PURPOSE: To minimize the oxide film formed at the time of decarburizing annealing and to provide superior magnetic properties by removing the oxide film after process annealing, lowering the dew point in the rear part of a soaking zone in a decarburizing annealing furnace, and also regulating the atmosphere in the above-mentioned rear part to a weak-oxidizing atmosphere.
CONSTITUTION: A hot rolled plate of silicon steel having a composition containing, by weight, 0.02-0.1% C, 2.5-4.0% Si, 0.01-0.10% Mn, and 0.005-0.06%, in total, of one or more elements among S, Se, and Al is subjected to two-time or more cold rollings and process annealing, and is formed to the final sheet thickness. Cold rolling is done after the removal of the oxide scale formed at the steel sheet surface in the above-mentioned process annealing stage. Subsequently, at the time of decarburizing annealing, the atmosphere is regulated to strong-oxidizing atmosphere in the fore half alone and to weak-oxidizing atmosphere in the back half, by which the formation of the oxide film is minimized. A difference in average dew point between the front part and the rear part of the soaking zone in the furnace at the time of decarburizing annealing is regulated to ≥10°C, preferably to ≥30°C.