Abstract: In order to promote lightweight process of heavy duty automobile, and optimizing product structure of domestic hot-rolled ultra-high strength steel, with low cost and high performance as basic principle, a composition design idea of low carbon content and an quantitative manganese added with microalloying elements Nb and Ti is proposed, and fine grain strengthening and precipitation strengthening are used, the 700 MPa grade ultra-high strength heavy duty automobile carriage strip is carried out on Tianjin Iron Hot-rolled Plate Company 1 750 mm semi-continuous hot rolling mill. The heating temperature is 1 250 ℃, the finishing temperature between 780～860 ℃, the coiling temperature between 500～620 ℃. The microstructure of steel is fine ferrite, small amounts of degenerated pearlite, and a lot of dispersed (Nb, Ti)C precipitates with the scale of 20 nm and below. The yield and tensile strengths of the steel plate are about 600 MPa and 700 MPa, respectively. The elongation is between 19%～22%. The steel plate has good cold bending property, hole expansion ability and low-temperature impact toughness, reach or over that of domestic and abroad. The carriage plate has excellent weldability. 20% structure weight loss can be achieved by replacing Q345, and its application effect is good.

Key words: C-Mn steel, Fine grain strengthening, Precipitation strengthening, Ultra-high strength steel