Metal-coated steel strip

Abstract

A method of forming an Al—Zn—Si—Mg alloy coating on a steel strip includes dipping steel strip into a bath of molten Al—Zn—Si—Mg alloy and forming a coating of the alloy on exposed surfaces of the steel strip. The method also includes controlling conditions in the molten coating bath and downstream of the coating bath so that there is a uniform Al/Zn ratio across the surface of the coating formed on the steel strip. An Al—Zn—Mg—Si coated steel strip includes a uniform Al/Zn ratio on the surface or the outermost 1-2 μm of the Al—Zn—Si—Mg alloy coating.

Claims

1. A method of forming an Al—Zn—Si—Mg alloy coating on a steel strip having no visual apparent ash mark defect, the method including: dipping the steel strip into a bath of molten Al—Zn—Si—Mg alloy containing, in % by weight, Zn: 30% to 60% Si: 0.3% to 3% Mg: more than 1.8% to and less than 3.0% Balance: Al and unavoidable impurities; forming a coating of the alloy on exposed surfaces of the steel strip; controlling Ca composition of the molten Al—Zn—Si—Mg alloy in the bath to be at least 100 ppm and less than 200 ppm, and controlling the rate of cooling of a coated steel strip after the coated steel strip leaves the bath to be greater than 10° C./s and less than 40° C./s while the coated strip temperature is between 400° C. and 510° C., so that there is a uniform Al/Zn ratio across the surface of the coating formed on the steel strip such that a variation in an Al/Zn ratio between two or more independent areas on a surface of the coating formed on the steel strip is less than 0.1.

2. The method defined in claim 1 includes controlling the Ca concentration of the molten coating bath to be less than 180 ppm.

3. The method defined in claim 1 wherein the Al—Zn—Si—Mg alloy includes less than 2.5% by weight Mg.

4. The method defined in claim 1 wherein the Al—Zn—Si—Mg alloy includes more than 1.2% by weight Si.

5. The method defined in claim 1 wherein the Al—Zn—Si—Mg alloy includes less than 2.5% by weight Si.

6. The method defined in claim 1 wherein the Al—Zn—Si—Mg alloy includes: Zn: 35% to 50% Si: 1.2% to 2.5% Mg: more than 1.8% to and less than 3.0% Balance: Al and unavoidable impurities.

7. The method defined in claim 1 includes controlling the Ca concentration of the molten coating bath to be at least 120 ppm.

8. The method defined in claim 1 including taking a sample from the molten coating bath and measuring the Ca concentration and the Mg concentration in the molten coating bath.

9. The method defined in claim 1 wherein the coating has uniform surface/sub-surface distribution of Mg.sub.2Si in the microstructure of the coating.

10. The method defined in claim 1 including controlling the cooling rate of the coated strip to be less than 35° C./s while the coated strip temperature is between 400° C. and 510° C.

11. The method defined in claim 1 wherein the coated strip has a coating mass of 50-200 g/m.sup.2.

12. The method defined in claim 1 wherein the Al—Zn—Si—Mg alloy is maintained molten in the coating bath at a temperature in a range of 595-610° C.

Description

DESCRIPTION OF DRAWINGS

(1) The present invention is described further by way of example with reference to the accompanying drawings of which:

(2) FIG. 1 is the above-described photograph of part of the surface of the Al—Zn—Si—Mg alloy coated steel strip from the plant trials captured under ideal viewing conditions; and

(3) FIG. 2 is a schematic drawing of one embodiment of a continuous production line for producing steel strip coated with an Al—Zn—Si—Mg alloy in accordance with the method of the present invention.

DESCRIPTION OF EMBODIMENT OF THE INVENTION

(4) With reference to FIG. 2, in use, coils of cold-rolled low carbon steel strip are uncoiled at an uncoiling station 1 and successive uncoiled lengths of strip are welded end to end by a welder 2 and form a continuous length of strip.

(5) The strip is then passed successively through an accumulator 3, a strip cleaning section 4 and a furnace assembly 5. The furnace assembly 5 includes a preheater, a pre-heat reducing furnace, and a reducing furnace.

(6) The strip is heat treated in the furnace assembly by careful control of process variables including: (i) to the temperature profile in the furnaces, (ii) the reducing gas concentration in the furnaces, (iii) the gas flow rate through the furnaces, and (iv) strip residence time in the furnaces (i.e. line speed).

(7) The process variables in the furnace assembly 5 are controlled so that there is removal of iron oxide residues from the surface of the strip and removal of residual oils and iron fines from the surface of the strip.

(8) The heat treated strip is then passed via an outlet snout downwardly into and through a molten bath containing an Al—Zn—Si—Mg alloy having a Ca concentration in a range of 100-200 ppm in a coating pot 6 and is coated with Al—Zn—Si—Mg alloy. The Al—Zn—Si—Mg alloy is maintained molten in the coating pot at a selected temperature in a range of 595-610° C. by use of heating inductors (not shown). Within the bath the strip passes around a sink roll and is taken upwardly out of the bath. The line speed is selected to provide a selected immersion time of strip in the coating bath to produce a coating having a coating mass of 50-200 g/m.sup.2 on both surfaces of the strip.

(9) After leaving the coating bath 6 the strip passes vertically through a gas wiping station (not shown) at which its coated surfaces are subjected to jets of wiping gas to control the thickness of the coating.

(10) The coated strip is then passed through a cooling section 7 and subjected to forced cooling at a selected cooling rate greater than 10° C./s but less than 40° C./s while the coated strip temperature is between 400° C. and 510° C. The cooling rate may be any suitable cooling rate at coated strip temperatures less than 400° C. or greater than 510° C.

(11) The cooled, coated strip is then passed through a rolling section 8 that conditions the surface of the coated strip.

(12) The coated strip is thereafter coiled at a coiling station 10.

(13) As discussed above, the applicant has conducted extensive research and development work in relation to Al—Zn—Si—Mg alloy coatings on steel strip which includes plant trials and the applicant noticed a defect on the surface of Al—Zn—Si—Mg alloy coated steel strip during plant trials. The plant trials were carried out with an Al—Zn—Si—Mg alloy having the following composition, in wt. %: 53Al-43Zn-2Mg-1.5Si—0.45Fe and incidental impurities. The applicant was surprised that the defect occurred. The applicant had not observed the defect in extensive laboratory work on Al—Zn—Si—Mg alloy coatings. Moreover, since noticing the defect in plant trials, the applicant has not been able to reproduce the defect in the laboratory. The applicant has not observed the defect on standard 55% Al—Zn alloy coated steel strip that has been available commercially in Australia and elsewhere for many years. Moreover, as discussed above, the applicant has found that the defect has a number of different forms, including streaks, patches, and a wood grain pattern, and severe examples of each of these forms of the defect are shown in FIG. 1.

(14) As is discussed above, the applicant has found that the above-described defect is due to variations in the Al/Zn ratio on the surface of Al—Zn—Si—Mg alloy coatings and may be due to a non-uniform distribution of Mg.sub.2Si in the microstructure of the of coatings and the invention includes controlling conditions in the molten coating bath and downstream of the coating bath so that there is a uniform Al/Zn ratio across the surface of the coating formed on the steel strip.

(15) The method of the invention includes controlling any suitable conditions in the molten coating bath and downstream of the coating bath so that there is a uniform Al/Zn ratio (in accordance with the definition on page 5) across the surface of the coating, i.e. on or within the outermost 1-2 μm of the coating cross section, formed on the steel strip.

(16) By way of example, the embodiment of the method of the invention described in relation to FIG. 2 includes controlling (a) the Ca concentration in the molten coating bath, (b) the Mg concentration of the molten coating bath, and (c) the rate of cooling the coated steel strip after the coated steel strip leaves the molten coating bath, as described above in the description of FIG. 2.

(17) It is noted that the invention is not confined to controlling this combination of conditions.

(18) Many modifications may be made to the present invention described above without departing from the spirit and scope of the invention.