HIGH-STRENGTH HOT-DIP GALVANIZED STEEL SHEET AND METHOD FOR PRODUCING THE SAME

Abstract

A high-strength, hot-dip galvanized steel sheet is provided that has excellent coating appearance, excellent coating adhesion during severe forming, and excellent LME crack resistance. The high-strength, hot-dip galvanized steel sheet comprises a steel sheet having a chemical composition containing predetermined steel components, the mass ratio of (Si + Cr) to Mn ((Si + Cr) / Mn) being not less than 0.25, the mass ratio of Si to Mn (Si / Mn) being less than 0.25, and the remainder being Fe and secondary impurities, and a zinc coating layer formed on the surface of the steel sheet in a coating quantity of not less than 20 g / m² and not more than 120 g / m² per surface of the steel sheet.In the production method, the maximum temperature T of the steel sheet in an annealing / heating furnace is greater than 700°C and not greater than 900°C, the dew point of the atmosphere in the heating furnace in the temperature range of the steel sheet is not less than 700°C and not greater than T°C and is not less than -20°C, and the atmosphere in the heating furnace contains hydrogen in an amount of not less than 3.0% by volume and not greater than 20.0% by volume and one or more selected from the group consisting of SO2 in an amount of not less than 0.1 ppm by volume and not greater than 3.0 ppm by volume and HCl in an amount of not less than 0.5 ppm by volume and not greater than 10.0 ppm by volume.

Description

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

[0001] The present invention relates to a high-strength galvanized steel sheet having excellent workability, which becomes increasingly important as tensile strength increases, and suitable for use as a building material or an automobile crash-resistant part, and a method for producing the same.

[0002] In recent years, there has been a strong demand for improved automobile crash safety and fuel efficiency, and the strength of steel sheets, which are used as parts materials, is increasing. Furthermore, as automobiles become more widespread on a global scale and are used for a variety of purposes in a wide variety of regions and climates, steel sheets, which are used as parts materials, are required to have high levels of corrosion prevention.

[0003] In general, hot-dip galvanized steel sheets are produced by using a thin steel sheet obtained by hot-rolling or cold-rolling a slab as a base material, and subjecting the base steel sheet to recrystallization annealing and hot-dip galvanizing treatment in an annealing furnace of a continuous hot-dip galvanizing line (hereinafter referred to as CGL). In the case of alloyed hot-dip galvanized steel sheets, they are produced by further subjecting the hot-dip galvanizing treatment to alloying treatment.

[0004] Here, the heating furnace types of the annealing furnace of a CGL include a direct fired furnace (DFF) type, a non-oxidizing furnace (NOF) type, and a radiant tube furnace (RTF) type. In recent years, the construction of CGLs equipped with all radiant tube heating furnaces, in which all heating furnaces are radiant furnaces, has been increasing for reasons such as ease of operation and reduced pick-up, which allows for the production of high-quality coated steel sheets at low cost. On the other hand, in the case of steel sheets containing easily oxidizable elements such as Si, Mn, and Cr, it is preferable to oxidize the steel under appropriate conditions immediately before reduction annealing. Unlike annealing furnaces in which a direct fired furnace or a non-oxidizing furnace is located upstream of the annealing furnace, an all-radiant tube annealing furnace does not have an oxidation step immediately before reduction annealing. Therefore, it is disadvantageous in terms of ensuring platability for steel sheets containing easily oxidizable elements such as Si, Mn, and Cr.

[0005] As a method for manufacturing a hot-dip galvanized steel sheet using a high-strength steel sheet containing a large amount of Si and Mn as a base material, Patent Documents 1 and 2 disclose a technique for internally oxidizing the surface layer of the base steel by increasing the dew point during the heating process in an annealing furnace. However, the techniques described in Patent Documents 1 and 2 assume that the area for controlling the dew point is the entire furnace interior, making it difficult to control the dew point and to achieve stable operation. Furthermore, when an alloyed hot-dip galvanized steel sheet is manufactured under unstable dew point control, variations in the distribution of internal oxides formed on the base steel sheet are observed, raising concerns about the occurrence of defects such as uneven plating wettability and uneven alloying in the longitudinal and width directions of the steel sheet.

[0006] In addition, Patent Document 3 discloses a method for producing a gas containing an oxidizing gas, H ２ O and O ２ Not only that, CO ２ A technique has been disclosed in which the Si concentration is also specified to internally oxidize the surface layer of the base steel immediately before plating, thereby suppressing external oxidation and improving the plating appearance. However, when a particularly large amount of Si is contained, as in Patent Document 3, the presence of internal oxides makes it easier for cracks to occur on the surface of the base steel sheet during processing, and the plating peeling resistance is deteriorated. Furthermore, deterioration of corrosion resistance is also observed. ２ However, there are concerns that problems such as contamination inside the furnace and carburization of the steel sheet surface may occur, resulting in changes in mechanical properties.

[0007] Furthermore, in recent years, high-strength hot-dip galvanized steel sheets and high-strength galvannealed steel sheets have been increasingly applied to locations that are subject to severe processing (hereinafter referred to as "severe processing"), and resistance to coating peeling during severe processing has become increasingly important. Specifically, when a coated steel sheet is bent at an angle exceeding 90°, or when the steel sheet is subjected to processing due to impact, it is required to suppress coating peeling in the processed portion.

[0008] In addition, if a steel sheet contains a large amount of Si, there is a concern that, during resistance welding, residual stress is generated near the weld, and zinc in the coating layer melts and diffuses into the grain boundaries, causing liquid metal embrittlement (LME), which may lead to intergranular cracking (LME cracking) in the steel sheet. In particular, when welding is performed with the welding electrode at a large inclination relative to the steel sheet surface, the residual stress increases, potentially causing cracking. Since residual stress is thought to increase as the strength of the steel sheet increases, there is a concern that LME cracking may occur as the strength of the steel sheet increases.

[0009] To achieve these properties, it is necessary not only to ensure the desired steel sheet structure, but also to more precisely control the structure and structure of the surface layer of the base steel directly below the coating layer, which has the potential to become the starting point for cracks during severe working, and to control the amount of Si added to the steel within a range that allows resistance weld cracking resistance (hereinafter also referred to as "LME cracking resistance"). However, such control is difficult with conventional technology, and it has not been possible to produce hot-dip galvanized steel sheets that have excellent coating adhesion and LME cracking resistance during severe working using Si-containing high-strength steel sheets as base materials in a CGL equipped with an all-radiant tube type heating furnace in its annealing furnace.

[0010] Republished No. 2014-102901 Patent Publication No. 2014-525986 Patent Publication No. 2006-233333

[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, which uses a steel sheet containing Si, Mn, and Cr as a base material and is manufactured in a CGL equipped with an all-radiant tube type heating furnace in an annealing furnace, and which has excellent coating appearance, coating adhesion during severe working, and LME cracking resistance, and a method for manufacturing the same.

[0012] The term "high strength" used in the high-strength galvanized steel sheet of the present invention means that the tensile strength is 780 MPa or more.

[0013] From the viewpoint of ensuring galvanic properties, it is effective to provide an oxidation process immediately before annealing in a heating furnace. However, there are issues with coating peelability and corrosion resistance during processing, and from the viewpoint of cost and operability, ensuring galvanic properties in an all-radiant tube heating furnace is highly desired. In response to this, conventionally, the dew point is raised by simply increasing the water vapor partial pressure throughout the annealing furnace, thereby oxidizing the interior of the surface layer of the steel sheet. During this process, surface diffusion and surface oxidation of easily oxidizable elements in the steel (hereinafter also referred to as surface enrichment) also occur simultaneously with internal oxidation. Therefore, in order to ensure the above-mentioned coating appearance and coating adhesion, it is necessary to efficiently suppress the surface enrichment. Therefore, the present inventors have intensively studied factors related to internal oxidation and surface enrichment, and have found that the presence of trace amounts of SO in the atmosphere during annealing can reduce the surface oxidation. ２ It has been found that it is extremely important to contain corrosive gases such as HCl and to control the dew point in the temperature range of 700°C or higher.

[0014] Furthermore, the present inventors investigated the required dew point at various steel sheet annealing temperatures, and as a result, they found that, when the maximum temperature of the steel sheet in the atmosphere inside the heating furnace is T°C, controlling the dew point from 700°C or higher to T°C or lower to -20°C or lower, and controlling the Cr concentration in addition to the Si concentration and Mn concentration in the steel, is effective in promoting oxidation of the surface layer of the steel sheet within a depth of 100 μm from the surface of the base steel sheet toward the center of the sheet thickness (hereinafter, this may be referred to as internal oxidation) and suppressing surface segregation.

[0015] Here, the reason why the maximum temperature T°C of the steel sheet in the atmosphere inside the heating furnace is set to 900°C or less is that if the maximum temperature T°C exceeds 900°C, it becomes difficult to suppress the surface segregation of Si, Mn, and Cr, and in addition, internal oxidation becomes excessive, which deteriorates the surface appearance and the plating adhesion during processing.

[0016] In addition, it was also clarified that LME cracking resistance can be improved by controlling the Si and Mn concentrations in the steel within appropriate ranges.

[0017] By subjecting a steel sheet having a predetermined composition to such treatment, selective surface oxidation of easily oxidizable elements such as Si, Mn, and Cr can be suppressed, and surface segregation of these elements can be suppressed, resulting in a high-strength hot-dip galvanized steel sheet that is excellent in coating appearance, coating adhesion during severe working, and LME cracking resistance. Excellent coating appearance means an appearance in which no bare areas or alloying irregularities are observed.

[0018] The high-strength hot-dip galvanized steel sheet obtained by the above method has an oxygen content of 0.030 g / m per side in the surface layer of the steel sheet immediately below the galvanized layer and within 100 μm from the surface of the base steel sheet. ２ 0.40g / m or more ２ The structure is as follows: Furthermore, the maximum length of internal oxides present in the surface layer portion of the steel sheet is 6.0 μm or less, and the number of internal oxides present in the surface layer portion of the steel sheet and having a length of 1.0 μm or more is 20 or less per 100 μm of the surface layer portion of the steel sheet in the width direction of the steel sheet. This makes it possible to achieve stress relaxation and crack prevention during bending in the surface layer of the base steel, and results in excellent coating appearance and coating adhesion during severe bending. The present invention is based on the above findings and has the following features. [1] A steel sheet having a composition containing, by mass%, C: 0.060% or more and 0.250% or less, Si: 0.10% or more and 0.80% or less, Mn: 1.50% or more and 3.50% or less, P: 0.020% or less, S: 0.0100% or less, Al: 0.100% or less, N: 0.0060% or less, and Cr: 1.0% or less, wherein the mass ratio of (Si+Cr) to Mn ((Si+Cr) / Mn) is 0.25 or more, the mass ratio of Si to Mn (Si / Mn) is less than 0.25, and the balance is Fe and unavoidable impurities, and a plating coating weight per side of 20 g / m ２ 120g / m or more ２A method for producing a high-strength hot-dip galvanized steel sheet having the following zinc coating layer, wherein, when a steel sheet is annealed and hot-dip galvanized in a continuous hot-dip galvanizing facility, the maximum temperature T of the steel sheet in an annealing heating furnace is higher than 700°C and not higher than 900°C, the dew point of the atmosphere in the heating furnace in a temperature range of the steel sheet temperature from 700°C to T°C is -20°C or higher, and the atmosphere in the heating furnace contains 3.0 vol% to 20.0 vol% hydrogen and 0.1 volppm to 3.0 volppm SO ２and 0.5 vol ppm to 10.0 vol ppm of HCl. [2] The method for producing a high-strength hot-dip galvanized steel sheet according to [1], wherein the steel sheet further contains, in mass %, one or more elements selected from the group consisting of Group A to Group E below. Group A: one or more of Ti, Nb, V, W, and Zr, in total, 0.200% or less Group B: one or more of Mo, Cu, Co, and Ni, in total, 0.01% or more and 0.5% or less Group C: B, 0.0003% or more and 0.0050% or less Group D: one or more of Sb and Sn, in total, 0.001% or more and 0.200% or less Group E: one or more of Ca, Mg, and REM, in total, 0.0001% or more and 0.0005% or less [3] The high-strength hot-dip galvanized steel sheet according to [1] or [2], wherein the steel sheet further contains, as the chemical composition, one or more groups selected from the following groups F to I, in mass %: Group F: Ta: 0.10% or less (not including 0%) Group G: One or more selected from Te: 0.10% or less (not including 0%), As: 0.10% or less (not including 0%), Hf: 0.10% or less (not including 0%) Group H: One or more selected from Bi: 0.20% or less (not including 0%), Pb: 0.20% or less (not including 0%) Group I: One or more selected from Zn: 0.10% or less (not including 0%), Ge: 0.10% or less (not including 0%), Sr: 0.10% or less (not including 0%), Cs: 0.10% or less (not including 0%) [4] A steel sheet having a composition containing, by mass%, C: 0.060% or more and 0.250% or less, Si: 0.10% or more and 0.80% or less, Mn: 1.50% or more and 3.50% or less, P: 0.020% or less, S: 0.0100% or less, Al: 0.100% or less, N: 0.0060% or less, and Cr: 1.0% or less, wherein the mass ratio of (Si+Cr) to Mn ((Si+Cr) / Mn) is 0.25 or more, the mass ratio of Si to Mn (Si / Mn) is less than 0.25, and the balance is Fe and unavoidable impurities, and wherein a plating coating weight per side is 20 g / m ２ 120g / m or more ２The oxygen content of the surface layer of the steel sheet directly under the zinc-coated layer within a depth of 100 μm from the surface of the base steel sheet in the direction of the center of the sheet thickness is 0.030 g / m per side ２ 0.40g / m or more ２ [5] A high-strength hot-dip galvanized steel sheet according to [4], wherein in a cross section of the steel sheet, the maximum length of internal oxides present in the surface layer portion of the steel sheet is 6.0 μm or less, and the number of internal oxides present in the surface layer portion of the steel sheet and having a length of 1.0 μm or more is 20 or less per 100 μm of the surface layer portion of the steel sheet in the width direction of the steel sheet. Group A: one or more of Ti, Nb, V, W, and Zr, in total, 0.200% or less Group B: one or more of Mo, Cu, Co, and Ni, in total, 0.01% or more and 0.5% or less Group C: B, 0.0003% or more and 0.0050% or less Group D: one or more of Sb and Sn, in total, 0.001% or more and 0.200% or less Group E: one or more of Ca, Mg, and REM, in total, 0.0001% or more and 0.0005% or less [6] The high-strength hot-dip galvanized steel sheet according to [4] or [5], wherein the steel sheet further contains, as the chemical composition, one or more groups selected from the following groups F to I, in mass %: Group F: Ta: 0.10% or less (not including 0%) Group G: One or more selected from Te: 0.10% or less (not including 0%), As: 0.10% or less (not including 0%), Hf: 0.10% or less (not including 0%) Group H: One or more selected from Bi: 0.20% or less (not including 0%), Pb: 0.20% or less (not including 0%) Group I: One or more selected from Zn: 0.10% or less (not including 0%), Ge: 0.10% or less (not including 0%), Sr: 0.10% or less (not including 0%), Cs: 0.10% or less (not including 0%)

[0019] According to the present invention, a high-strength hot-dip galvanized steel sheet having excellent coating appearance, coating adhesion during severe working, and LME cracking resistance can be obtained.

[0020] Fig. 1 is a structural diagram of a test material for evaluating LME cracking resistance. The upper diagram in Fig. 2 is a plan view of a plate assembly with a welded portion, and the lower diagram is a drawing showing a cross section in the plate thickness direction after cutting the plate assembly with a welded portion at the cutting position shown in the upper diagram.

[0021] Hereinafter, embodiments of the present invention will be described, but the present invention is not limited to the following embodiments.

[0022] First, we will explain the annealing atmosphere conditions, which determine the structure of the substrate steel sheet surface directly below the coating layer, and are the most important requirement of the present invention. During the heating process of annealing, the dew point of the atmosphere is controlled to −20°C or higher when the steel sheet temperature in the heating furnace is in the temperature range of 700°C or higher to T°C or lower. Here, T°C is the maximum temperature of the atmosphere in the annealing furnace, and 700°C < T ≦ 900°C.

[0023] In order to perform a recrystallization treatment of the strain imparted by the usual cold rolling, the maximum temperature of the steel sheet during the heating process of annealing must exceed 700°C. On the other hand, if the temperature exceeds 900°C, it becomes difficult to suppress the surface segregation of Si, Mn, and Cr. In addition, excessive oxidation occurs in the surface layer of the steel sheet within a depth of 100 μm from the surface of the substrate steel sheet toward the center of the sheet thickness, degrading the surface appearance and the coating adhesion during processing. Therefore, the maximum temperature of the steel sheet must be above 700°C and not more than 900°C.

[0024] The temperature range in which the steel sheet temperature is 700°C or higher and T°C or lower (700°C<T≦900°C) during the heating process will be described.

[0025] In order to suppress the formation of surface oxides, it is most important to control the dew point at steel sheet temperatures of 700°C or higher, where the diffusion rate of elements is high. In this case, in order to efficiently internally oxidize Si, Mn, and Cr and suppress surface segregation, it is necessary to control the dew point to −20°C or higher in the temperature range of 700°C or higher and T°C or lower (700°C < T ≦ 900°C). If the dew point is lower than −20°C, suppression of surface segregation will be insufficient, resulting in deterioration of the coating appearance and coating adhesion. While there is no particular upper limit for the dew point, if the dew point is too high, the operating costs of the humidification equipment will increase. Therefore, the dew point is preferably +30°C or lower, more preferably lower than +20°C.

[0026] Here, in the temperature range below 700°C, the surface diffusion of easily oxidizable elements such as Si, Mn, and Cr is very small due to the low temperature, and surface segregation is suppressed (or does not occur). Therefore, the wettability of the molten zinc with the steel sheet is not impaired. Therefore, there is no need to particularly control the dew point in the temperature range below 700°C.

[0027] The hydrogen concentration in the heating furnace atmosphere during annealing must be 3.0 vol% or more and 20.0 vol% or less. If the hydrogen concentration in the heating furnace atmosphere in the temperature range of 700°C or more is too high, the amount of diffusible hydrogen remaining in the steel will be too high. As a result, hydrogen embrittlement will occur, resulting in impaired workability. On the other hand, if the hydrogen concentration in the heating furnace atmosphere is too low, the steel sheet surface will be insufficiently reduced, becoming inactive and leading to poor plating. For these reasons, the hydrogen concentration in the temperature range of 700°C or more is set to 3.0 vol% or more and 20.0 vol% or less.

[0028] The atmosphere in the heating furnace during annealing is 0.1 vol ppm or more and 3.0 vol ppm or less of SO ２ It is necessary to contain at least one of SO and HCl in an amount of 0.5 vol ppm or more and 10.0 vol ppm or less. Although the detailed reason is not clear, it is presumed that when these corrosive gases are present in appropriate amounts, they promote internal oxidation rather than surface oxidation of Si, Mn, and Cr, improving not only the coating adhesion but also the length of the internal oxides present in the surface layer of the steel sheet, as described below, thereby improving corrosion resistance after processing. The improving effect of these corrosive gases is similar to that of SO ２ This becomes apparent when the concentration of CO is 0.1 vol ppm or more and the concentration of HCl is 0.5 vol ppm or more. ２ If the SO 2 concentration exceeds 3.0 vol ppm and the HCl concentration exceeds 10.0 vol ppm, the deterioration of the furnace body inside the heating furnace may be accelerated. ２ In the case of HCl, the content must be 0.1 vol ppm or more and 3.0 vol ppm or less, and in the case of HCl, the content must be 0.5 vol ppm or more and 10.0 vol ppm or less. The balance of the atmosphere in the heating furnace during annealing is nitrogen, CO, CO ２ It may also contain gases such as

[0029] These trace amounts of SO ２The concentration of corrosive gases such as HCl and the like can be controlled by adjusting the amount of gas when the gas containing these corrosive gases is introduced directly into the furnace. ２ Applying a liquid containing SO4 or HCl, adjusting the amount, and further adjusting the H ２ SO ４ It can also be controlled by adjusting the concentration of SO and HCl. ２ It is important to control the concentration of corrosive gases such as SO and HCl. ２ The method for controlling the concentration of corrosive gases such as HCl is not limited to the above method.

[0030] Next, the steel composition of the high-strength hot-dip galvanized steel sheet to which the present invention is directed will be described. The composition is expressed in mass %.

[0031] C: 0.060% or more and 0.250% or less C is an element effective in increasing the strength of steel sheets. It contributes to this increase by forming martensite, a hard phase in the steel structure. Depending on the manufacturing method, C also contributes to increased strength by forming fine alloy compounds or alloy carbonitrides with carbide-forming elements such as Nb, Ti, V, and Zr. To achieve these effects, the C content is set to 0.060% or more. Furthermore, from the viewpoint of consistently achieving a tensile strength (TS) of 780 MPa or more, the C content is preferably set to 0.090% or more. On the other hand, if the C content exceeds 0.250%, the martensite becomes excessively hard, and even if inclusions and the amount of hydrogen in the steel are controlled, the bending workability tends to be unimproved. Therefore, the C content is set to 0.250% or less.

[0032] Si: 0.10% or more and 0.80% or less Si is an element that contributes to high strength mainly through solid solution strengthening. Its ductility declines relatively little relative to the increase in strength, contributing not only to strength but also to an improved balance between strength and ductility. Improved ductility leads to improved bendability. On the other hand, excessive Si addition expands the liquid phase stability region of zinc to the lower temperature side, thereby degrading LME cracking resistance and facilitating the formation of Si-based oxides on the steel sheet surface, which may result in ungalvanized areas. Therefore, it is sufficient to add only the amount necessary to ensure strength, and the Si content is set to 0.10% or more. Furthermore, from the viewpoints of LME cracking resistance and galvanizability, the Si content is set to 0.80% or less. Preferably, the Si content is set to 0.70% or less.

[0033] Mn: 1.50% or more and 3.50% or less Mn is an effective element that contributes to high strength through solid solution strengthening and martensite formation. To achieve this effect, the Mn content is set to 1.50% or more. Preferably, the Mn content is 1.80% or more. On the other hand, if the Mn content exceeds 3.50%, unevenness in the steel structure is likely to occur due to Mn segregation, leading to reduced workability. In addition, Mn is likely to oxidize externally as an oxide or composite oxide on the steel sheet surface, which may cause bare spots. Therefore, the Mn content is set to 3.50% or less.

[0034] P: 0.020% or less P is an effective element that contributes to increasing the strength of steel sheet through solid solution strengthening, but it also affects galvanizability. It particularly deteriorates wettability with steel sheet and slows the alloying rate of the coating layer, which has a significant effect on high-alloy systems used to obtain high-strength steel sheet. Therefore, the P content is set to 0.020% or less, and more preferably 0.010% or less. There is no particular lower limit, but a P content of less than 0.0001% leads to reduced production efficiency and increased dephosphorization costs during the manufacturing process, so the P content is preferably set to 0.0001% or more.

[0035] S: 0.0100% or less S tends to form sulfide-based inclusions in steel. In particular, when a large amount of Mn is added to increase strength, MnS-based inclusions tend to form. This not only impairs bendability, but also causes hot embrittlement, adversely affecting the manufacturing process, so it is preferable to reduce the S content as much as possible. In the present invention, the S content is set to 0.0100% or less. There is no particular lower limit, but an S content of less than 0.0001% leads to reduced production efficiency and increased costs in the manufacturing process, so it is desirable to set the S content to 0.0001% or more.

[0036] Al: 0.100% or less Al is added as a deoxidizer. To obtain this effect, a content of 0.001% or more is preferable. On the other hand, if the Al content exceeds 0.100%, inclusions are likely to form during the manufacturing process, deteriorating bendability. Therefore, the Al content should be 0.100% or less, preferably 0.080% or less as sol. Al in the steel.

[0037] N: 0.0060% or less If the N content exceeds 0.0060%, excess nitrides are formed in the steel, which reduces workability and may also lead to deterioration of the surface properties of the steel sheet. Therefore, the N content is set to 0.0060% or less, preferably 0.0050% or less. From the viewpoint of purifying the microstructure and improving ductility, it is preferable that the N content be as low as possible. However, this leads to reduced production efficiency and increased costs in the manufacturing process, so the N content is set to 0.0001% or more.

[0038] Cr: 1.0% or less Cr is an element that contributes to high strength by improving hardenability and facilitating the formation of martensite. Cr is added to adjust strength as a substitute for C, Si, and Mn. However, like Si, Cr easily forms Cr-based oxides on the steel sheet surface, which may cause unplated areas. Therefore, it is sufficient to add only the amount necessary to ensure strength, and from the perspective of platability, the Cr content is set to 1.0% or less. The Cr content is preferably 0.7% or less. Although there is no particular lower limit, the Cr content is set to 0.05% or more in order to stably control internal oxidation.

[0039] The above-mentioned composition may contain the following optional components. When the optional elements listed below are contained in an amount less than the lower limit listed below, the optional components are considered to be included as inevitable impurities.

[0040] Contains one or more elements selected from the following groups A to E, in mass percent: Group A: One or more of Ti, Nb, V, W, and Zr, totaling 0.200% or less Group B: One or more of Mo, Cu, Co, and Ni, totaling 0.01% to 0.5% Group C: B, 0.0003% to 0.0050% Group D: One or more of Sb and Sn, totaling 0.001% to 0.200% Group E: One or more of Ca, Mg, and REM, totaling 0.0001% to 0.0005% Ti, Nb, V, W, and Zr form carbides and nitrides (sometimes carbonitrides) with C and N. These fine precipitates contribute to increasing the strength of the steel plate. In particular, by precipitating in soft ferrite, the strength is increased, and the effect of reducing the difference in strength with martensite contributes to improving not only bendability but also stretch flangeability. Furthermore, these elements have the effect of refining the structure of hot-rolled coils, and by refining the steel structure after subsequent cold rolling and annealing, they also contribute to increasing strength and improving workability such as bendability. From the viewpoint of achieving this effect, it is preferable to contain at least one of Ti, Nb, V, W, and Zr in total at 0.005% or more. However, excessive addition increases the deformation resistance during cold rolling, inhibiting productivity, and the presence of excessive or coarse precipitates reduces the ductility of ferrite, thereby reducing the ductility and bendability of the steel sheet. For this reason, the upper limit of the total content of at least one of Ti, Nb, V, W, and Zr is set to 0.200%.

[0041] Mo, Cu, Co, and Ni are elements that contribute to high strength by improving hardenability and facilitating the formation of martensite. To achieve these effects, it is preferable to contain at least one of Mo, Cu, Co, and Ni in a total amount of 0.01% or more. Excessive addition of Mo, Cu, Co, and Ni leads to saturation of the effects and increased costs, and Cu induces cracks during hot rolling, causing surface defects. Therefore, it is preferable to limit the total amount of at least one of Mo, Cu, Co, and Ni to 0.5% or less. Note that Ni has the effect of suppressing the occurrence of surface defects due to the addition of Cu, so it is preferable to add it simultaneously when adding Cu. In particular, it is preferable to contain at least half the amount of Cu.

[0042] B is also an element that contributes to high strength by improving hardenability and facilitating the formation of martensite. A lower limit is set for B to obtain the effect of suppressing the formation of ferrite that occurs during the annealing and cooling process, and an upper limit is set because even if B is added in excess, the effect of increasing strength saturates and, instead, the hardenability becomes excessive, which can lead to disadvantages such as cracking of the weld during welding. Therefore, when B is contained, the content is preferably 0.0003% or more and 0.0050% or less.

[0043] Since Sb and Sn are elements that suppress decarburization, denitrification, deboronization, etc., and are effective in suppressing a decrease in the strength of the steel sheet, it is preferable to contain at least one of Sb and Sn in a total amount of 0.001% or more. However, excessive addition of these elements deteriorates the surface properties, so the upper limit is preferably set to 0.200% in total of the at least one element.

[0044] Adding small amounts of Ca, Mg, and REM has the effect of spheroidizing the shape of sulfides and improving the bendability of the steel sheet. On the other hand, adding excessive amounts of these elements causes excessive formation of sulfides and oxides in the steel, which reduces the workability, particularly the bendability, of the steel sheet. Therefore, it is preferable that the total content of one or more of Ca, Mg, and REM is 0.0005% or less. There is no particular lower limit for the content, but it is preferable that the total content of one or more of Ca, Mg, and REM is 0.0001% or more. The above-mentioned composition may further contain the following components as optional components. Group F Ta: 0.10% or less (not including 0%) Group G One or more selected from Te: 0.10% or less (not including 0%), As: 0.10% or less (not including 0%), Hf: 0.10% or less (not including 0%) Group H One or more selected from Bi: 0.20% or less (not including 0%), Pb: 0.20% or less (not including 0%) Group I One or more selected from Zn: 0.10% or less (not including 0%), Ge: 0.10% or less (not including 0%), Sr: 0.10% or less (not including 0%), Cs: 0.10% or less (not including 0%) Group F [Ta: 0.10% or less (not including 0%)] Ta, like the elements in Group A, is an effective element for increasing the strength of steel sheet, and may be added as needed. Although the effect of improving strength can be obtained by adding 0.005% or more of Ta, in order to prevent an increase in costs, when Ta is added, the Ta content is set to 0.10% or less. - Group G [one or more elements selected from Te: 0.10% or less (excluding 0%), As: 0.10% or less (excluding 0%), and Hf: 0.10% or less (excluding 0%)] Like the elements in Group C, Te, As, and Hf are all elements used to control the morphology of sulfides. - Te: 0.10% or less (excluding 0%) When Te is added at 0.001% or more, the morphology of sulfides can be controlled and ductility and toughness can be improved, but in order to prevent an increase in costs, when Te is added, the Te content is set to 0.10% or less.As: 0.10% or less (excluding 0%). The addition of 0.001% or more of As can control the morphology of sulfides and improve ductility and toughness. However, to prevent an increase in costs, the As content, if present, is set to 0.10% or less. Hf: 0.10% or less (excluding 0%). The addition of 0.01% or more of Hf can control the morphology of sulfides and improve ductility and toughness. However, to prevent an increase in costs, the Hf content, if present, is set to 0.10% or less. H group [one or more elements selected from Bi: 0.20% or less (excluding 0%) and Pb: 0.20% or less (excluding 0%)] Both Bi and Pb are elements that suppress grain boundary segregation and improve ductility and toughness. When Bi and Pb are contained, each of them is set to exceed 0%. Bi: 0.20% or less (excluding 0%) When Bi is contained in an amount of 0.001% or more, grain boundary segregation can be suppressed and ductility and toughness can be improved. Bi also has the effect of improving machinability and smoothness of cut edges, and has the effect of improving delayed fracture resistance of cut edges. When Bi is contained, the Bi content is set to 0.20% or less to prevent an increase in costs. Pb: 0.20% or less (excluding 0%) When Pb is contained in an amount of 0.001% or more, grain boundary segregation can be suppressed and ductility and toughness can be improved. Pb also has the effect of improving machinability and smoothness of cut edges, and has the effect of improving delayed fracture resistance of cut edges. When Pb is contained, the Pb content is set to 0.20% or less to prevent an increase in costs. Group I [one or more elements selected from Zn: 0.10% or less (excluding 0%), Ge: 0.10% or less (excluding 0%), Sr: 0.10% or less (excluding 0%), and Cs: 0.10% or less (excluding 0%)] Zn, Ge, Sr, and Cs are elements that increase strength without significantly affecting mechanical properties or surface quality. When Zn, Ge, Sr, or Cs is contained, each is set to be greater than 0%. Zn: 0.10% or less (excluding 0%) Even if Zn is contained in an amount of 0.001% or more, it does not significantly affect mechanical properties or surface quality. From the viewpoint of preventing an increase in costs, when Zn is contained, the Zn content is set to 0.10% or less.Ge: 0.10% or less (excluding 0%) Even if Ge is contained in an amount of 0.001% or more, it does not have a significant effect on mechanical properties or surface quality. To prevent an increase in costs, if Ge is contained, the Ge content is set to 0.10% or less. Sr: 0.10% or less (excluding 0%) Even if Sr is contained in an amount of 0.001% or more, it does not have a significant effect on mechanical properties or surface quality. To prevent an increase in costs, if Sr is contained, the Sr content is set to 0.10% or less. Cs: 0.10% or less (excluding 0%) Even if Cs is contained in an amount of 0.001% or more, it does not have a significant effect on mechanical properties or surface quality. To prevent an increase in costs, if Cs is contained, the Cs content is set to 0.10% or less.

[0045] In the steel sheet (hereinafter, sometimes referred to as base steel sheet, substrate steel sheet, base steel, or base steel), the balance other than the above-mentioned component composition is Fe and inevitable impurities.

[0046] The mass ratio of (Si + Cr) to Mn ((Si + Cr) / Mn) is 0.25 or more. To obtain excellent plating properties, it is important to control elements that are easily oxidized in steel (Cr has this effect in addition to Si). To suppress the external oxidation of Mn, it is necessary to form a composite oxide of Si and Cr with Mn inside the steel sheet. If the (Si + Cr) / Mn) ratio is less than 0.25, sufficient amounts of composite internal oxides of Si, Cr, and Mn are not formed in the surface layer of the steel sheet within 100 μm from the surface of the substrate steel sheet, causing surface concentration of these elements and resulting in plating defects. For this reason, the (Si + Cr) / Mn) ratio is set to 0.25 or more.

[0047] The mass ratio of Si to Mn (Si / Mn) is less than 0.25. Si increases strength with relatively little decrease in ductility, contributing to an improvement in the balance between strength and ductility. On the other hand, Si expands the liquidus stability region of zinc to lower temperatures, thereby degrading LME cracking resistance. However, by controlling the weight ratio of Si to Mn (Si / Mn) to less than 0.25, the deterioration of LME cracking resistance due to increased Si concentration can be mitigated. While the mechanism is unclear, it is believed that an increase in Mn concentration suppresses the expansion (shift) of the liquidus stability region of zinc to lower temperatures that occurs with increased Si concentration. Therefore, the Si / Mn ratio is set to less than 0.25.

[0048] The balance is Fe and unavoidable impurities.

[0049] The high-strength galvanized steel sheet of the present invention has a coating weight of 20 g / m on one side of the surface of the steel sheet. ２ 120g / m or more ２ It has a zinc plating layer of 20 g / m ２ If the density is less than 120 g / m, it becomes difficult to ensure corrosion resistance. ２ If it exceeds this limit, the plating adhesion will deteriorate.

[0050] Furthermore, the high-strength galvanized steel sheet of the present invention has an oxygen content of the surface layer of the steel sheet immediately below the galvanized layer within a depth of 100 μm from the surface of the substrate steel sheet toward the center of the sheet thickness, which is measured by the method described in the Examples, of 0.030 g / m per side. ２ 0.40g / m or more ２ From the viewpoint of obtaining fine inner oxides, it is desirable that the oxygen-containing oxides contain as a main component at least one element selected from Fe, Si, Mn, Al, P, B, Nb, Ti, Cr, Mo, and V. Furthermore, in the cross section of the steel sheet, the maximum length of the inner oxides present in the surface layer portion of the steel sheet is 6.0 μm or less, and the number of inner oxides present in the surface layer portion of the steel sheet and on grain boundaries having a length of 1.0 μm or more is 20 or less per 100 μm of the surface layer portion of the steel sheet in the width direction of the steel sheet.

[0051] In hot-dip galvanized steel sheets containing Si and a large amount of Mn added to the steel, in order to achieve good appearance and satisfactory coating adhesion during processing, it is necessary to control the amount and size of internal oxides in the surface layer of the steel substrate directly below the coating layer, which may become the starting point for cracks during severe processing. Therefore, in the present invention, dew point control was performed to control the oxygen potential of the atmosphere in the annealing furnace during the annealing process described below. By controlling the dew point and increasing the oxygen potential, easily oxidizable elements such as Si, Mn, and Cr are internally oxidized before coating. As a result, the activity of Si, Mn, and Cr in the surface layer of the steel substrate is reduced, suppressing external oxidation and leading to improved coating appearance and adhesion. This improvement effect is achieved when the oxygen content in the surface layer of the steel sheet within 100 μm from the surface of the substrate steel is 0.030 g / m per side. ２ The lower limit is 0.030 g / m per side. ２ On the other hand, if the oxygen content is 0.40 g / m per side, ２ If the amount exceeds 0.40 g / m, this effect saturates and in some cases it may cause cracks during severe processing. ２ Let's say.

[0052] After careful investigation of the conditions for internal oxides present in the surface layer of the steel sheet, which serve as crack initiation sites, it was found that internal oxides present in the surface layer of the steel sheet with a width of 0.1 μm or more and a length of 1.0 μm or more contribute to cracking, and that the longer the length, the more significant the impact of cracking. Therefore, in the cross section of the steel sheet, the maximum length of internal oxides present in the surface layer of the steel sheet is set to 6.0 μm or less. If the internal oxide length exceeds 6.0 μm, the internal oxide becomes a crack initiation site during working, significantly deteriorating adhesion and corrosion resistance after working. On the other hand, internal oxides with a width of less than 0.1 μm or a length of less than 1.0 μm have little effect on cracking during severe working, and therefore do not affect adhesion and corrosion resistance after working. There is no particular lower limit for the maximum length of internal oxides present in the surface layer of the steel sheet, but in the method of the present invention, it is approximately 0.3 μm, and preferably 0.3 μm or more.

[0053] Furthermore, in the cross section of the steel sheet, among the internal oxides present in the surface layer portion of the steel sheet, the number of internal oxides having a length of 1.0 μm or more is set to 20 or less per 100 μm of the surface layer portion in the width direction of the steel sheet. Even if the number of internal oxides having a length of 1.0 μm or more exceeds 20 per 100 μm of the surface layer portion in the width direction of the steel sheet, the internal oxides will become crack initiation points during processing, similar to when the maximum length of the internal oxides exceeds 6.0 μm, and adhesion and corrosion resistance after processing will deteriorate. Therefore, the number of internal oxides having a length of 1.0 μm or more is set to 20 or less per 100 μm of the surface layer portion in the width direction of the steel sheet. There is no particular lower limit specified, but if there are less than 4 internal oxides, the oxygen content of the surface layer portion of the steel sheet: 0.030 g / m per side ２ Since it is difficult to satisfy the above, it is desirable to have four or more.

[0054] Next, a method for producing a high-strength galvanized steel sheet of the present invention will be described. The production method of the present invention includes a casting step, a hot rolling step, a pickling step, a cold rolling step, an annealing step, and a galvanizing step. Each step will be described below.

[0055] The casting step is a step of casting a steel having the above-described chemical composition to produce a steel material. The steel used in the production method of the present invention is preferably cast under conditions in which the molten steel flow rate at the solidification interface in the vicinity of the mold meniscus is 16 cm / sec or more.

[0056] Production of Steel Material (Slab (Cast)) The steel used in the production method of the present invention is what is generally called a slab produced by a continuous casting method, but this is for the purpose of preventing macrosegregation of alloy components, and it may also be produced by an ingot casting method or a thin slab casting method.

[0057] From the viewpoint of controlling inclusions, continuous casting is preferably performed under conditions in which the molten steel flow rate at the solidification interface near the mold meniscus is 16 cm / sec or more. "Near the mold meniscus" refers to the interface between the powder used during continuous casting in the mold and the molten steel. In ingot casting, it is desirable to sufficiently float inclusions during solidification and discard the portion where the floated aggregates are used in the next process.

[0058] The hot rolling process is a process in which the steel material after the casting process is hot rolled.

[0059] After the steel slab has been produced, in addition to the conventional method of cooling it to room temperature and then reheating it, it is also possible to easily load the hot slab into a heating furnace without cooling it to near room temperature and then hot roll it, or to hot roll it immediately after slightly reheating it, or to hot roll it while maintaining the high temperature after casting.

[0060] There are no particular restrictions on the hot rolling method, but it is desirable to carry out the hot rolling under the following conditions.

[0061] The heating temperature of the steel slab is preferably in the range of 1100°C or higher and 1350°C or lower. This is because precipitates present in the steel slab tend to coarsen, which is disadvantageous when ensuring strength through precipitation strengthening, for example. Alternatively, the coarse precipitates may act as nuclei and adversely affect microstructure formation in the subsequent annealing process. Furthermore, scaling off bubbles and defects on the slab surface by heating reduces cracks and irregularities on the steel sheet surface, achieving a smooth steel sheet surface is beneficial for product quality. From this perspective, the slab heating temperature is specified. To achieve this effect, the slab heating temperature is preferably 1100°C or higher. On the other hand, if the slab heating temperature exceeds 1350°C, coarsening of austenite grains occurs, which coarsens the steel structure of the final product and reduces the strength and bendability of the steel sheet. Therefore, a preferred upper limit of the slab heating temperature is specified as 1350°C or lower.

[0062] In the hot rolling process, which includes rough rolling and finish rolling, steel slabs are generally made into sheet bars by rough rolling and then into hot-rolled coils by finish rolling, but depending on the mill capacity, etc., such divisions do not matter as long as the specified size is achieved.

[0063] The following hot rolling conditions are recommended:

[0064] Finish rolling temperature: 800°C or higher, 950°C or lower. Setting the finish rolling temperature to 800°C or higher aims to homogenize the structure obtained in the hot-rolled coil, thereby ensuring a uniform structure in the final product. A non-uniform structure leads to reduced bendability. On the other hand, a finish rolling temperature above 950°C increases the amount of oxide (scale) generated, roughening the interface between the base steel and the oxide, degrading the surface quality after pickling and cold rolling. Furthermore, coarsening of the grain size can lead to reduced strength and bendability of the steel sheet, similar to the case of coarsening of the structure of a steel slab.

[0065] In order to refine and homogenize the structure of the hot-rolled coil (hot-rolled sheet) after the hot rolling, cooling is started within 3 seconds after the end of finish rolling, and the hot-rolled coil (hot-rolled sheet) is cooled at an average cooling rate of 10 to 250°C / s in the temperature range of [finish rolling temperature] to [finish rolling temperature - 100]°C, and is preferably wound into a coil in the temperature range of 450 to 700°C.

[0066] The pickling process is a process in which the steel sheet after the hot rolling process is pickled. Scale is removed from the steel sheet surface by pickling. Pickling conditions may be set appropriately.

[0067] The cold rolling step is a step of cold rolling the steel sheet after the pickling step.

[0068] The cold rolling reduction is preferably 20% or more and 80% or less. A reduction of 20% or more allows a uniform and fine steel structure to be obtained in the subsequent annealing process, so a reduction of 20% or more is preferred. If the cold rolling reduction is less than 20%, the steel is likely to become coarse grained during annealing and also to have a non-uniform structure, which raises concerns about reduced strength and workability in the final product, as mentioned above. As for the upper limit, a high reduction not only reduces productivity due to the rolling load, but may also result in poor shape, so a reduction of 80% or less is preferred. Pickling may be performed after cold rolling.

[0069] In the annealing step, annealing is performed in a controlled atmosphere in the heating furnace as described above.

[0070] The galvanizing step is carried out, for example, by immersing the steel sheet in a hot-dip galvanizing bath. The hot-dip galvanizing treatment may be carried out by a conventional method, and the coating weight per side is adjusted to fall within the above range.

[0071] After the galvanizing treatment, the galvanized steel sheet may be subjected to alloying treatment as needed by holding the galvanized steel sheet in a temperature range of 450 to 580°C for about 1 to 60 seconds.

[0072] Molten steel having the chemical compositions shown in Tables 1-1 and 1-2 was melted in a converter and continuously cast into slabs. The chemical compositions are in mass percent, with the remainder consisting of Fe and unavoidable impurities. The slabs were heated to 1200°C and hot-rolled at a finish rolling temperature of 840°C and a coiling temperature of 550°C to form hot-rolled coils with a thickness of 2.8 mm. The hot-rolled coils were cold-rolled to a thickness of 1.6 mm with a cold-rolling reduction of 50%. The cold-rolled steel sheets were annealed in an annealing furnace atmosphere under the conditions shown in Table 2, cooled to 600°C at an average cooling rate of 3°C / sec to 520°C, held at 520°C for 50 seconds, and then galvanized to produce high-strength hot-dip galvanized steel sheets. All samples except No. 39 were then subjected to an alloying treatment.

[0073]

[0074]

[0075] Samples were taken from the plated steel sheets obtained as described above, and the appearances were visually observed to evaluate the platability (surface quality) and the plating characteristics. Furthermore, a tensile test was conducted to measure the tensile strength (TS). The evaluation methods were as follows.

[0076] (1) Oxygen Content of the Steel Sheet Surface Layer Within 100 μm Depth from the Surface of the Base Steel Sheet Directly Below the Coating Layer in the Center Direction of Sheet Thickness To measure the oxygen content directly below the coating layer, only the coating layer was stripped using a hydrochloric acid or alkaline solution containing an inhibitor to prevent dissolution of the base steel, and the oxygen content was then measured using the "Impulse Furnace-Infrared Absorption Method." However, since it is necessary to subtract the amount of oxygen contained in the raw material (i.e., the steel sheet before annealing), in the present invention, the surface layers on both sides of the high-strength steel sheet after continuous annealing are polished to a depth of 100 μm or more to measure the oxygen concentration in the steel, and this measured value is defined as the amount of oxygen contained in the raw material, OH. In addition, the oxygen concentration in the steel throughout the entire thickness direction of the high-strength steel sheet after continuous annealing is measured, and this measured value is defined as the amount of oxygen after oxidation, OI. Using the thus obtained amount of oxygen after oxidation, OI, of the steel sheet, and the amount of oxygen originally contained in the raw material, OH, the difference between OI and OH (= OI - OH) was calculated, and the value converted to the amount per unit area on one side (g / m ２ ) was taken as the oxygen content.

[0077] Furthermore, 10 mm x 10 mm specimens were cut from the cross section after plating removal, embedded in resin, and then mirror-finished to prepare specimens for cross-sectional observation. Using an SEM, a cross section of 20 μm in the width direction was observed at 5000x magnification, with five fields per level. The maximum length (μm) of the internal oxides in each observation field and the number of internal oxides with a length of 1.0 μm or more were measured, and the average values ​​were used as the maximum length of the internal oxides and the number of internal oxides with a length of 1.0 μm or more for that level. Only oxides with an oxide width of 0.1 μm or more were counted for the number of internal oxides.

[0078] (2) Surface Properties (Appearance) The appearance of the produced hot-dip galvanized steel sheets was visually observed, and those with no unplated defects were rated as "○: Pass, Excellent", those with unplated defects were rated as "×: Fail", and those with no unplated defects but uneven plating appearance were rated as "△: Pass, Fair". Note that unplated defects refer to areas with a diameter of 50 μm or more where no plating is present and the steel sheet is exposed. The results obtained are also shown in Table 2.

[0079] (3) Coating Adhesion [Alloyed Hot-Dip Galvanized Steel Sheet] In this example, a hot-dip galvanized steel sheet was bent 90°, and cellophane tape was pressed against the processed section to transfer the peeled material to the cellophane tape. The amount of peeled material on the cellophane tape was measured as the Zn count using an X-ray fluorescence method. Measurement conditions were a mask diameter of 30 mm, a fluorescent X-ray acceleration voltage of 50 kV, an acceleration current of 50 mA, and a measurement time of 20 seconds. In particular, considering the possibility of in-plane unevenness (variation) in adhesion, adhesion was measured at five locations (30 locations in total) along a 6-m longitudinal length of the manufactured hot-dip galvanized steel sheet, every 1 m along the longitudinal direction of the coil: ¼, ½ (center), ¾, and 50 mm from the edge of the steel sheet. The highest Zn count among these was used to evaluate the galvanizability according to the following criteria. In this invention, the following rankings, ⊚ or ∘, were considered acceptable. ⊚ (Pass, better): Zn count less than 6,000. ◯ (pass, excellent): The Zn count is 6000 or more and less than 8000. × (fail): The Zn count is 8000 or more.

[0080] [Unalloyed hot-dip galvanized steel sheet] The coating adhesion of the hot-dip galvanized steel sheet was evaluated by a ball impact test. After the steel sheet was attached to a mold having holes of 3 / 8 and 1 / 2 inch diameter under the conditions of a ball weight of 2.8 kg and a drop height of 1 m, the ball impact test was carried out, tape was peeled off the processed area, and the presence or absence of peeling of the coating layer was visually judged and marked according to the following criteria. Here, ２ Peeling below the level was rated as minor peeling, and anything greater than that was rated as minor peeling. ⊚ (pass, better): No peeling of the plating layer under any of the conditions ◯ (pass, excellent): Minor peeling with 3 / 8 inch diameter △ (fail): Peeling with 3 / 8 inch diameter, no plating peeling with 1 / 2 inch diameter × (fail): Peeling of the plating under any of the conditions (4) Corrosion resistance after processing Test pieces were prepared by the same processing as in the plating peel resistance test, without tape peeling, and were tested under the following standard conditions using a degreasing agent: FC-E2011, a surface conditioner: PL-X, and a chemical conversion coating agent: Palbond (registered trademark) PB-L3065, all manufactured by Nihon Parkerizing Co., Ltd., until a chemical conversion coating adhesion weight of 1.7 to 3.0 g / m was achieved. ２<Standard conditions> Degreasing step: treatment temperature of 40°C, treatment time of 120 seconds Spray degreasing, surface adjustment step: pH 9.5, treatment temperature of room temperature, treatment time of 20 seconds Chemical conversion treatment step: temperature of chemical conversion treatment solution of 35°C, treatment time of 120 seconds The surfaces of the test pieces that had been subjected to the above chemical conversion treatment were electrodeposited using electrodeposition paint V-50 manufactured by Nippon Paint Co., Ltd. to a film thickness of 25 μm, and then subjected to the following corrosion test. <Salt Spray Test (SST)> For the galvannealed steel sheets of the test pieces subjected to chemical conversion treatment and electrodeposition coating, cut scratches reaching the plating were made with a cutter on the surface of the bent portion and on the ball impact portion of the galvannealed steel sheets. Then, these test pieces were subjected to a 240-hour salt spray test using a 5 mass% NaCl aqueous solution in accordance with the neutral salt spray test specified in JIS Z2371:2000. Then, a tape peel test was performed on the crosscut scratches, and the maximum peel width on both sides of the cut scratches was measured. Symbols (◎, ○, ×) were assigned according to the following criteria. If this maximum peel width was 2.0 mm or less, the corrosion resistance in the salt spray test could be evaluated as good. ◎: The maximum overall width of the bulge from the cut flaw is 2.0 mm or less (good) ○: The maximum overall width of the bulge from the cut flaw is more than 2.0 mm and 2.5 mm or less (pass) ×: The maximum overall width of the bulge from the cut flaw is more than 2.5 mm (fail) (5) Evaluation of LME cracking resistance Test pieces 2 were cut out from the hot-dip galvanized steel sheet to a length of 150 mm in the longitudinal direction and 50 mm in the transverse direction, with the direction perpendicular to the rolling (TD) as the longitudinal direction and the rolling direction as the transverse direction. Test pieces 2 were cut out to the same size, and the coating weight of the hot-dip galvanized layer per side was 50 g / m ２A test specimen 2 was stacked with a test hot-dip galvanized steel sheet (thickness: 1.6 mm, TS: 980 MPa class) 1 to form a sheet assembly. This sheet assembly was assembled so that the hot-dip galvanized layer of the test specimen 2 was aligned with the hot-dip galvanized layer surface of the commercially available hot-dip galvanized steel sheet. As shown in Figure 1 , this sheet assembly was fixed to a fixing base 5 via a 2.0 mm thick spacer 3 at a 5° inclination, the maximum inclination expected for some part shapes. The spacer 3 was a pair of steel plates measuring 50 mm in the longitudinal direction, 45 mm in the transverse direction, and 2.0 mm thick. The longitudinal end faces of each of the pair of steel plates were aligned with both transverse end faces of the sheet assembly. Therefore, the distance between the pair of steel plates constituting the spacer 3 was 60 mm. The fixing base 5 was a single plate with a hole in the center.

[0081] Next, using a servomotor-operated single-phase AC (50 Hz) resistance welding machine, the sheet assembly was pressed with a pair of electrodes 4 (tip diameter: 6 mm) while being deflected. Resistance welding was performed under conditions of a pressure of 3.5 kN, a hold time of 0.10 or 0.16 seconds, and a welding current and welding time such that the nugget diameter 7 of the weld was 5.9 mm (i.e., the welding current and welding time were appropriately adjusted for each sheet assembly so that the nugget diameter 7 was 5.9 mm). This produced a sheet assembly with a weld. The pair of electrodes 4 pressed the sheet assembly from above and below in the vertical direction, and the lower electrode 4 pressed the test piece through a hole in the fixture 5. During pressing, the lower electrode 4 of the pair of electrodes 4 was fixed to the fixture 5 so that the lower electrode 4 was in contact with a plane extending from the surface where the spacer 3 and the fixture 5 met, and the upper electrode 4 was movable. The upper electrode 4 was also in contact with the center of the test hot-dip galvanized steel sheet 1.

[0082] The hold time refers to the time from when the welding current is finished flowing to when the electrodes start to be released. The nugget diameter 7 refers to the distance between the ends of the nugget in the longitudinal direction of the sheet assembly, as shown in Figure 2.

[0083] Next, as shown in Figure 2, the sheet assembly with the weld was cut to include the weld (nugget), and the cross section of the weld was observed with an optical microscope (200x magnification). The resistance weld crack resistance properties of the weld were evaluated according to the following criteria. Here, the upper diagram in Figure 2 is a plan view of the sheet assembly with the weld, and the cutting position is indicated by line segment 8. The lower diagram in Figure 2 is a drawing showing a cross section in the sheet thickness direction of the sheet assembly after cutting, and schematically shows cracks (cracking) that occurred in the test specimen. Note that if cracks occur in the test hot-dip galvanized steel sheet 1, the stress in the test specimen 2 will be dispersed, preventing an appropriate evaluation. For this reason, data in which no cracks occurred in the test hot-dip galvanized steel sheet 1 was used as an example.

[0084] If the evaluation below was "◯" or "◎", the resistance weld crack resistance characteristics of the welded part were judged to be good and excellent, respectively, and if it was "×", the resistance weld crack resistance characteristics of the welded part were judged to be poor.

[0085] ⊚: No cracks of 0.1 mm or more in length were observed with a hold time of 0.10 seconds.

[0086] ◯: Cracks of 0.1 mm or more in length are observed at a hold time of 0.10 seconds, but no cracks of 0.1 mm or more in length are observed at a hold time of 0.16 seconds.

[0087] ×: A crack of 0.1 mm or more in length was observed at a hold time of 0.16 seconds.

[0088] (6) Tensile Test JIS No. 5 tensile test pieces (JIS Z2201) were taken from the plated steel sheets in the direction perpendicular to the rolling direction, and tensile tests were performed at a constant tensile speed (crosshead speed) of 10 mm / min. The tensile strength was determined by dividing the maximum load in the tensile test by the initial cross-sectional area of ​​the parallel part of the test piece. The thickness of the plated steel sheets used in calculating the cross-sectional area of ​​the parallel part was the thickness including the plating.

[0089] (7) Analysis method of furnace gas The furnace gas was collected from the annealing furnace and analyzed by ion chromatography. ２ The analysis was carried out three times, and the average value was used as the concentration of the gas inside the furnace.

[0090]

[0091] As is clear from Table 2, the hot-dip galvanized steel sheets produced by the method of the present invention are high-strength steel sheets containing easily oxidizable elements such as Si, Mn, and Cr, yet they also have good coating appearance and are excellent in coating adhesion and LME cracking resistance. On the other hand, the comparative examples are inferior in at least one of coating appearance, coating adhesion, LME cracking resistance, and tensile strength.

[0092] The high-strength hot-dip galvanized steel sheet of the present invention is excellent in coating appearance, coating adhesion, and LME cracking resistance, and can be used as a surface-treated steel sheet for reducing the weight and increasing the strength of automobile bodies. In addition to automobiles, the steel sheet can also be used in a wide range of fields, such as home appliances and building materials, as a surface-treated steel sheet that imparts rust prevention properties to base steel sheets.

[0093] REFERENCE SIGNS LIST 1 Test hot-dip galvanized steel sheet 2 Test piece 3 Spacer 4 Electrode 5 Fixing base 6 Nugget 7 Nugget diameter 8 Line indicating cutting position 9 Crack (fracture)

Claims

1. A steel sheet having a composition containing, by mass%, C: 0.060% or more and 0.250% or less, Si: 0.10% or more and 0.80% or less, Mn: 1.50% or more and 3.50% or less, P: 0.020% or less, S: 0.0100% or less, Al: 0.100% or less, N: 0.0060% or less, and Cr: 1.0% or less, with a mass ratio of (Si+Cr) to Mn ((Si+Cr) / Mn) being 0.25 or more, a mass ratio of Si to Mn (Si / Mn) being less than 0.25, and the balance being Fe and unavoidable impurities, with a coating weight of 20 g / m per side on the surface of the steel sheet. ２ 120g / m or more ２ A method for producing a high-strength hot-dip galvanized steel sheet having the following zinc-coated layer, comprising the steps of: when subjecting a steel sheet to annealing and hot-dip galvanizing treatment in a continuous hot-dip galvanizing facility, the maximum temperature T of the steel sheet in an annealing heating furnace is higher than 700°C and not higher than 900°C, the dew point of the atmosphere in the heating furnace in a temperature range in which the steel sheet temperature is 700°C or higher and T°C or lower is -20°C or higher, and the atmosphere in the heating furnace contains hydrogen of 3.0 vol% or higher and 20.0 vol% or lower and SO of 0.1 volppm or higher and 3.0 volppm or lower ２ and 0.5 vol ppm or more and 10.0 vol ppm or less of HCl.

2. The method for producing a high-strength hot-dip galvanized steel sheet according to claim 1, wherein the steel sheet further contains, in mass%, one or more elements selected from the following groups A to E: Group A: one or more of Ti, Nb, V, W, and Zr, total of 0.200% or less; Group B: one or more of Mo, Cu, Co, and Ni, total of 0.01% to 0.5%; Group C: B, 0.0003% to 0.0050%; Group D: one or more of Sb and Sn, total of 0.001% to 0.200%; Group E: one or more of Ca, Mg, and REM, total of 0.0001% to 0.0005%.

3. The method for producing a high-strength hot-dip galvanized steel sheet according to claim 1 or 2, wherein the steel sheet further contains, in mass %, one or more elements selected from the following groups F to I as the chemical composition: Group F: Ta: 0.10% or less (excluding 0%) Group G: One or more elements selected from Te: 0.10% or less (excluding 0%), As: 0.10% or less (excluding 0%), Hf: 0.10% or less (excluding 0%) Group H: One or more elements selected from Bi: 0.20% or less (excluding 0%), Pb: 0.20% or less (excluding 0%) Group I: One or more elements selected from Zn: 0.10% or less (excluding 0%), Ge: 0.10% or less (excluding 0%), Sr: 0.10% or less (excluding 0%), Cs: 0.10% or less (excluding 0%) 4. A steel sheet having a composition containing, by mass%, C: 0.060% to 0.250%, Si: 0.10% to 0.80%, Mn: 1.50% to 3.50%, P: 0.020% or less, S: 0.0100% or less, Al: 0.100% or less, N: 0.0060% or less, and Cr: 1.0% or less, with a mass ratio of (Si+Cr) to Mn ((Si+Cr) / Mn) of 0.25 or more, a mass ratio of Si to Mn (Si / Mn) of less than 0.25, and the balance consisting of Fe and unavoidable impurities, with a coating weight of 20 g / m per side on the surface of the steel sheet. ２ 120g / m or more ２ The oxygen content of the steel sheet surface layer within a depth of 100 μm from the surface of the base steel sheet directly under the zinc-coated layer in the sheet thickness center direction is 0.030 g / m per side ２ 0.40g / m or more ２ in a cross section of the steel sheet, a maximum length of internal oxides present in a surface layer portion of the steel sheet is 6.0 μm or less, and the number of internal oxides present in the surface layer portion of the steel sheet and having a length of 1.0 μm or more is 20 or less per 100 μm of the surface layer portion of the steel sheet in the width direction of the steel sheet.

5. The high-strength hot-dip galvanized steel sheet according to claim 4, further comprising, in mass%, one or more elements selected from the following groups A to E: Group A: at least one of Ti, Nb, V, W and Zr, total of 0.200% or less, Group B: at least one of Mo, Cu, Co and Ni, total of 0.01% to 0.5% or less, Group C: B, 0.0003% to 0.0050%, Group D: at least one of Sb and Sn, total of 0.001% to 0.200%, Group E: at least one of Ca, Mg and REM, total of 0.0001% to 0.0005%.

6. The high-strength hot-dip galvanized steel sheet according to claim 4 or 5, wherein the steel sheet further contains, as the chemical composition, one or more elements selected from the following groups F to I, in mass %: Group F: Ta: 0.10% or less (excluding 0%) Group G: One or more elements selected from Te: 0.10% or less (excluding 0%), As: 0.10% or less (excluding 0%), Hf: 0.10% or less (excluding 0%) Group H: One or more elements selected from Bi: 0.20% or less (excluding 0%), Pb: 0.20% or less (excluding 0%) Group I: One or more elements selected from Zn: 0.10% or less (excluding 0%), Ge: 0.10% or less (excluding 0%), Sr: 0.10% or less (excluding 0%), Cs: 0.10% or less (excluding 0%)