What is difference between Thermo Mechanically Treated bars and Cold Twisted bars?

COLD TWISTED BARS: The Cold Twisted Deformed Bars (CTD Bars) and the Thermo Mechanically Treated Bars (TMT Bars) are rolled in the same process i.e. hot rolling in a temperature of 1200oC to 1000oC drop. CTD Bar is entered into cooling bed from the final pass (roll) then kept for natural cooling and being twisted in the room temperature. The bar is stretched to some extent during twisting by which, the inside grains are in crystle forms, are crushed and thereby, the bars attains the required strength TATA TISCON rebars are 'hot rolled' from steel billet and subjected to PLC controlled on line thermo - mechanical treatment in three successive stages: (a) Quenching - The hot rolled bar leaving the final mill stand is rapidly quenched by a special water spry 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 to the surface causing tempering the outer martensitic layer into a structure called Tempered Martensite. The core still remains austenitic at the stage. (c) Atomospheric Cooling - This takes place on the cooling bed where austenitic core transformed into ductile ferrite pearlite structure. Thus the final structure consists of an optimum combination of strong outer layer (tempered martensite) with ductile core (ferrite pearlite). This gives TATA TISCON its unique combination of higher strength and ductility.