Use of the Press Hardening Technology for Treatment of TRIP Steel
Multiphase TRIP steels are able to absorb impact-deformation energy and therefore they are used as safety components in vehicle construction. One of the ways how to produce these precision sheet metal parts is the press hardening technology. Besides specific alloying which in most cases is done with manganese and silicon, the properties of TRIP steels are also determined by the appropriate heat treatment parameters. The key processing parameter is the holding time at the temperature of the bainitic transformation, at which not only the formation of bainite but also the stabilization of retained austenite take place. This holding time is technologically demanding for use in industry. Therefore, the possibility of processing this type of steel by press hardening technology has been experimentally verified.
Material-technology modelling was used to build models corresponding to the press hardening temperature profile. These models were tested on low-alloy CMnSi steels. In the experimental program, the effect of the tool temperature in the range from RT to 550°C, as well as the influence of different cooling rates on the structure development and mechanical properties was investigated. Mixed structures from ferritic-martensitic to ferritic-martensitic-bainitic with different volume fraction of retained austenite were obtained. Tensile strengths ranged from 730 to 1300 MPa with elongations from 24 to 9%.
Key words: TRIP steel, retained austenite, press hardening