Dr Stan Zurek - Magnetics technical specialist
What are electrical steels?
As the name suggests, electrical steels are manufactured specifically for electrical applications. They are produced in long coils as relatively thin sheets; the thickness depends on application and is most usually between 0.23 and 1 mm. The thickness is dictated mainly by the eddy current losses generated during magnetisation of the sheet, but also partly by the specialised mechanical rolling that is needed to obtain optimum magnetic performance.
Thinner sheets are more expensive to manufacture, but they result in lower losses. On the other hand, thicker sheets allow more steel to be packed into a given volume – the so-called “stacking factor”. This means that, for any design using electrical steels, a balance always has to be struck between performance, efficiency and price.
Where are electrical steels used?
The size of any electrical machine (motor, generator, transformer or even relay) is mostly determined by the size of the magnetic core. Some of the largest can be found in power plant generators, where the magnetic core alone can weigh hundreds of tonnes. In such a design the price of magnetic material will clearly be a very important factor, and the material that best combines low cost with good magnetic properties is iron.
Electrical steels comprise mostly iron – over 94% alloy content. The next most important constituent is silicon, the addition of which increases the electrical resistivity of the alloy, thereby lowering eddy current losses. In contrast to structural steels, in electrical steels the presence of carbon is undesirable, as it increases losses. Carbon content is, therefore, kept as low as possible.
Unfortunately, the silicon in electrical steels makes them harder and more brittle, which creates problems with cutting and punching out the intricate laminations that are typically used in magnetic cores. (See Fig. 1). In order to help with mechanical processing, aluminium and manganese can also be added to the steel in small amounts (less than 0.5 %).
There are two main types of electrical steel: grain-oriented and non-oriented.
Grain-oriented electrical steel (GOES)
The process for producing GOES was developed by American researcher Norman P Goss in the 1930s. A sheet of steel is rolled in a particular way (with so-called critical thickness reduction), which creates seeds of grains that are aligned with the direction of rolling. During the subsequent thermal treatment and re-crystallisation, grains grow from the seeds until they fill the whole volume. (See Fig. 2).
Each grain is therefore aligned and the magnetic properties are maximised along the rolling direction, which is also referred to as the direction of “easy magnetisation”. (Magnetisation at 90º to this direction is “hard” and at 60º is “hardest” – a direct result of the grain orientation). This means that GOES is very useful in applications where the magnetic flux has to be guided in a specific direction, for instance in a limb of a transformer (Fig. 3).
GOES usually contain around 96.5% iron, 3% silicon and 0.5% other elements. The silicon reduces the losses and helps in aligning the grain growth. From a magnetic point of view the optimum amount of silicon is actually 6.5%, where the losses are even lower, the permeability reaches maximum and the steel does not exhibit magnetostriction, which is the main cause of acoustic noise generated by transformers. However, steel with 6.5% silicon content is so brittle that it cannot be mechanically cut or punched, so it needs to be cut with lasers or by using other specialised techniques. An alternative approach is to cut regular 3% silicon material and then infuse it with more silicon after cutting.
Over the years there have been many modifications and improvements to the manufacturing process for GOES, leading most importantly to the reduction of losses by a factor of ten or so. The production process currently used has over 20 steps, which means that GOES is much more expensive than construction steel. It is however still far cheaper than other magnetic alternatives like nickel and cobalt alloys.
One of the ways in which the magnetic performance of GOES has been enhanced is by the addition of a special coating layer that induces tensile stress on the surface of the sheet. This improves the permeability of the GOES and reduces losses. The result is that steel manufactured in this way can work at higher magnetisation (B) than the more conventional grades. This process is often referred to by its patented commercial name “Hi-B”.
There have also been laboratory trials of so-called “double oriented” steel, which has two orthogonal “easy” directions, achieved by appropriate grain growth. The production process for this is, however, even more expensive and the material has yet to find a volume commercial application.
The prevailing thicknesses for commercially used GOES are 0.35 mm (cheapest), 0.27 mm and 0.23 mm (most expensive). The price is typically a few US dollars per kilogram.
Non-oriented electrical steel (NOES)
In NOES the grains are much smaller, usually less than 0.1 mm. They are not oriented along any specific direction. Quite the opposite – they should be distributed as randomly as possible to provide loading symmetry. This is because the NOES are used in cores of rotating machines where magnetic flux rotates and changes its position all the time in the “teeth” of the laminations (see also Fig. 1). The manufacturing process is simpler than for GOES and the steel is therefore cheaper – usually less than half the price.
Because NOES is produced in thicker sheets (0.35 mm and more) punching is more difficult and there are more mechanical deformations and stresses introduced during cutting. This degrades the magnetic performance and cannot be always tolerated. In these cases, laminations must be annealed after cutting to improve their magnetic properties. As a result the NOES are available in two main types: fully-processed which do not require re-annealing, and semi-processed for which final thermal treatment is necessary.
Another important factor is also the silicon content. The addition of silicon slightly reduces the saturation magnetisation. This is important in rotating machines, in which the teeth (see also Fig. 1) may work under saturated conditions. The mechanical torque of rotating machine is proportional to the square of magnetisation so even a small improvement in magnetisation is justified for the resulting significant gains in torque. For this reason, when mechanical performance is of prime importance, the silicon content is often reduced to much lower levels (sometimes even below 0.5%). This improves the torque, but of course results in higher eddy current losses, which can only be reduced by decreasing the thickness of the laminations used.
NOES is often used for small cheap transformers for domestic appliances, where the price of the device is a crucial factor.
Who makes electrical steels?
There are only around 20 manufacturers in the world that make electrical steels. Among these are ThyssenKrupp (Germany/France), Cogent Power (UK/Sweden), Arcelor Mittal (Brazil), AK Steel (US), Bau (China), Tata (India) and Stalprodukt (Poland). Out of these only a handful are capable of making the best quality GOES, whereas most produce conventional GOES and NOES.
Importance of electrical steels
Despite the current economic crisis, the demand for GOES continues to be very high. This is perceived to be a result of rapid development in Asian countries and the growing need for replacements for ageing equipment throughout Europe and the US.
A single high-power generator requires hundreds of tons of electrical steel. The energy produced by the generator must be then transformed, with large transformers containing yet more electrical steel, to higher voltages. It is then distributed and transformed with more transformers back to medium voltage and subsequently with even more transformers to low voltage for the final user. It is estimated that 70% of all electrical energy is consumed in electrical motors, which again are built from electrical steels.
In practice ALL the electrical energy from any house socket was transformed several times by transformers that use electrical steel. So next time when you are munching on toast that has been made in an electric toaster and drinking coffee made with water from an electric kettle, spare a moment to think about the electrical steels that make it all possible.
Fig. 1. Rotor and stator laminations of induction motor
Fig. 2. Uncoated surface of grainoriented electrical steel with grains up to 20 mm long
Fig. 3. Each part of transformer core laminations can be cut so that it is aligned with the preferred rolling direction