JP2754127B2 - P-containing high-strength galvannealed steel sheet - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a P-containing high strength galvannealed steel sheet.

[0002]

2. Description of the Related Art From the viewpoint of emission control, alloyed hot-dip galvanizing using a high-strength steel plate as a base for an outer or inner plate of an automobile for the purpose of responding to a demand for lighter automobiles which has recently become stronger. The demand for using steel sheets is increasing. Among the high-strength steel sheets, the P-containing high-strength steel sheets have a problem that a P oxide is generated at an interface between a plating layer and a ground iron and inhibits plating wettability, so that plating adhesion is reduced.

[0003]

SUMMARY OF THE INVENTION The present invention solves the above-mentioned problems and provides a P-containing high-strength galvannealed steel sheet having good plating adhesion.

[0004]

SUMMARY OF THE INVENTION In order to meet the needs for increasing the strength of automotive steel sheets, it is necessary to improve the plating wettability of steel sheets having a high P content. Then, as a result of studying the plating layer structure, the content of P was 0.01 to
0.2% high strength steel sheet as base metal
Iron layer with a P content of 0.01% or less in the layer, component composition in the second layer
A P-containing high-strength alloyed hot-dip galvanized steel sheet having a plating layer of Al ≦ 0.5% , Mn 0.5-10.0%, and residual Zn.

[0005]

FIG. 1 shows a P-containing high-strength galvannealed steel sheet having good plating adhesion according to the present invention. As shown in FIG. 1, the base material 3 is a high-strength steel plate having a P content in the range of 0.01 to 0.2%, and is a cold-rolled steel plate having a plate thickness of 0.2 to 2.0 mm. Here, if the P content is 0.01% or less, sufficient strength cannot be secured to meet the need for reducing the weight of automobiles, and if the P content is 0.2% or more, the steel sheet becomes brittle. Not preferred. In order to reduce the weight of automobiles, it is necessary that the thickness be at least 2 mm or less. If the thickness is less than 0.2 mm, the strength of the steel sheet is not ensured.
mm.

The symbol 2 shown in FIG. 1 represents a P content of 0.01%.
The following pure iron layer is necessary for improving plating adhesion. In this manufacturing process, first, an iron oxide film is formed in an oxidized zone in an annealing furnace of a continuous hot-dip plating line by several thousand Å. Since the substance such as P is hardly diffused in the iron oxide film, the P concentration in the iron oxide film becomes low. By reducing this, a pure iron layer having a low P concentration on the steel sheet surface can be formed.

However, when an iron oxide film is formed, the combustion air ratio in the oxidation zone is such that an iron oxide film sufficient to form a pure iron layer having a P content of 0.01% or less is formed. 0.
If it is less than 0.9, an oxide film cannot be formed. On the other hand, if the combustion air ratio exceeds 1.2%, the iron oxide film formed in the oxidation zone is too thick and cannot be reduced in the next reduction zone or plating bath. Therefore, plating wettability is hindered. Therefore, the combustion air ratio of the oxidation zone is 0.9 to 1.2.
It is necessary to adjust to the range.

Since the thickness of the iron oxide film is not uniform in some places,
When the thickness of the iron oxide film after annealing becomes 200 ° or less, the iron oxide film is partially reduced to produce a pure iron portion. Pure iron part is P
Is diffused from the inside of the steel sheet to the surface, and the P concentration increases. or,
If the iron oxide film thickness after annealing becomes 1000 ° or more, it cannot be completely reduced in the plating bath, and the oxide film layer remains under the plating layer, thereby inhibiting alloying. Therefore, the iron oxide film thickness immediately before the immersion in the plating bath should be adjusted to be in the range of 200 to 1000 °. By reducing the remaining iron oxide film in the plating bath, the formation of P oxide is prevented, and an alloyed hot-dip galvanized steel sheet having good plating adhesion can be produced. However, if iron oxide that cannot be reduced in the plating bath remains, the adhesion of the plating decreases, so that the reducing power of the plating bath needs to be high in order to reduce the iron oxide film in the plating bath. .

Conventionally, it was thought that when the iron oxide film disappeared in the plating bath, the iron oxide film was turned into metallic iron by a reduction reaction. When the iron oxide film is contained, the process in which the iron oxide film disappears in the plating bath is completely different. It was found that the higher the Mn concentration in the plating bath, the higher the reduction rate of the iron oxide film, because the plating bath depends on the rate of infiltration and diffusion into the iron oxide film. Especially in the bath M
When the n concentration exceeds 0.5%, the peeling speed of the iron oxide film is dramatically improved. Therefore, according to the knowledge obtained this time, a bath component suitable as a plating bath having a high reducing property is: Al ≦ 0.5
% , Mn 0.5 to 10.0%, and residual Zn. Thus composition of the plating layer of code 1 shown in FIG. 1 Al ≦ 0.5
% , Mn 0.5 to 10.0%, and residual Zn.

[0010]

FIG. 3 shows an example of a conventional method and an example of manufacturing using a plating line having a plating adhesion of the product of the present invention. That is, FIG. 3 is a diagram showing the relationship between the Mn concentration in the Zn bath and the powdering peeling width. It can be seen that the peeling speed of the iron oxide film is dramatically improved particularly when the concentration in the Zn bath exceeds 0.5%. . In addition, the test method evaluated by the powdering test by 60 degree V bending.

[0011]

As described above, according to the present invention, a P-containing high-strength alloyed hot-dip galvanized steel sheet having extremely excellent plating adhesion can be obtained.

[Brief description of the drawings]

FIG. 1 is a view showing a P-containing high-strength galvannealed steel sheet having good plating adhesion according to the present invention;

FIG. 2 is a view showing a process of peeling an iron oxide film in a plating bath containing Mn;

FIG. 3 is a diagram showing a relationship between a Mn concentration in a bath and a powdering peeling width.

[Explanation of symbols]

 1 plating layer 2 pure iron layer 3 base steel plate

──────────────────────────────────────────────────続 き Continued on front page (58) Field surveyed (Int.Cl. ⁶ , DB name) C23C 2/00-2/40

Claims (1)

(57) [Claims] 1. A high-strength steel sheet having a P content of 0.5 to 0.2% as a base material, and a P content of 0.1 in the first layer on the steel sheet surface.
01% or less pure iron layer, component composition Al ≦ 0.5 in second layer
% , Mn 0.5 to 10.0%, and a P-containing high-strength galvannealed steel sheet having a plating layer of Zn.