New Extremely Low Carbon Bainitic High-Strength Steel Bar Produced Without Heat Treatment Having Excellent Machinability and Toughness

摘要

Low alloy steels such as AISI 4137 have been used for many kinds of machine components, which have been usually manufactured by a machining followed by a quenching and tempering. It is preferable that these components can be produced without a heat treating in terms of energy and cost savings. A non heat-treated high strength steel for machine structural use has been newly developed through a thermo-mechanical precipitation control process (hereafter, TPCP). The TPCP is a technology for controlling the strength of steel by a precipitation hardening with an extremely low carbon bainitic microstructure. The carbon content of the steel is decreased below 0.02mass% for realizing the proper microstructure, which improves both machinability and notch toughness. In order to make the microstructure bainitic and to obtain an effective precipitation hardening, some micro-alloying elements are added. This advanced technology was applied to the manufactur-ing of the steel. The developed steel showed higher yield strength and better notch toughness than those of a quenched and tempered AISI 4137 steel. It was also proved that the steel had an excellent machinability because of the extremely low carbon bainitic microstructure.