Abstract: Two low-carbon alloyed structural steel of different compositions were investigated via hot-rolling tests and thermal simulation tests for the effects of their chemical compositions, original microstructures and partial heating process on the ultra-fine austenitic grains. The results indicated that the process of warm rolling and cold deformation is most beneficial to enabling the ferritic/pearlitic structure being hot-rolled as original one to obtain ultra-fine austenite with grain size about 1μm. Moreover, adding properly the alloying elements Nb, V and Ti and increasing heating rate can also be helpful to austenitic grain refining, and the ultra-refining effect is not obvious if the heating rate is more than 100°C/s. But, the austenitic grain refining is obvious with homogeneous grain size if the steel is predeformed before heating.