0.2 mm Grade B20R065 CRGO Grain-Oriented Silicon Steel Mother Coil

Reading the Grade Designation

In the grain-oriented electrical steel designation used for these products, the letter B identifies grain-oriented material, the following two digits give the nominal thickness in hundredths of a millimetre, and the trailing three digits give the maximum specific total loss in hundredths of a watt per kilogram. On that basis grade B20R065 is a 0.20 mm grain-oriented steel with a guaranteed maximum core loss of 0.65 W/kg measured at 1.7 T and 50 Hz. Grades in this family are characterised by high magnetic permeability in the rolling direction, achieved through a cold-rolling and high-temperature secondary recrystallisation process.

Requirements for individual grain-oriented materials, including loss limits and magnetic polarisation values, are specified in IEC 60404-8-7, and flat-rolled fully processed grain-oriented silicon-iron specifications are also covered by ASTM A876. Chinese production follows GB/T 2521.1 for cold-rolled grain-oriented electrical steel.

Magnetic and Physical Properties

Thin 0.20 mm material is selected where core loss dominates the loss budget or where the transformer operates at elevated frequency. The penalty is a higher stacking factor loss and greater handling sensitivity: the sheet is more easily bent, and bending damage reduces permeability locally. Build factor for thin high-permeability grades is typically specified at 0.96 or above.

Mother Coil, Slitting and Cutting

Mother coils are supplied in widths that are subsequently slit to the lamination widths required for the core. Slitting quality matters: burr height and edge deformation both raise local loss and can cause interlaminar short circuits after stacking. Cut-to-length lines must keep the strip flat and avoid tension that introduces residual stress, because stress raises loss in grain-oriented steel in proportion to the stress level and its angle to the rolling direction.

During handling, coils should be stored indoors on a dry floor, with the axis horizontal and protected from moisture so that the coating does not rust. Edge rust and coating damage are the most common causes of rejected material at incoming inspection.

Core Building Practice

B20R065 is normally built into wound cores or step-lap mitered cores. Wound cores from thin strip give a continuous grain path with no joints, which minimises no-load loss and exciting current; step-lap mitered joints reduce the air-gap reluctance at the corners. Typical design flux density for distribution transformer cores built from this grade sits between 1.6 T and 1.7 T, leaving margin below the knee of the magnetisation curve so that no-load current and noise stay controlled.

Annealing after cutting relieves the cutting stress and restores loss levels. Where the core is not re-annealed, the designer must account for the additional loss introduced by shearing.

Incoming Inspection and Acceptance

Acceptance testing should combine a certificate review with physical checks: thickness and thickness uniformity, width and burr height, coating adhesion, and magnetic loss verified by the Epstein method or by a single sheet tester for production control. Repeatability between the two methods differs, so the method used for acceptance must be stated in the purchase specification. Coils should be identified by heat number so that core-loss results can be traced back to the melting and rolling route.

Frequently Asked Questions

Q: What does the 065 in B20R065 mean?
A: The trailing digits give the guaranteed maximum specific total loss, so 065 means 0.65 W/kg measured at 1.7 T and 50 Hz.

Q: Why choose 0.20 mm strip instead of 0.23 mm or 0.27 mm?
A: Thinner strip reduces eddy-current loss within the sheet, which lowers total core loss; the trade-off is a lower build factor and more delicate handling.

Q: Which standards apply to the magnetic testing?
A: Material requirements are specified in IEC 60404-8-7, with loss measurement by the Epstein frame method of IEC 60404-2 or the single sheet tester of IEC 60404-3.

Q: Does bending a lamination damage the grade?
A: Yes. Plastic deformation degrades grain orientation and raises local loss, so cut laminations must be handled flat and re-annealed if they have been stressed.

Q: How should mother coils be stored?
A: Indoors, dry, with the coil axis horizontal and the coating protected from moisture; edge rust and coating damage are the main reasons material is rejected on arrival.

Q: Is the stacking factor the same as sheet thickness?
A: No. Stacking factor accounts for the coating and for the small gaps between laminations, so the magnetic cross-section is always less than the geometric cross-section.