Method for manufacturing high-strength hot-dip galvanized steel sheets

Abstract

A method for manufacturing a high-strength hot-dip galvanized steel sheet is provided, which prevents deterioration of the appearance quality of the steel sheet, has excellent resistance to LME cracking, ductility, and bending properties, and can suppress deterioration of delayed fracture resistance caused by hydrogen embrittlement. A method for manufacturing a high-strength hot-dip galvanized steel sheet, wherein hot-dip galvanizing is performed after continuous annealing, wherein, in the front end of a direct-fired heating furnace, the steel sheet is heated to 400°C or higher and 670°C or lower in an atmosphere containing 1,000 volume ppm or more of O2 and 1,000 volume ppm or more of H2O, and in the rear end of a direct-fired heating furnace, the steel sheet is heated to 600°C or higher and 700°C or lower in an atmosphere containing 500 volume ppm or less of O2, and in a heating and holding furnace, the H2O concentration in the furnace atmosphere satisfies 5,000 volume ppm or higher and 40,000 volume ppm or lower, H2 concentration 2 volume% or higher and 20 volume% or lower, and the ratio of the partial pressure of H2O (PH2O) to the partial pressure of H2 (PH2), log(PH2O / PH2), is -1.1 or higher and 0.5 or lower, and the steel sheet is heated to 650°C or higher Maintain at 900℃ or lower for at least 90 seconds.

Description

Technology Field

[0001] The present invention relates to a method for manufacturing a hot-dip galvanized steel sheet having excellent resistance to welding fracture and resistance to delayed fracture. Background Technology

[0002] Recently, there has been a strong demand for improved fuel efficiency in automobiles from the perspective of protecting the global environment. Additionally, there is a strong demand for enhanced safety in automobiles to ensure passenger safety in the event of a collision. To meet these demands, it is necessary to achieve both lightweighting and high strength in vehicle bodies; consequently, the thinning of walls through increased strength is being actively pursued in hot-dip galvanized steel sheets, which serve as the material for automotive parts. However, given that most automotive parts are manufactured by forming and processing steel sheets, these sheets require excellent formability in addition to high strength.

[0003] While there are various methods to increase the strength of hot-dip galvanized steel sheets, solid solution strengthening via Si addition, in addition to the utilization of martensite through C addition, is a method that can achieve high strength without significantly compromising the formability of the sheet. Meanwhile, in the manufacture of automotive parts, press-formed components are often assembled by resistance welding (spot welding). If the steel sheet contains a large amount of C or Si, there is a concern that Liquid Metal Embrittlement (LME) may occur during resistance welding. This is because residual stress is generated near the weld zone, causing the zinc in the plating layer to melt and diffuse into the grain boundaries, resulting in grain boundary fracture (LME fracture) in the steel sheet. In particular, if welding is performed with the electrode angled relative to the steel sheet, residual stress increases, raising concerns about fracture formation. Since residual stress is thought to increase with the increase in steel plate strength, there is concern about the occurrence of LME fracture accompanying the increase in steel plate strength.

[0004] In addition, it is known that as the strength of steel increases, delayed fracture caused by hydrogen embrittlement becomes more likely to occur, and this tendency is particularly pronounced in high-strength steel with a tensile strength of 1180 MPa or higher. Furthermore, delayed fracture is a phenomenon in which, when high-strength steel is subjected to static load stress (load stress less than its tensile strength) and a certain amount of time has elapsed, sudden brittle fracture occurs without any apparent plastic deformation. While such delayed fracture is often caused by hydrogen that has penetrated the steel sheet due to corrosion caused by the service environment, hydrogen that has penetrated the steel sheet during the annealing process of a Continuous Galvanizing Line (CGL) also degrades the mechanical properties of steel sheets, particularly those with a tensile strength exceeding 980 MPa, and causes brittle fracture.

[0005] As described above, there is a demand for high-strength steel sheets that have excellent resistance welding fracture resistance (hereinafter also simply referred to as "LME fracture resistance") and suppress the deterioration of mechanical properties caused by hydrogen in the steel.

[0006] Conventionally, as a method for improving non-plating defects occurring 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 on the surface layer of the steel sheet in an atmosphere containing H2 with a dew point of 5°C or higher. However, when heated to 700°C or higher in an atmosphere containing O2, the amount of oxidation of the steel sheet is high, and oxide adheres to the inside of the furnace during reduction annealing, which has the problem of impairing the appearance quality of the steel sheet.

[0007] Patent Document 2 discloses a method for 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 for reducing the oxides on the surface layer of the steel sheet in an atmosphere containing H2O and H2 with a dew point of 5°C or higher and 500 volume ppm or higher and 5000 volume ppm or lower. Patent Document 3 similarly discloses a method for oxidizing the surface of Si-added steel by increasing the air ratio of a direct-fired furnace (DFF) and reducing the oxides on the surface layer of the steel sheet log(P H2O / P H2 A method of reduction in an atmosphere where the ) is -3.4 or higher and -1.1 or lower has been disclosed. In these methods, although the amount of oxidation of the steel sheet can be adjusted and good appearance quality can be secured, there is a problem in that sufficient LME fracture resistance or delayed fracture resistance cannot be obtained because a large amount of hydrogen that penetrated into the steel during annealing remains. Prior art literature

[0008] Japanese Patent Publication No. 5652219 Japanese Patent Publication No. 6052270 Japanese Patent Publication No. 6172297 The problem to be solved

[0009] The present invention aims to provide a method for manufacturing a high-strength hot-dip galvanized steel sheet that prevents the deterioration of the appearance quality of the steel sheet caused by oxides in the furnace during reduction annealing, which occur when the amount of oxidation of the steel sheet is excessive, and has excellent resistance to LME cracking and ductility, while simultaneously suppressing the deterioration of the resistance to delayed fracture caused by hydrogen embrittlement. means of solving the problem

[0010] The inventors ensure the appearance quality of the steel sheet by optimizing the O2 concentration and temperature during oxidation of the steel sheet according to the Si concentration and Mn concentration contained in the steel sheet to suppress excessive oxidation, and also the H2O concentration, H2 concentration and log(P H2O / P H2By optimizing the resistance welding fracture characteristics, it was discovered that the deterioration of the delayed fracture characteristics caused by hydrogen embrittlement can be suppressed, thereby completing the present invention.

[0011] The present invention is based on the above understanding. That is, the gist of the present invention is as follows.

[0012] [1] After hot rolling a slab containing, in mass%, C: 0.05% or more and 0.30% or less, Si: 0.45% or more and 2.0% or less, and Mn: 1.0% or more and 4.0% or less, the temperature T calculated from the following formula (1) C A method for manufacturing a high-strength hot-dip galvanized steel sheet comprising: a hot rolling process in which the steel is wound into a coil at a temperature of (°C) or lower and acid-cleaned; a cold rolling process in which cold rolling is performed on the hot-rolled sheet obtained from the hot rolling process; and a method for continuously annealing the cold-rolled steel sheet obtained from the cold rolling process in an annealing furnace having a direct-fire type heating furnace and a radiant tube type heating and preservation furnace, and then performing hot-dip galvanizing.

[0013] In the above direct-fire type heating furnace, in the early stage, a steel plate is heated from 400°C to 670°C in an atmosphere containing 1,000 volume ppm or more of O2 and 1,000 volume ppm or more of H2O, and

[0014] In a later stage, the steel plate is heated to 600°C or higher and 700°C or lower in an atmosphere containing 500 volume ppm or less of O2, and

[0015] In the annealing furnace having the above-mentioned heating and preservation furnace, the H2O concentration in the furnace atmosphere is 5,000 volume ppm or more and 40,000 volume ppm or less, and the H2 concentration is 2 volume% or more and 20 volume% or less.

[0016] Partial pressure of H2O (P H2O ) and partial pressure of H2 (P H2 The ratio of ) log(P H2O / P H2A method for manufacturing a high-strength hot-dip galvanized steel sheet by maintaining the steel sheet temperature at 650°C or higher and 900°C or lower for 90 seconds or more in an atmosphere satisfying -1.1 or higher and 0.5 or lower.

[0017] T C =-30([Si]+[Mn])+775...(1)

[0018] [Si] is the Si content (mass%) contained in the steel sheet

[0019] [Mn] is the Mn content (mass%) contained in the steel plate

[0020] [2] A method for manufacturing high-strength hot-dip galvanized steel sheets as described in [1], in which hot-dip galvanizing is performed on the steel sheet, followed by alloying treatment.

[0021] [3] A method for manufacturing high-strength hot-dip galvanized steel sheets described in [1] and [2], having an additional cooling and heating process after heating and holding in a radiant tube-type heating and holding furnace, cooling from the final holding temperature in the annealing to a temperature of 150 to 350°C under conditions of an average cooling rate of 10°C / second or more, then heating to a temperature of 350 to 600°C and holding for 10 to 600 seconds.

[0022] [4] Partial pressure of H2O (P H2O ) and partial pressure of H2 (P H2 The ratio of ) log(P H2O / P H2 A method for manufacturing high-strength hot-dip galvanized steel sheets described in any of [1] to [3], which is an atmosphere satisfying -0.99 or higher and 0.5 or lower.

[0023] [5] Partial pressure of H2O (P H2O ) and partial pressure of H2 (P H2 The ratio of ) log(P H2O / P H2 A method for manufacturing high-strength hot-dip galvanized steel sheets described in any of [1] to [4], which is an atmosphere satisfying -0.9 or more and 0.5 or less.

[0024] [6] Partial pressure of H2O (P H2O ) and partial pressure of H2 (P H2 The ratio of ) log(P H2O / P H2 A method for manufacturing high-strength hot-dip galvanized steel sheets described in any of [1] to [5], which is an atmosphere satisfying -0.7 or more and 0.5 or less. Effects of the invention

[0025] According to the present invention, a high-strength steel plate can be provided in which excellent resistance to weld fracture in the welded part and good appearance quality are obtained, and hydrogen in the steel, which is a factor in the deterioration of the resistance to delayed fracture, is sufficiently reduced. Brief explanation of the drawing

[0026] Figure 1 is a structural diagram of a test material for evaluating LME fracture resistance. The upper drawing of FIG. 2 is a plan view of a sheet assembly, and the lower drawing is a cross-section in the thickness direction after cutting the sheet assembly at the cutting position shown in the upper drawing. Specific details for implementing the invention

[0027] (Form for carrying out the invention)

[0028] Hereinafter, embodiments of the present invention will be described.

[0029] In addition, in the following description, the unit of each element content of the composition of the Si-containing slab and the content of each element of the composition of the plating layer is "mass%", and unless otherwise specifically stated, it is simply expressed as "%". Also, in this specification, a numerical range indicated by "~" means a range that includes the values ​​described before and after "~" as lower and upper limits. Also, in this specification, "high strength" of a steel plate means that the tensile strength TS of the steel plate, measured in accordance with JIS Z 2241 (2011), is 590 MPa or higher.

[0030] First, the composition of the Si-containing slab will be explained.

[0031] <Slab Components>

[0032] Si: 0.45% or more and 2.0% or less

[0033] Silicon is an effective element for achieving high strength in steel sheets because it has a significant effect (solid solution strengthening ability) of increasing the strength of steel through solid solution without significantly impairing workability. On the other hand, silicon is also an element that adversely affects resistance weld cracking characteristics in the weldment. When adding silicon to achieve high strength in steel sheets, an addition of 0.45% or more is required. Furthermore, if the silicon content is less than 0.45%, no particular problem arises regarding resistance weld cracking characteristics in the weldment, so there is insufficient need to apply the present invention. On the other hand, if the silicon content exceeds 3.0%, hot and cold rollability are significantly reduced, which adversely affects productivity or causes a decrease in the ductility of the steel sheet itself. Therefore, silicon is added within a range of 0.45% or more and 3.0% or less. The silicon content is preferably 0.7% or more, and more preferably 0.9% or more. In addition, the Si content is preferably 2.5% or less, more preferably 2.0% or less.

[0034] C: 0.30% or less

[0035] C improves the workability of the steel sheet by forming martensite or the like as a steel structure. When C is included, in order to obtain good weldability and resistance to LME cracking, it is preferable to keep the C content at 0.8% or less, and more preferable to keep it at 0.30% or less. Although there is no specific lower limit for C, in order to obtain good workability, it is preferable to keep C at 0.03% or more, and more preferable to keep it at 0.05% or more.

[0036] Mn: 1.0% or more, 4.0% or less

[0037] Manganese is an element that strengthens steel through solid solution strengthening to increase strength, enhances quenchability, and promotes the formation of retained austenite, bainite, and martensite. These effects are manifested by containing 1.0% or more of Mn. On the other hand, if the Mn content is 4.0% or less, the above effects are obtained without causing an increase in cost. Therefore, it is preferable to have a Mn content of 1.0% or more, and 4.0% or less. It is more preferable to have a Mn content of 1.8% or more. Furthermore, it is more preferable to have a Mn content of 3.3% or less.

[0038] Regarding the following components, their content is not limited, but the preferred range is as follows.

[0039] P: 0.1% or less (excluding 0%)

[0040] By suppressing the P content, a decrease in weldability can be prevented. Additionally, by preventing P from segregating at grain boundaries, the deterioration of ductility, bendability, and toughness can be prevented. Furthermore, if a large amount of P is added, the crystal grain size increases as ferrite transformation is promoted. Therefore, it is desirable to keep the P content 0.1% or less. The lower limit of P is not specifically limited, but due to production technology constraints, it is greater than 0% and is typically 0.001% or more.

[0041] S: 0.03% or less (excluding 0%)

[0042] It is desirable to keep the sulfur content at 0.03% or less, and more desirable to keep it at 0.02% or less. By suppressing the sulfur content, a decrease in weldability is prevented, and a decrease in ductility during hot rolling is prevented, thereby suppressing hot cracking and significantly improving surface properties. Furthermore, by suppressing the sulfur content, the formation of coarse sulfides as impurity elements is prevented, which can prevent a decrease in the ductility, bendability, and stretch flangeability of the steel sheet. These problems become significant when the sulfur content exceeds 0.03%, so it is desirable to reduce the sulfur content as much as possible. The lower limit of sulfur is not specifically limited, but due to production technology constraints, it is greater than 0% and is typically 0.001% or more.

[0043] Al: 0.1% or less (excluding 0%)

[0044] Since Al is thermodynamically the most oxidizable, it oxidizes prior to Si and Mn, thereby suppressing the oxidation of Si and Mn on the outermost surface layer of the steel sheet and promoting the oxidation of Si and Mn within the steel sheet. This effect is obtained when the Al content is 0.01% or higher. On the other hand, if the Al content exceeds 0.1%, the cost increases. Therefore, when adding it, it is desirable to keep the Al content 0.1% or lower. The lower limit of Al is not specifically limited and is greater than 0%, typically 0.001% or higher.

[0045] N: 0.010% or less (excluding 0%)

[0046] It is preferable to keep the N content at 0.010% or less. By keeping the N content at 0.010% or less, it is possible to prevent N from forming coarse nitrides with Ti, Nb, and V at high temperatures, thereby preventing the effect of increasing the strength of the steel sheet through the addition of Ti, Nb, and V from being compromised. Furthermore, keeping the N content at 0.010% or less also prevents a decrease in toughness. Additionally, keeping the N content at 0.010% or less prevents slab cracking and surface scratches from occurring 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. The lower limit of the N content is not particularly limited, but is greater than 0% due to production technology constraints, and is typically 0.0005% or more.

[0047] The composition of the components may additionally, optionally, contain one or more types selected from the group consisting of B: 0.005% or less, Ti: 0.2% or less, Cr: 1.0% or less, Cu: 1.0% or less, Ni: 1.0% or less, Mo: 1.0% or less, Nb: 0.20% or less, V: 0.5% or less, Sb: 0.200% or less, Ta: 0.1% or less, W: 0.5% or less, Zr: 0.1% or less, Sn: 0.20% or less, Ca: 0.005% or less, Mg: 0.005% or less, and REM (Rare Earth Metal): 0.005% or less.

[0048] B: 0.005% or less

[0049] B is an effective element for improving the quenchability of steel. To improve quenchability, it is desirable to have a B content of 0.0003% or more, and more desirable to have a B content of 0.0005% or more. However, since excessive addition of B lowers formability, it is desirable to keep the B content at 0.005% or less.

[0050] Ti: 0.2% or less

[0051] Ti is effective for precipitation strengthening of steel. Although there is no specific lower limit for Ti, it is desirable to have it at least 0.005% to obtain the effect of strength adjustment. However, if Ti is added excessively, the hard phase becomes excessive and formability decreases; therefore, when adding Ti, it is desirable to keep the Ti amount at 0.2% or less, and more desirable to keep it at 0.05% or less.

[0052] Cr: 1.0% or less

[0053] It is desirable to have a Cr content of 0.005% or more. By making the Cr content 0.005% or more, quenchability is improved, and the balance between strength and ductility can be improved. When adding Cr, it is desirable to keep the Cr content 1.0% or less to prevent cost increases.

[0054] Cu: 1.0% or less

[0055] It is desirable to have a Cu content of 0.005% or more. By making the Cu content 0.005% or more, the formation of the residual γ phase can be promoted. In addition, when adding Cu, it is desirable to have a Cu content of 1.0% or less in order to prevent cost increases.

[0056] Ni: 1.0% or less

[0057] It is desirable to have a Ni content of 0.005% or more. By making the Ni content 0.005% or more, the formation of the residual γ phase can be promoted. In addition, when adding Ni, it is desirable to keep the Ni content 1.0% or less to prevent cost increases.

[0058] Mo: 1.0% or less

[0059] It is desirable to have a Mo content of 0.005% or more. By making the Mo content 0.005% or more, the effect of strength adjustment can be obtained. More preferably, the Mo content should be 0.05% or more. In addition, when adding Mo, it is desirable to have a Mo content of 1.0% or less from the perspective of preventing cost increases.

[0060] Nb: 0.20% or less

[0061] The effect of improving strength is obtained by including 0.005% or more of Nb. In addition, when including Nb, it is desirable to keep the amount of Nb 0.20% or less to prevent cost increases.

[0062] V: 0.5% or less

[0063] The effect of improving strength is obtained by including V at a content of 0.005% or more. In addition, when V is included, it is desirable to keep the V amount at 0.5% or less to prevent cost increases.

[0064] Sb: 0.200% or less

[0065] Sb may be included for the purpose of suppressing nitriding, oxidation, or decarburization in the region extending to a depth of tens of microns from the steel sheet surface caused by oxidation. By suppressing nitriding and oxidation on the steel sheet surface, Sb prevents a decrease in the amount of martensite formed on the steel sheet surface, thereby improving the fatigue properties and surface quality of the steel sheet. To obtain this effect, it is desirable to have an Sb content of 0.001% or more. On the other hand, to obtain good toughness, it is desirable to have an Sb content of 0.200% or less.

[0066] Ta: 0.1% or less

[0067] The effect of improving strength is obtained by including Ta in an amount of 0.001% or more. In addition, when Ta is included, it is desirable to keep the amount of Ta 0.1% or less to prevent cost increases.

[0068] W: 0.5% or less

[0069] The effect of improving strength is obtained by including W at a content of 0.005% or more. In addition, when W is included, it is desirable to keep the amount of W at 0.5% or less to prevent cost increases.

[0070] Zr: 0.1% or less

[0071] The effect of improving strength is obtained by including 0.0005% or more of Zr. In addition, when including Zr, it is desirable to keep the amount of Zr 0.1% or less in order to prevent cost increases.

[0072] Sn: 0.20% or less

[0073] Sn is an element effective in suppressing the decrease in strength of steel by inhibiting denitrification, de-decomposition, etc. To obtain this effect, it is desirable to have an amount of 0.002% or more. On the other hand, to obtain good impact resistance, it is desirable to keep the amount of Sn at 0.20% or less.

[0074] Ca: 0.005% or less

[0075] By containing 0.0005% or more of Ca, the form of the sulfide can be controlled, thereby improving ductility and toughness. In addition, from the perspective of obtaining good ductility, it is desirable to keep the amount of Ca at 0.005% or less.

[0076] Mg: 0.005% or less

[0077] By containing 0.0005% or more of Mg, the form of sulfides can be controlled, thereby improving ductility and toughness. Additionally, when Mg is included, it is desirable to keep the Mg amount at 0.005% or less to prevent cost increases.

[0078] REM: 0.005% or less

[0079] By containing 0.0005% or more of REM, the form of the sulfide can be controlled, thereby improving ductility and toughness. In addition, when REM is included, it is desirable to keep the amount of REM 0.005% or less to obtain good toughness.

[0080] In the Si-containing slab of the present embodiment, the remainder other than the above components is Fe and unavoidable impurities. Here, the Si-containing steel sheet may be either a cold-rolled steel sheet or a hot-rolled steel sheet.

[0081] Hot Rolling

[0082] The hot rolling process is a process in which, after hot rolling the aforementioned slab, the temperature T calculated from the following formula (1) C It is a process of acid cleaning by winding into a coil at a temperature of (°C) or lower.

[0083] The technical significance of the hot rolling process is explained. In conventional hot rolling, oxygen diffuses from the oxide scale into the interior of the steel sheet during the cooling process after rolling is completed and the sheet is coiled. Consequently, internal oxides of Si or Mn are formed inside the sheet rather than on the surface. However, since the internal oxides of Si or Mn formed after rolling are uneven, they cause appearance defects, such as uneven plating adhesion when molten plating is performed in a subsequent CGL, or uneven alloying after alloying treatment. Therefore, it is important to suppress the formation of internal oxides during hot rolling. To suppress the formation of internal oxides of Si or Mn, it is effective to lower the coiling temperature after rolling. Furthermore, when using steel with a high content of Si or Mn formed as oxides, it is necessary to lower the coiling temperature even further.

[0084] As a result of conducting further investigations, it was found that by controlling the amount of internal oxidation in the longitudinal center and the widthwise center of the coil (the sum of Si internal oxide and Mn internal oxide formed in the steel plate surface layer within 10 μm from the steel plate surface immediately below the scale of the hot-rolled plate; the amount of internal oxidation is defined as the amount of oxygen at the longitudinal and widthwise center of the coil after rolling) to 0.10 g / m² or less, the internal oxidation of Si or Mn becomes more uniform, and even if a molten plating treatment is performed thereafter, the occurrence of non-uniformity in plating adhesion or non-uniformity in appearance after alloying treatment can be further suppressed. Here, using steel with varying Si and Mn content, hot rolling was performed, and as a result of investigating the amount of internal oxidation in the longitudinal center and the widthwise center of the coil formed after cooling, the temperature T calculated from the following formula (1) C By winding into a coil at a temperature of (°C) or lower, the total amount of Si internal oxide and Mn internal oxide formed during the hot rolling process can be controlled to 0.10 g / m² or less.

[0085] Tc=-30([Si]+[Mn])+775...(1)

[0086] Here, Tc is the coiling temperature after rolling, and [Si] and [Mn] are the Si content and Mn content in the steel, respectively. Additionally, Tc is preferably 400℃ or higher.

[0087] In addition, although the heating temperature before hot rolling and the finishing temperature of hot rolling are not particularly limited, from the perspective of microstructure control, it is desirable to heat the slab to 1100–1300°C and complete the finishing rolling at 800–1000°C.

[0088] In the present invention, after the above rolling, acid cleaning is performed to remove scale. The acid cleaning method is not particularly limited, and a standard method may be adopted.

[0089] Cold Rolling Process

[0090] The cold rolling process is a process of performing cold rolling on the hot-rolled plate obtained from the above hot rolling process. The conditions for cold rolling are not particularly limited, and for example, it is acceptable to cold roll the cooled hot-rolled plate with a predetermined reduction rate of 30 to 80%.

[0091] Annealing Process

[0092] The annealing process of the present invention comprises, for a cold-rolled sheet obtained from the cold rolling process, a process of oxidizing the steel sheet using a direct-fire heating furnace having two or more separated zones, and a process of reducing the oxidized steel sheet using a radiant tube-type heating furnace or a preservation furnace.

[0093] First, I will explain the direct-fire heating furnace (oxidation annealing process of steel plates).

[0094] Adding C, Si, or Mn is effective for realizing high strength and high workability of steel. However, when using steel sheets to which these elements have been added, oxides of Si and Mn are formed on the surface of the steel sheet during the annealing process (oxidation treatment + reduction annealing) performed before hot-dip galvanizing, making it difficult to ensure plating properties. Therefore, it is effective to oxidize Si and Mn within the steel sheet and prevent the oxidation of these elements on the surface of the steel sheet; however, as mentioned above, in the present invention, it is essential to suppress internal oxidation formed after hot rolling from the perspective of plating adhesion and non-uniformity of alloying. Even in cases where the formation of internal oxidation after hot rolling is minimal, by strictly controlling the annealing conditions (oxidation treatment conditions + reduction annealing conditions) before performing hot-dip galvanizing, Si and Mn are oxidized within the steel sheet, thereby improving plating properties. Furthermore, the reactivity between the plating and the steel sheet can be increased, and plating adhesion can be improved. Furthermore, in the annealing process, an oxidation treatment is performed to oxidize Si and Mn within the steel sheet and prevent oxidation on the steel sheet surface. In particular, it is necessary to obtain an amount of iron oxide exceeding a certain threshold through the oxidation treatment. Subsequently, it is effective to perform reductive annealing, hot-dip plating, and, if necessary, alloying treatment.

[0095] To obtain a sufficient amount of iron oxide, it is necessary to manage the heating atmosphere and temperature. Atmosphere control is achieved by controlling the air ratio of the direct-fire furnace. A direct-fire furnace heats steel plates by directly applying a burner flame—generated by mixing air with fuel such as coke oven gas (COG), a byproduct gas of steel mills—to the surface of the steel plate. By increasing the air ratio to increase the proportion of air relative to fuel, unreacted oxygen remains in the flame, making it possible to promote the oxidation of the steel plate using this oxygen. Here, in addition to coke oven gas, natural gas, hydrogen gas, or ammonia gas may also be used as fuel for the direct-fire furnace. The oxidation products generated when these fuels are burned include CO, CO2, H2O, and NO X There are others. In addition, N2 from the combustion air is also present in the atmosphere.

[0096] On the other hand, if the steel sheet is excessively oxidized, oxides peel off during the subsequent reduction annealing process, causing a phenomenon called "pickup" where the sheets adhere to the rolls. If pickup occurs on the rolls, it significantly impairs the appearance of the galvanized steel sheet. Therefore, the process of oxidizing the steel sheet using a direct-fire furnace requires having two or more separated zones and heating in two or more different atmospheres. Next, the front and rear sections of the heating zone will be explained.

[0097] Heating zone shear

[0098] Heating a steel plate from 400°C to 670°C in an atmosphere containing 1,000 volume ppm or more of O2 and 1,000 volume ppm or more of H2O

[0099] In the heating zone, the air ratio is adjusted so that the atmosphere has an O2 concentration of 1,000 volume ppm or more and an H2O concentration of 1,000 volume ppm or more, and the cold-rolled steel sheet is heated. Here, if the O2 concentration is less than 1,000 volume ppm and the H2O concentration is less than 1,000 volume ppm, the oxidation of the steel sheet becomes insufficient. On the other hand, when the O2 concentration is 1,000 volume ppm or more and the H2O concentration is 1,000 volume ppm or more, the influence of the O2 and H2O concentrations on the oxidation of the steel sheet is small, and the influence of the steel sheet temperature becomes large, so no upper limit is specifically set. Preferably, from the perspective of equipment deterioration, it is desirable that the O2 concentration be 10,000 volume ppm or less and the H2O concentration be 10,000 volume ppm or less. The steel sheet is heated so that its temperature is in the range of 400°C or more and 670°C or less. If the temperature of the steel plate is less than 400°C, the oxidation of the steel plate is insufficient, and if it exceeds 670°C, the oxidation of the steel plate becomes excessive, resulting in the aforementioned pickup of the roll. Therefore, in the present invention, it is an essential condition to heat the steel plate so that its temperature is in the range of 400°C or higher and 670°C or lower.

[0100] Rear end of the heating zone

[0101] Heating a steel plate to 600°C or higher and 700°C or lower in an atmosphere containing 500 volume ppm or less of O2

[0102] The rear end of the heating zone is an important requirement in the present invention to suppress the aforementioned roll pickup and obtain a beautiful surface appearance free from compression damage. To prevent the occurrence of the pickup phenomenon, it is important to reduce a portion of the surface (surface layer) of the steel sheet that has been oxidized. To perform this reduction treatment, the air ratio at the rear end of the heating zone is adjusted so that the atmosphere of O2 is 500 volume ppm or less, and the steel sheet that has passed through the front end of the heating zone is heated. Here, if O2 exceeds 500 volume ppm, the oxidation of the steel sheet becomes excessive, and the aforementioned roll pickup occurs. The steel sheet is heated so that its temperature is in the range of 600°C or higher and 700°C or lower. If the temperature of the steel sheet is below 600°C, the reduction of a portion of the steel sheet surface (surface layer) is insufficient, and if it exceeds 700°C, a portion of the steel sheet surface (surface layer) is not reduced, oxidation is accelerated, and the aforementioned roll pickup may occur. Therefore, in the present invention, it is an essential condition to heat the steel plate so that its temperature is in the range of 600°C or higher and 700°C or lower.

[0103] Next, we will explain the radiant tubular heating furnace or the retention furnace (reduction annealing process for steel plates).

[0104] As described above, adding C, Si, or Mn is effective for realizing high strength and high workability of steel. However, when using steel sheets with particularly high amounts of C or Si, there is a concern that the zinc in the plating layer will melt and diffuse into the grain boundaries, causing LME to occur and resulting in grain boundary fracture (LME fracture) in the steel sheet. Furthermore, it is known that delayed fracture caused by hydrogen embrittlement becomes more likely to occur with increasing strength of the steel. Regarding such delayed fracture, while it is often caused by hydrogen that has penetrated the steel sheet due to corrosion caused by the operating environment, hydrogen that has penetrated the steel sheet during the annealing process of the CGL also causes deterioration in the resistance to delayed fracture, particularly in steel sheets with a tensile strength exceeding 980 MPa.

[0105] To solve these problems, it is important to control the atmosphere during the reduction annealing process (oxidation treatment + reduction annealing) performed before hot-dip galvanizing. Although the mechanism is not clear, controlling the atmosphere during reduction annealing reduces the dissolved Si or Mn surrounding the internal oxide layer of Si or Mn formed. Additionally, since C is oxidized by H2O in the atmosphere and released into the furnace as CO gas, the C concentration on the surface layer of the steel sheet decreases. Consequently, because a region lacking dissolved C and Si, which are the causes of LME fracture, is formed on the surface layer, it is thought that LME fracture is less likely to occur. Furthermore, regarding hydrogen that has entered the steel sheet, it is thought that the presence of an internal oxide layer of Si or Mn on the surface layer of the steel sheet allows the internal oxides of Si or Mn formed on the surface layer to disperse within the plating layer when alloying the plating layer with the underlayer steel. Consequently, dehydrogenation from the steel sheet after manufacturing is promoted, thereby obtaining good resistance to delayed fracture.

[0106] For reductive annealing, radiant tubular heating or retention can be used. At this time, by controlling the atmospheric H2O concentration to between 5,000 volume ppm and 40,000 volume ppm, LME cracking can be suppressed and dehydrogenation can be promoted. If the H2O concentration is less than 5,000 volume ppm, the resistance to LME cracking and the dehydrogenation promotion effect cannot be considered sufficient. On the other hand, if the H2O concentration exceeds 40,000 volume ppm, equipment damage is a concern, so it is desirable to keep it 40,000 volume ppm or less. Here, the difference in H2O concentration between the upper and lower parts of the furnace needs to be 2,000 volume ppm or less. If the difference in H2O concentration between the upper and lower parts of the furnace exceeds 2,000 volume ppm, Si or Mn in the steel do not oxidize internally but oxidize externally, which impairs plating properties and may cause non-plating defects. In addition, there are cases where the LME fracture resistance or dehydrogenation promotion effect is insufficient because a sufficient internal oxide layer is not formed.

[0107] The H2 concentration during reductive annealing also significantly affects the formation of the internal oxide layer. The H2 concentration needs to be 2 volume% or more to 20 volume% or less. In addition, the partial pressure of H2O (P H2O ) and partial pressure of H2 (P H2 The ratio of ) needs to satisfy the following relationship. If the H2 concentration is less than 2 volume%, the reduction of the oxidized steel sheet is insufficient, which may result in non-plating defects or impair plating adhesion when hot-dip galvanizing is performed. On the other hand, if the hydrogen concentration exceeds 20 volume%, a large amount of hydrogen remains in the steel sheet; even if dehydrogenation is promoted, the amount of hydrogen remaining in the steel increases, which may prevent the attainment of good delayed fracture resistance characteristics. Regarding the formation of the internal oxide layer, the partial pressure of H2O (P H2O ) and partial pressure of H2 (P H2 The ratio of ) has an influence. To obtain good LME fracture resistance or a dehydrogenation promoting effect, log(P H2O / P H2) needs to be greater than or equal to -1.1 and less than or equal to 0.5. log(P H2O / P H2 If ) is less than -1.1, a sufficient internal oxide layer is not formed, and in some cases, good resistance to LME fracture or a dehydrogenation promoting effect may not be obtained. On the other hand, log(P H2O / P H2 If ) exceeds 0.5, equipment damage is a concern, so log(P H2O / P H2 It is desirable that ) be 0.5 or less.

[0108] In addition, regarding the bendability required for the formability of high-strength steel sheets, log(P H2O / P H2 It was found that increasing ) is effective. Although the mechanism is not clear, it is thought to be due to the improved formability resulting from the reduction of hydrogen in the steel sheet, and the change in deformation dispersion ability caused by the existence of a surface layer with relatively good formability due to the presence of an internal oxide layer. log(P H2O / P H2 By making ) greater than -1.1, bendability is also improved, but log(P H2O / P H2 Flexibility is further improved by making ) -0.99 or higher, it may be made -0.90 or higher, or it can be further improved by making it -0.7 or higher. Also, in any case, log(P H2O / P H2 The upper limit of ) is preferably 0.5.

[0109] Furthermore, regarding the reducing annealing atmosphere, it is preferable to use N2 for all substances other than H2O and H2 from a cost perspective. In addition, NO X I SO X There may be contamination with , CO, CO2, etc.

[0110] The temperature of the reduction annealing needs to be 650°C or higher and 900°C or lower. If the temperature is below 650°C, the formation of the internal oxide layer, which is necessary for improving resistance to LME fracture or promoting dehydrogenation, may be insufficient. Additionally, if the temperature exceeds 900°C, damage to the annealing furnace body is a concern, so it is desirable to keep the temperature below 900°C.

[0111] The reducing atmosphere described above may be satisfied in part or all of the furnace. If only part of the furnace satisfies the atmosphere described above, the annealing time in the specified atmosphere must be at least 90 seconds. If annealing is performed in the specified atmosphere for at least 90 seconds, the atmosphere for reducing annealing does not need to be controlled in all of the furnace.

[0112] <Cooling and Heating Process>

[0113] The cooling and heating process is a process in which, after reduction annealing, the material is cooled from the final holding temperature of the reduction annealing to a cooling temperature of 150 to 350°C under conditions of an average cooling rate of 10°C / second or more, then heated to a reheating temperature of 350 to 600°C, and held at the said temperature for 10 to 600 seconds. By performing this cooling and heating process, mechanical properties can be further enhanced. Furthermore, in the present invention, the cooling and heating process is not an essential process, so it may be performed as needed.

[0114] If the cooling rate from the final holding temperature in reduction annealing is less than 10°C / sec, pearlite is formed, and TS×EL and hole expandability are reduced. Therefore, it is desirable that the cooling rate from the final holding temperature in reduction annealing be 10°C / sec or higher. Here, the final holding temperature in reduction annealing refers to the temperature at which a steel sheet, which has been annealed within a range satisfying the requirements for the annealing temperature, hydrogen concentration, dew point, and holding time of the reduction annealing, deviates from at least one of the requirements.

[0115] At a cooling temperature higher than 350°C, the temperature of the plating bath rises during the subsequent molten plating process, which may accelerate the generation of dross that impairs surface appearance quality. Therefore, it is desirable for the cooling temperature to be 350°C or lower. Mechanical properties can be improved by lowering the cooling temperature to 350°C or lower. Furthermore, if the cooling temperature is lower than 150°C, the austenite almost completely transforms into martensite during cooling, reducing the amount of untransformed austenite. Therefore, it is desirable for the cooling temperature to be in the range of 150 to 350°C. Regarding the cooling method, any cooling method such as gas jet cooling, mist cooling, water cooling, or metal quenching may be used as long as the target cooling rate and cooling stop temperature (cooling temperature) can be achieved.

[0116] Here, after cooling to the cooling temperature, if possible, the material may be heated to the reheating temperature and held for 10 seconds or more. By holding for 10 seconds or more, the martensite formed during cooling is tempered and becomes tempered martensite. As a result, hole expandability is improved, and the untransformed austenite that did not transform into martensite during cooling is stabilized, and finally, a sufficient amount of retained austenite is obtained, which may result in improved ductility.

[0117] In addition, when reheating, if the reheating temperature exceeds 600°C, the untransformed austenite at the time of cooling cessation transforms into pearlite, and ultimately, no more than 3% of the area of ​​residual austenite is obtained. If the holding time during reheating is less than 10 seconds, the stabilization of the austenite becomes insufficient, and if it exceeds 600 seconds, the untransformed austenite at the time of cooling cessation transforms into bainite, and ultimately, no sufficient amount of residual austenite is obtained. Therefore, the temperature for reheating is set to a range of 350 to 600°C, and the holding time in that temperature range is set to 10 to 600 seconds.

[0118] <Hot-dip galvanizing process>

[0119] After performing hot-dip galvanizing on a steel plate, an alloying treatment may be performed. The hot-dip galvanizing process is a process of performing hot-dip galvanizing treatment on an annealed plate after an annealing process in a hot-dip galvanizing bath containing 0.12 to 0.22 mass% of Al.

[0120] In the present invention, the Al concentration in the zinc plating bath is set to 0.12 to 0.22 mass%. If the concentration is less than 0.12 mass%, an Fe-Zn alloy phase is formed during plating, which may result in deterioration of plating adhesion or non-uniformity of the appearance. If the concentration exceeds 0.22 mass%, the Fe-Al alloy phase formed at the plating / base steel interface during plating becomes thick, which deteriorates weldability. Additionally, because there is a large amount of Al in the bath, a large amount of Al oxide film is formed on the surface of the plated steel sheet, which may impair not only weldability but also appearance.

[0121] When performing alloying treatment, the Al concentration in the plating bath is preferably 0.12 to 0.17 mass%. If it is less than 0.12 mass%, an Fe-Zn alloy phase is formed during plating, which may result in deterioration of plating adhesion or non-uniformity of the appearance. If it is greater than 0.17 mass%, a thick Fe-Al alloy phase is formed at the plating / base iron interface during plating, which acts as a barrier to the Fe-Zn alloying reaction, causing the alloying temperature to rise and the mechanical properties to deteriorate.

[0122] Other conditions during hot-dip galvanizing are not limited, but for example, the hot-dip galvanizing bath temperature may be in the normal range of 440 to 500°C, and the steel sheet may be immersed in the plating bath at a plate temperature of 440 to 550°C, and the amount of coating may be adjusted by gas wiping, etc.

[0123] <Alloying Process>

[0124] The alloying treatment process is a process of performing alloying treatment on a steel sheet after the hot-dip galvanizing process at a temperature range of 450 to 550°C for 10 to 60 seconds.

[0125] The degree of alloying (Fe concentration in the plating layer) after alloying treatment is not particularly limited, but a degree of alloying of 7 to 15 mass% is preferred. If it is less than 7 mass%, the η phase remains and press formability is poor, and if it exceeds 15 mass%, plating adhesion is poor.

[0126] Examples

[0127] After melting the steel with the chemical composition shown in Table 1, it was made into a slab by continuous casting.

[0128] After heating these slabs to 1200°C, hot rolling was performed at a finishing temperature of 890°C to obtain a plate thickness of 2.6 mm. The slabs were then wound into coils at the winding temperatures shown in Table 2, cooled, and then the black scale was removed by acid cleaning to obtain hot-rolled plates. At this time, the amount of internal oxidation of Si and / or Mn in the center of the coil along the longitudinal direction and the width direction was measured using the method shown below.

[0129]

[0130]

[0131] <Amount of internal oxidation after hot rolling>

[0132] The amount of internal oxidation was measured by the "impulse furnace melt-infrared absorption method." The oxygen concentration in each steel was measured before and after polishing a 10 mm × 70 mm area of ​​the surface layer (center of the coil (both the widthwise center and the lengthwise center)) on both sides of the hot-rolled plate to 10 µm. Furthermore, from the difference between these measurements, the amount of oxygen per unit area on one side existing in the 10 µm region from the steel plate surface was calculated and defined as the amount of internal oxidation of Si and / or Mn (g / m²). It was confirmed that the internal oxide formed on the surface layer of the hot-rolled plate was an oxide of Si and / or Mn by embedding the hot-rolled plate in resin, polishing the cross-section, and then observing with a Scanning Electron Microscope (SEM) and performing elemental analysis using EDS (Energy Dispersive X-ray Spectrometer). The amount of internal oxidation is shown in Table 3.

[0133] Subsequently, after cold rolling to a thickness of 1.2 mm, annealing and hot-dip galvanizing treatments were performed in a CGL. The furnace front section was heated using a direct-fire furnace equipped with a nozzle mix-type burner under the conditions shown in Table 2. Subsequently, the furnace rear section was heated using a direct-fire furnace equipped with a premix-type burner under the conditions shown in Table 2. In addition, the oxidation initiation temperature was set to 300°C. Since the oxidation initiation temperature does not particularly affect the plating appearance, an oxidation atmosphere below 400°C may be used. Reduction annealing was performed in a radiant tube-type heating and holding furnace under the conditions shown in Table 2, followed by cooling. Next, hot-dip galvanizing treatment was performed using a zinc bath at 460°C containing 0.135% Al, after which the amount per unit area was adjusted to approximately 50 g / m² by gas wiping. Under some conditions, alloying treatment was performed.

[0134] Next, the appearance of the high-strength hot-dip galvanized steel sheet obtained above was evaluated, and its tensile properties were investigated. In addition, resistance to LME fracture, dehydrogenation behavior, and damage to the furnace were evaluated. The measurement and evaluation methods are described below.

[0135] <Appearance>

[0136] The appearance of the steel sheet was visually inspected, and "◎" was given if there were no appearance defects such as non-plating, indentation damage due to pickup phenomenon, or uneven alloying; "○" was given if there were minor appearance defects but within an acceptable range as a product; and "×" was given if there were clear uneven alloying, non-plating, or indentation damage. If the above evaluation was "○" or "◎", it was determined that the appearance was good.

[0137] Tensile properties

[0138] The test was performed using a JIS No. 5 test specimen with the rolling direction as the tensile direction, in accordance with the method in accordance with JIS Z2241. A TS(MPa)×EL(%) of 8000(MPa·%) or higher was judged to be good.

[0139] <LME Fragility>

[0140] A test specimen cut from a hot-dip galvanized steel sheet with the rolling direction perpendicular (TD) as the longer side and the rolling direction as the shorter side, measuring 150 mm in the longer direction × 50 mm in the shorter direction, was stacked with a test hot-dip galvanized steel sheet (sheet thickness 1.6 mm, TS: 980 MPa class) cut to the same size, having a plating adhesion amount of 50 g / m² per side of the hot-dip galvanized layer. This sheet was assembled so that the hot-dip galvanized layer of the test specimen and the hot-dip galvanized layer surface of the commercially available hot-dip galvanized steel sheet were aligned. As shown in Fig. 1, this sheet was fixed to a holder through a spacer with a thickness of 2.0 mm at a 5° angle, which is the maximum angle of inclination expected in some parts shapes. The spacer is a pair of steel plates measuring 50 mm in the long direction × 45 mm in the short direction × 2.0 mm in thickness, and is arranged so that the long direction cross-section of each of the pair of steel plates aligns with both short direction cross-sections of the plate set. Therefore, the distance between the pair of steel plates constituting the spacer is 60 mm. The fixing member is a single plate with a hole drilled in the center.

[0141] Next, using a single-phase AC (50 Hz) resistance welder with a servo motor pressurizing type, resistance welding was performed on a plate with a bent state while pressing the plate with a pair of electrodes (tip diameter: 6 mm), with a pressing force of 3.5 kN, a hold time of 0.10 seconds or 0.16 seconds, and welding current and welding time conditions such that the nugget diameter of the welded part becomes 5.9 mm (i.e., the welding current and welding time are appropriately adjusted so that the nugget diameter becomes 5.9 mm for each plate) to form a plate with a welded part. At this time, the pair of electrodes presses the plate from the top and bottom in the vertical direction, and the lower electrode presses the test specimen through the hole of the holder. During pressing, the lower electrode and the holder are fixed so that the lower electrode of the pair of electrodes contacts a plane extending the surface where the spacer and the holder meet, and the upper electrode is made movable. In addition, the upper electrode was positioned to contact the center of the test hot-dip galvanized steel sheet. Also, hold time refers to the time from when the welding current has been fully flowed until the electrode begins to open. Also, nugget diameter refers to the distance of the nugget end in the long direction of the plate, as shown in FIG. 2.

[0142] Next, as shown in FIG. 2, the welded plate with the weld attached was cut to include the weld (nugget), the cross-section of the weld was observed using an optical microscope (200x magnification), and the resistance-resistant weld cracking characteristics in the weld were evaluated according to the following criteria. Here, the upper drawing of FIG. 2 is a plan view of the welded plate with the weld attached and indicates the cutting location. The lower drawing of FIG. 2 is a cross-section in the plate thickness direction of the plate after cutting and schematically shows the cracks that occurred in the test specimen. Furthermore, if cracking occurs in the hot-dip galvanized steel plate for testing, the stress of the test specimen is dispersed, and an appropriate evaluation cannot be made. For this reason, data in which no cracking occurred in the hot-dip galvanized steel plate for testing was adopted as an example.

[0143] If the evaluation below is "○" or "◎", the resistance-resistant weld cracking characteristics in the weldment are judged to be good and excellent, respectively, and if it is "×", the resistance-resistant weld cracking characteristics in the weldment are judged to be inferior.

[0144] ◎: No cracks longer than 0.1 mm are detected at a hold time of 0.10 seconds.

[0145] ○: A crack with a length of 0.1 mm or more is detected at a hold time of 0.10 seconds, but a crack with a length of 0.1 mm or more is not detected at a hold time of 0.16 seconds.

[0146] ×: A crack with a length of 0.1 mm or more is confirmed at a hold time of 0.16 seconds.

[0147] Dehydrogenation behavior

[0148] A rectangular test specimen with a major axis length of 30 mm and a minor axis length of 5 mm was taken from the center of the width of the hot-dip galvanized steel sheet, and the plating layer of the test specimen was removed using a router. Then, hydrogen analysis was performed immediately using a temperature-removal analysis device under conditions of an analysis start temperature of 25°C, an analysis end temperature of 300°C, and a heating rate of 200°C / hour, and the amount of hydrogen released (mass ppm / min), which is the amount of hydrogen released from the surface of the test specimen at each temperature, was measured. The sum of the amounts of hydrogen released from the analysis start temperature to 300°C was calculated as the amount of diffusible hydrogen in the steel. Here, a diffusible hydrogen amount in the steel of 0.10 mass ppm or less was designated as "Good" (◎), and 0.30 mass ppm or less was designated as "Pass" (○). In addition, based on experience, when the amount of diffusible hydrogen in steel exceeds 0.30 mass ppm, the delayed fracture resistance of the steel sheet often deteriorates, so 0.30 mass ppm or more was marked as “×”. Dehydrogenation behavior was judged to be superior in the cases of “◎” and “○”.

[0149] Roche Damage

[0150] The damage to the furnace was evaluated by visual inspection to see whether discoloration was observed on the steel shell (SUS310S) inside the annealing furnace. Here, if no discoloration was observed on the steel shell, it was marked "○" and it was determined that no furnace damage occurred. If discoloration was clearly observed, it was marked "×" and it was determined that furnace damage occurred.

[0151] Methods for Evaluating Flexibility

[0152] A rectangular specimen measuring 25 × 100 mm was cut from a manufactured galvanized steel sheet, with the short side parallel to the rolling direction. Subsequently, a 90° V-bending test was performed to form a ridge when bent in the rolling direction. The streak speed was set to 50 mm / min, and a deterministic pressing was applied by pushing the specimen against a die with a load of 10 tons for 5 seconds. The test was conducted by varying the tip radius (R) of the V-shaped punch in 0.5 steps, and the area near the specimen ridge was observed using a 20x magnification lens to check for the presence or absence of cracks. R / t was calculated from the minimum R at which no cracks occurred and the thickness of the specimen (t mm, using a value rounded to the hundreds digit from the thousands digit), and this was used as an indicator of bendability. The smaller the R / t, the better the bendability. Here, R / t less than 1.0 was designated as very good “◎+”, less than 1.5 as good “◎”, less than 2.0 as pass “○”, less than 4.0 as average “△”, and 4.0 or higher as “×”.

[0153] The results obtained from the above are shown in Table 3 along with the manufacturing conditions.

[0154]

[0155] From Table 3, the example of the present invention, despite being a high-strength hot-dip galvanized steel sheet containing C, Si, and Mn, exhibits excellent resistance to LME cracking, good plating appearance, and low diffusible hydrogen content in the steel sheet, allowing for good resistance to delayed fracture, low damage to the furnace, and excellent ductility and bendability. On the other hand, the comparative example manufactured outside the scope of the present invention is inferior in one or more of the following: resistance to LME cracking, plating appearance, diffusible hydrogen content in the steel sheet, and damage to the furnace.

[0156] Industrial applicability

[0157] The high-strength hot-dip galvanized steel sheet obtained by the manufacturing method of the present invention has excellent appearance quality and resistance to welding fracture, and at the same time can suppress the deterioration of resistance to delayed fracture caused by hydrogen embrittlement, and can be used as a surface-treated steel sheet for making the vehicle body itself lighter and stronger. Explanation of the symbols

[0158] 1: Hot-dip galvanized steel sheet for testing 2 : Test piece 3 : Spacer 4: Electrode 5 : Fixing bracket 6 : Nugget 7 : Nugget diameter 8 : Cutting line

Claims

Claim 1 After hot rolling a slab containing, in mass%, C: 0.05% or more and 0.30% or less, Si: 0.45% or more and 2.0% or less, Mn: 1.0% or more and 4.0% or less, P: greater than 0% and 0.1% or less, S: greater than 0% and 0.03% or less, Al: greater than 0% and 0.1% or less, N: greater than 0% and 0.010% or less, and the remainder being Fe and unavoidable impurities, the temperature T calculated from the following formula (1) C A method for manufacturing a high-strength hot-dip galvanized steel sheet having a tensile strength of 590 MPa or more as measured in accordance with JIS Z 2241 (2011), comprising: a hot rolling process of obtaining a hot-rolled sheet by winding it into a coil at a temperature of (°C) or lower and acid cleaning; a cold rolling process of performing cold rolling on the hot-rolled sheet obtained from the hot rolling process; and performing hot-dip galvanizing after continuously annealing the cold-rolled steel sheet obtained from the cold rolling process in an annealing furnace having a direct-fire type furnace and a radiant tube type heating and preservation furnace, wherein in the direct-fire type furnace, the steel sheet is heated from 400°C to 670°C in an atmosphere containing 1000 volume ppm or more of O2 and 1000 volume ppm or more of H2O in the early stage, and in the later stage, O2 is 500 volume ppm or less In an atmosphere containing a steel plate, the steel plate is heated to 600°C or higher and 700°C or lower, and in the annealing furnace having the heating and preservation furnace, the H2O concentration in the atmosphere inside the furnace is 5,000 volume ppm or higher and 40,000 volume ppm or lower, the H2 concentration is 2 volume% or higher and 20 volume% or lower, and the partial pressure of H2O (P H2O ) and partial pressure of H2 (P H2 The ratio of ) log(P H2O / P H2 A method for manufacturing a high-strength hot-dip galvanized steel sheet by maintaining the steel sheet temperature at 650°C or higher and 900°C or lower for 90 seconds or more in an atmosphere satisfying an atmosphere of -1.1 or higher and 0.5 or lower. C =-30([Si]+[Mn])+775 ···(1)[Si] is the Si content (mass%) contained in the steel sheet, [Mn] is the Mn content (mass%) contained in the steel sheet Claim 2 A method for manufacturing a high-strength hot-dip galvanized steel sheet according to claim 1, wherein a steel sheet is subjected to hot-dip galvanizing, and then an alloying treatment is performed. Claim 3 A method for manufacturing a high-strength hot-dip galvanized steel sheet according to claim 1, further comprising a cooling and heating process in which, after heating and holding in a radiant tube-type heating and holding furnace, the temperature is cooled from the final holding temperature in the annealing to a temperature of 150 to 350°C under conditions of an average cooling rate of 10°C / second or more, then heated to a temperature of 350 to 600°C and held for 10 to 600 seconds. Claim 4 A method for manufacturing a high-strength hot-dip galvanized steel sheet according to claim 2, further comprising a cooling and heating process in which, after heating and holding in a radiant tube-type heating and holding furnace, the temperature is cooled from the final holding temperature in the annealing to a temperature of 150 to 350°C under conditions of an average cooling rate of 10°C / second or more, then heated to a temperature of 350 to 600°C and held for 10 to 600 seconds. Claim 5 In claim 1, the partial pressure of H2O (P H2O ) and partial pressure of H2 (P H2 The ratio of ) log(P H2O / P H2 A method for manufacturing high-strength hot-dip galvanized steel sheets in an atmosphere satisfying -0.99 or higher and 0.5 or lower. Claim 6 In paragraph 2, the partial pressure of the H2O (P H2O ) and partial pressure of H2 (P H2 The ratio of ) log(P H2O / P H2 A method for manufacturing high-strength hot-dip galvanized steel sheets in an atmosphere satisfying -0.99 or higher and 0.5 or lower. Claim 7 In paragraph 3, the partial pressure of the H2O (P H2O ) and partial pressure of H2 (P H2 The ratio of ) log(P H2O / P H2 A method for manufacturing high-strength hot-dip galvanized steel sheets in an atmosphere satisfying -0.99 or higher and 0.5 or lower. Claim 8 In paragraph 4, the partial pressure of the above H2O (P H2O ) and partial pressure of H2 (P H2 The ratio of ) log(P H2O / P H2 A method for manufacturing high-strength hot-dip galvanized steel sheets in an atmosphere satisfying -0.99 or higher and 0.5 or lower. Claim 9 In any one of claims 1 to 8, the partial pressure (P) of H2O H2O ) and partial pressure of H2 (P H2 The ratio of ) log(P H2O / P H2 A method for manufacturing high-strength hot-dip galvanized steel sheets in an atmosphere satisfying -0.9 or higher and 0.5 or lower. Claim 10 In any one of claims 1 to 8, the partial pressure (P) of H2O H2O ) and partial pressure of H2 (P H2 The ratio of ) log(P H2O / P H2 A method for manufacturing high-strength hot-dip galvanized steel sheets in an atmosphere satisfying -0.7 or higher and 0.5 or lower. Claim 11 In claim 9, the partial pressure of H2O (P H2O ) and partial pressure of H2 (P H2 The ratio of ) log(P H2O / P H2 A method for manufacturing high-strength hot-dip galvanized steel sheets in an atmosphere satisfying -0.7 or higher and 0.5 or lower.

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