超高强度钢热冲压的氢致延迟断裂机理和预测

Hot stamping technology has been wildly used in car and manufacturing industries. However, hydrogen permeation may happen during the production process or in service, which induces the degradation of mechanical properties and delayed fracture, i.e. hydrogen embrittlement (HE), that brings potential safety hazard for passengers. To study the HE of hot stamping parts, the following research works will be carried out: First, the HE susceptibility of hot stamping steel is studied based on a series of experiments and those influencing factors of HE are analyzed; Second, the mechanism of HE is studied based on the combination analysis of the hot stamping process and the post production process. Third, the effects of hydrogen on elastic-plastic behavior of hot stamping parts are discussed and the revised constitutive model and fracture criterion are built. Fourth, a numerical simulation scheme based on FEM is developed, and the process optimization is carried out to decrease the HE susceptibility. Some innovation points will be put forward in this project. First, the hydrogen evolution during the hot stamping process and its diffusion and concentration during the post-production process are combined studied to disclosure the mechanism of HE. Second, the effects of stress status on HE are discussed, and the constitutive model and the fracture criterion covering wider range of stress triaxiality are presented. Third, the interrelationship between hydrogen diffusion and plastic deformation is studied and a hydrogen- mechanical coupling solution scheme is offered, in which a stabilization method is developed to overcome the elastic snap-back instability during the crack initiation. Forth, the Q&P hot forming technology is introduced and an optimized hot stamping process with high strength, high ductility and low HE susceptibility is explored. The proposed project work would explain why HE happens for hot stamping steel, and how it works under the HE status; Moreover, it would provide the reference to the research of hydrogen induced microstructure evolution at elevated temperatures.

超高强度钢热冲压成形技术在汽车和制造行业得到广泛应用，然而在生产或使役过程中，氢的渗入会导致产品机械性能下降以致发生延迟断裂现象（亦称氢脆），带来严重的安全隐患。本项目针对含氢热冲压钢，开展氢脆敏感性评价与影响因素分析；通过热冲压生产过程与生产后过程联合分析揭示氢脆现象发生的机理；构建力学模型并发展相应的数值求解方案；开展抗氢脆工艺优化与验证。项目重点关注初始稳定状态的氢在热冲压过程中受热力相变作用下的形态演化，以及热冲压结束后氢在材料内部的扩散与聚集；考虑不同应力状态下的弹塑性行为，建立适用于大应力三轴度变化范围的本构模型与断裂模型；设计氢-力耦合求解方案，发展稳定化算法以克服裂纹萌生时的弹性回跳失稳；结合Q&P热成形技术，探索具有高强度、高韧性、低氢脆敏感性的热冲压成形工艺。项目成果可解释热冲压钢发生氢脆的原因，描述其氢脆力学行为；并为高温下氢致材料微观结构的演化研究提供借鉴。

超高强度钢热冲压成形技术在制造业特别是汽车行业得到广泛应用，然而在生产或服役过程中，氢的渗入会导致产品的机械性能严重下降（氢脆），甚至发生延迟断裂现象，带来严重的安全隐患。. 项目针对热冲压超高强钢的氢脆和延迟开裂现象，开展了如下工作：（1）搭建了氢脆实验平台；（2）开展了充氢实验及充氢后的力学性能实验；（3）梳理了热冲压钢氢脆性能的影响因素与影响规律；（4）开展了氢脆试样的微观组织表征研究，在此基础上分析氢脆机理；（5）研究了热冲压裸板与铝硅镀层板的氢脆性能差异，并给出了合理的解释；（6）结合热冲压实际生产条件，研究消除和降低氢脆现象的控制技术。. 项目取得了如下成果：（1）证实了热冲压超高强钢具有高氢脆敏感性，充氢后材料的强度和塑性均显著下降；（2）梳理了影响氢脆的主要因素与影响规律，包括氢含量、塑性应变、应力场、材料组织状态等；（3）发现了氢脆的可逆与不可逆现象，当氢浓度较低时氢脆现象可逆，而当氢浓度较高时氢致损伤不可逆；（4）揭示了氢渗入对断裂形式的影响，材料由韧性断裂转变为脆性断裂，断面处形成氢脆层；（5）发现热冲压钢的氢致裂纹一般起源于原始奥氏体晶界或板条马氏体界面等缺陷位置，并沿垂直于板条马氏体方向进行扩展；（6）揭示了具有铝硅镀层的热冲压材料更易发生氢脆的原因，在于铝硅镀层在加热过程中发生的形态转变与合金化过程导致产生额外的氢且后续难以逸散；（7）针对氢难以追踪的难题，设计了氢微印试验，通过氢与银之间的置换反应后还原的银粒子间接表示氢的含量和位置；（8）结合热冲压生产，提出了降低氢脆风险的质量控制技术，包括加热温度控制技术、模具设计技术、成形工艺控制技术等。. 本项目揭示了热冲压钢的氢脆现象及背后的机理，为该领域的科学研究提供了有益的参考；同时为工业生产中如何预防和降低氢脆危害提供了思路和方法。项目取得了预期的效果。