JPH06172953A - Production of si-containing high strength galvannealed steel sheet - Google Patents

Abstract

PURPOSE:To provide the method for stably producing an Si-contg. high strength galvannealed steel sheet. CONSTITUTION:At the time of continuously applying galvanizing to a high strength steel sheet having 0.2 to 2.0% Si content and successively executing heating and alloying, it is oxidized in an oxidizing zone in an atmosphere having 0.9 to 1.2 combustion air ratio. In the subsequent reducing zone, it is reduced so that an iron oxidized film will be allowed to remain in the range of 200 to 2000Angstrom thickness, and after that, hot-dip plating treatment is executed by using a galvanizing bath contg. 0 to 10% Al and 1.0 to 10% Mn, and the balance Zn. Furthermore, at the time of executing the galvanizing treatment, the steel sheet is applied with vibration, and successively, heating and alloying treatment is executed, by which the Si-contg. high strength galvannealed steel sheet can stably be produced.

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 Si-containing high strength galvannealed steel sheet.

[0002]

2. Description of the Related Art From the viewpoint of exhaust gas regulations, an alloy based on a high strength steel plate as an outer plate or an inner plate of an automobile for the purpose of responding to the recent growing demand for weight reduction of automobiles and preventing corrosion. The demand for using hot-dip galvanized steel sheets is increasing. Among the high-strength steel sheets, the high Si-containing high-strength steel sheet has a problem of poor plating property. As a conventional technique for improving the plating property, according to Japanese Patent Application Laid-Open No. 55-122865, the thickness of the oxide film on the steel surface is 400 in a non-oxidizing furnace.
A method is known in which after oxidizing so as to have a thickness of 10,000 Å, it is annealed in an atmosphere containing hydrogen and plated. The purpose of this method is to positively generate an iron oxide film in the oxidation zone to suppress the formation of Si oxide which hinders the plating adhesion, and to improve the plating adhesion.

However, it is practically difficult to control the reduction time of the iron oxide film in this conventional technique. If the reduction time is too long, the surface concentration of Si is caused, and if it is too short, the steel surface is strengthened with iron. Since the film remains, there is a problem that the defective plating property cannot be completely eliminated after all, and there is a problem that the production of Si oxide cannot be completely suppressed by this technique. Therefore, a method of performing pre-plating before annealing has been proposed as disclosed in Japanese Patent Laid-Open No. 2-38549.
However, since the pre-plating method requires plating equipment,
It cannot be adopted if there is no space. In addition, the installation of pre-plating equipment raises the problem of increased production costs.

[0004]

DISCLOSURE OF THE INVENTION The present invention proposes a method for producing a high Si-containing high-strength hot-dip galvanized steel sheet having good plating properties without installing new equipment such as pre-plating equipment. is there.

[0005]

[Means for Solving the Problems] As a result of intensive studies on plating treatment of high strength steel sheet, the following findings were obtained. The prior art predicts and controls the reduction completion time of iron oxide, and immediately after the reduction is completed, the steel sheet is immersed in the plating bath. This is based on the assumption that the generation of Si oxide is suppressed as long as the iron oxide film is present on the surface of the steel sheet, however small the iron oxide film thickness is. However, in reality, it is difficult to predict the reduction end time, and the iron oxide film is unevenly distributed on the surface of the steel sheet. There is a problem in that the location where there is no iron oxide film is generated, and the generation of Si oxide starts from that location, so that the defective plating is not completely resolved. Therefore, as a result of intensive studies on the annealing conditions and plating bath components, the present inventors have obtained the following findings.

An iron oxide film is positively generated in the oxidation zone.
Since Si does not diffuse in the iron oxide, generation of Si oxide is suppressed while the surface of the steel sheet is completely covered with the iron oxide film. However, in order to completely cover the steel plate surface, 200Å
The above oxide film thickness is required. So iron oxide film is 200
It is possible to completely prevent the generation of Si oxide by stopping the reduction at about 1000 Å and reducing the remaining iron oxide film in the plating bath. However, in order to reduce the iron oxide film in the plating bath, it is necessary to improve the reducing power of the plating bath. It has been considered that the simplest method for increasing the reducing power is to increase the Al concentration in the plating bath, but as a result of detailed study, the following findings were found this time.

When Mn is contained in the plating bath, the plating bath penetrates into the iron oxide film and peels off the iron oxide film by the process shown in FIG. That is, as shown in FIG. 1, the iron oxide film 2 on the surface of the steel sheet 1 shows a state 3 in which the plating bath containing Mn diffuses and penetrates into the iron oxide film. As a result, the state 4 in which the iron oxide film is peeled off is performed. On the other hand, the conventional process in which the iron oxide film disappears in the plating bath is completely different from the finding that it was thought to be due to the reduction reaction.

Applying this knowledge, vibration is applied to the steel sheet during plating in order to promote the peeling of the oxide film. Since the disappearance process of the iron oxide film is not a reduction reaction but peeling, the iron oxide film that is just on the steel plate due to vibration is dissipated into the plating bath, as shown in FIG. Promotes peeling of iron oxide film. As a result, the disappearance rate of the iron oxide film is improved. The present invention has been completed based on the above findings.

[0009]

The present invention will be described in detail below. The cause of plating failure is Si-M generated on the steel plate surface in the reduction zone during annealing.
It is an n complex oxide. The Si concentration in steel that can suppress the formation of Si oxide by the method of the present invention is in the range of up to 2.0%, and when the Si concentration becomes 0.2%, the steel sheet itself cannot have sufficient strength. The applicable range of the method of the present invention is 0.2 to 2.0%. First, an iron oxide film of several thousand liters is produced in the oxidation zone of a continuous hot dip galvanizing line. Since Si hardly diffuses in the iron oxide film, the generation of Si oxide is suppressed.

However, the combustion air ratio of the oxidation zone at the time of forming the iron oxide film must be 0.9 or more and less than 0.9 in order to form the iron oxide film sufficient to suppress the formation of Si oxide. In the case of, the oxide film cannot be formed. On the other hand, if the combustion air ratio exceeds 1.5%, the iron oxide film thickness formed in the oxidation zone is too thick to be reduced completely in the next reduction zone or in the plating bath, and the oxide layer becomes the plating layer. Since it remains underneath, it impairs plating adhesion. Therefore, it is necessary to adjust the combustion air ratio in the oxidation zone to the range of 0.9 to 1.2.

The iron oxide film thickness is nonuniform depending on the location, and when the iron oxide film thickness is 200 Å or less, the iron oxide film has pinholes. Therefore, when the iron oxide film thickness after reduction is 200 Å or less, Si oxide is generated in the pinhole portion, and therefore the iron oxide film thickness after reduction is 20 in order to suppress the formation of Si oxide.
Should be greater than 0Å. The iron oxide film thickness after annealing is 2
If it exceeds 000Å, the reduction cannot be completed in the plating bath, and the oxide film layer remains under the plating layer, which hinders alloying. Therefore, the iron oxide film thickness just before the immersion of the plating bath is 2
It should be adjusted to be in the range of 0 to 1000Å.

Since the oxide film is reduced in the plating bath, it is necessary to use a plating bath having a high reducing power. According to the knowledge obtained by the present inventors, the plating method uses Al 0-10.
This is a method in which a plating bath containing 0% and Mn of 1.0 to 10.0% and the balance Zn is used, and at the same time vibration is applied to the steel sheet. By using this method, it is possible to stably manufacture a steel sheet with improved plating properties. As a method of applying ultrasonic waves, there are a method of installing a sound wave source outside the plating bath and a method of installing a vibrator inside the plating bath. The oscillator of sound waves is not particularly limited.

[0013]

[Examples] FIG. 2 shows the pass rate when manufactured by using a conventionally used continuous hot dip galvanizing line as an example. The definition of product acceptance rate is as follows. If the product is defective in plating or if the product's powdering property is inspected and the peeling width is more than 3 mm, the product is rejected.
The pass rate is defined as the number of coils that did not fail when 100 coils with a weight of 20 t or more were passed. Implementation conditions and evaluation results in actual line Table 1 and Table 2
Shown in.

[0014]

[Table 1]

[0015]

[Table 2]

[0016]

As described above, according to the method of the present invention, S
The i-containing high-strength steel sheet has an extremely excellent effect that can significantly improve the production efficiency when producing the steel sheet.

[Brief description of drawings]

FIG. 1 is a diagram showing a process of peeling an iron oxide film in a plating bath containing Mn according to the present invention.

FIG. 2 is a diagram showing the relationship between the Mn concentration in the bath and the product acceptance rate.

[Explanation of symbols]

 1 Steel plate 2 Iron oxide film 3 State where plating bath diffuses and penetrates into iron oxide film 4 State where iron oxide film peels off

Claims (1)

[Claims] 1. When a high-strength steel sheet having a Si content of 0.2 to 2.0% is continuously galvanized and subsequently heat-alloyed, a combustion air ratio in an oxidation zone is obtained. 0.
After being oxidized in an atmosphere of 9 to 1.2 and reduced in the subsequent reduction zone so that the iron oxide film thickness remains in the range of 200 to 2000Å, Al 0 to 10%, Mn
Characteristically, hot-dip galvanizing treatment is performed using a zinc plating bath containing the balance of 1.0 to 10% and the balance is Zn, vibration is applied to the steel sheet when the hot dip galvanizing treatment is further performed, and then heat alloying treatment is performed. A method for producing a high-strength, hot-dip galvannealed steel sheet containing Si.