Electrical steel

Electrical steel is iron with about three per cent silicon in it, rolled to a thin sheet, and it is one of the highest-tonnage functional materials there is — on the order of ten million tonnes a year, essentially all of it inside transformers and motors.

The silicon does two things and gives up a third. It roughly quadruples the electrical resistivity, which suppresses the eddy currents that would otherwise waste energy heating the core. It reduces magnetostriction, the slight change in dimension a material undergoes when magnetised, which is what a transformer's hum is. And it makes the steel brittle and hard to roll, which is why the silicon content stops at around three and a half per cent rather than going higher — the metallurgy would like more and the rolling mill cannot cope.

It comes in two families that are different products. Grain-oriented steel has its crystals aligned and is used in transformers, where the field always runs the same way. Non-oriented steel is deliberately isotropic and is used in motors, where the field rotates.

Iron is magnetically anisotropic: it magnetises far more easily along the cube edge of its crystal lattice than along any other direction. In an ordinary steel the crystals point every way and that advantage averages out.

Grain-oriented steel is a hundred-year effort to stop it averaging out. Norman Goss found in 1934 that a particular sequence of cold rolling and annealing produces a sheet in which almost every grain has its easy axis lying along the rolling direction — the Goss texture. A core built from that sheet, with the flux running along the rolling direction, magnetises with substantially less loss than one built from random material.

The process that achieves it is one of the more remarkable pieces of industrial metallurgy. A fine dispersion of manganese sulfide or aluminium nitride particles is deliberately introduced to pin the grain boundaries during annealing; a small number of correctly oriented grains escape the pinning and grow enormously, consuming their neighbours, until the sheet is made of a few very large grains all pointing the right way. It is abnormal grain growth used as a manufacturing tool, and it took decades to control.

A motor cannot use it. The field in a rotating machine sweeps around the stator, so a sheet that is excellent in one direction and poor at right angles to it is worse than one that is mediocre in every direction. Non-oriented steel is processed to be as isotropic as possible, which is a different specification arrived at by leaving out most of the cleverness.

Cast, hot rolled, then cold rolled to final thickness — typically 0.23 to 0.35 millimetres for transformer grades and 0.35 to 0.65 for motor grades — with the anneal between passes doing the metallurgical work.

The grain-oriented route adds a high-temperature box anneal, often more than a day at over 1,100 °C in hydrogen, during which the secondary recrystallisation happens and the sulfur or nitrogen introduced earlier is removed again.

The sheet is then coated with an insulating layer, usually a magnesium silicate glass film formed during the anneal plus a phosphate topcoat, so the laminations do not conduct to one another when stacked. That coating also puts the steel into tension as it cools, which further reduces loss — the coating is a functional part of the material rather than a finish on it.

Laser or mechanical domain refinement is the last refinement: scribing fine lines across the surface subdivides the magnetic domains and cuts loss by a further ten per cent or so, and is why the best transformer steel is scored with faint stripes.

Grain-oriented electrical steel is made by a small number of producers worldwide, needs a plant that costs on the order of a billion dollars, and takes years to bring to specification. It is not a commodity steel and cannot be substituted at short notice.

That became visible when grid investment and electric-vehicle motor demand rose at the same time: transformer lead times went from months to years across Europe and North America from 2022, and electrical steel supply was a named part of the constraint. A grid connection queue is partly a materials queue, which is not how it is usually described.

Amorphous metal cores — iron-boron-silicon ribbon quenched so fast it never crystallises — have roughly a third the core loss and have been available since the 1980s. They have taken a small share, because the ribbon is thin, brittle and awkward to handle, and because the loss saving is paid back over decades rather than years. It is a good illustration of a materially better answer losing to the incumbent on manufacturability.