根据KRC和LFG模型提出的Fe-C合金的奥氏体相变机制, 系统地计算了过冷奥氏体的相变驱动力, 从热力学的角度分析了过冷奥氏体分解生成纳米级渗碳体颗粒的可能性, 并且在热轧后超快速冷却的条件下, 发现热轧亚共析钢的组织中存在大量纳米级渗碳体弥散分布的区域, 渗碳体的尺寸在十到几十纳米, 实现了在无微合金元素添加的条件下渗碳体的纳米级析出. 此外, 在过冷奥氏体组织中先共析铁素体附近存在大量的富C区, 根据平衡浓度计算, 局部C的摩尔分数可达到0.04-0.08, 这部分高浓度的奥氏体分解析出纳米级渗碳体的倾向性更大.

In order to meet both the development requirements for reduction in cost and strengthening, recently,the research of precipitation of cementites, as the most economical and common precipitates in steels, has drawn wide attention in the field of precipitation strengthening again, because if cementites could be effectively refined to the scale of a few nanometers, it could also generate very strong precipitation strengthening effects to replace the strengthening role of the precipitates of micro-alloying elements. However, the cementites in hypoeutectoid steels usually form lamellar pearlite structure in near-equilibrium conditions, unable to form the precipitation of nanoscale particles, and they tend to be coarsened significantly at high temperatures after hot rolling. Therefore,the non-equilibrium precipitation of cementites only could be realized by increasing cooling rate after hot rolling, and the thermodynamic feasibility for the formation of nanoscale cementite precipitates during cooling has to be determined. In this work, according to the austenitic transformation mechanism of KRC and LFG models in Fe—C alloys, the transformation driving force of undercooled austenite was calculated systematically in a thermodynamic view, and the effect of ultra fast cooling (UFC) after hot rolling on the precipitation behavior ofnano-scale cementite particles was investigated. Based on the calculation results, the driving force of degenerated pearlitic transformation is the most negative in the three transformation mechanisms, at the same undercooled temperature, which theoretically indicates that the degenerated pearlitic transformation of undercooled austenite can easily occur to form cementite and ferrite with the equilibrium concentrations. In practical manufacturing, the diffusion of carbon atoms could be restrained by decreasing temperature in short time in the application of UFC, as a result that cementites would most likely dispersed in the form of nano-scaled particles directly, rather than being fully grown up into lamellar pealites. Due to the UFC, a large number of dispersed nano-scaled cementite areas were found in the microstructure of hot-rolled hypoeutectoid experiment steels, where the size of the cementites was within the range of ten to tens nanometers. The precipitation of nano-scaled cementites was realized without the micro-alloying elements. Moreover, there were a lot of carbon-rich areas in the microstructure of undercooled austenite, based on the equilibrium concentration calculation, in which the local mole fraction of carbon could be from 0.04 to 0.08, and this part of austenite with the high carbon concentration was apt to decompose and form likely the precipitation of nano-scaled cementites.