低密度、高强塑性积钢的强韧化机制与内氧化行为研究

Low-density and high strength-plastic product Fe-Mn-Al-C system steel was considered as an effective way to realize the lightweight and safety of the automobile. However, for the Fe-Mn-Al-C steel with high Al, high C and medium Mn, the fundamental research on the strength-toughen mechanisms and high-temperature internal oxidation behavior of the steel still needs to be carried out. In this work, the low-density and high strength-plastic product Fe-Mn-Al-C system steel with Fe-Mn-Al-C high Al, high C and medium Mn was developed using multi-alloying method. The physical and chemical behavior of the steel during the solidification process was studied by means of thermal-physical simulation method. The phase transformation characteristics and the crystal structure of the phase composition of the system during solidification process were studied. The relationship between the internal oxidation behavior and the crack in the cast slab was analyzed by studying the oxidation characteristics of the cast steel billet. The calculation model about the stacking fault energy was build and the strength-toughen mechanisms and its key technologies of the system were deeply investigated by studying the stacking fault characteristics, the twinning characteristics, the dislocation characteristics, and the precipitation characteristics of the κ carbide in the process of solid solution treatment and deformation process of the system. Therefore, the theoretical principle to control the solidification microstructure and the internal oxidation behavior of the cast slab, to control the microstructure in the hot deformation, and to increase the properties of the system could be build.

低密度、高强塑积Fe-Mn-Al-C系钢被认为是实现汽车轻量化和安全性的有效途径。但对高含Al、C和中Mn该系钢的强韧化机制与高温内氧化行为的基础研究尚需深入开展。本项目拟采用多元合金化手段，开发出具有高Al、和中Mn和高C的Fe-Mn-Al-C低密度、高强塑性积钢。采用热物理模拟手段，对该系钢在凝固过程中的冶金物理化学行为进行研究；系统研究该系钢在凝固过程的相变特征及组成相的晶体结构；通过对该系钢铸坯氧化特性的研究，分析其内氧化行为及其与铸坯裂纹间的关联关系；通过该系钢在固溶过程以及形变过程中的层错特征、孪生特征、位错特征及κ碳化物的析出特征的研究，构建该系钢的层错能计算模型并深入探讨该系钢的强韧化机制及控制关键。为低密度、高强塑性积Fe-Mn-Al-C系钢的凝固组织及铸坯氧化特性的控制、热变形过程的组织结构与性能控制、为进一步提高该系钢的性能提供理论基础。

节能与安全性是车辆、船舶、航空航天及军事等领域追求的主要目标之一，降低上述领域所用材料的密度并提高其强度和强塑积是实现上述目标的有效途径。Fe-Mn-Al-C低密度钢因其具有的比常规钢低10%~15%的密度，并兼具高强度和塑性及强塑积等特性，有望成为制造上述领域所用零部件的备选材料，因此成为国内外的研究热点。本项目从成分设计、氧化特性、κ-碳化物与金属间化合物特性、氢致强塑性损伤等角度对该类钢的组织结构和性能及其强韧化和氢致强塑性损伤机制进行了研究。研究发现，当奥氏体基Fe-xMn(x=20, 28)-yAl(y=8,9,10)-1C低密度钢的氧化温度由1050℃提高到1100℃并氧化5~25h时，氧化速率加快且其氧化动力学由两阶段抛物线转为一阶段抛物线。Mn/Al比的降低使得氧化层中生成的具有高保护作用的Al2O3的温度提高到更高温度且氧化层中Al2O3的含量降低。具有适量κ-碳化物含量和少量铁素体的奥氏体基Fe-28Mn-10Al-1C低密度钢的强度、塑性、比强度和抗氧化性能优于几乎完全由奥氏体构成的Fe-28Mn-8Al-1C低密度钢和较高铁素体含量的Fe-20Mn-10Al-1C低密度钢。通过降低上述具有综合性能优良的Fe-28Mn-10Al-1C低密度钢的C含量，提高Mn含量，并采用Si替代部分Al且增加0.5%Mo而设计的Fe-30Mn-9Al-1Si-0.5Mo-0.9C低密度钢在经1050℃固溶和500~550℃时效12h后时发生了DO3（Fe3Al）向B2（FeAl）相的转变以及β-Mn沿奥氏体/铁素体界面的析出现象，提高了该钢的强度但却显著降其塑性。时效过程中析出的κ-碳化物以及形变过程中形成的交叉微带是该钢具有优良强度和塑性配合及高强塑积的主要原因。对于铁素体基Fe-8.3Mn-(9.5, 12)Al-1C钢而言，氢的存在对其强度和塑性，特别是塑性的损伤较大，断裂特征由氢引入前的韧性断裂转变为氢引入后的准解理断裂。Al含量的提高对降低氢致强度和塑性损伤有利。κ-碳化物的存在以及奥氏体中的退火孪晶是造成氢致上述低密度钢强度和塑性损伤的主要原因。含Al低密度钢中析出的沿奥氏体晶界析出的κ-碳化物是造成该类钢热轧边裂的主要原因。上述研究结果对提高Fe-Mn-Al-C低密度钢的强度和塑性及其强塑积，控制其氧化和氢致损伤具有重要的科学意义。