Abstract: The industrial test and lab rolling tests were carried out for two low carbon steels.The samples with different microstructures were obtained and the strengthening mechanism and microstructures through which the products could obtain good mechanical properties were investigated.Introducing 5%-10% bainite in the matrix of ultra-fine ferrite could make the tensile strength increase about 20 MPa.The yield strength and tensile strength could be increased to more than 520 MPa and 620 MPa,respectively,through addition of 0.02% Nb and suitable TMCP process refinement strengthening was enhanced and precipitation strengthening was also introduced.The strengthening mechanisms for 500 MPa ultra-fine grain steels should be grain refinement strengthening,transformation strengthening and precipitation strengthening.