【作者】 景财年；

【导师】 王作成；

【作者基本信息】 山东大学 ， 材料加工工程， 2007， 博士

【摘要】 TRIP钢由于TRIP效应使钢板在获得高强度的同时又不损失塑性，这种强化机制使其具有优异的性能，在汽车业中应用可节能减重、降低成本而又不失安全性，应用前景广阔。传统CMnSi-TRIP钢用硅元素来稳定残余奥氏体，但硅含量高于1％时钢板表面易产生稳定的氧化物，使钢板涂镀能力变差。用铝替代硅可减少氧化物提高钢板涂镀能力和表面质量，易于实现热镀锌退火生产。CMnAl-TRIP钢作为新钢种，对其进行系统研究具有重要的理论意义和实用价值。本文设计了三种不同成分的CMnAl-TRIP钢，利用热膨胀模拟、X射线衍射、扫描电镜、透射电镜和光学电镜、拉伸试验等测试方法，较为系统的研究了试验钢的组织性能，发现了最佳的热处理参数，讨论了热处理工艺参数和化学成分等因素对组织性能影响的原因，研究了残余奥氏体的力学稳定性。实验结果表明，铝替代硅后使CMnAl-TRIP钢的Ac₁、Ac₃温度升高，0.05P钢Ac₁为750℃，0.1P和0.1PSi两种钢Ac₁都为757℃，三种试验钢加热至1100℃也未获得单一奥氏体区；在冷却至贝氏体区等温前会发生先共析铁素体的生成，使双相区奥氏体数量减少，剩余奥氏体的碳含量升高，并且高双相区退火温度更有利于先共析铁素体的形成。热模拟试验发现0.05P钢在770℃双相区退火时奥氏体及其碳含量分别为25％和0.54％，在820℃时分别为35％和0.40％，在870℃分别达到45％和0.31％。但双相区退火温度对残余奥氏体及其碳含量的影响并不呈单调变化趋势，在820℃双相区退火试样的残余奥氏体含量为14％，碳含量为1.36％，都为最高值；热模拟数据显示相变动力随贝氏体等温温度的升高而升高，但奥氏体向贝氏体转变的数量却呈下降趋势。0.05P钢420℃等温时和450℃等温时贝氏体相变动力学差异不大，500℃等温时在30s热膨胀曲线有一明显的下降过程，推断为奥氏体发生了渗碳体的分解；0.05P钢在680℃和720℃退火时的Avrami指数n分别为1.011和0.77，低于理想的Avrami指数（2～4）。通过不同双相区退火温度试验发现三种试验钢在不同退火温度下都含有大于10％的残余奥氏体，残余奥氏体含量在800℃至890℃呈上升趋势；铁素体含量随双相区退火温度的升高而降低，贝氏体含量则随双相区退火温度的升高而升高；抗拉强度随双相区退火温度的变化同残余奥氏体的变化相似，屈服强度随双相区退火温度的升高而升高；延伸率则相反；研究发现选择双相区退火温度使铁素体和奥氏体比为65％∶35％时可获得最佳的力学性能，0.05P钢在820℃有最高值39.3％，0.1 P钢和0.1 PSi钢分别在820℃和770℃有最高值36.1％和36.6％，强塑积都高于22000 MPa％，颈缩时的刀值都大于0.2。通过贝氏体等温试验发现在贝氏体较少时间保温组织中会形成马氏体，残余奥氏体含量较少；随贝氏体保温时间的延长，贝氏体转变充分，马氏体含量减少而残余奥氏体及其碳含量增加；450℃等温300s时发现渗碳体的析出，而在高温500℃时30s就已发生奥氏体的分解，造成残余奥氏体及其碳含量的降低；试样抗拉强度随贝氏体保温时间的延长而下降，延伸率和屈服强度则随保温时间的延长而升高；450℃等温时试样力学性能优于其它温度，强塑积最高，0.05P钢在450℃保温60s时抗拉强度为646.5MPa，延伸率达到39.3％，强塑积为25400MPa％，和传统CMnSi-TRIP钢相当，能满足使用要求。实际生产中可围绕820℃等温2min和450℃保温60s来设计热镀锌退火生产线的热处理工艺参数和板速等其它相关参数。对比合金成分对组织性能的影响发现磷、硅、铜的添加使0.1P钢和0.1PSi钢具有高的抗拉强度、屈服强度和低的延伸率，抗拉强度的提高是磷固溶强化和TRIP效应综合作用的结果；磷的添加提高了奥氏体的淬透性，在冷却至贝氏体等温时减少了先共析铁素体的生成，使残余奥氏体含量升高，残余奥氏体碳含量降低，降低了残余奥氏体的力学稳定性，使应变后期瞬时n值降低，导致延伸率的降低；由于钢中铝含量较高，试验中磷、硅含量对贝氏体转变的影响很小。铜元素提高了残余奥氏体含量，但使其碳含量降低。含铜钢抗拉强度的提高是由于TRIP效应、铜的固溶强化作用以及更多的贝氏体组织共同作用的结果。分析瞬时n值和残余奥氏体的稳定性发现瞬时n值特征与TRIP效应有关，不同化学成分、热处理工艺参数下试样的瞬时n值特征都取决于残余奥氏体的力学稳定性；残余奥氏体的力学稳定性低时，应变初期发生较快的马氏体相变，对应较强的TRIP效应和高的瞬时n值；应变后期由于较少的马氏体相变，TRIP效应弱，瞬时n值低，导致均匀延伸率和延伸率的降低；而残余奥氏体力学稳定性高时在整个应变阶段都可持续地发生马氏体相变，特别是在应变后期能够保证TRIP效应，维持较高的瞬时n值，使试样具有高的延伸率；研究TRIP钢性能特点时，不仅要考虑残余奥氏体的含量，更为重要的是要考虑残余奥氏体的稳定性。分析不同热处理工艺参数和化学成分对残余奥氏体力学稳定性的影响，发现残余奥氏体碳含量是决定其力学稳定性的主要因素，不同贝氏体等温温度和保温时间下贝氏体转变程度不同，造成残余奥氏体中碳含量的差异而影响其稳定性；同时残余奥氏体的晶粒细化作用也能提高其力学稳定性；在应力作用下残余奥氏体向马氏体的相变受材料层错能的影响，铝元素能提高材料的层错能，减少马氏体形核率，使残余奥氏体力学稳定性提高。结合试验数据，给出了CMnAl-TRIP钢残余奥氏体碳含量和马氏体形核率α之间的估算公式。更多还原

【Abstract】 In order to meet the automobile industry’s need for weight reduction and safety improvement, the application of advanced high strength steel sheets such as TRIP steel have been examined for suspension and structural part. TRIP steel is the most beneficial steel for increasing the strength of steel without deteriorating the strength and elongation balance. Generally, TRIP steel can reduce the weight and save the fuel without losing the safety. However, the steel containing high silicon elements can not be applied automotive parts due to low surface condition. In this study, aims to promote the surface quality substitute silicon by aluminum base on 0.15C-1.5 Mn-1.5Al-TRIP steel, the effects of alloying elements, heat treatment condition, such as intercritical annealing（IA）and isothermal bainitic transformation（IBT） parameters on microstructure and mechanical properties were investigated, by means of X-ray diffraction（XRD）, dilatometric simulation, scanning electron microcopy （SEM）, transmission electron microcopy （TEM）, optical microstructure （OM） and tensile testing.The experimental results show that the three steels have higher Ac₁ Ac₃ temperature than conventional TRIP steels, with 750℃of Ac₁ for 0.05P steel and 757℃for the others. The Ac₃ is not found for the steels until heating up to 1100℃. Before cooling to the IBT zone, the pro-eutectoid ferrite is found and it affects the volume fraction and carbon content of the austenite which formed in the intercritical zone, the higher IA temperature is favor the formation of pro-eutectoid ferrite.Through dilatometric simulation, we find that 0.05P steel has volume fraction and carbon content of the retained austenite of 25% and 0.54% at 770℃, 35% and 0.40% at 820℃, 45% and 0.31% at 870℃, respectively. But the tendency is not monotony with the IA temperature, the volume fraction and carbon content of the retained austenite arrives the highest at 820℃, with the number of 14% and 1.36%, respectively. The bainite transformation kinetics is increase with the increase of IBT temperature, which the cementite is found at 500℃holding for 30s. The 0.05P steel had the Avrami content of 1.011 at 680℃and 0.77 at 720℃, which are lower than the ideal Avrami content around 2-4.The three steels have more than 10% volume fraction of retained austenite, with the lowest amount at 800℃and the highest at 770℃, showing the increase tendency during 800℃to 890℃. The volume fraction of ferrite decreases with the increase of IA temperature, while the bainite shows the reverse tendency. The tensile strength is similar to the tendency of retained austenite; yield strength is increase and elongation were decrease with the increase of IA temperature. The IA temperature gives ferrite and austenite fraction of 65%:35% is the best selection, in which the 0.05P and 0.1P steel have the elongation of 39.3% and 36.1% at 820℃. The product of tensile strength and elongation were greater than 22000 MPa%, with the n value greater than 0.2 at necking.The martensite is founded at shorter IBT holding time, at the same time, the volume fraction of retained austenite was little. During the IBT holding, the retained austenite is carbon enriched through the bainitic transformation and can resist the following cooling, then the martensite is diminished and retained austenite is increased. The cementite is founded at 450℃for 300s, while it appears at 30s for 500℃.The cementite appears reduced the volume fraction of retained austenite and its carbon content, which would harm the mechanical properties. All the samples show the highest of the product of tensile strength and elongation at 450℃, and 0.05P steel with the tensile strength of 646.5MPa, elongation of 39.3% and product of tensile strength and elongation of 25400 MPa % at 450℃for 60s, respectively. In industrial manufacture, the IBT parameters of 450℃for 60s can be selected for the continual galvannealing lines and the galvanizing rigs.P and Cu can increase the volume fraction of retained austenite but low its carbon content. The retained austenite fraction is increased by the increment of P content from 0.05wt% to 0.1 wt%. The volume fraction of retained austenite fraction reaches 13% for 0.05P steel and 15% for 0.1P steel after isothermal holding for 60 seconds at IBT temperature. The increase of tensile strength by the increment of P content is ascribed to the higher fraction of strain-induced transformed martensite as well as the solid-solution hardening effect of P. Despite of the increase of retained austenite fraction, the elongation of TRIP-aided steel is deteriorated by the increment of P content. It originates from the lower mechanical stability of retained austenite in 0.1P steel, which brings about rapid strain-induced martensite transformation and thus inhibits persistent work hardening during deformation.Compared with the instantaneous n value and the mechanical stability of retained austenite, there are all affected by the TRIP effect. If the mechanical stability of retained austenite is low, most transforms to martensite at early stage of deform and shows the stronger TRIP effect and higher instantaneous n value; the TRIP effect is weak at the late stage of deform and shows lower instantaneous n value, together with low uniform and total elongation. When the mechanical stability of retained austenite is high, it can continually transform to martensite with high instantaneous n value during whole deform. Not only the volume fraction of retained austenite, but also the mechanical stability is considered for TRIP steel.The carbon content of retained austenite is the mainly factor influence its mechanical stability, the heat treatment parameters and the alloying affect the mechanical stability through the carbon content of the retained austenite, the refined retained austenite grain size also increases the stability. The instinct stacking-fault energy affects the nucleation of martesite as well as the retained austenite transforms during deform, aluminum increases the instinct stacking-fault energy and promotes the stability of retained austenite.更多还原