Designing of the formation process of recyclable eco-steels

这项工作的目的是提出指导方针，选择适合回收的钢（生态钢），并建议化学成分和生产工艺，以满足所需的机械性能。主要结果如下：1)得到了钢的化学成分、热处理与力学性能之间的数值关系。针对Fe-C-Si-Mn等适合循环利用的简单化学成分钢，采用回归分析方法，得到了其力学性能（应力-应变曲线、YS、TS、均匀伸长率、断裂伸长率）与化学成分和热处理的关系。利用所得到的方程，建立了计算任意热处理后任意简单化学成分钢的力学性能的方法。2)提出了一种预测材料力学性能的新方法——力学热力学，并对其应用进行了验证。指出决定材料强度的主要因素是结构自由能。我们认为材料的强度与结构自由能的平方根成正比。当相变产生微观组织时，结构自由能是相变驱动力的一定比例。因此，利用相变驱动力来预测材料的强度是可能的。这就是机械热力学。结果表明，在珠光体钢中，流变应力与转化驱动力的平方根之间存在良好的线性关系。所得的相关系数远高于显微组织观察所得的相关系数。最后推导了贝氏体和马氏体钢相变驱动力与强度的关系。

This work was aimed to present the guide line to select steels suitable for recycle (eco-steel) and suggest chemical compositions and production process to satisfy the required mechanical properties. The main results obtained were as follows :1) The numerical relationship between the chemical compositions and thermal treatment and mechanical properties of steels were obtained.About the simple chemical composition of steels such as Fe-C-Si-Mn which are suitable for recycle, the relationships between the mechanical properties (stress-strain curve, YS,TS,uniform elongation, fracture elongation) and chemical compositions and thermal treatments were obtained using regression method. Using the obtained equations, the method to calculate the mechanical properties of any given simple chemical compositions of steels after any thermal treatments was established.2) A new method to predict mechanical properties, mechano-thermodynamics, was proposed and the usage of it was proved.It was noted that the main factor which determines the strength of materials was structural free energy. It was considered out that the strength of materials is proportional to the square root of the structural free energy. When the microstructure is produced by phase transformation, the structural free energy is a certain fraction of the transformation driving force.Therefore it is possible to predict the strength of materials using the transformation driving force. This was maned mechano-thermodynamics. It was shown in pearlite steels that good linear relationships exist between the flow stress and the square root of the transformation driving force. The obtained correlation factor was much higher than that obtained from the observation of microstructure. Finally the relationships between the transformation driving force and strength were derived for bainite and martensite steels.