Ladle Refining Furnace : A state-of-the-art method for producing high quality construction grade steel specially with Iron ore as the raw material
Much has been said about the efficacy of the Ladle Refining Furnace in the production of construction grade steel. In fact, though it is by no means a new technology, it has recently found more favour with steel manufacturers who have embraced the Ladle Refining furnace method with more enthusiasm than ever before.
Let us consider the reasons for the increased popularity of the Ladle Refining Furnace.
What is ladle refining furnace?
In the process of steelmaking, once the steel is made in the basic oxygen 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. Additional heat is required to carry out these operations. Hence the ladle is provided with a heat source which heats the liquid metal and compensates thermal losses. Thus the furnace in which the quality of liquid steel is improved by refining and heating is called a ladle refining furnace (LRF).
In the field of “Process Metallurgy”, refining the steel in the ladle is also called “Secondary Metallurgy”.
LRF helps to produce better quality steel in the induction route
Best TMT bars in India are produced as per the standard IS:1786. The standard mentions the right chemical composition and associated physical properties to produce construction quality steel. Sulphur (S) and phosphorous (P) are two crucial elements in steel whose proportions need to be controlled to make high quality steel. Both these elements, if present in higher proportions, reduce the 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 removes dissolved gases. All these operations help produce better quality TMT bars / steel.
High quality TMT bars, specially of 500D or 550D grade bars, which are mostly needed for large scale infrastructure projects, can be easily produced through the induction furnace + LRF route.
LRF helps in improving the quality of steel
The LRF refined steel process is efficient in making superior quality steel making via:
Desulphurization of steel
Removal of oxide inclusions to make steel cleaner
Homogenization of the temperature as well as of the composition of molten steel by gentle stirring
Degassing of steel through inert gas purging to achieve low gas content (oxygen, nitrogen and hydrogen)
Saving on Ferro Alloys consumption by better recovery and maintaining composition in narrow range
Improving grain refinement in the microstructure
Increase in productivity as melting furnace gets emptied earlier at low temperature
Acting as a buffer by holding molten steel for sequencing on the CCM
Induction furnace with LRF is at par with Blast furnace Route
To understand this, we have to study the steelmaking process of both the routes.
The blast furnace route steelmaking may be described as BF > BOF > LRF > CCM > Rolling. The Blast furnace receives iron ore as a raw material and reduces it to produce high carbon hot metal. The hot metal is refined in BOF with the use of oxygen.
Direct injection of oxygen 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 leads to the formation of 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.
The final chemistry adjustment is done at LRF station. Thus the use of LRF is inevitable for BF/BOF steel making route.
During the process of BOF operation, a large amount of 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.
The 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. A 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, the 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 the induction furnace route of steelmaking, use of steel scrap can be completely avoided. Steel scrap is not very harmful for steelmaking but sometimes bad quality scrap may create the problem of tramp elements.
Considering the processes discussed above and equipments involved in steelmaking, it can be said that in both the routes, BF and IF, the steel at CCM if is reaching after processing at LRF, then the steel produced using both the routes will be of the same quality and there will be no difference.
More companies are now producing steel through the IF route using LRF
The Ladle Refining Furnace (LRF) technology has existed for a very long time and the steel produced both in the Blast Furnace and Electric arc furnace route is also refined in the LRF.
Till about 15 years ago the steel in the induction furnace route was primarily 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/ Fe 500 grade of IS:1786 which allows for higher sulphur and phosphorous levels.
However since around 2003, we are seeing increased use of Coal based Sponge Iron (produced using Iron Ore) as the raw material for Induction furnace. Normally iron ore is responsible for P in steel and coal is responsible for S in steel. 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 ( for 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 and dams, the 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 ( 0.04 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 more and more plants using sponge iron as major raw material have started installing an LRF along with induction furnace.
At present, the LRF is the most effective method of producing high quality construction grade steel. Through the induction route specially when iron ore is used as raw material.
It is the ideal method for making high quality steel for producing “D” grade TMT bars as per the IS:1786 norms.