Improvement on room-temperature ductility of 6.5Â wt.% Si steel by stress-relief annealing treatments after warm rolling

- The thin sheets used in the study are fabricated by strip casting and rolling.
- Ordered phase region for 6.5%Si steel should be extended down to about 400 °C.
- Ductility of 6.5%Si steel is significantly improved after 350 °Cannealing.
- Qualities of cold rolled 6.5%Si steel are optimized by stress-relief annealing.

In our previous work, we demonstrated that thin sheets of 6.5 wt.% Si steel could be fabricated by cold rolling with deformation induced disordering (DID). However, because the warm rolled sheets possessed high hardness, edge cracks were still formed in the cold rolled sheets. Therefore, hardness of warm rolled sheets needed to be decreased to further improve the ductility at room temperature. In the present work, disordered 6.5 wt.% Si sheets with the thickness of 0.35 mm were fabricated by strip casting, hot rolling and warm rolling. Stress-relief annealing treatments were carried out at temperatures from 300 °C to 550 °C, and influences of annealing temperatures on the ductility of warm rolled sheets were investigated. It had been observed that the room-temperature bending properties experienced ″over-hill″ changes, although dislocation densities in the annealed specimens were indeed decreased with the increase of annealing temperatures. The specimen annealed at 350 °C exhibited the lowest macro Vickers hardness and highest fracture deflection among all specimens, with a certain amount of dimples having been observed on the fracture surfaces after bending. Ordered domains could be observed when the annealing temperature was higher than 400 °C, and they grew with the increase of annealing temperatures. The integrated effect of decrease in dislocation density and suppression of re-ordering was realized at 350 °C, which was believed to be responsible for the highest room-temperature ductility, the best surface quality, and the least edge cracking for the cold rolled sheets.