Patent ID: 8888934

Claim:
A method for producing a formed steel part having a predominantly ferritic-bainitic structure, (a) providing a primary material in the shape of one of a steel blank or a pre-formed steel part, comprising in % by weight: C: 0.02-0.6%, Mn: 0.5-2.0%, Al: 0.01-0.06%, Si: max. 0.4%, Cr: max. 1.2%, P: max. 0.035%, S: max. 0.035%, and optionally one or more of the elements from the group consisting of Ti, Cu, B, Mo, Ni, N, with the proviso that Ti: max. 0.05%, Cu: max. 0.01%, B: 0.0008-0.005%, Mo: max. 0.3%, Ni: max. 0.4%, N: max. 0.01%, and the remainder as iron and unavoidable impurities; (b) heating and maintaining the primary material at a heating temperature (TA) lying between an Ac1 and Ac3 temperature of the steel, such that the steel has a two-phase mixed structure of austenite and ferrite; (c) placing the primary material having said two-phase mixed structure of austenite and ferrite into a press-form tool and press forming the primary material into the formed steel part; (d) bringing the formed steel part to a bainite forming temperature (TB), which is above a martensite starting temperature (MS), however below a pearlite transformation temperature of the steel, from which the primary material is produced; (e) maintaining the formed steel part at the bainite forming temperature (TB) for an austempering period (tB) in a substantially isothermic manner, thereby transforming austenite of said two-phase mixture to bainite while avoiding the transformation of austenite into martensite during step (e); and (f) cooling the formed steel part to room temperature after the end of the austempering period (tB), wherein a structure of the cooled steel part consists predominantly of ferrite and bainite, a martensite content thereof being less than 5%, and wherein residual austenite contents of up to 10% may be present.