1. What is ladle refining furnace?
In steelmaking, once the steel is made in Blast furnace, arc furnace or induction furnace, it is transferred into the ladle for further refining, chemistry adjustment, inclusion modifications, etc. All these treatments are carried out in the ladle itself. Additional heat is required to carry out these operations. Hence, ladle is provided with a heat source which heats the liquid metal and compensates thermal losses. Thus, the furnace in which quality of liquid steel is improved by refining and heating in the ladle, is called ladle refining furnace. In the field of “Process Metallurgy”, refining the steel in ladle is also called “Secondary Metallurgy”.
2. How does LRF help produce better quality TMT bars/ steel thru the induction route?
TMT bars are produced as per the standard IS 1786. The standard mentions chemical composition and associated physical properties to produce quality steel. Sulphur and phosphorous are two crucial elements in steel to control. Both these elements, if present in higher proportion, reduce ductility of steel and create inclusions. With the help of LRF, the steel is refined in terms of S and P in the induction route. Additionally, LRF treatment helps float inclusions in steel and remove dissolved gases. All these operations help produce better quality TMT bars / steel. High quality TMT bars, specially of 500D or 550D grade, mostly needed for infrastructure projects, can be easily produced thru the induction furnace + LRF route.
3. Companies claim that if Iron Ore is used as raw material, then Induction furnace with LRF will produce steel quality ( construction grade TMT bars) at par with Blast furnace. Is this correct?
Yes, this is correct. To understand this, we have to study the process of steelmaking of both the routes. The blast furnace route steelmaking may be described as BF BOF LRF CCM Rolling. Blast furnace receives iron ore as a raw material and reduces it to produces high carbon hot metal. The hot metal is refined in BOF with the use of oxygen gas. Direct injection of oxygen gas into the metal refines the steel in terms of C, Si, Mn, P, etc. but also increases oxygen level in the bath to as high as 800-850 ppm. This act leads to form various metallic and non-metallic inclusions in the liquid steel. To remove the dissolved gas, to float inclusions and to reduce S, the steel is taken to LRF and treated to produce clean steel. Final chemistry adjustment is done at LRF station. Thus, use of LRF is inevitable for BF/BOF steel making route. During the process of BOF operation, large amount is heat is generated due to oxidation of various elements. This increases the bath temperature. To reduce this temperature, about 10-15% scrap is added in BOF as a coolant.
Induction furnace route of steelmaking may be described as Rotary kiln (sponge iron) Induction furnace LRF CCM Rolling. Induction furnace accepts sponge iron as raw material to produce steel. The charge mix may contain sponge iron as high as 85-90%. Sponge iron is produced using iron ore. Along with sponge iron about 10-15% pig iron is added in induction furnace. Pig iron is produced using iron ore. High carbon bath is required to take care of unreduced iron oxide present in the sponge iron. Once the melting is over, steel is taken to LRF station where it is refined in terms of sulphur and phosphorous. LRF also helps float inclusions and remove gases. At the end of LRF treatment, final chemistry is obtained by adding ferro alloys and the liquid steel is sent to the caster.
It is important to note that BOF inevitably requires about 10-15% steel scrap in steel making process where as in induction furnace route of steelmaking, use of steel scrap can be completely avoided. Steel scrap is not harmful for steelmaking but, sometimes bad quality scrap may create the problem of tramp elements.
Looking at above processes and equipments involved in steelmaking, it can be said that in both the routes, BF and IF, the steel at CCM is reached after processing at LRF. Hence, the steel produced using both the routes are same in quality and there is no difference.
4. If LRF as a refining technology has existed for so long, why was it not used extensively by the induction steel producer? Why suddenly we are seeing more; more companies producing steel though the IF route using LRF?
Yes, Ladle Refining Furnace (LRF) technology has existed for a very long time; the steel produced both in the Blast Furnace and Electric arc furnace route is also refined in the LRF. The steel in the induction furnace route till about 15 years back wasprimarily produced using scrap as the raw material. Steel produced using scrap typically has low Sulphur; Phosphorus and therefore there was no need to further bring Sulphur; Phosphorus down using LRF. Also the induction route TMT was of Fe415/ Fe500 grade of IS 1786 which allows for higher sulpher and phosphorous levels.
However since around 2003, we are seeing increased use of Sponge Iron (Iron Ore) as the raw material for Induction furnace. This results into higher Sulphur; Phosphorus levels in the steel, typically more than the specified limits of IS 1786 ( even Fe500) . Thus, there is a need to bring Sulphur; Phosphorus down if steel ( TMT bars as per IS 1786) is produced using Sponge Iron as the raw material. With the focus of the Government on building critical infrastructure in the country including Ports, Bridges, Dams etc and more ; more construction of high rise building in urban areas, we are seeing a shift towards the use of 500D grade TMT bars.
These bars have much lower sulphur and Phosphorus limits ( .04 max each ) and therefore the refining of steel to bring the sulphur and Phosphorus down becomes almost mandatory ( especially if the raw material is iron ore ). That is the reason plants using sponge iron as major raw material have started installing LRF while those using scrap as major raw material may still continue to be without LRF for some more time.