Die steel with high thermal fatigue resistance and high temperature impact toughness and preparation method thereof

A steel plate comprises the following components in percentage by weight: 0.20%-0.30% of C, 0.50%-0.70% of Si, 0.70%-0.90% of Mn, less than or equal to 0.015% of P, less than or equal to 0.015% of S, 5.50%-6.50% of Cr, 2.00%-2.20% of Mo, 0.50%-0.70% of V, 0.005%-0.01% of Mg, 0.01%-0.03% of Zn, 2%-4% of Zn/Mg, 15%-25% of Si/(Mg + Zn) and the balance of Fe and inevitable impurities. The production method of the steel plate comprises the steps of smelting, continuous casting, slab heating, rolling, straightening, slow cooling, pretreatment and heat treatment. The thickness of the die steel ranges from 11 mm to 120 mm; the room-temperature hardness is 41-43 HRC, and the hardness difference of the same plate is less than or equal to 2 HRC; at the temperature of 300 DEG C, the hardness of the core part is 420-430 HV; at the temperature of 450 DEG C, the hardness of the core part is 340-350 HV; at the temperature of 600 DEG C, the hardness of the core part is 280-290 HV; at the temperature of 750 DEG C, the hardness