Strengthening of 350kgf/mm² Grade Maraging Steel through Special Thermomechanical Treatment

Abstract

When the conventional thermomechanical treatment (TMT) was applied to the 10Ni-18Co-14Mo maraging steel, the austenite grain was refined to below l0μ but the precipitation in austenite could not be suppressed, thus resulting in the rather lower strength. This study has been conducted to achieve the nominal 350kgf/mm² tensile strength by optimizing the thermomechanical processing variables and by modifying the chemical composition of the steel. In order to obtain the microstructure with fine grain and no coarse precipitates formed in austenite, the special thermomechanical treatment (STMT) has been developed. STMT consisted of multipass rolling from the solution treatment temperature to 1000°C, interrupting the rolling and air cooling to the temperature which is below the nose temperature of precipitation start curve and just above the recrystallization temperature, re-rolling quickly by sufficient amount of reduction, and quenching below room temperature. This interrupted rolling is essential to suppress the strain-induced precipitation. When STMT was applied to two steels, the nominal microstructure was not obtained for the 10Ni-18Co-14Mo steel but was successfully obtained for the 10Ni-18Co-12Mo-1Ti steel. It is, however, essential to choice the correct re-rolling temperature and to employ the adequate amount of reduction, and by optimizing these rolling conditions, the tensile strength of 363kgf/mm² was achieved in the 10Ni-18Co-12Mo-1Ti maraging steel.