Processing and properties of C–Mn steel with dual-phase microstructure

Abstract

Effect of applied thermomechanical rolling and heat treatment on dual-phase and multi-phase microstructure formation in steel 15 (formerly 15GA) and achieved mechanical properties was studied. Thermomechanical rolling was made in full austenitic range. After rolling steel was held in the air for 145–250 s in α + γ temperature range then quenched in water. In that way dual-phase microstructure was created in rolled plate of the thickness 12 mm. The heat treatment of steel was based on annealing in α + γ range at temperature 1063 K for 3600 s, quenching in water and tempering at temperature 913 K for 5400 s. Microstructures after these treatments were composed of ferrite matrix and bainite–martensite–austenite islands (BMA). Volume fraction of islands was in the range 19–37% for steel after thermomechanical treatment and 22–29% after heat treatment. The effect of volume fraction of BMA islands and residual austenite within those islands on mechanical properties of steel was observed. High tensile properties were achieved: after thermomechanical rolling: TS = 910 MPa, YS = 760 MPa, El = 14.4% and after water quenching from α + γ temperature range and tempering: TS = 510 MPa, YS = 328 MPa, El = 37.27%.