Abstract: The effect of isothermal temperature, isothermal holding time, austenitizing temperature and original structure on the microstructure and mechanical properties of 30CrMnSiNi2A steel are studied by using orthogonal experimental method. A new heat-treatment process is suggested, which can ensure the carbon content of steel within upper and lower limit to fulfill the technical requirements. The reason for the suggested process of improving the strength and toughness of the steel is discussed. The twin martensite between lath martensite and bainite and the long lath martensite are the main reasons for the decrease of KIC when the steel is isothermally quenched at 240℃-280℃.

Key words: 30CrMnSiNi2A steel, heat-treatment, isothermal quenching