WO2023181390A1

WO2023181390A1 - Hot-dip galvannealed steel sheet manufacturing method

Info

Publication number: WO2023181390A1
Application number: PCT/JP2022/014582
Authority: WO
Inventors: 聖太郎寺嶋; 達也中垣内; 裕美吉冨; 克弥星野; 友輔奥村
Original assignee: Ｊｆｅスチール株式会社
Priority date: 2022-03-25
Filing date: 2022-03-25
Publication date: 2023-09-28
Also published as: JP7480928B2; JPWO2023181390A1

Abstract

The present invention provides a hot-dip galvannealed steel sheet that has a beautiful external appearance without defects such as bare spots, has excellent plating adhesion, and also has excellent hydrogen embrittlement resistance.　A manufacturing method for this hot-dip galvannealed steel sheet involves: heating a steel sheet having a composition containing, in mass percentages, 0.10-2.00%, inclusive, of Si and 1.0-5.0%, inclusive, of Mn to at least 600°C in an oxidation atmosphere including 1000-30000 volumetric ppm, inclusive, of O2; holding the steel sheet after the oxidation step for at least 20 s in a reduction atmosphere having a hydrogen concentration that is greater than 8 vol% and at most 30 vol%, at 700°C or higher; holding the steel sheet for 20-300 s, inclusive, in an isothermal atmosphere having a hydrogen concentration of 0.2-8 vol%, inclusive, at 750°C or higher; cooling the steel sheet; immersing the steel sheet in a molten zinc bath to obtain a hot-dip galvanized steel sheet; performing an alloying process on the hot-dip galvanized steel sheet to obtain a hot-dip galvannealed steel sheet; and, after cooling the hot-dip galvannealed steel sheet to a cooling stop temperature that is at most a Ms point, holding the steel sheet for at least 30 s at a temperature that is at least the cooling stop temperature and is 100-450°C, inclusive, in a reheating atmosphere having a hydrogen concentration of at most 0.2 vol%.

Description

Manufacturing method of alloyed hot-dip galvanized steel sheet

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

In recent years, from the perspective of global environmental conservation, there has been a strong demand for improvements in fuel efficiency in order to reduce _CO2 emissions from automobiles. Along with this, there is an active movement to reduce the weight of car bodies by thinning the walls of car body parts, and there is a growing need for higher strength steel plates, which are materials for car body parts.

Addition of elements with solid solution strengthening ability, such as Si and Mn, is effective in increasing the strength of steel sheets. These elements are more easily oxidized than Fe.

Generally, alloyed hot-dip galvanized steel sheets use a thin steel sheet obtained by hot-rolling or cold-rolling a slab as a base steel sheet, and the base steel sheet is processed using a continuous hot-dip galvanizing line (CGL). It is manufactured by annealing in an annealing furnace, followed by hot-dip galvanizing and alloying. When producing an alloyed hot-dip galvanized steel sheet using a high-strength steel sheet containing a large amount of easily oxidizable elements such as Si and Mn mentioned above as a base steel sheet, the easily oxidizable elements in the base steel sheet are oxidized during annealing. and concentrates on the surface of the base steel sheet, forming oxides on the surface of the base steel sheet. This oxide reduces the wettability of the surface of the base steel sheet with molten zinc, and can cause deterioration in appearance such as non-plating and deterioration in plating adhesion. Furthermore, since oxides exist between the base steel sheet and the hot-dip galvanizing, there is a risk that the adhesion of the plating will deteriorate. Hereinafter, "plating properties" will be used to refer to both plating appearance and plating adhesion.

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

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 an H ₂ concentration of 9% or higher.

Patent Document 3 discloses a technique for reducing the amount of hydrogen in a hot rolled steel sheet by controlling the annealing temperature and hydrogen concentration in an annealing furnace.

Patent Document 4 discloses a technique for changing the ratio of water vapor partial pressure to hydrogen partial pressure in a heating zone and a soaking zone.

In addition, Patent Document 5 discloses that a steel plate after annealing and plating is subjected to post-heating under predetermined conditions to reduce hydrogen in the steel, and high-strength molten zinc with excellent plating properties and hydrogen embrittlement resistance is used. A method of manufacturing a plated steel sheet is disclosed.

JP2016-53211A Japanese Patent Application Laid-Open No. 54-130443 Patent No. 3266008 specification Patent No. 5811841 specification JP2020-45568A

In the method described in Patent Document 1, there is room for improvement in plating properties when the amount of Mn relative to the amount of Si added to the steel is more than a predetermined amount. Further, in Patent Document 1, in order to reduce the oxidized steel plate, the steel plate is maintained at a high temperature in a reducing atmosphere containing hydrogen, and at this time, hydrogen in the furnace atmosphere penetrates into the steel. Afterwards, the steel is plated with hydrogen penetrating it, but since the diffusion of hydrogen in the plating layer is significantly slower than in the steel, there is a risk that diffusible hydrogen may remain in the steel and cause hydrogen embrittlement. There is.

Further, Patent Documents 2 and 3 are both techniques for suppressing blistering (plating blistering) of hot rolled steel sheets, and are insufficient to improve the delayed fracture resistance of high strength cold rolled steel sheets. Further, in the method of Patent Document 3, when the steel plate is oxidized and reduced to ensure plating properties, the reduction is insufficient. Further, although the dew point is not specified in Patent Document 3, there is a risk that the steel plate may be oxidized depending on the balance between the dew point and the hydrogen concentration.

In the technique of Patent Document 4, the ratio of water vapor partial pressure to hydrogen partial pressure is changed by changing the dew point, and the case where the hydrogen concentration changes is not taken into consideration.

Further, 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 dew point. Since the method described in Patent Document 5 does not include an oxidation-reduction step, plating properties may be impaired depending on the amounts of Si and Mn contained in the steel. For example, in Example 1 of Patent Document 5, a cold-rolled steel sheet having 1.25% by mass of Si and 2.67% by mass of Mn as steel components has a dew point of -30°C without going through a pre-process such as an oxidation-reduction process. Annealed in an atmosphere and hot-dip galvanized. In this method, there is a risk that defects such as non-plating occur and the surface appearance is impaired, and as in the present invention described later, an appropriate balance between hydrogen embrittlement resistance and surface appearance is not sufficiently considered.

As mentioned above, in the production of high-strength alloyed hot-dip galvanized steel sheets, a method for achieving both plating properties and hydrogen embrittlement resistance by specifying in detail the appropriate atmospheric conditions and heat treatment conditions according to the steel sheet components is still under development. It has not been.

Therefore, an object of the present disclosure is to provide an alloyed hot-dip galvanized steel sheet that has a beautiful surface appearance free from defects such as unplatedness, excellent plating adhesion, and also has excellent hydrogen embrittlement resistance.

As a result of extensive studies, the inventors found that by controlling the atmosphere during heating of the steel sheet and also performing a prescribed heat treatment on the steel sheet after plating and alloying, they were able to achieve a beautiful surface appearance and excellent plating adhesion. It has been discovered that it is possible to produce an alloyed hot-dip galvanized steel sheet that has the following properties and also has excellent hydrogen embrittlement resistance. In particular, by making the hot-dip galvanized layer an alloyed hot-dip galvanized layer with a high hydrogen diffusion rate and then subjecting it to a predetermined heat treatment, hydrogen in the steel can be significantly reduced.

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 plate having a composition of Si: 0.10% or more and 2.00% or less and Mn: 1.0% or more and 5.0% or less in mass%,
An oxidation step of heating to 600° C. or higher in an oxidizing atmosphere containing O ₂ :1000 volume ppm or more and 30000 volume ppm or less;
A reduction step in which the steel plate after the oxidation step is held at 700° C. or higher in a reducing atmosphere with a hydrogen concentration of more than 8% by volume and not more than 30% by volume for 20 seconds or more;
A soaking step in which the steel plate after the reduction step is held at 750° C. or higher in a soaking atmosphere with a hydrogen concentration of 0.2 volume % or more and 8 volume % or less for 50 seconds or more and 300 seconds or less;
a cooling step of cooling the steel plate after the soaking step;
A plating step in which the cooled steel sheet is immersed 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;
After cooling the alloyed hot-dip galvanized steel sheet to a cooling stop temperature below the Ms point, it is heated for 30 seconds at a temperature above the cooling stop temperature and above 100°C and below 450°C in a reheating atmosphere with a hydrogen concentration of 0.2% by volume or below. A cooling-reheating step of holding the above,
A method for producing an alloyed hot-dip galvanized steel sheet.

[2] The component composition satisfies [Si]/[Mn] of 0.23 or more,
The atmosphere in the soaking step has a dew point of -20°C or more and +20°C or less,
The method for producing an alloyed hot-dip galvanized steel sheet according to [1] above.
Here, [Si] and [Mn] respectively indicate the content (mass %) of Si and Mn in the above component composition.

[3] The component 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 an alloyed hot-dip galvanized steel sheet according to [1] above.
Here, [Si] and [Mn] respectively indicate the content (mass %) of Si and Mn in the above component composition.

[4] The alloyed hot-dip galvanized steel sheet according to any one of [1] to [3] above, wherein the soaking atmosphere in the soaking step has a hydrogen concentration of 0.2% by volume or more and 5% by volume or less. Production method.

[5] In the cooling step, the steel plate after the soaking step is cooled at an average cooling rate of 10 from 600°C to 900°C 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. [6] The method for producing an alloyed hot-dip galvanized steel sheet according to any one of [1] to [4] above, wherein the component composition is cooled to 300° C. or more and 500° C. or less at a temperature of 300° C. or more and 500° C. or less. 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 according to any one of [1] to [5] above, containing Al: 0.003% or more and 2.000% or less and N: 0.0100% or less, with the remainder consisting of Fe and inevitable impurities. Method for manufacturing alloyed hot-dip galvanized steel sheet.

[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 an alloyed hot-dip galvanized steel sheet according to [6] above, containing at least one selected from Bi: 0.0200% or less and REM: 0.0200% or less.

[8] In the cooling-reheating step, the alloyed hot-dip galvanized steel sheet according to any one of [1] to [7], wherein the cooling stop temperature is (Ms point -50 ° C.) or lower. Production method.

[9] The method for producing an alloyed hot-dip galvanized steel sheet according to [8] above, wherein the cooling stop temperature is (Ms point −100° C.) or lower.

According to the present disclosure, it is possible to provide an alloyed hot-dip galvanized steel sheet that has a beautiful surface appearance free from defects such as unplatedness, excellent plating adhesion, and also has excellent hydrogen embrittlement resistance.

FIG. 3 is a diagram showing an example of atmospheric dew point measurement positions in a split annealing furnace.

Conventionally, it has been difficult to achieve both a beautiful surface appearance, excellent plating adhesion, and hydrogen embrittlement resistance. Oxidation-reduction of steel sheets is effective for improving the plating properties of Si and Mn-containing steels. In order to complete the reduction of Fe oxide before plating, annealing in a high hydrogen concentration atmosphere is essential, and a large amount of hydrogen inevitably enters the steel. When the hydrogen concentration during annealing is low, Fe reduction is not completed, and Fe oxide remaining on the steel sheet surface causes deterioration of plating properties. Therefore, in the present 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 that does not re-oxidize Fe. This makes it possible to reduce hydrogen that has once penetrated into the steel.

Hereinafter, embodiments of the present disclosure will be described. Note that the present disclosure is not limited to the following embodiments.

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

Furthermore, in this specification, the expression "high strength" of the steel plate means that the tensile strength of the steel plate is 340 MPa or more.

First, the appropriate range of the composition of the base steel sheet and the reason for its limitation will be explained.

Si: 0.10% or more and 2.00% or less Si is a solid solution strengthening element and contributes to increasing the strength of the steel plate. Furthermore, since it also has the effect of suppressing carbide formation and making it easier to obtain retained austenite, it is effective in achieving both strength and ductility of steel sheets. 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%, hot rollability and cold rollability will be greatly reduced, which may adversely affect productivity or even reduce the ductility of the steel sheet itself. . Furthermore, the formation of Si oxide on the surface of the steel sheet becomes significant, and good plating properties may not be obtained. Therefore, the amount of Si is 0.10% or more and 2.00% or less. The amount of Si is preferably 0.25% or more. Moreover, the amount of Si is preferably 1.70% or less.

Mn: 1.0% or more and 5.0% or less Mn has the effect of solid solution strengthening the steel to increase its strength, increasing hardenability, and promoting the formation of retained austenite, bainite, and martensite. It is an element. Such an effect is produced by adding 1.0% or more of Mn. 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 formation of Mn oxides on the surface of the steel sheet during plating will be insufficiently suppressed. , good plating properties may not be obtained. Therefore, the amount of Mn is set to 1.0% or more and 5.0% or less. The amount of Mn is more preferably 1.5% or more, and even more preferably 2.0% or more. Moreover, the amount of Mn is more preferably 4.0% or less, and even more preferably 3.5% or less.

The component composition of the base steel plate according to the present embodiment requires that it contain Si and Mn in a predetermined amount and ratio range. Others may be freely selected according to the design of mechanical properties and are not particularly limited. However, in order to obtain a steel plate with a tensile strength of 340 MPa or more, it is preferable to have the following composition.

C: 0.05% or more and 0.40% or less C is an effective element for increasing the strength of a 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 in an amount of 0.05% or more. Further, in order to obtain good weldability, the amount of C is preferably 0.40% or less. The amount of C is more preferably 0.07% or more. Further, the amount of C is preferably 0.35% or less.

P: 0.001% or more and 0.100% or less By suppressing the P content, better weldability can be obtained. Furthermore, it is possible to prevent P from segregating at grain boundaries, making it possible to particularly improve ductility, bendability, and toughness. Moreover, by suppressing the content of P, ferrite transformation can be suppressed and crystal grain size can be prevented from becoming coarse. Therefore, the amount of P is preferably 0.100% or less. The amount of P is more preferably 0.050% or less. The lower limit of P is not particularly limited. Due to production technology constraints, the P amount may be more than 0%, and may be 0.001% or more.

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

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

N: 0.0100% or less (not including 0%)
The amount of N is preferably 0.0100% or less. By controlling the amount of N to 0.0100% or less, it is possible to better prevent N from forming coarse nitrides with Ti, Nb, and V at high temperatures, and to increase the strength of steel sheets by adding Ti, Nb, and V. This can prevent loss of effectiveness. Further, by controlling the amount of N to 0.0100% or less, better toughness can be obtained. Furthermore, by setting the N amount to 0.0100% or less, it is possible to prevent slab cracking and surface flaws from occurring during hot rolling. Therefore, the amount of N is preferably 0.0100% or less, 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 production technology constraints, and may be 0.0005% or more.

The component composition may further optionally contain a predetermined amount of at least one selected from the following element groups.

B: 0.0100% or less B is an effective element for improving the hardenability of steel. In order to improve hardenability, the amount of B is preferably 0.0001% or more, more preferably 0.0005% or more. In order to obtain better moldability, the amount of B is preferably 0.0100% or less, 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 strength, it is preferably 0.001% or more. In order to obtain better moldability, when adding Ti, the amount of Ti is preferably 0.200% or less, more preferably 0.060% or less.

Nb: 0.200% or less By adding Nb, the effect of improving strength can be obtained. In order to obtain this effect, the amount of Nb is preferably 0.001% or more, more preferably 0.005% or more. Further, by setting the content to 0.200% or less, cost increases can be prevented. Therefore, the amount of Nb is preferably 0.200% or less, more preferably 0.060% or less.

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

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

V: 0.100% or less By adding the amount of V, the effect of improving strength can be obtained. In order to obtain this effect, the V amount is preferably 0.001% or more, more preferably 0.005% or more. Further, by setting the content to 0.100% or less, an increase in cost can be prevented. Therefore, the V amount is preferably 0.100% or less, 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 suitable range and tensile strength within a more suitable range. . In order to obtain such effects, the Cu content is preferably 0.005% or more, more preferably 0.020% or more. Further, when adding Cu amount, from the viewpoint of preventing cost increase, the Cu amount is preferably 1.00% or less, more preferably 0.20% or less.

Cr: 1.00% or less By adding Cr, hardenability can be improved and the balance between strength and ductility can be improved. In order to obtain this effect, it is preferable that the Cr content is 0.001% or more. Furthermore, when adding Cr, the amount of Cr is preferably 1.00% or less, more preferably 0.80% or less, from the viewpoint of preventing cost increases.

Ni: 1.00% or less By adding Ni, hardenability can be improved and the balance between strength and ductility can be improved. In order to obtain this effect, it is preferable that the Ni content be 0.005% or more. Furthermore, when adding Ni, the amount of Ni is preferably 1.00% or less, more preferably 0.80% or less, from the viewpoint of preventing cost increases.

Mo: 0.50% or less By adding Mo, the effect of adjusting strength can be obtained. In order to obtain this effect, the amount of Mo is preferably 0.005% or more, more preferably 0.01% or more. Furthermore, when Mo is added, the content is preferably 0.50% or less, more preferably 0.45% or less, from the viewpoint of preventing cost increases.

Ta: 0.100% or less By adding Ta, the effect of improving strength can be obtained. In order to obtain this effect, it is preferable to contain Ta in an amount of 0.001% or more. Further, when Ta is contained, the amount of Ta is preferably 0.100% or less from the viewpoint of preventing cost increases. The amount of Ta is more preferably 0.050% or less.

W: 0.500% or less By adding W, the effect of improving strength can be obtained. In order to obtain this effect, the amount of W is preferably 0.001% or more, more preferably 0.003% or more. Moreover, when containing W, the amount of W is preferably 0.500% or less, more preferably 0.450% or less, from the viewpoint of preventing cost increases.

Zr: 0.020% or less By adding Zr, the ultimate deformability of the steel plate can be improved, and the stretch flangeability can be improved. In order to obtain this effect, the Zr amount is preferably 0.0005% or more, more preferably 0.0010% or more. Furthermore, when containing Zr, the amount of Zr is preferably 0.020% or less from the viewpoint of preventing cost increases. The amount of Zr is more preferably 0.010% or less.

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

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

Zn: 0.020% or less By adding Zn, the ultimate deformability of the steel plate can be improved and the stretch flangeability can be improved. In order to obtain this effect, it is preferable that the amount of Zn is 0.001% or more. Furthermore, when containing Zn, the amount of Zn is preferably 0.020% or less from the viewpoint of preventing cost increases. The amount of Zn is more preferably 0.010% or less.

Co: 0.020% or less By adding Co, the ultimate deformability of the steel plate can be improved, and the stretch flangeability can be improved. In order to obtain this effect, it is preferable that the amount of Co is 0.001% or more. Further, when Co is contained, the amount of Co is preferably 0.020% or less from the viewpoint of preventing cost increases. The amount of Co is more preferably 0.010% or less.

Ce, Se, Te, Ge, As, Sr, Cs, Hf, Pb, Bi, REM: 0.0200% or less By adding these elements, the ultimate deformability of the steel plate is improved and stretch flangeability is improved. You can obtain the effect of In order to obtain this effect, it is preferable that at least one of these elements be 0.0001% or more. On the other hand, from the viewpoint of preventing cost increases, 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 according to this embodiment other than the above-mentioned components consists of Fe and inevitable impurities.

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

Next, a method for manufacturing an alloyed hot-dip galvanized steel sheet according to an embodiment of the present disclosure will be described.

First, a steel plate 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 produce a cold-rolled steel plate.

Next, before the steel plate having the above-mentioned composition is immersed in a hot-dip galvanizing bath, the steel plate is subjected to recrystallization annealing including an oxidation process, a reduction process, and a soaking process. In one example, cold rolled steel sheet is fed to a CGL. Although the configuration of the CGL is not particularly limited, in one example, the CGL includes a continuous annealing furnace in which a heating zone, a soaking zone, and a cooling zone are arranged in this order, hot-dip galvanizing equipment provided downstream of the cooling zone, and It has an alloying furnace installed downstream of the hot-dip galvanizing equipment. In one example, a steel plate is transported inside a continuous annealing furnace through a heating zone, a soaking zone, and a cooling zone in order, recrystallization annealing is performed on the steel sheet, and hot-dip galvanizing equipment is used to discharge the steel sheet from the cooling zone. The hot-dip galvanized steel sheet is subjected to hot-dip galvanizing to obtain a hot-dip galvanized steel sheet, and then, using an alloying furnace, the hot-dip galvanized steel sheet is subjected to alloying treatment to obtain an alloyed hot-dip galvanized steel sheet.

[Oxidation process]
By generating an iron oxide layer on the surface of the steel sheet in the oxidation step and then generating reduced iron in the reduction step, the formation of Si and Mn oxides on the surface of the steel sheet can be suppressed and excellent plating properties can be obtained. In one example, the oxidation step is performed in the heating zone of the CGL.

Oxidizing atmosphere containing 1000 volume ppm or more and 30000 volume ppm or less of O ₂ By setting the O ₂ concentration of the oxidizing atmosphere in the oxidation step to 1000 volume ppm or more, oxidation of the steel sheet is promoted. If the O ₂ concentration in the oxidizing atmosphere is less than 1000 ppm by volume, the steel sheet will not be oxidized sufficiently, and the above effects will not be obtained. On the other hand, if the O ₂ concentration in the oxidizing atmosphere exceeds 30,000 ppm by volume, the steel sheet will be excessively oxidized, and unreduced iron oxide will remain in the subsequent reduction step, causing deterioration in plating properties. Although the remainder of the oxidizing atmosphere is not particularly limited, in one example, it consists of N ₂ , CO, CO ₂ , H ₂ O, and inevitable impurities, and the ratio thereof is not particularly limited.

Steel plate temperature: Heating to 600° C. or higher In order to promote oxidation of the steel plate in the oxidation step, the steel plate temperature needs to be 600° C. or higher. If the steel plate temperature is less than 600°C, oxidation will be insufficient and the above effects will not be obtained. The steel plate temperature in the oxidation step is preferably 650°C or higher. Although the upper limit of the steel plate temperature in the oxidation step is not particularly limited, it is preferably 900° C. or lower. By setting the steel plate temperature to 900°C or less, excessive oxidation of the steel plate can be more effectively prevented, unreduced iron oxide can be more effectively prevented from remaining in the reduction process, and plating properties can be further improved. Can be done. Note that the steel plate temperature is based on the temperature of the steel plate surface. The same applies to the following various steps.

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

Steel plate temperature: 700°C or higher If the steel plate temperature in the reduction step is less than 700°C, the reduction rate is slow and unreduced iron oxide may remain. The steel plate temperature in the reduction step is preferably 750°C or higher. The upper limit of the steel plate temperature in the reduction process is not particularly limited, but in order to better prevent the deterioration of the furnace body, the steel plate temperature in the reduction process is preferably 950°C or less.

Hydrogen concentration: more than 8% by volume and not more than 30% by volume The higher the hydrogen concentration in the reducing atmosphere, the faster the reduction of iron oxide. However, as the hydrogen concentration increases, the amount of hydrogen solidly dissolved in the steel increases, leading to deterioration of hydrogen embrittlement resistance. If the hydrogen concentration in the reducing atmosphere is 8% by volume or less, the rate of reduction of iron oxide will be insufficient. On the other hand, if the hydrogen concentration in the reducing atmosphere exceeds 30% by volume, 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 of the reducing atmosphere is set to 8% by volume or more and 30% by volume or less. The hydrogen concentration of the reducing atmosphere is preferably 20% by volume or less, more preferably 18% by volume or less. Further, the hydrogen concentration of the reducing atmosphere is preferably 12% by volume or more.

Holding time: 20 seconds or more The longer the holding time in a reducing atmosphere, the more advantageous it is to complete the reduction of iron oxide. If the holding time in the reducing atmosphere is less than 20 seconds, the reduction of iron oxide may not be completed. Therefore, the holding time in the reducing atmosphere is set to 20 seconds or more. The holding time under the reducing atmosphere is preferably 25 seconds or more. The upper limit of the retention time in a reducing atmosphere is not particularly limited. From the viewpoint of productivity, the holding time in a reducing atmosphere is preferably 150 seconds or less.

[Soaking process]
In the soaking process, the reduction process in which iron oxide is reduced in a hydrogen-rich atmosphere aims to reduce hydrogen solidly dissolved inside the steel sheet.

Hydrogen concentration: 0.2% by volume or more and 8% by volume or less Since the reduction of the steel plate has been completed in this process, it is maintained in a soaked atmosphere with a lower hydrogen concentration than in the reduction process to reduce hydrogen solidly dissolved inside the steel plate. can do. In order to obtain this effect, the hydrogen concentration needs to be 8% by volume or less. Further, the hydrogen concentration is preferably 5% by volume or less. On the other hand, if the hydrogen concentration is less than 0.2% by volume, it is difficult to uniformly control the inside of the furnace, and there is a risk that the reduced iron will be reoxidized. Therefore, the hydrogen concentration in the soaking atmosphere is preferably 0.2% by volume or more, more preferably 0.5% or more.

Steel plate temperature: 750°C or higher If the steel plate temperature in the soaking step is less than 750°C, there is a risk that the reduced iron layer on the surface of the steel plate will be reoxidized. Furthermore, the recrystallization of the steel sheet structure may be insufficient, or the volume fraction of ferrite may be excessive, making it impossible to obtain the required material quality. The steel plate temperature in the heating step is preferably 780°C or higher. Although the upper limit of the steel plate temperature in the soaking step is not particularly limited, it is preferably 950° C. or lower from the viewpoint of production equipment. The steel plate temperature in the soaking step is more preferably 900° C. or lower.

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

Dew point of reduction process and soaking process The dew point of the atmosphere in the reduction process and soaking process is not particularly limited, but from the viewpoint of industrial ease of handling, it is preferably -50°C or higher, and should be +20°C or lower. is preferred. 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, the dew point inside the furnace can be more easily controlled and adverse effects on the furnace body can be suitably avoided.

In addition, by setting the atmospheric dew point of the reduction step and soaking step to an appropriate range according to the value of [Si]/[Mn], which is the mass % ratio of Si and Mn contained in the composition of the steel sheet, Surface appearance and hydrogen embrittlement resistance can be further improved. Note that [Si] and [Mn] respectively indicate the content (% by mass) of Si and Mn in the component composition. Below, preferable atmospheric dew points will be explained separately for cases where [Si]/[Mn] is 0.23 or more and cases where it is less than 0.23.

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

・[Si]/[Mn]<0.23
・Ambient dew point in the reduction process is less than -20°C ・Ambient dew point in the soaking process is less than -20°C If [Si]/[Mn] is less than 0.23, the amount of Mn is higher than the amount of Si in the steel. The oxides formed in the surface layer of the steel sheet are mainly Mn oxides. In this case, by setting the dew point of the reducing atmosphere and the atmospheric dew point of the soaking process within the above range, it is possible to suppress the formation of a single Mn oxide on the outermost surface of the steel sheet after the reduction of iron oxide is completed. This further improves plating wettability and provides a more excellent surface appearance. In order to obtain this effect, the dew point of the reduction step is preferably lower than -20°C, more preferably lower than -25°C. Further, the dew point of the soaking step is preferably less than -20°C, more preferably less than -25°C.

As described above, in the reduction step and the soaking step, the hydrogen concentrations of the atmospheres used are different from each other. Also, the dew points used in the reduction step and the soaking step may also be different. In the reduction step and the soaking step, the method of varying the hydrogen concentration and dew point of the atmosphere is not particularly limited. For example, a method is to divide the furnace that performs both processes, use the furnaces connected via seal rolls, and separately inject gas controlled to the desired hydrogen concentration and dew point into each divided furnace. be. Further, 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 or the like before inputting it into the furnace.

In one example, the above-mentioned reduction step and soaking step are performed using a furnace in which the soaking zone of the CGL is divided into a first stage and a second stage, which are connected via a seal roll. That is, as an example of a CGL that can be suitably applied to the method for manufacturing an alloyed hot-dip galvanized steel sheet according to the present disclosure, a continuous annealing furnace in which a heating zone, a soaking zone, and a cooling zone are arranged in this order, and a continuous annealing furnace in which a heating zone, a soaking zone, and a cooling zone are arranged in this order, and a continuous hot-dip galvanizing facility, the soaking zone having a first soaking zone and a second soaking zone, the first soaking zone and the second soaking zone being connected via a seal roll; Examples include plating equipment. In this way, according to the CGL which has a soaking zone in which the first soaking zone and the second soaking zone are connected via a seal roll, the movement of the atmosphere is blocked between the first soaking zone and the second soaking zone, thereby reducing the reduction. It is possible to control the hydrogen concentration and dew point of the atmosphere in the process and soaking process independently of each other. Therefore, like the method for manufacturing an alloyed hot-dip galvanized steel sheet according to the present disclosure, it can be suitably applied to a method in which the hydrogen concentration and dew point of the atmosphere in the reduction step and the soaking step are controlled independently from each other.

Note that the method for monitoring the hydrogen concentration and dew point in the atmosphere inside the furnace is not particularly limited. For example, measurement can be performed by providing an atmosphere measurement port that can guide the gas inside the furnace to the outside at a necessary position in each furnace and connecting it to a hydrogen concentration meter and a dew point meter. Also, for example, if the reduction process and soaking process are carried out in separate furnaces as described above, it is possible to install independent atmosphere measurement ports at three locations in the upper, middle, and lower parts of each furnace. It is possible to know the distribution state of hydrogen concentration in the height direction inside the reactor. FIG. 1 shows an example of atmospheric dew point measurement positions in a split annealing furnace. In one example, an example of a CGL soaking area is schematically shown. As shown in FIG. 1, the steel strip 6 is supplied to a split annealing furnace in which a first stage soaking zone 1 and a second stage soaking zone 2 are connected via an atmosphere sealing zone 3, and the above-mentioned reduction process and soaking process are performed. At this time, the dew point measurement positions 6 can be independently provided at three locations in each furnace: an upper part, a middle part, and a lower part. By combining this with the above control method, the furnace atmosphere composition can be continuously controlled.

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

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

In addition, in order to better prevent the amount of hydrogen in the steel from increasing during cooling, it is preferable to cool the steel sheet from a temperature of 600° C. or higher and 950° C. or lower at an average cooling rate of 10° C./s or higher. The cooling start temperature is more preferably 700°C or higher. Moreover, it is more preferable that the average cooling rate is 15° C./s or more. In order to cool the steel plate temperature in the subsequent plating process to the same level as the plating bath temperature, the cooling stop temperature is preferably 500°C or less. Further, by setting the cooling stop temperature to 300° C. or higher, excessive martensitic transformation can be prevented and the strength of the steel plate can be further improved. Therefore, it is preferable to cool to 300°C or more and 500°C or less at the above average cooling rate.

[Plating process]
Next, the steel plate after the cooling process is immersed in a hot-dip galvanizing bath to obtain a hot-dip galvanized steel plate. In one example, the plating process is performed using CGL's hot dip galvanizing equipment.

The conditions for immersion in the hot-dip galvanizing bath are not particularly limited, and a general method may be used. The hot-dip galvanizing bath consists of Al, Zn, and inevitable impurities, and its composition is not particularly specified, but in one example, the Al concentration in the bath may be 0.05% by mass or more and 0.190% by mass or less. could be. If the Al concentration in the bath is 0.05% by mass or more, the generation of bottom dross can be more suitably prevented. Moreover, if the Al concentration in the bath is 0.190% by mass or less, the generation of top dross can be more suitably prevented. Also from the viewpoint of cost, it is preferable that the Al concentration in the bath is 0.190% by mass or less. The plating bath temperature is also not particularly specified, but may be 440°C or higher and 500°C or lower.

The amount of plating deposited per side is not particularly limited, but in one example, it is 25 g/m ² or more and 80 g/m ² or less. If the amount of plating deposited per side is 25 g/m ² or more, corrosion resistance is particularly good, and control of the amount of plating deposited is particularly easy. Further, if the amount of plating deposited per side is 80 g/m ² or less, the plating adhesion is particularly good. The method of adjusting the coating amount is not particularly limited, but it can be adjusted by using gas wiping and adjusting the gas pressure and the distance between the wiping nozzle and the steel plate.

[Alloying process]
The hot-dip galvanized steel sheet is subjected to alloying treatment to obtain an alloyed hot-dip galvanized steel sheet. The hydrogen diffusion rate in the hot-dip galvanized layer mainly consisting of the η phase before alloying treatment is significantly slower than that in the steel, which prevents the release of hydrogen in the steel during the subsequent cooling-reheating process. If the hot-dip galvanized layer is an alloyed hot-dip galvanized layer, the hydrogen diffusion rate in the alloyed hot-dip galvanized layer is much faster than that in the hot-dip galvanized layer, so the release of hydrogen in the steel is promoted. Therefore, in order to significantly reduce hydrogen in the steel in the subsequent cooling-reheating and holding process, it is important to perform an alloying process prior to the cooling-reheating and 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 a temperature of 440° C. or higher. Further, the alloying treatment may be performed while maintaining the steel sheet temperature at a temperature of 600° C. or lower. The alloying treatment may be performed by holding the hot-dip galvanized steel sheet at the above temperature for 5 seconds or more and 60 seconds or less.

The alloyed hot-dip galvanized layer after alloying preferably has an alloying degree (Fe content in the hot-dip galvanized layer) of 7% by mass or more. Moreover, it is preferable that the alloyed hot-dip galvanized layer after alloying has an alloying degree of 15% by mass or less. By setting the degree of alloying of the alloyed hot-dip galvanized layer to 7% by mass or more, it is possible to prevent the η phase from remaining in the alloyed hot-dip galvanized layer and to more appropriately remove hydrogen in the steel during the reheating and holding process. can be reduced. In addition, by setting the degree of alloying of the alloyed hot-dip galvanized layer to 15% by mass or less, the formation of Γ phase at the interface between the alloyed hot-dip galvanized layer and the base steel sheet can be more effectively prevented. Good plating adhesion can be obtained.

[Cooling-reheating holding process]
Next, after cooling the alloyed hot-dip galvanized steel sheet to a cooling stop temperature below the Ms point, a cooling-reheating step is performed in which the alloyed hot-dip galvanized steel sheet is held at a temperature above the cooling stop temperature and between 100° C. and 450° C. for 30 seconds or more. In order to obtain excellent hydrogen embrittlement resistance, it is necessary to cool the alloyed hot-dip galvanized steel sheet after the alloying process to below the Ms point and then reheat it to further reduce hydrogen in the steel.

Cooling stop temperature: Below Ms point Austenite has a larger amount of hydrogen in solid solution than ferrite, but the diffusion rate of hydrogen is higher in ferrite with BCC structure and martensite with BCT structure than in austenite with FCC structure. . Therefore, by performing reheating after transforming austenite containing more dissolved hydrogen into martensite, it is possible to more efficiently reduce hydrogen in steel. Therefore, the cooling stop temperature in the cooling-reheating process is set to be below the Ms point at which martensitic transformation begins. Furthermore, the greater the degree of supercooling relative to the Ms point, the more the martensitic transformation is promoted, and the reheating can be performed with less untransformed austenite, which is advantageous for reducing hydrogen in the steel. Therefore, the cooling stop temperature in the cooling-reheating step is more preferably (Ms point -50°C) or lower, and even more preferably (Ms point -100°C or lower). Although the lower limit of the cooling stop temperature is not particularly limited, it is preferably set to 20°C or higher because a cooling stop temperature of less than 20°C requires a high heat extraction ability in the cooling zone, leading to an increase in cost.

Atmospheric hydrogen concentration during reheating: 0.2% by volume or less In order to promote release of diffusible hydrogen in the steel to the outside of the system during reheating and holding, it is advantageous to lower the atmospheric hydrogen concentration. It is preferably 0.2% by volume or less, more preferably 0.1% by volume or less. The lower limit of the hydrogen concentration is not particularly limited, but since hydrogen gas is inevitably included in the atmosphere, it may be, for example, 0.00001% by volume or more.

Reheating temperature: Cooling stop temperature or higher and 100°C or higher and 450°C or lower After cooling is stopped, reheating is performed to a reheating temperature higher than the cooling stop temperature in order to promote release of hydrogen in the steel. In order to sufficiently obtain the effect of reducing hydrogen in steel, it is necessary to set the reheating temperature to 100° C. or higher. On the other hand, if the reheating temperature exceeds 450° C., there is a risk that the plating properties will deteriorate. Therefore, the reheating temperature is set to 100°C or more and 450°C or less. The reheating temperature is preferably 200°C or higher. The reheating temperature is preferably 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 steel will be 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 50 seconds or more. The upper limit of the holding time at the reheating temperature is not particularly limited, but from the viewpoint of productivity, the holding time at the reheating temperature is preferably 300 seconds or less.

Note that manufacturing conditions other than those described above can be determined by conventional methods.

According to the above manufacturing method, it is possible to provide an alloyed hot-dip galvanized steel sheet having a tensile strength of preferably 340 MPa or more. The tensile strength of the alloyed hot-dip galvanized steel sheet is more preferably 500 MPa or more, and still more 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 an alloyed hot-dip galvanized 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 that the crosshead displacement speed Vc is 1.67×10 ⁻¹ mm/s, and TS is measured.

According to the above manufacturing method, it is possible to provide an alloyed hot-dip galvanized steel sheet with a reduced amount of diffusible hydrogen. The amount of diffusible hydrogen in the alloyed hot-dip galvanized steel sheet produced by this production method is preferably 0.30 wt. ppm or less, more preferably 0.20wt. ppm or less. Note that the upper limit of the amount of diffusible hydrogen in the alloyed hot-dip galvanized steel sheet is not particularly limited, but is, for example, 0.01 wt. It can be more than ppm.

Here, the amount of diffusible hydrogen is measured as follows. A 5×30 mm test piece is cut out from an alloyed hot-dip galvanized steel sheet, and the alloyed hot-dip galvanized layer on the surface of the test piece is removed using a router (precision grinder). Immediately, hydrogen analysis is performed using a temperature programmed desorption analyzer 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 released hydrogen at each temperature is measured. Among these, the cumulative value of the amount of released hydrogen in the range of 210° C. from the analysis start temperature is determined as the amount of diffusible hydrogen in the steel.

Using cold-rolled steel sheets with a thickness of 1.4 mm having the composition shown in Table 1, alloyed hot-dip galvanized steel sheets were manufactured under the conditions shown in Tables 2-1 and 2-2, and the following evaluations were conducted. Ta. The evaluation results are shown in Table 2-1 and Table 2-2. As the equipment, a CGL was used in which a front-stage soaking zone and a rear-stage soaking zone were connected by a seal roll. Various manufacturing conditions not listed in Tables 2-1 and 2-2 were as follows.
・Plating bath composition: 0.13wt.% Al-added Zn bath ・Plating bath temperature: 460°C
・Plating coverage range: 40-60gm ^-2
・Alloying degree range: 8.0 to 14.0% by mass
・Hydrogen concentration during reheating and holding: 0.1% by volume

The tensile strength (TS) was measured by the method described above.

Plating property The plating appearance of the alloyed hot-dip galvanized steel sheet was evaluated by the method shown below.

<Exterior>
The plating surface of the alloyed hot-dip galvanized steel sheet was visually observed, evaluated and ranked according to the following criteria. Those with

ranks

1 and 2 were set as the preferred range of the present invention.
Appearance Rank No plating or uneven appearance: 1
No unplatedness, but uneven appearance: 2
With non-plating: 3

<Amount of diffusible hydrogen in steel sheet>
The amount of diffusible hydrogen in steel was determined according to the method described above, and evaluated based on the following criteria. Those with

ranks

1 and 2 were set as the preferred range of the present invention.
Amount of diffusible hydrogen in steel (wt.ppm) Rank 0.20 or less: 1
More than 0.20 and less than 0.30: 2
More than 0.30: 3

<Hydrogen embrittlement resistance>
As an evaluation of hydrogen embrittlement resistance, the occurrence of cracks in resistance spot welds was evaluated. A 30×100 mm test piece was cut out from an alloyed hot-dip galvanized steel sheet. A welded test piece was prepared by sandwiching spacers with a thickness of 2 mm between both ends of the test piece, and joining the spacers at the center by spot welding. At this time, an inverter DC resistance spot welding machine was used for spot welding, and a dome-shaped electrode made of chromium copper and having a tip diameter of 6 mm was used. The pressurizing force was 380 kgf, the current application time was 16 cycles/50 Hz, and the holding time was 5 cycles/50 Hz. The welding current value was set to form a nugget diameter corresponding to the tensile strength of each steel plate. When the tensile strength was less than 1250 MPa, the nugget diameter was 3.8 mm, and when the tensile strength was 1250 MPa or more, the nugget diameter was 4.8 mm. The distance between the spacers at both ends was 40 mm, and the steel plate and the spacer were secured together by welding in advance. After being left for 24 hours after welding, the spacer portion was cut off. The cross section of the weld nugget was observed and evaluated based on the following criteria. Those with

ranks

1 and 2 were set as the preferred range of the present invention.
Crack observation results Rank: No cracks: 1 (particularly excellent in hydrogen embrittlement resistance)
Only microcracks of 100 μm or less occur: 2 (excellent hydrogen embrittlement resistance)
Cracks exceeding 100 μm: 3 (poor hydrogen embrittlement resistance)

Among the alloyed hot-dip galvanized steel sheets shown in Tables 2-1 and 2-2, the examples of the present invention have better plating properties than the comparative examples, and the amount of diffusible hydrogen in the steel is sufficiently reduced, resulting in excellent hydrogen embrittlement resistance. I can see that

1 First stage soaking zone 2 Second stage soaking zone 3 Atmosphere sealing zone 4 Roll 5 Steel strip 6 Dew point measurement position

Claims

A steel plate having a composition containing Si: 0.10% or more and 2.00% or less and Mn: 1.0% or more and 5.0% or less in mass%,
An oxidation step of heating to 600° C. or higher in an oxidizing atmosphere containing O 2 :1000 volume ppm or more and 30000 volume ppm or less;
A reduction step in which the steel plate after the oxidation step is held at 700° C. or higher in a reducing atmosphere with a hydrogen concentration of more than 8% by volume and not more than 30% by volume for 20 seconds or more;
A soaking step in which the steel plate after the reduction step is held at 750° C. or higher in a soaking atmosphere with a hydrogen concentration of 0.2 volume % or more and 8 volume % or less for 20 seconds or more and 300 seconds or less;
a cooling step of cooling the steel plate after the soaking step;
A plating step in which the cooled steel sheet is immersed 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;
After cooling the alloyed hot-dip galvanized steel sheet to a cooling stop temperature below the Ms point, it is heated for 30 seconds at a temperature above the cooling stop temperature and above 100°C and below 450°C in a reheating atmosphere with a hydrogen concentration of 0.2% by volume or below. A cooling-reheating step of holding the above,
A method for producing an alloyed hot-dip galvanized steel sheet.
The component composition satisfies [Si]/[Mn] of 0.23 or more,
The atmosphere in the soaking step has a dew point of -20°C or more and +20°C or less,
A method for producing an alloyed hot-dip galvanized steel sheet according to claim 1.
Here, [Si] and [Mn] respectively indicate the content (mass %) of Si and Mn in the above component composition.
The component 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,
A method for producing an alloyed hot-dip galvanized steel sheet according to claim 1.
Here, [Si] and [Mn] respectively indicate the content (mass %) of Si and Mn in the above component composition.
The soaking atmosphere in the soaking step has a hydrogen concentration of 0.2% by volume or more and 5% by volume or less,
A method for producing an alloyed hot-dip galvanized steel sheet according to any one of claims 1 to 3.
In the cooling step, the steel plate after the soaking step is cooled at an average cooling rate of 10°C/s from 600°C to 900°C in an atmosphere with a hydrogen concentration of 0.5% by volume or more and 30% by volume or less and a dew point of 0°C or less. The method for manufacturing an alloyed hot-dip galvanized steel sheet according to any one of claims 1 to 4, which comprises cooling to 300°C or more and 500°C or less in the above steps.
The component composition further includes, 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 alloying melt according to any one of claims 1 to 5, containing Al: 0.003% or more and 2.000% or less and N: 0.0100% or less, with the remainder consisting of Fe and inevitable impurities. Method of manufacturing galvanized steel sheet.
The component composition further includes, 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 an alloyed hot-dip galvanized steel sheet according to any one of claims 1 to 6, containing at least one selected from Bi: 0.0200% or less and REM: 0.0200% or less.
The method for producing an alloyed hot-dip galvanized steel sheet according to any one of claims 1 to 7, wherein in the cooling-reheating step, the cooling stop temperature is (Ms point -50°C) or lower.
The method for producing an alloyed hot-dip galvanized steel sheet according to claim 8, wherein the cooling stop temperature is (Ms point -100°C) or lower.