Abstract: The effects of heat treatment process parameters of the step austempering in room-temperature machine oil on the structure and properties of the low alloy bainite ductile iron for milling ball have been studied.The optimum process parameters of austenitizing temperature were 930-950 ℃,the temperature for taking out of oil was 180-220 ℃,the time interval from oil to furnace was 60 s,the isothermal temperature was 220-240 ℃,and the isothermal time was 1-1.5 h.The matrix microstructure of the material consisted of lower bainite,a small amount of martensite,retained austenite and carbide.For the obtained milling ball and impact sample,the impact toughness was more than 14 J/cm2 and hardness was 52-56 HRC.