高强度钢板热成形本构理论与实验分析

热成形(热冲压)过程中硼钢的热、力、相变耦合关系是研究热成形理论的基础, 同 时也是决定热成形工艺及数值模拟准确性的关键因素. 对热成形硼钢进行高温拉伸及淬火实 验: 硼钢板材试样在奥氏体化(950℃)后保温一定时间, 然后在连续冷却的同时施 加拉伸力, 记录此过程中力、位移、膨胀量及温度的变化. 通过对不同冷却速率及不同拉伸 力情况下上述物理量的变化规律及微观组织性能的分析, 研究硼钢相变过程中的热、力、相 变耦合关系. 建立了硼钢相变过程中的热、力、相变耦合模型. 通过引入混合定律对热成形 过程中的多相材料热力学参数和力学性能进行等效分析; 对热成形应变组成及其形成机理 进行了分析, 引入了相变体积应力及相变塑性应力等新概念. 硼钢高温流动应力采用修改的 Norton-Hoff形式, 并通过实验确定了流动应力的材料常数. 在此基础上将热、力、相变耦 合关系引入热成形本构方程中, 分别建立了高强度钢板热成形的全量形式及增量形式本构方 程. 对U形零部件热成形过程进行了数值模拟, 并与实验结果进行比较, 结果证明建 立的本构理论的有效性.

CONSTITUTIVE THEORY AND EXPERIMENT ANALYSIS OF HOT FORMING FOR HIGH STRENGTH STEEL

Thermal-mechanical-transformation coupled relations of boron steel are investigated in the hot forming (HFS, hot stamping) process by tensile and quenching experiments at high temperature. In the experiments, plate specimens of boron steel are austenitized for five minutes at 950℃, then formed in tension and quenched. The force, displacement, expansion and temperature in the experimental process are measured. Based on the analysis of the above physical quantities variation and the specimen's microstructure, thermal-mechanical-transformation coupled relations of boron steel are researched and the thermal-mechanical-transformation coupled constitutive models are developed. The multi-phase mixed relationship is introduced to analyze the effective thermo-mechanical parameters and mechanical properties of multi phases during the hot forming. The components of strain and their evolved mechanism for hot forming are investigated. The phase-transformation volume stress and phase-transformation plastic stress are defined and expressed to explain the mechanism of thermal-mechanical-transformation coupled relations. The modified Norton-Hoff flow stress is applied to describe the flow properties of boron steel at high temperatures and the material parameters is obtained by analyzing the measured data of high-temperature tensile tests. Based on the above research, the thermal-mechanical-transformation coupled models are introduced into the constitutive equations of hot forming, and the integrated and incremental constitutive equations are developed, respectively. Numerical simulation of U-shaped hot forming process is implemented and compared with the experimental results. The results prove the validity of the developed constitutive equations.