Doststeels | MANUFACTURING

State-of-the-art Processes

Steel billets are heated in a reheating furnace and rolled through a sequence of rolling stands which progressively reduce the billet to the final size and shape of the reinforcing bar. The THERMEX Process takes over and the bar is subjected to heat treatment in three successive stages. The first stage of ‘QUENCHING’ begins when the hot rolled bar leaves the final mill stand and is rapidly quenched by a special water spray system. This converts the surface layer of the bar to a hardened structure called ‘Martensite’ while the core remains austenitic. The second stage of ‘Self tempering’ begins when the bar leaves the quenching box with a temperature gradient through its cross section, the temperature of the core being higher than that of the surface.

This allows heat to flow from the core to the surface, resulting in tempering of the surface, giving a structure called ‘Tempered Martensite’ which is strong and tough. The core is still austenitic at this stage. The third stage of ‘Atmospheric Cooling’ takes place on the cooling bed, where the austenitic core is transformed to a ductile ferrite-pearlite core.

Bar Quenching Process

Difference between bar quenching process and conventional process

The DSL Bar Quenching Process

The DSL Bar Quenching Process In Bar quenching process, the rebar properties are attained by regulating the microstructure of a strong tempered martensite ring with a ductile core which enables the use of lower carbon and manganese contents. Combination of this microstructure and lower alloy content gives better rebar ductility, more strength and good weld ability. Then rapid Air-cooling on the automatic cooling bed at lower temperatures results in a thin and compact mill scale on the rebar surface.

Conventional Process

In the conventional Hot Rolling Process, the properties are achieved by regulating the chemical composition. Use of higher carbon and manganese content though increase the strength but may impair rebar ductility and / or weld ability. The slow air-cooling results in a thick but flak mill scale layer on the rebar surface. The microstructure of the rebar is ferrite – paralytic, typical of any medium carbon steel.

5 Stages of Manufacturing

The manufacturing process comprises of 5 main stages

Furnace
The reheating furnace heats the billets up to 1150o Celsius for optimum rolling of the billets into rebars.
Stands
The 18 horizontal and vertical rolling stands then roll the square billets into perfect round bars by the end of the last stand, before quenching.
Pomini Quenching System
PLC based Pomini Quenching System douses the bar with controlled water pressures over specified areas. This integral process ensures the best quality of the finished product as quenching causes the atomic structure within the steel bar to re-align for the best ratio of strength and elongation.
Cooling Bed
The natural air draft cooling bed is the hottest part of the process line where the finished rebar is allowed to attain ambient temperature over time.
Bundling and Binding
Automatic bundling and binding station wraps the bars into finished bundles, ready to be lifted by magnetic crane and onwards to dispatch.

Water Treatment Plant

Dost Steels Ltd houses its own water treatment plant which recycles the entire water being used for all stages of manufacturing. This plays a big role in saving our environment and ensures waste particles such as iron oxide and oil sludge do not mix into our precious surroundings causing them harm.

DSL also uses a Heat Recuperator for efficiency and recycling of all waste heat and exhaust flue gases to be recovered and reused in conjunction with fresh inputs. This greatly reduces the negative environmental impact due to the steel bar manufacturing process.