低密度中锰高铝汽车用钢变形行为及物理模型的研究

The trend of modern automobiles is weight reduction, energy saving, gas emission reduction and increase of safety. As the addition of Al can reduce the weight of the steels remarkably, low density Fe-Mn-Al-C steels with high tensile strength and elongation have received much attention in the high performance steel development in recent years. Up till now, the investigations on low density steels with a relatively high Al(>8%) concentrate on high Mn contents. Since it is prone to form cracks during casting and hot rolling when the steels contain high Mn, reduction in Mn concentration would benefit to the applications of low density Fe-Mn-Al-C steels. In the project, a novel ferrite, austenite and carbide triplex steel with medium Mn and relatively high Al has been proposed and the deformation behaviors and microstructural evolution during deformation will be investigated and the deformation features of each constituent phase would be characterized. The effects of grain size and volume fraction of ferrite and the size and volume fraction of carbides on the strain hardening behaviors of the steels will be clarified and the mechanisms of strain hardening will be elucidated. Meanwhile, constituent models will be built based on the deformation mechanisms and the micro-stress and strain distribution among the constituent phases will be analyzed quantitatively and the prediction and optimization can be realized. The fulfillment of the present project will provide a new way for the alloy and microstructure design as well as fabrication of low density steels.

现代汽车的发展方向是减重、节能和提高安全性。由于添加铝可使钢的强度显著降低，高强度高塑性Fe-Mn-Al-C低密度钢近年来是高性能钢研究开发的热点之一。目前，对高铝(>8%)的Fe-Mn-Al-C低密度钢的研究均限于高的锰含量(20-30%)。由于锰含量过高导致在铸锭和热轧时易出现裂纹，降低锰含量将有利于低密度Fe-Mn-Al-C钢的应用。本项目提出一种新型的中锰高铝铁素体+奥氏体+碳化物复相钢，对其变形行为和变形过程中的微观组织演变进行研究，深入了解变形过程中各组成相的变形特征，明确不同铁素体晶粒尺寸和相比例、碳化物尺寸和体积分数对应变硬化行为的影响，揭示应变硬化的微观机制。同时，从变形机制出发，建立应力应变物理模型，定量分析各相之间的应力应变分配及相互协调，实现实验钢力学行为的预测和优化。本项目的实施将为高铝低密度钢的研究提供一种新的思路，为低密度钢的合金成分设计、制备及应用提供依据。

在钢中添加较多的铝含量，可以降低材料的密度，开发低密度钢是汽车用钢轻量化的发展方向之一。本项目采用热力学方法设计了新型中锰高铝低密度合金体系，通过合金设计和组织调控，使 Fe-Mn-Al-C系中锰高铝低密度钢的抗拉强度超过1.0 GPa，塑性约为40%，达到了项目提出的目标。明确了合金元素和热处理制度对组织组成的影响规律，揭示了多尺度复相组织调控机理。对Fe-Mn-Al-C系低密度钢变形过程中的组织演变进行了研究，明确了变形机制及对变形行为的影响规律。在几何必须位错和统计储存位错的基础上，结合金属材料一般流变应力模型，提出了适用于低密度钢不均匀塑性变形的应力-应变模型，并经实验验证。该模型可为低密度钢的组织设计提供依据，同时丰富了塑性变形理论。对实验钢在不同变形温度和应变速率下的变形行为进行了研究，分析了热变形过程中各物相的变形机制、κ碳化物的动态析出行为，建立了热加工图，可为中锰高铝低密度钢的热加工提供理论依据。首次用高屈强比的中锰高铝汽车钢进行冷冲压试验，制备的U形件表面质量好，无回弹，具有良好的冷冲压性能，说明本项目的研究成果具有潜在的工业应用前景。