Characterization and physical based modeling of the bake-hardening effect in dual-phase steels: Development of a through-process modeling approach for the material behavior

汽车行业受到不断挑战的推动，不断提高对未来汽车的安全和燃油经济性要求。然而，这意味着需要改进钢种，以满足汽车结构和安全部件的更高标准。双相(DP)钢具有高的屈服强度和良好的成形性，且生产成本适中，因此在汽车上得到了广泛的应用。最近，有人提出在DP钢中使用烘烤硬化(BH)效应作为一种额外的强化机制，以提高屈服强度。这种强化发生在车身在板材成形操作和车身组装之后的油漆烘焙过程中。在汽车工程中，材料行为的计算机模拟已经成为产品和工艺开发中不可或缺的一部分。对于这些模拟在工业实践中的适用性来说，提供能够预测每个加工步骤中的材料行为的模型是必不可少的。使用这种全过程建模方法，可以使用计算机模拟来调整每个过程步骤中的过程参数，以获得最终的机械性能。该项目旨在为DP钢中的BH效应开发新的基于物理的方法。这将通过结合理论模型对DP钢中BH效应的实验表征的一系列进展来实现。实验研究将集中于影响DP钢中BH效应的不同因素。显然，在工业化生产的DP钢上，不同的组织特征(例如，晶粒度、马氏体体积分数和形态)、预加载条件(单轴、双轴和平面应变)以及时效条件将被研究到纳米级。计划实验的实验结果将用于DP钢中BH效应的理论建模，这将与TU Wien合作完成。理论模型将涵盖从纳米到宏观的范围。对于铁素体静态应变时效动力学和马氏体回火效应的物理模拟，将采用一种新的理论方法。基于物理的内状态变量(ISV)方法将被用于微观组织演化的建模以及流动应力的建模。此外，还将建立描述屈服点现象的应力-应变曲线模型。最后，拟议工作的综合成果将产生一个先进的软件工具，用于对DP钢中的BH效应进行全过程建模。

The automotive industry is driven by the continuous challenge to improve safety and fuel economy requirements for future vehicles. This implies, however, the need for improved steel grades to meet higher standards for automotive structural and safety parts. Dual-phase (DP) steels are widely used in automotive applications because they combine high yield strength and good formability at moderate production costs. Recently, it has been proposed to use the bake-hardening (BH) effect as an additional strengthening mechanism in DP steels to benefit from increased yield strength. This strengthening occurs during paint baking of the car body after sheet forming operations and car body assembly. In automotive engineering, computer simulations of material behavior have become an integral part in the product and process development. For the applicability of these simulations in industrial practice it is essential to have models provided which are able to predict the material behavior during each step of processing. With this through-process modeling approach, computer simulations can be used to adjust the process parameters in each process step in order to achieve the final mechanical properties. The project aims at developing new physical-based approaches for the BH effect in DP steels. This will be achieved through a series of advances in experimental characterization of the BH effect in DP steels combined with theoretical modeling. The experimental investigations will focus on the different factors affecting the BH effect in DP steels. Explicitly, on industrially produced DP steels different microstructure characteristics (e.g. grain size, martensite volume fraction and morphology), pre-loading conditions (uni-axial, bi-axial and plane strain), and ageing conditions will be investigated down to the nanoscopic scale. The experimental results from the planned experiments are to be used in the theoretical modeling of the BH effect in DP steels which will be accomplished in cooperation with TU Wien. The theoretical modeling will cover the range from the nanoscopic to the macroscopic scale. For the physically-based modeling of the static strain-ageing kinetic in ferrite and the tempering effects in martensite, a new theoretical approach will be employed. The physical-based internal-state-variable (ISV) approach will be utilized for the modeling of microstructure evolution as well as for the flow-stress modeling. Moreover, a model for stress-strain curves describing the yield-point phenomenon will be developed. Finally, the integrated outcome of the proposed work will result in an advanced software tool for through-process modeling of the BH effect in DP steels.