PROCESS FOR PRODUCING NONDIRECTIONAL ELECTRICAL STEEL SHEET EXCELLENT IN CLOSE ADHESION OF INSULATING FILM

摘要

A process for producing nondirectional electrical steel sheets used as iron cores of electromagnetic equipment, such as motors, generators and small transformers, wherein the condition of annealing cold rolled steel sheets are properly controlled to form a dense surface oxide layer, thus producing nondirectional electrical steel sheets improved in close adhesion of insulating film. The process comprises the steps of: reheating a slab having a composition comprising by weight C: not more than 0.05 %, Si: not more than 3.50 %, Mn: not more than 1.5 %, P: not more than 0.15 %, S: not more than 0.015 %, and Al: not more than 1.0 % or further comprising at least one member selected among Sn: 0.03 to 0.30 %, Sb: 0.03 to 0.30 %, N: 0.03 to 1.0 %, and Cu: 0.03 to 0.50 % with the balance consisting of Fe and unavoidable impurities; hot rolling the reheated slab; optionally annealing the hot rolled slab; subjecting the resultant sheet to pickling and cold rolling; conducting low-temperature annealing in the temperature range of from 750 to 850 DEG C for 30 sec to 5 min in a humid atmosphere having a dew point of 25 to 65 DEG C; conducting high-temperature annealing in the temperature range of from 800 to 1070 DEG C for 10 sec to 3 min in a dry atmosphere having a dew point of 0 DEG C or below; applying an insulating film to the sheet; and subjecting the laminate to heat treatment for curing.