Method for producing a formed steel part having a predominantly ferritic-bainitic structure

Abstract

In a method to produce formed steel parts a primary steel material is provided, which (in % by weight) comprises 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 of the 'Ti, Cu, B, Mo, Ni, N' group, 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. The primary material is heated through at a heating temperature (TA) lying between the Ac1 and the Ac3 temperature, such that at best incomplete austenitising of the primary material takes place, is placed into a press-form tool and formed therein into the formed steel part. The formed steel part is then heated to a bainite forming temperature (TB), which is above the martensite starting temperature (MS), however below the pearlite transformation temperature of the steel. After cooling, it is maintained for an austernpering period (tB) at the bainite forming temperature (TB) in a substantially isothermic manner, until the foamed steel part has produced a structure consisting predominantly of ferrite and bainite, the martensite content thereof being <5%, wherein residual austenite contents of <10% may be present. The formed part is then cooled to room temperature.