Latest developments on steel front
January, 1999
THE advent of thermo mechanically treated (TMT) steel bars and corrosion resistance steel (CRS) has heralded a new era of economy in the construction industry in India. Round plain steel ruled over this industry up to sixties while Tor steel took over the scene in seventies and maintained its supremacy till nineties. 1997 and 1998 has now seen an extensive use of TMT steel and corrosion resistance steel thereby ushering in greater economy and longer life for RCC structures.
Reinforced cement concrete (RCC) today has become an integral part of every structure, be it a multi-storeyed building, a tunnel, a flyover, a TV tower, a bridge, a nuclear reactor, an industrial unit or a residential apartment. With RCC so much coming under use, the engineers are constantly striving to devise ways and means to reduce its cost and bring economy in construction. Among its constituent materials, RCC has steel as the costliest material. Therefore, the focus has been more on steel than on any other constituent material.
Steel alone accounts for 30 to 40% of the cost of RCC. It can be in the form of mild steel, medium tensile steel, high tensile steel, deformed bars, high tensile steel wires, hard-drawn steel wire fabric, twisted steel fabric and so on but the mild steel and the deformed bars have remained under most common use.
Initially, conventional mild steel used to be put as reinforcement in RCC works. In 1967, tor steel (trade name for deformed bars) was introduced in India and due to its advantageous properties, it completely replaced the mild steel except a few situations where acute bending was required in higher diameters of steel bars. Tor steel, called grade Tor-40, with its characteristic strength of 415N/sq mm proved to be much economical than the conventional mild steel having a strength of 250 N/sq. mm only. While there has been a very small difference in their price-range, Tor steel is almost 65% stronger than mild steel and thus directly affects economy in RCC works.
Since 1967, about 40 million tonnes of steel has been saved in India only because of replacement of mild steel by the Tor-40 steel. This occurred because the quantity of Tor-40 steel used to be almost 40 per cent lesser than that of mild steel for the same structure. At the present cost of steel, the saving amounts to Rs 50,000 crore. Working out the figures for the whole of the world is a mind — boggling exercise.
Research and development continued even after the discovery of Tor-40 steel. Aim has been to discover still better quality of steel without ignoring the fact that steel for reinforcement has to be basically a low-carbon steel and an alloy of iron and carbon. Higher the yield strength of steel, lower will be the steel requirement and thus lower will be the cost of steel and RCC. Efforts in this direction have borne fruit. Recently, more superior grades of steel have been discovered.
Superior grades of steel thus developed are known as Tor-50 and Tor-55. These grades with their characteristic strengths of 500 N/sq mm and 550 N/sq mm, respectively are much stronger than Tor 40 grade steel and mild steel. Use of Tor-50 and Tor-55 steel permits a larger spacing among bars avoiding congestion as these bars possess a higher yield strength and a lesser number of them is required.
While Tor-40 steel brings a saving of 35% cost when used in place of round steel, Tor-50 and Tor-55 grades can bring an additional saving of 10% and 14%, respectively. For a four storeyed building, the saving that can be made by using superior grades of steel is shown in table.
Thermo mechanically treated steel known as TMT steel can be described as a new-generation-high-strength steel having superior properties such as weldability, strength, ductility and bendability meeting highest quality standards at international level. Under thermo mechanical treatment of bars, the steel bars are made to pass through a specially designed water cooling system where these are kept for such a period that outer surface of bars becomes colder while the core remains hot. This creates a temperature gradient in the bars. When the bars are taken out of the cooling system, the heat flows from the core to the outer surface causing further tempering of steel bars thereby helping them in attaining higher yield strength of steel.
To decide the percentage of carbon content in steel has been a major challenge for the engineers. While a certain minimum carbon content in steel is essential to achieve the required strength, an excess of carbon content threatens its property of weldability. In TMT bars, this problem has been eliminated. In these bars, the carbon content can be restricted to 0.2% to attain weldability and at the same time no strength is lost on this account. The joints can be welded by ordinary electrodes and no extra precautions are required. SAIL and RINL (Rastriya Ispat Nigam Limited) are doing a good job in producing TMT bars.
Another advantage of TMT bars is their tough surface providing high yield strength and a soft core providing excellent ductility. Strength, weldability and ductility are such properties which declare TMT steel highly economical and safe for use. An additional advantage of TMT steel is that a twisting operation is included in Tor steel, which subjects the bars to torsional stresses making them less corrosion resistant while TMT bars are free of such stresses thus having superior corrosion resistance.
A new development on steel front is production of corrosion resistant steel, called CRS. Carbon content in this steel is restricted to 0.18%, Mangnese is absent, silicon is 0.45% and the percentage of corrosion resistant elements such as chromium is as high as 1.5%. The smaller is the diameter of steel, the greater are the chances of its getting corroded. Corrosion of steel occurs mostly due to chloride ion effect or carbonation. Instead of conventional method of providing metallic coatings to steel, a change in the metallurgy of steel itself is finding its way now. SAIL and TISCO have taken great strides in this direction.
Further research towards production of fusion bound epoxy coated reinforcement steel and corrosion resistant low alloy steel is under way. To meet with the increasing demand of electrical industry SAIL has intensified its efforts to produce sophisticated silicon steel. Interesting results in this direction are expected soon.
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