JP2002030403A - Hot dip galvannealed steel sheet and its production method - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a hot dip galvannealed steel sheet excellent in ductility, in which the base material is excellent in local ductility, and plating wettability and powdering properties are improved and to provide its production method. SOLUTION: This steel sheet has a composition containing, by mass, 0.05 to 0.2% C, 0.02 to 0.70% Si, 0.5 to 3.0% Mn, 0.005 to 0.10% P, <=0.1% S, 0.10 to 2.0% Al and <=0.01% N, also satisfying Si(%)+Al(%)>=0.5, and the balance Fe with inevitable impurities, the base material contains an austenitic phase of >=1% by volume, further, in the plating film, the concentration of Fe is 8 to 15 mass%, also, the average Γ phase thickness in the plating film is <=2 μm, the maximum Γ1 phase length in the thickness direction is <=1.5 μm, and the relation of the maximum Γ1 phase length/the Γ phase thickness <=1.0 is satisfied.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an alloyed hot-dip galvanized steel sheet using a base material having excellent local ductility, having a high rust-preventing ability, and having good powdering properties. .

[0002]

2. Description of the Related Art In recent years, hot-dip galvanized steel sheets have been used in large quantities in the fields of home appliances, building materials, and automobiles, but especially in terms of economic efficiency, rust prevention function, and performance after painting. Plated steel sheets are widely used.

[0003] On the other hand, as for the base material performance, various machines and devices are strongly promoted to have high performance and light weight at the same time.
Many high strength technologies have been developed. However,
With the increase in strength, the ductility of the steel sheet is impaired, so that there is a problem that workability is deteriorated.

To cope with such a problem, Japanese Unexamined Patent Publication No. Hei 5-70886 discloses a steel sheet having good hole-expanding properties and truly good workability at the time of pressing. In addition, a material containing retained austenite in steel and having significantly improved local ductility has been proposed.

On the other hand, the performance of a steel sheet as a product is as follows.
It is required to have high strength, excellent workability, and economically high rust prevention ability. Therefore, a material obtained by subjecting the base material to galvannealing has come to be required.

[0006] In order to manufacture such a plated steel sheet, first, the base material is usually subjected to an appropriate degreasing and washing step.
Alternatively, preheat the steel sheet in a weakly oxidizing or reducing atmosphere without performing degreasing cleaning, then anneal the steel sheet in a hydrogen + nitrogen reducing atmosphere, and then cool the steel sheet to near the plating temperature and then melt it. It is manufactured by immersion in a zinc bath. In addition, galvannealed steel sheets are
Thereafter, the material temperature is continuously raised to 500 to 600 ° C in a heat treatment furnace.
By heating for 3 to 60 seconds, a Fe-Zn alloy plating layer is formed. The plating layer at this time is made of Fe-Zn
And generally has an average Fe concentration of 8 to 12
% By mass. The coating weight of the plating is usually 20 to
A 70 g / m ^2, it is difficult to manufacture in conventional means is the following this range, also generally supply since those above this range it is difficult to secure powdering resistance of the plating layer It has not been.

[0007] When a material containing retained austenite is used as a base material of such an alloyed hot-dip galvanized steel sheet, the following problem arises that cannot be improved under ordinary conditions.

First, in a steel sheet to which Si and Al are added, oxides of Si and Al are concentrated on the surface during reduction annealing, and the wettability during hot-dip plating is significantly deteriorated. Regarding this point, various improvement methods have been proposed for steels to which Si alone has been added even in the past.
As shown in the publication, the steel sheet is pre-oxidized and the S
An improvement method for reducing the concentration of i-oxide has been proposed.

[0009] However, with respect to steel materials to which Si and Al are added in a complex manner, Al is oxidized even if the oxygen concentration is lower than Si, so that a more improved technique is required.
Of course, improvement in wettability when Al and Si are added in a complex manner has not yet been achieved.

Further, at the time of alloying, there is a problem that operability is deteriorated due to delay of alloying. As to these wettability and alloying problems, no improvement method has been practically proposed so far.
i, No base alloyed hot-dip galvanized steel sheet having a composition containing retained austenite in Al composite added steel has been manufactured.

[0011]

An object of the present invention is to provide an alloyed hot-dip galvanized steel sheet mainly used for automobiles and a method for producing the same.

More specifically, an object of the present invention is to provide a steel sheet containing a retained austenite excellent in local ductility, in particular, to reduce the plating wettability and to eliminate the retained austenite during alloying. To prevent
An object of the present invention is to provide an alloyed hot-dip galvanized steel sheet having improved powdering properties and excellent ductility, and a method for producing the same.

[0013]

Means for Solving the Problems We first set S
i, hot-dip galvanizing was performed on the residual austenite-containing material to which Al composite was added, and the wettability at that time was examined.

First, when a heat treatment is performed on a steel containing Si and Al in a specific reduction annealing heat pattern, 1% by volume or more of retained austenite remains, excellent in local ductility and high strength. It was confirmed that a steel plate could be manufactured.

The sum of the Si content in steel and the Al content in steel was 0.5% or more, and the C content in steel was preferably 0.05% or more and 0.20% or less. The amount of Si was 0.02% or more, and was approximately 0.3%. The Al content was 2.0% or less.

Next, this material is heated to 780.degree.
First, reduction annealing at 70 ° C or lower, and then 550 ° C or lower 35
Cooling is performed in a low-temperature holding temperature range of 0 ° C. or more, and after staying in this temperature range for 20 s or more, cooling to room temperature is performed.
It was confirmed that a steel material containing at least volume% of retained austenite could be produced.

For these materials, the heat pattern
When performing reduction annealing of the alloy, in order to ensure wettability,
The components of Si and Al, which are problem elements in steel, are used alone or
When we investigated the wettability when adding the composite,
If Si is higher than 0.70%, wettability cannot be ensured. It was also found that if the content of Al is higher than 2.0%, wettability cannot be ensured.

Further, a study was also conducted on a material containing both components in steel. However, it was found that the respective effects on the wettability occurred independently.
It was found that a material having 70% or less and Al: 2.0% or less can ensure wettability.

In this manner, wettability can be ensured, and
This has led to the definition of material components for which the required amount of retained austenite is present. These steel materials are highly ductile plating materials that can be plated.
The goal is to alloy this plated material into an alloyed hot-dip galvanized steel material.

Therefore, the usual alloying temperature of 460 ° C. to 60 ° C.
When alloying was performed at about 0 ° C., it was found that the alloying in a high-temperature region extremely deteriorated the workability. It is presumed that the reason was that the disappearance of the retained austenite phase progressed due to the alloying treatment in the high temperature region.

Therefore, the present inventors have conducted intensive studies in order to limit the region in which the retained austenite phase does not disappear. As a result, the austenitic phase was remarkably reduced in those treated for more than 5 seconds in a temperature range of 550 ° C. to 600 ° C. It was found to decrease.

From the above, it is possible to manufacture a GA steel sheet containing 1% by volume or more of retained austenite having good material properties by not performing the treatment for more than 5 seconds in the temperature range of 550 ° C. to 600 ° C. during alloying. It has become possible.

GA containing these retained austenite phases
When a steel sheet (alloyed hot-dip galvanized steel sheet, the same applies hereinafter) was pressed, there was a problem that extremely high workability and extremely high powdering resistance were required. Therefore, alloying was performed in various temperature ranges, and a GA steel sheet having a Fe concentration in the coating of 8 to 15% was prepared, and a detailed investigation was performed.

The film is formed in a range of 0.01 to 1.00 from the cross-sectional direction.
% Etched with thin nital solution of SE
As a result of observing the M and observing the structure of the cross section, and examining the relationship with the powdering resistance, the ratio between ( _{1) the} phase thickness and ( ₁₎ the maximum length of the ₁ phase shows that the correlation with the powdering property is good. I understood.

The ratio of the maximum length of _one phase / the thickness of the phase is 1.0
When the value exceeds 1.0, the powdering property rapidly deteriorates.
If it is less than 10, it was found that the powdering property was good.

When the conditions for producing a film meeting these conditions were also investigated in detail, it was first found that there was a correlation with the alloying temperature. Furthermore, it was found that the steel having a high Si content can satisfy the above-mentioned coating conditions even at a high alloying temperature, and also has a good effect when the Fe concentration in the coating is low.

Therefore, regarding the upper limit of the alloying temperature, the relationship between Si in steel and Fe concentration in the film is formulated,
The inventors have found that it is possible to determine the structure of the alloy layer in the coating, and have obtained the following equation 2 through experiments.

[0028] The value of TA1 (℃) ≦ 530 + 100 × [Si] -0.25 × [Fe] ····· ( Formula 2) (gamma ₁ phase of maximum length / gamma phase thickness) 1.0 In the case of a film structure exceeding the above, the reason why remarkable deterioration of the powdering property occurs is unknown, but can be estimated as follows.

First, the Γ phase is uniformly formed on the steel sheet base material side.
You. Above this (coating side)₁Phase, δ₁Generated in phase order
Γ₁The phases are not generated homogeneously but grow locally
There is a fast part. Γ₁The phase is the hardest alloy phase in the film
Therefore, when the film is processed,
It is a phase that can not follow the turn. Therefore, around this phase,
Distortion will occur, but Γ₁The longer the phase
However, the distortion in the vicinity will increase. This distortion
Only occurs at the interface with the adjacent alloy phase.
Γ phase and δ₁It occurs at the interface with the phase.₁phase
The second largest strain occurs at the interface with the poorly workable Γ phase
It is presumed that. This distortion is ₁Adhesion with phase
If the limit is exceeded, cracks will occur at the interface. turtle
Cracks are the starting point of film peeling and may cause powdering.
Therefore, these series of phenomena are powdering properties and
Will be relevant.

[0030] From the above facts, taking the maximum length and gamma phase ratio of gamma _1-phase, which is estimated to be related to powdering resistance. In the present invention, Equation 2 is set to design the film. The reason for this is not clear, but the phenomena are as follows: increase in alloying temperature, increase in Fe concentration in the film. for, although one which facilitates the growth of the gamma _1-phase, increases in the steel amount of Si, because it is intended to inhibit the growth of gamma _1-phase, those empirically region defining this is by the formula 2 is there.

Γ phase, Γ ₁ for these parameters
The details of the impact on growth of the phase are unknown, but Γ
It is presumed that the phase ₁ is characterized by a higher growth rate and local growth than the normal phase Γ. This trend, the Fe concentration in the film is increased, by the difference between the phases of growth becomes clear, the ratio of the maximum length / gamma phase thickness of gamma _1-phase increases. Moreover, alloying at a high temperature, but would promote the growth of the alloy phase, than gamma phase, more growth gamma _1-phase, to facilitate, as the alloying at a high temperature, the maximum length of the gamma _1-phase The sa / Γ phase thickness increases.

These parameters are considered to be effective in the direction of deteriorating the powdering property. For Si in steel, there is a tendency to suppress local growth of Γ ₁ phase,
_{Since one} phase is grown on average, the maximum length is reduced when the Fe diffusion amounts are equal. That is, the maximum length / gamma phase thickness of gamma _1-phase is reduced, powdering property is good. From these facts, it has become possible to improve the powdering property by creating Expression 2 and following it.

From the above, it is possible to manufacture an alloyed hot-dip galvanized steel sheet containing 1% by volume or more of the retained austenite phase and having good powdering properties.
The present invention has been completed.

Here, the present invention is as follows. (1) An alloyed hot-dip galvanized steel sheet in which a plating film is provided on the surface of a steel sheet base material, wherein the chemical composition of the base material is, by mass%, C: 0.05% or more and 0.20% or less, and Si: 0.02%. that's all
0.70% or less, Mn: 0.50% or more and 3.0% or less, P: 0.005
% To 0.10% or less, S: 0.1% or less, sol.Al: 0.10% to 2.0%, N: 0.01% or less, and Si (%) +
Al (%) ≧ 0.5, the balance being Fe and unavoidable impurities, the base material contains 1% or more by volume of an austenite phase, and the plating film further comprises:
Fe concentration is 8% by mass or more and 15% by mass or less, and
An alloyed hot-dip galvanized steel sheet characterized by having an average phase thickness of 2 μm or less and a maximum thickness in the thickness direction of the plating film of ₁ μm or less, and further satisfying the following formula 1.

The maximum gamma ₁ Ainaga of / gamma phase thickness ≦ 1.0 · · · the steel sheet having a chemical composition of (Formula 1) (2) above (1), wherein, 750 ° C.
The reduction annealing is performed at a temperature of not less than 350 ° C. and a temperature of 350 ° C. or less and 550 ° C. or less.
K) After holding for 20 s or more and then performing hot-dip galvanizing, the temperature is 460 ° C. or more and less than 600 ° C.
Alloying at an alloying temperature (TA1) that satisfies the following condition, wherein the residence time (tA2) at an alloying temperature of 550 ° C. or more and less than 600 ° C. is zero or 5 s or less. Manufacturing method of hot-dip galvanized steel sheet.

TA1 (° C.) ≦ 530 + 100 × [Si] −0.25 × [Fe] (Formula 2) TA1: alloying temperature, [Si]: amount of Si in steel, [Fe]: coating Medium F
e concentration

[0037]

BEST MODE FOR CARRYING OUT THE INVENTION In the present invention, the reason why the chemical composition of the base material and the heat treatment conditions are specified as described above will be described below. In this specification, “%” for defining the chemical composition is “% by mass” unless otherwise specified.

Regarding the chemical composition of the base material:
5% or more and 0.20% or less. The lower limit is the amount of C necessary for precipitating the retained austenite phase stably. If the amount is lower than 0.05%, the amount of C in the austenite phase is reduced, so that the production becomes unstable and the production becomes difficult. C
Although the upper limit of the amount is 0.20%, it is advisable to increase the amount of C in order to manufacture a high-tensile steel sheet, but if C is too high when performing welding, there is a problem that welding cannot be performed.
The upper limit was 0.20%.

When the C content is increased, it is possible to easily increase the strength, but at the same time, the ductility is deteriorated. Therefore, the residual austenite phase is stably contained, and the ductility is very high. Considering that it is high, the amount of C is
It is preferable to control at 0.08% or more and 0.15% or less.

The content of Si is specified to be 0.02% or more and 0.70% or less. The lower limit has the function of promoting the growth of the austenite phase and assisting the enrichment of C in the formed austenite phase. Below this concentration, stable generation of the retained austenite phase is small, and the lower limit is made 0.02%. The upper limit is set to 0.70%. When the amount of Si exceeds this, as described above, a specific heat pattern (temperature range) is taken at the time of reduction annealing, so that the Si oxide is concentrated on the surface. When performing hot-dip plating, wettability cannot be ensured. Therefore, the Si concentration in steel is specified as described above. In consideration of the fact that alloying in a later step occurs promptly, it is more preferable that the Si content be 0.02% or more and 0.50% or less.

Mn is specified to be 0.50% or more and 3.0% or less. Mn is an element for stabilizing the austenite phase, and has an effect of stabilizing the austenite phase when added at a lower limit of 0.50% or more. However, if the addition amount is too large, the steel sheet becomes brittle, so the upper limit is made 3.0%.
Further, when the amount of added Mn increases, the manufacturing cost of the steel sheet increases. Therefore, the preferable range is about 2.5%, and the appropriate range of the Mn amount is 0.50% or more and 2.5% or less.

[0042] P is defined as not less than 0.005% and not more than 0.10%. Since P is an element inevitably mixed, it is preferable that P is low, but in order to adjust the content to less than 0.005%,
Due to high costs, the lower limit is 0.005%. 0.10%
If it is excessive, the ductility of the steel sheet deteriorates, so it is specified as 0.10% or less.

S is specified to be 0.1% or less. S
Is also contained as an impurity in steel, so it is better to have a low concentration. When the content exceeds 0.1%, Mn
Since the precipitation of S becomes conspicuous and not only impairs the ductility of the steel sheet but also consumes Mn which is a stabilizing element of the austenite phase as a precipitate, the S content is specified to be 0.1% or less. . There is no specific lower limit, but
Normally, up to about 0.001% of a material can be manufactured without any particular problem.

The content of Al is specified to be 0.10% or more and 2.0% or less. Al is an element that increases the amount of C in the austenite phase by increasing the volume fraction of the ferrite phase, similarly to Si. Such an effect is exhibited at 0.10% or more. The upper limit is 2.0%, but 2.0%
When it exceeds, a large number of inclusions increase in the steel, and the ductility is deteriorated.

Al can increase the amount of C in austenite by the same effect as Si, but in fact, by adding both Si and Al, the volume ratio of ferrite can be effectively increased, It is possible to increase the amount of C in austenite to stabilize the austenite phase.

From these facts, in order to keep the retained austenite in the steel at 1% by volume or more, Si (%) +
It is necessary to satisfy Al (%) ≧ 0.5. When the amounts of Si and Al satisfy the above formula, it is possible to manufacture a steel material in which retained austenite remains at 1% by volume or more.

N is defined to be 0.01% or less. N in steel is also an unavoidable impurity, and the lower the content, the better. If the N content exceeds 0.01%, AlN is likely to be generated and Al will be consumed.
The lower limit is not specified, but usually up to about 0.0002%,
It can be manufactured without any particular problem.

In the present invention, the chemical composition of the base material is Fe and impurities other than the above-mentioned components, but in addition to N and S as unavoidable impurities, Ni, Co, Cu, Cr and the like are not more than 0.2% in total. Permissible.

Next, the steps of a method for producing a galvannealed steel sheet by subjecting the base material to hot dip galvanizing in a hot dip galvanizing production line will be described in order. First, the base material that is a steel sheet may be a steel sheet that has been subjected to pickling after hot rolling or a material obtained by cold rolling a hot-rolled steel sheet, as long as it has the above composition.

Since a steel sheet is often coated with an oil such as a rolling oil or an antirust oil, it is preferable to perform degreasing as a pretreatment for hot-dip plating. As a degreasing method, alkali degreasing, electrolytic degreasing in an alkali, and the like are often used, and 5 to 20% sodium hydroxide is often used in a normal production line. Degreasing is not required when the hot-rolled black scale is directly reduced and plated in a CGL facility, or when the black scale is pickled and descaled in a hot-dip plating facility.

In order to burn oil on the surface of the steel sheet, degreasing can be omitted. As described above, the present invention can be applied to any steel sheet without any particular problem.

As the next step, preheating is performed. Here, there are a gas heating method using a burner, a heating method using a radiant tube, and the like, but there is no problem using either type of furnace.

However, since the total amount of Si and Al is at least 0.5% or more, first the surface is oxidized to prevent preferential oxidation of the oxides of Si and Al on the surface, and the subsequent reduction annealing is performed. Thus, it is preferable to generate reduced iron in order to ensure wettability. The target oxidation amount at this stage is S
i: 0.2% or less, or Al: 1.0% or less.
If it is ² g / m ² or more, sufficient plating is possible.

Also, Si: exceeds 0.2%, or
When the content of Al exceeds 1.0%, it is preferable to generate iron oxide of 0.5 g / m ² or more. Next, reduction annealing is performed. The reduction annealing is performed by first raising the temperature to a temperature of 780 ° C or more and 870 ° C or less. Here, in order to produce a ferrite + austenite two-phase structure, it is necessary to heat to a temperature range from the Ac ₁ transformation point to the Ac ₃ transformation point, and if it is too low, it takes too much time to re-dissolve cementite, On the other hand, if it is too high, the volume fraction of austenite will increase too much, and the C content in austenite will decrease.

From the above, the upper and lower limits of the temperature are set to 870 ° C.
And 780 ° C. For the subsequent heat pattern, it is preferable to gradually cool to 700 ° C.,
Also, from 700 ° C to 350 ° C to 550 ° C in the next temperature range.
The cooling rate to the temperature is preferably about 50 ° C./s. However, these cooling patterns often cannot be realized with hot-dip plating equipment.

However, if the retained austenite can be stably generated and the ferrite volume concentration can be adjusted to an appropriate value, there is no need to stick to the above-mentioned heat pattern. For example, even if the material is cooled linearly from 780 ° C. to 500 ° C. at 20 ° C./s, it is sufficiently possible to adjust the retained austenite to 1% by volume or more.

The treatment at 350 ° C. to 550 ° C. is specified as 20 s or more. First, in this temperature range, enrichment of C is promoted while austenite is transformed into bainite. 5
Above 50 ° C., no bainite transformation occurs, while 35
When the temperature is lower than 0 ° C., lower bainite is formed, and C enrichment in austenite does not sufficiently occur. Therefore, 350 ° C
It is specified as not less than 550 ° C.

The residence time in this temperature range is specified as 20 s or more. This time is a necessary time for sufficiently enriching C in austenite, and is preferably 60 s or more.

The temperature range of the overaging treatment is preferably not less than the bath temperature, and at least not less than 10 ° C. lower than the bath temperature, in order to perform hot-dip plating in the next step. For example, when the bath temperature is 450 ° C., the overaging temperature is preferably 440 ° C. or higher. Considering that the volume fraction of retained austenite is reduced in the subsequent alloying treatment, the preferred range of the residence time in this temperature range is 60 s or more. However, the longest residence time is often about 90 s as the substantial residence time.

After passing through such a pretreatment step, subsequently, hot-dip plating is performed. The hot-dip plating method at this time is according to the conventional method, and the Al concentration in the bath is 0.08% to 0.1%.
It is often performed at about 6%, and the plating bath temperature is 440
It is often performed at about 480 to 480 ° C. After the plating, the basis weight is adjusted by gas wiping, and then the alloy enters the alloying furnace.

Depending on the alloying at this time, Fe in the film
The concentration is 8% or more and 15% or less. Regarding the lower limit, if the Fe concentration is lower than this, η-Zn remains on the surface, so that the paintability, weldability and flaking properties deteriorate. Regarding the upper limit, when the iron concentration in the film is higher than 15%, the thickness of the Γ phase exceeds the specified amount of 2.0 μm, and the deterioration of the powdering property becomes remarkable. Therefore,
The iron concentration in the film is specified to be 8% or more and 15% or less. Preferably it is 8 to 11%.

The characteristics of the plating film after alloying are as follows:
Phase thickness: 2.0 μm or less, maximum Γ ₁ Phase length: 1.5 μm
Hereafter, it is defined that the following expression 1 is satisfied. In the maximum gamma ₁ Ainaga of / gamma phase thickness ≦ 1.0 · · · (Equation 1) The present invention, to the gamma phase thickness and 2.0μm or less, beyond which, powdering degradation significantly This is to increase. Therefore, it is specified to be 2.0 μm or less. A preferable range of the powdering property is 1.0 μm or less in a more preferable range.下限 The lower limit of the phase thickness is not particularly defined, but the range that can be confirmed by TEM or the like is 0.
It is considered that about 01 μm is appropriate.

( ₁ ) _One phase is defined by the maximum length. Γ _One phase usually has non-uniform growth,
It has a needle-like shape and grows locally. A crack is generated between the locally grown portion and the Γ phase, and serves as a starting point for peeling of the film. Therefore, it is necessary to define the maximum length of ₁ phase.

In the present invention, the maximum length of _one phase is 1.5 μm
When the value is not more than m, the powdering property is remarkably deteriorated as in the case of the limitation of the phase thickness when the value exceeds this value. Therefore,
It is specified as 1.5 μm or less. Preferably it is 1.0 μm or less.

[0065] Further, in the above-mentioned range, the ratio of maximum length / gamma phase thickness of gamma _1-phase, it was confirmed that the powdering property is remarkably deteriorated when it exceeds 1.0. For this cause, details are unclear, the ratio most distortion is concentrated part of the gamma _1-phase and the gamma phase thickness, and the values defined by the formula 1 is consistent with greater than 1.0 regions, by distortion at the interface between the gamma phase and gamma ₁ phase at such regions are concentrated, the strain energy exceeds the adhesion force of the interface, a crack occurs believed to cause powdering.

[0066] Thus, the ratio of the gamma ₁ maximum length / gamma phase thickness is 1.0 or less. In order to manufacture an alloyed hot-dip galvanized steel sheet having the above-described film structure and base material structure, the alloying temperature (TA1) is set to 460 ° C or higher and lower than 600 ° C, and the alloying temperature is set to 550 ° C or higher and lower than 600 ° C. Dwell time (tA2) is zero or 5 s or less.

TA1 (° C.) ≦ 530 + 100 × [Si] −0.25 × [Fe] (Formula 2) TA1: alloying temperature, [Si]: amount of Si in steel, [Fe]: in film F
e concentration The range of the alloying temperature (TA1) is 460 ° C or more and less than 600 ° C. If the amount is less than the lower limit, the alloying speed is low, the productivity is low, and a substantial alloying process cannot be performed. Regarding the upper limit, if alloying is performed at a higher temperature,
Since the phase appears in a very large amount, the powdering property is significantly deteriorated and does not become a product.

Further, 550 is set within this alloying temperature range.
The residence time in the range of not lower than 600 ° C and not higher than 0 ° C is defined as zero or 5 seconds or less. In the range of 550 ° C. or more and less than 600 ° C., the austenite phase in steel becomes unstable and decomposes. When the austenite phase in the steel is decomposed, the high strength and ductile properties are lost. However, when the alloying temperature is high, the alloying speed is high, and it is particularly advantageous for a material having a high Si content in steel. When staying in this temperature range, the austenite phase does not disappear and the time for maintaining the material properties is specified as 5 s or less.

As described above, according to the present invention, it is possible to manufacture a GA steel sheet containing 1% by volume or more of the retained austenite phase. The reason why the retained austenite phase is 1% by volume or more is that if the amount is less than this, ductility cannot be improved. Although the upper limit is not specified, the base material component specified above is often a material that generates at most 15% by volume or less of retained austenite.

Finally, the upper limit of the alloying temperature is given by the following equation (2).
Although it is stipulated, the reasons for the limitation are as follows. Equation 2
Is an empirical formula for setting the ratio of the maximum length of the Γ ₁ phase / the thickness of the １．０ phase to 1.0 or less in the above-described film structure.
The concentration has the effect of not deteriorating the powdering property even when the alloying temperature is raised, and the higher the Fe concentration, the lower the powdering property.

From the above, it can be empirically determined that the upper limit of the alloying temperature is defined by Equation 2 and the deterioration of the powdering property can be suppressed. Thus, according to the present invention, by defining the base material and setting the conditions of reduction annealing, hot-dip plating and alloying, it is possible to produce a GA steel sheet which is rich in ductility and excellent in film adhesion. became.

[0072]

EXAMPLE A steel material having the chemical composition shown in Table 1 was melted in a laboratory, hot-rolled and cold-rolled to obtain a 1.4 mm thick base steel sheet.

From these steel plates, a width of 80 mm and a length of 200 mm
The cold-rolled steel sheet was cut out using a hot-dip plating simulator (manufactured by Resca Co., Ltd.), and the preheating was raised to 550 ° C in air or nitrogen at 550 ° C at 15 ° C / s, and the holding time was 2 seconds. After cooling to 200 ° C., reduction annealing was performed in a 10% hydrogen-nitrogen atmosphere (dew point −60 ° C. or less) according to a predetermined heat pattern shown in Table 2, followed by hot-dip plating.

The hot-dip plating bath used was an Fe saturated bath with an Al concentration of 0.12%, and the bath temperature was 460 ° C. The alloying treatment was performed in the simulator using an infrared heating furnace immediately after plating. The heat pattern at this time is described in Table 3.

The properties of the alloyed hot-dip galvanized steel sheet obtained as described above were measured in the following manner.
Are shown together. The evaluation method of each characteristic at that time was as follows.

(1) Tensile test The obtained galvannealed steel sheet (GA) was JIS5
The specimen was processed to the size of a tensile test specimen, and a tensile test was performed.

(2) Hole expansion test The same GA was cut into a 70 mm square, and the clearance was measured.
For a test piece punched out with a hole of 0.1 mm and diameter of 10 mm, a 33 mm３３ punch was pressed in with a 36.5 mm 内径 inner diameter die pressed with a wrinkle holding force of 3 tons.
The hole diameter at the limit of crack initiation was measured.

(3) Measurement of Austenite Amount After the film was removed by dissolution in acid, the amount of retained austenite was measured by X-ray reflection intensity measurement.

(4) Measurement of Powdering Property A disc having a diameter of 60 mm was punched out of a GA steel plate, a cylindrical cup was press-molded with a die having a punch diameter of 30 mm and a die shoulder R3 mm, and peeling with an adhesive tape was performed on the outer surface of the cup wall. Then, the total peeling weight of the plating was measured.

(5) Measurement of ( ₁ ) Phase Thickness and ( ₁ ) Maximum Length of _One Phase The interface between the coating and the base material was polished from the cross-sectional direction, etched with a 0.05% nitric acid-alcohol solution (a nital solution) for 1 minute, and then subjected to an electron microscope. The thickness of the Γ phase was measured at 10 places, and the average value was defined as the Γ phase thickness. Was measured also gamma ₁ phase thickness by the same method to measure the length of 30 or more gamma _1-phase, and the maximum length of the average of up to 5 points.

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[Table 1]

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[Table 2]

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[Table 3]

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As is clear from the above description, according to the present invention, an alloyed hot-dip galvanized steel sheet which can ensure excellent powdering properties, ductility, and workability can be obtained, and as a material for automobiles. It is particularly suitable and has great significance in practical use of the present invention.

Claims (2)

[Claims] An alloyed hot-dip galvanized steel sheet in which a plating film is provided on the surface of a base material of a steel sheet, wherein the base material has a chemical composition in mass% of C: 0.05% to 0.20%, Si:
0.02% to 0.70%, Mn: 0.50% to 3.0%, P: 0.005% to 0.10%, S: 0.1% or less, so
l. Al: 0.10% or more and 2.0% or less, N: 0.01% or less, and satisfying Si (%) + Al (%) ≧ 0.5 and the balance consisting of Fe and impurities. And 1% or more by volume of an austenite phase. Further, the plating film has an Fe concentration of 8% by mass or more and 15% by mass or less, and the Δ phase average thickness of the plating film is 2 μm.
Hereinafter, the maximum gamma ₁ Ainaga in the thickness direction: A is 1.5μm or less, galvannealed steel sheet, characterized by further satisfying the following equation 1. Maximum Γ ₁ phase length / Γ phase thickness ≤ 1.0 (Equation 1) 2. The steel sheet having the chemical composition according to claim 1 is subjected to reduction annealing at 750 ° C. or more and 870 ° C. or less, and then at a low temperature holding temperature (TK) of 350 ° C. or more and 550 ° C. or less.
After holding the residence time (tK) for 20 s or more, and then performing hot-dip galvanizing, an alloying treatment is performed at an alloying temperature (TA1) that is not less than 460 ° C and less than 600 ° C and satisfies the following equation 2. When the alloying temperature is 550 ° C or higher and 600 ° C
A method for producing an alloyed hot-dip galvanized steel sheet, wherein the residence time (tA2) is less than or equal to 5 s or less. TA1 (° C.) ≦ 530 + 100 × [Si] −0.25 × [Fe] (Formula 2) TA1: alloying temperature, [Si]: amount of Si in steel, [Fe]: F in film
e concentration