Influence of Manganese and Boron Alloying and Processing Conditions on the Microstructure and the Mechanical Properties of 0.4% Carbon Steels

The performance of three steels with 0.4% carbon content was evaluated in the warm forging condition and benchmarked against the as-hot-deformed and hardening and tempering condition. The steels include a plain carbon steel S45C, a carbon-manganese steel 40Mn5 and a carbon-manganese-boron-alloyed steel 15B41. Such lean alloyed steels are preferred by automotive companies for component manufacture. In the as-hot-rolled condition, the steel with a ferrite pearlite microstructure shows, a good strength and lowest impact toughness. In the hardened and tempered condition at 650 °C, a tempered martensite microstructure with a moderate improvement in strength and significant enhancement in impact toughness, than the hot-rolled steel, is obtained. The steel subjected to warm forging at 760 °C, shows a ferrite pearlitic microstructure with a moderate improvement in strength but with the highest impact toughness level. The microstructure evolution at the various processing conditions has been brought out and correlated with the mechanical properties. The relative merits in processing the steels at the different processing conditions show that warm forging is an effective means to achieve the highest strength toughness combination in a ferrite pearlitic matrix.