| Compared to austenitic stainless steel(ASS),ferritic stainless steel(FSS)has superior thermal conductivity,lower linear expansion and excellent resistance to stress corrosion cracking.Super FSS with 27%Cr is widely used in Europe and America as the pipeline,heat exchanger and condenser whose corrosion resistance(especially pitting and crevice corrosion resistance)is much better than original FSS,equivalent to super ASS and nickel-base alloy while retaining a lower fabrication cost.The high purity smelting for super FSS is difficult,and due to the fact that the microstructure of super FSS with high chromium and molybdenum in the whole temperature range is ferrite,it is impossible to refine the microstructure by phase transformation which results in coarse as cast and annealed grains.Meanwhile,inclusions with sharp edges also deteriorate the properties of the steel and limit its application.In this experiment,effects of cerium on the microstructure,mechanical properties and corrosion resistance of 27%Cr super FSS have been studied.In this experiment,the ultra-low carbon and nitrogen smelting of super FSS was realized by vacuum refining and niobium-titanium stabilization,and the addition of rare earth Ce after pre-deoxidation with Al+SiCa could effectively achieve deoxidation,desulfurization and microalloying.In addition,the yield rate of Ce would increase with the increase of the Ce additions.The results of inclusions and microstructures analysis showed that Ce elements can significantly modify inclusions and refine ferrite grains.The main inclusions in super FSS without Ce addition were sharp edged(TiNb)N·Al2O3.When the content of Ce was 0.0230%,(TiNb)N·Al2O3 were modified into(TiNb)N·CeAlO3.When the Ce content is 0.0498%,inclusions in steels were controlled as mainly spherical Ce2O3 and Ce2O2S.Continuing to increase Ce content,inclusions such as complex Ce-O-P and Ce-O-P-As in steels also generated in addition to Ce2O3 and Ce2O2S.With increasing Ce content,the number of inclusions per unit area would increase,the average diameter would decrease firstly and then increase.After adding Ce,the grain size of as cast and recrystallized grain were refined.The optimium solid-solution system for super FSS is heat preservation at 1080? for 5min followed by water quenching.Mechanical test showed that with increasing Ce content,the strength,plasticity and toughness of super FSS were enhanced firstly and weakened finally,and when the Ce content was 0.0498%,the mechanical properties of super FSS got the best.The mismatch between Ce2O3,Ce2O2S and ?-Fe formed during solidification of molten steel is very low which could be used as nucleating agent to improve the solidification structure.Meanwhile,Ce atom segregating at grain boundary could inhibit grain growth and grain refinement will improve the strength,hardness and plasticity of the super FSS.Rare earth Ce can promote the spheroidization and refinement of inclusions,change the fracture mode of materails and improve the toughness.When the Ce content was excessive,the size of recrystallized grains and inclusions would increase which deteriorated mechanical properties of super FSS.The results of uniform corrosion test showed that the loss of super FSS was the highest in 40%H2SO4 at 60?.The addition of Ce could effectively improve the uniform corrosion resistance of super FSS,Ce(OH)4 would form in the rust layer and inhibited further corrosions of the super FSS.The results of pitting experiment showed that effects of Ce on the refinement and spheroidization of inclusions made the diameter of inclusions smaller than the critical diameter of pitting nucleation which improved pitting corrosion resistance of super FSS.With the increase of Ce content,the passive film impedance of super FSS increased firstly and then decreased,reaching a maximum of 3.24E+0.5 cm2. |
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