Determination of forming limits of the martensitic chromium steels in hot forming processes

计划研究项目的目的是对马氏体铬钢的热成形进行数值模拟，考虑高温下的成形极限、产生的显微组织及其对产品性能的影响。在钣金成形中，马氏体铬钢的热成形与锰硼钢的热冲压相比具有各种优势，例如较低的临界冷却速率、较高的断裂伸长率（断裂应变）以及较高的强度和更好的材料耐腐蚀性。在热成形过程中，工件温度、刀具温度和加工时间之间存在复杂的相互作用，这些相互作用对工件的微观组织、最终零件的性能和残余应力以及零件的变形产生重大影响，因此对热成形过程的数值模拟和工艺极限的预测尤为重要。特别是，与锰硼钢相比，马氏体铬钢的材料成本更高，因此模拟工艺设计对于节省资源、测试从而降低成本至关重要。计划开发的用于马氏体铬钢热成形数值模拟的材料模型将基于DFG资助的锰硼钢（22MnB5）热冲压研究项目中开发的材料模型进行。为了使材料模型适应马氏体铬钢及其显微组织行为，需要对材料进行全面的表征。为了确定金属板的成形极限，使用成形极限图是目前最先进的方法。由于材料的流动行为和变形能力取决于温度，在室温下获得的常规成形极限图不能用于热成形过程。为了预测马氏体铬钢在热成形过程中的变形能力以及成形温度对变形能力的影响，本项目的进一步目标是确定温度相关的成形极限图并在数值模拟中实现。这样可以对成形极限和冷却后产生的部件性能进行实际评估。通过热成形验证件对数值模型进行了验证。

The aim of the planned research project is numerical simulation of the hot forming of a martensitic chromium steel taking into account the forming limit at elevated temperatures, the resulting microstructure and their effects on the product properties. In the sheet metal forming, hot forming of martensitic chromium steels has various advantages over hot stamping of manganese-boron steels, such as a lower critical cooling rate, a higher elongation at break (fracture strain) along with higher strengths and a better corrosion resistance of the material. During the hot forming processes, complex interactions between the workpiece temperature, the tool temperature and the process time exist which significantly influence the microstructure, the final component properties and the residual stresses as well as the component distortion thus numerical modelling and prediction of the process limits is of particular importance for this process. In particular, the higher material costs of martensitic chromium steels compared to the manganese-boron steels make a simulative process design vital in order to save resources, tests and thus reduce the costs. The planned development of a material model for the numerical simulation of hot forming of martensitic chromium steels will be carried out based on a material model which was developed in the DFG funded preceding research projects for hot stamping of the manganese-boron steel (22MnB5). A comprehensive material characterization is required in order to adapt the material model to the martensitic chromium steels and their microstructural behavior.For the determination of the forming limits of sheet metals, the use of forming limit diagrams is state of the art. As the flow behavior and the deformability of a material are dependent on the temperature, conventional forming limit diagrams obtained at room temperature cannot be used for hot forming processes. In order to predict the deformability of martensitic chromium steels as well as the influence of the forming temperature on the deformability in the hot forming process, the further objective of this project lies in the determination of temperature-dependent forming limit diagrams and their implementation in the numerical simulation. This allows a realistic evaluation of the forming limit and the resulting component properties after cooling. The numerical model is validated with the help of hot formed demonstrator components.