A Novel Tri-Phase Lightweight Press-Hardening Steel Exhibiting Both Excellent Mechanical Properties and Outstanding Oxidization Resistance

A novel Si-Al alloyed press hardening steel (PHS) with the ferrite(α)-austenite(γ)-martensite(α′) triplex microstructure was developed to have marginal oxidization after the hot forming in air, and the resultant ultimate tensile strength (UTS) of 1620 MPa and total elongation (TE) of 14% were achieved simultaneously at reduced density. Both performances are superior to the existing PHSs. Particularly, the baking at 170 °C leads to remarkable increases in both strength and ductility together with the transition of tensile fracture from the brittle interfacial cracking to the ductile one. This is because austenite and ferrite were hardened due to the segregation of C atoms into geometry necessary dislocations (GND) but martensite softened due to the loss of supersaturated C atoms during the baking. Atomic probe tomography examination reveals that some C atoms segregated to the α/α′ interfaces during the baking for increased cohesive energy of the phase interface, thus hindering the interfacial cracking. In addition, the employed Al/Si alloying affords stronger oxidization resistance than both Al/Cr and Si/Cr because they are more rapidly oxidized than Cr to form the dense Al2O3/SiO2 layers for earlier protection.