KR20240152885A - Method for manufacturing high strength hot dip galvanized steel sheet - Google Patents

Abstract

Provided is a method for manufacturing a high-strength hot-dip galvanized steel sheet which prevents deterioration of the appearance quality of the steel sheet, has excellent LME crack resistance, 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 which is subjected to hot-dip galvanization after continuous annealing, wherein, at a front end of a direct-fired heating furnace, the steel sheet is heated from 400°C to 670°C in an atmosphere containing 1,000 ppm by volume or more of O ₂ and 1,000 ppm by volume or more of H ₂ O, and at a rear end of the direct-fired heating furnace, the steel sheet is heated from 600°C to 700°C in an atmosphere containing 500 ppm by volume or less of O ₂ , and in a heating and preservation furnace, the H ₂ O concentration of the atmosphere inside the furnace is from 5,000 ppm by volume or more to 40,000 ppm by volume or less, the H ₂ concentration is from 2% by volume or more to 20% by volume or less, and the ratio of the partial pressure of H ₂ O (P _H2O ) to the partial pressure of H ₂ (P _H2 ) log (P _H2O /P _H2 ) is -1.1. In an atmosphere that satisfies the criteria of 0.5 or less, preservation is performed at a temperature of 650℃ or higher and 900℃ or lower for 90 seconds or longer.

Description

Method for manufacturing high strength hot dip galvanized steel sheet

The present invention relates to a method for manufacturing a hot-dip galvanized steel sheet having excellent resistance to welding cracking and delayed fracture.

Recently, from the viewpoint of protecting the global environment, there has been a strong demand for improving the fuel efficiency of automobiles. In addition, from the viewpoint of ensuring the safety of passengers in the event of a collision, there has also been a strong demand for improving the safety of automobiles. In order to meet these demands, it is necessary to achieve both weight reduction and high strength in automobile bodies, and in the case of hot-dip galvanized steel sheets, which are materials for automobile parts, thinning due to high strength is actively being promoted. However, since most automobile parts are manufactured by forming and processing steel sheets, these steel sheets are required to have excellent formability in addition to high strength.

There are various methods for increasing the strength of hot-dip galvanized steel sheets, but as a method for achieving high strength without significantly damaging the formability of hot-dip galvanized steel sheets, in addition to utilizing martensite by adding C, solid solution strengthening by adding Si can be mentioned. Meanwhile, in the manufacture of automobile parts, press-formed parts are often assembled by resistance welding (spot welding). If a lot of C or Si is added to the steel sheet, there is a concern that residual stress is generated near the weld during resistance welding, and the zinc in the plating layer melts and diffuses into the grain boundaries, causing liquid metal embrittlement (LME), resulting in intergranular cracking (LME cracking) in the steel sheet. In particular, if welding is performed with the welding electrode at an angle to the steel sheet, there is a concern that residual stress may increase and cracking may occur. Since residual stress is thought to increase with the increase in strength of the steel plate, the occurrence of LME cracking accompanying the increase in strength of the steel plate is a concern.

In addition, it is also known that delayed fracture due to hydrogen embrittlement becomes more likely to occur as the strength of steel increases, and this tendency is particularly prominent in high-strength steels with a tensile strength of 1180 MPa or more. In addition, delayed fracture refers to a phenomenon in which, when a high-strength steel is subjected to a static load stress (load stress less than the tensile strength), brittle fracture occurs suddenly with almost no apparent plastic deformation after a certain period of time. Regarding such delayed fracture, it is often caused by corrosion caused by the usage environment and hydrogen that has penetrated into the steel sheet, but hydrogen that has penetrated into the steel sheet during the annealing process of a continuous galvanizing line (CGL) also deteriorates the mechanical properties of steel sheets with a tensile strength exceeding 980 PMa, causing brittle fracture.

As described above, there is a demand for a high-strength steel plate having excellent resistance welding fracture characteristics (hereinafter also simply referred to as “LME fracture resistance”) and suppressing deterioration of mechanical properties caused by hydrogen in the steel.

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 to 700°C or higher in an atmosphere containing O ₂ , and reducing the oxides on the surface layer of the steel sheet in an atmosphere containing H ₂ having a dew point of 5°C or higher. However, when heated to 700°C or higher in an atmosphere containing O ₂ , the amount of oxidation of the steel sheet increases, and there is a problem in that oxides adhere to the inside of the furnace during reduction annealing, thereby deteriorating the appearance quality of the steel sheet.

Patent Document 2 discloses a method for oxidizing the surface of Si-added steel by heating the steel 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 having a dew point of 5°C or higher and 500 volume ppm or higher and 5,000 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 steel sheet surface in an atmosphere in which log(P _H2O /P _H2 ) is -3.4 or higher and -1.1 or lower. In these methods, since the oxidation amount of the steel sheet can be adjusted, good appearance quality can be secured, however, there is a problem that sufficient LME brittleness resistance or delayed fracture resistance cannot be obtained because a large amount of hydrogen that has penetrated into the steel during annealing remains.

Japanese Patent No. 5652219 Japanese Patent No. 6052270 Japanese Patent No. 6172297

The present invention aims to provide a method for manufacturing a high-strength hot-dip galvanized steel sheet, which prevents deterioration of the appearance quality of a steel sheet due to adhesion of oxides inside the furnace to the steel sheet during reduction annealing when the oxidation amount of the steel sheet is excessive, and which has excellent LME crack resistance and ductility and, at the same time, suppresses deterioration of delayed fracture resistance caused by hydrogen embrittlement.

The present inventors have found that by optimizing the O ₂ concentration and temperature during oxidation of a steel sheet according to the Si concentration and Mn concentration contained in the steel sheet, thereby suppressing excessive oxidation, thereby securing the appearance quality of the steel sheet, and further optimizing the H ₂ O concentration, H ₂ concentration, and log(P _H2O /P _H2 ) during reduction annealing, it is possible to obtain excellent resistance welding cracking characteristics and at the same time suppress deterioration of delayed fracture characteristics due to hydrogen embrittlement, thereby completing the present invention.

The present invention has been made based on the above recognition. That is, the gist of the present invention is as follows.

[1] A method for manufacturing a high-strength hot-dip galvanized steel sheet, comprising: a hot-rolling step of 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, and then winding the slab into a coil at a temperature T _C (°C) calculated from the following formula (1) and acid-cleaning it; a cold-rolling step of cold-rolling the hot-rolled sheet obtained in the hot-rolling step; and a cold-rolling step of continuously annealing the cold-rolled steel sheet obtained in the cold-rolling step in an annealing furnace having a direct-fired heating furnace and a radiant tube-type heating and holding furnace, and then hot-dip galvanizing it.

In the above direct-fired heating furnace, in the early stage, the steel plate is heated to a temperature of 400°C or higher and 670°C or lower in an atmosphere containing 1000 ppm by volume or more of _O2 and 1000 ppm by volume or more of _H2O .

In the later stage, the steel plate is heated to 600℃ or higher and 700℃ or lower in an atmosphere containing 500 volume ppm or less of _O2 ,

In the annealing furnace having the above heating and preservation, the _H2O concentration of the atmosphere inside the furnace is 5,000 to 40,000 volume ppm, the _H2 concentration is 2 to 20 volume%,

A method for manufacturing a _high -strength _hot -dip galvanized steel sheet, comprising maintaining the _steel sheet at a temperature of 650° _C or higher and 900°C or lower for 90 seconds or longer in an atmosphere in which the ratio of the partial pressure of H2O (P H2O) to the partial pressure of H2 (P H2) log (P _H2O /P _H2 ) satisfies -1.1 or more and 0.5 or less.

T _C = -30([Si] + [Mn]) + 775 ···(1)

[Si] is the Si content (mass%) included in the steel plate.

[Mn] is the Mn content (mass%) included in the steel plate.

[2] A method for manufacturing a high-strength hot-dip galvanized steel sheet as described in [1], which comprises performing hot-dip galvanizing on a steel sheet and then performing an alloying treatment.

[3] A method for manufacturing a high-strength hot-dip galvanized steel sheet as described in [1] to [2], further comprising a cooling and heating process in which, after heating and preservation in a radiant tube-shaped heating and preservation holding furnace, the steel sheet is cooled from the final preservation and holding temperature in the annealing to a temperature of 150 to 350°C under the condition that the average cooling rate is 10°C/sec or more, and then heated to a temperature of 350 to 600°C and preserved and held for 10 to 600 seconds.

[4] A method for manufacturing a high-strength hot _- dip galvanized steel sheet as described in any one of [1] to [3], wherein the _atmosphere satisfies the ratio log(P _H2O /P _H2 ) of the partial pressure of _H2O (P _H2O ) and the partial pressure of H2 (P H2) of -0.99 or more and 0.5 or less.

[5] A method for manufacturing a high-strength _hot- dip galvanized _steel sheet as described in any one of [1] to [4], wherein the atmosphere satisfies the ratio log(P _H2O /P _H2 ) of the partial pressure of _H2O (P _H2O ) and the partial pressure of H2 (P H2) of -0.9 or more and 0.5 or less.

[6] A method for manufacturing a high-strength _hot- dip galvanized _steel sheet as described in any one of [1] to [5], wherein the atmosphere satisfies the ratio log(P _H2O /P _H2 ) of the partial pressure of _H2O (P _H2O ) and the partial pressure of H2 (P H2) of -0.7 or more and 0.5 or less.

According to the present invention, it is possible to provide a high-strength steel plate having excellent weld crack resistance and good appearance quality in a welded portion, and sufficiently reducing hydrogen in the steel, which is a factor in deterioration of delayed fracture resistance.

Figure 1 is a structural diagram of a test material for evaluating LME breakage.
The upper drawing of Fig. 2 is a plan view of a sheet assembly for attaching a welded portion, and the lower drawing is a drawing showing a cross-section in the sheet thickness direction after cutting the sheet assembly for attaching a welded portion at the cutting position shown in the upper drawing.

(Form for carrying out the invention)

Hereinafter, embodiments of the present invention will be described.

In addition, in the description below, the units of the content of each element of the component composition of the Si-containing slab and the content of each element of the component composition of the plating layer are both "mass%", and are simply expressed as "%" unless otherwise specified. In addition, in this specification, the numerical range indicated using "∼" means a range that includes the numerical values described before and after "∼" as the lower limit and the upper limit. In addition, in this specification, "high strength" of the steel plate means that the tensile strength TS of the steel plate measured in accordance with JIS Z 2241 (2011) is 590 MPa or more.

First, the composition of the Si-containing slab is explained.

<Slab component>

Si: 0.45% or more and 2.0% or less

Si is an effective element for achieving high strength of steel plates because it has a large effect of increasing the strength of steel by solid solution (solid solution strengthening ability) without significantly damaging workability. On the other hand, Si is also an element that adversely affects the resistance welding cracking characteristics in welded areas. When adding Si to achieve high strength of steel plates, the addition of 0.45% or more is necessary. Furthermore, when Si is less than 0.45%, there is no particular problem with the resistance welding cracking characteristics in welded areas, and therefore there is little need to apply the present invention. On the other hand, when the Si content exceeds 3.0%, the hot rolling properties and cold rolling properties are greatly reduced, which adversely affects productivity or causes a reduction in the ductility of the steel plate itself. Therefore, Si is added in a range of 0.45% or more and 3.0% or less. The Si content is preferably 0.7% or more, and more preferably 0.9% or more. In addition, the amount of Si is preferably 2.5% or less, more preferably 2.0% or less.

C: 0.30% or less

C improves the workability of steel sheets by forming martensite and the like as a strong structure. When containing C, in order to obtain good weldability and LME crack resistance, the C content is preferably 0.8% or less, more preferably 0.30% or less. The lower limit of C is not particularly limited, but in order to obtain good workability, it is preferably 0.03% or more, more preferably 0.05% or more.

Mn: 1.0% or more and 4.0% or less

Manganese is an element that has the effect of strengthening steel by solidification and increasing its strength, as well as increasing quenchability, and promoting the formation of retained austenite, bainite, and martensite. These effects are expressed by containing 1.0% or more of manganese. On the other hand, when the manganese content is 4.0% or less, the above effect is obtained without causing an increase in cost. Therefore, the manganese content is preferably 1.0% or more, and more preferably 4.0% or less. The manganese content is more preferably 1.8% or more. Furthermore, the manganese content is more preferably 3.3% or less.

As for the following components, their content is not limited, but the preferred range is as follows.

P: 0.1% or less (excluding 0%)

By suppressing the P content, the deterioration of weldability can be prevented. In addition, by preventing P from segregating at grain boundaries, the deterioration of ductility, bendability, and toughness can be prevented. In addition, if a large amount of P is added, the crystal grain size also increases by promoting ferrite transformation. Therefore, it is preferable that the P amount be 0.1% or less. The lower limit of P is not particularly limited, and is more than 0% due to constraints in production technology, and is usually 0.001% or more.

S: 0.03% or less (excluding 0%)

It is preferable that the S content be 0.03% or less, and more preferably 0.02% or less. By suppressing the S content, not only can the deterioration of weldability be prevented, but also the deterioration of ductility during hot rolling can be prevented, hot cracking can be suppressed, and the surface quality can be significantly improved. Furthermore, by suppressing the S content, it is possible to prevent the deterioration of the ductility, bendability, and stretch flangeability of the steel sheet by forming coarse sulfides as impurity elements. These problems become significant when the S content exceeds 0.03%, and it is preferable to reduce the S content as much as possible. The lower limit of S is not particularly limited, and is more than 0% due to constraints in production technology, and is usually 0.001% or more.

Al: 0.1% or less (excluding 0%)

Since Al is thermodynamically the most easily oxidized, it oxidizes before Si and Mn, and has the effect of suppressing oxidation of Si and Mn in the outermost layer of the steel plate and promoting oxidation of Si and Mn in the interior of the steel plate. This effect is obtained when the Al content is 0.01% or more. On the other hand, if the Al content exceeds 0.1%, the cost increases. Therefore, when adding, it is preferable that the Al content be 0.1% or less. The lower limit of Al is not particularly limited, and is more than 0%, and is usually 0.001% or more.

N: 0.010% or less (excluding 0%)

It is preferable that the N content be 0.010% or less. By setting the N content to 0.010% or less, it is possible to prevent N from forming coarse nitrides with Ti, Nb, and V at high temperatures, thereby impairing the effect of increasing the strength of the steel sheet by adding Ti, Nb, and V. In addition, by setting the N content to 0.010% or less, it is also possible to prevent a decrease in toughness. In addition, by setting the N content to 0.010% or less, it is possible to prevent the slab from cracking or 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, and is usually 0.0005% or more due to constraints in production technology.

The composition may additionally optionally contain one or more 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.

B: 0.005% or less

B is an effective element for improving the quenching property of steel. In order to improve the quenching property, the B content is preferably 0.0003% or more, and more preferably 0.0005% or more. However, since excessive addition of B reduces the formability, the B content is preferably 0.005% or less.

Ti: 0.2% or less

Ti is effective in strengthening the steel by precipitation. The lower limit of Ti is not particularly limited, but in order to obtain the effect of strength adjustment, it is preferable to set it to 0.005% or more. However, if Ti is added excessively, the hard phase becomes excessive and the formability deteriorates, so when adding Ti, the Ti amount is preferably 0.2% or less, and more preferably 0.05% or less.

Cr: 1.0% or less

It is desirable that the Cr content be 0.005% or more. By setting the Cr content to 0.005% or more, the quenching property can be improved and the balance between strength and ductility can be improved. When adding Cr, it is desirable that the Cr content be 1.0% or less from the viewpoint of preventing cost increases.

Cu: 1.0% or less

It is preferable that the amount of Cu be 0.005% or more. By setting the amount of Cu to 0.005% or more, the formation of residual γ phase can be promoted. In addition, when adding Cu, it is preferable that the amount of Cu be 1.0% or less from the viewpoint of preventing cost increase.

Ni: 1.0% or less

It is preferable that the amount of Ni be 0.005% or more. By setting the amount of Ni to 0.005% or more, the formation of residual γ phase can be promoted. In addition, when adding Ni, from the viewpoint of preventing cost increase, it is preferable that the amount of Ni be 1.0% or less.

Mo: 1.0% or less

It is preferable that the Mo amount be 0.005% or more. By setting the Mo amount to 0.005% or more, the effect of strength adjustment can be obtained. The Mo amount is more preferably 0.05% or more. In addition, when adding Mo, from the viewpoint of preventing cost increase, the Mo amount is preferably 1.0% or less.

Nb: 0.20% or less

The effect of improving strength is obtained by containing Nb at 0.005% or more. In addition, when containing Nb, it is preferable to keep the amount of Nb to 0.20% or less from the viewpoint of preventing cost increases.

V: 0.5% or less

The effect of strength improvement is obtained by containing V at 0.005% or more. In addition, when containing V, it is preferable to keep the amount of V to 0.5% or less from the viewpoint of preventing cost increase.

Sb: 0.200% or less

Sb can be contained from the viewpoint of suppressing decarburization in a region of a depth of several tens of microns from the steel plate surface caused by nitriding, oxidation, or oxidation of the steel plate surface. Sb prevents a decrease in the amount of martensite formed on the steel plate surface by suppressing nitriding and oxidation of the steel plate surface, thereby improving the fatigue properties and surface quality of the steel plate. In order to obtain this effect, the Sb amount is preferably 0.001% or more. On the other hand, in order to obtain good toughness, the Sb amount is preferably 0.200% or less.

Ta: 0.1% or less

The effect of improving strength is obtained by containing Ta at 0.001% or more. In addition, when containing Ta, it is preferable to keep the Ta amount to 0.1% or less from the viewpoint of preventing cost increase.

W: 0.5% or less

The effect of improving strength is obtained by containing W at 0.005% or more. In addition, when containing W, it is preferable to keep the amount of W to 0.5% or less from the viewpoint of preventing cost increases.

Zr: 0.1% or less

The effect of improving strength is obtained by containing Zr at 0.0005% or more. In addition, when containing Zr, it is preferable to keep the Zr amount to 0.1% or less from the viewpoint of preventing cost increase.

Sn: 0.20% or less

Sn is an effective element for suppressing denitrification, decomposition, etc., and thus suppressing the reduction in strength of steel. To obtain this effect, it is desirable to set the Sn content at 0.002% or more. On the other hand, to obtain good impact resistance, it is desirable to set the Sn content at 0.20% or less.

Ca: 0.005% or less

Ca can control the form of sulfides by containing 0.0005% or more, thereby improving ductility and toughness. In addition, from the viewpoint of obtaining good ductility, it is preferable that the amount of Ca be 0.005% or less.

Mg: 0.005% or less

Mg can control the form of sulfide by containing 0.0005% or more, thereby improving ductility and toughness. In addition, when containing Mg, it is preferable to keep the amount of Mg to 0.005% or less from the viewpoint of preventing cost increases.

REM: 0.005% or less

REM can control the form of sulfides by containing 0.0005% or more, thereby improving ductility and toughness. In addition, when containing REM, it is preferable to set the REM amount to 0.005% or less from the viewpoint of obtaining good toughness.

The Si-containing slab of the present embodiment comprises, in addition to the above components, the remainder being Fe and unavoidable impurities. Here, the Si-containing steel sheet may be either a cold-rolled steel sheet or a hot-rolled steel sheet.

<Hot Rolling>

The hot rolling process is a process in which the slab described above is hot rolled, then wound into a coil at a temperature T _C (℃) or lower calculated from the following formula (1), and then acid washed.

The technical significance of the hot rolling process is explained. In normal hot rolling, after rolling is completed and coiled into a coil, oxygen diffuses from the oxide scale into the inside of the steel sheet during the cooling process, so that internal oxides of Si or Mn are formed inside rather than on the surface of the steel sheet. However, since the internal oxides of Si or Mn formed after rolling are formed unevenly, when hot-dip plating is performed in a subsequent CGL, they cause appearance defects such as uneven plating adhesion or uneven alloying after alloying. Therefore, it is important to suppress the formation of internal oxides in hot rolling. In order to suppress internal oxides of Si or Mn, it is effective to lower the coiling temperature after rolling. In addition, when using steel with a high content of Si or Mn that forms as oxides, it is necessary to further lower the coiling temperature.

As a result of further investigation, it was found that by controlling the internal oxidation amount (the sum of Si internal oxide and Mn internal oxide generated in the steel plate surface layer within 10 ㎛ from the steel plate surface immediately below the scale of a hot-rolled sheet; the internal oxidation amount is expressed as the oxygen amount in the longitudinal and transverse central positions of the coil after rolling) in the longitudinal central portion of the coil and also in the transverse central portion to 0.10 g/㎡ or less, the internal oxidation of Si and Mn becomes more uniform, and even if a hot-dip galvanizing process is performed thereafter, the occurrence of unevenness in plating adhesion and unevenness in appearance after alloying can be further suppressed. Here, by using steel with changed Si and Mn contents, hot rolling was performed, and the internal oxidation amounts in the longitudinal central portion of the coil formed after cooling and also in the widthwise central portion were investigated, it was found that by winding the coil at a temperature T _C (℃) or lower calculated from the following formula (1), the total of Si internal oxide and Mn internal oxide formed in the hot rolling process can be controlled to 0.10 g/㎡ or lower.

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

Here, Tc is the coiling temperature after rolling, [Si] and [Mn] are the Si content and Mn content in the steel, respectively. In addition, Tc is preferably 400°C or higher.

In addition, the heating temperature before hot rolling and the finishing temperature of hot rolling are not particularly limited, but from the viewpoint of tissue control, it is preferable to heat and crack the slab at 1100 to 1300°C and complete the finishing rolling at 800 to 1000°C.

In the present invention, after the above rolling, acid washing is performed to remove scale. The acid washing method is not particularly limited, and a conventional method may be adopted.

<Cold rolling process>

The cold rolling process is a process of cold rolling the hot rolled sheet obtained in the above hot rolling process. The conditions for cold rolling are not particularly limited, and for example, it is sufficient to cold roll the cooled hot rolled sheet at a predetermined reduction ratio of 30 to 80%.

<Annealing process>

The annealing process of the present invention comprises a process of oxidizing a steel sheet obtained by the cold rolling process using a direct 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.

First, we will explain the direct fire heating furnace (oxidation annealing process for steel plates).

In order to realize high strength and high workability of the steel, it is effective to add C, Si or Mn. However, if a steel sheet to which these elements have been added is used, Si and Mn oxides are generated on the surface of the steel sheet during the annealing process (oxidation treatment + reduction annealing) performed before hot-dip galvanizing treatment, making it difficult to secure plating properties. Therefore, it is effective to oxidize Si or Mn inside the steel sheet and prevent oxidation of these elements on the surface of the steel sheet, but as described above, in the present invention, it is essential to suppress internal oxidation formed after hot rolling from the viewpoint of unevenness of plating adhesion or alloying. Even in cases where the formation of internal oxidation after hot rolling is small, by strictly controlling the annealing conditions (oxidation treatment conditions + reduction annealing conditions) before hot-dip galvanizing treatment, Si and Mn are oxidized inside the steel sheet, thereby improving the plating properties, and further increasing the reactivity between the plating and the steel sheet, thereby improving the plating adhesion. And, in the annealing process, in order to oxidize Si and Mn inside the steel plate and prevent oxidation on the surface of the steel plate, oxidation treatment is performed. In particular, it is necessary to obtain a certain amount or more of iron oxide through oxidation treatment. After that, it is effective to perform reduction annealing, hot-dip plating, and alloying treatment as needed.

In order to obtain a sufficient amount of iron oxide, it becomes necessary to manage the heating atmosphere and temperature. The atmosphere is controlled by controlling the air ratio of the direct-fired heating furnace. The direct-fired heating furnace directly applies a burner flame that mixes fuel such as coke oven gas (COG), a by-product gas of the steel mill, and air to the surface of the steel plate to heat the steel plate. By increasing the air ratio and increasing the ratio of air to fuel, unreacted oxygen remains in the flame, and the oxygen can promote oxidation of the steel plate. Here, in addition to coke oven gas, natural gas, hydrogen gas, ammonia gas, etc. may be used as the fuel of the direct-fired heating furnace. Oxidation products generated when these fuels are burned include CO, CO ₂ , H ₂ O, NO _X , etc. In addition, N ₂ in the combustion air is also present in the atmosphere.

On the other hand, if the steel sheet is excessively oxidized, the oxide is peeled off in the subsequent reduction annealing process and attached to the roll, causing a phenomenon called pickup. If pickup occurs in the roll, it greatly deteriorates the appearance of the galvanized steel sheet. Therefore, the process of oxidizing the steel sheet using a direct heating furnace requires two or more separated areas and heating in two or more different atmospheres. Next, the front end of the heating zone and the back end of the heating zone are explained.

Heating shear

The steel plate is heated to a temperature of 400℃ or higher and 670℃ or lower in an atmosphere containing 1000 volume ppm or more of _O2 and 1000 volume ppm or more of _H2O .

In the heating zone front, the air ratio is adjusted so that the _O2 is 1000 ppm by volume or more and the _H2O is 1000 ppm by volume or more, and the cold rolled steel sheet is heated. Here, if the _O2 is less than 1000 ppm by volume and the _H2O is less than 1000 ppm by volume, the oxidation of the steel sheet becomes insufficient. On the other hand, if the _O2 is 1000 ppm by volume or more and the _H2O is 1000 ppm by volume or more, the effect of the _O2 and _H2O concentrations on the oxidation of the steel sheet is small, and the effect of the temperature of the steel sheet becomes large, so the upper limit is not specifically set. Preferably, from the viewpoint of deterioration of the equipment, the _O2 is 10000 ppm by volume or less and the _H2O is 10000 ppm by volume or less. The steel sheet is heated so that the temperature is in the range of 400°C or more and 670°C or less. If the temperature of the steel plate is lower than 400℃, the oxidation of the steel plate is insufficient, and if it exceeds 670℃, the oxidation of the steel plate becomes excessive, resulting in pickup by the aforementioned roller. Therefore, in the present invention, it is essential to heat the steel plate so that the temperature is in the range of 400℃ or higher and 670℃ or lower.

After the heating zone

The steel plate is heated to a temperature of 600℃ or higher and 700℃ or lower in an atmosphere containing 500 volume ppm or less of _O2.

The rear end of the heating zone is an important requirement in the present invention for suppressing the aforementioned roll pickup and obtaining a beautiful surface appearance without pressing damage, etc. In order to prevent the occurrence of the pickup phenomenon, it is important to reduce a part (surface layer) of the surface of the steel plate that has been oxidized. To perform such reduction treatment, the air ratio is adjusted so that the atmosphere of _O2 is 500 volume ppm or less at the rear end of the heating zone, and the steel plate 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 plate becomes excessive, and the aforementioned pickup by the roll occurs. The steel plate is heated so that the temperature is in the range of 600°C or more and 700°C or less. If the temperature of the steel plate is less than 600°C, the reduction of a part (surface layer) of the surface of the steel plate is insufficient, and if it exceeds 700°C, a part (surface layer) of the surface of the steel plate is not reduced, oxidation is promoted, and the aforementioned pickup by the roll may occur. Therefore, in the present invention, it is essential to heat the steel plate so that the temperature is in the range of 600℃ or higher and 700℃ or lower.

Next, we will explain about radiant tube type heater and preservation furnace (reduction annealing process of steel plate).

As described so far, it is effective to add C, Si or Mn to realize high strength and high workability of steel. However, there is concern that when using steel sheets with a large amount of C or Si added, the zinc in the plating layer melts and diffuses into the grain boundaries, causing LME, resulting in grain boundary cracking (LME cracking) in the steel sheet. In addition, it is also known that delayed fracture due to hydrogen embrittlement becomes more likely to occur as the strength of the steel increases. Although such delayed fracture is often caused by corrosion caused by the usage environment and hydrogen that has penetrated into the steel sheet, hydrogen that has penetrated into the steel sheet during the CGL annealing process also causes deterioration in the delayed fracture resistance of steel sheets, especially those with a tensile strength exceeding 980 PMa.

In order to solve these problems, it is important to control the atmosphere of the reduction annealing during the annealing process (oxidation treatment + reduction annealing) performed before hot-dip galvanizing. Although the mechanism is not clear, by controlling the atmosphere of the reduction annealing, the dissolved Si or Mn around the internal oxide layer of Si or Mn formed decreases. In addition, since C is oxidized by _H2O in the atmosphere and released into the furnace as CO gas, the C concentration in the surface layer of the steel sheet decreases. As a result, an area deficient in dissolved C and Si, which causes LME cracking, is formed on the surface layer, so it is thought that LME cracking is difficult to occur as a result. In addition, with respect to hydrogen that has penetrated the steel sheet, since the internal oxide layer of Si or Mn similarly exists on the surface layer of the steel sheet, when alloying the plating layer and the base steel, the internal oxide of Si or Mn formed on the surface layer of the steel sheet is dispersed in the plating layer, and thus dehydrogenation from the steel sheet after manufacturing is promoted, so it is thought that good delayed fracture resistance is obtained.

For reduction annealing, radiant tube-shaped heating or preservation can be used. At this time, by controlling the H ₂ O concentration of the atmosphere to 5000 ppm by volume or more and 40000 ppm by volume or less, LME cracking can be suppressed and dehydrogenation can be promoted. If the H ₂ O concentration is less than 5000 ppm by volume, the LME cracking resistance or the dehydrogenation promotion effect cannot be said to be sufficient. On the other hand, if the H ₂ O concentration exceeds 40000 ppm by volume, there is a concern about equipment damage, so it is preferable that it is 40000 ppm by volume or less. Here, the difference in the H ₂ O concentration between the upper and lower parts of the furnace must be 2000 ppm by volume or less. If the difference in _H2O concentration between the upper and lower parts of the furnace exceeds 2000 ppm by volume, Si or Mn in the steel may not be internally oxidized but may be externally oxidized, which may impair plating properties and cause plating defects. In addition, if a sufficient internal oxide layer is not formed, LME crack resistance or dehydrogenation promotion effect may not be sufficient.

The formation of the internal oxide layer is also greatly affected by the H ₂ concentration during reduction annealing. The H ₂ concentration must be 2 vol% or more and 20 vol% or less. In addition, the ratio of the partial pressure of H ₂ O (P _H2O ) to the partial pressure of H ₂ (P _H2 ) must satisfy the following formula (2). If the H ₂ concentration is less than 2 vol%, the reduction of the oxidized steel sheet is insufficient, and when hot-dip galvanizing is performed, a non-plating defect may occur or the plating adhesion may be impaired. On the other hand, if the hydrogen concentration exceeds 20 vol%, a large amount of hydrogen may remain in the steel sheet, and even if dehydrogenation is promoted, the amount of hydrogen remaining in the steel may increase, and good delayed fracture resistance may not be obtained. The ratio of the partial pressure of H ₂ O (P _H2O ) to the partial pressure of H ₂ (P _H2 ) affects the formation of the internal oxide layer. In order to obtain good LME breakage resistance or dehydrogenation promotion effect, log(P _H2O /P _H2 ) needs to be -1.1 or more and 0.5 or less. If log(P _H2O /P _H2 ) is less than -1.1, a sufficient internal oxide layer is not formed, and good LME breakage resistance or dehydrogenation promotion effect may not be obtained. On the other hand, if log(P _H2O /P _H2 ) exceeds 0.5, there is a concern about equipment damage, so it is preferable that log(P _H2O /P _H2 ) is 0.5 or less.

In addition, it was found that increasing log(P _H2O /P _H2 ) is effective for the bendability required for the formability of high-strength steel plates. Although this mechanism is not clear, it is thought to be due to the effect of improving formability by reducing hydrogen in the steel plate and the change in strain distribution ability due to the presence of a layer with relatively good formability on the surface due to the presence of an internal oxide layer. By setting log(P _H2O /P _H2 ) to -1.1 or more, bendability is also improved, but by setting log(P _H2O /P _H2 ) to -0.99 or more, bendability is further improved, and it may be set to -0.90 or more, and can be further improved by setting it to -0.7 or more. In any case, the upper limit of log(P _H2O /P _H2 ) is preferably 0.5 or less.

In addition, for the reduction annealing atmosphere, it is preferable to use N ₂ from a cost standpoint, other than H ₂ O and H ₂ . In addition, there may be mixing of NO _X , SO _X , CO, CO ₂ , etc.

The temperature of reduction annealing must be 650℃ or higher and 900℃ or lower. If it is lower than 650℃, the formation of an internal oxide layer, which is necessary for improving LME crack resistance or promoting dehydrogenation, may be insufficient. In addition, if it exceeds 900℃, there is concern about damage to the furnace body of the annealing furnace, so it is preferable that it is lower than 900℃.

The reducing atmosphere described above may be satisfied in part or in whole within the furnace. In the case where part satisfies the atmosphere described above, the annealing time in the designated atmosphere is required to be 90 seconds or longer. If annealing is performed in the designated atmosphere for 90 seconds or longer, the atmosphere of the reducing annealing may not be controlled in whole within the furnace.

<Cooling heating process>

The cooling and heating process is a process in which, after reduction annealing, the final holding temperature in reduction annealing is cooled to a cooling temperature of 150 to 350°C under the condition of an average cooling rate of 10°C/sec or more, and then the process is heated to a reheating temperature of 350 to 600°C and held at that temperature for 10 to 600 seconds. By performing this cooling and heating process, the mechanical properties can be further improved. In addition, in the present invention, the cooling and heating process is not an essential process, and may be performed as needed.

When the cooling rate from the final holding temperature in reduction annealing is less than 10°C/sec, pearlite is generated, which reduces TS×EL and hole expandability. Therefore, the cooling rate from the final holding temperature in reduction annealing is preferably 10°C/sec or more. Here, the final holding temperature in reduction annealing refers to the temperature at which a steel sheet annealed in a range satisfying the requirements of the annealing temperature, hydrogen concentration, dew point, and holding time of the reduction annealing deviates from at least one of the requirements.

When the cooling temperature reaches a temperature higher than 350°C, in the subsequent molten plating process, the temperature of the plating bath rises, which may promote the generation of dross that deteriorates the surface appearance quality. Therefore, the cooling temperature is preferably 350°C or lower. By setting the cooling temperature to 350°C or lower, the mechanical properties can be improved. In addition, when the cooling temperature falls below 150°C, most of the austenite transforms into martensite during cooling, and the amount of untransformed austenite decreases. Therefore, the cooling temperature is preferably in the range of 150 to 350°C. As for the cooling method, any cooling method such as gas jet cooling, mist cooling, water cooling, or metal quench may be used as long as the target cooling rate and cooling stop temperature (cooling stop temperature) can be achieved.

Here, after cooling to the cooling temperature, in some cases, heating to the reheating temperature may be performed and kept for 10 seconds or more. By keeping for 10 seconds or more, the martensite generated during cooling is tempered to become tempered martensite. As a result, hole expandability is improved, and also, the untransformed austenite that was not transformed into martensite during cooling is stabilized, and finally, a sufficient amount of retained austenite is obtained, so that ductility is improved in some cases.

In addition, in the case of reheating, if the reheating temperature exceeds 600°C, the undeformed austenite at the time of cooling stop transforms into pearlite, and ultimately, retained austenite of 3% or more in area ratio is not obtained. If the preservation and holding time at the time of reheating is less than 10 seconds, austenite stabilization becomes insufficient, and furthermore, if it exceeds 600 seconds, the undeformed austenite at the time of cooling stop transforms into bainite, and ultimately, a sufficient amount of retained austenite is not obtained. Therefore, the temperature at the time of reheating is set to be in the range of 350 to 600°C, and the preservation and holding time in that temperature range is set to be 10 to 600 seconds.

<Hot-dip galvanizing process>

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

In the present invention, the Al concentration in the zinc plating bath is set to 0.12 to 0.22 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 occurrence of uneven appearance. If it exceeds 0.22 mass%, a thick Fe-Al alloy phase is formed at the plating/base iron interface during plating, which may result in deterioration of weldability. In addition, since there is a large amount of Al in the bath, a large amount of Al oxide film is formed on the surface of the plating steel sheet, which may result in deterioration of not only weldability but also appearance.

When alloying treatment is performed, the Al concentration in the plating bath is preferably 0.12 to 0.17 mass%. When 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 occurrence of uneven appearance. When it exceeds 0.17 mass%, the Fe-Al alloy phase formed at the plating/base iron interface during plating becomes thick and acts as a barrier to the Fe-Zn alloying reaction, which may result in a high alloying temperature and deterioration of mechanical properties.

There are no other restrictions on the conditions during hot-dip galvanizing, but for example, the hot-dip galvanizing bath temperature is usually in the range of 440 to 500°C, and the steel sheet is immersed in the plating bath at a plate temperature of 440 to 550°C, and the adhesion amount can be adjusted by gas wiping, etc.

<Alloying Process>

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

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

Example

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

These slabs were heated to 1200℃, hot rolled at a finishing temperature of 890℃ to a plate thickness of 2.6mm, coiled at the coiling temperature shown in Table 2, cooled, and then acid-washed to remove black scale to produce hot-rolled sheets. At this time, the internal oxidation amount of Si and/or Mn in the longitudinal direction of the coil and the central portion in the width direction was measured by the method shown below.

<Internal oxidation amount after hot rolling>

The internal oxidation amount is measured by the "impulse furnace melting-infrared absorption method." The oxygen concentration in each steel was measured before and after polishing 10 μm in a 10 mm x 70 mm area of the surface layers on both sides of the hot-rolled sheet (the center of the coil (the center in the width direction and also the center in the length direction)). In addition, from the difference between those measured values, the oxygen amount per unit area on one side existing in an area of 10 μm from the steel sheet surface was obtained, and this was taken as the internal oxidation amount (g/m2) of Si and/or Mn. That the internal oxide formed on the surface layer of the hot-rolled sheet was oxide of Si and/or Mn was confirmed by observation with a SEM and elemental analysis with an EDS (energy dispersive X-ray spectrometer) after embedding the hot-rolled sheet in a resin and polishing the cross-section. The internal oxidation amount is shown in Table 3.

Next, after cold rolling to a cold rolled sheet having a thickness of 1.2 mm, annealing and hot dip galvanizing in a CGL were performed. The front end of the heating furnace was heated under the conditions shown in Table 2 by a direct-fired heating furnace having a nozzle-mix type burner. Next, the rear end of the heating furnace was heated under the conditions shown in Table 2 by a direct-fired heating furnace having a premix type burner. In addition, the oxidation initiation temperature was set to 300°C. Since the oxidation initiation temperature does not particularly affect the plating appearance, an oxidizing atmosphere of less than 400°C may be used. Reduction annealing was performed in a radiant tube-type heating and preservation furnace under the conditions shown in Table 2, and then cooling was performed. Subsequently, hot dip galvanizing was performed using a 460°C zinc bath containing 0.135% Al as shown in Table 2, and then the amount per unit area was adjusted to approximately 50 g/㎡ by gas wiping. In some conditions, alloying treatment was performed.

Continuing, the appearance of the high-strength hot-dip galvanized steel sheet obtained by the above was evaluated and the tensile properties were investigated. In addition, the LME crack resistance, dehydrogenation behavior, and damage to the furnace were evaluated. The measurement method and evaluation method are described below.

<Appearance>

The appearance of the steel plate was visually observed, and if there were no appearance defects such as galling, pressing damage due to pick-up phenomenon, or uneven alloying, it was given an "◎" rating. If there were slight appearance defects but they were within the acceptable range as a product, it was given an "○" rating. If there was clear uneven alloying, galling, or pressing damage, it was given an "×" rating. If the above evaluation was "○" or "◎", the appearance was judged to be good.

<Tensile properties>

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

<My LME Fragility>

A test piece cut from a hot-dip galvanized steel sheet to a lengthwise 150 mm × lengthwise 50 mm, with the rolling direction perpendicular (TD) as the long side and the rolling direction as the short side, was made into a board by overlapping a test hot-dip galvanized steel sheet (plate thickness: 1.2 mm, TS: 980 MPa class) whose hot-dip galvanized layer has a plating adhesion amount per side of 50 g/m2 and which was cut to the same size. This board was assembled so that the hot-dip galvanized layer of the test piece and the hot-dip galvanized layer surface of a commercial hot-dip galvanized steel sheet were aligned. As shown in Fig. 1(A), this board was fixed to a fixture at an incline of 5°, which is the maximum incline assumed for some component shapes, via a spacer having a thickness of 2.0 mm. 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 each of the long-side cross sections of the pair of steel plates is arranged so that it matches both short-side cross sections of the plate. Therefore, the distance between the pair of steel plates forming the spacer is 60 mm. The fixing member is a single plate with a hole in the center.

Next, using a single-phase AC (50 Hz) resistance welder pressurized by a servo motor, the plate was pressed by a pair of electrodes (tip diameter: 6 mm) while the plate was bent, and resistance welding was performed under the conditions of a pressing force of 3.5 kN, a hold time of 0.10 s or 0.16 s, and a welding current and welding time that made the nugget diameter of the weld 5.9 mm (i.e., the welding current and welding time were appropriately adjusted so that the nugget diameter was 5.9 mm for each plate) to obtain a plate with a welded portion attached. At this time, one pair of electrodes pressed the plate from above and below in the vertical direction, and the lower electrode pressed the test piece through the hole of the fixture. During pressurization, the lower electrode and the fixture were fixed so that the lower electrode of the pair of electrodes contacted a plane extending from the surface where the spacer and the fixture contacted, and the upper electrode was movable. In addition, the upper electrode was made to contact the center of the test hot-dip galvanized steel plate. In addition, the hold time refers to the time from when the welding current is fully applied until the electrode starts to open. In addition, the nugget diameter refers to the distance of the end (10) of the nugget in the longitudinal direction of the plate, as shown in Fig. 2.

Next, as shown in Fig. 2, the weld attachment plate was cut to include the weld (nugget), the cross-section of the weld was observed with an optical microscope (200x), and the resistance 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 weld attachment plate and shows the cut position. The lower drawing of Fig. 2 is a drawing showing a cross-section in the plate thickness direction of the plate after cutting, and schematically shows a crack that occurred in the test piece. In addition, when a crack occurs in the test hot-dip galvanized steel sheet, the stress in the test piece is dispersed, and an appropriate evaluation cannot be performed. Therefore, data in which a crack does not occur in the test hot-dip galvanized steel sheet was adopted as an example.

If the evaluation below is “○” or “◎”, the resistance welding cracking characteristics in the weld zone are judged to be good or excellent, respectively, and if it is “×”, the resistance welding cracking characteristics in the weld zone are judged to be poor.

◎: No cracks longer than 0.1mm are observed at a hold time of 0.10 seconds.

○: A crack longer than 0.1 mm is observed at a hold time of 0.10 seconds, but no crack longer than 0.1 mm is observed at a hold time of 0.16 seconds.

×: A crack with a length of 0.1mm or longer is observed at a hold time of 0.16 seconds.

<Dehydrogenation behavior>

A rectangular test piece with a major axis length of 30 mm and a minor axis length of 5 mm was collected from the central portion of the width of a hot-dip galvanized steel sheet, the plating layer of the test piece was removed with a Leutor, and immediately thereafter, a temperature-rising and desorption analyzer was used to perform hydrogen analysis under the conditions of an analysis start temperature of 25°C, an analysis end temperature of 300°C, and a heating rate of 200°C/hour. The released hydrogen amount (mass ppm/min), which is the amount of hydrogen released from the surface of the test piece at each temperature, was measured. The sum of the released hydrogen amounts from the analysis start temperature to 300°C was calculated as the diffusible hydrogen amount in the steel. Here, a steel with a diffusible hydrogen amount of 0.10 mass ppm or less was rated as "good" (◎), and a steel with a diffusible hydrogen amount of 0.30 mass ppm or less was rated as "passed" (○). 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 plate often deteriorates, so 0.30 mass ppm or more was rated as “×”. Dehydrogenation behavior was judged to be excellent in cases of “◎” and “○”.

<Loche Damage>

Damage to the furnace was assessed visually to see if discoloration was observed in the iron shell (SUS310S) inside the annealing furnace. Here, if no discoloration was observed in the iron shell, it was marked as “○” and judged as not causing damage to the furnace. If discoloration was clearly observed, it was marked as “×” and judged as causing damage to the furnace.

<Method for evaluating bendability>

From the manufactured galvanized steel sheet, a 25×100mm rectangular test piece was cut out so that the short side was parallel to the rolling direction. Then, a 90° V-bending test was performed so that the ridge line would be the ridge line when bent in the rolling direction. The streak speed was 50mm/min, and a final pressing was performed by pressing against a die with a load of 10 tons for 5 seconds. The test was performed by changing the tip R of the V-shaped punch in various steps of 0.5, and the presence or absence of cracks (breaks) was confirmed by observing the vicinity of the ridge line of the test piece with a 20x lens. R/t was calculated from the minimum R at which no cracks occurred and the plate thickness of the test piece (tmm, using a value from the thousandth to the rounded hundredth), and this was used as an index of bendability. The smaller R/t is, the better the bendability. Here, R/t less than 1.0 was considered very good "◎＋", less than 1.5 was considered good "◎", less than 2.0 was considered pass "○", less than 4.0 was considered average "△", and 4.0 or more was considered "×".

The results obtained above, together with the manufacturing conditions, are shown in Table 3.

From Table 3, the examples of the present invention, despite being high-strength hot-dip galvanized steel sheets containing C, Si, and Mn, have excellent LME crack resistance, good plating appearance, and a small amount of diffusible hydrogen in the steel sheet, so that good delayed fracture resistance can be expected, and damage to the furnace is also small, and ductility and bendability are also excellent. On the other hand, comparative examples manufactured outside the scope of the present invention are inferior in at least one of LME crack resistance, plating appearance, amount of diffusible hydrogen in the steel sheet, and damage to the furnace.

Industrial applicability

The high-strength hot-dip galvanized steel sheet obtained by the manufacturing method of the present invention has excellent appearance quality, weld crack resistance, and at the same time, can suppress deterioration of delayed fracture resistance due to hydrogen embrittlement, and can be used as a surface-treated steel sheet for making the body of an automobile lightweight and high-strength.

1: Hot-dip galvanized steel sheet for testing
2: Test piece
3 : Spacer
4: Electrode
5 : Fixed stand
6 : Nugget
7 : Nugget diameter
8 : Cutting line

Claims (6)

A method for manufacturing a high-strength hot-dip galvanized steel sheet, comprising: a hot-rolling step of 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, and then winding the slab into a coil at a temperature T _C (°C) calculated from the following formula (1) and acid washing, a cold-rolling step of cold-rolling the hot-rolled sheet obtained in the hot-rolling step, and continuously annealing the cold-rolled steel sheet obtained in the cold-rolling step in an annealing furnace having a direct-fired heating furnace and a radiant tube-type heating and holding furnace, and then hot-dip galvanizing it,
In the above direct-fired heating furnace, in the early stage, the steel sheet is heated from 400°C to 670°C in an atmosphere containing 1000 ppm by volume or more of _O2 and 1000 ppm by volume or more of _H2O , and in the later stage, the steel sheet is heated from 600°C to 700°C in an atmosphere containing 500 ppm by volume or less of _O2 .
In the annealing furnace having the above heating and preservation, the _H2O concentration of the atmosphere inside the furnace is 5,000 to 40,000 volume ppm, the _H2 concentration is 2 to 20 volume%,
A method for manufacturing a _high -strength _hot -dip galvanized steel sheet, comprising maintaining the _steel sheet at a temperature of 650° _C or higher and 900°C or lower for 90 seconds or longer in an atmosphere in which the ratio of the partial pressure of H2O (P H2O) to the partial pressure of H2 (P H2) log (P _H2O /P _H2 ) satisfies -1.1 or more and 0.5 or less.
T _C = -30([Si] + [Mn]) + 775 ···(1)
[Si] is the Si content (mass%) included in the steel plate.
[Mn] is the Mn content (mass%) included in the steel plate. In the first paragraph,
A method for manufacturing a high-strength hot-dip galvanized steel sheet, comprising applying hot-dip galvanization to a steel sheet and then performing an alloying treatment. In paragraph 1 or 2,
A method for manufacturing a high-strength hot-dip galvanized steel sheet, comprising an additional cooling and heating process in which, after heating and preservation in a radiant tube-shaped heating and preservation holding furnace, the steel sheet is cooled from the final preservation and holding temperature in the annealing to a temperature of 150 to 350°C under the condition of an average cooling rate of 10°C/sec or more, and then heated to a temperature of 350 to 600°C and preserved and held for 10 to 600 seconds. In any one of claims 1 to 3,
A method for manufacturing a high-strength hot-dip galvanized steel sheet, wherein the atmosphere satisfies the ratio log(P _H2O /P _H2 ) of the partial pressure of _H2O (P _H2O ) and the partial pressure of _H2 (P _H2 ) of -0.99 or more and 0.5 or less. In any one of claims 1 to 4,
A method for manufacturing a high-strength hot-dip galvanized steel sheet, wherein the atmosphere satisfies the ratio log(P _H2O /P _{H2) of the partial pressure of H2O (P H2O) and the partial pressure of H2 (P H2 ) of -0.9 or} more and 0.5 or less. In any one of claims 1 to 5,
A method for manufacturing a high-strength hot-dip galvanized steel sheet, wherein the atmosphere satisfies the ratio log(P _H2O /P _{H2) of the partial pressure of H2O (P H2O) and the partial pressure of H2 (P H2 ) of -0.7 or} more and 0.5 or less.