Method for producing high-strength hot-dip galvanized steel sheet

Abstract

Provided is a method for producing a high-strength hot-dip galvanized steel sheet which has excellent LME cracking resistance while ensuring the appearance quality of the steel sheet, and in which the deterioration of delayed fracture resistance characteristics due to hydrogen embrittlement is suppressed. This method for producing a hot-dip galvanized steel sheet is a method in which a steel sheet containing a prescribed amount of Si is subjected to oxidation treatment, then reduction annealing is performed, and then hot-dip galvanizing is performed. The oxidation treatment is carried out in an atmosphere containing N2 and 500 ppm by volume or more of O2 at a temperature of 500-800 DEG C (inclusive). The reduction annealing is performed by means of a first reduction annealing of a front stage, in which the reduction annealing is held at a predetermined temperature for a predetermined time in an atmosphere containing 5.0-25 vol% of hydrogen gas with a dew point of-45 DEG C or more and + 20 DEG C or less and a remainder of N2, and a second reduction annealing of a rear stage, in which the reduction annealing is held at a predetermined temperature for a predetermined time in an atmosphere containing 5.0-25 vol% of hydrogen gas with a dew point of-45 DEG C or more and + 20 DEG C or less and the remainder of N2. The steel sheet is held at a predetermined temperature for a predetermined time in an atmosphere having a dew point of-10 DEG C to + 20 DEG C inclusive and containing 2.0 vol% to 8.0 vol% inclusive of hydrogen and a remainder of N2. In addition, the hydrogen gas concentration is adjusted so that the hydrogen gas concentration in the first reduction annealing in the front stage is higher than the hydrogen gas concentration in the second reduction annealing in the rear stage.

Description

Technical Field

[0001] This invention relates to a method for manufacturing high-strength hot-dip galvanized steel sheets with excellent resistance to resistance welding cracking and delayed fracture. Background Technology

[0002] In recent years, from the perspective of protecting the Earth's environment, there has been a strong demand to improve the fuel efficiency of automobiles. Furthermore, from the perspective of ensuring occupant safety in the event of a collision, there is also a strong demand to improve vehicle safety. To meet these requirements, it is necessary to simultaneously achieve lightweighting and high strength in automobile bodies. For hot-dip galvanized steel sheets, which are the raw materials for automobile components, there is an active push to achieve thinner walls through high strength. However, since most automobile components are manufactured by forming steel sheets, these steel sheets, in addition to requiring high strength, also require excellent formability.

[0003] There are various methods to improve the strength of hot-dip galvanized steel sheets. Among these methods, those that achieve high strength without significantly compromising the formability of the steel sheet include the effective utilization of martensite through the addition of carbon (C) and the solid solution strengthening through the addition of silicon (Si). On the other hand, in the manufacture of automotive parts, stamped components are mostly assembled by resistance welding (spot welding). If large amounts of C and Si are added to the steel sheet, during resistance welding, residual stress is generated near the weld joint, causing the zinc coating to melt and diffuse into the grain boundaries, resulting in liquid metal embrittlement (LME). This leads to grain boundary cracking (LME cracking), which is a concern. Especially when welding is performed with the welding electrode at an angle to the steel sheet, the residual stress increases, potentially causing cracking. Residual stress is believed to increase with increasing steel sheet strength; therefore, the occurrence of LME cracking accompanying the increase in steel sheet strength is a concern.

[0004] Furthermore, it is known that delayed fracture due to hydrogen embrittlement becomes more likely to occur with increasing steel strength, especially for high-strength steels with tensile strengths above 1180 MPa. It should be noted that delayed fracture refers to the phenomenon where high-strength steel, under static load stress (less than the tensile strength), suddenly fractures after a certain period without any apparent plastic deformation. This delayed fracture is mostly caused by hydrogen intrusion into the steel sheet due to corrosion from the service environment. However, hydrogen intrusion during the annealing process of a continuous hot-dip galvanizing line (CGL) can also degrade the mechanical properties of steel sheets, particularly those with tensile strengths exceeding 980 MPa, leading to brittle fracture.

[0005] As mentioned above, high-strength steel sheets with excellent resistance to resistance welding cracking (hereinafter also referred to as "LME cracking resistance") and suppression of the deterioration of delayed fracture characteristics due to hydrogen in the steel are required.

[0006] Conventionally, as a method to improve the non-plating defects in Si-added steel, Patent Document 1 discloses a method of oxidizing the surface of Si-added steel by heating it to 700°C or higher in an atmosphere containing O2, and reducing the oxide layer on the steel sheet surface in an atmosphere containing H2 with a dew point of 5°C or higher. Patent Document 2 discloses a method of oxidizing the surface of Si-added steel by heating it to 600°C or higher and 850°C or lower in an atmosphere containing O2, and reducing the oxidized steel sheet in an atmosphere containing H2O and H2 with a dew point of 5°C or higher and 5000 ppm by volume or less. Patent Document 3 similarly discloses a method of oxidizing the surface of Si-added steel by increasing the air ratio of a direct-fired furnace (DFF), and reducing the oxide layer on the steel sheet in an atmosphere containing H2O and H2 with a dew point of 5°C or higher and 5000 ppm by volume or less. H2O / P H2 Methods for reducing oxides on the surface of steel plates in an atmosphere above -3.4°C and below -1.1°C. However, while these methods can adjust the amount of oxidation in the steel plate and ensure good appearance quality, they have the following problems: due to the suppression of LME cracking in Si-added steel and the large amount of hydrogen remaining in the steel after annealing, sufficient resistance to LME cracking and delayed fracture characteristics cannot be obtained.

[0007] Existing technical documents

[0008] Patent documents

[0009] Patent Document 1: Japanese Patent No. 5652219

[0010] Patent Document 2: Japanese Patent No. 6052270

[0011] Patent Document 3: Japanese Patent No. 6172297 Summary of the Invention

[0012] The problem that the invention aims to solve

[0013] Therefore, the object of the present invention is to provide a high-strength hot-dip galvanized steel sheet that can ensure the appearance quality of the steel sheet, has excellent resistance to LME cracking, and can suppress the deterioration of delayed fracture resistance caused by hydrogen embrittlement.

[0014] Methods for solving problems

[0015] The inventors discovered that by appropriately suppressing the O2 concentration and temperature during steel plate oxidation, the appearance quality of the steel plate can be ensured. Furthermore, by optimizing the H2O and H2 concentrations during reduction annealing, the resistance to resistance welding cracking characteristics are excellent, while the deterioration of the resistance to delayed fracture caused by hydrogen embrittlement can be suppressed, thus completing this invention.

[0016] This invention is based on the above-mentioned insights. Specifically, the main structure of this invention is as follows.

[0017] [1] A method for manufacturing a high-strength hot-dip galvanized steel sheet, comprising: oxidizing a steel sheet containing 0.45% to 2.0% (by mass%) of Si, followed by reduction annealing, and then hot-dip galvanizing.

[0018] In the above oxidation treatment, the steel plate is oxidized in an atmosphere containing N2 and O2 at a temperature range of 500°C to 800°C.

[0019] The reduction annealing described above is carried out in different atmospheres in the first and second stages. In the first reduction annealing stage, the steel plate is held in an annealing atmosphere with a dew point of -45°C to +20°C, containing 5.0% to 25% hydrogen by volume and the balance being N2, at a temperature of 650°C to 900°C for 20 seconds to 150 seconds.

[0020] In the second reduction annealing stage, the steel sheet after the first reduction annealing is placed in an atmosphere with a dew point of -10°C to +20°C, containing 2.0% to 8.0% by volume of hydrogen, with the balance being N2, and the hydrogen concentration is adjusted such that the hydrogen concentration in the first reduction annealing stage is set to H2a and the hydrogen concentration in the second reduction annealing stage is set to H2b, with H2a > H2b. The atmosphere is then held at a temperature of 700°C to 950°C for 30 seconds to 300 seconds before hot-dip galvanizing.

[0021] [2] The manufacturing method of high-strength hot-dip galvanized steel sheet according to [1], wherein the reduction annealing is performed using an annealing furnace that is divided into two or more sections in the direction of steel sheet travel and is capable of annealing in two or more different atmospheres.

[0022] [3] The manufacturing method of high-strength hot-dip galvanized steel sheet according to [1] or [2], wherein after hot-dip galvanizing the steel sheet, alloying treatment of the hot-dip galvanized layer is performed.

[0023] [4] The method for manufacturing high-strength hot-dip galvanized steel sheet according to any one of [1] to [3], wherein the above-mentioned oxidation treatment is performed in the heating process of the steel sheet as a pre-reduction annealing process.

[0024] [5] The method for manufacturing high-strength hot-dip galvanized steel sheet according to [4] wherein the above-mentioned oxidation treatment is carried out in a temperature range of at least 50°C above 500°C and below 800°C.

[0025] [6] A method for manufacturing high-strength hot-dip galvanized steel sheet according to any one of [1] to [5], wherein, in the above-mentioned oxidation treatment, a direct-fired heating furnace (DFF) is used, and the air ratio of at least a portion of the atmosphere inside the heating furnace is set to 1.0 or more, thereby oxidizing the surface of the steel sheet.

[0026] [7] The method for manufacturing high-strength hot-dip galvanized steel sheet according to any one of [1] to [6], wherein the oxidation treatment is performed using a direct-fired heating furnace that is divided into two or more sections in the direction of steel sheet travel and is capable of oxidation in two or more atmospheres.

[0027] In the first heating zone at the front of the heating furnace, when the air ratio for the temperature range in which the above oxidation treatment is performed is set to α, the temperature is heated to a temperature of T1 (°C) or higher, calculated by the following formula (1), under the condition that the average heating rate at 200°C or higher is 10°C / second or higher and 50°C / second or lower.

[0028] In the second heating zone at the rear of the heating furnace, the steel plate that has passed through the first heating zone is heated to a temperature above T2 (°C) calculated by the following formula (2) under the conditions that the air ratio is ≤0.9 and the average heating rate is 5°C / second or more and 30°C / second or less when the temperature is higher than T1 (°C).

[0029] T1=28.2[Si]+7.95[Mn]-86.2α+666 ---(1)

[0030] T2 = T1 + 30 ---(2)

[0031] Here, [Si] is the Si content (mass%) in the steel plate, and [Mn] is the Mn content (mass%) in the steel plate.

[0032] [8] A method for manufacturing high-strength hot-dip galvanized steel sheet according to any one of [1] to [7], wherein the sheet contains N2 and O2 at 500 ppm or more by volume, and further contains CO, CO2, H2O, and NO. X The above oxidation treatment is carried out in one or more atmospheres.

[0033] [9] The method for manufacturing high-strength hot-dip galvanized steel sheet according to any one of [1] to [5] and [8], wherein the above-mentioned oxidation treatment is performed using a radiant tube furnace.

[0034]

[10] The method for manufacturing high-strength hot-dip galvanized steel sheet according to any one of [1] to [5], [8], [9], wherein the above-mentioned reduction annealing is performed using a radiant tube heating soaking furnace.

[0035]

[11] The method for manufacturing high-strength hot-dip galvanized steel sheet according to any one of [1] to

[10] , wherein, in the second reduction annealing, the annealing atmosphere contains 2.0% by volume or more and less than 5.0% by volume of hydrogen and the balance of N2.

[0036]

[12] The method for manufacturing high-strength hot-dip galvanized steel sheet according to any one of [1] to

[11] , wherein, in the first reduction annealing, the annealing atmosphere contains 5.0% by volume or more and 12% by volume or less of hydrogen and the balance N2, and in the second reduction annealing, the annealing atmosphere contains 2.0% by volume or more and less than 3.0% by volume of hydrogen and the balance N2.

[0037] Invention Effects

[0038] According to the present invention, a high-strength hot-dip galvanized steel sheet can be provided, which has excellent resistance to LME cracking in the welded part, good appearance quality, and significantly reduced hydrogen in the steel that is a deteriorating factor for resistance to delayed fracture. Attached Figure Description

[0039] Figure 1 This is a structural diagram of the test material used to evaluate resistance to LME cracking.

[0040] Figure 2 The top image is a plan 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 it at the cutting position shown in the top image. Detailed Implementation

[0041] The embodiments of the present invention will be described below.

[0042] It should be noted that in the following description, the content of each element in the composition of the Si-containing steel sheet and the content of each element in the coating composition are all expressed in terms of mass%, unless otherwise specified. Furthermore, in this specification, the numerical range indicated by "~" refers to the range including the values ​​listed before and after "~" as the lower and upper limits. Additionally, in this specification, high strength steel sheet means that the tensile strength TS of the steel sheet measured according to JIS Z 2241 (2011) is 590 MPa or higher.

[0043] Here, steel plate refers to steel plate manufactured by cold rolling or hot rolling. Steel plates are typically manufactured by cold rolling or hot rolling, but in this invention, the manufacturing method of the steel plate is not particularly limited.

[0044] First, the composition of the Si-containing steel plate will be explained.

[0045] Si: 0.45% or more and 2.0% or less

[0046] Si is an effective element for achieving high strength in steel sheets because it significantly increases the strength of steel through solid solution without greatly impairing workability (solid solution strengthening ability). On the other hand, Si also negatively affects the resistance to resistance weld cracking characteristics of welded parts. When adding Si to achieve high strength in steel sheets, it needs to contain 0.45% or more. Furthermore, when the Si content is less than 0.45%, the resistance to resistance weld cracking characteristics of welded parts do not present a significant problem, making the application of this invention unnecessary. On the other hand, when the Si content exceeds 2.0%, hot-rolling and cold-rolling properties decrease significantly, negatively impacting productivity or reducing the ductility of the steel sheet itself. Therefore, Si is contained in the range of 0.45% or more and 2.0% or less. The Si content is preferably 0.7% or more, more preferably 0.9% or more. Furthermore, the Si content is preferably 1.8% or less, more preferably 1.6% or less.

[0047] The Si-containing steel sheet of this embodiment requires that it contain Si within the above-mentioned range, but other components are permissible as long as they fall within the composition range typically found in steel sheets, and there are no particular restrictions. However, when the Si-containing steel sheet of this embodiment is required to achieve a high strength with a tensile strength (TS) of 590 MPa or higher, the following composition is preferred.

[0048] C: Below 0.3%

[0049] C improves the workability of steel sheets by forming martensite and other components that form the steel structure. When C is present, to obtain good weldability, the C content is preferably 0.3% or less, more preferably 0.25% or less. There is no particular limitation on the lower limit of C, but to obtain good workability, C is preferably 0.03% or more, more preferably 0.05% or more.

[0050] Mn: 1.0% or more and 4.0% or less

[0051] Mn has the function of strengthening steel through solid solution treatment, increasing its strength, improving hardenability, and promoting the formation of retained austenite, bainite, and martensite. This effect is manifested when the Mn content is 1.0% or more. On the other hand, when the Mn content is 4.0% or less, the above effects can be obtained without increasing costs. Therefore, the Mn content is preferably 1.0% or more, preferably 4.0% or less, more preferably 1.8% or more, and even more preferably 3.3% or less.

[0052] P: Below 0.1% (excluding 0%)

[0053] By suppressing the phosphorus (P) content, a decrease in weldability can be prevented. Furthermore, P segregation at grain boundaries can be prevented, thus preventing deterioration in ductility, flexibility, and toughness. Additionally, a large addition of P promotes the ferrite phase transformation, thereby increasing the grain size. Therefore, the P content is preferably set to 0.1% or less. There is no particular lower limit for the P content; due to limitations in production technology, it is generally greater than 0%, typically 0.001% or more.

[0054] S: Below 0.03% (excluding 0%)

[0055] The sulfur (S) content is preferably set to 0.03% or less, more preferably 0.02% or less. By suppressing the S content, it is possible to prevent a decrease in weldability and a decrease in ductility during hot rolling, thereby suppressing hot rolling cracking and significantly improving surface properties. Furthermore, by suppressing the S content, it is possible to prevent a decrease in the ductility, bendability, and elongation flangeability of the steel sheet due to the formation of coarse sulfides as impurity elements. These problems become significant when the S content exceeds 0.03%, and the S content is preferably reduced as much as possible. There is no particular lower limit for the S content, but it is generally greater than 0% due to limitations in production technology, and is typically 0.0001% or more.

[0056] N: less than 0.010% (excluding 0%)

[0057] The nitrogen (N) content is preferably set to 0.010% or less. By setting the N content to 0.010% or less, it is possible to prevent the formation of coarse nitrides by N with Ti, Nb, and V at high temperatures, thereby preventing damage to the high-strength effect of the steel sheet caused by the addition of Ti, Nb, and V. In addition, setting the N content to 0.010% or less can also prevent a decrease in toughness. Furthermore, setting the N content to 0.010% or less can prevent slab cracking and surface defects during hot rolling. The N content is preferably 0.005% or less, more preferably 0.003% or less, and even more preferably 0.002% or less. There is no particular limitation on the lower limit of the N content, but it is generally greater than 0% due to limitations in production technology, and is usually 0.0005% or more.

[0058] Al: Below 0.1% (excluding 0%)

[0059] Al is thermodynamically the most readily oxidized, thus oxidizing before Si and Mn. It inhibits the oxidation of Si and Mn on the outermost layer of the steel sheet and promotes their oxidation within the steel sheet. This effect is achieved when the Al content is 0.01% or higher. On the other hand, an Al content exceeding 0.1% leads to increased costs. Therefore, when added, the Al content is preferably set to 0.1% or lower. There is no particular lower limit for Al; it is generally greater than 0%, typically 0.001% or higher.

[0060] The composition may also optionally contain one or more of the following groups: B: less than 0.005%, Ti: less than 0.2%, Cr: less than 1.0%, Mo: less than 1.0%, Cu: less than 1.0%, Ni: less than 1.0%, Nb: less than 0.20%, Sb: less than 0.200%, V: less than 0.5%, Ta: less than 0.1%, W: less than 0.5%, Zr: less than 0.1%, Sn: less than 0.20%, Ca: less than 0.005%, Mg: less than 0.005%, and REM (Rare Earth Metal): less than 0.005%.

[0061] B: Below 0.005%

[0062] Boron (B) is an effective element for improving the hardenability of steel. To improve hardenability, the B content is preferably 0.0003% or more, and more preferably 0.0005% or more. However, excessive addition of B reduces formability, so the B content is preferably 0.005% or less.

[0063] Ti: below 0.2%

[0064] Ti is effective in precipitation strengthening of steel. There is no particular lower limit for the amount of Ti, but it is preferable to set it to 0.005% or more in order to achieve the effect of adjusting strength. However, when Ti is added excessively, the hard phase becomes too large and the formability decreases. Therefore, when Ti is added, the Ti content is preferably set to 0.2% or less, and more preferably 0.05% or less.

[0065] Cr: less than 1.0%

[0066] The Cr content is preferably set at 0.005% or higher. By setting the Cr content at 0.005% or higher, hardenability can be improved, and the balance between strength and ductility can be better balanced. When Cr is added, from the viewpoint of preventing cost increases, the Cr content is preferably set at 1.0% or lower.

[0067] Mo: 1.0% or less

[0068] The Mo content is preferably set to 0.005% or more. By setting the Mo content to 0.005% or more, the strength can be adjusted. The Mo content is more preferably set to 0.05% or more. In addition, when adding Mo, from the viewpoint of preventing cost increases, the Mo content is preferably 1.0% or less.

[0069] Cu: below 1.0%

[0070] The Cu content is preferably set to 0.005% or more. Setting the Cu content to 0.005% or more promotes the formation of the residual γ phase. Furthermore, when Cu is added, from the viewpoint of preventing cost increases, the Cu content is preferably set to 1.0% or less.

[0071] Ni: below 1.0%

[0072] The Ni content is preferably set to 0.005% or more. Setting the Ni content to 0.005% or more promotes the formation of the residual γ phase. Furthermore, when Ni is added, from the viewpoint of preventing cost increases, the Ni content is preferably set to 1.0% or less.

[0073] Nb: below 0.20%

[0074] By including 0.005% or more of Nb, the strength can be improved. Furthermore, when Nb is included, from the viewpoint of preventing cost increases, the Nb content is preferably set to 0.20% or less.

[0075] Sb: below 0.200%

[0076] From the viewpoint of suppressing nitriding, oxidation, or decarburization in the region extending from the steel plate surface to a depth of tens of micrometers due to oxidation, Sb may be included. Sb prevents the reduction of martensite formation on the steel plate surface by suppressing nitriding and oxidation, thereby improving the fatigue properties and surface quality of the steel plate. To achieve this effect, the Sb content is preferably set to 0.001% or more. On the other hand, to obtain good toughness, the Sb content is preferably set to 0.200% or less.

[0077] V: Below 0.5%

[0078] By including 0.005% or more of vitamin V, the strength can be improved. Furthermore, from the viewpoint of preventing cost increases, when vitamin V is included, the V content is preferably set to 0.5% or less.

[0079] Ta: Below 0.1%

[0080] By including 0.001% or more of Ta, the strength can be improved. Furthermore, when Ta is included, from the viewpoint of preventing cost increases, the Ta content is preferably set to 0.1% or less.

[0081] W: Below 0.5%

[0082] By including 0.005% or more of W, the strength can be improved. Furthermore, from the viewpoint of preventing cost increases, the W content is preferably set to 0.5% or less when W is included.

[0083] Zr: below 0.1%

[0084] By including 0.0005% or more of Zr, the strength can be improved. Furthermore, when Zr is included, from the viewpoint of preventing cost increases, the Zr content is preferably set to 0.1% or less.

[0085] Sn: below 0.20%

[0086] Sn is an effective element for inhibiting denitrification and deboration, thereby suppressing the reduction of steel strength. To achieve this effect, a content of 0.002% or more is preferred. On the other hand, to obtain good impact resistance, the Sn content is preferably set to 0.20% or less.

[0087] Ca: below 0.005%

[0088] By including 0.0005% or more of Ca, the morphology of sulfides can be controlled, and ductility and toughness can be improved. Furthermore, from the viewpoint of obtaining good ductility, when Ca is present, the Ca content is preferably set to 0.005% or less.

[0089] Mg: less than 0.005%

[0090] By including 0.0005% or more of Mg, the morphology of sulfides can be controlled, and ductility and toughness can be improved. Furthermore, from the viewpoint of preventing cost increases, the Mg content is preferably set to 0.005% or less when Mg is included.

[0091] REM: below 0.005%

[0092] By including 0.0005% or more of REM, the morphology of sulfides can be controlled, and ductility and toughness can be improved. Furthermore, from the viewpoint of obtaining good toughness, the REM content is preferably set to 0.005% or less when REM is included.

[0093] In this embodiment, the balance of the Si-containing steel sheet, other than the components described above, is Fe and unavoidable impurities. Here, the Si-containing steel sheet can be either cold-rolled steel sheet or hot-rolled steel sheet.

[0094] Next, we will explain the oxidation treatment, reduction annealing, and hot-dip galvanizing.

[0095] It should be noted that, in this invention, the temperatures specified in the oxidation treatment, reduction annealing, and cooling after annealing are all "steel plate temperatures".

[0096] The manufacturing method of the present invention is a method for manufacturing hot-dip galvanized steel sheets containing 0.45% to 2.0% Si, which involves oxidation treatment, reduction annealing, and hot-dip galvanizing. In the oxidation heating process, the steel sheet is oxidized by heating it in a specified oxidizing atmosphere, thereby generating Fe oxide on the surface of the steel sheet. The subsequent reduction annealing process consists of a process of reducing the oxidized steel sheet in different atmospheres in the preceding and following stages.

[0097] In the first reduction annealing stage before reduction annealing, the steel sheet is recrystallized and annealed in a reducing atmosphere of Fe with a specified hydrogen concentration and dew point. This reduces the Fe oxide generated in the aforementioned oxidation heating process, forming a reduced iron layer on the surface of the steel sheet. In the second reduction annealing stage after reduction annealing, the steel sheet is recrystallized and annealed in a reducing atmosphere of Fe with a specified hydrogen concentration and dew point. This forms a low-solution Si / C layer on the surface of the steel sheet to improve resistance to LME cracking, while simultaneously releasing hydrogen dissolved in the steel. After reduction annealing, the steel sheet is cooled to a specified temperature and then immersed in a hot-dip galvanizing bath for hot-dip galvanizing. The manufacturing method of the present invention includes manufacturing an alloyed hot-dip galvanized steel sheet by alloying the hot-dip galvanized layer after hot-dip galvanizing.

[0098] In this invention, the oxidation heating process and the subsequent reduction annealing process are typically carried out in a continuous annealing furnace having, from the inlet side, an oxidation zone (a region for oxidation treatment), a reduction zone (a region for first reduction annealing), a soaking zone (a region for second reduction annealing), and a cooling zone.

[0099] The manufacturing method of the present invention will be described below in the order of oxidation treatment, reduction annealing (first reduction annealing, second reduction annealing, and cooling after annealing), and hot-dip galvanizing.

[0100] First, let's explain the oxidation process.

[0101] In this oxidation process, the steel sheet is oxidized in an atmosphere containing N2 and O2 at a volume ppm or higher in an oxidation step set between 500°C and 800°C. In this invention, by oxidizing the steel sheet in this oxidation process and then reducing it in the subsequent reduction annealing, a reduced iron layer is formed on the surface of the steel sheet, preventing Si and Mn from diffusing to the surface of the steel sheet and oxidizing, thereby ensuring plating integrity.

[0102] Setting the O2 concentration in the oxidation atmosphere to above 500 ppm by volume can promote the oxidation of the steel sheet. When the O2 concentration is less than 500 ppm by volume, the oxidation of the steel sheet becomes incomplete, forming oxides of Si and Mn, which reduces the plating properties.

[0103] In oxidation treatment, setting the steel plate temperature (the highest temperature the steel plate can reach) above 500℃ can promote the oxidation of the steel plate. When the steel plate temperature is below 500℃, oxidation becomes insufficient, forming oxides of Si and Mn, which reduces the plating properties.

[0104] On the other hand, when the steel plate temperature (the highest temperature the steel plate reaches) exceeds 800°C, the oxidation of the steel plate becomes excessive. In the subsequent reduction annealing (first process), reduction is incomplete, and during this process, oxides peel off, causing a pickup phenomenon that adheres to the rollers. When this pickup occurs on the rollers, indentations are created on the steel plate, severely damaging the appearance of the galvanized steel plate.

[0105] Next, we will explain reduction annealing.

[0106] Reduction annealing needs to be carried out in different atmospheres at the beginning and end.

[0107] In the first reduction annealing stage, the oxidized steel plate is held in an atmosphere with a dew point of -45°C to +20°C, a hydrogen concentration of 5.0% to 25% by volume, and a balance of N2, at a temperature of 650°C to 900°C for 20 to 150 seconds.

[0108] A reduced iron layer is formed on the steel plate surface by reducing the Fe oxide formed during the oxidation treatment in the reducing atmosphere of the first reduction annealing, preventing Si and Mn from diffusing to the steel plate surface and oxidizing, thus ensuring plating integrity. In the subsequent second reduction annealing using a low hydrogen concentration atmosphere, reduction is almost non-existent; therefore, the reduction of Fe oxide must be completed in the first reduction annealing. Furthermore, by controlling the temperature, dew point, and hydrogen concentration of the reduction annealing, a layer with low concentrations of dissolved Si and dissolved C, which negatively impact LME resistance, can be formed on the surface. The mechanism is not yet clear, but Si is known to be easily oxidized, even in a Fe reducing atmosphere. Especially when the H₂O concentration is high, it is believed that oxides are formed inside the steel plate, reducing the amount of dissolved Si around it. C is oxidized by H₂O in the atmosphere and released into the furnace as CO gas, thus reducing the C concentration on the steel plate surface. These factors are believed to improve LME crack resistance.

[0109] When the annealing temperature of the steel sheet in the first reduction annealing process is below 650°C, reduction becomes insufficient, and Fe oxide adheres to the rolls, causing defects in the steel sheet. Furthermore, in the subsequent second reduction annealing process, the Fe oxide is not substantially reduced, thus preventing plating. On the other hand, annealing temperatures exceeding 900°C have a significant impact on the furnace body. Therefore, the annealing temperature of the steel sheet is set between 650°C and 900°C.

[0110] Regarding the dew point of the atmosphere in the first reduction annealing process, if the dew point is to be below -45°C, equipment for lowering the dew point is required, increasing costs. On the other hand, if the dew point exceeds +20°C, there are concerns about damage to the furnace body. Therefore, the dew point is set to be above -45°C and below +20°C. Furthermore, regarding hydrogen concentration, a higher hydrogen concentration results in faster reduction of Fe oxide; however, a higher hydrogen concentration also makes it easier for hydrogen to remain in the steel. When the hydrogen concentration is less than 5.0% by volume, reduction becomes insufficient; on the other hand, when the hydrogen concentration exceeds 25% by volume, the reduction effect saturates, and a large amount of hydrogen dissolves in the steel, making it difficult to sufficiently reduce the hydrogen content in the steel in the subsequent second reduction annealing process. Therefore, the hydrogen concentration is set to be above 5.0% by volume and below 25% by volume. Moreover, from the above perspective, a hydrogen concentration of 10% by volume or more is more preferable. From the perspective of operating costs, a hydrogen concentration of 20% by volume or less is preferred, and more preferably 15% by volume or less.

[0111] Furthermore, the low-solid-solution Si and C layer used to improve LME crack resistance is more easily formed at high dew points and low hydrogen concentrations. Therefore, the dew point is preferably -20°C or higher, more preferably -5°C or higher. The hydrogen concentration is preferably 15% by volume or lower. The low-solid-solution Si and C layer used to improve LME resistance is mainly formed in the second reduction annealing process. Preferably, a certain amount of the low-solid-solution Si and C layer is also formed in the first reduction annealing process by setting the dew point to -20°C or higher.

[0112] In the first reduction annealing process, if the holding time at 650°C or above and 900°C is less than 20 seconds, reduction is not fully completed. On the other hand, reduction can be fully completed with a holding time of 150 seconds or less; therefore, holding times exceeding 150 seconds actually reduce productivity. The low-solid-solution Si / C layer used to improve resistance to LME cracking is more easily formed with longer holding times. Hydrogen in the steel saturates at approximately 20 seconds, so the holding time has little effect. For these reasons, the holding time in the first reduction annealing process at 650°C or above and 900°C is set to 20 seconds or more and 150 seconds or less.

[0113] In the second reduction annealing stage, the steel sheet after the first reduction annealing process is held at a temperature of 700°C to 950°C for 30 seconds to 300 seconds in an atmosphere with a dew point of -10°C to +20°C, a hydrogen concentration of 2.0% by volume to 8.0% by volume, including the balance N2, and with the hydrogen concentration adjusted such that H2a is greater than H2b when the hydrogen concentration in the first reduction annealing stage is set to H2a and the hydrogen concentration in the second reduction annealing stage is set to H2b. In this second reduction annealing stage, simultaneously: a low-solution Si / C layer is formed on the surface of the steel sheet that has been reduced in the first reduction annealing stage to improve resistance to LME cracking; and hydrogen is released from the steel sheet by maintaining it in a low-hydrogen atmosphere.

[0114] As mentioned earlier, the surface low-solution Si and C layer used to improve LME crack resistance is more likely to be formed when the dew point is high and the hydrogen concentration is low.

[0115] Therefore, from the viewpoint of fully forming a surface layer of low-solid-solid Si and C, the dew point in the second reduction annealing process is preferably -10°C or higher, more preferably 0°C or higher. On the other hand, when the dew point exceeds +20°C, the reduced Fe formed in the first reduction annealing process may sometimes oxidize again, hindering plating performance, and dew point control is also difficult, raising concerns about potential impact on the furnace body. Therefore, the dew point is set to -10°C or higher and +20°C or lower.

[0116] Similarly, from the viewpoint of fully forming a surface layer of low-solid-solid Si and C, the hydrogen concentration in the second reduction annealing process is preferably 8.0% by volume or less, more preferably less than 5.0% by volume. Furthermore, regarding the hydrogen concentration in the steel sheet, the lower the hydrogen concentration, the easier it is for the hydrogen dissolved in the steel sheet during the first reduction annealing process to be released. However, it is difficult to uniformly control the hydrogen concentration in the furnace to less than 2.0% by volume, as there is a concern about oxidation of the steel sheet in areas with low hydrogen concentration. Therefore, the hydrogen concentration is set to 2.0% by volume or more. It should be noted that the annealing atmosphere in the second reduction annealing process includes a residual amount of N2.

[0117] Furthermore, when the annealing atmosphere in the first reduction annealing contains 5.0% to 12% hydrogen and the balance N2, it is more preferable that the annealing atmosphere in the second reduction annealing contains 2.0% to 3.0% hydrogen and the balance N2. By optimizing the hydrogen concentration of the annealing atmosphere in the first and second reduction annealings respectively, the hydrogen content in the steel sheet can be further reduced.

[0118] In the second reduction annealing process, if the annealing temperature of the steel plate is below 700℃, the surface layer of low-solid-solution Si and C cannot be fully formed, and dehydrogenation cannot be promoted. On the other hand, if the annealing temperature exceeds 950℃, it has a significant impact on the furnace body. Therefore, the annealing temperature of the steel plate is set between 700℃ and 950℃.

[0119] The holding time in the second reduction annealing process is set to be between 30 and 300 seconds. Less than 30 seconds is insufficient to form a sufficiently low-solid-solid Si / C layer on the surface. On the other hand, exceeding 300 seconds can sometimes reduce productivity.

[0120] Here, the hydrogen concentration needs to be adjusted so that H2a > H2b when the hydrogen concentration in the first reduction annealing stage is set to H2a and the hydrogen concentration in the second reduction annealing stage is set to H2b. The reason for this is to reduce the amount of hydrogen that penetrates into the steel during the first reduction annealing stage during the second reduction annealing stage.

[0121] In this invention, a high concentration of hydrogen is required to reduce the iron oxide generated during oxidation treatment in the first reduction annealing step of reduction annealing. This makes it difficult to fully form a surface low-solid-solution Si / C layer to improve resistance to LME cracking, and hydrogen is largely dissolved in the steel. Therefore, oxidation-reduction and the sufficient formation of the surface low-solid-solution Si / C layer are important, as is the dehydrogenation balance of hydrogen in the steel. For this purpose, it is necessary to optimize the conditions of the first and second reduction annealing steps of reduction annealing as described above. For optimization, it is preferable to use an annealing furnace that is divided into two or more sections in the direction of steel sheet travel and is capable of annealing in two or more different atmospheres.

[0122] The aforementioned oxidation treatment is preferably performed during the heating process of the steel sheet, which is a pre-reduction annealing process. This is because the oxidation treatment can be carried out efficiently at an optimal temperature that varies depending on the steel composition.

[0123] Furthermore, it is preferable to perform the aforementioned oxidation treatment within a temperature range of at least 50°C, between 500°C and 800°C. Here, "temperature range" refers to the temperature range during which the oxidation treatment is performed while heating from 500°C to 800°C. For example, when performing oxidation treatment within a temperature range of 550–600°C, the temperature range is 50°C. Similarly, when performing oxidation treatment within a temperature range of 450–600°C, the temperature range between 500°C and 800°C is 100°C. The reason for this is to ensure a more uniform oxidation treatment to obtain the amount of reduced iron required for improved plating properties. While the reason why the oxidation treatment becomes more uniform when performed within a temperature range of 50°C or higher is not yet clear, it is believed that the oxidation behavior of the steel sheet varies with the temperature range, and therefore, starting oxidation at a lower temperature with a slower oxidation rate allows for more uniform oxidation.

[0124] Here, the furnace used for the oxidation zone to perform oxidation treatment can be either a direct-fire type (direct-fire type furnace (DFF)) with a direct-fire burner or a radiant tube type with an atmosphere-controlled chamber.

[0125] Direct-fire heating refers to a method of heating steel plates by directly contacting the steel plate surface with a burner flame that mixes and burns fuels such as coke oven gas (COG), a byproduct gas of steelmaking, with air. Compared to radiant heating, direct-fire heating of steel plates occurs much faster, offering advantages such as shorter furnace lengths and increased production line speeds. Furthermore, when the air-to-fuel ratio is increased to at least 1.0 in the furnace atmosphere, unburned oxygen remains in the flame, which can promote oxidation of the steel plate surface. Therefore, the oxygen concentration in the atmosphere can be controlled by adjusting the air-fuel ratio. Fuels that can be used in direct-fire heating include COG, liquefied natural gas (LNG), hydrogen, and ammonia. Thus, in direct-fire heating, oxidation products such as CO, CO2, H2O, and NO are present in the oxidizer as fuel gas components, depending on the type of fuel used. X The oxidation process involves N2 and O2 (at least 500 ppm by volume), as well as CO, CO2, H2O, and NO. X It can be carried out in one or more atmospheres.

[0126] Radiant tube heating refers to the method of heating steel plates through the radiant heat of heated tubes. Compared with direct-fired burners, this method has a slower heating rate. Therefore, although the furnace length is longer, it has advantages such as easier maintenance and repair.

[0127] In any method, as long as sufficient oxidation of the steel plate can be obtained, and as long as part or all of the heating furnace is controlled to the specified atmosphere, it is acceptable. In particular, in the radiant tube method, due to the slow heating rate, it is also possible to cover a portion of the furnace with a chamber and control only that portion to the specified atmosphere.

[0128] When using a direct-fire method in this oxidation process, the oxidation process can be carried out in a direct-fire furnace that is divided into two or more sections and capable of oxidation in two or more atmospheres. The reason for this is that effective in-furnace dehydrogenation is effective for forming a low-solution Si and C layer on the surface required for resistance to LME cracking during the subsequent reduction annealing, and for promoting dehydrogenation that affects resistance to delayed fracture characteristics.

[0129] When the steel sheet is over-oxidized during this oxidation process, the oxides peel off during the subsequent reduction annealing, causing adhesion to the rollers. This adhesion severely damages the appearance of the galvanized steel sheet. In the first reduction annealing stage of the subsequent reduction annealing, this adhesion is more likely to occur when the hydrogen concentration is below 10% by volume. This reduces the reducing power of the atmosphere, making adhesion particularly likely in the early stages of the reduction annealing. In the first reduction annealing stage of the subsequent reduction annealing, when the hydrogen concentration is greater than 10% by volume, it is not necessary to perform such separation into two or more sections. Maintaining a hydrogen concentration below 10% by volume in the first reduction annealing stage of the subsequent reduction annealing is an important condition for obtaining a beautiful surface appearance free of indentations and other defects.

[0130] When using a direct-fired heating furnace with zones divided into two or more sections in the oxidation heating process, in the first heating zone of the first section, the aforementioned cold-rolled sheet is heated to a temperature above the heating temperature T1 (°C) calculated by the following formula (1) under the condition that the air ratio of the direct-fired heating furnace is set to α, and the average heating rate at 200°C or above is 10°C / second or more and 50°C / second or less. It should be noted that T1 is preferably 750°C or less.

[0131] T1=28.2[Si]+7.95[Mn]-86.2α+666 ---(1)

[0132] Wherein, [Si]: mass percentage of Si in steel, [Mn]: mass percentage of Mn in steel, α: air ratio of direct-fired furnace.

[0133] Here, the reasons for determining the above formula (1) will be explained.

[0134] To suppress the oxidation of Si and Mn on the surface of the steel sheet before hot-dip galvanizing, it is important to form internal oxidation of Si and Mn. In the pre-oxidation heating process, an active oxidation treatment can be performed to generate iron oxides that serve as the oxygen supply source for the internal oxidation of Si and Mn.

[0135] To obtain a sufficient amount of iron oxide, the heating atmosphere and temperature need to be managed. Atmosphere control is achieved by adjusting the air ratio in a direct-fired furnace. Increasing the air ratio, thus increasing the proportion of air relative to fuel, leaves unreacted oxygen in the flame, which can be used to promote the oxidation of the steel plate.

[0136] Furthermore, the heating temperature can be varied depending on the Si and Mn content. To suppress the oxidation of Si and Mn on the surface of the steel plate, Si and Mn can be oxidized internally. As the Si and Mn content increases, the amount of oxygen required for internal oxidation also increases. Therefore, the higher the Si and Mn content, the higher the oxidation temperature must be. In particular, it is known that adding Si to steel inhibits the oxidation reaction of iron; therefore, the higher the Si content, the higher the oxidation temperature must be.

[0137] Regarding the above formula (1), the influence of Si content, Mn content and air ratio of direct-fired heating furnace on the outlet side temperature (heating temperature T1) of heating furnace is analyzed by multiple regression analysis, and the result is obtained based on the results.

[0138] For the reasons mentioned above, it is preferable to heat in the first heating zone to a temperature T1 (°C) or higher as calculated by the above formula (1). However, regarding the upper limit of the air ratio α when heating in the first heating zone, from the purpose of suppressing excessive iron oxidation reaction and preventing subsequent adhesion, it is preferably 1.5 or less. In addition, when the air ratio is lower, the oxidizing power of the atmosphere is weaker, and even if formula (1) is satisfied, sometimes it is not possible to ensure a sufficient amount of oxidation. Therefore, the above air ratio α is preferably 0.9 or higher.

[0139] Furthermore, in the first heating zone, the average heating rate above 200°C can be set to 10–50°C / second. When the average heating rate exceeds 50°C / second, the heating time in the first heating zone becomes shorter, thus failing to form a sufficient amount of iron oxide. On the other hand, when the average heating rate is less than 10°C / second, heating requires a longer time, reducing production efficiency. Additionally, due to the formation of excessive iron oxide, Fe oxides peel off in the reducing atmosphere furnace during the subsequent reduction annealing, causing adhesion. Therefore, the average heating rate above 200°C is set to 10–50°C / second.

[0140] The average heating rate (°C / second) here is obtained by dividing the difference between the heating end temperature (the highest temperature reached by the steel plate (above T1)) (°C) and the heating start temperature (above 200°C) by the heating time (s).

[0141] In the second heating zone of the rear section, the cold-rolled plate after the first heating zone of the front section is heated in a direct-fired heating furnace under the conditions of an air ratio ≤0.9 and an average heating rate of 5℃ / second or more and 30℃ / second or less when the temperature is higher than T1 to a temperature above T2 (℃) calculated by the following formula (2).

[0142] T2 = T1 + 30 ---(2)

[0143] The second heating zone can prevent adhesion even when the hydrogen concentration in the furnace is reduced during the subsequent reduction annealing process, thus obtaining a beautiful surface appearance without indentations.

[0144] To prevent adhesion, it is important to reduce a portion (surface layer) of the once-oxidized steel plate surface. For this reduction, the air ratio of the burner in the direct-fired furnace needs to be controlled below 0.9. By reducing the air ratio and thus lowering the O2 concentration, the surface layer of iron oxides is partially reduced, preventing direct contact between the furnace rollers and the iron oxides during the subsequent reduction annealing process, thereby preventing adhesion. This reduction reaction is difficult to occur when the air ratio exceeds 0.9; therefore, the air ratio is set below 0.9. Furthermore, for stable combustion in the direct-fired furnace, an air ratio of 0.7 or higher is preferred.

[0145] When the heating temperature of the second heating zone in the latter part is lower than T2 as expressed by equation (2), the reduction reaction is difficult to occur, and the effect of suppressing adhesion cannot be obtained. In addition, in order to reduce unnecessary heating costs, T2 is preferably below 750°C.

[0146] Furthermore, in the second heating zone, the average heating rate above T1 is preferably set to 5°C / second or more and 30°C / second or less. When the average heating rate exceeds 30°C / second, the heating time in the second heating zone becomes shorter, thus failing to achieve a sufficient amount of iron oxide reduction reaction. On the other hand, when the average heating rate is less than 5°C / second, heating requires a longer time, reducing production efficiency. It should be noted that "average heating rate above T1" refers to the average heating rate from above T1 until the temperature reached in the second heating zone. The average heating rate (°C / second) here is obtained by dividing the difference between the heating end temperature (maximum temperature reached by the steel plate) (°C) and the heating start temperature by the heating time (s).

[0147] Furthermore, as mentioned above, the oxidation treatment is preferably carried out in an environment containing N2 and O2 at a concentration of 500 ppm or more by volume, and also containing CO, CO2, H2O, and NO. X The oxidation process is carried out in one or more atmospheres. The reason is unclear, but it is believed that the oxidation treatment of the steel plate is more stable by including these gases.

[0148] The aforementioned oxidation treatment is sometimes preferably carried out using a radiant tube furnace. When a controllable atmosphere chamber for oxidation treatment is provided in the radiant tube furnace, it has the advantages of better maintainability and the ability to suppress fluctuations in the width direction compared to a direct-fired furnace.

[0149] Furthermore, the aforementioned reduction annealing is preferably performed using a radiant tube furnace. Additionally, a radiant tube soaking furnace is preferred. This is because it is easier to control the reducing atmosphere within the furnace, and it also offers advantages in terms of equipment cost.

[0150] Hot-dip galvanizing refers to the process of hot-dip galvanizing annealed plates after reduction annealing in a hot-dip galvanizing bath containing 0.12 to 0.22% by mass of Al.

[0151] In this invention, the Al concentration in the zinc plating bath is preferably 0.12–0.22% by mass. When it is less than 0.12% by mass, an Fe-Zn alloy phase is formed during plating, sometimes resulting in poor adhesion of the coating or uneven appearance. When it exceeds 0.22% by mass, the Fe-Al alloy phase formed at the coating / steel substrate interface is thicker during plating, leading to poor weldability. In addition, due to the high Al content in the plating bath, a large amount of Al oxide film is formed on the surface of the coated steel sheet, sometimes impairing not only weldability but also appearance.

[0152] Sometimes alloying is performed after hot-dip galvanizing. The present invention is also effective in this case.

[0153] When alloying is performed, the Al concentration in the plating bath is preferably 0.12–0.17% by mass. When it is less than 0.12% by mass, an Fe-Zn alloy phase is formed during plating, which may sometimes lead to poor adhesion of the coating or uneven appearance. When it exceeds 0.17% by mass, the Fe-Al alloy phase formed at the coating / steel substrate interface during plating is thicker, which becomes an obstacle to the Fe-Zn alloying reaction. Therefore, the alloying temperature may be higher and the mechanical properties may deteriorate.

[0154] Other conditions during hot-dip galvanizing are not restricted. For example, the steel plate can be immersed in the galvanizing bath at a temperature of 440 to 500°C or at a plate temperature of 440 to 550°C, and the amount of adhesion can be adjusted by means of gas wiping.

[0155] As an alloying treatment, the hot-dip galvanized steel sheet is heated for 10 to 60 seconds in the temperature range of 450 to 550°C.

[0156] There are no particular restrictions on the degree of alloying (Fe concentration in the coating) after alloying treatment, but an alloying degree with an Fe concentration of 7 to 15% by mass is preferred. When it is less than 7% by mass, the η phase remains, resulting in poor stamping formability; when it exceeds 15% by mass, the coating adhesion is poor.

[0157] Example

[0158] [Example 1]

[0159] 1.2 mm cold-rolled sheets with the chemical composition shown in Table 1 were annealed and hot-dip galvanized on CGL. Oxidation heating was performed in a direct-fire furnace with a nozzle-mixing burner under the conditions shown in Table 2. It should be noted that the oxidation start temperature was set at 300°C. The oxidation start temperature has no particular effect on the coating appearance, so an oxidizing atmosphere below 300°C can also be used. Reduction annealing was performed in a radiant tube furnace divided into two zones by sealed rollers under the conditions shown in Table 2, followed by cooling. Then, hot-dip galvanizing was performed using a 460°C galvanizing bath containing 0.135% by mass Al, and the adhesion was adjusted to approximately 50 g / m² per side by gas wiping. 2 Alloying was performed under certain conditions. In Table 2, the holding time shown in the table was performed at the highest temperature reached by the steel sheet in the first reduction annealing, and the holding time shown in the table was performed at the highest temperature reached by the steel sheet in the second reduction annealing.

[0160]

[0161]

[0162] Next, the appearance of the high-strength hot-dip galvanized steel sheet obtained above was evaluated, and its tensile properties were investigated. Furthermore, its resistance to LME cracking, dehydrogenation behavior, and damage to the furnace body were evaluated. The measurement and evaluation methods are shown below.

[0163] <Appearance>

[0164] By visually inspecting the appearance of the steel plate, defects such as lack of plating, indentations caused by adhesion, or uneven alloying are marked with "◎". Minor defects that are within acceptable limits for a finished product are marked with "〇". Obvious uneven alloying, lack of plating, or indentations are marked with "×". When the above evaluation is "〇" or "◎", the appearance is judged to be good.

[0165] <Tension Properties>

[0166] With the rolling direction as the stretching direction, JIS No. 5 test piece was used, and the stretching was performed according to the method of JIS Z2241.

[0167] <Resistance to LME cracking>

[0168] A test piece was 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. This piece was compared with a hot-dip galvanized sheet cut to the same size, with a coating adhesion of 50g / m² per single side. 2The test samples were prepared by stacking hot-dip galvanized steel sheets (1.2 mm thick, TS: 980 MPa grade) into a panel assembly. This assembly was made so that the hot-dip galvanized layer of the test piece was in contact with the hot-dip galvanized layer of the commercially available hot-dip galvanized steel sheet. For example... Figure 1 As shown, the plate assembly is fixed to the mounting platform at a maximum tilt angle of 5°, determined by the shape of some components, with 2.0mm thick shims between them. The shims are a pair of steel plates, each 50mm long, 45mm short, and 2.0mm thick, with the long side end face of each plate aligned with the short side end faces of the plate assembly. Therefore, the distance between the pair of steel plates constituting the shims is 60mm. The mounting platform is a plate with a hole in the center.

[0169] Next, using a servo motor-driven pressure-applied single-phase AC (50Hz) resistance welding machine, resistance welding was performed while bending the plate assembly under pressure applied by a pair of electrodes (tip diameter: 6mm). The conditions were: pressure: 3.5kN; holding time: 0.10 seconds or 0.16 seconds; weld nugget diameter: 5.9mm; welding current and welding time (i.e., the welding current and welding time were appropriately adjusted to ensure a weld nugget diameter of 5.9mm for each plate assembly). This produced a plate assembly with a welded section. At this time, a pair of electrodes applied pressure to the plate assembly vertically from top to bottom, with the lower electrode applying pressure to the test piece through a hole in the mounting plate. During pressure application, the lower electrode and the mounting plate were fixed so that the contact pad between the lower electrode and the mounting plate extended plane, while the upper electrode was movable. Furthermore, the upper electrode was brought into contact with the center of the test hot-dip galvanized steel plate.

[0170] It should be noted that the holding time refers to the time from when the welding current stops flowing until the electrodes are turned on. Additionally, the weld nugget diameter refers to... Figure 2 The distance from the end 10 of the melt core along the long side of the plate assembly shown.

[0171] Next, as Figure 2 As shown, the aforementioned plate assembly with welded sections was cut to include the welded section (weld nugget), and the cross-section of the welded section was observed using an optical microscope (200x). The resistance to resistance welding cracking characteristics of the welded section were evaluated according to the following criteria. Here, Figure 2 The image above is a plan view of the plate assembly with welded sections, showing the cut-off position. Figure 2 The figure below shows a cross-section along the thickness of the cut plate assembly, schematically illustrating the cracks that occurred on the test piece. It should be noted that when cracks occurred in the hot-dip galvanized steel sheet used in the test, the stress in the test piece was dispersed, making proper evaluation impossible. Therefore, data from the hot-dip galvanized steel sheet used in the test, where no cracks occurred, were used as an example.

[0172] When the following evaluation is “〇” or “◎”, it is judged that the resistance welding crack resistance characteristics of the welded part are good and excellent, respectively. When it is “×”, it is judged that the resistance welding crack resistance characteristics of the welded part are poor.

[0173] ◎: No cracks longer than 0.1 mm were observed during the holding time of 0.10 seconds.

[0174] ○: Cracks longer than 0.1 mm were observed when the holding time was 0.10 seconds, but no cracks longer than 0.1 mm were observed when the holding time was 0.16 seconds.

[0175] ×: A crack with a length of more than 0.1 mm was observed when the holding time was 0.16 seconds.

[0176] <Dehydrogenation behavior>

[0177] A strip-shaped test piece with a major axis length of 30 mm and a minor axis length of 5 mm was cut from the center of the width of a hot-dip galvanized steel sheet. After removing the coating of the test piece using a Leutor, hydrogen analysis was immediately performed using a heating-desorption analyzer at an analysis start temperature of 25°C, an analysis end temperature of 300°C, and a heating rate of 200°C / hour. The amount of hydrogen released from the surface of the test piece at each temperature was measured, i.e., the hydrogen release amount (mass ppm / min). The total amount of hydrogen released from the analysis start temperature to 300°C was calculated as the diffusible hydrogen content in the steel. Here, the case where the diffusible hydrogen content in the steel is below 0.01 mass ppm is marked as best "◎++", the case where the diffusible hydrogen content in the steel is below 0.06 mass ppm is marked as very good "◎+", the case where the diffusible hydrogen content in the steel is below 0.10 mass ppm is marked as good "◎", and the case where the diffusible hydrogen content in the steel is below 0.30 mass ppm is marked as acceptable "〇". Furthermore, based on experience, when the diffusible hydrogen content in steel exceeds 0.30 ppm by mass, the delayed fracture resistance of the steel plate generally decreases; therefore, values ​​above 0.30 ppm by mass are marked with an "×". Regarding dehydrogenation behavior, "◎" and "〇" indicate excellent performance.

[0178] <Furnace body damage>

[0179] Damage to the furnace body is evaluated by visually observing whether the iron sheet (SUS310S) inside the annealing furnace changes color. Here, a case where no color change is observed is marked as "〇", indicating no damage to the furnace body. A case where a significant color change is observed is marked as "×", indicating damage to the furnace body.

[0180] The results obtained above, along with the manufacturing conditions, are shown in Table 3.

[0181]

[0182] According to Table 3, although the present invention uses a high-strength hot-dip galvanized steel sheet containing Si, it exhibits excellent resistance to LME cracking, a good coating appearance, and low diffusible hydrogen content in the steel sheet, thus promising good resistance to delayed fracture and minimal damage to the furnace body. On the other hand, comparative examples manufactured outside the scope of this invention show poorer resistance to LME cracking, coating appearance, diffusible hydrogen content in the steel sheet, and damage to the furnace body in any one of these aspects.

[0183] [Example 2]

[0184] Similarly, 1.2 mm cold-rolled sheets with the chemical composition shown in Table 1 were annealed and hot-dip galvanized on CGL. Oxidation heating was performed in a direct-fire type furnace with a nozzle-mixing burner, divided into two zones, according to the conditions shown in Table 4. Reduction annealing was performed in a radiant tube type furnace, divided into two zones by sealed rollers, according to the conditions shown in Table 4, followed by cooling. Then, hot-dip galvanizing was performed using a 460°C bath containing 0.135% Al, and the adhesion was adjusted to approximately 50 g / m² by gas wiping. 2 Alloying was performed under certain conditions. In Table 4, the holding time shown in the table was performed at the highest temperature reached by the steel sheet in the first reduction annealing, and the holding time shown in the table was performed at the highest temperature reached by the steel sheet in the second reduction annealing.

[0185] Next, the appearance of the high-strength hot-dip galvanized steel sheet obtained above was evaluated in the same manner as in Example 1, and its tensile properties were investigated. Furthermore, its resistance to LME cracking, dehydrogenation behavior, and damage to the furnace body were evaluated.

[0186]

[0187] The results obtained above are shown in Table 5.

[0188]

[0189] According to Table 5, although the present invention uses a high-strength hot-dip galvanized steel sheet containing Si, it exhibits excellent resistance to LME cracking, a good coating appearance, and low diffusible hydrogen content in the steel sheet, thus promising good resistance to delayed fracture and minimal damage to the furnace body. Furthermore, by using a direct-fired furnace with the oxidation heating process divided into two zones, a high level of balance is achieved between LME cracking resistance, coating appearance, and reduction of diffusible hydrogen in the steel.

[0190] [Example 3]

[0191] Similarly, 1.2 mm cold-rolled sheets with the chemical composition shown in Table 1 were annealed and hot-dip galvanized on CGL. Oxidation heating was performed in a direct-fired furnace with a nozzle-mixing burner, divided into two zones, according to the conditions shown in Table 6. Reduction annealing was performed and cooled in a radiant tube furnace, divided into two zones by sealed rollers, according to the conditions shown in Table 6. The hydrogen concentration in both the first and second reduction annealing processes within the reduction annealing furnace was kept low.

[0192] Next, after hot-dip galvanizing in a 460°C bath containing 0.135% Al, the adhesion amount was adjusted to approximately 50 g / m² by gas wiping. 2 Alloying was performed under certain conditions. In Table 6, the holding time shown in the table was performed at the highest temperature reached by the steel sheet during the first reduction annealing, and the holding time shown in the table was performed at the highest temperature reached by the steel sheet during the second reduction annealing.

[0193] Next, the appearance of the high-strength hot-dip galvanized steel sheet obtained above was evaluated in the same manner as in Example 1, and its tensile properties were investigated. Furthermore, its resistance to LME cracking, dehydrogenation behavior, and damage to the furnace body were evaluated.

[0194]

[0195] The results obtained above are shown in Table 7.

[0196]

[0197] According to Table 7, although the present invention uses a high-strength hot-dip galvanized steel sheet containing Si, it exhibits excellent resistance to LME cracking, a good coating appearance, and low diffusible hydrogen content, thus promising good resistance to delayed fracture and minimal damage to the furnace. Furthermore, by reducing the hydrogen concentration in the annealing furnace, diffusible hydrogen in the steel can be reduced to its limit.

[0198] Industrial availability

[0199] The high-strength hot-dip galvanized steel sheet obtained by the manufacturing method of the present invention has excellent appearance quality and resistance to resistance welding cracking characteristics, and can suppress the deterioration of delayed fracture resistance characteristics caused by hydrogen embrittlement. It can be used as a surface-treated steel sheet for making the automobile body itself lighter and stronger.

[0200] Symbol Explanation

[0201] 1. Experimental hot-dip galvanized steel sheet

[0202] 2 Test pieces

[0203] 3 gaskets

[0204] 4 electrodes

[0205] 5. Fixed platform

[0206] 6. Melting core

[0207] 7. Melting core diameter

[0208] 8. Cutting the wire

Claims

1. A method for manufacturing a high-strength hot-dip galvanized steel sheet, comprising: oxidizing a steel sheet containing 0.45% to 2.0% (by mass%) of Si; then reducing and annealing the steel sheet; and finally hot-dip galvanizing the steel sheet. In the oxidation treatment, the steel sheet is oxidized in an atmosphere containing N2 and O2 at a temperature range of 500°C to 800°C. The reduction annealing is carried out in different atmospheres in the first and second stages. In the first reduction annealing stage, the steel plate is held in an annealing atmosphere with a dew point of -45°C to +20°C, containing 5.0% to 25% hydrogen by volume and the balance N2, at a temperature of 650°C to 900°C for 20 seconds to 150 seconds. In the second reduction annealing stage, the steel sheet after the first reduction annealing is placed in an atmosphere with a dew point of -10°C to +20°C, containing 2.0% to 8.0% by volume of hydrogen, with the balance being N2, and the hydrogen concentration is adjusted such that the hydrogen concentration in the first reduction annealing stage is set to H2a and the hydrogen concentration in the second reduction annealing stage is set to H2b, with H2a > H2b. The atmosphere is then held at a temperature of 700°C to 950°C for 30 seconds to 300 seconds before hot-dip galvanizing.

2. The method for manufacturing high-strength hot-dip galvanized steel sheet according to claim 1, wherein, The reduction annealing uses an annealing furnace that is divided into two or more sections in the direction of the steel plate's travel and is capable of annealing in two or more different atmospheres.

3. The method for manufacturing high-strength hot-dip galvanized steel sheet according to claim 1 or 2, wherein, After hot-dip galvanizing the steel plate, the hot-dip galvanized layer is alloyed.

4. The method for manufacturing high-strength hot-dip galvanized steel sheet according to any one of claims 1 to 3, wherein, The oxidation treatment is performed in the heating process of the steel plate, which is a preceding process to reduction annealing.

5. The method for manufacturing high-strength hot-dip galvanized steel sheet according to claim 4, wherein, The oxidation treatment is carried out in a temperature range of at least 50°C above 500°C and below 800°C.

6. The method for manufacturing high-strength hot-dip galvanized steel sheet according to any one of claims 1 to 5, wherein, In the oxidation process, a direct-fired furnace (DFF) is used, and the air ratio of at least a portion of the atmosphere inside the furnace is set to 1.0 or higher, thereby oxidizing the surface of the steel plate.

7. The method for manufacturing high-strength hot-dip galvanized steel sheet according to any one of claims 1 to 6, wherein, The oxidation process uses a direct-fired heating furnace that is divided into two or more sections in the direction of steel plate travel and is capable of oxidation in two or more atmospheres. In the first heating zone at the front of the heating furnace, when the air ratio for the temperature range in which the oxidation treatment is performed is set to α, the temperature is heated to a temperature of T1 (°C) or higher, calculated by the following formula (1), under the condition that the average heating rate at 200°C or higher is 10°C / second or higher and 50°C / second or lower. In the second heating zone at the rear of the heating furnace, the steel plate that has passed through the first heating zone is heated to a temperature above T2 (°C) calculated by the following formula (2) under the conditions that the air ratio is ≤0.9 and the average heating rate is 5°C / second or more and 30°C / second or less when the temperature is higher than T1 (°C). T1=28.2[Si]+7.95[Mn]-86.2α+666 ---(1) T2 = T1 + 30 ---(2) Here, [Si] is the Si content (mass%) in the steel plate, and [Mn] is the Mn content (mass%) in the steel plate.

8. The method for manufacturing high-strength hot-dip galvanized steel sheet according to any one of claims 1 to 7, wherein, It contains N2 and O2 at a volume concentration of over 500 ppm, and also includes CO, CO2, H2O, and NO. X The oxidation process is carried out in one or more atmospheres.

9. The method for manufacturing high-strength hot-dip galvanized steel sheet according to any one of claims 1 to 5 and 8, wherein, The oxidation process is performed using a radiant tube furnace.

10. The method for manufacturing high-strength hot-dip galvanized steel sheet according to any one of claims 1 to 5, 8, and 9, wherein, The reduction annealing was performed using a radiant tube heating homogenizer.

11. The method for manufacturing high-strength hot-dip galvanized steel sheet according to any one of claims 1 to 10, wherein, In the second reduction annealing, the annealing atmosphere contains 2.0% by volume and less than 5.0% by volume of hydrogen, with the balance being N2.

12. The method for manufacturing high-strength hot-dip galvanized steel sheet according to any one of claims 1 to 11, wherein, In the first reduction annealing, the annealing atmosphere contains 5.0% by volume and 12% by volume of hydrogen, with the balance being N2. In the second reduction annealing, the annealing atmosphere contains 2.0% by volume and 3.0% by volume of hydrogen, with the balance being N2.

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