Transformation Delay and Texture Memory Effect of Columnar Grained Cast Slab in Low Grades Non-oriented Electrical Steels

The cast slab of low grades non-oriented electrical steels experiences twice diffusional solid phase transformation and demonstrates the features of strong morphological memory referring to columnar structure and texture memory referring to the preferred texture at room temperature. In this paper, electron backscatter diffraction (EBSD), quasi-insitu observation of heating samples, and dilatometry are used to study and analyze the two kinds of memory phenomena. The results show that the cast slab consists of about 70% coarse columnar grains and 30% small equiaxed grains. Many small equiaxed grains show Σ3 misorientations with columnar grains indicating K-S orientation relationship obeyed during phase transformation. Coarse columnar grains show a typical ||growing axis orientation which is the same as solidified columnar grains. The quasi-insitu observation shows that transformation of columnar grained ferrite to austenite is very sluggish and columnar grained ferrite can still be seen even at a superheating degree of 176°C for 1 hour. Dilatometry measurement indicates that the starting transformation temperatures for a columnar-grain-dominant sample and a small-equiaxed-grain-dominant sample are similar, whereas their transformation extents are quite different with columnar grained sample showing a low dilatational amount due to insufficient transformation. It is most likely that the coarse columnar grains in cast slab are retained and untransformed high temperature δ-ferrite are not subjected to twice complete transformations. These retaining columnar grains in low grades of electrical steels can be used to improve magnetic properties through optimizing processing parameters.