Abstract: Owing to the increasing surface quality of plastic products, such as plastic medical supplies and resin lenses, the demands of plastic molds have increased. Corrosion and wear are the most important failure behaviors of plastic molds; therefore, best-quality plastic mold materials should feature high hardness and corrosion resistance. Super martensitic stainless steels show the optimum combination of strength, hardness, and wear and corrosion resistance after appropriate heat treatment. Therefore, they are the most mainstream materials in the field of high-grade die steel, especially AISI 420. Herein, AISI 420 steels with different average particle sizes and roundness of spheroidized microstructures were treated by different quenching and tempering procedures. Hardness test, scanning electron microscope, and X-ray powder diffraction were then used to research the impact of the spheroidized microstructure on the quenching and tempering characteristics. Additionally, the differences in corrosion resistance were investigated using a potentiodynamic polarization test and soaking corrosion in 3.5% NaCl solution. The results show that small and diffuse spheroidized microstructures increase the solution degree of the C element in AISI 420 steel during quenching, improving the hardening capacity, but increasing the amount of retained austenite simultaneously. Smaller-sized Cr-rich carbides enable the AISI 420 steel to dissolve more Cr element in the austenitizing procedure; therefore, the Cr content of the matrix is higher after quenching and tempering, which reduces the probability of the chromium-depleted area and shows better pitting resistance. Fewer large-sized, undissolved carbides reduce the probability of pitting nucleation in a corrosive environment and improve the corrosion resistance of AISI 420 steel. After tempering at 250℃, AISI 420 shows excellent corrosion resistance and higher hardness. While the steel exhibits the highest hardness, it also bring about the greatest damage to corrosion resistance when tempered at 480℃.