Bluescope Steel Limited

Ross McDowall Smith

Qiyang Liu

Joe Williams

Wayne Andrew Renshaw

Aaron Kiffer Neufeld

Bryan Andrew Shedden

David James Nolan

Geoff Tapsell

Red rust staining of Al/Zn coated steel strip in 'acid rain' or “polluted” environments can be minimised by forming the coating as an Al—Zn—Si—Mg alloy coating with an OT:SDAS ratio greater than a value of 0.5:1, where OT is the overlay thickness on a surface of the strip and SDAS is the measure of the secondary dendrite arm spacing for the Al-rich alpha phase dendrites in the coating. Red rust staining in “acid rain” or “polluted” environments and corrosion at cut edges in marine environments can be minimised in Al—Zn—Si—Mg alloy coatings on steel strip by selection of the composition (principally Mg and Si) and solidification control (principally by cooling rate) and forming MgSi phase particles of a particular morphology in interdendritic channels.

Corrosion protection with Al/Zn-based coatings

Red rust staining of Al/Zn coated steel strip in 'acid rain' or 'polluted' environments can be minimised by forming the coating as an Al&#x2014;Zn&#x2014;Si&#x2014;Mg alloy coating with an OT:SDAS ratio greater than a value of 0.5:1, where OT is the overlay thickness on a surface of the strip and SDAS is the measure of the secondary dendrite arm spacing for the Al-rich alpha phase dendrites in the coating. Red rust staining in &#x201c;acid rain&#x201d; or &#x201c;polluted&#x201d; environments and corrosion at cut edges in marine environments can be minimised in Al&#x2014;Zn&#x2014;Si&#x2014;Mg alloy coatings on steel strip by selection of the composition (principally Mg and Si) and solidification control (principally by cooling rate) and forming MgSi phase particles of a particular morphology in interdendritic channels.

A method of forming a coating of an Al&#x2014;Zn&#x2014;Si&#x2014;Mg alloy on a steel strip to form an Al&#x2014;Zn&#x2014;Mg&#x2014;Si coated steel strip is disclosed. The method includes the steps of dipping steel strip into a bath of molten Al&#x2014;Zn&#x2014;Si&#x2014;Mg alloy and forming a coating of the alloy on exposed surfaces of the steel strip and cooling the coated strip with cooling water. The cooling step includes controlling the p H of cooling water to be in a range of pH 5-9. Particular embodiments focus on Al&#x2014;Zn&#x2014;Si&#x2014;Mg alloys that contain the following elements in % by weight: Zn: 2 to 19, Si: 0.01 to 2, Mg: 1 to 10, and Balance Al and unavoidable impurities.

Method of producing metal-coated steel strip

An Al&#x2014;Zn&#x2014;Si&#x2014;Mg alloy coated strip that has MgSi phase particles that are &#x2264;2 &#x3bc;m and have a globular shape. A method of forming an Al&#x2014;Zn&#x2014;Si&#x2014;Mg alloy coated strip comprises (a) heat treating a solidified coating to facilitate globularisation of MgSi phase particles in the coating and/or (b) changing the coating bath chemistry to form intermetallic compound phases that act as nucleation sites for MgSi phase particles with the result that small MgSi particles form on solidification of the coating.

Metal-coated steel strip

A steel strip having a coating of an aluminium-zinc-silicon alloy on at least one surface of the strip is disclosed. The strip is characterised in that the aluminium-zinc-silicon alloy contains less than 1.2 wt. % silicon and also contains magnesium. A method of forming a coating of an aluminium-zinc-silicon alloy on a steel strip is also disclosed. The method includes moving steel strip upwardly through a coating pot containing a bath of an aluminium-zinc-silicon alloy and having an opening in a bottom wall of the pot and forming a coating of the alloy on the strip. The method is characterized by minimizing residence time of steel strip in contact with the aluminium-zinc-silicon alloy bath in the pot.

A method of treating an Al/Zn-based alloy coated product that includes an Al/Zn-based alloy coating on a substrate is disclosed. The method includes the steps of rapid intense heating of the alloy coating for a very short duration, and rapid cooling of the alloy coating, and forming a modified crystalline microstructure of the alloy coating.

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Cordeaux Heights, Australia

Ross McDowall Smith