High strength IF steel sheets with sufficient formability had been extensively used in automotive industry.In this paper,a new type of high strength cold-rolled IF steel with higher carbon and niobium contents was studied.Thermal plastic and continuous annealing were performed on thermo-mechanical simulator.The transformation points were tested by thermal expansion apparatus.Optical microscopy and transmission election microscope (TEM) were used to analyze the microstructure and the secondary precipitates of the steel.The results showed,the ductibility temperature range was from 950℃ to 1250℃ and the transformation points were 887℃ and 913℃ respectively.The grain size of this steel was smaller than that of conventional high strength IF steel.At the mean time,there were many fine Nb(C,N) precipitates distributed in the intra-granular regions and the PFZ (precipitate free zone) were formed in the neighborhood of grain boundaries.Due to the unique micro-structural feature,the yield strength and the yield ratio of the steel were decreased while the tensile strength was increased.With the increasing of the annealing temperature,the strength decreased,the total elongation A50,r-value at 15% strain and n-value were all increased.In order to obtain the favorable mechanical properties,the skin-pass rolling rate should be chosen at 0.6-0.8%. High strength IF steel sheets with sufficient formability had been extensively used in automotive industry.In this paper, a new type of high strength cold-rolled IF steel with higher carbon and niobium contents was studied.Thermal plastic and continuous annealing were performed on thermo- mechanical simulator. The transformation points were tested by thermal expansion apparatus. Optical microscopy and transmission election microscope (TEM) were used to analyze the microstructure and the secondary precipitates of the steel. The results showed, the ductibility temperature range was from 950 ° C to 1250 ℃ and the transformation points were 887 ℃ and 913 ℃ respectively. Grain size of this steel was smaller than that of conventional high strength IF steel. At the time, there were many fine Nb (C, N) precipitates distributed in the intra -granular regions and the PFZ (precipitate free zone) were formed in the neighborhood of grain boundaries. Due to the unique micro-structural feature, the yiel d strength and the yield ratio of the steel were decreased while the tensile strength was increased. When the increasing of the annealing temperature, the strength decreased, the total elongation A50, r-value at 15% strain and n-value were all increased. In order to obtain the favorable mechanical properties, the skin-pass rolling rate should be chosen at 0.6-0.8%.