Bluescope Steel Limited

Nippon Steel Corporation

Nippon Steel Coated Sheet Corporation

Joe Williams

Qiyang Liu

Wayne Andrew Renshaw

Ross McDowall Smith

Aaron Kiffer Neufeld

David James Nolan

Bryan Andrew Shedden

Nobuyuki Shimoda

Shiro Fujii

Cat Tu

Jason Hodges

Shuichi Kondo

Takashi Hirasawa

Ross Mcdowall Smith

Robert Ian Scott

An Al—Zn—Si—Mg alloy coated strip that has MgSi particles in the coating microstructure is disclosed. The distribution of MgSi particles is such that the surface of the coating has only a small proportion of MgSi particles or is at least substantially free of any MgSi particles.

Metal-coated steel strip

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

An Al&#x2014;Zn&#x2014;Si&#x2014;Mg alloy coated strip that has MgSi particles in the coating microstructure is disclosed. The distribution of MgSi particles is such that the surface of the coating has only a small proportion of MgSi particles or is at least substantially free of any MgSi particles.

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 an Al&#x2014;Zn&#x2014;Si&#x2014;Mg alloy coating on a steel strip includes 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. 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&#x2014;Zn&#x2014;Mg&#x2014;Si coated steel strip includes a uniform Al/Zn ratio on the surface or the outermost 1-2 &#x3bc;m of the Al&#x2014;Zn&#x2014;Si&#x2014;Mg alloy coating.

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.

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.

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Woonona, Australia

Joe Williams