Abstract: Core cooling specification of different diameter bars was formulated, based on the measured temperature change of rotor steel quenching process. The microstructure and mechanical properties of the G50 ultra-high strength steel after austenitizing and cooling at different cooling rates were studied. The results show that the cooling specimens equivalent ø430 mm and ø500 mm bars core form martensite and lower bainite complex structure, their tensile strength is higher than that of the oil quenched specimens, the yield strength is lower, but the impact property is not significantly affected. The core specimen equivalent ø650 mm rod forms more than 60% lower bainite, further reduces the yield strength, but obviously rises up the impact prooperty, showing that the steel can maintain high toughness in slower cooling rate.

Key words: ultra-high strength steel, cooling specification, toughness