汽车尾气净化器用高铝不锈钢凝固特性和调控机制研究

Systematical and profound studies on solidification behaviors and characteristics of steels are important theoretical foundations for developing of solidification technologies and regulating of solidification process. This project is evoked by the difficulties on regulating of solidification and deformation of high aluminum Fe-Cr-Al stainless steel, founded in the earlier researches on development of high aluminum stainless steel for automobile gas purifier substrate. Multi-scale melting and solidification trials with samples of 50kg, 1kg, 10mm and 4mm in diameter will be carried out in the project. To study the solidification characteristics from different sides and levels, various experimental and analysis methods reported in literatures and developed by our team will be applied comprehensively in the research, combined with analyzing of solidification theory and simulation of professional software Procast. The studies will focus on: 1) Important solidification characteristics of high aluminum stainless steel, such as the characteristics of structure evolution, migration and distribution of solute elements, precipitation of precipitates, contraction, high temperature ductility, etc.; 2) Interactions of solidification structure, distribution and segregation of solutes, and precipitates, and influences of external factors, such as casting temperature, rate of solidification and cooling, sample morphologies, etc., on the solidification characteristics; 3) Regulation mechanism for solidification structure evolution, distribution of solutes, precipitates, high temperature ductility, and internal stress and strain in ingots. The final goals of the research are to provide foundations for solidification technology development of high aluminum stainless steel applied as automobile gas purifier substrate and to promote the progress of air pollution disposal.

钢铁凝固行为和特性的深入研究是凝固成形技术研发和凝固过程有效调控的重要理论基础。本项目拟在前期汽车尾气净化器载体用高铝不锈钢研发的基础上，针对Fe-Cr-Al系高铝不锈钢凝固成形调控困难问题，综合采用文献报道和本梯队自主研发的实验方法和分析技术，结合凝固理论和Procast专业软件，对50kg、1kg、直径10mm和4mm等多尺度试样凝固行为和特性进行系统深入分析。主要研究：1）高铝不锈钢凝固过程中组织演变特性、溶质组元迁移和分布特性、析出物析出特性、凝固收缩特性、高温塑性等重要凝固特性；2）凝固组织、溶质组元分布与偏析、析出物析出之间的交互作用，浇铸温度、冷却速度和试样尺寸等外部因素对凝固特性的影响机制；3）高铝不锈钢凝固过程中组织演变、组元分布、析出物、高温塑性及铸锭内部应力应变调控机制。本研究将为汽车尾气净化器用高铝不锈钢凝固技术研发和质量控制提供理论基础，促进大气污染治理进展。

FeCrAl不锈钢具有优异的抗高温氧化性能和低的热膨胀系数，是金属蜂窝型汽车尾气净化器载体的理想材料。本项目以凝固成形技术的研发和对凝固过程的有效调控为目标，对FeCrAl不锈钢凝固行为和特性进行了深入研究。.采用高温差示扫描量热（DSC）实验与热力学计算相结合的方法对高铝不锈钢凝固及冷却过程中相变进行了研究。结合Thermo-Calc软件计算和DSC实验验证，确定了适合FeCrAl不锈钢的液、固相线计算公式。采用Thermo-Calc软件计算了Fe-(18~21)Cr-(3~5)Al-(0~0.03)C-(0~0.2)Si-(0~0.2)Mn多元系相图，狭窄的两相区温度区间使FeCrAl不锈钢凝固时柱状晶异常发达，而L→铁素体的单一凝固相变使FeCrAl不锈钢组织粗大。结合DSC和X射线衍射实验结果发现凝固过程中Cr7C3与σ相是否析出分别取决于体系中C含量与Si含量，FeCrAl不锈钢在常温下的平衡相组成为αFe+AlN+Cr23C6+α’。铬碳化物和σ相析出是影响FeCrAl不锈钢高温塑性和中温塑性的主要因素。.从凝固条件及合金成分两个方面对凝固组织形貌与元素偏析进行分析。研究结果表明，随着冷速的增大，液固界面形貌转变如下：平界面→树枝晶界面→胞晶界面→六边形胞晶界面→高速胞晶界面。Al含量的增加会降低枝晶-胞晶转变及胞晶-枝晶转变临界拉速。Al和Cr在FeCrAl不锈钢中偏析程度较低。CAFE法对FeCrAl不锈钢铸锭凝固组织模拟结果表明，随着冷却强度的增加，等轴晶区减小，柱状晶区扩大。.研究了不同工艺条件下稀土La对FeCrAl不锈钢的合金化作用及La、Ti处理对FeCrAl不锈钢凝固组织的影响。结果表明采用真空感应炉合金化+普通电渣的La合金化方案最佳。采用La、Ti处理可以细化FeCrAl不锈钢的凝固组织。.高铝不锈钢的高温塑性及调控机制的研究表明，FeCrAl不锈钢在一定拉速与温度下会发生动态应变时效。含Ti铸锭中的TiN夹杂很容易诱发裂纹，不利于铸锭高温力学性能的改善。铬碳化物析出是FeCrAl不锈钢高温第三脆性区形成的主要因素，添加稀土镧可以提高FeCrAl不锈钢高温抗拉强度，并且消除FeCrAl不锈钢高温第三脆性区。.上述研究初步揭示了汽车尾气净化器用高铝FeCrAl不锈钢凝固特性及调控机制，为FeCrAl不锈钢研发提供了良好的理论指导。