Process for the production of grain oriented electrical steel strip starting from thin slabs
AssigneeAcciali Speciali Terni S.p.A.
InventorsStefano Fortunati, Stefano Cicale', Giuseppe Abbruzzese
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Abstract
In the production of grain oriented electrical steel sheet, controlling the condition of thin slab continuous casting results in advantageous solidification structures and precipitates. The steel has an initial content of carbon less than 300 ppm and an initial content of acid-soluble aluminum higher than that normally used for said type of steel. During the final processing steps, the annealed sheet is nitrided through a limited amount of nitrogen. This, in turn, renders the process for controlling the grain dimensions much less critical and results in a constant quality product.
Claims16
A process for the production of silicon steel strip, in which a silicon steel is continuously cast, high temperature annealed, hot rolled, cold rolled in a single step or in a plurality of steps with intermediate annealings to form a cold rolled strip;
- continuously treating the cold rolled strip to obtain primary annealing and decarburization;
- coated with annealing separator and box annealed to attain final secondary recrystallization, said process being characterized by the following steps:
- (i) continuous casting a thin slab of the following composition:
- 2 to 5.5 wt % Si, 0.05 to 0.4 wt % Mn, <250 ppm (S+(5.04× Se)), 30 to 130 ppm N, 0.05 to 0.35 wt % Cu, 15 to 300 ppm C, and 200 to 400 ppm Al, the remainder being iron and minor impurities, and having a thickness between 40 and 70 mm, utilizing a) a casting speed of 3 to 5 m/min, b) a steel temperature during the casting of less than 30° C. higher than the liquidus temperature of the steel, c) a cooling speed so as to obtain a complete solidification within 30 to 100 s, d) a mold oscillation amplitude between 1 and 10 mm, and e) an oscillation frequency between 200 and 400 cycles per minute;
- (ii) equalizing the thus obtained slabs and hot rolling them, after which the strip cooling is delayed for at least 5 seconds after the strip leaves the last rolling stand;
- (iii) directly sending the strip to the cold rolling, avoiding the usual annealing step;
- (iv) cold rolling in a single step or a plurality of steps, if necessary, with intermediate annealing, with a reduction ratio in the last step of at least 80%;
- (v) continuously annealing the cold rolled strip for decarburization and primary recrystallization, for a total time of 100 to 350 s, at a temperature comprised between 850 and 1050° C. in a wet nitrogen/hydrogen atmosphere, with a pH2O/pH2 comprised between 0.3 and 0.7;
- (vi) coating the strip with annealing separator, coiling it and box annealing the coils in an atmosphere having the following compositions during the heating-up:
- a) hydrogen mixed with at least 30% vol nitrogen up to 900° C., b) hydrogen mixed with at least 40% vol nitrogen up to 1100-120020 C. and c) and thereafter maintaining the coils at this temperature in pure hydrogen.
Dependencies are inferred from the claim text.