WO1988008886A1

WO1988008886A1 - A process and an apparatus for hot dip coating a steel strip

Info

Publication number: WO1988008886A1
Application number: PCT/SE1988/000236
Authority: WO
Inventors: Alf Göran Helmer ENGBERG
Original assignee: SSAB Svenskt StÅl Ab
Priority date: 1987-05-05
Filing date: 1988-05-05
Publication date: 1988-11-17
Also published as: AU1720088A; SE8701837D0

Abstract

In a continuous process in a coating apparatus, a steel strip (18) is continuously immersed in a bath of molten metal that essentially comprises aluminium and zinc (melting pot 11). The strip (18) is rapidly cooled to about 200°C and the coating is worked by passing a pair of work rolls (15). Then, the coating is held at about 200°C for some minutes only, which overages the coating. Thus, the process of coating, working and overaging is carried out in a continuous process.

Description

A process and an apparatus for hot dip coating a steel strip

This invention refers to a continuous process for coating a steel strip comprising continuously immersing the strip in a bath of molten aluminium and zinc and cooling the strip to a temperature below 210 C. The invention relates also to a coating apparatus for applying to a steel strip a ductile coating that essentially comprises aluminium and zinc, comprising in series, a hot dip station with a pot (11) with molten metal a cooling station (12-14) , and a thermal treatment station (16) for the strip with the solidified coating. BACKGROUND AND PRIOR ART

It is known from US-A 4287008 a method of making ductile coating of alumimium and zinc by thermal treatment followed by a slow cooling. The time for the overageing treatment is in the magnitude of 24 hours. Such a long treatment is expensive. SUMMARY OF THE INVENTION

It is an object of the invention to reduce the time for making an aluminium-zinc coating ductile and to make it possible to overage the coating in line with the hot dip coating of the steel strip.

This is accomplished by the in-line steps of first working the solidified coating and then thermally treating the worked coating to overage it. BRIEF DESCRIPTION OF THE DRAWINGS

Fig. 1 shows schematically a hot dip coating apparatus according to the invention.

Fig. 2 is a time-temperature diagram on an example of treating a steel strip in accordance with the invention. Fig. 3 is a diagram corresponding to Fig. 2 but showing another example in accordance with the invention.

In Fig. 1 there is a pot 11 with molten metal, three conventional cooling units 12-14, a pair of working rolls 15, a thermal treatment unit 16 and a cooling unit 17. The steel strips 18 that is being treated is conveyed over a plurality of lead rollers that have not been designated reference numerals. The units and devices shown in Fig. 1 are conventional per se and are therefore not described in detail. Devices like snouts, protection hoods, jet finishing nozzles and like parts of the conventional technique are not illustrate In other words, in Fig. 1 are schematically shown a coating station 11, a cooling station 12-14, a working station 15, a thermal treating station 16, and a cooling station 17.

The molten metal in the pot 11 comprises essentially 20-80% Al and 80-20% Zn and includes 1-2% Si. Its analysis can for example be 20-80% Al, 1-2% Si, balance Zn. Due to the melting points, the temperature of the bath can suitably be about 600°C for a bath of this analysis.

In the example of the invention illustrated in Fig. 2, the steel strip is cooled in the cooling units 12-14 to about 200 C and the solidified coating is then plastically deformed when passing the pair of work rolls 15 shown in Fig. 1. Then, the strip 18 passes through the thermal treatment unit 16 which takes about 100 seconds and overages the coating so that the ductility of the coating is substan- tially increased. Then, the strip is cooled in the cooling unit 17 to ambient temperature.

As described above, the coating is worked substantially at the overaging temperature. It might be advantageous to work the coating at such an elevated temperature since less force is required. It might also be advantageous to apply a sequence of reversed bending by using more than two rolls. It should be noted that preferably only the coating should be worked (plastically deformed) , not the steel.

The cooling in the cooling units 12-14 should be comparatively fast; in the temperature range 320 C-200°C it should be faster than 8°C/s, preferably higher than 10°C/s. Thus, quenching does not seem to be necessary; gas cooling can be used.

In the example of the invention illustrated in Fig. 3, the steel strip 18 is cooled substantially to ambient temperature, i.e. to a temperature under 50 C, before the coating is worked by rolling in the pair of rolls 15 in Fig. 1. The steel strip 18 is then heated to about 200 C and maintained at this temperature for about 100 seconds so that it becomes overaged. Then, the steel strip 18 is cooled to the ambient temperature.

In both examples above, the coating is thus worked before the ageing and overageing at an elevated temperature. When Al-Zn alloys are aged and overaged, a sequential dispersion of various phases takes place; among the phases is a stable Zn-phase. One explanation of the advantageous effect of the combination of plastic deformation and subsequent ageing at elevated temperature may be that dislocations result from the plastic deformation and that the dislocations are favourable to the nucleation of the stable Zn-phase.

As described the ageing temperature can preferably be about 200°C. If the temperature is lower, the time necessary to overage the coating will be longer. The temperature may not be considerably higher since the ductility will then not increase as a result of the thermal treatment unless the cooling rate above about 205 C is below 1 C per minute. The temperature should be between 150 C and 210 C and preferably between 190 C and 205 C. The duration of the thermal treatmen should be in the range 1-5 minutes and it could be in the range of 1-3 minutes.

For an alloy coating comprising 55% Al, 1.5% Si, balance Zn the following equation seems to be valid:

where tmm. is the req-uired minimum period of ageing in seconds and T is the temperature of ageing in K (degrees Kelvin) . Generally the combination of working and thermal treatment in accordance with the invention seems to reduce the required period of time for ageing by about 90%.

Claims

1. A continuous process for coating a steel strip comprising continuously immersing the strip in a bath of molten aluminium and zinc and cooling the strip to a temperature below 210 C c h a r a c t e r i z e d by the in-line steps of working the solidified coating and thermally treating the worked coating to overage it.

2. A process according to claim 1 c h a r a c t e r i z e d in that the strip is overaged at a temperature between 150 C and 210 C.

3. A process according to claim 2 c h a r a c t e r i z e d in that the strip is overaged at a temperature between 190 C and 205 C.

4. A process according to any one of the preceding claims c h a r a c t e r i z e d in that the strip is thermally treated 1-5 minutes to overage it.

5. A process according to claim 4 c h a r a c t e r i z e d in that the strip is thermally treated 1-3 minutes to overage it.

6. A process according to any one of the preceding claims c h a r a c t e r i z e d in that the working is carried out substantially at the overageing temperature.

7. A process according to any one of the preceding claims, c h a r a c t e r i z e d in that the coating is worked by rolling.

8. A coating apparatus for applying to a steel strip a ductile coating that essentially comprises aluminium and zinc, comprising in series, a hot dip station with a pot (11) with molten metal a cooling station (12-14) , and a thermal treatment station (16) for the strip with the solidified coating, c h a r a c t e r i z e d by a working station (15) between the cooling station (12-14) and the thermal treatment station (16) for working the solidified coating.

9. An apparatus according to claim 8 c h a r a c t e r i z e d in that the working station comprises a pair of rolls (15) .

10. An apparatus according to claim 9 c h a r a c t e r i z e d in that the thermal treatment station (16) is so adapted to the hot dip station that the strip will be thermally treated a time that is between 1 and 5 minutes.