Low-yield-ratio and high strength thick plate and preparation process thereof

The invention provides a low-yield-ratio and high strength thick plate and a preparation process thereof. The thick plate comprises the following components in percentage by weight: 0.05-0.10 percent of C, 0.20-0.30 percent of Si, 1.60-1.80 percent of Mn, 0.05-0.07 percent of Nb, 0.30-0.40 percent of Ni, 0.30-0.40 percent of Cu, 0.15-0.30 percent of Cr, 0.20-0.40 percent of Mo, not more than 0.02 percent of Ti, not more than 0.045 percent of Al and the balance of iron and impurities. The preparation process adopts a rolling and cooling control process on the basis of the component design. The invention successfully solves the problems of contradiction and difficult accordancy of high strength and low yield ratio of a building thick plate, has simple process and wide application prospect, can effectively improve the yield and reduce the production cost and is suitable for industrialized large production and the fields of high-rise buildings, bridges, pipelines, and the like.