Industrial Production of 780 MPa Grade Tempering Steel Plates for Cold Forming and for Hot-Strip Rolling

Leveled steel plates were taken from hot- rolled strip steel with tensile strength of 780 MPa, which had been widely used in building cranes and concrete pump trucks by cold forming. In the present work, the plates were tempered on a production line, and their mechanical properties, welding performance, cold forming property and microstructures were investigated. The results show that the strength and hardness of the steel increased significantly after the tempering, while the impact toughness and elongation rate decreased slightly. The difference between the mechanical properties in transverse direction and these in longitudinal direction decreased due to the tempering. The mechanical properties of the welded joints were improved showing reduced strength-loss after welding. The cold forming performance was also greatly improved. No significant change in phase constituents was observed due to the tempering, however, some ferrites grains were coarsened and their dislocation density decreased associated with increasing of cellular dislocation. Increased precipitates containing Ti, Nb, Mo were observed, resulting in the precipitation hardening.