Microstructural Control by Dynamic Recrystallization and Phase Transformation

采用无间隙(IF)钢和一种铁素体不锈钢，研究了动态再结晶(DRX)和动态相变(DPT)对钢铁的细化作用。主要研究结果总结如下。1和2的结果是IF钢的结果，3和4的结果是后者的结果1。在很长一段时间内，DRX奥氏体呈现出多步回复过程，不会完全软化。IF钢中细小的DRX奥氏体晶导致了细化的铁素体晶粒的演化。即使经过较长时间的DRX奥氏体退火，铁素体晶粒也几乎不长大。这是由于DRX基质中生长的颗粒稳定存在的结果。奥氏体晶内高密度位错亚结构上析出细小的碳氮化物。这会导致奥氏体铁素体晶粒的稳定，随后在快速冷却过程中会演化出细小的铁素体晶。亚稳奥氏体相铁素体钢的应力-应变曲线显示出一个尖锐的应力峰和快速的加工软化，然后在高应变下出现稳定的流动。功的软化是变形过程中奥氏体相(伽马)向铁素体(α)相变的结果，即动态相变。然而，这种伽马到阿尔法的转换并不是静态发生的。动态相变形成的铁素体晶粒度随应变速率和温度的升高而减小，导致室温硬度增加。

The effects of dynamic recrystallization (DRX) and dynamic phase transformation (DPT) on the grain refinement of iron and steels were studied by using interstitial free (IF) steels and a ferritic stainless steel. The main results are summarized as follows. The result of 1 and 2 are for IF steels, and those of 3 and 4 for the latter steel.1. The DRX austenite shows multiple-step restoration processes and do not go to full softening for a long period of annealing time. Finer DRX austenite grains in IF steels lead to the evolution of finer ferrite grains compared with usual grains. Ferrite grains scarcely grow even after annealing of DRX austenite for a long period of time. These results from the stable existence of growing grains in DRX matrices.2. Fine carbonenitrides precipitate on high density dislocation substructures developed in austenitic DRX grains. This can result in the stabilization of austenitic DRX grains, followed by the evolution of fine ferrite grains during repid cooling.3. Stress-strain curves of a crominum ferritic steel in the metastable austenite phase show a sharp stress peak and a rapid work softening, followed by a steady state flow at hihg strains. The work softening results from austenite (gamma) to ferrite (alpha) phase transformation during deformation ; namely, this is termed dynamic phase transformation. This kind of gamma to alpha transformation, however, does not statically take place.4. Ferritic grain sizes evolved by dynamic phase transformation decrease with increasing strain rate and temperature, leading to increase in room temperature hardness.