Using infrared carbon-sulfur analysis and EBSD technology, the carbon content and texture changes of Nb-containing Hi-B steel after different decarburizing annealing processes were studied.
Cold-rolled grain-oriented silicon steel is a silicon-iron soft magnetic material with a {110} <001> texture (Goss texture). Its production equipment is complex, and the production process requirements are strict. It is known as an "artwork" in steel materials. In recent years, researchers at home and abroad have been committed to exploring new low-temperature solid solution inhibitors. Studies have shown that the nanoscale Nb(C,N) precipitates formed in Nb-containing grain-oriented silicon steel not only have lower solid solution temperature and stability but also can effectively inhibit the recrystallization of austenite and the growth of primary recrystallization grains when used as inhibitors. This is consistent with the production trend of grain-oriented silicon steel towards low-temperature slab heating and thin specifications.
On the other hand, the carbon content in grain-oriented silicon steel is closely related to its magnetic properties. To ensure that grain-oriented silicon steel has suitable microstructure evolution during hot rolling, cold rolling, and annealing, and to obtain the best texture and magnetic properties, the carbon content in the steel before decarburizing annealing needs to be maintained at 0.04%~0.07%. At the same time, to ensure that the final product of grain-oriented silicon steel has a single ferrite phase to eliminate the magnetic aging phenomenon, the carbon content in the steel plate after decarburizing annealing generally does not exceed 50×10⁻⁶~60×10⁻⁶. After decarburizing annealing, the cold-rolled steel strip (thickness is usually within 0.3 mm) will undergo primary recrystallization. The {111}<112> and {111}<110> oriented grains in grain-oriented silicon steel can promote the abnormal growth of Goss nuclei and improve their magnetic properties, while the rotated copper type {112}<110> and {001} oriented grains are not conducive to the occurrence of secondary recrystallization. Therefore, the number and distribution of these oriented grains after primary recrystallization will have a great impact on the secondary recrystallization of grain-oriented silicon steel. However, at present, there is a lack of systematic and in-depth research on the decarburization process parameters and texture evolution after primary recrystallization in Nb-containing Hi-B steel.
Based on this, this paper takes Hi-B steel with a Nb content of 0.028% as the research object to investigate the influence of decarburizing annealing conditions on the carbon content, primary recrystallization texture, and grain size of the steel, and analyzes the orientation relationship between {111}<112> and {111}<110> grains and Goss grains.
Under the conditions of a protective atmosphere of 33%H2+67%N2 (volume fraction) and a dew point of 45℃, the decarburization effect of Nb-containing Hi-B steel is the best when annealed at 850℃ for 180s, and the carbon content in the steel is about 0.0037%.
After decarburizing annealing, the Nb-containing Hi-B steel samples have been completely recrystallized. The texture is composed of strong {411}<148>, {111}<112>, and {111}<110> texture components, and a small number of Goss grains are scattered between the {111}<112> or {411}<148> grains.
When the annealing time is 180s, as the decarburization temperature increases from 830℃ to 850℃, the number of Σ9 and Σ13b grain boundaries that are beneficial to the abnormal growth of Goss grains in the Nb-containing Hi-B steel sample first increases and then decreases, that is, a maximum value appears after annealing at 840℃; when the decarburization temperature is 850℃, the number of Σ9 and Σ13b grain boundaries increases with the extension of the holding time.
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