Method of producing low iron loss, low-noise grain-oriented silicon steel sheet, and low-noise stacked transformer

A stacked transformer made of a grain oriented silicon steel sheet and having a noise difference (dBi-dB'i) less than 5 dB, said sheet capable of being produced by a method which comprises:

• preparing a finish-annealed grain oriented silicon steel sheet;
• irradiating the surface of said grain oriented silicon steel sheet with an electron beam directed along scan paths which cross the rolling direction of said steel sheet,
• said irradiating having been applied to said sheet at a scanning speed v(cm/s) and a spacing L(cm) in the rolling direction, with an electron beam having a beam diameter d(cm) generated by a current I.sub.b (mA) and an acceleration voltage V.sub.k (kV);
• wherein said beam has been applied with a surface energy density .alpha. (J/cm.sup.2) on the surface of said steel sheet as determined by the following formula (1), and is about 0.16 J/cm.sup.2 or more,
• and wherein said surface energy density .alpha. (J/cm.sup.2) and a surface energy density .beta. (J/cm.sup.2) on the scan paths meet the approximate condition of the following formula (3):
• .alpha.=(V.sub.k .multidot.I.sub.b)/(L.multidot.v) (1)
• .beta.=(V.sub.k .multidot.I.sub.b)/(d.multidot.v) (2)
• 0.6-0.06.beta..ltoreq..alpha..ltoreq.0.90-0.08.beta. (3).

A stacked transformer having a noise difference (dBi-dB'i) less than 5 dB, said transformer comprising finish-annealed grain oriented silicon steel sheet with reduced iron loss, said sheet being capable of being produced by a method comprising the steps of:

• preparing said finish-annealed grain oriented silicon steel sheet;
• and
• irradiating the surface of said sheet with an electron beam directed along scan paths which cross the rolling direction of said steel sheet, said electron beam being applied to said sheet at a scanning speed v (cm/s) and a spacing L (cm) in the rolling direction, said electron beam being generated by a current I.sub.b (mA) and an acceleration voltage V.sub.k (kV) and having a beam diameter d (cm), wherein said beam is applied with a surface density .alpha. (J/cm.sup.2) of at least about 0.16 J/cm.sup.2 on the surface of said sheet and a surface density .beta. (J/cm.sup.2) on said scan paths according to the following formulas:
• .alpha.=(V.sub.k .multidot.I.sub.b)/(L.multidot.v);
• .beta.=(V.sub.k .multidot.I.sub.b)/(d.multidot.v);
• and
• 0. 6-0.06.beta..ltoreq..alpha..ltoreq.0.90-0.08.beta..