The thermal flattening of grain oriented silicon steel which is in the
semi-processed condition has improved magnetic properties after a stress
relief anneal by using a low temperature and high tension flattening
anneal. The flattening process is conducted at a temperature between
1000.degree. to 1435.degree. F. (540.degree. to 780.degree. C.) with a
tension selected to produce a yield strength/tension ratio from above 5 to
about 20 and preferably from 7 to 13. The yield strength of the material
will vary depending on the length of the time at peak temperature but are
typically from 400 to 4000 psi (29,200 to 292,000 gm/cm.sup.2). The
material as thermally flattened will have at least about 10% stress. After
a stress relief anneal above about 1450.degree. F. (785.degree. C.), the
material has significantly improved core loss compared to conventional
thermally flattened material. The material is particularly suited for
wound transformer core applications.
