Modeling bainitic transformations during press hardening

冲压硬化是生产汽车安全相关零部件的一项重要加工技术。零件在规定的刀具温度低于550°C的刀具中以奥氏体态进行热成形，然后在该刀具内进行负荷淬火。这项技术可以成形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，形成马氏体或贝氏体微组织，机械载荷、内应力、化学成分和相变动力学之间存在强烈的耦合。在本项目中，我们使用约400到500°C的刀具，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。由于工艺条件的不同，贝氏体是在高应力下由预应变奥氏体形成的。在这个跨越规模的项目中，我们的目标是以自下而上的方法从根本上理解贝氏体相变。我们从用相场方法在最小尺度上对贝氏体片和片层的理论描述开始。这些相锋生长预测的结果与介观尺度上的中间建模步骤相关联，该步骤追求对涉及塑性变形的典型体积元素中的相变动力学和材料性质的预测。用粗粒相场模型和晶体塑性方法相结合来预测相分数。握手与基于平均场概念的宏观描述相结合，结合动力学模型，允许对相变动力学和机械载荷影响下的力学行为进行理论指导的预测。在这里，基于有限元的模型允许预测加工过程中整个工件的力学行为。将热-化学-机械耦合模拟水平三部曲与压力硬化过程中贝氏体形成的实验研究进行了比较。使用EBSD+EDX，HR-EPMA和TEM的先进电子显微镜被用于定量微结构动力学评估。这允许直接验证不同尺度上的模拟预测，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自下而上地了解贝氏体相变，特别是在外加应力的影响下，并为高强度钢板组件高级冲压硬化过程中工艺步骤的理论指导改进铺平道路。重点将集中在合金元素(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.