JPH10265925A - Production of galvannealed steel sheet excellent in plating adhesion - Google Patents

Abstract

PROBLEM TO BE SOLVED: To produce a plated steel sheet excellent in plating adhesion by using, as material, a steel sheet prepared by hot rolling at specific cooling temp., applying hot dip galvanizing to the steel sheet, subjecting the resultant plated steel sheet to temp. raise at specific temp. rise rate up to a specific temp. and to holding at the temp. to carry out alloying treatment, and forming a Zn-Fe alloy plating layer of specific iron content. SOLUTION: Coiling temp., at the time of hot rolling for a steel sheet as material, such as cold rolled steel sheet and hot rolled steel plate, is regulated to >=600 deg.C to optimize steel sheet characteristic before plating. This steel sheet is passed through a hot dip galvanizing bath at prescribed temp. to undergo hot dipping. Subsequently, the resultant hot dipped steel sheet is subjected to temp. raise at >=30 deg.C/sec temp. rise rate up to 470-510 deg.C and held at this temp. to undergo alloying treatment. By this procedure, a Zn-Fe alloy plating layer of 8-11 wt.% iron content is formed while preventing the formation of intermetallic compound Γ-phase deteriorating plating adhesion. Further, it is preferable that, after alloying treatment, cooling is applied to the plated steel sheet down to 420 deg.C at >=30 deg.C/sec cooling rate to prevent the development of ζ-phase.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing a galvannealed steel sheet having excellent plating adhesion, and more particularly to a technique for producing a steel sheet frequently used as a rust-preventive steel sheet for automobiles.

[0002]

2. Description of the Related Art At present, zinc-based hot-dip or electroplated steel sheets have been developed and put into practical use as rust-resistant steel sheets for automobiles from the viewpoint of excellent sacrificial corrosion protection. Above all, galvannealed steel sheets are inexpensive to manufacture and have high corrosion resistance, and are therefore frequently used in actual vehicles among zinc-based coated steel sheets. However, alloyed hot-dip galvanized steel sheets, after hot-dip galvanizing the steel sheet, are usually subjected to alloying treatment at a temperature in the vicinity of 500 ° C., and the interface between the two by interdiffusion between the ground iron and the zinc, which is the plating layer. In addition, the method has a problem that the adhesion between the plating layer and the steel sheet is essentially lower than that of the galvanized steel sheet which has been subjected to electroplating, since it is produced by producing an intermetallic compound of Zn-Fe.

[0003] Therefore, when manufacturing a hot-dip galvanized or alloyed hot-dip galvanized steel sheet, a measure to improve the adhesion by adding an appropriate amount of Al to a plating bath is considered. Some zinc-based hot-dip galvanized steel sheets are usually manufactured using an Al-containing zinc bath.
However, the above measures alone are not sufficient to always ensure the adhesion of the plating layer required as a steel sheet for automobiles, for example, strict control of the Al concentration in the hot-dip galvanizing bath, and control of the alloying temperature. Optimization has been carried out.
Furthermore, recently, in Japanese Unexamined Patent Publication No. 1-279738, rapid heating is performed for 2.0 seconds or less until the sheet temperature reaches 470 ° C. or more in the course of temperature rise in the alloying furnace after the completion of the weight control. After the completion of alloying, the plate temperature is 420 ° C in the cooling process
It is disclosed that rapid cooling is performed to the following temperature range in 2 seconds or less, and a technique for securing good adhesion by rapid heating and rapid cooling during alloying has been proposed.

[0004]

In the case of alloyed hot-dip galvanized steel sheets, previous studies have shown that there is a close relationship between the adhesion of the coating layer and the phase structure of the plating. It is known that, when the abundance of the intermetallic compound Г phase having a high iron content formed at the interface with, increases, the adhesiveness deteriorates. As described above, the plating layer in the alloyed hot-dip galvanized steel sheet is formed by interdiffusion between the galvanized layer and the steel sheet, and therefore, in a normal method, even if Al is present in the plating bath. As a result, a Г phase is inevitably generated at the interface between the plating layer and the steel sheet. further,
Even if the alloying conditions such as the alloying temperature, the heating rate during alloying, and the cooling rate are optimized and the Г phase formation can be suppressed to some extent, the formation cannot be completely suppressed. Therefore, although the plating adhesion of the product steel sheet is relatively good, it cannot be said that it has reached a sufficient level.

[0005] In view of such circumstances, an object of the present invention is to provide a method for producing an alloyed hot-dip galvanized steel sheet having more excellent plating adhesion than before.

[0006]

Means for Solving the Problems The inventors of the present invention have conducted intensive studies to achieve the above-mentioned object, and in order to produce an alloyed hot-dip galvanized steel sheet having excellent plating adhesion, it is necessary to take measures only after plating. There is a limit, and it has been found that the adhesion can be dramatically improved by optimizing the properties of the steel sheet before plating.

That is, according to the present invention, in producing an alloyed hot-dip galvanized steel sheet, the winding temperature of the steel sheet as a raw material during hot rolling is set to 600 ° C. or more, and after hot-dip galvanizing, 30 ° C. /
An alloying treatment was performed by raising the temperature to 470 to 510 ° C. at a rate of temperature increase of at least sec and maintaining the temperature in this temperature range, and the iron content was 8 to 1
Provided is a method for producing an alloyed hot-dip galvanized steel sheet having excellent plating adhesion, characterized by obtaining a 1 wt% Zn-Fe alloy plating layer. In this case, it is preferable to further cool to 420 ° C. at a cooling rate of 30 ° C./sec or more after the alloying treatment because the plating adhesion can be further improved.

In the present invention, the winding temperature of the steel sheet as the raw material is limited, this is combined with the heating rate during the alloying of the plating layer, and the cooling rate after the alloying is also specified. The appearance of Γ phase can be suppressed at the time of formation. As a result, it has become possible to manufacture an alloyed hot-dip galvanized steel sheet having significantly superior plating adhesion as well as controlling the iron content during plating.

[0009]

BEST MODE FOR CARRYING OUT THE INVENTION Hereinafter, the present invention will be described in more detail. As described above, in order to ensure good adhesion of the alloyed hot-dip galvanized steel sheet, it is necessary to minimize the Г phase that appears at the interface between the plated layer and the steel sheet during the alloying treatment. The inventor first investigated the relationship between the amount of the Г phase and the alloying conditions, and found out the alloying conditions for obtaining excellent adhesion to the galvannealed steel sheet as described below.

[0010] With respect to the alloying temperature, it is necessary to raise and maintain the sheet temperature at the time of leaving the plating bath in the range of 470 to 510 ° C, preferably in the range of 480 to 490 ° C. Is required. In alloying in a temperature range other than 470 to 510 ° C., it is difficult to obtain good adhesion even if other conditions such as a heating rate are changed. When the alloy is formed at a temperature lower than 470 ° C., a ζ phase is easily generated in the surface phase of the plating. Since the ζ phase has a small solid solubility limit of iron,
When present in -Fe alloy layer surface, the diffusion of iron suppression from base steel as compared to the case of [delta] ₁ Single-phase, resulting in surface iron content of is Г phase is likely to generate elevated. The appearance of the ζ phase on the plating surface not only promotes the 生成 phase formation, but is disadvantageous from the viewpoint of affecting other performance, such as deterioration of flaking property during press working and deterioration of slidability. Become. It is also clear from the phase diagram that the Г phase is likely to be formed when the alloying temperature is higher than 510 ° C., and the alloying temperature should not exceed 510 ° C.

In the present invention, the alloying treatment is performed within the above-mentioned temperature range. However, it is also very important to control the iron content in the coating layer for suppressing the Г phase. It is necessary to control the iron content during plating to 8 to 11 wt%. If the iron content is within the above range, when the iron content is less than 8 wt%, an unalloyed η phase remains on the plating surface layer, which adversely affects various properties such as corrosion resistance and coating film adhesion. If it exceeds 11 wt%, a large amount of Г phase will be generated at the interface between the plating layer and the steel sheet, and it will be difficult to ensure good adhesion. The iron content in the plating is mainly governed by the plating thickness, the alloying treatment temperature and the treatment time. Alloying temperature of 470-5
In the range of 10 ° C., the alloying treatment time is 10 to 20 sec.
Is appropriate. If the alloying treatment time is longer than 30 sec, the iron content exceeds 11 wt%, which is not preferable. As described above, in order to produce an alloyed hot-dip galvanized steel sheet having good adhesion, it is necessary to keep the iron content within the above range by appropriately adjusting the alloying treatment time.

Next, the inventor has stated that as a condition that greatly affects the plating adhesion, it is necessary to raise the temperature and cooling rate during alloying to a certain value or more, and to perform high-speed heating and high-speed cooling. Also found. As the heating rate, after adjusting the amount of deposition performed following plating, heating at a heating rate of 30 ° C./sec or more at the time of heating to the alloying temperature minimizes the Г phase generation during alloying. It becomes possible to suppress. The reason is as follows. That is, when the rate of temperature rise during alloying is low, the ζ phase is likely to be generated at 470 ° C.
The residence time in the low-temperature region of less than becomes longer, alloying proceeds in a state where the ζ phase is generated, and the Г phase is easily generated at the interface between the plating layer and the steel sheet for the same reason as described above. That's why. Further, even if alloying is successfully completed, if the cooling rate is not sufficiently high, a ζ phase is likely to be generated. In other words, since it passes through the temperature region where the ζ phase can exist for a long time, the transformation from the δ ₁ phase to the ζ phase occurs, and the Г phase is easily generated for the same reason as described above. . Therefore, the cooling rate during alloying is also 30 ° C./sec.
It is desirable to cool at the above-mentioned high cooling rate.

By the way, it has not been enough to secure sufficient adhesiveness only by specifying the heating rate and the cooling rate during the alloying described above. Therefore, the inventor
Focusing on optimizing the properties of the steel sheet before plating, it has been found that by combining this with the above-mentioned heating rate and cooling rate during alloying, the adhesion can be significantly improved. The optimization of the properties of the steel sheet can be made by winding the steel sheet at a certain temperature or higher during hot rolling. That is, it has been clarified that by winding at a temperature of 600 ° C. or higher, the adhesion of the alloyed galvanized steel sheet is remarkably improved. The reason is considered as follows. When the winding temperature of the steel sheet during hot rolling is set to 600 ° C. or higher, a grain boundary oxide layer is formed below the iron-based oxide layer generated during hot rolling. This grain boundary oxide layer is
Oxides are present in the form of oxides dispersed in the surface layer of the steel sheet. This grain boundary oxide layer significantly suppresses the alloying element such as Mn added as a component in the steel from being concentrated on the surface during the annealing or raising the temperature in the hot-dip galvanizing production process, and during the subsequent alloying treatment. It has the effect of improving the iron diffusion rate.
Therefore, for the above-mentioned reason, by defining the winding temperature at the time of hot rolling, the alloying speed is increased as a whole, and the effect of increasing the temperature increasing speed at the time of the alloying treatment is more effective.

Specific means for increasing the temperature at the time of alloying to 30 ° C./sec or more include gas heating, induction heating, and the like. In the present invention, the means is not particularly limited as long as the heating rate can be maintained at 30 ° C./sec or more. Further, after the alloying treatment, the cooling rate up to 420 ° C. is increased by 30 ° C./sec.
In order to ensure the above, means such as spraying water and cooling such as gas cooling, mist cooling or fog cooling, etc., is mentioned. The means is not particularly limited.

[0015] The present invention is realized by defining the winding temperature during hot rolling, but the process from hot rolling to hot-dip coating is not particularly limited. Any of a cold-rolled steel sheet and a hot-rolled steel sheet may be used.

[0016]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be described below with reference to embodiments. Ti
-Nb-based ultra-low carbon mild steel sheets were produced on actual lines at each winding condition temperature shown in Table 1. This material was pickled and cold-rolled on a laboratory scale, and subjected to alkaline electrolytic degreasing and hydrochloric acid pickling, then annealed under the following conditions, and hot-dip galvanized using a vertical hot-dip coating apparatus in the laboratory. . (Annealing conditions) Atmosphere: 5% H ₂ -N ₂ Dew point: -40 ° C Heating rate: 10 ° C / sec Annealing temperature: 800 ° C Cooling time: 20 sec. However, cooling after annealing was performed by blowing an atmosphere gas onto the steel sheet. In addition, the temperature of the steel sheet was measured by a thermocouple attached to the steel sheet, and the cooling rate immediately before the infiltration of the plating bath, at which the cooling rate was the slowest, was detected. (Hot-dip plating conditions) Plating bath concentration: Al: 0.14 wt%, Fe: 0.04 wt%, Pb: 0.008 wt% Bath temperature: 475 ° C Penetration plate temperature: 475 ° C Immersion time: 1 sec. Coating weight: 50 g / m ² Next, more galvanized steel sheet manufactured by the method, and charged into a heating furnace of direct energization method performs alloying at a constant temperature, and cooled by blowing nitrogen gas . Adjustment of the heating rate was performed by appropriately changing the input power, and adjustment of the cooling rate was performed by appropriately changing the flow rate of the nitrogen gas. Table 1 shows that the rate of temperature increase, the alloying temperature, the alloying time at a plate temperature of 420 ° C. or more, and the plate temperature after the alloying treatment were 420 ° C.
The cooling rate up to ° C. is also described.

[0017]

[Table 1]

The plating layer of the alloyed hot-dip galvanized steel sheet obtained as described above was dissolved in hydrochloric acid containing an inhibitor, and the iron content in the plating layer was analyzed by ICP emission spectroscopy. Shown in Further, as a performance test of the plating layer, a sample size: width 40 mm × length 1
The plating stripped from the tape when the 00 mm sample was bent back by 90 degrees was subjected to a powdering test for X-ray fluorescence analysis as the amount of plating peeling, and the cps (count / sec) as an index, and this cps value is also shown in Table 1.

[0019]

As shown in Table 1, the winding temperature during hot rolling, the cooling rate after annealing, the rate of temperature rise during alloying,
By optimizing the alloying temperature, the cooling rate, and the iron content in the plating layer, the plating adhesion of the galvannealed steel sheet can be significantly improved. ADVANTAGE OF THE INVENTION According to this invention, it has become possible to manufacture an alloyed hot-dip galvanized steel sheet that is much more excellent in plating adhesion than in the past.

Claims (2)

[Claims] In the production of an alloyed hot-dip galvanized steel sheet, the winding temperature during hot rolling of a steel sheet as a material is set to 600 ° C.
As described above, after hot-dip plating, an alloying process is performed to raise the temperature to 470 to 510 ° C. at a rate of 30 ° C./sec or higher and to maintain the temperature in this temperature range, so that Zn—F having an iron content of 8 to 11 wt% is obtained.
A method for producing an alloyed hot-dip galvanized steel sheet having excellent plating adhesion, characterized by obtaining an e-alloy plating layer. 2. After the alloying treatment, the temperature is increased up to 420 ° C.
The method for producing a galvannealed steel sheet having excellent plating adhesion according to claim 1, wherein the steel sheet is cooled at a cooling rate of 0 ° C / sec or more.