Study On The Relationship Between Transformation Structure And Strength And Toughness Of New Secondary Hardening Steel

High alloy secondary-hardening ultra-high strength steel is a kind of ultra-high strength steel that has strength and toughness matching rate, the higher corrosion resistance and the lower cost. In recent years, the secondary-hardening ultra-high strength steel that has excellent mechanical properties obtains widely focus and research. The steel as preferred bearing structure unit that was extensively applied to the fields of aeronautics and astronautics, military defense and the key areas of science and technology. But it has some more prominent problems which include high alloy content, complex precipitated phase and organization evolution, and the hot working processes. These questions restricted this materials further development and application. Therefore, research on the internal relations between change organization and strong-toughness of new secondary-hardening ultra-high strength steel has important theoretical significance and practical application value.The paper has used the experimental method of SEM, OM and physical and chemical phase analysis to observe and analyze the microstructure and morphology of different system. Studying(Mo,W)6C carbide and (Nb,W)(C,N) carbonitride effects on strong-toughness of steel in the process of austenitizing. The results show that lower austenitizing temperature sample of undissolved (Mo,W)6C makes martensitic matrix of lower C, Mo and W content reducing M2C carbides precipitation in the process of tempering except damage impact toughness. So it weakens the secondary reinforcement effect. On the contrary, reduced undissolved (Mo,W)6C in higher austenitizing temperature and (Nb,W)(C,N) carbonitride refining grains improve the secondary reinforcement effect. The organization of this steel is obviously coarsen after 1100℃ austenitizing that leads the mechanical properties go down.Under those conditions of a temperature range of 850-1200℃, true strain 0.9 and strain rate range of 0.01-1 OS-1, the hot temperature flow curves were measured and hot deformed microstructures were observed. The results show that the flow stress and peak strain were reduced by increasing deformed temperature or decreasing strain rate. The temperature of full dynamic recrystallization increases by the strain rate rising. Through calculation, the hot deformation activation energy Q of tested steel is 489.712 KJ/mol, and the hot deformation equation is derived on the base of the experiments.Research on hot ductility behavior of the steel under the range of deformation temperature(900-1200℃) and strain rate(0.1S-1), this paper observes and analyzes the microstructure of the steel after deformation. The results show that the tested steel has excellent hot ductility. The reduction of tested steel were more than 70% during the deform temperature 900-1200℃.Under the different deform temperature, the peak stress exhibits a linear decreasing with the increasing temperature. In the process of hot tensile, M6C carbide distribution has a promoting effect on dynamic recrystallisation at a lower deformation temperatrure.The mechanical properties of the steel are Rm≥2000MPa, R0.2≥1700MPa, AKU2≥60J,KIC≥110MPa(?) after 1070℃ x90minOQ,1060℃×75minOQ,-73℃×2hAC, 520℃×5hAC (peak aging treatment) treatment. It has the best strong-toughness match. At underaged condition, the secondary-hardening effect isn’t strong enough due to the unsufficient precipitates. In overaging state, when 560℃×5hAC aging treatment appearing high temperature tempering brittleness reduced the mechanical properties of steel. The increase of the aging temperature coarsens precipitated phases and damages mechanical properties of the steel.