Effect Of Elements W And RE On Microstructure And Mechanical Properties Of Ferritic Stainless Steel

Ferritic stainless steels are widely used in automobile industry,solid oxide fuel cells,kitchen appliances and other fields due to their lower cost,relatively higher thermal conductivity and lower thermal expansion,excellent anti-oxidation ability and good corrosion resistance.In this thesis,ferritic stainless steel(444)for the automobile exhaust system has been used as experimental material and a systematic investigation has been carried out.Particularly,the effects of W and rare earth(RE)additions to 444-type ferritic stainless steel have been studied on mechanical properties,formability and high temperature properties.Addition of RE in ferritic stainless steel has an effect on spherization and metamorphism for inclusions.Also,RE can promote the precipitation of residual deleterious elements from ferritic stainless steel and effectively reduce the amounts of sulfur and the residual elements in steel such that the steel performance can be improved.By conducting recrystallization annealing experiment and observing the microstructure and measuring the hardness,it is showed that the hardness and recrystallization temperature of ferrite stainless steel are increased and grains can be refined by adding W and RE.The mechanical properties of several kinds of ferritic stainless steel(with addition of W and/or RE)were measured and the results show that both yield strength and tensile strength of ferrite stainless steel can be improved by adding W and RE,while the elongation to failure will be reduced.The mechanical properties of ferritic stainless steel are affected by solid solution strengthening,grain refinement strengthening and second phase precipitation strengthening by adding W.X-ray diffraction technique was used to measure the cold rolling texture of 444-type ferritic stainless steel after different amount of rolling deformation.When cold rolling reduction ratio is lower than 60%,the cold rolling texture tends to concentrate on {111}<112>and the volume fraction of y-fiber texture is also increased with increasing the deformation amount.When the cold rolling reduction ratio is over 60%,{111}<112>texture is changed into {111}<110>by rotation along ND.The hot-rolling,cold-rolling and annealing textures of these ferritic stainless steel were measured and the results shows that a partial y-fiber texture can be fored in ferrite stainless steel during hot rolling after adding of RE.Afterwards,a strong y-fiber texture in subsequent cold rolling process can be produced.Addition of W to ferritic stainless steel has adverse effect on the deep drawability and can lower r value.Formability has a close relation to the strain hardening exponent n,anisotropy ?r and strain hardening coefficients K value.Addition of RE has a beneficial effect on the overall formability,while W addition can only improve material strength and n value.High-temperature mechanical properties of several types of W and/or RE added ferritic stainless steel have also been measured.The results show that the solid solution temperature of Laves phase Fe2Nb in 444-type ferritic stainless steel can be increased and thus lead precipitation of Laves phase Fe2Nb by adding RE.High melting-point Laves phase(Fe,Cr)2(Nb,W)with higher stability can be formed by adding W,which can strengthen the ferritic matrix at elevated temperature and thus improve the high-temperature strength.