Non-Oriented Silicon Steel Classification: Grades, Process and Applications

Classification of Silicon Steel by Silicon Content

Silicon electrical steel is a soft magnetic alloy of iron and silicon, and its silicon content is the first classification criterion. Low-silicon grades contain roughly 0.5 to 2.5 percent silicon and offer better mechanical strength; they are mainly used for motor and generator cores, which is why they are commonly called motor silicon steel. High-silicon grades contain approximately 2.5 to 4.5 percent silicon and give lower core loss and higher permeability, but the material becomes more brittle and harder to punch. High-silicon grades are preferred for transformer cores and are commonly called transformer silicon steel. The boundary between the two groups is not a strict line: large motors are frequently built from high-silicon grades when the loss saving justifies the more difficult punching, and some small transformers use low-silicon grades for cost reasons.

Classification by Production Process: Hot-Rolled and Cold-Rolled

The second classification criterion is the rolling process. Hot-rolled silicon steel was the traditional product, but it suffers from uneven thickness, poor surface quality and inconsistent magnetic properties. Cold-rolled silicon steel is produced with precise thickness control, better surface finish and higher magnetic performance, and has almost completely replaced the hot-rolled product in new equipment. China phased out hot-rolled silicon steel production in the late 1990s under the national cold-replacing-hot policy, and today the vast majority of electrical steel consumed worldwide is cold-rolled.

Cold-Rolled Grades: Non-Oriented and Grain-Oriented

Cold-rolled silicon steel is divided into two families according to grain texture. Cold-rolled non-oriented steel, abbreviated CRNGO, has a random grain orientation so that its magnetic properties are similar in all directions in the sheet plane; it is used where the flux direction changes, such as in rotating machines. Cold-rolled grain-oriented steel, abbreviated CRGO, has grains aligned in the rolling direction by a controlled secondary recrystallisation process, giving very low loss in that direction; it is used where the flux path is fixed, such as transformer cores. Within CRGO, the high-permeability Hi-B grades with laser domain refinement achieve the lowest losses, for example 0.65 to 0.85 W/kg at 1.7 T and 50 Hz in the 0.20 to 0.27 mm thickness range.

Common CRNGO Grades and Their Typical Use

The table lists representative CRNGO grades and their typical applications. In the grade designation, the first number is the nominal thickness in hundredths of a millimetre, and the second number is one hundred times the maximum guaranteed core loss at 1.5 T and 50 Hz, for example 50W470 means 0.50 mm thickness and a maximum loss of 4.70 W/kg.

How to Choose the Right Grade

Selecting a silicon steel grade starts with the operating frequency and the required loss budget. For 50 or 60 Hz rotating machines, CRNGO between 0.35 and 0.65 mm is used, and thinner material is chosen only when losses or noise must be reduced. For transformer cores operating at grid frequency, CRGO is the standard choice because the flux is unidirectional. In both families, a higher performance grade lowers energy loss but increases material cost, so the decision is normally based on the capitalised value of the losses over the equipment lifetime. National and international standards to reference are GB/T 2521.1 for non-oriented electrical steel, GB/T 2521.2 for grain-oriented electrical steel, and IEC 60404-8-1 and IEC 60404-8-7 for the classification of magnetic materials.

Common Misunderstandings

Two points are often confused. First, non-oriented steel is not a low-quality version of grain-oriented steel; it is a different product family optimised for rotating fields, and using CRGO in a motor would actually increase loss because the rotating flux cannot follow the rolling direction. Second, the thickness of the lamination is a design choice, not a direct indicator of grade quality: a 0.20 mm CRNGO grade is not necessarily better than a 0.50 mm CRNGO grade for a motor, because stacking factor, cost and punching behaviour all change with thickness.

FAQ

What is the difference between non-oriented and grain-oriented silicon steel?

Non-oriented steel has random grain orientation and near-isotropic magnetic properties, suited to rotating machines; grain-oriented steel has aligned grains in the rolling direction, giving very low loss in that direction, and is used for transformer cores.

What silicon content is used in transformer silicon steel?

Grain-oriented transformer steel typically contains about 3.0 to 3.5 percent silicon, which balances low loss, high permeability and workability after the final annealing process.

Why was hot-rolled silicon steel replaced by cold-rolled steel?

Cold-rolled material offers uniform thickness, better surface quality and superior magnetic properties, and it can be produced as precisely oriented grain structures. China required the phase-out of hot-rolled silicon steel in the late 1990s.

How is the grade number such as 50W470 interpreted?

50 indicates a nominal thickness of 0.50 mm, W identifies it as a non-oriented grade, and 470 means the maximum guaranteed core loss is 4.70 W/kg at 1.5 T and 50 Hz, per GB/T 2521.1.

Can low-silicon steel be used in transformer cores?

Small and low-cost transformers occasionally use low-silicon or non-oriented grades, but grain-oriented high-silicon steel is the standard for distribution and power transformers because of its far lower no-load loss at 50 or 60 Hz.