Process for the production of grain oriented magnetic steel sheets

Abstract

A process for the production of grain oriented magnetic steel sheets comprises through-heating slabs containing more than 0.005% C, 2.5 to 6.5% Si, 0.03 to 0.15% Mn, 0.010 to 0.050% S, 0.010 to 0.035% Al, 0.0045 to 0.0120% N, and 0.020 to 0.300% Cu, the balance being iron and residual impurities, to a temperature which is lower than the solubility temperature T.sub.1 of magnesium sulfide and higher than the solubility temperature T.sub.2 of copper sulfide, T.sub.1 and T.sub.2 being dependent on the silicon content. The through-heated slabs are then hot roughed, followed by hot finish rolling at an initial temperature of at least 960.degree. C. and a final rolling temperature of 880.degree. C. to 1,000.degree. C., to produce a hot rolled strip having a thickness in the range of 1.5 to 7 mm. During this last step, at least 60% of the total nitrogen content precipitates in the form of coarse AlN particles. Thereafter, the hot rolled strips are annealed at temperature of 880.degree. C. to 1,150.degree. C., followed by cooling at a cooling rate higher than 15 .degree. K./sec to induce further precipitation of AlN and copper sulfide particles. Thereafter the strip is cold rolled in one or more cold rolling stages to a final strip thickness of 0.1 mm to 0.5 mm, subjected to recrystallization and decarburization annealing in a wet atmosphere containing H.sub.2 and N.sub.2. A separating agent containing mainly MgO is then applied to both surfaces of the strip. The strip is then high temperature annealed, an insulating coating is applied and the strip is subjected to a final annealing.