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

In addition to the thermal stresses the bending and straightening operation s in a curved type slab caster cause mechanical strains and stresses in the temperature region between 700 degreesC and 1200 degreesC. Many investigat ions have shown a breakdown of the hot ductility behaviour of several steel grades due to the austenite / ferrite transformation and due to the precip itation of non-metallic inclusions in the temperature range mentioned above . The arrangement of the two phases - austenite and ferrite - as well as th e arrangement of non-metallic inclusions in the austenite matrix play an im portant role. The formation of ferrite is observed as thin ferritic films a long the austenite grain boundaries. The movement of the interphase boundar y has been modeled. The coupled processes, carbon diffusion and interface m igration, 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 distributi on has been analyzed in this composite.