JP7480928B2 - Manufacturing method of galvannealed steel sheet - Google Patents

Description

The present disclosure relates to a method for manufacturing alloyed hot-dip galvanized steel sheet.

In recent years, from the viewpoint of protecting the global environment, there has been a strong demand for improving fuel efficiency to reduce _CO2 emissions from automobiles. Accordingly, there has been a growing trend to reduce the weight of automobile bodies by making the body parts thinner, and there is an increasing need for high-strength steel sheets, which are materials for automobile body parts.

To increase the strength of steel plate, it is effective to add elements with solid solution strengthening properties, such as Si and Mn. These elements are more easily oxidized than Fe.

In general, alloyed hot-dip galvanized steel sheets are manufactured by using thin steel sheets obtained by hot-rolling or cold-rolling a slab as a base steel sheet, annealing the base steel sheet in an annealing furnace of a continuous hot-dip galvanizing line (CGL), and then performing hot-dip galvanizing and alloying processes. When alloyed hot-dip galvanized steel sheets are manufactured using high-strength steel sheets containing large amounts of easily oxidizable elements such as Si and Mn as the base steel sheet, the easily oxidizable elements in the base steel sheet are oxidized during annealing and concentrated on the surface of the base steel sheet, forming oxides on the surface of the base steel sheet. These oxides can reduce the wettability of the base steel sheet surface with molten zinc, causing deterioration of the appearance such as non-coating and deterioration of the coating adhesion. In addition, the presence of oxides between the base steel sheet and the hot-dip galvanizing may cause deterioration of the coating adhesion. Hereinafter, the term "coating property" is used to refer to both the coating appearance and the coating adhesion.

Patent Document 1 discloses a method for manufacturing hot-dip galvanized steel sheet using a high-strength steel sheet containing a large amount of Si as the base steel sheet, in which an oxide film is formed on the surface of the steel sheet and then reduction annealing is performed.

Patent Document 2 discloses a technique for promoting dehydrogenation in a hot-rolled steel sheet at a temperature of 450° C. or higher and 550° C. or lower and at an _H2 concentration of 9% or higher.

Patent document 3 discloses a technology for reducing the amount of hydrogen in hot-rolled steel sheets by controlling the annealing temperature and hydrogen concentration in an annealing furnace.

Patent document 4 discloses a technology for changing the ratio of water vapor partial pressure to hydrogen partial pressure in the heating zone and the isotropic zone.

Furthermore, Patent Document 5 discloses a method for producing a high-strength hot-dip galvanized steel sheet that has excellent plateability and resistance to hydrogen embrittlement by subjecting the steel sheet to post-heating under specified conditions after annealing and plating processes in order to reduce hydrogen in the steel.

JP 2016-53211 A Japanese Patent Application Laid-Open No. 54-130443 Patent No. 3266008 Patent No. 5811841 JP 2020-45568 A

In the method described in Patent Document 1, there was room for improvement in plating properties when the amount of Mn relative to the amount of Si added to the steel was equal to or greater than a predetermined amount. In Patent Document 1, the steel sheet is kept at high temperature in a reducing atmosphere containing hydrogen in order to reduce the oxidized steel sheet, and at this time, hydrogen in the furnace atmosphere penetrates into the steel. The steel is then plated with hydrogen still in the state where it has penetrated, but because hydrogen diffuses much slower in the plating layer than in the steel, diffusible hydrogen remains in the steel, which may cause hydrogen embrittlement.

In addition, both Patent Documents 2 and 3 are technologies for suppressing blisters (plating swelling) in hot-rolled steel sheets, but are insufficient for improving the delayed fracture resistance of high-strength cold-rolled steel sheets. In addition, in the method of Patent Document 3, when the steel sheet is oxidized and reduced to ensure galvanic properties, the reduction is insufficient. In addition, Patent Document 3 does not specify the dew point, but there is a risk that the steel sheet will oxidize depending on the balance between the dew point and the hydrogen concentration.

In the technology of Patent Document 4, the ratio of water vapor partial pressure to hydrogen partial pressure is changed by changing the dew point, and no consideration is given to what happens when the hydrogen concentration changes.

In addition, in Patent Document 5, the only regulation regarding the atmosphere in the annealing process is the hydrogen concentration, and there is no mention of moisture in the furnace, such as the dew point. The method described in Patent Document 5 does not include an oxidation-reduction process, so depending on the amount of Si and Mn contained in the steel, galvanization may be impaired. For example, in Example 1 of Patent Document 5, a cold-rolled steel sheet having 1.25 mass% Si and 2.67 mass% Mn as steel components is annealed in an atmosphere with a dew point of -30°C without going through a previous process such as an oxidation-reduction process, and then hot-dip galvanization is performed. This method has the risk of generating defects such as non-plating, which impairs the surface appearance, and does not sufficiently consider the appropriate balance between hydrogen embrittlement resistance and surface appearance, as described later in the present invention.

As described above, in the production of high-strength galvannealed steel sheets, a method has not yet been developed that precisely specifies the appropriate atmospheric conditions and heat treatment conditions according to the steel sheet composition, thereby achieving both coatability and hydrogen embrittlement resistance.

Therefore, the present disclosure aims to provide a galvannealed steel sheet that has a beautiful surface appearance free of defects such as bare spots, excellent plating adhesion, and also has excellent resistance to hydrogen embrittlement.

After extensive research, the inventors discovered that by controlling the atmosphere during heating of the steel sheet and also subjecting the steel sheet after plating and alloying to a prescribed heat treatment, it is possible to manufacture a galvannealed steel sheet that has a beautiful surface appearance, excellent plating adhesion, and excellent resistance to hydrogen embrittlement. In particular, by forming the galvannealed layer into a galvannealed layer with a high hydrogen diffusion rate and then subjecting the steel sheet to a prescribed heat treatment, it is possible to significantly reduce hydrogen in the steel.

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

[1] A steel sheet having a composition, in mass%, of Si: 0.10% or more and 2.00% or less, and Mn: 1.0% or more and 5.0% or less,
An oxidation step of heating to 600° C. or higher in an oxidizing atmosphere containing O ₂ : 1,000 ppm by volume or more and 30,000 ppm by volume or less;
A reduction process in which the steel sheet after the oxidation process is held at 700° C. or higher in a reducing atmosphere having a hydrogen concentration of more than 8 vol.% and not more than 30 vol.% for 20 s or more;
a soaking step of holding the steel sheet after the reduction step in a soaking atmosphere having a hydrogen concentration of 0.2 vol% to 8 vol% at 750°C or higher for 50 s to 300 s;
A cooling process of cooling the steel sheet after the soaking process;
A plating step of immersing the cooled steel sheet in a hot-dip galvanizing bath to obtain a hot-dip galvanized steel sheet;
An alloying step of performing an alloying treatment on the hot-dip galvanized steel sheet to obtain an alloyed hot-dip galvanized steel sheet;
a cooling-reheating step of cooling the galvannealed steel sheet to a cooling stop temperature equal to or lower than the Ms point, and then holding the steel sheet at a temperature equal to or higher than the cooling stop temperature and equal to or higher than 100° C. and equal to or lower than 450° C. for 30 s or more in a reheating atmosphere having a hydrogen concentration of 0.2 vol.% or less;
The method for producing a galvannealed steel sheet comprising the steps of:

[2] The composition of the composition satisfies [Si]/[Mn] of 0.23 or more,
The atmosphere in the soaking step has a dew point of −20° C. or higher and +20° C. or lower.
The method for producing the galvannealed steel sheet according to [1] above.
Here, [Si] and [Mn] respectively indicate the contents (mass%) of Si and Mn in the above-mentioned composition.

[3] The composition satisfies [Si]/[Mn] less than 0.23,
The atmosphere in the reduction step has a dew point of less than −20° C.,
The atmosphere in the soaking step has a dew point of less than −20° C.
The method for producing the galvannealed steel sheet according to [1] above.
Here, [Si] and [Mn] respectively indicate the contents (mass%) of Si and Mn in the above-mentioned composition.

[4] A method for manufacturing an alloyed hot-dip galvanized steel sheet described in any one of [1] to [3], wherein the soaking atmosphere in the soaking process has a hydrogen concentration of 0.2 volume % or more and 5 volume % or less.

[5] The method for producing a galvannealed steel sheet according to any one of [1] to [4], wherein in the cooling step, the steel sheet after the soaking step is cooled in an atmosphere having a hydrogen concentration of 0.5 vol% or more and 30 vol% or less and a dew point of 0°C or less from 600°C or more and 900°C or less to 300°C or more and 500°C or less at an average cooling rate of 10°C/s or more. [6] The composition further comprises, in mass%,
C: 0.05% or more and 0.40% or less,
P: 0.001% or more and 0.100% or less,
S: 0.0200% or less,
The method for producing a galvannealed steel sheet according to any one of the above [1] to [5], comprising: Al: 0.003% or more and 2.000% or less; N: 0.0100% or less; and the balance consisting of Fe and unavoidable impurities.

[7] The component composition further comprises, in mass%,
B: 0.0100% or less,
Ti: 0.200% or less,
Nb: 0.200% or less,
Sb: 0.200% or less,
Sn: 0.200% or less,
V: 0.100% or less,
Cu: 1.00% or less,
Cr: 1.00% or less,
Ni: 1.00% or less,
Mo: 0.50% or less,
Ta: 0.100% or less,
W: 0.500% or less,
Zr: 0.020% or less,
Ca: 0.0200% or less,
Mg: 0.0200% or less,
Zn: 0.020% or less,
Co: 0.020% or less,
Ce: 0.0200% or less,
Se: 0.0200% or less,
Te: 0.0200% or less,
Ge: 0.0200% or less,
As: 0.0200% or less,
Sr: 0.0200% or less,
Cs: 0.0200% or less,
Hf: 0.0200% or less,
Pb: 0.0200% or less,
The method for producing a galvannealed steel sheet according to the above [6], further comprising: at least one selected from Bi: 0.0200% or less and REM: 0.0200% or less.

[8] A method for manufacturing an alloyed hot-dip galvanized steel sheet described in any one of [1] to [7], wherein in the cooling-reheating process, the cooling stop temperature is (Ms point - 50°C) or lower.

[9] A method for manufacturing an alloyed hot-dip galvanized steel sheet described in [8], wherein the cooling stop temperature is (Ms point - 100°C) or less.

According to the present disclosure, it is possible to provide a galvannealed steel sheet having a beautiful surface appearance free of defects such as uncoated areas, excellent coating adhesion, and excellent resistance to hydrogen embrittlement.

FIG. 2 is a diagram showing an example of an atmosphere dew point measurement position in a split-type annealing furnace.

Conventionally, it has been difficult to achieve both beautiful surface appearance and excellent plating adhesion, as well as hydrogen embrittlement resistance. Oxidation-reduction of steel sheets is effective in improving the plating properties of Si- and Mn-containing steels. In order to complete the reduction of Fe oxides before plating, annealing in a high hydrogen concentration atmosphere is essential, which inevitably results in a large amount of hydrogen penetrating into the steel. If the hydrogen concentration during annealing is low, Fe reduction is not completed, and the Fe oxides remaining on the steel sheet surface lead to deterioration of plating properties. Therefore, in this disclosure, annealing is divided into a reduction process and a soaking process, and after completing the reduction at a high hydrogen concentration, the hydrogen concentration in the soaking process is reduced as necessary to the minimum level at which Fe is not reoxidized, making it possible to reduce the hydrogen that has once penetrated into the steel.

The following describes embodiments of the present disclosure. Note that the present disclosure is not limited to the following embodiments.

In the following description, the units for the content of each element in the steel composition and the content of each element in the plating layer composition are all "mass %", and are simply indicated as "%" unless otherwise specified. The units for hydrogen concentration are all "volume %", and are simply indicated as "%" unless otherwise specified. In this specification, a numerical range expressed using "~" means a range that includes the numerical values written before and after "~" as the lower and upper limits.

In addition, in this specification, a steel plate being "high strength" means that the tensile strength of the steel plate is 340 MPa or more.

First, we will explain the appropriate range of the chemical composition of the base steel plate and the reasons for its limitations.

Si: 0.10% to 2.00% Si is a solid solution strengthening element and contributes to high strength of steel sheet. It also has the effect of suppressing carbide generation and facilitating the production of retained austenite, so it is effective in achieving both strength and ductility of steel sheet. In order to obtain such an effect, the Si content needs to be 0.10% or more. On the other hand, if the Si content exceeds 2.00%, the hot rolling property and cold rolling property are greatly deteriorated, which may adversely affect productivity or may even deteriorate the ductility of the steel sheet itself. Furthermore, the formation of Si oxide on the steel sheet surface becomes significant, and good plating properties may not be obtained. Therefore, the Si content is 0.10% to 2.00%. The Si content is preferably 0.25% or more. The Si content is preferably 1.70% or less.

Mn: 1.0% or more and 5.0% or less Mn is an element that strengthens steel by solid solution strengthening, improves hardenability, and promotes the formation of retained austenite, bainite, and martensite. Such effects are achieved by adding Mn at 1.0% or more. On the other hand, if the Mn content exceeds 5.0%, not only will the cost increase, but even if the manufacturing method according to the present embodiment is used, the inhibition of the formation of Mn oxides on the steel sheet surface during plating may be insufficient, and good plating properties may not be obtained. Therefore, the Mn content is set to 1.0% or more and 5.0% or less. The Mn content is more preferably set to 1.5% or more, and even more preferably set to 2.0% or more. Moreover, the Mn content is more preferably set to 4.0% or less, and even more preferably set to 3.5% or less.

The essential component composition of the base steel sheet according to this embodiment is that it contains Si and Mn in a specified amount and ratio range. The rest may be freely selected according to the design of the mechanical properties and is not particularly limited. However, in order to obtain a steel sheet with a tensile strength of 340 MPa or more, the following component composition is preferable.

C: 0.05% or more and 0.40% or less C is an element effective in increasing the strength of steel plate, and contributes to increasing the strength by forming martensite, which is one of the hard phases of the steel structure. For this purpose, it is preferable to contain C at 0.05% or more. In addition, in order to obtain good weldability, it is preferable to set the C content to 0.40% or less. It is more preferable to set the C content to 0.07% or more. It is also preferable to set the C content to 0.35% or less.

P: 0.001% or more and 0.100% or less Better weldability can be obtained by suppressing the P content. Furthermore, it is possible to prevent P from segregating to grain boundaries, and to particularly improve ductility, bendability, and toughness. In addition, it is possible to suppress ferrite transformation and prevent the grain size from becoming coarse by suppressing the P content. Therefore, it is preferable that the P content is 0.100% or less. It is more preferable that the P content is 0.050% or less. The lower limit of P is not particularly limited. Due to production technology constraints, the P content may be more than 0% and may be 0.001% or more.

S: 0.0200% or less (excluding 0%)
The amount of S is preferably 0.0200% or less, and more preferably 0.0150% or less. By suppressing the amount of S, it is possible to prevent a decrease in weldability and a decrease in ductility during hot rolling, suppress hot cracking, and significantly improve surface properties. Furthermore, by suppressing the amount of S, it is possible to prevent the formation of coarse sulfides, and obtain better ductility, bendability, and stretch flangeability. Therefore, it is preferable that the amount of S is 0.0200% or less. It is more preferable that the amount of S is 0.0100% or less. The lower limit of S is not particularly limited, and may be more than 0% due to constraints on production technology, and may be 0.0001% or more.

Al: 0.003% or more and 2.000% or less Since Al is thermodynamically most easily oxidized, it oxidizes before Si and Mn, and has the effect of suppressing the oxidation of Si and Mn at the outermost layer of the steel sheet and promoting the oxidation of Si and Mn inside the steel sheet. This effect is obtained when the Al content is 0.003% or more. On the other hand, from the viewpoint of cost, the Al content is preferably 2.000% or less. Therefore, when added, the Al content is preferably 0.003% or more and 2.000% or less. The Al content is more preferably 0.010% or more.

N: 0.0100% or less (excluding 0%)
The N content is preferably 0.0100% or less. By setting the N content to 0.0100% or less, it is possible to more suitably prevent N from forming coarse nitrides with Ti, Nb, and V at high temperatures, and to prevent the effect of increasing the strength of the steel plate by adding Ti, Nb, and V from being impaired. In addition, by setting the N content to 0.0100% or less, better toughness can be obtained. Furthermore, by setting the N content to 0.0100% or less, it is possible to prevent slab cracks and surface defects from occurring during hot rolling. Therefore, the N content is preferably 0.0100% or less, and more preferably 0.0050% or less. The lower limit of the N content is not particularly limited, and may be more than 0% due to constraints on production technology, and may be 0.0005% or more.

The composition may further optionally contain a predetermined amount of at least one element selected from the following group of elements:

B: 0.0100% or less B is an element effective for improving the hardenability of steel. In order to improve the hardenability, the B content is preferably 0.0001% or more, and more preferably 0.0005% or more. In order to obtain better formability, the B content is preferably 0.0100% or less, and more preferably 0.0050% or less.

Ti: 0.200% or less Ti is effective for precipitation strengthening of steel. The lower limit of Ti is not particularly limited, but in order to obtain the effect of adjusting the strength, it is preferable to set it to 0.001% or more. When Ti is added to obtain better formability, the amount of Ti is preferably set to 0.200% or less, and more preferably set to 0.060% or less.

Nb: 0.200% or less The addition of Nb can improve the strength. To achieve this effect, the Nb content is preferably 0.001% or more, and more preferably 0.005% or more. In addition, by setting the Nb content to 0.200% or less, it is possible to prevent an increase in costs. Therefore, the Nb content is preferably 0.200% or less, and more preferably 0.060% or less.

Sb: 0.200% or less Sb can be added for the purpose of suppressing excessive decarburization of the steel sheet surface, preventing a decrease in the amount of martensite formed, and improving the fatigue properties and surface quality of the steel sheet. In order to obtain such effects, the Sb content is preferably 0.001% or more. On the other hand, in order to obtain better toughness, the Sb content is preferably 0.200% or less. The Sb content is more preferably 0.060% or less.

Sn: 0.200% or less Sn is an element that is effective in suppressing decarburization, denitrification, etc., and suppressing the decrease in strength of steel. To obtain such effects, it is preferable that the Sn content be 0.002% or more. On the other hand, in order to obtain better impact resistance, it is preferable that the Sn content be 0.200% or less. It is more preferable that the Sn content be 0.060% or less.

V: 0.100% or less The addition of V can improve the strength. To achieve this effect, the V content is preferably 0.001% or more, and more preferably 0.005% or more. In addition, by setting the V content to 0.100% or less, it is possible to prevent an increase in costs. Therefore, the V content is preferably 0.100% or less, and more preferably 0.060% or less.

Cu: 1.00% or less The amount of Cu is an element that increases hardenability, and is an effective element for setting the area ratio of the hard phase within a more preferable range and setting the tensile strength within a more preferable range. In order to obtain such effects, the amount of Cu is preferably 0.005% or more, and more preferably 0.020% or more. In addition, when adding the amount of Cu, from the viewpoint of preventing an increase in cost, the amount of Cu is preferably 1.00% or less, and more preferably 0.20% or less.

Cr: 1.00% or less The addition of Cr can improve the hardenability and improve the balance between strength and ductility. In order to obtain this effect, it is preferable that the Cr content is 0.001% or more. In addition, when Cr is added, from the viewpoint of preventing an increase in costs, the Cr content is preferably 1.00% or less, and more preferably 0.80% or less.

Ni: 1.00% or less The addition of Ni can improve the hardenability and improve the balance between strength and ductility. In order to obtain this effect, it is preferable that the Ni content is 0.005% or more. In addition, when Ni is added, from the viewpoint of preventing an increase in cost, the Ni content is preferably 1.00% or less, and more preferably 0.80% or less.

Mo: 0.50% or less The addition of Mo can provide the effect of adjusting strength. To obtain this effect, the Mo content is preferably 0.005% or more, and more preferably 0.01% or more. When Mo is added, from the viewpoint of preventing an increase in cost, it is preferably 0.50% or less, and more preferably 0.45% or less.

Ta: 0.100% or less The addition of Ta can provide an effect of improving strength. In order to obtain this effect, it is preferable to contain Ta at a content of 0.001% or more. In addition, when Ta is contained, it is preferable to set the Ta content to 0.100% or less from the viewpoint of preventing an increase in cost. It is more preferable to set the Ta content to 0.050% or less.

W: 0.500% or less The addition of W can provide an effect of improving strength. In order to obtain this effect, the W content is preferably 0.001% or more, and more preferably 0.003% or more. In addition, when W is contained, from the viewpoint of preventing an increase in cost, the W content is preferably 0.500% or less, and more preferably 0.450% or less.

Zr: 0.020% or less The addition of Zr can improve the ultimate deformability of the steel sheet and improve the stretch flangeability. In order to obtain this effect, the Zr content is preferably 0.0005% or more, and more preferably 0.0010% or more. In addition, when Zr is contained, from the viewpoint of preventing an increase in cost, the Zr content is preferably 0.020% or less. The Zr content is more preferably 0.010% or less.

Ca: 0.0200% or less By containing 0.0005% or more of Ca, the morphology of sulfides can be controlled, and ductility and toughness can be further improved. In order to obtain better ductility, the Ca content is preferably 0.0200% or less. The Ca content is more preferably 0.0100% or less.

Mg: 0.0200% or less By including 0.0005% or more of Mg, the form of sulfides can be controlled and ductility and toughness can be improved. In order to obtain better ductility, the Mg content is preferably 0.0200% or less. The Mg content is more preferably 0.0100% or less.

Zn: 0.020% or less The addition of Zn can improve the ultimate deformability of the steel sheet and improve the stretch flangeability. In order to obtain this effect, it is preferable that the Zn content is 0.001% or more. In addition, when Zn is contained, it is preferable that the Zn content is 0.020% or less from the viewpoint of preventing an increase in cost. It is more preferable that the Zn content is 0.010% or less.

Co: 0.020% or less The addition of Co can improve the ultimate deformability of the steel sheet and improve the stretch flangeability. In order to obtain this effect, it is preferable that the Co content is 0.001% or more. In addition, when Co is contained, it is preferable that the Co content is 0.020% or less from the viewpoint of preventing an increase in cost. It is more preferable that the Co content is 0.010% or less.

Ce, Se, Te, Ge, As, Sr, Cs, Hf, Pb, Bi, REM: 0.0200% or less By adding these elements, it is possible to obtain the effect of improving the ultimate deformability of the steel sheet and improving the stretch flangeability. In order to obtain this effect, it is preferable that at least one of these elements is 0.0001% or more. On the other hand, from the viewpoint of preventing an increase in cost, when at least one of these elements is added, the content of each is preferably 0.0200% or less.

The remainder of the composition of the base steel plate in this embodiment other than the above components consists of Fe and unavoidable impurities.

The thickness of the base steel plate in this embodiment is not particularly limited, but can generally be 0.5 mm or more and 3.2 mm or less.

Next, a method for manufacturing a galvannealed steel sheet according to one embodiment of the present disclosure will be described.

First, a steel sheet having the above-mentioned composition is manufactured according to a standard method. In one example, a steel slab having the above-mentioned composition is hot-rolled and cold-rolled to manufacture a cold-rolled steel sheet.

Next, before immersing the steel sheet having the above-mentioned composition in a hot-dip galvanizing bath, the steel sheet is subjected to recrystallization annealing including an oxidation step, a reduction step, and a soaking step. In one example, the cold-rolled steel sheet is supplied to a CGL. The configuration of the CGL is not particularly limited, but in one example, the CGL has a continuous annealing furnace in which a heating zone, a soaking zone, and a cooling zone are arranged in this order, a hot-dip galvanizing facility provided downstream of the cooling zone, and an alloying furnace provided downstream of the hot-dip galvanizing facility. In one example, the steel sheet is transported in the continuous annealing furnace through the heating zone, the soaking zone, and the cooling zone in this order, and recrystallization annealing is performed on the steel sheet, and the hot-dip galvanizing facility is used to hot-dip galvanize the steel sheet discharged from the cooling zone to obtain a hot-dip galvanized steel sheet, and then an alloying furnace is used to perform an alloying treatment on the hot-dip galvanized steel sheet to obtain an alloyed hot-dip galvanized steel sheet.

[Oxidation process]
By forming an iron oxide layer on the surface of the steel sheet in the oxidation step and then forming reduced iron in the reduction step, it is possible to suppress the formation of Si and Mn oxides on the steel sheet surface and obtain excellent galvanic properties. In one example, the oxidation step is performed in the heating zone of the CGL.

Oxidizing atmosphere containing _O2 at 1000 volume ppm or more and 30000 volume ppm or less By setting the _O2 concentration of the oxidizing atmosphere in the oxidation step to 1000 volume ppm or more, the oxidation of the steel sheet is promoted. If the _O2 concentration of the oxidizing atmosphere is less than 1000 volume ppm, the oxidation of the steel sheet is insufficient and the above effect cannot be obtained. On the other hand, if the _O2 concentration of the oxidizing atmosphere is more than 30000 volume ppm, the oxidation of the steel sheet becomes excessive, and unreduced iron oxide remains in the subsequent reduction step, which causes deterioration of plating. The remainder of the oxidizing atmosphere is not particularly limited, but in one example, it is composed of _N2 , CO, _CO2 , _H2O and unavoidable impurities, and the ratio thereof is not particularly limited.

Steel sheet temperature: heated to 600°C or higher In order to promote oxidation of the steel sheet in the oxidation process, the steel sheet temperature needs to be 600°C or higher. If the steel sheet temperature is less than 600°C, oxidation will be insufficient and the above effect will not be obtained. The steel sheet temperature in the oxidation process is preferably 650°C or higher. There is no particular upper limit to the steel sheet temperature in the oxidation process, but it is preferably 900°C or lower. By keeping the steel sheet temperature at 900°C or lower, excessive oxidation of the steel sheet can be more suitably prevented, and unreduced iron oxide can be more suitably prevented from remaining in the reduction process, thereby further improving the galvanizing property. The steel sheet temperature is based on the temperature of the steel sheet surface. The same applies to the various processes described below.

[Reduction process]
As described above, excellent plating properties can be obtained by generating iron oxide in the oxidation step and then reducing the iron oxide to generate a reduced iron layer. In the subsequent soaking step, the atmosphere has a low hydrogen concentration, so the reduction reaction rate is slow. Therefore, it is necessary to complete the reduction of the iron oxide in the reduction step.

Steel sheet temperature: 700°C or higher If the steel sheet temperature in the reduction step is less than 700°C, the reduction rate is slow and unreduced iron oxide may remain. The steel sheet temperature in the reduction step is preferably 750°C or higher. There is no particular upper limit to the steel sheet temperature in the reduction step, but in order to more suitably prevent deterioration of the furnace body, the steel sheet temperature in the reduction step is preferably 950°C or lower.

Hydrogen concentration: more than 8 vol% and 30 vol% or less The higher the hydrogen concentration in the reducing atmosphere, the faster the reduction of iron oxide. However, the higher the hydrogen concentration, the more hydrogen dissolved in the steel, which leads to deterioration of hydrogen embrittlement resistance. If the hydrogen concentration in the reducing atmosphere is 8 vol% or less, the reduction rate of iron oxide becomes insufficient. On the other hand, if the hydrogen concentration in the reducing atmosphere is more than 30 vol%, the reduction rate becomes saturated and it becomes difficult to sufficiently reduce the amount of hydrogen in the steel in the next soaking step. Therefore, the hydrogen concentration in the reducing atmosphere is set to 8 vol% or more and 30 vol% or less. The hydrogen concentration in the reducing atmosphere is preferably set to 20 vol% or less, more preferably 18 vol% or less. In addition, the hydrogen concentration in the reducing atmosphere is preferably set to 12 vol% or more.

Holding time: 20 seconds or more The longer the holding time in the reducing atmosphere, the more advantageous it is for completing the reduction of the iron oxide. If the holding time in the reducing atmosphere is less than 20 seconds, the reduction of the iron oxide may not be completed. For this reason, the holding time in the reducing atmosphere is set to 20 seconds or more. The holding time in the reducing atmosphere is preferably set to 25 seconds or more. There is no particular upper limit to the holding time in the reducing atmosphere. From the viewpoint of productivity, the holding time in the reducing atmosphere is preferably set to 150 seconds or less.

[Heat soaking process]
In the soaking process, the amount of hydrogen dissolved in the steel sheet is reduced during the reduction process in which iron oxide reduction is carried out in a high hydrogen atmosphere.

Hydrogen concentration: 0.2 vol% or more and 8 vol% or less Since the reduction of the steel sheet is completed in this process, the steel sheet is kept in a soaking atmosphere with a lower hydrogen concentration than in the reduction process, and hydrogen dissolved in the steel sheet can be reduced. To achieve this effect, the hydrogen concentration needs to be 8 vol% or less. It is also preferable that the hydrogen concentration is 5 vol% or less. On the other hand, if the hydrogen concentration is less than 0.2 vol%, it is difficult to uniformly control the inside of the furnace, and there is a risk of reoxidization of the reduced iron. Therefore, the hydrogen concentration of the soaking atmosphere is preferably 0.2 vol% or more, and more preferably 0.5% or more.

Steel sheet temperature: 750°C or higher If the steel sheet temperature in the soaking step is less than 750°C, the reduced iron layer on the steel sheet surface may be reoxidized. In addition, the recrystallization of the steel sheet structure may be insufficient, or the volume fraction of ferrite may become excessive, making it impossible to obtain the required material. The steel sheet temperature in the heating step is preferably 780°C or higher. There is no particular upper limit to the steel sheet temperature in the soaking step, but from the viewpoint of production equipment, it is preferably 950°C or lower. The steel sheet temperature in the soaking step is more preferably 900°C or lower.

Holding time: 20 s or more and 300 s or less If the holding time in the soaking step is less than 20 s, hydrogen in the steel may not be sufficiently reduced. On the other hand, if the holding time exceeds 300 s, a large amount of Si and Mn oxides may be formed on the outermost surface of the steel sheet, which may lead to deterioration of platability. Therefore, the holding time is set to 20 s or more and 300 s or less. The holding time is preferably set to 50 s or more. In addition, the holding time is preferably set to 200 s or less.

Dew point of reduction step and soaking step The dew point of the atmosphere in the reduction step and soaking step is not particularly limited, but is preferably −50° C. or higher and +20° C. or lower from the viewpoint of industrial ease of handling. By setting the dew point to −50° C. or higher, the equipment cost for maintaining the atmosphere can be further reduced. By setting the dew point to +20° C. or lower, it becomes easier to control the dew point in the furnace and adverse effects on the furnace body can be suitably avoided.

In addition, by setting the atmospheric dew point in the reduction process and the soaking process in an appropriate range according to the value of [Si]/[Mn], which is the mass percentage ratio of Si to Mn contained in the steel sheet's composition, the surface appearance and hydrogen embrittlement resistance can be further improved. [Si] and [Mn] respectively indicate the contents (mass%) of Si and Mn in the composition. Below, the preferable atmospheric dew points are explained separately for the case where [Si]/[Mn] is 0.23 or more and the case where it is less than 0.23.

· [Si]/[Mn]≧0.23 or more · Atmospheric dew point in soaking step is −20°C or more +20°C or less When [Si]/[Mn] is 0.23 or more, the oxides formed in the steel sheet surface layer are mainly Si-Mn composite oxides. In this case, by setting the atmospheric dew point in the soaking step in the above range, the formation of Si-Mn composite oxides in the steel sheet surface layer after the reduction of iron oxide is completed is promoted, and the formation of oxides in the outermost surface of the steel sheet can be suppressed. This further improves the plating wettability and provides a better surface appearance. In addition, the effect of promoting the reduction of hydrogen in the steel in the cooling-reheating step described later is also obtained. In order to obtain these effects, the dew point in the soaking step is preferably −20°C or more, more preferably −15°C or more.

[Si] / [Mn] < 0.23
Atmospheric dew point in the reduction step is less than -20°C Atmospheric dew point in the soaking step is less than -20°C When [Si]/[Mn] is less than 0.23, the amount of Mn is greater than the amount of Si in the steel, so that oxides formed on the surface layer of the steel sheet are mainly oxides of Mn alone. In this case, by setting the atmospheric dew points in the reduction atmosphere and the soaking step to the above ranges, it is possible to suppress the formation of oxides of Mn alone on the outermost surface of the steel sheet after the reduction of iron oxide is completed. This further improves the plating wettability, and a more excellent surface appearance can be obtained. In order to obtain this effect, the dew point in the reduction step is preferably less than -20°C, more preferably less than -25°C. In addition, the dew point in the soaking step is preferably less than -20°C, more preferably less than -25°C.

As described above, the hydrogen concentration of the atmosphere used in the reduction step and the soaking step is different from each other. In addition, the dew point used in the reduction step and the soaking step may also be different. In the reduction step and the soaking step, the method of making the hydrogen concentration and dew point of the atmosphere different is not particularly limited. For example, there is a method of dividing the furnace in which both steps are performed, using furnaces connected via a seal roll, and separately feeding gases controlled to the target hydrogen concentration and dew point into each divided furnace. In addition, the method of controlling the dew point in the gas is not particularly limited, but for example, there is a method of humidifying at least one of N ₂ gas or H ₂ gas by bubbling before feeding into the furnace.

In one example, the reduction step and the soaking step described above are performed using a furnace in which the soaking zone of the CGL is divided into an upstream and a downstream zone and connected via a seal roll. That is, an example of a CGL that can be suitably applied to the manufacturing method of the galvannealed steel sheet according to the present disclosure is a continuous hot-dip galvanizing apparatus having a continuous annealing furnace in which a heating zone, a soaking zone, and a cooling zone are arranged in this order, and a hot-dip galvanizing facility provided after the cooling zone, in which the soaking zone has an upstream soaking zone and a downstream soaking zone, and the upstream soaking zone and the downstream soaking zone are connected via a seal roll. In this way, according to a CGL having a soaking zone in which the upstream soaking zone and the downstream soaking zone are connected via a seal roll, the movement of the atmosphere between the upstream soaking zone and the downstream soaking zone is blocked, so that the hydrogen concentration and the dew point of the atmosphere in the reduction step and the soaking step can be controlled independently of each other. Therefore, it can be suitably applied to a method of controlling the hydrogen concentration and the dew point of the atmosphere in the reduction step and the soaking step independently of each other, such as the manufacturing method of the galvannealed steel sheet according to the present disclosure.

The method of monitoring the hydrogen concentration and dew point in the atmosphere in the furnace is not particularly limited. For example, an atmosphere measurement port capable of guiding the gas in the furnace to the outside is provided at a necessary position in each furnace, and can be measured by connecting it to a hydrogen concentration meter and a dew point meter. In addition, for example, when the reduction process and the soaking process are performed in separate furnaces as described above, if independent atmosphere measurement ports are provided at three positions at the top, middle, and bottom of each furnace, the distribution state of the dew point and hydrogen concentration in the height direction in the furnace can be known. Figure 1 shows an example of an atmosphere dew point measurement position in a separate annealing furnace. In one example, an example of a soaking zone of a CGL is shown typically. As shown in Figure 1, a steel strip 6 is supplied to a separate annealing furnace in which a front soaking zone 1 and a rear soaking zone 2 are connected via an atmosphere seal zone 3, and the above-mentioned reduction process and soaking process are performed. In this case, the dew point measurement position 6 can be provided independently at three positions at the top, middle, and bottom of each furnace. By combining this with the above-mentioned control method, the atmosphere composition in the furnace can be continuously controlled.

[Cooling process]
Next, the steel sheet after the soaking step is cooled. The cooling conditions are not particularly limited, but preferably, the steel sheet after the soaking step is cooled from 600° C. to 950° C. at an average cooling rate of 10° C./s or more to 300° C. to 500° C. in a cooling atmosphere having a hydrogen concentration of 0.5 vol.% to 30 vol.% and a dew point of 0° C. or less. In one example, the cooling step is performed in a cooling zone of a CGL.

By cooling in a cooling atmosphere with a hydrogen concentration of 0.5 vol.% or more and a dew point of 0°C or less after the soaking process, it is possible to effectively prevent the steel sheet surface from being reoxidized during cooling. By setting the hydrogen concentration of the cooling atmosphere to 30 vol.% or less, it is possible to more effectively prevent the amount of hydrogen in the steel from increasing during cooling. The hydrogen concentration of the cooling atmosphere is more preferably 5 vol.% or more. Furthermore, the hydrogen concentration of the cooling atmosphere is more preferably 20 vol.% or less. The dew point of the cooling atmosphere is more preferably -30°C or less.

In order to more effectively prevent the amount of hydrogen in the steel from increasing during cooling, it is preferable to cool the steel sheet from a steel sheet temperature of 600°C to 950°C at an average cooling rate of 10°C/s or more. It is more preferable to set the cooling start temperature to 700°C or more. It is also more preferable to set the average cooling rate to 15°C/s or more. In order to cool the steel sheet temperature in the subsequent plating process to the same temperature as the plating bath temperature, it is preferable to set the cooling stop temperature to 500°C or less. In addition, by setting the cooling stop temperature to 300°C or more, it is possible to prevent excessive martensitic transformation and further improve the strength of the steel sheet. Therefore, it is preferable to cool the steel sheet to 300°C to 500°C at the above average cooling rate.

[Plating process]
The steel sheet after the cooling step is then immersed in a hot-dip galvanizing bath to obtain a hot-dip galvanized steel sheet. In one example, the galvanizing step is performed using a hot-dip galvanizing facility of CGL.

The conditions for immersion in the hot-dip galvanizing bath are not particularly limited, and may be performed by a general method. The hot-dip galvanizing bath is composed of Al, Zn, and unavoidable impurities, and the composition is not particularly specified, but in one example, the Al concentration in the bath may be 0.05 mass% or more, and may be 0.190 mass% or less. If the Al concentration in the bath is 0.05 mass% or more, the occurrence of bottom dross can be more suitably prevented. Also, if the Al concentration in the bath is 0.190 mass% or less, the occurrence of top dross can be more suitably prevented. From the viewpoint of cost, it is preferable to set the Al concentration in the bath to 0.190 mass% or less. The plating bath temperature is not particularly specified, but may be 440°C or more and 500°C or less.

The coating weight per side is not particularly limited, but in one example, it is 25 g/ ^m2 or more and 80 g/ ^m2 or less. If the coating weight per side is 25 g/ ^m2 or more, the corrosion resistance is particularly good and the control of the coating weight is particularly easy. If the coating weight per side is 80 g/ ^m2 or less, the coating adhesion is particularly good. There is no particular limit to the method for adjusting the coating weight, but it can be adjusted by using gas wiping and adjusting the gas pressure and the distance between the wiping nozzle and the steel sheet.

[Alloying process]
A hot-dip galvanized steel sheet is subjected to an alloying treatment to obtain a galvannealed steel sheet. The hydrogen diffusion rate in the hot-dip galvanized layer, which is mainly composed of the η phase before the alloying treatment, is significantly slower than that in the steel, which prevents the hydrogen in the steel from being released in the subsequent cooling and reheating process. If the hot-dip galvanized layer is used as an alloyed hot-dip galvanized layer, the hydrogen diffusion rate in the hot-dip galvanized layer is much faster than that in the hot-dip galvanized layer, which promotes the release of hydrogen in the steel. Therefore, in order to significantly reduce hydrogen in the steel in the subsequent cooling and reheating holding process, it is important to perform the alloying process prior to the cooling and reheating holding process.

The conditions for the alloying treatment are not particularly limited. For example, the alloying treatment may be performed by maintaining the steel sheet temperature at 440°C or higher. The alloying treatment may also be performed by maintaining the steel sheet temperature at 600°C or lower. The alloying treatment may be performed by maintaining the hot-dip galvanized steel sheet at the above temperature for 5 s to 60 s.

The alloying degree of the galvannealed layer after alloying is preferably 7% by mass or more (Fe content in the galvannealed layer). In addition, the alloying degree of the galvannealed layer after alloying is preferably 15% by mass or less. By setting the alloying degree of the galvannealed layer to 7% by mass or more, it is possible to prevent the η phase from remaining in the galvannealed layer, and more suitably reduce hydrogen in the steel during the reheating-holding process. In addition, by setting the alloying degree of the galvannealed layer to 15% by mass or less, it is possible to more suitably prevent the formation of the Γ phase at the interface between the galvannealed layer and the base steel sheet, and more suitable plating adhesion can be obtained.

[Cooling-reheating holding process]
Next, the galvannealed steel sheet is cooled to a cooling stop temperature equal to or lower than the Ms point, and then subjected to a cooling-reheating step in which the steel sheet is held at a temperature equal to or higher than the cooling stop temperature and equal to or higher than 100° C. and equal to or lower than 450° C. for 30 s or more. In order to obtain excellent hydrogen embrittlement resistance, the galvannealed steel sheet after the alloying step needs to be cooled to the Ms point or lower and then reheated to further reduce hydrogen in the steel.

Cooling stop temperature: Ms point or lower Austenite dissolves a larger amount of hydrogen than ferrite, while the diffusion rate of hydrogen is higher in ferrite of BCC structure and martensite of BCT structure than in austenite of FCC structure. Therefore, by transforming austenite containing more dissolved hydrogen into martensite and then reheating, hydrogen in steel can be reduced more efficiently. Therefore, the cooling stop temperature in the cooling-reheating process is set to the Ms point at which martensite transformation begins or lower. In addition, the greater the degree of undercooling relative to the Ms point, the more martensite transformation is promoted, and reheating can be performed in a state where there is less untransformed austenite, which is advantageous for reducing hydrogen in steel. Therefore, the cooling stop temperature in the cooling-reheating process is more preferably (Ms point - 50 ° C) or lower, and even more preferably (Ms point - 100 ° C) or lower. There is no particular limit to the lower limit of the cooling stop temperature, but it is preferably set to 20 ° C or higher because a cooling stop temperature of less than 20 ° C requires a high heat extraction capacity in the cooling zone, which leads to increased costs.

Atmospheric hydrogen concentration during reheating: 0.2 vol% or less In order to promote the release of diffusible hydrogen in the steel to the outside of the system during reheating, it is advantageous to lower the atmospheric hydrogen concentration. It is preferably 0.2 vol% or less, and more preferably 0.1 vol% or less. There is no particular lower limit for the hydrogen concentration, but since hydrogen gas is unavoidably contained in the atmosphere, it may be, for example, 0.00001 vol% or more.

Reheating temperature: equal to or higher than the cooling stop temperature and equal to or higher than 100°C and equal to or lower than 450°C After cooling is stopped, reheating is performed to a reheating temperature equal to or higher than the cooling stop temperature in order to promote the release of hydrogen in the steel. In order to fully obtain the effect of reducing hydrogen in the steel, the reheating temperature needs to be 100°C or higher. On the other hand, if the reheating temperature exceeds 450°C, there is a risk of deterioration of platability. Therefore, the reheating temperature is set to 100°C or higher and equal to or lower than 450°C. The reheating temperature is preferably set to 200°C or higher. The reheating temperature is preferably set to 400°C or lower.

Holding time: 30 seconds or more If the holding time at the reheating temperature is less than 30 seconds, the effect of reducing hydrogen in the steel becomes insufficient. Therefore, the holding time at the reheating temperature is set to 30 seconds or more. The holding time at the heating temperature is preferably set to 50 seconds or more. There is no particular upper limit to the holding time at the reheating temperature, but from the viewpoint of productivity, the holding time at the reheating temperature is preferably set to 300 seconds or less.

Manufacturing conditions other than those mentioned above can be determined by conventional methods.

According to the above-mentioned manufacturing method, it is possible to provide a galvannealed steel sheet having a tensile strength of preferably 340 MPa or more. The tensile strength of the galvannealed steel sheet is more preferably 500 MPa or more, and further preferably 980 MPa or more. Here, the tensile strength (TS) is measured in accordance with JIS Z 2241 as follows. A JIS No. 5 test piece is taken from the galvannealed steel sheet so that the longitudinal direction is perpendicular to the rolling direction of the steel sheet. Using the test piece, a tensile test is performed under the condition of a crosshead displacement speed Vc of 1.67×10 ⁻¹ mm/s, and TS is measured.

According to the above manufacturing method, it is possible to provide a galvannealed steel sheet with a reduced amount of diffusible hydrogen. The amount of diffusible hydrogen in the galvannealed steel sheet manufactured by this manufacturing method is preferably 0.30 wt. ppm or less, more preferably 0.20 wt. ppm or less. The upper limit of the amount of diffusible hydrogen in the galvannealed steel sheet is not particularly limited, but may be, for example, 0.01 wt. ppm or more.

Here, the amount of diffusible hydrogen is measured as follows. A 5 x 30 mm test piece is cut out from the galvannealed steel sheet, and the galvannealed layer on the surface of the test piece is removed using a router (precision grinder). Immediately, a thermal desorption analyzer is used to perform hydrogen analysis at an analysis start temperature of 25°C, an analysis end temperature of 300°C, and a heating rate of 200°C/hr, and the amount of hydrogen released at each temperature is measured. Of these, the cumulative amount of hydrogen released in the range from the analysis start temperature to 210°C is calculated as the amount of diffusible hydrogen in the steel.

Using cold-rolled steel sheets with a thickness of 1.4 mm and having the chemical composition shown in Table 1, galvannealed steel sheets were produced under the conditions shown in Tables 2-1 and 2-2, and the following evaluations were carried out. The evaluation results are shown in Tables 2-1 and 2-2. The equipment used was a CGL in which the upstream and downstream uniforming zones were connected by seal rolls. Various production conditions not shown in Tables 2-1 and 2-2 were as follows.
・Plating bath composition: Zn bath with 0.13 wt.% Al added ・Plating bath temperature: 460°C
・Plating weight range: 40 to 60 gm ^-2
Alloying degree range: 8.0 to 14.0 mass%
Hydrogen concentration during reheating: 0.1% by volume

Tensile strength (TS) was measured using the method described above.

Plating Property The plating appearance of the galvannealed steel sheets was evaluated by the method described below.

<Appearance>
The plating surface of the galvannealed steel sheet was visually observed and evaluated and ranked according to the following criteria, with ranks 1 and 2 falling within the preferred range of the present invention.
Appearance Rank No plating or uneven appearance: 1
No unplated areas, but uneven appearance: 2
Unplated: 3

<Amount of diffusible hydrogen in steel sheet>
The amount of diffusible hydrogen in the steel was determined according to the above-mentioned method and evaluated according to the following criteria. Those ranked 1 and 2 were determined to be within the preferred range of the present invention.
Diffusible hydrogen content in steel (wt. ppm) Rank 0.20 or less: 1
More than 0.20 and 0.30 or less: 2
Over 0.30: 3

<Hydrogen embrittlement resistance>
The hydrogen embrittlement resistance was evaluated by evaluating the occurrence of cracks in the resistance spot welded portion. A test piece of 30 x 100 mm was cut out from the galvannealed steel sheet. A spacer having a plate thickness of 2 mm was sandwiched between both ends of the test piece, and the center between the spacers was spot-welded to prepare a welded test piece. In this case, an inverter DC resistance spot welding machine was used for spot welding, and a dome-shaped electrode made of chromium copper with a tip diameter of 6 mm was used. The pressure was 380 kgf, the current time was 16 cycles/50 Hz, and the holding time was 5 cycles/50 Hz. The welding current value was set as a condition for forming a nugget diameter according to the tensile strength of each steel sheet. The nugget diameter was 3.8 mm when the tensile strength was less than 1250 MPa, and 4.8 mm when the tensile strength was 1250 MPa or more. The spacer interval at both ends was 40 mm, and the steel sheet and the spacer were previously bound by welding. After leaving it for 24 hours after welding, the spacer portion was cut off. The cross section of the welded nugget was observed and evaluated according to the following criteria. Ranks 1 and 2 were within the preferred range of the present invention.
Crack observation results Rank No cracks: 1 (particularly excellent resistance to hydrogen embrittlement)
Only microcracks of 100 μm or less occurred: 2 (Excellent resistance to hydrogen embrittlement)
Cracks larger than 100 μm: 3 (poor hydrogen embrittlement resistance)

Of the alloyed hot-dip galvanized steel sheets shown in Tables 2-1 and 2-2, it can be seen that the examples of the present invention have superior coating properties compared to the comparative examples, and the amount of diffusible hydrogen in the steel is sufficiently reduced, resulting in excellent resistance to hydrogen embrittlement.

1 Front stage equalizing zone 2 Rear stage equalizing zone 3 Atmosphere sealing zone 4 Roll 5 Steel strip 6 Dew point measurement position

Claims (9)

A steel sheet having a composition containing, in mass%, Si: 0.10% or more and 2.00% or less, and Mn: 1.0% or more and 5.0% or less,
An oxidation step of heating to 600° C. or higher in an oxidizing atmosphere containing O ₂ : 1,000 ppm by volume or more and 30,000 ppm by volume or less;
A reduction process in which the steel sheet after the oxidation process is held at 700° C. or higher in a reducing atmosphere having a hydrogen concentration of more than 8 vol.% and not more than 30 vol.% for 20 s or more;
a soaking step of holding the steel sheet after the reduction step in a soaking atmosphere having a hydrogen concentration of 0.2 vol% to 8 vol% at 750°C or higher for 20 s to 300 s;
A cooling process of cooling the steel sheet after the soaking process;
A plating step of immersing the cooled steel sheet in a hot-dip galvanizing bath to obtain a hot-dip galvanized steel sheet;
An alloying step of performing an alloying treatment on the hot-dip galvanized steel sheet to obtain an alloyed hot-dip galvanized steel sheet;
a cooling-reheating step of cooling the galvannealed steel sheet to a cooling stop temperature equal to or lower than the Ms point, and then holding the steel sheet at a temperature equal to or higher than the cooling stop temperature and equal to or higher than 100° C. and equal to or lower than 450° C. for 30 s or more in a reheating atmosphere having a hydrogen concentration of 0.2 vol.% or less;
The method for producing a galvannealed steel sheet comprising the steps of: The composition satisfies [Si]/[Mn] of 0.23 or more,
The atmosphere in the soaking step has a dew point of −20° C. or higher and +20° C. or lower.
The method for producing the galvannealed steel sheet according to claim 1.
Here, [Si] and [Mn] respectively indicate the contents (mass%) of Si and Mn in the above-mentioned composition. The composition satisfies [Si]/[Mn] less than 0.23,
The atmosphere in the reduction step has a dew point of less than −20° C.,
The atmosphere in the soaking step has a dew point of less than −20° C.
The method for producing the galvannealed steel sheet according to claim 1.
Here, [Si] and [Mn] respectively indicate the contents (mass%) of Si and Mn in the above-mentioned composition. The soaking atmosphere in the soaking step has a hydrogen concentration of 0.2 vol% or more and 5 vol% or less.
The method for producing a galvannealed steel sheet according to any one of claims 1 to 3. The method for producing a galvannealed steel sheet according to any one of claims 1 to 4, wherein in the cooling step, the steel sheet after the soaking step is cooled from 600°C to 900°C at an average cooling rate of 10°C/s or more in an atmosphere with a hydrogen concentration of 0.5% by volume to 30% by volume and a dew point of 0°C or less. The composition further comprises, in mass%,
C: 0.05% or more and 0.40% or less,
P: 0.001% or more and 0.100% or less,
S: 0.0200% or less,
The method for producing a galvannealed steel sheet according to any one of claims 1 to 5, comprising: Al: 0.003% or more and 2.000% or less; N: 0.0100% or less; and the balance consisting of Fe and unavoidable impurities. The composition further comprises, in mass%,
B: 0.0100% or less,
Ti: 0.200% or less,
Nb: 0.200% or less,
Sb: 0.200% or less,
Sn: 0.200% or less,
V: 0.100% or less,
Cu: 1.00% or less,
Cr: 1.00% or less,
Ni: 1.00% or less,
Mo: 0.50% or less,
Ta: 0.100% or less,
W: 0.500% or less,
Zr: 0.020% or less,
Ca: 0.0200% or less,
Mg: 0.0200% or less,
Zn: 0.020% or less,
Co: 0.020% or less,
Ce: 0.0200% or less,
Se: 0.0200% or less,
Te: 0.0200% or less,
Ge: 0.0200% or less,
As: 0.0200% or less,
Sr: 0.0200% or less,
Cs: 0.0200% or less,
Hf: 0.0200% or less,
Pb: 0.0200% or less,
The method for producing a galvannealed steel sheet according to any one of claims 1 to 6, comprising: at least one selected from Bi: 0.0200% or less and REM: 0.0200% or less. A method for producing an alloyed hot-dip galvanized steel sheet according to any one of claims 1 to 7, wherein in the cooling-reheating process, the cooling stop temperature is equal to or lower than (Ms point - 50°C). A method for producing alloyed hot-dip galvanized steel sheet as described in claim 8, wherein the cooling stop temperature is (Ms point - 100°C) or less.

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