JPH11269666A - Surface treated steel sheet having excellent resistance weldability - Google Patents

Abstract

PROBLEM TO BE SOLVED: To make it possible to obtain a high-reliability joint in resistance welding, such as spot welding, by forming lower plating layers consisting of an Al material and upper plating layers consisting of a Cr material or Mo material on both sides of a steel sheet. SOLUTION: The lower plating layers 2 consisting of the Al or an alloy consisting essentially of the Al are formed on both sides of the steel sheet 1. The upper plating layers 5 consisting of the Cr or an alloy consisting essentially of the Cr or Mo or an alloy consisting essentially of the Mo are formed on the lower plating layers 2. The thickness of the lower plating layers 2 is substituted to >=5 to <=30 μm and the thickness of the upper plating layers 5 to >=0.3 to <=5.0 μm. When the welding of the steel sheets 1 to each other is executed by pressing welding electrodes 3 to the steel sheet surfaces via the upper plating layers 5 by spot welding, the reaction between the lower plating layers 2 and the welding electrodes 3 is suppressed and the wear of the front ends of the electrodes is extremely little. The diameter of a nugget 4 is kept constant and the strength of the supporting welding is kept nearly constant as well.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a surface-treated steel sheet having excellent resistance weldability used as a material for automobiles and the like.

[0002]

2. Description of the Related Art As a material for a fuel tank of an automobile, a Pb-Sn-based alloy-plated steel sheet has hitherto been used. However, recently, from the standpoint of environmental protection, regulations on Pb have become more stringent, and in response to this,
Al-based alloy-plated steel sheets such as Al-Si-based steel sheets are being used instead of Pb-Sn-based alloy-plated steel sheets.

On the other hand, in the field of automobiles and the like, resistance welding such as spot welding and seam welding has been frequently used as a method of welding a surface-treated steel sheet (plated steel sheet).
This method is often used for welding fuel tank materials. In the case of resistance welding, for example, in the case of spot welding, as shown in FIG.
The two coated steel sheets are sandwiched between the welding electrodes 3 and pressurized. Then, by applying a current, Joule heat is generated between the coated steel sheets to form a nugget (melted portion) 4 in the overlapped portion. And a method of welding plated steel sheets to each other.

[0004]

However, Al or A
When resistance-welding a steel sheet plated with an alloy containing l as a main component, for example, in the case of spot welding, with an increase in the number of welding spots, from the plating layer of Al or an alloy containing Al as a main component, Al infiltrates (diffuses) on the side of the welding electrode (usually made of Cu-Cr alloy), and C
A u-Al-Fe-based intermetallic compound layer is generated. This intermetallic compound is a very hard and brittle compound having a Vickers hardness of 700 or more. Therefore, it is easy to peel off from the tip of the welding electrode due to the pressure at the time of spot welding and to drop it.
At the tip of the welding electrode after the lack of the intermetallic compound, the generation of the same intermetallic compound is repeated, so that the tip of the welding electrode is consumed. As a result, the life of the welding electrode is significantly reduced as compared with a case where a normal cold-rolled steel sheet is spot-welded. Further, since the tip diameter of the electrode increases due to the consumption of the welding electrode, the current density during spot welding decreases. As a result, the nugget diameter decreases with an increase in the number of welding points, and the joining strength also decreases accordingly. When the current density becomes less than a certain current density, no nugget is formed.

As one method for preventing the consumption of the welding electrode, a method of removing a plating layer of Al or an alloy containing Al as a main component on the side in contact with the welding electrode can be considered. However, Al or Al formed on the steel sheet surface
The plating layer of an alloy containing as a main component is important for improving the corrosion resistance of the steel sheet. Therefore, easily removing such a plating layer causes a reduction in corrosion resistance. Further, removing the plated layer of Al or an alloy containing Al as a main component, once formed, causes an increase in the number of steps in the assembling process and a decrease in productivity.

As another method for preventing the consumption of the welding electrode, metal plating having a sufficient electrical conductivity that does not easily react with the welding electrode (Cu—Cr alloy) used for resistance welding and does not impair the weldability. A method of coating the layer on a plating layer of Al or an alloy containing Al as a main component is considered. As an example of this, for example,
As disclosed in Japanese Patent No. 246182, Ni, Fe is deposited on a plating layer of Al or an alloy containing Al as a main component.
Alternatively, a method has been considered in which a plating layer of an alloy containing these elements as main components is coated to suppress wear of the welding electrode. However, in this method, a plating layer of another metal, for example, a plating layer of Cr, Mo or an alloy mainly containing these, is coated on a plating layer of Al or an alloy mainly containing Al to form a welding electrode. It does not specify how to reduce the wear on steel.

Therefore, in the present invention, a metal plating layer which does not easily react with a welding electrode (Cu-Cr alloy) used for resistance welding and has sufficient electric conductivity so as not to impair weldability is mainly made of Al or Al. An object of the present invention is to enable normal resistance welding while suppressing consumption of a welding electrode by using a surface-treated steel sheet coated on a plating layer of an alloy as a component.

[0008]

In order to achieve the object, the surface-treated steel sheet of the present invention has a structure in which Al or an alloy containing Al as a main component is formed on one or both sides of a plating layer formed on both sides of the steel sheet. And a metal plating layer hardly reacting with the welding electrode. The present inventors,
On the plating layer of Al or an alloy containing Al as a main component formed on the surface of the steel sheet, a plating layer which is hard to react with a Cu-Cr alloy usually used as a material of a welding electrode and has excellent electric conductivity. We have devised a coating method and studied a metal suitable for it. As a result, Cr, M
When the plating layer of o and an alloy containing these as the main components is formed on the plating layer of Al or the alloy containing Al as the main component, the consumption of the welding electrode is suppressed, and good resistance welding can be performed. They have found and completed the present invention.

Here, the gist of the present invention is that "(1) Al or a lower plating layer of an alloy containing Al as a main component formed on both sides of a steel sheet, and the plating layer on one or both sides of the steel sheet. Cr or C formed on
A surface-treated steel sheet having excellent resistance weldability, characterized by having an upper plating layer of an alloy containing r as a main component.

(2) A lower plating layer formed of Al or an alloy containing Al as a main component formed on both surfaces of a steel plate, and a Mo layer formed on the plating layer on one or both surfaces of the steel plate.
Alternatively, a surface-treated steel sheet excellent in resistance weldability, comprising an upper plating layer of an alloy containing Mo as a main component. (3) In the above (1), A formed on both surfaces of the steel plate
The thickness of a lower plating layer of an alloy containing l or Al as a main component is 5 μm or more and 30 μm or less, and Cr or an alloy containing Cr as a main component formed on the plating layer on one or both surfaces of a steel plate A surface-treated steel sheet having excellent resistance weldability, characterized in that the thickness of the upper plating layer is from 0.3 μm to 5.0 μm.

(4) In the above (2), the thickness of the lower plating layer of Al or an alloy containing Al as a main component formed on both surfaces of the steel sheet is 5 μm or more and 30 μm or less;
Resistance weldability characterized in that the thickness of the upper plating layer of Mo or an alloy containing Mo as a main component formed on the plating layer on one or both surfaces of the steel sheet is 0.3 μm or more and 5.0 μm or less. Excellent surface treated steel sheet. ".

[0012]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS A surface-treated steel sheet according to the present invention will be specifically described below with reference to the drawings. In resistance welding, for example, in the case of spot welding, as shown in FIG. 2, the welding electrode 3 is pressed against the steel material surface via the plating layer 5 of Cr, Mo, or an alloy containing these as a main component. Therefore, the reaction between the plating layer 2 of Al or an alloy containing Al as a main component and the welding electrode 3 is suppressed. That is, since infiltration (diffusion) of Al into the welding electrode is suppressed, the fragile Cu-Al-F is formed at the tip of the welding electrode.
e The generation of intermetallic compounds is suppressed, and the wear at the tip of the electrode is extremely reduced as compared with the case where the plating layer 5 of Cr, Mo or an alloy containing these as a main component is not coated. This is because Cr, Mo, or an alloy containing these as a main component is a metal that does not form a brittle intermetallic compound with the Cu—Cr alloy that is the material of the welding electrode. As a result, even when spot welding is performed continuously (dotting), the electrode tip diameter does not increase, so that the contact area between the electrode tip and the plated steel sheet is kept almost constant, and the welding current density at the nugget forming part is also reduced. It is kept almost constant. Therefore, since the nugget diameter is kept constant, the strength of the spot weld is also kept almost constant.

On the other hand, when a conventional plated steel sheet of Al or an alloy containing Al as a main component is spot-welded, the diameter of the electrode tip increases with the wear of the electrode tip, and the electrode comes into contact with the plated steel sheet. Because the area of the electrode tip increases,
The current density of the nugget forming portion decreases as the number of welding points increases. As a result, the diameter of the nugget decreases with an increase in the number of hit points, and the joining strength also decreases, so that no nugget is formed below a certain welding current density.

As a plating layer of Al or an alloy containing Al as a main component formed on the surface of the steel sheet, in addition to pure Al,
Alloys such as Al-Si, Al-Mn, and Al-Zn are conceivable. In addition, the thickness is 5 μm or more and 30 μm
It is preferable to set the following. This is because, in order to maintain sufficient corrosion resistance, the plating thickness is preferably 5 μm or more, and if the plating thickness is more than 30 μm, Cr, Mo, or an alloy containing these as a main component is formed thereon. This is because even if the plating layer is coated, the effect of suppressing electrode wear is not recognized. When the plating thickness is more than 30 μm, the effect of suppressing electrode wear is not recognized because the plating layer of Al or an alloy mainly composed of Al destroys the upper plating layer during spot welding and makes direct contact with the tip of the welding electrode. It is thought that it is because. The plating layer of Al or an alloy containing Al as a main component is usually formed by a hot-dip plating method, but may be formed by a method such as an electroplating method, a vapor deposition method, or a thermal spraying method.

As a metal plating layer formed on a plating layer of Al or an alloy containing Al as a main component, the ease of plating, corrosion resistance, cost, etc. are comprehensively evaluated.
It is necessary to form a plating layer of r, Mo, or an alloy containing these elements as main components. For example, Cr-Zn is optimal. The thickness is desirably smaller than the thickness of the plating layer of Al or an alloy containing Al as a main component.
Specifically, the thickness is preferably 0.3 μm or more and 5.0 μm or less. This is because if the thickness is less than 0.3 μm, no effect on suppressing electrode wear is recognized. this is,
If the thickness of the plating layer is too small, the plating layer is broken at the time of spot welding, and the welding electrode directly contacts the lower Al plating layer or an Al-based alloy plating layer. Also,
If the thickness is more than 5.0 μm, a considerable time is required for coating, which leads to a decrease in productivity and an increase in cost. When the upper metal plating layer is provided on one side of the plating layer of Al or an alloy containing Al as a main component, it is preferable to form the upper metal plating layer by an electroplating method. However, a method such as hot-dip plating, vapor deposition, or thermal spraying may be employed. In addition, it is sufficient that the upper metal plating layer is basically coated only on one side in contact with the welding electrode, but it may be coated on both sides if necessary.

This effect is similarly observed in other resistance welding, for example, seam welding, and is effective in suppressing electrode wear. Further, the metal plating layer coated on the plating layer of Al or an alloy containing Al as a main component,
This is effective not only for improving resistance weldability but also for improving workability such as formability. That is, when processing the surface-treated steel sheet of the present invention in a state where the coating layer is in contact with the mold, the plating layer of Al or an alloy containing Al as a main component is avoided from directly contacting the mold. Alternatively, the plating layer of the alloy containing Al as a main component does not adhere to the mold. Further, since the thin metal plating layer has a smaller coefficient of friction than the plating layer of Al or an alloy containing Al as a main component, the press workability is also improved.

[0017]

The present invention will be described more specifically with reference to the following examples. -Example 1-15 μm thick A on both sides of a cold-rolled steel sheet having a thickness of 0.8 mm.
The l-Si alloy plating layer was coated by a hot-dip plating method. Next, on one surface, the thickness of Cr, Cu, Mo, S was changed by 0.1 μm in the range of 0.1 to 1.0 μm.
The n plating layer was coated by an ion plating method. Spot welding was performed by arranging these surface-treated steel sheets in combination of the same kind of plated steel sheets so that the welding electrode side was the side coated with Cr, Cu, Mo, and Sn plating.

A DC type spot welding machine (70 kVA) is used for the spot welding, and Cu-Cr is used as the welding electrode.
Using a dome-shaped electrode (40R-6φ) made of alloy, pressing force: 2156 kN, welding current: 9.5 kA, conduction time;
Evaluation of continuous hitting property up to 150 points was performed under the condition of 0.2 ms (12 cycles). The relationship between the number of welding points and the electrode tip diameter (the diameter of the portion where the welding electrode contacts the plated steel sheet) when the thicknesses of Cr, Cu, Mo, and Sn plating are 0.1 μm and 0.5 μm, respectively, This is shown in FIGS. In addition, the electrode tip diameter was investigated by sandwiching pressure-sensitive paper between the electrodes. When the thickness of the metal plating layer coated on the Al—Si alloy plating layer is 0.1 μm, FIG.
As shown by, the increase in the electrode tip diameter was worse when the metal plating layer was coated, and no effect of suppressing electrode wear was observed. On the other hand, when the thickness of the metal plating layer is 0.5 μm, as shown in FIG.
In the case of o-plating, the electrode tip diameter hardly increased, and an effect of suppressing electrode wear was observed. As a result of the experiment, Cr, Mo
In the case of plating, the electrode tip diameter did not increase when the plating thickness was 0.3 μm or more, and an effect of suppressing electrode wear was recognized.
Instead of Al-Si plating, pure Al, Al-Mn, A
The same result as described above was obtained by using l-Zn plating. Similar results were obtained when Cr-Zn plating was used instead of Cr plating. Furthermore, a plating of, for example, Cr was formed by an electroplating method, and the same evaluation was performed. The results were the same as above. -Example 2-In Example 1, the thickness of the Al-Si alloy plating layer was set to 1
The same investigation was performed by changing 10 μm at a time in the range of 0 to 50 μm. As a result, when the thickness of the Al—Si alloy plating layer is greater than 30 μm, even if the Cr or Mo plating layer is coated in the thickness range of 0.3 to 5.0 μm, the effect of suppressing electrode wear is recognized. I couldn't.

[0019]

As described above, in the present invention,
The welding electrode and Al or Al are mainly formed by forming a plating layer of Cr, Mo or an alloy mainly containing these on the plating layer of Al or an alloy mainly containing Al formed on the surface of the steel sheet. This prevents direct contact with a plating layer of an alloy as a component, suppresses generation of a brittle intermetallic compound, and suppresses consumption of a welding electrode. Thereby, the life of the welding electrode is significantly improved as compared with the conventional material. Further, since the nugget is also generated stably, the variation in joining strength is reduced, and a highly reliable joint can be obtained in resistance welding such as spot welding.

[Brief description of the drawings]

FIG. 1 is a sectional view showing a general spot welding method.

FIG. 2 is a sectional view showing a spot welding method of the present invention.

FIG. 3 is a graph showing the relationship between the number of welding points and the electrode tip diameter.

FIG. 4 is a graph showing the relationship between the number of welding points and the electrode tip diameter.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 ... Steel plate 2 ... Plating layer of Al or the alloy which has Al as a main component 3 ... Welding electrode 4 ... Nugget (melting part) 5 ... Thin metal plating layer

 ──────────────────────────────────────────────────の Continued on the front page (72) Inventor Kazumi Nishimura 20-1 Shintomi, Futtsu-shi, Chiba Nippon Steel Corporation Technology Development Division

Claims (4)

[Claims] 1. Al or Al formed on both sides of a steel sheet
Resistance welding comprising: a lower plating layer of an alloy mainly composed of: and an upper plating layer of Cr or an alloy mainly composed of Cr formed on one or both surfaces of the steel sheet on the plating layer. Surface treated steel sheet with excellent properties. 2. Al or Al formed on both sides of a steel sheet
Welding comprising: a lower plating layer of an alloy mainly composed of: and an upper plating layer of Mo or an alloy mainly composed of Mo formed on one or both surfaces of the steel sheet on the plating layer. Surface treated steel sheet with excellent properties. 3. The surface-treated steel sheet according to claim 1,
The thickness of the lower plating layer of Al or an alloy containing Al as a main component formed on both surfaces of the steel plate is 5 μm or more and 30 μm or less, and Cr or Cr formed on the plating layer on one or both surfaces of the steel plate A surface-treated steel sheet having excellent resistance weldability, wherein the thickness of an upper plating layer of an alloy containing Cr as a main component is 0.3 μm or more and 5.0 μm or less. 4. The surface-treated steel sheet according to claim 2,
The thickness of the lower plating layer of Al or an alloy containing Al as a main component formed on both surfaces of the steel plate is 5 μm or more and 30 μm or less, and Mo or Mo formed on the plating layer on one or both surfaces of the steel plate A surface-treated steel sheet having excellent resistance weldability, wherein the thickness of an upper plating layer of an alloy containing Mo as a main component is from 0.3 μm to 5.0 μm.