Modeling bainitic transformations during press hardening

冲压硬化是汽车安全相关部件生产的一项重要加工技术。零件在低于550°C的规定刀具温度下在奥氏体状态下热成形，然后在该刀具内进行负载淬火。这种技术可以制成由超高强度钢制成的复杂形状的零件。根据淬火条件的不同，会形成马氏体或贝氏体组织，并在机械载荷、内应力、化学成分和相变动力学之间形成强耦合。在这个项目中，我们重点研究了部分或贝氏体挤压硬化过程中奥氏体到贝氏体的转变，使用的工具温度约为400至500℃。由于加工条件的原因，贝氏体是由高应力下的预应变奥氏体形成的。在这个规模桥接项目中，我们的目标是通过自下而上的方法对潜在的贝氏体转换有一个基本的理解。我们从用相场方法在最小尺度上对贝氏体血小板和束状体进行理论描述开始。这些相前沿生长预测的结果与中尺度水平上的中间建模步骤有关，该步骤追求在涉及塑性变形的代表性体积单元中预测转化动力学和材料特性。相分数预测采用粗粒度相场模型结合晶体塑性方法。基于平均场概念与动力学模型相结合的握手宏观描述，可以在理论指导下预测转化动力学和机械载荷影响下的力学行为。在这里，基于有限元的模型允许在加工过程中预测整个工件的力学行为。热-化学-机械耦合模型水平的三部曲与贝氏体形成在压力硬化过程中的实验研究进行了比较。本项目采用EBSD+EDX、HR-EPMA和TEM等先进电子显微镜进行微观结构动力学定量评价。这允许在不同尺度上直接验证模拟预测，并允许改进理论描述，以及将实验设置和条件与多尺度建模方法联系起来。该项目的主要目标是发展对贝氏体相变的自下而上的理解，特别是在外加应力的影响下，并为高强度钢板部件在高级压力硬化过程中的加工步骤的理论指导改进设置路径。重点是合金元素（C, Si, Cr）的作用，塑性效应对界面动力学和沉淀的影响，以及与热力学一致模型的联系。

Press hardening is an important processing technology for the production of safety relevant components for automotive applications. The parts are hot formed in the austenitic state in a tool with defined tool temperature below 550 °C and afterwards quenched under load inside this tool. This technology allows forming of complex shaped parts made of ultrahigh strength steels. Depending on quenching conditions martensitic or bainitic microstructures form, with a strong coupling between mechanical load, internal stresses, chemical composition and phase transformation kinetics. In this project, we focus on the austenite-to-bainite transformation during a partial or bainitic press hardening process, using tools of about 400 to 500 °C. Due to processing conditions, the bainite forms from a pre-strained austenite under high stresses. In this scale-bridging project we aim for a fundamental understanding of the underlying bainitic transformations in a bottom-up approach. We start from the theoretical description of bainitic platelets and sheaves on the smallest scales using phase field methods. The outcome from these phase front growth predictions is connected to an intermediate modeling step on the mesoscale level, which pursues the prediction of transformation kinetics and material properties in representative volume elements, involving plastic deformations. Phase fractions are predicted using coarser grained phase field models combined with a crystal plasticity approach. Handshaking with macroscopic descriptions based on a mean field concept combined with kinetic models allow for a theory-guided prediction of transformation kinetics and the mechanical behavior under the influence of mechanical load. Here, finite element based models allow to predict the mechanical behavior of entire work pieces during processing. The trilogy of thermo-chemo-mechanically coupled modeling levels is compared to experimental investigations of bainite formation during press hardening. Advanced electron microscopy using EBSD+EDX, HR-EPMA and TEM are applied within this project for quantitative microstructure kinetics evaluation. This allows to directly validate the simulation predictions on different scales and allows for improvements of the theoretical descriptions, as well as to link experimental setups and conditions to the multi-scale modeling approach. The main goal of the project is to develop a bottom-up understanding of bainitic transformations in particular under the influence of applied stresses, and to set the path for a theory-guided improvement of processing steps during advanced press hardening of high strength steel sheet components. A focus will be on the role of alloying elements (C, Si, Cr), the influence of plastic effects on interface kinetics and precipitation, and the link to thermodynamically consistent models.