JP2653315B2 - Manufacturing method of galvanized steel sheet with excellent weldability - Google Patents

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 galvanized steel sheet which is excellent in weldability, especially in continuous spotting in spot welding.

[0002]

2. Description of the Related Art In recent years, in order to enhance the durability of an automobile body, the introduction of galvanized steel sheet into the body has been promoted. As is well known, the body of an automobile is assembled by welding, mainly spot welding. However, the spot weldability of a galvanized steel sheet, particularly the continuous hitting property, is significantly inferior to that of a normal steel sheet, resulting in an increase in the cost of the welding tip.

It is well known that it is effective to form a thick oxide film on the surface of a plating layer in order to improve the continuous hitting property of a zinc-based plated steel sheet. The oxide film has a higher melting point than the zinc-based metal forming the plating layer, and this intervenes between the plating layer and the electrode chip to prevent chip wear. As a method for forming a thick oxide film on the surface of the plating layer, a method of performing anodic oxidation in an alkaline solution is disclosed in JP-A-1-149996. Cathodic electrolysis is performed using a solution containing an oxidizing agent. A method for performing anodic electrolysis is described in JP-A-2-47294 and JP-A-2-47.
293, JP-A-2-190496 and the like.

[0004]

However, these methods require the installation of an anodic oxidation cell or a cathodic oxidation cell at the time of implementation, and require enormous equipment costs. Furthermore, even after the cell is installed, the cost of electric power increases due to the electrolysis, and the plated steel sheet produced becomes extremely expensive.

[0005] An object of the present invention is to provide a method for producing a galvanized steel sheet having excellent continuous hitting properties in spot welding at low cost.

[0006]

Means for Solving the Problems If an oxide film is formed thickly on the surface of the plating layer of a zinc-based plated steel sheet, as described above, the spot weldability is improved. Is active, and the surface of the plating layer is oxidized even in a normal zinc-based plated steel sheet. However, the oxide film is formed immediately after plating, and prevents oxidation in a thin state, so that sufficient spot weldability cannot be ensured.

The present inventors consider that it is effective to utilize the active force originally possessed by the plating layer in order to manufacture a zinc-based plated steel sheet having excellent spot weldability at low cost. Research continued on specific means. as a result,
It was found that it is effective to grind and activate the plating layer surface of the plated steel sheet.

FIG. 1 shows the case where the plating layer surface of the hot-dip galvanized steel sheet is ground and the case where it is not ground.
The result of having investigated the oxidation state of the surface by EPMA is shown.
As is clear from the figure, the newly formed oxide film on the surface after grinding is extremely thicker than the oxide film formed immediately after plating. The reason is that the zinc plating surface having a relatively high flatness is made uneven by grinding, and the surface area of the plating increases.

When the high melting point oxide film is formed on the surface of the plating layer, even if the plating layer is heated to a high temperature during welding, the diffusion of the plating layer into the electrode chip is prevented, and the diameter of the electrode is reduced. Expansion can be suppressed.

FIG. 2 shows the contact resistance when two hot-dip Zn-coated steel sheets are stacked and pressed with an electrode tip, with and without grinding the plating layer surface of the plated steel sheets. . The pressure was 250 kgf and the electrode was a double R type (5.0 mm diameter). By grinding the surface of the plating layer and newly forming a thick oxide film on the surface, the contact resistance increases. Particularly, the contact resistance is higher when both surfaces of the plated steel sheet are ground and the oxide film is formed again on both surfaces than when single-side grinding is performed. As a result, the decrease in the current density is suppressed, and the continuous spotting property in spot welding is improved as shown in Table 1.

[0011]

[Table 1]

The method of the present invention is based on the above findings. After a steel sheet is subjected to zinc plating, the surface of the plating layer is activated by grinding it with 0.1 to 16 g / m ² and the activation is performed. By re-forming an oxide film on the surface thus formed using the activity of the plating layer, a galvanized steel sheet having excellent continuous spotting properties in spot welding can be manufactured at low cost.

[0013]

In the method of the present invention, the galvanized steel sheet includes any of a hot-dip coated steel sheet and an electroplated steel sheet. Regarding the plating composition, in addition to pure Zn,
Including those to which alloying elements such as Ni, Al, and Mn are added, and those that are alloyed by heat treatment after plating are also included.

The plating layer surface of the zinc-based plated steel sheet can be ground using a general-purpose grinding means such as a roll brush, and can be performed not only in the atmosphere but also while contacting the plating layer surface with water. By this grinding, the surface of the plating layer is activated, and a thick oxide film is formed on the ground surface by the active force of the plating layer, thereby improving the continuous spotting property in spot welding. Further, the plated steel sheet may be heated to promote oxidation of the ground surface. The heating temperature in that case is preferably 400 ° C. or less in consideration of the melting point of the zinc-based metal.

When the grinding amount is less than 0.1 g / m ² , the surface of the plating layer is not sufficiently activated and a newly formed oxide film is insufficient, so that the grinding amount is 0.1 g / m ² or more. However,
Excessive grinding leads to a lack of a plating layer. Since common adhesion amount of the zinc-based plated steel sheet is 20 to 120 g / m ^2, in order to leave also sufficient plating layer after grinding, the upper limit of the amount of grinding is set to 16g / m ^2. Particularly preferred grinding amount is 1 to 5
g / m ² .

FIG. 3 is a cross-sectional view schematically showing a laminated structure of a galvanized steel sheet manufactured by the method of the present invention. By grinding the surface of the plating layer 2 laminated on the surface of the base material 1, a thick oxide film 3 is formed on the surface.
This oxide film 3 improves continuous spotting properties in spot welding. FIG. 4 shows the spot welding.

When the galvanized steel sheet is a double-sided steel sheet, grinding may be performed on either one side or both sides. Also in the case of single-side grinding, if the oxide film 3 thickly formed on the ground surface is brought into contact with the electrode tip 4, the electrode tip 4
Diffusion of metal to the surface is suppressed. However, the contact resistance is large in the case of double-side grinding, and the overall continuous hitting property is superior in the case of double-side grinding.

[0018]

Embodiments of the present invention will be described below.

The thickness is 0.8 mm and the amount of adhesion is 60 per side.
In the production of a double-sided hot-dip Zn-plated steel sheet of g / m ² , the surface of the plated layer after plating was dry-ground in the air, and the spot weldability of the plated steel sheet after the grinding was examined. For grinding,
Using a grinding machine in which three nylon brush rolls having wire diameters of 0.8 mm, 0.6 mm, and 0.4 mm were arranged in tandem, SiC was also used as an abrasive. Table 2 shows the grinding conditions. Further, the spot weldability was evaluated by the number of spots (the number of weldings that can continue sound welding) when spot welding was repeated under the conditions shown in Table 3. A similar survey was conducted with a sheet thickness of 0.
8 mm, double-sided electric Z with an adhesion amount of 30 g / m ² per side
This was also performed for n-plated steel sheets. The survey results are shown in Tables 4 and 5.

[0020]

[Table 2]

[0021]

[Table 3]

[0022]

[Table 4]

[0023]

[Table 5]

As can be seen from Tables 4 and 5, the number of continuous dots exceeds 10,000 in the welding of the base material.
It decreases sharply to 0 times and 1800 times by electroplating. But,
When the surface of the plating layer is ground, the continuous hitting property is greatly improved, and the improvement effect is larger in the case of double-side grinding. However, when the grinding amount is small, the improvement effect is small. In addition,
In the case of single-side grinding, as shown in FIG. 4, the ground surface was brought into contact with the electrode tip.

[0025]

As is apparent from the above description, the method for producing a zinc-based plated steel sheet according to the present invention can greatly improve the continuous hitting property in spot welding, and has a great effect in reducing the cost of electrode tips. Further, since a simple means of grinding the surface of the plating layer is used, the equipment cost for implementation is low, the running cost is low, and the production cost of the plated steel sheet is not significantly affected.

[Brief description of the drawings]

FIG. 1 is a graph showing the effect of grinding a plating layer surface on the formation of an oxide film.

FIG. 2 is a table showing the effect of grinding of a plating layer surface on contact resistance during spot welding.

FIG. 3 is a schematic view showing a laminated structure of a galvanized steel sheet manufactured by the method of the present invention.

FIG. 4 is a schematic view showing spot welding of a galvanized steel sheet manufactured by the method of the present invention.

[Explanation of symbols]

 1 Base material 2 Plating layer 3 Oxide film

Claims (1)

(57) [Claims] After a zinc-based plating is applied to a steel sheet, the surface of the plating layer is activated by grinding it with 0.1 to 16 g / m ² ,
A method for producing a galvanized steel sheet excellent in weldability, characterized in that an oxide film is newly formed on the activated surface by using the activity of a plating layer.