Production of silicon steel sheet stock having the property of high surface resistivity

Claim 1

A PROCESS OF TREATING FERROUS MAGNETIC SHEET GAUGE STOCK TO GIVE IT A HIGH SURFACE RESISTIVITY WHICH COMPRISES TREATING THE CLEAN SURFACES OF THE FERROUS STOCK WITH A WATER SOLUTION OF PHOSPHORIC ACID CONTAINING SUBSTANTIALLY BETWEEN 7% AND 50% BY WEIGHT OF THE ACID, METERING THE QUANTITY OF RESIDUAL SOLUTION REMAINING ON THE SURFACE OF THE STOCK TO A THIN, NON-FLOWING FILM OF SUBSTANTIALLY UNIFORM THICKNESS, AND SUBJECTING THE STOCK TO AN OPEN ANNEALING HEAT TREATMENT IN AN AT MOSPHERE NON-REDUCING TO PHOSPHATES AT A TEMPERATURE OF FROM SUBSTANTIALLY 400*F. TO AND INCLUDING ANNEALING TEMPERATURES ABOVE 1000*F. FOR DEVELOPING MAGNETIC PROPERTIES, TO THE EXTENT OF CAUSING THE PHOSPHORIC ACID TO REACT WITH THE IRON OF SAID FERROUS MAGNETIC SHEET STOCK AND FORM A RESIDENT INSULATIVE PHOSPHATE COATING THEREON HAVING A RESISTANCE OF ATLEAST 1 OHM PER SQUARE CENTIMETER AT A PRESSURE OF 50 POUNDS PER SQUARE INCH AND A SPACE FACTOR OF AT LEAST SUBSTANTIALLY 90%.