A thermomechanical process to achieve mechanical properties comparable to those of quenched-tempered medium-C steel

In quenching-tempering (QT), limiting factors such as hardenability, temper embrittlement, quenchant pollution, and quenching distortion and cracks have to be considered. In order to avoid these limitations, we propose a thermomechanical process to fabricate spheroidized cementite in ferrite matrix in a medium-C steel with mechanical properties that are comparable to QT, which is based on multi-pass caliber rolling at start rolling temperature (SRT) in two-phase α+γ region. In this study, the effect of the SRT (in the range from 720 °C to 820 °C) on microstructures, mechanical properties, and impact fracture behavior were investigated by using 45 steel as experimental material. Results indicate that the rolling can result in a fibrous “ferrite + pearlite” microstructure. If the SRT is at 740 °C to 760 °C, then the multi-sized ferrite grains with intensive α-fiber texture (<101>//RD) and the completely spheroidized cementite distributed in the fine ferrite grain region can be formed. The spheroidization of cementite is attributed to the deformation-induced divorced eutectoid transformation. The mechanical properties significantly exceeded the requirements of the Chinese National Standard GB/T 699−1999 for quenched-tempered 45 steel. When the SRT decreased from 780 °C to 820 °C to 720 °C to 760 °C, the Charpy impact absorbed energy (AKV) increased from approximately 50 J to 100 J, and the impact fracture behavior changes from dimple along with some splits into quasi-cleavage. This thermomechanical process is a potential alternative to QT of medium-C steels.