Dual phase steels with 0.03% carbon and various manganese contents of 1.4%, 1.9% and 2.3% were made in laboratory, and their mechanical properties and ageing characteristics were studied.
In the water-quenched dual phase steels, as manganese content increased, work hardening rate and strength-ductility combination increased. Due to the higher manganese content, dual phase steels with ferrite plus martensite structures were able to be obtained by slow air cooling instead of water quenching. It was found in this study that the critical cooling rate for the formation of dual phase structure was about 40℃/sec for 1.4% Mn content, 5℃/sec for 1.9% Mn content, and below 0.1℃/sec for 2.3% Mn content, respectively. Mechanical properties of dual phase steels made by slow cooling were superior to those of water-quenched steels, because slow cooling made ferrite matrix cleaner by decreasing the solute carbon content.
With the ageing treatment of water-quenched dual phase steels, overageing was retarded with increased manganese content. The specimens with higher manganese content were cooled slowly, which decreased carbon content in ferrite matrix. In the initial ageing stage the smaller solute carbon content resulted in smaller increase of yield stress. The activation energy for ageing was estimated as 27,000 ± 3,000 cal/mole in 1.4% Mn dual phase steel. This value is higher than that for carbon diffusion in α-iron, 20,100 cal/mole. This difference seems to be attributed to the attraction between carbon and manganese.
