Coated steel sheet with thin aluminium alloy coating and coating method thereof

A coated steel sheet with an aluminium alloy coating for hot stamping, comprising a substrate steel sheet and the aluminium alloy coating coated on at least one surface thereof,

• wherein the aluminium alloy coating comprises:
• a FeAlSi inhibitive layer adjacent to the substrate steel sheet, and
• an Al alloy layer outside the FeAlSi inhibitive layer,
• wherein a thickness of the FeAlSi inhibitive layer is no more than 60% of a total coating thickness of the aluminium alloy coating, and the thickness of the FeAlSi inhibitive layer is 1.5˜6 μm,
• wherein, at a location within 2 μm from an interface between the FeAlSi inhibitive layer and the substrate steel sheet to an interior of the substrate steel sheet, diameters of Kirkendall voids are no more than 2.5 μm,
• wherein a number of Kirkendall voids with a diameter of no less than 0.5 μm and no more than 2.5 μm does not exceed 15 per 35 μm, and
• wherein the number of Kirkendall voids is determined by the following:
• in a field of view of an SEM image of a vertical cross-section at the location, the number of Kirkendall voids within a length of 35 μm are counted, and the diameter of each Kirkendall void is determined by:
• in the field of view, a longest diameter and a shortest diameter of the Kirkendall void are measured and the diameter of the Kirkendall void is equal to half of a sum of the longest diameter and the shortest diameter.

A coating method for coating a thin aluminium alloy coating onto at least one surface of a substrate steel sheet for hot stamping, comprising:

• a) pre-treating the substrate steel sheet before coating;
• b) after heating the pre-treated substrate steel sheet, cooling it to a predetermined temperature in the range of 610˜650° C.;
• c) dipping the substrate steel sheet cooled to the predetermined temperature in step b) in a heated coating solution for 2˜7 seconds for hot-dip coating, wherein the composition of the coating solution comprises by weight:
• 9%˜12% Si, and the balance of Al or Al alloy and unavoidable impurities;
• in the process, the temperature of the coating solution is higher than the predetermined temperature and maintained at 630˜670° C.;
• d) after the substrate steel sheet is out of the coating solution and before the coating solution on the at least one surface of the substrate steel sheet solidifies, removing excessive coating solution on the at least one surface by air knife purge to control a coating thickness on the at least one surface;
• and
• e) cooling the substrate steel sheet to room temperature to obtain a coated steel sheet with a thin aluminium alloy coating,
• wherein the substrate steel sheet comprises the following constituents by weight:
• 0.05˜0.45% C, 0.5˜10% Mn, 0˜0.01% B, 0˜0.4% Nb+Ti+V, 0.01˜2% Si, 0.01˜2% Al, 0.01˜5% Cr+Ni+Mo+Cu and 0˜2% Cr, 0˜2% Ni, 0˜2% Mo and 0˜2% Cu, and the balance of Fe and unavoidable impurity elements,
• wherein the aluminium alloy coating comprises:
• a FeAlSi inhibitive layer adjacent to the substrate steel sheet;
• and an Al alloy layer outside the FeAlSi inhibitive layer,
• wherein a thickness of the FeAlSi inhibitive layer is no more than 60% of the total coating thickness of the aluminium alloy coating, and the thickness of the FeAlSi inhibitive layer is 1.5˜6 μm,
• wherein, at a location within 2 μm from an interface between the FeAlSi inhibitive layer and the substrate steel sheet to an interior of the substrate steel sheet, diameters of Kirkendall voids are no more than 2.5 μm, wherein a number of Kirkendall voids with a diameter of no less than 0.5 μm and no more than 2.5 μm does not exceed 15 per 35 μm, and
• wherein the number of Kirkendall voids is determined by the following:
• in a field of view of an SEM image of a vertical cross-section at the location, the number of Kirkendall voids within a length of 35 μm are counted, and the diameter of each Kirkendall void is determined by:
• in the field of view, a longest diameter and a shortest diameter of the Kirkendall void are measured and the diameter of the Kirkendall void is equal to half of a sum of the longest diameter and the shortest diameter.