METAL COATED STEEL STRIP

Abstract

A steel strip that has a coating of an Al—Zn—Si alloy that contains 0.3-10 wt. % Mg and 0.005-0.2 wt. % V.

Claims

1. A cold rolled steel strip that has a corrosion-resistant coating of an Al—Zn—Si—Mg alloy providing galvanic protection, the coating containing the following ranges in % by weight of the elements Al, Zn, Si, Mg and V: Al: 45 to 60%; Zn: 35 to 60%; Si: 1.2 to 2.5%; Mg: 1.0 to 3.0%; and V: 0.03 to 0.2%, wherein the V is present in the range of 0.03 to 0.2% as a deliberate alloy addition and not as an unavoidable impurity.

2. The steel strip defined in claim 1 wherein the alloy coating contains less than 0.15 wt. % V.

3. The steel strip defined in claim 1 wherein the alloy coating contains less than 0.1 wt. % V.

4. The steel strip defined in claim 1 wherein the alloy coating contains other elements present as unavoidable impurities and/or as deliberate alloy additions.

5. The steel strip defined in claim 1 wherein the alloy coating is a single layer.

6. A cold formed end-use product comprising the steel strip defined in claim 1.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0056] The present invention is described further by way of example with reference to the accompanying drawings, of which:

[0057] FIG. 1 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; and

[0058] FIG. 2 is an Anodic Tafel plot showing a comparison of coating alloys, including an embodiment of an alloy coating in accordance with the present invention.

DETAILED DESCRIPTION

[0059] With reference to FIG. 1, in use, coils of cold rolled 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.

[0060] 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 preheat reducing furnace, and a reducing furnace.

[0061] The strip is heat treated in the furnace assembly 5 by careful control of process variables including: (i) 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).

[0062] 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.

[0063] 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 held 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 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. Both surfaces of the strip are coated with the Al—Zn—Si—Mg alloy as it passes through the bath.

[0064] 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.

[0065] The coated strip is then passed through a cooling section 7 and subjected to forced cooling.

[0066] The cooled, coated strip is then passed through a rolling section 8 that conditions the surface of the coated strip.

[0067] The coated strip is thereafter coiled at a coiling station 10.

[0068] As is indicated above, the present invention is based on research work carried out by the applicant on the known 55% Al—Zn—Si alloy coating on steel strip which found that magnesium and vanadium enhance specific aspects of corrosion performance of the coated steel strip.

[0069] The research work included accelerated corrosion testing and outdoor exposure testing in acidic and marine environments for extended time periods.

[0070] The Anodic Tafel plot in FIG. 2 illustrates the results of a part of the research work showing a comparison of alloy layers in neutral pH 0.1M NaCl. The plot shows the logarithm of the current density (“J”—in A/cm.sup.2) against the electrode potential (in Volts) for 3 alloy compositions. The plot shows the results of research work on coatings of (a) the known 55% Al—Zn—Si alloy (“AZ”), (b) an Al—Zn—Si—Zn alloy containing Ca (“AM(Ca)”), and (c) an Al—Zn—Si—Zn alloy containing V in accordance with one embodiment of the present invention (“AM(V)”).

[0071] The plot of FIG. 2 compares the corrosion performance of the alloy coatings (a), (h), and (c). The plot and other results obtained by the applicant indicate that:

[0072] (a) the AM(V) alloy coating of the present invention had a lower corrosion current at a given corrosion potential than the other alloy coatings (1.5-2 times improvement of AM(V) over AM(Ca)):

[0073] (b) the AM(V) alloy coating of the present invention had more noble corrosion potential compared to AM(Ca) (+0.03 V and +0.11 V respectively);

[0074] (c) the AM(V) alloy coating of the present invention had more noble pitting potential compared to AM(Ca) (+0.04 V and +0.18 V respectively); and

[0075] (d) the AM(V) alloy coating of the present invention had significantly lower oxidative current under anodic polarisation—compared to AM(Ca), at −025 V, the oxidative current is about 20000 times less for AM(V).

[0076] These improvements in the resistance for anodic dissolution of the alloy layer imply that upon exposure of the alloy coating of the present invention to corrodants (salt, acid, and dissolved oxygen) the metallurgical phase will corrode at a slow rate and the mode of corrosion will be generalised and less prone to localised and pitting corrosion mode. These properties will impart a longer life in an end-use product, as it will be rendered less likely to red rust staining, metal coating blistering and substrate perforation.

[0077] Many modifications may be made to the present invention as described above without departing from the spirit and scope of the invention.