Metal-coated steel strip

An Al—Zn—Si—Mg alloy coated steel strip comprising a coating of an Al—Zn—Si—Mg alloy, the Al—Zn—Si—Mg alloy coating comprising:

• 40 to 60 wt % aluminum;
• 40 to 60 wt % zinc;
• 0.3 to 3 wt % silicon;
• and 0.3 to 10 wt % magnesium;
• a thickness greater than 7 μm and less than 30 μm and a coating thickness variation being no more than 40% in a 5 mm diameter section of the coating;
• and
• a microstructure comprising Mg2Si particles,
• wherein the surface of the coating has no more than 10 wt % of Mg2Si particles to lower risk of mottling defect on the surface.

A hot-dip coating method for forming a coating of a corrosion-resistant Al—Zn—Si—Mg alloy on a steel strip with lower risk of mottling defect on the surface of the coating, the method comprising:

• passing a steel strip through a hot dip coating bath that comprises aluminum, zinc, silicon, and magnesium to form an alloy coating on the steel strip having, in weight %, 40 to 60% aluminum;
• 40 to 60% zinc;
• 0.3 to 3% silicon;
• and 0.3 to 10% magnesium, and
• cooling the coated steel strip exiting the hot dip coating bath during solidification of the coating at a cooling rate of less than 80° C./sec and greater than 11° C./sec for coating masses up to 75 grams per square meter of strip surface per side and at a cooling rate of less than 50° C./sec and greater than 11° C./sec for coating masses of 75-100 grams per square meter of strip surface per side,
• wherein the coating having a coating thickness greater than 7 μm and less than 30 μm and a coating thickness variation being no more than 40% in a 5 mm diameter section of the coating, and
• wherein the coating having a microstructure comprising Mg2Si particles such that the surface of the coating has no more than 10 wt % of Mg2Si particles to lower risk of mottling defect on the surface.