Effect of the Initial Columnar‐Grained Inhomogeneity of Electrical Steels on the Transformation Temperature

Abstract

The effects of various initial structures and heating and cooling rates on the columnar‐grained transformation temperature are studied by a thermal dilatometer and electron backscatter diffraction. The results show that deformation can reduce the thermal hysteresis more effectively than decrease the heating rate, and the phase transformation hysteresis of the deformed sample is the smallest during heating, followed by the columnar‐grained sample with a slow heating rate, whereas the columnar‐grained sample with a fast heating rate has the largest phase transformation hysteresis. Moreover, the dilatational amount changes greatly with cooling rate. The coarse columnar grains can severely restrain dilatation. The smaller dilatational amount is related to the incomplete phase transformation of the columnar grains and also related to the suppression of the dilatational amount of the small grains by the surrounding columnar grains. In contrast, the largest dilatational amount is caused by the relatively sufficient phase transformation. In addition, the changes of the transformation temperature and the dilatational amount are mainly induced by the grain size effect and the inhomogeneity of the structures, but {100} texture can affect the uniformity of grain size.

Application of Transformation Treatment to Commercial Low-Grade Electrical Steels under Different Processing Conditions

“…Moreover, it is seen that initial microstructure, rolling reduction, final heating temperature and cooling rate also show their effect and needed to be further optimized. Furthermore, although the silicon content in [1] is as low as 0.35%, much lower than those in this study (0.72-1%Si), according to Thermo-Calc calculation, the addition of high 0.14%P increases its A 1 -A 3 to a higher value than the steels containing 0.64%Si [8] and 2# sheets in this study.…”

“…The commercial products are subjected only to recrystallization annealing in the third stage. Based on our previous quasi-in situ observation of the coarse columnar grains structure in cast slabs, it is confirmed that many columnar grains are retained δ ferrite and the transformation from δ ferrite to austenite is delayed or even suppressed in low-grade electrical steels, showing morphological and texture memory effects of {100} columnar grain structure [1,7,8]. We then suggest a new way of increasing {100} texture in hot-rolled bands to improve magnetic properties of final products, namely the so-called metastable ferrite hot rolling method [7].…”

“…Figure 2 shows the EBSD maps of initial cast slab structures of samples 1# (1%Si) and (0.72%Si). The silicon contents of these two steels are higher than that of 0.65%Si in the l grade electrical steel [8]. It is seen that many {100}-oriented columnar grains (red color) e Theoretically twice transformation of δ → γ →  should occur during continuous casting equiaxed grain structure should be resulted.…”

“…Theoretically twice transformation of δ → γ → α should occur during continuous casting and equiaxed grain structure should be resulted. However, it is confirmed by quasi-in situ EBSD mapping [7,8] that transformation delay and suppression occur, leading to many untransformed regions in columnar grains. In contrast, small equiaxed grains with normally {111} orientations (blue color) are transformed grains.…”