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Electrical Steels

• satyendra
• April 25, 2013
• 2 Comments
• CGO, CRGO, CRNGO, eltrical sheets, GOES, HGO, hyatersis, lamination, magnetic proerties, NOES,

Electrical Steels

Electrical steel is an important functional material which is used in the cores of generators, motors, transformers, and other power supply and conversion systems. In particular, low iron loss has been required in recent years as a means of contributing to a reduction in CO2 emissions into the atmosphere by achieving higher efficiency, and hence, energy savings in these devices.

Electrical steel is a kind of special steel which is tailored to show certain specific magnetic properties such as small hysteresis area (small energy dissipation per cycle or low core loss) and high permeability. It is also called lamination steel, silicon (Si) steel, Si-electrical steel, or transformer steel. The steel contains specific percentage of Si in it which is responsible for its unique property.  In mild steel there is much loss in electrical energy due to hysteresis and eddy current and hence use of mild steel is uneconomical when it is used in the electrical devices. The hysteresis loss is shown in Fig 1.  The hysteresis loss is proportional to the area of the respective loops shown in the figure.

Fig 1 Magnetic hysteresis loop of electric steel and mild steel

Electrical steels have special physical properties which make them suitable for application in the production of electric equipments and appliances with rotating magnetic fields.  The utilization of the fully processed steels is also widespread for construction of electrical static devices. Electrical steels are used in the stacked cores of transformers and motors, which are rarely seen by the ordinary people.

First silicon-iron alloys designed for the application in the electrical transformers were produced in the early 1900s. Si content of the first commercial products has been in the range of 1 % to 4 %, and since this beginning, a maximum amount of around 3 % Si was recognized as the best compromise between magnetic and mechanical performances of the electrical steels.

Ferro-magnetic materials are classified as hard magnetic materials, such as permanent magnets, which externally supply magnetic flux semi-permanently once being magnetized, and soft magnetic materials, such as those used in electromagnet cores, which cease to supply flux when the electric current passing through the coil is stopped. Iron-based soft magnetic materials are used in the cores of electrical equipments such as transformers, generators, and motors. Reducing losses generated from materials of these applications contributes directly to the improving of the energy conversion efficiency. Further, since electrical steels are essentially a metallic material with electrical conductivity, the application frequency range is at the most 100 kHz from the direct current range, even when their thickness is reduced to 0.1 mm.

Electrical steel is a poly-crystalline material, and these crystals or grains show a strong magnetic anisotropy. They develop an entirely different magnetic behaviour depending on the direction of the magnetic field. In the 1930s Norman Goss invented a process of cold-rolling and heat-treatment for electrical steels which improved the magnetic properties significantly along the rolling direction of the sheet. This process orders the grains in the direction of the best magnetic characteristics and achieves a high degree of preferred crystal orientations. The ordered direction of crystals or grains is termed ‘Goss texture’ characterizing cold-rolled-grain-oriented (CRGO) electrical steel. Power transformer cores are made of this product which is called electrical steel.

In electrical steels, the iron loss under alternating magnetic fields is reduced to the absolute minimum by applying advanced metallurgical treatments. Types of electrical steel are either non oriented electrical steel (NOES) or grain oriented electrical steel (GOES). The NOES can be fully processed or semi processed.

Non-oriented, fully processed electrical steel has varying Si levels which range from 0.5 % to 3.25 % Si. It has uniform magnetic properties in all directions. This type of electrical steel does not need recrystallization processes to develop its properties. The low Si alloy grades provide better magnetic permeability and thermal conductivity. For high alloy grades, better performance is expected in high frequencies, with very low losses. This type of steel is good for the magnetic circuits in motors, transformers, and electrical system housing. This fully processed type provides difficulty in punchability due to a completed annealing process. Organic coatings are added to improve lubrication in the punching process.

Non-oriented semi-processed electrical steels are largely non-Si alloyed steel and are annealed at low temperatures after the final cold rolling. The end-user, however, has to provide the final stress-relief annealing according to the intended application of the steel. The punchability of this type of the electrical steel is better than the non-oriented fully processed type, so organic coatings are not needed. Non-oriented semi-processed grades are good core materials for small rotors, stators, and small power transformers.

The NOES is the mostly used material among all soft magnetic materials. The NOES is functional material for the generation of energy as well as for the use of electrical energy in electrical machines and components. There is no alternative material for the NOES which constitutes around 75 % to 80 % of the demand of electrical steels.

The relevant magnetic properties (magnetization behaviour and magnetic losses) of the NOES are determined by the intensities of the texture components and the inhomogeneity of the micro-structure of the finally processed material (grain size distribution, precipitations, and internal stresses). The number of the processing steps and the process parameters differ remarkably for the NOES compared to GOES. The processing steps after casting comprises of hot rolling, cold rolling and final annealing. Product and process development in the field of NOES is characterized, like for the GOES, by optimization of the magnetic properties and other physical properties for special application areas as well as by the developments.

GOES is composed of iron with 3 % Si content with grains oriented to deliver high permeability and low energy loss. Grain-oriented grades have strong crystallographic properties. This type undergoes a recrystallization process resulting in an enhanced grain structure which shows better magnetic properties in the rolling direction of the sheet. Grain-oriented steels are mostly used for non-rotating applications, such as transformers.

The thickness of GOES is mainly in the range of 0.23 mm to 0.35 mm, while that of NOES is 0.2 mm to 0.65 mm. With both steel types, a thin insulation coating is painted and baked on the sheet surface to reduce the eddy currents generated under alternating magnetic fields.

A distinctive characteristic of GOES is the fact that a component, called inhibitor, which inhibits grain growth in the steel sheet, is introduced in the steel. Specifically, with this technology, products with excellent magnetic properties  are obtained by strongly suppressing grain growth by fine dispersion of the inhibitor in the steel, performing high temperature, long sustained annealing (termed final annealing) in a condition which maintains a fine grain structure, and selectively promoting rapid growth of grains in this grain structure, and selectively promoting rapid growth of grains in this fine structure which possess a specific orientation favourable to magnetic properties, in a process termed secondary recrystallization.

Accordingly the important points for manufacturing process are (i) a technique which causes a fine dispersive precipitation of the inhibitor in steel, (ii) a technique which enables fine control of the grain structure, and (iii) a technique for selectively promoting the growth of grains with the specified orientation.

Production of electrical steels

The development of the production routes in the last century has been amazing following the target of continuous cost reduction and yield improving, reduction in energy consumption and production time, shortening and compaction of the processing cycles. The old conventional cycle to produce GOES products is shown in Fig 2. The cycle, based on the ‘Inherent Inhibition’ strategy, is very long and complex and the process parameters along all cycle are to be very strictly controlled to achieve the desired quality of the product.

Fig 2 Schematic view of grain oriented electrical steel sheets

A schematic link between the various processing steps of the conventional route and the metallurgical parameters which regulate the microstructure evolution along the cycle in view of the achievement of the final products is shown in the Fig 3.

Fig 3 Conventional production process for GOES

The progresses in improving the magnetic properties of GOES which came with the introduction of HiB (high permeability) technology (higher cold reduction rate supported by stronger inherent grain growth inhibition by fine AlN precipitation) substantially are based on the same processing strategy. Different producers adopted also other variants, but all of them are based on the same concept of regulating the microstructure evolution along the process, slowing the grain boundaries movement by second phases, and segregating elements.

The new opening toward innovative production cycles was achieved by the so called ‘acquired Inhibition technologies’ which have allowed for the production of high grades of GOES adopting relatively ‘low slab reheating temperature’.  Fig 4 shows the conceptual simplification of the new production strategy and the most important and immediate advantages achievable by the producers.

Fig 4 Low temperature slab reheating process for GOES

In the Fig 2 also shows the innovative cycle based on the ‘acquired Inhibition; strategy in the case of the adoption of thin slab casting technology which allows for a significant compaction of the cycle and cost reduction as well as additional metallurgical opportunities for the products compared to the conventional technology.

Looking at the future development trend of the GOES production technologies the driving force towards cycle compaction and rationalization in view of cost and time.

GOES is the material used as magnetically active media in the core of electric transformers, due to its high magnetic permeability and its low ‘core losses’ (power lost as heat due to dissipative phenomena active in the material during the magnetization process). The main parameters which characterize the performance of the material in the application are polarization at fixed value of maximum magnetizing field and the lost power due to dissipative phenomena occurring during magnetization process, for a fixed maximum induction. Characterizations are performed at a fixed frequency depending on the geographical area of the final application.

The material grades nowadays available are classified in two large classes (standard grain oriented (CGO) and high permeability grain oriented (HGO), which are historically produced with two different technologies. The magnetic characteristics are strongly anisotropic if measured at different angles respect the rolling direction. Such a magnetic behaviour is related to a highly an-isotropous distribution of grains orientations in the material, which from the metallurgical point of view, is constituted by large grains (much larger than thickness value) ranging from few mm to few cm and crystallograpically oriented with the easy magnetization direction of the lattice  aligned within few angular degrees less than 10 deg in ‘standard grain oriented’ (CGO) and less than 5 degree in ‘super oriented’(HGO)) with the rolling direction (Goss orientation {110} <001>) as shown in Fig 5.

Fig 5 Cubic crystal with {110}<001> orientation and typical HGO grain structure

The metallurgical and technological innovations in the field of GOES in the recent years have been of tremendous impact on the industrial production of these materials. Due to the rationalization of the production cycles (especially in hot area) GOES manufacturing has become more feasible, allowing new steel organizations to produce this product. By the new methods based on the acquired inhibition strategy especially associated with thin slab casting technologies it is now possible to produce all the grades (CGO and HGO) by adopting practically the same processing route and chemical composition with great advantages in terms of production costs. One important result of the innovative production strategies is that, differently from the past, the magnetic properties of the products can be tailored for the specific application in a continuous mode ranging from the old conventional CGO to the top grades of HGO products.

The new production routes give further improve chances for enlargement of the range of product grades which can be now offered to transformer manufacturers as given below.

• In the direction of lower dynamic core losses making feasible the production of higher Si content and lower strip thicknesses, mainly due to (i) improved capacity to handle higher ‘grain growth Inhibition’ level necessary to control the microstructure evolution after very high cold rolling rates, in view of the regulation of the oriented secondary recrystallization, and (ii) improved capacity to control the parameters influencing the brittleness of silicon-iron coils (hot band microstructure, surface and edges defects etc.). This allows the production of grades with 0.18 mm of thickness and lower.
• In the direction of grades having excellent magnetic properties (HGO) at higher thickness in respect to conventional products. This is mainly possible by the metallurgical chance to regulate texture and grain structure after primary recrystallization at final thickness with very low C content in the alloy. Such a condition avoids the need for decarburization, which represent the production bottle-neck in case of high thickness strips increasing the cost of production. The negative influence of high thickness on core losses is balanced by sharp texture achievable.

6.5 % silicon steel

Si is added to electrical steel sheets in order to increase their resistivity. In particular, because the eddy current loss in iron cores rises rapidly as the frequency increases, Si addition is extremely effective in improving the high frequency magnetic properties of electrical steel sheets. It is also known that the magnetostriction of steel sheets is changed by adding Si to Fe, and reaches zero at a Si content of 6.5 %.

Thus, high Si electrical steel sheets, and particularly 6.5 % Si steel sheets, display extremely good high fre­quency magnetic properties. However, the ductility of steel sheets decreases as the Si content increases and the material shows remarkable embrittlement when the Si content exceeds 3.5 %, making cold rolling dif­ficult. For this reason, production of high Si electrical steel sheet with added Si contents exceeding 3.5 % at the industrial level had been difficult.

Electrical steel is an iron alloy of iron which can have from zero percent to 6.5 % silicon but normally has Si content upto 3.2 % (higher concentrations normally provoke brittleness during cold rolling). Manganese and aluminum can be added upto 0.5 %. Si significantly increases the electrical resistivity of the steel, which decreases the induced eddy currents and narrows the hysteresis loop of the material, thus lowering the core loss. However due to the Si, the grain structure hardens and embrittles the steel, which adversely affects the workability of the steel, especially during rolling. When alloying, the concentration levels of carbon, sulphur, oxygen and nitrogen are to be kept low since these elements indicate the presence of carbides, sulphides, oxides and nitrides in the steel. These compounds, even in particles sizes as small as one micrometer in diameter, increase hysteresis losses and decrease magnetic permeability. The presence of carbon has a more detrimental effect than sulphur or oxygen. Carbon also causes magnetic aging when it slowly leaves the solid solution and precipitates as carbides, thus resulting in an increase in power loss over time. For this reason, the carbon level is kept to 0.005 % or lower. The carbon level can be reduced by annealing the steel in a decarburizing atmosphere, such as hydrogen.

To solve this technical problem, JFE Steel estab­lished the world’s first continuous production technology for 6.5 % Si steel sheets using chemical vapour deposition (CVD). Fig 6 shows the principle of the manufacturing process for 6.5 % Si steel sheets by CVD. First, a low Si steel thin sheet, which is produced easily by cold rolling, is selected as the base material. This is heated to a high temperature in a non-oxidizing atmosphere. When SiCl4 is supplied to the high temperature sheet surface, the Fe in the steel sheet and the Si in the SiCl4 gas undergo mutual substitution, and Si penetrates into the steel sheet. A Si-enriched layer is formed in the surface layer of the sheet by this chemical reaction. The Si is then diffused to the interior of the steel sheet by high temperature soaking in a non-oxidizing atmosphere, finally resulting in a steel sheet with a uniform 6.5 % Si content.

Fig 6 Production process for 6.5 % steel

Magnetic gradient high Si steel sheet

Gradient function materials, which are characterized by a continuous change in composition in the thickness direction, have been developed mainly in the fields of heat-resistant and thermo-electrical materials. The authors discovered that electrical steel sheets having a Si content distribution (gradient) in the sheet thickness direction display unique magnetic properties. A magnetic gradient high Si steel sheet with new magnetic properties, which cannot be realized in conventional electrical steel sheets, has been successfully developed by controlling the concentration distribution pattern in the sheet thickness direction. In particular, in the high frequency region, the new sheet possesses a low core loss property exceeding that of 6.5 % Si steel sheets.

As in the production of 6.5 % Si steel sheets, this new magnetic gradient high Si steel sheet is produced by a process of siliconizing by CVD, followed by diffusion treatment using the continuous siliconizing line. In the production of magnetic gradient high Si steel sheets, the product with the desired Si concentration distribution is obtained by controlling the amount of siliconizing during formation of the Si-enriched layer in the sheet surface layer and the siliconizing rate, and then controlling the temperature and treatment time during high temperature soaking in the non-oxidizing atmosphere. As shown in Fig 6, the magnetic gradient high Si steel sheets obtained by this process have a concentration distribution pattern in which the Si concentration increases continuously from the sheet centre to the surface layer, and have a 6.5 % Si composition in the sheet surface layer with extremely high magnetic permeability.