FIELD: manufacturing technology.
SUBSTANCE: invention relates to a part of a steel sheet or steel tape coated with a press on an aluminum base, suitable for varnishing and contact spot welding, and a method of making said part and can be used for production of automobiles. Said part has a coating comprising a base layer based on aluminum and containing silicon. Said part has zone I of mutual diffusion in transition area between steel sheet and main coating layer. Thickness of the mutual diffusion zone I is determined using the following formula: I [mcm] < 1/35 × weight of coating layer on both sides of steel part [g/m2] + 19/7. At zone of mutual diffusion there formed is a zone containing Fe2SiAl2 and FeAl2 intermetallic phases, having average overall thickness of 8–50 mcm. Surface layer of aluminum oxide and/or hydroxide with average thickness of at least 0.05 mcm to maximum 5 mcm is located on the zone with said intermetallic phases. Said method of making the discussed part involves depositing a coating on steel sheet or steel tape by hot dip method, comprising a base layer based on aluminum and containing silicon. Steel sheet or steel tape with applied main layer of coating is subjected to anodizing or plasma oxidation and/or treatment with hot water at temperature of at least 90 °C and/or steam treatment at temperature of at least 90 °C to form on surface of main layer surface layer containing oxide and/or aluminum hydroxide and having thickness of at least 0.05 mcm to maximum 0.5 mcm. Then steel plate or steel strip is heated at least partially to temperature above austenitization temperature, subject to molding, then cooled at a rate which is at least partially above critical rate of cooling.
EFFECT: obtaining a part with excellent varnishing and weldability characteristics, in particular resistance to spot welding.
14 cl, 3 dwg, 1 tbl
