JPH07258886A - Electrogalvannealed steel sheet excellent in spot weldability - Google Patents

Abstract

PURPOSE:To maintain electric current required for the formation of the prescribed diameter of a nugget without being changed from that in the initial stage of welding even in the case of continuous use without exchanging electrodes and to improve the efficiency of spot welding by forming an oxidized film contg. Ni oxides on an electrogalvannealed layer. CONSTITUTION:A steel sheet in which a 1st layer on the surface is constituted of electrogalvanizing, and the top layer is constituted of an oxidized film contg. Ni oxides having 50 to 500Angstrom thickness is formed. This steel sheet can be obtd., after the production of an electoplated steel sheet of a single layer or multiple layers of ordinary Zn-Ni or Zn-Fe, by the method of coating the plated face with am oxidizing nickel nitrate soln. or executing cathodic electrolysis or the like. At the time of using this steel sheet for an automobile field or the like, the productivity of spot welding is improved as well as corrosion resistance, and it shows industrially high effect.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a zinc-based electroalloy-plated steel sheet having excellent spot weldability.

[0002]

2. Description of the Related Art Conventionally, Zn--Ni or Z
n-Fe-based zinc-based electroalloy-plated steel sheet is ordinary electrolytic zinc
Since it has better corrosion resistance than galvanized steel,
It is often used in the field of motor vehicles. However, plating adhesion
As the amount increases, the spot weldability decreases, so
There is a strong demand for improved contact. Therefore, with conventional technology
As disclosed in Japanese Patent Laid-Open No. 1-149996.
Single- or multi-layer plating of zinc or alloyed zinc
Steel sheet with layers in an aqueous solution containing acid or alkali
Is anodized at 0.03 to 3.0 g / m on the surface of the steel sheet. ² 
Although steel sheets and the like formed with the oxide of are proposed,
However, zinc-based electroalloy plating produced on an industrial scale
In the case of steel plate, if the coating amount increases, the
The continuous weldability was often variable.

[0003]

According to the present invention, the first layer is a zinc-based electroalloy plating, and the upper layer thereof has a thickness of 50 to 500Å.
Is a zinc-based electroalloy-plated steel sheet excellent in spot weldability, which is an oxide film containing Ni oxide.

[0004]

The present invention will be described in detail below. When continuously welding ordinary zinc-based electroalloy-plated steel sheets 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 hard and brittle. Make a melting point alloy. 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, the number of continuous welding spots is reduced. However, a schematic diagram is shown in FIG. In the figure, reference numeral 1 is a steel plate, 2 is a zinc-based electroalloy plating film, and 3 is an oxide film containing Ni oxide having a film thickness of 50 to 500 Å. In the case of the steel sheet of the present invention, FIG.
As described above, even with the electrode after continuous welding, the current required for forming a predetermined nugget diameter is almost the same as that at the beginning of welding, and the number of continuous welding points is improved.

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
By containing, the melting point of the alloy layer of the electrode is increased and softening / wear of the electrode is suppressed. In addition, even if the electrode wears or expands slightly and the current density during welding decreases, the resistance between the steel sheets is high, and the high resistance of the steel sheet surface narrows the conduction path of the welding current that flows during welding. It seems that the necessary calorific value can be secured.

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 that continuous spot 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,
Since the press formability required for a zinc-based electroalloy-plated steel sheet is also affected, the optimum Ni oxide thickness is 60-
It is 300Å.

The zinc-based electroalloy-plated steel sheet of the present invention means Zn-Ni or Zn-Fe single-layer and double-layer plated steel sheets. Apply the nickel nitrate solution to the steel plate,
Alternatively, it can be manufactured by a method such as cathodic electrolysis.

[0008]

EXAMPLES Examples of the present invention are shown in Table 1 together with comparative examples.
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 Current flowing 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 √t × minimum current value that exceeds the plate thickness), 1 dot / 2 seconds speed, and 30 seconds pause for every 20 dots Continuous welding end: 100 dots Each time, a 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√t × plate thickness was ended. The current was applied and the value was calculated.

[0009]

[Table 1]

[0010]

INDUSTRIAL APPLICABILITY As described above, the present invention provides a zinc-based electroalloy-plated steel sheet excellent in spot weldability and has a great industrial effect.

[Brief description of drawings]

FIG. 1 is a schematic view of a zinc-based electroalloy-plated steel sheet of the present invention,

FIG. 2 shows an example of the results of measuring the relationship between the current value and the nugget diameter during continuous welding for the Zn-Ni electroalloy-plated steel sheets without Ni oxide of the present invention example and the comparative example shown in the examples. It is a figure.

[Explanation of symbols]

 1 Steel plate 2 Zinc-based electric alloy plating film 3 Oxide film containing Ni oxide

Claims (1)

[Claims] 1. The first layer is zinc-based electroalloy plating,
A zinc-based electroalloy-plated steel sheet excellent in spot weldability, characterized in that an upper layer thereof is an oxide film containing Ni oxide having a thickness of 50 to 500Å.