分区控制模具温度的硼钢板热成形过程相变机理及高温变形特性研究

The traditional hot stamping process of boron-manganese steel can significantly reduce vehicle weight and improve body strength and collision resistance. But more and more attention is being paid to the issue of matching between mechanical properties of component and safety of body-in-white. A novel hot stamping process based on tailor-heated segmented dies to achieve final components with demanded mechanical property distribution is proposed. Phase transformation mechanism and deformation behavior at elevated temperature are most important key issues of the novel process. By means of SEM and XRD et al., the mathematical model of the stress-induced phase transformation will be set up, and the quantitative relationship between stress state and phase transformation kinetics will be obtained, which in turn will reveal the effect of deformation history such as temperature, strain rate, plastic strain on TTT curves. Quantitative characterization between stress state and phase transformation plasticity coefficient will be explored, therefore, the impact mechanism of stress state on phase transformation plasticity will be revealed. Based on phenomenological method of continuum and isotropic damage assumption, combined with ductile fracture criteria, and by means of FEM and optimization algorithms, the mathematical model reflecting the relationship between damage value and FLD of Nakazima test will be built. The analytical formulas of heat transfer coefficient related to pressure, temperature will be obtained. Therefore, the mathematical model of heat transfer of tailor-heated segmented dies will be set up. Consequently, it will lay a foundation of theory and application for die structure design, cooling channel/heating unit layout and tool material selection, which are necessary to obtain final components with demanded mechanical property distribution.

传统硼钢热冲压工艺在降低车重的同时能够显著提高车身强度和抗碰撞性。但是零部件力学性能与车身安全相匹配的问题，愈来愈受到重视。项目提出基于模具温度分区控制的新型热冲压工艺，实现零件力学性能按需分布。针对硼钢的相变机理及高温变形特性，拟借助SEM、XRD等手段，建立应力诱导相变的数学模型，获得应力状态与组织转变动力学的定量关系，揭示变形温度、应变速率、塑性应变对TTT曲线的影响规律；探索应力状态与各相相变塑性参数关系的定量表征，揭示应力状态对相变塑性的影响机理。基于连续介质的唯象方法，结合韧性断裂准则，依靠数值模拟方法和优化方法建立损伤值与Nakazima实验中FLD关系的数学模型。得到边界换热系数与单位面积接触压力、模具/硼钢板温度等因素关系的解析公式，建立温度分区控制的模压传热过程数学模型，为模具结构设计、冷却流道/加热元件布置、模具选材，进而实现零件的力学性能按需分布奠定理论与应用基础。

随着能源和环境问题的日益突出，汽车轻量化成了倡导国家节能减排政策的一项重要举措；同时对轿车的安全性、可靠性、舒适性等要求的提高，促使寻找新的替代材料及新的成形工艺。汽车轻量化及新型制造工艺已成为汽车工业发展中的一项关键性研究课题。. 本项目主要研究内容包括：（1）硼钢各相高温下本构模型研究；（2）硼钢高温成形过程扩散型及非扩散型相变机制研究；（3）硼钢高温状态摩擦磨损机理研究；（4）基于连续介质损伤力学的硼钢高温状态成形极限研究；（5）接触传热及温度控制。. 重要结果：（1）建立了硼钢在奥氏体、铁素体+珠光体、贝氏体和马氏体组织时中高温状态下本构关系模型；（2）获得了应力状态与组织转变动力学定量关系，明确了各相相变塑性参数与变形温度、应力状态的定量表征；（3）揭示了高温成形过程中微观粗糙表面几何形貌对摩擦系数的影响规律；（4）基于连续损伤力学及韧性断裂准则，利用有限元数值分析方法获得了高温Nakazima实验FLD；（5）得到了边界换热系数与单位面积接触压力、模具表面温度、硼钢板的温度等因素关系的解析公式。. 在以上研究成果的基础上发表SCI收录论文8篇，EI收录论文13篇，获批发明专利2项。在深入研究基于模具温度分区控制的新型热冲压工艺基础上，解决了其应用的关键科学问题，对实现零件的力学性能按需分布，提升汽车轻量化以及安全性有着重要的理论和现实意义。