High-strength galvannealed steel sheet and method for manufacturing the same

Abstract

The present invention provides a galvannealed steel sheet having a tensile strength of 780 MPa or more and excellent release properties of diffusible hydrogen from the steel sheet, and a method for producing the same. On a steel sheet with a specified composition, the coating weight per side is 20 g / m 2 More than 120g / m 2 The steel sheet has the following alloyed hot-dip galvanized layer, and the oxygen content of the surface layer of the steel sheet within 100 μm from the surface of the base steel sheet directly below the galvanized layer toward the center of the sheet thickness is 0.030 g / m per side 2 The number of crack intersections in the coating layer is less than 500 / mm on a surface parallel to the surface of the steel sheet at a depth of M [μm] (1≦M≦5) from the surface of the zinc coating layer. 2 The high-strength galvannealed steel sheet has the above-mentioned diffusible hydrogen content in the steel of 0.30 mass ppm or less and a tensile strength of 780 MPa or more.

Description

[Technical Field]

[0001] The present invention relates to a high-strength galvannealed steel sheet having excellent ability to release diffusible hydrogen from steel, and a method for producing the same. [Background technology]

[0002] In recent years, efforts have been made to reduce CO2 emissions by improving automobile fuel efficiency. One effective way to improve fuel efficiency is to reduce the weight of the vehicle body by using thinner steel sheets, which are used to make the body. However, this thinning also reduces crashworthiness. Therefore, to maintain crashworthiness, there is a demand for higher strength steel sheets themselves, and the same demand exists for hot-dip galvanized steel sheets, which have rust-resistant properties.

[0003] However, new problems have arisen with the increase in the strength of steel plates. One of these is delayed fracture, a phenomenon in which, when high-strength steel plates are subjected to static load stress (load stress less than the tensile strength), they suddenly fracture due to brittleness, with little apparent plastic deformation, after a certain period of time.

[0004] One of the causes of delayed fracture is thought to be hydrogen embrittlement due to diffusible hydrogen in steel. Hot-dip galvanized steel sheets are generally produced by using hot-rolled or cold-rolled steel sheets as base materials, recrystallizing the base steel sheets in the annealing furnace of a continuous galvanizing line (CGL), and then hot-dip galvanizing the steel sheets. Galvannealed steel sheets are produced by further alloying treatment after hot-dip galvanizing. Among these, CGL annealing requires a reducing atmosphere containing hydrogen to suppress oxidation of the steel sheet surface, which causes bare spots, and to obtain a good coating appearance. However, hydrogen in this atmosphere penetrates the steel sheet and remains as diffusible hydrogen, degrading its delayed fracture resistance. In particular, hydrogen in the steel tends to remain after annealing in high-strength steel sheets with a tensile strength of 780 MPa or more, resulting in a significant decrease in delayed fracture resistance. This is because high-strength steel sheets with a tensile strength of 780 MPa or more require the formation of hard structures such as martensite or bainite to achieve the required strength, and for this to occur, the austenite phase must be generated during the annealing process.However, the austenite phase is more likely to absorb large amounts of hydrogen than the ferrite phase, and because the hydrogen diffusion rate is slow, once hydrogen is absorbed during the annealing process, it is difficult to release it during the cooling process.Furthermore, when hot-dip galvanizing is applied, the hydrogen permeation rate through the galvanized layer is even slower, making it more likely that diffusible hydrogen will remain in the steel, which has been an issue.

[0005] Therefore, in order to improve delayed fracture resistance, it is necessary to release diffusible hydrogen that has been incorporated into the steel during the manufacturing process, particularly in high-strength hot-dip galvanized steel sheets. For example, Patent Document 1 proposes a method in which hydrogen-containing steel is heated at a predetermined temperature through a baking treatment, causing the hydrogen to diffuse and be released from the steel surface. Furthermore, Patent Documents 2 and 3 propose methods for releasing hydrogen through a coating layer, in which a certain number of cracks are formed in the coating, and hydrogen in the steel sheet is released through the cracks to the outside of the steel sheet. [Prior art documents] [Patent documents]

[0006] [Patent Document 1] Japanese Patent Application Publication No. 7-173646 [Patent Document 2] Japanese Patent Application Publication No. 6-33213 [Patent Document 3] International Publication No. 2018 / 124157 Summary of the Invention [Problem to be solved by the invention]

[0007] However, in the baking treatment of Patent Document 1, in the case of hot-dip galvanized steel sheets, it is often difficult to release hydrogen through the coating layer because the coating layer is thick. Furthermore, if the baking treatment temperature is increased to promote hydrogen release, problems arise such as changes in the properties of the coating layer and the mechanical properties of the steel sheet.

[0008] Furthermore, the method of forming cracks during plating as in Patent Documents 2 and 3 has the problem that even if cracks are formed in the same manner as in Patent Documents 2 and 3, the rate of hydrogen release varies greatly depending on the manufacturing method.

[0009] The present invention has been made in view of the above circumstances, and an object of the present invention is to provide a high-strength galvannealed steel sheet having a tensile strength of 780 MPa or more, good release of diffusible hydrogen in steel, and excellent resistance to delayed fracture. Here, the delayed fracture resistance can be evaluated by the method described in the Examples, and when the evaluation criteria of the Examples are satisfied, the delayed fracture resistance is judged to be excellent. [Means for solving the problem]

[0010] The present inventors have found the following regarding hydrogen release from high-strength galvannealed steel sheets. They found that hydrogen release is affected not only by the presence or absence of cracks in the coating, but also by their morphology, and that it is particularly important that there be a certain number of crack intersections near the interface between the steel sheet and the coating. Furthermore, they found that a coating layer with a large number of crack intersections can be formed by optimizing the annealing atmosphere before hot-dip galvanizing and the cooling rate after alloying. The present invention was made based on these findings, and the gist of the present invention is as follows. [1] A high-strength galvannealed steel sheet having a coating layer on a base steel sheet, The composition of the base steel sheet is, in mass%, C: 0.06% or more and 0.30% or less, Si: 0.01% or more but less than 1.50%, Mn: 1.5% or more and 3.5% or less, P: 0.1% or less (not including 0%), S: 0.03% or less (not including 0%), sol.Al: 0.1% or less (not including 0%), N: 0.007% or less (not including 0%), O: 0.003% or less (not including 0%), the mass ratio of Si to Mn (Si / Mn) is 0.25 or less, and the balance is Fe and unavoidable impurities, The coating weight per side of the base steel sheet is 20 g / m 2 More than 120g / m 2 The following alloyed hot-dip galvanized layer is provided: The oxygen content of the surface layer of the steel sheet within 100 μm from the surface of the base steel sheet directly below the zinc coating layer toward the center of the sheet thickness is 0.030 g / m per side 2 is less than The number of intersections of cracks in the coating layer is 500 / mm on a surface parallel to the surface of the steel sheet at a depth M [μm] (1≦M≦5) from the surface of the zinc coating layer. 2 or more, the amount of diffusible hydrogen in the steel is 0.30 mass ppm or less, and the tensile strength is 780 MPa or more. [2] The high-strength galvannealed steel sheet according to [1], wherein the base steel sheet further contains, in mass %, one or more components selected from the following groups A to D: Group A Nb: 0.05% or less (excluding 0%) Ti: 0.08% or less (excluding 0%) V: 0.2% or less (excluding 0%) W: 0.15% or less (excluding 0%) Zr: 0.15% or less (excluding 0%), one or more selected from the above; Group B Cr: 1.0% or less (excluding 0%) Ni: 1.0% or less (excluding 0%) Cu: 1.0% or less (excluding 0%) Mo: 1.0% or less (excluding 0%) Co: 1.0% or less (excluding 0%) B: 0.005% or less (excluding 0%), one or more selected from the above; Group C Ca: 0.005% or less (excluding 0%) Mg: 0.005% or less (excluding 0%) REM: 0.005% or less (excluding 0%), one or more selected from the following; Group D Sn: 0.2% or less (excluding 0%) Sb: 0.2% or less (excluding 0%). [3] The high-strength galvannealed steel sheet according to [1] or [2], wherein the base steel sheet further contains, in mass %, one or more groups selected from the following groups E to H as the chemical composition: Group E Ta: 0.10% or less (excluding 0%); F group Te: 0.10% or less (excluding 0%) As: 0.10% or less (excluding 0%) Hf: 0.10% or less (excluding 0%), one or more selected from the following; G group Bi: 0.20% or less (excluding 0%) Pb: 0.20% or less (excluding 0%), one or more selected from the following; H group Zn: 0.10% or less (excluding 0%) Ge: 0.10% or less (excluding 0%) Sr: 0.10% or less (excluding 0%) Cs: 0.10% or less (excluding 0%), one or more selected from the following. [4] A high-strength galvannealed steel sheet according to any one of [1], [2] and [3], wherein the steel structure in the range from the surface of the base steel sheet to a depth of 1 / 8 to 3 / 8 of the sheet thickness, in terms of area %, has a total area ratio of martensite, bainite and retained γ of 30% or more. [5] A high-strength galvannealed steel sheet according to any one of [1], [2] and [3], wherein the steel structure in the range from the surface of the base steel sheet to a depth of 1 / 8 to 3 / 8 of the sheet thickness has a total area ratio of martensite, bainite and retained γ of 50% or more in area %, and the tensile strength is 980 MPa or more. [6] A high-strength galvannealed steel sheet according to any one of [1], [2] and [3], wherein the steel structure in the range from the surface of the base steel sheet to a depth of 1 / 8 to 3 / 8 of the sheet thickness has a total area ratio of martensite, bainite and retained γ of 70% or more in area %, and the tensile strength is 1180 MPa or more. [7] A high-strength galvannealed steel sheet according to any one of [1], [2] and [3], wherein the steel structure in the range from the surface of the base steel sheet to a depth of 1 / 8 to 3 / 8 of the sheet thickness has a total area ratio of martensite, bainite and retained γ of 85% or more in area %, and the tensile strength is 1310 MPa or more. [8] A high-strength galvannealed steel sheet according to any one of [1], [2] and [3], wherein the steel structure in the range from the surface of the base steel sheet to a depth of 1 / 8 to 3 / 8 of the sheet thickness has a total area ratio of martensite, bainite and retained γ of 90% or more in area %, and the tensile strength is 1470 MPa or more. [9] A method for producing a high-strength galvannealed steel sheet having a composition according to any one of [1], [2], or [3], When annealing and hot-dip galvanizing treatments are performed on a steel sheet in a continuous hot-dip galvanizing facility, the dew point of the atmosphere in the annealing heating furnace in a temperature range where the steel sheet temperature in the annealing heating furnace is 700°C or higher is -40°C or lower, and the atmosphere in the heating furnace contains at least one of SO2 of 0.1 volppm or more and 3.0 volppm or less and HCl of 0.5 volppm or more and 10.0 volppm or less in addition to 20.0 vol% or less of hydrogen, A method for producing a high-strength galvannealed steel sheet, in which the cooling rate from the alloying treatment of the zinc-coated layer to 250°C is 5°C / sec or more, and after cooling to room temperature, one or both of the following steps (1) and (2) are carried out: (1) After cooling to room temperature, the mixture is kept at room temperature for 48 hours or more. (2) After cooling to room temperature, the mixture is reheated to a temperature of 50°C or higher and 400°C or lower and maintained at this temperature for 0.1 hours or longer. [Effects of the Invention]

[0011] According to the present invention, a high-strength galvannealed steel sheet having excellent delayed fracture resistance can be obtained due to improved release of diffusible hydrogen from the steel. The high-strength galvannealed steel sheet obtained by the present invention is suitable for structural members such as automobile parts, and by applying it to such applications, it is possible to reduce the weight of the vehicle body and thereby improve fuel efficiency. DETAILED DESCRIPTION OF THE INVENTION

[0012] The present invention is directed to a high-strength galvannealed steel sheet having a hot-dip galvanized layer on at least one side of a substrate steel sheet (base steel sheet) and being alloyed after hot-dip galvanizing. The term "high strength" means that the tensile strength TS of the steel sheet measured in accordance with JIS Z2241 (2011) is 780 MPa or more.

[0013] The chemical composition of the base steel sheet (base steel sheet) and the reasons for its limitations will be explained below. In the following explanation, "%" representing the content of the component elements in the base steel sheet means "mass %" unless otherwise specified. Tensile strength is referred to as TS. Furthermore, "steel sheet" may include hot-dip galvanized steel sheet and galvannealed hot-dip galvanized steel sheet.

[0014] C: 0.06% or more and 0.30% or less C has the effect of improving workability by forming martensite or the like as a steel structure, but in order to obtain good weldability, the C content must be 0.30% or less, and more preferably 0.25% or less.In order to obtain good workability, the C content must be 0.06% or more, and preferably 0.09% or more.

[0015] Si: 0.01% or more and less than 1.50% Silicon is an effective element for achieving high strength in steel sheets because it has a significant effect of increasing the strength of steel through solid solution (solid solution strengthening ability) without significantly impairing workability. However, silicon is also an element that adversely affects the resistance weld cracking resistance of welds. When silicon is added to steel sheets to increase their strength, a content of 0.01% or more is required. On the other hand, if the Si content is 1.50% or more, the amount of Si concentrated on the surface of the substrate steel sheet during CGL annealing increases, and Si oxides that cause unplated defects are formed on the surface of the substrate steel sheet, making it difficult to achieve good galvanizability. Therefore, it is necessary to contain Si in a range of less than 1.50%. From this perspective, the Si content is preferably 0.80% or less, and more preferably 0.60% or less.

[0016] Mn: 1.5% or more and 3.5% or less Mn is an element that strengthens steel through solid solution strengthening, improves hardenability, and promotes the formation of retained γ, bainite, and martensite. These effects are achieved when the Mn content is 1.5% or more. Therefore, the Mn content must be 1.5% or more, and preferably 1.8% or more. On the other hand, if the Mn content is 3.5% or less, the above effects can be obtained without increasing costs. Therefore, the Mn content must be 3.5% or less, and is preferably 3.3% or less.

[0017] P: 0.1% or less (excluding 0%) By reducing the P content, it is possible to prevent a decrease in weldability, and furthermore, it is possible to prevent P from segregating at grain boundaries, thereby preventing deterioration in ductility, bendability, and toughness. Furthermore, if a large amount of P is contained, the ferrite transformation is promoted, which increases the grain size. Therefore, the P content must be 0.1% or less. There is no particular lower limit for the P content, but it is usually preferable to set it to 0.001% or more due to production technology constraints.

[0018] S: 0.03% or less (excluding 0%) It is preferable to reduce the S content as much as possible. By suppressing the S content, it is possible to prevent a decrease in weldability, prevent a decrease in ductility during hot rolling, suppress hot cracking, and significantly improve surface properties. Furthermore, by suppressing the S content, it is possible to prevent a decrease in the delayed fracture resistance, ductility, bendability, and stretch flangeability of the steel sheet due to the formation of coarse sulfides as an impurity element. Since the problems caused by S become significant when the S content exceeds 0.03%, the S content must be 0.03% or less, and preferably 0.02% or less. From the viewpoint of improving delayed fracture resistance, the S content is preferably 0.01% or less, and more preferably 0.003% or less. There is no particular limitation on the lower limit of the S content, but it is usually preferable to set it to 0.0001% or more due to constraints on production technology.

[0019] N: 0.007% or less (excluding 0%) By limiting the N content to 0.007% or less, it is possible to prevent N from forming coarse nitrides with Ti, Nb, and V at high temperatures, thereby preventing the effect of adding Ti, Nb, and V to increase the strength of the steel sheet from being impaired. Furthermore, by limiting the N content to 0.007% or less, it is possible to prevent a decrease in toughness. Furthermore, by limiting the N content to 0.007% or less, it is possible to prevent the occurrence of slab cracks and surface defects during hot rolling. Therefore, the N content must be limited to 0.007% or less, preferably 0.005% or less, more preferably 0.003% or less, and even more preferably 0.002% or less. There is no particular lower limit for the N content, but it is usually preferable to set it to 0.0005% or more due to constraints on production technology.

[0020] Sol.Al: 0.1% or less (excluding 0%) Because Al is thermodynamically the easiest to oxidize, it oxidizes before Si and Mn, suppressing the oxidation of Si and Mn at the outermost surface of the substrate steel sheet and promoting the oxidation of Si and Mn inside the steel sheet. This effect is achieved when the sol.Al content is 0.01% or more. On the other hand, if the sol. Al content exceeds 0.1%, the cost increases. Therefore, if sol. Al is contained, the sol. Al content must be 0.1% or less. Although there is no particular lower limit for the sol.Al content, it is preferable to set it to 0.001% or more because removing sol.Al at the impurity level also leads to increased costs. Also, as mentioned above, it is preferable that the sol.Al content be 0.01% or more.

[0021] O: 0.003% or less (excluding 0%) O is an element that forms oxide-based inclusions such as Al2O3, SiO2, CaO, MgO, (Al,Ca)-O, and (Si,Mn)-O in steel, and the formation of these inclusions degrades delayed fracture resistance. To minimize this adverse effect on delayed fracture resistance, the O content must be 0.003% or less. There is no particular lower limit for the O content, but the currently industrially feasible lower limit is approximately 0.0005%.

[0022] The mass ratio of Si to Mn (Si / Mn) is 0.25 or less If the mass ratio of Si to Mn (Si / Mn) is large, Si-based oxides are more likely to form on the surface of the substrate steel sheet during annealing, making it more likely that oxide-induced coating defects will occur. To prevent such coating defects, the mass ratio of Si to Mn (Si / Mn) needs to be 0.25 or less. From the viewpoint of preventing coating defects, it is more preferable that the mass ratio of Si to Mn (Si / Mn) be 0.20 or less. In addition to the above-mentioned composition, the steel sheet preferably further contains, in mass %, one or more elements selected from the following groups A to D. The composition of the elements in groups A to D and the effects of their inclusion will be explained below.

[0023] Group A [one or more elements selected from Nb: 0.05% or less (excluding 0%), Ti: 0.08% or less (excluding 0%), V: 0.2% or less (excluding 0%), W: 0.15% or less (excluding 0%), Zr: 0.15% or less (excluding 0%)] Nb, Ti, V, W, and Zr are all elements effective in increasing the strength of the base steel sheet and can be added as needed. Nb is an element that can obtain a fine structure even with a small amount of addition, and can achieve high strength without impairing toughness. Nb: 0.05% or less (excluding 0%) Although the effect of improving strength can be obtained by adding 0.005% or more of Nb, from the viewpoint of preventing an increase in cost, the Nb content is preferably 0.05% or less. Ti: 0.08% or less (excluding 0%) Ti is an element effective for precipitation strengthening of steel. Although there is no particular lower limit for Ti, it is preferably 0.005% or more in order to obtain the effect of adjusting strength. However, if Ti is added in excess, the hard phase becomes excessively large and formability deteriorates. Therefore, when Ti is contained, the Ti content is preferably 0.08% or less, and more preferably 0.05% or less. V: 0.2% or less (excluding 0%) When V is contained in an amount of 0.005% or more, the effect of improving strength can be obtained. However, from the viewpoint of preventing an increase in costs, when V is contained, the V content is preferably 0.2% or less. W: 0.15% or less (excluding 0%) When W is contained in an amount of 0.005% or more, the effect of improving strength can be obtained. However, from the viewpoint of preventing an increase in costs, when W is contained, the W content is preferably 0.15% or less. Zr: 0.15% or less (excluding 0%) When Zr is contained in an amount of 0.0005% or more, the effect of improving strength can be obtained. However, from the viewpoint of preventing an increase in costs, when Zr is contained, the Zr content is preferably 0.15% or less.

[0024] B group [one or more elements selected from Cr: 1.0% or less (excluding 0%), Ni: 1.0% or less (excluding 0%), Cu: 1.0% or less (excluding 0%), Mo: 1.0% or less (excluding 0%), Co: 1.0% or less (excluding 0%), and B: 0.005% or less (excluding 0%)] Cr, Ni, Cu, Mo, Co and B are all elements that improve the hardenability of the steel sheet. Cr: 1.0% or less (excluding 0%) When Cr is contained in an amount of 0.005% or more, hardenability is improved and the balance between strength and ductility can be improved. However, from the viewpoint of preventing an increase in costs, the Cr content is preferably set to 1.0% or less. Ni: 1.0% or less (excluding 0%) When Ni is contained in an amount of 0.005% or more, it is possible to promote the formation of a residual γ phase. However, from the viewpoint of preventing an increase in costs, when Ni is contained, the Ni content is preferably 1.0% or less. Cu: 1.0% or less (excluding 0%) When Cu is contained in an amount of 0.005% or more, it is possible to promote the formation of the residual γ phase. However, from the viewpoint of preventing an increase in costs, when Cu is contained, the Cu content is preferably 1.0% or less. Mo: 1.0% or less (excluding 0%) When Mo is contained in an amount of 0.005% or more, the strength adjusting effect is obtained, and this effect is particularly enhanced when the Mo content is 0.05% or more. However, from the viewpoint of preventing an increase in costs, the Mo content is preferably 1.0% or less. Co: 1.0% or less (excluding 0%) Co is an element that is effective in improving stretch flangeability by spheroidizing the shape of inclusions and improving the ultimate deformability of steel sheets. To improve stretch flangeability, the Co content is preferably 0.005% or more, and more preferably 0.010% or more. However, excessive Co content generates a large amount of coarse precipitates and inclusions, which reduces bendability, so the Co content is preferably 1.0% or less. B: 0.005% or less (excluding 0%) B is an element effective in improving the hardenability of steel. To improve hardenability, the B content is preferably 0.0003% or more, and more preferably 0.0005% or more. However, since an excessive B content reduces formability, the B content is preferably 0.005% or less.

[0025] C group [one or more elements selected from Ca: 0.005% or less (excluding 0%), Mg: 0.005% or less (excluding 0%), and REM: 0.005% or less (excluding 0%)] Ca, Mg and REM (rare earth elements) are all elements used as deoxidizers. Ca: 0.005% or less (excluding 0%) When Ca is contained in an amount of 0.0005% or more, the morphology of sulfides can be controlled and ductility and toughness can be improved. However, from the viewpoint of obtaining good ductility, when Ca is contained, the Ca content is preferably 0.005% or less. Mg: 0.005% or less (excluding 0%) When Mg is contained in an amount of 0.0005% or more, the morphology of sulfides can be controlled and ductility and toughness can be improved. However, from the viewpoint of preventing an increase in costs, when Mg is contained, the Mg content is preferably 0.005% or less. REM: 0.005% or less (excluding 0%) When REM is contained in an amount of 0.0005% or more, the morphology of sulfides can be controlled and ductility and toughness can be improved. However, from the viewpoint of obtaining good toughness, when REM is contained, the REM content is preferably 0.005% or less.

[0026] Group D [one or more elements selected from Sn: 0.2% or less (excluding 0%) and Sb: 0.2% or less (excluding 0%)] Sb and Sn are elements that suppress decarburization, denitrification, deboronization, etc., and are effective in suppressing a decrease in the strength of the steel sheet. Therefore, when Sb and Sn are contained, their contents are each set to more than 0%. Sn: 0.2% or less (excluding 0%) Sn is an element that is effective in suppressing denitrification, deboronation, etc., and thus suppressing a decrease in the strength of steel. To obtain this effect, the content of Sn is preferably 0.002% or more. On the other hand, when Sn is contained, in order to obtain good impact resistance, the Sn content is preferably 0.2% or less. Sb: 0.2% or less (excluding 0%) Sb can be added to suppress nitriding and oxidation of the substrate steel surface, or decarburization of the substrate steel surface in a region of several tens of microns caused by oxidation. By suppressing nitriding and oxidation of the substrate steel surface, Sb prevents a decrease in the amount of martensite formed on the substrate steel surface, improving the fatigue properties and surface quality of the substrate steel. To achieve this effect, the Sb content is preferably 0.001% or more. On the other hand, to obtain good toughness, the Sb content is preferably 0.2% or less.

[0027] In addition to the above-mentioned chemical composition, the base steel sheet preferably further contains, by mass %, one or more elements selected from the following groups E to H. The chemical compositions of groups E to H and the effects of their inclusion will be explained below. Group E [Ta: 0.10% or less (excluding 0%)] Ta, like the elements of group A, is an element effective in increasing the strength of the steel sheet and can be added as needed. Although the effect of improving strength can be obtained by adding 0.005% or more of Ta, from the viewpoint of preventing an increase in costs, if Ta is added, the Ta content is set to 0.10% or less.

[0028] F group [one or more elements selected from Te: 0.10% or less (excluding 0%), As: 0.10% or less (excluding 0%), and Hf: 0.10% or less (excluding 0%)] Te, As and Hf, like the elements of group C, are all elements used to control the morphology of sulfides. Te: 0.10% or less (excluding 0%) When Te is contained in an amount of 0.001% or more, the morphology of sulfides can be controlled and ductility and toughness can be improved. However, from the viewpoint of preventing an increase in costs, when Te is contained, the Te content is set to 0.10% or less. As: 0.10% or less (excluding 0%) When As is contained in an amount of 0.001% or more, the morphology of sulfides can be controlled and ductility and toughness can be improved. However, from the viewpoint of preventing an increase in costs, when As is contained, the As content is set to 0.10% or less. Hf: 0.10% or less (excluding 0%) When Hf is contained in an amount of 0.01% or more, the morphology of sulfides can be controlled and ductility and toughness can be improved. However, from the viewpoint of preventing an increase in costs, if Hf is contained, the Hf content is set to 0.10% or less.

[0029] Group G [one or more elements selected from Bi: 0.20% or less (excluding 0%) and Pb: 0.20% or less (excluding 0%)] Both Bi and Pb are elements that suppress grain boundary segregation and improve ductility and toughness. When Bi and Pb are contained, their respective contents should exceed 0%. Bi: 0.20% or less (excluding 0%) When Bi is contained in an amount of 0.001% or more, it can suppress grain boundary segregation and improve ductility and toughness. Bi also has the effect of improving machinability and improving the smoothness of cut edges, and it also acts to improve the delayed fracture resistance of cut edges. When Bi is contained, the Bi content is set to 0.20% or less to prevent an increase in costs. Pb: 0.20% or less (excluding 0%) When Pb is contained in an amount of 0.001% or more, it can suppress grain boundary segregation and improve ductility and toughness. Pb also has the effect of improving machinability and improving the smoothness of cut edges, and it also acts to improve the delayed fracture resistance of cut edges. If Pb is contained, the Pb content is set to 0.20% or less to prevent an increase in costs.

[0030] H group [one or more elements selected from Zn: 0.10% or less (excluding 0%), Ge: 0.10% or less (excluding 0%), Sr: 0.10% or less (excluding 0%), Cs: 0.10% or less (excluding 0%)] Zn, Ge, Sr, and Cs are elements that increase strength without significantly affecting mechanical properties or surface quality. When Zn, Ge, Sr, or Cs is contained, the content of each must be greater than 0%. Zn: 0.10% or less (excluding 0%) Even if Zn is contained in an amount of 0.001% or more, it does not have a significant effect on mechanical properties or surface quality. From the viewpoint of preventing an increase in costs, if Zn is contained, the Zn content is set to 0.10% or less. Ge: 0.10% or less (excluding 0%) Even if Ge is contained in an amount of 0.001% or more, it does not have a significant effect on mechanical properties or surface quality. From the viewpoint of preventing an increase in cost, if Ge is contained, the Ge content is set to 0.10% or less. Sr: 0.10% or less (excluding 0%) Even if the Sr content is 0.001% or more, it does not have a significant effect on mechanical properties or surface quality. To prevent an increase in cost, if Sr is contained, the Sr content is set to 0.10% or less. Cs: 0.10% or less (excluding 0%) Even if the Cs content is 0.001% or more, it does not have a significant effect on mechanical properties or surface quality. To prevent an increase in costs, if Cs is contained, the Cs content is set to 0.10% or less.

[0031] The composition of the substrate steel sheet (base steel sheet) is made up of the above-mentioned components, with the remainder being Fe and unavoidable impurities.

[0032] Next, the production conditions of the present invention will be explained. In the present invention, a base steel sheet (cold-rolled steel sheet or hot-rolled steel sheet) having the above-mentioned components is introduced into continuous hot-dip galvanizing equipment, and after continuous annealing in the equipment, hot-dip galvanizing is performed, and further alloying treatment is performed to obtain a galvannealed steel sheet.

[0033] In general, continuous hot-dip galvanizing equipment is composed of an annealing furnace and a hot-dip galvanizing device located downstream of the annealing furnace, and the hot-dip galvanizing device is equipped with 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 continuous hot-dip galvanizing equipment, a general continuous hot-dip galvanizing line (CGL) configured to continuously perform a series of processes including heating, cooling, hot-dip galvanizing, and hot-dip galvanizing alloying treatment can be applied. The base steel sheet introduced into the continuous hot-dip galvanizing line is annealed while passing through an annealing furnace which is provided with a heating zone, a soaking zone, and a cooling zone in this order. The specific annealing conditions are as follows:

[0034] During the annealing heating process, the atmospheric dew point in the heating furnace in the temperature range where the base steel sheet temperature is 700°C or higher is set to -40°C or lower. The atmosphere in the heating furnace contains 20.0 vol% or less hydrogen, as well as at least one of SO2 at 0.1 volppm to 3.0 volppm and HCl at 0.5 volppm to 10.0 volppm.

[0035] Dew point of the atmosphere inside the heating furnace in the temperature range of 700°C or higher: -40°C or lower At steel sheet temperatures of 700°C or higher, surface segregation due to diffusion of Si and Mn increases. In this case, if the atmospheric dew point is set to -40°C or lower, the formation of internal oxides in the surface layer of the substrate steel sheet, i.e., at a depth of up to 100 μm from the surface of the substrate steel sheet, can be suppressed. This makes it possible to suppress cracks originating from oxides during severe deformation, improving formability. Furthermore, the formation of Si oxides and Mn oxides on the surface of the substrate steel sheet can also be suppressed, thereby suppressing the occurrence of coating defects due to oxides. To fully achieve these effects of suppressing the formation of oxides on the surface layer and surface of the substrate steel sheet, a temperature of -45°C or lower is more preferable.

[0036] Hydrogen concentration in the atmosphere inside the heating furnace: 20.0 vol% or less The hydrogen concentration in the heating furnace atmosphere during annealing must be 20.0 vol% or less. If the hydrogen concentration in the heating furnace atmosphere in the temperature range of 700°C or higher is too high, the amount of diffusible hydrogen in the steel increases, making it difficult to release hydrogen. As a result, there is a problem of reduced delayed fracture resistance due to hydrogen embrittlement. For these reasons, the hydrogen concentration in the temperature range of 700°C or higher must be 20.0 vol% or less. On the other hand, if the hydrogen concentration in the heating furnace atmosphere is 3.0 vol% or more, the surface of the base steel sheet is sufficiently reduced and activated, resulting in good galvanizability. Therefore, it is preferable that the hydrogen concentration in the heating furnace atmosphere be 3.0 vol% or more.

[0037] The atmosphere inside the heating furnace must contain at least one of the following: SO2 concentration: 0.1 volppm or more and 3.0 volppm or less; or HCl concentration: 0.5 volppm or more and 10.0 ppm or less. One of the important requirements of the present invention is that the atmosphere in the heating furnace contains at least one of SO2 and HCl at the above-mentioned concentrations. The atmosphere in the heating furnace during annealing must contain at least one of SO2 and HCl at 0.1 volppm to 3.0 volppm and 0.5 volppm to 10.0 volppm. While the exact reason is unclear, the presence of corrosive gases such as SO2 and HCl in appropriate amounts reduces the grain size of the surface layer of the substrate steel sheet after recrystallization annealing. This also increases the number of cracks and intersections during plating, which are important for the release of diffusible hydrogen from steel, when a certain level of stress is applied to the coating layer. During the alloying process of the galvanized layer, a rapid Fe-Zn reaction known as an outburst reaction occurs at the grain boundaries of the steel sheet, forming an alloy phase with a high Fe concentration and low ductility in the coating layer near the grain boundaries. Furthermore, because this alloy phase has low ductility, cracks may form when a certain level of stress is applied to the coating layer, potentially serving as the initiation point for cracks during plating. When the grain size of the steel sheet surface decreases due to the presence of corrosive gases, an outburst reaction occurs, increasing the density of grain boundaries on the steel sheet surface. As a result, the spacing between the formation of low-ductility alloy phases also decreases. This is thought to increase the likelihood of cracks in the coating layer due to tensile stress applied to the coating layer during cooling of the steel sheet after alloying, resulting in an increase in the number of crack intersections. The improvement effect of this corrosive gas is apparent when SO2 is 0.1 ppm or more and HCl is 0.5 ppm or more. However, SO2 exceeding 3.0 ppm and HCl exceeding 10.0 ppm may accelerate the deterioration of the furnace body. Therefore, the SO2 concentration should be 0.1 volppm to 3.0 volppm, and the HCl concentration should be 0.5 volppm to 10.0 ppm. The remainder of the furnace atmosphere during annealing may contain gases such as nitrogen, CO, and CO2.

[0038] The concentrations of these trace amounts of corrosive gases such as SO2 and HCl can be controlled by adjusting the amount of gas containing these corrosive gases that is introduced directly into the furnace. It can also be controlled by applying a liquid containing H2SO4 or HCl to the substrate steel sheet before it enters the furnace, or by diluting this liquid with water. The key is to control the concentrations of trace amounts of corrosive gases such as SO2 and HCl, and methods for controlling the concentrations of these trace amounts of corrosive gases such as SO2 and HCl are not limited to the methods described above.

[0039] In the present invention, the substrate steel sheet is preferably annealed at a maximum temperature of 700°C or higher and 900°C or lower in order to recrystallize the strain imparted by rolling. At temperatures of 700°C or higher, iron oxide on the surface of the substrate steel sheet is sufficiently reduced, resulting in a good coating appearance, so the maximum temperature of the substrate steel sheet is preferably 700°C or higher. Furthermore, by setting the temperature to 900°C or lower, surface segregation of Si, Mn, and Cr can be suppressed, resulting in a good coating appearance, so the maximum temperature of the substrate steel sheet is preferably 900°C or lower.

[0040] In the present invention, the base steel sheet that has been continuously annealed under the above-described conditions is cooled and then immersed in a hot-dip galvanizing bath to undergo hot-dip galvanizing treatment. The cooling temperature is preferably 200 to 520°C, and the sheet is heated as necessary before being immersed in the hot-dip galvanizing bath. The bath temperature of the hot-dip galvanizing bath is generally about 440 to 500°C. The hot-dip galvanizing bath is not particularly limited, but may, for example, contain an Al content of 0.10 to 0.23% by mass, and further contain one or more elements selected from Pb, Sb, Si, Sn, Mg, Mn, Ni, Cr, Co, Ca, Cu, Li, Ti, Be, Bi, and REM in a total amount of 0 to 3.5% by mass, with the balance consisting of Zn and unavoidable impurities. In order to prevent changes in the temperature of the coating bath, the temperature of the base steel sheet before coating (immersion sheet temperature) is preferably set to the coating bath temperature or higher and to the coating bath temperature + 50°C or lower.

[0041] After the above-mentioned hot-dip galvanizing treatment, a galvannealing treatment is further performed to form a galvannealed hot-dip galvanized layer. The alloying treatment is preferably performed in a temperature range of 480°C or higher and 570°C or lower. If the alloying temperature is lower than 480°C, the Zn-Fe alloying rate becomes excessively slow, making alloying extremely difficult. On the other hand, if the alloying temperature exceeds 570°C, untransformed austenite may transform into pearlite, resulting in a decrease in TS and El. The alloying treatment is more preferably performed in a temperature range of 490°C or higher and 560°C or lower, and even more preferably in a temperature range of 490°C or higher and 530°C or lower. The coating weight of galvannealed steel sheet (GA) is 20 to 120 g / m per side. 2 The coating weight can be adjusted by performing gas wiping after hot dip galvanizing.

[0042] After the above-mentioned alloying treatment of zinc plating, the steel sheet is cooled to 50°C or less. Furthermore, the average cooling rate from the alloying temperature to 250°C is set to 5°C / s or more. The reason for this will be explained below.

[0043] Average cooling rate: 5℃ / s or more One of the important requirements of the present invention is that the average cooling rate from the end of the alloying treatment to 250°C is 5°C / s or more, which allows the number of crack intersections in the coating necessary for the release of diffusible hydrogen from the steel to be achieved. Because the thermal expansion coefficient of the galvannealed hot-dip coating layer is higher than that of the base steel sheet, tensile stress is generated in the coating layer during cooling, which is thought to lead to the formation of cracks in the galvannealed hot-dip coating layer. When the average cooling rate is 5°C / s or more, the temperature difference between the surface coating layer and the base steel sheet increases, sufficiently increasing the tensile stress in the coating layer, accelerating crack formation and achieving the number of crack intersections necessary for hydrogen release. As a result, the release of diffusible hydrogen from the steel is improved, and the amount of diffusible hydrogen in the steel can be reduced to 0.30 mass ppm. On the other hand, when the average cooling rate is less than 5°C / s, the temperature difference between the coating layer and the steel sheet during cooling is smaller than when the cooling rate is faster, which is thought to reduce the tensile stress in the coating layer and suppress crack formation. The average cooling rate is preferably 7°C / s or more, and more preferably 10°C / s or more. As a cooling method after the hot dip galvanizing alloying treatment, gas jet cooling, mist cooling, water cooling, air cooling, etc. can be appropriately applied.

[0044] The galvannealed steel sheet cooled to 50°C or less may be rolled at a predetermined elongation rate. The elongation rate in this rolling is preferably 0.05% or more and 1.00% or less. By setting the elongation rate in this rolling to 0.05% or more, the dimensional accuracy of the steel sheet can be adjusted. On the other hand, if the elongation rate in this rolling exceeds 1.00%, the YS increases and the dimensional accuracy during forming decreases. The elongation rate in this rolling is more preferably 0.70% or less and 0.10% or more. The rolling may be performed online in a device connected to the continuous hot-dip galvanizing facility, or may be performed offline from the continuous hot-dip galvanizing facility. The target elongation (e.g., 0.05% to 1.00%) may be achieved in a single rolling run, or may be achieved by multiple rolling runs. As the rolling, temper rolling is generally carried out, but rolling by a method such as processing with a leveler may also be used as long as it can impart an elongation rate equivalent to that of temper rolling.

[0045] The galvannealed steel sheet after cooling is either held at room temperature for 48 hours or more, or reheated and held as described below, or both, to release diffusible hydrogen in the steel sheet. Holding at room temperature for a certain period of time or more allows diffusible hydrogen in the steel sheet to be released through cracks during plating, thereby improving delayed fracture resistance. In order to sufficiently release diffusible hydrogen in the steel sheet at room temperature, when holding at room temperature, the holding time must be 48 hours or more. The holding time is more preferably 96 hours or more, and even more preferably 144 hours or more. There is no particular upper limit to the holding time, but from the viewpoint of productivity, it is preferably 1,800 hours or less. In the present invention, room temperature refers to a temperature of 0°C to 40°C.

[0046] When reheating a galvannealed steel sheet, it is necessary to cool it to room temperature, reheat it to a temperature range of 50°C to 400°C, and maintain the temperature for 0.1 hours or more. Reheating and maintaining the temperature allows diffusible hydrogen in the steel sheet to be released through cracks in the coating in a shorter time than maintaining the temperature at room temperature, thereby improving delayed fracture resistance. To fully achieve the effect of promoting the release of diffusible hydrogen from the steel sheet, the temperature must be 50°C or higher, preferably 80°C or higher. On the other hand, temperatures above 400°C may remelt the coating layer, resulting in deterioration of the appearance. Therefore, the temperature must be 400°C or lower, preferably 200°C or lower. The temperature here refers to the maximum temperature reached during the heat retention, and may be constant or variable as long as it is within the range of 50°C to 400°C. The heat retention time must be 0.1 hours or longer to fully achieve the effect of promoting the release of diffusible hydrogen from the steel sheet. The heat retention time is preferably 0.5 hours or longer, more preferably 1.0 hour or longer. There is no particular upper limit to the heat retention time, but from the viewpoint of productivity, it is preferably 48 hours or less.

[0047] The manufacturing conditions other than those mentioned above can be determined by ordinary methods.

[0048] Next, the high-strength galvannealed steel sheet obtained by the above-described manufacturing method will be described. First, the steel sheet of the present invention produced by the above-described method can have a total amount of hydrogen released at 25° C. or higher and 300° C. or lower of 0.30 mass ppm or less when measured by temperature ramp analysis. As a result, a high-strength galvannealed steel sheet having excellent delayed fracture resistance can be obtained.

[0049] The high-strength galvannealed steel sheet produced by the present invention has a TS of 780 MPa or more. Furthermore, when further increasing the strength, the TS can be increased to 980 MPa or more. The TS is measured in accordance with JIS Z2241 as follows. A JIS No. 5 test piece is taken from the hot-dip galvanized steel sheet so that the longitudinal direction is perpendicular to the rolling direction of the steel sheet. Using this test piece, a crosshead displacement rate Vc of 1.67 × 10 -1A tensile test is carried out under the condition of mm / s, and TS is measured.

[0050] The structure of the steel sheet is not particularly limited, but in order to ensure a tensile strength of 780 MPa or more, the following steel sheet structure is preferable. Note that the following steel sheet structure is the steel structure in the range of 1 / 8 to 3 / 8 thickness depth from the surface of the steel sheet. That is, it is preferable that the steel sheet structure has a total area ratio of martensite, bainite and retained γ (retained austenite) of 30% or more, thereby obtaining a steel sheet having a tensile strength of 780 MPa or more. Furthermore, by making the total area ratio of martensite, bainite, and retained γ 50% or more, a steel sheet having a tensile strength of 980 MPa or more can be obtained, and by making the total area ratio of martensite, bainite, and retained γ 70% or more, a steel sheet having a tensile strength of 1180 MPa or more can be obtained. Furthermore, by setting the total area ratio of martensite, bainite, and retained γ to 85% or more, a steel sheet having a tensile strength of 1310 MPa or more can be obtained, and by setting the total area ratio of martensite, bainite, and retained γ to 90% or more, a steel sheet having a tensile strength of 1470 MPa or more can be obtained. The steel structure can be identified by etching the polished cross section of the steel sheet with acid or the like, followed by observation with an optical microscope or scanning electron microscope (SEM). Because the structure of high-strength steel sheet is complex and fine, it is preferable to use an SEM, which allows for more detailed observation of the microstructure. Steel sheet polishing can be performed by cutting the cross section, embedding it in resin, mechanically polishing it with abrasive paper, or the like, and then finish-polishing it with diamond paste or oxide particles. Electrolytic polishing or ion polishing may also be used for finish polishing. Furthermore, etching with nital at a concentration of 1 vol% to 5 vol% can produce steps on the cross section that are suitable for observing the steel structure, so a nital concentration of 1 vol% to 5 vol% is preferred. Because the structure of high-strength steel sheets is fine, it is preferable to perform SEM observation at a magnification of 1000 to 3000 times. Furthermore, to obtain sufficient representativeness in calculating the area ratio of each structure, it is preferable to use SEM photographs of three or more fields of view, and more preferably five or more fields of view. On the other hand, if the number of fields of view is too large, SEM observation takes too much time, so the number of fields of view of the SEM is preferably 10 or less.

[0051] Here, the size and abundance of each structure are not particularly limited, but the following sizes and abundances can be exemplified as one embodiment of the present invention. The aspect ratio represents the ratio of the length of the major axis to the length of the minor axis, which is perpendicular to the major axis, i.e., the length of the major axis / the length of the minor axis. The thickness represents the length of the minor axis, and the circle equivalent diameter represents the diameter of a perfect circle having the area of ​​each structure calculated from an SEM photograph. Tempered martensite Aspect ratio ≦8, equivalent circle diameter ≦30μm Distribution density of carbides inside the structure: 0.10 to 12 particles / μm 2 Fresh martensite and residual γ Lump: Aspect ratio ≦8, Equivalent circle diameter: 3~30μm Granular: Aspect ratio ≦8, Equivalent circle diameter: 0.40 μm or more, less than 3 μm Plate or film: aspect ratio over 8, thickness: 0.10 to 8 μm Bainite Film or plate: aspect ratio greater than 8, thickness ≦8μm Lumped: Aspect ratio ≦8, equivalent circle diameter ≦30μm Distribution density of carbides within the structure: 0.10 to 6 particles / μm in all forms 2 carbide Granular: Aspect ratio ≦8, Equivalent circle diameter: 0.01 μm or more, less than 0.40 μm Film-like: Aspect ratio greater than 8, Equivalent circle diameter: 0.01 μm or more, less than 0.10 μm

[0052] The high-strength galvannealed steel sheet of the present invention has a coating weight of 20 g / m per side on the surface of the base steel sheet. 2More than 120g / m 2 It has a zinc plating layer of 20g / m 2 If the coating weight is less than 120g / m, not only is the corrosion resistance likely to decrease, but it is also difficult to control the coating weight. 2 If it exceeds this value, the plating adhesion is likely to decrease. The high-strength galvannealed steel sheet of the present invention has a good surface appearance according to the evaluation criteria described in the examples.

[0053] The high-strength galvannealed steel sheet of the present invention has an oxygen content of the surface layer of the substrate steel sheet immediately below the galvanized layer, within 100 μm from the surface of the substrate steel sheet toward the center of the sheet thickness, of 0.030 g / m per side, as measured by the method described in the Examples. 2 The oxygen content in the surface layer of the substrate steel sheet is derived from oxides present in the surface layer of the substrate steel sheet. A large amount of oxides present in the surface layer of the substrate steel sheet means that there is also a large amount of oxide formed on the surface of the substrate steel sheet, which causes coating defects due to oxides. Therefore, from the perspective of preventing coating defects, the oxygen content should be 0.030 g / m per side. 2 In order to obtain a better plating appearance, the oxygen content must be less than 0.020 g / m per side. 2 It is preferable that it is less than 10 ...

[0054] The high-strength galvannealed steel sheet of the present invention has a crack intersection number of 500 / mm on a plane parallel to the surface of the galvanized steel sheet at a depth M [μm] (1≦M≦5) from the surface of the galvanized layer. 2This is the above, and is an important requirement of the present invention. The number of intersections is measured by observing the surface after reducing the thickness from the surface of the zinc-coated layer to between 1 μm and 5 μm, which makes it possible to observe the morphology of cracks that are likely to be connected to the surface of the substrate steel sheet. If the thickness is reduced to more than 5 μm, the substrate steel sheet may be exposed due to the influence of unevenness on the surface of the substrate steel sheet. On the other hand, if the thickness is reduced by less than 1 μm, there is a high possibility that cracks that are not connected to the surface of the substrate steel sheet will be observed, which will result in measuring cracks that are not related to hydrogen release and causing a misjudgment of hydrogen release properties. If the number of intersections of the cracks measured in this way is 500 points / mm 2 The above conditions are necessary to obtain sufficient hydrogen desorption properties. The thickness reduction of the coating layer can be performed by measuring the thickness of the sample before, during, and after polishing, and mechanically polishing the sample so that the reduced thickness is 1 μm to 5 μm. The reduced thickness is calculated from the difference in the thickness of the sample before and after polishing. The method for measuring the thickness of the sample is not particularly limited as long as it can provide an accuracy of 0.1 μm or better. For example, a contact-type thickness gauge or a non-contact laser-based thickness gauge can be used. The method for reducing the coating layer is also not particularly limited. For example, Ar sputtering can be used. Note that a "crack intersection" refers to a point where three or more cracks originate from one point. In the production of high-strength galvannealed steel sheets, cracks in the coating layer may be crushed on the coating surface after the alloying treatment due to contact with a roll or temper rolling. In this case, if the cracks have a large number of intersections and are sufficiently connected, even if some of the cracks are crushed, hydrogen can be released from the remaining cracks, and good hydrogen desorption properties can be achieved. On the other hand, if there are many isolated cracks, the cracks will collapse, eliminating the pathways for hydrogen release, and hydrogen desorption is likely to deteriorate. From this perspective, the number of intersections of cracks on the plating surface observed after thickness reduction is 500 points / mm 2 If the number of intersections of the cracks is equal to or greater than 700, the cracks connected to the surface of the substrate steel sheet are sufficiently connected to each other, and good hydrogen release properties can be obtained. 2 More preferably, 1000 points / mm 2Although there is no particular upper limit to the number of intersections of cracks, in order to prevent a decrease in coating adhesion, it is set to 5000 points / mm 2 It is preferable to do the following: The high-strength galvannealed steel sheet according to the present invention has a diffusible hydrogen content of 0.30 ppm by mass or less. If the diffusible hydrogen content in the steel exceeds 0.30 ppm by mass, delayed fracture resistance becomes insufficient, so the diffusible hydrogen content in the steel must be 0.30 ppm by mass or less. The diffusible hydrogen content in the steel is preferably 0.15 ppm by mass or less, and more preferably 0.05 ppm by mass or less. By performing cooling after annealing and galvannealing under the conditions described in the production method of the present invention, the diffusible hydrogen content in the steel can be reduced to 0.30 ppm by mass or less.

[0055] The thickness of the hot-dip galvanized steel sheet produced in the present invention is not particularly limited, but is usually about 0.3 mm or more and 2.8 mm or less. [Example]

[0056] Steels having the chemical compositions shown in Table 1 were melted and cast slabs were obtained. These slabs were then hot-rolled, pickled, and cold-rolled to form cold-rolled steel sheets having a thickness of 1.2 mm. These cold-rolled steel sheets were used as the base steel sheets for the galvannealed steel sheets. In a CGL equipped with an all-radiant tube (ART) type annealing furnace, the steel sheets were annealed under the conditions shown in Table 2, then hot-dip galvanized (coating composition: Zn-0.14 mass% Al) and gas wiped to a coating weight of approximately 50 g / m per side. 2 After the alloying treatment, the samples were cooled under the conditions shown in Table 2, and some samples were reheated and kept at the same temperature. The surface of the zinc coating layer of the galvannealed steel sheets obtained in this manner was polished to a depth of 1 μm to 5 μm from the surface of the coating layer, and the number of intersections of cracks in the coating layer visible in the observed images was counted when observed under an SEM. Furthermore, the thickness of the zinc coating layer, the amount of diffusible hydrogen in the steel sheet, and the delayed fracture resistance were measured using the measurement and evaluation methods described below. Furnace gas analysis, and evaluation of the tensile strength and structure of the steel sheet were also performed. The results, along with the manufacturing conditions, are shown in Table 2.

[0057] Analysis method for furnace gas Gas inside the annealing furnace was collected and the SO2 and HCl concentrations were determined by ion chromatography. The analysis was carried out three times, and the average value was used as the furnace gas concentration.

[0058] Galvanized layer thickness measurement A 10mm x 10mm specimen was cut from the width center of the galvannealed steel sheet obtained using the method described above. It was embedded in resin so that the plated surface was perpendicular, and polished with waterproof abrasive paper. It was then finish-polished with 1μm diamond abrasive grains. After polishing, the specimen was etched with 0.05% nital for 30 seconds, and the cross section was observed using an SEM at 400x magnification. For SEM observations, secondary electron images were acquired at an accelerating voltage of 10kV. Five consecutive fields were observed from the center of the specimen, and the plating thickness was measured at six equal positions (five locations) in each field. The average thickness value obtained using the above procedure at all 25 locations was taken as the thickness of the zinc plating layer.

[0059] Measurement of thickness reduction of the zinc coating layer and number of intersections of cracks in the coating layer A 20mm x 20mm specimen was cut from the width center of the galvannealed steel sheet and degreased with alcohol. The plating surface was then mechanically polished with 3μm diamond abrasive grains, followed by finish polishing with 1μm and 0.25μm diamond abrasive grains to reduce the thickness of the plating layer. The thickness reduction was calculated from the difference in thickness of the specimen before and after polishing, and adjusted so that the thickness reduction after polishing was between 1μm and 5μm. After polishing, the specimen was etched with 1% nital for 30 seconds and observed under an SEM at 500x magnification. Backscattered electron images were observed at an accelerating voltage of 15kV. Nine consecutive fields of view (3x3) were observed from the center of the specimen, and the number of crack intersections in the plating was counted in each field. A crack intersection was defined as a point where three or more cracks originated. The sum of the number of intersections in the nine fields of view was divided by the area of ​​the observation field to determine the area (mm). 2 The number of intersections per

[0060] Measurement of diffusible hydrogen content in steel sheets A rectangular test piece measuring 30 mm in length and 5 mm in width was taken from the center of the width of the galvannealed steel sheet. The coating layer was removed using a router. Hydrogen analysis was performed using a thermal desorption analyzer under conditions of starting temperature 25°C, ending temperature 300°C, and heating rate 200°C / hour. The amount of released hydrogen (mass ppm), which is the amount of hydrogen released from the surface of the test piece at each temperature, was measured. The sum of the hydrogen amounts detected at temperatures below 300°C was taken as the diffusible hydrogen content in the steel sheet. A diffusible hydrogen content of 0.05 mass ppm or less was evaluated as excellent (◎), a diffusible hydrogen content of more than 0.05 mass ppm and less than 0.15 mass ppm was evaluated as good (○), and a diffusible hydrogen content of more than 0.15 mass ppm and less than 0.30 mass ppm was evaluated as acceptable (△). Experience has shown that a diffusible hydrogen content in steel exceeding 0.30 mass ppm often leads to a decrease in delayed fracture resistance; therefore, a diffusible hydrogen content exceeding 0.30 mass ppm was evaluated as poor (×).

[0061] Evaluation of delayed fracture resistance A rectangular specimen measuring 100 mm in length and 20 mm in length was cut from the galvannealed steel sheet perpendicular to the rolling direction. A 15 mm diameter punched hole with a 12.5% ​​clearance was drilled at the center of the specimen's major and minor axes. Tensile tests were performed on these specimens to evaluate their delayed fracture resistance based on the presence or absence of delayed fracture through the punched hole. To prevent the release of diffusible hydrogen from the steel due to aging, the time from cutting the rectangular specimen from the galvannealed steel sheet to the start of the delayed fracture tensile test (tensile speed: 10 mm / min) was limited to within 10 minutes. The maximum stress at which no cracks (here, "cracks" refers to fractures under tensile stress) occurred after 100 hours of loading was defined as the critical stress. Delayed fracture resistance was evaluated as the ratio of the critical stress to the yield stress. The evaluation criteria for delayed fracture resistance were as follows: when the critical stress / yield stress was 1.10 or more, it was given an excellent "◎", when it was less than 1.10 and 1.05 or more, it was given a good "〇", when it was less than 1.05 and 1.00 or more, it was given a fair "△", and when it was less than 1.00, it was given a poor "×". Note that the delayed fracture resistance evaluated in delayed fracture tests is generally lower (disadvantageous) for steel plates with higher strength.

[0062] Measurement of the oxygen content in the surface layer of the steel sheet within 100 μm from the surface of the base steel sheet directly below the coating layer toward the center of the sheet thickness To measure the oxygen content directly below the coating layer, only the coating layer was stripped with alkali, and then the oxygen content was measured using the "impulse furnace infrared absorption method." However, since it is necessary to subtract the amount of oxygen contained in the raw material (i.e., the steel sheet before annealing), in the present invention, the surface layer of both sides of the high-strength steel sheet after continuous annealing is polished to a depth of 100 μm or more to measure the oxygen concentration in the steel, and this measurement value is taken as the amount of oxygen contained in the raw material, OH. In addition, the oxygen concentration in the steel throughout the thickness direction of the high-strength steel sheet after continuous annealing is measured, and this measurement value is taken as the amount of oxygen after oxidation, OI. Using the amount of oxygen after oxidation of the steel sheet obtained in this way, OI, and the amount of oxygen originally contained in the raw material, OH, the difference between OI and OH (= OI - OH) was calculated, and further converted into the amount per unit area on one side (g / m 2 ) was taken as the oxygen content.

[0063] Evaluation of plating appearance The appearance of the coating on galvannealed hot-dip galvannealed steel sheets was visually inspected for the presence or absence of uncoated defects and the V-shaped scale patterns that are a symptom of these defects. Observations were carried out by sampling A4-sized steel sheets from any location near the center of the coil width in the stationary part of the coil, excluding the leading and trailing ends, and visual inspection was carried out on both the front and back surfaces of the steel sheets, with N=5 samples being inspected. A rating of excellent (◎) was given when no defects were found in the observed area; good (○) when scale patterns were found but no uncoated defects were found; fair (△) when scale patterns and fewer than five uncoated defects larger than 0.5 mm were found; and poor (×) when five or more uncoated defects larger than 0.5 mm were found.

[0064] Tensile test JIS Z2241 No. 5 test pieces were taken from the galvannealed steel sheet in the direction perpendicular to the rolling direction (with the sheet width direction being the tensile direction), and these test pieces were subjected to a tensile test in accordance with JIS Z2241 (2011) to measure the tensile strength (TS).

[0065] Observation and measurement of the substrate steel sheet structure The total area ratio of martensite, bainite, and retained γ in the substrate steel sheet microstructure was measured as follows. A sample was cut out so that the observation surface was a thickness cross section (L cross section) parallel to the rolling direction of the substrate steel sheet. This observation surface was polished with diamond paste and then finish-polished with alumina. The observation surface of the sample was then etched with 3 vol% nital to reveal the microstructure. The steel microstructure was observed within a depth range of 1 / 8 to 3 / 8 of the sheet thickness on this observation surface, and five fields of view were observed at 3000x magnification using an SEM. The total area of ​​martensite, bainite, and retained γ was calculated from the obtained microstructure images, and the area ratio was calculated by dividing this total area by the measured area for the five fields of view. The average of these values ​​was used as the total area ratio of martensite, bainite, and retained γ. The martensite, bainite, retained γ, and other microstructures were identified as follows. Martensite There are two types of martensite: tempered martensite and fresh martensite. Tempered martensite Tempered martensite is the gray or dark gray area that is close to black in an SEM photograph. Tempered martensite has a blocky morphology with boundaries at the interfaces with other structures such as prior gamma grain boundaries and ferrite. However, tempered martensite may contain other structures such as bainite inside, resulting in a concave shape. Tempered martensite contains many carbides inside, but depending on the plane orientation, there may be only a small amount of carbides. Fresh martensite Fresh martensite is the gray or white area in the SEM image. Fresh martensite is in the form of blocks, granules, plates, or films, and does not contain carbides. Bainite Bainite is the dark gray region in the SEM photograph. Bainite appears as films, plates, or blocks formed by connecting some or all of these adjacent regions, and contains a small amount of carbides. Bainite that has been tempered after formation may contain coarse carbides. Retained austenite (retained γ) The retained γ is a region that has the same color and morphology as the above-mentioned fresh martensite. Note that because it is not possible to distinguish between retained γ and fresh martensite based on the contrast in the SEM photograph, it is identified as a region that combines these two structures. In order to ensure the strength of the steel sheet, it is necessary to control the total area ratio of the martensite, bainite, and retained γ, but the remainder may include, but is not limited to, the structures shown below. ferrite Ferrite is the black area in the SEM photograph. Ferrite has a massive morphology and contains almost no carbides. Bainitic ferrite contains almost no carbides inside and has similar mechanical properties to ferrite, so it belongs to the ferrite group. Ferrite may contain granular or massive fresh martensite, granular or massive retained γ, or both. Fresh martensite and retained γ contained within ferrite are not included in the ferrite area fraction, but are treated as the area fraction of fresh martensite or retained γ. carbide Carbides are the white areas in SEM photographs. Carbides are in the form of granules or films. Carbides are mainly formed as fine particles inside ferrite, martensite, and bainite, but their area ratios are small and can be ignored. Therefore, the area ratio of carbides is not excluded from the area ratio of each structure containing carbides, but is included in the area ratio of each structure. Organizations other than those mentioned above Each of the above structures may contain nitrides such as TiN, carbonitrides such as (Nb,Ti)(C,N), sulfides such as MnS and CaS, and oxides such as Al2O3 and SiO2, with a total area ratio of a few percent. Since the area ratios of these are small and can be ignored, the area ratios of these nitrides, carbonitrides, sulfides, or oxides are included in the area ratio of each structure containing them. Pearlite may also be included. In the case of pearlite, the area ratio of pearlite is calculated separately.

[0066] According to Tables 1 and 2, the galvannealed steel sheets of the present invention have a beautiful surface appearance with no uncoated areas, and also have excellent release properties of diffusible hydrogen in the steel sheet and delayed fracture resistance. [Table 1] [Table 2]

Claims

1. A high-strength galvannealed steel sheet having a plating layer on a base steel sheet, The composition of the base steel sheet is expressed as mass %. C: 0.06% or more and 0.30% or less, Si: 0.20% or more and less than 3.00%; Mn: 1.5% or more and 3.5% or less, P: 0.1% or less (excluding 0%) S: 0.03% or less (excluding 0%), sol. Al: 0.1% or less (excluding 0%), N: 0.007% or less (excluding 0%), O: 0.003% or less (excluding 0%) Cr: 1.0% or less (excluding 0%); a mass ratio of Si, Mn and Cr (Si+Cr) / Mn of 0.25 or more; Furthermore, as an optional component, it contains, in mass %, one or more components selected from the following groups A to H, Group A Nb: 0.05% or less (excluding 0%) Ti: 0.08% or less (excluding 0%) V: 0.2% or less (excluding 0%) W: 0.15% or less (excluding 0%) Zr: 0.15% or less (excluding 0%), one or more selected from the following: Group B Ni: 1.0% or less (excluding 0%) Cu: 1.0% or less (excluding 0%) Mo: 1.0% or less (excluding 0%) Co: 1.0% or less (excluding 0%) B: 0.005% or less (excluding 0%), one or more selected from the following: Group C Ca: 0.005% or less (excluding 0%) Mg: 0.005% or less (excluding 0%) REM: 0.005% or less (excluding 0%), one or more selected from the following: Group D Sn: 0.2% or less (excluding 0%) Sb: 0.2% or less (excluding 0%), one or more selected from the following: Group E Ta: 0.10% or less (excluding 0%); F group Te: 0.10% or less (excluding 0%) As: 0.10% or less (excluding 0%) Hf: 0.10% or less (excluding 0%), one or more selected from the following: G group Bi: 0.20% or less (excluding 0%) Pb: 0.20% or less (excluding 0%), one or more selected from the following: H group Zn: 0.10% or less (excluding 0%) Ge: 0.10% or less (excluding 0%) Sr: 0.10% or less (excluding 0%) Cs: 0.10% or less (excluding 0%), one or more selected from the following the balance being Fe and unavoidable impurities; The coating weight per side of the base steel sheet is 20 g / m ２ 120g / m or more ２ The following alloyed hot-dip galvanized layer is provided: The oxygen content of the surface layer of the steel sheet within 100 μm from the surface of the base steel sheet directly under the zinc plating layer toward the center of the sheet thickness is 0.030 g / m per side ２ is less than The number of intersections of cracks in the coating layer is 500 / mm on a surface parallel to the surface of the steel sheet at a depth M [μm] (1≦M≦5) from the surface of the zinc coating layer. ２ That's all, A high-strength galvannealed steel sheet having a diffusible hydrogen content of 0.30 mass ppm or less and a tensile strength of 780 MPa or more.

2. The high-strength galvannealed steel sheet according to claim 1, wherein a steel structure in a range from a surface of the base steel sheet to a depth of ⅛ to ⅜ of the sheet thickness has a total area ratio, in area %, of martensite, bainite and retained γ of 30% or more.

3. The high-strength galvannealed steel sheet according to claim 1, wherein a steel structure in a range from a surface of the base steel sheet to a depth of ⅛ to ⅜ of the sheet thickness has a total area ratio of martensite, bainite, and retained γ of 50% or more in area %, and the tensile strength is 980 MPa or more.

4. The high-strength galvannealed steel sheet according to claim 1, wherein a steel structure in a range from a surface of the base steel sheet to a depth of ⅛ to ⅜ of the sheet thickness has a total area ratio of martensite, bainite, and retained γ of 70% or more in area %, and the tensile strength is 1180 MPa or more.

5. The high-strength galvannealed steel sheet according to claim 1, wherein a steel structure in a range from a surface of the base steel sheet to a depth of ⅛ to ⅜ of the sheet thickness has a total area ratio of martensite, bainite, and retained γ of 85% or more in area %, and the tensile strength is 1310 MPa or more.

6. The high-strength galvannealed steel sheet according to claim 1, wherein a steel structure in a range from a surface of the base steel sheet to a depth of ⅛ to ⅜ of the sheet thickness has a total area ratio of martensite, bainite, and retained γ of 90% or more in area %, and the tensile strength is 1470 MPa or more.

7. A method for producing a high-strength galvannealed steel sheet having the composition according to claim 1, comprising: When annealing and hot-dip galvanizing treatments are performed on a base steel sheet in a continuous hot-dip galvanizing facility, the dew point of the atmosphere in the heating furnace in the temperature range of the base steel sheet in the annealing heating furnace is -40°C or lower in a temperature range of 700°C or higher, and the atmosphere in the heating furnace contains 20.0 vol% or less of hydrogen and 0.1 volppm or more and 3.0 volppm or less of SO ２ or 0.5 vol ppm to 10.0 vol ppm of HCl, A method for producing a high-strength galvannealed steel sheet, comprising: cooling a galvannealed layer from an alloying treatment to 250°C at a rate of 5°C / sec or more; and, after cooling to room temperature, performing either or both of the following steps (1) and (2): (1) After cooling to room temperature, the mixture is kept at room temperature for 48 hours or more. (2) After cooling to room temperature, the mixture is reheated to a temperature of 50° C. or higher and 400° C. or lower and maintained at this temperature for 0.1 hours or longer.