Study On The Microstructure And Performance Of Austenitic Valve Steeel By Micro-Alloying

On the basis of 5Cr21Mn9Ni4N (21-4N) steel and by micro-alloying technique with adding microelement Rare Earth (RE), niobium and vanadium, the 21-4NRE and 6Cr21Mn10MoVNbN austenitic valve steels were successfully produced. Product yield of the 21-4NRE steel is 5% higher than 21-4N steel, with some improvement of each property. With product yield of the steel reaches at 60%, the properties of 6Cr21Mn10MoVNbN steel reach to the same international standard, and it supplies a domestic gap. The 6Cr21Mn10MoVNbN steel had been produced and sold over a thousand ton, which substitutes for imported goods. Proceeds of sale for the steel had reached to a hundred million RMB with good economic effect and social efficiency. Influences of RE on microstructure, inclusions and grain growth of 21-4N steel were systematically studied, thus the optimal mass percent of RE was obtained and the functionary mechanisms of RE were preliminarily investigated. The hot deformation behaviors and microstructure evolution of 6Cr21Mn10MoVNbN steel were systematically analyzed by thermomechanical compression test and hot rolling solid solution test, and the best technical parameters were obtained. The mechanical properties, oxidation resistance and wear resistance of two valve steels were systematically studied, and the concerned mechanisms were also analyzed.The analysis on the microstructure of steel 21-4N alloyed by RE showed that: Solid solution amount of RE in 21-4N steel is about 10-6～10-5 order of magnitude. Appropriate RE (20%wt) added in 21-4N steel can obviously refine the microstructure and light the extent of segregation phenomenon under as-cast condition, make precipitated phase under finished product state distribute evenly with nearly the same size and the amount of precipitated phase in the grain increases, and, effectively metamorphose inclusions and improve the morphology of inclusions. The precipitated mechanism of M7C3 interstitial phase under as-cast condition may be cellular reaction, and appropriate RE can decrease cellular structures. Precipitated phase under finished product state of steel 21-4N alloyed by RE is M23C6. The 21-4N steel has the feature of sharp austenite grain coarsening after solid solution treatment at the temperature above 1150℃. RE added in the steel has the benefit to restrain grain growth and increase grain growth activation energy. The optimal amount of RE added in 21-4N steel is 0.2%wt.The results of thermomechanical compression test for 6Cr21Mn10MoVNbN steel showed that: 6Cr21Mn10MoVNbN steel under the present deformation conditions can take place dynamic recrystallization of austenite. Deformation conditions have remarkable influence on the resistance for deformation and the grain size of dynamic recrystallization. Miura H. modified model by bulging mechanism of grain boundary can apply to the mechanism for the nucleation of a dynamic recrystallization grain. With the beginning precipitated time early than dynamic recrystallization or competitive happen with dynamic recrystallization, M(C,N) phase mainly precipitate under the process of hot deformation. Its precipitate belongs to common precipitate and has priority in grain boundary. The precipitated mechanism of M7C3 phase, precipitated under cooling process after hot deformation, may be cellular reaction, which is one of reason to induce the crack of hot deformation. M7C3 phase mainly precipitate in grain boundary and dislocation, and its precipitate is effected with deformation conditions and cooling system. The microstructure evolution of single-pass hot deformation process is obtained through the analysis of the microstructures.The hot rolling solid solution test for 6Cr21Mn10MoVNbN steel showed that: Grain coarsening temperature of solid solution treatment is 1150℃. The mechanism of precipitate phase coarsening is Ostwald coarsening. At the temperature from 1050℃to 1100℃, the activation energy value for grain growth is 189.9kJ/mol. The grain growth kinetics equation is: D=D0+2.08×106 t0.368exp(-189856/RT). The best temperature range of hot deformation for 6Cr21Mn10MoVNbN steel is 1150～950℃. Cooling velocity ought to air cooling, and strain is not suitable too great when the temperature of hot rolling is too high. The best...