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

Abstract

Provided is a high-strength galvannealed steel sheet having a tensile strength of 780 MPa or more and excellent hydrogen detachability. A high-strength galvannealed steel sheet having a base steel sheet having a prescribed composition and a galvannealed layer formed on the surface of the base steel sheet, and in the galvannealed layer, a crack extending from the surface of the galvannealed layer to the interface of the base steel sheet and the galvannealed layer and connected at the interface to a grain boundary that is a grain boundary of the base steel sheet and in which an oxide exists, the density of the crack being 10 or more per mm, and the tensile strength being 780 MPa or more.

Description

Technical Field

[0001] This invention relates to high-strength molten galvanized steel sheet and its manufacturing method, particularly to high-strength molten galvanized steel sheet with excellent hydrogen desorption properties suitable for applications involving automotive parts and its manufacturing method. Background Technology

[0002] In recent years, with increasing awareness of environmental protection, there has been a strong demand to improve fuel efficiency to reduce CO2 emissions from automobiles. Consequently, the trend towards lightweighting automobiles by increasing the strength of body materials to achieve thinner walls has become increasingly active. Therefore, high-strength steel sheets are now being widely used in rust-resistant galvanized steel sheets.

[0003] On the other hand, the increasing strength of steel plates has also brought about new challenges. One of these is delayed fracture. This is the phenomenon where a high-strength steel plate, under static load stress (load stress below tensile strength), suddenly undergoes brittle fracture after a certain period of time, with almost no visible plastic deformation.

[0004] In the case of steel plates, it is known that this delayed fracture occurs due to hydrogen embrittlement of the steel at stress concentration points caused by residual stress and stress concentrations during the stamping process. It is believed that the hydrogen causing this embrittlement is, in most cases, hydrogen that intrudes into the steel from the external environment and diffuses.

[0005] In the manufacturing process of galvanized steel sheet, galvanizing is performed after the steel sheet has undergone heat annealing. This heat annealing is carried out in a non-oxidizing or reducing atmosphere containing hydrogen to suppress oxidation of the steel sheet surface. Therefore, the manufacturing process of galvanized steel sheet results in a large amount of diffusible hydrogen in the steel sheet, which sometimes contributes to delayed fracture. In addition to delayed fracture, diffusible hydrogen also degrades various properties of the steel sheet, such as ductility and porosity.

[0006] As a treatment for releasing (removing) hydrogen from steel that has penetrated it, baking treatment is known (e.g., Patent Document 1). Baking treatment is a method of heating hydrogen-impregnated steel at a specified temperature to allow the hydrogen to diffuse and be released (removed) from the surface of the steel.

[0007] Patent documents 2 and 3 propose a method to release hydrogen from the steel plate to the outside of the steel plate by forming a certain number of cracks in the coating.

[0008] Existing technical documents Patent documents Patent Document 1: Japanese Patent Application Publication No. 7-173646 Patent Document 2: Japanese Patent Application Publication No. 6-33213 Patent Document 3: International Publication No. 2018 / 124157 Summary of the Invention

[0009] The problem that the invention aims to solve However, in the baking process described in Patent Document 1, when using molten galvanized steel sheets, the thick coating often makes it difficult to release hydrogen through the coating. Furthermore, increasing the baking temperature to promote hydrogen release can also lead to changes in the properties of the coating.

[0010] In addition, in the method of forming cracks in the coating in Patent Document 2 or 3, although hydrogen removal is promoted, there is a problem that even if cracks are formed in the same way as in Patent Document 2 or 3, the rate of hydrogen removal varies greatly depending on the steel plate being manufactured.

[0011] The present invention was made in view of the above circumstances, and its object is to provide a high-strength fused galvanized steel sheet with a tensile strength of 780 MPa or more and excellent hydrogen removal properties.

[0012] Methods for solving problems The inventors of this application conducted a detailed comparative study on high-strength molten galvanized steel sheets with different hydrogen desorption rates, which utilize cracks formed within the coating to promote hydrogen desorption, and discovered the following insights.

[0013] In high-strength molten galvanized steel sheets where hydrogen desorption was facilitated, cracks formed within the coating. In steel sheets with slower hydrogen desorption rates, many cracks, although reaching the interface between the coating and the base steel sheet, stopped forming at the grain surface of the base steel sheet (see [reference]). Figure 2 That is, such as Figure 2 As shown, although the crack 6 formed in the steel plate with a relatively slow hydrogen desorption rate continues from the surface of the molten zinc layer 2 to the interface between the base steel plate 1 and the molten zinc layer 2, it stops at the grain surface of the base steel plate 1.

[0014] On the other hand, in steel plates with a relatively fast hydrogen desorption rate, cracks extend to the interface between the coating and the base steel plate, and further reach the grain boundaries (grain boundaries) of the base steel plate, where oxides are present (see...). Figure 1 That is, such as Figure 1 As shown, for steel sheets with relatively fast hydrogen removal rates, there are cracks 5 within the molten zinc layer 2 that extend from the surface of the molten zinc layer 2 to the interface between the base steel sheet 1 and the aforementioned molten zinc layer 2, and are connected to grain boundaries 4 of the base steel sheet 1 at the interface between the base steel sheet 1 and the molten zinc layer 2, where oxides 3 are present. Furthermore, the more cracks that reach the grain boundaries of the base steel sheet, the faster the hydrogen removal rate of the steel sheet.

[0015] This invention is based on the above insights, and its main points are as follows.

[0016] [1] A high-strength molten galvanized steel sheet, comprising a base steel sheet and a molten galvanized layer formed on the surface of the base steel sheet. The aforementioned base steel plate contains, by mass percent: C: 0.050% to 0.400% Si: 0.30% to 2.50% Mn: 1.80% or more, less than 6.00% P: 0.001% to 0.100% S: 0.0001% to 0.0200%, and Al: 0.01% to 2.00% The balance consists of Fe and unavoidable impurities. Within the aforementioned molten zinc coating, there are cracks that extend from the surface of the molten zinc coating to the interface between the base steel plate and the aforementioned molten zinc coating, and are connected to the grain boundaries of the base steel plate at the interface between the base steel plate and the aforementioned molten zinc coating, and to the grain boundaries of the base steel plate where oxides are present, and the density of the aforementioned cracks in the cross-section of the steel plate is 10 cracks / mm or more. The tensile strength of the aforementioned high-strength molten galvanized steel sheet is above 780 MPa.

[0017] [2] The high-strength galvanized steel sheet as described in [1], wherein the aforementioned base steel sheet further contains, by weight %, a portion selected from: N: 0.0005% to 0.0100% Ti: 0.005% to 0.200% Nb: 0.005% to 0.200% B: 0.0003% to 0.0050% Ni: 0.005% to 1.000% Cr: 0.005% to 1.000% V: 0.005% to 0.500% Mo: 0.005% to 1.000% Co: 0.001% to 0.010% Cu: 0.005% to 1.000% Sn: 0.002% to 0.200% Sb: 0.005% to 0.100% Ta: 0.001% to 0.010% W: 0.005% to 0.100% Ca: 0.0005% to 0.0050% Mg: ≥0.0005% ≤0.0050%, and REM: One or more of the following: above 0.0005% and below 0.0050%.

[0018] [3] The high-strength galvanized steel sheet as described in [1] or [2], wherein the aforementioned base steel sheet further contains, by mass%, one or more of groups A to C below. Group A Selected from Te: exceeding 0% and below 0.10% As: exceeding 0% and below 0.10%, and Hf: One or more of the following: exceeding 0% and below 0.10%; Group B Selected from Bi: exceeding 0% and below 0.20%, and Pb: One or more of the following: exceeding 0% and below 0.20%; Group C Selected from Zn: greater than 0% and less than 0.10% Ge: More than 0% and less than 0.10% Sr: greater than 0% and less than 0.10%, and Cs: One or more of the following: exceeding 0% and below 0.10%.

[0019] [4] The method for manufacturing high-strength fused galvanized steel sheet as described in any one of [1] to [3] above, wherein, The base steel plate was annealed and cooled in an atmosphere containing H2 with a dew point above -30℃ and below +20℃. Then, after the molten zinc plating process, alloying treatment is carried out. During the subsequent cooling process, a tension of 1.5 kgf / mm was applied to the steel plate. 2 above.

[0020] [5] The manufacturing method of high-strength molten galvanized steel sheet as described in [4], wherein in the atmosphere of the aforementioned annealing treatment, H2 is 2 vol% or more and 30 vol% or less, and the balance is one or more of N2, H2O, CO, CO2, O2 and unavoidable impurities, and the annealing temperature of the aforementioned annealing treatment is 700°C or more and 950°C or less.

[0021] It should be noted that in this specification, all percentages representing the content of steel components are by mass. Furthermore, in this invention, "high-strength fused galvanized steel sheet" refers to alloyed fused galvanized steel sheet with a tensile strength (TS) of 780 MPa or higher.

[0022] Invention Effects According to the present invention, it is possible to provide a high-strength fused galvanized steel sheet with a tensile strength of 780 MPa or more and excellent hydrogen removal properties.

[0023] By applying the high-strength molten galvanized steel sheet of the present invention to, for example, automotive structural components, it is possible to achieve improved fuel efficiency based on vehicle body lightweighting. Attached Figure Description

[0024] [ Figure 1 ] Figure 1 This is a schematic diagram illustrating a crack extending from the surface of the molten zinc layer to the interface between the base steel plate and the molten zinc layer, and further connecting to the grain boundary of the base steel plate at the interface between the base steel plate and the molten zinc layer, where an oxide is present.

[0025] [ Figure 2 ] Figure 2 This is a schematic diagram illustrating a crack that, although it extends from the surface of the molten galvanized layer to the interface between the base steel sheet and the molten galvanized layer, stops at the grain surface of the base steel sheet. Detailed Implementation

[0026] The present invention will be described based on the following embodiments. However, the present invention is not limited to the following embodiments.

[0027] One embodiment of the present invention relates to a high-strength molten galvanized steel sheet comprising a base steel sheet and a molten galvanized layer formed on the surface of the base steel sheet.

[0028] (1) First, the composition of the base steel plate is explained. The base steel plate has the following composition: 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.

[0029] C: 0.050% to 0.400% Carbon (C) is an austenite-forming element and is effective in improving the strength and ductility of the base steel sheet (annealed sheet). A C content of 0.050% or higher ensures the strength of the base steel sheet; therefore, a C content of 0.050% or higher is preferred. A C content of 0.060% or higher is more desirable. On the other hand, excessive C content leads to significant hardening of the weld and heat-affected zone, deterioration of the mechanical properties of the weld, and reduction in spot weldability and arc weldability. Therefore, a C content of 0.400% or lower is preferred. A C content of 0.300% or lower is more preferably 0.200% or lower.

[0030] Si: 0.30% to 2.50% Si is a ferrite-forming element and is also effective in improving the solid solution strengthening and work hardening ability of ferrite in the base steel plate. To achieve the above effects, a Si content of 0.30% or more is required. On the other hand, if the Si content exceeds 2.50%, the above effects become saturated. Therefore, the Si content is set at 0.30% to 2.50%.

[0031] Mn: 1.80% or more, 6.00% or less Mn is an austenite-forming element and is effective in ensuring the strength of the base steel sheet. It is difficult to ensure strength when the Mn content is below 1.80%. On the other hand, if the Mn content exceeds 6.00%, its effect on strength becomes saturated. Therefore, the Mn content is set between 1.80% and 6.00%. The Mn content is preferably 2.00% or more. Furthermore, the Mn content is preferably 5.50% or less.

[0032] P: 0.001% to 0.100% Phosphorus (P) is an unavoidable impurity element that causes embrittlement by segregating at grain boundaries, thus degrading impact resistance. This trend becomes significant if the P content exceeds 0.100%, therefore the P content is set below 0.100%, preferably below 0.050%. It should be noted that lower P content is generally preferred, but from a refining cost perspective, a P content of 0.001% or higher is recommended.

[0033] S: 0.0001% to 0.0200% Sulfur (S) segregates at grain boundaries, causing embrittlement of the steel during hot working, and exists as sulfides, reducing its local deformation capacity. Therefore, the S content is set to 0.0200% or less. On the other hand, considering production technology limitations, 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.

[0034] Al: 0.01% to 2.00% Al is added as a deoxidizer, but its effect is insufficient when the content is below 0.01%. 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 sheet 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.

[0035] The balance other than the above components can be Fe and unavoidable impurities.

[0036] The composition of the aforementioned base steel plate may optionally contain components selected from N: 0.0005% to 0.0100%, Ti: 0.005% to 0.200%, Nb: 0.005% to 0.200%, B: 0.0003% to 0.0050%, Ni: 0.005% to 1.000%, Cr: 0.005% to 1.000%, V: 0.005% to 0.500%, Mo: 0.005% to 1.000%, and Co: 0. One or more of the following: Cu: 0.005% to 1.000%, Sn: 0.002% to 0.200%, Sb: 0.005% to 0.100%, Ta: 0.001% to 0.010%, W: 0.005% to 0.100%, Ca: 0.0005% to 0.0050%, Mg: 0.0005% to 0.0050%, and REM: 0.0005% to 0.0050%.

[0037] N: 0.0005% to 0.0100% Nitrogen (N) is an element that degrades the aging resistance of steel. In particular, if the N content exceeds 0.0100%, the degradation of aging resistance becomes significant. Lower N content is preferred, but excessive denitrification due to N content below 0.0005% increases production costs. Therefore, in the case of N content, the N content is preferably set to 0.0005% to 0.0100%. The N content is further preferably 0.0010% or more. Furthermore, the N content is even more preferably 0.0070% or less.

[0038] Ti: 0.005% to 0.200% Ti is an effective precipitation strengthening element for steel. Furthermore, by containing Ti, a relatively hard ferrite is formed, thereby reducing the hardness difference with the hard second phase (martensite or retained austenite), ensuring good tensile flangeability. This effect is achieved when the Ti content is 0.005% or higher. On the other hand, if the Ti content exceeds 0.200%, the area ratio of hard martensite becomes excessive, increasing microporosity at the martensite grain boundaries, which in turn promotes crack propagation and reduces formability. Therefore, when Ti is present, the Ti content is preferably set to 0.005% or higher and 0.200% or lower. The Ti content is more preferably 0.010% or higher. Furthermore, the Ti content is more preferably 0.100% or lower.

[0039] Nb: 0.005% to 0.200% Nitrogen (Nb) is an effective precipitation strengthening element for steel. Furthermore, similar to the effect of Ti, the presence of Nb forms a harder ferrite, thereby reducing the hardness difference with the hard second phase (martensite or retained austenite) and ensuring good tensile flangeability. This effect is achieved when the Nb content is 0.005% or higher. On the other hand, if the Nb content exceeds 0.200%, the area ratio of hard martensite becomes excessive, increasing microporosity at the martensite grain boundaries, which in turn promotes crack propagation and reduces formability. It also contributes to increased costs. Therefore, when Nb is present, the Nb content is preferably set to 0.005% or higher and 0.200% or lower. The Nb content is more preferably 0.010% or higher. Furthermore, the Nb content is more preferably 0.100% or lower.

[0040] B: 0.0003% to 0.0050% Boron (B) inhibits the formation and growth of ferrite from austenite grain boundaries, thus controlling the microstructure and can be included 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%, the formability decreases. Therefore, when B is included, the B content is preferably set to 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.

[0041] Ni: 0.005% to 1.000% Ni is an element that stabilizes retained austenite, which is effective in ensuring good ductility. Furthermore, it is an element that increases the strength of steel through solid solution strengthening. These effects are achieved when the Ni content is 0.005% or higher. On the other hand, if the Ni content exceeds 1.000%, the hard martensite becomes excessively large, increasing microporosity at the martensite grain boundaries, thereby promoting crack propagation and reducing bending and tensile flangeability. It also contributes to increased costs. Therefore, when Ni is present, the Ni content is preferably set to 0.005% to 1.000%. The Ni content is more preferably 0.010% or higher. Furthermore, the Ni content is more preferably 0.500% or lower.

[0042] 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 contribute to a balance between strength and ductility, and can therefore be included as needed. However, excessive inclusion of any of these elements can lead to excessively large hard martensite, increased microporosity at the grain boundaries, crack propagation, and reduced formability. It also increases costs. Therefore, when including these elements, the Cr content is preferably 0.005% to 1.000%, the V content is preferably 0.005% to 0.500%, the Mo content is preferably 0.005% to 1.000%, and the Co content is preferably 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.

[0043] Cu: 0.005% to 1.000% Cu is an effective strengthening element for steel. This effect is achieved when the Cu content is 0.005% or higher. On the other hand, if the Cu content exceeds 1.000%, the hard martensite becomes too large, the microporosity at the martensite grain boundaries increases, and consequently, crack propagation is promoted, reducing formability. Therefore, when Cu is present, the Cu content is preferably set to 0.005% to 1.000%. The Cu content is more preferably 0.010% or higher. Furthermore, the Cu content is more preferably 0.500% or lower.

[0044] Sn: 0.002% to 0.200%; Sb: 0.005% to 0.100% From the viewpoint of suppressing decarburization in the tens of μm region of the steel plate surface caused by nitriding and oxidation, Sn and Sb can be included as needed. Suppressing such nitriding and oxidation prevents a decrease in the martensite area ratio at the steel plate surface, effectively ensuring strength and material stability. On the other hand, excessive inclusion of either element leads to a decrease in toughness. Therefore, when Sn is included, the Sn content is preferably set to 0.002% to 0.200% or less. Similarly, when Sb is included, the Sb content is preferably set to 0.005% to 0.100% or less. The Sn content is further preferably 0.005% or more. Furthermore, the Sn content is further preferably 0.100% or less. The Sb content is further preferably 0.010% or more. Furthermore, the Sb content is further preferably 0.080% or less.

[0045] Ta: 0.001% to 0.010%; W: 0.005% to 0.100% Ta and W, like Ti and Nb, form carbides and carbonitrides, contributing to increased strength. Furthermore, it is believed that Ta and W partially dissolve in Nb carbides and Nb carbonitrides, forming composite precipitates such as (Nb,Ta)(C,N), which significantly suppresses precipitate coarsening and stabilizes the contribution of precipitation strengthening to strength. Therefore, it is preferable to contain one or more of Ta and W. In the case of Ta, the above-mentioned effect is obtained when the Ta content is 0.001% or more, and in the case of W, the above-mentioned effect is obtained when the W content is 0.005% or more. On the other hand, even if more than 0.010% Ta or more than 0.100% W is contained, the aforementioned effect saturates, and the alloy cost increases. Therefore, when Ta is contained, the Ta content is preferably set to 0.001% or more and 0.010% or less. Similarly, when W is contained, the W content is preferably set to 0.005% or more and 0.100% or less.

[0046] Ca: ≥0.0005% ≤0.0050%, Mg: ≥0.0005% ≤0.0050%, REM: ≥0.0005% ≤0.0050% Ca, Mg, and REM are each effective elements for shaping sulfides into spherical forms and for mitigating the adverse effects of sulfides on porosity (stretch flange properties). However, excessive content of any of these elements can lead to an increase in inclusions and other defects, causing surface and internal defects. Therefore, when these elements are present, their respective contents are preferably set to 0.0005% to 0.0050%. It should be noted that REM (rare earth metals) is a collective term for 17 elements, including Sc, Y, and the lanthanides. REM can contain one or more of these 17 elements, and the REM content referred to here is the total content of these elements.

[0047] It should be noted that if the contents of N, Ti, Nb, B, Ni, Cr, V, Mo, Co, Cu, Sn, Sb, Ta, W, Ca, Mg, and REM, which are described as optional components above, are below the lower limit, the component is considered to be contained as an unavoidable impurity.

[0048] The composition of the aforementioned base steel plate may also optionally contain one or more groups selected from the following groups A to C.

[0049] Group A Selected from Te: exceeding 0% and below 0.10% As: exceeding 0% and below 0.10%, and Hf: One or more of the following: above 0% and below 0.10% Group B Selected from Bi: exceeding 0% and below 0.20%, and Pb: One or more of the following: above 0% and below 0.20% Group C Selected from Zn: greater than 0% and less than 0.10% Ge: More than 0% and less than 0.10% Sr: greater than 0% and less than 0.10%, and Cs: One or more of the following: exceeding 0% and below 0.10% Group A [selected from one or more of the following: Te: more than 0% and less than 0.10%, As: more than 0% and less than 0.10%, and Hf: more than 0% and less than 0.10%] Te, As, and Hf are all elements used for speciation control of sulfides. When Te, As, and Hf are present, their respective contents can be set to exceed 0%.

[0050] Te: More than 0% and less than 0.10% By including 0.001% or more of Te, the morphology of sulfides can be controlled, thereby improving ductility and toughness. From the viewpoint of achieving this effect, when Te is included, the Te content is preferably set to 0.001% or more. However, from the viewpoint of preventing cost increases, when Te is included, the Te content is preferably set to 0.10% or less.

[0051] As: More than 0% and less than 0.10% By including 0.001% or more of As, the morphology of sulfides can be controlled, thereby improving ductility and toughness. From the viewpoint of achieving this effect, when As is included, the As content is preferably set to 0.001% or more. However, from the viewpoint of preventing cost increases, when As is included, the As content is preferably set to 0.10% or less.

[0052] Hf: Above 0% and below 0.10% By including 0.01% or more of Hf, the morphology of sulfides can be controlled, thereby improving ductility and toughness. From the viewpoint of achieving this effect, when Hf is included, the Hf content is preferably set to 0.01% or more. However, from the viewpoint of preventing cost increases, when Hf is included, the Hf content is preferably set to 0.10% or less.

[0053] Group B [selected from one or more of Bi: greater than 0% and less than 0.20%, and Pb: greater than 0% and less than 0.20%] Both Bi and Pb are elements that suppress grain boundary segregation and improve ductility and toughness. When Bi and Pb are present, their respective contents can be set to be greater than 0%.

[0054] Bi: More than 0% and less than 0.20% By containing 0.001% or more Bi, grain boundary segregation can be suppressed, thereby improving ductility and toughness. From the viewpoint of achieving this effect, when Bi is present, the Bi content is preferably set to 0.001% or more. Furthermore, Bi improves machinability and the smoothness of the cut surface, thus enhancing the resistance of the cut surface to delayed fracture. However, from the viewpoint of preventing cost increases, when Bi is present, the Bi content is preferably set to 0.20% or less.

[0055] Pb: above 0% and below 0.20% By including 0.001% or more of Pb, grain boundary segregation can be suppressed, thereby improving ductility and toughness. From the viewpoint of achieving this effect, when Pb is included, the Pb content is preferably set to 0.001% or more. Furthermore, Pb improves machinability and the smoothness of the cut surface, thus enhancing the resistance of the cut surface to delayed fracture. However, from the viewpoint of preventing cost increases, when Pb is included, the Pb content is preferably set to 0.20% or less.

[0056] Group C [selected from one or more of the following: Zn: more than 0% and less than 0.10%; Ge: more than 0% and less than 0.10%; Sr: more than 0% and less than 0.10%; Cs: more than 0% and less than 0.10%] Zn, Ge, Sr, and Cs are all elements that can improve strength without significantly affecting mechanical properties and surface quality. When Zn, Ge, Sr, and Cs are present, their respective contents can be set to exceed 0%.

[0057] Zn: greater than 0% and less than 0.10% By including 0.001% or more of Zn, strength can be improved without significantly affecting mechanical properties and surface quality. From the viewpoint of achieving this effect, the Zn content is preferably set to 0.001% or more when containing Zn. However, from the viewpoint of preventing cost increases, the Zn content is preferably set to 0.10% or less when containing Zn.

[0058] Ge: More than 0% and less than 0.10% By including 0.001% or more of Ge, strength can be improved without significantly affecting mechanical properties and surface quality. From the viewpoint of achieving this effect, the Ge content is preferably set to 0.001% or more when containing Ge. However, from the viewpoint of preventing cost increases, the Ge content is preferably set to 0.10% or less when containing Ge.

[0059] Sr: greater than 0% and less than 0.10% By including 0.001% or more of Sr, strength can be improved without significantly affecting mechanical properties and surface quality. From the viewpoint of achieving this effect, when Sr is included, the Sr content is preferably 0.001% or more. However, from the viewpoint of preventing cost increases, when Sr is included, the Sr content is preferably set to 0.10% or less.

[0060] Cs: More than 0% and less than 0.10% By including 0.001% or more of Cs, strength can be improved without significantly affecting mechanical properties and surface quality. From the viewpoint of achieving this effect, the Cs content is preferably set to 0.001% or more when Cs is included. However, from the viewpoint of preventing cost increases, the Cs content is preferably set to 0.10% or less when Cs is included.

[0061] (2) Next, the cracks (crazing) in the molten zinc layer will be explained.

[0062] The high-strength molten galvanized steel sheet of the present invention has cracks in the molten galvanized layer that extend from the surface of the molten galvanized layer to the interface between the base steel sheet and the molten galvanized layer, and are connected to the grain boundaries of the base steel sheet at the interface between the base steel sheet and the molten galvanized layer. Oxides are present at the grain boundaries connected to the aforementioned cracks.

[0063] Cracks formed within the molten zinc coating extend from the surface of the molten zinc coating to the interface between the base steel plate and the molten zinc coating. It is speculated that the cracks within the molten zinc layer are diffusion paths of hydrogen detaching from the base steel sheet and extending to its surface. Therefore, in order for hydrogen to diffuse into the atmosphere instead of remaining within the molten zinc layer, the cracks need to extend from the surface of the molten zinc layer to the interface between the base steel sheet and the molten zinc layer.

[0064] Cracks formed within the molten zinc layer connect with the grain boundaries of the base steel plate at the interface between the base steel plate and the molten zinc layer. The path for hydrogen to diffuse at high speed from the base steel sheet to its surface is the grain boundaries of the base steel sheet. Therefore, cracks within the molten zinc layer continue from the surface of the molten zinc layer to the interface between the base steel sheet and the molten zinc layer, as... Figure 1 The diagram also shows that by connecting with the grain boundaries of the base steel plate at the interface between the base steel plate and the molten zinc layer, high-speed hydrogen diffusion from the base steel plate through the molten zinc layer to the atmosphere can be achieved.

[0065] Oxides are present in the grain boundaries of the base steel plate connected to the crack. The presence of oxides at grain boundaries reduces their strength, making them more susceptible to becoming the initiation point for cracks (crazing) in the molten zinc plating layer. Furthermore, oxides act as diffusion pathways for hydrogen, promoting its diffusion. Therefore, the more oxide-rich the grain boundaries, the easier it is to introduce cracks that serve as hydrogen diffusion pathways. Figure 1 This is a schematic diagram of a crack 5 that extends from the surface of the molten galvanized layer 2 to the interface between the base steel plate 1 and the molten galvanized layer 2, and is connected at the interface between the base steel plate 1 and the molten galvanized layer 2 to a grain boundary 4 of the base steel plate 1, where oxides 3 are present. The high-strength molten galvanized steel sheet of the present invention has a specified density... Figure 1 The cracks shown are as depicted.

[0066] The density of the aforementioned cracks is 10 or more per mm. The cracks within the molten zinc layer serve as diffusion paths for hydrogen; therefore, to promote hydrogen removal, the crack density needs to be 10 cracks / mm or higher. More preferably, the crack density is 15 cracks / mm or higher. There is no particular upper limit to the crack density, but if the crack density is too high, the adhesion of the molten zinc layer deteriorates; therefore, the crack density is preferably below 50 cracks / mm.

[0067] To confirm cracks within the molten zinc plating layer, a cross-section for observation of the sample is prepared using focused ion beam (FIB), and the prepared cross-section is observed using a scanning electron microscope (SEM). The observation cross-section can be any of a vertical or inclined cross-section; from the viewpoint of ease of FIB cross-section preparation and SEM observation, an inclined cross-section is preferred. When performing SEM observation on a non-inclined sample, the inclination angle of the FIB cross-section is preferably 45°, as this ensures that the vertical dimension of the observed image matches the actual cross-section dimension. A certain degree of coarse cracks can also be observed by embedding the sample in resin and grinding it. However, with this method, the plating layer is ground up during grinding, burying fine cracks, thus often making it impossible to observe fine cracks. Furthermore, it is difficult to confirm whether the cracks are connected to the grain boundaries of the base steel plate. Therefore, in this invention, a method is adopted to prepare an observation cross-section for the sample using FIB and to perform SEM observation on the aforementioned cross-section. At least 20 observation sites are randomly selected from the steel plate, and the observation cross-section is prepared in a manner that allows observation of the steel plate cross-section with a width of at least 30 μm. Since cracks may sometimes be located off-center, the width of the cross-section to be observed is preferably 5 mm or more in total. Within this field of view, the number of cracks and whether they are connected to the grain boundaries of the base steel plate are confirmed. Furthermore, the presence of oxides in the grain boundaries of the steel plate is identified by elemental analysis of the grain boundary portion using energy-dispersive X-rays (EDS) mounted in a SEM. In this invention, grain boundaries containing 10% or more oxygen by mass are considered to contain oxides. The number of cracks extending from the surface of the molten zinc layer to the interface between the base steel plate and the molten zinc layer, and connected to the grain boundaries of the base steel plate containing oxides at the interface, is counted, and the crack density (cracks / mm) is calculated. Cracks sometimes branch within the molten zinc layer; each branched crack is counted as one crack based on the number of cracks before the branch. Furthermore, a crack connected to a grain boundary containing oxides refers to a crack that, when measured along the grain boundary starting from the point of connection between the crack and the grain boundary, is connected to a grain boundary containing oxides within 0.5 μm from the aforementioned starting point. For information on the measurement of crack density, please refer to the description in the examples.

[0068] (3) Next, the manufacturing method of high-strength molten galvanized steel sheet will be explained.

[0069] The high-strength fused galvanized steel sheet of the present invention can be manufactured, for example, by the following method. First, a steel billet having the above-described composition is heated in a hot rolling process and then subjected to rough rolling and finish rolling to produce a hot-rolled steel sheet. Then, after removing the oxide scale from the surface of the hot-rolled steel sheet by a pickling process, it is cold-rolled as needed in a cold rolling process. There are no particular limitations on the process from hot rolling to cold rolling; for example, known manufacturing methods can be used.

[0070] Next, the hot-rolled steel sheet or, if necessary, the cold-rolled steel sheet is annealed and cooled to produce a base steel sheet (annealed sheet). Then, the base steel sheet undergoes a plating process (molten galvanizing), alloying treatment, and cooling. It should be noted that, in this invention, to obtain a high-strength molten galvanized steel sheet free from defects such as unplated areas, any of the following methods is preferably implemented: a redox method in which an Fe oxide layer is pre-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 and the precipitation of Si oxide to the steel sheet surface is suppressed by controlling the atmosphere during annealing.

[0071] The atmosphere used in the annealing process is set to contain H2. Furthermore, the dew point of the atmosphere used in the annealing process is set to +20°C or lower. If the dew point of the atmosphere exceeds +20°C, the steel plate surface becomes more prone to oxidation, or the coating wettability deteriorates, resulting in defects such as uncoated areas. The aforementioned dew point is preferably +18°C or lower, more preferably +15°C or lower. Additionally, the aforementioned dew point is set to -30°C or higher. When the dew point of the atmosphere is below -30°C, oxides that would otherwise form at grain boundaries, becoming the initiation point of cracks in the molten zinc coating and a diffusion path for hydrogen in the base steel plate, will not form. Furthermore, when the dew point is -30°C or higher, it also has the effect of suppressing the precipitation of Si oxides, which cause a decrease in coating wettability, onto the surface of the base steel plate, and suppressing uncoated defects. The aforementioned dew point is preferably -25°C or higher, more preferably -20°C or higher.

[0072] To suppress oxidation of the steel sheet surface, annealing is preferably performed in an atmosphere containing 2 vol% to 30 vol% H2, with the balance being any one or more of N2, H2O, CO, CO2, and O2, as well as unavoidable impurities. If the H2 concentration in the atmosphere is less than 2 vol%, oxidation occurs on the steel sheet surface, the wettability of the coating deteriorates, coating defects occur, and the corrosion resistance of the steel sheet deteriorates. On the other hand, if the H2 concentration in the atmosphere exceeds 30 vol%, excess hydrogen penetrates into the steel sheet during annealing, and even if the dehydrogenation rate after coating treatment is rapid, the reduction of hydrogen concentration will take time. The H2 concentration in the atmosphere during annealing is more preferably 3 vol% or more. Furthermore, the H2 concentration in the atmosphere during annealing is more preferably 5 vol% or less.

[0073] In the above annealing process, the annealing temperature is not particularly limited, but is preferably between 700°C and 950°C. When the annealing temperature is below 700°C, the reduction of the natural oxide film on the steel plate surface is insufficient, the wettability of the coating decreases, and defects such as incomplete coating occur. If the annealing temperature exceeds 950°C, excess hydrogen penetrates into the steel plate, increasing the concentration of diffusible hydrogen in the steel plate. A more preferred annealing temperature is 730°C or higher, and even more preferred is 750°C or higher.

[0074] Because the base steel sheet in this invention contains a significant amount of Si, the Si in the base steel sheet is oxidized during annealing due to the trace amounts of H2O in the atmosphere, forming Si-containing oxides on the surface of the base steel sheet. This reduces the wettability of molten zinc with the base steel sheet, resulting in defects such as incomplete coating. One method to address this problem is a redox method: 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, improving the wettability of the coating. Another method involves increasing the H2O concentration (dew point) in the annealing atmosphere to a certain extent during annealing, causing the Si in the base steel sheet to oxidize internally and inhibiting the precipitation of Si oxides to the surface of the base steel sheet, thus improving the wettability of the coating. In this invention, high-strength molten zinc-coated steel sheets without defects such as incomplete coating can be obtained using any method.

[0075] In redox methods, methods for pre-oxidizing the base steel sheet include atmosphere oxidation and flame-based oxidation; any method can be used as long as Fe can be oxidized. In atmosphere oxidation, an atmosphere with an O2 concentration of 0.1 vol% to 4.0 vol% is desirable. Flame-based oxidation is preferably carried out under conditions where the ratio of combustible gas to air supplied to the flame burner, i.e., the air ratio, is 1.00 to 1.50. The air ratio is a value obtained by dividing the volume of air supplied to the flame burner by the volume of air that reacts completely with the combustible components in the combustible gas. Under these conditions, the iron oxide layer formed on the surface of the steel sheet is reduced to metallic iron during annealing. As a result, a high-strength fused galvanized steel sheet with improved wettability between molten zinc and the base steel sheet and free from defects such as incomplete coating 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. As a result, high-strength fused galvanized steel sheets with reduced coating wettability, suppressed precipitation of Si oxides onto the base steel sheet surface, and no defects such as uncoated sheets can be obtained.

[0076] After annealing, the steel is cooled. The cooling conditions are not particularly limited. For example, it can be cooled to below 550°C at any cooling rate. Then, molten galvanizing and alloying are performed, followed by cooling. The conditions for molten galvanizing and alloying are not particularly limited; for example, known conditions can be used. Molten galvanizing can be performed, for example, by immersing the base steel sheet in a zinc bath at 440–550°C. The zinc bath contains Zn and Al, as well as unavoidable impurities; its composition is not particularly specified. For example, the Al concentration in the bath can be set to 0.05% by mass or more and 0.30% by mass or less. Furthermore, alloying can be performed, for example, by heating the steel sheet after molten galvanizing to an alloying temperature of 450–600°C.

[0077] To generate cracks within the molten zinc layer that extend from the surface of the molten zinc layer to the interface between the base steel sheet and the molten zinc layer, and are connected to the grain boundaries of the base steel sheet at the interface, the tension applied to the steel sheet after alloying treatment and cooling to below 200°C needs to be set to 1.5 kgf / mm. 2 The aforementioned tension is preferably 1.8 kgf / mm. 2 The above, more preferably 2.0 kgf / mm 2The above describes the process. Metals contract when cooled, but tension acts as a resistance to this contraction, causing stress to concentrate locally, particularly at grain boundaries. The base steel sheet, due to its high ductility, is not damaged, while the molten galvanized layer, with its lower ductility compared to the base steel sheet, allows cracks to form starting from the stress concentration points. This allows the aforementioned cracks to form within the molten galvanized layer. It should be noted that the upper limit of the aforementioned tension is not specifically limited; however, excessive tension may lead to uneven elongation of the steel sheet in the width direction or necking. Therefore, a tension of 20.0 kgf / mm is preferred. 2 The following should be noted: Using 1 kgf = 9.80665 N as a reference, the above 1.5 kgf / mm... 2 This can be converted to 14.709975 N / mm 2 .

[0078] It should be noted that the cooling conditions for the aforementioned cooling are not particularly limited. For example, cooling to below 150°C is possible at a cooling rate of 60°C / min to 600°C / min. Furthermore, the method for applying the aforementioned tension to the steel plate is not particularly limited. For example, a method of applying tension using tension rollers installed in the production line during the cooling process after alloying treatment can be cited. Additionally, the aforementioned tension can be calculated, for example, by dividing the total load (kgf) of the load sensors on both sides of the tension roller by the cross-sectional area of ​​the steel plate (= plate thickness (mm) × plate width (mm)) (mm²). 2 And thus obtained.

[0079] The high-strength fused galvanized steel sheet of the present invention can be obtained through the above processes. The diffusible hydrogen content in the high-strength fused galvanized steel sheet of the present invention varies depending on the characteristics of the steel sheet, but is preferably 0.40 ppm by mass or less after 20 days from the date of manufacture. Furthermore, the hydrogen removal rate calculated by measuring the diffusible hydrogen content within 3 days after manufacture and after 20 days is preferably 0.005 ppm by mass / day or more. It should be noted that the aforementioned diffusible hydrogen content can be measured using the method described in the examples. In addition, in the present invention, excellent hydrogen removal performance means that the aforementioned hydrogen removal rate is 0.005 ppm by mass / day or more.

[0080] The high-strength fused 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. Furthermore, the upper limit of the aforementioned tensile strength is not particularly limited; as an example, the aforementioned tensile strength can be 2400 MPa or less.

[0081] It should be noted that the high-strength molten galvanized steel sheet of the present invention is typically manufactured by subjecting the steel raw material to conventional steelmaking, casting, hot rolling, and other processes. However, for example, it can also be manufactured by using thin slab continuous casting or further thin strip continuous casting, thereby omitting part or all of the hot rolling process.

[0082] Example Steel with the composition shown in Table 1, the balance being Fe and unavoidable impurities, was smelted in a converter and produced into slabs using continuous casting. The resulting slabs were heated to 1200°C and hot-rolled to thicknesses of 2.3–4.5 mm, then wound. Next, the hot-rolled steel sheets were pickled, and a portion were cold-rolled. Then, they were annealed and cooled in a furnace with adjustable atmosphere according to the conditions shown in Table 2. In some cases, oxidation treatment was performed before annealing. It should be noted that in the "Oxidation Method" column of Table 2, oxidation treatment performed under the above-described atmosphere is denoted as "Atmosphere," oxidation treatment performed under the above-described flame-based oxidation is denoted as "Flame," and no oxidation treatment is denoted as "-". Subsequently, a zinc plating bath containing 0.13–0.19% by mass Al (zinc plating bath temperature: 465°C) was used for melt galvanizing, followed by alloying treatment at 520°C and cooling to below 200°C to obtain melt-galvanized steel sheet (alloyed melt-galvanized steel sheet). It should be noted that during the aforementioned cooling process, tension rollers installed in the production line were used to apply the tension shown in Table 2 to the steel sheet.

[0083] The surface appearance, crack density in the molten zinc layer, diffusible hydrogen content and hydrogen desorption rate, and tensile strength of the above-mentioned molten galvanized steel sheet were evaluated.

[0084] [Surface Appearance] Test pieces measuring 230mm × 350mm were cut from any location on the manufactured molten galvanized steel sheet. The number of defects such as uncoated areas and uneven color were visually counted, and evaluated according to the following criteria. ◎ and 〇 were rated as having excellent surface appearance according to the following criteria.

[0085] <Judgment Criteria> ◎: Defect-free department 〇: Slight unevenness in tone △: Uneven defects ×: Uncoated areas exist. [Crack density within the molten zinc coating] Twenty sections were randomly selected from test pieces cut from the manufactured molten galvanized steel sheets. Using focused ion beam (FIB), SEM observation sections with a 45° tilt angle were fabricated within a 50 μm width. SEM observation was performed at 5000x magnification, and elemental analysis of the grain boundaries of the steel sheets was conducted based on SEM-EDS. The number of cracks in the observed sections that extended from the surface of the molten galvanized layer to the interface between the base steel sheet and the molten galvanized layer, and were connected to oxide grain boundaries within the grain boundaries of the base steel sheet, was counted. This number was then divided by the overall surface line length of the observed section to determine the crack density. A crack density of 10 cracks / mm or higher was considered acceptable.

[0086] [Diffusible hydrogen content and hydrogen removal rate in steel plates] Test pieces measuring 30 mm in length and 5 mm in width were collected from the manufactured molten galvanized steel sheet. After the molten galvanized layer was removed by grinding, the diffusible hydrogen content in the steel was measured. The measurement used a heated hydrogen removal analysis method, with a heating rate of 200 °C / hr. It should be noted that hydrogen detected below 300 °C is considered diffusible hydrogen. The diffusible hydrogen content within 3 days and after 20 days of manufacturing the molten galvanized steel sheet was measured, and the result was divided by the number of days to obtain the hydrogen removal rate. A hydrogen removal rate of 0.005 ppm / day or higher was considered acceptable. Furthermore, the diffusible hydrogen content after 20 days was preferably 0.40 ppm or lower.

[0087] Tensile strength The tensile test was conducted using JIS 5 test pieces, which were collected in a direction perpendicular to the rolling direction of the steel plate, in accordance with JIS Z 2241:2011, and the tensile strength (TS) was determined.

[0088] The results are shown in Table 2.

[0089] [Table 1] [Table 2] The alloyed molten galvanized steel sheets of the present invention all have a TS of 780 MPa or higher, exhibiting excellent hydrogen removal properties. On the other hand, the comparative examples show insufficient hydrogen removal properties.

[0090] Industrial availability According to the present invention, a high-strength alloyed fused galvanized steel sheet with high strength (tensile strength of 780 MPa or more) and excellent hydrogen efflux properties can be obtained. By applying the high-strength alloyed fused galvanized steel sheet of the present invention to, for example, automotive structural components, improved fuel efficiency based on vehicle body lightweighting can be achieved.

[0091] Explanation of reference numerals in the attached figures 1. Base material steel plate 2. Hot-dip galvanized layer 3 oxides 4. Grain boundary 5. Cracks (cracks connected to the grain boundaries of oxides in the base steel plate) 6. Cracks (cracks that have stopped at the grain surface of the base steel plate).

Claims

1. A high-strength galvanized steel sheet, comprising a base steel sheet and a galvanized layer formed on the surface of the base steel sheet. The base steel plate contains, by mass%,: C: 0.050% to 0.400% Si: 0.30% to 2.50% Mn: 1.80% or more, less than 6.00% P: 0.001% to 0.100% S: 0.0001% to 0.0200%, and Al: 0.01% to 2.00% The balance consists of Fe and unavoidable impurities. Within the molten zinc coating, there are cracks that extend from the surface of the molten zinc coating to the interface between the base steel plate and the molten zinc coating, and are connected at the interface between the base steel plate and the molten zinc coating to grain boundaries of the base steel plate where oxides are present, and the density of the cracks in the cross-section of the steel plate is 10 cracks / mm or more. The tensile strength of the high-strength molten galvanized steel sheet is above 780 MPa.

2. The high-strength galvanized steel sheet as described in claim 1, wherein, The base steel plate further contains, by weight percent, a selection from: N: 0.0005% to 0.0100% Ti: 0.005% to 0.200% Nb: 0.005% to 0.200% B: 0.0003% to 0.0050% Ni: 0.005% to 1.000% Cr: 0.005% to 1.000% V: 0.005% to 0.500% Mo: 0.005% to 1.000% Co: 0.001% to 0.010% Cu: 0.005% to 1.000% Sn: 0.002% to 0.200% Sb: 0.005% to 0.100% Ta: 0.001% to 0.010% W: 0.005% to 0.100% Ca: 0.0005% to 0.0050% Mg: ≥0.0005% ≤0.0050%, and REM: One or more of the following: above 0.0005% and below 0.0050%.

3. The high-strength galvanized steel sheet as described in claim 1 or 2, wherein, The base steel plate further contains, by weight percent, one or more components selected from groups A to C below. Group A Selected from Te: exceeding 0% and below 0.10% As: exceeding 0% and below 0.10%, and Hf: One or more of the following: exceeding 0% and below 0.10%; Group B Selected from Bi: exceeding 0% and below 0.20%, and Pb: One or more of the following: exceeding 0% and below 0.20%; Group C Selected from Zn: greater than 0% and less than 0.10% Ge: More than 0% and less than 0.10% Sr: greater than 0% and less than 0.10%, and Cs: One or more of the following: exceeding 0% and below 0.10%.

4. The method for manufacturing high-strength fused galvanized steel sheet according to any one of claims 1 to 3, wherein, The base steel plate was annealed and cooled in an atmosphere containing H2 with a dew point above -30℃ and below +20℃. Then, after the molten zinc plating process, alloying treatment is carried out. During the subsequent cooling process, a tension of 1.5 kgf / mm was applied to the steel plate. 2 above.

5. The method for manufacturing high-strength fused galvanized steel sheet as described in claim 4, wherein, In the annealing atmosphere, H2 is 2 vol% to 30 vol%, and the balance is one or more of N2, H2O, CO, CO2, and O2, as well as unavoidable impurities. The annealing temperature is 700°C to 950°C.

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