Next high strength alloys for light weighting

The excellent mechanical properties of high-strength low-alloyed steels are the result of advanced heat treatment and thermomechanical treatment procedures. One such technique is the Q&P process. The process is characterized by rapid cooling to a region between the Ms and Mf temperatures, subsequent heating to just below the Ms and isothermal holding to achieve carbon partitioning between the super-saturated martensite and austenite and to facilitate stabilisation of retained austenite. Combining Q&P with other processes opens additional opportunities for applications of AHSS steels. To demonstrate the benefits of the procedure, 42SiCr steel was processed in a technology chain involving internal high pressure hot forming, press hardening and the thermomechanical Q&P process. A complex-shaped part was selected as a demonstration product. Feedstock in the form of a tube was formed by internal pressure of nitrogen gas in a die cavity. The die wall at the ambient temperature had a quenching effect on the workpiece. After reaching the required temperature the semi-product is carried to the furnace where partitioning is carried out. After stepwise optimization of processing parameters, products with martensitic structure and a small amount of bainite were obtained. In all locations of the product which were tested, the ultimate strength exceeded 1950 MPa and elongation reached 15 %. These procedures clearly demonstrate the potential of modern High Strength Steels for Light Weighting.