Product Description

Non-oriented electrical steels are iron-silicon alloys in which magnetic properties are practically the same in any direction in the plane of the material. Standard grades are available, with the advantages of our proprietary DI-MAX® processing that enhance our product's magnetic properties. Material is available fully and semi-processed, depending on the grade.

DI-MAX grades have superior permeability at high inductions, low average core loss, and good gauge uniformity. In addition, cold finishing plus strip annealing produces a smooth surface resulting in excellent flatness and a high stacking factor. Applications include high-efficiency motors, large and small transformers, generators, lighting ballasts and ignition coils.

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Product Applications

Available Grades

Available GradesDownloads

DI-MAX® M-15 - M-47

DI-MAX® M-13





Product Details

Surface Insulations

Standard Surface Finish (C-0)

Unless otherwise specified by the customer, all AK Steel non-oriented electrical steel grades are supplied with a very thin surface film (equivalent to ASTM Type C-0 coating) resulting from the DI-MAX processing for best die life. This smooth, tightly adherent film provides sufficient surface insulation resistance for many applications in small apparatus. The insulation resistance of this surface is usually enhanced by a stress-relief anneal and is not adversely affected by oils. However, this type of insulation provides less protection against rust than other available surface insulations. For rust protection and additional insulation resistance for larger apparatus, it is suggested that a suitable coating be applied.

C-3 Insulation

This organic varnish coating (equivalent to ASTM Type C-3 coating) is applied over the natural oxide (C-0) surface. It provides very high levels of surface insulation resistance as well as protection against rusting. C-3 insulation also has a beneficial effect on die life. While it is suitable at normal operating temperatures of electrical apparatus, it will not withstand the heat of stress-relief annealing. It withstands oil and is suitable for oil-immersed and air-cooled cores.

C-4 Insulation

This inorganic surface treatment (equivalent to ASTM Type C-4 coating) is produced by special chemical and thermal processing of the steel surface. C-4 insulation is best for punched laminations where only a moderate degree of surface insulation is required, but good die life and minimum effect on lamination factor are very important. It provides thorough surface coverage with a very thin film that affords a moderate amount of surface insulation. It is not harmed by ordinary stress-relief annealing temperatures, retaining adequate surface insulation characteristics for many applications when annealed in recommended neutral atmospheres. C-4 insulation is unaffected by oils, making it useful for either oil-immersed or air-cooled apparatus. This special AK Steel insulation provides some protection against rusting.

C-5 Insulation

This inorganic-based surface insulation (equivalent to ASTM Type C-5 coating) is suggested for use where superior insulation is required after a stress-relief anneal. It provides a uniform, high-resistance insulation for the more severe requirements of large electrical apparatus. C-5 insulation has a minimum effect on lamination factor. It can be exposed to ordinary stress-relief annealing temperatures without impairment of its superior insulation resistance when specified protective annealing atmospheres are used. In addition, C-5 insulation is not affected by oils, and provides some rust resistance. A small amount of organic material is contained in C-5 insulation to enhance die life relative to the standard surface finish and C-4 insulation. C-5 insulation is especially useful in large transformers made with stacked, flat laminations and in large motors and generators.



All magnetic tests are made on Epstein specimens. Test pieces are sheared with sharp shears to avoid burr and excessive strain. Test piece dimensions are specified by ASTM Method A343. Core losses of fully-processed as-sheared (AS) specimens represent the magnetic quality of mill annealed material except for the effect of strains introduced in shearing the strips.

When core loss tests for grading purposes are made on semi-processed quality-annealed (QA) specimens, strips are annealed to develop the inherent magnetic properties of the material. This annealing is similar to annealing that must be performed by the customer on semi-processed grades.

Flux Density Determinations

Flux density is calculated from the net cross-sectional area of the specimen. Net area is determined from the actual weight of the specimen and the assumed density values (as specified by ASTM). Net area can be approximated by multiplying gross area of the compacted core by its lamination factor, expressed as a decimal. Flux density is calculated from voltage measurements made on an open secondary winding using an average-responsive (rectifier-type) AC voltmeter.

Test Circuit

Magnetic data are obtained on a magnetic circuit having negligible joint effects. The exciting winding is uniformly distributed along the circuit. In all tests for AC magnetic properties, the flux wave form is sinusoidal. If a design calls for a substantial departure from these test conditions, suitable allowances must be made in applying the test data.

Core Loss and Exciting Current Measurement

Core loss values are determined using a wattmeter. The method eliminates the influence of copper loss and voltage drop in the test windings from the core loss measurement. Exciting current values are measured using a true RMS-reading ammeter.


General Procedures

Both stress-relief and final process annealing can be accomplished by batch or continuous annealing. However, proper atmosphere control, annealing time and annealing temperature are necessary for the grade being processed.

Batch Annealing

This type of annealing is especially adapted to relatively low-volume annealing operations. However, it is more easily controlled and more flexible than continuous methods. Stacked flat laminations, wound cores or stamped pieces can be annealed in batches. A covered annealing box or on open trays with an adequate furnace atmosphere may be used. Annealing trays and plates should be flat. Plates and charge covers, if used, should be made of low-carbon material to prevent carbon contamination. For the same reason, punching lubricants should be removed before annealing. Load the charge compactly on the plate. It is preferred to orient the laminations or cores in a manner that radiant heat can strike a portion of the edges. This facilitates heat transfer and reduces heating time. Better atmosphere control and reduced gas utilization can be attained by using a box or annealing cover. When an inner cover is not used, the furnace should be flushed with a protective gas to diminish the effect of leaks. Oxidation impairs the magnetic properties (especially at high flux densities) and must be avoided. Annealing box covers should fit closely. The original atmosphere should be flushed completely before heating. To avoid distortion of the material, cooling should be performed slowly under protective atmosphere to a temperature of 600 to 800 °F (316 – 427 °C). For charges weighing only a few thousand pounds, the cooling rate should not be more than 50 °F (28 °C) per hour. Even slower cooling is advisable for heavier furnace charges.

Continuous Annealing

Continuous annealing methods are especially adapted to high volume production. They can be used effectively to either stress relieve or process anneal electrical steels. As in batch annealing, adequate precautions must be taken to assure proper atmosphere control. Continuous annealing furnaces must be adequately flushed with protective atmosphere to prevent oxidation. Usually, oil can be removed from the material by burn-off in a pre-anneal chamber. Care should be taken in loading and stacking to prevent the possibility of distortion. This is especially important if stamped pieces are stacked on edge. Because of the continuous nature of this process, the material is brought to temperature quickly. This precludes stacking cores or laminations in heavy charges. The mass of the parts must be small enough so that the annealing temperature can be reached quickly. Special attention must be paid to stacking procedures and furnace capacity. Semi-processed material requires longer soak times and good circulation of the decarburizing atmosphere to all parts of the charge.

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