Prehardening plastic die steel plate and manufacture method thereof

The invention relates to a prehardening plastic die steel plate and a manufacture method thereof. The steel plate comprises, by weight, C 0.28 to 0.40, Si 0.20 to 0.80, Mn 0.60 to 1.0, Cr 1.40 to 2.2, Mo 0.30 to 0.50, P no more than 0.030, S no more than 0.030, Al 0.01 to 0.08, Ti 0.01 to 0.05 and B 0.0010 to 0.0050, with the balance being Fe and unavoidable impurities. The manufacture method comprises a first step of smelting, that is, carrying out tapping and casting when components in liquid steel meet requirements so as to obtain steel ingots, a second step of hot rolling, that is, rolling the steel ingots into steel plates with a thickness of 60 to 200 mm, a width of 300 to 1500 and a length of 2000 to 8000 mm, a third step of cooling, that is, cooling the steel plates to a temperature below 550 DEG C, wherein, slow cooling time is no less than 24 hours, and a fourth step of tempering, that is, arranging the steel plates in a tempering furnace for tempering at a temperature of 400 to 600 DEG C, carrying