低碳钢在A_(e3)温度之上的形变诱导铁素体(一种马氏体)的相变研究

INVESTIGATION ON DEFORMATION INDUCED FER-RITE (A KIND OF MARTENSITE) TRANSFORMATION ABOVE A_(e3) TEMPERATURE IN A LOW CARBON STEEL

通过对低碳钢Q235的单向压缩实验,研究了应变、应变速率和变形温度(高于奥氏体铁素体平衡转变温度Ae3)对形变诱导铁素体相变的影响,通过光学显微镜、扫描电子显微镜和X射线衍射仪研究了热变形试样的微观组织结构,利用纳米压痕仪测定了形变诱导铁素体和先共析铁素体的纳米压痕硬度和弹性模量.结果表明,形变诱导铁素体相变可以在Ae3 温度之上发生且应变速率和应变越大,相变越容易.在名义应变ε=80%,应变速率ε=20s-1的条件下形变诱导铁素体相变上限温度为945℃(Ae3+98℃).同时发现一个有趣的现象是,在870-920℃区间内变形时,随变形温度下降,应力上升;而在830-870℃区间变形时,随变形温度的下降,整体应力反而下降.与先共析铁素体X射线衍射峰比较,形变诱导铁素体X射线衍射峰明显向小角度方向漂移,形变诱导铁素体的纳米压痕硬度和弹性模量亦明显大于先共析铁素体. 实验表明,这种形变诱导铁素体本质上是一种马氏体.

A series of unidirectional compression tests of a low carbon steel Q235 were performed on a Gleeble 3500 thermal simulator, and the influences of strain, strain rate and deformation temperature （above the isothermal transformation temperature Ae3 from austenite to ferrite） on deformation induced ferrite transformation （DIFT） have been examined. The microstructure, nanoindentation hardness and the elastic modulus of deformation induced ferrite （DIF） and proeutectoid ferrtie were determined by optical microscope, scanning electron microscope （SEM）, X-ray diffraction （XRD） and nanoindentation techniques. The results show that DIFT can take place above Ae3 temperature, the higher the strain rate and the strain, the more favorable for DIFT. When ε=80% and ε=20 s^-1, the upper limit temperature of DIFT can be elevated to 945℃（Ae3＋98℃）. An interesting phenomenon is found that when the deformation temperature is between 870-920℃, the total stress is increased with deformation temperature decreasing. However, when deformed between 830-870℃, the total stress is decreased with deformation temperature decreasing. Compared with the diffraction peak of proeutectoid ferrite, the diffraction peak of DIF was shift to a low angle in XRD analysis, and both the nanoindentation hardness and elastic modulus of DIF are much higher than those of proeutectoid ferrite, which proved DIF to be a kind of martensite.