Effects of new heat treatment process on microstructure and mechanical properties of low carbon Nb-Ti micro-alloyed steels

The test steels were quenched to room temperature using a new heat treatment process, and the effects of the reheating temperature and relaxation-quenching process on microstructure and mechanical properties of low carbon Nb-Ti micro-alloyed steels were investigated. The results showed that the number of precipitates decreased, while the mean size increased with the size distribution getting even nonuniform. The martensitic lath bundle size growed and the size of each martensitic lath decreased with the increase of the reheating temperature. Compared with the sample quenched directly, the number of precipitates in the sample reduced, which was reheated to 850°C and then relaxed by slowly cooling to 700°C before quenching. Its mean diameter increased and the size distribution was more uneven. When the sample was reheated to 900°C and also relaxed by slowly cooling to 700°C before quenching, (Nb, Ti)x(CN)y continued to precipitate on the surface of intrinsic granular precipitates. The best reheating temperature was 900°C. Moreover, the tensile strength of the sample which was air-cooled about 20 s increasesd 24 MPa and reached its maximum value.