Effect of caliber rolling reduction ratios on the microstructure and mechanical properties of 45 medium carbon steel

Tempered medium carbon steel with poor low-temperature toughness always limits its practical applications, while temper forming process can realize the strengthening-toughening aim by changing the microstructure. In this work, a grade 45 medium carbon steel was deformed with different rolling reduction ratios of 28%, 52%, 78%, and 82% by caliber rolling at the temperature of 500oC, respectively. The texture intensity, high angle boundaries density, and elongated fibrous grains are gradually increased, as well as the tensile strength, fracture elongation, and low-temperature impact toughness are also improved with the increase of caliber rolling reduction ratio, and strengthening-toughening aim of 45-grade steel has been realized at high caliber rolling reduction ratio. Fibrous grains, //RD texture, grain boundaries type, and carbides bimodal distribution resulted in the formation of crack bifurcation and delamination cracks during the tensile testing and impact testing, which can obviously decrease the ductile-brittle transition temperature and improve the low-temperature impact toughness. The caliber rolled 45 steel at the rolling reduction ratio of 82% obtains superior mechanical properties with the tensile strength of 964MPa, uniform elongation of 0.18, and low-temperature impact toughness of 176J at −40oC, which can effectively extend the practical low-temperature service range of 45-grade steel.