Diffusional phase transformations in steel - theory and application to continuous casting

Abstract
In addition to the thermal stresses the bending and straightening operations in a curved type slab caster cause mechanical strains and stresses in the temperature region between 700°C and 1200°C. Many investigations have shown a breakdown of the hot ductility behaviour of several steel grades due to the austenite / ferrite transformation and due to the precipitation of non-metallic inclusions in the temperature range mentioned above. The arrangement of the two phases - austenite and ferrite - as well as the arrangement of non-metallic inclusions in the austenite matrix play an important role. The formation of ferrite is observed as thin ferritic films along the austenite grain boundaries. The movement of the interphase boundary has been modeled. The coupled processes, carbon diffusion and interface migration, have been taken into account. This procedure allows an estimation of the ferrite film thickness. In the second part of the paper, a two-dimensional micromechanical study is performed assuming a small amount of ferrite. The stress strain distribution has been analyzed in this composite.