Improvement of Formability by Moderate Temperature Forming and Thermo-mechanical Process and Examination of Fatigue Properties of TRIP-aided Dual-phase Steel Sheets.

TRIP辅助双相钢(TDP钢)已经开发出来，通过减轻车辆重量来提高安全性和燃油效率。本研究的目的是提高TDP钢的拉伸成形性和拉伸凸缘能力，并检验其疲劳强度。在20ﾟC~200ﾟC的中温成形条件下，TDP钢的拉伸成形性和拉伸法兰性能得到改善，残余奥氏体区的ms温度越低，其成形性越大，最佳成形温度越低。α+伽马区的热机械处理可获得大量的残余奥氏体相和细小的组织，但提高了残余奥氏体相的mS温度，特别是对低锰钢和低硅素钢。TDP钢由于残余奥氏体应变诱发相变和母体与第二相强度差异产生的内应力，产生了很大的疲劳硬化。疲劳硬化强度随试验温度的升高而降低。与常规双相钢相比，在100200ﾟC下获得了更高的疲劳极限，其原因正在调查中。

TRIP-aided dual-phase steels (TDP steels) have been developed to improve safety and fuel efficiency by reducing vehicle weight. The purpose of the present investigation is to improve stretch-formability and stretch-flange-ability and to examine fatigue strength of TDP steels.The stretch-formability and stretch-flangeability of TDP steels were improved by moderate temperature forming at 20ﾟC to 200ﾟC. The lower Ms temperature of the retained austenite, the greater these formability and the lower the optimum forming temperature. Thermo-mechanical process in alpha+gamma region gave a significant amount of retained austenite and fine microstructure, but increased the retained austenite Ms temperature, particularly for low manganese and low sili-con steels.Great fatigue hardening appeared for TDP steels owing to strain induced transformation of the retained austenite and internal stress resulting from a difference of strength between matrix and second phase. The fatigue hardening decreased with increasing testing temperature. Higher fatigue limit compared to that of a conventional dual-phase steel was obtained at 100 to 200ﾟC. The reason is under investigation.