JPH04276057A - Manufacture of high si-containing high tensile strength galvannealed steel sheet having good plating adhesion - Google Patents

Abstract

PURPOSE:To offer a method for manufacturing a high Si-contg. high tensile strength galvannealed steel sheet having good plating adhesion. CONSTITUTION:A high tensile strength steel sheet contg. Si in the range of >=0.2% is oxidized at 0.9 to 1.2 combustion air ratio for suppressing the surface concentration of Si by the oxidized film of iron, is reduced so as to regulate the thickness of the oxidized film of iron to <=500Angstrom by a reducing zone contg. hydrogen, is subjected to hot dip plating treatment by using a galvanizing bath contg. Mn and Al which are reducing elements in the range of 0.01 to 0.2% and the balance Zn for reducing the remaining oxidized film and is furthermore subjected to heating and alloying treatment. By this manufacturing method, plating treatment can be executed in a state where surface concentration is perfectly suppressed owing to the presence of the oxidized film of iron till the dipping of the steel sheet into the plating bath, by which the galvannealed steel sheet having good plating adhesion can be obtd.

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 high-Si-containing, high-strength alloyed hot-dip galvanized steel sheet with good coating adhesion.

0002

2. Description of the Related Art In recent years, efforts have been made to reduce the weight of automobiles from the viewpoint of exhaust gas regulations. One effective way to reduce the weight of a car body is to reduce the thickness of the outer panels, but in order to ensure safety, it is necessary to increase the strength of the panels accordingly. . When a high-Si-containing high-strength steel plate is used as an outer plate, poor plating properties become a problem in hot-dip plating. As a conventional technique for improving plating properties, according to Japanese Patent Application Laid-Open No. 55-122865, the thickness of an oxide film on the steel surface was increased to 4.
A known method is to oxidize the film to a thickness of 00 to 10,000 Å, then annealing it in an atmosphere containing hydrogen, and plating. According to this method, the surface concentration of Si can be suppressed by the iron oxide film, and the generation of silicon oxide that inhibits plating adhesion can be suppressed, so that the plating adhesion can be improved. However, in the conventional technology, it is practically impossible to adjust the reduction time of the iron oxide film; if the reduction time is too long, Si will concentrate on the surface, and if it is too short, the iron oxide film will form on the steel surface. remains, so there is a problem that the poor plating quality cannot be completely resolved after all.

0003

[Problems to be Solved by the Invention] As a result of extensive research into plating treatments for high-strength steel sheets, the following findings were obtained. The conventional technique predicts the reduction time of iron oxide and immerses the steel plate in the plating bath immediately after the reduction is completed. This is based on the assumption that no matter how thin the iron oxide film becomes, as long as iron oxide exists on the surface of the steel sheet, surface concentration of Si will be suppressed. However, in reality, the iron oxide film is unevenly distributed on the surface of the steel sheet, and when the oxide film becomes thin, Si gradually begins to concentrate on the surface before it completely disappears, making it impossible to completely eliminate plating defects. There is a problem with this. After oxidizing in a direct-fired furnace with a combustion air ratio of 0.9 to 1.2, the steel plate is reduced to an oxide film of 500 Å or less in a reduction furnace containing hydrogen, and then the steel plate is treated with Mn of 0.01 to 2%.
The remaining oxide film is reduced by the Mn and Al in the bath by hot dipping using a galvanizing bath containing 0.01 to 2% Al and the balance Zn. In the method of the present invention, an oxide film of several hundred angstroms of iron exists on the surface of the steel sheet until it is immersed in the plating bath, so surface concentration of Si can be completely suppressed, resulting in very good plating adhesion. You can get steel plates. Plating adhesion is a very important property in hot-dip galvanized steel sheets, which are often used for automobile steel sheets, because it directly affects corrosion resistance. The present invention has been accomplished based on the above findings.

0004

[Means for Solving the Problems] The gist of the present invention is to provide a steel plate containing Si in a range of 0.2% or more with a combustion air ratio of 0.9%.
~1.2 After oxidation in a direct-fired furnace and reduced in a hydrogen-containing reduction furnace, the steel plate was reduced to 0.01 to 2% Mn and 0.0% Al.
A high-Si-containing, high-tensile-alloyed hot-dip galvanized steel sheet with good plating adhesion, characterized in that it is hot-dipped using a galvanizing bath consisting of 1 to 2% Zn and the balance is Zn, and is further heat-alloyed. This is the manufacturing method.

[0005]

[Operation] The present invention will be explained in detail below. The method of the present invention is S
In order to suppress the surface concentration of iron, an oxide film of iron is generated in a direct-fired furnace, and the oxide film is reduced to less than 500 Å in a reduction furnace, and the aluminum and manganese in the plating bath are removed. The remaining oxide film is reduced. The advantage of this method is that since an iron oxide film exists on the surface of the steel plate until the steel plate is immersed in the plating bath, plating can be performed with the surface concentration of Si completely suppressed, resulting in improved plating adhesion. The advantage is that it is possible to obtain extremely high-quality steel sheets. However, the combustion air ratio in a direct-fired furnace or non-oxidizing furnace must be 0.9 or more to generate a sufficient iron oxide film to suppress Si, and if it exceeds 1.2, the oxidation of the iron produced Since the film thickness is too thick and cannot be completely reduced, oxidation should be performed within the range of 0.9 to 1.2. Furthermore, the steel plate is heated and alloyed. The method of the present invention completely suppresses the surface enrichment of Si, so it can be heated and alloyed. However, when heat-alloyed using the conventional method, the defective plating area spreads, and the corrosion resistance deteriorates considerably. I end up. The heating temperature at this time is 300℃~
A range of 900°C is preferred. Both Mn and Al in the bath are
0.01% or more is necessary to reduce the iron oxide film, and if it exceeds 2%, the alloying rate during heating alloying will be reduced, so it should be in the range of 0.01 to 2%.

[0006]

[Examples] Examples are shown in Table 1. The test method is as follows (
1) Plating adhesion test Plating adhesion was evaluated by a powdering test using 60° V bending. ◎: 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 3mm (2) Corrosion resistance test Salt water spray test according to JIS Z 2371 200
Comparative investigation of the amount of plate thickness reduction conducted continuously for 0 hours. ◎: The amount of decrease in plate thickness is minimal.○: The amount of decrease in plate thickness is small.△: The amount of decrease in plate thickness is large. (3) Plating wettability test Plating wettability was evaluated based on the degree of occurrence of poor plating properties in the actual line. ○: No plating defects △: Pinhole-like plating defects ×: Plating defects with a diameter of 1 mm or more 1 to 5 in Table 1 indicate the Si concentration in the steel sheet from 0.1 to 0.5.
%, 6 to 10 is a non-oxidizing furnace middle plate temperature of 3
00 to 800°C, 11 to 12 are non-oxidizing furnace air ratios changed to 0.95 to 1.2, 13
- 17 are those in which the reduction furnace plate temperature was changed from 300 to 800°C, 18 to 21 are those in which the reduction time is changed to 10 seconds to 2 minutes, and 22 to 25 are those in which the hydrogen concentration in the reduction furnace is changed to 20 to 80%. For 26 to 30, the plate temperature in the heating alloying furnace was changed to 3.
00 to 700°C, 31 to 36, the heating rate of the heating alloying furnace was 5 to 60°C/sec, 37 to 41, the aluminum concentration in the bath, and 42 to 50, the manganese concentration in the bath. thing. 51 uses HOT material, 52-5
4 is a comparative example. No. 52 does not contain Mn in the bath, No. 53 does not contain Al in the bath, and No. 54 has a low combustion air ratio, resulting in poor plating adhesion, corrosion resistance, and plating wettability.

[0007]

[Table 1A]

[0008]

[Table 1B]

[0009]

[Effects of the Invention] The method of the present invention has the following effects. Si is immersed in a plating bath with the iron oxide film remaining.
Since the plating treatment can be carried out in a state where the surface concentration of .

Claims (1)

[Claims] Claim 1: After oxidizing a steel plate containing Si in a range of 0.2% or more in a direct fire furnace with a combustion air ratio of 0.9 to 1.2,
After reducing the steel plate in a reduction furnace containing hydrogen, the Mn content of the steel plate was reduced to 0.0.
High Si content with good plating adhesion, characterized by performing hot-dip plating using a galvanizing bath consisting of 1% to 2% Al, 0.01 to 2% Al, and the balance Zn, and further heat alloying treatment. A method for manufacturing high tensile strength alloyed hot-dip galvanized steel sheet.