Metal-coated steel strip

A hot-dip coating method to reduce mottling defect on a surface of an alloy coating of a coated steel strip, the method comprising:

• passing a steel strip through a hot dip coating bath that contains aluminium, zinc, silicon, and magnesium to form an alloy coating on the steel strip, wherein the alloy coating comprises, in weight %, 40% to 60% aluminium;
• 40% to 60% zinc;
• 0.3% to 3% silicon;
• and 0.3% to 10% magnesium, and
• controlling a short range coating thickness variation of the alloy coating to be no more than 40% in a 5 mm diameter section of the coating;
• controlling solidification of the alloy coating, including:
• cooling the coated steel strip at a cooling rate of:
• greater than 11° C./sec and less than 80° C./sec for coating masses up to 75 grams per square meter of strip surface per side, or
• greater than 11° C./sec and less than 50° C./sec for coating masses of 75-100 grams per square meter of strip surface per side,
• wherein the alloy coating has a microstructure comprising Mg2Si particles, and
• wherein the method controls the distribution of the Mg2Si within the coating microstructure such that the surface of the alloy coating has no more than 10 wt % of Mg2Si particles to thereby reduce mottling defect on the surface of the alloy coating.