Steel Rebar Production process is initiated by charging the scrap used as raw material into the electric-arc furnaces. The charged raw materials are turned into liquid steel by being melted down in the electric-arc furnaces. The steel in the liquid state is then transferred to the ladle furnaces and the necessary alloying procedures are completed in here to obtain the targeted quality.
In the final stage, the liquid steel is transferred to the continuous casting machines and turned into steel billets (semi-finished goods) in required sizes. Upon assessment of the quality control analyses, steel billets are approved and delivered to the rolling mills.
Rolling Mill Process
Steel billet is charged into the re-heating furnace to obtain the required rolling temperature prior to its delivery to the rolling stand. The heating process is a part of the procedure for acquiring the required mechanical properties. After the cooling bed, where the material is quenched down to the annealing temperature, the bars are cut into the required lengths for the completion of mechanical and chemical laboratory tests and then packed in bundles for dispatch.
Objectives of the QTR Process
• To create a layer of martensite at bar surface with a predetermined depth by partial water quenching.
• To tamper the crude martensite formed at the surface by residual heat flowing from core to the surface during final air cooling.
Three Stages of the Process
Steel bar leaving the last stand of the rolling mill is submitted to a special heat treatment involving three stages.
1. Water Quenching Process
The first stage consists of drastic water cooling applied to the bar it leaves the last finishing stand. The efficiency of water cooling equipment used in this stage has to be so high in order to produce a very hard cooling on the rod surface, faster than the critical rate of martensite, thus obtaining a surface layer of crude martensite.
2. Tempering Stage
In the second stage, the bar leaves the quenching line and is exposed to the air. The heat flow from rod core heats up the quenched surface by conduction and the martensite formed in the first stage is therefore subjected to self-tempering which assures adequate ductility while maintaining high yield strength levels.
3. Final Cooling Stage
The third stage occurs on the roller conveyor and it consists of a semi-isothermal transformation of the non-transformed austenite at the bar core.