A new approach for predicting forming limit curves (FLCs) at elevated temperatures was proposed herein. FLCs are often used to predict failure and determine the optimal forming parameters of automotive parts. First, a graphical method based on a modified maximum force criterion was applied to estimate the FLCs of 22MnB5 boron steel sheets at room temperature using various hardening laws. Subsequently, the predicted FLC data at room temperature were compared with corresponding data obtained from Nakazima’s tests to obtain the best prediction. To estimate the FLC at elevated temperatures, tensile tests were conducted at various temperatures to determine the ratios of equivalent fracture strains between the corresponding elevated temperatures and room temperature. FLCs at elevated temperatures could be established based on obtained ratios. However, the predicted FLCs at elevated temperatures did not agree well with the corresponding FLC experimental data of Zhou et al. A new method was proposed herein to improve the prediction of FLCs at elevated temperatures. An FLC calculated at room temperature was utilized to predict the failure of Nakazima’s samples via finite element simulation. Based on the simulation results at room temperature, the mathematical relationships between the equivalent ductile fracture strain versus stress triaxiality and strain ratio were established and then combined with ratios between elevated and room temperatures to calculate the FLCs at different temperatures. The predicted FLCs at elevated temperatures agree well with the corresponding experimental FLC data.

本文提出了一种预测高温下的成形极限曲线（FLC）的新方法。FLC通常用于预测故障并确定汽车零件的最佳成形参数。首先，使用基于修改后的最大力准则的图形方法，使用各种硬化定律在室温下估算22MnB5硼钢板的FLC。随后，将室温下的预测FLC数据与从Nakazima的测试中获得的相应数据进行比较，以获得最佳预测。为了评估高温下的FLC，在各种温度下进行了拉伸试验，以确定相应的高温和室温之间的等效断裂应变之比。可以基于获得的比率建立高温下的FLC。然而，在高温下预测的FLC与Zhou等人的相应FLC实验数据不一致。本文提出了一种新的方法来改善高温下FLC的预测。在室温下计算的FLC通过有限元模拟用于预测Nakazima样品的失效。根据室温下的模拟结果，建立了等效延性断裂应变与应力三轴性和应变比之间的数学关系，然后结合高温和室温之间的比值来计算不同温度下的FLC。高温下的预测FLC与相应的实验FLC数据非常吻合。