The implementation of press-hardening applications and the utilization of hardenable steels are promising alternatives for optimized part geometries with complex shapes and no springback issues. Boron-based hot-forming steels (between 0.001% and 0.005% boron) have been in use since the 1990s for body-in-white construction. A typical minimum temperature of 850 °C must be maintained during the forming process (austenitization) followed by a cooling rate greater than 50 °C/s to ensure that the desired mechanical properties are achieved. Two types of press-hardening or hot forming applications are currently available:
During Direct Hot-Forming, all deformation of the blank is done in the high temperature austenitic range followed by quenching. Indirect Hot-Forming preforms the blank at room temperature to a high percentage of the final part shape followed by additional high temperature forming and quenching. The final microstructure of HF steel is similar to Martensite. Stress-strain curves after quenching are similar to martensitic (MS) steels.
Additional information on Hot Forming is located in Section 3.B.4. of the Advanced High-Strength Steels Application Guidelines. Current production grades of HF steels and automotive applications:
HF 340/480 As-received room temperature
HF 1050/1500 Heat treated after forming A-pillar, B-pillar, cross beam
HF 1200/1900 Heat treated after forming