PURPOSE: To form an intergranular oxide layer at the surface part of a steel sheet, to allow microcrack to occur in this surface part, and to improve workability by subjecting a sheet of steel, in which respective contents of C, Si, Mn, Al, N, and Fe are specified, to annealing under the prescribed conditions and then to cold working.
CONSTITUTION: A steel having a composition consisting of, by weight, ≤0.25% C, 0.05-1.5% Si, 0.05-2.5% Mn, 0.003-0.075% Al, 0.003-0.012% N, and the balance Fe is refined. Annealing is applied to the sheet of this steel under the prescribed atmosphere to allow intergranular oxidation to proceed, by which an intergranular oxide layer is formed at the surface part. Further, cold working is applied to this sheet to allow microcrack to occur in the above intergranular oxide layer.
KONDO TOSHIHIRO
TANAKA TERUO
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