There has been a rising demand in recent years for TMT rebars with higher strength and better ductility for use in different types of applications. The latest IS 1786: 2008 has addressed these requirements and incorporated changes that were necessary in order to meet latest technological developments in manufacturing steel bars and steel wire.
Following are some of the important modifications incorporated in the fourth revision:
- Fe 600 - a new strength grade has been introduced.
- For elongation, two categories for each grade have been introduced (except for Fe 600).
- 'Percentage total elongation at maximum force' has been introduced as a new parameter.
- Nominal sizes have been rationalized.
|Chemical Composition of Fe 600 bars|
|Sulphur and Phosphorus||0.075|
For any grade, including Fe 600, it is not imperative to add micro alloying elements. In case of strengthening elements like Nb, V, B, Ti being used individually or in combination, the total contents shall not exceed 0.30 percent.
|SL No.||Property||Fe 600|
|1||0.2 percent proof stress/yield stress, Min., N/sq. Mm||600|
|2||Elongation, percent, Min., on gauge length 5.65xSq root of A, where A is the cross sectional area of the test piece.||10|
|3||Tensile Strength, Min.||6 percent more than the actual 0.2 percent proof stress/yield stress but not less than 660.0 N/sq. mm|
The durability of concrete structures depends on how well the concrete can withstand chemical and physical aberrations and protect its embedded reinforcement against corrosion. Environmental exposure, especially to air salinity, impurities in ground water and presence of acid particles in the air are common factors that cause corrosion. The CRSD (Corrosion Resistance Steel with Fe500D properties) rebar from Tata TISCON is an answer to this problem and has been a major innovation breakthrough to help builders and architects.
Tata TISCON CRSD rebars come with a special protective chemistry that creates a shield against corrosion. This protects the life of RCC structures like buildings, bridges, dams and industrial works.
|Tata TISCON CRS - Mechanical Properties|
|IS:1786 Fe500D||Tata TISCON CRSD *|
|Yield Stress, YS (min, N/mm2)||500||500|
|Ultimate Tensile Strength, UTS (min, N/mm2)||565||580|
*Typical values for 90% of batches
CRSD - Process Technology
The innovative technology used in making Tata TISCON CRSD rebars involves diminishing the Phosphorus (P) content in steel by selecting corrosion resistant elements. Produced using Thermo Mechanical Treatment (TMT) route Tata TISOCN CRSD rebars are compliant with the chemical composition of Fe500D grade. Corrosion resistant qualities in the resulting microstructure are increased due to:
- Elimination of residual torsional stresses
- A thin oxide film developing on the steel surface that helps plug holes in the oxide layer
- Presence of self-tempered lathe martensitic layer on surface that prevents corrosion attack
- Consistent spreading of corrosion resistant elements from core to surface
|Chemical composition of Tata TISCON CRS (in %)|
|Carbon (max)||Sulphur (max)||Phosphorus (max)||Copper (min)|
Advantages of Tata TISCON CRSD
- Greater Corrosion resistance makes it more long-lasting
- High yield strength together with better ductility and bendability
- More convenience in material handling and transportation
- No extra operations required during fabrication
- Easy to weld
- Can be bend and rebend using very small mandrel
Tata Tiscon 500d, a product from Tata steel, is the new generation high strength ribbed reinforcement bar. It is different from ordinary bars in its method of manufacturing and consequently, in its combination of properties.
Tata Tiscon 500D rebars are 'hot rolled' from steel billets and subjected to PLC controlled on-line thermo-mechanical treatment in three successive stages:
The hot rolled bar leaving the final mill stand is rapidly quenched by a special water spray system. This hardens the surface of the bar to a depth optimised for each section through formation of martensitic rim while the core remains hot and austenitic.
(b) Self Tempering
When the bar leaves the quenching box, the core remains hot compared to the surface, allowing heat to flow from the core to the surface, causing tempering of the outer martensitic layer into a structure called 'Tempered Martensite' The core still remains austenitic at this stage.
(c) Atmospheric Cooling
This takes place on the cooling bed where the austenitic core is transformed into a ductile ferrite-pearlite structure. Thus the final structure consists of an optimum combination of a strong outer layer (tempered martensite) with a ductile core (ferrite-pearlite). This gives TATA TISCON 500D its unique combination of higher strength and ductility.
|Chemical Composition of 500D bars|
|Chemical properties||Unit||IS:1786 Fe 500||IS:1786 Fe 500D||Tata Tiscon 500D*|
|Carbon||%||0.300 max||0.250 max||0.250 max|
|Carbon equivalent (C + Mn/6)||%||0.420 max||0.420 max||0.400 max|
|Sulphur||%||0.055 max||0.040 max||0.035 max|
|Phosphorus||%||0.055 max||0.040 max||0.035 max|
|S and P||%||0.105 max||0.075 max||0.070 max|
* As obtained in 90% of the heats
|Mechanical properties||Unit||IS:1786 Fe 500||IS:1786 Fe 500D||Tata Tiscon 500D*|
|Yiels Stress(YS)||N/mm2||500 min||500 min||540 min|
|Ultimate Tensile Strength (UTS)||N/mm2||545 min||565 min||600 min|
|UTS/YS)||Ratio||1.08 min||1.10 min||1.12 min|
|Elongation||%||12 min||16 min||18 min|
* As obtained in 90% of the heats