JPS5827356B2 - High corrosion resistance surface treated steel sheet - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a highly corrosion-resistant surface-treated steel sheet, and the present invention relates to a highly corrosion-resistant surface-treated steel sheet that has been improved so that it has particularly excellent paintability and weldability, and also has high corrosion resistance including the painted base surface. This is what I am trying to propose.

Recently, the use of so-called surface-treated steel products, in which steel sheets are subjected to chemical conversion treatment or further painting treatment, has expanded, and the high degree of corrosion resistance has improved surface properties including paintability, welding workability, and other workability. This has also come to be strongly requested.

In other words, while automobile steel sheets are a typical example of the above-mentioned applications, in recent years, coupled with the rapid development and maintenance of automobile road networks, it has become necessary to guarantee safe high-speed driving of automobiles, especially in cold regions and mountainous regions. This is to sufficiently protect the vehicle body from the corrosive effects of antifreeze agents, such as rock salt, calcium chloride, etc., which are sprayed to prevent snow accumulation and other icing on the road.

In this way, ensuring rust prevention properties for automobile steel sheets has been considered in conjunction with the welding process, and in recent years, galvanized single-sided rust prevention treated steel sheets have been used in large quantities. By using an anti-corrosion treated surface, the outer surface is a non-plated surface, resulting in excellent weldability and paintability.

However, considering the recent international nature of automobile demand,
Regulations by foreign laws and regulations concerning corrosion protection cannot be relaxed; for example, in Canada, in 1981, cars were required to have a warranty of 1.5 years free of surface rust and 5 years free of pitting corrosion, and the same is true in the United States. However, even stricter laws are being considered.

Therefore, in addition to countermeasures against corrosion from the back side of the vehicle body, corrosion from the outside side becomes a serious problem.

In other words, the concern about corrosion on the outside surface is due to damage (chipping) that occurs when the paint film is hit by stones, sand, rock salt, etc. thrown up from the road surface sprayed with anti-freeze when the car is driving, which reaches the base material. and
Moisture infiltrates through this damaged area and antifreeze agents (salt) act on it, causing corrosion to progress.

As a countermeasure against electrolytic corrosion from both the inside and outside of the vehicle body, it is conceivable to use a steel plate with thick zinc plating applied to both sides of the steel plate.

However, since the adhesion of the paint film applied to the galvanized surface is not good, it is not only easy to cause the paint film to peel off due to stone splashes mentioned above, or even to cause paint film blistering due to corrosion. Corrosion of thick zinc will significantly impair weldability.

The inventors have conducted intensive research to solve the above-mentioned problems with automotive exterior steel sheets, and as a result, they have developed a method to reduce the thickness of the zinc-based plating on one side, and selectively reduce the thickness of the zinc-based plating on both sides. It has been found that if the material is alloyed with the surface of the base steel by a heat treatment, and the remaining surface is left at its normal thickness, it can be advantageously adapted to the intended purpose. 3. In other words, a coating made by alloying a thin zinc-based plating layer is much more advantageous than normal zinc-based plating in terms of workability when welding this type of exterior steel plate, and also has The alloyed zinc-based plating film not only has temporary rust prevention properties as it is, but also has sufficiently good paint film adhesion, and is excellent in paintability and substrate corrosion resistance after painting.

Therefore, zinc-based plating (non-alloyed) is applied to the back side of automotive exterior steel sheets that are expected to have a high degree of corrosion resistance in the plating film itself. By applying a thin zinc-based plating to the surface side and subjecting this to alloying treatment, a surface-treated steel sheet that is extremely convenient as an exterior steel sheet for automobiles can be obtained.

This invention has a zinc-based electroplated anti-corrosion coating on both sides, and one side has a thin alloyed zinc coating with a thickness in the range of 0.5 to 2097 m that has been alloyed with the surface of the base steel by selective heat treatment. This is a highly corrosion-resistant surface-treated steel plate that serves as the base surface for painting.

To practice this invention, the zinc-based electroplated anti-corrosion coating is Zn, Zn-Ni or Zn-At plating, and the electroplated anti-corrosion coating on the other side opposite to the base surface to be painted is Zn-Ni or Zn-At. N
i or Zn-At plating, 10~40g/
There are various embodiments such as having a plating thickness and having the electroplated anticorrosive coating on the other side opposite the painted base surface be Zn plating and having a plating thickness in the range of 40 to 100 g/rrl. .

Generally speaking, the surface-treated steel sheet of the present invention can be easily manufactured by, for example, electroplating zinc on both sides of a steel sheet, and then applying heat treatment to only one side to form an alloy.

In addition to this zinc electroplating, electroplating of alloys of zinc and nickel, aluminum, etc., or composite electroplating typified by zinc and aluminum, can be applied, and in this invention, zinc-based Expressed in bulk with electroplating.

Even in the case of flattened electroplating, the plating thickness is determined depending on the requirements for corrosion resistance, and in particular, the thickness of the plating on one side that is to be alloyed with the surface of the base metal used as the coating base surface of this invention is 0.5.
The range should be 9/rr1 or more and 20117m or less, and if it is too thin (less than 0.5g/lri'), it will be necessary to temporarily prevent rust during the painting process and under the paint film. This is because both corrosion resistance is unsatisfied, and on the other hand, if the weight exceeds 20 g/-, there will be problems in weldability and paintability.

On the other hand, depending on the anti-corrosion function required on the back side of the surface-treated steel sheet, for example, the degree that is commonly used for the single-sided galvanized steel sheet mentioned above, for example, in the case of Zn plating, it is 40 to 150 among 20 to 150. 100g/m range, and has better corrosion resistance than Zn plating.
For Ni plating and Zn-kl plating, regardless of whether it is alloy plating or composite plating, 20 g/
- degree is sufficient, but it is unavoidable that a compromise must be made due to the conflicting conditions between corrosion resistance and weldability.

In this way, in practice, it is advantageous to adapt the so-called differential thickness plating, in which the plating thickness is different on both sides of the steel plate.
It is clear that the electroplating method using a radial cell is suitable for this purpose.

The zinc-based electroplating on both sides can be applied to each side of the steel sheet in combinations as shown in the following example.

Next, the alloying treatment may be performed by a commonly used gas burner method, induction heating method, or resistance heating method.

In this case, alloying may occur in a part of the thick coating on the opposite back side, but if particularly high workability is required, alloying can be prevented by air cooling or the like.

The degree of alloying applied to the outer thin coating may be determined depending on the required quality performance, and for example, in the case of zinc electroplating, the degree of alloying is preferably such that the Fe concentration is between 10 and 12.

Examples of the present invention will be described below.

Plate thickness 1. 1Of on the outside of Omm cold rolled steel plate according to the following conditions! /m, 40g/lr on the back side? Electrolytic galvanizing with a thickness of : was applied, and the outer surface was then heated using a gas burner method at a heating temperature of 400'C for 2.5 minutes.

At this time, the opposite side was air-cooled to prevent alloying.

The weldability of the plated steel sheet according to the present invention was investigated using a double-sided electrogalvanized steel sheet of s 40.9/- as a comparison material.

The results are shown in Table 1. This weldability test was carried out at a in Figure 1.
Continuous spot welding was performed on the overlapping surfaces whose joining directions are individually shown in =d, and evaluation was made by the number of consecutive welds.

In the figure, 1 is a cold-rolled steel sheet, 2 is a galvanized coating, and 3 is an alloyed galvanized coating, and the arrows indicate the positions of the welding electrodes.

As is clear from Table 1, the weldability of the surface-treated steel sheet according to the present invention is superior to that of combination d, which is a double-sided galvanized steel sheet, in any of the combinations a to c of the joined overlapping surfaces, and The effect of the chemical treatment is remarkable.

In addition, the number of continuous dots in the case of different plating thicknesses was investigated by overlapping the plates in Figure 1, and the results shown in Table 2 were obtained.

Next, after coating the plated steel sheets according to the present invention and comparative materials, they were subjected to a DuPont impact test, and further after DuPont processing, 50%
A 0-hour salt spray test was conducted to examine the adhesion of the paint film and the corrosion resistance after painting.

The coating conditions were as follows: The sample material was pretreated with Percolene Z (a surface conditioner manufactured by Nippon Parker Co., Ltd.), and Bonderite 31 was applied.
14 (phosphate treatment agent manufactured by Nippon Parker Co., Ltd.) at a liquid temperature of 50°
As is clear from Table 3, the alloyed galvanized surface has superior coating film adhesion and post-painting corrosion resistance compared to the galvanized surface and cold-rolled steel sheet.

As described above, the present invention can significantly improve the corrosion resistance of the substrate after painting without impairing weldability, and exhibits excellent performance when applied to surface-treated steel sheets such as those for the exterior of automobiles.

[Brief explanation of drawings]

1A, 1B and 1C are illustrations of various types of steel sheets according to the present invention, and FIG. 1D is an explanatory view of a general spot welding procedure for overlapping steel sheets for comparison. 1...Cold rolled steel sheet, 2...Zinc-based electroplating coating, 3...Zinc-based electroplating alloying treatment coating.

Claims (1)

[Claims] 1. A zinc-based electroplated anti-corrosion coating on both sides, one side of which is alloyed with a thickness of 0.5 to 2097 m through selective heat treatment with the surface of the base steel. Highly corrosion-resistant surface-treated steel sheet with thin zinc coating. 2 Zinc-based electroplated anticorrosive coating is Zn, Zn-N
1. The surface-treated steel sheet according to 1, which is plated with i or Zn-At. 3. The surface-treated steel sheet according to 1 or 2, wherein the electroplated anticorrosive coating on the other side opposite to the painted base surface is Zn plating, and has a plating thickness in the range of 40 to 100 gyrri'. 4 The electroplated anticorrosion coating on the other side opposite to the painted base surface is Zn-Ni or Zn-At plating, and 1
The surface-treated steel sheet according to 1 or 2, having a plating thickness of 0 to 40 g/rrl.