| As the ferritic stainless steel used in automobile exhaust systems, the microstructure and properties of 00Cr12Ti ferritic stainless steel directly related to the life of exhaust system. It has been found that Nb addition in the steel remarkably increases the microstructure and mechanical properties. During the aging, Nb(C,N), (Nb,Ti)C and Fe2Nb were precipitated out, which can avoid precipitation of M23C6 carbides, increased the resistant performance of intergranular corrosion. In this thesis, based on previous research about 00Cr12Ti with ultra-low carbon and nitrogen, the ferritic stainless steel are designed by adding different content of Nb, smelted by vacuum arc furnace. The change of the microstructural and mechanical properties of cast steel and aging stainless steel with different temperature and different aging time are investigated by means of optical microscope, SEM, EPMA, X-ray diffraction and mechanical property test, then the resistant performance of intergranular corrosion of cast steel and aging steel are studied. From those investigations the following conclusions can be drawn:1. After adding Nb in the ferritic stainless steel, as increasing of the countent of Nb, the grain of the cast steel is refined, TiN, Cr23C6 and (Cr,Fe)7C3 precipitated in the matrix, and other precipitation such as NbC, (Nb,Ti)C and Fe2Nb are precipitated. As the lattice parameter of TiC is approximate with NbC and NbN, usually they precipitated together. As stablished elements, Nb and Ti are very important for the mechanical propertie of ferritic stainless steel, for the specimen not added Nb, more TiN precipitated in the matrix, which increased the hardness of the steel, in the orther hand, when added Nb in the steel, with the content of Nb increases, the hardness and Yield strength are decreased gradually.2. When aging at 920℃with 3 minutes, recrystallization is sufficient. As rised of the temperature and prolong of aging time, the grain size gradually increased and the hardness and yield strength decreased. Besides, the hardness and yield strength of 0.3% Nb steel are littler than the steel with 0.5% Nb added, this is mainly because effect of strengthening of the precipitation containing Nb.3. With adding of Nb in the steel, the position of the diffraction peak moved to the right wholly. Cr7C3 is precipitated in the steel when no Nb added, in the other side, when adding Nb in the steel, the precipitation of Cr7C3 can be restrained, meanwhile, other precipitation such as Fe3Nb3C, Fe2Nb and (Nb,Ti)(C,N) are precipitated. By EPMA we can know that as rised of the temperature, the content of solid solution of Nb reduced and the content of Nb in the second phase particles is rised. And the content of solid solution of Ti does not reduce below 950℃, and some particles contain Ti solided when the temperature is above 950℃. By SEM we can know that the shape of TiN is regula squareness, and the size is decreased with the addition of Nb.4. The corrosion rate and corrosion depth of the steel not aging are more serious than the steels containing Nb. Intergranular corrosion are serious for all the steels through organization observation and EPR detection. For the steels after aging, when no Nb added, intergranular corrosion occurred whether sensitization or not. In orther hand, when Nb added in the steel, even sensitized specimen are not have intergranular corrosion, this means added of Nb in the ferritic stainless steel can increase the properties of intergranular corrosion resistance. From the result of EPR, for the as-cast steel, the Ra of unsensitized specimen is less than that of sensitized specimen. For the steels after aging, when no Nb added, whether sensitization or not the Ra are more than the steel with Nb added. The result is the same with that by means of test in copper-copper sulfate-0.5% sulfuric acid. |
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