Abstract: The important development direction of gun barrel steel is to use the secondary hardening effect for ensuring a high temperature strength. This type of steels is generally of low toughness. The dissolution and precipitation behaviors of carbides as well as their effects on the toughness of a new 30Cr2MoV secondary hardened gun barrel steel were studied to improve the steel toughness. The carbides were characterized by using scanning electron microscope, transmission electron microscope, carbon replica and phase analysis technologies. The results show that, as the quenching temperature is raised from 850 to 1 050 ℃, the dissolution quantity of carbides increases, then so does the precipitation kinetics of secondary hardened phase. At the quenching temperature of 950 ℃ (slightly lower than the total solution temperature of MC carbides), undissolved MC at the austenite grain boundaries can effectively inhibit the growth of original austenite grains, and the impact energy remains high (107 J). As the tempering temperature increases from 600 to 700 ℃ after quenching at 950 ℃, the dissolution of M₃C and the precipitation of M₂C increase, the precipitation of M₇C₃ starts at a temperature slightly lower than 650 ℃, the content of MC remains almost unchanged, and the total content of carbides is decreased by about 14%. The impart fractures show quasi-cleavage, cleavage and ductile fracture, respectively, at the tempering temperatures of 600, 625 and 650 ℃. The cleavage fracture at 625 ℃ may be correlated with the transition from M₃C to M₂C, which causes intragranular strengthening and grain boundary weakening, segregation of impurity elements at grain boundaries and coarsening of carbides.

Key words: gunbarrelsteel, carbide, secondaryhardeningsteel, dissolution, precipitation