Abstract: A low carbon high strength steel with lower yield strength reached 700 MPa was developed through controlling rolling and controlling cooling,and its microstructures and strengthening mechanisms were analyzed.The results show the microstructure of this steel is ferrite and the high yield strength of steel is caused by fine grain ferrite and fine microalloy carbonitrides particles precipitated in ferrite.The amount of precipitation-strengthening is 250 MPa,this is the two or four times higher than that of hot-rolled microalloyed steels in the past.Good elongation is duo to high dislocation density and fine distribution of carbide particles.