel sheet increases, and the occurrence of gray dot defects and black dot defects cannot be appropriately suppressed. Therefore, the holding time should be 20 seconds or more, preferably 30 seconds or more. Also, the holding time should be 200 seconds or less, preferably 100 seconds or less. Note that this holding time refers to the time when the steel sheet 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 during the soaking heat treatment process to 0.010% by volume or less. The reasons for ammonia being contained in the atmosphere of 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 a new high-purity gas from the rear to the front (from the outlet side to the inlet side of the soaking zone) in the advancing direction of the steel sheet. By reducing the ammonia concentration in the soaking zone to further suppress the formation of nitride of B, it is possible to further suppress appearance defects caused by gray dot defects and black dot defects. Therefore, it is preferable to set the ammonia concentration in the atmosphere to 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 molten zinc plating bath to perform a molten zinc plating treatment. The reaching temperature of the cooling is set to 200 to 520°C, and it is preferable to heat it as necessary and then immerse it in the molten zinc plating bath. The bath temperature of the molten zinc plating bath is generally about 440 to 500°C. The molten zinc plating bath is not particularly limited. For example, it can have an Al content of 0.10% by mass or more and 0.23% by mass or less, and further contain a total of 0% by mass or more and 3.5% by 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 Zn and unavoidable impurities. Also, in order to prevent changes in the bath temperature of the plating bath, the temperature of the steel sheet before plating (immersion plate temperature) is preferably not lower than the plating bath temperature and not higher than the plating bath temperature + 50°C.

[0056] After the above-described hot-dip galvanizing treatment, an alloying treatment of the galvanized layer may be further performed to form an alloyed hot-dip galvanized layer. The alloying treatment is preferably carried out in a temperature range of 480°C or higher and 570°C or lower. 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 carried out 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 preferably 20 to 80 g / m per side. 2 The coating weight of the plating can be adjusted by performing gas wiping or the like after hot-dip galvanizing.

[0057] After hot-dip galvanizing as described above, or after further performing an alloying treatment, it is cooled to a temperature of room temperature or higher and 350°C or lower. From the viewpoint 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 viewpoint 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 viewpoint of ensuring high TS and improving ductility, after hot-dip galvanizing or after further performing an alloying treatment, the cooling rate to 350°C is preferably 3°C / s or more and 20°C / s or less. 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 performing an alloying treatment, the average cooling rate to 50°C is preferably 5°C / s or more. On the other hand, due to production technology constraints, the average cooling rate to 50°C is preferably 40°C / s or less. Also, this average cooling rate to 50°C is more preferably 7°C / s or more and 30°C / s or less. 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 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 become the objects of transactions 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. The elongation rate of this rolling is preferably 0.05% or more and 1.00% or less. By setting the elongation rate of this rolling to 0.05% or more, 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. The elongation rate of this rolling is more preferably 0.70% or less and 0.10% or more. The above rolling may be performed online in an apparatus continuous with the continuous hot-dip galvanizing facility, or may be performed offline with respect to the continuous hot-dip galvanizing facility. Also, the target elongation rate (for example, 0.05% or more and 1.00% or less) may be set by a single rolling, or the target elongation rate may be set by performing multiple rollings. In addition, as the above rolling, temper rolling is generally performed, 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 cooling to 350°C or lower after hot-dip galvanizing or after alloying treatment, and performing the above rolling as necessary, 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, it is preferable to perform heating as necessary after cooling and hold (heat-treat) 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 those described above 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. When further increasing the strength, the TS can be 780 MPa or more, and further 980 MPa or more. The measurement of TS is carried out as follows in accordance with JIS Z2241. From the hot-dip galvanized steel sheet, a JIS No. 5 test piece is taken so that the longitudinal direction is perpendicular to the rolling direction of the steel sheet. Using this test piece, a tensile test is carried out under the condition that the crosshead displacement speed Vc is 1.67×10 -1 mm / s, and the TS is measured. In addition, the 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 having the component composition shown in Table 1 (the balance being Fe and inevitable impurities) was melted in a converter and continuously cast into a steel slab. This steel slab was heated to 1250°C and rough-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. 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 to 4 in a CGL, then hot-dip galvanized under the conditions shown in Tables 2 to 4, and some of the steel sheets were further subjected to an alloying treatment after hot-dip galvanizing and cooled to 50°C or lower. Thereafter, temper rolling was carried out at an elongation rate of 0.1% to obtain a high-strength hot-dip galvanized steel sheet (GI) and a high-strength alloyed hot-dip galvanized steel sheet (GA). For those without the alloying treatment, the columns for the alloying treatment in Tables 2 and 3 were marked as "-".

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

[0062] Regarding the high-strength hot-dip galvanized steel sheet and high-strength alloyed hot-dip galvanized steel sheet obtained as described above, the tensile properties and plating appearance were evaluated as follows. The results are shown in Tables 2 - 4 together with the manufacturing conditions. ·Tensile strength The tensile test was carried out in accordance with JIS Z2241. From the obtained steel sheet, a JIS No. 5 test piece was taken so that the longitudinal direction was perpendicular to the rolling direction of the steel sheet. Using this test piece, a tensile test was conducted under the condition of a crosshead displacement speed Vc: 1.67×10 -1 mm / s, and the TS was measured.

[0063] ·Plating appearance The appearance of the galvanized 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 with an area of 1000 mm in the coil width direction × 1000 mm in the coil longitudinal direction, and was performed on 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-galvanized areas were found, it was rated 0 points. The higher the score of the steel sheet, the better the galvanized 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, and there were no minute plating unevennesses that could be a sign of these defects, and the surface property was particularly beautiful, it was rated 3+ points.

[0064] As shown in Tables 2 to 4, all of the hot-dip galvanized steel sheets of the invention examples had good galvanized appearances. Also, a high strength of TS: 590 MPa or more was achieved, and a hot-dip galvanized steel sheet that can achieve both high strength and excellent plating quality has been 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] [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 high-purity gas in the soaking zone. Using the steel materials of steels B, O, A, М, R, 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 5 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 as the measurement method. As shown in Table 5, 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.

[0070]

Table 5

Claims

1. In mass percent, C: 0.050% or more and 0.300% or less, Si: 0.80% or less, Mn: 2.30% or more and 3.50% or less, P: 0.100% or less, S: 0.0100% or less, sol. Al: 1.00% or less, N: 0.0200% or less, B: 0.0001% or more and 0.0050% or less and [%Mn] / [%Si] is 3.0 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: 5 vol. % or more, a dew point: −20° C. or less, and oxygen: 400 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 200 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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