JPH05263210A - Production of hot-dip zinc-aluminum alloy-plated steel sheet excellent in spot weldability - Google Patents

Abstract

PURPOSE:To provide a hot-dip zinc-aluminum alloy-plated steel sheet excellent in spot weldabillity by dipping a steel sheet in a galvanizing bath having specified contents of Al and Ti, heat-treating the steel sheet to cause alloying and diffusing the alloy so that the Fe content in the plating layer is specified. CONSTITUTION:A steel sheet contg. 3.0-7.0wt.% Al, 0.001-2.0wt.% Ti and the balance Zn is dipped in a hot dipping bath to provide a zinc plating layer on the steel sheet surface. The plated steel sheet is then placed in a heating furnace, heat-treated at about 450-650 deg.C for about 30 to 90 sec and alloyed, and the alloy is diffused so that the Fe content in the plating layer is controlled to >=5wt.%. A hot-dip zinc-aluminum alloy-plated steel sheet excellent in spot weldability is obtained in this way.

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 hot-dip zinc / aluminum alloy plated steel sheet having excellent spot weldability, and a method for producing a hot-dip zinc / aluminum alloy plated steel sheet having excellent spot weldability. .

[0002]

2. Description of the Related Art Recently, the demand for surface-treated steel sheets has been remarkably increasing in order to prolong the period of use in automobiles, household electric appliances, building materials and the like.

Among various surface-treated steel sheets, Zn-5 wt% Al hot-dip galvanized steel sheet is particularly excellent in corrosion resistance and workability as compared with general hot-dip galvanized steel sheet. It is beginning to be used primarily in building materials of the type that guarantees that rust will not occur.

However, since the Zn-5 wt% Al hot-dip galvanized steel sheet has a melting point of 400 ° C. or less, which is lower than that of a general zinc plating layer, the plating layer is not formed during spot welding. It is easily melted and the contact area of the steel plate surface is expanded, so that the current density is reduced.

As a countermeasure for this, it is necessary to increase the current density, and there is a problem that the spot weldability is inferior due to a short electrode life, and this improvement is required.

[0006]

DISCLOSURE OF THE INVENTION In view of the problems of the conventional surface-treated steel sheet described above, the present invention has conducted earnest research by the present inventors, and as a result, imparts excellent spot weldability to the surface-treated steel sheet. Therefore, a method for producing a hot dip zinc-aluminum alloy-plated steel sheet in which Fe is diffused in the plating layer has been developed.

[0007]

The features of the method for producing a hot-dip zinc-aluminum alloy-plated steel sheet excellent in spot weldability according to the present invention are that Al 3.0-7.0 wt% and Ti 0.001 ~ 2.0w
This is to immerse the steel sheet in a hot dip plating bath containing t% and the balance Zn, and then to perform heat treatment for diffusing the Fe content in the plating layer to 5 wt% or more.

The method for producing a hot-dip galvanized aluminum alloy-plated steel sheet having excellent spot weldability according to the present invention will be described in detail below.

First, in the method for producing a hot-dip zinc-aluminum alloy-plated steel sheet having excellent spot weldability according to the present invention, the components contained in the plating bath used and the component proportions will be described.

The content of Al in the plating bath is 3.0 wt%
If it is less than 7, good corrosion resistance cannot be obtained, and 7.
If the content exceeds 0 wt%, the mutual diffusion between the plating layer and the steel sheet is not completely performed. Therefore, A in the plating bath
The l content is 3.0 to 7.0 wt%.

The content of Ti in the plating bath is 0.001w.
If it is less than t%, the effect of increasing the mutual diffusion rate between the plating layer and the steel sheet is small, the mutual diffusion between the plating layer and the steel sheet is not completely performed, and if it is contained in excess of 2.0 wt%. A large amount of intermetallic compound called dross is generated in the plating bath and is taken into the plating layer to deteriorate the quality of the plating layer. Therefore, the Ti content in the plating bath is set to 0.001 to 2.0 wt%.

Further, the Fe content in the plating layer is 5 wt%
If it is less than Fe, Fe does not diffuse to the surface of the plating layer, so that the effect of improving spot weldability cannot be achieved. Therefore, the Fe content in the plated layer is set to 5 wt% or more.

That is, the inventor of the present invention immerses a steel sheet in a hot dip zinc-aluminum alloy plating bath to form a plating layer on the surface of the steel sheet, and then heat-treats it so that the Fe content of the steel sheet in the plating layer is 5 wt%. The above-mentioned diffusion improves the spot weldability.

In the method for producing a hot-dip zinc-aluminum alloy-plated steel sheet having excellent spot weldability according to the present invention, other than the above-described components contained in the plating bath to be used, it may be for other purposes, or Even if an impurity element is contained, there is no particular problem as long as the effect of the present invention can be recognized.

Next, the heat treatment of the steel sheet having the plating layer in the method for producing a hot-dip zinc-aluminum alloy-plated steel sheet having excellent spot weldability according to the present invention will be described.

At present, most continuous hot-dip galvanizing facilities are equipped with a heating furnace for heat-alloying a plated layer after plating in order to produce a galvannealed steel sheet. In this heating furnace, the furnace length is 30 m to 50 m, and the maximum temperature of the steel sheet in the furnace is 450 to 650.
The heating time is 30 seconds to 1 minute 3
It is 0 seconds. This is sufficient to form a diffusion alloy in the zinc plating layer (Al content in the plating bath: 0.1 to 0.2 wt%) and the base steel sheet, but as the Al content increases, the diffusion alloying rate increases. As it becomes slower, for example, 5 wt% A
The plating containing 1 does not have the effect of diffusing Fe in the steel sheet into the plating layer.

Therefore, it is conceivable to lengthen the heating equipment and extend the heating time. However, since the equipment remodeling cost becomes huge and it is of no practical significance, the equipment is not remodeled.
Paying attention to the fact that the diffusion rate of Fe in the plating layer can be increased by improving the contained components and component ratios of the plating bath, and Ti described above in 0.001 to 2.
By including a trace amount of 0 wt%, the heat diffusion treatment can be sufficiently performed even in the present hot dip galvanizing line equipment.

[0018]

[Examples] Examples of a method for producing a hot-dip zinc-aluminum alloy-plated steel sheet having excellent spot weldability according to the present invention will be described.

[0019]

【Example】

Specimen: Plate thickness 0.8 mm, aluminum killed steel plate Plating bath: Bath temperature 460 ° C. Immersion time 2 seconds Heat treatment: Heating temperature 600 ° C. Heating time 60 seconds Plating weight: 100 g / m ² Table 1 shows the plating bath. The content and ratio of the components, the presence or absence of heat treatment, the Fe content in the plating layer, the diffusivity, and the weldability are shown.

[0020]

[Table 1]

Diffusivity: The surface appearance of the plating layer was visually determined. Judgment Criteria ○ Alloyed to the surface (silver white) △ Partially alloyed (partly silver white) × Not alloyed (metallic luster) Weldability: Electrode shape ・ ・ ・ 16φ dome shape Pressing force ・ ・ ・ 220kgf Welding Time: 12 cycles (60 Hz) Welding current: Under welding conditions of 11 kA, two galvanized steel sheets were combined to perform a spot welding continuous spot test, and the nugget diameter after shear tension was measured. 4.
The electrode life was defined as the dot when the distance reached 5 mm. Judgment Criteria ○ 2000 points or more △ 1000 to 2000 points × 1000 points or less Even if titanium-containing steel is used as a steel other than the aluminum killed steel described above, the same effect can be obtained.

[0022]

As described above, the method for producing a hot-dip zinc / aluminum alloy-plated steel sheet having excellent spot weldability according to the present invention has the above-described structure, and therefore has a high spot-weldability. It is possible to manufacture an alloy-plated steel sheet.

Claims (1)

[Claims] 1. Al 3.0-7.0 wt%, Ti 0.001-2.0 w
After the steel plate was dipped in a hot dip plating bath containing t% and the balance Zn, heat treatment was performed to diffuse the Fe content in the plating layer to 5 wt% or more, which was excellent in spot weldability. Method for manufacturing hot dip zinc / aluminum alloy plated steel sheet.