CRGO vs. CRNGO: Understanding Grain-Oriented and Non-Oriented Electrical Steel Coils

The field of electrical engineering and power transmission would not be where it is without the use of specialised materials, even if this fact is not often acknowledged. Two critical materials that form the backbone of efficient transformers and electric motors are cold rolled grain oriented (CRGO) and cold rolled non-oriented (CRNGO) steel coils. Although they are both silicon-iron alloys that undergo cold rolling, their magnetic properties and applications differ significantly. Let’s delve into the specifics of each.

What are CRGO and CRNGO steel coils?

Cold Rolled Grain Oriented Steel Coil (CRGO): This specialized electrical steel is manufactured with a highly controlled process. This process aligns the crystalline structure (grains) predominantly in the direction of rolling. The orientation of the grains is deliberate, resulting in significantly superior magnetic properties (specifically, much lower core loss and higher permeability) when magnetized along the rolling direction (the “easy” axis). However, magnetic properties are markedly poorer in the perpendicular direction, and this is the main difference between the two. Where the magnetic flux path is well-defined and unidirectional, CRGO is primarily used in transformer cores.

Cold Rolled Non-Oriented Steel Coil (CRNGO): The random crystalline grain structure within the plane of the material is a feature of this electrical steel, as the name suggests. Consequently, its magnetic properties (core loss, permeability) are relatively consistent. This is true in all directions parallel to the sheet surface. This isotropic magnetic behaviour makes CRNGO the material of choice for applications where the direction of the magnetic flux changes constantly, such as in rotating machinery.

Unique Advantages & Key Features

Product specifications and variations

Both CRGO and CRNGO are available in coil form. They can subsequently be slit or cut into sheets/laminations. These sheets/laminations have precise dimensions. The following are the key specifications:

Thickness: For both types, the range is usually from around 0.23mm to 0.35mm, with specific grades adapted to suit frequency and power needs (thinner is usually used for higher frequencies).

Silicon content: CRGO typically has a higher silicon content (around 3%) than standard CRNGO grades, which contributes to its higher electrical resistivity and lower core losses. The amount of CRNGO silicon used varies a lot depending on the required magnetic performance grade.

Grades: The available grades of both are defined by specific maximum core loss (W/kg at a defined flux density and frequency, e.g. W1.5/50) and minimum permeability levels. The absolute lowest core losses commercially available are offered by higher-grade CRGO.

Coatings and Insulation.

Both types of coating are vital as they serve a number of purposes. These include electrical insulation between laminations (reducing eddy currents), corrosion protection and, sometimes, providing surface lubrication during stamping.

CRGO coatings (examples):

· Glass-type/Lite-Carlite: This is frequently employed for wound cores, for instance, toroidal cores in distribution transformers.

· Carlite 3 is perfect for use in stacked cores in power and distribution transformers.

· D-Coating (Allegheny Ludlum): This is comparable to Glass/Lite-Carlite, which is used for wound cores.

· T-Coating (Allegheny Ludlum): Stacked cores are the intended application.

CRNGO Coatings: It is applied after the final annealing process. These are adapted to the manufacturing process (e.g. weldability, punchability) and final application requirements (e.g. thermal stability, chemical resistance). Common types include inorganic coatings (C-3, C-4 and C-5) and organic coatings.

Manufacturing processes: The Path to Alignment

CRGO Manufacturing: Achieving the desired grain orientation requires a complex thermomechanical process, which is a process that involves the application of heat and mechanical force to a material.

1. Hot rolling to intermediate thickness is the process.

2. Cold rolling to final thickness, which involves a significant reduction.

3. Decarburisation annealing: Preventing magnetic aging is made possible by the removal of carbon in a wet hydrogen/nitrogen atmosphere, which is crucial.

4. Application of an annealing separator. This is often MgO-based.

5. Final Purification Annealing (High-Temperature Box Annealing): Performed at temperatures near 1200°C. Secondary recrystallization only occurs at this critical step, where grains oriented with their easy axis parallel to the rolling direction grow exceptionally large. This creates a highly oriented structure. The separator also forms the glass film insulation coating during this stage.

6. Application of final insulating coating (if required beyond the glass film).

7. Temper rolling/stretcher leveling for flatness.

CRNGO Manufacturing: While also involving cold rolling and annealing, the process is geared towards achieving a random grain structure:

1. Hot rolling.

2. Cold rolling to final gauge (usually less reduction than CRGO).

3. Continuous Annealing: Performed at lower temperatures (typically ~800-1100°C) in a wet hydrogen/nitrogen atmosphere. This anneal recrystallizes the steel but does not promote secondary recrystallivation and strong orientation. The primary goals are recrystallization, decarburization (removing carbon to prevent magnetic aging), and sometimes nitriding for certain high-permeability grades. This annealing is often done on continuous lines.

4. Application of the tailored insulating coating.

5. Temper rolling/stretcher leveling for flatness and surface quality.

Application scenes:

The ideal applications for these magnets are dictated by their distinct magnetic properties:

CRGO core applications:

Transformer cores (dominant use): This is especially true for large power, distribution and instrument transformers. The transformer’s efficiency is enhanced by its ultra-low core loss in the rolling direction, which reduces energy wastage as heat. The defined flux path in transformer cores perfectly exploits its directional superiority, which is a key feature of transformer design.

Wound cores: Toroidal cores, C-cores and other wound configurations are used primarily in distribution transformers. These benefit from specific CRGO coatings.

Static devices: In situations where magnetic flux is largely unidirectional.

CRNGO core applications:

Electric motor cores (dominant use): Used in laminations for stators and rotors across all sizes, from tiny appliance motors to massive industrial motors and the rapidly growing electric vehicle (EV) motor market. The constantly rotating magnetic field requires isotropic properties.

Generator cores: The requirements are comparable to those of motors, with the need for consistent performance in all directions.

Small transformers: This is especially true when flux paths are less directional or cost is a major factor (CRGO is generally more expensive).

Rotating machines: Devices where the direction of the magnetic field changes quickly.

Electromagnetic devices: The following items are included: solenoids, actuators, relays, ignition coils and lighting ballasts.

Conclusion: Choosing the Right Steel for the Magnetic Job

Within the electromagnetic world, CRGO and CRNGO electrical steel coils fulfil different functions, despite both being essential, high-performance materials.

In situations where your application requires the absolute lowest core losses and highest permeability in one specific direction, opt for CRGO (Grain Oriented). This is primarily recommended for static transformer cores where the magnetic flux path is well-defined and unidirectional. The higher cost is justified by the significant energy savings in transformer operation.

Select CRNGO (Non-Oriented) if your application demands reliable, uniform magnetic performance in all directions within the plane of the material. This is vital for the rotating magnetic fields found in electric motors, generators, and various other electromagnetic devices. These dynamic applications are ideal for this material because it is isotropic, easy to manufacture, and generally inexpensive.

It is crucial for engineers, designers, and procurement specialists to understand the differences between materials, including grain structure, magnetic anisotropy vs. isotropy, manufacturing nuances, and optimal application areas. This will help them to select the right material to maximise efficiency, performance, and cost-effectiveness in electrical and electromechanical systems. Continued advances in both CRGO and CRNGO technologies are driving improvements in energy efficiency across the global power grid and countless motor-driven applications.