Investigation On Corrosion Resistance Properties And Formabilities Of Ferritic Stainless Steel Used As Aumotive Exhaust System

Ferritic stainless steel （FSS） has good resistance of corrosion, lower linearexpansion coefficient and low-cost, which is widely used as the automotive exhaustsystem. In recent years, the higher service life requirements and emission temperatureof automobile exhaust system put forward the higher requirements for local corrosionresistance of cold end and high temperature oxidation resistance of hot end. Thelocalized corrosion resistance of ferritic stainless steels used as cold end of exhaustsystem was mainly improved by adding alloy elements such as chromium,molybdenum and copper, the representative material was00Cr19NbTi. The mainmeans to improve the performance of high temperature oxidation resistance ofautomobile exhaust manifold were as follow, the first was adding molybdenum andrare earth cerium with higher chromium and another was to increase the amount ofsilicon, the representative material are00Cr18NbTi and00Cr14Nb. Due to thecomplicated processing and high content silicon is detrimental to the formability,improve the formability of00Cr14Nb ferritic stainless steel becoming the keyresearch. Based on the above issues, the effect of copper and molybdenum on thelocalized corrosion in chloride of00Cr19NbTi FSS was studied, the content of copperwas optimized, while the effect of molybdenum and rare earth cerium additions on themechanism of increasing the high temperature oxidation resistance of00Cr18NbTiFSS, and the effect of alloy element and processing technology on the forming limitcurves of FSS with lower chromium and higher silicon00Cr14Nb FSS wereinvestigated by means of scanning electron microscopy, transmission electronmicroscopy, X-ray Photoelectron Spectroscopy, electron backscatter diffractiontechnology, X-ray diffraction technique, and mechanical properties testing and testequipment, the main conclusions are as follows:Based on the00Cr19NbTi FSS used as cold end of automobile exhaust system, acertain content of the copper and molybdenum added individually could improve thepitting corrosion resistance. No stress corrosion cracking （SCC） was observed in thisFSS with the copper or molybdenum single added, the SCC susceptibility increased with both of copper and molybdenum additions, the optimum content of copper andmolybdenum are0.20wt%and1.63wt%. The solid solution content of copper inferrite is～0.20%and the pitting potential of copper is higher than the matrix, whichimproves the pitting corrosion resistance of the surface passive film. Continue to addthe copper content to0.50wt%, the pitting corrosion resistance and stress corrosionresistance decreased rapidly because of the over-saturated copper precipitations. Theaging behavior of the ferritic stainless steel with0.50wt%copper and1.63wt%molybdenum added at600℃indicated that the ε-Cu phases continuously precipitatedand grew which promoted the preferential pitting corrosion of the surrounding matrix.Meanwhile, the selective dissolution and repassivation of Mo element in the matrixpromoted the second corrosion grooves parallel to stress and the initiation of SCC wascreated. When the aging time was short, ε-Cu phases was small and mainly distributedin the ferrite grain, the propagation of pitting corrosion was easy along the ferritegrain and the SCC fracture mode was mostly transgranular. With the increasing ofaging time, the size of ε-Cu precipitations increased and mostly dispersed to the grainboundaries. So, pitting corrosion at ε-Cu precipitations in grain boundaries was easierthan in the matrix of ferrite, the SCC fracture mode changed from mostlytransgranular to mostly intergranular.Based on00Cr18NbTi FSS used as the high temperature end of automotiveexhaust system,1.90%molybdenum added improved the high temperature oxidationresistance of FSS, the result showed that alloying element molybdenum facilities theprecipitation of Fe₂（Nb, Mo） Laves phases, and mainly concentrated in the grainboundaries, especially the Fe₂（Nb, Mo） Laves phases pinning to the triple boundaries,inhibited the bulk diffusion of iron and chromium cations from the matrix andeffectively avoided the rapid oxidation. Fe₂（Nb, Mo） Laves phases enriched in theinterface between the scale and matrix which could effectively reduce the oxidationrate of the stainless steel, while Fe₂（Nb, Mo） Laves phase were hard and brittle,resulting in the spallation of oxide scale.The mechanism of niobium and cerium on the high temperature oxidation wasanalyzed by changing the content of niobium and cerium, the results showed that theoptimum content of niobium and cerium were0.26wt%and0.11wt%, respectively.The improvement of high temperature oxidation resistance was attributed to thesolution strengthening of niobium which increases the bonding strength of theinterface between the substrate and the oxide film. The cerium oxide promotes theformation of Cr₂O₃films, deposited in the cavities surrounding the matrix and rapidlyformed the Cr₂O₃films to prevent the formation of large cavities, which couldimprove the adhesion between the oxide film and substrate and finally increases thehigh temperature resistance of investigated stainless steels.Based on low cost00Cr14Nb FSS used as exhaust manifold, the effect ofcomposition and process conditions on the forming limit curves of FSS was investigated. The results showed that small change of strain hardening value n withthe micro-alloying and processing changing, so the FLD0changed on the plane straincondition was negligible. With the increasing of strength coefficient K value, theforming limit curves of the bulging area increased. Forming limit curves in thetensile-compression area was mainly depended on the plastic strain ratio r value, theheight of curve in the tensile-compression area increases with the r value increasing.Because the interstitial atoms carbon and nitride could be totally fixed by dualstabilized niobium and titanium, the high intensity {111} α texture was obtained andconducived to the formation of γ recrystallization texture. Cold rolling texture withouthot band annealing exhibited a strong α texture, the texture orientation was mainlybetween in {001}<110>～{112}<110>. When cold rolled after hot band annealing,cold rolling texture gradually tended to {111}<110> and had a strong intensity.{111}crystal grains had a higher deformation stored energy and the nucleis appeared in the{111} region. The Σ13b coincidence site lattice grain boundaries were higher in thefinal sheet with hot band annealing showed the growth of {111}<112> orientation wasthe result of selective growth. The formation of γ fiber texture was attributed to thecombined effect of “oriented nucleation” and “selective growth”. With the increasingof annealing temperature, the grain size was also increased, the maximum texture wasclose to {111} pixel and the {111}<112> orientation intensity was the highest, whilethe r value came up to the highest and the final sheets obtained the maximum limitstrain value in the tensile-compression area.