High-strength alloyed hot-dip galvanized steel sheet and manufacturing method therefor

Abstract

Provided is an alloyed hot-dip galvanized steel sheet which has a tensile strength of 780 MPa or more and which has excellent release properties of diffusible hydrogen in the steel sheet. Also provided is a method for manufacturing the same.　The high-strength alloyed hot-dip galvanized steel sheet comprises a steel sheet having a predetermined component composition with an alloyed hot-dip galvanizing layer disposed thereon, where the coating weight of the galvanizing layer is 20 g / m2 to 120 g / m2 per steel sheet side. A surface layer of the base steel sheet, which is directly beneath the galvanizing layer and extending up to 100 μm from the surface of the steel sheet in the direction toward the midpoint of the thickness of the steel sheet, has an oxygen content of 0.030 g / m2 or more per side. A plane that is at a depth M [μm] (1 ≤ M ≤ 5) from the surface of the galvanizing layer and that is parallel to the surface of the steel sheet includes intersections of cracks in the galvanizing layer in the amount of at least 100 / mm2. The galvanized steel sheet has a diffusible hydrogen content of 0.30 mass ppm or less, and a tensile strength of 780 MPa or more.

Description

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

[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.

[0002] In recent years, CO emissions have increased due to improvements in automobile fuel efficiency. ２ Efforts to reduce emissions are underway. Reducing the weight of vehicle bodies by thinning the steel sheets used to make them is an effective way to improve fuel efficiency, but 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 high-strength steel plates undergo sudden brittle fracture after a certain period of time when subjected to static load stress (load stress less than the tensile strength), with little apparent plastic deformation.

[0004] One of the causes of this delayed fracture is thought to be hydrogen embrittlement due to diffusible hydrogen in steel. Hot-dip galvanized steel sheets are generally produced by using a hot-rolled steel sheet or a cold-rolled steel sheet as a base material, recrystallizing the base steel sheet in an annealing furnace of a continuous hot-dip galvanizing line (CGL), and then hot-dip galvanizing the base steel sheet. Galvannealed steel sheets are produced by hot-dip galvanizing followed by an alloying treatment. 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 into the steel sheet and remains as diffusible hydrogen, thereby degrading the delayed fracture resistance. In particular, in high-strength steel sheets with a tensile strength of 780 MPa or more, hydrogen in the steel tends to remain after annealing, significantly degrading the 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 purpose, an austenite phase must be generated in the annealing process, but 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 in the annealing process, it is difficult to release it during the cooling process. Furthermore, when hot-dip galvanizing is applied, the hydrogen permeation rate in 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 introduced 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 a steel sheet containing hydrogen is heated at a predetermined temperature through a baking treatment to diffuse the hydrogen and release it 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 from the cracks to the outside of the steel sheet.

[0006] JP-A-7-173646 JP-A-6-33213 International Publication No. 2018 / 124157

[0007] However, in the baking treatment of Patent Document 1, in the case of a hot-dip galvanized steel sheet, 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 methods of forming cracks during plating as described in Patent Documents 2 and 3 have the problem that even if cracks are formed in the same manner as in Patent Documents 2 and 3, the rate at which hydrogen is released varies greatly depending on the manufacturing method.

[0009] The present invention has been made in view of the above circumstances, and aims to provide a high-strength galvannealed steel sheet having a tensile strength of 780 MPa or more, good release of diffusible hydrogen in the steel, and excellent delayed fracture resistance. Here, the delayed fracture resistance can be evaluated by the method described in the examples, and the steel sheet is judged to have excellent delayed fracture resistance when it satisfies the evaluation criteria of the examples.

[0010] The present inventors have discovered the following about hydrogen release from high-strength galvannealed steel sheet. They found that hydrogen release is affected not only by the presence or absence of cracks in the coating but also by the morphology of the cracks, and that it is particularly important that a certain number of crack intersections exist near the interface between the steel sheet and the coating. They further 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 is summarized as follows: [1] A high-strength galvannealed steel sheet having a coating layer on a substrate steel sheet, the components of which are, in mass %, C: 0.06% to 0.30%, Si: 0.20% to 3.00%, Mn: 1.5% to 3.5%, P: 0.1% or less (excluding 0%), S: 0.03% or less (excluding 0%), sol. The steel sheet contains 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, with the balance being Fe and unavoidable impurities, and the steel sheet has a coating weight of 20 g / m on each side. ２ 120g / m or more ２ The steel sheet has an alloyed hot-dip galvanized layer having an oxygen content of 0.030 g / m per side within 100 μm from the surface of the base steel sheet directly under the galvanized layer toward the center of the sheet thickness. ２ or more, and the number of intersections of cracks in the plating layer is 100 / mm on a surface parallel to the sheet surface of the steel sheet at a depth M [μm] (1≦M≦5) from the surface of the zinc plating layer. ２a diffusible hydrogen content in the steel of 0.30 ppm by mass or less, and a tensile strength of 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 elements selected from the following groups A to D: Group A: one or more 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%); Group B: one or more selected from 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%); 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 elements selected from the following groups E to H as the chemical composition: Group E Ta: 0.10% or less (not including 0%) Group F One or more elements selected from Te: 0.10% or less (not including 0%), As: 0.10% or less (not including 0%), Hf: 0.10% or less (not including 0%) Group G One or more elements selected from Bi: 0.20% or less (not including 0%), Pb: 0.20% or less (not including 0%) Group H One or more elements selected from Zn: 0.10% or less (not including 0%), Ge: 0.10% or less (not including 0%), Sr: 0.10% or less (not including 0%), Cs: 0.10% or less (not including 0%) [4] The high-strength galvannealed steel sheet according to any one of [1], [2], and [3], wherein the steel structure in a 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] The high-strength galvannealed steel sheet according to any one of [1], [2], and [3], wherein the steel structure in a 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 fraction of martensite, bainite, and retained γ of 50% or more in area %, and the tensile strength is 980 MPa or more. [6] The high-strength galvannealed steel sheet according to any one of [1], [2], and [3], wherein the steel structure in a 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 fraction of martensite, bainite, and retained γ of 70% or more in area %, and the tensile strength is 1180 MPa or more. [7] The high-strength galvannealed steel sheet according to any one of [1], [2], or [3], wherein the steel structure in a range from the surface of the substrate steel sheet to a depth of 1 / 8 to 3 / 8 of the sheet thickness has a total area fraction of martensite, bainite, and retained γ of 85% or more, in terms of area %, and the tensile strength is 1310 MPa or more. [8] The high-strength galvannealed steel sheet according to any one of [1], [2], or [3], wherein the steel structure in a range from the surface of the substrate steel sheet to a depth of 1 / 8 to 3 / 8 of the sheet thickness has a total area fraction of martensite, bainite, and retained γ of 90% or more, in terms of 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], wherein, when a steel sheet is annealed and hot-dip galvanized in a continuous hot-dip galvanizing facility, the dew point of the atmosphere in the annealing furnace is -20°C or higher in a temperature range in which the steel sheet temperature in the annealing furnace is 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 or more of HCl, wherein the cooling rate from the end of alloying treatment of the zinc coating layer to 250°C is 5°C / sec or more, and the coating is held at room temperature for 48 hours or more.

[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.

[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 substrate steel sheet (base steel sheet) and the reasons for limiting it will be explained below. In the following explanation, "%" representing the content of the component elements of the substrate 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 is preferably 0.30% or less, 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: 0.20% or more, but less than 3.00% Si 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. Furthermore, Si is an easily oxidizable element and has the effect of promoting oxide formation in the surface layer of the substrate steel sheet in an atmosphere with a high oxygen potential. To achieve these effects of increasing the strength of the steel sheet and promoting internal oxide formation in the surface layer of the substrate steel sheet, the Si content must be 0.20% or more. On the other hand, if the Si content is 3.00% or more, Si oxides that cause unplated defects are formed not only in the surface layer of the substrate steel sheet but also on the surface of the substrate steel sheet during CGL annealing, making it difficult to achieve good galvanizability. Therefore, the Si content must be less than 3.00%. From this perspective, the Si content is preferably 2.00% or less, and more preferably 1.50% 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 residual γ, bainite, and martensite. These effects are achieved by including 1.5% or more of Mn. Therefore, the Mn content must be 1.5% or more, and more preferably 1.8% or more. On the other hand, if the Mn content is 3.5% or less, the above effects can be achieved without increasing costs. Therefore, the Mn content must be 3.5% or less, and preferably 3.3% or less.

[0017] P: 0.1% or less (excluding 0%) By suppressing the P content, it is possible to prevent a decrease in weldability, and further, it is possible to prevent P from segregating at grain boundaries, thereby preventing deterioration of 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 constraints on production technology.

[0018] S: 0.03% or less (excluding 0%) It is preferable to reduce the S content as much as possible. By reducing the S content, deterioration in weldability can be prevented, and deterioration in ductility during hot rolling can be prevented, thereby suppressing hot cracking and significantly improving surface properties. Furthermore, by reducing the S content, deterioration in delayed fracture resistance, ductility, bendability, and stretch flangeability of the steel sheet due to the formation of coarse sulfides as an impurity element can be prevented. 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. The lower limit of the S content is not particularly limited, 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 setting 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 increasing the strength of the steel sheet due to the addition of Ti, Nb, and V from being impaired. Furthermore, by setting the N content to 0.007% or less, it is possible to prevent a decrease in toughness. Furthermore, by setting the N content to 0.007% or less, it is possible to prevent the occurrence of slab cracks and surface defects during hot rolling. For this reason, the N content must be set to 0.007% or less, more preferably 0.005% or less, even more preferably 0.003% or less, and particularly 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 most easily oxidized, it oxidizes before Si and Mn, suppressing the oxidation of Si and Mn at the outermost layer of the substrate steel sheet and promoting the oxidation of Si and Mn inside the substrate steel sheet. This effect is achieved when the sol. Al content is 0.01% or more. On the other hand, a sol. Al content exceeding 0.1% increases costs. Therefore, when 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, removing sol. Al at the impurity level also increases costs, so it is preferable to set it to 0.001% or more. As mentioned above, the sol. Al content is preferably 0.01% or more.

[0021] O: 0.003% or less (excluding 0%) O is an element that reduces the amount of Al in steel. ２ O ３ , SiO ２ O is an element that forms oxide-based inclusions such as CaO, MgO, (Al,Ca)—O, and (Si,Mn)—O, and the generation of these inclusions deteriorates delayed fracture resistance. In order to minimize such adverse effects on delayed fracture resistance, the O content must be 0.003% or less. There is no particular restriction on the lower limit of the O content, but currently, the industrially feasible lower limit is about 0.0005%.

[0022] Cr: 1.0% or less (excluding 0%) Cr is an element that enhances hardenability and can generate the desired amount of quenched martensite or tempered martensite, thereby improving the balance between strength and ductility. It is also an easily oxidizable element, and like Si, it has the effect of promoting internal oxide formation in the surface layer of the substrate steel sheet in an atmosphere with a high oxygen potential. To achieve these effects, the Cr content is preferably 0.05% or more. However, like Si, excessive Cr content can form Cr-based oxides that cause unplated defects not only in the surface layer of the substrate steel sheet but also on the surface of the substrate steel sheet during CGL annealing, making it difficult to obtain a good plating appearance. Therefore, from the perspective of galvanizability, the Cr content must be 1.0% or less, and preferably 0.7% or less.

[0023] The mass ratio of Si, Mn, and Cr (Si + Cr) / Mn is 0.25 or more. When the mass ratio of Si, Mn, and Cr (Si + Cr) / Mn is 0.25 or more and the oxygen potential of the annealing atmosphere is sufficiently high, internal oxides are formed in the surface layer of the substrate steel sheet during annealing, suppressing oxide formation on the surface of the substrate steel sheet. This is due to the effect of Si and Cr, which are easily oxidized. Suppressing oxide formation on the surface of the substrate steel sheet improves the coating appearance. On the other hand, when the (Si + Cr) / Mn ratio is less than 0.25, sufficient amounts of Si and Cr composite oxides with Mn are not formed in the surface layer of the substrate steel sheet within 100 μm from the surface of the substrate steel sheet, and oxide formation on the surface of the substrate steel sheet cannot be fully suppressed. This makes it more likely that poor coating appearance will occur due to uncoated areas. From these perspectives, in order to suppress poor coating appearance, the mass ratio of Si, Mn, and Cr (Si + Cr) / Mn needs to be 0.25 or more. From the viewpoint of suppressing plating defects, it is more preferable that (Si+Cr) / Mn is 0.30 or more.

[0024] In addition to the above-described chemical composition, the base steel sheet preferably further contains, in mass %, one or more elements selected from the following groups A to D. The chemical compositions of groups A to D and the effects of their inclusion are explained below. 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%), and Zr: 0.15% or less (excluding 0%)] Nb, Ti, V, W, and Zr are all effective elements for increasing the strength of the base steel sheet and can be added as needed. Nb is an element that can achieve a fine structure even with a small amount of content and can achieve high strength without impairing toughness. Nb: 0.05% or less (excluding 0%). Although the strength can be improved by adding 0.005% or more of Nb, the Nb content is preferably 0.05% or less to prevent cost increases. Ti: 0.08% or less (excluding 0%) Ti is an element effective for precipitation strengthening of steel. While the lower limit of Ti is not particularly limited, it is preferably 0.005% or more to obtain the effect of adjusting strength. However, excessive addition of Ti results in an excessively large hard phase and reduced formability. 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%) V is contained in an amount of 0.005% or more to obtain the effect of improving strength. However, in order to prevent an increase in cost, when V is contained, the V content is preferably 0.2% or less. W: 0.15% or less (excluding 0%) W is contained in an amount of 0.005% or more to obtain the effect of improving strength. However, in order to prevent an increase in cost, 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.

[0025] Group B [at least one element selected from 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%)] Ni, Cu, Mo, Co, and B are all elements that improve the hardenability of the steel sheet. Ni: 1.0% or less (excluding 0%). Although the Ni content can promote the formation of residual γ phase by containing 0.005% or more, in order to prevent an increase in costs, when Ni is contained, the Ni content is preferably 1.0% or less. Cu: 1.0% or less (excluding 0%). Although the Cu content can promote the formation of residual γ phase by containing 0.005% or more, in order to prevent an increase in costs, when Cu is contained, the Cu content is preferably 1.0% or less. Mo: 1.0% or less (excluding 0%). The strength adjustment effect can be obtained by including 0.005% or more of Mo. This effect is particularly enhanced when the Mo content is 0.05% or more. However, from the viewpoint of preventing cost increases, the Mo content is preferably 1.0% or less. Co: 1.0% or less (excluding 0%). Co is an element that spheroidizes the shape of inclusions and improves the ultimate deformability of the steel sheet, thereby effectively improving stretch flangeability. 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, reducing bendability, so the Co content is preferably 1.0% or less. B: 0.005% or less (excluding 0%). B is an element that is effective in improving the hardenability of steel. In order 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.

[0026] Group C [At least one element 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%). Ca can be contained in an amount of 0.0005% or more to control the morphology of sulfides and improve ductility and toughness. 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%). Mg can be contained in an amount of 0.0005% or more to control the morphology of sulfides and improve ductility and toughness. 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.

[0027] Group D [At least one element 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 respective contents are set to be greater than 0%. Sn: 0.2% or less (excluding 0%) Sn is an element that suppresses denitrification, deboronization, etc., and is effective in suppressing a decrease in the strength of the steel sheet. To achieve this effect, a content of 0.002% or more is preferred. On the other hand, when Sn is contained, in order to obtain good impact resistance, the Sn content is preferably 0.20% or less. Sb: 0.2% or less (excluding 0%) Sb can be contained from the perspective of suppressing nitriding or oxidation of the substrate steel sheet surface, or decarburization of a region of several tens of microns on the surface of the substrate steel sheet caused by oxidation. Sb suppresses nitriding and oxidation of the surface of the substrate steel sheet, thereby preventing a decrease in the amount of martensite formed on the surface of the substrate steel sheet and improving the fatigue properties and surface quality of the substrate steel sheet. 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.

[0028] In addition to the above-mentioned chemical composition, the base steel sheet preferably further contains, in 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 in group A, is an element effective in increasing the strength of the steel sheet and can be contained as needed. The effect of improving strength can be obtained by containing 0.005% or more of Ta, but from the viewpoint of preventing an increase in costs, if Ta is contained, the Ta content is set to 0.10% or less.

[0029] Group F [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%)] Like the elements of group C, Te, As, and Hf 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, it is possible to control the morphology of sulfides and improve ductility and toughness. However, in order to prevent 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, it is possible to control the morphology of sulfides and improve ductility and toughness. However, in order to prevent 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 form of sulfides can be controlled and ductility and toughness can be improved. However, from the viewpoint of preventing an increase in costs, when Hf is contained, the Hf content is set to 0.10% or less.

[0030] 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 contents are each set to more than 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 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 cost increases. 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. Furthermore, Pb has the effect of improving machinability and smoothness of the cut end surface, and also acts to improve the delayed fracture resistance of the cut surface. When Pb is contained, the Pb content is set to 0.20% or less from the viewpoint of preventing an increase in cost.

[0031] 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%), and 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, each is set to 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 significantly affect mechanical properties or surface quality. To prevent cost increases, when 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 significantly affect mechanical properties or surface quality. In order to prevent an increase in costs, if Ge is contained, the Ge content is set to 0.10% or less. - Sr: 0.10% or less (excluding 0%) Even if Sr is contained in an amount of 0.001% or more, it does not have a significant effect on mechanical properties or surface quality. In order to prevent an increase in costs, if Sr is contained, the Sr content is set to 0.10% or less. - Cs: 0.10% or less (excluding 0%) Even if Cs is contained in an amount of 0.001% or more, it does not have a significant effect on mechanical properties or surface quality. In order to prevent an increase in costs, if Cs is contained, the Cs content is set to 0.10% or less.

[0032] The composition of the substrate steel sheet (base steel sheet) is such that the remainder other than the above-mentioned components is Fe and unavoidable impurities.

[0033] Next, the manufacturing conditions of the present invention will be described. 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 a continuous hot-dip galvanizing facility, and after continuous annealing in the facility, hot-dip galvanizing is performed, and further alloying treatment is performed to obtain a galvannealed steel sheet.

[0034] In general, continuous hot-dip galvanizing equipment is composed of an annealing furnace and a hot-dip galvanizing device located downstream of the annealing furnace. The hot-dip galvanizing device includes a hot-dip galvanizing bath and a snout connected to the steel strip outlet side of the annealing furnace, with its tip immersed in the hot-dip galvanizing bath. A typical continuous hot-dip galvanizing line (CGL) configured to continuously perform a series of processes including heating, cooling, hot-dip galvanizing, and hot-dip galvanizing alloying can be used as such continuous hot-dip galvanizing equipment. The base steel sheet introduced into the continuous hot-dip galvanizing equipment is annealed while passing through an annealing furnace having a heating zone, a soaking zone, and a cooling zone in this order. Specific annealing conditions are as follows:

[0035] During the heating process of annealing, the atmospheric dew point in the temperature range in the heating furnace where the temperature of the base steel sheet is 700°C or higher is set to -20°C or higher. In addition, the atmosphere in the heating furnace contains 20.0 vol% or less of hydrogen and 0.1 volppm to 3.0 volppm of SO. ２ and 0.5 vol ppm to 10.0 vol ppm of HCl.

[0036] Dew point of the atmosphere in the heating furnace in a temperature range of 700°C or higher: −20°C or higher. At steel sheet temperatures of 700°C or higher, surface segregation due to the diffusion of Si and Mn increases. In this case, setting the atmospheric dew point to −20°C or higher promotes the formation of internal oxides on the surface of the substrate steel sheet, i.e., at a depth of up to 100 μm from the surface. This can suppress the formation of oxides on the surface of the substrate steel sheet. This can suppress the occurrence of bare spots caused by oxides on the surface of the substrate steel sheet, improving the coating appearance. Furthermore, internal oxides formed on the surface of the substrate steel sheet are known to promote the release of diffusible hydrogen from the substrate steel sheet, thereby improving the hydrogen release rate. To fully achieve these effects of suppressing the formation of oxides on the surface of the substrate steel sheet and improving the hydrogen release rate, a temperature of −15°C or higher is more preferable.

[0037] Hydrogen concentration in the heating furnace atmosphere: 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 hydrogen release difficult. 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 substrate 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.

[0038] The atmosphere inside the heating furnace is SO ２ It is necessary to contain at least one of the following: a concentration of 0.1 vol ppm or more and 3.0 vol ppm or less in the atmosphere inside the heating furnace; or a HCl concentration of 0.5 vol ppm or more and 10.0 ppm or less in the atmosphere inside the heating furnace. ２ The inclusion of at least one of HCl and / or HCl is one of the important requirements of the present invention. The atmosphere in the heating furnace during annealing is 0.1 vol ppm to 3.0 vol ppm of SO ２ and HCl in an amount of 0.5 vol ppm or more and 10.0 vol ppm or less. ２When corrosive gases such as HCl and HCl are present in appropriate amounts, the grain size of the surface layer of the base steel sheet after recrystallization annealing decreases, and when a certain level of stress is applied to the coating layer, the number of cracks formed during coating and the number of intersections become more likely to increase. These cracks are important for the release of diffusible hydrogen in steel. During the alloying process of the zinc coating 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 form when a certain level of stress is applied to the coating layer, and serve as the starting point for crack initiation during coating. When the grain size decreases due to the presence of corrosive gases, the spacing between the formation of the low-ductility alloy phase also decreases. Therefore, it is thought that tensile stress applied to the coating layer during cooling of the steel sheet after alloying process makes the coating layer more susceptible to cracking, increasing the number of crack intersections. This improvement effect of corrosive gases is due to the presence of SO. ２ This becomes apparent when the concentration of SO2 is 0.1 ppm or more and the concentration of HCl is 0.5 ppm or more. ２ If the concentration of SO exceeds 3.0 ppm and the concentration of HCl exceeds 10.0 ppm, there is a possibility that deterioration of the furnace body inside the heating furnace will be accelerated. ２ In the case of HCl, the content must be 0.1 vol ppm or more and 3.0 vol ppm or less, and in the case of HCl, the content must be 0.5 vol ppm or more and 10.0 ppm or less. The balance of the atmosphere in the heating furnace during annealing is nitrogen, CO, CO ２ It may also contain gases such as

[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 a temperature 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] These trace amounts of SO ２The concentration of corrosive gases such as HCl and the like can be controlled by adjusting the amount of gas when introducing the gas containing these corrosive gases directly into the furnace. ２ Applying a liquid containing SO4 or HCl and adjusting the amount, and then ２ It can also be controlled by adjusting the concentration of SO4 or HCl. ２ It is important to control the concentration of corrosive gases such as SO and HCl. ２ The method for controlling the concentration of corrosive gases such as HCl is not limited to the above method.

[0041] 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 a temperature 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 maximum 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.

[0042] In the present invention, the base steel sheet continuously annealed under the above conditions is cooled and then immersed in a hot-dip galvanizing bath to undergo hot-dip galvanizing. The cooling temperature is preferably 200 to 520°C, and the steel sheet is heated as needed before immersion in the hot-dip galvanizing bath. The bath temperature of the hot-dip galvanizing bath is generally about 440 to 500°C. The hot-dip galvanizing bath is not particularly limited, but may be, for example, one having an Al content of 0.10% by mass to 0.23% by mass, and further containing 0% by mass to 3.5% by mass of one or more elements selected from Pb, Sb, Si, Sn, Mg, Mn, Ni, Cr, Co, Ca, Cu, Li, Ti, Be, Bi, and REM, with the balance being Zn and unavoidable impurities. In addition, in order to prevent changes in the temperature of the coating bath, it is preferable that the temperature of the base steel sheet before coating treatment (immersion sheet temperature) be equal to or higher than the coating bath temperature and equal to or lower than the coating bath temperature + 50°C.

[0043] After the above-mentioned hot-dip galvanizing treatment, a galvannealing treatment is further carried out to form a galvannealed 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 lower than 480°C, the Zn-Fe alloying rate becomes excessively slow, making alloying extremely difficult. On the other hand, if the alloying temperature exceeds 570°C, untransformed austenite may transform into pearlite, resulting in a decrease in TS and El. The alloying treatment is more preferably carried out in a temperature range of 490°C or higher 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 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.

[0044] 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.

[0045] Average cooling rate: 5°C / s or more. This is one of the important requirements of the present invention. By setting the average cooling rate from the end of the alloying treatment to 250°C to 5°C / s or more, the number of crack intersections in the coating required for releasing diffusible hydrogen from the steel can be obtained. 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 cause cracks to form in the coating of 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 required 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.

[0046] The galvannealed steel sheet cooled to 50°C or below 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 an apparatus connected to a continuous hot-dip galvanizing facility, or may be performed offline from the continuous hot-dip galvanizing facility. Furthermore, the target elongation rate (e.g., 0.05% or more and 1.00% or less) may be achieved in a single rolling pass, or the target elongation rate may be achieved by performing multiple rolling passes. Note that, although temper rolling is generally performed as the rolling, rolling by a method such as processing using a leveler may also be used as long as it can impart an elongation rate equivalent to that of temper rolling.

[0047] The galvannealed steel sheet is held at room temperature for 48 hours or more. This allows diffusible hydrogen in the steel sheet to be released through cracks during plating, improving delayed fracture resistance. In the present invention, room temperature refers to 0°C to 40°C. In order to fully achieve the effect of promoting the release of diffusible hydrogen in the steel sheet, 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 1800 hours or less. Furthermore, the release of diffusible hydrogen from the steel sheet may be carried out at a temperature range of 50°C to 400°C instead of room temperature. In this case, the release of diffusible hydrogen can be completed in a shorter time than when held at room temperature.

[0048] The manufacturing conditions other than those mentioned above can be determined by ordinary methods.

[0049] Next, the high-strength galvannealed steel sheet obtained by the above-mentioned production method will be described. First, by the above-mentioned production method, the steel sheet of the present invention can have a total hydrogen release of 0.30 mass ppm or less at 25°C or higher and 300°C or lower, as measured by temperature ramp analysis. As a result, a high-strength galvannealed steel sheet excellent in delayed fracture resistance can be obtained.

[0050] The high-strength galvannealed steel sheet produced in 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 －１ A tensile test is carried out under the condition of 1 / 1000 mm / s to measure TS.

[0051] The structure of the steel sheet is not particularly limited, but in order to ensure a tensile strength of 780 MPa or more, it is preferable to have a steel sheet structure as described below. Note that the steel sheet structure described below is a steel sheet structure in the range of ⅛ to ⅜ thickness depth from the surface of the steel sheet. That is, the steel sheet structure preferably has a total area ratio of martensite, bainite, and retained γ (retained austenite) of 30% or more, thereby obtaining a steel sheet with 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 with 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 with a tensile strength of 1180 MPa or more can be obtained. Furthermore, by setting the total area ratio of martensite, bainite, and residual γ to 85% or more, a steel sheet with a tensile strength of 1310 MPa or more can be obtained, and by setting the total area ratio of martensite, bainite, and residual γ to 90% or more, a steel sheet with a tensile strength of 1470 MPa or more can be obtained. The steel microstructure 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 microstructure of high-strength steel sheets is complex and fine, it is preferable to use an SEM, which allows for more detailed microstructural observation. The steel sheet can be polished 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. Furthermore, finish-polishing can also be performed by electrolytic polishing or ion polishing. Furthermore, etching with nital at a concentration of 1 vol% to 5 vol% can produce steps on the cross-section suitable for observing the steel microstructure, so the nital concentration is preferably 1 vol% to 5 vol%. 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, in order 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.

[0052] Here, the size and abundance of each structure are not particularly limited, but as one embodiment of the present invention, the following sizes and abundances can be exemplified. 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, circle equivalent diameter ≦30 μm Distribution density of carbides inside the structure: 0.10 to 12 / μm ２ Fresh martensite and residual γ Lumpy: Aspect ratio ≦8, equivalent circle diameter: 3 to 30 μm Granular: Aspect ratio ≦8, equivalent circle diameter: 0.40 μm or more, less than 3 μm Plate or film: Aspect ratio greater than 8, Thickness: 0.10 to 8 μm Bainite Film or plate: Aspect ratio greater than 8, Thickness ≦8 μm Lumpy: Aspect ratio ≦8, equivalent circle diameter ≦30 μm Distribution density of carbides within the structure: 0.10 to 6 / 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

[0053] 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. ２ 120g / m or more ２ It has a zinc plating layer of 20 g / m ２ If the coating weight is less than 120 g / m, not only is the corrosion resistance liable to decrease, but it is also difficult to control the coating weight. ２ The high-strength galvannealed steel sheet of the present invention has a good surface appearance according to the evaluation criteria described in the examples.

[0054] The high-strength galvannealed steel sheet of the present invention has an oxygen content of a surface layer portion 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. ２The 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. ２ In the above cases, the formation of oxides on the surface of the substrate steel sheet is suppressed, and a good coating appearance is obtained. ２ In the above cases, the effect of promoting the release of diffusible hydrogen in the steel sheet by the oxides is sufficiently obtained. Therefore, from the viewpoint of improving the coating appearance and promoting hydrogen release, the oxygen content is set to 0.030 g / m per side. ２ Furthermore, from the viewpoint of obtaining a better plating appearance and a hydrogen release promoting effect, the oxygen content must be 0.040 g / m per side. ２ It is preferable that this is equal to or greater than this.

[0055] The high-strength galvannealed steel sheet of the present invention has a crack intersection number of 100 / mm on a plane parallel to the sheet surface of the galvanized steel sheet at a depth M [μm] (1≦M≦5) from the surface of the galvanized layer. ２ This 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 1 μm or more and 5 μm or less, 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 performance. If the number of intersections of the cracks measured in this way is 100 points / mm ２The above conditions are necessary to obtain sufficient hydrogen desorption properties. The thickness reduction of the plating layer can be performed by measuring the thickness of a sample piece before, during, and after polishing, and mechanically polishing the sample piece parallel to the sheet surface of the galvanized steel sheet 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 piece before and after polishing. The method for measuring the thickness of the sample piece 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 plating 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 are generated from one point. In the production of high-strength galvannealed steel sheets, cracks in the plating layer may be crushed on the plating 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 to each other, hydrogen can be released from the uncrushed cracks even if some of the cracks are crushed, and it is believed that good hydrogen desorption properties can be achieved. On the other hand, if there are many isolated cracks, the cracks collapse, eliminating the pathways for hydrogen release, which is thought to lead to a deterioration in hydrogen release properties. From this perspective, it is considered that the number of intersections of cracks on the plating surface observed after thickness reduction is 100 points / mm ２ If this is the case or higher, it is believed that cracks connected to the surface of the base steel sheet are sufficiently connected, and good hydrogen release properties can be obtained. The amount of diffusible hydrogen in the steel of the high-strength galvannealed steel sheet according to the present invention is 0.30 ppm by mass or less. If the amount of diffusible hydrogen in the steel exceeds 0.30 ppm by mass, the delayed fracture resistance becomes insufficient, so the amount of diffusible hydrogen in the steel needs to be 0.30 ppm by mass or less. The amount of diffusible hydrogen 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 galvanizing alloying treatment under the conditions described in the production method of the present invention, the amount of diffusible hydrogen in the steel can be made 0.30 ppm by mass or less.

[0056] 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.

[0057] Steel having the chemical composition shown in Table 1 was melted and cast, and the resulting slab was hot-rolled, pickled, and cold-rolled to form a cold-rolled steel sheet with a thickness of 1.2 mm. This cold-rolled steel sheet was used as the base steel sheet for the galvannealed steel sheet. In a CGL equipped with an all-radiant tube (ART) annealing furnace, the steel sheet was annealed under the conditions shown in Table 2, and then hot-dip galvanized (coating composition: Zn-0.14 mass% Al) and gas-wiped to a coating weight per side of approximately 50 g / m. ２ The alloying treatment was carried out. After the alloying treatment, the steel sheets were cooled under the conditions shown in Table 2 and held at room temperature for a predetermined time. The surface of the galvanized layer of the obtained galvannealed steel sheets was polished to a depth of 1 μm to 5 μm from the surface of the galvanized layer, and the number of intersections of cracks in the galvanized layer observed in the observed image was counted. The thickness of the galvanized layer and the amount of diffusible hydrogen in the steel sheets were measured, and the delayed fracture resistance was evaluated using the measurement and evaluation methods shown below. Analysis of the furnace gas and evaluation of the tensile strength and structure of the steel sheets were also carried out. The above results, along with the manufacturing conditions, are shown in Table 2.

[0058] Analysis method of furnace gas: Furnace gas was collected from the annealing furnace and analyzed by ion chromatography. ２ The analysis was carried out three times, and the average value was used as the concentration of the gas inside the furnace.

[0059] Measurement of Zinc Plating Layer Thickness: A 10 mm x 10 mm sample was cut from the width center of the galvannealed steel sheet obtained by the above method. The sample was embedded in resin with the plated surface perpendicular to the plated surface, then polished perpendicular to the plated surface with waterproof abrasive paper, followed by finish polishing with 1 μm diamond abrasive grains. After polishing, the sample was etched with 0.05% nital for 30 seconds, and the cross section was observed with an SEM at 400x magnification. For SEM observation, secondary electron images were acquired at an accelerating voltage of 10 kV. The observation field consisted of five consecutive fields from the center of the sample, and the plating thickness was measured at six equal positions (five locations) in each field. The average thickness of all 25 locations obtained by the above procedure was taken as the thickness of the zinc plating layer.

[0060] Measurement of zinc plating layer thickness reduction and number of crack intersections in the plating layer. A 20 mm x 20 mm sample was cut from the width center of the galvannealed steel sheet and degreased with alcohol. The plating surface was then mechanically polished with diamond abrasive grains of 3 μm diameter, followed by finish polishing with diamond abrasive grains of 1 μm diameter and 0.25 μm diameter diamond abrasive grains to reduce the plating layer thickness. The thickness reduction was calculated from the difference in thickness of the sample before and after polishing, and adjusted so that the thickness reduction of the plating layer after polishing was in the range of 1 μm to 5 μm. After polishing, etching was performed with 1% nital for 30 seconds, and SEM observation was performed at 500x magnification. For SEM observation, backscattered electron images were observed at an accelerating voltage of 15 kV. The observation field was 9 consecutive fields of view (3 fields x 3 fields) from the center of the sample, and the number of crack intersections in the plating was counted in each field. The intersection of the cracks is defined as the point where three or more cracks have occurred from one point, and the area (mm ２ The number of intersections per

[0061] Measurement of diffusible hydrogen content in steel sheet: A rectangular test piece measuring 30 mm in major axis length and 5 mm in minor axis length was taken from the width center of a galvannealed steel sheet. The coating layer of the test piece was removed using a router. Hydrogen analysis was performed using a temperature-programmed hydrogen release analyzer under conditions of an analysis start temperature of 25°C, an analysis end temperature of 300°C, and a heating rate of 200°C / hour. The amount of released hydrogen (ppm by mass), which is the amount of hydrogen released from the surface of the test piece at each temperature, was measured. The sum of the amounts of hydrogen detected at 300°C or less was taken as the diffusible hydrogen content in the steel sheet. Here, a diffusible hydrogen content in the steel of 0.05 ppm by mass or less was evaluated as excellent (◎), a diffusible hydrogen content of more than 0.05 ppm by mass and less than 0.15 ppm by mass was evaluated as good (○), and a diffusible hydrogen content of more than 0.15 ppm by mass and less than 0.30 ppm by mass was evaluated as fair (△). From experience, when the amount of diffusible hydrogen in steel exceeds 0.30 ppm by mass, the delayed fracture resistance property is often reduced, and therefore, steels having a diffusible hydrogen content of more than 0.30 ppm by mass were rated as poor ("x").

[0062] Evaluation of Delayed Fracture Resistance: Strip-shaped test specimens with a major axis length of 100 mm and a minor axis length of 20 mm were taken from the galvannealed steel sheet in the direction perpendicular to the rolling direction. A punched hole with a diameter of 15 mm and a clearance of 12.5% ​​was drilled at the center of the major and minor axes of the test specimens. The test specimens were subjected to tensile tests, and their delayed fracture resistance was evaluated based on whether or not delayed fracture occurred through the punched hole. To prevent the release of diffusible hydrogen in the steel due to aging, the time from taking the strip-shaped test specimens from the galvannealed steel sheet to starting the delayed fracture tensile test (tensile speed: 10 mm / min) was limited to within 10 minutes. The loading time in the tensile test was a maximum of 100 hours. The maximum stress at which no cracks (here, "cracks" refers to fractures under tensile stress loading) occurred after 100 hours of loading was defined as the critical stress, and the delayed fracture resistance was evaluated by the ratio of the critical stress to the yield stress. The evaluation criteria for delayed fracture resistance were as follows: when 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 "◯", 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 the delayed fracture test is generally lower (disadvantageous) for steel sheets with higher strength.

[0063] 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 measured value is taken as the amount of oxygen contained in the raw material, OH. In addition, the oxygen concentration in the steel throughout the entire thickness direction of the high-strength steel sheet after continuous annealing is measured, and this measured value is taken as the amount of oxygen after oxidation, OI. Using the amount of oxygen after oxidation, OI, of the steel sheet obtained in this way 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.

[0064] Evaluation of Coating Appearance The coating appearance of the galvannealed steel sheets was visually observed to check for the presence or absence of uncoated defects and the V-shaped scale patterns that appear as a symptom of such defects. For the observation, an A4-sized steel sheet was sampled from any location near the center of the coil width in the stationary part, excluding the leading and trailing ends of the coil, and visual observation was made on the front and back surfaces of the steel sheet, N=5. When no defects were observed in the observed area, the rating was excellent (◎); when scale patterns were observed but no uncoated defects were observed, the rating was good (○); when scale patterns and fewer than five uncoated defects larger than 0.5 mm were observed, the rating was fair (△); and when five or more uncoated defects larger than 0.5 mm were observed, the rating was poor (×).

[0065] Tensile Test JIS Z2241 No. 5 test pieces were taken from the galvannealed steel sheet in the direction perpendicular to the rolling direction (so that the sheet width direction was 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).

[0066] Observation and Measurement of Base Steel Sheet Structure The total area ratio of martensite, bainite, and retained γ in the base steel sheet structure 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 base 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 structure. The steel structure was observed at a depth of 1 / 8 to 3 / 8 of the sheet thickness on this sample observation surface, and five fields of view were observed at a magnification of 3000x using an SEM. The total area of ​​martensite, bainite, and retained γ was determined from the obtained structure image, 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 is a gray or dark gray region close to black in an SEM photograph. Tempered martensite has a blocky morphology with boundaries at prior γ grain boundaries and interfaces with other structures such as ferrite. However, tempered martensite may contain other structures such as bainite, resulting in a concave shape. Tempered martensite contains many carbides, but depending on the plane orientation, there may be only a small amount of carbides. Fresh martensite is a gray or white region in an SEM photograph. Fresh martensite is blocky, granular, plate-like, or film-like and does not contain carbides. Bainite Bainite is a dark gray region in an SEM photograph. Bainite is either a film-like or plate-like morphology, or a blocky morphology in which some or all of these adjacent regions are connected, and contains a small amount of carbides. It may contain coarse carbides that have been tempered after production. Retained austenite (retained γ) Retained γ is a region that has the same color and shape 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 SEM photographs, it is identified as a region that combines these two structures.To ensure the strength of steel plates, the total area ratio of martensite, bainite, and residual γ must be controlled. However, the remainder may include the following structures, but is not limited to these. Ferrite: Ferrite is the black area in SEM images. Ferrite has a blocky morphology and contains almost no carbides. Bainitic ferrite contains almost no carbides and has similar mechanical properties to ferrite, so it belongs to the ferrite group. Ferrite may contain granular or blocky fresh martensite, granular or blocky residual γ, or both. Fresh martensite and residual γ contained within ferrite are not included in the area ratio of ferrite, but are treated as the area ratio of fresh martensite or residual γ. Carbide: Carbide is the white area in SEM images. Carbide has a granular or film-like morphology. Although fine carbides mainly form inside ferrite, martensite, and bainite, their area ratios are small and negligible. 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. Structures other than the above Each of the above structures includes nitrides such as TiN, carbonitrides such as (Nb,Ti)(C,N), sulfides such as MnS and CaS, and Al. ２ O ３ , SiO ２ These oxides may be contained in a total area ratio of a few percent. Since these area ratios are small and can be ignored, the area ratios of these nitrides, carbonitrides, sulfides, or oxides are included in the area ratios of each structure containing them. Pearlite may also be contained. In the case of pearlite, the area ratio of pearlite is calculated separately.

[0067] 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.

Claims

1. A high-strength galvannealed steel sheet having a coating layer on a substrate steel sheet, the composition of the substrate steel sheet being, in mass %, C: 0.06% or more and 0.30% or less, Si: 0.20% or more but less than 3.00%, 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%), and Cr: 1.0% or less (not including 0%), the mass ratio of Si, Mn, and Cr (Si+Cr) / Mn being 0.25 or more, with the balance being Fe and unavoidable impurities, and the coating weight per side of the substrate steel sheet being 20 g / m ２ 120g / m or more ２ The steel sheet has an alloyed hot-dip galvanized layer having an oxygen content of 0.030 g / m per side within 100 μm from the surface of the base steel sheet directly under the galvanized layer toward the center of the sheet thickness. ２ or more, and the number of intersections of cracks in the plating layer is 100 / mm on a surface parallel to the sheet surface of the steel sheet at a depth M [μm] (1≦M≦5) from the surface of the zinc plating layer. ２ a diffusible hydrogen content in the steel of 0.30 ppm by mass or less and a tensile strength of 780 MPa or more.

2. The high-strength galvannealed steel sheet according to claim 1, wherein the base steel sheet further contains, in mass %, one or more elements selected from the following groups A to D: Group A: one or more 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%); Group B: one or more selected from 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%); 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 claim 1 or 2, wherein the base steel sheet further contains, in mass %, one or more elements selected from the following groups E to H as part of its chemical composition: Group E: Ta: 0.10% or less (not including 0%) Group F: One or more elements selected from Te: 0.10% or less (not including 0%), As: 0.10% or less (not including 0%), Hf: 0.10% or less (not including 0%) Group G: One or more elements selected from Bi: 0.20% or less (not including 0%), Pb: 0.20% or less (not including 0%) Group H: One or more elements selected from Zn: 0.10% or less (not including 0%), Ge: 0.10% or less (not including 0%), Sr: 0.10% or less (not including 0%), Cs: 0.10% or less (not including 0%) 4. A high-strength galvannealed steel sheet according to any one of claims 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 30% or more in terms of area %.

5. A high-strength galvannealed steel sheet according to any one of claims 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, in area %, of martensite, bainite and retained γ of 50% or more, and the tensile strength is 980 MPa or more.

6. A high-strength galvannealed steel sheet according to any one of claims 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, in area %, of martensite, bainite and retained γ of 70% or more, and the tensile strength is 1180 MPa or more.

7. A high-strength galvannealed steel sheet according to any one of claims 1, 2 and 3, wherein the steel structure in the range from the surface of the substrate 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 claims 1, 2 and 3, wherein the steel structure in the range from the surface of the substrate 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 the chemical composition as defined in any one of claims 1, 2 and 3, wherein, when a base steel sheet is annealed and hot-dip galvanized 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 at 700°C or higher is -20°C or higher, and the atmosphere in the heating furnace contains 20.0 vol% or less of hydrogen and 0.1 volppm to 3.0 volppm of SO ２ or 0.5 vol ppm or more and 10.0 vol ppm or less of HCl, wherein the cooling rate from the end of alloying treatment of the zinc coating layer to 250°C is 5°C / sec or more, and after cooling, the zinc coating layer is held at room temperature for 48 hours or more.