铆螺钢不变形和变形条件下连续冷却的相变行为

在Gleeble-1500热模拟试验机上进行铆螺钢的热模拟实验,通过研究其不变形条件下和与实验室轧机轧制变形量相一致的变形条件下的连续冷却相变行为,建立了相应的静态和动态CCT图,通过扫描电镜(SEM)对其组织进行观测。结果表明,由于形变增加了形核位置和能量,加速了相变,在热变形的CCT图中,变形使铁素体、珠光体和贝氏体转变线向高温区的左移。最快的冷却速度获得了全部的马氏体组织;随冷却速度降低,粒状贝氏体、多边形铁素体和珠光体组织形成;快冷抑制了铁素体和珠光体形成,使硬度增高;硅、锰和铬合金元素使CCT图中的珠光体和贝氏体转变线右移;在变形条件下以3.3℃/s~16.7℃/s的冷速冷却时,能够得到多边形铁素体、粒状贝氏体和残余奥氏体组织。由于组织中残余奥氏体的存在,有助于产生相变诱发塑性(TRIP)效应,铆螺钢实际轧制时可能能够获得满意的冷镦性能。

Continuous cooling transformation behaviour in the undeformed and deformed conditions of cold heading steel

Thermomechanical simulations were performed by a Gleeble-1500 thermomechanical simulator in cold heading steel. The continuous cooling transformation (CCT) behaviors of this steel were investigated in the undeformed and deformed (the ther-momechanical processing schedule was consistent with laboratorial processing) conditions, respectively, and the corresponding static (without hot deformation) and dynamic (with hot deformation) CCT diagrams were constructed. The microstructures were observed by scanning electron microscopy (SEM). The results showed that hot deformation shifted the ferrite, pearlite and bain-ite transformati0n left to high temperature region in dynamic CCT diagram, because deformation increased nucleation sites and en-ergy and accelerated the transformation. The fastest cooling rate produced fully martensitic microstructure. As the cooling rate decreased, granular bainite, polygonal ferrite and pearlite were formed. A rapid cooling rate depressed the formation of ferrite and pearlite, which resulted in higher hardness. Alloying elements of Si, Mn and Cr made the pearlite and bainite curves of CCT diagrams move to the right. Polygonal ferrite, granular bainite and retained austenite could be obtained at cooling rates of 3.3℃/ s～16.7℃/s in the deformed condition, and this might contribute to a satisfactory TRIP effect due to the presence of retained austenite in the microstructure.