Pre-hardened mirror plastic die steel and smelting method thereof

The invention relates to pre-hardened mirror surface plastic die steel and a smelting method thereof, the die steel is composed of the following elements in percentage by mass: 0.20-0.25% of C, 0.50-0.70% of Si, 1.80-2.00% of Mn, less than or equal to 0.005% of S, less than or equal to 0.015% of P, 3.0-5.0% of Cr, 0.30-0.50% of Mo, 0.01-0.03% of Ni, less than or equal to 0.01% of Al, less than or equal to 0.005% of N, 00008-0.0015% of B and the balance of Fe and inevitable impurities, and the chemical components also need to meet the following formulas: (1) B = 0.043% + 0.625% of S-0. 128% of C-0.0031% of Cr-0.0036% of Mo; (2) Al is equal to-0.034% to 0.167% of C, 5.69% of S, 0.696% of Ti and 9.87% of N; the smelting method comprises the following steps: KR desulfurization-130t converter-LF-RH-slab continuous casting-casting blank slow cooling; the Rockwell hardness of the prepared extra-thick steel plate for the plastic mold is HRC28-34, the maximum section hardness difference is smaller than or equal to 3 H