Process for production of grain oriented electrical steel sheet having excellent magnetic properties

A process for producing a grain oriented electrical steel sheets having excellent magnetic properties, comprising the steps of:

• heating a slab comprising, in terms of by weight, 0.025 to 0.075% of C, 3.4 to 5.0% of Si, 0.015 to 0.080% of sol. Al, 0.0030 to 0.013% of N, 0.014% or less of (S+0.405 Se) and 0.05 to 0.8% of Mn, sol. Al (%)/Si (%) being 0.0080 or more, with the balance consisting of Fe and unavoidable impurities at a temperature below 1280.degree. C.;
• hot-rolling the heated slab;
• subjecting the hot-rolled steel sheet to cold rolling including final rolling with a reduction ratio of 80% or more once or at least twice with intermediate annealing between cold rolling;
• subjecting the final cold-rolled steel sheet to decarbonization annealing with regulating the average diameter of primary recrystallized grains of the steel sheet subjected to decarbonization annealing to 18 to 35 .mu.m in a period between the completion of the decarbonization annealing and the initiation of final annealing;
• coating the steel sheet subjected to decarbonization annealing with an annealing separator and subjecting the coated steel sheet to final annealing wherein the final annealing is effected in such a manner that the partial pressure of nitrogen, P.sub.N2 (%), in an annealing atmosphere in a final annealing furnace is 12.5% or more in a steel sheet temperature range of from 900.degree. C. to 1150.degree. C. in the heating stage of the final annealing;
• and
• subjecting the steel sheet to nitriding to cause the steel sheet to absorb 0.0010% by weight or more of nitrogen in a period between the completion of the hot rolling and the initiation of secondary recrystallization in the final annealing.