PURPOSE: To improve shape and to minimize iron-loss value by heat-treating a steel strip in the temp. range of 400W700°C while applying a tension producing an elongation of ≤1% to the steel strip to carry out flattening and then by subjecting the steel strip to annealing at 600W1,200°C in the temp. range higher than that of the preceding heat treatment by ≥200°C without causing tensile elongation.

CONSTITUTION: In subjecting a grain-oriented silicon steel strip after high-temp. batch annealing to flattening annealing in a continuous annealing furnace, the steel strip is first heat-treated in the temp. range of 400W700°C with the application of a tension producing an elongation of ≤1% and flattened. Subsequently, the above strip is subjected to annealing at 600W1,200°C in the temp. range higher than that of the preceding heat treatment by at least 200°C for a period necessary to relieve stress sufficiently without practically causing the elongation of strip.