JPH0681096A - Production of si-containing high strength galvannealed steel sheet good in plating adhesion - Google Patents

Abstract

PURPOSE:To reduce the cost of production by subjecting a high strength steel sheet having specified P concn. to alloying under heating under specified conditions. CONSTITUTION:At the time of continuously galvanizing a high strength steel sheet having 0.2 to 2.0% P content and successively executing alloying under heating, it is oxidized in an atmosphere having 0.9 to 1.2 combustion air ratio, is reduced in the subsequent reducing zone so that the thickness of an iron oxide film will remain in the range of 200 to 2000Angstrom , is thereafter subjected to galvanizing treatment by using a galvanizing bath contg. <0.05% Al and 0.05 to 1.0% Mn, and the balance Zn, and is furthermore subjected to alloying treatment under heating. In this way, the Si-contg. high strength galvannealed steel sheet can be produced without increasing the equipment for preplating or the like.

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 regulation, an alloy based on a high-strength steel plate as an outer or 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 an oxide film on the steel surface is 400 in an oxidation-free 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 iron is oxidized on the steel surface. Since the film remains, there is a problem that the 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 property 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 two new findings were found. (1) The reduction rate of iron oxide in the Mn-added bath is higher than that in the Al-added bath. (2) In the Mn-added bath, if the Al concentration exceeds 0.05%, the reduction reaction rate of the iron oxide is significantly reduced. Therefore, in the method of the present invention, a suitable bath component is Al 0.0
A zinc plating bath containing less than 5%, Mn 0.05 to 1.0%, and the balance Zn. The present invention has been completed based on the above findings.

[0008]

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, when the iron oxide film is formed, the combustion air ratio in the oxidation zone needs to be 0.9 or more in order to form an iron oxide film sufficient to suppress the formation of Si oxide. The oxide film cannot be formed. On the other hand, when the combustion air ratio exceeds 1.5%, the iron oxide film formed in the oxidation zone is too thick to be completely reduced in the next reduction zone and 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, a suitable bath component is Al less than 0.05%, Mn 0.
It is a zinc plating bath containing 05 to 1.0% and the balance Zn.

[0010]

[Examples] Table 1 shows an example of an example manufactured by using a conventionally used continuous hot dip galvanizing line. The test method is as follows: (1) Appearance of plating ◎: Alloyed uniformly, good appearance. ○: There is unevenness in alloying. X: There is a defective plating part.

(2) Powdering resistance test The plating adhesion was evaluated by a powdering test by bending at 60 ° V. ◎: Peeling amount 0 mm ◯: Peeling amount more than 0 mm, 1 mm or less △: Peeling amount more than 1 mm, 3 mm or less ×: Peeling amount more than 3 mm

(3) Corrosion resistance test A salt spray test according to JIS Z 2371 was continuously conducted for 1000 hours to carry out a comparative examination of sheet thickness reduction amounts. ◎: Minimal reduction in thickness ○: Small reduction in thickness △: Large reduction in thickness

[0013]

[Table 1]

1 to 5 in Table 1 indicate that the P concentration in the steel sheet is 0.
2 to 2.0%, 6 to 14 change the oxidation zone air ratio to 0.95 to 1.5, and 15 to 24 the iron oxide film thickness on the steel plate surface at the snout outlet side. In the example of changing from 200 to 2000, 25 to 29 have an Al concentration in the plating bath of 0 to 0.
In the case of changing to 05%, 30 to 39, the Mn concentration in the bath is 0.1
Example of changing to ~ 1.0%, 40 is an example applied to Hot material,
42-44 are comparative examples. 42 is an example of insufficient plating in the plating bath due to insufficient Mn concentration, resulting in poor plating performance. 43 is insufficient reduction in the plating bath due to high Al concentration in the bath, resulting in poor plating performance. No. 44 is an unsuitable example because the air ratio of the oxidation zone is low.

[0015]

According to the method of the present invention, a P-containing high-strength steel sheet can be manufactured without installing new equipment such as pre-plating equipment.

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 then reduced so that the iron oxide film thickness remains in the range of 200 to 2000Å in the subsequent reduction zone, Al is less than 0.05%, M
Production of a Si-containing high-strength hot-dip galvanized steel sheet characterized by performing hot-dip galvanizing treatment using a galvanizing bath containing n of 0.05 to 1.0% and the balance Zn, and further heat alloying treatment. Method.