Microstructure and Mechanical Properties of Low, Medium and High Si Press Hardening Steels subjected to Q&PTuesday (27.09.2016) 15:30 - 15:45 Part of:
While press hardening steels are of great and growing importance for lightweight construction in the automotive industry their use is constrained due to their limited ductility. To address this weakness, press hardening steels (22MnB5) with modified Si contents were subjected to different Q&P heat treatments during the quenching step of the hot stamping process with a Gleeble thermomechanical simulator. Thermocalc (DICTRA) calculations were used to determine the minimal duration for partitioning to ensure short processing times suitable for industrialization. Mechanical properties were characterized by tensile tests and plate bending tests (VDA 238-100), microstructure by optical as well as transmission electron microscopy and residual austenite content (RA) by microdiffraction.
As expected, the high Si alloy displayed the highest RA content as well as a superior product of ultimate tensile strength (UTS) and total elongation (TEL) throughout all Q&P heat treatments. Its tensile test performance was unmatched and significantly stronger than of reference 22MnB5 press hardening steel. Regarding the product of yield strength (YS) and TEL, the high Si variant still showed the best combination but was closely followed by the medium and low Si alloys. Considering the yield strength of > 1100 MPa for most alloys and conditions the bending angles were significantly improved compared to the fully hardened 22MnB5 reference. However, in contrast to elongation properties, bending was not positively influenced by RA. Since they generally contained less RA, the medium and low Si alloys exhibited better and more stable bending angles after Q&P heat treatment.
In conclusion, medium and low Si Q&P steels for press hardening may pose an interesting set of properties for the automotive industry as they feature a good combination of strength (YS) and ductility (bending angle and elongation) without compromising on coatability.