Critical Infrastructure drives rising demand for “D” Grade TMT Bars
The consumption of steel in India is on the rise. With the increasing requirement for developing reliable critical infrastructure projects and top quality high-rise constructions, the requirement of high-quality steel, especially construction-grade TMT bars, is also registering an ever-increasing demand.
Owing to these two factors, there has been a shift in the demand for construction-grade TMT bars. While the normal 500/550 grade steel TMT bars were commonly used earlier, at present the more ductile 500D/550D grade steel TMT bars have seen a significant surge in demand.
This is because specific properties of 500D and 550D grade steel TMT bars makes them highly suitable for constructing stronger, more stable infrastructure.
Both the 500D and 550D TMT bars have lower levels of Sulphur and Phosphorus specifications in BIS (0.04max, 0.04 max respectively), which makes them stronger and resilient to wear and tear than the previously used grades of steel.
Based on this the IS: 1786 Defines Fe500 and Fe500D as two different grades of TMT bars.
The overall difference in the chemical and physical properties of Fe500 and Fe500D grades of TMT bars are as follows:
As per IS:1786, Fe500 steel contains S+P 0.105% collectively and individually 0.055% each. On the other hand, Fe500D steel contains S+P 0.075% collectively and individually 0.04% each. The Elongation norm for Fe500 steel is 12% and Fe500D steel is 16%.
Fe500D TMT bars are also known as “D” grade TMT bars. In both Fe500D and Fe550D, “D” stands for ductility.
Due to their different properties, it has been found that in some applications it is better to use “D” grade TMT bars against normal TMT Bars.
The structures which require more ductility such as skyscrapers, stadiums, flyovers, airports, malls, multiplexes, dams, etc. demand “D” grade TMT bars.
The “D” grade TMT bars are more suitable in these conditions because of these special properties.
“D” grade TMT bars are hence much more reliable for the purpose of constructing such large and heavy structures so that they may withstand maximum stress and undergo the least possible damage.
Even Developers of critical infrastructure projects today prefer to use “D” grade TMT bars. The reason for this is that “D” grade TMT bars are very suitable for use in construction of ‘Critical Infrastructure’. Let us consider the structures that are defined as ‘Critical Infrastructure’.
Normally, the term critical infrastructure is used by the government to describe assets that are essential for the functioning of a society and economy. Commonly associated facilities for ‘Critical Infrastructure’ are hospitals, road and transport, water supply systems, schools and colleges, aviation, etc.
The buildings required for these projects have to be strong, long-lasting and safe. It is also important that they should stand still and remain stable in the event of natural calamities such as earthquakes and floods.
This may only be ensured by using high strength TMT bars with sufficient ductility. Fe500D or Fe550D TMT bars impart the required strength to the structure and meet the requirements necessary to ensure public safety. Hence, 500D or 550D grade TMT is preferable for use in developing critical infrastructure in India at present.
Another important property that the steel used to make best TMT bars for construction is that it should have lower Sulphur (S) and Phosphorus(P) levels.
S and P, both these elements are detrimental to the quality of steel. Both of them reduce the ductility of steel and increase brittleness. Additionally, Sulphur also leads to the defect ‘hot shortness’ wherein material tears off during hot rolling.
Phosphorous, on the other hand, is usually responsible for the issue known as ‘cold shortness’ which increases ductile brittle transition temperature. Hence, these elements should be as low as possible in the steel which is being used to make Fe500D and Fe550D TMT bars.
Can Sulphur and Phosphorus be reduced when steel is produced through the Induction Furnace route?
Induction furnaces use a silica lining. Silica is acidic in nature. Dephos and Desulph require high basicity slag, which is not possible to achieve in a Silica lined Induction furnace.
Hence, both Dephos and Desulph are carried out in the Ladle Refining Furnace (LRF).
In the field of steelmaking, ladle refining is known as secondary metallurgy. LRF is regularly used for desulphurization, final chemistry adjustment, inclusion floatation, removal of dissolved gases, temperature increment, etc. LRF is operated with highly basic reducing slag.
Dephosphorization of steel requires basic and oxidizing slag. Such slag making is not possible in induction furnace as it is operated with acidic lining. In such conditions after melting is over, phosphorous can be removed from steel by making basic and oxidizing slag in the ladle as it uses basic lining. Once dephos is over, the slag is removed from the ladle and the routine LRF process can be carried out. This is commonly called double slag practice. Thus, when LRF is coupled with the induction furnace, additional dephos operation can be carried out prior to desulph operation. This technology is also called ELdFOS process.
Using this IF – LRF route of steelmaking, one can reduce the phosphorous content from 0.1% to 0.03% and the Sulphur content from 0.07 to 0.03%. This requires an average process time of 45-50 minutes in the case of normal construction grade ( IS:1786 ) steelmaking.
Steel producers producing steel through Induction Furnace + Ladle refining route claim that their Best Quality TMT bars in India is equivalent to that of TMT bars produced through the BF-BOF route. Let us consider how far this claim is right.
The process flow of blast furnace route to produce TMT as per IS:1786 is blast furnace > Basic Oxygen Furnace (BOF) > Ladle Refining Furnace (LRF) > Continuous Casting Machine > Rolling mill > TMT bars.
In the blast furnace route refining in terms of C, P and Si is carried out in BOF. Reduction of Sulphur, removal of gases and chemistry adjustment is carried out in LRF. From LRF the material of required chemistry with quality is obtained and it is sent to the caster.
However, it is important to note that only by using an induction furnace alone, dephos is not possible. To produce the desired quality of steel, the induction furnace is coupled with a conventional ladle refining furnace where Dephos and desulph is carried out. The process flow to produce Best TMT Bars as per IS:1786 in an induction furnace is as follows – Melting in Induction Furnace > Refining in Ladle Furnace with double slag practice > Continuous Casting Machine > Rolling mill > TMT bars.
In this method of production, the steel that goes to the caster is refined using a ladle furnace in terms of P and S, and gas content. The final chemistry is obtained at the LRF station. In both the routes i.e. BF-BOF as well as IF, the steel that goes to the caster is refined using a Ladle Refining Furnace. This makes sure that the quality of steel in both routes remains the same. This may be expressed as Q (BF) = Q (IF)