【作者】 舒俊；

【导师】 徐洲；

【作者基本信息】 上海交通大学 ， 材料学， 2013， 博士

【摘要】 铁素体不锈钢具有良好的耐腐蚀性能，线膨胀系数低，同时成本低廉，被广泛应用于汽车排气系统。近年来，随着汽车使用寿命要求和排气温度的不断升高，对汽车排气系统冷端耐局部腐蚀性能和热端高温抗氧化性提出了更高的要求。排气系统冷端用不锈钢主要通过合金元素铬、钼和铜的添加来提高其耐局部腐蚀性能，代表材料为00Cr19NbTi。汽车排气歧管用不锈钢提高高温抗氧化性能的方式主要为两种，一是在铬含量较高的情况下添加钼和稀土元素铈，二是通过硅含量来增加其高温抗氧化性，代表材料分别为00Cr18NbTi和00Cr14Nb。由于排气歧管加工形状复杂，且硅添加量的增加会对铁素体不锈钢的成形性不利，所以提高00Cr14Nb铁素体不锈钢的成形性能成为研究的关键。本文借助于扫描电镜（SEM）、透射电镜（TEM）、X射线光电子谱（XPS）、电子背散射衍射技术（EBSD）、X-ray技术以及力学性能检测等分析技术和测试设备，研究了铜和钼对铁素体不锈钢在氯离子腐蚀介质中的局部腐蚀能力的影响，并且优化了铜的加入量。另外，在汽车排气系统高温端材料00Cr18NbTi的基础上通过合金元素钼和稀土铈的添加，研究了其对高温氧化的作用机理，同时还研究了合金元素和工艺条件对低铬高硅00Cr14Nb铁素体不锈钢成品板成形极限曲线的影响，其主要结论如下：在汽车排气系统用00Cr19NbTi铁素体不锈钢的基础上单独加入一定量的铜和钼能提高其抗点蚀性能，研究结果表明单独添加铜和钼铁素体不锈钢在氯离子腐蚀介质中具有良好的耐应力腐蚀性能，铜和钼同时添加会增加铁素体不锈钢的应力腐蚀敏感性，最佳铜和钼的添加量分别为0.2%和1.63%。因为铜在铁素体中的固溶量～0.20%，铜的电位高，能提高不锈钢表面钝化膜的耐点蚀能力。继续添加铜含量至0.50%，由于过饱和铜的析出，不锈钢点蚀抗力和耐应力腐蚀性能明显下降。同时研究了含铜0.50%和钼1.63%的铁素体不锈钢在600℃下的时效行为，结果表明：随着时效时间的增加，ε-Cu相不断析出和长大，促进了其周围基体的优先腐蚀，钼的快速腐蚀促进了点蚀坑尖端腐蚀沟槽的产生，在应力的作用下应力腐蚀裂纹萌生。时效时间较短时，ε-Cu析出相小且主要分布在晶内，点蚀沿晶内扩展容易，应力腐蚀开裂方式主要以穿晶型开裂为主；随着时效时间的增加，ε-Cu相的尺寸明显长大，并且在晶界的分布明显，裂纹在晶界上的扩展比晶内更容易，应力腐蚀开裂方式以沿晶开裂为主。在汽车排气管热端用铁素体不锈钢00Cr18NbTi的基础上添加1.90%的合金元素钼可提高不锈钢的高温抗氧化性，研究表明合金元素钼促进了Fe₂（Nb, Mo）Laves相的析出，且主要在晶界富集，尤其是其钉扎在三叉晶界阻止了基体中铁和锰离子由晶界向外扩散，从而有效的抑制了快速氧化。Fe₂（Nb, Mo） Laves相在氧化膜和不锈钢基体界面处富集，能有效的降低不锈钢的氧化速率，同时Fe₂（Nb, Mo） Laves相硬而脆，与氧化膜结合力不强，导致了氧化膜的剥落。另外，通过改变铈和铌的含量分析了铌和铈在高温氧化中的作用机理，研究结果表明：铌和铈的最佳添加量分别为0.26%和0.11%，其抗氧化性的提高主要是由于铌含量的降低增加了基体和氧化膜界面的结合强度，稀土铈的氧化物促进了Cr₂O₃膜的形成并且其氧化物沉积在不锈钢基体形成的空腔里并快速在其周围生成Cr₂O₃膜，阻止了空腔的形成，提高了氧化膜与不锈钢基体的粘附性，从而增加了不锈钢的高温抗氧化性能。以经济型排气歧管用00Cr14Nb铁素体不锈钢为基础，研究了成分和工艺对铁素体不锈钢成形极限曲线的影响规律，研究结果表明：铁素体不锈钢中微合金化和工艺对其应变硬化指数n值变化很小，其表现在成形极限曲线平面应变条件下FLD0的变化也很小；随着强度系数K值的增加，拉-拉区（胀形区）曲线有所增加；成形极限曲线拉-压区（压延区）主要受塑性应变比r值的影响，随着r值的增加曲线高度也在增加。铌钛双稳定有效固定了基体里固溶的碳氮原子，可以获得强度高的{111} α织构，有利于再结晶γ织构的形成。热轧不退火直接进行冷轧时，冷轧织构表现出强烈的α织构，织构取向主要集中在{001}<110>～{112}<110>之间。经热轧退火后冷轧时，冷轧态织构逐渐偏向{111}<110>且强度较高，{111}晶粒具有较高的形变储存能，获得了优先生长。热轧退火后冷轧成品板中∑13b重位点阵晶界较多，说明了{111}<112>取向晶粒的长大是选择性长大的结果。再结晶γ织构的形成主要是“定向形核”和“选择性生长”综合作用的结果。随着退火温度的增加，晶粒尺寸也在不断增加，再结晶织构逐渐回到了{111}轴，{111}<112>取向强度最高，平均塑性应变比r值最大，成品板在成形极限曲线拉-压区获得了最高的极限应变值。更多还原

【Abstract】 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.更多还原