DE1966807C3 - Process to improve the weldability and corrosion resistance of galvanized steel - Google Patents

Description

Zinc coated steel is widely used in industry, for example in the automotive industry. However, difficulties arise in welding zinc coated steel sheets on. In industry, zinc coated steel sheets are usually welded by a resistance spot welding process welded, the implementation of which it is necessary to use higher welding currents than when welding uncoated steel apply. The consequence of the higher welding current is an alloy of the zinc with the electrode material, whereby the service life of the welding electrodes is noticeably reduced. An attempt at this The problem to be solved was to add iron to the zinc coating (compare for example the article by H. A. J a h η 1 e in "British Welding Journal", 15 (3) 1968 on pages 113-119).

The invention now relates to a method for improving weldability and corrosion resistance of galvanized steel, which is characterized in that on the zinc layer by vacuum evaporation an iron layer is deposited as a cover layer.

This iron layer improves the weldability and the corrosion resistance in a manner unexpected per se of galvanized steel.

The iron is here preferably to temperatures above its melting point by a Electron beam gun heated. Usually at such temperatures iron is opposite its storage container very corrosive, but the heating with the electron beam reduces the Corrosion problems as the iron is only melted where the electron beam hits is focused.

Preferably, a steel strip is passed over cooled rollers through a vacuum chamber so that it running adjacent to at least one iron evaporation crucible disposed within the vacuum chamber is. In this way, the steel strip can be adjacent to alternately arranged zinc and Run iron evaporation crucibles located inside the vacuum chamber. Alternatively, the Steel strip can also be galvanized before it enters the vacuum chamber.

The outermost iron layer, which has considerable corrosion resistance, preferably has a thickness of at least 0.5 μm, z. B. up to 4 μπι.

A thickness of 4 to 80% of that of the zinc metal layer is particularly preferred. Thus, the Zinc layer should be essentially twice as thick as the iron layer.

Thus, if the thickness of the zinc metal layer is about 2 to 3 μπι, then that is the Iron metal layer expediently 1 µm. If a zinc metal layer with a thickness of 20 to 25 μπι is applied to the ferrous metal substrate, e.g. B. by ίο hot dipping, then the thickness of the ferrous metal layer 3 μπι amount.

The layers can consist of only one zinc and only

consist of an iron layer, or of a number of (usually alternating) zinc and

is iron layers, with the iron layer being the outermost Layer represents.

The invention is illustrated in examples.

example 1

As shown in the drawing, a Steel strips 1 with opposite surfaces 2, 3 through an inlet seal 4 into a vacuum chamber 5 introduced and removed again through an outlet seal 6. The pressure in the vacuum chamber 5 becomes

2s held by means (not shown) vacuum at 10 ~ ⁴ Torr.

The strip 1 runs on its migration through the vacuum chamber 5 over rollers 7 to 12, wherein the Rollers 9 through 12 by water or other means

.ίο be cooled. Strip 1 runs during the hike between the rollers 8 and 9 over a first horizontal path 13, with a surface 2 located at the bottom. When wandering between the rollers 7 and 11, he runs over a second one horizontal path 14 with its surface 3 at the bottom.

When hiking over the first horizontal path 13, the surface 2 runs above and adjacent to successively arranged zinc evaporation and iron evaporation crucibles 15, 16. When hiking over the second horizontal path 14, the surface 3 runs above and adjacent to successively arranged zinc evaporation and iron evaporation crucibles 15.16. In this way, each of the opposite sides of the strip 1 is successively coated with zinc and iron layers, the iron layer being the outermost layer. The zinc in the crucibles 15 is heated electrically heated immersion heater by means (not shown) while the iron (ie at about 100 ° C above its melting point) in the crucibles 16 electron gun (not shown) by a to about 1630 ⁰ C is heated. Since the iron is very reactive at this temperature and is corrosive to the container material, is

vs the heating with the electron beam appropriate, because here the iron is only melted at the point on which the electron beam is focused.

The thickness of the zinc and iron layers in this Case is 2.5 or 1.5 μπι, can in the required Wise can be varied by the heat input into the various crucibles 15 to 18 is controlled.

If a thick layer of iron, e.g. B. with 4 μπι, is to be deposited, then it is necessary for each of the three horizontal paths 13, 14 three

fts to provide iron evaporation crucible 16, the Strip 1 between each of the crucibles 16 is cooled by moving over water-cooled rollers (not shown) is directed. If this does not happen then the does

high temperature of the iron vapor that the zinc comes to melt

Using zinc / iron coated steel, 20,000 spot welds are carried out, before a deterioration in the welding quality or the need to adjust the welding electrode tips occurs

Example 2

The procedure is as in Example 1, with the exception that the steel strip 1 already has zinc layers has on its surfaces 2 and 3 before it enters the vacuum chamber 5, since it is a hot-dip galvanized steel strip. Accordingly, the zinc evaporation crucibles 15 are omitted.

In order to determine the corrosion resistance imparted by the outermost iron layer, a number is used of samples of the hot-dip galvanized steel each with the dimensions 15, 24x10.16 cm and each with zinc coatings exposed to a thickness of 25 μm in a given industrial environment. The through the Changes in weight due to corrosion are measured at 3-month intervals. One of the Sample has no iron layers (i.e. it is a conventional hot-dip galvanized steel object), while the other samples have outermost iron layers of varying thickness on the opposite one Have sides of steel. The results obtained are shown in Table 1.

Table 1

l'robc

sample Weight loss 9 (g / m ² ) 6.4 6th 12th months Months months 6.0 Hot-dip galvanized steel 3.8 4.5 5.2 without iron with about 0.5 μm iron 3.6 4.4 with about 1 µm iron 2.8 3.6

with about 2 μm iron
with about 3 μm iron
with about 4 pm iron

Moru

2.7
2.6
1.0

H 12

3.5 3.5 0.8

4.8 4.6 1.6

ίο It can be seen that with increasing thickness the Iron layer the corrosion rate (weight loss) is reduced.

To investigate the spot welding behavior, spot welds with a number of

Samples of the hot-dip galvanized steel were carried out. One the specimen has no iron layers (i.e. it is conventional hot-dip galvanized steel, while the other samples have outermost iron layers of varying thickness. The number of

: o Spot welds made with each sample before the welding electrode had to be trimmed was determined. The received The results are shown in Table 2.

Table 2

sample

Number of spot welds

Hot-dip galvanized steel without iron 1,700

2,300 2,800

6 700

7,200 12,000 14,000 20,000

reuerverzir.Kier aiani onne
with about 1.0 μm iron
with about 1.5 μm iron
with about 2.0 μm iron
with about 2.5 μm iron
with about 3.0 μm iron
with about 3.5 μm iron
with about 4.0 μπι iron

It can be seen that the greater the thickness of the iron layer, the better the spot welding behavior will.

1 sheet of drawings

Claims (3)

Patent claims: 1. Process to improve weldability and corrosion resistance of galvanized Steel, characterized in that on the zinc layer by vacuum evaporation Iron layer is deposited as a top layer. 2. The method according to claim 1, characterized in that several layers of alternating Iron and zinc are deposited, with the innermost layer being zinc and the outermost layer is made of iron or zinc. 3. The method according to claim 1 or 2, characterized in that one or more zinc layers can be applied by hot-dip galvanizing.