Steel Making Notes
Steel Making
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1. Introduction to Steel Production
Steel making is the process of producing steel from iron ore and scrap. Steel is an alloy of iron and carbon, with the carbon content typically between 0.2% and 2.1% by weight. Modern steel production involves two main routes: the Basic Oxygen Furnace (BOF) route and the Electric Arc Furnace (EAF) route.
Key Fact
Steel is the world's most important engineering and construction material, with global production reaching over 1.8 billion metric tons annually. About 70% of steel is produced via the BOF route, while 30% comes from EAFs.
Correct Answer: Steel is an iron-carbon alloy with controlled carbon content
The key distinction is that steel is an alloy of iron with controlled amounts of carbon (typically 0.2-2.1%) and often other alloying elements, while pure iron or pig iron has higher carbon content and more impurities.
2. Steel Production Routes
There are two main routes for steel production, each with distinct characteristics and applications:
Basic Oxygen Furnace (BOF)
- Uses 70-80% liquid hot metal (from blast furnace) and 20-30% scrap
- Oxygen is blown through the molten metal to reduce carbon content
- Process takes about 40 minutes per heat
- Produces high-quality steel for automotive and construction
- Higher capital costs but lower operating costs
Electric Arc Furnace (EAF)
- Uses almost 100% scrap metal (or alternative iron sources)
- Electric arcs melt the scrap (temperatures up to 3000°C)
- Process takes about 90 minutes per heat
- More flexible for specialty steel production
- Lower capital costs but higher energy costs
Correct Answer: Electric Arc Furnace (EAF)
EAFs use almost 100% scrap metal as their raw material, while BOFs use primarily liquid hot metal from blast furnaces (70-80%) with only 20-30% scrap.
3. Basic Oxygen Furnace (BOF) Process
The Basic Oxygen Steelmaking process is the dominant steel production method, capable of producing 200-400 tons of steel in a single heat lasting about 40 minutes.
Charging
Scrap metal (20-30%) is loaded into the furnace first, followed by hot metal (70-80%) from the blast furnace at about 1300°C.
Blowing
A water-cooled lance blows 99.5% pure oxygen at supersonic speeds onto the molten bath. This initiates several reactions:
- Decarburization: C + ½O₂ → CO
- Silicon oxidation: Si + O₂ → SiO₂
- Phosphorus removal: 2P + 5O → P₂O₅
Flux Addition
Lime (CaO) is added to form slag with impurities:
- SiO₂ + CaO → CaSiO₃ (slag)
- P₂O₅ + 3CaO → Ca₃(PO₄)₂ (slag)
Correct Answer: 0.04-0.08%
The BOF process reduces carbon content from about 4% in hot metal to 0.04-0.08% in the final steel product. Further adjustments can be made during secondary metallurgy.
4. Electric Arc Furnace (EAF) Process
The Electric Arc Furnace is the primary method for recycling steel scrap, accounting for about 30% of global steel production.
Furnace Charging
Scrap steel is loaded into the furnace using a large bucket. Different grades of scrap are carefully selected to achieve the desired steel composition.
Melting
Three graphite electrodes create electric arcs with temperatures up to 3000°C, melting the scrap within 50-60 minutes. Chemical energy is often supplemented by oxy-fuel burners.
Refining
Oxygen is lanced to remove impurities, and fluxes are added to form slag. The carbon content is adjusted by adding carbon or oxygen blowing.
Energy Efficiency
Modern EAFs consume about 350-400 kWh per ton of steel, compared to 20-25 GJ/ton for integrated steelmaking (BF-BOF route). The use of scrap reduces energy requirements by 70-75% compared to primary production.
Correct Answer: Lower energy consumption and CO₂ emissions
EAFs use about 70-75% less energy than BOFs because they melt scrap rather than reduce iron ore, resulting in significantly lower CO₂ emissions (0.4-0.6 tons CO₂/ton steel vs 1.8-2.2 tons for BOF).
5. Secondary Metallurgy Processes
After primary steelmaking, most steel undergoes secondary metallurgy to achieve precise composition and quality requirements.
Ladle Furnace
- Provides precise temperature control
- Allows for composition adjustments
- Enables deoxidation and desulfurization
- Typical treatment time: 30-45 minutes
Vacuum Degassing
- Removes hydrogen to prevent flaking
- Reduces nitrogen and oxygen content
- Essential for high-grade steels
- Processes: RH, VD, VOD
Correct Answer: Vacuum oxygen decarburization (VOD)
VOD combines vacuum treatment with oxygen blowing to achieve carbon contents below 0.01%, which is essential for producing ultra-low carbon stainless steels and electrical steels.
6. Continuous Casting
Continuous casting revolutionized steel production by directly converting molten steel into semi-finished products (slabs, blooms, billets).
Tundish
Molten steel from the ladle flows into a tundish that distributes it to the molds while maintaining steady flow and removing inclusions.
Water-Cooled Mold
The mold gives the initial shape and forms a solid shell while the interior remains liquid. Mold oscillation prevents sticking.
Secondary Cooling
Water sprays complete solidification while supporting the strand with rollers. Cooling rates affect microstructure and quality.
Yield Improvement
Continuous casting improved yields from 80-85% with ingot casting to 95-98% by eliminating ingot molds and primary mills, while also reducing energy consumption by about 30%.
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