High-strength hot-dip galvanized steel sheet and method for producing same

Abstract

Provided is a high-strength hot-dip galvanized steel sheet which has a tensile strength of 780 MPa or more and excellent hydrogen desorption properties.　Specifically provided is a high-strength hot-dip galvanized steel sheet which comprises a base material steel sheet and a hot-dip galvanization layer that is formed on the surface of the base material steel sheet, wherein: the base material steel sheet has a specific component composition; the hot-dip galvanization layer internally comprises cracks that extend from the surface of the hot-dip galvanization layer to the interface between the base material steel sheet and the hot-dip galvanization layer, and are continuous with a crystal grain boundary, in which an oxide is present, of the base material steel sheet at the interface between the base material steel sheet and the hot-dip galvanization layer; the density of the cracks is 10 per mm or more; and the tensile strength is 780 MPa or more.

Description

High-strength hot-dip galvanized steel sheet and method for manufacturing the same

[0001] The present invention relates to a high-strength hot-dip galvanized steel sheet and a manufacturing method thereof, and more particularly to a high-strength hot-dip galvanized steel sheet having excellent hydrogen desorption properties suitable for use in automotive components, and a manufacturing method thereof.

[0002] In recent years, with the growing awareness of protecting the global environment, ２ There is a strong demand for improved fuel efficiency to reduce emissions. Accordingly, there has been a growing trend to reduce the weight of vehicle bodies by increasing the strength of vehicle body materials to make them thinner. As a result, high-strength steel sheets, including hot-dip galvanized steel sheets with corrosion resistance, are beginning to be widely used.

[0003] On the other hand, new problems have arisen with the increase in the strength of steel plates. One of these is delayed fracture. This is a phenomenon in which, when high-strength steel plates are subjected to static load stress (load stress below the tensile strength), they suddenly undergo brittle fracture after a certain period of time, with little or no apparent plastic deformation.

[0004] It is known that delayed fracture occurs in steel plates due to residual stresses created when the steel is pressed into a specified shape and hydrogen embrittlement of the steel at stress-concentrated areas. The hydrogen that causes hydrogen embrittlement is thought to be hydrogen that has infiltrated and diffused into the steel from the external environment in most cases.

[0005] In the manufacturing process of hot-dip galvanized steel sheets, the steel sheets are subjected to hot-dip galvanization after heat annealing. This heat annealing is performed in a non-oxidizing atmosphere containing hydrogen or a reducing atmosphere to suppress oxidation of the steel sheet surface. Therefore, a large amount of diffusible hydrogen is contained in the steel sheet during the manufacturing process of hot-dip galvanized steel sheets, which can be a cause of delayed fracture. In addition to delayed fracture, diffusible hydrogen also deteriorates various properties of the steel sheet, such as ductility and hole expandability.

[0006] Baking is known as a process for releasing (desorbing) hydrogen that has penetrated into a steel material (see, for example, Patent Document 1). Baking is a method in which a steel material into which hydrogen has penetrated is heated at a predetermined temperature, thereby diffusing the hydrogen and causing it to be released (desorbed) from the surface of the steel material.

[0007] Patent Documents 2 and 3 propose a method of forming a certain number of cracks in the coating, thereby releasing hydrogen from the steel sheet through the cracks.

[0008] JP-A-7-173646 JP-A-6-33213 International Publication No. 2018 / 124157

[0009] 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 such as changes in the properties of the coating layer arise.

[0010] Furthermore, in the method of forming cracks during plating as disclosed in Patent Documents 2 and 3, although hydrogen desorption is promoted, there is a problem in that even if cracks are formed in the same manner as in Patent Documents 2 and 3, the hydrogen desorption rate varies greatly depending on the steel sheet produced.

[0011] The present invention has been made in view of the above circumstances, and has an object to provide a high-strength hot-dip galvanized steel sheet having a tensile strength of 780 MPa or more and excellent hydrogen desorption properties.

[0012] The present inventors conducted a detailed comparative study of high-strength hot-dip galvanized steel sheets in which hydrogen desorption is promoted by cracks formed in the coating layer, among steel sheets with different hydrogen desorption rates, and as a result, they discovered the following findings.

[0013] In the high-strength hot-dip galvanized steel sheets in which hydrogen desorption was promoted, cracks were formed in both coating layers. In the steel sheets with a relatively slow hydrogen desorption rate, many of the cracks reached the interface between the coating layer and the base steel sheet, but the cracks stopped at the grain surface of the base steel sheet (see Figure 2). That is, as shown in Figure 2, a crack 6 formed in the steel sheet with a relatively slow hydrogen desorption rate continued from the surface of the hot-dip galvanized layer 2 to the interface between the base steel sheet 1 and the hot-dip galvanized layer 2, but stopped at the grain surface of the base steel sheet 1.

[0014] On the other hand, in steel sheets with a relatively fast hydrogen desorption rate, cracks continued to the interface between the galvanized layer and the base steel sheet, and further reached the grain boundaries of the crystal grains of the base steel sheet, and oxides were present at the grain boundaries where the cracks reached (see FIG. 1 ). That is, as shown in FIG. 1 , steel sheets with a relatively fast hydrogen desorption rate had cracks 5 in the hot-dip galvanized layer 2 that continued from the surface of the hot-dip galvanized layer 2 to the interface between the base steel sheet 1 and the hot-dip galvanized layer 2, and further connected to grain boundaries 4 of the base steel sheet 1 at the interface between the base steel sheet 1 and the hot-dip galvanized layer 2, where oxides 3 were present at the grain boundaries. The steel sheets with a larger number of cracks that reached the grain boundaries of the base steel sheet had a faster hydrogen desorption rate.

[0015] The present invention has been made based on the above findings and provides the following: [1] A high-strength hot-dip galvanized steel sheet comprising a base steel sheet and a hot-dip galvanized layer formed on a surface of the base steel sheet, wherein the base steel sheet contains, by mass%, C: 0.050% to 0.400%, Si: 0.30% to 2.50%, Mn: 1.80% to 6.00%, P: 0.001% to 0.100%, S: 0.0001% to 0.0200%, and Al: 0.01% to 2.00%, with the balance being Fe and unavoidable impurities; a high-strength hot-dip galvanized steel sheet having cracks in the hot-dip galvanized layer, the cracks extending from the surface of the hot-dip galvanized layer to the interface between the base steel sheet and the hot-dip galvanized layer, and further connecting to grain boundaries of the base steel sheet at the interface between the base steel sheet and the hot-dip galvanized layer, the grain boundaries having oxides present at the grain boundaries, the density of the cracks being 10 cracks / mm or more in a cross section of the steel sheet; and a tensile strength of 780 MPa or more. [2] The base steel sheet further contains, in mass%, N: 0.0005% or more and 0.0100% or less, Ti: 0.005% or more and 0.200% or less, Nb: 0.005% or more and 0.200% or less, B: 0.0003% or more and 0.0050% or less, Ni: 0.005% or more and 1.000% or less, Cr: 0.005% or more and 1.000% or less, V: 0.005% or more and 0.500% or less, Mo: 0.005% or more and 1.000% or less, Co: 0.001% or more and 0.010% or less, Cu: 0.005% or more and 1.000% or less, Sn: 0.002% or more and 0.200% or less, Sb: 0.005% or more and 0.100% or less, [3] The high-strength hot-dip galvanized steel sheet according to [1], further comprising one or more elements selected from the group consisting of Ta: 0.001% or more and 0.010% or less, W: 0.005% or more and 0.100% or less, Ca: 0.0005% or more and 0.0050% or less, Mg: 0.0005% or more and 0.0050% or less, and REM: 0.0005% or more and 0.0050% or less. [3] The high-strength hot-dip galvanized steel sheet according to [1] or [2], wherein the base steel sheet further comprises, in mass %, one or more elements selected from the group A to C below.Group A: one or more selected from Te: more than 0% and 0.10% or less, As: more than 0% and 0.10% or less, and Hf: more than 0% and 0.10% or less; Group B: one or more selected from Bi: more than 0% and 0.20% or less, and Pb: more than 0% and 0.20% or less; Group C: one or more selected from Zn: more than 0% and 0.10% or less, Ge: more than 0% and 0.10% or less, Sr: more than 0% and 0.10% or less, and Cs: more than 0% and 0.10% or less. [4] A method for producing a high-strength hot-dip galvanized steel sheet according to any one of [1] to [3] above, wherein the base steel sheet has a dew point of -30°C or more and +20°C or less, and H. ２ The steel sheet is subjected to annealing treatment in an atmosphere containing Zn, followed by cooling, and then hot-dip galvanizing treatment is performed, followed by alloying treatment, and then cooling, and the tension applied to the steel sheet is 1.5 kgf / mm ２ [5] The method for producing a high-strength hot-dip galvanized steel sheet is as described above. ２ is 2 vol% or more and 30 vol% or less, and the balance is N ２ , H ２ O, CO, CO ２ , O ２ and unavoidable impurities, and the annealing temperature of the annealing treatment is 700°C or higher and 950°C or lower.

[0016] In this specification, all percentages representing the contents of steel components are by mass %. Furthermore, in the present invention, the "high-strength hot-dip galvanized steel sheet" refers to a galvannealed steel sheet having a tensile strength (TS) of 780 MPa or more.

[0017] According to the present invention, it is possible to provide a high-strength hot-dip galvanized steel sheet having a tensile strength of 780 MPa or more and excellent hydrogen desorption properties.

[0018] By applying the high-strength hot-dip galvanized steel sheet of the present invention to, for example, automobile structural members, it is possible to reduce the weight of the vehicle body and thereby improve fuel economy.

[0019] Fig. 1 is a schematic diagram showing a crack that continues from the surface of a hot-dip galvanized layer to the interface between a base steel sheet and the hot-dip galvanized layer and further connects to a grain boundary of the base steel sheet at the interface between the base steel sheet and the hot-dip galvanized layer, where an oxide exists at the grain boundary. Fig. 2 is a schematic diagram showing a crack that continues from the surface of the hot-dip galvanized layer to the interface between the base steel sheet and the hot-dip galvanized layer but stops at the grain surface of the base steel sheet.

[0020] The present invention will be described based on the following embodiments, but the present invention is not limited to the following embodiments.

[0021] A high-strength hot-dip galvanized steel sheet according to one embodiment of the present invention comprises a base steel sheet and a hot-dip galvanized layer formed on the surface of the base steel sheet.

[0022] (1) First, the chemical composition of the base steel sheet will be described. The base steel sheet has a chemical composition containing C: 0.050% to 0.400%, Si: 0.30% to 2.50%, Mn: 1.80% to 6.00%, P: 0.001% to 0.100%, S: 0.0001% to 0.0200%, and Al: 0.01% to 2.00%, with the balance being Fe and unavoidable impurities.

[0023] C: 0.050% or more and 0.400% or less C is an austenite-forming element and is effective in improving the strength and ductility of the base steel sheet (annealed sheet). The effect of ensuring the strength of the base steel sheet is exerted when the C content is 0.050% or more, so the C content is set to 0.050% or more. The C content is preferably 0.060% or more. On the other hand, if the C content is excessive, the weld and heat-affected zone will harden significantly, the mechanical properties of the weld will deteriorate, and spot weldability, arc weldability, etc. will decrease. Therefore, the C content is set to 0.400% or less. The C content is preferably 0.300% or less, and more preferably 0.200% or less.

[0024] Si: 0.30% or more and 2.50% or less Si is a ferrite-forming element and is also an element effective in improving the solid solution strengthening and work hardening ability of ferrite in the base steel sheet. To obtain these effects, a Si content of 0.30% or more is necessary. On the other hand, if the Si content exceeds 2.50%, the above effects saturate. Therefore, the Si content is set to 0.30% or more and 2.50% or less.

[0025] Mn: 1.80% or more and 6.00% or less Mn is an austenite-forming element and is an element effective in ensuring the strength of the base steel plate. If the Mn content is less than 1.80%, it is difficult to ensure strength. On the other hand, if the Mn content exceeds 6.00%, the effect on strength saturates. Therefore, the Mn content is set to 1.80% or more and 6.00% or less. The Mn content is preferably 2.00% or more. Furthermore, the Mn content is preferably 5.50% or less.

[0026] P: 0.001% or more and 0.100% or less P is an inevitable impurity element, and segregates at grain boundaries to cause embrittlement and deteriorate impact resistance. When the P content exceeds 0.100%, this tendency becomes significant, so the P content is set to 0.100% or less, preferably 0.050% or less. Note that the lower the P content, the better, but from the viewpoint of refining costs, the P content is set to 0.001% or more.

[0027] S: 0.0001% or more and 0.0200% or less S segregates at grain boundaries, embrittling steel during hot working, and also exists as sulfides, reducing local deformability. Therefore, the S content is set to 0.0200% or less. On the other hand, due to constraints in production technology, the S content is set to 0.0001% or more. Therefore, the S content is set to 0.0001% or more and 0.0200% or less. The S content is preferably 0.0100% or less, and more preferably 0.0050% or less.

[0028] Al: 0.01% or more and 2.00% or less Al is added as a deoxidizer, but if the content is less than 0.01%, the effect is not fully exerted. Therefore, the Al content is set to 0.01% or more. On the other hand, if the Al content exceeds 2.00%, the risk of steel slab cracking during continuous casting increases, reducing manufacturability. Therefore, the Al content is set to 0.01% or more and 2.00% or less. The Al content is preferably 0.02% or more. Furthermore, the Al content is preferably 1.20% or less, more preferably 1.00% or less, and even more preferably 0.10% or less.

[0029] The balance other than the above components may be Fe and unavoidable impurities.

[0030] The composition of the base steel sheet may further optionally include N: 0.0005% or more and 0.0100% or less, Ti: 0.005% or more and 0.200% or less, Nb: 0.005% or more and 0.200% or less, B: 0.0003% or more and 0.0050% or less, Ni: 0.005% or more and 1.000% or less, Cr: 0.005% or more and 1.000% or less, V: 0.005% or more and 0.500% or less, Mo: 0.005% or more and 1.000% or less, Co: 0.001% or more and 0.002% or less, It may contain one or more selected from the group consisting of Cu: 0.005% or more and 1.000% or less, Sn: 0.002% or more and 0.200% or less, Sb: 0.005% or more and 0.100% or less, Ta: 0.001% or more and 0.010% or less, W: 0.005% or more and 0.100% or less, Ca: 0.0005% or more and 0.0050% or less, Mg: 0.0005% or more and 0.0050% or less, and REM: 0.0005% or more and 0.0050% or less.

[0031] N: 0.0005% or more and 0.0100% or less N is an element that deteriorates the aging resistance of steel. In particular, when the N content exceeds 0.0100%, the deterioration of aging resistance becomes significant. The lower the N content, the better, but excessive denitrification when the N content is less than 0.0005% increases production costs. Therefore, when N is contained, the N content is preferably 0.0005% or more and 0.0100% or less. The N content is more preferably 0.0010% or more. Furthermore, the N content is more preferably 0.0070% or less.

[0032] Ti: 0.005% or more and 0.200% or less Ti is an element effective for precipitation strengthening of steel. In addition, the inclusion of Ti forms relatively hard ferrite, thereby reducing the difference in hardness with the hard second phase (martensite or retained austenite), and ensuring good stretch flangeability. These effects are achieved when the Ti content is 0.005% or more. On the other hand, if the Ti content exceeds 0.200%, the area ratio of hard martensite becomes excessive, increasing microvoids at the martensite grain boundaries, further promoting crack propagation and deteriorating formability. Therefore, when Ti is contained, the Ti content is preferably 0.005% or more and 0.200% or less. The Ti content is more preferably 0.010% or more. Furthermore, the Ti content is more preferably 0.100% or less.

[0033] Nb: 0.005% or more and 0.200% or less Nb is an element effective for precipitation strengthening of steel. Similar to the effect of Ti, the inclusion of Nb forms relatively hard ferrite, thereby reducing the difference in hardness with the hard second phase (martensite or retained austenite), and ensuring good stretch flangeability. These effects are achieved when the Nb content is 0.005% or more. On the other hand, if the Nb content exceeds 0.200%, the area ratio of hard martensite becomes excessive, increasing microvoids at the martensite grain boundaries, further promoting crack propagation and reducing formability. This also increases costs. Therefore, when Nb is contained, the Nb content is preferably 0.005% or more and 0.200% or less. The Nb content is more preferably 0.010% or more. The Nb content is more preferably 0.100% or less.

[0034] B: 0.0003% or more and 0.0050% or less B has the effect of suppressing the formation and growth of ferrite from austenite grain boundaries and allows for structural control, so it can be added as needed. This effect is achieved when the B content is 0.0003% or more. On the other hand, if the B content exceeds 0.0050%, formability decreases. Therefore, when B is contained, the B content is preferably 0.0003% or more and 0.0050% or less. The B content is more preferably 0.0005% or more. Furthermore, the B content is more preferably 0.0030% or less.

[0035] Ni: 0.005% or more and 1.000% or less Ni is an element that stabilizes retained austenite and is effective in ensuring good ductility. Furthermore, it is an element that increases the strength of steel through solid solution strengthening. These effects are obtained when the Ni content is 0.005% or more. On the other hand, if the Ni content exceeds 1.000%, the hard martensite becomes excessive, increasing microvoids at the martensite grain boundaries, and further promoting crack propagation, resulting in reduced bendability and stretch flangeability. It also increases costs. Therefore, when Ni is contained, the Ni content is preferably 0.005% or more and 1.000% or less. The Ni content is more preferably 0.010% or more. The Ni content is more preferably 0.500% or less.

[0036] Cr: 0.005% to 1.000%; V: 0.005% to 0.500%; Mo: 0.005% to 1.000%; Co: 0.001% to 0.010%. Cr, V, Mo, and Co each have the effect of improving the balance between strength and ductility, and can be added as needed. However, excessive inclusion of any of these elements results in excessive hard martensite, increasing microvoids at martensite grain boundaries, and further crack propagation, resulting in reduced formability. This also increases costs. Therefore, when these elements are included, it is preferable that the Cr content be 0.005% to 1.000%; the V content be 0.005% to 0.500%; the Mo content be 0.005% to 1.000%; and the Co content be 0.001% to 0.010%. The Cr content is more preferably 0.010% or more. Furthermore, the Cr content is more preferably 0.800% or less. The V content is more preferably 0.010% or more. Furthermore, the V content is more preferably 0.100% or less. The Mo content is more preferably 0.010% or more. Furthermore, the Mo content is more preferably 0.500% or less.

[0037] Cu: 0.005% or more and 1.000% or less Cu is an element effective in strengthening steel. This effect is obtained when the Cu content is 0.005% or more. On the other hand, if the Cu content exceeds 1.000%, the hard martensite becomes excessive, microvoids at the martensite grain boundaries increase, and crack propagation progresses, resulting in reduced formability. Therefore, when Cu is contained, the Cu content is preferably 0.005% or more and 1.000% or less. The Cu content is more preferably 0.010% or more. Furthermore, the Cu content is more preferably 0.500% or less.

[0038] Sn: 0.002% or more and 0.200% or less, Sb: 0.005% or more and 0.100% or less. Sn and Sb can be added as needed to suppress decarburization of a region of several tens of micrometers in the surface layer of the steel sheet, which occurs due to nitriding or oxidation of the steel sheet surface. Suppressing such nitriding and oxidation prevents a decrease in the area ratio of martensite on the steel sheet surface, which is effective in ensuring strength and material stability. On the other hand, excessive inclusion of either element leads to a decrease in toughness. Therefore, when Sn is added, the Sn content is preferably 0.002% or more and 0.200% or less. Furthermore, when Sb is added, the Sb content is preferably 0.005% or more and 0.100% or less. The Sn content is more preferably 0.005% or more. Furthermore, the Sn content is more preferably 0.100% or less. The Sb content is more preferably 0.010% or more. The Sb content is more preferably 0.080% or less.

[0039] Ta: 0.001% or more and 0.010% or less, W: 0.005% or more and 0.100% or less. Like Ti and Nb, Ta and W contribute to high strength by forming carbides and carbonitrides. In addition, Ta and W partially dissolve in Nb carbides and Nb carbonitrides, forming composite precipitates such as (Nb, Ta)(C, N), which significantly suppresses coarsening of precipitates and is thought to have the effect of stabilizing the contribution of precipitation strengthening to strength. For this reason, it is preferable to contain one or more of Ta and W. These effects are achieved when the Ta content is 0.001% or more, and when the W content is 0.005% or more. On the other hand, if the Ta content exceeds 0.010% or the W content exceeds 0.100%, the effects saturate and the alloy cost increases. Therefore, when Ta is contained, the Ta content is preferably 0.001% or more and 0.010% or less. When W is contained, the W content is preferably 0.005% or more and 0.100% or less.

[0040] Ca: 0.0005% or more and 0.0050% or less, Mg: 0.0005% or more and 0.0050% or less, REM: 0.0005% or more and 0.0050% or less. Ca, Mg, and REM are each effective elements for spheroidizing the shape of sulfides and improving the adverse effects of sulfides on hole expandability (stretch flangeability). However, excessive content of any of these elements causes an increase in inclusions, leading to surface and internal defects. Therefore, when these elements are contained, it is preferable that the content of each of these elements be 0.0005% or more and 0.0050% or less. REM (rare earth metal) is a collective term for 17 elements, including Sc, Y, and lanthanoid elements. REM can contain one or more of these 17 elements, and the REM content here refers to the total content of these elements.

[0041] In addition, when the content of N, Ti, Nb, B, Ni, Cr, V, Mo, Co, Cu, Sn, Sb, Ta, W, Ca, Mg, or REM, which are described as optional components above, is less than the lower limit, the component is considered to be contained as an unavoidable impurity.

[0042] The chemical composition of the base steel sheet may further optionally contain one or more elements selected from the following groups A to C. Group A: one or more elements selected from Te: more than 0% and 0.10% or less, As: more than 0% and 0.10% or less, and Hf: more than 0% and 0.10% or less Group B: one or more elements selected from Bi: more than 0% and 0.20% or less, and Pb: more than 0% and 0.20% or less Group C: one or more elements selected from Zn: more than 0% and 0.10% or less, Ge: more than 0% and 0.10% or less, Sr: more than 0.10% or less, and Cs: more than 0% and 0.10% or less

[0043] Group A [one or more selected from Te: more than 0% and 0.10% or less, As: more than 0% and 0.10% or less, and Hf: more than 0% and 0.10% or less] Te, As, and Hf are all elements used to control the morphology of sulfides. When Te, As, and Hf are contained, the content of each can be more than 0%.

[0044] Te: More than 0% and 0.10% or less 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. From the viewpoint of obtaining such effects, when Te is contained, the Te content is preferably 0.001% or more. However, from the viewpoint of preventing an increase in costs, when Te is contained, the Te content is preferably 0.10% or less.

[0045] As: More than 0% but not more than 0.10% 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. From the viewpoint of obtaining such effects, when As is contained, the As content is preferably 0.001% or more. However, from the viewpoint of preventing an increase in costs, when As is contained, the As content is preferably 0.10% or less.

[0046] Hf: more than 0% and 0.10% or less 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. From the viewpoint of obtaining such effects, when Hf is contained, the Hf content is preferably 0.01% or more. However, from the viewpoint of preventing an increase in costs, when Hf is contained, the Hf content is preferably 0.10% or less.

[0047] Group B [one or more selected from Bi: more than 0% but not more than 0.20% and Pb: more than 0% but not more than 0.20%] Bi and Pb are elements that suppress grain boundary segregation and improve ductility and toughness. When Bi and Pb are contained, their respective contents can be set to more than 0%.

[0048] Bi: More than 0% but not more than 0.20% Bi can suppress grain boundary segregation and improve ductility and toughness by containing 0.001% or more of Bi. From the viewpoint of obtaining such effects, when Bi is contained, the Bi content is preferably 0.001% or more. Bi also has the effect of improving machinability and improving the smoothness of the cut end surface, and has the effect of improving the delayed fracture resistance of the cut surface. However, when Bi is contained, from the viewpoint of preventing an increase in cost, the Bi content is preferably 0.20% or less.

[0049] Pb: More than 0% and 0.20% or less When Pb is contained in an amount of 0.001% or more, grain boundary segregation can be suppressed and ductility and toughness can be improved. From the viewpoint of obtaining such effects, when Pb is contained, the Pb content is preferably 0.001% or more. Furthermore, Pb has the effect of improving machinability and improving the smoothness of the cut end surface, and has the effect of improving the delayed fracture resistance of the cut surface. However, when Pb is contained, from the viewpoint of preventing an increase in cost, the Pb content is preferably 0.20% or less.

[0050] Group C [one or more selected from Zn: more than 0% and 0.10% or less, Ge: more than 0% and 0.10% or less, Sr: more than 0% and 0.10% or less, and Cs: more than 0% and 0.10% or less] Zn, Ge, Sr, and Cs are elements that increase strength without significantly affecting mechanical properties or surface quality. When Zn, Ge, Sr, and Cs are contained, the content of each can be more than 0%.

[0051] Zn: More than 0% and 0.10% or less When Zn is contained in an amount of 0.001% or more, the strength can be increased without significantly affecting the mechanical properties or surface quality. From the viewpoint of obtaining such an effect, when Zn is contained, the Zn content is preferably 0.001% or more. However, from the viewpoint of preventing an increase in cost, when Zn is contained, the Zn content is preferably 0.10% or less.

[0052] Ge: More than 0% and 0.10% or less When Ge is contained in an amount of 0.001% or more, the strength can be increased without significantly affecting the mechanical properties or surface quality. From the viewpoint of obtaining such an effect, when Ge is contained, the Ge content is preferably 0.001% or more. However, from the viewpoint of preventing an increase in cost, when Ge is contained, the Ge content is preferably 0.10% or less.

[0053] Sr: More than 0% but not more than 0.10% When Sr is contained in an amount of 0.001% or more, the strength can be increased without significantly affecting the mechanical properties or surface quality. From the viewpoint of obtaining such an effect, when Sr is contained, the Sr content is preferably 0.001% or more. However, from the viewpoint of preventing an increase in cost, when Sr is contained, the Sr content is preferably 0.10% or less.

[0054] Cs: More than 0% and 0.10% or less When Cs is contained in an amount of 0.001% or more, the strength can be increased without significantly affecting the mechanical properties or surface quality. From the viewpoint of obtaining such an effect, when Cs is contained, the Cs content is preferably 0.001% or more. However, from the viewpoint of preventing an increase in costs, when Cs is contained, the Cs content is preferably 0.10% or less.

[0055] (2) Next, cracks in the hot-dip galvanized layer will be described. The high-strength hot-dip galvanized steel sheet of the present invention has cracks in the hot-dip galvanized layer that extend from the surface of the hot-dip galvanized layer to the interface between the base steel sheet and the hot-dip galvanized layer and further connect to grain boundaries of the base steel sheet at the interface between the base steel sheet and the hot-dip galvanized layer, and oxides are present at the grain boundaries connected to the cracks.

[0056] Cracks formed in the hot-dip galvanized layer continue from the surface of the hot-dip galvanized layer to the interface between the base steel sheet and the hot-dip galvanized layer. It is presumed that cracks in the hot-dip galvanized layer serve as diffusion paths for hydrogen desorbed from the base steel sheet to the surface of the steel sheet. Therefore, in order for hydrogen to diffuse into the atmosphere without remaining within the hot-dip galvanized layer, it is necessary for the cracks to continue from the surface of the hot-dip galvanized layer to the interface between the base steel sheet and the hot-dip galvanized layer.

[0057] Cracks formed in the hot-dip galvanized layer are connected to the grain boundaries of the base steel sheet at the interface between the base steel sheet and the hot-dip galvanized layer. The pathway for high-speed diffusion of hydrogen from within the base steel sheet to the surface of the steel sheet is the grain boundaries of the base steel sheet. Therefore, since the cracks in the hot-dip galvanized layer continue from the surface of the hot-dip galvanized layer to the interface between the base steel sheet and the hot-dip galvanized layer and also connect to the grain boundaries of the base steel sheet at the interface between the base steel sheet and the hot-dip galvanized layer as shown in Figure 1, high-speed diffusion of hydrogen from within the base steel sheet through the hot-dip galvanized layer to the atmosphere is possible.

[0058] Oxides are present at the grain boundaries of the base steel sheet where cracks are connected. The presence of oxides at grain boundaries reduces the strength of the grain boundaries, making them more likely to become the starting point for cracks in the hot-dip galvanized layer. Furthermore, oxides serve as hydrogen diffusion paths, promoting hydrogen diffusion. Therefore, the more grain boundaries where oxides are present, the more likely cracks that serve as hydrogen diffusion paths are introduced. FIG. 1 is a schematic diagram showing cracks 5 that extend from the surface of the hot-dip galvanized layer 2 to the interface between the base steel sheet 1 and the hot-dip galvanized layer 2, and further connect to grain boundaries 4 of the base steel sheet 1 at the interface between the base steel sheet 1 and the hot-dip galvanized layer 2, where oxides 3 are present at the grain boundaries. The high-strength hot-dip galvanized steel sheet of the present invention has cracks such as those shown in FIG. 1 at a predetermined density.

[0059] The crack density is 10 cracks / mm or more. Because the cracks in the hot-dip galvanized layer act as hydrogen diffusion paths, the crack density must be 10 cracks / mm or more to promote hydrogen desorption. The crack density is more preferably 15 cracks / mm or more. There is no particular upper limit to the crack density, but if the crack density is too high, the adhesion of the hot-dip galvanized layer will deteriorate, so the crack density is preferably less than 50 cracks / mm.

[0060] To identify cracks in the hot-dip galvanized layer, a cross section of the sample is prepared for observation using a focused ion beam (FIB) and the prepared cross section is observed using a scanning electron microscope (SEM). The cross section for observation may be either a vertical cross section or an inclined cross section, but an inclined cross section is preferred from the viewpoint of ease of FIB cross section processing and SEM observation. When observed using an SEM without tilting the sample, the vertical dimensions of the observed image match the actual cross section dimensions, so the inclination angle of the FIB cross section is preferably 45°. Somewhat thick cracks can also be observed by embedding the sample in resin and polishing it. However, with this method, thin cracks are often buried by the plating during polishing, making them impossible to observe. Furthermore, it is difficult to determine whether the cracks are connected to the grain boundaries of the base steel sheet. Therefore, in the present invention, a cross section of the sample for observation is prepared using an FIB and then observed using an SEM. At least 20 observation locations are randomly selected from the steel sheet, and the observation cross section is processed so that the steel sheet cross section can be observed with a width of 30 μm or more. The width of the cross section to be observed is preferably 5 mm or more in total, since cracks may be somewhat unevenly distributed. Within that field of view, the number of cracks and whether or not they are connected to the grain boundaries of the base steel sheet are confirmed. Furthermore, the presence or absence of oxides at the grain boundaries of the steel sheet is determined by elemental analysis of the grain boundary portion using an energy dispersive X-ray spectrometer (EDS) mounted on the SEM. In the present invention, grain boundaries containing 10 mass % or more of oxygen are defined as grain boundaries containing oxides. Then, the number of cracks extending from the surface of the hot-dip galvanized layer to the interface between the base steel sheet and the hot-dip galvanized layer and connected to the grain boundaries containing oxides at the interface between the base steel sheet and the hot-dip galvanized layer is counted, and the density of the cracks (cracks / mm) is calculated. Cracks may branch within the hot-dip galvanized layer, but the number of branched cracks is counted as 1. Furthermore, a crack connected to a grain boundary where oxides exist means a crack that connects to a grain boundary where oxides exist within 0.5 μm from the starting point when measured along the grain boundary from the connection point between the crack and the grain boundary.For the measurement of crack density, see the description in the Examples.

[0061] (3) Next, a method for producing a high-strength hot-dip galvanized steel sheet will be described. The high-strength hot-dip galvanized steel sheet of the present invention can be produced, for example, by the following method. First, a steel slab having the above-described chemical composition is heated in a hot-rolling process, and then subjected to rough rolling and finish rolling to produce a hot-rolled steel sheet. Thereafter, scale on the surface of the hot-rolled steel sheet is removed in a pickling process, and the hot-rolled steel sheet is cold-rolled as needed in a cold-rolling process. The steps from the hot-rolling process to the cold-rolling process are not particularly limited, and for example, a known production method can be used.

[0062] Next, the hot-rolled steel sheet, or a cold-rolled steel sheet that has been cold-rolled as needed, is annealed and cooled to produce a base steel sheet (annealed sheet). The base steel sheet is then plated (hot-dip galvanized), alloyed, and then cooled. In the present invention, in order to obtain a high-strength hot-dip galvanized steel sheet free of defects such as bare spots, it is preferable to use either an oxidation-reduction method in which an Fe oxide layer is formed on the steel sheet surface before annealing and then reduced to metallic Fe during annealing, or a method in which Si is oxidized inside the steel sheet by controlling the atmosphere during annealing to suppress precipitation of Si oxide on the steel sheet surface.

[0063] The atmosphere in the annealing treatment was H ２ The atmosphere used in the annealing treatment should contain the above-mentioned elements. The dew point of the atmosphere used in the annealing treatment should be +20°C or lower. If the dew point of the atmosphere exceeds +20°C, the surface of the steel sheet is more susceptible to oxidation, which deteriorates plating wettability and causes defects such as bare spots. The dew point is preferably +18°C or lower, more preferably +15°C or lower. The dew point of the atmosphere should be -30°C or higher. If the dew point of the atmosphere is lower than -30°C, oxides that can serve as crack initiation points for the hot-dip galvanized layer and as diffusion paths for hydrogen in the base steel sheet are not formed at the grain boundaries. If the dew point is -30°C or higher, the precipitation of silicon oxides on the surface of the base steel sheet, which can cause a decrease in plating wettability, is suppressed, thereby also having the effect of suppressing bare spots. The dew point of the atmosphere should be -25°C or higher, more preferably -20°C or higher.

[0064] The annealing treatment is carried out in a manner that suppresses oxidation of the steel sheet surface by adding 2 vol% to 30 vol% of H. ２ and the remainder is N ２ , H ２ O, CO, CO ２ , O ２ It is preferable to carry out the process in an atmosphere containing at least one of the above and unavoidable impurities. ２ If the concentration is less than 2 vol%, the surface of the steel sheet is oxidized, the plating wettability is deteriorated, plating defects occur, and the corrosion resistance of the steel sheet is deteriorated. ２ If the concentration exceeds 30 vol%, excessive hydrogen penetrates into the steel sheet during annealing, and it takes time to reduce the hydrogen concentration even if the dehydrogenation rate after plating is fast. ２ The concentration is more preferably 3 vol % or more. ２ The concentration is more preferably 5 vol % or less.

[0065] In the annealing treatment, the annealing temperature is not particularly specified, but is preferably 700°C or higher and 950°C or lower. If the annealing temperature is lower than 700°C, the reduction of the natural oxide film on the steel sheet surface is insufficient, which reduces plating wettability and causes defects such as bare spots. If the annealing temperature exceeds 950°C, excess hydrogen penetrates into the steel sheet, increasing the diffusible hydrogen concentration in the steel sheet. The annealing temperature is more preferably 730°C or higher, and even more preferably 750°C or higher.

[0066] Since the base steel sheet in the present invention contains a large amount of Si, the trace amount of H contained in the atmosphere during the annealing treatment ２ O oxidizes Si in the base steel sheet, forming oxides containing Si on the surface of the base steel sheet. This reduces the wettability of the base steel sheet with molten zinc, resulting in defects such as unplated areas. One method for addressing this problem is the oxidation-reduction method, in which an Fe oxide layer is formed on the surface of the base steel sheet before annealing, and then the iron oxide layer is reduced to metallic iron during annealing, thereby improving plating wettability. In addition, the H ２In one method, the O concentration, i.e., the dew point, is increased to a certain level or more to oxidize the Si in the base steel sheet inside the base steel sheet, thereby suppressing the precipitation of Si oxide on the surface of the base steel sheet and improving the plating wettability. In the present invention, either method can be adopted in order to obtain a high-strength hot-dip galvanized steel sheet free from defects such as bare spots.

[0067] In the oxidation-reduction method, methods for pre-oxidizing the base steel sheet include atmospheric oxidation and oxidation by flame, but any method may be used as long as it can oxidize Fe. ２ An atmosphere with a concentration of 0.1 vol% or more and 4.0 vol% or less is desirable. Flame oxidation is preferably performed under conditions where the air ratio, which is the ratio of the combustible gas to air sent to the flame burner, is 1.00 or more and 1.50 or less. The air ratio is the value obtained by dividing the volume of air sent to the flame burner by the volume of air that reacts exactly with the combustible components in the combustible gas. Under these conditions, the iron oxide layer formed on the steel sheet surface is reduced to metallic iron during the annealing treatment. As a result, the wettability of the molten zinc to the base steel sheet is improved, and a high-strength hot-dip galvanized steel sheet free of defects such as bare spots can be obtained. Furthermore, when the iron oxide layer is reduced to metallic iron, oxygen released from the Fe oxide oxidizes Si inside the steel sheet, forming oxides at the grain boundaries of the base steel sheet. This suppresses the precipitation of Si oxide on the surface of the base steel sheet, which reduces the wettability of the coating, and a high-strength hot-dip galvanized steel sheet free of defects such as bare spots can be obtained.

[0068] After annealing, the steel sheet is cooled. The cooling conditions are not particularly limited. For example, the steel sheet is cooled to 550°C or less at an arbitrary cooling rate. Thereafter, the steel sheet is subjected to hot-dip galvanizing, alloying, and then cooled. The conditions for the hot-dip galvanizing and alloying treatments are not particularly limited, and known conditions can be used. The hot-dip galvanizing can be performed, for example, by immersing the base steel sheet in a galvanizing bath at 440 to 550°C. The galvanizing bath contains Zn, Al, and unavoidable impurities. The components are not particularly specified, but, for example, the Al concentration in the bath can be 0.05% by mass or more and 0.30% by mass or less. The alloying treatment can be performed, for example, by heating the steel sheet after hot-dip galvanizing to an alloying temperature of 450 to 600°C.

[0069] In order to generate cracks in the hot-dip galvanized layer that extend from the surface of the hot-dip galvanized layer to the interface between the base steel sheet and the hot-dip galvanized layer and further connect to the grain boundaries of the base steel sheet at the interface between the base steel sheet and the hot-dip galvanized layer, the tension applied to the steel sheet when cooling the steel sheet to 200°C or less after the alloying treatment is 1.5 kgf / mm ２ The tension must be 1.8 kgf / mm or more. ２ More than 2.0 kgf / mm is preferable. ２ The above is more preferable. Metal contracts when cooled, and tension acts as a resistance to contraction, making it easier for stress to concentrate locally at grain boundaries, etc. The base steel sheet is highly ductile and therefore does not break, but the hot-dip galvanized layer has lower ductility than the base steel sheet, so cracks occur at the points where stress concentrates. This allows the above-mentioned cracks to form in the hot-dip galvanized layer. There is no particular upper limit to the tension, but excessive tension may cause the steel sheet to stretch unevenly in the sheet width direction or cause drawing. For this reason, the tension should be set to 20.0 kgf / mm ２ It is preferable that the above-mentioned 1.5 kgf / mm ２ is 14.709975N / mm, assuming 1kgf = 9.80665N. ２ can be converted into

[0070] The cooling conditions for the cooling are not particularly limited. For example, the steel sheet can be cooled to 150°C or less at a cooling rate of 60°C / min to 600°C / min. The method for applying tension to the steel sheet is not particularly limited. For example, a method of applying tension using a bridle roll installed in the line in the cooling step after the alloying treatment can be mentioned. The tension can be calculated, for example, by multiplying the total value of the loads (kgf) of the load cells on the left and right sides of the tension-applying roll of the bridle roll by the cross-sectional area of ​​the steel sheet (= sheet thickness (mm) × sheet width (mm)) (mm ２ ) is obtained by dividing by

[0071] The high-strength hot-dip galvanized steel sheet of the present invention is obtained through the above steps. The amount of diffusible hydrogen in the steel of the high-strength hot-dip galvanized steel sheet of the present invention varies depending on the steel sheet properties, but is preferably 0.40 mass ppm or less 20 days after production. Furthermore, the hydrogen desorption rate calculated by measuring the diffusible hydrogen amount within 3 days after production and the diffusible hydrogen amount 20 days after production is preferably 0.005 mass ppm / day or more. The diffusible hydrogen amount can be measured by the method described in the Examples. Furthermore, in the present invention, having excellent hydrogen desorption properties means that the hydrogen desorption rate is 0.005 mass ppm / day or more.

[0072] The high-strength hot-dip galvanized steel sheet of the present invention has a tensile strength of 780 MPa or more, preferably 980 MPa or more, and more preferably 1180 MPa or more. The upper limit of the tensile strength is not particularly limited, but, for example, the tensile strength may be 2400 MPa or less.

[0073] The high-strength hot-dip galvanized steel sheet of the present invention is typically produced by subjecting a steel material to ordinary processes such as steelmaking, casting, hot rolling, etc. However, it may also be produced by employing a production method such as thin slab continuous casting or strip casting, omitting part or all of the hot rolling process.

[0074] Steels having the chemical compositions shown in Table 1, with the balance consisting of Fe and unavoidable impurities, were melted in a converter and formed into slabs by continuous casting. The obtained slabs were heated to 1200°C, hot-rolled to thicknesses of 2.3 to 4.5 mm, and then coiled. The obtained hot-rolled steel sheets were then pickled, and some of them were cold-rolled. Thereafter, they were annealed in an atmosphere-controllable furnace under the conditions shown in Table 2 and cooled. In some conditions, an oxidation treatment was performed before the annealing treatment. In the "Oxidation Method" column in Table 2, those that underwent the above-mentioned atmospheric oxidation as the oxidation treatment are marked with "Atmosphere," those that underwent the above-mentioned flame oxidation are marked with "Flame," and those that did not undergo oxidation treatment are marked with "-." Subsequently, the steel sheet was subjected to hot-dip galvanizing treatment in a galvanizing bath containing 0.13 to 0.19 mass % of Al (galvanizing bath temperature: 465°C), then subjected to alloying treatment at 520°C, and cooled to 200°C or less to obtain a hot-dip galvanized steel sheet (galvannealed hot-dip galvanized steel sheet). During the cooling, a tension shown in Table 2 was applied to the steel sheet using a bridle roll provided in the line.

[0075] The hot-dip galvanized steel sheets produced as above were evaluated for surface appearance, crack density in the hot-dip galvanized layer, amount of diffusible hydrogen in the steel, hydrogen desorption rate, and tensile strength.

[0076] [Surface appearance] Test pieces measuring 230 mm x 350 mm were cut out from any position on the produced hot-dip galvanized steel sheet, and the number of defects such as unplated areas and uneven color tone was visually counted and evaluated according to the following criteria. According to the following criteria, ◎ and ◯ were evaluated as excellent surface appearance. <Evaluation criteria> ◎: No defects ◯: Slight uneven color tone △: Concave and concave defects ×: Unplated areas

[0077] [Crack Density in the Hot-Dip Galvanized Layer] Twenty locations were randomly selected from test specimens cut from the manufactured hot-dip galvanized steel sheet, and cross sections for SEM observation, 50 μm wide and at an inclination angle of 45°, were processed using a focused ion beam (FIB). These cross sections were observed under SEM at 5000x magnification, and elemental analysis of the grain boundaries of the steel sheet was performed using SEM-EDS. Of the cracks present in the observed cross sections, the number of cracks extending from the surface of the hot-dip galvanized layer to the interface between the base steel sheet and the hot-dip galvanized layer and connecting to grain boundaries in the base steel sheet where oxides existed was counted. The crack density was calculated by dividing this number by the surface line length of the steel sheet across the entire observed cross section. A crack density of 10 cracks / mm or more was considered acceptable.

[0078] [Amount of Diffusible Hydrogen in Steel Sheet and Hydrogen Desorption Rate] Test pieces measuring 30 mm in length and 5 mm in width were taken from the produced hot-dip galvanized steel sheets, and the hot-dip galvanized layer was ground off and then the amount of diffusible hydrogen in the steel was measured. The measurement was performed using thermal desorption analysis at a heating rate of 200°C / hr. The hydrogen detected at 300°C or below was defined as diffusible hydrogen. The amount of diffusible hydrogen within 3 days and after 20 days from the production of the hot-dip galvanized steel sheet was measured, and the value divided by the number of days elapsed was defined as the hydrogen desorption rate. A hydrogen desorption rate of 0.005 mass ppm / day or more was considered acceptable. It is preferable that the amount of diffusible hydrogen after 20 days is 0.40 mass ppm or less.

[0079] [Tensile Strength] A tensile test was performed in accordance with JIS Z 2241:2011 using a JIS No. 5 test piece obtained by sampling so that the tensile direction was perpendicular to the rolling direction of the steel sheet, and the tensile strength (TS) was measured.

[0080] The results obtained are shown in Table 2.

[0081]

[0082]

[0083] The galvannealed steel sheets of the invention examples all had a TS of 780 MPa or more and were excellent in hydrogen desorption properties, whereas the comparative examples had insufficient hydrogen desorption properties.

[0084] According to the present invention, a high-strength galvannealed steel sheet having high strength (tensile strength of 780 MPa or more) and excellent desorption properties of diffusible hydrogen can be obtained. By applying the high-strength galvannealed steel sheet of the present invention to, for example, automotive structural members, it is possible to reduce the vehicle body weight and thereby improve fuel economy.

[0085] 1 Base steel sheet 2 Hot-dip galvanized layer 3 Oxide 4 Grain boundary 5 Crack (crack connecting to grain boundary where oxide of base steel sheet exists) 6 Crack (crack stopping at grain surface of base steel sheet)

Claims

1. A high-strength hot-dip galvanized steel sheet comprising a base steel sheet and a hot-dip galvanized layer formed on the surface of the base steel sheet, wherein the base steel sheet contains, by mass%, C: 0.050% or more and 0.400% or less, Si: 0.30% or more and 2.50% or less, Mn: 1.80% or more and 6.00% or less, P: 0.001% or more and 0.100% or less, S: 0.0001% or more and 0.0200% or less, and Al: 0.01% or more and 2.00% or less, with the balance being Fe and unavoidable impurities. a high-strength hot-dip galvanized steel sheet having cracks in the hot-dip galvanized layer, the cracks extending from the surface of the hot-dip galvanized layer to the interface between the base steel sheet and the hot-dip galvanized layer, and further connecting to grain boundaries of the base steel sheet at the interface between the base steel sheet and the hot-dip galvanized layer, the grain boundaries having oxides present at the grain boundaries, the density of the cracks being 10 cracks / mm or more in a cross section of the steel sheet; and a tensile strength of 780 MPa or more.

2. The base steel plate further contains, in mass%, N: 0.0005% or more and 0.0100% or less, Ti: 0.005% or more and 0.200% or less, Nb: 0.005% or more and 0.200% or less, B: 0.0003% or more and 0.0050% or less, Ni: 0.005% or more and 1.000% or less, Cr: 0.005% or more and 1.000% or less, V: 0.005% or more and 0.500% or less, Mo: 0.005% or more and 1.000% or less, Co: 0.001% or more and 0.010% or less, Cu: 0.005% or more and 1.000% or less, Sn: 0.002% or more and 0.200% or less, Sb: 0.005% or more and 0.100% or less, 2. The high-strength hot-dip galvanized steel sheet according to claim 1, comprising one or more elements selected from the group consisting of Ta: 0.001% or more and 0.010% or less, W: 0.005% or more and 0.100% or less, Ca: 0.0005% or more and 0.0050% or less, Mg: 0.0005% or more and 0.0050% or less, and REM: 0.0005% or more and 0.0050% or less.

3. The high-strength hot-dip galvanized 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 A to C: Group A: one or more elements selected from Te: more than 0% and 0.10% or less, As: more than 0% and 0.10% or less, and Hf: more than 0% and 0.10% or less; Group B: one or more elements selected from Bi: more than 0% and 0.20% or less, and Pb: more than 0% and 0.20% or less; Group C: one or more elements selected from Zn: more than 0% and 0.10% or less, Ge: more than 0% and 0.10% or less, Sr: more than 0.10% or less, and Cs: more than 0% and 0.10% or less.

4. A method for producing a high-strength hot-dip galvanized steel sheet according to any one of claims 1 to 3, wherein the base steel sheet is treated with a galvanized steel having a dew point of -30°C or higher and +20°C or lower and H ２ The steel sheet is subjected to annealing treatment in an atmosphere containing Zn, followed by cooling, and then hot-dip galvanizing treatment is performed, followed by alloying treatment, and then cooling, and the tension applied to the steel sheet is 1.5 kgf / mm ２ This is the above-mentioned method for producing a high-strength hot-dip galvanized steel sheet.

5. The annealing treatment is carried out in an atmosphere of H ２ is 2 vol% or more and 30 vol% or less, and the balance is N ２ , H ２ O, CO, CO ２ , O ２ and unavoidable impurities, and the annealing temperature of the annealing treatment is 700°C or higher and 950°C or lower.

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