Raw Material Preparation
Sourcing: The primary raw materials are iron ore, coal (transformed into coke), and limestone (or dolomite).
Beneficiation: Iron ore undergoes a beneficiation process to increase its iron content and remove impurities. Fines are agglomerated into pellets and sinter for efficiency.
Coke Production: Coal is converted into coke, which serves as a fuel and a reducing agent in the blast furnace.
Iron Making (Blast Furnace)
Iron ore, coke, and limestone are charged into a blast furnace.
Hot air is blown into the furnace, burning the coke and producing carbon monoxide.
Carbon monoxide reduces the iron ore to molten iron (also known as "hot metal" or pig iron).
Limestone reacts with impurities to form slag, a lighter molten waste product that floats on top of the molten iron and is removed.
Steel Making (Primary and Secondary Refining)
Primary Steelmaking: Molten iron from the blast furnace is transferred to a steelmaking furnace, typically a Basic Oxygen Furnace (BOF) or an Electric Arc Furnace (EAF).
BOF: Oxygen is blown through the molten iron, oxidizing carbon and other impurities, which are removed as slag or gases. Scrap steel may also be added.
EAF: Scrap steel (and sometimes direct reduced iron) is melted using high-power electric arcs.
Secondary Steelmaking: The raw steel then undergoes further refining to achieve precise chemical compositions and remove any remaining impurities. This can involve techniques like:
Ladle Refining: Heating and stirring the steel in a ladle to distribute alloying elements evenly and remove impurities.
Vacuum Degassing: Removing dissolved gases from the steel.
Alloy Addition: Specific alloying elements (e.g., manganese, chromium, nickel) are added to give the steel desired properties like strength, ductility, and corrosion resistance.
Continuous Casting
The refined molten steel is poured into a continuous casting machine.
It flows into water-cooled molds, where it rapidly solidifies into continuous strands.
These strands are then cut into semi-finished shapes like billets (smaller square cross-section) or blooms (larger cross-section). Billets are typically used for producing bars.
Hot Rolling
The billets are reheated in a furnace (typically to 1000-1250°C, or 1832-2282°F) to make them malleable.
They are then passed through a series of rolling mill stands. These stands have progressively smaller and shaped rollers that reduce the cross-section of the billet and elongate it, forming the desired shape and size of the steel bar. This process is crucial for establishing the basic physical properties and dimensions of the bars.
Thermo-Mechanical Treatment (for TMT Bars)
This is a critical stage for producing TMT bars and involves three key steps:
Quenching: As the hot-rolled bar exits the final rolling mill stand, it is rapidly cooled by a water spray system (often called a "Thermex System"). This rapid cooling hardens the outer surface of the bar, forming a hard martensitic rim, while the inner core remains hot and austenitic (ductile).
Self-Tempering: After quenching, the bar leaves the water spray system. The heat from the still-hot inner core transfers to the hardened outer surface. This self-tempering process tempers the martensitic layer, making it less brittle and forming a 'tempered martensite' structure. The core remains austenitic at this point.
Atmospheric Cooling: Finally, the bars are laid on a cooling bed and allowed to cool naturally in the ambient air. During this gradual cooling, the austenitic core transforms into a ductile ferrite-pearlite structure. The final product has a unique microstructure: a strong, tough outer layer (tempered martensite) and a soft, ductile inner core (ferrite-pearlite). This combination gives TMT bars their characteristic high strength and flexibility.
Finishing Processes
Straightening: The cooled bars are straightened to ensure dimensional accuracy.
Cutting to Length: The bars are cut to specified lengths using shears or friction saws.
Surface Treatment (Optional) Depending on requirements, bars might undergo surface treatments like pickling or coating.
Quality Inspection: Throughout the entire process, stringent quality control measures are implemented. Samples undergo chemical analysis, mechanical testing (tensile strength, bend tests), and non-destructive inspection (e.g., eddy current flaw detection) to ensure compliance with international standards and customer specifications.
Bundling and Packing: The finished steel bars are bundled and prepared for dispatch.
