Microstructural development in a hot rolled multi-phase steel

The present work describes the development of microstructures in multi-phase steels with minimum ultimate tensile strength (UTS) in the range of 750–800 MPa. A relatively leaner composition was designed to achieve the strength, and with two compositions, the UTS values of about 750 MPa and 800 MPa were achieved. In both cases, multi-phase microstructures were developed, which consisted of ferrite, bainite and martensite. Bainite was the major phase in these microstructures. The cooling rate played a vital role. Ferrite was observed to form as the major phase along with islands of bainite at a cooling rate of 2 °C/s or less than that. The desired amount of bainite phase could be obtained at and above a cooling rate of 20 °C/s. A double step cooling strategy was applied to achieve the desired multi-phase microstructure. After the completion of hot rolling at around 850 °C, a fast cooling to a temperature of 580 °C and holding there for a duration of 5–12 s led to the formation of ample bainite in the microstructure, followed by transformation of remaining austenite into martensite during the second step cooling at 250 °C.