Steel is an essential element of any civil or construction activity like Houses, Multi Storeys, Townships, Malls, Airports, Ports, Dams, Bridges etc. Steel not only provides strength to any construction but also provides durability and long life.
Transformation in Steel Industry
Each transformation of steel industry was driven by the growing market need for steel bars with higher strength and elongation. The 21st century has witnessed a rapid increase in the cost of steel products and also brought greater focus on safety and quality norms of construction. This has spurred the demand for development for TMT bars with properties of high strength and elongation to provide cost savings in the construction by reducing consumption.
Steel researchers across the world have been working hard to develop steel products to meet these specific market needs. This research has led to development of TMT steel Bars with yield strength of 500 N/mm²,
550 N/mm² and 600 N/mm². These bars, because of higher strength than traditional steel bars, are required in lesser quantity, thus, reducing the overall cost of structure significantly.
The BIS code for IS 1786 (High Strength Deformed Steel Bars And Wires for concrete Reinforcement Specification), IS 13920 (Ductile Detailing of Reinforced concrete structures subjected to Seismic forces – Code of Practice) and IS 456 (Plain And Reinforced Concrete – Code of Practice) incorporated necessary changes to include these latest developments in the field of steel technology.
IS 1786 under its latest revision groups the Steel Bars under the following grades :
a) Fe 415, Fe 415D, Fe 415S
b) Fe 500, Fe 500D, Fe 500S
c) Fe 550, Fe 550D
d) Fe 600
Reinforcing steel is produced through one or combination of processes like hot rolling after micro alloying, hot rolling followed by cold working or hot rolling followed by Thermo Mechanical Treatment (TMT process).
TMT Technology
TMT technology involves rapid cooling of hot rolled bars rolled out of a finishing rolling mill from a temperature of 950-1050°C to 550-600°C using high pressure cool water spray system. The bars after quenching are cooled under controlled atmospheric conditions to impart properties based on size and grade being rolled. The three major steps of TMT process are as under :
QUENCHING : The hot rolled bar coming out of the finishing rolling mill stand is passed through a quenching box where it is rapidly quenched using high pressure cool water spray system. The surface of the bar gets hardened to the depth optimized for each
1. section through formation of ‘Martensitic’ rim while the core remains hot and ‘Austenitic’.
2. SELF TEMPERING : The bar coming out of the quenching box thus has a hot core compared to the cool outer surface thus allowing the heat to flow from the inner core to the outer surface causing tempering of the outer martensitic rim into “Tempered Martensitic” structure. The core still remains “Austenitic” at this stage.
3. ATMOSPHERIC COOLING : The bar is then finished on a Automatic rake type controlled Cooling Bed where uniform atmospheric cooling of each bar takes place under controlled conditions resulting in the transformation of the hot “Austenitic” core of the bar into “Ferrite Pearlite” structure. The resultant final structure of the Bar thus produced consists of a hard and strong outer layer (Tempered Martensitic) with a soft and ductile inner core (Ferrite Pearlite) structure.
