Effect of Zr/Mg Treatment on Microstructure and Properties of Low Carbon Steel with Ti

For microalloyed steels with low carbon content and Ti content of around 0.08%, OM, SEM, and TEM techniques were employed to investigate the effects of Zr, Mg andZr/Mg composite treatments on their dynamic continuous cooling transformation, as well as the hot-rolled microstructure and mechanical properties. The results indicate that: in the continuous cooling transformation curve, the addition of Zr expands the ferrite region, while the addition of Mg suppresses ferrite formation and promotes bainite formation. In the hot-rolling tests, for steel with added Zr and Mg, the grain size of the hot-rolled microstructure is finer. There is no significant difference in the yield strength between this steel and the steel without addition of Zr, Mg, but the impact energy at -20 ℃ increases from 64 J to 229 J. The steel without addition of Zr, Mg contains a large amount of square TiN, whereas after adding Zr and Mg, the inclusions are mainly nearly circular oxide composite precipitates. The addition of Zr reduces the precipitation amount of inclusions but could refine the inclusion sizes. The addition of Mg increases the number of inclusions, and the composite treatment effect of Zr and Mg is consistent with that of adding Zr alone. Fine TiC with the size of 4 nm-10 nm precipitates in the steel, which could play a role in precipitation strengthening.