Hot-Formed Previously Welded Steel Part with very High Mechanical Resistance and Production Method

A method of fabricating a welded steel part comprising:

• providing at least a first and a second steel sheet each having a steel substrate and a pre-coating which includes an AlSiFe intermetallic alloy layer in contact with the steel substrate, the first and second steel sheets having different compositions or thicknesses, a composition of the steel substrate of the first and the second steel sheets comprising the following elements, expressed in percent by weight:
• 0.10%≦C≦0.5%;
• 0.5%≦Mn≦3%;
• 0.1%≦Si≦1%;
• 0.01%≦Cr≦1%;
• Ti≦0.2%;
• Al≦0.1%;
• S≦0.05%;
• P≦0.1%;
• and
• 0.0002%≦B≦0.010%,
• the balance being iron and unavoidable impurities from processing;
• laser butt welding the first and the second steel sheets at respective peripheral edges of the first and second steel sheets by using a filler wire over at least a portion of the length of the welded zone;
• then
• heating the welded blank to give the weld metal zone a microstructure that contains no ferrite;
• wherein the filler wire has a filler wire composition compensating for presence of aluminum in the AlSiFe intermetallic alloy layer in the weld metal zone.

A method for the fabrication of a welded steel part comprising providing at least a first steel sheet with a pre-coating including an intermetallic alloy layer;

• providing at least a second steel sheet with a pre-coating including an intermetallic alloy layer, the first and second steel sheets having different compositions or thicknesses;
• forming a welded blank by butt laser welding the at least first and the at least second steel sheets to form a weld metal zone and form a bond between the at least first and the at least second sheets, wherein a filler metal wire is used over at least a portion of the welded metal zone;
• heating the welded blank in the an austenitic range followed by hot forming and then cooling to form the welded steel part;
• wherein a carbon content over at least a portion of the weld metal zone is between 1.27 and 1.59 times the carbon content of whichever of the first or second steel sheet has a higher carbon content and wherein a composition of the steel substrate of the at least first or second sheet having a higher carbon content comprises carbon expressed in percent by weight of 0.1%≦C≦0.5% by weight, and wherein a carbon content of the filler wire is 0.6%≦C≦1.5%, by weight.