Abstract: The influence of Relaxation-Precipitation-Controlling Transformation(RPC) technique on the microstructure of low carbon steels was studied by thermal simulation. The results indicated that the ultra-fine bainite/martensite composite microstructure was obtained. With the strain increasing the refinement was promoted. As the final rolling temperature increases the refinement was hindered. When the precipitation speed during relaxing is comparable to the dislocation evolution rate, the best processing is obtained.