Trip-aided bainitic-ferritic sheet steel: A critical assessment of alloy design and heat treatment

Thin sheet steel grades with a bainitic and/or tempered martensitic matrix containing retained austenite are member of the newly developed third generation advanced high strength steels (AHSS). Due to the high strength of the bainitic and/or martensitic matrix and an appropriate amount of strain induced martensite formed from retained austenite, these steels possess a favorable combination of high deep drawability and stretch flangeability. Such steel grades with a minimum tensile strength of 1180 MPa were laboratory-processed to investigate their response of microstructure and mechanical properties to varying alloying additions and applied heat treatments. Particular attention was devoted to the development of a steel grade with a reduced Si content (Si < 1.0 mass-%) and the transfer of this alloying concept to large scale production. The newly designed grade has been produced successfully on a continuous annealing line resulting in attractive tensile properties combined with low edge crack susceptibility.