Abnormal grain growth with preferred orientation in non-oriented silicon steel: a quasi-in situ study

Abnormal grain growth (AGG) involving orientation selectivity during isothermal annealing in non-oriented silicon steels (NGO) was comparatively investigated by quasi-in situ observation. Remarkable abnormal growth of {100} grains occurred in strip-cast NGO after isothermal annealing. Similar AGG tendencies but with relatively small grain size and random orientation were observed in the suction-cast NGO and hot-rolled NGO. The abnormal grain growth process can be divided into three stages: (1) incubation stage where the seed grains germinated; (2) rapid development stage with seed grain growth rate of ~ 1.5 μm/s; (3) perfection stage with grain growth rate of ~ 0.4 μm/s, slowed down by coarse grains and the sheet surface. Grain growth kinetic can be well represented under three segments instead of constant linear regression. The kinetics for normal grain growth and AGG were D  =  D₀ + 4.43 × 10⁻⁶t^1.789 and D  =  D₀ + 5.95 × 10⁻⁶t^2.565, respectively. The selective AGG with preferred orientation showed strain dependence. With increase in prior deformation, the trend of AGG weakened and the orientation became more random. Inhomogeneous micro-strain distribution played a key role for initiation and propagation of AGG with preferred orientation in the strip-cast NGO. Sub-rapid solidification and slight deformation during strip casting process typically generated high level of strain inhomogeneity, which acted as an additional driving force to induce AGG by enhanced grain boundary migration. The preferred orientation of AGG may be related to orientation dependence of solidification and grain growth behaviors.