Herein, we revisited the influence of Nb and Mo on the microstructure, tensile properties, and bendability of hot-rolled and tempered martensitic steels with > 1 GPa. To produce martensitic steels with good ductility, we utilized three different heat treatments, i.e., hot-rolled followed by air cooling (HR), HR followed by austenzing and water quenching (HR-RQ), and tempering followed by air cooling after HR-RQ (HR-RQ-T).
In the HR state, Nb addition introduces yield strength (YS) gain (~100 MPa) through grain size reduction and Orowan strengthening of Nb-rich precipitates. Meanwhile, Mo addition provides further YS increment of 120 MPa by solid-solution strengthening and grain refinement. Tempering provides ─ˇYS = 180 MPa without grain size change, resulting from hampering of dislocation movement by C, i.e., Taylor hardening.