Local Laser Annealing of Cold Rolled Steel

Heat treatment by annealing can be used to restore the ductility of a cold worked steel. During annealing, the material is held at an elevated temperature for some time and the cold worked material will undergo a sequence of recovery, recrystallization and grain growth, which eventually reduces the hardness and improves ductility. Annealing is, however, usually performed as a bulk process on entire components, for example by placing them in a furnace. Significantly more precision can be achieved if a laser beam is used as the heat source. The accuracy of this method is high enough to allow annealing of regions of millimeter size and in arbitrary patterns. The material’s microstructure can be modified locally, making laser annealing a useful tool in manufacturing of functionally graded steels.

Processes of cold work and annealing, phenomena occurring before and under local laser annealing of cold rolled steel

Project and collaborators

The project combines laser annealing experiments with level set-based simulations of the microstructure evolution and is a collaboration with researchers at École Nationale Supérieure d’Arts et Métiers in Paris.

Multiscale Modeling of Recrystallization

Recrystallization (RX) is one of the main mechanisms involved in grain structure evolution. This might be a relatively slow (quasi-static) process or fast-proceeding dynamic process, driven by plastic deformation of the material. In both scenarios, RX proceeds by growth of new grains of low dislocation density which consume the surrounding cold-worked and high-energy microstructure, effectively lowering the level of stored energy in the material.

In this 5-year project, funded by the Swedish Science Council and the Crafoord foundation, a multiscale simulation is established, combining crystal plasticity modeling – to trace evolution of plasticity and texture – with a vertex model, tracing the grain structure evolution. The model is applied to, for example, large scale simulations of sheet metal rolling. Recrystallization, grain growth, particle (Zener) drag on migrating grain boundaries, plastic slip activity and texture evolution are examples of features which are all included in the model.

Grain structure evolution during recrystallization

Project

PhD student: Ylva Mellbin, Division of Solid Mechanics, Lund University.