A mechanism for secondary recrystallization in high-purity silicon iron

Various anomalous texture developments have been investigated in C-free and AlN-free electrical sheets, and new two-dimensional (2D) primary growth modeling has been carried out to understand them. In one-stage cold-rolled sheets, the primary $(001)$ texture pole intensity (TPI) increases with decreasing bulk sulfur content and sheet thickness. The high-index $\left(\mathrm{h}\mathrm{k}\mathrm{l}\right)$ and $(001)$ TPIs increase concurrently with increasing annealing time. In other case, the $(001)$ TPI increases to a maximum after which it rather decreases. In two-stage cold-rolled sheets showing the primary $\left(001\right)+(110)$ texture, the $(001)$ TPI increases with decreasing sheet thickness and second-stage cold-rolling ratio (CRR). The final $(001)$ grain size is determined by the combination between sheet thickness, bulk sulfur content, CRR, heating rate, and MnS precipitation reaction. New 2D primary growth modeling supports strongly that the higher surface energy grains as well as the lower surface energy ones can grow, although their growth is transient and peripheral. Several unique concepts implied in the 2D growth maps well explain the various anomalous texture developments. The $(111)$ island grains in electrical sheets fully (0 0 1)-textured after final annealing are always consumed with further annealing, regardless of the grain size. However, the $(110)$ island grains may grow or not, which depends on the grain size and the number of neighboring ones. Fully developed $(110)[001]$ Goss or $(001)$ texture can be obtained from electrical sheets of the same composition, depending on the processing variables. Fully $(001)$-textured electrical sheets of high magnetic properties were developed through the experimental results and the 2D growth modeling.

High-grade non-oriented electrical steel was successfully processed by controlling texture using strip casting technology. Remarkable abnormal grain growth of {100} grains occurred in the as-cast strip after isothermal annealing. This phenomenon is considered to be the synergistic effect involving the size advantage of coarse columnar grains and surface energy, where the normal grain growth was inhibited by thermal grooving in the surface layer. Special rotation path was identified during the cold rolling process and metastable {118} 〈110〉–{113} 〈110〉 texture dominated the deformation texture. The final annealed sheet exhibited strong Cube texture together with significantly superior magnetic properties.

A partial explanation of the phenomenon of abnormal growth of Goss-oriented grains in silicon steel is proposed. It follows from an observation that texture evolves toward Goss orientation if grains of low surface energy have a high probability of growth. A corresponding simulation starts with primary recrystallization texture and results in a strong Goss texture. To explain the development of early secondaries in subsurface layers, an option is considered that in addition to capillary forces, the growth is caused by surface-induced lattice instabilities.

With increasing heating rate, the final main texture of inhibitor-free 3% Si–Fe alloy strips was roughly changed from the {110}〈001〉 texture to the {110}〈uvw〉 + the {110}〈uvw〉 texture and subsequently to the {100}〈uvw〉 + the {111}〈uvw〉 texture, while magnetic induction decreased from 2.03 to 1.40 T. Such a dependence on heating rate is deeply related to the change in surface segregation kinetics of sulfur that governs the surface-energy-induced selective growth. The strips cold-rolled without any lubricant showed mainly the {110}〈001〉 and the {110}〈uvw〉 grains after final annealing, irrespective of heating rate. The final texture of the strips cold-rolled with a lubricant was very sensitive to the heating rate. This may be due to the nucleation behavior of the {110} nuclei that are formed from two kinds of cold rolling textures.

Decarburising annealing in the two-phase region is a well-known method to provide abnormal columnar grain growth in non-oriented electrical steel. Previous investigations have revealed that columnar-grained material produced in this way has an increased intensity of the “random cube” {1 0 0}〈0 $v$ $w$〉 and reduced {1 1 1}〈u $v$ $w$〉 deformation texture components in comparison with the same material with a typical equiaxed microstructure. In the present paper, the kinetics of the microstructure and texture development during the columnar grain growth is investigated by means of electron backscatter diffraction with emphasis on the early stages of the process. It has been found that texture changes reflect the character of the columnar grain growth when with further propagation of the columnar grains from the surface to the midplane the global texture becomes similar to the one of the near surface region in primary recrystallised material.

Decarburising annealing in the two-phase region is a known method to provide columnar grain growth in silicon non-oriented electrical steel that can improve magnetic properties. This method includes long-term preliminary annealing in vacuum and subsequent decarburising annealing in a wet hydrogen atmosphere (J. Mater. Eng. Perform. 5 (1996) 316). In the present paper an alternative method to produce the columnar microstructure in low and medium silicon non-oriented electrical steel without preliminary vacuum annealing is described. The influence of applied annealing temperature regimes and atmospheres on the development of the material microstructure are studied under the conditions of industrial continuous annealing. The dependence of the final material texture on the aforementioned conditions is presented.