2. Rolling of Electrical Steel

 

Starting material for getting steel is iron ore. It is then converted to hot metal in a blast furnace. Further treatment is done in steel melting shop to get required quality. Final product is obtained as slab, having a width of 150–210 mm with thickness of 150 mm.

This slab is carried to Hot Rolling Mill in the ambient temp and is further heated in a furnace, then rolled to thinner section, such as 2 -10 mm. It takes a form of coil. This coil is transported to cold rolling mill in cold condition where it is reduced in cold rolling mill, between 0.2 to 3 mm. There are various types of cold rolling mills. They may be 4-high, 6-high, 20-High mill. Mainly, rolling takes place between two working rolls. Rest are back up rolls to give support the working rolls. Back up rolls give support to maintain uniform thickness, rigidity to work roll, maintaining proper shape of final product.

The property of cold rolled sheet varies with hot rolled sheet. Cold rolling induces increase in strength, ductility, hardness elongation in grain size etc. Hence, after cold rolling, the coil is further put is a heating furnace to bring down hardness, and formation of uniform grain size. This is called annealing. Then again it is passed through skin pass mill, where surface of sheet becomes hard. It enables the user for accomplishing further processing in day to day work.

Electrical Steel is manufactured in cold-rolled strips less than 2 mm thick. Slabs of Electrical Steel are rolled at high temperatures into heavy gauge coils. Coils are then acid pickled to remove scale. The material is then cold rolled to final gauges in oil form and annealed. Flat-rolled Electrical Steels are produced to meet magnetic property specifications rather than specific chemical composition. Magnetic characteristics are of first importance and are dependent on processing as well as on chemical composition.

Silicon is the primary alloying element in Electrical Steel. It is added because it increases the volume resistivity of the steel and thereby reduces the eddy current component of core loss. Silicon is more effective in this respect than any other element which may be conveniently added. Silicon has an added benefit in that it affects the grain structure of the steel and thus gives somewhat improved core loss by its reduction of the hysteresis component in non-oriented Electrical Steels (CRNGO). Additionally, certain levels of silicon must be maintained to avoid a phase change and thus aid the crystal orientation process in “Oriented Electrical Steels (CRGO).

Depending upon the type of product, the other main alloying elements added to Electrical Steel are Aluminum and Manganese. Each of these usually is added in amounts less than 1.0% and more often between 0.1 and 0.5%. These elements are added mainly for their metallurgical effect rather than for any physical effect such as volume resistivity. They also favorably affect grain structure of the steel, thereby contributing to the lowering of the hysteresis component of the core loss. Other elements are present in Electrical Steels but are essentially impurities and are found only in residual amounts. Carbon is one element that changes in content from that present in the melt to that in the final product. Special heat treatments are given during mill processing to lower carbon content of the fully processed material to very low values. This removal of carbon occurs during annealing of the semi-processed grades by the customer.

CRGO & CRNGO are both Electrical Steel grades used for stampings & cores of electrical Transformers & other electrical appliances. CRNGO is Cold Rolled Non Grain Oriented and thickness available is 0.50 mm as compared to 0.27 mm of CRGO, hence the losses in CRNGO are more than four times of CRGO.

More on CRNGO and CRGO in the next Chapter……!!!