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

Abstract

Provided is a high-strength hot-dip galvanized steel sheet having excellent plating appearance, plating adhesion during strong processing, and LME cracking resistance. A high-strength hot-dip galvanized steel sheet and a method for manufacturing the same, the high-strength hot-dip galvanized steel sheet having, on the surface of a steel sheet having a component composition containing a prescribed steel component, ((Si + Cr) / Mn) of 0.25 or more and (Si / Mn) of less than 0.25, with the remainder comprising Fe and unavoidable impurities, a zinc plating layer having a plating layer adhesion amount of 20-120 g / m < 2 > per surface, the plating layer adhesion amount being greater than or equal to 20 g / m < 2 > and less than or equal to 120 g / m < 2 > per surface, in this method for producing a high-strength hot-dip galvanized steel sheet, the maximum temperature (T) of the steel sheet in an annealing furnace is higher than 700 DEG C and not more than 900 DEG C, the dew point of the atmosphere in the furnace in the temperature range where the steel sheet temperature is 700 DEG C or more and T DEG C or less is-20 DEG C or more, and the dew point of the atmosphere in the furnace in the temperature range where the steel sheet temperature is not less than 700 DEG C and not more than 20.0 vol% of hydrogen gas is not less than 20 vol% and not more than 20.0 vol%. And one or more elements selected from the group consisting of 0.1 ppm by volume or more and 3.0 ppm by volume or less of SO2 and 0.5 ppm by volume or more and 10.0 ppm by volume or less of HCl.

Description

Technical Field

[0001] This invention relates to a high-strength galvanized steel sheet with excellent processability, suitable for use in building materials and automotive collision-resistant components, where higher tensile strength equates to greater importance, and a method for manufacturing the same. Background Technology

[0002] Recently, there has been a strong demand to improve the crash safety and fuel efficiency of automobiles, which is driving the development of high-strength steel sheets used as raw materials for components. Furthermore, the widespread use of automobiles worldwide, in diverse regions and climates, and for various purposes, necessitates high rust resistance in the steel sheets used as raw materials for components.

[0003] Typically, hot-dip galvanized steel sheets are manufactured by using thin steel sheets obtained through hot rolling or cold rolling of slabs as the base material, and then subjecting the base material to recrystallization annealing and hot-dip galvanizing in the annealing furnace of a continuous galvanizing line (CGL). In the case of alloyed hot-dip galvanized steel sheets, further alloying treatment is performed after the hot-dip galvanizing process.

[0004] Here, the types of heating furnaces used for annealing in CGLs include direct-fired furnaces (DFF), non-oxidizing furnaces (NOF), and radiant tube furnaces (RTF). In recent years, due to the ease of operation and reduced pick-up rates, which allow for the production of high-quality coated steel sheets at low cost, the construction of CGLs equipped with all-radiant tube furnaces (where all heating furnaces are radiant) is increasing. On the other hand, for steel sheets containing easily oxidizable elements such as Si, Mn, and Cr, oxidation under appropriate conditions is preferable before reduction annealing. Unlike annealing furnaces with direct-fired or non-oxidizing furnaces located before reduction annealing, radiant tube annealing furnaces do not have an oxidation process before reduction annealing. Therefore, for steel sheets containing easily oxidizable elements such as Si, Mn, and Cr, this is disadvantageous in ensuring coating properties.

[0005] As a method for manufacturing hot-dip galvanized steel sheets using high-strength steel sheets containing large amounts of Si and Mn as the base material, Patent Documents 1 and 2 disclose a technique for internal oxidation of the steel substrate surface by increasing the dew point during the heating process in the annealing furnace. However, in the techniques described in Patent Documents 1 and 2, the dew point is controlled over the entire furnace, making dew point control difficult and stable operation challenging. Furthermore, when manufacturing alloyed hot-dip galvanized steel sheets under unstable dew point control, fluctuations in the distribution of internal oxides formed in the base steel sheet are observed, potentially leading to defects such as uneven coating wettability (fluctuations) and uneven alloying (fluctuations) in the length and width directions of the steel sheet.

[0006] Furthermore, Patent Document 3 discloses a technique that specifies not only H2O and O2 as oxidizing gases, but also the concentration of CO2, thereby causing internal oxidation of the steel substrate surface before plating, inhibiting external oxidation, and improving the appearance of the plating. However, when containing a particularly large amount of Si, as in Patent Document 3, the presence of internal oxides can easily cause cracks on the surface of the base steel plate during processing, leading to deterioration of the plating's peel resistance. Additionally, deterioration in corrosion resistance has been confirmed. Furthermore, CO2 can cause furnace contamination and carburization of the steel plate surface, raising concerns about changes in mechanical properties.

[0007] Furthermore, recently, the application of high-strength hot-dip galvanized steel sheets and high-strength alloyed hot-dip galvanized steel sheets to areas requiring stringent processing (hereinafter referred to as "heavy processing") is being promoted, and the coating peeling resistance during heavy processing is receiving attention. Specifically, it is required to suppress coating peeling from the processed areas when the coated steel sheet is bent at angles exceeding 90° and further bent into acute angles, or when impact is applied and the steel sheet is subjected to processing.

[0008] Furthermore, when a large amount of Si is added to the steel sheet, during resistance welding, residual stress is generated near the weld area, causing the zinc coating to melt and diffuse into the grain boundaries, leading to liquid metal embrittlement (LME). There is concern that LME cracks may form in the steel sheet. This is especially true when welding is performed with the welding electrode significantly tilted relative to the steel sheet surface, potentially increasing residual stress and causing cracks. It is believed that residual stress increases with the strength of the steel sheet, therefore there is concern about the potential for LME cracks associated with increased steel strength.

[0009] To achieve these characteristics, it is necessary not only to ensure the desired steel sheet microstructure but also to closely control the microstructure and structure of the steel substrate layer immediately beneath the coating, which could potentially become the initiation point for cracks during intensive processing. Furthermore, the amount of Si added to the steel must be controlled to maintain resistance to resistance welding cracking characteristics (hereinafter also referred to as "LME crack resistance"). However, in the prior art, such control is difficult, making it impossible to manufacture hot-dip galvanized steel sheets with excellent coating adhesion and LME crack resistance during intensive processing using Si-containing high-strength steel sheets as the base material in a CGL system equipped with a fully radiant tubular heating furnace in the annealing furnace.

[0010] Existing technical documents

[0011] Patent documents

[0012] Patent Document 1: Japanese Re-publication No. 2014-102901

[0013] Patent Document 2: Japanese Patent Publication No. 2014-525986

[0014] Patent Document 3: Japanese Patent Application Publication No. 2006-233333 Summary of the Invention

[0015] The problem that the invention aims to solve

[0016] The present invention was made in view of the above circumstances, and its object is to provide a high-strength hot-dip galvanized steel sheet manufactured in a CGL equipped with a fully radiant tubular heating furnace in an annealing furnace, using steel sheet containing Si, Mn or Cr as the base material, exhibiting excellent coating appearance and coating adhesion during heavy processing, as well as resistance to LME cracking, and a method thereof.

[0017] It should be noted that the high strength of the high-strength galvanized steel sheet of the present invention refers to a tensile strength of 780 MPa or higher.

[0018] Methods for solving problems

[0019] From the perspective of ensuring coating quality, setting up an oxidation process before annealing in the heating furnace is effective. However, problems exist regarding coating peeling and corrosion resistance during processing. Furthermore, from the perspective of cost and operability, ensuring coating quality in a full-radiation tube furnace is urgently needed. Conventionally, this has been addressed by simply increasing the overall water vapor partial pressure within the annealing furnace to raise the dew point, causing oxidation of the steel sheet's surface and interior. During this process, surface diffusion and surface oxidation (hereinafter also referred to as surface enrichment) of easily oxidizable elements in the steel occur simultaneously with internal oxidation. Therefore, to ensure the aforementioned coating appearance and adhesion, it is necessary to efficiently suppress this surface enrichment. Consequently, the inventors conducted in-depth research on factors related to internal oxidation and surface enrichment, and found that it is extremely important to contain trace amounts of corrosive gases such as SO2 and HCl in the annealing atmosphere and to control the dew point within a temperature range above 700°C.

[0020] Furthermore, the inventors investigated the required dew point at various steel plate annealing temperatures and found that, when the maximum temperature of the steel plate in the furnace atmosphere is set to T℃, controlling the dew point above 700℃ and below T℃ to -20℃ or higher, and controlling the Cr concentration in addition to controlling the Si and Mn concentrations in the steel, is effective in promoting oxidation (hereinafter, sometimes referred to as internal oxidation) of the surface layer of the steel plate within a depth of 100μm from the surface of the base steel plate toward the center of the plate thickness and inhibiting surface enrichment.

[0021] The reason for setting the maximum temperature T of the steel plate in the furnace atmosphere below 900°C is that when the maximum temperature T exceeds 900°C, it becomes difficult to suppress the surface enrichment of Si, Mn, and Cr, and internal oxidation becomes excessive, which reduces the surface appearance and the adhesion of the coating during processing.

[0022] In addition, it was clarified that resistance to LME cracking can be improved by controlling the Si and Mn concentrations in the steel to an appropriate range.

[0023] By treating steel sheets with specified compositions in this way, the selective surface oxidation of easily oxidized elements such as Si, Mn, and Cr can be suppressed, and the surface enrichment of these elements can be inhibited. This results in high-strength hot-dip galvanized steel sheets with excellent coating appearance, coating adhesion during heavy processing, and resistance to LME cracking. It should be noted that excellent coating appearance refers to an appearance that does not indicate uncoated or uneven alloying.

[0024] The high-strength hot-dip galvanized steel sheet obtained by the above method has the following structure: the oxygen content of the surface layer of the steel sheet, which is immediately below the zinc coating and extends within 100 μm from the surface of the base steel sheet, is 0.030 g / m² per single side. 2 Above and 0.40g / m 2Furthermore, the maximum length of the internal oxides present in the surface layer of the aforementioned steel plate is 6.0 μm or less, and the number of internal oxides present in the surface layer of the aforementioned steel plate with a length of 1.0 μm or more is 20 or less per 100 μm of the steel plate width direction of the surface layer. This achieves stress relief and crack prevention during bending processing of the steel base surface, as well as excellent coating appearance and coating adhesion during heavy processing.

[0025] Based on the above insights, the present invention is characterized as follows.

[0026] [1] A method for manufacturing a high-strength hot-dip galvanized steel sheet, wherein a coating with an adhesion amount of 20 g / m² on each side is formed on the surface of the steel sheet. 2 Above and 120g / m 2 The following method describes a method for producing high-strength hot-dip galvanized steel sheet with a zinc coating, wherein the steel sheet comprises, 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, Cr: 1.0% or less, 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 composed of Fe. The manufacturing method, which involves annealing and hot-dip galvanizing of steel plates in a continuous hot-dip galvanizing equipment, includes unavoidable impurities. The highest temperature T of the steel plate in the annealing furnace is above 700°C and below 900°C. The dew point of the atmosphere inside the furnace within the temperature range of 700°C above and T°C below is -20°C or higher. In addition to containing 3.0% by volume or more and 20.0% by volume of hydrogen, the atmosphere inside the furnace also contains one or more components selected from the group consisting of SO2 (0.1% by volume ppm to 3.0% by volume) and HCl (0.5% by volume ppm to 10.0% by volume).

[0027] [2] The manufacturing method of high-strength hot-dip galvanized steel sheet according to [1], wherein the steel sheet further contains, by mass%, one or more groups selected from the following groups A to E.

[0028] Group A: One or more of Ti, Nb, V, W, and Zr: total less than 0.200%.

[0029] Group B: One or more of Mo, Cu, Co, and Ni: totaling 0.01% or more but less than 0.5%

[0030] Group C: B: Above 0.0003% and below 0.0050%

[0031] Group D: One or more of Sb and Sn: Total percentage above 0.001% and below 0.200%

[0032] Group E: One or more of Ca, Mg, and REM: Total ≥0.0001% and ≤0.0005%

[0033] [3] The high-strength hot-dip galvanized steel sheet according to [1] or [2], wherein the steel sheet further contains, by mass %, one or more of the following groups F to I as the composition of the above components.

[0034] Group F: Ta: below 0.10% (excluding 0%)

[0035] Group G: Selected from one or more of the following: Te: less than 0.10% (excluding 0%), As: less than 0.10% (excluding 0%), and Hf: less than 0.10% (excluding 0%).

[0036] Group H: Selected from one or more of Bi: less than 0.20% (excluding 0%) and Pb: less than 0.20% (excluding 0%).

[0037] Group I: Selected from one or more of the following: Zn: less than 0.10% (excluding 0%), Ge: less than 0.10% (excluding 0%), Sr: less than 0.10% (excluding 0%), and Cs: less than 0.10% (excluding 0%).

[0038] [4] A high-strength hot-dip galvanized steel sheet, wherein the coating on the surface of the steel sheet has an adhesion amount of 20 g / m² per single side. 2 Above and 120g / m 2 The following zinc coating is applied to a steel sheet with the following composition by mass percentage: C: 0.060% to 0.250%, Si: 0.10% to 0.80%, Mn: 1.50% to 3.50%, P: 0.020%, S: 0.0100%, Al: 0.100%, N: 0.0060%, Cr: 1.0%, with a Si+Cr to Mn mass ratio ((Si+Cr) / Mn) of 0.25 or higher, and a Si to Mn mass ratio (Si / Mn) of less than 0.25. The balance consists of Fe and unavoidable impurities. The oxygen content of the surface layer of the steel sheet immediately below the zinc coating, extending 100 μm from the surface of the base steel sheet towards the center of the sheet thickness, is 0.030 g / m² per side. 2 Above and 0.40g / m 2In the following, in the cross-section of the steel plate, the maximum length of the internal oxide present in the surface layer of the steel plate is 6.0 μm or less, and the number of internal oxides with a length of 1.0 μm or more present in the surface layer of the steel plate is 20 or less per 100 μm of the steel plate width direction of the surface layer of the steel plate.

[0039] [5] The high-strength hot-dip galvanized steel sheet according to [4], wherein the steel sheet further contains, by mass%, one or more of groups A to E below.

[0040] Group A: One or more of Ti, Nb, V, W, and Zr: total less than 0.200%.

[0041] Group B: One or more of Mo, Cu, Co, and Ni: totaling 0.01% or more but less than 0.5%

[0042] Group C: B: Above 0.0003% and below 0.0050%

[0043] Group D: One or more of Sb and Sn: Total percentage above 0.001% and below 0.200%

[0044] Group E: One or more of Ca, Mg, and REM: Total ≥0.0001% and ≤0.0005%

[0045] [6] The high-strength hot-dip galvanized steel sheet according to [4] or [5], wherein the steel sheet further comprises, by mass%, one or more groups selected from the F to I groups below as the above-mentioned components.

[0046] Group F: Ta: below 0.10% (excluding 0%)

[0047] Group G: Selected from one or more of the following: Te: less than 0.10% (excluding 0%), As: less than 0.10% (excluding 0%), and Hf: less than 0.10% (excluding 0%).

[0048] Group H: Selected from one or more of Bi: less than 0.20% (excluding 0%) and Pb: less than 0.20% (excluding 0%).

[0049] Group I: Selected from one or more of the following: Zn: less than 0.10% (excluding 0%), Ge: less than 0.10% (excluding 0%), Sr: less than 0.10% (excluding 0%), and Cs: less than 0.10% (excluding 0%).

[0050] Invention Effects

[0051] According to the present invention, high-strength hot-dip galvanized steel sheets with excellent coating appearance, coating adhesion during heavy processing, and resistance to LME cracking can be obtained. Attached Figure Description

[0052] Figure 1 This is a structural diagram of the test material used to evaluate resistance to LME cracking.

[0053] Figure 2 The top image is a top view of the plate assembly with welded parts, and the bottom image is a cross-section in the thickness direction of the plate assembly after cutting the plate assembly with welded parts at the cutting position shown in the top image. Detailed Implementation

[0054] The embodiments of the present invention will be described below. It should be noted that the present invention is not limited to the following embodiments.

[0055] First, the annealing atmosphere conditions, which are the most important element of this invention and determine the structure of the base steel plate surface immediately below the coating, will be explained.

[0056] During the annealing heating process, the dew point of the atmosphere in the heating furnace, where the steel plate temperature is above 700℃ and below T℃, is controlled to be above -20℃. Here, T℃ is the highest temperature of the atmosphere in the annealing furnace, where 700℃ < T ≤ 900℃.

[0057] To achieve recrystallization of the strain imparted by the conventional cold rolling process, the maximum temperature of the steel sheet during the annealing heating process needs to be above 700°C. On the other hand, above 900°C, it becomes difficult to suppress the surface enrichment of Si, Mn, and Cr, and excessive oxidation occurs in the surface layer of the steel sheet within a depth of 100 μm from the base steel sheet surface towards the center of the sheet thickness, reducing the surface appearance and the adhesion of the coating during processing. Therefore, the maximum temperature of the steel sheet needs to be above 700°C but below 900°C.

[0058] The temperature range for the steel plate during the heating process is described as being above 700℃ and below T℃ (700℃<T≤900℃).

[0059] In suppressing the formation of surface oxides, controlling the dew point at temperatures above 700°C is crucial for steel plates where the diffusion rate of elements increases. At this temperature, to ensure efficient internal oxidation of Si, Mn, and Cr and suppress surface enrichment, the dew point needs to be controlled to above -20°C within the temperature range of 700°C to below T°C (700°C < T ≤ 900°C). When the dew point is below -20°C, the suppression of surface enrichment becomes insufficient, leading to a decrease in the appearance and adhesion of the coating. While there is no specific upper limit for the dew point, excessively high dew points increase the operating costs of humidification equipment; therefore, a dew point below +30°C is preferred, and more preferably below +20°C.

[0060] Here, within a temperature range below 700°C, due to the low temperature, the surface diffusion of easily oxidized elements such as Si, Mn, and Cr becomes very minimal, suppressing (or preventing) surface enrichment. Therefore, it does not hinder the wettability of molten zinc with the steel sheet. Thus, dew point control is not required within a temperature range below 700°C.

[0061] The hydrogen concentration in the furnace during annealing needs to be between 3.0% and 20.0% by volume. If the hydrogen concentration in the furnace atmosphere is too high within the temperature range above 700°C, the amount of diffusible hydrogen remaining in the steel becomes excessive. This results in hydrogen embrittlement and impaired workability. On the other hand, if the hydrogen concentration in the furnace atmosphere is too low, the reduction of the steel surface becomes insufficient, resulting in inactivity and poor plating. For these reasons, the hydrogen concentration within the temperature range above 700°C is set to between 3.0% and 20.0% by volume.

[0062] The atmosphere inside the furnace during annealing needs to contain at least one of SO2 at a concentration of 0.1 ppm to 3.0 ppm by volume and HCl at a concentration of 0.5 ppm to 10.0 ppm by volume. The detailed rationale is uncertain, but it is speculated that the presence of these corrosive gases in appropriate amounts promotes internal oxidation compared to surface oxidation of Si, Mn, and Cr. This not only improves coating adhesion but also reduces the length of internal oxides present on the surface of the steel plate (described later), thus improving post-processing corrosion resistance. The improvement effect brought about by these corrosive gases is evident when SO2 is 0.1 ppm by volume or higher and HCl is 0.5 ppm by volume or higher. However, when SO2 exceeds 3.0 ppm by volume and HCl exceeds 10.0 ppm by volume, it may promote the deterioration of the furnace body. Therefore, the concentration needs to be set at 0.1 ppm to 3.0 ppm by volume in the case of SO2, and at 0.5 ppm to 10.0 ppm by volume in the case of HCl. In addition, the remaining part of the atmosphere in the heating furnace during annealing may contain gases such as nitrogen, CO, and CO2.

[0063] The concentration of these trace amounts of corrosive gases such as SO2 and HCl can be controlled by adjusting the amount of gas introduced directly into the furnace. Alternatively, it can be controlled by coating the steel plate with a liquid containing H2SO4 and HCl before it enters the furnace and adjusting the amount, or further adjusting the concentration of H2SO4 and HCl in the liquid. In short, controlling the concentration of trace amounts of corrosive gases such as SO2 and HCl is important, and the methods for controlling these concentrations are not limited to those described above.

[0064] Next, the steel composition of the high-strength hot-dip galvanized steel sheet, which is the subject of this invention, will be described. It should be noted that the composition is expressed as a percentage by mass.

[0065] C: Above 0.060% and below 0.250%

[0066] Carbon (C) is an effective element for increasing the strength of steel sheets, contributing to strength increase by forming martensite, one of the hard phases in the steel structure. Furthermore, depending on the manufacturing method, it also contributes to strength increase 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. Additionally, from the viewpoint of stabilizing the tensile strength (TS) at 780 MPa or more, the C content is preferably 0.090% or more. On the other hand, when the C content exceeds 0.250%, the martensite becomes excessively hardened, and even if inclusions and hydrogen content in the steel are controlled, there is a tendency that bending workability cannot be improved. Therefore, the C content is set to 0.250% or less.

[0067] Si: 0.10% or more and 0.80% or less

[0068] Si is an element that primarily contributes to increased strength through solid solution strengthening. Compared to the increase in strength, the decrease in ductility is relatively small, contributing not only to increased strength but also to a better balance between strength and ductility. Improved ductility is related to improved bending performance. On the other hand, excessive addition of Si expands the liquidus stability region of zinc towards the low-temperature side, thus deteriorating resistance to LME cracking and easily forming Si-based oxides on the steel surface, sometimes causing problems with plating. Therefore, only the amount needed to ensure strength should be added, with a Si content of 0.10% or more. Furthermore, from the viewpoint of LME crack resistance and plating properties, the Si content should be 0.80% or less. Preferably, the Si content is 0.70% or less.

[0069] Mn: 1.50% or more and 3.50% or less

[0070] Mn is effective as an 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, when it exceeds 3.50%, Mn segregation and other factors can easily lead to unevenness in the steel microstructure, resulting in reduced workability. Furthermore, Mn is prone to external oxidation on the steel surface in the form of oxides or complex oxides, sometimes causing problems with plating. Therefore, the Mn content is set to 3.50% or less.

[0071] P: below 0.020%

[0072] Phosphorus (P) is an effective element that contributes to the high strength of steel sheets through solid solution strengthening, but it also affects plating properties. In particular, it leads to a deterioration in wettability with the steel sheet and a delay in the alloying speed of the coating, especially in high-alloy systems where high-strength steel sheets are obtained. Therefore, the P content is set to 0.020% or less, more preferably 0.010% or less. While no specific lower limit is specified, a content less than 0.0001% leads to reduced production efficiency and increased dephosphorization costs during manufacturing; therefore, a P content of 0.0001% or more is preferred.

[0073] S: Below 0.0100%

[0074] Sulfur (S) readily forms sulfide inclusions in steel. Especially when large amounts of manganese (Mn) are added for increased strength, MnS inclusions are easily formed. This impairs bending properties and, in addition, causes hot brittleness, negatively impacting the manufacturing process. Therefore, it is preferable to minimize S content. In this invention, the S content is set to 0.0100% or less. While no specific lower limit is specified, a content less than 0.0001% leads to reduced production efficiency and increased costs during manufacturing; therefore, the S content is preferably set to 0.0001% or more.

[0075] Al: below 0.100%

[0076] Al is added as a deoxidizer. To achieve its effect, it is preferable to contain 0.001% or more. On the other hand, when the Al content exceeds 0.100%, inclusions are easily formed during the manufacturing process, which deteriorates the bending properties. Therefore, it is preferable that the Al content is 0.100% or less, and more preferably 0.080% or less based on sol.Al in the steel.

[0077] N: below 0.0060%

[0078] When the nitrogen (N) content exceeds 0.0060%, in addition to generating excessive nitrides in the steel, which reduces its workability, it also leads to the 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 improving ductility by making the microstructure cleaner, a very low N content is preferred, but since this leads to reduced production efficiency and increased costs in the manufacturing process, the N content is set to 0.0001% or more.

[0079] Cr: less than 1.0%

[0080] Cr is an element that improves hardenability, readily forms martensite, and thus contributes to high strength. Cr is added as a substitute for C, Si, and Mn to adjust strength. On the other hand, like Si, Cr readily forms Cr-based oxides on the surface of steel sheets, sometimes causing a lack of plating. Therefore, only the amount needed to ensure strength is added; from a plating perspective, the Cr content is set to 1.0% or less. The Cr content is preferably 0.7% or less. While no specific lower limit is specified, the Cr content is set to 0.05% or more to stably control internal oxidation.

[0081] The above composition may include the following components as optional components. It should be noted that when the following optional elements are contained in amounts below the lower limit, the optional components are included as unavoidable impurities.

[0082] It contains, by mass%, one or more groups selected from A to E below.

[0083] Group A: One or more of Ti, Nb, V, W, and Zr: total less than 0.200%.

[0084] Group B: One or more of Mo, Cu, Co, and Ni: totaling 0.01% or more but less than 0.5%

[0085] Group C: B: Above 0.0003% and below 0.0050%

[0086] Group D: One or more of Sb and Sn: Total percentage above 0.001% and below 0.200%

[0087] Group E: One or more of Ca, Mg, and REM: Total ≥0.0001% and ≤0.0005%

[0088] Ti, Nb, V, W, and Zr form carbides and nitrides (sometimes carbonitrides) with C and N. The formation of fine precipitates contributes to the increased strength of the steel sheet. In particular, the strength is improved by precipitation in soft ferrite, and by reducing the strength difference with martensite, it contributes not only to improved bending properties but also to improved flange extension. Furthermore, these elements have the effect of refining the microstructure of hot-rolled coils, and by refining the microstructure of the steel after subsequent cold rolling and annealing, they also contribute to increased strength and improved workability such as bending properties. From the viewpoint of achieving 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 will increase the deformation resistance during cold rolling, hindering productivity. In addition, the presence of excessive or coarse precipitates reduces the ductility of ferrite, thus reducing the ductility and bending properties of the steel sheet. Therefore, the upper limit for the total amount of one or more of Ti, Nb, V, W, and Zr is set to 0.200%.

[0089] Mo, Cu, Co, and Ni are elements that improve hardenability, facilitate martensite formation, and thus contribute to high strength. To achieve these effects, it is preferable to contain at least one of Mo, Cu, Co, and Ni in total of 0.01%. Excessive addition of Mo, Cu, Co, and Ni can lead to saturation of the effect, increased cost, and, moreover, Cu can induce cracks during hot rolling, thus causing surface defects. Therefore, it is preferable to contain at least 0.5% of Mo, Cu, Co, and Ni in total. It should be noted that Ni, since it has the effect of suppressing surface defects caused by the addition of Cu, is preferably added simultaneously with Cu. Ni containing at least half the amount of Cu is particularly preferred.

[0090] Boron (B) is also an element that improves hardenability, facilitates martensite formation, and thus contributes to high strength. Regarding B, a lower limit is set to achieve an inhibitory effect on ferrite formation during annealing cooling. Furthermore, since even excessive addition would saturate the strength-enhancing effect, leading to excessive hardenability and potential disadvantages such as weld cracks during welding, an upper limit is set. Therefore, when B is present, the content is preferably set to 0.0003% or more and 0.0050% or less.

[0091] Sb and Sn are effective elements in inhibiting decarburization, denitrification, and deboronization, and thus in suppressing the reduction of steel plate strength. Therefore, it is preferable to contain one or more of Sb and Sn in total of 0.001% or more. However, excessive addition will reduce surface properties, so it is preferable to set the upper limit of the total amount of one or more of them to 0.200%.

[0092] Adding small amounts of Ca, Mg, and REM can spherize the sulfides and improve the flexibility of the steel sheet. On the other hand, excessive addition leads to the formation of excessive sulfides and oxides in the steel, reducing the workability, especially the flexibility, 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. Furthermore, there is no specific lower limit for the content, but it is preferable to contain a total of 0.0001% or more of one or more of Ca, Mg, and REM.

[0093] The above-mentioned composition may further include the following components as optional components.

[0094] Group F: Ta: below 0.10% (excluding 0%)

[0095] Group G: Selected from one or more of the following: Te: less than 0.10% (excluding 0%), As: less than 0.10% (excluding 0%), and Hf: less than 0.10% (excluding 0%).

[0096] Group H: Selected from one or more of Bi: less than 0.20% (excluding 0%) and Pb: less than 0.20% (excluding 0%).

[0097] Group I: Selected from one or more of the following: Zn: less than 0.10% (excluding 0%), Ge: less than 0.10% (excluding 0%), Sr: less than 0.10% (excluding 0%), and Cs: less than 0.10% (excluding 0%).

[0098] Group F [Ta: below 0.10% (excluding 0%)]

[0099] Like the elements in Group A, Ta is an effective element for improving the strength of steel plates and can be included as needed. While strength can be increased by including more than 0.005% Ta, from the perspective of preventing cost increases, the Ta content should be set below 0.10% when included.

[0100] • Group G [selected from one or more elements among Te: less than 0.10% (excluding 0%), As: less than 0.10% (excluding 0%), and Hf: less than 0.10% (excluding 0%)]

[0101] Te, As, and Hf, like the elements in group C, are all elements used for the speciation control of sulfides.

[0102] Te: Below 0.10% (excluding 0%)

[0103] By including more than 0.001% Te, the morphology of sulfides can be controlled, and ductility and toughness can be improved. However, from the point of view of preventing cost increases, the Te content is set to less than 0.10% when Te is included.

[0104] • As: less than 0.10% (excluding 0%)

[0105] By including more than 0.001% As, the morphology of sulfides can be controlled, and ductility and toughness can be improved. However, from the point of view of preventing cost increases, the As content is set to less than 0.10% when As is included.

[0106] • Hf: Below 0.10% (excluding 0%)

[0107] By including more than 0.01% Hf, the morphology of sulfides can be controlled, and ductility and toughness can be improved. However, from the point of view of preventing cost increases, the Hf content is set to less than 0.10% when Hf is included.

[0108] • Group H [selected from one or more elements selected from Bi: less than 0.20% (excluding 0%) and Pb: less than 0.20% (excluding 0%)]

[0109] Both Bi and Pb are elements that suppress grain boundary segregation and improve ductility and toughness. In the presence of Bi and Pb, their values ​​are set to be greater than 0%.

[0110] •Bi: 0.20% or less (excluding 0%)

[0111] By including more than 0.001% Bi, grain boundary segregation can be suppressed, and ductility and toughness can be improved. In addition, Bi improves machinability and the smoothness of the cut surface, thus enhancing the resistance to delayed fracture of the cut surface. To prevent cost increases, the Bi content is set to 0.20% or less when Bi is included.

[0112] • Pb: below 0.20% (excluding 0%)

[0113] By including 0.001% or more of Pb, grain boundary segregation can be suppressed, and ductility and toughness can be improved. In addition, Pb improves machinability and the smoothness of the cut surface, thus enhancing the resistance to delayed fracture of the cut surface. To prevent cost increases, the Pb content is set to 0.20% or less when including Pb.

[0114] • Group I [selected from one or more elements selected from Zn: less than 0.10% (excluding 0%), Ge: less than 0.10% (excluding 0%), Sr: less than 0.10% (excluding 0%), and Cs: less than 0.10% (excluding 0%)]

[0115] Zn, Ge, Sr, and Cs are all elements that do not significantly affect mechanical properties or surface quality but increase strength. In the case of Zn, Ge, Sr, and Cs, they are set to be more than 0%.

[0116] • Zn: less than 0.10% (excluding 0%)

[0117] Regarding Zn, even a content of 0.001% or higher has no significant impact on mechanical properties or surface quality. From the perspective of preventing cost increases, the Zn content should be set below 0.10% when Zn is present.

[0118] Ge: 0.10% or less (excluding 0%)

[0119] Regarding Ge, even a content of 0.001% or higher has no significant impact on mechanical properties or surface quality. From the perspective of preventing cost increases, the Ge content should be set below 0.10% when Ge is present.

[0120] • Sr: Below 0.10% (excluding 0%)

[0121] Regarding Sr, even a content of 0.001% or higher has no significant impact on mechanical properties or surface quality. From the perspective of preventing cost increases, the Sr content should be set below 0.10% when Sr is present.

[0122] • Cs: Below 0.10% (excluding 0%)

[0123] Regarding Cs, even a content of 0.001% or higher has no significant impact on mechanical properties or surface quality. From the perspective of preventing cost increases, the Cs content should be set below 0.10% when Cs is present.

[0124] In the steel plate (hereinafter, sometimes referred to as the base steel plate, matrix steel plate, steel base, or parent steel base), the balance other than the above-mentioned composition is Fe and unavoidable impurities.

[0125] The mass ratio of (Si+Cr) to Mn ((Si+Cr) / Mn) is 0.25 or higher.

[0126] To achieve excellent plating properties, it is important to control the amount of easily oxidized elements in the steel (besides Si, Cr also has this effect). From the perspective of suppressing the external oxidation of Mn, it is necessary to form a composite oxide of Si, Cr, and Mn inside the steel sheet. When the ((Si+Cr) / Mn) ratio is less than 0.25, a sufficient amount of Si, Cr, and Mn composite internal oxide cannot be formed in the surface layer of the steel sheet within 100 μm from the base steel sheet surface, resulting in surface enrichment of these elements and poor plating. Therefore, the ((Si+Cr) / Mn) ratio is set to 0.25 or higher.

[0127] The mass ratio of Si to Mn (Si / Mn) is less than 0.25.

[0128] Regarding Si, the decrease in ductility is relatively small compared to the increase in strength, contributing not only to the improvement in strength but also to the improvement of the balance between strength and ductility. On the other hand, Si expands the liquidus-stable region of zinc towards the low-temperature side, thus deteriorating resistance to LME cracking. However, by controlling the weight ratio of Si to Mn (Si / Mn) to less than 0.25, the deterioration of resistance to LME cracking caused by the increase in Si concentration can be mitigated. The mechanism is not yet clear, but it is believed that when the Mn concentration increases, it suppresses the expansion (shift) of the liquidus-stable region of zinc towards the low-temperature side that accompanies the increase in Si concentration. Therefore, the Si / Mn ratio is set to less than 0.25.

[0129] In addition, the balance consists of Fe and unavoidable impurities.

[0130] The high-strength galvanized steel sheet of the present invention has a coating adhesion amount of 20g / m² on each single side of the steel sheet surface. 2 Above and 120g / m2 The following zinc coatings have a strength of less than 20 g / m². 2 At that time, it is difficult to ensure corrosion resistance. On the other hand, exceeding 120g / m 2 Over time, the adhesion of the coating deteriorates.

[0131] Furthermore, in the high-strength galvanized steel sheet of the present invention, the oxygen content of the surface layer of the steel sheet, which is located immediately below the zinc coating and extends 100 μm from the surface of the base steel sheet towards the center of the sheet thickness, as measured by the method described in the embodiments, is 0.030 g / m² per single side. 2 Above and 0.40g / m 2 From the viewpoint of obtaining fine internal oxides, it is preferable that the aforementioned oxygen-containing oxides have at least one element selected from Fe, Si, Mn, Al, P, B, Nb, Ti, Cr, Mo, and V as their main component. Furthermore, in the cross-section of the steel plate, the maximum length of the internal oxides present in the surface layer of the steel plate is 6.0 μm or less, and the number of internal oxides present in the surface layer of the steel plate on grain boundaries having a length of 1.0 μm or more is 20 or less per 100 μm of the steel plate surface layer in the width direction.

[0132] In hot-dip galvanized steel sheets containing Si and a large amount of Mn, to achieve good appearance and coating adhesion during processing, it is necessary to control the amount and size of internal oxides in the steel substrate surface layer immediately below the coating, which could potentially become crack initiation points during heavy processing. Therefore, in this invention, dew point control is performed during the annealing process (described later) to control the oxygen potential of the atmosphere in the annealing furnace. By controlling the dew point to increase the oxygen potential, easily oxidizable elements such as Si, Mn, and Cr undergo internal oxidation before coating. As a result, the activity of Si, Mn, and Cr in the steel substrate surface layer decreases, external oxidation is suppressed, and the appearance and adhesion of the coating are improved. This improvement is achieved when the oxygen content in the steel substrate surface layer within 100 μm from the base steel sheet is 0.030 g / m² per side. 2 The above conditions become significant, therefore the lower limit is set at 0.030 g / m² per single side. 2 That's all. On the other hand, even if the oxygen content exceeds 0.40 g / m² per unit surface... 2 The effect exists in this way, but it is also saturated, and depending on the situation, it may also cause cracks during heavy processing. Therefore, the upper limit is set at 0.40 g / m. 2 .

[0133] A detailed investigation was conducted into the conditions under which internal oxides exist in the surface portion of the steel plate, which can become crack initiation sites. The results showed that internal oxides in the surface portion of the steel plate with a width of 0.1 μm or more and a length of 1.0 μm or more contribute to crack formation, and the longer the length, the more significant the crack initiation effect. Therefore, the maximum length of internal oxides present in the surface portion of the steel plate is set to 6.0 μm or less in the cross-section of the steel plate. When the length of internal oxides exceeds 6.0 μm, the internal oxides become crack initiation sites during processing, and the sealing and post-processing corrosion resistance are significantly deteriorated. On the other hand, internal oxides that satisfy either a width of less than 0.1 μm or a length of less than 1.0 μm have little effect on crack formation during heavy processing, and therefore have no effect on sealing and post-processing corrosion resistance. The lower limit of the maximum length of internal oxides present in the surface portion of the steel plate is not specifically specified, but in the method of the present invention, it is approximately 0.3 μm, preferably 0.3 μm or more.

[0134] Furthermore, in the cross-section of the steel plate, the number of internal oxides present in the surface layer of the steel plate with a length of 1.0 μm or more is set to 20 or less per 100 μm of the steel plate width direction in the surface layer. If the number of internal oxides with a length of 1.0 μm or more exceeds 20 per 100 μm of the steel plate width direction in the surface layer, similar to the case where the maximum length of the internal oxides exceeds 6.0 μm, the internal oxides become the initiation point for cracks during processing, and the sealing and corrosion resistance after processing deteriorate. Therefore, the number of internal oxides with a length of 1.0 μm or more is set to 20 or less per 100 μm of the steel plate width direction in the surface layer. While no specific lower limit is specified, a lower limit of less than 4 makes it difficult to meet the oxygen content requirement of 0.030 g / m² per single side for the surface layer of the steel plate. 2 Therefore, it is preferable to set it to 4 or more.

[0135] Next, the manufacturing method of the high-strength galvanized steel sheet of the present invention will be described. The manufacturing method of the present invention includes a casting process, a hot rolling process, a pickling process, a cold rolling process, an annealing process, and a galvanizing process. Each process will be described below.

[0136] The casting process refers to the process of casting steel having the above-mentioned composition to produce steel raw material. The steel used in the manufacturing method of the present invention is preferably cast under conditions where the molten steel flow rate at the solidification interface near the meniscus of the mold is 16 cm / s or higher.

[0137] Steel raw material (slab (cast sheet)) manufacturing

[0138] The steel used in the manufacturing method of the present invention is steel commonly referred to as slab steel manufactured by a continuous casting method, but its purpose is to prevent macroscopic segregation of alloy composition. It can also be manufactured by ingot casting, thin slab casting, etc.

[0139] From the perspective of controlling inclusions, in the case of continuous casting, it is preferable to cast under conditions where the molten steel flow rate at the solidification interface near the meniscus of the mold is 16 cm / s or higher. "Near the meniscus of the mold" refers to the interface between the powder and molten steel used in continuous casting within the mold. In the case of ingot casting, it is preferable to allow inclusions to fully float to the surface during solidification and discard the area where these inclusions accumulate for use in the next process.

[0140] The hot rolling process is the process of hot rolling the steel raw material after the casting process.

[0141] After manufacturing the steel billet, in addition to the existing method of temporarily cooling it to room temperature and then reheating it, the following methods can also be used without problems: hot rolling in a warm sheet state without cooling it to near room temperature; hot rolling immediately after a small amount of reheating; or hot rolling while maintaining a high temperature after casting.

[0142] There are no specific provisions regarding the hot rolling method, but it is preferred to carry out the hot rolling under the following conditions.

[0143] The preferred heating temperature for the steel billet is between 1100°C and 1350°C. This is because precipitates present in the billet tend to coarsen, which is detrimental, for example, when strength is ensured through precipitation strengthening. Alternatively, coarse precipitates may act as nuclei, negatively impacting microstructure formation during subsequent annealing. Furthermore, heating removes bubbles and defects from the slab surface, reducing surface cracking and unevenness, resulting in a smooth surface that improves product quality. Therefore, the slab heating temperature is specified based on this viewpoint. To achieve this effect, a slab heating temperature of 1100°C or higher is preferred. On the other hand, when the slab heating temperature exceeds 1350°C, austenite grains coarsen, leading to a coarser microstructure in the final product and reduced strength and flexibility of the steel sheet. Therefore, 1350°C or lower is specified as the preferred upper limit for the slab heating temperature.

[0144] In the hot rolling process, which includes roughing and finishing, steel billets are usually rolled into thin slabs through roughing and into hot-rolled coils through finishing. However, depending on the capacity of the rolling mill, such a distinction is not strictly enforced, as long as the billet reaches the specified dimensions, there is no problem.

[0145] The following conditions are recommended for hot rolling.

[0146] Finishing rolling temperature: above 800℃ and below 950℃

[0147] The purpose of setting the finishing rolling temperature above 800°C is to achieve a uniform microstructure in the hot-rolled coil, resulting in a more uniform microstructure in the final product. Inhomogeneous microstructure reduces flexibility. On the other hand, when the finishing rolling temperature exceeds 950°C, the amount of oxide (scale) formed increases, the interface between the steel matrix and the oxide becomes rough, and the surface quality deteriorates after pickling and cold rolling. Furthermore, the grain diameter becomes larger, which, similar to the coarsening of the billet microstructure, leads to a decrease in the strength and flexibility of the steel plate.

[0148] For hot-rolled coils (hot-rolled plates) after the above hot rolling is completed, in order to refine and homogenize the structure, it is preferable to start cooling within 3 seconds after finishing rolling, and to cool at an average cooling rate of 10 to 250°C / s in the temperature range of [finishing rolling temperature] to [finishing rolling temperature - 100]°C, and to roll them into coils in the temperature range of 450 to 700°C.

[0149] Pickling is a process performed on steel sheets after hot rolling. Pickling removes oxide scale from the surface of the steel sheet. Appropriate pickling conditions are sufficient.

[0150] The cold rolling process is a process of cold rolling steel plates that have undergone pickling.

[0151] The reduction rate of cold rolling is preferably 20% or more and 80% or less. By setting the reduction rate to 20% or more, a uniform and fine steel structure can be obtained in the subsequent annealing process; therefore, a reduction rate of 20% or more is preferred. When the cold rolling reduction rate is less than 20%, the steel is prone to becoming coarse-grained and uneven in structure during annealing, which, as mentioned above, raises concerns about reduced strength and processability of the final product. Regarding the upper limit, under high reduction rates, in addition to the reduction in productivity caused by rolling load, there is also the possibility of shape defects; therefore, a reduction rate of 80% or less is preferred. It should be noted that pickling can also be performed after cold rolling.

[0152] In the annealing process, the atmosphere inside the heating furnace is controlled as described above.

[0153] The galvanizing process is carried out, for example, by immersion in a hot-dip galvanizing bath. Hot-dip galvanizing can be performed using conventional methods, adjusting the coating thickness on each side to the range described above.

[0154] After galvanizing, the zinc coating can also be alloyed as needed. In this case, the galvanized steel sheet is held at a temperature of 450–580℃ for about 1–60 seconds.

[0155] Example

[0156] Molten steel with the compositions shown in Tables 1-1 and 1-2 was smelted in a converter and manufactured into slabs by continuous casting. The composition is % by mass, with the balance being Fe and unavoidable impurities. The slab was heated to 1200°C and hot-rolled at a finishing temperature of 840°C and a coiling temperature of 550°C to produce hot-rolled coils with a thickness of 2.8 mm. The hot-rolled coils were then cold-rolled into cold-rolled steel sheets with a thickness of 1.6 mm using a cold rolling reduction rate of 50%. The cold-rolled steel sheets were annealed in the annealing furnace atmosphere described in Table 2, cooled to 600°C at an average cooling rate of 3°C / s, held at 520°C for 50 seconds, and then galvanized to produce high-strength hot-dip galvanized steel sheets. Next, except for No. 39, alloying treatment was performed.

[0157]

[0158] Samples are cut from the coated steel sheets obtained through the above processes. The appearance is visually observed to evaluate the coating properties (surface characteristics), and the coating characteristics are also evaluated. Furthermore, a tensile test is performed to determine the tensile strength (TS). The evaluation method is as follows.

[0159] (1) The oxygen content of the surface layer of the steel plate, which is immediately below the coating and extends from the surface of the base steel plate towards the center of the plate thickness, within a depth of 100 μm.

[0160] To determine the oxygen content immediately beneath the coating, the coating is peeled off using a hydrochloric acid or alkaline solution containing an inhibitor without dissolving the base steel. The oxygen content is then determined using a pulse furnace-infrared absorption method. However, it is necessary to subtract the oxygen content in the original material (i.e., the steel sheet before annealing). Therefore, in this invention, the surface layer of both sides of the continuously annealed high-strength steel sheet is ground to at least 100 μm, and the oxygen concentration in the steel is measured. This measured value is taken as the oxygen content (OH) in the original material. In addition, the oxygen concentration in the steel throughout the thickness direction of the continuously annealed high-strength steel sheet is measured, and this measured value is taken as the oxygen content (OI) after oxidation. Using the oxygen content (OI) of the steel sheet obtained in this way and the original oxygen content (OH) in the original material, the difference between OI and OH (=OI-OH) is calculated. This difference is then further converted to the amount per unit area on one side, and the resulting value (g / m²) is calculated. 2 () as oxygen content.

[0161] Subsequently, 10mm × 10mm sample pieces were cut from the cross-section after the coating was peeled off, embedded in resin, and then mirror-finished to create a sample for cross-sectional observation. Using SEM at 5000x magnification, five fields of view with a width of 20μm were observed at each horizontal level. The maximum length (μm) of the internal oxides and the number of internal oxides with a length of 1.0μm or more were measured in each field of view. These values ​​were averaged, and the average value was used as the maximum length of the internal oxides and the number of internal oxides with a length of 1.0μm or more at that level. At this point, only oxides with a width of 0.1μm or more were considered in the count of internal oxides.

[0162] (2) Surface properties (appearance)

[0163] Visually inspect the appearance of the manufactured hot-dip galvanized steel sheets. A condition completely free of uncoated defects is rated "○: Qualified, Excellent"; a condition with uncoated defects is rated "×: Unqualified"; and a condition without uncoated defects but with uneven coating appearance is rated "△: Qualified, Pass". It should be noted that uncoated defects refer to areas of the steel sheet with no coating and exposed areas equivalent to a diameter of 50μm or larger.

[0164] The results are shown in Table 2.

[0165] (3) Coating adhesion

[0166] [Alloyed hot-dip galvanized steel sheet]

[0167] In this embodiment, cellophane tape is pressed against a processing section formed by bending a hot-dip galvanized steel sheet at 90° to transfer the peeling material to the cellophane tape. The amount of peeling material on the cellophane tape is determined by fluorescence X-ray diffraction in the form of Zn count. The measurement conditions are set as follows: mask diameter 30 mm, accelerating voltage of fluorescence X-ray 50 kV, accelerating current 50 mA, and measurement time 20 seconds. In particular, considering the possibility of uneven (fluctuating) adhesion within the surface, for any length of 6 m of the manufactured hot-dip galvanized steel sheet, adhesion is measured at 5 locations every 1 m along the length of the roll, at 1 / 4, 1 / 2 (center), 3 / 4, and 50 mm from the edge of the steel sheet (a total of 30 locations). Based on the highest Zn count, the coating performance is evaluated according to the following criteria. In this invention, the following grades ◎ or 〇 are considered acceptable.

[0168] ◎(Qualified, even better): Zn count less than 6000.

[0169] 〇 (Qualified, Excellent): Zn count is above 6000 and less than 8000.

[0170] × (Unacceptable): Zn count is above 8000.

[0171] [Unalloyed hot-dip galvanized steel sheet]

[0172] The adhesion of the coating on hot-dip galvanized steel sheets was evaluated using a falling ball impact test. With a ball weight of 2.8 kg and a drop height of 1 m, the steel sheet was installed in a mold with holes of 3 / 8 inch and 1 / 2 inch diameters, and the falling ball impact test was performed. Tape was peeled off the machined parts, and the presence or absence of coating peeling was visually assessed and marked according to the following standards. Here, 1 mm... 2 The following peeling is set as micro-peeling, with a thickness of 1mm. 2 The above stripping is defined as stripping.

[0173] ◎(Qualified, Superior): No coating peeling under any conditions

[0174] 〇 (Acceptable, Excellent): Minor peeling present in 3 / 8-inch caliber cases.

[0175] △(Failure): Peeling occurs in 3 / 8-inch diameter cases, but no plating peeling occurs in 1 / 2-inch diameter cases.

[0176] × (Unacceptable): Coating peeling occurs under any conditions.

[0177] (4) Corrosion resistance after processing

[0178] The same processing as the coating peel resistance test was performed. Test pieces were prepared without tape peeling. Using degreasing agent FC-E2011 (manufactured by PALBOND, Japan), surface conditioner PL-X, and chemical conversion treatment agent PALBOND (registered trademark) PB-L3065, the coating adhesion was chemically converted to a level of 1.7–3.0 g / m² under the following standard conditions. 2 Chemical transformation is carried out in a manner that facilitates the process.

[0179] <Standard Conditions>

[0180] • Degreasing process: processing temperature is 40℃, processing time is 120 seconds; • Spray degreasing and surface conditioning process: pH 9.5, treatment temperature room temperature, treatment time 20 seconds; • Chemical conversion treatment process: The temperature of the chemical conversion treatment solution is 35℃, and the treatment time is 120 seconds.

[0181] On the surface of the test pieces that underwent the above chemical conversion treatment, an electrodeposition coating of V-50 manufactured by Nippon Paint Co., Ltd. was applied with a film thickness of 25 μm for corrosion testing.

[0182] <Salt spray test (SST)>

[0183] For the test pieces that have undergone chemical conversion treatment and electrodeposition coating, the bent surfaces of alloyed hot-dip galvanized steel sheets, and the drop ball impact areas of hot-dip galvanized steel sheets, after introducing cutting defects that reach the coating with a cutting tool, the test pieces are subjected to a 240-hour salt spray test using a 5% (w / w) NaCl aqueous solution according to the neutral salt spray test specified in JIS Z2371:2000. Following this, a tape peel test is performed on the intersecting cutting defect areas, and the maximum total peel width (combining the left and right sides of the cutting defect area) is measured. Markings (◎, ○, ×) are used as the reference. When the maximum total peel width is 2.0 mm or less, the corrosion resistance in the salt spray test can be evaluated as good.

[0184] ◎: The total width of the maximum bulge from the cutting defect is less than 2.0 mm (good).

[0185] 〇: The total width of the maximum bulge from the cutting defect exceeds 2.0mm but is less than 2.5mm (acceptable)

[0186] ×: The total width of the maximum bulge from the cutting defect exceeds 2.5mm (unacceptable).

[0187] (5) Evaluation of resistance to LME cracking

[0188] Test piece 2, cut from a hot-dip galvanized steel sheet with the rolling right-angle direction (TD) as the long side and the rolling direction as the short side, measuring 150mm in the long side and 50mm in the short side, was compared with a piece of the same size, with a coating adhesion of 50g / m² on each single side of the hot-dip galvanized layer. 2 The test hot-dip galvanized steel sheets (1.6mm thick, TS: 980MPa grade) 1 were stacked to form a plate assembly. This assembly was created by bonding the hot-dip galvanized layer of the test piece 2 to the hot-dip galvanized layer of a commercially available hot-dip galvanized steel sheet. For example... Figure 1 As shown, the plate assembly is fixed to the mounting platform 5 with a maximum tilt of 5°, which is conceived in the shape of a portion of the component, separated by a spacer 3 with a thickness of 2.0 mm. The spacer 3 is a pair of steel plates with a long side of 50 mm × a short side of 45 mm × a thickness of 2.0 mm, and the long side end faces of each pair of steel plates are aligned with the short side end faces of the plate assembly. Therefore, the distance between the pair of steel plates constituting the spacer 3 is 60 mm. The mounting platform 5 is a plate with a hole in the center.

[0189] Next, resistance welding was performed using a single-phase AC (50Hz) resistance welding machine with a servo motor pressurization method. While bending the plate assembly under pressure applied by a pair of electrodes 4 (front diameter: 6mm), the welding current and welding time were adjusted appropriately for each plate assembly to achieve a welded section. The plate assembly was produced by applying a pressure of 3.5kN, a holding time of 0.10 seconds or 0.16 seconds, and a weld nugget diameter 7 of 5.9mm. At this time, the pair of electrodes 4 applied pressure to the plate assembly vertically, with the lower electrode 4 applying pressure to the test piece through a hole in the fixing platform 5. During pressure application, the lower electrode 4 was fixed to the fixing platform 5 by contacting the plane extending the contact surface between the spacer 3 and the fixing platform 5, while the upper electrode 4 was movable. Additionally, the upper electrode 4 was brought into contact with the center of the test hot-dip galvanized steel plate 1.

[0190] It should be noted that the holding time refers to the time from the end of welding current flow to the start of electrode opening. Additionally, the weld nugget diameter is 7... Figure 2 The figure shows the distance between the ends of the melt core along the length of the plate assembly.

[0191] Next, as Figure 2 As shown, the aforementioned plate assembly with the welded portion was cut to include the weld (weld nugget). The cross-section of the welded portion was observed using an optical microscope (200x magnification), and the resistance to resistance weld cracking characteristics of the welded portion were evaluated according to the following criteria. Here, Figure 2 The image above is a top view of the plate assembly with welded sections, with line segment 8 indicating the cutting position. Figure 2 The figure below shows a cross-section along the thickness of the cut plate assembly, schematically illustrating the cracks (fissures) that occurred in the test piece. It should be noted that, in the case of cracks occurring in the test hot-dip galvanized steel plate 1, the stress dispersion in test piece 2 prevented proper evaluation. Therefore, data from the test hot-dip galvanized steel plate 1, where no cracks occurred, were used as an example.

[0192] When the evaluation below is “〇” or “◎”, the resistance weld crack resistance characteristics of the welded part are judged as good or excellent, respectively. When the evaluation below is “×”, the resistance weld crack resistance characteristics of the welded part are judged as poor.

[0193] ◎: No cracks longer than 0.1 mm were detected during a holding time of 0.10 seconds.

[0194] ○: A crack longer than 0.1 mm was detected when the holding time was 0.10 seconds, but no crack longer than 0.1 mm was detected when the holding time was 0.16 seconds.

[0195] ×: A crack longer than 0.1 mm was detected during a holding time of 0.16 seconds.

[0196] (6) Tensile test

[0197] JIS5 tensile test pieces (JISZ2201) were cut from the galvanized steel sheet at a right angle to the rolling direction and subjected to tensile testing at a fixed tensile speed (crosshead speed) of 10 mm / min. The tensile strength was calculated by dividing the maximum load during the tensile test by the initial cross-sectional area of ​​the parallel portion of the test piece. The sheet thickness used in the calculation of the cross-sectional area of ​​the parallel portion included the galvanized coating thickness.

[0198] (7) Analysis methods for furnace gases

[0199] Gas was collected from the annealing furnace, and SO2 and HCl were quantified by ion chromatography. Three analyses were performed, and the average value was taken as the gas concentration inside the furnace.

[0200]

[0201] As shown in Table 2, the hot-dip galvanized steel sheet manufactured by the method of the present invention, although a high-strength steel sheet containing easily oxidizable elements such as Si, Mn, and Cr, also exhibits a good coating appearance and excellent coating adhesion and resistance to LME cracking. On the other hand, in the comparative examples, any one or more of the following aspects—coating appearance, coating adhesion, resistance to LME cracking, and tensile strength—were poor.

[0202] Industrial availability

[0203] The high-strength hot-dip galvanized steel sheet of this invention exhibits excellent coating appearance, coating adhesion, and resistance to LME cracking, making it suitable as a surface-treated steel sheet for achieving both lightweight and high-strength automotive body construction. Furthermore, besides automotive applications, it 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 resistance to the raw steel sheet.

[0204] Symbol Explanation

[0205] 1. Experimental hot-dip galvanized steel sheet

[0206] 2 Test pieces

[0207] 3 spacers

[0208] 4 electrodes

[0209] 5. Fixed platform

[0210] 6. Melting core

[0211] 7. Melting core diameter

[0212] 8 represents the line segment at the cutting position.

[0213] 9. Cracks (fissures)

Claims

1. A method for manufacturing a high-strength hot-dip galvanized steel sheet, wherein a coating with an adhesion amount of 20 g / m² on each side is formed on the surface of the steel sheet. 2 Above and 120g / m 2 The following method describes a high-strength hot-dip galvanized steel sheet with a zinc coating, wherein the steel sheet comprises, 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, Cr: 1.0% or less, 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 consists of Fe and unavoidable impurities. In this manufacturing method, When annealing and hot-dip galvanizing steel plates are carried out in a continuous hot-dip galvanizing equipment, the highest temperature T of the steel plate in the annealing furnace is higher than 700℃ and lower than 900℃. The dew point of the atmosphere in the furnace within the temperature range of 700℃ and T℃ is higher than -20℃. In addition to containing 3.0% by volume and 20.0% by volume of hydrogen, the atmosphere in the furnace also contains one or more of the following: SO2 selected from 0.1% by volume and 3.0% by volume of ppm and HCl selected from 0.5% by volume and 10.0% by volume of ppm.

2. The method for manufacturing high-strength hot-dip galvanized steel sheet according to claim 1, wherein, The steel plate, by mass percentage, also contains one or more of the following groups A to E. Group A: One or more of Ti, Nb, V, W and Zr: total less than 0.200%; Group B: One or more of Mo, Cu, Co, and Ni: totaling 0.01% or more and 0.5% or less; Group C: B: Above 0.0003% and below 0.0050%; Group D: One or more of Sb and Sn: Total percentages of 0.001% and 0.200% or less; Group E: One or more of Ca, Mg and REM: Total ≥0.0001% and ≤0.0005%.

3. The method for manufacturing high-strength hot-dip galvanized steel sheet according to claim 1 or 2, wherein, The steel plate, by mass percentage, also contains one or more components selected from groups F to I below as the composition. Group F: Ta: below 0.10% (excluding 0%); Group G: Selected from one or more of the following: Te: less than 0.10% (excluding 0%), As: less than 0.10% (excluding 0%), and Hf: less than 0.10% (excluding 0%). Group H: Selected from one or more of Bi: less than 0.20% (excluding 0%) and Pb: less than 0.20% (excluding 0%); Group I: Selected from one or more of the following: Zn: less than 0.10% (excluding 0%), Ge: less than 0.10% (excluding 0%), Sr: less than 0.10% (excluding 0%), and Cs: less than 0.10% (excluding 0%).

4. A high-strength hot-dip galvanized steel sheet, wherein the coating thickness on each side of the steel sheet is 20 g / m². 2 Above and 120g / m 2 The following zinc coating is applied to a steel sheet with the following composition by mass percentage: C: 0.060% to 0.250%, Si: 0.10% to 0.80%, Mn: 1.50% to 3.50%, P: 0.020%, S: 0.0100%, Al: 0.100%, N: 0.0060%, Cr: 1.0%, with a Si+Cr to Mn mass ratio ((Si+Cr) / Mn) of 0.25 or higher, a Si to Mn mass ratio (Si / Mn) of less than 0.25, and the balance consisting of Fe and unavoidable impurities. The oxygen content of the surface layer of the steel plate, immediately below the zinc coating and extending 100 μm from the surface of the base steel plate towards the center of the plate thickness, is 0.030 g / m² per single side. 2 Above and 0.40g / m 2 the following, In the cross-section of the steel plate, the maximum length of the internal oxide present in the surface layer of the steel plate is less than 6.0 μm, and the number of internal oxides with a length of more than 1.0 μm present in the surface layer of the steel plate is less than 20 per 100 μm of the steel plate width direction.

5. The high-strength hot-dip galvanized steel sheet according to claim 4, wherein, The steel plate, by mass percentage, also contains one or more of the following groups A to E. Group A: One or more of Ti, Nb, V, W and Zr: total less than 0.200%; Group B: One or more of Mo, Cu, Co, and Ni: totaling 0.01% or more and 0.5% or less; Group C: B: Above 0.0003% and below 0.0050%; Group D: One or more of Sb and Sn: Total percentages of 0.001% and 0.200% or less; Group E: One or more of Ca, Mg and REM: Total ≥0.0001% and ≤0.0005%.

6. The high-strength hot-dip galvanized steel sheet according to claim 4 or 5, wherein, The steel plate, by mass percentage, also contains one or more components selected from groups F to I below as the composition. Group F: Ta: below 0.10% (excluding 0%); Group G: Selected from one or more of the following: Te: less than 0.10% (excluding 0%), As: less than 0.10% (excluding 0%), and Hf: less than 0.10% (excluding 0%). Group H: Selected from one or more of Bi: less than 0.20% (excluding 0%) and Pb: less than 0.20% (excluding 0%); Group I: Selected from one or more of the following: Zn: less than 0.10% (excluding 0%), Ge: less than 0.10% (excluding 0%), Sr: less than 0.10% (excluding 0%), and Cs: less than 0.10% (excluding 0%).

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