Abstract A new steel chemical composition is combined with a new press hardening process, in which die-quenching is interrupted by opening the forming tool to permit slow cooling of the hot formed part through the anisothermal bainitic ferrite transformation. This promotes carbon partitioning to austenite before the forming tool is re-closed and die-quenching is resumed to near-ambient temperature. The final microstructure is predominantly bainitic ferrite with dispersions of martensite and up to 11 % retained austenite. Retained austenite can undergo stress induced transformation to martensite in an automobile crash event. The steel exhibits up to 25 % elongation and 930 MPa tensile strength. In contrast to traditional cold formable Transformation Induced Plasticity assisted steels, where retained austenite is consumed during work hardening of cold forming, here, the desired microstructure is achieved after hot forming meaning the retained austenite is more uniformly distributed within the formed part, which enhances energy absorption. The new steel chemical composition is carefully designed to provide optimal microstructural evolution within the constraints of the new press hardening process, yet relatively lean and manufacturer friendly. The new press hardening process is energy efficient as secondary heating is not required since retarded cooling through the bainitic ferrite transformation is provided by residual heat accumulation of the newly developed titanium alloy forming tool. Development of the new technology is demonstrated by press hardening experiments, tensile testing, microstructural analysis, transversal & axial crush testing of formed parts and numerical simulation of crush testing, including a new modelling technique that more accurately simulates deformation of hot versus cold formed parts. Results show a 22 % increase to energy absorption under axial crushing compared to traditional cold formed Transformation Induced Plasticity assisted steels owing to greater work hardening capacity in formed radii of the part, which are shown to be exposed to the highest stresses during crushing.

中文翻译：

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通过非等温贝氏体铁素体转变的 TRIP 辅助冲压硬化钢

摘要 一种新的钢化学成分与一种新的压力硬化工艺相结合，其中通过打开成型工具来中断模具淬火，使热成型零件通过等温贝氏体铁素体转变缓慢冷却。这促进了在成形工具重新闭合和模具淬火恢复到接近环境温度之前碳分配为奥氏体。最终的显微组织主要是贝氏体铁素体，其中含有马氏体和高达 11% 的残余奥氏体。在汽车碰撞事件中，残留奥氏体会发生应力诱导转变为马氏体。该钢具有高达 25% 的伸长率和 930 MPa 的抗拉强度。与传统的冷成型相变诱导塑性辅助钢相比，在冷成型加工硬化过程中消耗残余奥氏体，这里，在热成型后获得所需的显微组织，这意味着残余奥氏体更均匀地分布在成型零件中，这增强了能量吸收。新钢的化学成分经过精心设计，可在新的压力硬化工艺的限制内提供最佳的微观结构演变，但相对精简且对制造商友好。由于新开发的钛合金成形工具的余热积累提供了通过贝氏体铁素体转变的延迟冷却，因此新的压力硬化工艺是节能的，因为不需要二次加热。压制硬化实验、拉伸试验、显微结构分析、横向和 成形零件的轴向挤压测试和挤压测试的数值模拟，包括一种新的建模技术，可以更准确地模拟热成形零件与冷成形零件的变形。结果表明，与传统的冷成型相变诱导塑性辅助钢相比，轴向挤压下的能量吸收增加了 22%，因为零件的成形半径具有更大的加工硬化能力，这表明零件在挤压过程中承受了最高的应力。