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

Abstract

We provide high-strength hot-dip galvanized steel sheets that have excellent coating appearance, coating adhesion during heavy processing, and LME cracking resistance. The steel sheet contains specified steel components, with a ((Si+Cr) / Mn) ratio of 0.25 or more, a (Si / Mn) ratio of less than 0.25, and the balance consisting of Fe and unavoidable impurities. The surface of the steel sheet is coated with a coating weight of 20 g / m2 per side. 2 More than 120g / m 2 A method for producing a high-strength hot-dip galvanized steel sheet having the following zinc coating layer, wherein the maximum temperature T of the steel sheet in an annealing furnace is higher than 700°C but not higher than 900°C, the dew point of the atmosphere in the heating furnace in the 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 hydrogen from 3.0 vol% to 20.0 vol% and one or more elements selected from the group consisting of SO2 from 0.1 volppm to 3.0 volppm and HCl from 0.5 volppm to 10.0 volppm.

Description

[Technical Field]

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

[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, high levels of corrosion prevention are required for steel sheets, which are used as parts materials.

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

[0004] Here, the heating furnace types of CGL annealing furnaces include direct-fired furnaces (DFF), non-oxidizing furnaces (NOF), and radiant tube furnaces (RTF). 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 due to the ease of operation and the reduced occurrence of pick-up, which allows for the production of high-quality coated steel sheets at low cost. On the other hand, for 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 with direct-fired or non-oxidizing furnaces installed upstream of the annealing furnace, all-radiant tube annealing furnaces do not include an oxidation step immediately before reduction annealing. Therefore, they are disadvantageous in terms of ensuring galvanizability for steel sheets containing easily oxidizable elements such as Si, Mn, and Cr.

[0005] Patent Documents 1 and 2 disclose a technique 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, in which the surface layer of the base steel is internally oxidized 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 in which the dew point is controlled is the entire furnace interior, making it difficult to control the dew point and to achieve stable operation. Furthermore, when a galvannealed 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 transverse directions of the steel sheet.

[0006] Patent Document 3 also discloses a technology that, by simultaneously specifying the concentration of oxidizing gases H2O and O2, as well as CO2, internally oxidizes the surface layer of the steel substrate immediately before plating, 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 the surface of the substrate steel sheet more susceptible to cracking during processing, resulting in a deterioration in plating peel resistance. Corrosion resistance has also been observed to deteriorate. Furthermore, CO2 can cause problems such as furnace contamination and carburization of the steel sheet surface, 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 used in areas that require severe processing (hereinafter referred to as "severe processing"), and resistance to coating peeling during severe processing has become increasingly important. Specifically, when coated steel sheets are bent at angles greater than 90°, or when the steel sheets are subjected to processing such as impact, it is necessary to suppress coating peeling in the processed areas.

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

[0009] To achieve these properties, it is necessary not only to ensure the desired steel sheet microstructure, but also to more precisely control the microstructure and structure of the surface layer of the steel immediately below the coating layer, which has the potential to become the initiation 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 with 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. [Prior art documents] [Patent documents]

[0010] [Patent Document 1] Republished Publication No. 2014-102901 [Patent Document 2] Special Publication No. 2014-525986 [Patent Document 3] Japanese Patent Application Laid-Open No. 2006-233333 Summary of the Invention [Problem to be solved by the invention]

[0011] The present invention has been made in view of the above circumstances, and aims 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 annealing furnace equipped with an all-radiant tube type heating 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. [Means for solving the problem]

[0013] From the viewpoint of ensuring galvanic properties, it is effective to provide an oxidation process immediately before annealing in a heating furnace. However, due to issues with coating peelability and corrosion resistance during processing, and also from the viewpoint of cost and operability, ensuring galvanic properties in an all-radiant tube heating furnace is highly desirable. In response to this, conventional methods simply increase the water vapor partial pressure throughout the annealing furnace to raise the dew point and oxidize the inner 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, to ensure the above-mentioned coating appearance and coating adhesion, it is necessary to efficiently suppress this surface enrichment. Therefore, the present inventors have extensively investigated the factors related to internal oxidation and surface enrichment and have found that it is extremely important to include trace amounts of corrosive gases, such as SO2 and HCl, in the annealing atmosphere and further control the dew point at temperatures above 700°C.

[0014] Furthermore, the inventors investigated the required dew point at various steel sheet annealing temperatures and 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 and Mn concentrations in the steel, are 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 internal oxidation also 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 the 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 deformation, and LME cracking resistance. Excellent coating appearance refers to an appearance in which no bare areas or alloying irregularities are visible.

[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 directly below the zinc coating layer, within 100 μm from the surface of the base steel sheet. 2 More than 0.40g / m 2 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 length in the width direction of the steel sheet in the surface layer portion 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 coating weight of 20 g / m per side on the surface of a steel sheet having a chemical 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 and a mass ratio of Si to Mn (Si / Mn) of less than 0.25, with the remainder consisting of Fe and unavoidable impurities. 2 More than 120g / m 2 A method for producing a high-strength hot-dip galvanized steel sheet having the following zinc plating layer, A method for producing a high-strength hot-dip galvanized steel sheet, wherein, when annealing and hot-dip galvanizing treatments are performed on a steel sheet in continuous hot-dip galvanizing equipment, the maximum temperature T of the steel sheet in the 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 the temperature range of the steel sheet being 700°C or higher and T°C or lower is -20°C or higher, and the atmosphere in the heating furnace contains, in addition to 3.0 vol% or higher and 20.0 vol% or lower, hydrogen, and one or more elements selected from the group consisting of 0.1 volppm or higher and 3.0 volppm or lower of SO2 and 0.5 volppm or higher and 10.0 volppm or lower 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 following groups A to E: Group A: Ti, Nb, V, W, and Zr, total of 0.200% or less Group B: Mo, Cu, Co, and Ni, total of 0.01% to 0.5% Group C: B 0.0003% to 0.0050% Group D: Sb and / or Sn, 0.001% to 0.200% in total Group E: Ca, Mg, and / or REM, total of 0.0001% to 0.0005% [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 elements selected from the following groups F to I, in mass %: F group Ta: 0.10% or less (excluding 0%) G group Te: 0.10% or less (excluding 0%) As: 0.10% or less (excluding 0%) Hf: 0.10% or less (excluding 0%), one or more selected from the following H group Bi: 0.20% or less (excluding 0%) Pb: 0.20% or less (excluding 0%), one or more selected from the following Group I Zn: 0.10% or less (excluding 0%) Ge: 0.10% or less (excluding 0%) Sr: 0.10% or less (excluding 0%) Cs: 0.10% or less (excluding 0%), one or more selected from the following [4] A coating weight of 20 g / m2 on the surface of a steel sheet having a chemical 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) 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. 2 More than 120g / m 2 It has the following zinc plating layer: The oxygen content of the surface layer of the steel sheet directly below the zinc coating layer within a depth of 100 μm from the surface of the base steel sheet toward the center of the sheet thickness is 0.030 g / m per side 2 More than 0.40g / m 2 or less, 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. [5] The high-strength hot-dip galvanized steel sheet according to [4], further containing, by mass %, one or more elements selected from the following groups A to E: Group A: Ti, Nb, V, W, and Zr, total of 0.200% or less Group B: Mo, Cu, Co, and Ni, total of 0.01% to 0.5% Group C: B 0.0003% to 0.0050% Group D: Sb and / or Sn, 0.001% to 0.200% in total Group E: Ca, Mg, and / or REM, total of 0.0001% to 0.0005% [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 elements selected from the following groups F to I, in mass %: F group Ta: 0.10% or less (excluding 0%) G group Te: 0.10% or less (excluding 0%) As: 0.10% or less (excluding 0%) Hf: 0.10% or less (excluding 0%), one or more selected from the following H group Bi: 0.20% or less (excluding 0%) Pb: 0.20% or less (excluding 0%), one or more selected from the following Group I Zn: 0.10% or less (excluding 0%) Ge: 0.10% or less (excluding 0%) Sr: 0.10% or less (excluding 0%) Cs: 0.10% or less (excluding 0%), one or more selected from the following [Effects of the Invention]

[0019] According to the present invention, a high-strength hot-dip galvanized steel sheet can be obtained that is excellent in coating appearance, coating adhesion during heavy working, and LME cracking resistance. [Brief explanation of the drawings]

[0020] [Figure 1] FIG. 1 is a structural view of a test piece for evaluating LME crack resistance. [Figure 2] The upper figure in FIG. 2 is a plan view of a plate assembly with a welded part, and the lower figure is a drawing showing a cross-section in the plate thickness direction after cutting the plate assembly with a welded part at the cutting position shown in the upper figure.

Embodiments for Carrying Out the Invention

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

[0022] First, the annealing atmosphere conditions that determine the structure of the surface of the base steel plate directly under the plating layer, which is the most important requirement in the present invention, will be described. During the heating process of annealing, the dew point of the atmosphere in the heating furnace is controlled to be -20°C or higher in the temperature range where the temperature of the steel plate is 700°C or higher and 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 the recrystallization treatment of the strain imparted by normal cold rolling, the maximum temperature of the steel plate during the heating process of annealing needs to exceed 700°C. On the other hand, when it exceeds 900°C, in addition to the difficulty of suppressing the surface enrichment of Si, Mn, and Cr, the oxidation of the steel plate surface layer within a depth of 100 μm or less from the surface of the base steel plate toward the center of the plate thickness also becomes excessive, deteriorating the surface appearance and the plating adhesion during processing. Therefore, the maximum temperature of the steel plate needs to exceed 700°C and be 900°C or lower.

[0024] Regarding the temperature range where the temperature of the steel plate is 700°C or higher and T°C or lower (700°C < T ≤ 900°C) during the heating process.

[0025] In order to suppress the formation of surface oxides, dew point control at a steel plate temperature of 700 °C or higher, where the diffusion rate of elements increases, is of utmost importance. At this time, in order to efficiently internally oxidize Si, Mn, and Cr and suppress surface enrichment, it is necessary to control the dew point in the temperature range of 700 °C or higher and T °C or lower (700 °C < T ≤ 900 °C) to -20 °C or higher. When the dew point is less than -20 °C, the suppression of surface enrichment becomes insufficient, leading to a decrease in plating appearance and plating adhesion. Although the upper limit of the dew point is not particularly specified, if the dew point is too high, the operating cost of the humidification equipment also becomes large. Therefore, the dew point is preferably +30 °C or lower, more preferably less than +20 °C.

[0026] Here, in the temperature range below 700 °C, due to the low temperature, the surface diffusion of easily oxidizable elements such as Si, Mn, and Cr becomes very small, and surface enrichment is suppressed (or does not occur). Therefore, the wettability between molten zinc and the steel plate is not inhibited. Thus, in the temperature range below 700 °C, it is not particularly necessary to control the dew point.

[0027] The atmosphere in the heating furnace during annealing needs to have a hydrogen concentration of 3.0 vol% or more and 20.0 vol% or less. If the hydrogen concentration in the atmosphere in the heating furnace in the temperature range of 700 °C or higher is too high, the amount of diffusible hydrogen remaining in the steel becomes too high. As a result, there is a problem that hydrogen embrittlement occurs and the workability is impaired. On the other hand, if the hydrogen concentration in the atmosphere in the heating furnace is too low, the reduction of the steel plate surface becomes insufficient and it becomes inactive, leading to a problem of plating defects. For the above reasons, the hydrogen concentration in the temperature range of 700 °C or higher is set to 3.0 vol% or more and 20.0 vol% or less.

[0028] The atmosphere in the heating furnace during annealing must contain at least one of 0.1 volppm to 3.0 volppm of SO2 and 0.5 volppm to 10.0 volppm of HCl. While the exact reasons are unclear, the presence of these corrosive gases in appropriate amounts is believed to promote internal oxidation of Si, Mn, and Cr rather than surface oxidation, improving coating adhesion and reducing the length of internal oxides present in the steel sheet surface layer (described below), thereby improving corrosion resistance after processing. The improvement effect of these corrosive gases is most pronounced when SO2 is 0.1 volppm or more and HCl is 0.5 volppm or more. However, SO2 exceeding 3.0 volppm and HCl exceeding 10.0 volppm may accelerate deterioration of the furnace body. Therefore, the SO2 concentration must be 0.1 volppm to 3.0 volppm, and the HCl concentration must be 0.5 volppm to 10.0 volppm. The balance of the atmosphere in the heating furnace during annealing may contain gases such as nitrogen, CO, and CO2.

[0029] The concentrations of these trace amounts of corrosive gases such as SO2 and HCl can be controlled by adjusting the amount of gas when the gas containing these corrosive gases is introduced directly into the furnace. Alternatively, the concentrations can be controlled by applying a liquid containing H2SO4 or HCl to the steel sheet before it enters the furnace and adjusting the amount of the liquid, or by adjusting the H2SO4 or HCl concentration of the liquid. The point is that it is important to control the concentrations of trace amounts of corrosive gases such as SO2 and HCl, and methods for controlling the concentrations of these trace amounts of corrosive gases such as SO2 and HCl are not limited to the methods described above.

[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, and contributes to this 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 not improve. 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 making it more likely for Si-based oxides to form 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 should be 0.10% or more. From the viewpoints of LME cracking resistance and galvanizability, the Si content should be 0.80% or less. Preferably, the Si content should be 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, and 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%, Mn segregation and other factors can easily cause unevenness in the steel structure, leading to reduced workability. In addition, Mn is easily oxidized externally as an oxide or composite oxide on the steel sheet surface, which can 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. While there is no particular lower limit, 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 can impair bendability, and S 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 achieve 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, but 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 tends to form Cr-based oxides on the steel sheet surface, which can 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 of the following groups A to E, by mass%. Group A: Ti, Nb, V, W, and Zr, total of 0.200% or less Group B: Mo, Cu, Co, and Ni, total of 0.01% to 0.5% Group C: B 0.0003% to 0.0050% Group D: Sb and / or Sn, 0.001% to 0.200% in total Group E: Ca, Mg, and / or REM, total of 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 the strength of steel sheets. In particular, their precipitation in soft ferrite increases its strength, reducing the strength difference with martensite and contributing to improved bendability and stretch flangeability. Furthermore, these elements refine the structure of hot-rolled coils, thereby refining the steel structure after subsequent cold rolling and annealing, thereby contributing to increased strength and improved workability, including bendability. To achieve this effect, it is preferable to contain at least 0.005% of one or more of Ti, Nb, V, W, and Zr in total. However, excessive addition increases deformation resistance during cold rolling, impeding productivity. Furthermore, the presence of excessive or coarse precipitates reduces the ductility of ferrite, thereby reducing the ductility and bendability of steel sheets. Therefore, the upper limit of the total content of at least one of Ti, Nb, V, W, and Zr is set at 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. Ni has the effect of suppressing the occurrence of surface defects caused by the addition of Cu, so it is preferable to add it at the same time as adding Cu. In particular, it is preferable to contain at least half the amount of Ni as 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 during the annealing and cooling process. However, an upper limit is set because excessive addition of B saturates the strength-enhancing effect and can lead to excessive hardenability, which can lead to disadvantages such as cracking of the weld during welding. Therefore, when B is added, 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, since excessive addition deteriorates the surface properties, the upper limit of the total amount of at least one of Sb and Sn is preferably set to 0.200%.

[0044] Adding small amounts of Ca, Mg, and REM has the effect of spheroidizing the shape of sulfides and improving the bendability of 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 be 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 be 0.0001% or more. The above-mentioned composition may further contain the following components as optional components. F group Ta: 0.10% or less (excluding 0%) G group Te: 0.10% or less (excluding 0%) As: 0.10% or less (excluding 0%) Hf: 0.10% or less (excluding 0%), one or more selected from the following H group Bi: 0.20% or less (excluding 0%) Pb: 0.20% or less (excluding 0%), one or more selected from the following Group I Zn: 0.10% or less (excluding 0%) Ge: 0.10% or less (excluding 0%) Sr: 0.10% or less (excluding 0%) Cs: 0.10% or less (excluding 0%), one or more selected from the following F group [Ta: 0.10% or less (excluding 0%)] Ta, like the elements of group A, is an element effective in increasing the strength of the steel sheet and can be added as needed. Although the effect of improving strength can be obtained by adding 0.005% or more of Ta, from the viewpoint of preventing an increase in costs, if Ta is added, the Ta content is set to 0.10% or less. 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%)] Te, As and Hf, like the elements of group C, are all elements used to control the morphology of sulfides. Te: 0.10% or less (excluding 0%) When Te is contained in an amount of 0.001% or more, the morphology of sulfides can be controlled and ductility and toughness can be improved. However, from the viewpoint of preventing an increase in costs, when Te is contained, the Te content is set to 0.10% or less. As: 0.10% or less (excluding 0%) When As is contained in an amount of 0.001% or more, the morphology of sulfides can be controlled and ductility and toughness can be improved. However, from the viewpoint of preventing an increase in costs, when As is contained, the As content is set to 0.10% or less. Hf: 0.10% or less (excluding 0%) When Hf is contained in an amount of 0.01% or more, the morphology of sulfides can be controlled and ductility and toughness can be improved. However, from the viewpoint of preventing an increase in costs, if Hf is contained, the Hf content is set to 0.10% or less. 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, their respective contents should exceed 0%. Bi: 0.20% or less (excluding 0%) When Bi is contained in an amount of 0.001% or more, it can suppress grain boundary segregation and improve ductility and toughness. Bi also has the effect of improving machinability and improving the smoothness of cut edges, and it also acts to improve the delayed fracture resistance of cut edges. When Bi is contained, the Bi content is set to 0.20% or less to prevent an increase in costs. Pb: 0.20% or less (excluding 0%) When Pb is contained in an amount of 0.001% or more, it can suppress grain boundary segregation and improve ductility and toughness. Pb also has the effect of improving machinability and improving the smoothness of cut edges, and it also acts to improve the delayed fracture resistance of cut edges. If Pb is contained, the Pb content is set to 0.20% or less to prevent an increase in costs. Ion group [one or more elements selected from Zn: 0.10% or less (excluding 0%), Ge: 0.10% or less (excluding 0%), Sr: 0.10% or less (excluding 0%), Cs: 0.10% or less (excluding 0%)] Zn, Ge, Sr, and Cs are elements that increase strength without significantly affecting mechanical properties or surface quality. When Zn, Ge, Sr, or Cs is contained, the content of each must be greater than 0%. Zn: 0.10% or less (excluding 0%) Even if Zn is contained in an amount of 0.001% or more, it does not have a significant effect on mechanical properties or surface quality. From the viewpoint of preventing an increase in costs, if Zn is contained, the Zn content is set to 0.10% or less. Ge: 0.10% or less (excluding 0%) Even if Ge is contained in an amount of 0.001% or more, it does not have a significant effect on mechanical properties or surface quality. From the viewpoint of preventing an increase in cost, if Ge is contained, the Ge content is set to 0.10% or less. Sr: 0.10% or less (excluding 0%) Even if the Sr content is 0.001% or more, it does not have a significant effect on mechanical properties or surface quality. To prevent an increase in cost, if Sr is contained, the Sr content is set to 0.10% or less. Cs: 0.10% or less (excluding 0%) Even if the Cs content is 0.001% or more, it does not have a significant effect on mechanical properties or surface quality. To prevent an increase in costs, if Cs is contained, the Cs content is set to 0.10% or less.

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

[0046] The mass ratio of (Si+Cr) to Mn ((Si+Cr) / Mn) is 0.25 or more To achieve excellent plating properties, it is important to control the elements that are easily oxidized in steel (Cr as well as Si have this effect). To prevent external oxidation of Mn, it is necessary to form a composite oxide of Si, Cr, and Mn inside the steel sheet. If the ((Si + Cr) / Mn) ratio is less than 0.25, sufficient amounts of composite internal oxide of Si, Cr, and Mn will not form in the surface layer of the steel sheet within 100 μm from the surface of the substrate steel sheet, resulting in surface segregation of these elements and poor plating performance. Therefore, the ((Si + Cr) / Mn) ratio must be 0.25 or higher.

[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 not only to strength but also to an improvement in the balance between strength and ductility. On the other hand, Si expands the liquidus stability region of zinc to the lower temperature side, thereby degrading LME cracking resistance. However, by controlling the weight ratio of Si to Mn (Si / Mn) to less than 0.25, it is possible to mitigate the deterioration of LME cracking resistance caused by an increase in Si concentration. Although the mechanism behind this is unclear, it is thought that an increase in Mn concentration suppresses the expansion (shift) of the liquidus stability region of zinc to the lower temperature side that occurs with an increase in 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 per side on the surface of the steel sheet.2 More than 120g / m 2 It has a zinc plating layer of 20g / m 2 If it is less than 120g / 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 0.030 g / m per side in a surface layer of the steel sheet within a depth of 100 μm from the surface of the base steel sheet in the direction of the sheet thickness center immediately below the galvanized layer, as measured by the method described in the Examples. 2 More than 0.40g / m 2 From the viewpoint of obtaining fine internal 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 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 on grain boundaries 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.

[0051] In hot-dip galvanized steel sheets containing Si and a large amount of Mn, 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 initiation point of 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, the oxygen potential is increased, and 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, 2If the amount exceeds this limit, the effect will saturate and in some cases it may cause cracks during severe deformation. Therefore, the upper limit is 0.40 g / m 2 Let's say.

[0052] After careful investigation of the conditions for internal oxides present in the surface layer of a steel sheet, which serve as crack initiation sites, we found that internal oxides present in the surface layer of a 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 processing, significantly deteriorating adhesion and corrosion resistance after processing. 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 processing and therefore do not affect adhesion and corrosion resistance after processing. Although there is no particular lower limit for the maximum length of internal oxides present in the surface layer of a steel sheet, 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 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 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 in the width direction of the steel sheet, the internal oxides will become crack initiation points during processing, as in the case where 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 in the width direction of the steel sheet. There is no particular lower limit, but if there are less than 4, the oxygen content of the surface layer 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 process is a process in which a steel having the above-described chemical composition is cast into a steel material. The steel used in the production method of the present invention is preferably cast under conditions in which the flow rate of the molten steel at the solidification interface in the vicinity of the mold meniscus is 16 cm / sec or more.

[0056] Steel material (slab (cast billet)) manufacturing The steel used in the manufacturing method of the present invention is what is generally called a slab, which is produced by a continuous casting method. This is for the purpose of preventing macrosegregation of alloy components, but the steel 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 where the molten steel flow rate at the solidification interface near the mold meniscus is 16 cm / sec or higher. "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 they have floated and collected before using it 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 a steel slab is preferably in the range of 1100°C or higher and 1350°C or lower. This is because precipitates present in a steel slab tend to coarsen, which is disadvantageous when, for example, ensuring strength through precipitation strengthening. Alternatively, the coarse precipitates may act as nuclei and adversely affect microstructure formation during the subsequent annealing process. Furthermore, heating can scale off bubbles and defects on the slab surface, thereby reducing cracks and irregularities on the steel sheet surface and achieving a smooth steel sheet surface, which 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 microstructure 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] Finishing rolling temperature: 800℃ to 950℃ By setting the finish rolling temperature at 800°C or higher, the aim is to homogenize the structure obtained in the hot-rolled coil, thereby ensuring a uniform structure in the final product. A non-uniform structure results in reduced bendability. On the other hand, if the finish rolling temperature exceeds 950°C, the amount of oxide (scale) generated increases, causing the interface between the base steel and the oxide to become rough, resulting in poor surface quality after pickling and cold rolling. Furthermore, the grain size becomes coarse, which, like the coarsening of the structure of a steel slab, can cause a decrease in the strength and bendability of the steel plate.

[0065] In order to refine and homogenize the structure of the hot-rolled coil (hot-rolled sheet) after the hot rolling, it is preferable to start cooling within 3 seconds after the end of finish rolling, cool the hot-rolled coil (hot-rolled sheet) in the temperature range of [finish rolling temperature] to [finish rolling temperature - 100]°C at an average cooling rate of 10 to 250°C / s, and coil it 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 for 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 grains are likely to become coarse during annealing, and the structure is likely to become non-uniform, 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. Note that 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. In this case, the galvanized steel sheet may be held in a temperature range of 450 to 580°C for about 1 to 60 seconds. [Example]

[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 / s to 520°C, and held at 520°C for 50 seconds. After this, the sheets were galvanized to produce high-strength hot-dip galvanized steel sheets. All steel sheets except for No. 39 were then subjected to an alloying treatment.

[0073] [Table 1-1]

[0074] [Table 1-2]

[0075] Samples were taken from the plated steel sheets obtained as described above, and the appearance was 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 are as follows.

[0076] (1) Oxygen content in the surface layer of the steel sheet within a depth of 100 μm from the surface of the base steel sheet directly below the coating layer toward the center of the sheet thickness To measure the oxygen content directly below the coating layer, only the coating layer was stripped 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 layer of both sides of the high-strength steel sheet after continuous annealing is polished to a depth of 100 μm or more to measure the oxygen concentration in the steel, and this measurement is taken as the amount of oxygen contained in the raw material, OH. In addition, the oxygen concentration in the steel throughout the thickness direction of the high-strength steel sheet after continuous annealing is measured, and this measurement is taken as the amount of oxygen after oxidation, OI. Using the amount of oxygen after oxidation of the steel sheet obtained in this way, OI, and the amount of oxygen originally contained in the raw material, OH, the difference between OI and OH (= OI - OH) was calculated, and further converted into the amount per unit area on one side (g / m 2 ) was taken as the oxygen content.

[0077] Furthermore, 10mm x 10mm 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 20μm cross-section was observed at 5000x magnification, with five fields of view per level. The maximum length (μm) of the internal oxide within each field of view and the number of internal oxides with a length of 1.0μm or greater were measured, and the average values were used to determine the maximum length of the internal oxide and the number of internal oxides with a length of 1.0μm or greater for that level. Only oxides with an oxide width of 0.1μm or greater were counted for the number of internal oxides.

[0078] (2) Surface condition (appearance) The appearance of the manufactured hot-dip galvanized steel sheets was visually inspected, and those with no uncoated defects were rated as "○: Pass, Excellent", those with uncoated defects were rated as "×: Fail", and those with no uncoated defects but uneven plating appearance were rated as "△: Pass, Fair". Uncoated defects are areas with a diameter of 50 μm or more where no plating is present and the steel sheet is exposed. The results are shown in Table 2.

[0079] (3) Plating adhesion [Galvannealed 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 Zn counts using a fluorescent X-ray analysis. Measurement conditions included 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 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: 1 / 4 of the sheet width, 1 / 2 (center), 3 / 4 of the sheet width, and 50 mm from the steel sheet edge. The highest Zn count among these was used to evaluate the galvanizability according to the following criteria. In this invention, a rating of ⊚ or ◯ was considered acceptable. ⊚ (pass, better): Zn count is less than 6000. ◯ (pass, excellent): Zn count is 6000 or more and less than 8000. × (fail): Zn count is 8000 or more.

[0080] [Unalloyed hot-dip galvanized steel sheet] The coating adhesion of hot-dip galvanized steel sheets was evaluated by a ball impact test. After the steel sheet was attached to a mold with 3 / 8 and 1 / 2 inch diameter holes 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 this level was considered to be minor peeling, and anything greater than this was considered to be peeling. ◎ (Pass, better): No peeling of the plating layer under any conditions Good (Good): Minor peeling at 3 / 8 inch diameter △ (Fail): Peeling occurred with 3 / 8 inch diameter, no plating peeling with 1 / 2 inch diameter × (Fail): Plating peeled off under all conditions (4) Corrosion resistance after processing Test pieces were prepared in the same manner as in the plating peel resistance test, but without tape peeling. Using Nihon Parkerizing Co., Ltd.'s degreasing agent: FC-E2011, surface conditioner: PL-X, and chemical conversion coating agent: Palbond (registered trademark) PB-L3065, the chemical conversion coating film was measured under the following standard conditions until the coating weight reached 1.7 to 3.0 g / m. 2 Chemical conversion treatment was carried out to achieve this. <Standard conditions> Degreasing process: Processing temperature 40℃, processing time 120 seconds Spray degreasing, surface conditioning process; pH 9.5, processing temperature room temperature, processing time 20 seconds Chemical conversion treatment process: Chemical conversion treatment solution temperature 35℃, treatment time 120 seconds The surfaces of the test pieces subjected to the above chemical conversion treatment were electrocoated using V-50 electrodeposition paint 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)> The above-mentioned test specimens, which had been subjected to chemical conversion treatment and electrodeposition coating, were cut with a cutter at the bent surface of the galvannealed steel sheet and at the ball impact area of the hot-dip galvanized steel sheet, reaching down to the plating. After this, the test specimens 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. The cross-cut flaws were then subjected to a tape peel test, and the maximum peel width on both sides of the flaw was measured. A symbol (◎, ○, ×) was 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 was evaluated as good. ◎: Maximum bulge width from cut scratch is 2.0 mm or less (good) 〇: Maximum bulge width from cut scratch is over 2.0mm and 2.5mm or less (passed) ×: Maximum bulge width from cut defect exceeds 2.5 mm (failure) (5) LME cracking resistance evaluation Test piece 2 was cut out from the hot-dip galvanized steel sheet, measuring 150 mm in the longitudinal direction and 50 mm in the transverse direction, with the transverse direction being the rolling direction (TD) as the longitudinal direction, and the rolling direction as the transverse direction. Test pieces of the same size were cut out, and the coating weight of the hot-dip galvanized layer on each side was 50 g / m 2The specimen was then stacked on a test hot-dip galvanized steel sheet (1.6 mm thick, TS: 980 MPa grade) 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 fixture 5 via a 2.0 mm thick spacer 3 at a 5° tilt, the maximum tilt expected for some part shapes. The spacer 3 was a pair of steel plates measuring 50 mm long x 45 mm short x 2.0 mm thick, and was positioned so that the longitudinal end faces of each of the pair of steel plates were aligned with the lateral end faces of the sheet assembly. Therefore, the distance between the pair of steel plates constituting the spacer 3 was 60 mm. The fixture 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 bending the sheet assembly. 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 that resulted in a nugget diameter 7 of 5.9 mm at the weld (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 specimen 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 it was in contact with the plane extending from the surface where the spacer 3 and fixture 5 met, and the upper electrode 4 was movable. The upper electrode 4 was also placed 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 begin 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 so as to include the weld (nugget), and the cross section of the weld was observed under 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 the cross section of the sheet assembly in the sheet thickness direction after cutting, and schematically shows cracks (fractures) 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 longer than 0.1 mm 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 perpendicular to the rolling direction and subjected to tensile tests at a constant tensile speed (crosshead speed) of 10 mm / min. The tensile strength was calculated 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 to calculate the cross-sectional area of the parallel part was the thickness including the plating.

[0089] (7) Analysis method of furnace gas The gas inside the annealing furnace was collected and the SO2 and HCl concentrations were determined by ion chromatography. The analysis was carried out three times, and the average value was used as the furnace gas concentration.

[0090] [Table 2]

[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. [Industrial Applicability]

[0092] The high-strength hot-dip galvanized steel sheet of the present invention has excellent 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. [Explanation of symbols]

[0093] 1. Test hot-dip galvanized steel sheet 2 test specimens 3 spacers 4 electrodes 5 Fixed base 6. Nuggets 7 Nugget diameter 8 Line indicating cutting position 9 Cracks

Claims

1. A coating weight of 20 g / m per side is applied to the surface of a steel sheet having a chemical 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. ２ 120g / m or more ２ a method for producing a high-strength hot-dip galvanized steel sheet having a zinc-plated layer as described below, wherein the oxygen content of a steel sheet surface layer portion immediately below the zinc-plated layer and within a depth of 100 μm from the surface of a substrate steel sheet in a sheet thickness center direction is 0.030 g / m 2 or more and 0.40 g / m 2 or less per side, the maximum length of internal oxides present in the steel sheet surface layer portion in a cross section of the steel sheet is 6.0 μm or less, and the number of internal oxides present in the steel sheet surface layer portion and having a length of 1.0 μm or more is 20 or less per 100 μm of length in the steel sheet width direction of the steel sheet surface layer portion, When annealing and hot-dip galvanizing treatments are performed on a steel sheet in a continuous hot-dip galvanizing facility, the maximum temperature T of the steel sheet in the 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 the temperature range of the steel sheet being 700°C or higher and T°C or lower is -20°C or higher, and the atmosphere in the heating furnace contains 3.0 vol% or higher but not higher than 20.0 vol% hydrogen and 0.1 volppm or higher but not higher than 3.0 volppm SO 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 content of 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 is 0.0003% or more and 0.0050% or less Group D: Sb and / or Sn, with a total content of 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. 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, as the component composition, one or more groups selected from the following groups F to I, in mass %: F group Ta: 0.10% or less (excluding 0%) G group Te: 0.10% or less (excluding 0%) As: 0.10% or less (excluding 0%) Hf: 0.10% or less (excluding 0%), one or more selected from the following H group Bi: 0.20% or less (excluding 0%) Pb: 0.20% or less (excluding 0%), one or more selected from the following Group I Zn: 0.10% or less (excluding 0%) Ge: 0.10% or less (excluding 0%) Sr: 0.10% or less (excluding 0%) Cs: 0.10% or less (excluding 0%), one or more selected from the following

4. A coating weight of 20 g / m per side is applied to the surface of a steel sheet having a chemical 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. ２ 120g / m or more ２ It has the following zinc plating layer: The oxygen content of the surface layer of the steel sheet within a depth of 100 μm from the surface of the base steel sheet directly under the zinc plating layer in the direction of the center of the sheet thickness is 0.030 g / m per side ２ 0.40g / m or more ２ or less, 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.

5. The high-strength hot-dip galvanized steel sheet according to claim 4, 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 in total of 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 is 0.0003% or more and 0.0050% or less Group D: Sb and / or Sn, with a total content of 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 claim 4 or 5, wherein the steel sheet further contains, as the component composition, one or more groups selected from the following groups F to I, in mass %: F group Ta: 0.10% or less (excluding 0%) G group Te: 0.10% or less (excluding 0%) As: 0.10% or less (excluding 0%) Hf: 0.10% or less (excluding 0%), one or more selected from the following H group Bi: 0.20% or less (excluding 0%) Pb: 0.20% or less (excluding 0%), one or more selected from the following Group I Zn: 0.10% or less (excluding 0%) Ge: 0.10% or less (excluding 0%) Sr: 0.10% or less (excluding 0%) Cs: 0.10% or less (excluding 0%), one or more selected from the following

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