JPH07197277A - Galvanized steel sheet having excellent spot weldability - Google Patents

Abstract

PURPOSE:To obtain a galvanized steel sheet having excellent spot weldability by forming an oxidized film contg. an Ni oxide of a specific thickness as an upper layer of a galvanized first layer. CONSTITUTION:The steel sheet 1 is subjected to galvanizing 2 as the first layer. The oxidized film 3 contg. the Ni oxide of a thickness of 50 to 500Angstrom is formed on the upper layer thereof. This oxidized film 3 is obtd. by forming a galvanizing film 2 by ordinary electroplating or hot dip coating, then applying a nickel nitrate soln. thereon or subjecting the film to electrolysis, etc. As a result, the resistance of the steel sheet is enhanced and nugget formation is accelerated in a spot welding. The improvement of number of continuous welding spots is thus stably obtd.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a galvanized steel sheet having excellent spot weldability.

[0002]

2. Description of the Related Art Galvanized steel sheets are used in many fields such as automobiles, building materials and home appliances because they are superior in corrosion resistance to cold rolled steel sheets and hot rolled steel sheets. However, since the spot weldability decreases as the amount of zinc coating adhered increases, there is a strong demand for improvement in weldability, especially in automotive applications. Therefore, as a conventional technique, Japanese Patent Application Laid-Open No. 1-1
As disclosed in Japanese Patent Publication No. 49996, a steel sheet having a single layer or multiple plating layers of zinc or alloyed zinc is anodized in an aqueous solution containing acid or alkali,
A steel plate is known in which 0.03 to 3.0 g / m ² of oxide is formed on the surface of the steel plate.

[0003]

However, in the case of a galvanized steel sheet produced on an industrial scale, the continuous weldability of the galvanized steel often varies only by controlling the oxide thickness on the surface of the plated steel sheet as in the prior art. There was a problem.

[0004]

SUMMARY OF THE INVENTION The present invention has been made in view of the above problems, and the gist of the invention is that the first layer is galvanized and the upper layer has a thickness of 50 to 50 mm.
A galvanized steel sheet having excellent spot weldability, which is an oxide film containing 500 liters of Ni oxide.

[0005]

The present invention will be described in detail below with reference to the drawings.
FIG. 1 is a schematic view of the galvanized steel sheet of the present invention. Code 1
Is a steel plate, 2 is a galvanized film, and 3 is an oxide film containing Ni oxide. When conventional galvanized steel sheets are continuously welded with the same electrode, the zinc in the plating layer melted by the heat generated during welding reacts with copper, which is the main component of the welding electrode tip, and is a hard and brittle low-melting alloy make. As a result, the wear of the electrodes and the expansion of the diameter proceed, so that the welding current density flowing between the steel plates decreases, and it becomes impossible to form a predetermined nugget diameter necessary for joining the steel plates. Therefore, conventionally, the number of continuous welding spots is reduced as shown in FIG. That is, FIG. 3 is a diagram showing an example of the result of measuring the relationship between the current value and the nugget diameter during conventional continuous welding. According to this figure, regarding the nugget diameter and welding current after 500, 1000, and 2000 continuous welding points, it is understood that when the number of welding points is 1000 and 2000, the nugget diameter rapidly decreases and the welding current decreases.

However, in the case of the steel sheet of the present invention, as shown in FIG. 2, the current required for forming the predetermined nugget diameter is almost the same as that at the beginning of welding even with the electrode after continuous welding, so that the number of continuous welding points is small. improves. That is, FIG. 2 is a diagram showing an example of the result of measuring the relationship between the current value and the nugget diameter during continuous welding according to the present invention. According to this, the number of continuous welding spots is 1000 as compared with the conventional comparative example shown in FIG.
And 2000, the nugget diameter is almost unchanged and the welding current can be maintained.

Although this mechanism is not clear, Ni is contained in the alloy layer of the electrode during continuous welding, and as shown in Table 1 of the examples, Ni oxide causes a marked increase in resistance of the steel sheet. It is thought to be affecting. That is, Ni
Including the alloy increases the melting point of the alloy layer of the electrode and suppresses electrode softening / wear damage.Also, electrode wear / expansion occurs slightly, and the resistance between steel sheets is high even when the current density during welding decreases. It is considered that the heat generation amount necessary for forming the nugget diameter can be secured due to the effect that the passage of the welding current flowing during welding is narrowed due to the high resistance of the steel sheet surface.

When Ni in the upper layer of the plating is present in a metallic state, the resistance of the steel sheet does not increase, so continuous weldability is not improved. Also, the thickness of Ni oxide is 50Å
Similarly, when the thickness is thinner, the resistance does not increase, so that the sufficient effect of improving the weldability cannot be exhibited. If the thickness of the Ni oxide is more than 500 Å, the resistance becomes too high, which may cause the failure of electric conduction during welding. In addition, as the thickness of Ni oxide increases, the coefficient of friction of the steel plate surface increases,
The optimum Ni oxide thickness is 60 to 3 because it also affects the press formability required for the galvannealed steel sheet.
It is 00Å.

The galvanized steel sheet of the present invention means electrogalvanized steel sheet and hot-dip galvanized steel sheet,
After the usual electroplating or hot dipping treatment, the nickel nitrate solution can be applied to a steel sheet or electrolyzed.

[0010]

EXAMPLES Examples of the present invention are shown in Table 1 together with comparative examples.

[0011]

[Table 1]

The continuous weldability was evaluated under the following conditions. Pressure: 200 kgf Electrode: Al ₂ O ₃ dispersion strengthened copper, tip diameter 4.5 mmφ,
Dome type energization time: 10 cycles Continuous welding condition: Nugget forming current I ₀ (nugget diameter is 4
Continuous welding under the conditions of 1.4 times the current value (Ia) of the minimum current value above √ × plate thickness), speed of 1 dot / 2 seconds, and 30 seconds pause at every 20 dots Continuous welding end: Every 100 dots The sample for measuring the nugget diameter was welded at a current value of 0.85 × Ia, and it was determined that the time when the nugget diameter became smaller than 4√ × plate thickness was terminated. The resistance of the steel plate was 1 A current at a pressure of 200 kgf. I asked for it.

[0013]

INDUSTRIAL APPLICABILITY As described above, the present invention provides a galvanized steel sheet excellent in spot weldability and has a great industrial effect.

[Brief description of drawings]

FIG. 1 is a schematic diagram of a galvanized steel sheet according to the present invention,

FIG. 2 is a diagram showing an example of a result of measuring a relationship between a current value and a nugget diameter during continuous welding of a galvanized steel sheet according to the present invention,

FIG. 3 is a diagram showing an example of a result of measuring a relation between a current value and a nugget diameter during continuous welding of a conventional galvanized steel sheet without Ni oxide.

[Explanation of symbols]

 1 Steel plate 2 Galvanized film 3 Oxide film containing Ni oxide

Claims (1)

[Claims] 1. A galvanized steel sheet excellent in spot weldability, wherein the first layer is galvanized and the upper layer is an oxide film containing Ni oxide having a thickness of 50 to 500 Å.