Around 1970, cold twisted deformed (CTD) was developed in Europe and sold under the brand name TOR steel. Many mills in Europe adopted this process with some automation of twisting and bundling facilities. However, this had a short life in Europe where it was developed. In this process, the yield strength is raised to desired levels by cold working or cold twisting but at the cost of ductility. The resultant total elongation values were a poor maximum of only 14% was specified for yield strength of 415 N/mm2 and still lower for the desired level of 500 N/mm2. Thus Europe was forced to abandon use of CTD bars within a few years of its development by about 1974 after which quenching technology was developed and subsequently hot rolled bars have been in use all over the world since the last 40 years.
TOR has also been a major reason for the scale of destruction in Pakistani natural disasters and calamities. A prime example of this is the 2005 earthquake in northern Pakistan which killed over a 100,000 people and left more than 3,500,000 people homeless. Had better quality of steel rebars been used, it just might have been possible for the scale of this destruction to tone down a bit.
Advantage of DSL Hot rolled bars over conventional cold twisted bars
High Strength and High Ductility
This combination results in added safety of the structure compared to very low safety in cold twisted steel bars due to low ductility.
Superior Bend Ability
The superior bend ability of the Dost Steels bar compared to a conventional cold twisted bar makes the bar easy to fabricate with.
High Corrosion Resistance
The precise control of the thermos-mechanical treatment process, results in a uniform and thick tempered martensite rim, completely free from internal stresses. This martensite rim improves the corrosion resistance of Dost Steels bar as well as the fatigue strength, when compared to traditional cold twisted bars.
Close Dimensional Tolerances
Dost Steels bar is supplied with mass/metre on the negative side of the specified tolerance on a weighted average basis. This further reduces consumption because of extra length per unit mass.
Better Heat Resistance
Dost Steels bar have better resistance to high temperatures in case of fire.
Superior Seismic Resistance
Due to its unique combination of strength and high ductility, Dost Steels bars carry higher fatigue resistance and can adequately support a seismic design. When evaluated for resistance to seismic loads on column joints in a RCC structure, the energy dissipation has been found to be almost same for each cycle indicating very high and uniform ductility maintained under such loads. Dost Steels bars meet the technical specification of new version of BS 4449-2005 (effective from January 2006) and ASTM 615A, ASTM 706A that cover the seismic zones requirements. This caters to the increasing demand of high ductility requirements in re-bars for Pakistan because nearly 75% of the country falls in the high seismic hazard category.
Dost Steels bars are higher strength rebars with elongation implying that you can economize on steel consumptions by about 15% without sacrificing safety. Secondly due to ease of working time is also saved at construction time
Enhanced Resistance to Discoloration and Rust
It is the change in colour of thin surface oxide layer due to atmospheric exposure without any negative effects on the material properties. Discoloration effects a thin surface layer only without penetration into the material and does not affect the main characteristic of the bar.
It is the result of severe reaction between material and the atmosphere leading to degradation of material properties. It results from the interaction of the rebar material with the corrosive environment and its effect penetrates into the material and thus degrades its properties.
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