This invention relates to a method and apparatus for working the surface of grain-oriented silicon steel to affect the domain size and reduce core losses. More particularly, this invention relates to providing localized compressive strains on the surface of grain-oriented silicon steel through a roll pass.
Grain-oriented silicon steel is conventionally used in electrical applications, such as power transformers, generators, and the like. Grain-oriented silicon steels of this type typically have silicon contents on the order of 2.8 to 4.5%. The silicon content of the steel in electrical applications, such as transformer cores, permits cyclic variation of the applied magnetic field with limited energy loss, which is termed core loss. It is desirable, therefore, in steels of this type to reduce core loss.
In the production of silicon steels of this type the steel is hot rolled and then cold rolled to final gauge by one or more cold-rolling operations with intermediate anneals. Thereafter the steel is typically decarburized, coated, as with a magnesium oxide coating, and then subjected to a final high temperature texture annealing operation wherein the desired secondary recrystallization is achieved.
It is known that core loss values of grain-oriented silicon steels may be reduced if the steel is subjected to any of various practices to induce localized strains in the surface of the steel. Such practices may be generally referred to as "scribing" and may be performed either prior to or after the final high temperature annealing operation. If the steel is scribed after the decarburization anneal but prior to the final high temperature texture anneal, then the scribing generally controls the growth of the secondary recrystallization grains to preclude formation of large grains and so results in reduced domain sizes. U.S. Pat. No. 3,990,923, issued Nov. 9, 1976, discloses methods wherein prior to the final high temperature annealing, a part of the surface is worked, such as by mechanical plastic working, local thermal treatment or chemical treatment.
If the steel is scribed after final texture annealing, then there is induced a superficial disturbance of the stress state of the texture annealed sheet so that the domain wall spacing is reduced. These disturbances typically are narrow, straight lines or scribes generally spaced at intervals equal to or less than the grain size of the steel. The scribe lines are typically transverse to the rolling direction and typically applied to only one side of the steel. U.S. Pat. No. 3,647,575, issued Mar. 7, 1972, discloses a method wherein watt losses are to be improved in cube-texture silicon-iron sheets after annealing and complete recrystallization. The method includes partially plastically deforming the sheet surface by providing narrowly spaced shallow grooves, such as by a cutter or abrasive powder jet. The sheet is preferably scribed on opposite sides in different orientations. U.S. Pat. No. 4,203,784, issued May 20, 1980, relates to producing a plurality of linear strains to grain-oriented steel having a glassy film after final texture annealing by forcibly moving a rotatable body having a convex roller shape in a transverse direction.
There have also been attempts to use grooved surface rollers during the cold rolling prior to final texture annealing to develop a desired grain orientation. U.S. Pat. No. 3,947,296, issued Mar. 30, 1976, discloses a process to produce cube-on-face grain orientation by cold rolling the hot-rolled band for at least 20% reduction using a roller with a grooved surface, then cold rolling with smooth rollers and thereafter decarburizing and final texture annealing. U.S. Pat. No. 4,318,758, issued Mar. 9, 1982, relates to producing a (hko)[001] texture by cold rolling the hot-roll band, coating and final texture annealing. Such practices are distinguishable from scribing techniques.
What is needed is a method and apparatus for scribing grain-oriented silicon steel wherein the scribe lines required to improve the core loss values of the steel may be applied in a uniform and efficient manner to result in uniform and reproducibly lower core loss values. A low cost scribing practice should be compatible with the conventional steps and equipment for producing grain-oriented silicon steels.