What is the difference between Mild steel, high yield strength deformed bars, cold twisted deformed bars and QST bars?
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Steel reinforcing bars are used to enhance the tensile strength of concrete though concrete is weak in tension, and its because both have the same coefficient of thermal expansion. Also, structural steel is used as such to construct steel buildings, which is more easier in erection and is having less strength to weight ratio compared to concrete. Also steel have 100% scrap value.
MILD STEEL REINFORCING BARS
The mild steel bars are smooth and round in shape having diameter ranges from 6mm to 50mm used for small scale construction works.
Mild steel is also known as low carbon steel having 0.1% to 0.25% of carbon. But mild steel is not considered as an alloy steel though it doesn’t contain other elements in it compared to different steel varieties. As carbon content increases, ultimate strength and hardness will increase, but ductility and toughness decreases. Though mild steel contains very little carbon, it is ductile and weldable but having low strength. Also, mild steel is highly prone to corrosion.
HIGH YIELD STRENGTH DEFORMED BARS
High yield strength deformed bars (HYSD) are manufactured under heat treatment followed by either heat rolled or cold twisted for shaping.
HYSD bars moderately resistant to corrosion and the manufacturing process itself cause surface flaws in it. It is heavily weighted, and its transportation cost is high. Residual stress is high and tensile stress is low in HYSD bars that cause deformation.
THERMO-MECHANICALLY TREATED BARS
TMT bars are having much more advantages than hysd bars and nowadays hysd bars are not using.
TMT bars are having a hardened outer core and soft inner core. These bars are passed through water after they get hot rolled, which makes the surface more firm, and the inner core stays warmer itself. Hence core became more ductile, and no deformation process employed like in hysd makes it more strengthened. TMT bars are having soft ferrite-pearlite core with a hard martensitic rim.TMT bars help to reduce steel consumption to 8 to 11% compared to hysd bars. TMT bars are having more corrosion resistive property and is highly flexible than hysd bars. It is highly earthquake resistant with high tensile strength and low residual strength. TMT bars can absorb about 400 to 600°C of heat and highly fire-resistant.
COLD TWISTED DEFORMED BARS
The specifications related to CTD bars are provided in IS: 1786–1985. They are commonly known as TOR steel bars.
TOR is a kind of high adherence steel. TOR bars have surface deformation formed by twisting the steel after lengthening. This imparts high yield strength to steel and furnishes legitimate proper bondage with concrete.TOR is a type of hysd steel, where the steel bars, subsequent to experiencing the required heat treatment process, are cold twisted and distorted.
QUENCHED AND SELF TEMPERED STEEL
These are very much similar to TMT bars where annealing is the process which is not conducted for QST steel manufacturing. These have adequate quality and malleability. QST steel bars are also having the same cross-sectional material of soft ferrite-pearlite core with a hard martensitic rim as that of TMT bars.